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Petrology and Emplacement of Reversely Zoned Gabbro-Diorite Plutons in the Smartville Complex, Northern California 总被引：2，自引：0，他引：2
Gabbroic plutons are part of the intrusive substructure of theSmartville Complex, a late Jurassic, rifted, ensimatic arc locatedin the northern Sierra Nevada of California. The plutons rangefrom unzoned, equant bodies of olivine gabbro less than 1 kmin diameter to elongate intrusions up to 25 km in length thatare reversly zoned from olivine gabbro cores to quartz dioriterims. The felsic rocks dip inward beneath the mafic core, indicatingthat this zoning reversal continues to depth. The zoned plutonshave relatively shallow keels. We interpret the reversed zoningas an emplacement feature, analogous to the compositional zoningin a zoned tephra sheet. It formed as a result of tapping analready zoned, deeper level magma chamber. Whether the originalzoning of the magma was concentric or stratiform cannot be readilydeduced. During emplacement, considerable amounts of cumulaterocks were mobilized. The mineralogy and geochemistry of the reversely zoned plutonsindicate that they contain two suites of rocks: a cumulate suiterepresented by olivine gabbro and olivine clinopyroxenite anda differentiated suite of non-cumulate olivine gabbros, gabbronorites,and diorites that lie along a compositional continuum and approximateliquid compositions. Plagioclase and olivine compositions inthe Smartville Complex cumulate suite are identical to thosein modern arc cumulates and are characteristic of the arc cumulatesuite. The differentiated rocks form a compositionally continuousseries that is geochemically very similar to a differentiatedsuite of arc tholeiitic basalts and andesites. Fractionationmodeling indicates that removal of mineral phases found in thecumulate gabbros from the mafic members of the differentiatedsuite can produce the lithologic variation seen in the zonedplutons. Plutons such as those in the Smartville Complex indicatethat there is a genetic link between cumulate rocks and a basalt-andesitefractionation trend in arcs, supporting the hypothesis thatarc andesites form by crystal fractionation. The gabbroic plutonsand related Alaska-type ultramafic complexes contain ultramaficcumulates that can rectify the discrepancy between the cumulatemode predicted by fractionation models and the observed modeof gabbroic cumulates in arcs. 相似文献
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Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb 总被引：36，自引：1，他引：35
The melting relations of five metamorphosed basalts and andesites(greenstones and amphibolites), collected from the late JurassicSmartville arc complex of California, were investigated experimentallyat 800–1000? C and 1, 3, and 6. 9 kb. Dehydration-melting(no water added) experiments contained only the water structurallybound in metamorphic minerals (largely amphiboles). They yieldedmildly peraluminous to metaluminous granodioritic to trondhjemiticmelts (Na/K is a function of starting composition) similar inmajor element composition to silicic rocks in modern oceanicarcs. The dehydration melts are water-undersaturated, with,and coexist with the anhydrous residual solid (restite) assemblageplagioclase + orthopyroxene + clinopyroxene + magnetite ? ilmen-ite,with plagioclase constituting 50% of the restite mode. In thedehydration-melting experiments at 3 kb the onset of meltingoccurred between 850 and 900 ? C, as amphibole and quartz brokedown to yield pyroxenes plus melt. Total pressure is greaterthan in the dehydration-melting experiments and has little effecton melt composition or phase relations. In the water-saturated (water added, so that experiments, meltsformed at 3 kb and above are strongly peraluminous, rich inCa and poor in Fe, Mg, Ti, and K. Their compositions are unlikethose of most silicic igneous rocks. These melts coexist withthe amphibole-rich, plagioclase-poor restite assemblage amphibole+ magnetite ? clinopyroxene ? plagioclase ? ilmenite. The highlyaluminous nature of the melts and the plagioclase-poor natureof the restite both reflect the substantial contribution ofplagioclase (along with quartz) to melts in high-pressure water-saturatedsystems. Water pressure equals Ptoul in the water-saturatedexperiments and has a profound effect on both melt compositionand phase relations. At 1 kb, the water-saturated experimentsyielded melt and mineral products with some characteristicsof the dehydration-melting experiments (no amphibole at highT), and some characteristics of the 3-kb, water-saturated experiments(amphibole plus melt coexisting at lower T, elevated Al, loweredFe). As pressure is increased from 3 to 6. 9 kb, the stabilityfields of both plagioclase and clinopyroxene decrease relativeto amphibole and the Al contents of the melts increase. These experiments have important implications for the petrogenesisof low-K silicic rocks in arcs. First, dehydration melting isa viable mechanism for the formation of these rocks; water-saturatedmelting is not. Second, because of the influence of rock compositionon melt composition, low-grade metamorphic and hydrothermalprocesses that alter the alkali contents and Na/ K in arc basementterranes may have a direct impact on the petrogenesis of silicicmagmas in arcs, particularly the formation of extremely low-Ktrondhjemites. Third, the experiments predict that anhydrous,pyroxene- and plagioclase-rich ‘granulitic’ restiteassemblages should develop as a result of partial melting inarc terranes. Such assemblages occur in at least two deeplyeroded arc complexes. 相似文献
