科研之友
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摘要
Xing' an Mongolian Orogenic Belt is located in the eastern part of the Central Asia Orogenic Belt, composed of the Paleozoic opposite subduction orogenic belt that includes a northern orogenic belt (NOB) and southern orogenic belt (SOB). The NOB extends ca. 550km from Xilinhot in the east to Airgin Sum in the west. The SOB outcrops from Ondor Sum in the east, via Bater, to Tugurige in the west. Songliao-Hunshandake block and oceanic crust called Ondor Sum Group located between NOB and SOB. In recent years, the Early Paleozoic structural deformation in the Ondor Sum-Hongqi area has been recognized, which suggests that a subduction of the Songliao-Hunshandake block beneath the North China craton. But, how to extend the subduction belt from Ondor Sum-Hongqi area to neighbor area and geochronology evidences related with the subduction is still unsolved. In Tugurige area, ca. 150km to the west of Ondor Sum-Hongqi area of the SOB, six tectonic units have been recognized, which includes, from north to south, fold belt of subducted oceanic crust, melange, fore-arc belt, island arc belt, back arc foreland basin and North China craton. In the fore-arc belt a deformation zone is found that is 20km long from east to west and 2 similar to 4km wide from north to south. The deformation zone is composed of the Ordovician Baiyunshan Formation including quartz schists, muscovite quartz schists, foliated marbles and quartz sandstones. Widespread south-dipping foliation, but penetrative inequality, can be observed in the zone. From north to south, the deformation zone can be divided into four strong and weak deformed sub-zones. The strong deformed sub-zone, with the thickness exceeded 450m, is characterized by two phase deformations, in which fabric analysis of quartz reveal change from dynamic recrystallization to static recrystallization. Analysis of microscopic photographs indicates that most quartz grains show different boundary shapes and subgrains show a bulgy recrystallization in weak deformed zone. The subgrain rotation recrystallization representing higher temperature can been observed from the samples of strong deformed zone. There are some static recrystallization grains, which show approximate equigranular, straight edge and triple point structure. According to these quartz fabric the temperature range of the strong deformed sub-zone is about 350 similar to 500 degrees C, which is near or higher than the muscovite closure temperature. So the muscovite age of strong deformed zone in the Baiyunshan Formation can represent the deformation age. A muscovite Ar/Ar isochron age of 440.3 +/- 7.2Ma has been acquired from the strong deformed sub-zone. Considering the sample located in fore-arc or island arc area of the SOB, which is a high strain zone of plate subduction, the age should suggest a deformation event related with the subduction during the late Early Paleozoic. The deformation zone, therefore, provides structural deformation evidence for the SOB in Tugurige area. The formation age of deformation zone correlates with the high-pressure metamorphic event age in Ondor Sum area, which suggests that two regions are affected by the late Early Paleozoic orogeny and the SOB extends from Ondor in the east to Tugurige in the west.
- 出版日期2015-1
- 单位北京大学
