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The concept of defrosting, heating and partially melting a crystal-rich, weakly mobile or immobile magma (magmatic mush) has gained wide support from volcanological studies. This process has been called on as a means to promote eruption of crystal-rich (monotonous) dacites and also to permit extraction of felsic magmas, which may accumulate in the upper crust and erupt as crystal-poor rhyolite or trachyte. Most studies of the latter type of defrosting call on a conjugate plutonic part of the system from which the felsic melt was extracted. Although petrographic and geochemical features of defrosting are well described for eruptive rocks, few studies have investigated petrographic and geochemical features of plutons that indicate defrosting. The Jurassic Ashland pluton is a tilted intrusive complex exposed in southern Oregon and northern California, USA. The central part of the pluton, quartz monzodiorite (QMD), displays petrographic features (crystal fragmentation, widespread resorption, local concentrations of refractory minerals) characteristic of partial melting. This unit is intruded and overlain by biotite granite, which displays plagioclase cores identical in composition to plagioclase in the underlying QMD, resorbed titanite cores in plagioclase, and local subhedral to anhedral cores of alkali feldspar in poikilitic alkali feldspar crystals. Hornblende and biotite in the biotite granite display enrichments in Ti and Nb relative to these minerals in QMD, negating fractional crystallization as a petrogenetic process. Instead, these high Ti and Nb contents reflect significant partial melting of titanite in the QMD, thereby releasing high field strength elements to the defrosted biotite granite melt. The biotite granite magmas are thus interpreted as the melt-rich products of defrosting caused by intrusion of mafic–intermediate magmas into the QMD magma column. [ABSTRACT FROM AUTHOR]

This study addresses the question of how and where arc magmas obtain their chemical and isotopic characteristics. The Wooley Creek batholith and Slinkard pluton are a tilted, mid- to upper-crustal part of a vertically extensive, late-Jurassic, arc-related magmatic system in the Klamath Mountains, northern California. The main stage of the system is divided into an older lower zone (c. 159 Ma) emplaced as multiple sheet-like bodies, a younger upper zone (c. 158–156 Ma), which is gradationally zoned upward from mafic tonalite to granite, and a complex central zone, which represents the transition between the lower and upper zones. Xenoliths are common and locally abundant in the lower and central zones and preserve a ghost stratigraphy of the three host terranes. Bulk-rock Nd isotope data along with ages and Hf and oxygen isotope data on zircons were used to assess the location and timing of differentiation and assimilation. Xenoliths display a wide range of εNd (whole-rock) and εHf (zircon), ranges that correlate with rocks in the host terranes. Among individual pluton samples, zircon Hf and oxygen isotope data display ranges too large to represent uniform magma compositions, and very few data are consistent with uncontaminated mantle-derived magma. In addition, zoning of Zr and Hf in augite and hornblende indicates that zircon crystallized at temperatures near or below 800 °C; these temperatures are lower than emplacement temperatures. Therefore, the diversity of zircon isotope compositions reflects in situ crystallization from heterogeneous magmas. On the basis of these and published data, the system is interpreted to reflect initial MASH-zone differentiation, which resulted in elevated δ18O and lowered εHf in the magmas prior to zircon crystallization. Further differentiation, and particularly assimilation–fractional crystallization, occurred at the level of emplacement on a piecemeal (local) basis as individual magma batches interacted with partial melts from host-rock xenoliths. This piecemeal assimilation was accompanied by zircon crystallization, resulting in the heterogeneous isotopic signatures. Magmatism ended with late-stage emplacement of isotopically evolved granitic magmas (c. 156 Ma) whose compositions primarily reflect reworking of the deep-crustal MASH environment. [ABSTRACT FROM AUTHOR]

We present microbeam major- and trace-element data from 14 monzodiorites collected from the Malaspina Pluton (Fiordland, New Zealand) with the goal of evaluating processes involved in the production of andesites in lower arc crust. We focus on relict igneous assemblages consisting of plagioclase and amphibole with lesser amounts of clinopyroxene, orthopyroxene, biotite and quartz. These relict igneous assemblages are heterogeneously preserved in the lower crust within sheeted intrusions that display hypersolidus fabrics defined by alignment of unstrained plagioclase and amphibole. Trace-element data from relict igneous amphiboles in these rocks reveal two distinct groups: one relatively enriched in high field strength element concentrations and one relatively depleted. The enriched amphibole group has Zr values in the range of ∼25–110 ppm, Nb values of ∼5–32 ppm, and Th values up to 2·4 ppm. The depleted group, in contrast, shows Zr values <35 ppm and Nb values <0·25 ppm, and Th is generally below the level of detection. Amphibole crystallization temperatures calculated from major elements range from ∼960 to 830 °C for all samples in the pluton; however, we do not observe significant differences in the range of crystallization temperatures between enriched (∼960–840 °C) and depleted groups (∼940–830 °C). Bulk-rock Sr and Nd isotopes are also remarkably homogeneous and show no apparent difference between enriched (εNdi = 0·1 to –0·1; 87Sr/86Sri = 0·70420–0·70413) and depleted groups (εNdi = 0·3 to –0·4; 87Sr/86Sri = 0·70424–0·70411). Calculated amphibole-equilibrium melt compositions using chemometric equations indicate that melts were highly fractionated (molar Mg# <50), andesitic to dacitic in composition, and were much more evolved than bulk lower continental crust or primitive basalts and andesites predicted to have formed from hydrous melting of mantle-wedge peridotite beneath an arc. We suggest that melts originated from a common, isotopically homogeneous source beneath the Malaspina Pluton, and differences between enriched and depleted trace-element groups reflect varying contributions from subducted sediment-derived melt and sediment-derived fluid, respectively. Our data demonstrate that andesites and dacites were the dominant melts that intruded the lower crust, and their compositions mirror middle and upper bulk-continental crust estimates. Continental crust-like geochemical signatures were acquired in the source region from interaction between hydrous mantle-wedge melts and recycled subducted sediment rather than assimilation and/or remelting of pre-existing lower continental crust. [ABSTRACT FROM AUTHOR]

