西藏南部印度-亚洲碰撞带岩石圈:岩石学-地球化学约束

On the Lithosphere of Indo-Asia Collision Zone in Southern Tibet:Petrological and Geochemical Constraints

拟以岩石学和地球化学的研究为基础,结合地球物理与构造地质学的研究成果,从一个侧面探讨青藏高原岩石圈、特别是印度-亚洲主碰撞带岩石圈结构、组成及今后进一步的研究方向.印度-亚洲主碰撞带具有青藏高原最厚的地壳,由初生地壳及再循环地壳两类不同性质的地壳构成;青藏巨厚地壳是由于构造增厚及地幔物质注入(通过岩浆作用)增厚两种机制形成的.碰撞以来藏南地壳加厚主要发生在约50～25Ma期间.青藏岩石圈地幔在地球化学和岩石学上是不均一的,至少存在3种地球化学端元:(1)新特提斯大洋岩石圈端元;(2)印度陆下岩石圈端元;(3)新特提斯闭合前青藏原有的岩石圈端元.在青藏高原还发现了一批壳幔深源岩石包体及高压-超高压矿物,对于认识青藏深部有重要的意义.可以识别出青藏高原现今存在3种岩石圈结构类型:第1种,增厚的岩石圈(帕米尔型);第2种,减薄的岩石圈(冈底斯型);第3种,加厚-减薄-再加厚的岩石圈(羌塘型).这3类岩石圈是否在时间上具有先后顺序,尚无明确的证据,需要在今后加以注意.研究表明,沿冈底斯带后碰撞钾质-超钾质火山活动,可能与新特提斯洋俯冲板片在后碰撞阶段的断离及印度大陆岩石圈向青藏的持续俯冲作用有关,但西段、中段与东段的动力学机制不相同.在青藏高原北部地区(羌塘、可可西里等地区),后碰撞钾质-超钾质火山活动,可能与波状外向扩展式的软流圈上隆引起的减压熔融有关.在高原北缘西昆仑、玉门等地区,其形成机制可能为大规模走滑断层引起的减压熔融.青藏高原后碰撞火成活动具有明显而有规律的时空迁移.同碰撞的林子宗火山活动在65Ma左右始于冈底斯南部,标志印度-亚洲大陆碰撞的开始.于45Ma左右火山活动向北迁移到羌塘-"三江"北段,开始了后碰撞火山活动;然后自内向外迁移,即北向可可西里、南向冈底斯(在冈底斯内部又自西向东)、东向西秦岭迁移;最后(6Ma以来),再分别向高原的西北、东北、东南三隅迁移.结合已有地球物理资料,一种可能的解释是它可能暗示由印度和亚洲大陆板块碰撞所诱发的深部物质(如中-下地壳、软流圈地幔物质)流动.

This paper discusses the composition and structure of the lithosphere of the Tibetan Plateau, especially of the main collision zone in southern Tibet upon the basis of petrological and geochemical studies, combining with geological and geophysical researches. The Indo-Asia main collision zone possess the thickest crust of the Tibetan Plateau, which consists of two dif- ferent types of the crust, juvenile crust and recycled crust. The thicken crust formed by two mechanism, both structural thickening and the inputs of the mantle materials into the crust via magmatism. The lithospheric mantle underneath the Tibetan Plateau is inhomogeneous in petrology and geochemistry. At least three mantle isotopic reservoirs may be distinguished from the heterogeneity of Tibetan magmatic sources： （1） a Neo-Tethyan, Indian Ocean （DUPAL-like） component, （2） an EM2-rich Indian subcontinental lithospheric mantle component, and （3） a primordial Tibetan lithospheric mantle component generated prior to the India-Asia collision, which can also be considered the pre-（India-Asia） collisional Asian lithospheric mantle component. Also, some mantle-and lower crust-derived xenoliths carried by volcanies, and the outcrops of high pressure-ultrahigh pressure mineral assemblages have been found on the Tibetan Plateau. Three structural types of the lithosphere of the Tibetan Plateau can be distinguished, i. e. , thickened lithosphere （Pamirs-type）, thinned lithosphere （Gangdese-type） and thickened-thinned-rethickened lithosphere （Qiangtang-type）. The temporal relations among these three structural types of the lithosphere, however, is unclear so far. The post- collisional potassic-ultrapotassic volcanism along the Gangdese was presumably related to slab break-off of the subducting Neo-Tethyan plate and the subduction of Indian continental lithosphere beneath the Lhasa Block, with different mechanism in western, middle and eastern segments, respectively. In the northern part of the plateau（the Qiangtang,the Hoh Nil, etc. ）, however, volcanism could be related to a wavelike outward propagation of upwelling asthenosphere. In the northern margin of the plateau （western Kunlun, Yumen, etc. ）, volcanism might be as a result of de compressive melting induced by large-scale strike-slip faulting. Migration of collisional and posteollisional volcanism with time shows a highly distinctive pattern. Initially, as an initial response to the India-Asia collision, igneous activity migrated northward be tween ca. 65 and 45 Ma, away from the Tsangpo collision suture. Between ca. 45 and 6 Ma, volcanic activity migrated outward from the plateau interior, implying wavelike outward propagation of upwelling asthenosphere. A third stage, still in progress, is marked by the migration of activity to northwestern, northeast-eastern, and southeastern peripheral regions of the plateau between 6 Ma and the present. Overall, such a highly distinctive pattern of activity can be interpreted to reflect lateral asthenospheric mantle flow or lower crust flow induced by the approach, and ensuing collision, of relatively thick （India and Eurasia） continental plates.