Based on comprehensive analysis of field work, petrological and geochemical characteristics, the authors study the sources of dolomitizing fluids and the models of dolomitization of the Lower Ordovician Penglaiba Formation in Tongguzibulong Outcrop, the northwestern margin of Tarim Basin, China. Four types of
dolomite are recognized: euhedral-subhedral powder crystallized dolomite, euhedral-subhedral fine crystallized dolomite, subhedral-xenotopic medium crystallized dolomite and subhedral-xenotopic coarse crystallized dolomite. Powder-fine crystallized dolomite is distributed in the lower part of Penglaiba Formation, and has cloudy center surrounded by clear rim. With residual sand texture, inter-crystal pores and inter-partical pores, medium-coarse crystallized dolomite is distributed in the upper part of Penglaiba Formation. The REE patterns of dolomite and contemporaneous limestone rich in LREE and deplete in HREE, present a trait of unobvious Ce anomaly and Eu negative anomaly, the values of C-O isotope locate in the scope of contemporaneous marine dolomite, and Eu negative anomaly. All these denote that the dolomitizing fluid is normal or slightly concentrated seawater. Besides, Powder-fine crystallized dolomite present a low value of Fe, Mn and a high value of Sr, Ba, formed by reflux seepage dolomitization in penesaline seawater. Cloudy center surrounded by clear rim texture and multi-rimmed texture are the results of over-dolomitization. Medium-coarse crystallized dolomite with residual grain texture, interbedded with sand limestone, are controlled by high frequency sea level change. With a higher value of Fe, Mn and a lower value of Sr, Ba when compared with powder-fine crystallized dolomite, medium-coarse dolomite formed by the early reflux seepage dolomitization and intensified by the subsequent burial recrystallization.