INSTITUTE OF METEORITICS, UNIVERSITY OF NEW MEXICO, ALBUQUERQUE, NM 87131, USA

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1 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 4 PAGES Ancient and Modern Subduction Zone Contributions to the Mantle Sources of Lavas from the Lassen Region of California Inferred from Lu Hf Isotopic Systematics LARS E. BORG 1, JANNE BLICHERT-TOFT 2 AND MICHAEL A. CLYNNE 3 1 INSTITUTE OF METEORITICS, UNIVERSITY OF NEW MEXICO, ALBUQUERQUE, NM 87131, USA 2 ÉCOLE NORMALE SUPÉRIEURE DE LYON, 69364, LYON CEDEX 7, FRANCE 3 US GEOLOGICAL SURVEY, 345 MIDDLEFIELD RD, MS 910, MENLO PARK, CA 94025, USA RECEIVED FEBRUARY 25, 2001; REVISED TYPESCRIPT ACCEPTED OCTOBER 29, 2001 INTRODUCTION The geochemical and isotopic variability of mantle wedge peridotites beneath continental volcanic arcs is poorly constrained. This is a result of the fact that most of what is known about the geochemistry of the mantle wedge is inferred from the composition of basaltic arc lavas, which, although derived from the wedge, are affected by variable contributions from the subducting slab and continental crust (e.g. Gill, 1981; Tatsumi et al., 1986; Hawkesworth et al., 1993, 1994). Despite these limitations there is significant evidence to suggest that mantle wedge peri- dotites are heterogeneous in composition and mineralogy. For example, it has been postulated that the compositions of mantle peridotites beneath the Cascade arc vary from compositions that are similar to sources for mid-ocean ridge basalt ( MORB), Hawaiian ocean-island basalt (OIB), and high-alumina-olivine tholeiites (HAOT) from the Basin and Range province (Hughes, 1990; Leeman et al., 1990; Bacon et al., 1997; Borg et al., 1997; Clynne & Borg, 1997). The restite mineralogy of the source regions for Cascade magmas is poorly constrained as well. For example, experiments conducted by Baker et al. (1994) found that calc-alkaline basalts from the Mt. Shasta region of California did not have garnet on their liquidus, and Reiners et al. (2000) were able to model trace element variations in basaltic lavas from the central Hafnium isotopic compositions have been determined on a suite of calc-alkaline and high-alumina-olivine tholeiitic lavas from the Lassen region of California and are used, in conjunction with previously published mineralogical, geochemical, and isotopic data, to constrain their petrogenesis. Positive correlation between ε Hf values and geochemical indices of the modern subduction component indicates that the isotopic compositions of the calc-alkaline lavas record addition of radiogenic Hf from the subducted slab. However, the addition of the modern subduction component increases the ε Hf values of most calc-alkaline lavas by <0 5 units over estimates of non-subduction enriched peridotites of the mantle wedge. The Lu Hf isotopic systematics of the Lassen lavas suggest that the calc-alkaline magmas have equilibrated with garnet at some point in their history, whereas the tholeiitic magmas have not. These observations require the two lava types to be derived from different sources. The isotopic variability of the Lassen lavas cannot be produced by mixing mantle sources inferred to be present in the eastern central Pacific and western USA with a modern subduction component. Instead, the isotopic variability is consistent with mixing of a depleted mantle source, a more fertile mantle source enriched by an ancient subduction component, and a modern subduction component. KEY WORDS: Hf isotopes; Cascade arc; subduction zone; calc-alkaline; tholeiitic Corresponding author. Telephone: (505) Fax: (505) lborg@unm.edu. Oxford University Press 2002

2 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 4 APRIL 2002 Cascades without garnet in their source. In contrast, considered to characterize lavas that are little evolved Guffanti et al. (1990) noted that the depth of the slab since being separated from their mantle source [e.g. beneath the southernmost Cascades was in the garnet Basaltic Volcanism Study Project (BVSP), 1981]. Primitive peridotite stability field. In addition, several studies of lavas are sparsely porphyritic and have simple minperidotite calc-alkaline lavas from the Cascade range have suggested eral assemblages (usually singly saturated) of unzoned the presence of garnet in their source regions based on phenocrysts with compositions in equilibrium with mantle rare earth element (REE) geochemistry of the lavas (Borg peridotites. Bulk-rock compositions have high contents et al., 1997; Conrey et al., 1997). of compatible elements and are also in equilibrium with This paper assesses the heterogeneity, mineralogy, and mantle peridotite. Two mineralogically and geoevolution of the sub-arc mantle in the southernmost chemically distinct types of primitive lavas are present in Cascades by examining the Hf isotopic systematics of a the Lassen area; HAOT [equivalent to low-potassium suite of calc-alkaline and tholeiitic lavas. This can be done olivine tholeiites of Bacon et al. (1997)] and com- because, although the modern subduction component is positionally diverse calc-alkaline basalts andesites (CAB). found to contribute radiogenic Hf to the mantle wedge, The andesitic lavas meet these primitive criteria, but the Hf isotopic composition of many Lassen lavas primandesites. have higher silica content (53 57% SiO 2 ) and thus are arily reflects the isotopic composition of their mantle These are similar to rocks found in other arcs source regions before the addition of the modern subor and are often referred to as high-magnesium andesites duction component. Before the Hf isotopic systematics sanukitoids (e.g. Tatsumi, 1981; Tatsumi & Ishizaka, of the Lassen lavas are discussed a review of the pertinent 1982; Yogodzinski et al., 1994). geologic, petrologic, and geochemical observations is Primitive HAOT lavas contain up to a few percent presented below. olivine phenocrysts. The olivines typically include chro- mian spinel, which is also occasionally present in the groundmass. The olivine and spinel are unzoned and REVIEW OF THE GEOCHEMISTRY have magnesian compositions with high Ni (Fo 86 88,Ni [ 2000). These compositions are in equilibrium with AND PETROLOGY OF PRIMITIVE the bulk composition of the lava and potentially in equilibrium with fertile peridotite (e.g. BVSP, 1981). LAVAS FROM THE LASSEN REGION Chromian spinels have magnesian compositions and fall OF THE CASCADE RANGE within the mantle array of Arai (1987). Olivine spinel The Lassen region is the southernmost segment of the equilibria indicate crystallization at temperatures apactive Cascade arc (Guffanti & Weaver, 1988) and is proaching 1300 C and elevated pressures (Clynne & located in northeastern California between the Klamath Borg, 1997). Sparse calcic plagioclase ([An 70 ) is present terrane and the northern Sierra Nevada. Volcanism in in a few primitive HAOT, but clinopyroxene is absent. the Lassen area can be characterized on two scales. The most striking feature of HAOT from the Lassen Between the large long-lived volcanic centers that pro- region is the limited range of major- and trace-element duce evolved rocks, regional volcanism has built a broad variability displayed by these lavas (Clynne, 1993; Bacon platform of small to intermediate-sized volcanoes that et al., 1997). Distinctive features of HAOT from the have short lifetimes. Typically, these volcanoes are monogenetic Lassen region are low SiO 2 (>48 wt %), FeO /MgO or are active over a relatively short period of of , high Al 2 O 3 (17 18 wt %), and low alkali time, from a few years to a few thousand years. The content, especially K 2 O(>0 2 wt % or less). HAOT overall result is construction of a broad platform of have high contents of compatible elements ( MgO [ volcanic rocks that in the vicinity of the active arc is 9 wt %, Ni [ 100 ppm and Cr [ 200 ppm). In >4 km thick. comparison with CAB, HAOT have lower concentrations The extensional Basin and Range province is expanding of large ion lithophile elements (LILE), higher concade westward and impinging upon the active Cas- centrations of heavy REE (HREE), lower LILE/high arc in northern California (Guffanti et al., 1990), field strength element (HFSE) values, and lower light with the result that normal faults are prominent in the REE (LREE)/HREE ( Fig. 1a). In general, their trace Lassen area. Linear arrays of small cinder cones and lava element characteristics are E-MORB-like. HAOT display flows that trend parallel to the normal faults demonstrate weak enrichments of Ba, Sr, and Pb relative to LILE or that magmas often exploit these weaknesses in the crust. LREE. Sr and Nd isotopic systematics of HAOT are A small subset of lavas erupted from monogenetic vents distinct from those of CAB, primarily in having slightly are the most primitive in the area, and further discussion higher 144 Nd/ 143 Nd ratios at equivalent 87 Sr/ 86 Sr ratios is limited to them. (Bacon et al., 1997; Borg et al., 1997). We define primitive lavas as those meeting a combination Bartels et al. (1991) have experimentally equilibrated a of physical and geochemical criteria generally magnesian HAOT composition with a spinel lherzolite 706

