Late stage oxide growth associated with hydrothermal alteration of the Western Granite, Isle of Rum, NW Scotland

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1 Article Volume 12, Number 1 6 January 2011 Q01001, doi: /2010gc ISSN: Late stage oxide growth associated with hydrothermal alteration of the Western Granite, Isle of Rum, NW Scotland M. S. Petronis Environmental Geology Program, Natural Resources Management Department, New Mexico Highlands University, Las Vegas, New Mexico 87701, USA (mspetro@nmhu.edu) B. O Driscoll School of Physical and Geographical Sciences, Keele University, William Smith Building, Keele ST5 5BG, UK J. Lindline Environmental Geology Program, Natural Resources Management Department, New Mexico Highlands University, Las Vegas, New Mexico 87701, USA [1] The Paleogene Western Granite, Isle of Rum, NW Scotland, represents a westward tilted, shallow level, felsic intrusion associated with the early growth stages of the classic early Paleogene Rum Igneous Centre. Rock magnetic data reveal an east to west variability in magnetic properties across the intrusion that is also reflected in petrographic and geochemical observations. It is argued that the subtle intraplutonic variations in oxide grain size and composition observed in the Western Granite might be a common feature of shallow epizonal igneous intrusions. As these mineralogical differences are not easily detectable without detailed rock magnetic study, this research underscores the need for comprehensive rock magnetic studies when interpreting the postemplacement history of shallow level plutons. Paleomagnetic data indicate that the eastern part of the intrusion represents the shallower part of the exposed intrusion as further evidenced by roof pendants of Lewisian Gneiss and abundant drusy (miarolitic) cavities indicative of shallow emplacement. In contrast, the western part represents a deeper ( 900 m) part of the intrusion as evidenced by the lack of roof pendants and existence of only minor drusy cavities. Rock magnetic data indicate that the dominant magnetic phase is low Ti titanomagnetite with some titanomaghemite. Magnetic grain sizes range from pseudosingle domain in the west and multidomain in the east with a slight increase in Ti content of the titanomagnetite phases in the east as revealed by Curie point estimates. We interpret these data to indicate that late stage (subsolidus) hydrothermal deuteric fluids associated with emplacement of the Stage 2 Rum Layered Suite produced secondary growth of Fe Ti oxide mineral phases along silicate grain boundaries in the eastern part of the intrusion. The secondary oxide growth resulted in a slightly coarser magnetic grain size fraction and an increase in Ti content of the titanomagnetite grains. The deeper structural levels of the western part of the intrusion and its distance from the contact of the Rum Layered Suite inhibited secondary oxide growth, and thus, this part of the intrusion preserves a primary magmatic oxide phase of a smaller magnetic grain size and near pure magnetite chemistry. Components: 14,100 words, 12 figures, 3 tables. Keywords: Western Granite; rock magnetism; Isle of Rum; hydrothermal alteration; British Tertiary Igneous Province; magnetic grain size. Copyright 2011 by the American Geophysical Union 1 of 32

2 Index Terms: 1527 Geomagnetism and Paleomagnetism: Paleomagnetism applied to geologic processes; 1519 Geomagnetism and Paleomagnetism: Magnetic mineralogy and petrology; 1518 Geomagnetism and Paleomagnetism: Magnetic fabrics and anisotropy; 8414 Volcanology: Eruption mechanisms and flow emplacement; 8486 Volcanology: Field relationships (1090, 3690). Received 2 June 2010; Revised 11 October 2010; Accepted 29 October 2010; Published 6 January Petronis, M. S., B. O Driscoll, and J. Lindline (2011), Late stage oxide growth associated with hydrothermal alteration of the Western Granite, Isle of Rum, NW Scotland, Geochem. Geophys. Geosyst., 12, Q01001, doi: /2010gc Introduction [2] Progress has been made in understanding the origin of remanent magnetization and magnetic anisotropy of shallow igneous rocks. We remain challenged, however, to fully develop a complete model for the processes associated with the initial oxide cooling and growth, and synemplacement to postemplacement changes to the magnetic mineralogy. These changes are relevant first order processes for any igneous intrusion and can have a major impact on the magnetic fraction of a material, remanence preservation, and magnetic anisotropy. The Isle of Rum Western Granite (Scotland) preserves a primary, dominantly single component magnetization that reflects remanence acquisition during rapid cooling [Petronis et al., 2009] yet preserves evidence of both primary magmatic and secondary hydrothermal titanomagnetite grains grown during or shortly after late stage crystallization. The rock magnetic experiments carried out here indicate that only the eastern part of the intrusion experienced a secondary growth of Fe Ti oxide mineral phases, while the deeper part of the intrusion preserves the primary magnetic (magmatic) mineral phases. An important implication of this study is the degree to which hydrothermal alteration affected the rock magnetic properties of the Western Granite. The differences revealed in magnetic grain size, bulk susceptibility, and Fe Ti oxide composition between western and eastern portions of the intrusion could have easily been interpreted as two separate, non related igneous intrusions. Comprehensive rock magnetic study, however, elucidates the hydrothermal effect and highlights the sensitivity of the Fe Ti oxides to subtle changes in temperature and chemistry and therefore provide a means to gauge (both mineralogically and magnetically) the cooling and alteration history the rocks have experienced. Postemplacement alteration of the oxide fraction, if not fully recognized by detailed rock magnetic study, can lead to an erroneous interpretation of the primary emplacement geometry and influence the potential interpretation of remanence in these intrusions. Here we investigate several interrelated concepts associated with shallow magma emplacement and oxide genesis: (1) the cooling rate and primary oxide growth, (2) the preservation or modification of the primary oxide phase under conditions of moderate alteration, and (3) the effect a late thermal history and associated chemical changes have on the resultant magnetic mineralogy. This case study demonstrates how improved interpretations of igneous emplacement can be achieved when rock magnetic techniques are fully employed and combined with detailed textural and chemical analysis. [3] Rock magnetic data, low field susceptibility measurements, whole rock geochemical data, and reflected light petrography of the Western Granite reveal intraplutonic discordance in magnetic behavior, trace element compositions, and Fe Ti oxide phase relations. These observations prompted detailed rock magnetic studies at the Institute of Rock Magnetism (University of Minnesota, USA) to fully characterize the magnetic mineralogy, domain states, grain size distributions, and mineral composition of these rocks. The purpose of this study attempts to better understand the magnetic mineralogy of the Western Granite and what processes (i.e., magmatic, metasomatic, hydrothermal) influenced its character. We propose that the Western Granite experienced a late stage hydrothermal alteration event that affected the eastern and structurally shallowest part of the pluton resulting in the growth of new magnetic mineral phases. We argue that the likely cause of the alteration of the Western Granite is related to its proximity to the younger Rum Layered Suite; the contact between both intrusions makes up most of the eastern and southern parts of the former body. It is suggested, in agreement with previous studies [Greenwood et al., 1992], that significant hydrothermal alteration of adjacent 2of32

