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.

Based on comprehensive analysis of field work, petrological and geochemical characteristics, this paper studies the characteristics and origin of giant patch dolomite of the Lower-Middle Ordovician HangGuletag Formation in Wuligezitag area, NE Tarim Basin, China. It can be seen in the field that the grayish yellow giant patch dolomite partially develops in the black thin-bedded micritic limestone, with an irregular boundary not controlled by sedimentation. The giant patches are several meters in size (the largest >10 m) which is larger than and quite different from the several-centimeter (decimeter) burrow-related dolomite or leopard fur dolomite. Microscopically, it consists of micro-finely and planare(s) dolomite crystals with a size near 100 μm. Besides, it is also characterized by its unique geochemical features: 1) δ¹³C_PDB (−1.34‰-−0.62‰) accords with the range of Early Ordovician seawater while δ¹⁸O_PDB (−8.01‰ -−4.79‰) is higher; 2) ⁸⁷Sr/⁸⁶Sr  (0.711000-0.711535, AVG=0.710863) is much higher than that of limestone and contemporaneous seawater; 3) average high Fe and Mn content (2001.32 and 601.73 μg/g, respectively) and low Sr and Ba (33.14 and 8.27 μg/g, respectively) content versus that of limestone; 4) dolomite has slightly negative Eu anomaly and similar REE composition and distribution patterns compared with limestone; 5) low order degree value (AVG=0.6). This study reveals that the dolomitization was related to fault and occurred in low-temperature and shallow-burial environment in Early-Middle Ordovician. Dolomitizing fluid was generated from the concentrated formation fluid in lower-middle member of HangGuletag Formation and underlying Tursaktag Group. Most Mg²⁺ came from the stabilization of quasi-stable carbonate minerals, and the transformation of clay minerals of (calcareous) mud in the limestone strata not only provided a little Mg²⁺ but caused the rise of ⁸⁷Sr/⁸⁶Sr. Dolomitizing fluid was likely to be richened in Xingdi-related normal fault and fractures which also acted as the main fluid pathways, and it finally caused the partial dolomitization of nearby limestone strata.