中亚造山带中段古亚洲洋北向平板俯冲过程:来自埃达克岩的证据

早三叠世是中亚造山带（CAOB）中部构造演化的关键时期，尽管该时期古亚洲洋在地表已经闭合，但残余的大洋板片仍在持续的挤压作用下继续俯冲，造山作用依然活跃。本文对中亚造山带中段林西地区的下三叠统幸福之路组火山岩地层进行了锆石U-Pb测年、岩石地球化学以及锆石原位Lu-Hf同位素研究。LA-ICP-MS锆石U-Pb定年结果显示了该地层的形成时代为247Ma，为早三叠世岩浆活动的产物。岩石学和地球化学研究表明，幸福之路组火山岩具有高SiO₂（64.10%~68.90%）、Al₂O₃（13.47%~17.50%），低MgO（0.51%~1.42%），轻稀土富集、重稀土亏损，无Eu异常（δEu平均值为1.09）以及高Sr（384×10^-6~956×10^-6，平均616×10^-6），低Y（5.66×10^-6~7.63×10^-6，平均6.51×10^-6）的特点，表明其为大洋板片熔融产生的典型埃达克岩。锆石原位Lu-Hf同位素分析结果显示其ε_Hf（t）为+10.8~+15.6，平均为+13.9。样品单阶段Hf地壳模式年龄（t_DM1）为264~462Ma，表明其岩浆源区的亏损特征。结合区域资料，我们将研究区中晚二叠世至晚三叠世划分为四个构造演化阶段：1）中-晚二叠世时期，特征为钙碱性岩浆岩及碰撞杂岩的发育；2）早-中三叠世时期，特征为广泛分布的典型埃达克岩，是古亚洲洋地表闭合后大洋板片继续北向平板俯冲的产物；3）230Ma左右开始持续10Myr，该时期是岩浆活动宁静期；4）220Ma至晚三叠世末，研究区进入区域性伸展，A型花岗岩、富钾钙碱性花岗岩类和超基性岩大量侵位，变质核杂岩及韧性剪切带也在此时产生。前三个阶段代表了完整的古亚洲洋大洋板块平板俯冲过程，而最后一个阶段标志着研究区地壳进入了新的演化阶段。

Northward plate subduction process of the Paleo-Asian Ocean in the middle part of the Central Asian Orogenic Belt: Evidence from adakites

The Early Triassic was a key period of geological evolution in middle part of the Central Asian Orogenic Belt (CAOB). Although the Paleo-Asian Ocean was closed at the surface during this period, the residual oceanic slabs continued to subduct under continuous compression, i.e., the orogeny was still active. In this paper, zircon U-Pb dating, petrological geochemistry, and in-situ zircon Lu-Hf isotope studies have been conducted on the Lower Triassic Xingfuzhilu Formation volcanic strata in the Linxi area of the middle part of CAOB. LA-ICP-MS zircon U-Pb dating results show that the formation age of Xingfuzhilu Formation is 247Ma, which is the product of Early Triassic magmatism. Petrology and geochemical study have shown that the volcanic rocks in the Xingfuzhilu Formation have high SiO2(64.10%~68.90%), Al2O3(13.47%~17.50%) and Sr (384×10-6~956×10-6, averaged at 616×10-6), low MgO (0.51%~1.42%) and Y (5.66×10-6~ 7.63×10-6, averaged at 6.51×10-6) contents; with a LREE-enrichment and HREE deficit, without a Eu anomaly (averaged δEu is 1.09), showing that it has typical characteristics of adakites that derived from the melting of residual oceanic slabs. The εHf(t) of in-situ zircon Hf isotopic analysis is between +10.8~+15.6 (averaged at +13.9). The single-stage Hf crustal model ages (tDM1) of the zirons are within the range of 264~462Ma, indicating the magma is from a depleted mantle. Based on regional published data, we divided the tectonic evolution of the study area into four stages from the Middle-Late Permian to the Late Triassic: 1) The Middle-Late Permian period, characterized by the development of calcareous-alkali magmatic rocks and collisional complexes; 2) The Early-Middle Triassic period, characterized by widely distributed typical adakite, which is the product of the continuous northward subduction of the oceanic slab after the surface closure of the Paleo-Asian Ocean; 3) It's last 10My from 230Ma, a period without any magmatic activities; 4) From 220Ma to the end of the Late Triassic, a regional extension period, characterized with large amounts of A-type granites, massive potassium-rich calcium alkaline granites and ultrabasic rocks emplacement, accompanied with metamorphic core complexes and ductile shear zones development. The first three stages represent the complete plate subduction process of the Paleo-Asian Ocean oceanic plate, while the last stage standardizes that the crustal of the study area has entered a new evolutionary stage.