Medium-pressure metamorphism of the Huangsong Group in the southeastern margin of the Xing'an-Mongolian Orogenic Belt,NE China and its tectonic implication: Evidence from petrology,mineralogy and ~(40)Ar/~(39)Ar geochronology

Анотація

This paper reports the petrography,mineral chemistry and biotite Ar-Ar dating data of the representative rocks from the Huangsong Group in the southeastern margin of the Xing'an-Mongolian Orogenic Belt,with the aim of constraining its type,pressuretemperature conditions,the timing of the metamorphisms and its tectonic implications. Petrographic observations indicate that the Huangsong Group is mainly composed of biotite-plagioclase gneiss,two mica-plagioclase schist,plagioclase-amphibole schist,and different types of mylonites. The former generally occurs as small area,implying a regional dynamothermal metamorphism,whereas the latter occurs as belt,revealing the overprinting and modification of a low-temperature dynamic metamorphism. In addition,the andalusite hornstones occurring around the intrusions reflect the contact metamorphism. The estimated P-T results indicate that the pressures and temperatures of the regional dynamo thermal metamorphism range from 5. 8 to 7. 5kb and 525 to 597℃,respectively. Its geothermal gradient varies from 21 to 27℃ / km, implying a medium-pressure metamorphism( epidote-amphibolite facies-low amphibolite facies). The 40 Ar /39 Ar dating of biotite from garnet-bearing two mica plagioclase schist is 239. 22 ± 3. 02 Ma,whereas one of biotite from biotite-mylonite is 193. 91 ± 2. 16 Ma. Combined with the closure temperature of biotite and previously published geochronological data,we conclude that the former implies that the medium-pressure metamorphism in the Huangsong Group took place during Late Permian-Early Triassic,whereas the latter represents the timing of the late low-temperature dynamic metamorphism,i. e.,Early Jurassic. Together with the coeval metamorphism,magmatism,and sedimentary formations as well as regional tectonic evolution history,we consider that Late Permian-Early Triassic medium-pressure metamorphism in the Huangsong Group is related to the collision and amalgamation between the Siberia and North China cratons and implies that the final closure of the eastern segment of the PaleoAsian Ocean had finished.

Класифікація

Схожі роботи