Petrological, chronological and geotectonic geological analysis of Precambrian metamorphic rock in Korean Peninsula shows that the remnants ( 〉3.4 Ga) of continental nuclei crust were formed in the Paleoarchean in the Rangrim Massif. In the massif, the main formation ages of continental crust range from 3.2 Ga to 2.5 Ga, its important growing period was 2.8 - 2. 5 Ga. The subsequent expansion period of the Rangrim Massif was 2. 4 - 2. 2 Ga. The division events occurred in 1.85 Ga and in the Late Paleoproterozoic-Early Mesoproterozoic, respectively. Since then the massif was relatively stable. However, the last division of the Rangrim Massif occurred at 793 Ma. 相似文献
Coarse-grained whiteschist, containing the assemblage: garnet+kyanite+phengite+talc+quartz/coesite, is an abundant constituent of the ultrahigh-pressure metamorphic (UHPM) belt in the Kulet region of the Kokchetav massif of Kazakhstan.
Garnet displays prograde compositional zonation, with decreasing spessartine and increasing pyrope components, from core to rim. Cores were recrystallized at T=380°C (inner) to 580°C (outer) at P<10 kbar (garnet–ilmenite geothermometry, margarite+quartz stability), and mantles at T=720–760°C and PH20=34–36 kbar (coesite+graphite stability, phengite geobarometer, KFMASH system reaction equilibria). Textural evidence indicates that rims grew during decompression and cooling, within the Qtz-stability field.
Silica inclusions (quartz and/or coesite) of various textural types within garnets display a systematic zonal distribution. Cores contain abundant inclusions of euhedral quartz (type 1 inclusions). Inner mantle regions contain inclusions of polycrystalline quartz pseudomorphs after coesite (type 2), with minute dusty micro-inclusions of chlorite, and more rarely, talc and kyanite in their cores; intense radial and concentric fractures are well developed in the garnet. Intermediate mantle regions contain bimineralic inclusions with coesite cores and palisade quartz rims (type 3), which are also surrounded by radial fractures. Subhedral inclusions of pure coesite without quartz overgrowths or radial fractures (type 4) occur in the outer part of the mantle. Garnet rims are silica-inclusion-free.
Type 1 inclusions in garnet cores represent the low-P, low-T precursor stage to UHPM recrystallization, and attest to the persistence of low-P assemblages in the coesite-stability field. Coesites in inclusion types 2, 3, and 4 are interpreted to have sequentially crystallized by net transfer reaction (kyanite+talc=garnet+coesite+H2O), and were sequestered within the garnet with progressively decreasing amounts of intragranular aqueous fluid.
During the retrograde evolution of the rock, all three inclusion types diverged from the host garnet P–T path at the coesite–quartz equilibrium, and followed a trajectory parallel to the equilibrium boundary resulting in inclusion overpressure. Coesite in type 2 inclusions suffered rapid intragranular H2O-catalysed transformation to quartz, and ruptured the host garnet at about 600°C (when inclusion P27 kbar, garnet host P9 kbar). Instantaneous decompression to the host garnet P–T path, passed through the kyanite+talc=chlorite+quartz reaction equilibrium, resulting in the dusty micro-assemblage in inclusion cores. Type 3 inclusions suffered a lower volumetric proportion transformation to quartz at the coesite–quartz equilibrium, and finally underwent rupture and decompression when T<400°C, facilitating coesite preservation. Type 4 coesite inclusions are interpreted to have suffered minimal transformation to quartz and proceeded to surface temperature conditions along or near the coesite–quartz equilibrium boundary. 相似文献
Graphitic cherts are interbedded within terrigenous sediments in the Cadomian orogenic belt of end-Proterozoic age. In the Armorican Massif (NW France), the graphitic cherts are of two types: massive cherts essentially composed of quartz (SiO2 > 96%) and with rare sedimentary structures; laminated cherts containing up to 3·4% Al2O3 and 92–98% SiO2. Sedimentary structures observed in the laminated cherts are indicative of a restricted hypersaline tidal or supratidal environment. The origins of both types of chert are to be found in the diagenetic processes of silification of terrigenous and mixed terrigenous-evaporitic facies. These processes, which could be mediated by the presence of organic matter, were controlled by the migration of the freshwater/saltwater mixing zone during periods of relative sea-level change. The proposed diagenetic origin for the cherts places a number of constraints on their use in the establishment of stratigraphic correlations. 相似文献
LA-ICP-MS U-Pb analyses performed on zircon grains from the Lizio granite yielded an emplacement age of 316 ± 6 Ma. Typical S-C structures show that the Lizio granite was emplaced contemporaneously with dextral shearing along the northern branch of the South Armorican Shear Zone and that it was therefore active at that time. 40Ar/39Ar analyses performed on muscovite grains yielded plateau dates ranging between 311.5 and 308.2 Ma. Muscovite chemistry is typical of primary magmatic muscovite, which precludes a late fluids-induced resetting of the K-Ar isotopic system. 40Ar/39Ar dates thus likely correspond to the cooling ages below the argon closure temperature. Considering the uncertainties on the measured ages, we can propose that either the Lizio granite cooled down quickly in less than a million of years or that it remained in a hot environment for several millions of years after its emplacement. This latter scenario could have been sustained by shear heating during dextral shearing along the northern branch of the South Armorican Shear Zone. 相似文献