The Significance of Ultrahigh-Pressure Metamorphism in the Tso Morari Complex, NW Himalaya

最近在喜马拉雅山脉西北部发现的始新世超高压（UHP）变质组合可能表明喜马拉雅-西藏造山运动期间印度大陆地壳在欧亚大陆下的有限俯冲。目前，这一观测的构造意义是有争议的，因为喜马拉雅超高压变质组合的分布尚未被详细绘制，因为变质作用的年龄是不精确的，因为超高压岩石包体及其周围环境之间的结构关系在很大程度上是不受约束的。迄今为止，在Tso Morari只发现了可能与印度板块有亲缘关系的基性榴辉岩；我们提出的工作的一个重要部分将是确定超高压组合是否发生在可以明确追溯到印度板块起源的岩性中。如果是这样，我们为超高压变质作用建立高精度日期的目标将确定大陆俯冲是发生在印度-欧亚大陆碰撞开始之后——受到独立手段的良好约束——还是发生在碰撞之前，就像最近提出的那样。我们还打算在始新世-中新世区间（造山带历史上最不为人所知的区间之一）对这一超高压地形的压力-温度-时间演化进行更全面的了解。将这些数据与详细的地图和主要构造的运动学分析结合起来，应该能更好地限制喜马拉雅-西藏系统中超高压岩石的挖掘过程。

Recently discovered, Eocene ultrahigh-pressure (UHP) metamorphic assemblages in thenorthwestern Himalayas may indicate the limited subduction of Indian continental crust beneath Eurasia during Himalayan-Tibetan orogenesis. At present, the tectonic significance of this observation is controversial because the distribution of Himalayan UHP metamorphic assemblages has not been mapped in detail, because the age of metamorphism is imprecisely known, and because the structural relationships among UHP rock packages and their surroundings are largely unconstrained. We propose an integrated geochronologic (U-Pb, Sm-Nd, and 40 Ar/39 Ar), Nd isotopic, petrologic, and structural study of the most recently discovered UHP terrain in the Himalayas: the Tso Morari complex of Ladakh, India. Thus far, the UHP-diagnostic mineral coesite has been found at Tso Morari only in mafic eclogite that may or may not be of Indian plate affinity; an important part of our proposed work will be to determine if UHP assemblages occur in lithologies that can be traced unequivocally to a provenance in the Indian plate. If so, our aim of establishing high-precision dates for UHP metamorphism will determine whether or not continental subduction occurred subsequent to the initiation of India-Eurasia collision - well-constrained by independent means - or prior to collision in an intraoceanic environment, as has been proposed recently. We also intend to develop a more comprehensive knowledge of the pressure-temperature- time evolution of this UHP terrain over the Eocene-Miocene interval, one of the most poorly understood intervals in the history of the orogen. The integration of such data with detailed maps and kinematic analysis of major structures should result in much better constraints on the processes responsible for the exhumation of UHP rocks in the Himalayan-Tibetan system.