Geochronological Study Of Alkaline Rocks In Panxi,SW China And Its Geological Implications

Alkaline rocks are often associated with extensional tectonism and regarded as the characteristic products occurring in continental margins or rift zones. They were usually generated in deep-large-fault zone and closely connected with basic/ultrabasic rocks in space.Therefore, alkaline rocks are of significance in tectonic petrology. At the beginning of the 1980s, geological and geophysical studies were performed in Panzhihua-Xichang paleo-rift zone. However, these studies only involved geology and petrochemistry of the ring alkali complexes. The geochronological study was conducted during the 1990s.Because layered intrusions in Panxi paleorift host large-scale economic V-and Ti-bearing magnetite deposits, some geochronological studies have been done on it since the 1960s'. The dating techniques include K-Ar, Ar-Ar, Rb-Sr, Sm-Nd and U-Pb dating etc., but different researchers provided different results on the same layered intrusions so that the contentious ages on the timing of layered rocks have been produced. The present paper analyzes the age of the nephline-pyroxenite in ultrabasic ring alkalic complexes of Jijie and Daxiangping by Ar/ Ar method. These data shows the age of nephline-pyroxenite in ultrabasic ring alkalic complexes of Jijie and Daxiangping rang from 214~ 193Ma, which is consistant with the late Triassic epoch, belonging to Indosinian period. Generally speaking, the emplacement of ultrabasic ring alkalic complexes occurred later than that of the gabbro ore bearing.Alkali-feldspar syenites from the Maomaogou area of Huili County in Panxi of Sichuan Province, SW China are sodium alkaline rock, the minerals of which are mainly composed of calcic ferroaugite, ferroamphibole and biotite.The age of zircon SHRIMP U-Pb dating is 224 ± 8Ma, suggesting that the Maomaogou alkali-feldspar syenite was formed in the late Triassic period. At the same time, two old age periods of relict cores of zircon are determined, they are the Archean age(2692-2818Ma) and Neoproterozoic age (622-691 Ma), respectively. These periods indicate that the cores derived from the crystal basement. Furthermore, the Archean age of zircon provides the first reliable SHRIMP U-Pb zircon datingevidence of >2.8G basement in the Xikang-Yunnan Axis of southwest of China.The Cida and Taihe layered intrusions in the Panxi paleorift were studied in this paper. The biotite 40Ar/39Ar dating and the zircon ELA-ICP-MS U-Pb dating for these layered intrusions have been performed. The ages obtained are as follows: the Cida rock body has biotite 40Ar/39Ar age of 256Ma; and the Taihe rock body has a zircon U-Pb age of 215Ma. These data suggest that the Cida and Taihe layered rock bodies were formed possibly in the late Variscan-Indosinian period about 256-215Ma ago.Further more, it is established that the ore-lean and low-grade layered gabbro bodies intruded earlier than ore-bearing and high-grade layered gabbro ones. Generally speaking, the emplacement of ultrabasic ring alkalic complexes occurred later than that of the gabbro ore bearing. It is believed that the timing of early intrusion emplacement(259-256Ma) is not the timing of pre-rift doming, but is consistent with the timing of rift during the late Permian, which is synchronous with the outpouring of Emeishan basalts during the late Permian-late Triassic Period(256-215Ma). Consequently, the whole process of layered gabbro magmatism after outpouring is closely related with the fault structure and mantle-plume activity.