陕西商南县陡岭岩群湘河片麻岩地球化学特征及成因探讨

南秦岭地区商南湘河一带发育元古代陡岭岩群(Ar_3Pt_1D)片麻岩,是区内晶质石墨矿的主要赋矿层位。该片麻岩系为一套以含榴黑云(矽线)斜长片麻岩、(石墨)黑云斜长片麻岩、混合岩化黑云斜长片麻岩为主,夹石墨片岩。该地层中(石墨)片麻岩的地球化学特征研究表明:其原岩为一套含碳质、富铝的泥质和粉砂质沉积岩建造,具有孔慈岩系的特点,物源偏向于来自于长英质岩石的沉积物,但不集中于长英质物源,而是混合型物源。大离子亲石原素富集,高场强元素亏损,Y、Yb含量低,具有典型的岛弧成因特征,推断其形成环境应为活动大陆边缘。稀土元素地球化学特征上显示原岩为沉积岩、粘土岩,也显示出成熟度较低的陆壳基底变质岩系的特征,反映了沉积环境更接近于大陆岛弧,碎屑岩的物质主要来自早元古代不成熟的地壳岩石。

Geochemical Characteristics and Petrogenesis of Xianghe gneiss in Douling rock group from ShangNan county, Shaanxi province

The Proterozoic Douling Group gneiss is the main ore-hosting horizon of crystalline graphite deposits in the Xianghe area of southern Qinling Mountains. The gneiss series is mainly composed of garnet biotite (sillimanite) plagioclase gneiss, (graphite) biotite plagioclase gneiss and migmatized biotite plagioclase gneiss with graphite schist. Geochemical characteristics of the (graphite) gneiss in this stratum indicate that the original rock is a set of carbonaceous, aluminium-rich argillaceous and silty sedimentary rocks with the characteristics of the Kongci series. The provenance of the gneiss tends to be sediments from felsic rocks, not felsic, but mixed ones. The LILE in the rock are enriched, but the HFSE are depleted, with lower Y and Yb contents, which has typical characteristics of island genesis. It is concluded that the formation environment should be active continental margin. Geochemical characteristics of REE show that the original rocks are sedimentary rocks and clay rocks, and that the lower maturity continental crustal basement metamorphic rocks series, reflecting that the sedimentary environment is closer to the continental island arc, and the clastic rocks mainly came from the immature crustal rocks of the early Proterozoic.