THE CORRELATION BETWEEN THE DOLOMITES AND THEIR FORMING CONDITIONS IN THE GUANWUSHAN FORMATION (MIDDLE DEVONIAN)GANXI, SICHUAN, CHINA

The Guanwushan Formation consists of three types of dolomite; medium crystal line dolomite which occurred in biostrome environment, micrite dolomite in lagoon and fine crystalline dolomite in tidal flat. The first type of dolomite, distributed in the lower part of Guanwushan Formation, is characterized as follow: 1 ) containing abundant reef-forming organisms and other fossils, 2 ) 100-500μm in size, euhedral rhombohedral crystal forms. 3 ) very good ordering degree and low CaCO₃ mole content, 4 ) low δ¹⁸O values and 5 ) dull intensity of cathodoluminescence. Second type of dolomite, distributed in middle part of Guanwushan Formation, is characterized as follow: 1 ) containing few fossils but a lot of pyrite, 2 ) 5-60μm in size, dominantly xenomorphic granular crystal form, 3 ) poor ordering degree and high CaCO₃mole content, 4 ) relatively high 618O values and 5 ) bright intensity of cathodoluminescence. Third type of dolomite distributed in the upper part of Guanwushan Formation, is characterized as follow: 1 ) containing some fossils, 2 ) 40-200μm in size, dominantly hypidiomorphic granular crystal form, 3 ) very good ordering degree and lower CaCO₃ mole content, 4 ) relatively low δ¹⁸O values, low δ¹³Cvalues and 5 ) medium intensity of cathodoluminescence. The micrite dolomite in the lagoon resulted from the dolomitization by the solution having a high Mg²⁺/Ca²⁺ ratio during penecontemporaneous stage. The medium crystalline dolomite in biostrome and the fine crystalline dolomite in tidal flat resulted from the dolomitization by meteoric ground waters mixing with sea water during the early diagenetic stage.