Magnetic susceptibility patterns in a Cordilleran granitoid:

Transcription

1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 103, NO. B3, PAGES , MARCH 10, 1998 Magnetic susceptibility patterns in a Cordilleran granitoid: The Las Tazas Complex, northern Chile Jeff Wilson 1 School of Geological Sciences, Kingston University, Kingston-upon-Thames Surrey, England, United Kingdom Abstract. The Las Tazas Complex is a ferromagnetic -type granitoid that crops out in the Coastal Cordillera of northern Chile. Magnetic susceptibility patterns across the complex are controlled by modal magnetite contents, rather than overall mineralogy, resulting an ambiguous correlation between susceptibility magnitude variation and lithological zoning. There is a good correlation between plagioclase magmatic fabric orientations and magnetic fabric orientation, demonstrating that anisotropy of magnetic susceptibility (AMS) measurements can accurately characterize magmatic fabric patterns in Cordilleran -type granitoids. However, there is no simple quantitative relationship between magmatic and magnetic fabric intensities. The magmatic and magnetic fabricshare a similar character, and it is clear that AMS fabrics are strongly influenced by the dominant plagioclase fabric across the Las Tazas Complex but cannot be directly equated in quantitative strain analysis. 1. Magnetic Susceptibility and Granites batholiths along the Cordilleran magmatic arcs around the Pacific basin, and are commonly referred to as -type intru- The study of low-field magnetic susceptibility in rocks has sions (see Pitcher [1993] for review). n contrast, ilmenitebeen established for a long time and has been applied to a bearin granites characteristically form during the early stages variety of rock types, from flow indication sediments to in continental collision and postorogenic uplift, cropping out structural trends in metamorphic belts (summarized by as scattered plutons and batholiths across most orogenic belts, Borradaile [1988] and Rochettet al. [1992]). However, its and are commonly referred to as S-type intrusions [Pitcher, application to the study of granite fabrics has become common 1993]. n the absence of magnetite the susceptibility patterns only in the past few years, and its validity in this field is still in paramagnetic S-type granites are controlled by biotite and actively debated. The majority of studies have concentrated amphibole [Rochettet al., 1992; Gleizes et al., 1993].. As on (1) the relationship between susceptibility magnitudes and major rock-forming minerals their modal abundance deterlithological zoning [e.g., Gleizes et al., 1993] and (2) the mine susceptibility magnitudes and closely relates to overall relationship between anisotropy of magnetic susceptibility lithological variation [e.g., Bouchez et al., 1990; Gleizes et (AMS) and magmatic-state fabrics [e.g., Bouchez and Gleizes, al., 1993]. As anisotropic minerals they also define the 1995]. Magnetic susceptibility patterns in ilmenite-bearing S- orientation of the magnetic susceptibility and magmatic-state type granites have been examined detail, as they display a fabrics in these rocks [Guillet et al., 1983; Bouchez et al., simple relationship between mineralogy and magnetic struc- 1990; Gleizes et al., 1993]. n contrast, the relationship ture. n contrast, few studies have examined magnetic between lithological zoning and susceptibility magnitudes has susceptibility patterns in the magnetite-bearing -type granites, been poorly documented in ferromagnetic -type granitoids, whose magnetic signatures are considerably more complex. because magnitudes directly relate to modal magnetite This contribution presents the first magnetic susceptibility contents, which are controlled by magmatic oxygen fugacity, study of an Andean intrusion and one of the first to examine rather than the dominant mineral assemblage [shihara, 1977; magnetic susceptibility patterns in a magnetite-bearing -type Pitcher, 1993]. n addition, susceptibility fabrics are congranitoid. New magnetic susceptibility data are compared trolled by preferred magnetite fabrics, which often display a with litho!ogical and structural observations to examine the complex relationship with the dominant magmatic fabric in the relationship between magmatic-state and magnetic fabrics and rock [Archanjo et al., 1995]. to assess the validity of magnetic susceptibility as a lithological mapping tool in Cordilleran granites. Granitoid intrusions can be subdivided on the basis of their 2. The Las Tazas Complex dominant oxide population into magnetite- or ilmenite-bearing The Las Tazas Complex crops out in the Cordillera de la lithologies [shihara, 1977]. Where magnetite is present, its Costa of northern Chile (Figure 1). t is a magnetite-bearing, ferromagnetic character will control susceptibility patterns. -type intrusion that was emplaced in the Andean magmatic Magnetite-bearingranitoids dominantly form during arc during the Early Cretaceous along the Atacama Fault subduction-related magmatism, cropping out as huge linear Zone, a km-long transcurrent fault zone whose activity dominated the arc at that time [Brown et al., 1993]. Now at Department of Geology, Rhodes University, Grahamstown, The Las Tazas Complex is highly elongate and flanked by South Africa. vertical mylonitic shear zones of the Atacama Fault Zone, Copyright 1998 by the American Geophysical Union. Paper number 97JB /98/97JB which form its eastern and western contacts. t consists of a larger, granodioritic northern pluton and a smaller, monzonitic southern pluton (Figure 2). Both display typical Cordilleran granitoid mineralogical assemblages, dominated by a

