华北东部晚中生代区域伸展背景下同构造花岗岩体的起源与就位

花岗质岩浆的起源、迁移及就位是研究大陆岩石圈流变学特性的重要方面。然而,板内伸展背景下同构造花岗岩体的岩浆来源、就位机制和岩浆流动与区域应力场的关系等问题缺乏系统性的总结。晚中生代期间华北板块东部逐渐变为区域伸展体制,同时中浅部地壳形成一系列的韧性剪切带、变质核杂岩和拆离断层,这些伸展构造往往伴有同剪切变形的花岗岩体。因此,华北东部是系统研究板内伸展背景下同构造花岗岩体的最佳区域。本文选取多个典型的同构造花岗岩体,进行综合分析。通过归纳总结这些同构造岩体的岩石地球化学和年代学资料,发现多数同构造岩体具有多个岩浆源区,且较早就位的中性岩席(单元)往往来自壳幔混合岩浆或新生下地壳的部分熔融,而较晚的酸性岩席(单元)则主要来源于古老下地壳的部分熔融。这一特点反映了同伸展岩体岩浆源区由深至浅的演化规律,也揭示了区域伸展背景下源自地幔的流体和热量是触发地壳部分熔融的重要因素。通过分析岩浆就位过程中围岩和岩体中形成的定向及变形组构,发现华北东部同伸展岩体的就位模式可分为三大类:以扁平岩床或岩基形式就位于中部地壳的水平韧性剪切带内;岩浆以近直立运移的方式形成长轴平行拆离断层的岩基,就位于变质核杂岩核部或拆离断层下盘;岩浆就位于再活化的先存断裂,通过膨胀作用、挤压围岩获得就位空间并使围岩变形,形成类似底辟作用的就位方式。剪切应力和浮力是影响岩浆运移方向的重要力学参数。岩浆自源区上升的过程中浮力起着主要控制作用,就位于韧性剪切带时剪切应力起着控制作用,就位于浅部地壳的脆-韧性过渡带时浮力的作用再次凸显。

Genesis and emplacement of syntectonic granitic plutons during Late Mesozoic extension in eastern North China

Much is known about the origin, migration, and emplacement of granitic magma with respect to the rheology of the continental lithosphere. However, magma sources, emplacement mechanisms, and the relationship between stress fields and melt flow in intraplate extensional settings are still disputed. During the Late Mesozoic, tectonic regime of the eastern North China Plate became gradually into regional extension, and a series of ductile shear zones, metamorphic core complexes, and detachment faults developed in the middle-shallow crust, accompanied by syntectonic granitic plutons. Thus, eastern North China is a favorable region for the study of syn-extensional formation of granites. Based on previous published data, we present a geochemical and geochronological investigation of typical syntectonic granites in this region. Results indicate that the syntectonic plutons had multiple magma sources, whereby magma of the earlier intermediate intrusive sheets was derived from the partial melting of juvenile lower crust or crust-mantle mixing, and later felsic sheets were derived from the partial melting of ancient lower crust. These findings reveal a deep-shallow evolution of the magma source for the syn-extensional plutons, and reveal that fluid and heat from the mantle triggered partial melting of the crust under regional extension. Three emplacement mechanisms for these syn-extensioanl plutons are inferred: (1) the plutons were emplaced in a horizontal ductile shear zone in the middle crust as tabular sills or batholiths; (2) the magma formed batholiths with the long-axis parallel to detachment faults by subvertical migration into the core of metamorphic core complexes or the footwall of detachment faults; and (3) the magma was located in reactivated pre-existing faults and, by injection under pressure into wallrock, emplacement space was produced by ballooning of magma and synchronous deformation of surrounding rocks, producing an emplacement mode similar to diapirism. Results also suggest that shear stress and buoyancy were important mechanical parameters affecting the orientation of magma ascent and migration. During the process of magma ascent from the source, the flow direction of the melt was controlled mainly by buoyancy; shear stress played a controlling role during magma intrusion into ductile shear zones; and buoyancy was a primary control during magma migration into the brittle-plastic transition zone in the shallow crust.