上扬子克拉通内新元古代A型花岗岩的发现及其地质意义

The discovery and geological implications of the Neoproterozoic A-type granites in the upper Yangtze craton

关键词:  上扬子克拉通  新元古代  A型花岗岩  岩石地球化学  SHRIMP锆石U-Pb测年  Rodinia超大陆裂解

Key words:  upper Yangtze craton  Neoproterozoic  A-type granite  lithogeochemistry  SHRIMP zircon U-Pb dating  breakup of the suppercontinent Rodinia

摘要:

处于上扬子克拉通内的峨边牛郎坝黑云母花岗岩一直以来很少被人重视和研究,本文首次报道了该岩体的SHRIMP锆石U-Pb定年和岩石地球化学数据。牛郎坝花岗岩以高硅(SiO₂>75%)、低钙(CaO=0.46~0.20%)、贫镁、富碱(Na₂O+K₂O=8.31~9.28%)、铝质(A/KNC=1.02~1.12)为特征;微量元素地球化学表现出强烈亏损Ba、Sr、Eu(δEu=0.05~0.08),富集Rb、Th、U。全岩样品的104*Ga/Al在2.6至2.9之间变化,高场强元素Zr、Y、Ga的含量较高,且没有明显的异常。主量元素和微量元素分析均表明牛郎坝花岗岩为铝质A₂亚型花岗岩特征。其Y/Nb=5.0~5.9,Nb/Ta=4.4~5.0,表明岩浆源区受到较强的陆壳组分混染作用。该花岗岩的SHRIMP锆石U-Pb测年结果为826±21.4Ma,与前人对扬子东南缘新元古代花岗岩的年龄测试结果基本一致,反映牛郎坝花岗岩也是该阶段泛扬子克拉通强烈岩浆活动的一部分。扬子克拉通内的牛郎坝A型花岗岩可能是新元古代中期在Rodinia超大陆裂解背景下与地幔柱构造相关的壳幔相互作用的产物。

Abstract:

Little has long been known for the Niulangba biotite granites in Ebian, Sichuan within the upper Yangtze craton. The purpose of this paper is to present, for the first time, our recent results of SHRIMP zircon U-Pb dating and relevant geochemical data. The Niulangba granites are characterized by higher SiO₂ (SiO₂ >75%) contents, lower Ca (CaO=0.46% to 0.20%) contents, depletion in Mg, enrichment in alkali (Na₂O + K₂O=8.31% to 9.28%) and Al (A/KNC=1.02 to 1.12). The geochemistry of the trace elements suggests the highly depletion in Ba, Sr, Eu (δEu=0.05 to 0.08) and enrichment in Rb, Th and U. The 104* Ga/Al ratios for the bulk-rock samples range between 2.6 and 2.9. The high field strength elements Zr, Y and Ga have higher contents, and no anomaly has been identified. It can be seen from the major and trace element analyses that the Niulangba granites belong to the aluminous A₂-subtype granites. The Y/Nb ratios ranging from 5.0 to 5.9 and Nb/Ta ratios ranging from 4.4 to 5.0 have disclosed that the magmatic source areas were once subjected to highly hybridization of continental compositions. The SHRIMP zircon U-Pb ages are dated at 826±21.4 Ma, in general agreement with the previous data for the Neoproterozoic granites from the southeastern Yangtze craton. This implies that the Niulangba granites should be assigned to part of the highly magmatic activity within the pan-Yangtze craton during this period. These granites may result from the mantle plume-associated crust-mantle interaction against the breakup of the suppercontinent Rodinia during the middle Neoproterozoic.