2 5258 WLSON: GRANTOD MAGNETC SUSCEPTBLTY Figure 1. Geological maps of (a) the Cordillera de la Costa and (b) the vicinity of the Las Tazas Complex. subhedral plagioclase framework, together with interstitial hornblende, biotite, alkali feldspar, quartz, and magnetite with or without clinopyroxene. Plagioclase crystals commonly display a preferred magmatic-state alignment, while the interstitial phases are nonaligned, demonstrating a low level of postcrystallization solid-state deformation [Paterson et al., 1989]. Magnetite is the dominant oxide mineral, forming anhedral polygonal to rounded crystals. The northern pluton is vertically sheeted and comprises five main granodioritic lithological units, labeled N1-N5, which are separated by well-defined internal contacts (Figure 2). Units N1-N4 display magmatic plagioclase foliations that increase in intensity westward across the pluton. The foliations dip subvertically and strike north-south (Figure 3). Unit N1 also displays elongate dioritic enclaves with a steep southward plunge, while unit N5 displays a subhorizontal magmatic plagioclase lineation, with a north-south trend (Figure 3). The southern pluton comprises a suite of quartz monzonites and quartz monzodiorites, which are grouped into four lithological units, S1 to S4 (Figure 2). Subvertical magmatic plagioclase foliations are sporadically identified across the pluton, striking 020 ø (Figure 3). 3. Magnetic Data Acquisition Seventy-six samples have been measured from 17 sites across the northern pluton, with a site spacing of -1 km (Figure 2). A further 57 samples have been measured from 13 sites across the southern pluton, with a site spacing of 1 km (Figure 2). Of the 30 sites sampled, 19 have also been analyzed geochemically (Tables 1 and 2). Two 25-mmdiameter cores have been sampled from each site, drilled in situ or from oriented blocks (Table 1). Each core has yielded two to three 22-mm-long samples, which have been analyzed by using a Kappabridge KLY-2 susceptometer (Geofyzika Brno), which works in low alternating field (+ 3.8x10-4 T; 920

3 WLSON' GRANTOD MAGNETC SUSCEPTBLTY unit N1 1 1 b umt. N2 de Peralillo Quebrada diorite Queeradd de Saladito ' 1 unit S1 : unit S3 unit S4. sample site 10. inferred contact. Quebrada ' 25 Vafillas. uebrada de.,[ 5- Guamang a & > ff., *12., 5km (a) 3km [ (b) Figure 2. Maps of (a) lithological zoning and site distribution and (b) average susceptibility (mm; 10-3 S) distribution within the Lad Tazas Complex.

