Fluid Inclusion and Isotope Studies of Calcite Veins in Shizhu Synclinorium, Central China: Record of Origin of Fluids and Diagenetic Conditions

Abstract

Calcite veins occur widely in carbonate strata in Shizhu synclinorium of the foreland fold belt settings of western Mid-Yangtze of central China, and proverbially, fluid activities cause the precipitation of calcites. This study is focused on the origin and timing of paleofluid activity events in Paleozoic and Mesozoic carbonate strata of Shizhu synclinorium using basin modeling technology and petrographical observation with the data from isotope and fluid-inclusion of these calcite veins. Two successive generations of fracture-filling calcite cements are distinguished based on microscopy and cathode luminescence. The older calcites (Stage 1) are moderate red luminescence, isometric texture and exhibit intense cleavage. The melting temperatures of two-phase aqueous inclusions in Stages 1 calcites have values between -8.9 and -18.9 C, corresponding to a range in salinity between 12.73 and 21.61 eq. wt% NaCl. Homogenization temperatures display a wide range (78.6-215.5 C) in each sampling stratum. Fluid inclusion microthermometric analysis indicates that there existed four to five fluid activity episodes. The δ18OV-PDB values of Stage 1 calcites (-12.94‰ to -6.77‰) are found to be more negative than those of host rocks (-10.81‰ to -4.38‰). The majority of the 87Sr/86Sr ratios of those calcites in each stratum vary in the range of coeval seawater. The significantly positive values of the calculated δ18O values of fluids-precipitating calcites (-0.41‰ to 14.42‰, SMOW) and 87Sr/86Sr ratios varying in the range of coeval seawater suggest fluids-forming calcites in each stratigraphic unit could be due to the involvement of fluids that originated from coeval seawater and evolved through different degrees of water rock interaction. The younger Stage 2 calcites have only been encountered in the central parts of a few thick veins. These calcites are characterized by an intensely zoned luminescence pattern, with bright red, dull red and non-luminescent zones, which is interpreted to have precipitated from shallower meteoric waters. Fluid-inclusion data combined with basin modeling indicate various episodes of evolved basinal fluids flow through formation fractures spanning a time frame from 135 to 50Ma, the veins probably reflect episodes of fluid flow that initiated by tectonic compression and uplift during Late Yanshanian and Early Himalayan orogeny.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016