Detail A petrologic investigation of deep-crustal and upper-mantle xenoliths from the Sierra Nevada, California: Constraints on lithospheric composition beneath continental arcs and the origin of cordilleran batholiths Bibliografi Author: Ducea, Mihai Nicolae Topik: GEOLOGY|GEOCHEMISTRY Bahasa: (EN )    ISBN: 0-591-96534-8     Penerbit: CALIFORNIA INSTITUTE OF TECHNOLOGY     Tahun Terbit: 1998     Jenis: Theses - Dissertation Fulltext: 9842298.pdf (0.0B; 0 download) Abstract I present evidence for a $/sim$100 km thick sequence of cogenetic rocks which together comprise the Sierra Nevada batholith of California (SNB). The SNB magmatism produced granitoids which reside in the Sierra Nevada upper-crust as well as deep-crust and upper-mantle mafic-ultramafic cumulates. Samples of the mafic-ultramafic sequence are preserved only as xenoliths in Miocene volcanics which erupted through the central part of the batholith. Sm-Nd and Rb-Sr mineral geochronologic analyses were performed on fresh, cumulate textured, eclogite and granulite facies xenoliths. All samples equilibrated between $/sim$35 and 100 km beneath the batholith and yield Sm-Nd mineral ages between 81 and 136 Ma, broadly coincident with the previously established period of most voluminous batholithic magmatism in the Sierra Nevada. The xenolith whole rock initial $/rm /sp[87]Sr//sp[86]Sr,//sp[143]Nd//sp[144]Nd,/ /sp[206]Pb//sp[204]Pb,//sp[207]Pb//sp[204]Pb,/ /sp[208]Pb//sp[204]Pb$ and the $/delta/sp[18]$O are similar to the ratios published for the outcrops of the central SNB. These xenoliths are magmatically related to the upper-crustal granitoids, as cumulates and/or restites. The SNB magmatism was a large scale process responsible for efficiently segregating a $/sim$30-35 km thick column of granitoids from a 70 km mafic-ultramafic root. Trace element data indicate that these garnet-rich assemblages were in equilibrium at depths of $>$40 km with SNB-like melts. The petrologic mechanisms responsible for 'distillation' of SNB granitoids are similar to those which led to the extraction of the granitoids that make up the continental cratonic nucleii. The xenolith isotopic compositions suggest that the compositional variability observed in the SNB is primarily inherited from the source rocks in the deep lithosphere. The isotopic ratios of the deep-seated residues/cumulates require the existence of three source components for the batholith: (1) accreted supracrustal, (2) Proterozoic deep-lithospheric, and (3) Phanerozoic, MORB-like. Xenoliths sampled in younger, Pliocene volcanics are all garnet-free and are dominated by spinel peridotites which equilibrated over the same depth interval as the garnet-rich assemblages from Miocene pipes. Silica-rich glass inclusions, trapped along grain boundaries of Pliocene peridotites have compositions similar to the eclogitic keel. These observations and geophysical constraints suggest that the thick, dense 'eclogitic' root foundered in the mantle between Mid-Miocene and Pliocene. Opini Anda Klik untuk menuliskan opini Anda tentang koleksi ini! Lihat Sejarah Pengadaan  Konversi Metadata   Kembali

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