4 5260 WLSON: GRANTOD MAGNETC SUSCEPTBLTY Figure 3. Lambert equal-area stereographic projections of (a) poles to plagioclase foliations within the northern pluton, (b) poles to plagioclase foliations across the southern pluton, and (c) plagioclase lineations across unit N5. Hz) with a sensitivity greater than 10-7 S. Susceptibilities of Flinn and T parameters (Pflinn<l, T>0) [Flinn, 1965; between 104 and 102 S and anisotropies down to 0.2 % can be measured [Rochett et al., 1992]. Fifteen oriented susceptibility magnitude measurements are taken from each sample, and a three-dimensional susceptibility ellipsoid is calculated from them by a classical eigenvector procedure [Harvey and Laxton, 1980] using a program of M. de Saint Blanquat (1996). Jelenik, 1981], reflecting the dominance of the magnetic foliation (Table 1). Ellipsoids across the southern pluton are generally more oblate than those across the northern pluton, although there is no systematic variation across either pluton (Figure 4). Large susceptibility magnitudes and anisotropies have resulted in well-defined magnetic foliation and lineation orientations in all sites across the Las Tazas Complex. The 4. Magnetic Susceptibility Patterns northern and southern plutons share a similar subvertical foliation with a 010ø-015 ø strike (Figures 5 and 6; Table 3). The Las Tazas Complex displays relatively high susceptibil- Lineations across the southern pluton have a subvertical ity magnitudes (Km), in the range 10-2 to 10-3 S (Figure 2 and plunge (Figure 6). n contrast, lineations across the northern Table 1). The northern pluton displays slightly lower suscep- pluton display a shallower, southward plunge (Figure 6), with tibility magnitudes than the southern pluton (Figure 2 and a mean of 42 ø toward 191 ø (Table 3). The plunge shallows Table 3). ndividualithological units cannot be distinguished, progressively eastward across the pluton along Quebrada de although there is a consistent symmetrical distribution of Guamanga and Quebrada de las Animas (Figure 5). magnitudes across the northern pluton, with the greatest magnitudes in the pluton core (Figure 2). All samples across the Las Tazas Complex display a significant magnetic anisotropy, P% (Table 1), which is generally higher across the 5. Discussion There is an ambiguous relationship between susceptibility northern pluton than the southern pluton (Figure 4, Tables 1 magnitude and lithological zoning across the Las Tazas and 3). Anisotropies increase northwards and westwards Complex. Susceptibility magnitudes do clearly distinguish across the northern pluton, though there is no systematic between northern pluton granodiorites and southern pluton variation across the southern pluton (Figure 4). Magnetic fabrics are dominated by the foliation component, F% (Table monzonites, or between lithological units within either pluton. Susceptibility magnitudes across the complex, 10 '2 to 10-3 S, 1). Magnetic foliation intensities mirror total anisotropies, are consistent with other ferromagnetic granitoids [e.g., increasing northward and westward across the northern Archanjo et al., 1994, 1995; Geoffroy et al., 1997], reflecting pluton. Magnetic ellipsoid shapes across the Las Tazas Complex are dominantly oblate (Figure 4), as defined by the the dominance of accessory magnetite contents, rather than modal contents of the major rock-forming minerals, confirm-

