Geochemical and Geochronological Evidences of Neoproterozoic Tectonic Evolution in Tarim Basin

The Tarim Basin in northwest China is a craton with complete Precambrian crystalline basement and well-developed Neoproterozoic sedimentary cover. Many Neoproterozoic volcanic rock outcrops were found around Tarim basin and their forming implied the Rodinia supercontinent assemblage and break-up. There are also some hypotheses for the nature of Tarim basement and central aeromagnetic anomaly. To test these hypotheses, lithogeochemical analyses of major elements, rare earth elements and trace elements and SHRIMP U-Pb zircon dating were performed for three samples of Precambrian basement rocks (QM1-1, migmatitic granite, from Well Qiman 1 in northern Tarim Basin, and TD2-1, granodiorite, from Well Tadong 2 and YD2-1, granodiorite, from Well Yingdong 2 in eastern Tarim Basin). The results show that the rock-forming ages are 1850.4 ± 9.1 Ma, 733.6 ± 5.7 Ma and 744.2 ± 4.5 Ma for QM1-1, TD2-1 and YD2-1, respectively. QM1-1 is measured to contain SiO2 and K2O of 70.12 and 5.90, respectively, higher than TD2-1 (SiO2, 63.91; K2O, 3.01) and YD2-1 (SiO2, 63.72; K2O, 3.16), and also contain Al2O3, CaO and NaO of 12.57, 2.61 and 1.86, respectively, lower than TD2-1 (16.24, 3.65 and 3.60) and YD2-1. All three samples exhibit a right-leaning light rare earth element distribution pattern, and represent relatively strong differentiation of LREE and low fractionation of heavy rare earth element. Generally, they have similar lithogeochemical characteristics of trace elements, and display strong enrichment of large ion lithophile elements (Rb, Ba, K and Pb) and relative depletion of high field strength elements (Nb, Ta, Sr, P and Ti) on the primitive mantle-normalized trace element spider diagrams. Their similarity in rare earth element and trace element distribution patterns indicates that the granodiorites (TD2-1, YD2-1) might be originated from recrystallization of the ancient crustal melt represented by QM1-1 after mixing with a higher proportion of mantle-derived melt. Consequently, the granodiorites inherited the lithogeochemical characteristics of the ancient continental crust. The rock-forming age and lithology indicate that TD2-1 and YD2-1 are products of the Neoproterozoic Rodinia supercontinent break-up and also prove the central aeromagnetic anomaly where they locate as a rift valley in the period. According to the rock-forming age of QM1-1 and the 1.8 Ga medium to shallow metamorphic rocks found commonly in the outcrops around the basin, as well as the duration of the Columbia supercontinent convergence, it is concluded that the Tarim oldland was merged during the Columbia supercontinent convergence and has undergone a unified tectonic evolution since then.