Mesozoic Tectonism and Magmatism in Anqing Area of Middle-Lower Yangtze River
Qiu Ruilong Dong Shuwen (Anhui Institute of Geological Science, Hefei)
The metallogenic belt of the Middle—Lower Yangtze River is limited within the shorterning foreland zone resulting from collision orogen belt between the North China and Yangtze massif during mesozoic, which plenty of magmatism and metallization (Cu、Fe、Au、S etc.) took place in a transitional period from collision shorterning at Indosinian (T_3) to rift—expanse at late Yanshanian (K_2). In Anqing area the tholeiitic magma had formed in terms of crust thickening and lower crust anatexis under the collision compression, which we call as Dongling diapir magmatite series. Then, calci—alkalic magma series of crust—mantle mix—melting, e. g. Yueshan magmatite series of the enrichment of alkali and silicon, has gradually become as main current of the magma during deep fault extension beginning and smooth lifting of mantle. The third one, alkali—lime volcanic rocks and A—type granite series produced (Dalongshan alkali—feldspar quartz syenite) when the extension could dominate in thc area. By the time of late Cretaceous and Tertiary period, the area has been controlled completely by the rift of the encircling pacific Ocean, the structural system changed into the form of a graben—horst and the bed sandwhiching alkali basalt constructed in the normal fault basin. The main intrusion of the magma happened in the time when the tectonic transition from regional compression to extension. Due to the differences of the forming environment and of magma composition, the collisional granites source from lower crust intruded upward as diapir, while passive intrusion is of the magma series of the crust—mantle mix—melting. The metalliations are concentrated in the later. The tectonic units might be divided into two parts in Anqing area: Dongling magma diapir sliding system and Yueshan—Baizishan thrust sheets. Three magma series are as follows: the low—Si ane Fe—rich diorite, the Si—rich and alkali—rich diorite and the alkali—lime magma series.