PETROLOGY AND GEOCHEMISTRY OF THE BAIJIANTAN-BAIKOUQUAN OPHIOLITIC MÉLANGES: IMPLICATIONS FOR GEOLOGY EVOLUTION OF WEST JUNGGAR, CHINA

The west Junggar, as a major component of the core part of the CAOB locating on south of the Tajin – Tarbahatai – Kujibai – Honguleleng ophiolitic belt (TTKH), is considered to be a Paleozoic orogenic belt resulting from the convergence of the Siberian and Kazakhstan-Junggar plates. This paper provide comprehensive geologic, petrographic, and geochemical data-set, discuss P-T conditions for metamorphic evolution of the ophiolitic mélanges based on thermodynamically calculated P-T pseudosections for garnet amphibolite, and interpret lithological characteristics of the west Junggar in a subduction- accretion scheme. The TTKH and Tangbale - Baijiantan-Baikouquan (TBB) ophiolitic belts, representing the relics of the Ordovician Oceanic floor, subducted to north under the Chingiz-Tarbahatai arc and to south under the Junggar plate, respectively. The Baijiantan- Baikouquan ophiolite mélanges, as the major part of the TBB, consist of serpentinized lherzolite, spinel-bearing serpentinite, metagabbro, garnet amphibolite, dolomite marble, pillow basalt, and abyssal radiolarian chert. The flatten REE patterns with positive Eu anomalies and insignificant depletion of HFSE relative to the primitive mantle suggest a MORB origin for metagabbro. Lherzolite samples form a Sm-Nd isotopic isochron with age of 474Ma and e_Nd(T) of +8.9. Lherzolite samples with positive e_Nd(T) values of +8.8 ~ +9.1 and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7037 ~ 0.7040 are rather homogeneous in Sr-Nd isotopic composition, while metagabbro samples show relatively wide Sr-Nd isotopic compositional ranges with e_Nd(T) of +5.9 ~ +11.0. The Sm-Nd isotopic isochron age (~380Ma) for garnet amphibolite samples, consisting with zircon U-Pb age (~385Ma) for metagabbro, represents a magmatic event prior to subduction. Thermodynamic calculation for garnet amphibolite displays a clockwise P-T path with peak metamorphic condition of ~15 kbar (520-560 °C, ~342Ma), which reflects a subduction channel condition. The obtained Rb-Sr isochron ages (335Ma, 333Ma) for metagabbro represent a metamorphic event during exhumation.