东昆仑东段三叠纪岩浆混合作用:以香加南山花岗岩基为例

香加南山花岗岩基位于东昆仑造山带东段,岩基主要岩石类型为花岗闪长岩。千瓦大桥-加鲁河一带花岗岩体为香加南山岩基的重要组成部分。香加南山花岗岩基含大量暗色微粒包体,包体中捕掳晶丰富。千瓦大桥-加鲁河一带花岗岩体寄主岩中斜长石和暗色微粒包体中捕掳晶斜长石具正常环带,An值震荡变化,角闪石和黑云母Mg O含量和Mg#值较低,具壳源特征;暗色微粒包体中基质斜长石具核边结构,核部和边部An值存在间断,角闪石和黑云母Mg O含量和Mg#值较高,具幔源特征。LA-ICP-MS锆石U-Pb同位素定年结果显示千瓦大桥花岗闪长岩、暗色微粒包体和加鲁河辉长岩的结晶年龄分别为251.0±1.9Ma、252.8±3.0Ma和221.4±3.3Ma。千瓦大桥花岗闪长岩和加鲁河花岗闪长岩富集轻稀土元素(LREE)和大离子亲石元素(LILE),亏损高场强元素(HFSE),具较低的Mg#和Nb/Ta比值;从千瓦大桥到加鲁河花岗闪长岩呈现出由准铝质中钾钙碱性系列向准铝-弱过铝质中钾-高钾钙碱性系列演化;暗色微粒包体和加鲁河辉长岩轻重稀土元素分异程度相对较低,具较高的Mg#和Nb/Ta比值。千瓦大桥花岗闪长岩和加鲁河花岗闪长岩分别为古特提斯演化俯冲阶段和后碰撞阶段幔源岩浆底侵新生地壳使其部分熔融产物。镁铁质岩浆注入长英质岩浆的混合作用形成了暗色微粒包体。岩浆混合过程中,如果岩浆不完全混合,混合岩浆中混入物质除了长英质岩浆的残留岩浆和捕掳晶,还应该有镁铁质岩浆与长英质岩浆之间的元素梯度差导致的物质扩散;如果岩浆为近完全混合,混合岩浆近似为镁铁质岩浆和长英质岩浆以一定比例二元混合。东昆仑东段晚古生代-早中生代幔源岩浆对花岗质岩浆的影响是一个持续的过程,从俯冲阶段早期流体交代地幔熔融,到俯冲阶段后期板片断离,然后同碰撞阶段板片断离的持续影响,再到后碰撞阶段加厚地壳的拆沉作用,由于地球动力学体制不同,导致幔源岩浆影响的大小和特征不同。

Triassic magma mixing and mingling at the the eastern section of Eastern Kunlun: A case study from Xiangjiananshan granitic batholith

Xiangjiananshan granitic batholith located in the eastern part of the Eastern Kunlun Orogenic Belt (EKOB) is mainly composed of granodiorit. Granite pluton in Qianwadaqiao-Jialuhe area is an important of Xiangjiananshan granitic batholith and contains a large number of mafic microgranular enclaves (MMEs), MMEs rich in xenocrysts. Plagioclase in the host rock and xenocrystals plagioclase in MMEs has osicillatory zonings; the contents of MgO and Mg^# values of amphibole and biotite are low, with the features of the crust source; plagioclases in matrix of MMEs have core-rim structures with an An value discontinuity; the contents of MgO and Mg^# values of amphibole and biotite are high, and they are characterized by mantle source. LA-ICP-MS zircon U-Pb dating suggests that the granodiorite, MMEs in Qianwadaqiao area and gabbro in Jialuhe area crystallized 251.0±1.9Ma, 252.8±3.0Ma and 221.4±3.3Ma ago, respectively. Granodiorite in Qianwadaqiao-Jialuhe area is geochemically featured by LILE enrichment and HFSE depletion, low Mg^# value and Nb/Ta ratio. The calc alkaline series granodiorite evolved gradually from metaluminous middle-K to weakly peraluminous middle-to high-K in geochemistry in the place from Qianwadaqiao area to Jialuhe area. The MMEs and the Jialuhe gabbro have unconspicuous differentiation between HREE and LREE, and high Mg^# value and Nb/Ta ratio. Inferred by the geochronological and geochemical data, it is believed that the granodiorites in Qianwadaqiao area and Jialuhe were the products of partial melting, triggered by underplating from mafic magma into juvenile crust in the EKOB, occurred in subduction and post-collision period of paleo-Tethyan respectively. And the MMEs are results of magma mixing and mingling between mafic and felsic magma. During the magma mixing and mingling, if the magma is mixing, the mixture should be material diffusion between mafic and felsic magma as a result of element difference as well as the residual felsic magma and xenocrysts; if the magma is mingling, the mingling magma would be composed of mafic and felsic magma in a certain volume proportion. The influence to the granitoid melts by the Late Palozoic-Early Mesozoic mantle-derived magma of Eastern Kunlun is a continuous process, which was from the mantle melting with fluid metasomatism in early subduction stage to the large-scale plate break-off in late subduction atage, from the continuous influence of plate break-off in syn-collision stage to delamination of thicken crust in post-collision stage. The influence degrees and features of mantle-derived magma were different in the different geodynamic system of the process.