Bulk-rock compositions are commonly used as proxies for melt compositions, particularly in silicic plutonic systems. However, crystal accumulation and/or melt loss may play an important role in bulk-rock compositional variability (McCarthy and Hasty 1976; McCarthy and Groves 1979; Wiebe 1993; Wiebe et al. 2002; Collins et al. 2006; Deering and Bachmann 2010; Miller et al. 2011; Vernon and Collins 2011; Lee and Morton 2015; Lee et al. 2015; Barnes et al. 2016a; Schaen et al. 2018). Recognizing and quantifying the effects of crystal accumulation and melt loss in these silicic systems is challenging. Hornblende-melt Fe/Mg partitioning relationships and hornblende (Hbl) chemometry are used here to test for equilibrium with encompassing bulk-rock and/or glass compositions from several plutonic and volcanic systems. Furthermore, we assess the extent to which these tests can be appropriately applied to Hbl from plutonic systems by investigating whether Hbl from the long-lived (~10 Ma) Tuolumne Intrusive Complex preserves magmatic crystallization histories. On the basis of regular zoning patterns, co-variation of both fast- and slow-diffusing trace elements, Hbl thermometry, and compositional overlap with volcanic Hbl we conclude that Hbl from plutons largely preserve records supporting the preservation of a magmatic crystallization history, although many compositional analyses yield calculated temperatures <750 °C, which is unusual in volcanic Hbl. Hornblende is only rarely in equilibrium with host plutonic bulk-rock compositions over a wide range of SiO2 contents (42–78 wt%). Hornblende chemometry indicates that the majority of Hbl from the plutonic systems investigated here is in equilibrium with melts that are typically more silicic (dacitic to rhyolitic in composition) than bulk-rock compositions. These results are consistent with crystal accumulation and/or loss of silicic melts within middle- to upper-crustal plutons. Although the processes by which melts are removed from these plutonic systems is uncertain, it is evident that these melts are either redistributed in the crust (e.g., leucogranite dikes, plutonic roofs, etc.) or are instead erupted. In contrast, Hbl from volcanic rocks is more commonly in equilibrium with bulk-rock and glass compositions. In most cases, where Hbl is out of equilibrium with its host glass, the glasses are more evolved than the calculated melts indicating crystallization from a less fractionated melt and/or mixed crystal populations. Where Hbl is not in equilibrium with volcanic bulk-rocks, the bulk-rock compositions are typically more mafic than the calculated melts. In some intermediate volcanic samples, the occurrence of wide-ranges of calculated melt compositions is indicative of magma mixing. The general absence of Hbl with temperatures <750 °C from volcanic systems suggests that magmatic mushes below this temperature are unlikely to erupt. Our results indicate that bulk-rock compositions of granitic plutonic rocks only rarely approximate melt compositions and that the possibility of crystal accumulation and/ or melt loss cannot be ignored. We suggest that detailed assessments of crystal accumulation and melt loss processes in magmatic systems are crucial to evaluating magma differentiation processes and discerning petrogenetic links between plutonic and volcanic systems. [ABSTRACT FROM AUTHOR]

Cumulate processes in granitic magma systems are thought by some to be negligible and by others to be common and widespread. Because most granitic rocks lack obvious evidence of accumulation, such as modal layering, other means of identifying cumulate rocks and estimating proportions of melt lost must be developed. The approach presented here utilizes major and trace element compositions of hornblende to estimate melt compositions necessary for zircon saturation. It then compares these estimates with bulk-rock compositions to estimate proportions of extracted melt. Data from three arc-related magmatic systems were used (English Peak pluton, Wooley Creek batholith, and Tuolumne Intrusive Complex). In all three systems, magmatic hornblende displays core-to-rim decreases in Zr, Hf, and Zr/Hf. This zoning indicates that zircon must have fractionated during crystallization of hornblende, at temperatures greater than 800 °C. This T estimate is in agreement with Ti-in-zircon thermometry, which yields a maximum T estimate of 855 °C. On the basis of this evidence, concentrations of Zr in melts from which hornblende and zircon crystallized were calculated by (1) applying saturation equations to bulk-rock compositions, (2) applying saturation equations to calculated melt compositions, and (3) using hornblende/melt partition coefficients for Zr. The results indicate that melt was lost during crystallization of the granitic magmas, conservatively at least as much as 40 %. These results are in agreement with published estimates of melt loss from other plutonic systems and suggest that bulk-rock compositions of many granitic rocks reflect crystal accumulation and are therefore inappropriate for use in thermodynamic calculations and in direct comparison of potentially consanguineous volcanic and plutonic suites. [ABSTRACT FROM AUTHOR]

