Provenance analyses of early Mesozoic sediments in the Ningwu basin: Implications for the tectonic–palaeogeographic evolution of the northcentral North China Craton

This paper reports results from detrital zircon U–Pb geochronology, Hf isotopic geochemistry, sandstone modal analysis, and palaeocurrent analysis of the early Mesozoic strata within the Ningwu basin, China, with the aims of constraining the depositional ages and sedimentary provenances and shedding new light on the Mesozoic tectonic evolution of the northcentral North China Craton (NCC). The zircons from early Mesozoic sandstones are characterized by three major populations: Phanerozoic (late Palaeozoic and early Mesozoic), late Palaeoproterozoic (with a peak at approximately 1.8 Ga), and Neoarchaean (with a peak at approximately 2.5 Ga). Notably, three Phanerozoic zircons in the Early Triassic Liujiagou Formation were found to have positive <i>ε</i>_Hf(<i>t</i>) values and characteristics typical of zircons from the Central Asian Orogenic Belt (CAOB). Therefore, the CAOB began to represent the provenance of sediment in the sedimentary basins in the northern NCC no later than the Early Triassic (261 Ma), implying that the final amalgamation of the NCC and CAOB occurred before the Early Triassic. The U–Pb geochronologic and Hf isotopic results show that the Lower Middle Triassic sediments were mainly sourced from the Yinshan–Yanshan Orogenic Belt (YYOB), and that a sudden change in provenances occurred, shifting from a mixed YYOB and CAOB source in the Middle Jurassic to a primarily YYOB source in the Late Jurassic. The results of the sandstone modal analysis suggest that the majority of the samples from the Lower Middle Jurassic rocks were derived from either Continental Block or Recycled Orogen sources, whereas all the samples from the Upper Jurassic rocks were derived from Mixed sources. The change in source might be ascribed to the southward subduction and closure of the Okhotsk Ocean and the resulting intense uplift of the YYOB during the Late Jurassic. This uplift likely represents the start of the Yanshan Orogeny.

本文报道了中国宁武盆地（Ningwu basin）早中生代地层的碎屑锆石U-Pb年代学（detrital zircon U–Pb geochronology）、Hf同位素地球化学（Hf isotopic geochemistry）、砂岩骨架成分分析（sandstone modal analysis）以及古流向分析（palaeocurrent analysis）结果，旨在约束其沉积时代与沉积物源区，并为华北克拉通（North China Craton, NCC）中北部的中生代构造演化提供新认知。
研究区早中生代砂岩中的锆石可划分为三大主要年龄组分：显生宙（包括晚古生代及早中生代）、古元古代晚期（峰值约1.8 Ga）以及新太古代（峰值约2.5 Ga）。值得注意的是，早三叠世刘家沟组中的3颗显生宙锆石具有正的εHf(t)值，具备中亚造山带（Central Asian Orogenic Belt, CAOB）锆石的典型特征。据此可知，中亚造山带最晚于早三叠世（261 Ma）即已成为华北克拉通北部沉积盆地的物源区之一，这意味着华北克拉通与中亚造山带的最终拼合发生在早三叠世之前。
U-Pb年代学与Hf同位素结果显示，中三叠统下部沉积物的主要物源为阴山-燕山造山带（Yinshan–Yanshan Orogenic Belt, YYOB）；中侏罗世时期物源发生突变，从中侏罗世的阴山-燕山造山带与中亚造山带混合供源，转变为晚侏罗世以阴山-燕山造山带为主的物源体系。砂岩骨架成分分析结果表明，大部分中侏罗统下部样品的物源为陆块或再旋回造山带，而所有上侏罗统样品的物源均为混合源。沉积物源的转变可能与晚侏罗世鄂霍次克洋向南俯冲闭合及由此引发的阴山-燕山造山带强烈隆升有关，该隆升事件或标志着燕山造山作用的起始。