华北东南缘五河杂岩的时代和岩石成因及其变质交代过程中的同位素行为

The detailed field investigations will be carried out on the Wuhe complex and related rocks within the Precambrian metamorphic basement distributed over the Bengbu uplift and adjacent areas at the southeastern margin of the North China Block in order to clarify their rock compositions and relationships between them. In light of petrological study, we will construct the evolutionary processes of various metamorphic rocks, in the mean time, peak conditions and P-T paths will be constrained by means of conventional mineral-pairs thermobarometry and modern petrological analysis such as quantitative pseudosection analysis, and Ti-in-zircon and Zr-in-rutile thermometers on the studied rocks in the region. The precursor and metamorphic ages of the rocks, and their P-T conditions and ages of partial melting will be defined by zircon and mineral Sr-Nd dating and amphibole Ar-Ar dating combined with thermobarometry and thermodynamic modeling. By a combined study of element and Sr-Nd-Pb-Mg-Hf isotope geochemistry, the rock assemblages of the Wuhe complex and their precursor nature and petrogenesis will be clarified. In the meantime, and their elemental and isotopic behavior during metamorphic processes, and implications for tracing the isotopic heterogeneity of the lower crust and deep carbonate recycling will be explored. Especially, this study will be focused on precursor ages and petrogenesis of meta-basic rocks (e.g., garnet granulite and garnet amphibolite) and various gneisses, and stages, characters and P-T conditions of partial melting in various metamorphic rocks in the region. Based on the obtained results and published data, we will reconstruct the P-T-t paths of the Wuhe complex, and further build the formation and tectonic evolution model on the Precambrian lower crust at the southeastern margin of the North China Block.

拟重点对华北陆块东南缘蚌埠隆起及相邻地区前寒武纪变质基底中五河杂岩及相关岩石进行详细的野外地质调查，查明其岩石组成及其相互关系。通过岩石学研究，重建不同类型变质岩的演化过程，并应用传统矿物对温压计和结合现代岩石学分析方法（包括相平衡模拟、锆石中Ti和金红石中Zr温度计等）来限定它们的峰期变质条件和P-T轨迹。通过元素和Sr-Nd-Pb-Mg-Hf同位素地球化学研究以及锆石U-Pb和矿物的Sr-Nd-Ar同位素定年，查明五河杂岩的岩石组成及其原岩性质和岩石成因，确定不同类型岩石（重点是石榴麻粒岩和石榴斜长角闪岩等变基性岩与各种片麻岩）的形成时代和变质时代以及部分熔融作用的期次、特点与P-T条件，探讨它们在变质交代过程中的元素和同位素行为及其对下地壳同位素不均一性和深部碳循环的意义。在此基础之上，揭示五河杂岩的变质过程与P-T-t轨迹，建立华北陆块东南缘前寒武纪下地壳的形成和构造演化模式。

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，开展了华北东南缘蚌埠隆起及相邻地区前寒武纪变质基底中五河杂岩及相关岩石（石榴斜长角闪岩、石榴麻粒岩、不同类型TTG片麻岩和花岗片麻岩及大理岩等）的变质岩石学、地球化学和同位素年代学研究。通过岩石学研究，重建了不同类型变质岩的演化过程，并应用传统地质温压计和结合现代岩石学分析方法（包括相平衡模拟等）来限定它们的峰期变质条件和P-T轨迹。其中，镁铁质麻粒岩的高压麻粒岩相阶段的代表性矿物为石榴子石+单斜辉石+ 斜长石+石英+金红石+富Ti角闪石，温压条件为850–900℃/1.5 GPa；近等温减压麻粒岩相阶段，代表性矿物为石榴子石+斜方辉石+斜长石±单斜辉石，温压条件为 ~900℃/1.1–1.2 GPa；角闪岩相退变质阶段，代表性矿物为角闪石+斜长石，温压条件为600–680℃/0.65–0.75 GPa。锆石U-Pb定年结果表明，高压麻粒岩相、中压麻粒岩相和角闪岩相变质时代分别为 ~1.90 Ga、~1.85 Ga和 ~1.78 Ga。通过元素和Sr-Nd-Pb-Hf同位素地球化学研究以及锆石U-Pb同位素定年，查明了五河杂岩的岩石组成及其原岩性质和岩石成因，确定了不同类型岩石（重点是不同类型片麻岩、石榴麻粒岩和石榴斜长角闪岩等变基性岩以及大理岩）的形成时代和变质时代以及部分熔融作用的期次、特点与P-T条件，探讨了它们在变质交代过程中的元素和同位素行为及其对下地壳Pb同位素不均一性的意义。在此基础之上，揭示了五河杂岩及相关岩石的变质演化过程与P-T-t轨迹；发现了2.6Ga前大洋俯冲和板块构造作用的岩石地球化学证据；建立了华北陆块东南缘前寒武纪下地壳的形成和构造演化模式；进一步证明五河杂岩属于胶-辽-吉带的西延，二者共同构成了华北克拉通东部一条古元古代碰撞造山带。