胶东早白垩世海阳岩体晶洞碱长花岗岩熔-流体作用的成因矿物学证据

GENETIC MINERALOGICAL EVIDENCE OF MELT-FLUID INTERACTION IN THE EARLY CRETACEOUS HAIYANG ALKALI FELDSPAR GRANITE,JIAODONG PENINSULA,EASTERN CHINA

胶东海阳岩体属于中生代早白垩世晚期岩浆活动的产物。该岩体主要岩性为似斑状中粗粒黑云角闪二长花岗岩、晶洞中粗粒正长—碱长花岗岩和晶洞似斑状中—细粒碱长花岗岩。其中,晶洞碱长花岗岩记录了明显的熔—流体作用痕迹,对其进行相关研究有助于深入认识花岗质熔体与流体的作用特征。海阳岩体内部广泛发育晶洞构造,晶洞内可见萤石及碳酸盐矿物,是宏观流体活动的证据;碱性长石和钠长石的共结结晶是熔—流体作用的岩相学证据;岩石还发育显微球粒结构,这是由于岩浆过冷却度急剧增大导致不同矿物相发生快速结晶,造成岩浆成分上的不混溶而形成钠长石和碱性长石的交生现象。碱长花岗岩和黑云角闪二长花岗岩锆石CL图像显示为岩浆锆石特征,U-Pb年龄分别为113±1 Ma和112±1 Ma。碱长花岗岩HY11-07和HY11-13号锆石轻稀土元素富集(ΣLREE为561.26×10^(-6)~1018.47×10^(-6)),Ce的正异常较弱(δCe为1.91~3.42),显示出受流体作用的趋势。总之,本次研究显示胶东海阳岩体中晶洞碱长花岗岩受到过岩浆中的流体作用。其可能过程是岩浆侵位后,其含有的大量流体及挥发分因快速减压而迅速逃逸,导致大量矿物的固相线温度快速提高,熔体过冷度急剧加大,造成海阳岩体大量晶洞构造和显微球粒结构的形成。

The Haiyang pluton in Jiaodong area belongs to the late magmatic products in early Cretaceous of Mesozoic era,which is mainly composed of porphyritic medium-coarse-grained biotite hornblende monzogranite,coarse-grained syenite and fine-grained alkali-feldspar granite.Among them,the voids in the miarolitic alkali feldspar granite have clear traces of melt-fluid interaction,and the study on them is helpful to understand the interaction characteristics of granitic melt and fluid.In Haiyang rock mass,crystal voids are widely developed with occurrence of fluorite and carbonate minerals in the crystal voids,which are the evidence of macroscopic fluid activity.The co-crystallization of alkaline feldspar and albite is petrographic evidence of melt-fluid interaction.Due to the rapid crystallization of different mineral phases and the rapid increase of magma supercooling degree,the microspherulite structure is developed in the rocks,which results in immiscibility of magma composition and the symbiosis of albite and alkaline feldspar.Zircon CL images of alkali feldspar granite and biotite hornblende monzonite show the characteristics of magmatic zircons,with U-Pb ages of 113±1 Ma and 112±1 Ma,respectively.Zircons HY11-07 and HY11-13 from alkali-feldspar granite are rich in LREE(ΣLREE range 561.26×10^(-6) to 1018.47×10^(-6)),and have weak positive Ce anomaly(δCe range 1.91-3.42),which indicates the fluid influence.In conclusion,this study shows that the Jingdong alkali-feldspar granite was subjected to the action of fluid from magma.The possible process is that a large number of fluids and volatiles in the magma rapidly escape due to rapid decompression,which leads to the rapid increase of solid phase temperature of a large number of minerals and the sharp increase of the undercooling degree of the melt,resulting in the formation of a large number of miarolitic structures and microspherulite structures in Haiyang rock mass.