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Russian Geology and Geophysics

2011 year, number 3

THE BULKA PERIDOTITE-GABBRO INTRUSION ( West Sayan ), A SYNCOLLISIONAL TYPE OF LAYERED INTRUSIONS

E.V. Borodinaa, A.E. Izokha, and A.A. Mongushb
a V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
b Tuvinian Institute of Complex Natural Resources Exploration, Siberian Branch of the Russian Academy of Sciences, ul. Internatsional'naya 117a, Kyzyl, 667007, Russia
Keywords: Layered intrusions, peridotite-gabbro complexes, fractional crystallization, modeling, parental melt, picrites, rare-earth elements
Pages: 307-319

Abstract

The Bulka layered intrusion, a standard intrusion of the Bulka complex, is localized in northeastern West Sayan. The layered series is composed of dunites, melanotroctolites, troctolites, olivine gabbro, gabbro, hornblende gabbro, olivine leucogabbro, olivine leucogabbronorites, leucogabbro, hornblende leucogabbro, leucotroctolites, anorthosites, and plagioclase-containing hornblendites. The Mg-number (Mg#) varies from 76 in peridotites (38 wt.% MgO) to 34 in anorthosites (2 wt.% MgO). From bottom to top of the section, the following sequence of crystallization of liquidus minerals is observed: Ol ⇒ Pl ⇒ Cpx ⇒ Opx ⇒ Amph + Mag. The petrochemical features of the massif rocks are consistent with the fractional crystallization of picritic parental magma (24 wt.% MgO, 11 wt.% FeO, Mg# = 80). The compositions of model cumulates determined by fractional crystallization modeling using the COMAGMAT 3.5 computer program at 1-2 kbar, QFM buffer, and 0.5 wt.% H2O in the melt are similar to those of the intrusion rocks. The REE and trace-element patterns for these rocks show a nonfractionated composition trend with LILE and HFSE depletion. Depleted LILE pattern is similar to that in N-MORB, which evidences that the parental magma was produced through the partial melting of oceanic lithospheric mantle probably similar to the adjacent ophiolites of the Kurtushiba belt. The HFSE depletion of the intrusion rocks might be due to the island-arc processes during the belt formation. The Bulka intrusion formed at the accretion-collision stage of magmatism as a result of the differentiation of picritic melt generated from oceanic lithospheric mantle with superposed island-arc processes.