Recent studies of arc volcanic systems have shown that major and trace element zoning in calcic amphibole yields information about magmatic processes such as fractional crystallization and mixing. Similar studies of plutonic amphibole are scant, yet hold the potential to yield comparable information. To that end, calcic amphibole from late-stage rocks of the English Peak plutonic complex (EPC; Klamath Mountains, northern California) was analyzed in situ, in textural context. The pluton's late stage consists of three nested intrusive units inwardly zoned from tonalite to granite. Bulk-rock compositions and U-Pb (zircon) ages are consistent either with internal fractional crystallization of a single magma batch or with episodic emplacement of successively evolved magmas, ± magma mixing. Major and trace element abundances and zoning patterns in hornblende (s.l.) are used to test these two interpretations, identify specific magmatic units, determine the temperature range of hornblende stability, and model magma crystallization. In each mapped unit, euhedral to subhedral hornblende displays prominent olive-brown core zones that crystallized at 880-775 °C. Cores are embayed and rimmed by green hornblende crystallized from 775-690 °C. These distinctions are preserved even in samples with moderate deuteric alteration. Some trace elements (Zr, Hf, Sr, Ti, V) decrease monotonically from core to rim, suggesting co-precipitation of hornblende with plagioclase, ilmenite, and zircon. Others (Ba, Rb) are approximately constant in highest-T core zones, then decrease, consistent with onset of biotite crystallization. In contrast, initial rim-ward decreases in Sc, Y, and REE change to near-constant values within olive-brown cores, a change modeled by a decrease in bulk partition coefficients (D) due to onset of biotite crystallization. These elements then increase in abundance in green rims, with as much as a fourfold enrichment. Such enrichments can result from resorption/ re-precipitation attending changing P and T during final emplacement, whereby trace elements in core zones were redistributed to the rims. Although hornblende compositions from the three zones are similar, outer-zone hornblende has higher Ti, Ba, Sc, and REE, whereas interior-zone hornblende has higher Mn. These differences are consistent with episodic ascent of compositionally similar but not identical magmas from a mid-crustal reservoir. Evidence for in situ magma mixing is lacking in hornblende. Core-to-rim decrease in Zr indicates hornblende and zircon crystallized together, at T as high as 880 °C. Because zircon saturation thermometry yields T estimates <720 °C for all EPC samples, many of the analyzed rocks are inferred to be cumulates. This study illustrates the utility of detailed major and trace element analysis of hornblende as a means to identify magmatic units and model petrogenetic processes in calc-alkaline granitic rocks. [ABSTRACT FROM AUTHOR]

The Wooley Creek batholith is a Late Jurassic, arc-related, calc-alkaline plutonic complex in the Klamath Mountain province of California. Post-emplacement tilting and erosion have exposed ~12 km of structural relief. The complex consists of an older (~159.1 Ma) lower zone (pyroxenite to tonalite) assembled by piecemeal emplacement of many magma batches, a younger (~158.2 Ma) upper zone (quartz diorite to granite), and a transitional central zone. In the lower zone, pyroxenes are too Fe rich to be in equilibrium with a melt whose composition was that of the host rock. Mass-balance calculations and simulations using rhyolite-MELTS indicate that these rocks are cumulates of pyroxenes and plagioclase ± olivine and accessory apatite and oxides. Percentages of interstitial melt varied from ~7.5-83%. The plagioclase/pyroxene ratios of cumulates vary considerably among the most mafic rocks, but are relatively uniform among quartz diorite to tonalite. This near-constant ratio results in compositional trends that mimic a liquid line of descent. In the upper zone, bulk-rock compositional trends are consistent with differentiation of andesitic parental magmas. Upward gradation from quartz dioritic to granitic compositions, modeled via mass-balance calculations and rhyolite-MELTS simulations, indicate that structurally lower parts of the upper zone are cumulates of hornblende and plagioclase ± biotite and accessory minerals, with 37-80% trapped melt. In contrast, the structurally higher part of the upper zone represents differentiated magma that escaped the subjacent cumulates, representing differentiated melt fractions remaining from 92-54%. The ratio of cumulate plagioclase/(plagioclase + mafic minerals) is ~0.48 among upper-zone cumulates, mimicking a liquid line of descent. The results suggest that compositional variation in many calc-alkaline plutons may be at least as representative of crystal accumulation as of fractional crystallization. If so, then the assumption that arc plutons geochemically resemble frozen liquids is dubious and should be tested on a case-by-case basis. Moreover, comparisons of plutonic rock compositions with those of potentially comagmatic volcanic rocks will commonly yield spurious results unless accumulation in the plutons is accounted for. [ABSTRACT FROM AUTHOR]