3 country rocks accompanied emplacement of the Rum Layered Suite. The results from this study show that subtle differences in magnetic mineralogy within a pluton can be used to reveal hydrothermal alteration processes that are difficult to recognize using standard petrographic and field observational methods. 2. Geological Setting [4] The Western Granite on the Isle of Rum (Inner Hebrides, NW Scotland) is the largest of several granitoid bodies around the margin of the Rum volcanic center, all of which are interpreted as part of the early felsic updoming and caldera collapse stage (Stage 1) that preceded the intrusion of the Rum Layered Suite (Stage 2 [Emeleus et al., 1996; Emeleus, 1997]) (Figure 1). A single 40 Ar/ 39 Ar age determination on biotite of ± 0.45 Ma provides the only age date constraining the emplacement of the Western Granite [Chambers et al., 2005]. Apparent geochemical similarities to rhyodacites and other rocks belonging to Stage 1 [Dunham, 1967; Donaldson et al., 2001] and crosscutting contacts with the younger Rum Layered Suite further support granite emplacement as early Paleogene. During Stage 1, central uplift on a major arcuate fault system, the Main Ring Fault (MRF) [Bailey, 1944], was accompanied by felsic and mixed felsic/mafic magmatism and the formation of a caldera which filled with felsic ash flows, tuffs and breccias [Emeleus, 1997]. The country rocks were strongly domed over the central complex, while uplift along the MRF brought masses of Lewisian gneiss and basal members of the Torridon Group to the present structural levels (Figure 1). Subsequent caldera subsidence resulted in the preservation of Jurassic sedimentary rocks and Paleocene basaltic lavas of the Eigg Lava Formation in the MRF (Figure 1). Shortly after motion along the MRF, several granites were intruded of which the Western Granite is the most voluminous (Figure 1). Emplacement of the Layered Suite (Stage 2) resulted in 15 of west side down tilting of the Western Granite as evidenced from recent paleomagnetic studies and anisotropy of magnetic susceptibility (AMS) data that reveal the intrusion has an overall domal shape [Petronis et al., 2009]. The likely ascent path of the magma was structurally controlled along the south to southeastern margin of the granite where it exploited a preexisting structure (Figure 1) [Petronis et al., 2009]. Excluding the variation in topographic relief, a simple calculation based on the inferred tilt of the intrusion and the distance between the eastern and western part of the pluton suggests that the western part represents a structural level 900 m deeper than the eastern part of the intrusion. 3. Western Granite [5] The texture and mineralogy of the Western Granite have previously been described by Emeleus et al. [1971], Emeleus [1997], and, more recently, by Petronis et al. [2009]. The section below is modified from Petronis et al. [2009] and the reader is referred to that paper for a more detailed description of the petrology. We report here only the salient observations. The Western Granite is largely a granophyric, fine grained granite dominated by quartz, alkali feldspar, and sodic plagioclase (combined quantities of up to 85 vol %). The granite texture varies considerably at the outcrop scale with abrupt grain size variations and frequent irregular concentrations of quartz and calcite filled drusy cavities [Emeleus, 1997]. Field mapping shows a central area in the northeast of the granite that contains abundant drusy cavities and finegrained rock, with the remainder of the granite being medium grained microgranite/granophyre lacking abundant drusy cavities [Emeleus, 1997]. Most quartz and alkali feldspar form radiate graphic intergrowths around plagioclase grains (Figures 2a and 2b), a feature observed in almost all samples examined in this and previous studies. Both feldspars usually exhibit turbid grain surfaces (Figures 2c and 2d), and partial alteration to aggregates of fine grained white mica is common. This effect is much more pronounced in the eastern part of the intrusion, while samples collected from the western part of the granite are fresher and do not display heavy alteration. Mafic phases are principally biotite and hornblende and make up less than 10 modal percent. The lithological variation within the Western Granite suggested that it was possibly a composite intrusion, although given the limited outcrop exposure, it is difficult to distinguish between coalesced magma bodies or patchy alteration of the intrusion based on hand sample and thin section observations alone. Importantly, no evidence of a fabric, magmatic or solid state, is observed in the field or in thin section, although a strong magnetic fabric carried by the shape pre- 3of32

4 Figure 1. The Isle of Rum. (a) Generalized geology of the Isle of Rum showing major rock types. (Modified from Petronis et al. [2009], after Emeleus [1997], reproduced by permission of the British Geological Survey NERC. All rights reserved. IPR/131 66CY.) (b) Simplified geologic map of the Isle of Rum depicting the major geologic units and structural features. (Modified from Petronis et al. [2009], after Emeleus [1994], with permission from Scottish Natural Heritage.) A A indicates approximate location of cross section shown in Figure 12. 4of32

5 Figure 2. Representative photomicrographs, taken in crossed polars, of typical Western Granite textures and mineralogy. (a) Coarse grained (euhedral) opaque oxide crystals in close proximity to altered biotite crystal, all set in a highly seriticized groundmass from a sample collected in the west of the intrusion. (b d) Images illustrating the intergrowths of quartz and plagioclase that comprise the typical granophyric texture of the Western Granite. The feldspar in these intergrowths is usually completely altered to fine grained aggregates of micaceous minerals. In Figure 2b, the small, highly birefringent crystals at the center of the image are zircon, from a sample in the west of the intrusion. In Figures 2c and 2d, note the euhedral outlines of highly altered (seriticized) alkali feldspar (in the case of Figure 2c, the crystal on the right has relict simple twinning present). ferred orientation of magnetite grains is preserved throughout the granite [Petronis et al., 2009]. 4. Magnetic Oxide Mineralogy [6] Characterization of the magnetic mineralogy in the Western Granite and its spatial variation across the exposed parts of the intrusion are the main focus of this paper. The Fe Ti oxide mineral phases provide important constraints on the cooling rate, temperature, and chemistry of the magma due to unstable ferrous ions within the melt [Lindsley, 1991]. These phases are often good indicators of system changes and can reflect the igneous and hydrothermal history as revealed by their composition, morphology, and textural relationships to the silicate mineral phases [Lindsley, 1991]. By examining the oxide minerals with reflected light microscopy, it is often possible to identify and constrain the thermal and alteration history which a particular rock has undergone [e.g., Speer and Becker, 1992, and references therein]. In turn, combining these observations with detailed rock magnetic data provides a powerful tool to fully characterize the cooling history and postemplacement alteration of the intrusion [e.g., Alva Valdivia et al., 2001; Lagrou et al., 2004; Uehara et al., 2010]. The distributions of opaque grains in the Western Granite are somewhat typical of many felsic to intermediate intrusions where subequant grains ( microns (mm) in size) occur predominantly as interstitial material in the granite silicate framework in volume concentrations of 0.5% 2.0% (Figures 2a and 3a). The opaque grains are typically Fe Ti oxides and are often very closely associated spatially with biotite and hornblende crystals [Black, 1954; Emeleus, 1997] (Figure 3b), a textural relationship seen in other shallow igneous intrusions [Speer and Becker, 1992; Stevenson et al., 2007]. The grains occur as roughly equidimensional crystals and as long slender rods up to 5of32

6 Figure 3. Reflected light photomicrographs of Western Granite samples. (a) Sample from the east of the intrusion containing high concentrations of fine grained apparently randomly distributed titanomagnetite. (b) Sample from the west of the intrusion illustrating relatively euhedral magnetite crystals in close spatial association with biotite. (c and d) Enlargement of ilmenite crystals, with characteristic exsolution texture (see text for details), in granite samples from the east of the intrusion. 100 mm in size and as very fine material distributed throughout the groundmass (Figure 3a). Based on petrographic inspection and energy dispersive X ray analysis [Petronis et al., 2009], Fe Ti oxides include an abundance of titanomagnetite and titanomaghemite that in some cases have altered to hemoilmenite or ilmeno hematite, and rare, unoxidized magnetite. The distribution of these phases is highly irregular. In some samples the rocks contain locally very high concentrations of oxides and in others irregular distributions of exclusively fine, less than 10 mm oxide grains localized along silicate grain boundaries. Many relatively coarse magnetite grains show evidence of apparent high temperature oxidation exsolution and the formation of abundant ilmenite lamellae with a classic trellis oxidation pattern (Figures 3c and 3d). This intergrowth pattern is identical in appearance to the well known Widmanstatten texture in iron meteorites. The oxyexsolved lamellae show sharp, well defined contacts with their titanomagnetite hosts and appear smooth in outline. The ilmenite lamellae are typically concentrated along cracks, around silicate inclusions, and along the titanomagnetite grain boundaries. These textural features are often interpreted as evidence that subsolidus oxidation and not magmatic oxidation exsolution is responsible for the formation of ilmenite lamellae from a primary titanomagnetite phase [Lindsley, 1991, see references therein]. These observations are discussed more fully below. 5. [7] Fourteen whole rock samples from the Western Granite were analyzed for major and trace elements via energy dispersive X ray fluorescence spectrometry at Colorado College, USA. Sample preparation consisted of reduction of each to a fine powder. For trace element analysis, an aliquot was compressed into a circular pellet; for major element analysis, an aliquot was fused into a glass disc with 6of32

