冈底斯带东段中生代-早新生代下地壳的组成与演化及其对大陆地壳生长的意义

The mechanism of crustal growth for the Gangdese belt in southern Tibetan Plateau remains a topic of much debate. The Mesozoic-Early Cenozoic lower crustal section in the eastern Gangdese belt contains important information related to its crust growth. This project will carry out an integrated study of petrology, mineralogy, geochronology, and geochemistry for the mafic-ultramafic cumulates, mafic and felsic granulites from this lower crustal section. Our aims are as follows: (1) determinating the crystallization ages of the mafic-ultramafic cumulates, and the protolith and metamorphic ages of the mafic and felsic granulites; (2) investigating the petrogenesis of the mafic-ultramafic cumulates, mafic and felsic granulites, and discussing the genetic relationships between the cumulates, granulites, and granitoids in this lower crustal section; (3) revealing the crustal accretional and reformational processes of the eastern Gangdese belt during the Mesozoic-Early Cenozoic; (4) constructing the formation and evolution model of the Mesozoic-Early Cenozoic lower crust of Gangdese belt; and (5) exploring the crustal growth mechanism of the Gangdese belt and its implications for the continental crustal growth.

青藏高原南缘冈底斯带的地壳生长机制还存在较大争议。冈底斯带东段的中生代-早新生代下地壳剖面提供了观察和认识其深部地壳物质组成与演化的窗口，对揭示冈底斯带的地壳生长机制具有重要意义。本项目重点对该下地壳剖面中的镁铁质-超镁铁质堆晶岩、镁铁质和长英质麻粒岩进行详细的岩石学、矿物学、年代学和地球化学研究，测定堆晶岩的结晶时代、麻粒岩的原岩和变质时代；分别查明堆晶岩、镁铁质和长英质麻粒岩的岩石成因，并分析它们与该剖面中的中生代-早新生代花岗岩类的成因联系；揭示冈底斯带中生代-早新生代下地壳的增生与改造过程；建立冈底斯带中生代-早新生代下地壳组成和演化模型，探讨冈底斯带的地壳生长机制及其对大陆地壳生长的意义。

青藏高原南缘冈底斯弧地壳的生长和演化机制还存在较大争议。本项目以冈底斯带东段林芝-卧龙地区出露的冈底斯弧深部地壳剖面为研究对象，开展了系统的野外地质、岩相学、年代学和地球化学组成研究。结果表明，晚白垩世冈底斯弧下地壳中的石榴石辉长岩、超镁铁质-镁铁质岩系列和花岗岩类，为富水和含水弧岩浆在95-85 Ma内经分离结晶作用形成。模拟计算的冈底斯弧下地壳地球物理属性和化学组成与现今平均大陆下地壳对等深度的地球物理属性和化学组成相似，反映大陆弧是大陆地壳生长的主要场所。冈底斯弧深部地壳剖面中的新生代（52-25 Ma）角闪岩相-麻粒岩相长英质变质岩的原岩包括有弧前沉积岩和175-65 Ma弧岩浆岩。这些变质岩可能是在印度-欧亚板块碰撞过程中构造剥蚀作用导致弧前沉积岩和弧岩浆岩带入下地壳深度发生高级变质作用形成。少部分地壳物质进入地幔深度发生熔融而交代岩石圈地幔。在~50 Ma时，特提斯洋壳的断离作用可能导致早期被交代的岩石圈地幔和下地壳物质发生熔融形成~50 Ma的辉长岩和高Sr/Y花岗岩。因此，本项目一方面揭示了冈底斯弧早期地壳生长机制，另一方面探讨了印度-欧亚板块陆陆碰撞过程冈底斯弧地壳的损失与改造过程。本项目研究成果对认识大陆地壳的生长与改造具有重要意义。