Metamorphism and tectonic mechanisms of subduction zones

形成在汇聚板块边缘的俯冲带由俯冲岩石圈板块和上部岩石圈板块组成,具有不对称的热结构。俯冲岩石圈板块具有冷的地温梯度,而上部岩石圈板块具有热的地温梯度。俯冲板块的变质作用发生在5~15℃/km地温梯度下,可进一步划分为冷俯冲板块型(5~10℃/km)和热俯冲板块型(10~15℃/km),即西阿尔卑斯型和古巴型。俯冲带上板块的变质作用发生15~50℃/km地温梯度下,可进一步划为冷地壳型(15~25℃/km)和热地壳型(25~50℃/km),统称为科迪勒拉型。冷俯冲板块的变质作用是以大洋和大陆地壳岩石深俯冲到地幔,发生低温/高压及超高压变质作用为特征。所形成的低温/高压和超高压变质岩具有顺时针型P-T轨迹,其折返过程是以近等温或升温降压和部分熔融为特征。热俯冲板块型变质作用发生在年轻板块的正常俯冲和古老板块的平缓俯冲过程中。从大洋岩石圈初始俯冲到成熟俯冲,俯冲板块的地温梯度由热到冷,从热俯冲型转变成冷俯冲型。热俯冲板块的变质岩可具有顺时针型,也可具有逆时针型P-T轨迹,可以发生高温和高压下的部分熔融,形成埃达克质岩浆岩。俯冲带上板块的冷地壳型变质作用发生在构造挤压导致的加厚地壳环境,加厚的下地壳发生高温、高压麻粒岩相和榴辉岩相变质作用,可具有顺时针和逆时针型P-T轨迹。加厚新生下地壳的部分熔融形成埃达克质岩浆和高密度的基性残留体(弧榴辉岩)。热地壳型变质作用发生在构造伸展导致的减薄地壳环境。由于强烈的幔源岩浆增生和软流圈上涌,下地壳发生高温或超高温麻粒岩相变质作用和部分熔融,所形成的变质岩可具有顺时针型或逆时针型P-T轨迹。在岩浆弧加厚地壳的伸展过程中,早先形成的高温和高压变质岩可以叠加超高温变质作用。俯冲带上板块的岩浆弧可能是超高温变质岩形成的最主要构造环境。上板块下地壳的部分熔融可以形成大体积的花岗岩,由此导致新生地壳组成和成分的分异,是大陆地壳生长和成熟的重要机制。大陆碰撞造山带的加厚下地壳具有冷的地温梯度,可以发生高压麻粒岩和榴辉岩相变质作用。这些高级变质岩具有顺时针型P-T轨迹,在其折返过程中叠加中压、高温,甚至超高温变质作用。碰撞造山带下地壳的长期部分熔融可以形成不同成分的壳源花岗岩。;Subduction zones at the convergent plate margins consist of the subducting and upper lithosphere plates, and have an asymmetric thermal structure. The subducting lithosphere plate has a clod thermal gradient (5~15℃/km), and the upper lithosphere plate has a hot thermal gradient (15~50℃/km). Metamorphism in the subducting plate can be divided into the cold subducting plate (5~10℃/km), and hot subducting plate (10~15℃/km) types, called the Western Alpine and Cuban types, respectively. The upper plate metamorphism is generally called the Cordilleran type, and further divided into the cold crust (15~25℃/km) and hot crust (25~50℃/km) types. Metamorphism of the cold subducting plate is characterized by oceanic and continental crustal rocks subducting into the deep mantle, and experiencing ultrahigh-pressure (UHP) metamorphism. The generated UHP rocks have clockwise pressure (P)-temperature (T) paths, and experienced isothermal or temperature-increasing and decompressional metamorphism and partial melting during their exhumation. Metamorphism of the hot subducting plate occurred during normal subduction of young lithosphere and spreading mid-oceanic ridge, and shallow or flat subduction of old lithosphere. From the initial to mature subduction of oceanic lithosphere, the subducting plate has increasing thermal gradient, and transforming from a hot subduction to a clod subduction. Metamorphic rocks in the hot subducting plate have clockwise and counterclockwise P-T paths, and experienced high-temperature (HT) and high-pressure (HP) metamorphism and partial melting to generate adakitic melts during their burial. The cold crust type of metamorphism occurred at a thickened crustal environment that resulted from of the upper plate compression. The thickened lower crustal rocks have clockwise and counterclockwise P-T paths, experienced HT and HP granulite-facies to eclogite-facies metamorphism and partial melting to generate adakitic melts and dense residues of arclogite. The hot crust type of metamorphism occurred at the thinning crustal environment that was induced by the upper plate extension. The lower crustal rocks undergone HT to ultra-high temperature (UHT) granulite-facies metamorphism and partial melting due to accretion of mantle-derived magma, and upper welling of asthenosphere, and with clockwise and counterclockwise P-T paths. During the extension of thickened crust of the upper plate, the early-generated HT and HP metamorphic rocks commonly overprinted UHT metamorphism. Therefore, the magmatic arcs of subduction zone upper plates were probably main tectonic environment of UHT metamorphism. The partial melting of lower crust of upper plate can produces a large volume of granites, and in turn results in differentiation of component and composition of juvenile crusts, forming a key mechanism of growth and maturation of continental crust. The thickened lower crust of continental collisional orogens has a cold thermal gradient, and is characterized by HP granulite-and eclogite-facies metamorphism. The HP rocks have clockwise P-T paths, and commonly overprinted medium-pressure and HT to UHT metamorphism. The prolonged partial melting of the thickened lower crust of collisional orogens resulted in formation of crust-derived granites with variable geochemical compositions.