华北克拉通南缘古元古代晚期小河A型花岗岩体成因及其地质意义

Petrogenesis and Geologic Implication of the Late Paleoproterozoic A-type Xiaohe Pluton along the Southern Margin of the North China Craton

华北克拉通南缘出露的古元古代花岗岩是研究前寒武纪地壳演化的重要载体。文章报道出露于华北克拉通南缘的小河花岗质岩体的锆石U-Pb年龄和地球化学组成,探讨其岩石成因和前寒武纪地壳演化意义。小河岩体的岩性为含黑云母二长花岗岩,根据其稀土元素特征可以分为两类,第一类表现为轻稀土元素富集、重稀土亏损的右倾配分模式,具有中等分异特征,第二类具有稀土总量较低,轻重稀土分馏不明显,中稀土亏损的稀土元素分布模式,具有高分异特征。锆石U-Pb年龄表明两种类型岩石形成于~1.80 Ga,为同一期次岩浆活动。两类岩石均具有富硅、富碱和弱过铝质特征,属于高钾碱钙质—钙碱质系列;富集Rb、Th、U、K等元素,亏损Ba、Sr、P和Ti等元素。两类岩石有相同的Nd-Hf同位素组成特征,其岩浆源区可能均为华北克拉通南缘晚太古宙基底物质。此类铝质A型岩浆作用可能是与伸展体制相关的岩浆—热事件,暗示在约1.80 Ga时华北克拉通南缘为碰撞/造山后的伸展环境。

Paleoproterozoic granitoids in the southern margin of the North China Craton are important carriers for studying the crustal evolution in Precambrian.In this study,we report zircon U-Pb ages and whole-rock geochemical compositions of the Xiaohe pluton exposed along the southern margin of the North China Craton in order to discuss its petrogenesis and significance for Precambrian crustal evolution.The Xiaohe pluton is composed of biotite-bearing monzogranite,geochemically,which can be divided into two categories according to its rare earth element characteristics.Type I shows the right dip distribution mode of light rare earth elements enrichment and heavy rare earth elements depletion,which has the characteristics of medium differentiation.Type II has a distribution pattern of rare earth elements with low total rare earth elements,insignificant fractionation of light and heavy rare earth elements,and depletion of middle rare earth elements,which has the characteristics of high differentiation.The zircon U-Pb dating on these two types of granite yielded~1.80 Ga ages for the emplacement time of the magmas,indicating both of them emplaced in single magmatic period.All the analyzed rocks are rich in silica and alkaline and weakly peraluminous,having high-K alkali-calcareous and calc-alkaline characteristics.They are enriched in Rb,Th,U,K,etc.,but depleted in Ba,Sr,P,and Ti.Both types granites have similar Nd-Hf isotopic compositions and their magmas should originate from partial melting of the Late Archean basement rocks along the southern margin of the North China Craton.The magmatism of A-type granite probably represents the tectonicsthermal event closely related to extension,implying a post-collisional/post-orogenic extensional setting along the southern margin of the North China Craton at ca.1.80 Ga.