五台岩群石榴云母片岩地球化学特征及其地质意义

摘要

五台岩群石咀亚群石榴云母片岩主要由石榴石、黑云母、白云母、斜长石、石英和钛铁矿等构成,少量样品含绿帘石、十字石、蓝晶石或斜方闪石.样品的SiO2为51.75％ ～69.06％,Al2O2为11.99％～20.39％,个别贫钠,稀土元素配分模式上显示轻稀土富集、重稀土相对亏损的样式,无明显Eu异常[(La/Yb)N =3.10～ 19.20,(Eu/Eu*)N=0.90～ 1.13],高场强元素和大离子亲石元素与世界典型页岩成分一致.元素间的协变关系指示粘土矿物控制了岩石的化学成分.通过原岩类型判别、重矿物优选堆积判别、物源判别和沉积构造环境判别等方法,恢复其原岩以页岩为主,无明显的重矿物优选堆积,沉积物主要来自五台地区同时代的TTG或/和基性岩石,沉积于弧盆环境,可能的沉积时代为2.53～2.50 Ga.结合THERMOCALC变质相平衡模拟和变质锆石SHRIMP U-Pb定年结果,得到其在～1.95 Ga经历了p-t轨迹为顺时针样式的蓝晶石型变质作用.%Garnet-mica schist from the Shizui Subgroup of Wutai Group is chiefly composed of garnet,biotite,muscovite,plagioclase,quartz and ilmenite,with a few samples containing epidote,staurolite,kyanite and ortho-amphibole.Samples have SiO2 51.75％～69.06％ and Al2O3 11.99％～20.39％ with a few samples being poor in sodium.Samples exhibit higher LREE and lower HREE in REE patterns without obvious Eu anomaly [(La/Yb)N=3.10～19.20,(Eu/Eu*)N =0.90～1.13],and their high strength element compositions and large ion lithophile element compositions are similar to those of classical shales in the world.Correlations between related elements imply that clay minerals dominate the rock bulk compositions.Based on a series of diagrams such as geochemical classification,heavy mineral accumulation,source rock and tectonic setting discrimination,the authors found that the protolith of the rocks was shales without obvious heavy mineral accumulation.The sediments mainly came from those coeval TTG rocks and/or basic rocks in Wutai area,and were deposited in an arc basin environment at about 2.53～2.50 Ga.Combined with the results of metamorphic equilibria modeling using THERMOCALC method and metamorphic zircon dating using SHRIMP method,the authors have found that the garnet-mica schist experienced a kyanite-type metamorphism with a clockwise p-t path at ～1.95 Ga.