SPATIAL DISTRIBUTION OF ANKERITE AND DOLOMITE IN THE XISHA ISLANDS: EVIDENCE FROM GEOCHEMISTRY

Dolomite and ankerite are widely distributed in the Xisha Islands. So far, seven layers of dolomite have been found in the well Xike 1. The seven layers of dolomite may be grouped into three zones, the shallow, the middle and the deep. The shallow zone is less than 600 meters in depth including the layer 1, layer 2 and layer 3 with a total thickness of 203 m; the middle zone includes the layer 4, layer 5 and layer 6 with a total thickness of 79 meters and occurs in a range of 600～1 000 m in depth; the deep zone includes the layer 7, 155 m in thickness buried underground more than 1000 meters in depth. The geochemical characteristics of major and trace elements of the seven layers of dolomite are studied in this paper, while diagenetic environment and genetic model discussed based on the newly acquired geochemical data combined with previous research results. Dolomite in the well Xike 1 was far beyond the affection of terrigenous materials, high content of SiO₂ may come from the volcanic activities surrounding the islands. It is evidenced that the dolomitization in the Xisha area happened in an oxidation diagenetic environment, but the intensity of oxidation is different in the shallow, middle and deep zones. The paleosalinity of sea water was high when the dolomites formed, and there are slight differences between the three zones. The content of strontium in the dolomite of the well Xike 1 is generally low, as it was strongly leached by fresh water in an environment of reef platform with relatively shallow water. Comparatively, the influence of atmospheric fresh water was strongest in the shallow zone, followed by those in the middle and deep zones. The diagenesis of dolomite in the Xisha Islands is influenced by many factors, such as high palaeosalinity seawater, atmospheric fresh water and water reflux infiltration.