张广才岭燕山早期白石山岩体成因与壳幔相互作用

Petrogenesis and crust-mantle interaction of early Yanshanian Baishishan pluton in Zhangguangcai Range.

出露于东北地区张广才岭的白石山岩体 ,其全岩 -矿物 Rb- Sr等时线年龄为 196± 4Ma,表明形成于中生代的燕山早期 ,而非传统认识上的印支期。岩体主岩和闪长质包体均具有低 ISr(≈ 0 .70 5 )和正 εNd(t) (+1.7～ +2 .2 )值的特点 ,反映岩体成因与地幔具有密切的联系。地质学、岩石学和地球化学的详细研究表明该岩体具有壳幔混合成因 ,闪长质包体是较基性的地幔岩浆进入主岩浆中淬火结晶而成 ,花岗质岩浆的源区主要为新生的地壳物质。动力学分析表明 ,本区在华北板块和西伯利亚板块碰撞拼合后 ,由于东侧大洋板块的俯冲及后续的岩石圈拆沉效应 ,导致软流圈地幔上隆及幔源岩浆的板底垫托 ,并进而造成先存和新生地壳的部分熔融和不同源区岩浆的混合作用。底垫的新生地壳是兴蒙造山带造山后晚期形成的。因此 ,古生代 -中生代是本区地壳生长的重要时期 。

The Baishishan pluton, located in Zhangguangcai Range, NE China,has a whole-rock-mineral Rb-Sr isochron of 196 +/-4 Ma, which suggests that this pluton emplaced at early stage of Mesozoic Yanshanian period, but not the Indosinian as previously thought. The rocks and their dioritic enclaves ore characterized by low I-Sr(approximate to0.705) and positive epsilon (Nd)(t)(+ 1.7 similar to + 2.2), suggesting that the generation of this pluton was closely related to the mantle. The detail studies of geology, petrology and geochemistry conclude that the pluton was the mixing product of crustal- and mantle-derived magmas. The dioritic enclaves were the quenched fragment of mantle-derived magma when it entered the host granitic magma, whose source was mainly a newly formed juvenile crust, Dynamic analyses indicate that t after the collision of North China and Siberian Cratons, the subduction of eastern-ocean plate and the following lithospheric delamination resulted in the upwelling of asthenospheric mantle and the underplating of mantle-derived magma, which led to the partial melting of pre-existed and juvenile crusts and following the mixing of these different magmas. The underplated juvenile crust was formed in the post-orogenic evolution of Xing'an Mongolian Orogenic Belt. Therefore, the Plaeozoic-Mesozoic was a main stage of crust growth in this region, and at the same time, this crustal growth was conducted in vertical tectonic regime.