A DISCUSSION OF THE ORIGIN OF YANGXIN DOLOMITE OF LOWER PERMIAN IN SOUTHWEST SICHUAN

Based on the lithological texture, optic features, cathodoluminescence, trace elements and inclusion temperature measurment the Yangxin series of the Lower Permian (which is exposed on Ermei, Hanwang and Baoxing of south-west Sichuan) with a cumulative thickness of approximate 100m combined with the occurrence of the dolomite and its vertical petrographic complex, horizontal distribution and the deposition and burial history, the author proposes that the dolomite in this region was originated from bioclastic limestone of beach facies which deposited under the control of the Ermeishan-Washan ancient fault during early stage. It is beneficial to form mixing dolomite by presh and marinewater because of it frequntly expose and submerge from sea level and sequentially form the diagenesis of uplifting and downgoing of earthcurst, folding and faulting in geohistorical process. Especially the Dongwu movent soon happened after the deposition of Yangxin series, it made the earth crust uprise, leading these strata to expose and denude, and the Baoxing, northwest portion of the region was the highest and denuded deeply while its southeast portion, the Hanwang and Daxin denuded less with 100-150m in remains thickness (the Lower Permian) more than that of Baoxing, it was the depression at that time, thus making the fresh water in west side of the highland flow along the Ermeishan- Wasan fault and its accompanying fractures towards the depression, wherever there reserved 5% of seawatcr in rocks, there would promote dolomitization to take place. Subsequently the volcanic eruption, basalt' s mass flowed out, and hydrothermal activity from eaeth crust in that region, these major geological events deeply influenced the strata in Ermei, Hanwang and Baoxing, changed the physical and chemical conditions of rocks increased the geotherm and the composition of pore water was also changed, and simultaneously provides the ions of various elements as calcium and magnesium etc. thus a favorable condition for continuous dolomitization was possessed. Owing to the long term subsidence of earth crust, the Yangxin series of the Lower Permian in that area deeply buried under several thousand meters and sustained a number of reworkings by deep burial, such as the increase of geothermal gradient composition change of pore water, transformation of organic matter into hydrocarbon etc. which also provides conditions for continuous dolomitiaztion. Until the later Indo-China movement, the dolomite was sustained such dynamic action as intensive compression and tension, making them eproduces such "strain textures" as tight contact between crystals in a zigzag form, not show crystal, but show sheet-connected orange in color under cathoaluminescence, a general extinction, and mechanical twincrystals etc. They are further to evolve into the present dolomite with extremely inhomogeneous in of silt fine to medium and coarse crystals, bu it is maine the medium-coarse crystals in which there coexist with the euhedral, semieuhedral and anhedral crystals the anhedral one is the main, this is a result of superposition of various diagenetic condition and various types dolomitizations, it made the dolomite crystals to enlarge the porosity to decrease and the reservoir capacity to worsen.