Genesis Of The Wenquan Ultramafic Rocks In East Kunlun Orogen, Northwest China

The East Kunlun Orogenic Belt(EKOB) is a composite orogenic belt which has undergone multi-stage cycles tectonic evolution, locataed in the western of Central Orogenic Belt. Focusing on the ocean-land tectonic cycles of Proto-Tethys Ocean and Paleo-Tethys Ocean, it primarily includes early Paleozoic ophiolite in Central East Kunlun Fault zone(CEKF) and late Paleozoic ophiolite in Buqingshan-A’nyemaqen tectonic belt, where the early Paleozoic ophiolite was discovered afterwards. Wenquan ultramafic rocks were regarded as Proterozoic ophiolites, and they are in the CEKF together with Qingshuiquan and Wutuo ophiolites. Through the studying of mineralogy, petrology, geochemistry and zircon U-Pb geochronology, this paper discusses the petrogenesis, tectonic environment and geological significance for tectonic evolution of the EKOB during Paleozoic period. Until now, some new points have been achieved as follows:1 、 Wenquan ultramafic rocks are consist of dunite, wehrlite, olivineclinopyroxenite and clinopyroxenite. Olivine in dunite has Fo of 91.1~92.0 and others in wehrlite and clinopyroxenite have Fo of 80~85; Cr-spinel in wehrlite and clinopyroxenite has Cr# of 79~91, Mg# of 9~30, Ti O2 of 0.09~2.11% and Al2O3 of 3.30~15.09%; Clinopyroxene in wehrlite, olivine-clinopyroxenite and clinopyroxenite is diopside and has the comparable Mg#(92~98)with those in ophiolitic cumulates. Combining the whole rock geochemical analysis, dunites belong to metamorphic peridotite and clinopyroxenite(olivine-clinopyroxenite and clinopyroxenite) and wehrlite belong to ultramafic cumulates in the SSZ-type ophiolite, neither Alaskan-type complex nor layered intrusion.2、The cumulates are most likely evolved from the high-Mg basaltic magmas which generated by the high degree partial melting of the depleted mantle, and undergoing fractional crystallization and crustal contamination. The remarked peritectic texture and cumulate texture in wehrlite and clinopyroxenite, the positive correlation between Fo and Ni O content in olivine and Mg# and Cr2O3 content in clinopyroxene, these three characters explain that the magma is mainly controlled by fractional crystallization. The magma source might have the contribution from crustal contamination as the evidence of enriched LILE, depleted HFSE, positive correlation of the covariant relationships(Ce/Nb-Th/Nb, Ta/Yb-Th/Yb and Ti O2/Yb-La/Yb),(Th/Nb)N>1 and Nb/La<1。3、Zircons from clinopyroxenite yield a weighted mean age of(331±2)Ma, which is nearly coeval with the formation age of Buqingshan late Paleozoic ophiolite. The age result reveals that the possible petrogenetic relationship between the Wenquan ultramafic rocks and the Buqingshan ophiolite. Therefore, Wenquan ultramafic rocks might be a part of residual oceanic crust of the Paleo-Tethys ocean, in other words, the remains of the Paleo-Tethys ocean exist in Central East Kunlun Fault zone.