西大别浒湾变质带洋壳和陆壳耦合俯冲的锆石U-Pb年龄记录及其动力学意义

西大别浒湾高压变质带是研究秦岭-大别-苏鲁造山带演化的关键区域.本文对该变质带熊店和学河两地的两个榴辉岩样品进行了LA-(MC)-ICPMS锆石U-Pb定年、微量元素分析及Hf同位素测定.熊店榴辉岩岩浆锆石得到的年龄为406±14Ma,具有高的εHf(t)值(εHf(t)=11.3±1.3),年轻的亏损地幔模式年龄(tDM=578±52Ma),其来源可能为亏损地幔,进而说明它们的原岩可能为古特提斯洋壳物质;学河榴辉岩岩浆锆石的年龄为703±8Ma,具有略低的εHf(t)值(εHf(t)=4.11±0.94),较老的亏损地幔模式年龄(tDM=1105±37Ma),其原岩可能为扬子克拉通新元古代裂谷岩浆作用产生的新生陆壳物质.这些结果表明浒湾地区存在原岩形成于新元古代和志留纪两个时期的榴辉岩.熊店榴辉岩中变质锆石的微量元素特征与岩浆锆石类似,可能为完全重结晶成因锆石,其206Ph/238U加权平均年龄为316±1Ma,代表了洋壳榴辉岩榴辉岩相峰期变质的最早时间.学河榴辉岩变质锆石以低Th/U、Nb/Ta比值为特征,其REE组成模式为不明显的Eu负异常,HREE呈平坦型.这些特征反映了这些锆石形成时出现了较大数量的石榴子石与金红石,而缺乏长石.根据锆石Ti温度计计算学河榴辉岩变质锆石形成的温度范围是704～741℃,与榴辉岩相变质温度一致.它们对应的206Pb/238U加权平均年龄为312±3Ma,可作为浒湾变质带榴辉岩相峰期变质年龄,这一结果表明浒湾变质带陆壳成因榴辉岩存在石炭纪榴辉岩相变质事件.空间上相近的古生代洋壳和新元古代陆壳具有相同的榴辉岩相变质年龄表明,浒湾变质带的洋壳和陆壳榴辉岩可能存在石炭纪的耦合俯冲作用.

Zircon U-Pb age recording couple subduction of oceanic and continental eclogites in the Huwan shear zone, western Dabie Mountains, and its geodynamic implications

The Huwan high-pressure ( HP) metamorphic zone is one of the key regions to investigate the evolution of the Qinling-Dabie-Sulu orogenic belt. Two eclogite samples from Xiongdian and Xuehe in this zone were selected for LA-ICPMS zircon U-Pb dating, trace element and Hf isotope analysis to constrain the eclogite-facies metamorphism and protolith ages and their origin. U-Pb and trace element analyses of the magmatic zircon cores in the Xiongdian eclogite constrain their protolith formation at 406 ± 14Ma. The zircons have high ε_(Hf)(t) values (ε_(Hf)(t) = 11. 3 ±1. 3) and young t_(DM) ages (t_(DM) =578 ±52Ma). It suggests that the protolith of the Xiongdian eclogite was derived from the depleted mantle, and thus confirmed its derivation from the Paleotethyan oceanic crust. In contrast, U-Pb and trace element analyses of the magmatic zircon cores in the Xuehe eclogite yield Neoproterozoic protolith age of 703 ±8Ma, suggesting that its protolith was continental crust derivation from the Yangtze Craton. The zircons in Xuehe eclogite have relatively low ε_(Hf)(t) values (ε_(Hf)(t) =4. 11 ±0. 94) and old t_(DM) ages (t_(DM) = 1105 ±37Ma). This demonstrates that the protolith of the Xuehe eclogite was probably derived from remelting of Neoproterozoic juvenile continental crust materials by rift magmatism in the northern margin of the Yangtze Craton. Both magmatic and metamorphic zircons in the Xiongdian eclogite have similar trace element characteristic. It suggests that the metamorphic zircons formed due to complete recrystallization, and the weighted average age of 316 ± 1 Ma is probably a maximum estimate of the peak eclogite-facies. The metamorphic zircons in the Xuehe eclogite are characterized by low Th/U ratios, insignificant negative Eu anomalies, relatively flat HREE patterns, and low Nb/Ta ratios. These signatures reflect that these zircons formed in the presence of garnet and rutile, and absence of feldspar, and thus under eclogite-facies conditions. The temperatures of the metamorphic zircons in the Xuehe eclogite were calculated of 704 ～ 741℃ from their Ti contents, consistent with their formation under eclogite-facies conditions. Their weighted mean ~(206)Pb/~(238)U age of 312 ± 3Ma is thus taken as the best estimate age of the eclogite-facies metamorphism in the Huwan shear zone. These results provide unambiguous evidence for the Carboniferous eclogite-facies metamorphism of both oceanic and continental rocks in this zone. The coexistence of the Paleozoic oceanic crust and Neoproterozoic continental crust with similar metamorphic ages in the Huwan shear zone implies that the HP continental rocks may have played a key role in exhumation and preservation of the oceanic rocks through buoyancy-driven uplift.