The Late Jurassic English Peak plutonic complex was emplaced in an upper crustal retro-arc setting in the central Klamath Mountains province, northern California. Emplacement of the main, central pluton was preceded by intrusion of two satellite bodies: the Uncles Creek pluton crystallized from H2O-rich quartz dioritic magma with hornblende as the liquidus mafic phase; in contrast, the Heiney Bar pluton is a c. 25km diameter body zoned from gabbro to granodiorite. Al-in-hornblende barometry from these two plutons indicates a stage of magma storage at c. 600- 500 MPa. The central English Peak pluton is a c. 15km diameter body composed of early and late stages. Early stage rocks range from gabbro to tonalite, with variable proportions of augite, orthopyroxene, hornblende and biotite. The early stage lacks discernible zoning and rock types vary at the outcrop scale. This diversity is reflected in bulk-rock compositions, which do not form a compositional array. The late-stage intrusion consists of three concentric units that are zoned from outer, more mafic rocks (quartz diorite, tonalite, quartz monzodiorite) to inner, compositionally evolved rocks (granodiorite and granite). Late-stage samples plot in smooth, typically linear arrays for most major and trace elements. Al-in-hornblende pressures indicate that late-stage hornblende cores grew in a reservoir at c. 400MPa and that rims grew at the level of final emplacement (c. 250 MPa). The mid-crustal reservoir was the site of late-stage magma evolution, including episodic magma mixing. Oxygen and Sr isotopes indicate initial evolution of English Peak pluton magmas in a deep crustal region of mixing, assimilation, storage, and homogenization (MASH zone), where they were contaminated by metasedimentary rocks. Thus, the English Peak pluton represents a crustalscale system, with mantle-derived magmas that differentiated near the Moho, storage and crystallization of satellite-pluton magmas in the middle crust (c. 600-500 MPa), development of a large, episodically recharged, magma chamber in the upper middle crust (c. 400 MPa) and final emplacement in the upper crust. [ABSTRACT FROM AUTHOR]

Hornblende in the Kuna Crest lobe (KCL) of the Tuolumne Intrusive Complex (TIC) and the upper zone of the Wooley Creek batholith (WCB) precipitated over a temperature range of ~835 to 700 °C, and thus has the potential to record magmatic processes. We measured trace element concentrations in hornblende from the WCB, from the KCL of the TIC, and from one sample from an adjacent interior unit of the TIC to compare and contrast magmatic processes in these two mid-crustal intrusions. In both systems the magmatic amphibole is magnesiohornblende in which Ti, Zr, Hf, Nb, Sr, Ba, and rare earth elements (REE) typically decrease from crystal interiors to rims, an indication of compatible behavior of these elements, and the size of the negative Eu anomaly decreases. In the Kuna Crest lobe, hornblende from individual mapped units differs in trace element abundances and zoning trends. Some samples contain at least two distinct hornblende populations, which is particularly evident in the shapes of REE patterns. In contrast, compositions of hornblende from all structural levels of the upper WCB and related dacitic roof-zone dikes form a single broad array and the REE patterns are essentially indistinguishable, regardless of rock type, from quartz diorite to granite. In the WCB, Zr/Hf ratios in hornblende are consistent with crystallization from a melt with chondritic Zr/Hf values. In contrast, most hornblende in the KCL has Zr/Hf values lower than expected from crystallization from a melt with chondritic values, suggesting that zircon fractionation occurred before and during crystallization of the hornblende. Simple fractional crystallization models indicate that REE, high field strength elements, Sr, and Ba were compatible in KCL and WCB magmas as hornblende grew; these trends require removal of hornblende + plagioclase + zircon ± ilmenite ± biotite. The uniform variations of trace element concentrations and patterns in the upper WCB and roofzone dikes indicates crystallization from a large magma body that was compositionally uniform; probably stirred by convection caused by influx of mafic magmas at the base of the zone (Coint et al. 2013a, 2013b; cf. Burgisser and Bergantz 2011). In contrast, in the KCL, each analyzed sample displays distinct hornblende compositions and zoning patterns, some of which are bimodal. These features indicate that each analyzed sample represents a distinct magma and that individual magmas were variably modified by fractionation and mixing. Hornblende trace element contents and zoning patterns prove to be powerful tools for identification of magma batches and for assessing magmatic processes, and thereby relating plutonic rocks to hypabyssal and volcanic equivalents. [ABSTRACT FROM AUTHOR]

