川中北部地区二叠系茅三段—吴一段铁白云岩成因机制研究OA

[Objective]At present,the iron dolomite genesis mechanism of the Maosan-Wuyi in north-central Sichuan is not clear.[Methods]Based on the rock-mineral characteristics of the Maosan-Wuyi,we combined rock TI-MA scanning,whole-rock X-ray diffraction analysis,microzonation in situ main trace elements,strontium isotopes fluid inclusions homogeneous temperature,and in situ U-Pb chronology of iron dolomite to study the mechanism of iron dolomite genesis.[Results]The study shows that:(1)the rock types of the Maosan and Wuyi are cherts,dolo-mites and tuffs.The tuffs mainly include mud crystal clastic,bright crystal clastic,and cloudy tuff;the dolomites are semi-autogenous-to-autogenous powdery-to-fine crystalline dolomite;the sedimentary tuffs are mainly composed of volcanic clasts,carbonaceous,calcium-magnesium,sand-grade clasts,and metallic minerals,among which the vol-canic clasts are glassy clasts,basalt,and other clasts,and the calcium-magnesium mainly consists of dolomite and calcite.(2)The average values of trace elements in the dolomite at the top of Maosan and Wuyi show extremely high Fe content(10 678.40×10-6),high Mn content(822.95×10-6),Si content(1 929.81×10-6),Al content(394.11×10-6),and high Na+K content(362.38×10-6),indicating that the fluids in the closed environment are characterized by high salinity and alkaline metal content.This also indicates that the rock-forming fluids in the confined environ-ment are characterized by high salinity and high alkaline metal content.The 87Sr/86Sr in the matrix of ankerite and mud crystal clastic tuffs are within the range of seawater from the same period in the Maokou and Wujiaping groups,and they also have low total rare earth element(REE)values and light REE(LREE)loss,consistent with the LREE loss pattern of seawater,indicating that the dolomitized diagenetic fluids and seawater of the same period have similar characteristics.(3)The U-Pb age of powdery-to-fine crystalline dolomite in the Maosan and Wuyi is 245.36±1.08 Ma,and the dolomitization mainly occurred in the Late Permian to Early Triassic.[Conclusions]The source of dolo-mitization fluids in the powdery-to-fine crystalline dolomite of the Maosan and Wuyi is the dewatering of clay minerals in the argillaceous tuffite and tuffaceous mudstone of the overlying Wuyi and Wuer.During this process,a large num-ber of Mg2+,Fe2+,Al3+,and Si4+-rich ions were precipitated and transported to the granular tuffs at the top of Maosan and Wuyi to form iron dolomite using the seawater residual from the Maokou Formation to Wujiaping Formation dur-ing the same period as the carrier.