7 Table 1. Whole Rock Major and Trace Element Geochemical Data for the Western Granite a Eastern Sites Western Sites Location WG1 WG3 WG10 WG23 WG30 WG34 WG13 WG15 WG16 WG18 WG20 WG21 WG27 WG29 SiO 2 (wt %) TiO Al 2 O FeO MnO MgO CaO Na 2 O K 2 O P 2 O Total LOI Rb (ppm) Sr Y Zr Nb La Ce Pr Nd Sm Gd Dy Yb Zn Ba Th a Major elements normalized to 100%. the admixture of a binder. Major and trace element data are presented in Table 1 and Figure 4. The Western Granite is peraluminous (mole percent oxide ratio Al 2 O 3 /CaO+K 2 O+Na 2 O > 1.1) to marginally metaluminous. Samples vary from intermediate to felsic compositions with whole rock SiO 2 ranging from to wt %, though the majority of samples have SiO 2 values greater than 68%. The chemistry of the Western Granite corresponds to a subalkaline, tholeiitic suite with FeO T / (FeO T +MgO) > 0.87 and has Rb Y+Nb and Nb Y variations characteristic of within plate granites. As a whole, it shows a narrow range in major element compositions, excepting Na 2 O and FeO T, which range from 1.76 to 4.05 and 2.80 to 7.29 wt %, respectively. The Western Granite displays a narrow range in trace element variability, as demonstrated on an ocean ridge granite normalized trace element diagram. Likewise, the suite displays a narrow range in rare earth element concentrations and overlapping chondrite normalized rare earth element patterns. A few trace elements, Ba, Rb, and Zr, occur in higher concentrations in the western portion relative to the eastern portion. These elements are compatible with alkali feldspar (Ba and Rb) and zircon (Zr), which were major fractionating phases (Figure 4a). Likewise, several elements, notably TiO 2, occur in lower proportions in the west relative to the east. [8] The samples show a wide range of loss on ignition values ( wt %), signifying variable hydrothermal alteration. We developed an isocon diagram [Grant, 1986] to evaluate hydrothermal chemical gains and losses between western and eastern portions of the Western Granite (Figure 4i). We used samples of intermediary Ba, Rb, and Zr content to eliminate fractionation and mineral accumulation influences. We selected sample WG20 to represent unaltered (C O ) Western Granite (from the west) and WG34 (C A )to represent altered Western Granite (from the east). Data for the major and minor components are plotted as oxide weight percents, while trace elements are plotted in 0.1 ppm. An isocon is drawn for constant alumina and constant silica through the origin. Most elements form a linear array along the isocon, suggesting they were immobile during hydrothermal alteration. However, as Figure 4i illustrates, some elements were geochemically decoupled during a hydrothermal event. Na 2 O, Zr, and Zn were lost 7of32

8 Figure 4. Geochemical data. (a) SiO 2 versus mole percent oxide ratio Al 2 O 3 /(CaO+Na 2 O+K 2 O) for the Western Granite. Alumina saturation classification of Shand [1947]. Open squares indicate western samples; open circles indicate eastern samples. Western Granite samples plotted on Irvine and Baragar s [1971] classification diagrams: (b) weight percent total alkalies (Na 2 O+K 2 O) versus SiO 2 and (c) ternary Alk FeO MgO plot of weight percent Na 2 O+K 2 O, FeO+0.9*Fe 2 O 3, and MgO dividing subalkaline rocks into tholeiitic and calc alkaline fields. Symbols are as in Figure 4a. (d and e) Granite tectonic discriminant diagrams after Pearce et al. [1984]. Trace elements are in ppm. Abbreviations are as follows: VAG, volcanic arc granite; ORG, ocean ridge granite; syn COLG, syncollision granite; WPG, within plate granite. (f) Selected Western Granite major element oxide (weight percent) and trace element (ppm) variation diagrams as a function of weight percent SiO 2. (g) Chondrite normalized rare earth element patterns for Western Granite samples. Chondrite values taken from Boynton [1984]. (h) Ocean ridge granitenormalized patterns for Western Granite samples. Ocean ridge granite values from Pearce et al. [1984]. (i) An isocon diagram comparing Western Granite samples WG20 (west) and WG34 (east). Oxides are plotted in weight percents, and trace elements are plotted in 0.1 ppm. An isocon is drawn for constant alumina and constant silica through the origin. Most elements form a linear array along the isocon, suggesting they were immobile during hydrothermal alteration. Except for alumina and silica, the labeled data points highlight elements that were gained or lost during hydrothermal alteration. C A, altered Western Granite; C O, unaltered Western Granite. 8of32

9 Figure 4. (continued) during hydrothermal alteration, while Sr and TiO 2 were gained. 6. Rock Magnetic Analytical Methods 6.1. Field Sampling [9] Samples were collected as oriented drill cores from 27 sites throughout the Western Granite using a gasoline powered drill with a nonmagnetic diamond tipped drill bit. All samples were collected from rocks that are confined to a relatively consistent topographic level within the intrusion and oriented using a magnetic and, when possible, a sun compass. At each site, from six to fourteen independent samples (typically eight) were drilled over an area of about 25 m 2. In the laboratory, each core sample was cut into 2.2 by 2.5 cm right cylinder specimens, using a diamond tipped, nonmagnetic saw blade with up to three specimens per sample obtained Rock Magnetic Experiments [10] To characterize the magnetic mineralogy, standard rock magnetic experiments were conducted at the University of New Mexico (UNM) and New Mexico Highlands University (NMHU) paleomagnetic rock magnetism laboratories. Equipment used at the UNM lab included a KLY 4S Kappabridge, 2G Enterprise superconducting rock magnetometer, Model SSM 1A Schonstedt spinner magnetometer, Model GSD 1 Schonstedt AF demagnetizer (modified to allow tuning of input current), dtech ARM unit, and home built static impulse magnets capable of 1 to 3 T peak fields. At the UNM lab, tests included (in sequential fashion): AMS, remanence studies, DC acquisition of a saturation isothermal remanent magnetization (IRM), DC demagnetization of the saturation IRM to yield a back field IRM (coercivity of remanence), and three component thermal demagnetization of IRM acquired in different fields. Equipment used at the NMHU laboratory included a MFK1 A multifunction Kappabridge with a CS4 attachment. At the NMHU laboratory, progressive reheating low field susceptibility versus temperature experiments from room temperature to 450 C were conducted. In addition, we carried out a suite of experiments at the Institute of Rock Magnetism at the University of Minnesota. These experiments utilized the Princeton Measurements Vibrating Sample Magnetometer and the KLY2 Kappabridge with a CS2 attachment to measure susceptibility versus temperature. Hysteresis experiments, using the Princeton Measurements Vibrating Sample Magnetometer were conducted at low (10 K to 100 K), room (273 K), and high (273 K to 800 K) temperatures. A brief description and purpose of each experiment is presented below Demagnetization Behavior [11] Alternating field (AF) and thermal (TH) demagnetization experiments provide a wealth of information on the magnetic mineralogy and magnetic domain size and reveal the presence of several 9of32

10 Figure 4. (continued) magnetic directional components. Orthogonal vector diagrams are projections of the horizontal and vertical components of the remanence vector during demagnetization and allow for multiple magnetic directional components to be resolved when their coercivity spectra do not fully overlap. The median destructive field (MFD) of the NRM during AF demagnetization provides information on the magnetic domain size, and the thermal unblocking temperature spectra during TH demagnetization aids with identifying the magnetic phase(s) carrying the remanence. 10 of 32

