塔里木及西南天山深源捕掳体的含水性：岩石圈流变学性质及其意义

How and why intra-plate strain becomes focused within discrete zones is a basic issue in geodynamics. Due to the remote effect of the Indo-Euasia collision, the Paleozoic orogens surrounding the Tarim block have been intensely deformed since the late Cenozoic, contrasting with the Tarim block itself that remains relatively stable. Despite the areas have been extensively studied, the controversy still remains as to the mechanism of the strain partitioning. One view is that the strain has been largely and preferentially absorbed by the pre-existing faults in the Paleozoic orogens, and little stress was transmitted across them into the core of the Tarim block. The other view, on the contrast, ascribes it to the stronger strength of the Tarim lithosphere than its surroundings. Therefore, the rheology of the deep lithosphere and its distribution hold the key to addressing the issue, and water is one of the key factors affecting rheological strengths. The project is to obtain water contents of the constituent minerals of deep-sourced xenoliths (peridotite, granulite and pyroxenite) from the Tarim and the southwestern Tianshan, to clarify the rheology of the deep lithosphere and its profile and hence to discuss its relationship with strain partitioning.

板内应变如何和为何集中于一些不连续带中是地球动力学研究中的基础问题之一。受印度-欧亚碰撞远程效应的影响，晚新生代以来，塔里木周缘古生代造山带遭受了强烈的变形，而塔里木却保持相对稳定。该区域研究程度虽高，但对应变分配机制的认识还存在争议。一种观点认为应变主要被造山带中先存的断层再活化所吸收，应力很难传达到塔里木陆块内部；另一种观点则认为塔里木岩石圈的力学性质比周缘造山带的强。因此，深入了解岩石圈流变学强度和分布是解决问题的关键，而水又是影响岩石圈流变学性质的关键因素之一。本项目拟通过对塔里木和西南天山深源捕掳体（橄榄岩、麻粒岩和辉石岩）中主要组成矿物水含量的测定，来了解深部岩石圈的有效粘滞度及其分布特征，探讨应变分配与流变学性质之间的关系。

晚新生代以来，塔里木周缘古生代造山带遭受了强烈的变形而塔里木却保持相对稳定。目前对应变分配机制的认识还存在争议，需要深入了解岩石圈流变学强度和分布，而水又是影响岩石圈流变学性质的关键因素之一。本项目对塔里木（西克尔和皮羌）和西南天山（托云）的橄榄岩和麻粒岩包体进行了系统的矿物水含量和地球化学的研究。橄榄岩的水含量的红外分析显示岩石圈地幔具有明显的横向变化：天山造山带具有高的水含量165±17ppm，与天山结合部的塔里木北缘具有低的水含量17±5ppm，而塔里木内部具有中等高的水含量118±20ppm。麻粒岩包体的水含量显示塔里木和天山下地壳具有类似低的水含量,分别为264-361ppm和243-314ppm。结合本研究得到的地幔水含量和温度范围，在相同差应力条件下，天山造山带地幔的有效粘滞度（~6×1017 Pa·s-1）比塔里木（~3×1020 Pa·s-1）低约三个数量级，塔里木内部和边缘具有近似的有效粘滞度。由于麻粒岩的主要组成斜长石具有低的水含量，其变化对下地壳的流变学强度影响有限。橄榄岩中矿物都具有类似的主量元素组成，且变化小。橄榄石的Mg值和尖晶石的Cr#都指示经历过非常小程度的部分熔融，水与橄榄石的Mg#值之间存在微弱的正相关性。单斜辉石微量元素显示不同程度的LREE富集，西克尔样品整体要比皮羌更富集LREE，但与矿物水含量之间也不存在明显的相关性。西克尔和皮羌的水含量明显差异可能反映了地幔柱相关的大火成岩省过程对塔里木内部岩石圈地幔的水化作用，而非之前认为的脱水。进一步，根据西克尔麻粒岩长石的水含量和水在长石与熔体之间的分配系数，粗略估计LIPs原始熔体的水含量约为4.2w.t.%。