The formation of continents involves a combination of magmatic and metamorphic processes. These processes become indistinguishable at the crust-mantle interface, where the pressure-temperature（P-T）conditions of（ul...The formation of continents involves a combination of magmatic and metamorphic processes. These processes become indistinguishable at the crust-mantle interface, where the pressure-temperature（P-T）conditions of（ultra） high-temperature granulites and magmatic rocks are similar. Continents grow laterally, by magmatic activity above oceanic subduction zones（high-pressure metamorphic setting）, and vertically by accumulation of mantle-derived magmas at the base of the crust（high-temperature metamorphic setting）. Both events are separated from each other in time; the vertical accretion postdating lateral growth by several tens of millions of years. Fluid inclusion data indicate that during the high-temperature metamorphic episode the granulite lower crust is invaded by large amounts of low H2O-activity fluids including high-density CO2 and concentrated saline solutions（brines）. These fluids are expelled from the lower crust to higher crustal levels at the end of the high-grade metamorphic event. The final amalgamation of supercontinents corresponds to episodes of ultra-high temperature metamorphism involving large-scale accumulation of these low-water activity fluids in the lower crust.This accumulation causes tectonic instability, which together with the heat input from the subcontinental lithospheric mantle, leads to the disruption of supercontinents. Thus, the fragmentation of a supercontinent is already programmed at the time of its amalgamation.展开更多
Until the middle of the 20th century,the continental crust was considered to be dominantly granitic.This hypothesis was revised after the Second World War when several new studies led to the realization that the conti...Until the middle of the 20th century,the continental crust was considered to be dominantly granitic.This hypothesis was revised after the Second World War when several new studies led to the realization that the continental crust is dominantly made of metamorphic rocks.Magmatic rocks were emplaced at peak metamorphic conditions in domains,which can be defined by geophysical discontinuities.Low to medium-grade metamorphic rocks constitute the upper crust,granitic migmatites and intrusive granites occur in the middle crust,and the lower crust,situated between the Conrad and Moho discontinuities,comprises charnockites and granulites.The continental crust acquired its final structure during metamorphic episodes associated with mantle upwelling,which mostly occurred in supercontinents prior to their disruption,during which the base of the crust experienced ultrahigh temperatures(>1000℃,ultrahigh temperature granulite-facies metamorphism).Heat is provided by underplating of mantle-derived mafic magmas,as well as by a massive influx of low H_(2)O activity mantle fluids,i.e.high-density CO_(2) and highsalinity brines.These fluids are initially stored in ultrahigh temperature domains,and subsequently infiltrate the lower crust,where they generate anhydrous granulite mineral assemblages.The brines can reach upper crustal levels,possibly even the surface,along major shear zones,where granitoids are generated through brine streaming in addition to those formed by dehydration melting in upper crustal levels.展开更多
文摘The formation of continents involves a combination of magmatic and metamorphic processes. These processes become indistinguishable at the crust-mantle interface, where the pressure-temperature（P-T）conditions of（ultra） high-temperature granulites and magmatic rocks are similar. Continents grow laterally, by magmatic activity above oceanic subduction zones（high-pressure metamorphic setting）, and vertically by accumulation of mantle-derived magmas at the base of the crust（high-temperature metamorphic setting）. Both events are separated from each other in time; the vertical accretion postdating lateral growth by several tens of millions of years. Fluid inclusion data indicate that during the high-temperature metamorphic episode the granulite lower crust is invaded by large amounts of low H2O-activity fluids including high-density CO2 and concentrated saline solutions（brines）. These fluids are expelled from the lower crust to higher crustal levels at the end of the high-grade metamorphic event. The final amalgamation of supercontinents corresponds to episodes of ultra-high temperature metamorphism involving large-scale accumulation of these low-water activity fluids in the lower crust.This accumulation causes tectonic instability, which together with the heat input from the subcontinental lithospheric mantle, leads to the disruption of supercontinents. Thus, the fragmentation of a supercontinent is already programmed at the time of its amalgamation.
文摘Until the middle of the 20th century,the continental crust was considered to be dominantly granitic.This hypothesis was revised after the Second World War when several new studies led to the realization that the continental crust is dominantly made of metamorphic rocks.Magmatic rocks were emplaced at peak metamorphic conditions in domains,which can be defined by geophysical discontinuities.Low to medium-grade metamorphic rocks constitute the upper crust,granitic migmatites and intrusive granites occur in the middle crust,and the lower crust,situated between the Conrad and Moho discontinuities,comprises charnockites and granulites.The continental crust acquired its final structure during metamorphic episodes associated with mantle upwelling,which mostly occurred in supercontinents prior to their disruption,during which the base of the crust experienced ultrahigh temperatures(>1000℃,ultrahigh temperature granulite-facies metamorphism).Heat is provided by underplating of mantle-derived mafic magmas,as well as by a massive influx of low H_(2)O activity mantle fluids,i.e.high-density CO_(2) and highsalinity brines.These fluids are initially stored in ultrahigh temperature domains,and subsequently infiltrate the lower crust,where they generate anhydrous granulite mineral assemblages.The brines can reach upper crustal levels,possibly even the surface,along major shear zones,where granitoids are generated through brine streaming in addition to those formed by dehydration melting in upper crustal levels.
