Trace Elements and Sr, Nd Isotope Geochemistry of the Lajimiao Norite-Gabbro from the North Qinling Belt

The Lajimiao norite-gabbro complex, as a part of the ophiolites on the southern side of the North Qinling belt, consists of gabbro and norite-gabbro. They were derived from different magma series: the gabbro was derived from tholeiitic magma series with higher TiO2, REE abundance and Fe3+/Fe2+ ratio ; norite-gabbro was derived from calc-alkali magma series with lower TiO2, Fe3+/Fe2+ ratio and REE abundance and much lower HREE abundance, which suggests that the source of the norite-gabbro magma was deeper and controlled by eclogite facies. Geochemical characteristics of both plutonic rocks are similar to those of island-arc basalts, such as relatively high contents of Ba, Pb and Sr and relatively low contents of Nb, Zr and N j.The Sr, Nd isotopic characteristics of the Lajimiao norite-gabbro complex are similar to those of ophiolites. Its εNd values are constant, about+2; whereas εst values have wide variation from - 6.4 to +31.2 and positively correlate with Na2O, H2O+ and CO2 contents and the Fe3+/Fe2+ ratio. The εNd- Nd/Th, εNd- La/Nb and εNd-Ba/Nb diagrams clearly show that there were significant components of terrigenous sediments in the mantle source of the Lajimiao norite-gabbro complex. It suggests that large amount of sediments had been carried into the mantle by the subducted ancient Qinling sea plate during the Palaeozoic.

北秦岭拉圾庙苏长辉长岩的痕量元素和Sr,Nd同位素地球化学

北秦岭拉圾庙苏长辉长岩体由辉长岩和苏长辉长岩组成。它们分属两个不同的岩浆系列:辉长岩属拉斑系列,TiO_2,Fe^(3+)/Fe^(2+)和REE丰度较高;苏长辉长岩属钙碱系列,低TiO_2,Fe^(3+)/Fe^(2+)和REE,特别是HREE。这说明后者源区较深并受榴辉岩相控制。两种岩石均有与岛弧玄武岩类似的地球化学特征,如较高的Ba,Pb,Sr和较低的Nb、Zr、Ni含量等。拉圾庙辉长岩和苏长辉长岩的Sr,Nd同位素特征与蛇绿岩类似,其ε_(Nd)值恒定在+2左右,ε^(Sr)变化范围较大(-6.4—+31.2),并与Na_2O,H_2O+,CO_2含量和Fe^(3+)/Fe^(2+)有正相关关系。在ε_(Nd—Nb/Th,ε_(Nd)-La/Nb和ε_(Nd)-Ba/Nb图中,清楚显示了该岩体的地幔源含较多的陆源沉积物组分。它说明古秦岭海板块在古生代俯冲时,携带有较多沉积物进入地幔。

秦岭拉鸡庙镁铁质岩体锆石LA-ICP-MS年代学研究

秦岭拉鸡庙镁铁质岩体位于北秦岭南缘,主要由辉长岩(80%)、苏长辉长岩(15%)和少量闪长岩等侵入杂岩组成。对采自该岩体闪长岩的锆石进行阴极发光图像、微区原位LA-ICP-MS微量元素分析和U-Pb定年。CL图像显示这些锆石可以分为两类,一类锆石呈长柱状,具有明显的岩浆生长环带;另一类则呈浑圆状,阴极发光图像复杂,部分颗粒岩浆生长环带较模糊,个别样品外围存在一窄的亮色环边,推测为后期地质事件影响的结果。对26颗锆石核部和生长边进行28次U-Pb同位素分析,获得两组^(206)Pb/^(238)U年龄,分别为973±60Ma和422±7Ma。分析结果显示,所有样品具有高的Th,U,REE含量,明显富集HREE,其Th/U比值普遍高于0.6,表明这些锆石应属于岩浆成因。其中,422±7Ma应该代表拉鸡庙镁铁质岩体的成岩时代,这可能与古生代扬子陆块或者是具有扬子板块属性的微陆块和华北陆块的碰撞有关,该碰撞导致了秦岭洋的闭合;而973±60Ma应为捕获锆石年龄,代表北秦岭早期与Rodinia超大陆拼合有关的岩浆事件。考虑到没有检测到典型的华北克拉通的年龄,推测元古代北秦岭更接近华南板块。