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6 5262 WLSON: GRANTOD MAGNETC SUSCEPTBLTY Table 2. Data Parameters Listed in Tables 1 and 3 Parameter K2m K3m P% F% L% Pflinn T Trend Plunge Strike Dip Dir. NS a(k0 a(k3) s.d.. Chem. Drilled Explanation average susceptibility ( Km/nS), values in 10-3 S average susceptibility in the Kmax direction ( K /ns), values in 10-3 S average susceptibility in the Ki,,t direction ( K2/ns), values in 10-3 S average susceptibility in the Kin, direction ( K3/ns), values in 10-3 S total anisotropy intensity, equal to 100[(Klm/K3m )- 1] magnetic foliation intensity, equal to 100[(K2m/K3m )- 1] magnetic lineation intensity, equal to 100[(Klm/K2m )- 1] Flinn parameter, equal to L %/F% T parameter, equal to 2(ln K2n,-lnK3m)/(ln K m-ln K3m)-i trend of magnetic lineation plunge of magnetic lineation strike of the magnetic foliation dip of the magnetic foliation dip direction of the magnetic foliation number of specimens in the site 95 % confidence angle on the orientation of Kj [Jelenik 1978] 95 % confidence angle on the orientation of K3 [Jelenik 1978] average standar deviations on Kn, measurements site also analyzed geochemically samples drilled in situ or block sampled ing that magnitudes do not directly relate to lithological [Ellwood and Whitney, 1980] and the preferential distribution variation. Magnetite contents do relate to geochemistry of grains through the rock [Hargreaves et al., 1991]. The across the Las Tazas Complex, illustrated by a positive correlation between Km and Fe203* wt % (Figure 7). However, this correlation is poorly defined, suggesting a complicated relationship. While magnetite reflects magma fugacity, Fe203* is shared between biotite, amphibole, magnetite, and other Fe-bearing accessories, whose relative abundance is a function of elemental content, water content, oxygen fugacity, and magma chamber dynamics [Pitcher, 1993]. This complicated relationship between magnetite content and lithology appears to be characteristic of magnetite-bearingranitoids [e.g., Archanjo et al., 1994, 1995; Geoffroy et al., 1997], and suggests that susceptibility magnitude measurement may be an unreliable tool for lithological mapping in Cordilleran -type granitoids. Field measurements of magmatic plagioclase foliations (Figure 3)clearly agree with laboratory magnetic foliation early cooling history of Cordilleran granitoids is dominated by plagioclase crystallization [Bryon et al. 1994], and magmatic fabrics evolve through the preferential growth and rotation of plagioclase grains under an applied stress. The close agreement between the magnetic and magmatic fabrics indicates that both have responded to the applied magmatic stress in a similar way, evolving a fabric through the preferential rotation and clustering of magnetite grains during the early stages of crystallization. While there is a close connection between magnetic and magmatic fabric orientations, their respective fabric intensities cannot be directly related. Magnetite fabric intensifies reflect a combination of geochemistry, crystallization history, and strain rate during crystallization [Tribe and D'Lemos, 1996]. Of these the geochemical contribution appears to be small, suggested by a poor correlation between fabric intensity and measurements (Figure 6). n addition, the steep southward magnetite content (Figure 8). Both magmatic and magnetic plunge of the enclave long axes across unit N 1 agrees with the fabric intensities increase westward across the northern steep southward plunge of the magnetic lineation within it, while the subhorizontal plagioclase linearion within unit N5 agrees with the subhorizontal magnetic lineation across it (Figures 3 and 6). This correlation of foliation and lineation data suggests a close, probably genetic, relationship between the magmatic and magnetic fabrics across the Las Tazas complex. The dominant magnetite magnetic fabric orientations result from the preferred alignment of magnetite grains pluton, suggesting that magnetite fabric formation was strongly influenced by the dominant plagioclase fabric, evolving from rotation and clustering of magnetite grains parallel to aligned plagioclase crystals. n contrast, plagioc lase fabric intensities do not correspond to magnetic fabric intensifies across the southern pluton and along the northward pluton, indicating that plagioclase fabrics have not controlled magnetite fabric development. The apparently variable Table 3. Average Magnetic Susceptibility Data for the Northern and Southern Plutons Pluton Km P % F % L% PFlinn T Trend Plunge Strike Dip Dir. NS Northern west 76 Southern west 57

7 WLSON: GRANTOD MAGNETC SUSCEPTBLTY sample site - fault - inferred contact 3km N! Quebrada de Salado /,0.25 '! ! x9 6 ll.i ^ Varillas Fault \ ' 1-4'13' J 0.5: 0.53 J + Portezuelo.33 Varilla '4'0' \ \ (a) (b) Figure 4. Maps of (a) magnetic anisotropy distribution and (b) Pflinn distribution.

8 5264 WLSON' GRANTOD MAGNETC SUSCEPTBLTY!! Quebrada J las Animals 6 1_;41 / '"- Quebmda de Guhm ga Varilla Fault ' / t r?o /' 'x? ', /', P rtezuelo %7 \\ Varilla 85 t Figure 5. Maps of magnetic structural data: (a) magnetic foliations and (b) magnetic lineations.

