The Paleozoic evolution of Central Tianshan: Geochemical and geochronological evidence

Abstract

A suite of Paleozoic granitoids in Central Tianshan was studied for both geochemistry and geochronology in an effort to constrain their origin and tectonic setting. We combined LA-ICP-MS dating of zircon, standard geochemical analyses and Hf-isotopic studies of zircon to develop our tectonic model. Based on our analysis, the granitoids formed in three distinctive stages: ~450–400Ma, ~370–350Ma and ca. 340Ma. The first stage (450–400Ma) granitoids exhibit metaluminous, magnesian, high-K to shoshonitic characteristics of I-type granitoids (arc-setting), that are enriched in LREE relative to HREE with high (La/Yb)CN values, show negative Eu anomaly and are depleted in Nb, Ta and Ti. This phase of granitoid emplacement was most likely related to the southward subduction of the Paleo-Tianshan Ocean beneath the Tarim block and the subsequent Central Tianshan arc. In contrast, the second stage granitoids (370–350Ma) are distinctly different and are classified as calc-alkaline or shoshonitic plutons with a weak positive Eu anomaly. Within the second stage granitoids, it appears that the earlier (~365Ma) granitoids fit within the A-type field whereas the younger (~352Ma) granitoids plot within the post-collisional potassic field. These granitoids formed during collisions between Central Tianshan and the Tuha terrane that occurred along the northern margin of Central Tianshan. Lastly, the ca. 340Ma granitoids are typical of volcanic arc granitoids again that probably formed during the northward subduction of the South Tianshan Ocean beneath the Central Tianshan landmass or the subsequent southward subduction of the residual Paleo-Tianshan Ocean.The Hf isotopic data of zircons from all the studied granitoids were pooled and yielded three prominent Hf TDMC model age populations: ca. 2400Ma, ca. 1400Ma and ca. 1100Ma. The Hf-data shows a significant input of juvenile crust in addition to crustal recycling. We interpret these three phases of juvenile crustal addition to phases of global growth of continental crust (~2400Ma), the addition of juvenile crust during the breakup of the Columbia supercontinent (~1400Ma) and the assembly of Rodinia (~1100Ma).