青藏高原南部拉萨地体的变质作用与动力学

拉萨地体位于欧亚板块的最南缘,它在新生代与印度大陆的碰撞形成了青藏高原和喜马拉雅造山带。因此,拉萨地体是揭示青藏高原形成与演化历史的关键之一。拉萨地体中的中、高级变质岩以前被认为是拉萨地体的前寒武纪变质基底。但新近的研究表明,拉萨地体经历了多期和不同类型的变质作用,包括在洋壳俯冲构造体制下发生的新元古代和晚古生代高压变质作用,在陆-陆碰撞环境下发生的早古生代和早中生代中压型变质作用,在洋中脊俯冲过程中发生的晚白垩纪高温/中压变质作用,以及在大陆俯冲带上盘加厚大陆地壳深部发生的两期新生代中压型变质作用。这些变质作用和伴生的岩浆作用表明,拉萨地体经历了从新元古代至新生代的复杂演化过程。(1)北拉萨地体的结晶基底包括新元古代的洋壳岩石,它们很可能是在Rodinia超大陆裂解过程中形成的莫桑比克洋的残余。(2)随着莫桑比克洋的俯冲和东、西冈瓦纳大陆的汇聚,拉萨地体洋壳基底经历了晚新元古代的(～650Ma)的高压变质作用和早古代的(～485Ma)中压型变质作用。这很可能表明北拉萨地体起源于东非造山带的北端。(3)在古特提斯洋向冈瓦纳大陆北缘的俯冲过程中,拉萨地体和羌塘地体经历了中古生代的(～360Ma)岩浆作用。(4)古特提斯洋盆的闭合和南、北拉萨地体的碰撞,导致了晚二叠纪(～260Ma)高压变质带和三叠纪(～220Ma)中压变质带的形成。(5)在新特提斯洋中脊向北的俯冲过程中,拉萨地体经历了晚白垩纪(～90Ma)安第斯型造山作用,形成了高温/中压型变质带和高温的紫苏花岗岩。(6)在早新生代(55～45Ma),印度与欧亚板块的碰撞,导致拉萨地体地壳加厚,形成了中压角闪岩相变质作用和同碰撞岩浆作用。(7)在晚始新世(40～30Ma),随着大陆的继续汇聚,南拉萨地体经历了另一期角闪岩相至麻粒岩相变质作用和深熔作用。拉萨地体的构造演化过程是研究汇聚板块边缘变质作用与动力学的最佳实例。

Metamorphism and Dynamics of the Lhasa Terrane, South Tibet

The Lhasa terrane in southern Tibet has long been accepted as the last crustal block accreted with Eurasia prior to its collision with the northward drifting of Indian continent in the Cenozoic. Thus, the Lhasa terrane is the key to studying the origin and evolution of the Tibetan Plateau. The Lhasa terrane experienced multistage metamorphism, which included the Neoproterozoic and Late Paleozoic HP metamorphism in the oceanic subduction realm, the Early Paleozoic and Early Mesozoic MP metamorphism in the continent-continent collisional zone, the Late Cretaceous HT/MP metamorphism in the mid-oceanic ridge subduction zone, and two stages of Cenozoic MP metamorphism in the thickened crust above the continental subduction zone. These metamorphic events and the associated magmatism suggest that the Lhasa terrane experienced a complex tectonic evolution from the Neoproterozoic to Cenozoic. (1) The crystalline basement of the North Lhasa terrane includes Neoproterozoic oceanic crustal rocks, representing probably the remnants of the Mozambique Ocean derived from the break-up of the Rodinia supercontinent. (2) The oceanic crustal basement of North Lhasa witnessed a Late Cryogenian (~650 Ma) HP metamorphism and an Early Paleozoic (~485 Ma) MP metamorphism in the subduction realm associated with the closure of the Mozambique Ocean and the final amalgamation of eastern and western Gondwana, suggesting that the North Lhasa terrane might have been partly derived from the northern segment of the East African Orogen. (3) The Lhasa and Qiangtang terranes witnessed Middle Paleozoic (~360 Ma) magmatism, suggesting an Andean-type orogeny that was derived from the subduction of the Paleo-Tethys Ocean. (4) The closure of Paleo-Tethys Ocean between the North and South Lhasa terranes and subsequent terrane collision resulted in the formation of Late Permian (~260 Ma) HP metamorphic belt and Triassic (220 Ma) MP metamorphic belt. (5) The South Lhasa terrane experienced Late Cretaceous (~90 Ma) Andean-type orogeny, characterized by the regional HT/MP metamorphism and coeval intrusion of the voluminous Gangdise batholith during the northward subduction of the Neo-Tethyan Ocean. (6) During the Early Cenozoic (55~45 Ma), the continent-continent collisional orogeny resulted in the thickening of the crust of the South Lhasa terrane and the generation of MP amphibolite-facies metamorphism and syn-collisional magmatism. (7) Following the continuous continent convergence, the South Lhasa terrane also experienced MP metamorphism during Late Eocene (40~ 30 Ma). The tectonic imprints from the Lhasa terrane provide excellent examples for understanding geodynamics of metamorphic processes along convergent plate boundaries.