西准噶尔南部晚石炭世中-酸性火山岩成因机制及其对准噶尔洋闭合时限的约束

Genetic mechanism of Late Carboniferous intermediate-acid volcanic rocks in southern West Junggar and its constraints on the closure of the Junggar Ocean

西准噶尔地区广泛分布石炭纪岩浆岩类,了解它们的成因和形成机制可为揭示准噶尔洋盆晚古生代构造格局和发展演化以及约束古大洋闭合时限提供重要依据。本文报道了西准噶尔南部哈拉阿拉特山地区晚石炭世中酸性火山岩的岩石学、锆石U-Pb年代学以及地球化学特征。岩石学研究表明其岩性主要为安山岩、英安质安山岩、流纹英安斑岩和流纹岩。LA-ICP-MS锆石U-Pb定年结果表明哈拉阿拉特山地区中酸性火山岩年龄为308~305 Ma,为晚石炭世中晚期。中酸性火山岩均相对富集轻稀土和大离子亲石元素,亏损Nb、Ta等高场强元素,富钠贫钾,属于低钾拉斑中钾钙碱性系列岛弧火山岩。其中安山岩和英安质安山岩具有较高SiO_(2)(56.15%~66.13%)、Al_(2)O_(3)(16.03%~17.94%)、Na_(2)O(3.44%~5.59%)、Sr((364~576)×10^(-6))含量和Na_(2)O/K_(2)O(3.20~6.40)、Sr/Y(33.5~55.6)比值,贫MgO(1.59%~2.68%)、Y((10.0~16.0)×10^(-6))和Yb((1.08~1.83)×10^(-6))的特征,并且具有Eu正异常(δEu=1.09~1.22),属于典型的埃达克岩,是经俯冲的洋壳板片在石榴角闪岩相发生部分熔融而成,且熔融产生的埃达克质熔体在上升过程中未与上覆地幔橄榄岩发生明显交代作用;流纹岩和流纹英安斑岩具有高的SiO_(2)(69.59%~75.03%)和全碱(w(Na_(2)O+K_(2)O)=7.81%~8.89%)、极低的TFe 2O 3(0.94%~1.57%)和MgO(0.12%~0.97%)含量以及弱负Eu异常(δEu=0.63~1.00)等特征,为准铝质I型流纹岩,是下地壳镁铁质岩石部分熔融的产物,很可能有少量幔源岩浆混入。综合本文数据并结合研究区大量前人已发表的岩浆岩类研究成果,认为西准噶尔南部地区在晚石炭世中晚期仍处于岛弧弧后盆地演化体系,准噶尔洋的闭合时限可能至少推迟至早二叠世早期之后。

Carboniferous magmatic rocks are exclusively discovered in West Junggar,and investigating their petrogenesis and geodynamic mechanisms can offer crucial insights into understanding the Late Paleozoic tectonic framework,evolutionary history,and the closure of the Junggar Ocean.This study presents petrological,geochronological,and geochemical data on Late Carboniferous intermediate-acid volcanic rocks,including andesite,dacitic andesite,rhyolitic dacite porphyry,and rhyolite in the Hala alate Mountain of southern West Junggar,aiming to elucidate their genetic mechanisms.LA-ICP-MS zircon U-Pb dating of andesite and rhyolite revealed crystallization ages of 308305 million years ago,indicating the presence of middle to late Late Carboniferous volcanic rocks in West Junggar.These volcanic rocks are geochemically characterized by enrichment in light rare earth elements and large ion lithophile elements,as well as depletion in high field strength elements(such as Nb,Ta).They exhibit high sodium content and low potassium content,falling within the low-K tholeiitic to medium-K calc-alkaline series,resembling island arc volcanic rocks.The andesite and dacitic andesite have high content of SiO_(2)(56.15%66.13%),Al_(2)O_(3)(16.03%17.94%),Na_(2)O(3.44%5.59%),Sr(364576)×10^(-6),high Na_(2)O/K_(2)O(3.206.40)and Sr/Y(33.555.6)ratios,and also exhibit low content of MgO(1.59%2.68%),Y(10.016.0)×10^(-6)and Yb(1.081.83)×10^(-6)with positive Eu anomalies(δEu=1.091.22),showing some features of typical adakite in arc settings,and are believed to be derived from the partial melting of subducted oceanic crust in garnet amphibolite-facies without significant interaction with the overlying mantle wedge.Rhyolitic dacite porphyry and rhyolite have high content of SiO_(2)(69.59%75.03%)and total alkali(Na_(2)O+K_(2)O:7.81%8.89%),and very low content of TFe 2O 3(0.94%1.57%)and MgO(0.12%0.97%)with weak negative Eu anomalies(δEu=0.631.00),indicating that they are metaluminous I-type rhyolites,and are probably originated from the partial melting of basaltic lower crust,with addition of a small amount of mantle-derived materials.The findings presented,combined with previously published research in the region,suggest that an island arc-back-arc basin evolutionary system persisted in the study area during the late Late Carboniferous.Furthermore,the closure of the Junggar Ocean in southern West Junggar likely occurred later than the early Early Permian.