原特提斯洋俯冲起始——来自北祁连早寒武世弧前岩浆作用的新证据

形成于弧前背景的岩浆岩可能记录了俯冲起始的重要证据。本文以北祁连走廊南山蛇绿混杂岩带新识别出的富铌辉长岩和斜长花岗岩为研究对象，在野外观察的基础上，结合岩石学、地球化学、同位素地球化学和年代学的综合研究，对原特提斯洋初始俯冲的时间进行约束，并探讨其形成过程及构造意义。锆石U-Pb定年结果显示富铌辉长岩样品锆石加权平均²⁰⁶Pb/²³⁸U年龄为512±4 Ma；两个斜长花岗岩样品的锆石加权平均²⁰⁶Pb/²³⁸U年龄分别为522±3 Ma和519±1 Ma。全岩地球化学结果显示富铌辉长岩的Nb含量为7.49×10^-6～10.80×10^-6,TiO₂含量为1.50%～2.08%,Nb/U值为11.9～13.4,(Nb/La)_N>0.5，明显高于岛弧玄武岩，其εNd(t)值介于+4.38～+5.78之间。斜长花岗岩样品K₂O含量(0.31%～1.66%)和K₂O/Na<...

Proto-Tethys Ocean subduction initiation: New evidence from the Early Cambrian forearc magmatism in the Northern Qilian Orogen

The magmatic rocks formed in the forearc setting are considered to possibly record key evidences for subduction initiation. In this paper, the niobium-rich gabbro and plagiogranite are newly reported in the Corridor Nan-shan ophiolitic melange zone of the North Qilian Orogen. Based on field observation, combined with comprehensive studies of petrology, geochemistry, isotope geochemistry and chronology, the time of initial subduction of proto-Tethys Ocean is constrained and its evolution model and tectonic implication are discussed. Zircon U-Pb dating results reveal a weighted average ²⁰⁶Pb/²³⁸U zircon age at 512±4 Ma for a niobium-enriched gabbro sample, and 522±3 Ma and 519±1 Ma for two plagiogranite samples. The niobium-rich gabbro samples have Nb contents of 7.49×10^-6~10.80×10^-6, TiO₂ of 1.50%~2.08%, Nb/U values of 11.9~13.4 and (Nb/La)_N > 0.5, which are obviously higher than those of island-arc basalts. εNd(t) values of Nb-enriched grabbro are between +4.38 and +5.78. Plagiogranite samples have low K₂O content (0.31%~1.66%) and low ratios of K₂O/Na₂O (0.05~0.43). The chondrite-normalized rare earth element pattern is relatively flat, with weakly positive Eu or negative anomalies. The plagioclase samples are relatively enriched in large ion lithophile elements such as Ba, Sr and U, and depleted in high field strength elements such as Nb and Ta. Combing their positive εNd(t) value (+3.35) and highly positive εHf(t) ranging (mostly between 6.2 to 12.9), we suggest an oceanic plagiogranite. The new data, along with the regional geology, indicate the Baijingsi plagiogranites are resulted from partial melting of the young and hot subducting oceanic slab at shallow depths in relatively high temperature during subduction initiation stage. Subsequently, the partial melting of subducting slab led to product of adakitic melts, which metasomatized or interacted with the upper mantle wedge peridotites, then the partial melting of metasomatized mantle peridotites gave rise to Nb-enriched basaltic magma. These data indicate that initial stage of subduction of the Prototethyan ocean (Paleo-Qilian ocean) occurred in the Early Cambrian.