Proto-Tethys Tectonic Evolution from Ordovician to Devonian in Southwestern Margin of Tarim Block, NW China

Geochemistry, zircon U-Pb dating and heavy mineral compositional analysis were carried out for the clastic rocks from Ordovician-Devonian drilling samples, to constrain the tectonic evolution of Proto-Tethys in southwestern Tarim. The zircon grains are divided into three genetic types, i.e. magmatic zircon, metamorphic zircon and trapped or residual zircon, recording the tectonic-thermal events in the periods of the 482-443 Ma, 438-425 Ma and 414-406 Ma since the Cambrian, as well as the Neoproterozoic rift magma events between 840 Ma and 750 Ma. The clastic rocks have high content of SiO₂ but relatively low content of Al₂O₃ and ^TFe₂O₃+MgO. They are characterized by right-dip chondrite-normalized REE pattern and flat HREE distribution, with weak to medium Eu negative anomaly. The ratios of trace elements, such as Th/U ratio, Cr/Zr ratio, Rb/Cs ratio, and La/Sc, suggests the terrigenous clastic components were derived from the felsic rocks in upper crust. It is assumed that the tectonic setting of sedimentary basin was characterized by continental island arc and active continental margin in late Ordovician. From the Early Silurian to Middle Devonian, the provenance was then predominated by continental island arc and passive continental margin, and mingled with active continental margin slightly. After the Late Devonian, it gradually changed over to evidential passive continental margin. It is indicated by the heavy minerals that the medium to acid magma activity became frequent while the basic magma activity became quiet during the time from Early Silurian to Middle Devonian, and the mountain uplifted massively since Late Devonian. Thus, the tectonic evolution of Proto-Tethys in south-western margin of Tarim could be divided into three stages, i.e. the northward subduction in Ordovician, the closure of back-arc ocean and folding orogeny in Silurian, and post-collision extension in Devonian.