不同板块构造背景下灰岩地球化学及Nd同位素对比研究：以青藏高原为例

The limestones are the best exposed sedimentary rocks as the sandstones and mudstones. Since the foundation of the plate tectonic theory, the international community has elucidated the geochemistry and Nd isotopes of the sandstones and mudstones under various plate tectonic settings, and has established well-known and generally-accepted geochemical and Nd isotopic models to distinguish the plate tectonic environments under that these sandstones and mudstones deposited. Moreover, the community has even well worked on the geochemistry of the cherts that are poorly exposed generally and has established quite mature geochemical models for the discrimination of the depositional-tectonic environments. However, a comprehensive study of the geochemistry and Nd isotopes of the limestones that formed under different plate tectonic setting is still lacking. A literature investigation indicates it is possible to set up geochemical and Nd isotopical models of the limestones for distinguishing the plate tectonic environments where the limestones deposited. In the Tibetan Plateau of western China, the limestones have been discovered to form at least six various plate tectonic environments, including foreland basin, forearc basin, backarc rift basin, passive continental margin, oceanic island, and continental interior. We will conduct systematic analyses on the major elements, trace elements (including the rare earth elements), and the Sm-Nd isotopes of these limestones collected from the Tibetan Plateau, in an attempt to elucadite the geochemistry and Nd isotopes of the limestones depositing in various plate tectonic environments, and establish the geochemical and Nd isotopical models of the limestones for discrimination of plate tectonic settings.

灰岩和砂岩、泥质岩石同为地表出露最广泛的沉积岩。自板块构造理论问世以来，国际上早已对形成于不同板块构造背景下的砂岩和泥质岩石进行了广泛和深入的地球化学-同位素对比研究，由此产生了获得广泛公认的砂岩和泥质岩石形成板块构造背景的主元素-微量元素地球化学判别模式；甚至对出露相对较少的硅岩，也形成了相当成熟的板块构造背景地球化学判别图解。然而，迄今仍缺乏对不同板块构造背景下灰岩地球化学对比研究。在青藏高原，可至少分辨出前陆盆地、弧前盆地、弧后裂谷盆地、被动大陆边缘、洋岛、内陆盆地等6种主要板块构造背景下产出的灰岩。拟通过对这6种灰岩系统采样并进行主元素、微量元素（含稀土元素）和Sm-Nd等同位素的对比研究，结合微观结构和副矿物组分和含量的研究，探讨不同板块构造背景下灰岩地球化学-同位素特征及其控制因素，建立灰岩形成板块构造背景的主元素-微量元素-Nd同位素判别模式/图解，争取填补国际上研究空白。

灰岩为地表出露最广泛的沉积岩之一，但地学界缺乏对不同板块构造背景下灰岩地球化学对比研究和灰岩板块构造背景地球化学判别图解。本项目系统对东羌塘雁石坪侏罗纪前陆盆地灰岩、日喀则弧前盆地日喀则群底部灰岩、措勤盆地改则中白垩弧后裂谷型灰岩、喜马拉雅地体冈巴晚白垩被动大陆边缘型灰岩、仁布洋岛灰岩或/和巴颜喀拉中二叠洋岛灰岩、可可西里第三纪内陆盆地风火山群灰岩夹层等6种的不同板块构造背景下形成的灰岩进行了系统的剖面研究和采样；同时加强了对青藏高原灰岩形成的大地构造环境进行了深入研究，新发现了洋底高原型灰岩，并对灰岩沉积的大地构造背景进行了进一步深化研究。对所有灰岩样品进行了切片观察，研究其微观结构和副矿物组分和含量，并选择样品进行了全面系统的主元素、微量元素、Nd同位素测试。同时对全球大洋盆地中沉积的灰岩沉积环境进行了详细的检索和追踪研究并系统检索了已有地球化学数据，建立了全球不同构造环境灰岩地球化学数据库。深入研究了不同环境下灰岩各种元素、特征元素比值以及同位素之间的相关性，运用地球化学指标，建立了灰岩形成板块构造背景的主元素-微量元素判别模式/图解，获得了不同板块构造背景下灰岩的地球化学均值，填补了国际研究空白。