The granitoid suites encountered by drilling in the northern South China Sea (SCS) remain important for understanding the evolution of the late Mesozoic Southeast Asian continental margin. They comprise a range of rock types including diorite, tonalite, granodiorite, monzogranite and syenogranite with SiO2spanning 56.4–76.8%. Newly acquired secondary ion mass spectrometry (SIMS) U–Pb ages of samples from 14 boreholes indicate two key magmatic episodes: Late Jurassic (161.6–148.2 Ma) and Early Cretaceous (136.5–101.7 Ma). Jurassic magmatism probably began in late Middle Jurassic time, documented by the dates of inherited zircons. The granitoids are dominated by metaluminous to weakly peraluminous I-type granites, are transitional between magnesian and ferroan, and encompass calc-alkaline, high-K calc-alkaline, and shoshonitic series. The geochemical signatures suggest that these granitoids were mostly generated in a normal continental arc environment. Notable features of the I-type samples are well-defined negative Nb–Ta–Ti anomalies typical of arc-related magmas. Taken together, the late Mesozoic arc granites of the SCS, the accretionary wedge of the Palawan terrane to the southeast, and the zone of lithospheric extension north of the SCS throughout Southeast China, define a southeast-to-northwest trench-arc-backarc architecture for the late Mesozoic Southeast Asian continental margin whose geodynamic setting is related to subduction of the Palaeo-Pacific slab beneath the Asian continent. Two key subduction episodes are recognized, one in Late Jurassic and the other in Early Cretaceous time. [ABSTRACT FROM PUBLISHER]

The structurally lowest parts of the Upper Nappe of the Helgeland Nappe Complex consist primarily of migmatitic quartzofeldspathic gneiss with intercalated calc-silicate gneiss. Detrital zircons in the gneiss indicate an Ordovician depositional age. Migmatization via biotite dehydration reactions occurred at ∼480 Ma at pressures of ∼700 MPa; some migmatites were mobilized into cross-cutting diatexites. Diverse gabbroic to granitic plutons and dikes of the Bindal Batholith were emplaced from 448-424 Ma. They range from calc-alkaline to alkalic and most have initial 87Sr/86Sr <0.707 and ϵNd from 0 to -4. Pressures of emplacement were ca. 300 MPa. Shear zones with reverse shear sense cut 437 Ma intrusions, suggesting that contraction accompanied magmatism. [ABSTRACT FROM AUTHOR]

Metasedimentary rocks of the Helgeland Nappe Complex and S-type granitic rocks of the Bindal Batholith are exposed on the island of Ylvingen. The Ylvingen plutonic rocks are part of a larger system dominated by S-type granodiorite exposed within the Vega Archipelago. Petrographic evidence and previously published P-T estimates indicate that the Vega-Ylvingen intrusive complex is tilted to the south and west. The results of bulk-rock thermodynamic modeling suggest that the Ylvingen host rocks were metamorphosed to ∼7-8 kbar and <550 °C along a clockwise P-T path. At ∼475 Ma, these host rocks were intruded by S-type magmas and underwent minor contact metamorphism. The modeled P-T path for an intrusive diatexite sample suggests that decompression from ∼7-8 kbar to <5 kbar was synmagmatic. These results indicate that crustal thickening, pluton emplacement, and exhumation occurred in rapid succession during the Ordovician Taconian orogeny. Petrographic observations and bulk rock compositions suggest that the granitic magmas are variably enriched in residual phases and accumulated plagioclase and biotite at the level of emplacement. [ABSTRACT FROM AUTHOR]

The Hortavær Complex in north-central Norway is a well documented example of the formation of an alkaline magmatic suite due to the assimilation of carbonate and calc-silicate rocks (Vogt 1916; Gustavson & Prestvik, 1979; Barnes et al. 2003, 2005). In addition to calcic pyroxene, aegirine occurs in veins near skarn-like assemblages in a zone where dioritic sheets were emplaced into syenite. Other vein minerals are biotite, albite, K-feldspar, calcite, and ilmenite. The aegirine is almost pure NaFeSi2O6. Laser-ablation ICP-MS analysis shows that the aegirine crystals are typically lower in trace element concentrations than the calcic clinopyroxene of the complex. Chondrite-normalized rare earth element (REE) patterns show a prominent cup shape, and light REE abundances range from ~ 0.1x to > 10x chondrites. [ABSTRACT FROM AUTHOR]

The Hortavær intrusive complex consists of a wide range of plutonic rocks, from gabbro (calcic) to alkali feldspar syenite (alkalic). Emplacement at 456 ± 8 Ma (U-Pb, titanite) was primarily as a series of dikes that range from tens of cm to many meters in width. Magma mingling structures are ubiquitous and magma mixing occurred locally. Therefore, the intrusive complex can be considered to be a zone of magma transfer and storage rather than a single pool of magma. The complex is characterized by CaO-rich rock compositions and by the presence of minerals typical of assimilation of carbonate-rich metasedimentary rocks, such as calcite, titanite, scapolite, nepheline, Ca-garnet, and idocrase. These features have been interpreted as the result of intense assimilation of host calc-silicate rocks. However, major and trace element compositions show inflections in the magmatic trend, which argues for crystal-liquid separation processes. Furthermore, some syenites are quartz bearing, which suggests a quartzofeldspathic component in the contaminant.When combined, these observations suggest that the Hortavær magmas evolved by assimilation-fractional crystallization of parental gabbroic magmas to evolved syenitic ones. Fractionation was initially dominated by separation of clinopyroxene, which was stabilized at the expense of olivine because of assimilation of carbonate components of the host rocks. The excessive fractionation of clinopyroxene forced melt compositions to alkali-rich, silica-undersaturated compositions, rather than SiO2-rich ones expected with olivine fractionation. Such assimilation was possible because of the open-system behavior of the intrusive complex (magma transfer zone) such that CO2 was lost before it could stop assimilation reactions. Development of evolved quartz syenitic magmas required a shift to assimilation of the quartzofeldspathic component of the host rocks, which occurred because carbonates are insoluble in such evolved magmas. [ABSTRACT FROM AUTHOR]

