|
Huaikun Li, Tianjin Institute of Geology and Mineral Resources (China)
Songnian Lu, Tianjin Institute of Geology and Mineral Resources (China)
Zhenqun Xiang, Tianjin Institute of Geology and Mineral Resources (China)
|
|
Recent study reveals many lines of evidence of the Late Palaeoproterozoic to Early Mesoproterozoic Columbia Supercontinental events, including high grade (amphibolite to granulite facies) metamorphism and rapakivi granitic magmatism in northwestern China. The granulite and other high grade metamorphic supracrustal rocks of the Milan Group in the Aketashitage area, southeastern margin of the Tarim Craton, occurs as lenses within granitic gneisses. Ages of the detrital zircons from the garnet-muscovite-quartz schist ranging from ca. 2884 Ma to ca. 2339 Ma imply that sedimentation took place in early Palaeoproterozoic. While the weighted average 207Pb/206Pb age of 1986 ¡À 29 Ma of 11 analyses is regarded as the metamorphic age based on CL images of the zircons. The age of garnet-two pyroxene granulite is 1968 ¡À 8 Ma that is consistent within error to the metamorphic age of the garnet-muscovite-quartz schist. Therefore a significant tectonic event that might be an orogeny related with the assembly of the Columbia supercontinent occurred on the southeastern margin of the Tarim Craton between ca. 2000 and ca. 1960 Ma. Palaeoproterozoic supracrustal rocks in the Quanji Massif on the northern margin of the Qaidam Basin to the southeast of the Tarim Craton across the Altyn Tagh are mainly composed of amphibolite to granulite facies metamorphic gneisses represented by the Dakendaban Group. Mafic granulites have been found in quartzo-feldspathic gneisses and comprise hypersthene, clinopyroxene, amphibole and plagioclase. Recently a zircon ID-TIMS U-Pb age of 1823¡À22 Ma was obtained for the granulite. Two zircon SHRIMP U-Pb ages of 1852¡À15 Ma and 1815¡À15 Ma for amphibolite were obtained. The amphibolite and the granulite occur as lenses in the quartzo-feldspathic gneisses.
Rapakivi granite in the Quanji Massif is unconformably overlain by Triassic strata in the northern part of the intrusion and has a fault contact with the Mesoproterozoic Wandonggou Group along the southern margin of the granite. Two U¨CPb zircon ages of 1776¡À33 Ma and 1763¡À53 Ma have been obtained for the rapakivi granite. The occurrence of rapakivi granite in the Quanji Massif suggests a rifting tectonic environment in the late Palaeoproterozoic similar to that of the North China Craton.
It is significantly different in timing of the high grade metamorphism between the Aketashitage area and the Quanji Masif, namely the former is ca. 100 ∼ 150 Ma older than the latter. On the other hand, the ages of the high grade metamorphism and the rapakivi granite in the Quanji Massif are coeval with those of the counterparts in the North China Craton. Therefore the Quanji massif was probably connected with, or a part of, the North China Craton in the Late Palaeoproterozoic; while the Tarim Craton was more likely adjacent to other ancient cratons.
|