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On the Iberia Abyssal Plain (Ocean Drilling Program Site 1070),gabbroic pegmatites and related rocks (127 ± 4 Ma, U–Pbzircon) intrude upper mantle that was subsequently exposed andserpentinized during Early Cretaceous non-volcanic rifting.The pegmatites include a 3–4 m dike or sill (the ‘main’pegmatite), numerous dikelets of 1–5 cm thickness, andclasts within the overlying ophicalcite breccia. Exclusive ofrodingitization, the main pegmatite contains 40–70% calcicandesine, 25–35% kaersutitic amphibole (Mg# 60–70),5–25% augite (Mg# 70–80) and 1–2% ilmenite.The dikelets are more magnesian (Mg# up to 82 in kaersutiteand 88 in augite). Most indications are that the high Mg#s inthe dikelets reflect igneous compositions. Isotopic and elementalchemistry indicate that the pegmatite-forming melt was enrichedin incompatible elements relative to normal mid-ocean ridgebasalt, but not as enriched as Azores basalts. The amphibole-bearingplagioclase peridotites of the Iberia Abyssal Plain are an appropriatesource for the pegmatite melts. A combination of decompressionaccompanying unroofing and heating from the upwelling asthenospherebeneath the developing rift caused P–T conditions in theamphibole-bearing lithosphere to exceed the dehydration-meltingsolidus ( 相似文献
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JICHA B. R.; SINGER B. S.; BROPHY J. G.; FOURNELLE J. H.; JOHNSON C. M.; BEARD B. L.; LAPEN T. J.; MAHLEN N. J. 《Journal of Petrology》2004,45(9):1845-1875
Major and trace element compositions and Sr, Nd, Pb, and Hfisotope ratios of Aleutian island arc lavas from Kanaga, Roundhead,Seguam, and Shishaldin volcanoes provide constraints on thecomposition and origin of the material transferred from thesubducted slab to the mantle wedge. 40Ar/39Ar dating indicatesthat the lavas erupted mainly during the last 相似文献
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We present results of dehydration melting experiments [3–15kbar, 810–950C f(O2) QFM (quartz-fayalite-magetite)and Ni-NiO] on two Fe-rich mixtures of biotite (37%), plagioclaseAn38 (27%), quartz (34%) and ilmenite (2%), which differ onlyin their biotite compositions (mg-number 23 and 0.4). Dehydrationmelting of metagreywackes of constant modal composition generatesa wide range of melt fractions, melt compositions and residualassemblages, through the combined effects of pressure, Fe/Mgratio and f(O2). Crystallization of garnet is the chief controlon melting behavior, and is limited by two reactions: (1) thebreakdown of garnet + quartz to orthopyroxene + plagioclaseat low P, and (2) the oxidation of garnet to magnetite + anorthite+ quartz (enstatite), which is sensitive to both f(O2) andP. Because of these reactions, melting of Mg-rich metagreywackesis rather insensitive to f(O2) but strongly sensitive to P;the converse is true for Fe-rich metagreywackes. Garnet crystallizationrequires that plagioclase break down incongruently, liberatingalbite. This increases the Na2O content of the melts and enhancesmelt production. Thus, melting of metagreywacke in a reducingdeep-crustal environment (with garnet stable) would producemore, and more sodic, melt than would garnet-absent meltingof the same source material in a relatively oxidizing, shallow-crustalenvironment. KEY WORDS: anatexis; metasediments; gneisses; granites; garnet
*Corresponding author. Telephone: 706-542-2394; fax: 706-542-2425; e-mail: alpatino{at}uga.cc.uga.edu 相似文献
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We performed vapor-absent melting and crystallization experimentson two bulk compositions that model metamorphic rocks containinga single hydrous phase: a biotite gneiss [37% bio (mg-number55), 34% qtz, 27% plg (An38), 2% ilm] and a quartz amphibolite[54% hbl (mg-number 60), 24% qtz, 20% plg (An38), 2% ilm]. Experimentswere performed at 3 and 5 kbar in internally heated pressurevessels (IHPV), and at 7, 10, 125 and 15 kbar in piston cylinderapparatus (PC), from the vapor-absent solidi to (at least) thetemperature at which the hydrous mineral disappeared. Dehydration-meltingbegins at similar temperatures in both bulk compositions, rangingfrom T850C at P = 3 kbar T930C at P = 15 kbar. The hydrousmineral disappears 50C above the solidus in both systems, exceptin IHPV experiments at f(O2) above Ni–NiO, in which biotitestability extends up to atleast 80C above the solidus. At theT at which the hydrous minerals disappear the biotite gneissproduces 2–3 times more melt than the quartz amphibolite(50–60 wt% vs 20–30 wt%). In both systems, variationsin melt productivity with P are controlled by three competingfactors: (1) the positive d P/dT slopes of the solidi, (2) decreasingH2O activity with increasing P at constant H2O content, and(3) Na2O activity, which increases with P concomitantly withbreakdown of plagioclase. Melt productivities at T = 920–950Care maximized at intermediate pressures (7 kbar). The biotitegneiss produces strongly peraluminous granitic melts (SiO2>70wt%) and residual assemblages of quartz norite (P>125 kbar)or garnet pyroxenite (P>125 kbar). The quartz amphiboliteproduces strongly peraluminous granodioritic melts (SiO2>70wt%) that coexist with clinopyroxene + orthopyroxene + plagioclase+ quartz at P>10 kbar)garnet. The results of coupled meltingand crystallization experiments on the quartz amphibolite suggestthat strongly peraluminous granitoid rocks (e.g. cordierite-bearingand two-mica granites) can be derived from melting of Al-poorprotoliths. KEY WORDS: dehydration-melting; biotite gneiss; amphibolite; felsic magmas
*Corresponding author 相似文献
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