Sandstones hosted by Pacific-type orogenic belts and related accretionary complexes carry records of their source intra-oceanic or continental magmatic arcs. Study of arc-derived sandstones is important for reconstructing tectonic evolution of Pacific-type convergent margins in case such arcs get partly or completely eroded during subduction. In this paper, we present first U-Pb detrital zircon ages and Hf-in-zircon isotope ratios and whole-rock geochemical and Nd data from Ordovician to early Silurian clastic sediments (sandstones) of the Itmurundy Pacific-type orogenic belt and its hosted accretionary complex (AC). The Itmurundy AC is located in the northern Balkhash area of central Kazakhstan and includes sandstones of the Itmurundy and Kazyk fms. (central segment) and Tyuretai Fm. (eastern segment). The U-Pb ages of detrital zircons from Group 1 sandstones show unimodal distributions with major peaks at ca. 467–456 Ma and the youngest clusters of ages from 442 to 439 Ma Ma suggesting post-Llandovery deposition. The distributions of the U-Pb ages of detrital zircons from Group 2 sandstones are polymodal with major peaks at ca. 462–461 Ma and subordinate peaks at 788, 992, 2472 Ma. The Group 2 zircons yielded two youngest clusters of U-Pb ages: 458 Ma and 424 Ma suggesting post-middle Ordovician and post-Ludlow maximum depositional ages. According to petrographic classifications, both Group 1 and 2 sandstones are poorly-sorted and poorly-rounded greywacke or litharenite. The chemical compositions imply that these sandstones are immature and weakly weathered sediments. The Group 1 greywackes are characterized by relatively low SiO₂ (63 wt%) compared to Group 2 greywackes (SiO₂ = 66 wt% in average). The major and trace element compositions of both groups of sandstones resemble those of supra-subduction andesitic to dacitic volcanic rocks. The positive values of whole-rock ɛNd(t) (+1 to + 5.3) and zircon ɛHf(t) (+9.8 to +17.5) obtained from the Group 1 greywackes suggest juvenile igneous rocks in the source area. On the contrary, the negative values of ɛNd(t) (−7.2) and variable values of zircon ɛHf(t) (−24.5 to +11.5) from the Group 2 sandstones suggest that their parental igneous rocks were derived from mantle sources possessing both juvenile and recycled characteristics. Conclusively, the greywacke sandstones of both groups formed by erosion of an Ordovician intra-oceanic volcanic arc once existed at an active margin of the Paleo-Asian Ocean, however those of Group 1 deposited in a fore-arc basin and trench, while those of Group 2 deposited in a back-arc basin.

由太平洋型造山带和相关增生杂岩所承载的砂岩带有其源洋内或大陆岩浆弧的记录。弧源砂岩的研究对于重建太平洋型辐合边缘的构造演化具有重要意义，以防此类弧在俯冲过程中被部分或完全侵蚀。在本文中，我们首先介绍了伊穆伦底太平洋型造山带及其宿主的奥陶纪至志留纪早期碎屑沉积物（砂岩）的 U-Pb 碎屑锆石年龄和 Hf-in-锆石同位素比以及全岩地球化学和 Nd 数据。增生复合体（AC）。Itmurundy AC 位于哈萨克斯坦中部巴尔喀什北部地区，包括 Itmurundy 和 Kazyk 组的砂岩。（中央段）和Tyuretai Fm。（东段）。来自第 1 组砂岩的碎屑锆石的 U-Pb 年龄显示出单峰分布，主峰位于 ca。467-456 Ma 和年龄从 442 到 439 Ma Ma 的最年轻星团，表明 Llandovery 后沉积。来自第 2 组砂岩的碎屑锆石的 U-Pb 年龄分布是多峰的，主要峰在 ca。462-461 Ma 和 788、992、2472 Ma 的次峰。第 2 组锆石产生了两个最年轻的 U-Pb 年龄簇：458 Ma 和 424 Ma，表明中奥陶世后和勒德洛后最大沉积年龄。根据岩石学分类，第 1 组和第 2 组砂岩都是分选差、圆度差的灰泥岩或岩屑岩。化学成分表明这些砂岩是未成熟和弱风化的沉积物。第 1 组灰船的特点是 SiO2 相对较低₂ (63 wt%) 与第 2 组灰船 (SiO ₂ = 平均 66 wt%）。两组砂岩的主量元素和微量元素组成类似于超俯冲安山质至英安质火山岩。从第一组灰泥中获得的全岩 ɛNd(t) (+1 至 +5.3) 和锆石 ɛHf(t) (+9.8 至 +17.5) 的正值表明源区有幼火成岩。相反，来自第 2 组砂岩的 ɛNd(t) (-7.2) 和锆石 ɛHf(t) 的变量值 (-24.5 至 +11.5) 的负值表明它们的母体火成岩来自具有两者的地幔源幼体和再生特性。综上所述，奥陶系洋内火山弧侵蚀形成的两组灰泥砂岩曾存在于古亚洲洋活动边缘，而第一组沉积于弧前盆地和海沟中，