3 BORG et al. PETROGENESIS OF LASSEN REGION LAVAS assemblage at a temperature of 1290 C and pressure of Olivine has a magnesian composition and high Ni ([Fo 88 ; kbar, consistent with the mineral geothermometry Ni [ 2000 ppm) in equilibrium with the bulk composition cited above. Clynne (1993) suggested that HAOT were of the lava and potentially in equilibrium with mantle derived by >10% partial melting of depleted sub- peridotite (e.g. BVSP, 1981). Chromian spinels have continental lithospheric peridotite that had been sub- magnesian compositions and fall on, or near, the mantle jected to Fe and Al enrichment by addition of pyroxene array of Arai (1987). Olivine spinel equilibria indicate through metasomatism. Baker et al. (1994) concluded that crystallization at [1250 C and elevated pressures HAOT at Mt Shasta represents a nearly anhydrous (Clynne & Borg, 1997). A few magnesian andesites also 6 10% partial melt of subcontinental mantle that last contain sparse chromian diopside with [0 5 wt % Cr 2 O 3 equilibrated near the base of the crust. Bacon et al. (1997) and mg-numbers [90 (Clynne, 1993). Primitive CAB lack attributed their weak subduction-related geochemical sig- plagioclase except as fine groundmass crystals. Primitive nature (elevated Sr, Ba, Pb, and Sr/P) to passage of CAB contain high contents of compatible elements ( MgO arc magmas through their mantle source region in the [ 8wt%,Ni[ 100 ppm and Cr [ 200 ppm, and Mesozoic era. whole-rock FeO /MgO = ). Primitive CAB are sparsely porphyritic and have simple In the Lassen region, CAB define a compositional and mineral assemblages. They contain up to >5% unzoned isotopic continuum (Figs 1 and 2). The ends of the olivine phenocrysts, most with included chromian spinel. continuum can be defined by the degree to which Sr is enriched over other similarly incompatible elements. This definition has petrogenetic significance because most of the geochemical and isotopic characteristics of Lassen basalts correlate with Sr enrichment ( Fig. 2). To make use of a considerable dataset for major elements and Sr, Borg et al. (1997) used the ratio (Sr/P) N as an analog to the commonly used LILE/LREE (e.g. Sr/Nd) to indicate the relative enrichment of Sr over LREE in arc rocks (Gill, 1981). The Sr enrichment is defined as normalized Fig. 1. (a) MORB-normalized incompatible element plots of representative HAOT and CAB. CAB display a range of compositions from low (Sr/P) N represented by LM (Φ) to high (Sr/P) N represented by LC (Χ). (b) Source mixing partial melting models from Borg et al. (1997) reproducing the compositions of the CAB. Numerical results of these models were presented in table 3 of Borg et al. (1997) and partition coefficients summarized in table 4 of Borg et al. (1997). Β, 5% non-modal partial melt of a source with ol: opx:cpx:spl:grt:amph = 56:12:6:4:7:15 melting in the proportions ol: opx:cpx:spl:grt:amph = 5:10:45:1:4:35. The trace element composition of this source is a mixture of 99 31% MORB-source + 0 6% slabderived fluid % Pacific sediment. Composition of MORB source, fluid, and sediment for all models from Borg et al. (1997), Stolper & Newman, (1994), and Ben Othman et al. (1989), respectively. Ε, 8% non-modal partial melt of a source with ol:opx:cpx:spl:grt: phl = 51:19:19:5:4:12 melting in the proportions ol:opx:cpx:spl:grt: phl = 11:17:55:1:1:15. The trace element composition of this source is a mixture of 99 6% OIB-source + 0 4% sediment. The models are compared with typical low (Sr/P) N CAB (LM ) and high (Sr/P) N CAB (LC ). (c) Sample LMM (Α) has an incompatible element pattern that is typical of most HAOT (Clynne, 1993), whereas LC (Ν) has higher LILE abundances and is representative of only a small fraction of HAOT. Ο, a mixture of 75% HAOT (LMM-279) and 25% CAB (LC ). Κ, results of 8% partial melting of a peridotite with ol:opx:cpx:spl = 55:15:25:5 in the proportions ol:opx:cpx:spl = 15:25:58:2. The incompatible element composition of the source is assumed to be represented by a mixture of 97% depleted mantle and 3% slab-derived fluid. It should be noted that the models are consistent with production of the high-lile HAOT lavas from the low-lile HAOT lavas both by the addition of a modern subduction component and by magma mixing with CAB. The low- LILE HAOT lavas cannot be derived from depleted mantle sources through the addition of a modern subduction component. 707

