The study of subduction-zone processes is a key to development of the plate tectonic theory.Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth’s surface and interior.By developing the subduction channel model into continental collision orogens,insights are provided into tectonic processes during continental subduction and its products.The continental crust,composed of felsic to mafic rocks,is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel.Part of the subcontinental lithospheric mantle wedge,composed of peridotite,is offscrapped from its bottom.The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow,resulting in varying extents of metamorphism,with heterogeneous deformation and local anatexis.All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust-and mantle-derived rocks in the subduction channels,resulting in different types of metamorphic rocks now exposed in the same orogens.The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment.The nature of premetamorphic protoliths dictates the type of collisional orogens,the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism.
A continuous flow method,by a combination of thermal conversion elemental analyzer(TC/EA)with isotope ratio mass spectrometry(MS),was developed to determine both H isotope composition and H2O concentration of ultrahigh-pressure(UHP)metamorphic rocks in the Dabie-Sulu orogenic belt.By using the developed step-heating technique,we have studied H2O concentration and H isotope composition of the different forms of water(structural OH and molecular H2O)in garnet.The quantitative measurements of H2O concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical outcrops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust.Therefore,by decompression dehydration at the contact between eclogite and gneiss,the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss.The measured maximum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500and 3500 ppm,respectively,under the peak UHP metamorphic conditions.
