燕山造山带后石湖山碱性环状杂岩体的成因与形成过程

后石湖山杂岩体是与垮塌破火山口有关的碱性环状杂岩体, 主要由呈环形分布的碱性火山岩、环状岩墙(斑状石英正长岩)、嵌套的中心复式岩株(晶洞碱长花岗岩和斑状碱长花岗岩)和锥状岩席(石英正长斑岩和花岗斑岩)组成.LA-ICPMS锆石U-Pb年代学分析表明, 斑状石英正长岩环状岩墙、石英正长斑岩和花岗斑岩锥状岩席的侵位年龄分别为119±3Ma、121±2Ma和121±2Ma.该环状杂岩体火山岩与侵入岩的形成年龄相近, 体现了它作为火山-侵入杂岩体的特征.斑状石英正长岩富碱(Na₂O+K₂O=10.0%~10.5%), K₂O含量较高(5.21%~5.42%), 具正的Eu异常(Eu/Eu*=1.05~1.40).碱长花岗岩和斑岩均具有富碱、高FeOtot/MgO、Ga/Al、Zr、Nb和REE值(Eu除外), 以及低Al₂O₃、CaO、MgO、Ba、Sr和Eu含量的特征, 都属于A型花岗岩质岩石.其中斑岩为铝质A型花岗岩, 具有高的初始岩浆温度(880~901℃).所有A型花岗质岩石均具有较富集的Nd同位素组成, ε_Nd(t)值变化于-13.9~-12.2之间.斑状石英正长岩是下地壳中-基性麻粒岩和片麻岩部分熔融产生的熔体与幔源玄武质岩浆混合, 后又发生单斜辉石分离结晶的产物; 碱长花岗岩源于上地壳长英质岩石部分熔融产生的熔体与幔源玄武质岩浆混合, 随后经历长石的分离结晶作用而成; 斑岩是受幔源岩浆底侵加热的上地壳长英质岩石的部分熔融产生的熔体, 并经历了长石的分离结晶作用而产生.该环状杂岩体的形成过程可以概括为: (1)火山爆炸性喷发形成大量的碱性火山熔岩和火山碎屑岩; (2)地下岩浆房空虚导致压力下降, 其顶板围岩失稳而沿火山口周围近直立的环状断裂垮塌, 形成塌陷的破火山口.与此同时, 下覆岩浆房的岩浆被动挤入环状断裂而形成斑状石英正长岩环状岩墙; (3)浅部地壳的长英质岩浆房过压, 促使其高温过碱质A型花岗质岩浆上升侵位形成了中心的斑状碱长花岗岩岩株, 这些岩浆的上涌导致上覆围岩产生倾角中-陡的、内倾的锥状裂隙, 为石英正长斑岩锥状岩席侵位提供了空间; (4)浅部岩浆房复活, 高温过碱质A型花岗质岩浆再度上升侵位形成被嵌套的晶洞碱长花岗岩岩株.同样, 这种岩浆的再度上侵导致上覆围岩产生了倾角较陡而内倾的锥状裂隙, 为花岗斑岩锥状岩席提供了侵位空间.后石湖山碱性环状杂岩体的形成是华北东部早白垩世与克拉通破坏相关的伸展构造体制下的产物, 这种构造体制可能与古太平洋板块的俯冲作用有关. 

The Origin and Evolution of the Houshihushan Alkaline Ring Complex in the Yanshan Orogenic Belt

The Houshihushan complex is an alkaline ring complex associated with a collapsed caldera, consisting of a circular screen of alkaline volcanic rocks and post-collapse resurgent intrusions including a ring dyke of porphyritic quartz syenite, a central composite hypabyssal intrusion of nested stocks of drusy alkali-feldspar granite and porphyritic alkali-feldspar granite, and cone sheets of quartz syenite porphyry and granite porphyry. Zircon LA-ICPMS U-Pb analyses yields mean 206Pb/238U ages of 119±3Ma for porphyritic quartz syenite, 121±2Ma for quartz syenite and 121±2Ma for granite porphyry, respectively. Volcanic rocks of the Houshihushan Ring Complex (HRC) have similar ages to those of the intrusive rocks, confirming it as a volcanic-intrusive complex. Porphyritic quartz syenites have high contents of Na₂O+K₂O (10.0%-10.5%) and K₂O (5.21%-5.42%) with positive Eu anomalies (Eu/Eu*=1.05-1.40). Alkali-feldspar granites and porphyries are characterized by enriched Na₂O+K₂O, FeOtot/MgO, Ga/Al, Zr, Nb and REE (except for Eu) and low abundance of Al₂O₃, CaO, MgO, Ba, Sr and Eu, indicative of A-type granitic rocks. The porphyries can be classified as aluminous A-type granites, and show high zircon saturation temperatures (880-901℃). All the A-type granites of the HRC posses negative ε_Nd(t) values from -13.9 to -12.2. Porphyritic quartz syenite magmas were derived from partial melting of intermediate to mafic granulites and gneisses in the lower crust that mixed with enriched mantle-derived basaltic magma, with subsequent differentiation of clinopyroxene. Alkali-feldspar granite magmas were produced by mixing of mantle-derived basaltic magmas with upper crustal felsic melts, with fractionation of feldspars. The petrogenetic processes of porphyritic magmas involved partial melting of quartzfeldspathic rocks at shallow crust depths coupled with differentiation of feldspars. We suggest that development of the HRC involved the following four-stage sequence: (1) massive alkaline laves and pyroclastics erupted explosively; (2) the subsided caldera formed because of loss of magma from an underlying magma chamber which reduced magma pressure and facilitated collapse of the roof of the magma chamber along near-vertical ring faults. Magma intruded passively up the opening ring-faults to form the ring dyke of porphyritic quartz syenite during caldera collapse; (3) the high-level magma chamer became overpressured, and hot peralkline A-type granite magma was emplaced as the central stock of porphyritic alkali-feldspar granite. The overlying crust was fractured to generate cone fractures that provided space for the ascent of felsic melts to form cone sheets of quartz syenite porphyry; (4) the chamber resurged and a cogenetic pluton was emplaced as the nested stock of drusy alkali-feldspar granite. Build-up of magma overpressure within the central source chamber imparted upward force to fracture the host rock and form new conical fractures. These fractures were filled with magma to form cone sheets of granite porphyry. The Houshihushan alkaline ring complex formed over a brief time period in an extensional setting related to destruction of the eastern North China Craton during Early Cretaceous, possibly associated with subduction of paleo-Pacific plate.