4 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 4 APRIL 2002 (Sr/P) N >1 in which the data are normalized to primitive a radiogenic Os component derived from the slab and mantle values of Sun & McDonough (1989). Good correlation mantle Os derived from the mantle wedge. However, between P and Nd (r 2 = 0 90) and between (Sr/ the low Os abundances of the high (Sr/P) N CAB make P) N and (Sr/Nd) N (r 2 = 0 82) demonstrates that these them particularly susceptible to contamination. Thus, ratios are analogous. It is important to note that the Borg et al. (2000) suggested a second possibility: that high variations in Sr/P are not simply the result of depletions (Sr/P) N CAB were contaminated by small amounts of in Sr and P associated with fractional crystallization of radiogenic Os from the crust. Other geochemical characteristics plagioclase and apatite. Lack of plagioclase phenocrysts of the rocks are not affected by this small and Eu anomalies in the whole-rock REE patterns sug- amount of crustal contamination. The Os isotopic compositions gests that variations in the abundances of Sr observed in of three HAOT lavas from the Lassen region the lavas do not reflect plagioclase fractionation. Likewise, were not reported by Borg et al. (2000). The Os isotopic a lack of apatite phenocrysts, as well as relatively constant compositions of these samples are similar to those of P/Zr ratios in the whole rocks indicate that P is unlikely HAOT to the north of the study area (Table 1), which to be controlled by apatite fractionation. Instead, the have been interpreted to be derived from the mantle variation in Sr/P ratios is the result of simultaneous lithosphere (Hart et al., 1997). increase in the abundance of Sr and accompanying Four conclusions derived from these studies provide a depletion in incompatible elements (including P and framework for a model in which to interpret and test the LREE). Except for a few evolved samples, there are good origin of the Hf isotopic systematics. First, unradiogenic correlations between (Sr/P) N and some other commonly Pb isotopic compositions and low 187 Os/ 188 Os ratios of used indicators of arc enrichment, such as Ba/Ta ( Fig. many CAB and low 187 Os/ 188 Os ratios of HAOT suggest 2a), illustrating the usefulness of (Sr/P) N as a geochemical that crustal contamination was minimal (Borg et al., 1997, indicator. 2000). This is supported by the primitive nature of CAB Primitive CAB display a range of incompatible element and HAOT. patterns (Fig. 1a). At one end of the spectrum are lavas Second, the origin of the geochemical variation of with low contents of incompatible elements, high ratios primitive CAB in the Lassen region can be explained by of LILE to HFSE abundances (e.g. Sr/P), high LREE/ source mixing between a relatively enriched component HREE and Sr and Nd isotopic values approaching those with a primitive mantle or Bulk Earth-like isotopic composition of MORB (Fig. 3). The most extreme lavas in this group (OIB-like) and a slab-derived fluid component are primitive magnesian andesites. These samples have with an arc geochemical signature and near MORB-like the largest proportion of slab-derived component isotopic composition (Borg et al., 1997). Low Cs/Rb ratios (Clynne, 1993; Bacon et al., 1997; Borg et al., 1997). The (<0 064) in all CAB and low Pb isotopic compositions other end of the spectrum is characterized by lavas in high (Sr/P) N CAB indicate that subducted sediment with higher abundances of incompatible elements, lower plays at most a minor role in their origin. The relative LILE/HFSE, lower LREE/HREE, and Sr, Nd, and Pb proportion of the slab-derived component in the Lassen isotopic ratios that are like those of OIB. These features magmas, as measured by (Sr/P) N, decreases from the and lower relative enrichment of Sr over LREE and P forearc to the backarc. led Borg et al. (1997) to propose that these samples are Third, Clynne & Borg (1997) showed that systematic the least influenced by a slab component. We use compositional differences between phenocrysts in HAOT (Sr/P) N = 3 3 to distinguish low and high (Sr/P) N samples, and CAB and correlations between the composition of but emphasize that the CAB display a continuum of spinels and the bulk composition of Lassen basalts reflect (Sr/P) N and that few of the samples analyzed for Hf have bulk chemical variability and relative fertility of their intermediate (Sr/P) N. mantle sources. They concluded that the relative source Clynne (1993) and Borg et al. (1997) discussed across- fertility decreases in the order HAOT low (Sr/P) N arc variation in the geochemistry of primitive lavas in CAB high (Sr/P) N CAB. Of the high (Sr/P) N CAB, the the southernmost Cascade range. Low (Sr/P) N basalts magnesian andesites are derived from the least fertile characterize the arc axis and backarc, and are sparsely mantle sources. Thus, the relative fertility of the CAB present in the forearc, whereas high (Sr/P) N basalts and magma source decreases from the backarc to the forearc. magnesian andesites characterize the forearc, and are Finally, Clynne (1993) proposed that HAOT and CAB extremely rare in the arc axis and backarc. have fundamentally different source regions and are not The Os isotopic compositions of some of the same linked by a common source or process, a conclusion that CAB samples reported here were determined by Borg was shared by Baker et al. (1994) and Bacon et al. (1997). et al. (2000). They found that the proportion of slab Thus, the origins of HAOT and CAB magmas and their component, as measured by (Sr/P) N, correlates with the Hf systematics are considered separately below. Rare Os isotopic composition of CAB ( Fig. 2d). This correlation basalt lavas have geochemical characteristics that are is interpreted to be the result of mixing between intermediate between HAOT and the CAB spectrum. 708

5 BORG et al. PETROGENESIS OF LASSEN REGION LAVAS Fig. 2. Plot of (Sr/P) N of CAB vs (a) Ba/Ta, (b) 87 Sr/ 86 Sr, (c) 207 Pb/ 204 Pb, (d) 187 Os/ 188 Os, (e) ε Nd, and (f ) ε Hf. Χ, CAB with (Sr/P) N >3 3; Φ, CAB with (Sr/P) N <3 3 (although we have divided CAB into two groups they should be interpreted as a continuum). Isotopic compositions correlate with (Sr/P) N, which is interpreted to be a geochemical indicator of the modern subduction component in CAB (see text). The origin of these rocks is unclear, but probably involves source or magma mixing between HAOT and CAB before phenocryst crystallization. Four of these samples, LC , LC86-951, LC , and LC , were analyzed here for Hf isotopic composition. RESULTS The samples used for this investigation represent a subset of samples for which mineral, major element, trace element, and isotopic compositions were determined (Clynne, 1993; Borg, 1995; Bacon et al., 1997; Borg et al., 1997, 2000; Clynne & Borg, 1997) and were chosen because they encompass the range of mineralogical and geochemical variability observed in the southernmost Cascades. The Hf isotopic data for CAB and HAOT lavas are reported in Table 1 along with analytical techniques and standards. The Hf isotopic compositions of the Lassen rocks range from 176 Hf/ 177 Hf of (ε Hf =+5 2) to (ε Hf =+12 2). In comparison, ε Nd values range from +1 3 to +6 5, and 87 Sr/ 86 Sr ratios range from to The Hf Nd isotopic systematics of the Lassen lavas are compared with lavas from the Pacific oceanic basin, the western USA, and island arcs in Fig. 3a and b. The Lassen lavas have Hf Nd isotopic systematics that are similar to worldwide island-arc volcanics (White & Patchett, 1984; Salters & Hart, 1991; Pearce et al., 1999) and fall in the Hf Nd mantle array defined by oceanic basalts. In fact, with the 709

6 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 4 APRIL 2002 Fig. 3. Isotopic compositions of Lassen CAB and HAOT. Χ, CAB with (Sr/P) N >3 3; Φ, CAB with (Sr/P) N <3 3; Α, typical HAOT; Ν, HAOT with small calc-alkaline geochemical signatures. Sr, Nd, and Hf isotopic data for Bulk Silicate Earth (BSE), MORB, worldwide OIB, and Hawaiian OIB (H-OIB), island-arc volcanics (IAV), western Great Basin (WGB), Basin and Range (B & R), and Sonoma Tolay volcanics (S-T) from White & Patchett (1984), Hegner & Tatsumoto (1987), White et al. (1987), Farmer et al. (1989), Chen et al. (1991, 1996), Salters & Hart (1991), Garcia et al. (1993, 1996), Leeman et al. (1994), Roden et al. (1994), Beard & Glazner (1995), Blichert-Toft & Albarède (1997, 1999), Salters & White (1998), Blichert-Toft et al. (1999), Jackson et al. (1999), Pietruszka & Garcia (1999) and J. Blichert-Toft (unpublished data, 2000). (a, b) Hf Nd isotope plot. The Hf Nd systematics of the Lassen lavas are similar to Hawaiian OIB. (c) Sr Hf isotope plot. MORB field estimated from range of Sr isotopic compositions observed in Juan de Fuca MORB and Hf isotopic compositions measured in N-MORB worldwide. Curves are mixing models for slab-derived fluid and depleted mantle. Intervals are 0 1, 0 5, and 2% fluid. Composition of slab-derived fluid from Stolper & Newman (1994) and Borg et al. (1997) (Sr 4681 ppm, Hf 27 ppm). Sr and Hf isotopic compositions are assumed to be Juan de Fuca MORB-like ( 87 Sr/ 86 Sr = ; 176 Hf/ 177 Hf = ; see text). The composition of depleted mantle is estimated from partial melting models of MORB (Borg et al., 1997; Sr 5 8 ppm; Hf 0 2 ppm). The Sr and Hf isotopic compositions of the mantle source for lower curve are estimated to be similar to average low (Sr/P) N lavas ( 87 Sr/ 86 Sr = and 176 Hf/ 177 Hf = ) and the Sr and Hf isotopic compositions for the upper curve are estimated to be similar to the CAB sample with the most radiogenic Hf isotopic composition (LB92-166; 87 Sr/ 86 Sr = and 176 Hf/ 177 Hf = ). (d) Hf Pb isotopic plot. Terrigeneous sediment ( Trg. seds ) estimated from Pb isotopic composition Gorda Basin sediments subducting at present (Church & Tilton, 1973; Church, 1976) and Hf isotopic compositions from modern sediments from juvenile terranes in the Canadian Cordillera (Vervoort et al., 1999). Curves are mixing models for slab-derived fluid and depleted mantle with different isotopic compositions. Intervals are 0 1, 0 5, and 2% fluid. Composition of slab-derived fluid from Stolper & Newman (1994) and Borg et al. (1997) (Pb 17 ppm; Hf 27 ppm), and Pb and Hf isotopic compositions are assumed to be MORB-like ( 206 Pb/ 204 Pb = 18 4; 176 Hf/ 177 Hf = ). The composition of depleted mantle is estimated from partial melting models of MORB (Borg et al., 1997; Pb 0 03 ppm; Hf 0 2 ppm). The Pb and Hf isotopic compositions for lower curve are estimated to be similar to average low (Sr/P) N CAB ( 206 Pb/ 204 Pb = and 176 Hf/ 177 Hf = ) and the Pb and Hf isotopic compositions for the upper curve are estimated to be similar to the CAB with the most radiogenic Hf isotopic composition (LB92-166; 206 Pb/ 204 Pb = and 176 Hf/ 177 Hf = ). It should be noted that, to reproduce the Sr Hf Pb isotopic systematics of the lavas, the mixing models require mantle source regions with variable Hf isotopic compositions. exception of a few samples, all of the Lassen lavas lie in 1997). Lavas from the Sonoma Tolay volcanic field in the Hf Nd field defined by Hawaiian OIB, and are also northwestern California are the only mafic lavas analyzed similar to some Basin and Range lavas from the western from the western USA that have Hf and Nd isotopic Great Basin. The western Great Basin lavas are identified compositions that are more radiogenic than the Lassen from other Basin and Range lavas by elevated Sr contents lavas (Beard & Johnson, 1997). and 87 Sr/ 86 Sr ratios (Leeman, 1970; Beard & Johnson, The Hf isotopic composition of the lavas correlates 1997). Other Basin and Range lavas, characterized by inversely with the many incompatible element abundances, lower Sr contents and 87 Sr/ 86 Sr ratios, have significantly such as LREE (Fig. 4a), but not with SiO 2 (Fig. lower Hf isotopic compositions for a given Nd isotopic 4b) or abundances of highly compatible elements such composition than the Lassen lavas (Beard & Johnson, as Ni ( Fig. 5). The Hf isotopic composition of CAB 710

