Petrogenesis and Tectonic Implications of the Early Triassic Nianzi Adakitic Granite Unit in the Yanshan Fold and Thrust Belt:New Constraints from U-Pb Geochronology and Sr-Nd-Hf Isotopes

The Nianzi granite unit,which includes the Nianzi,Xiaolianghou and Xiawopu granitic intrusions,is a significant component of the northern part of the North China Craton(NCC)and is situated in the Yanshan fold and thrust belt(YFTB).However,there is still debate regarding the tectonic evolutionary history of the YFTB during the late Permian to Triassic period,specifically regarding the timing of subduction and collision between the NCC and the Paleo-Asian Ocean.The Nianzi granite unit exhibits unique petrological,geochronological and geochemical signatures that shed light on the tectonic evolutionary history of the YFTB.This study presents detailed petrology,whole-rock geochemistry,together with Sr-Nd isotopic,zircon U-Pb dating and Lu-Hf isotopic data of the granites within the Nianzi granite unit.Our findings demonstrate that the granites primarily consist of subhedral K-feldspar,plagioclase,quartz,minor biotite and hornblende,with accessory titanite,apatite,magnetite and zircon.Zircon U-Pb dating indicates that the Xiaolianghou granite was emplaced at 247.5±0.62 Ma.Additionally,the adakitic characteristics of the Nianzi,Xiawopu and Xiaolianghou granitic intrusions,such as high Sr and Ba contents and high ratios of Sr/Y and(La/Yb)N,combined with negative Sr-Nd and Lu-Hf isotopes(87Sr/86Sr)i=0.705681–0.7057433,εNd(t)=−21.98 to−20.97,zirconεHf(t)=−20.26 to−9.92,as well as the I-type granite features of high SiO_(2),Na_(2)O and K_(2)O/Na_(2)O ratios,enriched Rb,K,Sr and Ba,along with depleted Th,U,Nb,Ta,P and Ti,suggest that the Nianzi granitic unit was mainly derived from the partial melting of a thickened lower crust containing hydrous,calc-alkaline to high-K calc-alkaline,mafic to intermediate metamorphic rocks.In light of these parameters,we further integrate our data with previous studies and conclude that the Nianzi granitic unit was generated in a post-collisional extensional environment during the Early Triassic.

Structural Evolution of the Eastern Qiulitagh Fold and Thrust Belt,Northern Tarim Basin,China

The eastern Qiulitagh fold and thrust belt （EQFTB） is part of the active Kuqa fold and thrust belts of the northern Tarim Basin. Seismic reflection profiles have been integrated with surface geologic and drill data to examine the deformation and structure style of the EQFTB, particularly the deformational history of the Dina 2 gas field. Seismic interpretations suggest that Dongqiu 8 is overall a duplex structure developed beneath a passive roof thrust, which generated from a tipline in the Miocene Jidike Formation, and the sole thrust was initiated from the same Jidike Formation evaporite zone that extends westward beneath the Kuqatawu anticline. Dongqiu 5 is a pop-up structure at the western part of the EQFTB, also developed beneath the Jidike Formation evaporite. Very gentle basement dip and steep dipping topographic slope in the EQFTB suggest that the Jidike Formation salt provides effective decoupling. The strong deformation in the EQFTB appears to have developed further south, in an area where evaporite may be lacking. Since the Pliocene, the EQFTB has moved farther south over the evaporite and reached the Yaken area. Restoring a balanced cross-section suggests that the minimum shortening across the EQFTB is more than 7800 m. Assuming that this shortening occurred during the 5.3 Ma timespan, the shortening rate is approximately 1.47 mm/year.

Tectono-sedimentary Evolution of Piggy-Back Basin:Example from Kuqa Fold and Thrust Belt,Northern Tarim Basin,Northwest China

The Baicheng （拜城） piggy-back basin was part of the intracontinental foreland basin system of southern Tianshan （天山） Mountains. It was formed on a thrust-and-fold belt induced by Mio-Pliocene collision between southern Tianshan Mountains and Tarim craton that controlled the thick synorogenic basin fills. Sedimentological analysis and a restored cross-section based on seismic data and field studies revealed three tectono-depositional sequences of synorogenic basin fills. （1） The Lower Miocene sequence （Jidike （吉迪克 ） Formation） was formed under alluvial-braided river-lacustrine environments, in response to geodynamic changes of the Kuqa （库车） fold and thrust belt from the embryonic foreland to a foredeep in the course of orogenic loading period. （2） The Upper Miocene sequence （Kangcun （康村） Formation） was developed in fluvial-delta and lacustrine environments, within a foredeep due to orogenic thrust. （3） The Pliocene sequence （Kuqa Formation） was formed in the Baicheng piggy-back basin that became a wedge-top depozone, thrusting in the Qiulitagh （秋里塔格） thrust belt that propagated progressively southward to the Yaken （牙肯） thrust belt.