11 Figure 4. (continued) 6.4. Isothermal Remanent Magnetization Acquisition [12] The acquisition of isothermal remanent magnetization (IRM) is acquired through the application of stepwise increasing uniaxial fields until saturation is reached. The experiments provide an important nondestructive tool for the investigation of the coercivity spectrum of the magnetic minerals present within a sample [Dunlop and Özdemir, 1997]. For magnetite, multidomain (MD) grains are characterized by rapid acquisition and saturation at low applied fields while single domain (SD) grains require a higher field to reach saturation. SD grains are said to be magnetically hard whereas MD grains tend to be magnetically soft [Dunlop and Özdemir, 1997]. Thus, an IRM acquisition curve, and the associated back field IRM, provide information on both the dominant domain state of the magnetic fraction as well as the composition of the material. Low coercivity phases such as magnetite (Fe 3 O 4 ) reach complete saturation by 300 mt while high coercivity phase such as hematite (Fe 2 O 3 ) and pyrrhotite (Fe 7 S 8 ) do not saturate until beyond 2.5 T fields [Dunlop and Özdemir, 1997]. 11 of 32

12 6.5. Three Component Isothermal Remanent Magnetization [13] Three component IRM experiments involved inducing different applied fields along the X, Y, and Z axis of the specimen [Lowrie, 1990]. The applied fields are chosen at the limits of the coercivity spectrum from a soft magnetic phase, like magnetite, to a hard magnetic phase, like hematite. In these experiments, we applied saturating fields of 3.0 T along the x axis, 0.3 T along the y axis, and 0.03 T along the Z axis. The specimens are then thermally demagnetized. The thermal demagnetization of the three orthogonal components of the composite SIRM provides an estimate of the relative quantities and unblocking temperature spectra of the magnetic phases present Hysteresis Measurements [14] In an attempt to further characterize the magnetic domain structure, hysteresis loops at standard and variable temperatures were measured on representative samples to aid with magnetic mineral identification and magnetic grain size determinations. Hysteresis loop parameters are useful in characterizing the intrinsic magnetic behavior of materials and are helpful with defining the origin of the natural remanence. Loops for SD materials are typically wider than loops for MD materials which reflect the higher coercivity and remanence in SD materials. The saturation magnetization (Ms) is the largest magnetization obtainable for a given material and is thus a proxy for the total amount of magnetic mineral present. The saturation remanence (Mr) is the remanent magnetization of the material after the saturating applied field is removed. The coercivity (Hc) and remanent coercivity (Hcr) are measures of magnetic stability or magnetic hardness and is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after saturation magnetization. The two ratios, Mr/Ms and Hcr/Hc, are commonly used as indicators of domain states and, indirectly, grain size [e.g., Dunlop and Özdemir, 1997]. For magnetite, high values of Mr/Ms (>0.5) indicate ( mm) SD grains, and low values (<0.05) are characteristic of ( mm) MD grains. The intermediate regions are referred to as pseudosingle domain (PSD) grains with values from 0.5 to Hcr/Hc is a less dependable parameter, but normally SD grains have a value <2, and MD grains should have values >4 [Day et al., 1977; Dunlop, 2002]. In this study, hysteresis loops and the associated parameters Mr/Ms, Hc, and Hcr were obtained. For a few select samples, we also examined the change of hysteresis loop parameters during warming from 10 K to 970 K (at intervals of 10 K) Temperature Variation of Low Field Susceptibility [15] Continuous low field susceptibility versus temperature measurements from room temperature to 700 C allowed for an evaluation of the magnetic mineral composition based on Curie point estimates and assisted with revealing mixtures of magnetic phases within a sample. Curie points were determined by measuring low field susceptibility versus temperature using a Kappabridge with a CS high temperature attachment and by induced moment versus temperature with a Princeton Measurements Vibrating Sample Magnetometer. For low field susceptibility versus temperature experiments, Curie points were estimated using either the inflection point [Tauxe, 1998] or Hopkinson peak methods [Moskowitz, 1981]. To avoid oxidation and chemical alteration of the magnetic phases, all experiments were conducted in an inert Ar atmosphere. 7. Rock Magnetic Results 7.1. Demagnetization Behavior and Susceptibility [16] AF and TH demagnetization of the NRM reveals a well defined characteristic remanent magnetization (ChRM) that decays near univectorally to the origin with less than 10% of the NRM remaining after treatment in 120 mt fields or by 580 C (Figure 5a). We see no variation between the general NRM decay between sites from the west and east parts of the intrusion; however, we note that the average NRM intensity from sites in the east are slightly higher, 1.8 A/m, than sites in the west, 0.6 A/m (Table 2). In addition, average median destructive fields (MDF) from sites in the east are 29.6 mt and sites in the west are 33.4 mt, although these values overlap at one standard deviation (Table 2). Low coercivity phases are characterized by MDF of the NRM, when it is essentially single component in nature, between 15 mt to 20 mt and moderate to higher coercivity phases are characterized by MDF of the NRM between about 25 mt to 70 mt. Of the eleven accepted sites in the west, eight are of the PSD grain size of moderate coercivity while of the ten accepted sites in the east, only four are PSD (Figure 5b). In general, magnetic grain size changes from MD in the east to PSD in the 12 of 32

13 Figure 5. Demagnetization behavior. (a) Representative in situ modified demagnetization diagrams [Zijderveld, 1967; Roy and Park, 1974]. Solid (open) symbols represent the projection onto the horizontal (true vertical) plane. AF demagnetization steps are given in mt, and thermal demagnetization steps are given in degrees Centigrade. Typically, AF and thermal demagnetization results from two specimens of the same sample are shown for comparison for some samples. Diagrams are designated by a site number (e.g., WG13), method of treatment (AF or TH), and rock type. Intensity (A/m) is shown along one axis for each sample; each tick equals indicated intensity. (b) Representative AF demagnetization behavior showing the MDF of the NRM which varies from 20 mt to 50 mt. Reprinted from Petronis et al. [2009], Cambridge Journals, reproduced with permission. 13 of 32

14 Table 2. Bulk Low Field Magnetic Susceptibility, NRM Intensity, and MDF Data British Grid Ordnance Survey Site Rock Type Km SI (E 3) NRM (A/m) MDF (mt) Grain Size Latitude (deg) Longitude (deg) Elevation (m) Eastern Sites WG1 WGS md WG2 WGS psd WG3 WG psd WG4 WG md WG10 WG md WG11 WG psd WG12 WG md WG22 WG psd WG23 WG psd WG24 WG md WG30 WG psd WG31 WG md WG32 WG md WG34 WG md Western Sites WG13 WG psd WG14 WG md WG15 WG psd WG16 WG psd WG17 WG psd WG18 WG psd WG19 WG md WG20 WG md WG21 WG md WG25 WG psd WG26 WG psd WG27 WG psd WG29 WG psd west. The magnetic susceptibly magnitude provides an estimate of the amount of magnetic material present within a sample and information on the grain size of the magnetic phase. It is often assumed that SD grains have a lower susceptibility then PSD and MD grains, though this is not always the case [Dunlop and Özdemir, 1997]. Bulk magnetic susceptibilities range from 68.6 E 3 SI to 4.36 E 3 SI. A general discordance in susceptibility magnitude is revealed (Table 2) with sites in the west yielding lower average bulk susceptibility values (16.7+/ 11.7 E 3 SI) than sites in the eastern part of the intrusion (37.1+/ 21.9 E 3 SI), consistent with the AF demagnetization results of a coarser magnetic fraction present in the eastern part of the intrusion Isothermal Remanent Magnetization Acquisition [17] Isothermal Remanent Magnetization (IRM) acquisition and back field IRM curves show a narrow spectrum of responses with all samples yielding steep acquisition and complete saturation by 0.10 to 0.25 T (Figure 6a). All samples were saturated up to 2.5 T applied field but do not show any evidence of a high coercivity phase (e.g., pyrrhotite, hematite). Back field IRM curves also show a limited range with coercivity of remanence values between 0.01 T and 0.05 T (Figure 6b). These data are consistent with a low Ti titanomagnetite phase Three Component Isothermal Remanent Magnetization [18] Response to thermal demagnetization of threeaxis IRMs from all sites reveal that the highcoercivity (3.0 T) IRMs are typically of the highest intensity and that they are fully unblocked by laboratory temperatures between 540 C to 580 C (Figure 7). The z axis (acquired at 0.03 T) component for all samples does not show a significant response to thermal demagnetization. For sites in the west, the x axis (acquired at 3.0 T) and y axis (acquired at 0.3 T) components show a rapid decrease of intensity between 150 C to 350 C and are fully unblocked between 560 C to 580 C. For 14 of 32

