扬子陆块西缘中元古代晚期元谋花岗岩的时代、成因及构造意义

Age, petrogenesis and tectonic implications of Late Mesoproterozoic Yuanmou Granite in the Western Yangtze Block, South China

关键词:  A型花岗岩  锆石U-Pb定年  中元古代晚期  扬子陆块西缘

Key words:  A-type granite  zircon U-Pb dating  late Mesoproterozoic  western margin of the Yangtze Block

摘要:

扬子陆块西缘晚中元古代地质演化史一直存在较大争议，本文选择以扬子西缘元谋杂岩中一套二长花岗岩为研究对象，开展岩相学、锆石U-Pb年代学、全岩地球化学等综合研究，为认识和理解扬子西缘晚中元古代地质演化提供支撑。两件元谋二长花岗岩样品的LA-ICP-MS锆石U-Pb年龄分别为1086±10 Ma（MSWD=1.4，n=50）和1099±10 Ma（MSWD=1.8，n=58）。所有样品具有高硅（SiO₂为69.44%~73.98%）、富碱（Na₂O+K₂O为6.11%~7.72%）、贫钙（CaO为0.39%~1.46%）、贫镁（MgO为0.52%~0.76%）、低钛（TiO₂为0.30%~0.59%）的特点，同时表现出强过铝质（A/CNK=1.19~1.35）及中钾钙碱性–钾玄岩系列特征。它们具有高的稀土元素总量（∑REE=211.60×10^-6~349.01×10^-6），呈现轻稀土元素富集和重稀土元素亏损（（La/Yb）_N=4.32~7.36）；富集Rb、U、Th等大离子亲石元素和Zr、Th、Hf等，亏损Nb、Ta、Ba等元素，并具有明显的负Eu异常（δEu=0.46~0.59），锆石饱和温度介于827~912℃之间，展示了A型花岗岩的属性。这些二长花岗岩可能是通过中上地壳的中酸性火成岩的部分熔融形成，结合前人的研究成果，它们最可能形成于弧后的伸展环境，综合扬子陆块周缘晚中元古代的岩浆记录，元谋杂岩中1.09 Ga二长花岗岩的形成应与扬子陆块开始参与Rodinia超大陆聚合有关。

Abstract:

There has always been considerable controversy in the late Mesoproterozoic geological evolution history of the western margin of the Yangtze Block. In this paper, a suite of monzonitic granites from the Yuanmou complex in the western margin of the Yangtze River has been selected as the research object for a comprehensive study of petrography, zircon U-Pb geochronology and whole-rock geochemistry, and it provides support for understanding the late-middle Proterozoic geological evolution of the western margin of the Yangtze.The LA-ICP-MS zircon U-Pb ages of two Yuanmou Monzogranite samples were 1086±10 Ma (MSWD=1.4, n=50) and 1099±10 Ma (MSWD=1.8, n=58), respectively. All the samples were characterized by high silicon (SiO₂=69.44%～73.98%), alkali-rich (K₂O+Na₂O=6.11%～7.72%), calcium-poor (0.39%～1.46%), magnesium-poor (0.52%～0.76%), and low titanium (TiO₂=0.30%～0.59%), at the same time, it shows strong peraluminous (A/CNK=1.19～1.35) and medium-k calc-alkaline-k-basalt series. They have a high total rare earth content (∑REE=211.60×10^-6～349.01×10^-6), a light rare earth enrichment and a heavy rare earth deficiency ((La/Yb)_N=4.32～7.36); Rb, u, Th and Zr, Th, HF were enriched, while Nb, Ta, Ba were depleted, and negative Eu anomalies were observed (δEu=0.46～0.59), the saturation temperature of zircon ranges from 827~912℃, indicating the properties of A-type granites，These monzogranite may have been formed by partial melting of intermediate-acid igneous rocks in the middle and upper crust, Combined with the research results of predecessors, they are most likely formed in the back-arc extensional environment, combined with the late-middle Proterozoic magmatic records of the margin of the Yangtze block, the formation of 1.09 Ga monzogranite in Yuanmou Complex is related to the initial participation of the Yangtze block in the Rodinia supercontinent convergence.