9 WLSON' GRANTOD MAGNETC SUSCEPTBLTY 5265 N n=76 n=76 Figure 6. Lambert equal-area stereographic projections of (a) poles to magnetic foliations across the northern pluton, (b) poles to magnetic foliations across the southern pluton, (c) magnetic lineations across the northern pluton and (d) magnetic lineations across the southern pluton. correspondence between plagioclase and magnetite fabric intensities indicates that they may be preserving different parts of the overall strain history. This possibility is reflected in the poorly defined correlation between fabric intensity and the resulting ellipsoid shape (Figure 9). Samples with higher fabric intensities tend to possess oblate ellipsoids, but the wide scatter suggests that the final ellipsoid shape reflects the end product of a more complex strain history, which may itself be complicated by magnetic interactions between grains [Gregoire et al., 1995]. As such the magmatic and magnetic fabrics within any given sample have formed by similar processes under the same stress state and share a similar character, but fabric intensities cannot be directly equated. This pattern is consistent with other ferromagnetic intrusions [e.g., Archanjo et al., 1994, 1995; Geoffroy et al., 1997] and suggests that AMS study can accurately characterize the 1 _ 4-// ! southern pluton, ' nørthern plutøn [i!! Magnetic susceptibility, 10-3 S Figure 7. Fe203* wt % against susceptibility magnitude.

10 5266 WLSON: GRANTOD MAGNETC SUSCEPTBLTY l l Susceptibility magnitude, 10-3 S Figure 8. Fabric intensity (P%) against and susceptibility magnitude (dominated by magnetite content). The lack of correlation illustrates that magnetic fabric intensities are not related to modal magnetite contents. orientations and overall character of magmatic fabrics in Cordilleran -type granitoids but should not be used for quantitative strain analysis in these rocks. 6. Conclusions rather than overall mineralogy. Magmatic-state plagioclase fabric orientations agree with AMS fabric orientations, demonstrating that AMS study can accurately characterize the orientations and overall character of magmatic fabrics in Cordilleran -type granitoids. However, there is no simple quantitative relationship between magmatic and magnetic fabric intensities. t is clear that AMS fabrics are strongly The Las Tazas Complex is a Cordilleran granitoid whose influenced by the dominant evolving plagioclase fabric but magnetic susceptibility patterns are controlled by accessory may be preserving different parts of the overall strain history. magnetite. Susceptibility magnitudes are a poor guide to As such, plagioclase and AMS fabrics should not be directly lithological zoning across the Complex, controlled by modal compared in Cordilleran -type granitoids during quantitative magnetite contents that reflect magmatic oxygen fugacity, strain analysis ll ll oblate ellipsoids prolate ellipsoids Magnetic anisotropy, P% Figure 9. Magnetic fabric intensity (P%) against and the resulting ellipsoid shape (T parameter). Magnetic ellipsoids become more oblate with greater magnetic fabric intensities.