7 BORG et al. PETROGENESIS OF LASSEN REGION LAVAS Table 1: Hf, Nd, Sr, Pb and Os isotopic composition of Lassen CA and HAOT lavas Sample Lava (Sr/P) N 176 Lu/ type 177 Hf 176 Hf/ ± ε Hf 143 Nd/ ε Nd 87 Sr/ Hf Nd Sr 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb 187 Os/ 188 Os LB CAB LC CAB LM CAB LB CAB LC CAB LB CAB LC CAB LC CAB LM CAB LB CAB LM CAB LC CAB LC CAB LM CAB LB CAB LC CAB LC CAB LB CAB LC CAB LC CAB LB CAB LB CAB LC CAB LB CAB LM CAB LC CAB LC CAB LB CAB LC CAB LC CAB LM CAB LB CAB LC CAB LC CAB LC CAB LC CAB LB CAB LC CAB LC CAB LC CAB LC HAOT LC HAOT LC HAOT LC HAOT LC HAOT LMM HAOT LC HAOT LM80-773A HAOT Hf isotope compositions were determined by multi-collector magnetic sector inductively coupled-plasma mass spectrometry using the VG model Plasma 54 in Lyon (Blichert-Toft et al., 1997). Hf chemical purification followed the method outlined by Blichert-Toft et al. (1997). Uncertainties reported on Hf measured isotope ratios are 2σ/n 1/2 analytical errors in last decimal place, where n is number of measured ratios. 176 Hf/ 177 Hf of the JMC-475 Hf standard was ±1 (Hf standard run every third sample). 176 Hf/ 177 Hf was normalized for mass fractionation to 179 Hf/ 177 Hf = ε Hf values were calculated relative to ( 176 Hf/ 177 Hf) CHUR(0) = (Blichert-Toft & Albarède, 1997). Results, analytical techniques, and uncertainties for Sr, Nd, Pb and Os analyses reported by Bacon et al. (1997), Borg et al. (1997, 2000, and unpublished data, 2000). ε Nd values were calculated relative to ( 143 Nd/ 144 Nd) CHUR(0) = Major element, trace element, and mineralogical data for these lavas have been given by Clynne (1993), Bacon et al. (1997), Borg et al. (1997) and Clynne & Borg (1997). 711

8 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 4 APRIL 2002 Fig. 4. ε Hf vs (a) La ppm and (b) SiO 2 wt %. Symbols as in Fig. 3. also varies with geochemical indices of the subduction component, such as (Sr/P) N, so that lavas with low (Sr/P) N also have low ε Hf values (Fig. 2f ). Assimilation fractional crystallization As discussed above, the lavas analyzed for Hf isotopic compositions in this study were chosen because they are the most primitive lavas observed in the Lassen region. As a result, they are expected to have undergone a minimal amount of differentiation and thus are likely to have had only minimal interaction with the continental crust. This conclusion is supported by the observation that many CAB and HAOT have mantle or near mantlelike 187 Os/ 188 Os isotopic ratios (Table 1; Hart et al., 1997; Borg et al., 2000; Borg et al., unpublished data, 2000). Furthermore, there is no correlation between Hf isotopic composition and SiO 2 (Fig. 4b) or between Hf isotopic composition and compatible element abundances such as Ni within the Lassen suite ( Fig. 5). Assimilation fractional crystallization (AFC) models presented in Fig. 5 demonstrate that Ni abundances decrease dramatically with a limited amount of differentiation, whereas Hf isotopic compositions require significantly more differentiation before they are affected. Thus, the Hf isotopic compositions of Lassen lavas with high Ni contents are unlikely to be strongly affected by AFC. It should be noted that several high (Sr/P) N samples with low Ni abundances have among the most radiogenic Hf isotopic compositions, suggesting that they have evolved by fractionation of small amounts of olivine with no measurable contamination ( Fig. 5). The Hf isotopic composition of two samples, LB and LC , may reflect significant crustal contamination. These samples have the lowest Hf and Nd isotopic compositions and the most Fe-rich olivines (Fo ) analyzed in the suite (Borg, 1995; Clynne & Borg, 1997). These samples are not used to model the isotopic composition of mantle source regions. PROCESSES THAT MODIFY Hf AND Nd ISOTOPIC COMPOSITIONS OF LASSEN MAGMAS The Hf and Nd isotopic compositions of the Lassen lavas may result from (1) interaction of primitive magmas with continental crust, (2) mixing between components derived from the mantle wedge and the subducting slab, and (3) the long-term isotopic variability of the mantle source regions. In this section, the effect that crustal differ- entiation and the addition of the subduction component to the mantle wedge has on the Hf Nd isotopic sys- tematics of the Lassen lavas is discussed. Addition of the subduction component to the mantle wedge Borg et al. (1997) argued that much of the geochemical variability observed in the Lassen CAB suite results from the variable contribution of the subduction component to the mantle source region. HAOT also have a weak subduction-related geochemical signature (Bacon et al., 1997). White & Patchett (1984) noted that the Hf isotopic compositions of many arc rocks are less radiogenic than MORB and concluded that this reflected contributions from a subduction component dominated by pelagic sediment. Thus, the Hf and Nd isotopic compositions of the Lassen lavas could be influenced by variable contributions from the subducted slab. Below we discuss the effect that variable contributions of the modern subduction component has on the compositions of CAB and HAOT. Effect of the subduction component on the Hf and Nd isotopic composition of CAB The trace element and isotopic signature of the subduction component depends on several factors including: 712