15 Figure 6. Representative normalized (a) IRM acquisition J/Jo, with normalized intensity, and (b) back field IRM demagnetization curves. The experiments provide a nondestructive tool to investigate the coercivity spectrum of the magnetic minerals present [Dunlop and Özdemir, 1997]. For low coercivity phases such as magnetite, multidomain (MD) grains are characterized by steep acquisition and saturation at low applied fields, while single domain (SD) grains require a higher field to reach saturation, with complete saturation by 300 mt. High coercivity phases such as hematite (Fe 2 O 3 ), pyrrhotite (Fe 7 S 8 ), and greigite (Fe 3 S 4 ) do not saturate until well beyond 1.0 T fields. The IRM acquisition curve and the associated back field IRM provide information on both the dominant domain state of the magnetic fraction as well as the composition of the material. All samples show a narrow spectrum of responses with all curves showing steep acquisition and complete saturation by 0.10 to 0.25 T. All samples were saturated up to 2.5 T applied field but do not show any evidence of a high coercivity phase(s). sites in the east, the X and Y components steadily decrease in value with increasing temperature with a rapid decrease in intensity and full unblocking between 560 C to 580 C Hysteresis Measurements [19] Hysteresis properties were measured in three experimental setups: (1) at room temperature (298 K), (2) during heating from 298 K to 970 K, and (3) on warming from 10 K to 100 K. During the heating and warming experiments, data were typically collected at 10 K increments The 298 K Hysteresis Results [20] Hysteresis data yield ambient temperature values of saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc) and remanence coercivity (Hr) that are typical of the MD to PSD grain size and these grain sizes vary spatially from the eastern to western part of the intrusion. In general, all samples yield steep acquisition and 15 of 32

16 Figure 7. Representative thermal demagnetization curves of three component orthogonal IRM [Lowrie, 1990]. IRM imparted along the X, Y, and Z axes of the specimen in 3.0, 0.3, and 0.03 T fields, respectively. The fields were chosen to capture the most common ferromagnetic minerals (hematite, goethite, pyrrhotite, and titanomagnetites). The thermal demagnetization of the three orthogonal components of the composite SIRM provides an estimate of the relative quantities and unblocking temperature spectra of the magnetic phases present, providing an estimate of the composition of the mineral phase(s) present within the sample. All sites reveal that the high coercivity (3.0 T) IRMs are typically of the highest intensity and are fully unblocked by laboratory temperatures between 540 C to 580 C, consistent with a low Ti magnetite phase. reach complete saturation by about 300 A/m; however, the shapes of the curves are variable. We see no evidence of wasp waist behavior indicative of a mixture of magnetic grain sizes. For sites in the west, the hysteresis loops are broader compared to the sites in the east consistent with a change in magnetic grain size estimates (Figure 8). Ms and Mr intensities in the eastern sites are significantly higher than in the western sites (Table 2). Hc values are somewhat higher for sites in the east with Hcr 16 of 32

17 Figure 8. Hysteresis loops aid with magnetic mineral identification and magnetic grain size determinations, are useful in characterizing the intrinsic magnetic behavior of materials, and are helpful with defining the origin of the natural remanence. Loops for SD materials are typically wider than loops for MD materials. The saturation magnetization (Ms) is the largest magnetization obtainable for a given material and is thus a proxy for the total amount of magnetic mineral present. The saturation remanence (Mrs) is the remanent magnetization of the material after the saturating applied field is removed. The coercivity (Hc or Bc) and remanent coercivity (Hcr or Bcr) are measures of magnetic stability or magnetic hardness and are the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after saturation magnetization. The two ratios, Mrs/Ms and Hcr/Hc, are commonly used as indicators of domain states and, indirectly, grain size. (a) Room temperature (25 C) hysteresis loops. Hysteresis data yield ambient temperature values of saturation magnetization (Ms), remanent magnetization (Mrs), coercivity (Hc), and remanence coercivity (Hr) that are typical of the MD to PSD grain size, and these grain sizes vary spatially from the eastern to western part of the intrusion. For sites in the west, the hysteresis loops are broader compared to the sites in the east, consistent with a change in magnetic grain size estimates. Ms and Mrs intensities in the eastern sites are significantly higher than in the western sites (Table 2). Hc values are somewhat higher for sites in the east with Hcr values similar across the intrusion. (b) Day plot of hysteresis properties for representative samples. On a Day plot [Day et al., 1977], which compares the ratios of Ms/Mrs versus Bcr/Bc, the eastern sites fall in the MD grain size field, and the western sites plot within the PSD field. PSD, pseudosingle domain; MD, multidomain. Thick solid line is a theoretical mixing curve of MD and uniaxial single domain (SD) magnetite; dashed line is a mixing curve of MD and cubic SD magnetite. Points indicate the percentage of SD grains in the mixture [after Dunlop, 2002]. 17 of 32

18 Figure 8. (continued) values similar across the intrusion. On a Day plot [Day et al., 1977], which compares the ratios of Ms/Mr versus Bcr/Bc, the eastern sites fall in the MD grain size field and for the western sites plot within the PSD field (Figure 8b). The magnetic grain size estimates are consistent with the higher susceptibility observed for sites in the east versus those in the west. Sites in the east yield Bcr/Bc values between 3.29 and 6.08 and Mr/Ms values of and sites in the west yield Bcr/Bc values between 1.72 and 3.71 and Mr/Ms values of (Figure 8b). Coercive forces are in the range 14.8 to 50.4 mt. The Mr/Ms and Hc values of sites in the west are consistent with a smaller grain size than sites in the east and all results fall on the hydrothermal trend defined by several authors [see Dunlop and Özdemir, 1997]. Using data from Dunlop and Özdemir [1997] sites in the west yield a magnetic grain size between 0.5 mm to 0.1 mm and sites in the east are between 1.0 mm to0.8mm Heating From 298 to 970 K [21] Representative samples from two sites in the western and three sites in the eastern part of the intrusion reveal the following behaviors in Bc, Mr/ Ms, Ms versus temperature during heating. A plot of Bc versus temperature (Figure 9a) yields a general decrease in Bc with increasing temperature. The two representative samples from the western part of the granite yield room temperature Bc values between 20 to 24 mt that decrease rapidly during heating. One sample, WG29, yields an abrupt decrease in Bc between 350 to 550 K with an increase in Bc to 650 K before decaying steadily at the Curie point. A similar pattern is observed during heating of WG16, although the initial decrease is not as abrupt as in WG29. Excepting two samples that show a sharp decrease in Bc around 400 K, the remaining samples, all from the eastern part of the intrusion, yield room temperature B C values between 4 to 12 mt that decrease 18 of 32

19 Figure 9. High temperature variations of hysteresis properties (see Figure 8 for explanation of abbreviations). (a) Bc, (b) Mrs/Ms, and (c) Ms versus temperature. steadily at the Curie point. We note that the secondorder phase transition [Verwey, 1939; Verwey et al., 1947], the point at which Bc increases to infinity, for samples in the west occurs about 20 K less than sites in the east (Figure 9a). A similar behavior is observed in a plot of Mr/Ms versus temperature (Figure 9b). A plot of Ms versus temperature (Figure 9c) yields a steady decrease in Ms with increasing temperature for all samples. Two of the three samples from the east (WG30, WG1) yield room temperature Ms values of 4.25 A/m and 2.5 A/m, respectively. The remaining three sites have room temperature Ms values less than 1 A/m. In general, the decay in Ms for all sites is nearly 19 of 32

