AN OUTLINE OF THE REGIONAL GEOLOGY OF ANHUI PROVINCE

An outline of the regional geology of Anhui is presented in this paper based on the new achievements in regional geological surveys. The post-late Archean strata, having a cumulative thickness up to over 60,000m, may be divided into 230 stratigraphic units. The entire province can be divided into the North China, Northern Huaiyang and Yangtze stratigra- phic regions. The geosynclinal sediments of the North China and Yangtze stratigraphic regions began to transform into platform sediments in Qingbai- kouian and Sinian times, while the Northern Huaiyang region continueed to maintain the characteristics of geosynclinal sediments prior to the Jurassic. The sediments of the entire province began to transform into fault depression or depression ones in the Jurassic. The Yangtze region changed relatively remarkably. There is a transitional zone of early Paleozoic deposition in the Jingxian-Dongzhi area, and there are noticeable differences between the stra- tigraphical and paleontological characteristics on both sides of the zone. Magmatic rocks include three genetic series: mantle-derived ultrabasic to intermediate-basic rocks, transitional anatectic intermediate to intermediate- acidic rocks and continental crust-reformed granite. Magmatic activity compri- ses seven cycles. The pre-Sinian geosynclinal stage consists of the Bengbu （late Archean）, Fengyang （early Proterozoic） and Wannan （middle Proterozoic and Qingbaikouian Period） cycles. In the early phase of each cycle, ultrabasic to intermediate-basic rock predominate, and in the late phase, intermediate-acidic rocks are dominant. Generally, eruption antedated intrution. The paraplatformal stage includes the Huoqiu and Indosinian cycles. Magmatic activity was relative- ly weak and mainly took place in the stage of the platform fold belt, and there was almost no volcanic activity. The rocks are dominated by intermedi- ate-acidic ones. The magmatic activity in the stage of continental-margin mo- bilization reached a climax in the middle-late Yanshanian times, generating intermediate and intermediate-acidic rocks. Volcanic activity took place fre- quently, and the volcanic rocks belong to alkaline basalt series but evolve towards acid and alkaline rocks. The Himalayan cycle gave rise to basaltic lavas. Regional metamorphism involved four phases, the Bengbu, Fengyang, Wannan （terminal middle Proterozoic and Qingbaikouian） and Variscan-Indo- sinian phases. The former two phases were accompanied by migmatization, which is characterized by first Na and then K metasomatism. A glaucophane +jadeite+quartz assemblage is present in the Zhangbaling glaucophane schist zone in central Anhui. Such Iow-temperature/high-pressure metamorphism might be related to a process of high pressure occurring along the Tancheng- Lujiang deep fracture in the Wannan phases. Anhui involves three tectonic units, the Sino-Korean paraplatform （con- sisting of the Huaihe platform depression and the Jianghua platform uplift）, Qinling geosynclinal system （referred to as the Northern Huaiyang geosyn- clinal fold belt in the province） and Yangtze paraplatform （composed of the Huaiyang and Jiangnan platform uplifts on both sides and the lower Yangtze platform depression in the middle）. The polycyclic characteristics of crustal development are conspicuous. Eight tectonic cycles may be recognized; they are the Bengbu, Fengyang, Wannan, Huoqiu, Caledonian, Variscan-Indosinian, Yanshanian, and Himalayan cycles. The fractures may be grouped into the E-W, NNE, NE, N-S and NW sets. In the easteen part of Anhui, a quite large proportion of nappe structure might originate from the NNE deep frac- ture. The NNE and E-W structures combine to control the formation and development of Meso-Cenozoic fault depressions （or depressions）. The develop- ment of these depressions and fault depressions progressed through four stages, i. e. formation of depression (J_1-2), development of fault depression （J₃-K₂）. cultimation of fault depression（K₂~2-E）, and decline of depression and fault de- pression （N）. To sum up, the process of geological evolution of Anhui can be divided into three major stages, pre-Sinian, Sinian to Triassic and post-Jurassic. The first stage was one of the geosynclinal development stages except in the Sino-Kor- ean paraplatform Where the basement had already consolidated before the middle Proterozoic. The geosyncline shifted following two systems: （1）it shift- ed towards all sides with the demal nucleus as the center （north）, and （2） it shifted unidirectionally towards southern Anhui in the shape of a chain made up of isolated island-like continental nuclei （south）. In the second stage, the paraplatform began to develop in most areas. In the Devonian, a unitary continent had been formed firstly in Anhui. In the Variscan-Indosinian phase, the northern Huaiyang area became a piedmont depression so that the uni- tary continent was dismembered. The Indosinian movement generated a plat- form-fold zone, and Anhui became a unitary continent again. Anhui began to enter the stage of development of the continental-margin mobile zone in the Jurassic, and the fault-block and basin-forming movement occupied the leading position.