9 BORG et al. PETROGENESIS OF LASSEN REGION LAVAS Fig. 5. Plot of Ni vs ε Hf. Curves 1 and 2 are assimilation fractional crystallization (AFC) models, and curve 3 is a fractional crystallization (FC) model. The ratios of the rate of assimilation/rate of crystallization (Ra/Rc) are 0 25 (curve 1) and 0 5 (curve 2). The parental magma has 350 ppm Ni and 2 8 ppm Hf. The assimilant has 5 ppm Ni and 4 1 ppm Hf and is typical of crustal rocks observed in the Lassen region (Borg & Clynne, 1998). The Hf isotopic composition of the crust is unconstrained so a relatively low ε Hf of 15 is assumed. The ε Hf of the parental magmas are varied. The fractionating assemblage is olivine (distribution coefficients in Table 2). Small diamonds represent fractionation intervals of 5%. The models demonstrate that most lavas with high Ni abundances are unlikely to have had their Hf isotopic compositions modified by AFC. It should be noted that some high (Sr/P) N CAB have radiogenic Hf isotopic compositions, but low Ni abundances, consistent with small amounts of FC. Symbols as in Fig. 3. Table 2: Partition coefficients sediment component is modeled simply as bulk Pacific basin sediment (Ben Othman et al., 1989). The model results are not strongly dependent on the type of sediment Sm Nd Lu Hf Ni (terrigeneous vs pelagic) because both types of sediment have similar abundances of Hf and Nd (Vervoort et al., olivine ). opx The slab-derived fluids calculated for Mariana trough cpx basalts by Stolper & Newman (1994) have elevated Sr/ spinel Nd and (Sr/P) N ratios and are strongly enriched in garnet incompatible elements such as Sr (4681 ppm), Pb (17 amph ppm), Nd (147 ppm), and Zr (985 ppm). Although Stolper & Newman (1994) did not estimate the abundance of Compiled by Borg et al. (1997). Hf, slab-derived fluids from the southern Cascades arc McKenzie & O Nions (1991). are estimated to be as high as 56 ppm (Grove et al., Calculated by Clynne & Borg (1997). 2002). These calculations belie a misconception that slabderived fluids are depleted in HFSE. Rather, slab-derived (1) the initial composition of the subducted oceanic basalt fluids have high LILE/HFSE ratios relative to most and sediment; (2) the basalt/sediment ratio contributing mantle and crustal rocks because they have very high to the subduction component; (3) the composition and LILE abundances. The presence of the subduction com- proportion of fluids or melts released from the slab before ponent in the source region of CAB can therefore reaching the forearc; (4) the phases that are present in potentially affect their Sr, Pb, Nd, and Hf isotopic com- the slab during dehydration melting beneath the forearc. positions. In the discussion presented below, the subduction comcomponent Stolper & Newman (1994) demonstrated that the fluid ponent is modeled to be a mixture of fluids derived from in Mariana trough magmas has unradiogenic dehydration of oceanic basalt and Pacific sediments. The 87 Sr/ 86 Sr (0 7026) and Pb isotopic compositions (e.g. composition of the fluid component used here is that 207 Pb/ 204 Pb = 15 56) and a radiogenic 143 Nd/ 144 Nd estimated by Stolper & Newman (1994), whereas the (0 5129) isotopic composition. Inverse correlations of 713

10 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 4 APRIL 2002 (Sr/P) N with 87 Sr/ Sr, Pb/ Pb, Nd/ 144 Nd, and isotopic composition of a MORB-like source with an ε Nd 176 Hf/ 177 Hf isotopic ratios of CAB (Fig. 2) indicate that value of +9 will only be lowered to +8 4 by the addition the subduction component in the southernmost Cascade of the maximum amount of average Pacific sediment arc is also characterized by unradiogenic Sr and Pb and (ε Nd = 6; Ben Othman et al., 1989) allowed by the radiogenic Nd (and Hf ). Mixing models demonstrate models. A similar calculation for Hf demonstrates that that the Sr and Pb isotopic variability observed in CAB the Hf isotopic composition of a MORB-like source will can be produced by mixing between peridotite with the only be lowered by 1 3 epsilon units by the addition of isotopic composition of the low (Sr/P) N lavas and a sediment with an ε Hf of 49 [corresponding to the most subduction component that is isotopically MORB-like unradiogenic sediment analyzed by Vervoort et al. (1999)]. (Bacon et al., 1997; Borg et al., 1997; Grove et al., 2002; The contribution of sediment to the Hf Nd isotopic Fig. 3c and d). The fact that lavas with relatively large composition of the Lassen source region is therefore subduction signatures have isotopic compositions that minimal and will be ignored in the following calculations approach MORB suggests that the contribution of either that estimate the isotopic effect that the addition of the terrigeneous or pelagic sediment to the isotopic composition subduction component has on the mantle wedge. of the lavas is either minimal and/or masked The proportion of Hf and Nd derived from fluids from by a proportionally large contribution of dehydration the slab is proportional to the Sr/P ratios of the lavas products of the subducted oceanic basalt. (Borg et al., 1997). The low (Sr/P) N lava source was The mixing models presented in Fig. 3c and d also calculated to inherit 1 2% of its Hf and Nd from the slabdemonstrate that simple binary mixing between a single derived fluids, whereas the highest (Sr/P) N lava source was mantle source and a slab-derived fluid will not reproduce calculated to have up to 45% of its Hf and 54% of its all the Sr, Pb, and Hf isotopic variability observed in Nd inherited from slab-derived fluids (Borg et al., 1997). CAB. To reproduce all of the isotopic variability observed These represent maximum values and are used to present in CAB at least three sources are required. This stems an extreme scenario for subduction control of the Hf from the fact that at a particular Sr or Pb isotopic and Nd isotopic compositions of CAB. The Hf and Nd composition there is a range of Hf (and Nd) isotopic isotopic composition of the CAB source region before compositions. This suggests that CAB are derived from fluxing with the subduction component is estimated by multiple mantle source regions with variable Hf (and Nd) subtracting the proportions of Hf and Nd inherited from isotopic compositions. It is therefore probable that the slab-derived fluids from the isotopic composition of the Hf and Nd isotopic variations observed in the CAB suite lavas. The slab-derived fluid is assumed to have a 143 Nd/ reflect the isotopic compositions of the mantle wedge 144 Nd ratio of average Juan de Fuca MORB ( ; to a large extent. To investigate this possibility, the Hegner & Tatsumoto, 1987; White et al., 1987). The contribution of slab-derived Hf and Nd is estimated 176 Hf/ 177 Hf ratio of the fluid component is estimated to using the results of source mixing partial melting models be using a regression of the Hf Nd isotopic presented by Borg et al. (1997) and in Fig. 1. compositions of primitive CAB and the 143 Nd/ 144 Nd ratio The contribution of Hf and Nd from the modern of average Juan de Fuca MORB. subduction component was assessed by Borg et al. (1997). The difference between the isotopic compositions cal- They modeled the incompatible element compositions culated for the part of the mantle wedge that has not been of CAB assuming that they were derived from peridotites enriched through the addition of the modern subduction with MORB to OIB-source incompatible element com- component and the measured Hf and Nd isotopic compositions positions that were enriched through variable input from of the Lassen lavas is presented in Fig. 6. The the subducted slab ( Fig. 3b). The subduction component calculations indicate that the Hf isotopic composition of was modeled as a mixture of slab-derived fluids with all but two of the lavas is within 2 epsilon units of the the composition based on the calculations of Stolper & non-subduction enriched peridotites, and that all but two Newman (1994) for Mariana arc basalts and Pacific basin of the low (Sr/P) N lavas are within 0 5 epsilon units. sediments measured by Ben Othman et al. (1989). From This suggests that most of the 6 5 epsilon unit variation these models the proportions of Hf and Nd derived from observed in the Hf isotopic composition of CAB is not the mantle wedge peridotite and fluids from subducted the result of variable contributions from the subduction oceanic crust and sediments were determined. component. In contrast, the Nd isotopic compositions of The proportion of Hf derived from sediment was the highest (Sr/P) N lava is estimated to be 6 epsilon units estimated to range from 1 to 2% and the proportion of higher than its non-subduction enriched peridotite source, Nd was estimated to range from 2 to 6% for the various whereas most low (Sr/P) N lavas are within 2 epsilon units. models. The relatively small sediment contribution to the Taken together, these results suggest that the Hf and Nd modern subduction component means that sediment will isotopic compositions of the low (Sr/P) N lavas are not not significantly contribute to the Hf and Nd isotopic strongly affected by the addition of the modern sub- compositions of the source regions. For example, the Nd duction component to the mantle wedge, whereas the 714