20 Figure 10. Low temperature variations of hysteresis properties (see Figure 8 for explanation of abbreviations). (a) Bc, (b) Mrs/Ms, and (c) Ms versus temperature. linear; however, sites WG30 and WG1 show an abrupt drop in Ms above 800 K. As observed with Bc, the Ms for the two sites from the west reach the phase transition at about 20 K lower than the sites from the eastern part of the intrusion (Figure 9c) Warming From 10 to 110 K [22] Six representative samples, three sites each from the west and east part of the intrusion, reveal the following behaviors in Bc, Mr/Ms, and Ms versus temperature experiments during warming from 10 K to 100 K (Figure 10). Four of the six sites yield near temperature independent behavior of Bc on warming (Figure 10a). Two sites (WG10 and WG13), however, show a strong temperature dependence of Bc on warming. On warming, WG10 initially increases slightly followed by a steady decrease to 100 K with only a slight increase in slope from 80 K to 100 K. WG13 yields a 20 of 32

21 complex warming curve with an initial rapid decrease in Bc from 10 K to 25 K followed by a steep increase between 25 K to 55 K with a plateau at 55 K to 65 K followed by a steady decrease in Bc between 65 K to 110 K. The results revealed by a plot of Mr/Ms versus temperature on warming indicate a near temperature independent behavior for four of six samples. Two western sites (WG13 Figure of 32

22 and WG16), yield a complex behavior characterized by a rapid drop in Mr/Ms from 10 K to 30 K. On continued warming, WG16 shows no temperature dependence on Mr/Ms, while for WG13 Mr/Ms increases up to about 60 K and then no further temperature dependence on warming to 100 K (Figure 10b). A plot of Ms versus temperature shows that sites WG26, WG24, WG1 yield temperature independent behavior; however, sites WG10, WG13, WG16 show a slight drop in Ms from 10 K to 50 K. On continued warming to 100 K, these samples show no change in Ms intensity (Figure 10c) Temperature Variation of Low Field Susceptibility [23] Low field continuous susceptibility versus temperature experiments yield a very narrow spectrum of response and reveal a slight increase in room temperature susceptibility of about 10 to about 30% after the complete cycling to 700 C with most samples fully reversible (Figure 11a). Using either the inflection point [Tauxe, 1998] or Hopkinson peak methods [Moskowitz, 1981], samples from the eastern sites yield inferred Curie points from 558 C to 566 C and western sites yield values between 562 C to 576 C, temperatures consistent with a low Ti titanomagnetite phase (Table 2). A few samples show an increase in susceptibility (a bump ) on heating between about 250 C to 350 C which is not present on the cooling curve suggesting the presence of a second ferromagnetic phase (Figure 11a). The bump may indicate that the oxidation of the Fe Ti oxide phase(s) is inhomogeneous, and during the heating experiment the mineral in some fashion homogenizes to a more susceptible phase as reflected by the modest increase in susceptibility [e.g., Hrouda et al., 2006] Progressive Repeated Heating Experiments [24] In order to more fully characterize the nature of the increase in susceptibility from 250 C to 350 C, we conducted progressive reheating experiments on two representative samples (WG10 and WG13; Figure 11b) [after Hrouda, 2003]. The bump between 250 C and 350 C was investigated to track the changes that occurred during heating by progressively heating and cooling a sample. All experiments were conducted in an argon atmosphere to limit the effects of oxidation during heating. This investigation involved the following procedure modified from Hrouda [2003]. The temperature variation of susceptibility was measured in the temperature interval from room temperature to 200 C and back to 40 C. In the second run, a new sample of the same volume was measured in the temperature interval from room temperature to 225 C and back to 40 C. The remaining runs progressively increase at 25 C intervals to a maximum temperature of 375 C following the same heating cooling procedure. Two additional steps were added from 400 C at 50 C intervals up 450 C. The relationship of the heating and cooling curves in individual runs was then evaluated. If the heating and cooling curves of a particular run are very near each other, it is likely that the heating in the respective temperature interval did not result in a change in the magnetic mineral phases. If the heating curves differ substantially, it is likely that the rock did undergo a change in the magnetic minerals or phases. During the heating experiment from 25 C to 250 C, all runs from the two samples Figure 11. Low field susceptibility versus temperature experiments. Continuous low field susceptibility versus temperature measurements from room temperature to 700 C allowed for an evaluation of the magnetic mineral composition based on Curie point estimates and assisted with revealing mixtures of magnetic phases within a sample. Curie points were estimated using either the inflection point [Tauxe, 1998] or Hopkinson peak methods [Moskowitz, 1981]. Pure magnetite has a Curie point of 580 C, which decreases near linearly with increasing Ti substitution to approximately 150 C for pure ilmenite. Curie points of other common minerals include hematite (675 ), pyrrhotite (320 ), and greigite ( 330 C) [Dunlop and Özdemir, 1997]. (a) Continuous low field susceptibility measurement during heating (gray lines) from 25 C to 700 C and cooling (black lines) to 40 C. Samples from the eastern sites yield inferred Curie points from 558 C to 566 C, and western sites yield values between 562 C and 576 C, temperatures consistent with a low Ti titanomagnetite phase. A few samples show an increase in susceptibility (a bump ) on heating between about 250 C and 350 C which is not present on the cooling curve, suggesting the presence of a second ferromagnetic phase (see Figure 11b). (b) Normalized low field susceptibility during progressive repeat heating experiments for samples WG10 and WG13. If the heating and cooling curves of a particular run are very near each other, it is likely that the heating in the respective temperature interval did not result in a change in the magnetic mineral phases. If the heating curves differ substantially, it is likely that the rock did undergo a change in the magnetic minerals or phases. Note that between 400 C and 450 C the mineral phase responsible for the bump has been fully altered to a new magnetic phase. 22 of 32

23 Figure 11. show an increase in susceptibility of 5% to 10% on cooling to 40 C (Figure 11b). The shape of the curves on heating up to 225 C are nearly reversible on cooling over the interval of 225 C to 150 C. On further cooling to 40 C, the susceptibility increased by 10% likely reflecting a slight mineralogical change to a more magnetic mineral phase. On heating beyond 250 C, the presence of the bump becomes strongly expressed with a plateau being reached as temperature increases. For WG13, the shape of the curve changes from a steep increase in susceptibility up to 250 C and a rapid decrease in susceptibility on further heating, likely reflecting the (continued) onset of mineral phase alteration. Though less pronounced, a similar pattern is observed for WG10. We argue that a mineralogical change occurs at or near 250 C for both samples. 8. Discussion 8.1. Reheating Low Field Susceptibility Versus Temperature Experiments [25] The conditions of the low field susceptibility versus temperature experiment often lead to changes 23 of 32