The dioritic Velfjord plutons (∼448 Ma) were emplaced into regional migmatitic metapelitic and metacarbonate rocks at mid-crustal levels, corresponding to pressures of ∼700 MPa. Exhumation to ∼400 MPa began while the migmatites were in a partly molten state. With increasing proximity to the plutons, regional stromatic migmatites change to diatexite, and diatexitic dikes are common within 500 m of the contacts. We interpret these relationships to indicate that heat from the plutons resulted in contact migmatization in a zone up to 1 km wide. Typical residual mineralogy in the diatexites is plagioclase + quartz + biotite + garnet + sillimanite ± K-feldspar, consistent with biotite dehydration melting. Pod- and dike-like leucosomes consist of two types: earlier high-K (granitic) ones with mineral assemblages identical to the migmatites and later low-K (tonalitic) ones in which sillimanite is sparse and garnet absent. The high-K leucosome magmas can be explained by biotite dehydration melting at 700 MPa. Within the aureole, mafic magmas were locally injected into, and hybridized with, the diatexites and the high-K leucosome magmas. In contrast, the low-K leucosomes are thought to result from local, late-stage remelting of H2O-saturated diatexite. The H2O-rich fluid was probably released from intergranular melt trapped in the diatexites during exhumation and solidification. Distinctive porphyritic ‘contact granites’ are common at pluton contacts. Although the mineral assemblage of these granites is identical to that of the diatexites, their isotopic compositions are distinct, with εNd and δ18O in the migmatites from −7·6 to −9·6 and from +10·9‰ to +13·5‰, and in the contact granites from −5·2 to −7·5 and from +9·6‰ to +12·3‰, respectively. Thus, the contact granites could have a source that is isotopically distinct from, but mineralogically similar to the diatexites, or they could result from mixing of magma similar to the high-K leucosomes with dioritic magmas. Mass balance calculations are consistent with the latter interpretation, with proportions of granitic to dioritic magmas from 7:1 to 7:3. Emplacement and solidification of the dioritic plutons provided zones of structural anisotropy along which high-K leucosome magmas and contact granite magmas collected. These magmas were injected by additional dioritic magma and further hybridized. Because the solidi of the plutons were several hundred degrees higher than that of the granitic magmas, the pluton walls acted as long-lived, hot, rigid surfaces along which magmas collected and migrated. [ABSTRACT FROM PUBLISHER]

This geochemical classification of granitic rocks is based upon three variables. These are FeO/(FeO + MgO) = Fe-number [or FeOtot/(FeOtot + MgO) = Fe*], the modified alkali–lime index (MALI) (Na2O + K2O – CaO) and the aluminum saturation index (ASI) [Al/(Ca – 1·67P + Na + K)]. The Fe-number (or Fe*) distinguishes ferroan granitoids, which manifest strong iron enrichment, from magnesian granitoids, which do not. The ferroan and magnesian granitoids can further be classified into alkalic, alkali–calcic, calc-alkalic, and calcic on the basis of the MALI and subdivided on the basis of the ASI into peraluminous, metaluminous or peralkaline. Because alkalic rocks are not likely to be peraluminous and calcic and calc-alkalic rocks are not likely to be peralkaline, this classification leads to 16 possible groups of granitic rocks. In this classification most Cordilleran granitoids are magnesian and calc-alkalic or calcic; both metaluminous and peraluminous types are present. A-type granitoids are ferroan alkali–calcic, although some are ferroan alkalic. Most are metaluminous although some are peraluminous. Caledonian post-orogenic granites are predominantly magnesian alkali–calcic. Those with <70 wt % SiO2 are dominantly metaluminous, whereas more silica-rich varieties are commonly peraluminous. Peraluminous leucogranites may be either magnesian or ferroan and have a MALI that ranges from calcic to alkalic. [ABSTRACT FROM PUBLISHER]

