Devonian to carboniferous tectonic evolution of the Kangguer Ocean in the Eastern Tianshan, NW China: Insights from three episodes of granitoids
Devonian to carboniferous tectonic evolution of the Kangguer Ocean in the Eastern Tianshan, NW China: Insights from three episodes of granitoids
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DOI：
10.1016/j.lithos.2019.105243
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发表时间：
2019-12
期刊：
影响因子：
3.5
通讯作者：
L. Du;Yunying Zhang;Zongying Huang;Xu-Ping Li;C. Yuan;Bin Wu;X. Long
中科院分区：
文献类型：
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作者：
L. Du;Yunying Zhang;Zongying Huang;Xu-Ping Li;C. Yuan;Bin Wu;X. Long
研究康古尔洋的构造演化对认识东天山构造格局具有重要意义。以往对康古尔洋的研究主要集中在晚石炭世至二叠纪的最终闭合。在康古尔带中发现了三期不同的花岗岩类：晚泥盆世（约1000年）; 360 Ma）正常I型花岗岩，早石炭世（约. 348 Ma）分馏I型花岗岩，和晚石炭世（约。317 - 302 Ma）埃达克质花岗岩，试图对这些岩石的来源提供约束，从而进一步解开了康古尔洋在此期间的演化。晚泥盆世花岗岩具钙碱性特征，Eu异常可忽略，（K2O + Na2O）/CaO比值较低，类似于I型花岗岩，不存在明显的分离结晶作用。它们显示出高SiO2（> 70重量%），具有低MgO（<0.83重量%）和Mg #（35 - 41）值、亏损Sr和Nd同位素值（（87Sr/86Sr）i = 0.7041; ε Nd（t）= 5.48 - 6.38）和年轻TDM2值（596 - 671 Ma），表明它们可能来自下地壳来源的年轻物质。早石炭世花岗岩的Sr、Nd同位素值与晚泥盆世花岗岩相似，但Eu、Sr明显亏损，Mg #（26 - 29）值较低，（K2O + Na2O）/CaO比值较高，表明它们也代表了较年轻的壳源岩浆，但经历了广泛的晶体分异。晚石炭世岩石具有高SiO2（63.7 - 69.0 wt. %），Sr/Y（17.4 - 83.3）、La/Yb（4.00 - 13.7）和Na_2O/K_2O（1.66 - 7.03）值以及低Y（4.46 - 12.2ppm）和Yb（0.50 - 1.10ppm）含量，指示埃达克岩的亲合性。上述地球化学特征及Sr、Nd同位素值与康古尔带板状埃达克岩和蛇绿岩相当，表明它们可能是由板状岩浆和地幔楔橄榄岩形成的。结合区域地质特征，提出康古尔洋片向北俯冲是晚泥盆世正常I型花岗岩形成的主要原因。随后，康古尔洋向南的回滚作用使早石炭世花岗岩进一步分离结晶。我们推断，晚石炭世埃达克质岩石的形成是由一次向北的平坦俯冲作用引起的.
Deciphering the tectonic evolution of the Kangguer Ocean is critical to understand the architecture of the Eastern Tianshan, NW China. Previous studies for the Kangguer Ocean mainly concentrated on final closure during the Latest Carboniferous to Permian. Three distinct episodes of granitoids have been recognized in the Kangguer belt: Late Devonian (ca. 360 Ma) normal I-type granites, Early Carboniferous (ca. 348 Ma) fractionated I-type granites, and Late Carboniferous (ca. 317–302 Ma) adakitic granites, attempting to provide constraints on the sources of these rocks, which further unravels the evolution of the Kangguer Ocean during this period. The Late Devonian granites exhibit calc-alkaline characteristics, with negligible Eu anomalies and low (K2O + Na2O)/CaO ratios, akin to I-type granites without significant fractional crystallization. They display high SiO2(>70 wt.%), with low MgO (<0.83 wt.%) and Mg#(35–41) values, together with depleted Sr and Nd isotopic values ((87Sr/86Sr)i= 0.7041; εNd(t) = 5.48–6.38) and juvenile TDM2values (596–671 Ma), suggesting that they were probably originated from the lower crustal-derived juvenile materials. The Early Carboniferous granites have similarly depleted Sr and Nd isotopic values to those of the Late Devonian samples, but display striking depletions in Eu and Sr and have lower Mg#(26–29) values and higher (K2O + Na2O)/CaO ratios, indicating that they also represent juvenile crustal-derived magmas, but experienced extensive crystal fractionation. The Late Carboniferous rocks exhibit high SiO2(63.7–69.0 wt.%), Sr/Y (17.4–83.3), La/Yb (4.00–13.7), and Na2O/K2O (1.66–7.03) values, and low Y (4.46–12.2 ppm) and Yb (0.50–1.10 ppm) contents, indicative of adakitic affinity. The above geochemical features, together with Sr and Nd isotopic values comparable to slab-derived adakites and ophiolitic rocks in the Kangguer belt, implying they were likely formed by slab-derived magmas and mantle wedge peridotites. Coupled with regional geology, a northward subduction of the Kangguer oceanic slab was suggested to interpret the formation of the Late Devonian normal I-type granites. Subsequently, the southward rollback of the Kangguer Ocean caused the generation of the Early Carboniferous granites with further fractional crystallization. We infer that a northward flat subduction resulted in formation the Late Carboniferous adakitic rocks.