全吉地块~1.8–1.7Ga富Nb正斜长角闪岩的岩浆-变质作用研究

The formation and evolution of the supercontinent Columbia (Nuna) is one of the significant scientific problems in Precambrian tectonic evolution studies. However, there are still different understandings and controversies for its final assembly and initial breakup times. The petrogenesis, tectonic settings and metamorphism properties of Nb-enriched mafic magmatic rocks can provide important constrains on different tectonic regimes of the ancient continental assembly or breakup. The Quanji Massif, interpreted as a fragment detached from the Tarim Craton, exposes two suits of Mesoproterozoic (~1.8–1.7 Ga) Nb-enriched mafic intrusive rocks that metamorphosed to amphibolite, which are not reported yet in China. This project will carry out petrological, petrographic, mineral chemical, geochronological and petrogeochemical studies on these two suits of Nb-enriched mafic intrusions in the Quanji Massif, to get information about the petrogenesis, intruding ages and tectonic settings of their protoliths, and metamorphic age, P–T type and P–T path of the regional metamorphism, in order to reveal the tectonic regimes for the Quanji Massif in early Mesoproterozoic time and provide substantial information for recognizing the relationship between ancient continents in northwest China and the global Columbia supercontinent.

全球Columbia (Nuna) 超大陆的形成与演化仍是前寒武纪构造演化研究的重大科学问题之一，其中，关于该大陆最终汇聚结束和初始裂解发生的时间，仍存在不同认识和争议。富Nb镁铁质岩浆岩的成因、构造环境和变质作用性质对古大陆的汇聚与裂解等构造体制的差别可以提供重要约束。全吉地块是一个具有克拉通性质的古大陆残片，发育两套我国还未见报道的中元古代（~1.8–1.7 Ga）富Nb镁铁质侵入岩，现已变质为斜长角闪岩。项目将对这两套富Nb的变质镁铁质侵入岩系统开展地质学调查、岩相学观察和矿物化学、年代学、岩石地球化学等研究，分析岩石成因和原岩岩浆产生的构造环境、侵入年龄，查明其区域变质作用年龄、P–T-类型和P–T轨迹，分析其地质动力学特征，为探讨全吉地块中元古代初期的构造体制和深化中国西北部古大陆响应全球Columbia超大陆汇聚或裂解事件联系的认识，提供更多参考资料。

项目按计划对全吉地块中出露的两套~1.8–1.7 Ga富Nb镁铁质侵入岩开展了系统的野外地质学、岩相学、年代学、地球化学以及变质作用特征研究，取得以下主要认识：.①富Nb斜长角闪岩岩瘤矿物成分主要为普通角闪石和斜长石，仍保存变余辉长结构。岩瘤的原岩侵位于1712±47 Ma，在472±29 Ma发生角闪岩相变质。原岩岩浆属于富Nb玄武岩，源自俯冲相关的熔体交代的地幔橄榄岩的部分熔融，经历了一定程度的分离结晶作用后于约1.7 Ga前快速上升，就位于岛弧相关环境。.②富Nb含石榴石斜长角闪岩岩墙中含有两个世代的角闪石+斜长石+石榴子石+石英的矿物组合。原岩于~1.80–1.78 Ga发生变质，变质作用类型为中P/T型角闪岩相，处于进变质作用阶段。原岩岩浆属于富Nb的玄武岩岩浆，起源于OIB型富集型地幔为主要占比的地幔端元，在经历高程度的结晶分异后，侵位于更靠近岛弧的构造环境。.③全吉地块在~1.80–1.78 Ga经历了洋-陆俯冲到弧-陆碰撞的构造体制转变，响应全球Columbia超大陆的汇聚过程到~1.7 Ga时仍没有结束。.