新疆甜水海地块种羊场石炭纪火山岩年代学和地球化学:岩石成因和地质意义

Geochronology and geochemistry of the Carboniferous volcanic rocks in Zhongyangchang from Tianshuihai terrane,Xinjiang: Petrogenesis and geological significance

甜水海地块西段的种羊场地区发育一套互层状产出的玄武岩-玄武安山岩-流纹岩,本文对其进行了岩石学、同位素年代学和地球化学研究。结果表明,流纹岩LA-ICP-MS锆石U-Pb定年获得三组年龄:343.5±4.1Ma表明火山岩的形成时代为早石炭纪,2439±26Ma和1988±36Ma说明甜水海地块存在前寒武纪结晶基底。其中玄武质岩石岩性从拉斑系列、钙碱性系列向碱性系列过渡,呈现出E-MORB(OIB)、大陆板内拉张和岛弧的混合特征,与典型弧后盆地Okinawa玄武岩有一定的差异,表明其可能是异常陆缘弧后盆地拉张裂解的产物。玄武质岩石和流纹岩的主量元素、稀土元素和微量元素比值对的差异表明它们不是同源岩浆演化的产物,玄武质岩石的源区为类似E-MORB(OIB)的岩石圈地幔,且发生了部分熔融,原始岩浆上升过程中经历了矿物分离结晶和地壳混染作用。流纹岩属于高硅高碱的钙碱性火山岩,是上地壳部分熔融的产物。种羊场早石炭纪火山岩可能代表了古特提洋西端早期扩张的记录,为西昆仑-喀喇昆仑地区晚古生代多岛洋格局提供了新的证据。

A suit of interbedding basalts,basaltic andesites and rhyolites distributes in Zhongyangchang located in the western part of Tianshuihai terrane. In this paper,the signatures of geology,petrology,geochronology and geochemistry characteristics on the rocks are researched. Three groups of ages are acquired by LA-ICP-MS zircon U-Pb dating of rhyolite. The age of 343. 5 ± 4. 1Ma indicates the volcanic rocks are formed during Early Carboniferous. The ages of 2439 ± 26 Ma and 1988 ± 36 Ma imply the existence of the Precambrian crystalline basement in Tianshuihai terrane. Lithologic characteristics of the basaltic rocks transit from tholeiitic series,calc-alkaline series to alkaline series. Additionally,the basaltic rocks which have some difference from typical continental marginal arc Okinawa Trough basalts,display E-MORB（ OIB）,continental within-plate basalt and island-arc basalt signatures,implying that the volcanic rocks in Zhongyangchang may form in abnormal back-arc basin in Tianshuihai terrane. The difference in major,trace element and REE normalized patterns of basaltic rocks and rhyolites suggests the different source. The source of basaltic rocks is similar to mantle source of E-MORB（ OIB） and occurred partial melting. When the primary magma ascent,the mineral fractionation took place,moreover,crustal contamination also occurred. The rhyolites are characterized by high-silicon,high-alkali and generated by partial melting of upper crust. The Early Carboniferous volcanic rocks in Zhongyangchang may be the record of the extension in early period for the western part of Palo-Tethys Ocean,providing new evidence for the pattern of poly-islands oceanic basin in Karakorum-Tianshuihai terrane.