11 WLSON: GRANTOD MAGNETC SUSCEPTBLTY 5267 Acknowledgments. The author would like to thank John Grocott and Pete Treloar for their constant supervision and Jean-Luc Bouchez for inspiration about the AMS technique analytical support at the Laboratoire Petrophysiquet Tectonique, Universit6 Paul Sabatier, Toulouse, France. n addition, many thanks to Eric Ferr6 and Jacquie D616ris for their patience and support. Also thanks to Don Tarling and Michael Jackson for helpful reviews. This research has been funded by a Kingston University research studentship. References Archanjo, C.J., J.L. Bouchez, M. Corsini, and A. Vauchez, The Pombal granite pluton: Magnetic fabric, emplacement and relationships with the Braziliano strike-slip setting of NE Brazil (Para a State), J. Struct. Geol., 16, , Arcbanjo, C.J., P. Launeau, and J.-L. Bouchez, Magnetic fabric versus magnetite and biotite shape fabrics of the magnetitebearing granite pluton of Gameleiras (northeast Brazil), Phys. Earth Planet. nter., 89, 63-75, Borradaile, J.G., Magnetic susceptibility, petrofabrics and strain, Tectonophysics, 156, 1-20, Bouchez, J.L., and G. Gleizes, Two-stage deformation of the Mont Louis-Andorra granite pluton (Variscan Pyrenees) inferred from magnetic susceptibility anisotropy, J. Geol. $oc. London, 152, , Bouchez, J.L., G. Gleizes, T. Djouadi, and P. Rochette, Microstructure and magnetic susceptibility applied to emplacement kinematics of granites: The example of the Foix pluton (French Pyrenees), Tectonophysics, 184, , Brown, M., F. Diaz, and J Grocott,., Displacement history of the Atacama Fault System 25ø00-27ø00 S, northern Chile, Geol. $oc. Am. Bull., 105, , Bryon, D.N., M.P. Atherton, and R.H. Hunter, The description of the primary textures of "Cordilleran" granitic rocks, Contrib. Mineral. Petrol., 117, 66-75, Ellwood, B.B., and J.A. Whitney, Magnetic fabric of the Elberton granite, Northeast Georgia, J. Geophys. Res., 85, , Flinn, D., On the symmetry principle and the deformation ellipsoid, Geol. Mag., 102, 36-45, Geoffroy, L., P. Olivier, and P. Rochette, Structure of a hypovolcanic acid complex inferred from magnetic susceptibly anisotropy measurements: The Western Redhills Granites (Skye, Scotland, Thulean gneous Province), Bull. Volcan., 59, , Gleizes, G., A. N6d61ec, J.L. Bouchez, A. Autran, and P. Rochette, Magnetic susceptibility of the Mont-Louis-Andorra ilmenite-type granite (Pyrenees): A new tool for the petrographic characterization and regional mapping of zoned granite plutons, J. Geophys. Res., 98, , Gregoire, V., M. de Saint Blanquat, A. N6d61ec, and J.L. Bouchez, Shape anisotropy versus magnetic interactions of magnetic grains: Experiments and application to AMS in granitic rocks, Geophys. Res. Lett., 22, , Guillet, P., Bouchez, J.L. and Wagner, J.J., Anisotropy of magnetic susceptibility and magnetic structures in the Gu6rande granite massif (France), Tectonics, 2, , Hargreaves, R.B., D. Johnson, and C.Y. Chan, Distribution anisotropy: The cause of AMS in igneous rocks?, Geophys. Res Lett., 18, , Harvey, P.K., and P.R. Laxton, The estimation of finite strain from the orientation distribution of passively deformed linear markers: Eigenvalue relationships, Tectonophysics, 165, 21-27, Hrouda, F., Magnetic anisotropy of rocks and its application in and geophysics, Geophys. Surv., 5, 37-82, shihara, S., The magnetite series and ilmenite series granitic rocks, Min. Geol., 27, , Jelenik, V., Statistical processing of anisotropy of magnetic susceptibility measured on groups of specimens, Stud. Geophys. Geod., 22, 50-62, Jelenik, V., Characterisation of the magnetic fabric of rocks, Tectonophysics, 79, T63-67, Paterson, S. R., R.H. Vernon, and O.T Tobisch, A review of criteria for the identification of magmatic and tectonic foliations in granitoids, J. Struct. Geol., 11, , Pitcher, W.S, The Nature and Origin of Granite, Blackie Acad. and Prof, New York, 321pp, Rochette, P., M. Jackson, and C. Aubourg, Rock magnetism and the interpretation of anisotropy of magnetic susceptibility, Rev. Geophys., 30, , Tribe,.R., and R.S D'Lemos, Significance of a hiatus in down temperature fabric development within syn-tectonic quartz diorite complexes, Channel slands, UK, J. Geol. Soc. London, 153, , J. Wilson, Department of Geology, Rhodes University, Grahamstown, 6140, South Africa ( wilson@rock.re.at. za) (Received January 14, 1997; revised October 14, 1997; accepted October 27, 1997.)