苏鲁混合岩化榴辉岩内副矿物原位地球化学和年代学研究

Determine the composition, source of hydrous melt and constrain the metamorphic P-T-t path during partial melting of ultrahigh-pressure (UHP) metamorphic rocks will provide important constraints to understand subduction-zone related processes including element and mass transfer, crust-mantle interaction and exhumation rate of deeply subducted continental crust. However, we still lack comprehensive knowledge concerning the composition and source of hydrous melts within natural UHP rock systems, and detailed P-T-t path from melting to melt crystallization is highly controversial. Based on high-precision lithological-structural mapping and in-situ analytical techniques of geochemistry and geochronology, this project plans to conduct in-situ major-trace element and Sr isotope analyses on epidote, as well as combined U-Pb dating and trace element analyses on titanite/rutile within migmatitic UHP eclogite (including leucosome, the host eclogite and country gneiss) from the Sulu orogenic belt. The results are used to: (1) quantitively constrain the compositions of hydrous melts liberated from UHP metamorphic rocks during exhumation; (2) trace the source of leucosomes within the migmatitic eclogite; and (3) establish the organic connections between pressure-temperature and timing information and thus to reconstruct the retrograde P-T-t path of partially melted eclogite. Ultimately, this project will contribute to better understand partial melting and the exhumation processes of deeply subducted UHP continental crust in a subduction channel.

厘定超高压变质岩内含水熔体成分、来源和熔融过程变质P-T-t轨迹可为深刻解译俯冲隧道内元素和物质迁移、壳–幔相互作用和深俯冲板片折返速率等关键科学问题提供重要约束。然而，关于天然超高压变质岩体系内含水熔体成分和来源的认识尚待深入，且熔融–熔体结晶阶段变质P-T-t轨迹特征仍有较大争议。本项目以高精度岩石-构造填图为指导，原位地球化学和年代学分析为手段，对苏鲁造山带将军山混合岩化超高压榴辉岩（包括浅色体、寄主榴辉岩和围岩片麻岩）内发育的绿帘石进行原位主微量元素和Sr同位素的剖面分析，并对榍石和金红石开展综合的U-Pb定年、微量元素和Zr温度测定工作，从而定量探讨超高压变质岩折返期间产生的含水熔体成分特征，准确示踪苏鲁混合岩化榴辉岩内浅色体来源，构建熔融–熔体结晶阶段变质温压条件和年代学之间的有机联系，重塑深熔榴辉岩退变质P-T-t轨迹，最终为全面理解超高压变质岩部分熔融和折返过程提供参考。

超高压变质岩的部分熔融深刻影响深俯冲陆壳的折返机制和壳-幔相互作用等前沿科学问题，因此是超高压变质作用研究的热点之一。然而，有关熔体成分、来源、熔融机制以及自熔体产生至结晶过程中变质P-T-t轨迹的认识仍在不断深入探索中。为此，本基金以苏鲁造山带仰口-将军山地区为研究区，以区内发育的混合岩化榴辉岩和片麻岩为研究对象，通过综合的岩石学、全岩和单矿物地球化学以及副矿物年代学研究，系统厘定出深俯冲陆壳熔融所产生的熔体来源和熔融机制，准确构建出折返过程的变质P-T-t-Melting轨迹，并定量估算了苏鲁带的折返和冷却速率。本基金研究表明，苏鲁深俯冲陆壳在折返过程发生过两期熔流体活动，分别为折返早期名义上无水矿物分解所致的超临界流体活动和折返后期多硅白云母分解所致的含水熔体活动。并且，苏鲁带中部的折返分两阶段进行，早期从超高压榴辉岩相至角闪岩相的折返过程为“快速”折返，速率为~7.5 km/Myr，而随后在中下地壳尺度的折返为“慢速”折返，速率为~0.87 km/Myr。因此，本基金研究结果为进一步提升我们对大陆深俯冲与超高压变质作用这一基础科学问题提供了重要的数据资料。