11 BORG et al. PETROGENESIS OF LASSEN REGION LAVAS Fig. 6. Plot of measured ε Hf and ε Nd isotopic compositions of CAB minus the isotopic compositions estimated for the mantle after the modern subduction component has been removed (see text). Diamonds represent Nd isotopic data, whereas triangles represent Hf isotopic data. Open symbols are low (Sr/P) N CAB; filled symbols are high (Sr/P) N CAB. ε measured ε mantle = 0 means that the isotopic composition of the lavas is equal to the isotopic composition of mantle before the addition of the modern subduction component. The Hf and Nd isotopic compositions of CAB with (Sr/P) N <2 are not strongly affected by the subduction component. Hf and Nd isotopic compositions of the high (Sr/P) N lavas are affected to some extent. As a result, the isotopic ratios of the low (Sr/P) N lavas are used in the following section to constrain the geochemical evolution of the mantle wedge beneath the southernmost Cascades. Effect of the subduction component on the Hf and Nd isotopic composition of HAOT The majority of Lassen HAOT, such as LMM81-297, have relatively flat incompatible element patterns with variable enrichments in Ba, Sr, and Pb relative to other equally incompatible elements (Clynne, 1993), indicative of a weak but variable arc geochemical signature (Bacon et al., 1997). Some HAOT, such as LC and LC86-951, have significantly higher incompatible element abundances that are suggestive of a larger calcalkaline input. These samples are considered to be HAOT lavas because they have appropriate mineral compositions (Clynne, 1993; Clynne & Borg, 1997) but have been classified as calc-alkaline by Bacon et al. (1997) based on incompatible element compositions. Partial melting models are unable to reproduce the compositions of typical HAOT lavas (e.g. LMM81-279) from an amphibole- and garnet-free depleted mantle source that has been enriched through the addition of a modern subduction component similar to that observed in CAB (Fig. 1c). Instead, a component is required that, on one hand, has elevated (Sr/P) N, La/Ta, and Ba/Ta ratios like the modern subduction component, but, on the other hand, has lower Rb/Ta, K/Ta, and LREE/ HREE ratios. The fact that HAOT lavas have 207 Pb/ 204 Pb isotopic ratios that are significantly elevated above both modern subducted sediments (Church & Tilton, 1973; Church, 1976) and the northern hemisphere reference line (Table 1) indicates that this component could not be added to their mantle source region in the modern subduction regime. Instead, elevated 207 Pb/ 204 Pb isotopic ratios of HAOT suggest that their source has been enriched in a subduction component containing a rel- atively large proportion of ancient sediment. Bacon et al. (1997) have argued that this component was added to the lithospheric mantle by calc-alkaline magmas, possibly during the Mesozoic era. Thus, the weak subduction signature observed in most HAOT is not a result of the presence of the modern subduction component. There- fore, the modern subduction component is expected to have little influence on the Hf and Nd isotopic com- positions of these HAOT. The LILE enrichment observed in a few of the HAOT lavas, such as LC , can be produced by mixing between LILE-depleted HAOT magmas and typical CAB magmas ( Fig. 1c). These lavas are likely to be produced by mixing of HAOT and CAB magmas before the onset of phenocryst crystallization (Clynne, 1993). These hybrid HAOT lavas are not used to distinguish petrogenetic 715

12 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 4 APRIL 2002 differences between HAOT and CAB and are not discussed further. ISOTOPIC EVOLUTION OF THE MANTLE BENEATH THE LASSEN REGION Because the Hf Nd isotopic compositions of the low (Sr/P) N CAB and HAOT lavas are least affected by contributions from the modern subduction component, they are used to constrain the isotopic composition of the mantle wedge before enrichment by the modern subduction component. Unique sources for CAB and HAOT magmas Evidence for garnet The Lassen CAB have LREE-enriched patterns indicated by chondrite-normalized La/Lu ratios that are as high as nine, whereas most HAOT lavas that lack calc-alkaline trace element signatures (see above) have chondrite- normalized La/Lu ratios of one or less. Clynne (1993) suggested that the elevated LREE/HREE ratios observed in CAB required them to have equilibrated with garnet at some point early in their history. Furthermore, Borg et al. (1997) could only model REE patterns of CAB if garnet was present in their source regions. In contrast, the lower La/Lu ratios of the HAOT lavas do not require garnet to be present. Simple batch partial melting models are consistent with the presence of garnet in the source region of CAB, and its absence in the source region of HAOT (Fig. 7). These models assume that the sources for CAB and HAOT have similar abundances of Lu and Hf. However, variations in the composition of chromian spinels in HAOT and CAB indicate that the source regions of these lavas are different (Borg & Clynne, 1997). The presence of garnet in the source regions of the low (Sr/ P) N CAB and HAOT can also be assessed using their Hf isotopic compositions. The advantage of this approach is that the composition of the source regions is calculated based on the Hf isotopic compositions of the lavas. The underlying assumption of these models is that the Lu/ Hf ratios and Hf isotopic compositions of the lavas reflect their parental melt compositions, and that their Hf isotopic compositions record the long-term Hf isotopic composition of their source regions. In other words, the Hf isotopic compositions of the lavas are not strongly affected by either AFC or the recent addition of Hf from the modern subduction component. These assumptions Fig. 7. Plot of Hf and Lu abundances of CAB and HAOT. Symbols as in Fig. 3. Curves represent batch non-modal partial melting models. Mineral modes and melting proportions are the same as those reported in Fig. 1. Partition coefficients are reported in Table 2. The source has three times depleted mantle values (McKenzie & O Nions, 1991; Hf 0 66 ppm, Lu 0 16 ppm). Diamonds on curves represent 1% melt intervals starting at 1%. Model demonstrates that Hf Lu systematics of CAB and HAOT are consistent with derivation from garnet-bearing and garnet-free sources. analyzed in this study and the Hf isotopic systematics calculated for the subduction component. The 176 Lu/ 177 Hf ratio of CAB and HAOT source regions is calculated based on the measured Hf isotopic compositions of the lavas in Fig. 8. The calculated 176 Lu/ 177 Hf ratio depends on the age of the mantle reservoir and its initial Hf isotopic composition (Beard & Johnson, 1993). As a result, calculations are made for sources having ages from 0 5 to 2 Ga and ranging from primitive (chondritic) mantle to depleted mantle compositions. Superimposed on this plot are non-modal batch partial melting models of Beard & Johnson (1993) that distinguish between garnet- and spinel-bearing source regions. The location of the boundaries of the garnet and the spinel peridotite fields depends on the amount of melting re- quired to produce the magmas, which is assumed to range from 1 to 20%. Thus, the amount of melting required to produce any given magma increases from left to right in Fig. 8. From Fig. 8 it is apparent that even 2 Ga peridotites are required to contain garnet to produce the measured 176 Lu/ 177 Hf ratios of CAB from their calculated sources. This is true even if the peridotite has a depleted mantle seem reasonable given the primitive nature of the lavas composition before further depletion at 2 Ga ( Fig. 8). 716

