北大别在俯冲和折返过程中的元素和同位素行为及其PTt轨迹

Previous researches suggested that the North Dabie Complex Zone (NDZ) experienced Triassic deep subduction as well as high-T metamorphism and multistage exhumation, resulting in the common formation of multiple decompression textures and the extremely rare preservation of former UHP metamorphic evidence. Moreover, elements and isotopic systems in minerals must have completely re-equilibrated or unequilibrated, and petrological and geochronological records for different metamorphic stages have partly or event completely removed from the eclogites and related high-grade rocks during retrogression. As a consequence, it is difficult to precisely recognize the metamorphic processes of the NDZ. Therefore, a combined study in the project will be carried on petrological geochemistry and geochronology of the NDZ by the further field investigation and the samples previously collected from the applicants. In light of petrological and element-geochemical study, the pressures and temperatures at different metamorphic stages will be defined by means of conventional thermobarometry and quantitative pseudosection analysis, and Ti-in-zircon and Zr-in-rutile thermometers on the eclogites and related rocks in the region. Element and isotope behavior of the NDZ during subduction and exhumation will be explored by elemental geochemistry of whole rocks and related minerals combined with zircon (and rutile) U-Pb dating, mineral Sr-Nd dating and amphibole Ar-Ar dating. Based on the results obtained from this project and published data, we will construct a precise metamorphic PTt path for the NDZ and further improve the multi-slice exhumation model for UHP rocks from different slices of deeply subducted continental crust in the Dabie orogen.

已有研究表明，北大别经历了三叠纪深俯冲以及高温变质作用和多阶段折返，表现出多期减压结构和极少保留超高压变质证据，并造成矿物的元素和同位素发生再平衡或不平衡以及不同变质阶段的岩石学和年代学等记录已部分或完全消失，严重影响了人们对北大别所经历过程的识别与准确认识。针对存在的问题，本项目拟通过详细野外调查与采样以及申请者已积累的样品，开展北大别的岩石地球化学和同位素年代学研究。通过岩石学和元素地球化学研究，应用相平衡以及锆石中Ti和金红石中Zr温度计等综合方法来限定北大别榴辉岩及相关岩石在不同变质阶段的PT条件。通过全岩和矿物的元素地球化学研究，结合锆石（和金红石）U-Pb定年、矿物的Sr-Nd同位素定年以及角闪石的Ar-Ar定年等，探讨北大别在俯冲和折返期过程中的元素和同位素行为。根据上述研究成果并结合已发表的资料，确定北大别的变质PTt轨迹，进一步修正或完善大别山超高压岩石的差异折返机制。

针对研究计划和拟解决的科学问题，本项目通过详细野外调查与采样以及申请者已积累的样品，开展了北大别不同地点榴辉岩、片麻岩和混合岩等的变质岩石学、地球化学和同位素年代学研究。通过岩相学研究，应用传统温压计、相平衡模拟以及锆石中Ti 和金红石中Zr 温度计等综合方法限定了北大别榴辉岩及相关岩石在不同变质阶段的P-T 条件，证明北大别榴辉岩经历了多阶段> 850 °C的高温变质作用和折返期间经历了多阶段麻粒岩相变质叠加（包括折返早期－表现为榴辉岩相变质之后绿辉石三阶段连续减压分解形成的麻粒岩相后成合晶：即透辉石+斜长石(Sy1)、透辉石+斜长石+紫苏辉石(Sy2) 和斜长石+紫苏辉石+角闪石+透辉石(Sy3)，对应的P-T条件分别为1.6–1.98 GPa/899–987 °C、1.1–1.3 GPa/850–890 °C和0.8–1.0 GPa/850–930 °C，以及碰撞后山根垮塌期间的麻粒岩相变质作用），重建了榴辉岩从俯冲到折返期间的完整P-T-t轨迹。同时，证明北大别榴辉岩及片麻岩经历了折返初期的减压脱水熔融和碰撞后燕山期有水加入的加热熔融（水致熔融）作用并确定了它们发生的时代（分别为206–209 Ma和130–143 Ma）和P-T条件（分别为800–830 °C/10–13 kbar和>750 °C/9.4–10 kbar）。通过元素和同位素地球化学研究，结合锆石（和金红石）U-Pb 定年、矿物的Sr-Nd 同位素定年以及角闪石的Ar-Ar 定年等，查明了北大别榴辉岩的原岩性质和岩石成因，探讨了北大别在俯冲和折返过程中的元素和同位素行为。此外，发现碰撞后形成的变质闪长岩，查明北大别存在两类不同产状、分别含蓝刚玉和红刚玉的片麻岩。因此，这些成果为进一步理解北大别的形成和演化过程以及深熔作用的期次与发生的条件提供了新的关键性证据与制约。