The late Eocene Harrison Pass pluton was emplaced in the transition zone between the infrastructure and suprastructure of the Ruby Mountains core complex. Emplacement was at ∼3 kbar pressure and was in two stages: early stage tonalitic to monzogranitic magmas, followed by late-stage monzogranites and mafic dikes. The early stage began with emplacement of biotite ± hornblende granodiorite of Toyn Creek, followed by the biotite monzogranite of Corral Creek. Quenched equivalents of these units are preserved as porphyritic dikes in the roof. Leucocratic monzogranite that forms cupolas in the roof zone represents the product of fractional crystallization of the early magmas. Al-in-hornblende barometry and the presence of magmatic epidote suggest that early stage magmas resided at a pressure of ∼5−6 kbar before emplacement in the upper crust. Compositional variation in the Toyn Creek and Corral Creek units is essentially linear, and can be explained by mixing of a tonalitic end member with a monzogranitic end member such as evolved samples of the monzogranite of Corral Creek. The tonalitic end member was itself a hybrid that formed by interaction of mafic magma with crustal melts. The monzogranitic end member is a crustal melt that escaped hybridization. Nd isotopic compositions of the early stage are heterogeneous and do not correlate with degree of differentiation, which is consistent with a compositionally heterogeneous felsic end member. Elemental variation in the early stage of the pluton is unusual because of its essentially linear trends, which suggest a single mixing event before emplacement in the upper crust. Late-stage activity consisted of three pulses of monzogranitic magma plus sparse mafic dikes. The largest and youngest of these pulses, the two-mica monzogranite of Green Mountain Creek, is distinct from all other units in the presence of restitic enclaves, low εNd, and high initial 87Sr/86Sr. Pod-like bodies of leucocratic biotite ± amphibole monzogranite and sheets and dikes of leucocratic two-mica monzogranite make up the other late-stage granites. These rocks display deep negative Eu anomalies and show wide, non-systematic concentrations of high field strength elements; however, their isotopic compositions are identical to those of the early stage rocks. They are thought to be small melt fractions of the lower to middle crust. Their elemental compositions are thought to result from the effects of residual plagioclase and accessory minerals. The variable and non-systematic isotopic compositions of all granitic units in the pluton suggest a heterogeneous source region, such as the Proterozoic Mojave province. [ABSTRACT FROM PUBLISHER]

The Velfjord plutons are mafic-intermediate plutons in the Bindal Batholith that were emplaced into pelitic and calcareous rocks of the Helgeland Nappe Complex of the Uppermost Allochthon at ∼448 Ma. Al-in-hornblende barometry suggests emplacement in the range of 6 to 8 kbar, which is consistent with the presence of magmatic epidote. Pelitic rocks in the aureoles underwent partial melting and leucosomes locally accumulated adjacent to the plutons to form ''contact granites''. The results of garnetaluminosilicate-silica-plagioclase (GASP) thermobarometry combined with a recently published petrogenetic grid for dehydration partial melting of pelites suggest a clockwise P-T-t path for the wall rocks. The GASP results, plus the presence of early kyanite, suggest initial melting in the 7 to 8 kbar range. Temperatures close to the plutons reached at least 750°C to 800°C. Nearly isothermal decompression to ∼4 to 5 kbar resulted in the hottest wall rocks reaching cordierite stability while they were still partly molten. Cooler wall rocks could not reach conditions of cordierite stability because of the positive slope of the cordierite-in equilibrium. The state of HNC crust prior to pluton emplacement cannot yet be determined. However, rapid exhumation of the plutons and their wall rocks suggests that the youngest plutons of the Bindal Batholith were probably emplaced at lower pressures. [ABSTRACT FROM AUTHOR]

The Priest pluton is part of the suite of 1.4-Ga granitic plutons in North America. The pluton is a corundum normative, peraluminous, K-feldspar megacrystic granodiorite to quartz monzonite. It is characterized by: (1) high Mg/Mg + Fe; (2) high abundances of Al[sub2]O[sub3], CaO, and large ion lithophile elements (e.g., Sr, Ba); and (3) low abundances of high field strength elements (e.g., Zr, Y). Major and trace element modeling suggests that variations in composition were produced by accumulation of K-feldspar and plagioclase, localized accumulation of plagioclase, biotite, and accessory minerals, and late-stage crystal fractionation of the residual melt. Plutons in the suite of 1.4 Ga, so-called "anorogenic" granitic plutons have been classified as A-type granites. Major and trace element abundances suggest that the Priest pluton is not A-type, but has more in common with S-type granites. In addition, typical tectonic classification techniques suggest that the pluton is either a volcanic-arc/syn-collisional or late orogenic granite. The pluton's intrusion was accompanied by highly partitioned, compressional deformation that is not typically associated with anorogenic, A-type plutons. The Priest pluton also is distinct from the nearby Sandia pluton, although the two plutons are nearly the same age. The Priest pluton has higher Mg number, alumina saturation index, and large ion lithophile element abundances as well as lower abundances of high field strength elements. The Priest pluton contains only biotite as its mafic phase, whereas the Sandia pluton contains biotite and hornblende. Differences in geochemistry between the Priest and Sandia plutons and the rest of the suite of 1.4 Ga granitic plutons suggest that the lower crust is a heterogeneous source region. Therefore, geochemical signatures that commonly are associated with particular tectonic settings may instead reflect heterogeneities in the source. [ABSTRACT FROM AUTHOR]

