太行山南段丰峪蚀变正长花岗岩LA-ICP-MS锆石U-Pb年龄及构造意义

Zircon U-Pb age of the altered syenogranite in Fengyu area of southern Taihang Mountain and its significance

在华北克拉通中东部的太行山南段丰峪一带,发现一处不整合于长城系大河组之下的蚀变正长花岗岩体。岩石类型为黑云母正长花岗岩,钾化、绢云母化蚀变明显。岩体低硅(SiO_2=67.46%～68.89%)、富碱(K_2O+Na_2O=8.09～9.62,K_2O/Na_2O远大于1),铝指数A/CNK=1.20～1.57,属过铝质、钙碱性A型花岗岩。岩石富集大离子亲石元素,显著亏损Nb、Ba、Ta、Sr、P、Ti元素,稀土元素含量高(192～774μg/g,平均483μg/g)。用LA-ICPMS技术测得锆石的207Pb/206Pb年龄为(1790±22)Ma(n=22,MSWD=0.80),这是迄今为止太行山南段长城系底界直接的控制年龄。岩石主、微量元素地球化学以及同位素年代学表明,丰峪蚀变正长花岗岩形成于板内伸展构造环境,为熊耳期三叉裂谷东北一支的存在提供了重要的岩浆岩石学证据,其所处的太行山区与熊耳群火山岩所处的豫西地区均为熊耳期三叉裂谷的组成部分,进而为太行山区与豫西地区的长城系对比提供了重要支撑。

An altered syenogranite pluton was found in Fengyu area,southern Taihang Mountain of central-eastern North China Craton,which unconformably underlies the Changcheng System.The rock type is biotite granite,which has undergone obvious Kfeldspar alteration and sericitization.The rocks are characterized by low SiO2(SiO2=67.46%-68.89%)and high K2O+Na2O(K2O+Na2O=8.09-9.62,K2O/Na2O-1)content,with aluminum index A/CNK being 1.20-1.57,showing features of peraluminous,calc-alkaline A-type granite.The pluton is also characterized by enrichment of LILE and depletion of Ba,Nb,Ta,Sr,P,Ti,as well as highΣREE values(192-774μg/g,483 ug/g om average).LA-ICP-MS zircon U-Pb dating of Fengyu altered syenogranite pluton yielded an 207Pb/206Pb age of(1790±22)Ma(n=22,MSWD=0.80),which is by far the most direct control of the lower limit age of Changcheng System in southern Taihang Mountain.Petrochemistry,rare earth elements,trace element geochemistry and isotopic geochronology all prove that Fengyu altered syenogranite pluton was formed in an intraplate extensional environment in the Paleoproterozoic period,thus providing an important magmatism evidence for the existence of northeast Xiongerrian trident continental rift;they also show that the Taihang Mountain area where Fengyu altered syenogranite pluton was formed and the western Henan Province where Xiong'er Group volcanic rocks were developed constitute part of the Xiongerrian trident continental rift,which thus provides an important support for the stratigraphic correlation of the Changcheng System between Taihang Mountain area and western Henan Province.