印度大陆俯冲前缘的可能位置:来自藏南和藏东活动热泉气体He同位素约束

Possible Location for Underthrusting Front of the Indus Continent: Constraints from Helium Isotope of the Geothermal Gas in southern Tibet and eastern Tibet

为了获得西藏碰撞造山带的深部信息 ,并约束印度大陆俯冲碰撞过程 ,系统研究了藏南地区活动热泉的氦同位素。本文资料和前人成果表明 ,青藏高原热泉气体 He同位素组成变化极大 ,R/ Ra介于 0 .0 17～5 .38之间 ,可分为两个不同的 He变化域 ,即幔源 He域 (R/ Ra:0 .11～ 5 .38;R为样品的 3He/ 4He值 ,Ra为空气的 3He/ 4He值 ,下同 )和壳源 He域 (R/ Ra:0 .0 17～ 0 .0 72 )。前者主要分布于东构造结附近的腾冲热海 (R/ Ra:0 .4 0～ 5 .38)、西构造结附近的狮泉河热田 (R/ Ra:0 .2 7～ 0 .30 )以及 89°E以东的拉萨热水活动带 (R/ Ra:0 .11～ 0 .98) ,后者集中于 89°E以西的昂仁热水活动区 (R/ Ra:0 .0 17～ 0 .0 72 )。东西构造结附近的热泉受走滑断裂控制 ,有不超过 5 0 %的幔源气贡献 ,而高原腹地内的热水活动则受 SN向裂谷控制 ,幔源 He域与壳源 He域以 89°E为界 ,分别横跨雅江缝合带。热泉氦同位素与深部地球物理探测资料综合分析表明 ,高原腹地的现代热水活动主要受上地壳内部成片出现的岩浆房或部分熔融层驱动 ,但 89°E以东地区有来自幔源熔浆的热和物质 (He气 )贡献。本文提出 ,印度大陆板块总体呈斜向俯冲态势 ,在 89°E以西 ,向北北东方向缓角度俯冲的板片可能已越过雅江缝合带 ,而在89°E以?

In order to get deep information on the Tibetan collisional orogen, and to constrain the orogeny processes during Indus—Asian continental collision, the isotopic geochemistry of helium emitted from geothermal springs in the southern Tibetan was systemically studied. Our researches and previously-published data show that the isotope composition of helium have a wide range (R/Ra = 0.017~5.38; R is 3He/4He value of sample, Ra is 3He/4He value of air), which defines two kinds of principal helium variation domains, i.e., mantle helium domain (R/Ra = 0.11~5.38) and crustal helium domain (R/Ra = 0.017~0.072). The former mainly distributes in the Tengchong Rehai geothermal field (R/Ra = 0.40~5.38) near the eastern Himalayan syntax, the Shiquanhe geothermal field (R/Ra = 0.27~0.30) near the western Himalayan syntax, and the Lhasa hydrothermal activity zone (R/Ra = 0.11~0.98) to the east of longitude 89°E. The latter mainly present in the Ngamring hydrothermal activity zone (R/Ra = 0.017~0.072) to the west of 89°E. Near the eastern and western syntaxes, where the hydrothermalism is controlled by large-scale strike-slip faults, there are <50% mantle-derive helium's contribution in geothermal helium. While in the hinterland of the plateau, where NS-trending rift control the development of hydrothermal zone, the value of R/Ra has obvious variation in east—west direction rather than in south—north direction. Both two helium domains are bounded by longitude 89°E in EW-direction, but all bestride the Yarlung Zangbu suture in SN-direction. Synthetic analysis on the helium isotopic data and available geophysical data shows that modern hydrothermal activity was mainly driven by magmatic chambers or partially molten layers occurring as patches in the upper crust, but the contribution of mantle heat and mass (helium) to spring gases has been recognized to the east of 89°E. As a consequence, we proposed that the Indus continent is underthresting obliquely northwards as a whole with an inconsistent underthrusting front, which shows different slab styles in two sides of 89°E. To the west of 89°E, the continental slab downgoing towards NNE in a gentle slope has probably passed the Yarlung Zangbu suture and reached to the Bangong—Nujing suture; while to the east of 89°E, underthrusting slab front, likely to tear down along the Yadong—Gulou rift valley, is downgoing in a steep slope, thus has not spanned the Yarlung Zangbu suture.