Geology and geochemistry of the Baijiantan-Baikouquan ophiolitic melanges: implications for geological evolution of west Junggar, Xinjiang, NW China

We report two newly identified Ordovician ophiolite belts in west Junggar, NW China: Tajin-Tarbahatai-Kujibai-Honguleleng (TTKH) and Tangbale-Baijiantan-Baikouquan (TBB) ophiolitic belts. These two ophiolitic belts provide constraints for the Palaeozoic reconstruction of Central Asia and the geological evolution of this region. The TTKH and TBB ophiolitic belts are dismembered parts of different ophiolitic belts which represent relics of Ordovician oceanic floor; they subducted to the north under the Chingiz-Tarbahatai arc and to the south under the Junggar plate, respectively. The Baijiantan-Baikouquan ophiolite melanges comprise the major part of the TBB. Flat rare Earth element (REE) patterns with positive Eu anomalies and insignificant depletion of high-field-strength elements (HFSE) relative to melts of primitive mantle suggest a mid-ocean-ridge basalt (MORB) origin for the metagabbro. Lherzolite samples define a Sm-Nd isotopic isochron with age of 474 Ma and (Nd(t)) of +8.9. Lherzolite samples with positive (Nd(t)) values of +8.8 to +9.1 and initial Sr-87/Sr-86 ratios of 0.7037-0.7040 are rather homogeneous in Sr-Nd isotopic composition, whereas metagabbro samples show wider Sr-Nd isotopic compositional ranges with (Nd(t)) of +5.9 to +11.0. The Sm-Nd isotopic isochron age (c. 380 Ma) for garnet amphibolite samples, consistent with a zircon U-Pb age (c. 385 Ma) for metagabbro, represents a magmatic event prior to subduction. Thermodynamic calculations for garnet amphibolite yield a clockwise pressure-temperature path with peak metamorphic condition of c. 15 kbar and 520-560 degrees C at 342 Ma, indicating a subduction-channel setting. The Rb-Sr isochron ages (335 Ma, 333 Ma) for metagabbro represent a metamorphic event during exhumation.