印度—亚洲俯冲带结构——岩浆作用证据

SUBDUCTION OF INDIA BENEATH TIBET:MAGMATISM EVIDENCE

在印度与亚洲大陆碰撞之后 ,两个大陆之间是否存在大陆俯冲是关系到高原地壳加厚、隆升等构造演化模式的重要问题。近 2 0年来以各种地球物理方法为主的深部探测结果揭示了青藏高原的岩石圈结构 ,表明印度向亚洲下部的俯冲是存在的 ,但是其俯冲的规模仍存在争议。不同观点认为印度岩石圈前缘已经到达班公—怒江缝合带的下部约 2 0 0km深度、俯冲在整个西藏岩石圈深部、或者仅仅越过雅鲁藏布江断裂。地热泉He同位素、碰撞后岩浆作用的年代学、岩石学与地球化学研究结果表明冈底斯带与高原北部地区具有相同的岩石圈地幔源区 ,并且存在印度板块在 13～ 2 5Ma之前就俯冲在冈底斯带西部的岩石学和地球化学证据 ,考虑到印度板块的持续向北运动 ,则岩浆作用支持印度岩石圈现今已经达到或者越过班公—怒江缝合带的俯冲模式。

The northward subduction of India plate beneath Tibet followed the onset of India collided with Asia is one of the most fundamental questions to the evolution of Tibetan plateau. But it is still in debate. Recently, increasing geophysical studies favor the model that India has subducted beneath Asia to the depth of 200 km beneath the plateau to the Banggong-Nujiang suture. Age, petrology and geochemistry of the wide-spreading postcollisional magmatism in Tibetan plateau allow us to reveal the composition and processes of the plateau's lithosphere after India collided with Asia around 55 Ma ago. Our results suggest that the Gangdese belt is geochemically a part of the north Tibet (Qiangtang, Kunlun, Hoh Kil). North Tibet has an undisturbed, ancient enriched upper mantle source region. Isotopic geochemistry evidence show that India has subducted beneath Gangdese to the north of Yarlung Zangpo suture 13～25 Ma ago. Taking the northward ongoing motion of India into account, we conclude that India plate should have subducted beneath Tibet to Bangong-Nujiang suture at present-day. This model agree very well with the results of helium geochemistry of spring water from southern Tibet and some geophysical survey got recently.