Early Paleozoic Magmatic Record From The Northern Margin Of The Tarim Craton: Further Insights On The Evolution Of The Central Asian Orogenic Belt

The Southern Tianshan Orogenic Belt (STOB) is an important region to understand theprolonged evolution of the Central Asian Orogenic Belt (CAOB). A comprehensive study ofzircon U-Pb ages and Lu-Hf isotopes, whole-rock elements and Sr-Nd isotopes was carried out fortwo intermediate-mafic intrusions in the Baicheng and Heiyingshan areas of Southwest Tianshan.LA-ICPMS zircon U-Pb dating yielded crystallization ages of431±3Ma (MSWD=0.073, n=31)and430±3Ma (MSWD=0.104, n=30) for the Baicheng diorites, and423.9±2.1Ma (MSWD=0.5,n=12) for the Heiyingshan gabbros. These data, in combination with other Late Silurian agespreviously reported for the intrusive suites from the STOB, indicate an Early Paleozoic magmaticevent in the region. In-situ zircon Hf isotope data on two samples from the Baicheng diorites showεHf(t) values of-2.4to4.9and0.2to6, and the Heiyingshan gabbros exhibit similar εHf(t) valuesfrom-1to1.8. The dioritic pluton exhibits Na-rich, arc-type geochemical and calc-alkalineaffinity, and shows trace element patterns characterized by enrichment in large ion lithophileelements (Rb, Ba, K and Sr) and depletion in high strength field elements (Nb, Ta, Ti, Zr and Hf).These features resemble those of arc-type igneous rocks. Combined with initial87Sr/86Sr ratios of0.7047to0.7064and εNd(t) of-1.6to-5.06, we infer that the magma from which the Baichengdioritic pluton formed was sourced from a mantle wedge which was subjected to metasomatism byfluids and melts related to subducted oceanic slab and the overlying sediments. The Heiyingshangabbro shows Na-rich nature and tholeiitic affinity. They not only display supra-subduction zoneaffinity, but also are geochemically similar to E-MORB, possible implying a back-arc basinsetting. The initial87Sr/86Sr ratios of the Heiyingshan samples range from0.7046to0.7051andεNd(t) values vary from1.76to5.19. We speculate that the upwelling of asthenosphere mantle tothe supra-subduction wedge, facilitated by subduction roll-back, provided the source for theHeiyingshan gabbroic rocks. Together with other geologic evidence, we correlate the generationof the Baicheng diorite with the southward subduction of the South-Tianshan Ocean during theEarly Paleozoic. The Heiyingshan gabbroic plutons were probably formed in a back-arc basin inresponse to the southward subduction of the Paleo-Tianshan Ocean. In combination with coevalarc-type magmatic rocks widely exposed in the southern margin of the Central Tianshan Block, adouble-sided subduction model is proposed to explain the evolution of the South-Tianshan Oceanin the Silurian.