大陆碰撞带深熔变质与花岗岩成因

Anatectic Metamorphism and Granite Petrogenesis in Continental Collision Zones

大陆碰撞带是地壳岩石发生深熔变质和花岗岩浆作用的重要场所,也是超大陆发生聚合和裂解过程的关键位置.对深熔变质作用的理解有助于揭示地壳分异过程和花岗岩成因等诸多问题,同时对威尔逊旋回过程中超大陆动力学的理解有重要启示.在大陆碰撞过程的不同阶段,其动力体制会从挤压变成拉张,地温梯度会由低变高,从而形成时空分布不同、矿物组合各异的变质岩和岩浆岩.在大陆碰撞和俯冲的挤压阶段,变质温压比值较低,只有硬碰撞引起的地壳加厚产生的高压麻粒岩相变质作用以及深俯冲导致的超高压变质作用,不会发生深熔变质作用.在大陆深俯冲结束之际,板块界面动力体制由挤压变为拉张,受到超高压变质的地壳会以岩片状沿俯冲隧道发生逆冲折返.在大陆碰撞后阶段,一旦碰撞带岩石圈地幔发生减薄,软流圈地幔上涌导致主动张裂,上覆地壳就会在高的地温梯度下发生巴肯型深熔变质作用,从而形成大量的花岗岩.因此,地壳深熔变质和花岗质岩浆的发育程度与岩石圈厚度的变化密切相关.虽然幔源镁铁质岩浆结晶分异作用是大洋俯冲带之上花岗岩形成的常见方式,但是地壳部分熔融才是大陆碰撞带花岗岩浆作用的典型方式.地壳岩石性质是形成不同类型花岗岩的关键,I型和S型花岗岩分别主要来自于变火成岩和变沉积岩的部分熔融,而A型花岗岩则起源于堆晶或残留体的部分熔融.脱水熔融和水化熔融是产生花岗岩浆的两种基本物理化学机制,二者可在同一区域内发生,构成脱水-水化耦合的部分熔融机制.大陆碰撞带作为古板块缝合带,也是构造薄弱带,易于发育大陆主动张裂,这是在碰撞后阶段深部陆壳脱水-水化耦合熔融直至浅部陆壳部分熔融的主要构造机制,结果形成花岗岩-混合岩-麻粒岩高温岩石组合.一般来说,超大陆聚合阶段与碰撞挤压背景下的变质作用相关,而超大陆裂解与主动张裂拉张体制下的变质作用相关.虽然大陆张裂未必都能成功,但是其中夭折张裂在业已汇聚板块边缘最为常见.因此,夭折大陆张裂与陆内深熔变质作用和花岗岩浆作用之间的关系最为密切,这也是正确认识超大陆聚合之后到裂解之前陆内地质过程与构造演化之间关系的关键.

Anatectic metamorphism develops from high-grade metamorphism to crustal anataxis,marking population of extensional tectonism in continental collision zones.It is much more related to breakup than assembly of supercontinents in both space and time.Deciphering the anatectic metamorphism is critical to understand the tectonic evolution of continental collision zones in the late stage.It is substantial to resolve not only petrological problems such as crustal differentiation and granite petrogenesis but also tectonic problems such as supercontinent geodynamics in Wilson cycles.In general,continental collision zones may change their dynamic regime from compression to extension and their geothermal gradient from low to high during their evolution from the early to late stages.This results in different types of metamorphism and magmatism with given occurrences in both time and space.Alpine type metamorphism is caused by compressional heating at low geothermal gradients during subduction of one lithosphere beneath the others,and Barrovian type metamorphism occurs at moderate geothermal gradients due either to compressional heating during collisional thickening of the crust or to extensional decompression during nearly isothermal exhumation of the deeply subducted crust.In contrast,Buchan type metamorphism is caused by extensional heating at high geothermal gradients during continental rifting.Because of the anatectic metamorphism,granites are produced together with migmatites and granulites in the post-collisional stage.Therefore,granitic magmatism is the endmember product of anatectic metamorphism consequential to thinning of the thickened lithospheric mantle.Both processes are closely associated with the change of lithospheric thickness along continental collision zones.Although granites above oceanic subduction zone are mainly produced by fractional crystallization of mantle-derived mafic magmas,partial melting of crustal rocks is the dominant way to generate granitic magmas in continental collision zones.In particular,the source nature of crustal rocks is a key to the composition of granites.Ⅰ-type and S-type granites are primarily derived from partial melting of metaigneous and metasedimentary rocks,respectively.In comparison,A-type granites are derived from partial melting of either cumulates or restites.Dehydration and hydration melting are two fundamental mechanisms to produce granitic magmas.They can occur simultaneously at the same zones,leading to dehydration melting in the deeper crust and hydration melting in the shallower crust.Upwelling of the asthenospheric mantle consequential to thinning of the lithospheric mantle is a common geodynamic mechanism for continental active rifting,giving rise to the dehydration-hydration coupled melting to produce granite-migmatite-granulite associations in continental collision zones.As a consequence,supercontinent assembly is associated with compressional metamorphism during continental collision,whereas supercontinent breakup is associated with extensional metamorphism during active rifting.Continent rifting may not succeed but fail,resulting in anatectic metamorphism and granitic magmatism at convergent plate margins.Therefore,the failed continental rifting is a key process in linking the types of regional metamorphism to the tectonic evolution of continental collision zones from supercontinent assembly to breakup.