13 BORG et al. PETROGENESIS OF LASSEN REGION LAVAS Fig. 8. Plot of measured 176 Lu/ 177 Hf of the lavas vs 176 Lu/ 177 Hf estimated for their source region using measured Hf isotopic compositions of the lavas. Symbols as in Fig. 3. The 176 Lu/ 177 Hf ratios of the sources are calculated from the Hf isotopic compositions of the lavas assuming the sources initially had Hf isotopic compositions that ranged from chondritic (CHUR; ε Hf = 0) to depleted mantle (DM; ε Hf =+7 6; Patchett et al., 1981) and are Ga and 2 Ga, respectively. Garnet and spinel peridotite fields are from Beard & Johnson (1993) and based on nonmodal partial melting models of Ottonello et al. (1984). Field boundaries are dependent on the percent of partial melting, which increases from left to right on the figure. Most CAB fall in the garnet peridotite field and most HAOT fall in the spinel peridotite field. CAB with a large contribution of the subduction com- Isotopic differences between CAB and HAOT sources ponent, i.e. the high (Sr/P) N lavas, fall very near the low The fact that the Hf and Nd isotopic compositions of (Sr/P) N lavas, further indicating that the model results low (Sr/P) N CAB and HAOT are least affected by do not depend on the size of the modern subduction contributions from the subducting slab permits comsignature. parisons between the isotopic compositions of their In general, the addition of a subduction-derived com- mantle sources. Furthermore, because the subduction ponent to the mantle wedge will have only a limited component in the Lassen region has unradiogenic Sr effect on the results of these models. This stems from the and Pb isotopic compositions, the Sr and Pb isotopic fact that, to affect the outcome of the models, the Hf compositions of low (Sr/P) N samples can be used to isotopic composition and Lu/Hf ratios of the mantle constrain the minimum Sr and Pb isotopic compositions wedge must be altered dramatically by the addition of of the mantle wedge before modern subduction zone the modern subduction component. For example, for a enrichment. typical Lassen CAB to fall in the spinel peridotite field Evidence for the presence of garnet in the source in Fig. 8, its Hf isotopic composition must be >9 epsilon region, as well as the depth of the subducted slab beneath units lower than the measured value. Furthermore, the the southern Cascade arc (Guffanti et al., 1990), suggests addition of the subduction component to the mantle that the CAB source region is in the asthenosphere. In wedge is unlikely to significantly change its Lu/Hf ratio contrast, the lack of evidence for the presence of garnet because bulk sediment and MORB, as well as fluids in the source region, combined with experimental eviderived from these components, are expected to have dence for equilibrium of HAOT at kbar (Bartels Lu/Hf ratios that are roughly similar to the mantle et al., 1991), suggests that the HAOT source region is in sources (e.g. Stolper & Newman, 1994; Vervoort et al., the lithosphere. The isotopic composition of the CAB 1999). source region is best represented by the range of isotopic In contrast to CAB, HAOT have significantly higher compositions of low (Sr/P) N CAB with small subduction 176 Lu/ 177 Hf ratios and therefore generally fall in the spinel geochemical signatures [i.e. (Sr/P) N <2; excluding LB91- peridotite field. In fact, the source region for HAOT 125] and is: 87 Sr/ 86 Sr = ; 208 Pb/ 204 Pb = must be melted >20% and be <0 5 Ga to contain garnet ; 207 Pb/ 204 Pb = ; 206 Pb/ 204 Pb = It is therefore likely that HAOT are derived from spinel ; ε Nd = +2 4 to +4 5; ε Hf = +7 5 to bearing peridotites, whereas the ultimate source of CAB The isotopic composition of the lithospheric is garnet bearing. portion of the mantle beneath the Lassen region is best 717

14 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 4 APRIL 2002 represented by the range of isotopic composition of in the eastern central Pacific or the western USA, suggesting HAOT (excluding hybrid HAOT-CAB samples) and is: the mantle source region beneath the south- 87 Sr/ 86 Sr = ; 208 Pb/ 204 Pb = ; ernmost Cascades is isotopically unique. 207 Pb/ 204 Pb = ; 206 Pb/ 204 Pb = ; ε Nd =+4 4 to +6 2; ε Hf =+9 7 to Evidence of the addition of an ancient subduction The Pb isotopic compositions of CAB and HAOT are indistinguishable. This suggests that the Pb isotopic component to mantle source regions compositions of the asthenospheric mantle wedge may The fact that low (Sr/P) N CAB and HAOT have very not be much higher than the compositions of the low (Sr/ small modern subduction geochemical signatures, yet P) have fairly radiogenic 207 Pb/ 204 N CAB. If this is true, then the Pb isotopic composition of Pb ratios, suggests that the mantle beneath the Lassen region is fairly constant. their mantle source regions have been enriched by the However, the Hf and Nd isotopic compositions of low addition of an ancient subduction component. This en- (Sr/P) N CAB are less radiogenic than the Hf and Nd richment could also account for the fact that both low isotopic composition of HAOT. The Sr isotopic comcompositions that are lower than depleted mantle values. (Sr/P) N CAB and HAOT have Hf and Nd isotopic position of the average low (Sr/P) N CAB may also be slightly higher than HAOT. Thus, despite similar Pb If this scenario is correct, then the isotopic systematics isotopic compositions CAB and HAOT appear to be of the ancient subduction component must be distinct derived from different mantle source regions with differacterized from the modern subduction component, being char- ent Hf, Nd, and Sr isotopic compositions. by low ε Hf and ε Nd and high 207 Pb/ 204 Pb ratios. Differences in the isotopic composition of the ancient and modern subduction component are consistent with a greater contribution of sediment in the ancient subduction component relative to the modern subduction com- Constraints on mantle sources ponent. The western USA has been the site of convergent Comparison with mantle sources present in the region plate tectonics since the Paleozoic, at least, so that the A central issue is whether the mantle sources beneath ancient subduction component could have been added the southernmost Cascades are similar to other mantle to the mantle at almost any time (e.g. Hooper et al., sources inferred to be present in the eastern Pacific and 1995; Rogers et al., 1995). However, there are intrusive the western USA and whether mixing of these mantle calc-alkaline rocks near the Lassen region emplaced sources can account for the inferred isotopic variability during Sierran magmatic events in the late Mesozoic era of the mantle source beneath the southernmost Cascades. (Gromme et al., 1967) and during Antler magmatic events The Pb isotopic compositions of the low (Sr/P) N CAB in the Devonian period (Hietanen, 1976). It is therefore and HAOT are more radiogenic than lavas from oceanic probable that ancient subduction zone enrichment of the environments in the eastern central Pacific region. More mantle could have occurred at these times. radiogenic Pb in the Lassen lavas relative to MORB We model the Hf, Nd, and Pb isotopic composition and Hawaiian OIB cannot reflect input of the modern of low (Sr/P) N CAB and HAOT sources to determine if subduction component because this component has unbetween their isotopic compositions are consistent with mixing radiogenic Pb isotopic compositions and is minimal in depleted mantle sources and a sediment-en- the source regions of these lavas (Borg et al., 1997). The riched subduction component at 0 1, 0 4, and 1 Ga. Sonoma Tolay lavas lie on the MORB end of the Lassen These models also illustrate the potential relationship Hf Nd isotopic array and may be representative of a between the low (Sr/P) N CAB and HAOT sources. The depleted mantle component in northwestern California. Hf and Nd isotopic compositions of the low (Sr/P) N CAB Although these lavas are characterized by radiogenic and HAOT source regions are calculated at various ages Pb isotopic compositions (average 206 Pb/ 204 Pb = 18 75; using the present-day isotopic compositions of the lavas 207 Pb/ 204 Pb = 15 60), they have lower 206 Pb/ 204 Pb ratios and estimates of 176 Lu/ 177 Hf and 147 Sm/ 144 Nd ratios of for a given 207 Pb/ 204 Pb ratio than the Lassen lavas (Table their source regions (see Fig. 9 caption). The mixing 1) and therefore are not representative of the Lassen models demonstrate that low (Sr/P) N CAB and HAOT mantle source. Continental basaltic lavas also have in- sources have isotopic compositions that are consistent appropriate isotopic compositions. For example, Basin with mixing between a MORB-source and a subduction and Range lavas have Nd isotopic compositions that are component characterized by low ε Hf and ε Nd and high too radiogenic ( Fig. 3b), whereas the western Great 207 Pb/ 204 Pb (Fig. 9a). It should be noted that as the age Basin lavas have Sr isotopic compositions that are too of subduction zone enrichment of the mantle source radiogenic ( 87 Sr/ 86 Sr = ; Farmer et al., 1989; regions increases, the estimated Nd isotopic compositions Beard & Glazner, 1995). Thus, the isotopic compositions of the sources spread away from the mixing line. This of the Lassen lavas are unlike those of other lavas observed suggests that the addition of the subduction component 718

