A record of post-collisional transition: evidence from geochronology and geochemistry of Palaeozoic volcanic rocks in the eastern Junggar, Central Asia

Abundant clues about the post-collisional transition in the southwestern margin of the Central Asian Orogenic Belt (CAOB) have been recorded by the late Palaeozoic volcanic rocks in the eastern Junggar region. These volcanic rocks provide a better window for understanding the magmatic and tectonic transition processes. This paper reports detailed petrology, whole-rock and Sr–Nd–Pb isotope geochemistry, and <i>in situ</i> zircon U–Pb dating analyses of these volcanic rocks from this region. Systematic geochemical and geochronological studies show that three stages of magmatism were recorded in this area, including ca. 407 Ma and ca. 340 Ma medium-to-high-K calc-alkaline series and ca. 320 Ma alkaline and tholeiite series. The ca. 407 Ma volcanic rocks have calc-alkaline compositions, strong enrichment in large-ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs), negative Nb anomalies, and light REE (rare earth element)-enriched patterns. The ca. 340 Ma volcanic rocks, which have similar calc-alkaline compositions and arc geochemical affinities but higher K content, have slightly lower Th/Yb and Ba/Nb values than typical island arc or continental margin volcanic suites. The compositions of the ca. 320 Ma volcanic rocks display a bimodal distribution; the ca. 320 Ma basaltic rocks are chemically similar to oceanic island basalt (OIB) but with slight depletion in Nb and Ta, wheres the acidic rocks show the features of A-type granite. The chemical compositions of these volcanic rocks indicate that the ca. 407 Ma magmatism was subduction related, while the ca. 340 Ma and ca. 320 Ma volcanic rocks were the product of post-collisional magmatism. This transition from ca. 407 Ma to ca. 320 Ma records slab subduction, slab break-off, and slab rollback and provides a plausible explanation for the complex spatial and temporal geochemical evolution of magmatism in eastern Junggar since the Devonian.

准噶尔东部地区的晚古生代火山岩，记录了中亚造山带（Central Asian Orogenic Belt, CAOB）西南缘碰撞后构造转换的丰富线索。这些火山岩为认识岩浆与构造转换过程提供了极佳的研究窗口。本文报道了该区域火山岩的详细岩石学、全岩Sr-Nd-Pb同位素地球化学以及原位锆石U-Pb定年分析结果。系统的地球化学与年代学研究表明，该区域记录了三期岩浆活动：约407 Ma和约340 Ma的中高钾钙碱性系列岩石，以及约320 Ma的碱性系列与拉斑玄武岩系列岩石。约407 Ma的火山岩具有钙碱性地球化学组成，强烈富集大离子亲石元素（large-ion lithophile elements, LILEs）、亏损高场强元素（high-field strength elements, HFSEs），呈现负Nb异常，且具轻稀土元素（rare earth element, REE）富集的配分模式。约340 Ma的火山岩具有相似的钙碱性组成与弧型地球化学亲和性，但钾含量更高，其Th/Yb与Ba/Nb值略低于典型岛弧或大陆边缘火山岩组合。约320 Ma的火山岩组成呈双峰式分布：其中的玄武质岩石化学组成与洋岛玄武岩（oceanic island basalt, OIB）相似，但Nb、Ta轻微亏损；酸性岩石则表现出A型花岗岩（A-type granite）的特征。上述火山岩的地球化学组成表明，约407 Ma的岩浆活动与俯冲作用相关，而约340 Ma和约320 Ma的火山岩为碰撞后岩浆作用的产物。从约407 Ma至约320 Ma的转换过程，记录了板片俯冲、板片折断与板片回撤过程，可为解释准噶尔东部自泥盆纪以来复杂的岩浆作用时空地球化学演化提供合理依据。