南阿尔金木纳布拉克地区长城系巴什库尔干岩群LA-ICP-MS锆石U-Pb定年及其地质意义

摘要

LA-ICP-MS zircon U-Pb dating yielded the protolith ages of 498~840Ma and 500~647Ma respectively for the two-mi⁃ca monzonitic gneiss in Hongliuquan Formation and the mica quartz schist in Zhasikansayi Formation from Bashikourgan rock group in Munabulake area, southern Altyn Mountains, indicating that the formation ages of the two rocks are Neoproterozoic rather than Mesoproterozoic. The zircon rims of the two-mica monzonitic gneiss exhibit HREE enrichment pattern, with Th/U ratios less than 0.1, showing the characteristics of metamorphic zircon. Zircon U-Pb dating yielded the metamorphic age of 498 ± 4Ma, which is identical to the peak metamorphic age of the HP granulite in this area and that of the HP-UHP metamorphic rocks in southern Al⁃tyn Mountains, suggesting that the rock should be the product of the deep subduction of southern Altyn Mountains. The two-mica monzonitic gneiss and two-mica quartz schist in this study and the HP granulite in Munabulake area recorded similar age information to the Altyn complex, implying that the three rocks belong to the Altyn complex rather than to the Bashikourgan rock group. There⁃fore, the Bashikourgan rock group should be disintegrated, and some of this rock group should be assigned to the southwest part of the Altyn complex. The reason for the disintegration of the Altyn complex was that the south Altyn subducted slab was exhumed and broken up at ca.450Ma, which resulted in the mantle magma upwelling. The upwelling mafic magma formed the present Munabu⁃lake mafic-ultramafic rock and separated the Altyn complex into two parts.%采用LA-ICP-MS微区原位锆石U-Pb同位素测定技术，测得木纳布拉克地区原划长城系巴什库尔干岩群红柳泉组的二云二长片麻岩和扎斯勘赛河组二云石英片岩的原岩形成年龄分别介于498~840Ma和500~647Ma之间，表明这2种岩石原岩的形成时代均为新元古代，而不是之前认为的中元古代。另外，红柳泉组二云二长片麻岩边部锆石表现为重稀土元素相对富集型稀土配分模式，其Th/U值大部分小于0.1，为变质锆石，测得其变质时代为498±4Ma，与该地区高压麻粒岩的峰期变质时代(486±5Ma)一致，且与南阿尔金阿尔金岩群中的高压-超高压岩石的峰期变质时代一致，推断其为阿尔金俯冲碰撞引发的高压-超高压变质作用的产物，属于南阿尔金高压-超高压变质带的一部分。通过原岩残留碎屑锆石的年代学对比，发现研究区内这2种岩石及已发现的高压麻粒岩均具有与阿尔金岩群相似的锆石年龄信息，表明本区红柳泉组和扎斯勘赛河组的这3种岩石不应再划归为中元古代的长城系巴什库尔干岩群，将其归属为阿尔金岩群更为合适。