松树沟橄榄岩及铬铁矿铂族元素和Re-Os同位素特征与意义

Peridotite that occurs in many orogenic belt recorded HP/UHP metamorphism, and the dynamics and evolution of the mantle before/during subduction processes. Thus, it has a great importance to understand the petrogenesis, protolith, evolution and metasomatism of the mantle. The investigation of PGE chemistry and Re-Os isotope systematics from the peridotite, particularly from the chromite, is so effective and important to solve these problems. Songshugou mylonization peridotites have a great significance to study Qinling orogen because of its special tectonic features. However, the petrogenesis and tectonic attribution of the peridotite massif is still controversial. Thus, we interested to study the Songshugou mylonitize peridotite massif. On the basis of field geological investigation and petrographical observation, we described and classified the peridotites, analyzed major and trace elements composition and mineral compositions, discriminate the chromite morphology and composition from different types of rocks. This study is mainly focused on the analysis of PGE content and Re-Os isotopic composition from different types of representative peridotites and chromites samples. Integration of the above study and results, confirm the petrogenesis and tectonic evolution of Songshugou peridotites, and as well as metasomatism processes related to melt/fluid during subuduction. Finally, we construct the formation and geological evolution of North Qinling Orogen.

造山带橄榄岩不仅能够记录与俯冲相关的高压/超高压变质作用，还可能记录了参与俯冲之前的地幔动力学过程。对它们的岩石成因、原岩属性、演化历史以及交代作用的研究具有重要意义。而对这些橄榄岩，尤其是对其中铬铁矿的铂族元素（PGE）和Re-Os同位素研究是解决这些问题的重要手段。松树沟橄榄岩特殊的构造位置使其成为探讨秦岭造山带演化的关键，但其成因、属性、是否经历了俯冲作用等问题仍然存在争议。因此，本项目以该岩体为研究对象，在野外考察和岩相学观察基础上，对橄榄岩进行详细分类，测定不同类型橄榄岩的矿物成分和全岩主、微量元素含量。确定不同岩石类型中铬铁矿的形态结构和成分差别。重点是选择具有代表性的橄榄岩样品及其中的铬铁矿进行PGE和Re-Os同位素分析。结合这些分析结果探讨松树沟橄榄岩的成因和经历的演化过程，以及在该过程中可能存在的熔体/流体的交代作用。为进一步理解北秦岭的形成历史和早期构造演化提供制约。

本项目在松树沟橄榄岩的属性、形成时代和构造演化等方面取得了以下重要成果：1、松树沟纯橄岩经历了高程度的部分熔融。中、细粒纯橄岩为粗粒纯橄岩不同程度糜棱岩化的结果。2、岩相学和地球化学研究表明方辉橄榄岩是纯橄岩与富Si熔体反应的交代成因。3、对橄榄岩和铬铁矿的Re-Os同位素研究表明松树沟橄榄岩的形成时代为新元古代。细粒纯橄岩具有相对高的Al2O3和CaO的含量、低的Os含量，相对高的Pd含量和187Os/188Os比值进一步支持了纯橄岩受到了后期的熔体/流体改造。4、与Oman和troodos蛇绿岩对比，结合围岩的变质基性岩的研究结果表明松树沟橄榄岩的属性为大洋岩石圈残片。5、松树沟橄榄岩中富镁角闪石具有切穿橄榄石和辉石的特征，说明是在后期的变质过程中形成。对橄榄岩矿物组合的P-T模拟显示其稳定域为680ºC-610ºC，与角闪岩相变质温度相似。6、对不同类型铬铁矿（部分蚀变贫铬铬铁矿、筛状富铬铬铁矿和铁铬铁矿、铬磁铁矿）的主、微量元素研究表明，这些铬铁矿的形成温度为 >600ºC-550ºC，也与角闪岩相变质条件相同。以上研究进一步支持了秦岭群在早古生代经历了整体俯冲，而松树沟的这些橄榄岩记录了大洋岩石圈形成到角闪岩相退变质的全过程。