【作者】 董锋；

【导师】 李曙光；

【作者基本信息】 中国科学技术大学 ， 地球化学， 2006， 硕士

【摘要】 大别—苏鲁造山带出露有大量的超高压榴辉岩和片麻岩，前人已经对出露地表的超高压变质岩全岩进行了大量的Pb同位素地球化学研究，揭示了南、北大别带分别是折返的俯冲上、下地壳。苏鲁带科学钻探的实施使我们获得了研究超高压变质带垂向Pb同位素的机会，从而对超高压变质带的岩石圈结构和超高压变质岩折返机制给予进一步制约。北淮阳地区出露的晚元古代辉石辉长岩被认为是扬子板块未参与深俯冲的部分，通过对他们的研究可以考察超高压变质岩原岩的地球化学性质。大别—苏鲁造山带内出露有大量的中生代岩浆岩，其中碰撞后基性—超基性岩浆岩反映了造山带上地幔的地球化学性质，可以帮助我们了解造山带深部岩石圈的性质，及深俯冲陆壳是否再循环进入地幔并对上地幔组成产生影响给予制约。本文主要对上述三种类型岩石进行Pb同位素地球化学研究。 1 大别山晚元古代镁铁质岩浆岩的初始Pb同位素组成 大别山榴辉岩的原岩主要是晚元古代镁铁质岩浆岩，了解其原岩初始Pb同位素组成是理解榴辉岩Pb同位素特征的基础。本文对北淮阳的晚元古代城冲和山七辉长岩体进行了全岩Pb同位素研究，它们是扬子陆块北缘未卷入深俯冲的晚元古镁铁质岩侵入体。Pb同位素研究表明，城冲和山七的初始Pb同位素比值(²⁰⁶Pb／²⁰⁴Pb_t=750Ma=16.68～17.09，²⁰⁷Pb／²⁰⁴Pb_t=750Ma=15.40～15.45，²⁰⁸pb／²⁰⁴Pb_t=750Ma=37.72～37.85)落在²⁰⁷Pb／²⁰⁴Pb-²⁰⁶Pb／²⁰⁴Pb图Pb同位素地幔演化线附近，略偏上，显示其Pb同位素主要源自800Ma左右的地幔并被上地壳物质混染。它的高²⁰⁸Pb／²⁰⁴pb表明混染地壳物质为具有高Th／U的古老地壳物质。 2 中国苏北大陆科学钻探100—2000m超高压变质岩普通Pb和Sr同位素研究—俯冲陆壳内脱耦的证据 本文首次对中国大陆科学钻探工程100—2000m榴辉岩中的绿辉石和片麻岩中的长石进行了普通Pb和Sr同位素研究。100～800m的样品Pb同位素比值(²⁰⁶Pb／²⁰⁴Pb=16.82-17.38，²⁰⁷Pb／²⁰⁴Pb=15.37-15.49，²⁰⁸Pb／^<sub>204Pb=37.21-37.72)较高，1600～2000m样品Pb同位素比值(²⁰⁶Pb／²⁰⁴Pb=16.05-16.47，²⁰⁷Pb／²⁰⁴Pb=15.22-15.29，²⁰⁸Pb／²⁰⁴Pb=36.68-37.48)较低。上，下岩片各自Pb同位素组成的均一性及相互之间的显著差异说明它们为两个不同岩片。800—1600m样品Pb同位素比值(²⁰⁶Pb／²⁰⁴Pb=16.27-16.73，²⁰⁷Pb／²⁰⁴Pb=15.27-15.38，²⁰⁸Pb／²⁰⁴Pb=36.73-更多还原

【Abstract】 Abundant ultra-high pressure metamorphic （UHPM） rocks, such as coesite-bearing eclogites and gneisses, occur in the Dabie-Sulu orogen. Based on Pb isotope geochemical study of the UHPM rocks exposed on the surface, the South and North Dabie zones are considered to be exhumed slices of the subducted upper and lower crust, respectively. This thesis will focus on the Pb isotope geochemistry of three kinds of rocks. First, the Chinese Continental Scientific Drilling project on the Su-Lu UHPM belt provides an opportunity to investigate the Pb isotopic composition of the UHPM belt along vertical directions, providing constraints on exhumation mechanism of the UHPM rocks. Second, the Chengchong and Shanqi gabbros in the North Huaiyang area are not involved in continental subduction and related UHP metamorphism, thus provide an opportunity to investigate the geochemical characters of protoliths of the UHPM rocks. Third, Mesozoic mafic intrusive rocks occur in Dabie-Sulu orogen widely, carrying geochemical information from their mantle source. Therefore, they will help us understand the geochemical characteristics of the lithospheric mantle underneath the Dabie orogen and test whether deeply subducted continental crust can be recycled into the upper mantle and the influence of the recycling on the composition of upper mantle.1 Initial Pb composition of the Neoproterozoic mafic intrusive rocks in the Dabie orogenTo understand the variation of Pb isotopic composition of UHPM eclgite, it is important to know Pb isotopic composition of the Neoproterozoic mafic igneous rocks in the Dabie orogen. Two Neoproterozoic mafic instrusions in the North Huaiyang, the Chengchong and Shanqi gabbro, were not involved in deep subduction. Initial Pb composition of the Chengchong and Shanqi gabbros (²⁰⁶Pb/²⁰⁴Pb_t=750Ma= 16.68～17.09, ²⁰⁷Pb/²⁰⁴Pb_t=750Ma= 15.40～15.45, ²⁰⁸Pb/²⁰⁴Pb_t=750Ma = 37.72～37.85) are close or a little above the Mantle line in Zartman and Doe’ Pb isotope evolution curve（Zartman and Doe, 1981）. This suggests that their source was mainly from mantle and contaminated by crustal material. The high ²⁰⁸Pb/²⁰⁴Pb character of the更多还原