15 BORG et al. PETROGENESIS OF LASSEN REGION LAVAS to the mantle was relatively recent. Alternatively, the inferred range of Hf and Nd isotopic compositions of the CAB and HAOT sources could reflect mixing between several depleted mantle sources and multiple subduction components with variable isotopic compositions. Although these models do not define the age of subduction zone enrichment conclusively, the models in which the subduction component was added to the mantle most recently seem to best fit the data. It should be noted that in most of the mixing models in Fig. 9, the low (Sr/P) N CAB and HAOT sources fall along a common mixing line. This suggests that the isotopic differences between CAB and HAOT sources are a result of an increased proportion of ancient subduction component in the CAB source. However, the compositions of chromian spinels in CAB and HAOT indicate that the two source regions are distinct (Clynne & Borg, 1997). These differences might reflect the style of meta- somatism in the lithosphere and asthenosphere. In the asthenospheric mantle, the subduction component might be added to the wedge by fluids coming off the slab, whereas in the lithospheric mantle, the subduction component might be added by magmas rising from the asthenosphere. Thus, this lithospheric mantle might, on average, be more fertile than the asthenospheric mantle. This model is consistent with the conclusion of Bacon et al. (1997) that the geochemical enrichment observed in HAOT in the southern Cascades reflects the passage of a calc-alkaline component to the mantle lithosphere. A PETROGENETIC MODEL FOR THE SOUTHERNMOST CASCADES Several observations constrain the petrogenetic model of the Lassen lava suite including the following: (1) the mantle underwent metasomatism through the addition of an ancient subduction component, possibly during Sierran magmatism; (2) the fertility of the mantle increases Fig. 9. Mixing models between an ancient subduction component and mantle peridotites demonstrating the potential relationship between CAB and HAOT. The calculated isotopic compositions of the source regions of CAB with (Sr/P) N <2 (Φ) and HAOT (Μ) at 0 1, 0 4, and 1 Ga are plotted. The Hf and Nd isotopic compositions of the low (Sr/P) N CAB and HAOT source regions are calculated at various ages using the present-day isotopic compositions of the lavas, and estimated 176 Lu/ 177 Hf and 147 Sm/ 144 Nd ratios of their source regions. The 176 Lu/ 177 Hf and 147 Sm/ 144 Nd ratios of the sources are in turn calculated from the compositions of the lavas using simple batch partial melting models and the partition coefficients in Table 2. Five percent melting is assumed for CAB and 10% for HAOT. The modal mineralogy used to calculate source compositions of low (Sr/P) N CAB and HAOT is ol:opx:cpx: grt = 35:35:26:4 and ol:opx:cpx = 35:35:30, respectively. Model results do not change significantly if more complex non-modal melt models are used. The present-day MORB source is assumed to have ε Hf =+16; ε Nd =+10, 207 Pb/ 204 Pb = 15 48, Hf 0 2 ppm, Nd 0 65 ppm, and Pb 0 03 ppm. The Hf and Nd isotopic composition of the MORB source at 0 1, 0 4, and 1 Ga is calculated assuming 176 Lu/ 177 Hf and 147 Sm/ 144 Nd ratios of and 0 21, respectively [depleted Earth values of McKenzie & O Nions (1991)]. The subduction component is assumed to have ε Hf = 9; ε Nd = 5, 207 Pb/ 204 Pb ratios of 15 7, Hf 27 ppm, Nd 147 ppm, and Pb 17 ppm. Element abundances for MORB-source and slab-derived fluid were calculated by Borg et al. (1997). The 207 Pb/ 204 Pb ratios of the MORB, CAB, and HAOT sources at 0 1 Ga are not adjusted from their present values for 235 U decay because their 235 U/ 204 Pb ratios are very low. Both low (Sr/P) N CAB and HAOT sources have isotopic compositions that are consistent with mixing between a depleted mantle source and an ancient subduction component. The CAB source appears to be more enriched in the ancient subduction component than the HAOT source. 719

16 JOURNAL OF PETROLOGY VOLUME 43 NUMBER 4 APRIL 2002 towards the backarc and the sources of the Lassen lavas decrease in fertility from HAOT, low (Sr/P) N CAB, to high (Sr/P) N CAB; (3) garnet is a residual phase in the CAB source, and absent in the HAOT source; (4) the modern subduction signature is present in CAB and mostly absent in HAOT and decreases from forearc to back arc. The petrogenetic model for the southernmost Cascades is summarized in Fig. 10. Before the onset of recent subduction, an ancient component, characterized by unradiogenic Hf and Nd, and radiogenic Pb, is added to the mantle now beneath the Lassen region by subduction. Similar 207 Pb/ 204 Pb isotopic ratios in low (Sr/P) N CAB and HAOT are consistent with this component being added to both the lithospheric and asthenospheric mantle. The Nd isotopic systematics of low (Sr/P) N CAB and HAOT suggest that this ancient subduction component was added to the mantle source regions during Sierran magmatism ( Fig. 9). HAOT are associated with the recent impingement of the Basin and Range Province on the Lassen region (Guffanti et al., 1990), which has caused melting of the lithosphere beneath the region. The isotopic composition of the HAOT source reflects interaction between lithospheric mantle and an ancient subduction component. Mixing models in Fig. 9 suggest that the HAOT mantle source contains a proportionally smaller amount of this ancient subduction component than the CAB source. Thus, the mineralogical and geochemical differences between CAB and HAOT reflect a variety of factors including: (1) the addition of the modern subduction component to the CAB source region; (2) differences in the depth of magma generation and corresponding presence or absence of garnet; (3) derivation from isotopically distinct source regions probably resulting from variable addition of an ancient subduction component to the mantle; (4) variable fertility of their peridotite Fig. 10. Schematic representation summarizing the petrogenesis of sources. Occasionally, CAB and HAOT magmas mix the Lassen lavas. Relative position of CAB, HAOT, and Sierran plutons based on field relations in southernmost Cascades. (a) Idealized crossforming hybrid magmas with mineralogical and geosection of the southernmost Cascades during Sierran magmatism. chemical characteristics that are intermediate. Figure depicts enrichment of the mantle by calc-alkaline magmas Like the HAOT source, the mantle source of CAB from the Sierra Nevada (gray diapirs). Both the lithospheric and has been enriched through the addition of an ancient asthenospheric portion of the mantle are enriched through the addition of this ancient subduction component at this time. (b) Idealized crosssubduction component. The range of CAB is produced section of the present southernmost Cascades. Corner flow of the by variations in (1) the proportion of modern subduction mantle wedge has distributed metasomatically enriched peridotites (gray component in the source region, and (2) changes in the field) throughout the mantle wedge. CAB (black diapirs) are produced by melting of garnet-bearing asthenospheric mantle enriched through relative fertility of the source region. A decrease of the the addition of an ancient subduction component. CAB in the forearc subduction signature is observed from west to east across have large subduction signatures because large quantities of fluids are the arc, suggesting that the arc signature reflects slab- released by dehydration of the modern subducting slab in this location. dehydration in the current tectonic setting (Borg et al., As the slab becomes progressively more dehydrated to the east (right) the modern subduction signature decreases in magnitude. In addition, 1997). Geochemical indices of the modern subduction peridotites of the mantle wedge may become progressively less fertile component and mineralogical indices of source fertility from east to west as magmas are removed. Thus, forearc lavas have correlate, suggesting that larger amounts of subduction larger subduction signatures and are derived from less fertile peridotites fluids are present in more refractory mantle sources in than the lavas of the arc axis and backarc. HAOT (white diapirs) are associated with Basin and Range faulting (curved lines) impinging on the forearc. We suggest that the bulk mantle wedge the area. They represent melts of garnet-free lithospheric mantle becomes increasingly refractory as melts are extracted enriched by ancient calc-alkaline magmas. 720