Wei Chunjing

Thermodynamic modelling of the phengite geobarometry

Using the updated models of activity-composition relations for solid solutions, P-T pseudosections and Si isopleths of phengite in the system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O) are calculated with the software THERMOCALC 323 for three bulk compositions of the average, Al-rich and K-rich metapelites. The Si isopleths of phengite in the calculated pseudosections are highly dependent on mineral assemblages. In the chlorite-and biotite-bearing di-and trivariant fields, and in the garnet + kyanite-bearing trivariant fields, the phengite can be used as a good geobarometer. But in the carpholite-present and chlorite-absent fields, the Si contents of phengite mainly indicate variance of temperature. For a given Si value, the phengite in kyanite-bearing assemblages shows higher pressures than in the K-feldspar-bearing assemblages. Addition of Na2O into the KFMASH leads to reduction of stability of the AFM phases. The Si isopleths of phengite in the mineral assemblages containing only one Na-phase are similar to those in the corresponding KFMASH assemblages. But in the assemblages with two or more Na-phases, the phengite Si isopleths differ remarkably from those in the KFMASH.

Implicaations of micro-compositions of garnet and biotite from high-grade meta-pelites

Abstract Based on detailed studies on the compositional zoning of garnet and biotite in pelitic rocks from the Jingshan group of granulite facies in north Jiaodong, P-T pseudosections with isopleths of Fe/(Fe+Mg) in garnet and biotite were calculated in the KF-MASH system for two representative rocks of sillimanite-garne-cordierite-biotite gneiss (Vbi/Vg>1) and sillimanite-garnet gneiss (Vbi/Vg 1 can nearly preserve their peak compositions, and biotites in garnet-rich rocks with Vbi/Vg<0.2 ...

Characteristics and evolution of the lower precambrian basement of the North China platform

Abstract The Lower Precambrian basement of the North China platform is composed of Lower, Upper Archaean and Lower Proterozoic metamorphic complexes. The Lower Archaean metamorphic complex, distributed mainly in the North Border sub-domain, consists of granulite, gneiss, amphibolite and pyroxenite. The metamorphic grade is up to granulite facies. The Upper Archaean metamorphic complex, distributed extensively in the whole region, is composed of gneiss, leptynite and amphibolite with minor granulite and magnetite quartzite. The metamorphic grade is predominated by amphibolite facies with minor granulite and greenschist facies locally. The Lower Proterozoic metamorphic complexes occur mainly in some peripheral and internal downwarped belts of the Archaean craton. The rock formations may clearly be divided into eugeosynclinal and miogeosynclinal facies. The metamorphic grade is chielfly of greenschist facies. According to the studies of some geological characteristics, the evolutional history of the Precambrian basement can be summarized as follows. At 3400–3500 Ma, a simatic protocrust might be present in the region. The first epoch of metamorphism accompanied by some charnockite intrusions occurred at about 2800–3000 Ma. This was followed by the second epoch of metamorphism accompanied by abundant intrusions of tonalite, trondhjemite and granodiorite at about 2500 Ma. The early Proterozoic tectonic settings were remarkably different from the Archaean as different geosynclines developed at the peripheries and interior of the Archaean craton. At about 1800 Ma, the basement of the North China platform was consolidated by the Luliang movement.

Electron microprobe monazite Th-Pb dating and its constraints on multi-stage metamorphism of low-pressure pelitic granulite from the Jingshan Group in the Jiaobei terrane

Monazites from low-pressure pelitic granulite of the Jingshan Group in the Jiaobei terrane were dated by the electron microprobe method. Three stages of metamorphic age at 1720±15 Ma, 1687±16 Ma and 1568±15 Ma were yielded by constructing age mapping for zoned monazites and PbO-ThO2* isochron diagram for unzoned monazites, respectively. The first age was interpreted as the age of an early amphibolite-facies metamorphism, the second age as the peak granulite-facies, and the last age as uplifting of the granulite unit. These ages are the first geochronologic data for the low-pressure pelitic granulites in the Jingshan Group, and thus are of important significance when discussing the tectono-metamorphic evolution of the Jiaobei terrane.