Two distinct basaltic suites characterize ∼1165- to 1120-Ma magmatism in west Texas. The ∼1163 Ma Pecos mafic intrusive complex consists of layered mafic and ultramafic rocks. Megacyclic units record repeated influx and mixing of tholeiitic magmas to yield a predominantly noritic body. Basaltic magmatism also accompanied emplacement of the 1120 ± 35-Ma Red Bluff granitic suite in the Franklin Mountains. In terms of their major elements, these basaltic rocks are transitional between tholeiitic and alkaline, but their low concentrations of Nb suggest a subduction-related origin. Except for their low Nb contents, the Franklin Mountains basaltic rocks are similar to coeval basaltic intrusive and volcanic rocks in the southwestern U.S.A. Of the numerous proposed tectonic settings, an extensional environment is preferred. Possible extensional associations include splays of the Midcontinent Rift, rifting caused by collision of a continental fragment during the Grenville orogeny, and extension related to collapse of over-thickened crust during the Grenville event. The low Nb contents of basalts associated with Franklin Mountains basaltic rocks are best explained as an artifact of older subduction, which modified the mantle source of the basalts hundreds of millions of years prior to their emplacement. [ABSTRACT FROM AUTHOR]

The Cornucopia stock, in the Blue Mountains of northeastern Oregon, is a small composite intrusion comprising five distinct intrusive units: a hornblende biotite tonalite, a biotite trondhjemite, and three cordierite-bearing two-mica trondhjemites. The stock was emplaced at shallow levels (<2 kbar) in island arc-related metasedimentary and metavolcanic rocks of the Wallowa terrane. The age of the intrusion is 116.8±1.2 Ma determined by 40Ar/39Ar incremental heating of biotite. These ages and volume constraints imply coeval emplacement of the tonalitic and trondjhemitic magmas. Tonalitic and trondhjemitic compositions span a narrow range of SiO2 content (65–74 wt %) and exhibit characteristics of a high-Al tonalite–trondhjemite–granitoid (TTG) suite, including light rare earth element (LREE) enrichment, low Y, Nb and Rb/Sr, and high Al2O3 and Sr. These compositions are consistent with an origin by 10–40% partial melting of a low-K tholeiitic source (with a relatively flat REE pattern), similar to metaigneous basement rocks of the Wallowa terrane, in equilibrium with a garnet pyroxene hornblendite residue. High Sr in the TTG rocks, lack of calculated residual plagioclase, and abundant calculated residual amphibole suggest that H2O in excess of that produced by amphibole dehydration was present at the site of melting. Conditions of partial melting are loosely constrained to pressures ≥10 kbar (residual garnet implied by REE abundances) and temperatures exceeding 900–950°C (magmatic temperatures based on apatite solubility). We suggest the Cornucopia tonalitic and trondhjemitic magmas formed by hydrous partial melting of lower island arc crust, probably as a result of underplating by, or intrusion of, mantle-derived basaltic magmas. Mantle melting may have been triggered by rapid uplift of Wallowa terrane crust, as the Blue Mountains terrane assemblage collided with the continental margin during Early Cretaceous time. Results of this study suggest that deformation and crustal melting associated with the accretion event were not strictly limited to the suture zone. [ABSTRACT FROM AUTHOR]

An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG (LOGIN to AGU's FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 94YCJ2454, Timescales of orogeny: Jurassic construction of the Klamath Mountains, B.R. Hacker, M.M. Donato, C.G. Barnes, M.O. McWilliams, and W.G. Ernst). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, DC 20009; $15.00. Payment must accompany order. Classical interpretations of orogeny were based on relatively imprecise biostratigraphic and isotopic age determinations that necessitated grouping apparently related features that may in reality have been greatly diachronous. Isotopic age techniques now have the precision required to resolve the timing of orogenic events on a scale much smaller than that of entire mountain belts. Forty-five new 40Ar/39Ar ages from the Klamath Mountains illuminate the deformation, metamorphism, magmatism, and sedimentation involved in the Jurassic construction of that orogen, leading to a new level of understanding regarding how preserved orogenic features relate to ancient plate tectonic processes. The new geochronologic relationships show that many Jurassic units of the Klamath Mountains had 200 Ma or older volcanoplutonic basement. Subsequent formation of a large ∼170 Ma arc was followed by contractional collapse of the arc. Collision with a spreading ridge may have led to large-scale NW-SE extension in the central and northern Klamaths from 167 to ∼155 Ma, coincident with the crystallization of voluminous plutonic and volcanic suites. Marked cooling of a large region of the central Klamath Mountains to below ∼350°C at ∼150 Ma may have occurred as the igneous belt was extinguished by subduction of colder material at deeper structural levels. These data demonstrate that the Klamath Mountains-and perhaps other similar orogens-were constructed during areally and temporally variant episodes of contraction, extension, and magmatism that do not fit classical definitions of orogeny. [ABSTRACT FROM AUTHOR]