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

新近发现的喜马拉雅西北部始新世超高压变质岩组合，可能反映了喜马拉雅-藏期造山过程中印度陆壳在欧亚大陆之下的有限俯冲。目前，由于喜马拉雅超高压变质岩组合的分布还没有得到详细的填图，变质作用的时代还不清楚，而且超高压岩包与其周围的构造关系在很大程度上是不受约束的，所以这一观测的构造意义仍存在争议。我们对喜马拉雅最新发现的超高压地体--印度拉达克左莫拉里杂岩--进行了综合的年代学(U-Pb、Sm-ND和40Ar/39Ar)、Nd同位素、岩石学和构造研究。到目前为止，超高压诊断矿物柯石英仅在左莫拉里的镁铁质榴辉岩中被发现，可能与印度板块亲缘，也可能不与印度板块亲缘；我们拟议的工作的一个重要部分将是确定超高压组合是否出现在可以明确追溯到印度板块物源的岩性中。如果是这样的话，我们建立超高压变质作用高精度年龄的目标将确定大陆俯冲是发生在印度-欧亚碰撞开始之后--受到独立手段的很好限制--还是像最近提出的那样，发生在大洋内部环境中的碰撞之前。我们还打算更全面地了解始新世-中新世期间这一超高压地形的压力-温度-时间演化，始新世-中新世是造山带历史上最鲜为人知的时期之一。将这些数据与主要构造的详细地图和运动学分析相结合，应该会对喜马拉雅-藏系超高压岩石折返过程产生更好的约束。

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.