Detrital zircon U-Pb-Hf isotopes and whole-rock geochemistry of Ediacaran - Silurian clastic sediments of the Uzbek Tienshan: sources and tectonic implications

The paper presents first high-precision data, U-Pb detrital zircon ages, whole-rock geochemistry, Hf-in-zircon and whole-rock Nd isotopes, from pre-Devonian (Ediacaran and Silurian) clastic sediments (sandstones) of the Tamdytau, Bukantau and Nuratau mountainous ranges of the Kyzylkum Desert and Nuratau Range in the western Uzbek Tienshan. The sediments form a turbidite-type complex associated with ocean plate stratigraphy units (oceanic pillow basalt, chert, siliceous mudstone and siltstone) and arc volcanic rocks. Four sandstone samples from the Tamdytau and Northern Nuratau Mts. (Besapan and Kaltadavan formations, respectively) yielded maximum depositional ages in the range of 570–540 Ma. These ages indicate the formation of pre-Devonian sedimentary units during a relatively narrow time interval from the latest Neoproterozoic (Ediacaran) to the earliest Cambrian, i.e. ca. 50–70 Myr. Five samples of turbidites from the Bukantau Mts. (Baimen Fm.) yielded a maximal depositional age of ca. 440 Ma, i.e. early Silurian (Llandovery). The petrographic, major and trace element compositions, as well as εNd(t) values ranging from -16 to -9 of those sandstones suggest their generally mature character and derivation from recycled orogens with a limited contribution of juvenile crust material. All sandstone samples yielded similar detrital zircon U-Pb age patterns characterized by major peaks at 650–570, 870–730, 1050–900 and 2400 Ma and by a smaller peak at ca. 1800 Ma. These patterns are similar to the U-Pb age spectra from the basement of the Tarim Craton. However, the Kyzylkum basement may contain a larger proportion of late Archaean rocks and igneous formations related to Ediacaran – earliest Cambrian orogenic events. All samples showing U-Pb detrital zircon age spectra with peaks at 650–570 Ma carry relatively large amounts of zircon grains with juvenile Hf isotope characteristics, i.e. positive εHf(t) (up to +10) suggesting their derivation from continental or Island arcs. The presence of exotic tectonic blocks composed of Ediacaran arc-type rocks hosted by the accretionary complex of the South Tienshan suggests that an extended arc system once existed at the southern convergent margin of the Turkestan Ocean. That arc system provided clastic material for the Palaeozoic sediments of the South Tienshan, but was probably destroyed by tectonic erosion during early Palaeozoic oceanic subduction.

本研究报道了乌兹别克斯坦西部天山克孜勒库姆沙漠区域塔姆迪套山（Tamdytau）、布坎套山（Bukantau）及努拉套山（Nuratau）的泥盆纪前（埃迪卡拉纪与志留纪）碎屑沉积岩（砂岩）的首批高精度数据，涵盖碎屑锆石U-Pb年代学、全岩地球化学、锆石原位Hf同位素及全岩Nd同位素数据。该套沉积岩形成于与洋板层序单元（洋壳枕状玄武岩、燧石、硅质泥岩及粉砂岩）及弧火山岩伴生的浊积岩复合体中。采自塔姆迪套山与努拉套山北部（分别对应贝萨潘组（Besapan）与卡尔塔达万组（Kaltadavan））的4件砂岩样品，其最大沉积年龄介于570~540 Ma之间，指示泥盆纪前沉积单元形成于新元古代晚期（埃迪卡拉纪）至寒武纪最早期的狭窄时间区间内，即约50~70百万年。采自布坎套山（白门组（Baimen Fm.））的5件浊积岩样品，其最大沉积年龄约为440 Ma，对应志留纪早期（兰多维列世（Llandovery））。该类砂岩的岩石学、主量与微量元素组成，以及εNd(t)值介于-16~-9之间的特征，均表明其整体成熟度较高，物源来自再旋回造山带，新生地壳物质贡献有限。所有砂岩样品均呈现相似的碎屑锆石U-Pb年龄谱，主要年龄峰集中于650~570 Ma、870~730 Ma、1050~900 Ma及2400 Ma，另有一处较小的年龄峰约为1800 Ma。该年龄谱与塔里木克拉通（Tarim Craton）基底的U-Pb年龄谱相似，但克孜勒库姆基底可能包含更高比例的晚太古代岩石以及与埃迪卡拉纪—寒武纪最早期造山事件相关的火成岩建造。所有呈现650~570 Ma年龄峰的碎屑锆石U-Pb年龄谱样品，均携带大量具有新生Hf同位素特征的锆石颗粒，即εHf(t)值为正（最高可达+10），指示其物源来自大陆弧或岛弧。南天山（South Tienshan）增生杂岩中产出由埃迪卡拉纪弧型岩石组成的异地构造岩块，这表明突厥洋（Turkestan Ocean）南缘汇聚带曾存在一个延展的弧系统。该弧系统曾为南天山古生代沉积提供碎屑物质，但在早古生代洋壳俯冲过程中可能因构造侵蚀作用被破坏。