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.
Plate subduction is an important mechanism for exchanging the mass and energy between the mantle and the crust,and the igneous rocks in subduction zones are the important carriers for studying the recycling of crustal materials and the crust-mantle interaction.This study presents a review of geochronology and geochemistry for postcollisional mafic igneous rocks from the Hong’an-Dabie-Sulu orogens and the southeastern edge of the North China Block.The available results indicate two types of the crust-mantle interaction in the continental subduction zone,which are represented by two types of mafic igneous rocks with distinct geochemical compositions.The first type of rocks exhibit arc-like trace element distribution patterns(i.e.enrichment of LILE,LREE and Pb,but depletion of HFSE)and enriched radiogenic Sr-Nd isotope compositions,whereas the second type of rocks show OIB-like trace element distribution patterns(i.e.enrichment of LILE and LREE,but no depletion of HFSE)and depleted radiogenic Sr-Nd isotope compositions.Both of them have variable zircon O isotope compositions,which are different from those of the normal mantle zircon,and contain residual crustal zircons.These geochemical features indicate that the two types of mafic igneous rocks were originated from the different natures of mantle sources.The mantle source for the second type of rocks would be generated by reaction of the overlying juvenile lithospheric mantle with felsic melts originated from previously subducted oceanic crust,whereas the mantle source for the first type of rocks would be generated by reaction of the overlying ancient lithospheric mantle of the North China Block with felsic melts from subsequently subducted continental crust of the South China Block.Therefore,there exist two types of the crust-mantle interaction in the continental subduction zone,and the postcollisional mafic igneous rocks provide petrological and geochemical records of the slab-mantle interactions in continental collision orogens.
Crustal subduction and continental collision is the core of plate tectonics theory. Understanding the formation and evolution of continental collision orogens is a key to develop the theory of plate tectonics. Different types of subduction zones have been categorized based on the nature of subducted crust. Two types of collisional orogens, i.e. arc-continent and continent-continent collisional orogens, have been recognized based on the nature of collisional blocks and the composition of derivative rocks. Arc-continent collisional orogens contain both ancient and juvenile crustal rocks, and reworking of those rocks at the post-collisional stage generates magmatic rocks with different geochemical compositions. If an orogen is built by collision between two relatively old continental blocks, post-collisional magmatic rocks are only derived from reworking of the old crustal rocks. Collisional orogens undergo reactivation and reworking at action of lithosphere extension, with inheritance not only in the tectonic regime but also in the geochemical compositions of reworked products(i.e., magmatic rocks). In order to unravel basic principles for the evolution of continental tectonics at the post-collisional stages, it is necessary to investigate the reworking of orogenic belts in the post-collisional regime, to recognize physicochemical differences in deep continental collision zones, and to understand petrogenetic links between the nature of subducted crust and post-collisional magmatic rocks. Afterwards we are in a position to build the systematics of continental tectonics and thus to develop the plate tectonics theory.
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.
