Supercontinent cycles, extreme metamorphic processes, and changing fluid regimes

摘要

The solid Earth supercontinent cycle is intimately related to a fluid cycle, and the fluid cycle has undergone secular changes throughout Earth history. We discuss the relationships between the supercontinental cycle with the timing and processes of ultrahigh-pressure (UHP) + ultrahigh-temperature (UHT) metamorphism and the release of fluids within the cycle. Two broad regions of contrasting fluid characteristics – water dominated and CO_2 dominated – are identified, and their relation to the assembly and dispersal of supercontinents is evaluated. Subduction zones at convergent margins are present at all stages of the supercontinent cycle, but their relative length and locations change throughout the cycle. One of the main effects of subduction is to bring oceanic lithosphere, overlying sediments that are not scraped off and accreted, and hydrous minerals deep into the mantle; these protoliths represent the source material for many of the fluids deep in themantle.We propose an integrated model of ridge subduction to explain the dynamics of UHP and UHT orogens. During ridge subduction, a slab window opens and is filled by upwelling mantle material. At moderate levels, near the base of the crust and in the lower domain, the slab window places hot asthenosphere against the base of the overlying plate in a region that would normally be cooled by the slab, leading to a belt of UHT metamorphic rocks at deep levels of the arc and forearc where hydrous fluids are absent and CO_2 dominates. Below this part, the geometry of the slab window is such that hot asthenosphere from the slab window is placed against hot asthenosphere of the overriding plate, so the largest effects are probably related to the loss of cooling by the descending slab, and the deprivation (temporarily) of slab-derived sediments and fluids. Ridge subduction is also associated with considerable disruption of the accretionary wedge, higher sedimentation rates in the trench, and changes in the style of subduction that lead to both greater growth of the wedge and subduction of sediments to greater depths. Many of these sediments ultimately are subducted, and form thin orogenic wedges deep in the mantle, hydrating the mantle and bringing sialic material to depth. Deeply subducted material undergoes high and UHP metamorphism; their extrusion and return to the surface became more common in the Phanerozoic Earth due to the lubricated channels provided by water within the subduction zone.