Sediment provenances of a Mesozoic intracontinental basin enclosed by multiple orogenic belts, Junggar Basin, NW China: insights from detrital ilmenite, Cr-spinel geochemistry, and zircon U–Pb geochronology

The Junggar Basin was an intracontinental basin enclosed by multiple orogenies in the Mesozoic. Unravelling the provenances development in the western Junggar Basin enclosed by the West Junggar Orogenic Belt (WJOB) and Tianshan Orogenic Belt (TSOB) would elucidate the exhumation history of these orogenic belts during the post-orogenic period. The TSOB includes Central Tianshan which includes a continental block with the Precambrian to early Palaeozoic metamorphic basements, while the WJOB includes Palaeozoic accretionary complexes. Given their contrasting geological histories, we use detrital garnets, metamorphic ilmenites, epidotes, and Precambrian zircons to distinguish the TSOB and detrital Cr-spinel to distinguish the WJOB. Combined with sedimentary analysis, palaeocurrents, heavy mineral data, seismic profiles, and zircon U-Pb geochronology, here we reconstruct a four-phase source-to-sink model. During the syn-rift phase, the southern basin was stably fed by the TSOB in the Triassic, evidenced by high Garnet-zircon index and stable single-peak Carboniferous detrital zircon age spectra. During the post-rift sag basin phase, the source-to-sink system experienced a significant change, because detrital zircon age spectra change into multiple peaks. During the transpressional basin phase, the WJOB replaced the TSOB as the dominant provenance in the late Middle Jurassic and the TSOB became the dominant provenance again in the Late Jurassic. Syndepositional volcanic materials provided a large amount of sediments. During the sag basin phase, lake transgression happened and the TSOB provenance could not reach the northern basin. We concluded that the WJOB experienced transpression and obvious denudation in the late Middle Jurassic, and the TSOB uplifted under transpression in the Late Jurassic. The transpression controlled the change of major provenances. The aridification in the Late Jurassic was recorded by the decrease in the Zircon-Tourmaline-Rutile index and the increase in epidotes and magnetites. The humidification in the Early Cretaceous was responded to by lake transgression and sediment recycling.

准噶尔盆地（Junggar Basin）是中生代时期被多个造山带围限的陆内盆地。厘清被西准噶尔造山带（West Junggar Orogenic Belt, WJOB）与天山造山带（Tianshan Orogenic Belt, TSOB）环绕的西准噶尔盆地物源演化特征，可为揭示两造山带后造山期的剥露历史提供关键依据。天山造山带包含中央天山陆块，其发育从前寒武纪至早古生代的变质基底；而西准噶尔造山带则以古生代增生杂岩为核心组成。鉴于二者地质演化历史存在显著差异，本研究采用碎屑石榴石、变质钛铁矿、绿帘石与前寒武纪锆石作为天山造山带的物源示踪指标，以碎屑铬尖晶石作为西准噶尔造山带的物源示踪指标。结合沉积分析、古流向数据、重矿物资料、地震剖面与锆石U-Pb年代学数据，本文重建了四阶段源汇系统演化模型。在裂谷期，三叠纪时期南部盆地稳定接收来自天山造山带的沉积物供给，该结论可通过高石榴石-锆石指数与稳定的单峰式石炭纪碎屑锆石年龄谱得到佐证。在裂谷后坳陷期，源汇系统发生显著转变，碎屑锆石年龄谱转变为多峰式分布。在压扭盆地期，中侏罗世晚期西准噶尔造山带取代天山造山带成为主要物源区，而晚侏罗世天山造山带重新占据主导物源地位。同沉积火山物质为该时期提供了大量沉积物供给。在坳陷期，湖侵作用发生，天山造山带的物源无法抵达北部盆地。本研究认为，中侏罗世晚期西准噶尔造山带经历了压扭作用与显著剥蚀，晚侏罗世天山造山带则在压扭作用下发生隆升。压扭作用控制了主要物源的时空更替。晚侏罗世的干旱化事件可通过锆石-电气石-金红石指数降低、绿帘石与磁铁矿含量升高得到记录。早白垩世的湿润化则对应湖侵作用与沉积物再旋回过程。