印支运动启动的时间:来自云南点苍山晚二叠世辉长岩和闪长岩的证据

Beginning of the Indosinian Orogeny:Insights from Late Permian Gabbro and Diorite in the Diancangshan Area of the Yunnan Province

印支运动最初定义为晚二叠世‒三叠纪华南板块与印支板块之间的碰撞造山事件。虽然前人通过不同的手段对其时限开展过广泛的研究,但是其开始时间仍然存在较大争议。本次研究在华南板块与印支板块碰撞带中段的点苍山地区,识别出了254~252 Ma的辉长岩和闪长岩。辉长岩显示出OIB型的地球化学特征,强烈富集轻稀土元素和强不相容元素,(^87Sr/^86Sr)i为0.706323,εNd(t)为−2.51,来源于OIB型石榴石地幔橄榄岩的部分熔融。闪长岩为高镁钙碱性的I型花岗岩,(^87Sr/^86Sr)i=0.710234,εNd(t)=−8.08,来源于亚碱性玄武岩质岩石圈地幔的部分熔融。综合前人研究,金沙江‒哀牢山‒Song Ma‒海南构造带的晚二叠世‒晚三叠世岩浆岩年龄频谱图显示出三个明显的峰值:251.7 Ma、246.7 Ma和230.1 Ma,分别代表最后一期弧岩浆岩、同碰撞高硅酸性岩和碰撞后岩浆岩。因此,250 Ma左右哀牢山古特提斯支洋盆或者弧后盆地基本闭合,华南板块与印支板块开始陆陆碰撞。

The Indosinian orogeny,which was originally defined as representative of the collision between the South China and Indochina Blocks,has attracted wide interest from international tectonic communities.The time of the beginning has long been a pending problem.In this study,we carried out field and geochemical investigations in the Diancangshan area in the central segment of the South China and Indochina collisional belt,and identified 254–252 Ma gabbro and diorite.The gabbro is enriched in LREEs,and strongly incompatible elements,showing OIB-like geochemical characteristics with(^87Sr/^86Sr)i=0.706323 andεNd(t)=−2.51.It was derived from partial melting of the OIB-like garnet-bearing mantle peridotite.The diorite resembles high-Mg calc alkaline I-type granites,characterized by(^87Sr/^86Sr)i=0.710234 andεNd(t)=−8.08,and likely formed by partial melting of subalkaline lithosphere mantle.Our compiled age data of the Late Permian to Triassic magmatic rocks along the Jinshajiang-Ailaoshan-Song Ma tectonic belt show three notable age peaks at 251.7 Ma,246.7 Ma and 230.1 Ma.These age pulses represent the last arc rock,syn-collisional high-Si felsic rocks and post-collisional magmatic rocks,respectively.The Ailaoshan ocean/back-arc basin closed at 250 Ma and the South China and Indochina collision(i.e.,the Indosinian orogeny)started subsequently.