24 in the magnetic phases due to the partially oxidizing environment of the sample holder, even though the experiments are conducted in an inert Argon atmosphere. The oxidation likely results from the presence of residual O 2 clinging to the grains within the sample. Overall, the low field susceptibility versus temperature experiments from the Western Granite show a slight increase in susceptibility magnitude on cooling. The increase in susceptibility (5% to 10%) on cooling reflects that a new magnetic phase (in this case low Ti magnetite) is generated by heating of a less magnetic phase resulting in the shift of the cooling curve. This behavior is indicative of low temperature oxidation of a single phase titanomagnetite into two phases magnetite and ilmenite [Irving, 1970; Marshall and Cox, 1972; Johnson and Atwater, 1977]. The low temperature oxidation explains the slight increase in the susceptibility on the cooling curve [Readman and O Reilly, 1970, 1972]. Titanomaghemite is metastable above 300 C and inverts to the more stable structure of magnetite and ilmenite on further heating [Dunlop and Özdemir, 1997]. This inversion, or phase change, results in a thermomagnetic curve which is significantly different in shape during heating than it is during cooling, a type of curve described as irreversible and one that is a very good indicator of the presence of titanomaghemite [Hrouda, 1982]. As discussed above, most sites analyzed in the Western Granite yield nearly reversible thermomagnetic curves indicative of a single magnetic phase being present within the samples. In all cases, Curie points are consistent with a low Ti titanomagnetite phase. A few samples, however, show a bump in the heating curve around 300 C that is not present on the cooling curve indicating that a significant mineralogical change occurred during the heating experiment. For two samples, WG10 (eastern) and WG13 (western), we investigated the origin of this effect during a series of progressive reheating experiments from 200 C to 450 C to better understand the nature of this bump (Figure 11b). [26] On heating from 200 C to 450 C the progressive reheating experiment for WG10 is nonreversible with a 10% increase in susceptibility after heating to 200 C. The susceptibility continues to increase slightly as the temperature increases up to 300 C. This indicates that even low heating temperatures result in the growth of a slightly more magnetic phase. On heating to 300 C, the cooling curve diverges from the heating curve and a decrease in susceptibility is observed (Figure 11b). This looping continues and becomes more pronounced up to 400 C and with further heating the bump is completely removed. We interpret this behavior to indicate that between 400 C 450 C a complete breakdown of the mineral phase responsible for the bump has been altered. WG13 reveals a similar behavior on heating from 200 C to 450 C the reheating experiment is nonreversible with a 10% increase in susceptibility after heating to 200 C. Yet on heating beyond 200 C, the cooling curve diverges from the heating curve with a decrease in the susceptibility forming the loop as seen in WG10 (Figure 11b). However, the susceptibility on complete cooling decreases from 10% above room temperature susceptibility at 200 C to less than 2% on cooling back to room temperature. Similar to WG10, on heating above 400 C the bump is completely removed, which we attribute to a complete break down of the mineral phase responsible for the increased susceptibility. Between the interval of 200 C to 400 C, exsolution of the single titanomagnetite into the two phase system of magnetite and ilmenite likely occurs and explains the increase in susceptibility and the nonreversible nature of the cooling curve. As discussed by Hrouda et al. [2006], the origin of the bump may indicate that the oxidation of the Fe Ti oxide is inhomogeneous. During the heating experiment the mineral begins to homogenize at 200 C and is completely exsolved into two phases by 400 C. Alternatively, the Curie point of pyrrhotite (310 C) lies between the interval of 200 C to 400 C and it is possible that some samples might contain a quantity of this iron sulfide mineral. In addition, some of the rock magnetic data are consistent with pyrrhotite being present in some samples. For example, in WG13 a very marked change in Bc versus warming from 10 K to 100 K at 30 K could be consistent with the 33 K low temperature transition reported for pyrrhotite (Figure 9) [Dekkers, 1988; Rochette et al., 1990] and a similar drop in Mr/Ms at 30 K is apparent in WG16 and WG13. The drop in Bc and Mr/Ms during the hightemperature hysteresis experiments (Figure 8) could be consistent with the reported Curie temperature of pyrrhotite (310 C). Finally, the unblocking of pyrrhotite between 200 C to 400 C could explain some of the changes in the low field susceptibility during heating. Although it is possible that some pyrrhotite exists in the Western Granite samples, we have found no evidence for iron sulfides in thin section, we see no systematic pattern of pyrrhotite being present in the likely hydrothermally altered rocks (see below), and the majority of the rock 24 of 32

25 magnetic experiments point toward titanomagnetite to titanomaghemite as the dominate magnetic phase in these rocks. We interpret the results from the progressive reheating experiments to indicate the following: [27] 1. The conditions within the sample holder, regardless if it is flooded with Ar, result in a net oxidizing environment allowing mineralogical changes to occur. A method to eliminate the residual O 2 clinging to the grains might involve preparing the samples within a vacuum chamber; although it would be difficult to fully clean residual O 2 from the grains. [28] 2. The breakdown of the Fe Ti oxide phase into two phases during the heating experiment occurs over an established interval between 200 C to 400 C for the Western Granite samples. The separation into two phases is complete by 400 C. Although not explored in this study, it might be possible to design an experiment with variable composition titanomaghemite to investigate if progressive low field susceptibility versus temperature experiments could be used as a tool to fingerprint the Ti content of the oxide phase within a sample. We suggest that the bump would shift to lower temperatures as the Ti content of the titanomaghemite increases Changes in Oxide Phase Magnetic Mineralogy [29] The evidence for postemplacement hydrothermal alteration has been recognized in various igneous intrusions of contrasting tectonic settings [Torsvik et al., 1983; Lapointe et al., 1986; Just et al., 2004], yet it is often difficult to distinguish between single or multiple episodes of alteration using petrologic techniques alone [e.g., Craw and Findlay, 1984]. In some instances, the variation of magnetic susceptibility is almost entirely associated with the oxidation state of the Fe Ti oxide mineral phase(s) and not the total iron oxide content and/or its grain size [Lapointe et al., 1986]. Therefore, the variation in magnetic properties is interpreted to be directly related to the alteration level seen in the rock with highly altered rock having the lower magnetic signal. Just et al. [2004] show that it is possible to separate multiple hydrothermal alteration stages with careful study of the rock magnetic properties and the geometry and orientation of magnetic fabrics. In their study of the Soultz granite, they demonstrated a clear association of a primary late magmatic alteration of magnetite and subsequent alteration phases consisting of relict magnetite and newly formed hematite and related the growth of these phases to the tectonic stresses following emplacement [Just et al., 2004]. A paleomagnetic study of the Helmsdale granite [Torsvik et al., 1983], northeast Scotland, revealed a multicomponent remanence dominated by two characteristic axes of magnetization: a primary remanence direction likely acquired in Upper Silurian to Lower Devonian and a second magnetization component partly carried by hematite that apparently formed through alteration of biotite and plagioclase. These and numerous other studies of felsic intrusive igneous rocks show the importance of mixing rock magnetic studies with detailed textural analysis. [30] Rock magnetic data from the Western Granite reveal a spatial pattern in magnetic mineralogy variation from the shallowest to deeper structural levels of the exposed pluton. The question remains as to the likely cause of this change in character and composition of the oxide mineral phase assemblages. As discussed extensively by Lindsley [1991], the texture and composition of the Fe Ti oxide minerals are sensitive to magma cooling rate, temperature and liquid chemistry chiefly because of the unstable ferrous ions within these minerals. Lindsley [1991] points out that oxides are good indicators of the changes in these conditions and tend to reflect the igneous, metamorphic, and hydrothermal history based on their composition, morphology, and textural relationships with the silicate matrix mineral phase. The initial composition and temperature stability fields of titanomagnetite in felsic igneous rocks are generally well understood [Buddington and Lindsley, 1964; Carmichael and Nicholls, 1967; Petersen, 1976; Johnson and Melson, 1978]. The Fe Ti oxide minerals and their relationship with the silicate matrix provides a means to assess the thermal and alteration history experienced by the rock and it is often possible to then relate these processes back to the magnetic properties of the host materials as we are attempting to do in this study. Depending on the subsolidus cooling history of an igneous rock, primary titanomagnetite minerals can follow one of two paths. If the material cools relatively quickly, the titanomagnetite will remain as a homogeneous single phase crystal, of small magnetic grain size, and likely preserve a primary thermoremanent magnetization (TRM). As shown by Petronis et al. [2009], the Western Granite preserves a primary, dominantly single component magnetization that reflects remanence acquisition during rapid cooling (e.g., a TRM). Alternatively, if cooling is slow, the primary titanomagnetite 25 of 32

