安徽铜陵角闪石堆积岩锆石U-Pb年代学、矿物化学及其对深部岩浆作用的启示

Zircon U-Pb Dating and Mineral Chemistry of the Amphibole Cumulates in Tongling,Anhui Province,and Its Implications for Deep Magmatism

长江中下游铜陵地区晚中生代岩浆活动强烈,壳幔互动频繁,此期间形成的岩体中含有大量角闪石巨晶和堆积岩,已有研究表明它们是底侵至下地壳的幔源岩浆在下地壳岩浆房中的演化产物。本次研究首次从角闪石堆积岩中分离出了岩浆锆石,并对其和寄主岩的锆石进行了LA-ICP-MS U-Pb定年。测年数据表明,堆积岩和其寄主岩的形成年龄值分别为144.67±0.41 Ma(MSWD=0.25)和137.27±0.41 Ma(MSWD=0.25)。堆积岩中锆石的发现一方面证实了区内下地壳岩浆房中的岩浆活动,另一方面限制了铜陵地区中生代幔源岩浆底侵以及壳幔相互作用的时代。角闪石矿物化学分析结果表明它们均为具较高Al_2O_3(13.52%～13.81%)、TiO_2(2.63%～2.98%)含量的韭闪石和镁绿钙闪石,但其较低的Cr(1.78×10^(-6)～3.12×10^(-6))、Ni(21.55×10^(-6)～26.10×10^(-6))含量和Mg值(68～69),暗示其非形成于原生幔源岩浆。综合分析认为,铜陵地区下地壳深位岩浆房中的岩浆可能为一套经历了一定演化的富水玄武—安山质岩浆,角闪石是其最后的主要晶出相。

The Tongling region in the Middle-Lower Yangtze River belt has experienced strong magmatic activities in the Late Mesozoic,and frequent interaction of crust and mantle resulted in the formation of large amount of amphibole cumulates or megacrysts.Voluminous study suggests that amphibole cumulates or megacrysts likely resulted from the evolution of the mantle underplated magma in the lower crust magma chamber.In this study,we separated zircons from the amphibole cumulate for the first time and conducted LA-ICP-MS U-Pb dating for these zircons and zircons from the host rocks.Dating analysis yielded isotopic ages of 144.67±0.41 Ma（MSWD=0.25）for cumulate and 137.27±0.41 Ma（MSWD=0.25）for host rock.The findings of zircons in the cumulate not only demonstrate the lower crust magmatism in the region but also provide constraint on the time of Mesozoic mantle-derived magma underplating and crust-mantle interaction in the Tongling region.Mineral chemistry analyses indicate that amphiboles are mainly pargasites or magnesiohastingsites,with high contents of Al2O3（13.52% ～13.81%）and TiO2（2.63%～2.98%）.The low Cr（1.78×10^-6～3.12×10^-6）,low Ni（21.55×10^-6～26.10×10^-6）,and low Mg number（68～69）of the amphibole imply they were not generated by primitive mantle-derived magma.Comprehensive analyses in the study indicate that the magma in the deep lower crust magma chamber might have evolved into basaltic-andesitic magma,with amphibole being the last major crystallization phase.