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Twenty samples of hornblendes from rocks of 14 plutonic unitsin the central Sierra Nevada and Inyo Mountains, California,have been studied in detail. Optical, density, single-crystaland powder X-ray diffraction, and major and minor element chemicaldata are reported. The compositions of the hornblendes show only limited correlationwith the chemistry of the rocks in which they occurred. Hornblendesfrom granitic rocks of the eastern Sierra Nevada and Inyo Mountainshave a wide range of tetrahedral aluminum content which is oftenas low as three-quarters of an atom per formula unit, whereashornblendes from younger granitic rocks elsewhere in the SierraNevada batholith contain more than one atom of tetrahedral aluminumper formula unit. Because an increase of aluminum in tetrahedralco-ordination is considered indicative of higher temperaturesof crystallization, the observed differences in the hornblendessuggest that older plutonic rocks of the batholith may havebeen metamorphosed regionally or may have been affected by widespreadhydrothermal action prior to consolidation of later graniticrocks. 相似文献
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An assemblage of pyroxenite, peridotite, and mafic granulitexenoliths contained in the toe of a 10 Ma trachybasalt flowremnant overlying Late Cretaceous granitoids indicates the presenceof a mafic-ultramafic complex beneath the Sierra Nevada batholith.Olivine-free pyroxenites that include orthopyroxenites, websterites,and clinopyroxenites are dominant. Primary igneous texturesare displayed by some pyroxenites, but commonly are masked byrecrystallization. Fe-rich harzburgites and lherzolites arerare. A few of the ultramafic xenoliths contain ovoid opaquepatches that are apparently pseudomorphs after garnet and havepyralspite garnet compositions. A pressure corresponding toa lower crustal depth of approximately 40 km has been determinedfrom two of these xenoliths using a garnet-orthopyroxene geobarometer.Abundant mafic granulites can be subdivided into those containing12 per cent or less A12O3 and chemically gradational with pyroxenitesand others containing more than 15 per cent A12O3 and showingconsiderable scatter on oxide variation diagrams. The high-aluminagranulite xenoliths have relatively low 87Rb/86Sr but high 87Sr/86Sr,whereas low-alumina and ultramafic xenoliths have a wide rangeof 87Rb/86Sr, but lower 87Sr/86Sr; the isotopic data indicatean age for the complex roughly the same as that of overlyinggranitoid plutons. However, the granitoids have initial 87Sr/86Srratios intermediate between the high-alumina and ultramaficxenoliths, suggesting that they may have resulted from mixingof basaltic magma, represented by the ultramafics, and crustalmaterials, with subsequent crystal fractionation. The trachybasaltmay represent a partial melt of the ultramafic rocks.Rocks analogousto the Chinese Peak xenoliths are exposed in the Giles complexof central Australia, a series of several deformed layered maficand ultramafic intrusions, emplaced in a granulite facies terrain.Contemporaneous development of mafic-ultramafic complexes andthe Sierra Nevada batholith may explain the present day thick({small tilde} 50 km) crust in this region 相似文献
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Biotites from plutonic recks of the central Sierra Nevada andInyo Mountains, California, have been examined and characterizedby powder X-ray diffraction and optical and chemical methods. Compositions of the biotites define a trend in the compositionaltriangle Fe+3 Fe+2Mg. When related to the experimentally studiedternary system KFe3+3AlSisO12H-1-KFe3+2 AlSi3O10(OH)2-KMg3AlSi3O10(OH)2and coupled with the estimated positions of biotite solid solutionsfor different oxygen buffers, the trend suggests that oxygenfugacities in magmas during biotite crystallization were slightlyhigher than those defined by the Ni-NiO buffer. The compositionaldata also suggest that magmas were ‘buffered’ withrespect to oxygen by oxides existing within the magmas themselves. Correlation between the Fe/(Fe+Mg) ratio, an inferred temperatureindicator, and other elements is generally poor, which suggeststhat factors other than temperature at the time of crystallizationexerted an important influence on compositions. 相似文献
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