BI-DIRECTIONAL SUBDUCTION OF THE SOUTH TIANSHAN OCEAN DURING THE LATE SILURIAN: MAGMATIC RECORDS FROM BOTH THE SOUTHERN CENTRAL TIANSHAN BLOCK AND NORTHERN TARIM CRATON

The Tianshan orogeny, situated in the southernmost of the Central Asian Orogenic Belt (CAOB), is a key area to understand the final accretion of the CAOB. One major issue in the Tianshan area is the subduction polarity of the South Tianshan Ocean. Magmatic rocks widely exposed in the orogen provide us a clue to solve this problem. A comprehensive study of zircon U-Pb dating, and whole rock geochemical analyses was carried out on the andesite rocks of the Bayanbulak formation and a monzonite pluton exposed on the southern margin of the Central Tianshan block (CTB) and a granodiorite in Oxidaban area on the northern margin of the Tarim Craton. LA-ICP-MS zircon U-Pb dating yield 423.2±4.5 Ma (MSWD=1.08, probability=0.37) for the andesite of the Bayanbulak formation and 423.9±2.9 Ma (MSWD=0.5, probability=0.97) for the monzonite in the southern part of the CTB, and 420.8±1.8 Ma (MSWD=0.072, probability=1.0) for the Oxidaban granodiorite in the northern Tarim Craton. All the samples exhibit Na-rich, calc-alkaline affinity. They display enrichment in rare earth element and large ion lithophile elements (Rb, Ba, Th, U, K and Sr), weak or no negative Eu anomalies, and depletion in high strength field elements (Nb, Ta, Ti and P), reflecting features of arc-type igneous rocks. This is further evidenced by the fact that all the samples plot into the subduction-related areas in various tectonic setting discrimination diagrams. The samples display high Mg# values ranging from 60.51 to 63.08 for the andesite, 48.76 to 51.85 for the monzonite, and 50.31 to 53.73 for the granodiorite, respectively, indicating a dominant magma derivation of mantle component for these igneous rocks. But the positive Zr and Hf anomalies also reflect minor continental contamination. These geochemical features suggest that these Late Silurian magmatic rocks formed in active continental margins. These data, in combination with other previously reported arc-type granitic rocks in the southern CTB and the northern Tarim Caron, support a bi-directional subduction model for the South Tianshan Ocean in the Late Silurian.