Alteration age of Emeishan basalt and its response to thermal events on the western margin of the Yangtze block

The Sichuan Basin, located on the western side of the Yangtze Craton, is a nearly rhomboid tectonic-sedimentary basin affected by large NE- and NW-trending faults. The characteristics of the basin were controlled mainly by the evolution of the Tethys tectonic domain. Volcanic rocks formed by the eruption of a large amount of basaltic magma over a short period of time were influenced by the activities of the mantle plume at Mount Emei during the Late Permian. These rocks are the main targets for exploration of the volcanic rock reservoir in the basin. When studying the volcanic rock reservoir in the Sichuan Basin and surrounding areas, the author discovered that the Emeishan basalts in typical sections included several lithologic assemblages, such as pyroxene basalt, trachyte basalt, and oblique porphyry basalt, all of which were affected by chloritization, silicification, carbonation, fluorite mineralization, and other hydrothermal alterations. In addition, the intensity of alteration and the distribution of amygdules showed a spatial coupling relationship of strong alteration and densely distributed amygdules near the fault and weaker alteration and sparsely distributed amygdules in locations away from the fault. Microscopic examination revealed that the hydrothermal minerals chlorite, calcite, and quartz found in the amygdules could have been filled multiple times and that the minerals showed gradual increase in crystallinity from the edge to center. The amygdules had a bead-like distribution along microcracks, indicating that this type of amygdules may not have been formed from normal hydrothermal crystallization of pores after the magmatic period. To explore the genesis of different forms of amygdules in the basalts of Mount Emei, a detailed analysis of the typical alteration minerals was carried out in combination with in situ rare earth element (REE) and U-Pb isotopic tests of the minerals. U-Pb isotopic dating showed that the calcite in different parts of the amygdules had four isotopic ages: 236.9 +/- 19.6, 184.2 +/- 13.2, 125.90 +/- 6.43, and 114.06 +/- 3.76 Ma. The in situ REEs of the alteration minerals showed that the cryptocrystalline chlorite altered from the matrix mafic minerals had partition characteristics similar to those of the Emeishan basalt. The total REEs of chlorites in amygdules decreased gradually from the edge to the center, showing the characteristics of a deep hydrothermal source mixed with formation water. The granular quartz in the cores of the amygdules showed a flat partition pattern with weak negative Ce and Eu anomalies, which was consistent with the regional hydrothermal fluid. The characteristics of the phosphate deposits were similar; the coarse-grained calcite in the cores of the amygdules is often associated with fluorite, apatite, and others, and the REEs showed a strong left-dipping feature of positive Ce anomaly, suggesting that the alteration fluid may have come from the deep mantle source. The results of this study preliminarily determined that the different U-Pb alteration ages of calcite are the constraints and responses of the Emeishan basalts to thermal events in the western margin of the Yangtze Plate. At the least, the Emeishan basalt experienced fluid superposition and transformation at different tectonic stages in the Late Indosinian, Early Yanshan, and Late Yanshan periods following its eruption at the end of the Paleozoic. The basalt reservoir at Mount Emei is mainly composed of secondary amygdules residual pores and secondary dissolution micropores caused by micro-scale structure-induced hydrothermal erosion. Regional tectonic-magmatic events acted on the basalt of Mount Emei, causing the basalt to undergo a series of transformations of structural fragmentation, hydrothermal dissolution, and alteration filling, which might have been the basic prerequisite for the formation of high-quality volcanic rock reservoir in the basin.