浙江临安指南山-东坑晚中生代侵入岩地球化学、岩石成因及构造环境

Geochemistry,Petrogenesis and Tectonic Environment of the Zhinanshan-Dongkeng Intrusive Rocks in the Late Mesozoic in Lin an,Zhejiang Province

在15万区域地质填图的基础上,对浙江临安晚中生代天目山火山盆地内的指南山-东坑侵入岩进行锆石U-Pb测年和地球化学分析,探讨火山喷发-侵入作用过程。指南山侵入岩主要由细中粒石英闪长岩胶结大量安山岩集块角砾组成,东坑侵入岩主要由外部中细粒石英二长岩和内部细粒正长花岗(斑)岩、中粗粒正长花岗岩组成。LA-ICP-MS锆石U-Pb测年显示,指南山细中粒石英闪长岩侵入时代为130.5±1.7 Ma,东坑细粒正长花岗(斑)岩和中粗粒正长花岗岩侵入时代为127.6±1.2 Ma和127.9±1.3 Ma,形成时代为华南中生代早白垩世火山活动的高峰期。岩石属准铝质–过铝质橄榄玄粗岩系列,具有中等-较高SiO_(2)、富碱、高K_(2)O/Na_(2)O、低P_(2)O_(5)、TiO_(2)特征,球粒陨石标准化稀土配分曲线具有弱右倾和强负铕异常的特征,富集K、Rb、Th、U,弱亏损Sr、Ba等大离子亲石元素,亏损-弱亏损Ti、P、Nb、Ta等高场强元素。天目山-莫干山一带火山盆地中花岗质火山岩与侵入岩具有密切成因关系,其母岩浆可能主要是由幔源岩浆底侵导致江南造山带地壳部分熔融形成的,受到大量富集岩石圈地幔物质混染,并且经历了较高程度的分异结晶演化作用,为华南地区早白垩世早期伸展拉张环境大规模喷发的产物。

On the basis of 150000 regional geological mapping,this work conducted zircon U-Pb dating and geochemical analysis of the Late Mesozoic intrusive rocks from Zhinanshan-Dongkeng in the Tianmushan volcanic basin,Lin an,Zhejiang Province,to explore the volcanic eruption-intrusion process.The Zhinanshan intrusive rocks are mainly composed of fine to medium grained quartz diorite cemented with a large number of andesite agglomerates and breccia.The Dongkeng intrusive rocks are mainly composed of medium to fine grained quartz monzonite on the outside,and fine grained syenogranite(porphyry)and medium to coarse grained syenogranite on the inside.LA-ICP-MS zircon U-Pb dating shows that the intrusive age of Zhinanshan fine to medium grained quartz diorite is 130.5±1.7 Ma,and that of Dongkeng fine-grained syenogranite(porphyry)and medium to coarse-grained syenogranite is 127.6±1.2 Ma and 127.9±1.3 Ma,respectively,which is the peak of Mesozoic Early Cretaceous volcanic activity in South China.The rocks belong to quasi-aluminous-peraluminous shoshonite series,characterized by medium to high SiO_(2),alkali-rich,high K_(2)O/Na_(2)O and low P_(2)O_(5)and TiO_(2).The normalized REE distribution curves of chondrite are characterized by weak right-inclim and strong negative Eu anomaly,enrichment of K,Rb,Th and U,weak depletion of large ion lithophile elements such as Sr and Ba,and weak depletion of high field strength elements such as Ti,P,Nb and Ta.There is a close genetic relationship between granitic volcanics and intrusion in the volcanic basin of Tianmushan-Moganshan area.Their primary magma might be formed by the partial melting of the crust in Jiangnan orogenic belt caused by the underplating of mantle-derived magma.It was contaminated by a large amount of enriched lithospheric mantle material,and might have experienced a high degree of fractional crystallization.They were the products of large-scale eruption in the extensional environment of Early Cretaceous in South China.