26 grains may undergo high temperature (>600 C) or deuteric oxidation to magnetite and ilmenite during postcumulus equilibration and result in a larger magnetic grain size fraction [Buddington and Lindsley, 1964; Haggerty, 1976]. Following the initial cooling history, and below temperatures of 600 C, two fundamental types of secondary alteration are commonly observed: (1) low temperature oxidation (<150 C) of titanomagnetite to titanomaghemite and (2) hydrothermal alteration (roughly 200 C 400 C) of the original titanomagnetite or the magnetite/ilmenite network. The Fe Ti oxides produced during the latter type of hydrothermal alteration are strongly dependent on the primary subsolidus cooling history of the rock (e.g., fast versus slow high temperature oxidation [Banerjee, 1991]). The composition and morphology of the secondary minerals formed by this process are strongly dependent on the prior degree of hightemperature oxidation experienced by the oxide mineral. Grains that have experienced little or no oxidation to magnetite/ilmenite follow the established crystallization process [see Buddington and Lindsley, 1964; Haggerty, 1976]. Those titanomagnetite grains, however, that have experienced substantial high temperature oxidation appear to follow one of two other alteration paths [Lindsley, 1991]. In the first case, ilmenite lamellae alone alter to form titanite (CaTiSiO 5 ), leaving a relict lamellar texture within the unaltered host grain. In a less common case, the secondary phase is leached out of the magnetite portion of the magnetite/ ilmenite network, leaving behind the remaining, unaltered ilmenite lamellae and ghost like lamellae in place of the magnetite portion of the original titanomagnetite grain [Lindsley, 1991]. Volumetrically, the secondary magnetite is much less important than primary magnetite, yet because of the very small magnetic grain size, SD to PSD, this form of magnetite may be an important contribution to the natural remanent magnetization and carry a geologically important and stable magnetization [Lindsley, 1991]. The Western Granite appears to preserve evidence of both primary magmatic and secondary, hydrothermal titanomagnetite grains grown during or shortly after late stage crystallization as evidenced by magnetite ilmenite exsolution lamellae in some oxides (Figures 3c and 3d). The rock magnetic data indicate that only the eastern part of the intrusion experienced a secondary growth of Fe Ti oxide mineral phases, while the deeper part of the intrusion preserves the primary magmatic mineral phases. We argue that the variations in oxide grain size and composition observed within the Western Granite might be a common feature of shallow igneous intrusions, although these oxide mineralogical changes are not easily detectable without detailed magnetic study General Rock Magnetic Properties [31] Rock magnetic data indicate that a ferromagnetic phase, likely low Ti titanomagnetite having a limited and spatially variable range of domain states and composition, occurs as the dominant mineral phase within the Western Granite (Table 3). The AF demagnetization response implies that the ChRM is carried by low to intermediate coercivity MD magnetite for most sites in the eastern part of the intrusion and PSD magnetite of somewhat higher coercivity for most sites located in the western part of the intrusion. The PSD grain size may in fact be an artifact of a mixture of SD and MD grain sizes (see below [Dunlop, 2002]). IRM acquisition curves are characteristic of a cubic phase (magnetite type curves) with both MD to PSD magnetitedominated behaviors (Table 3). The IRM reached remanence saturation at a typical applied field of 300 mt, indicating the prevalence of magnetically soft magnetite with no evidence of high coercivity phases (e.g., pyrrhotite, hematite) present at applied fields of up to 2.5 T. Back field demagnetization of SIRM yield coercivity values typical of a low Ti titanomagnetite phase and further confirm the presence of magnetically soft titanomagnetite in the samples. Three component isothermal remanent magnetization results show a limited range of laboratory unblocking temperature spectra consistent with low Ti titanomagnetite, as well as a small fraction of coarse grained titanomaghemite as evidenced by laboratory unblocking spectra between 150 C and 300 C; the drop in intensity near 300 C seen in WG20 and WG26 may reflect the presence of pyrrhotite. Room temperature hysteresis data reveal that samples from the west typically yield wider loops than then those from the east. A Day plot [Day et al., 1977] of the hysteresis parameters further demonstrates that a variation in domain size is spatially evident as samples from the west are generally of the PSD size while those from the east are MD grain size (Table 3). High and low temperature hysteresis experiments also support these interpretations. As noted by Dunlop [2002], for many rocks PSD grain size inferred from the Day type plot could be well explained by admixtures of larger MD size grains and smaller SD size particles. In this scenario, the PSD grains from the west could represent a 10% to 30% mixture of uniaxial to cubic single domain 26 of 32

27 Table 3. Summary of Rock Magnetic, Field, and Petrographic Observations and Geochemical Data a Magnetic Mineralogy Trace Elements Drusy Cavities Silicate Alteration Magnetic Grain Size Curie Domain Millimeters Point Location Km SI (E 3) NRM (A/m) MDF (mt) Eastern 37.1 ± ± ± 17.9 MD C Moderate Many TMT Elevated in K2O, Ba, Rb, Zr, and Yb Western 16.7 ± ± ± 19.9 PSD C Low Few TMT and TMHT Depleted in K 2 O, Ba, Rb, Zr, and Yb a Km, mean susceptibility in SI units; NRM, natural remanent magnetization intensity in A/m; MFD, medium destructive field of the NRM in mt; MD, multidomain; PSD, pseudosingle domain; TMT, titanomagnetite; TMHT, titanomaghemite. magnetite grains with multidomain grains. The presence of single domain grains in the west further supports our interpretation that this part of the intrusion preserves the primary magnetic phases of a small magnetic grain size (Figure 8b). Temperature variation of low field susceptibility experiments yield a narrow spectrum of results with average Curie point estimates from eastern sites of 560 C and western sites of 570 C (Table 3). The Curie temperature of end member magnetite has been shown to decrease with increasing ulvöspinel content (i.e., Ti content) [Readman and O Reilly, 1972; Nishitani and Kono, 1983; Moskowitz, 1987]. Temperature versus low field susceptibility experiments therefore provide a quantitative means to estimate mineral composition based on the Curie point when a single magnetic phase is present within the sample (e.g., titanomagnetite). Using the equations from Akimoto [1962] and the titanomagnetite solid solution series, where Fe 3 x Ti x O 4 (x = titanium content), the eastern sites have an average Ti content of 0.04 and western sites yield 0.02, values that are consistent with a low Ti titanomagnetite phase (Table 3) Postemplacement Hydrothermal Alteration [32] During the late stage of emplacement, hydrothermal alteration of shallow intrusions results commonly from magmatic fluids exsolving from the crystallizing magma and interacting with the crystallized mineral phases [Pariso and Johnson, 1989; Kontny et al., 2003]. If there is more water present than can be incorporated into magmatic hydrous crystals at the end stage of crystallization, water bearing magmas must evolve a separate aqueous phase. Sometimes these fluids migrate into fractures within the coalescing body and precipitate quartz and alkali feldspar rich veinlets or pegmatites. Other times, a final aqueous phase and the volatiles separate from the silicate melt and generates drusy cavities. Residual water rich fluids that remain in plutons after crystallization of quartz and feldspars are capable of reacting with the magmatic minerals [Pariso and Johnson, 1989; Kontny et al., 2003]. These hydrothermal fluids are thought to circulate along a large scale convective path with flow focused upward and outward from the source material at depth. The hot chemically reactive solutions pass through and around the crystallized mush along grain boundaries and minute fracture networks shifting the composition of the material by adding, removing, and/or redis- 27 of 32

28 Figure 12. Interpretive diagram depicting inferred postemplacement hydrothermal deuteric alteration and secondary oxide growth associated with the evolution of the Western Granite. Line of cross section shown in Figure 1. (a) The emplacement of the ultrabasic rocks at a shallow level caused northwest side down tilting of the granite. Alkali feldspar and plagioclase feldspar accumulate in the lower levels of the pluton, resulting in relatively higher abundances of the compatible elements K 2 O, Ba, Rb, Zr, and Yb. Hydrothermal deuteric fluids convect upward from a source at depth and migrate to shallow levels of the pluton, feldspars experience sericitization, and mafic silicates become chloritized. Small circulating arrows represent convecting deuteric fluids. (b) Focused deuteric fluids from the younger Rum Layered Suite, possibly mixed with meteoric waters, resulted in a secondary growth of Fe Ti oxide phases along silicate grain boundaries resulting in an increased magnetic grain size and increased Ti content of the oxides in the eastern part of the intrusion. The western part of the intrusion preserves fine grained magmatic low Ti titanomagnetite grains of the PSD grain size. Black circles indicate schematically the area affected by secondary oxide growth associated with hydrothermal deuteric alteration. tributing elements. This intrapluton deuteric alteration typically converts plagioclase to sericite, hornblende to secondary biotite, and primary magmatic biotite to chlorite, evidence of which is observed in the Western Granite. [33] We argue that hydrothermal deuteric alteration affected the Western Granite and impacted not only the silicate phases but Fe Ti oxide phases as well and hence the rock magnetic properties. Our preferred interpretation of the postemplacement history of the Western Granite is shown in Figure of 32