Retrograde P-T Path for Triassic very low-grade metapelite from Hongcan Deep Well in Songpan-Aba area

Songpan-Aba area is located in the southwestern of China (Fig.1). Hongcan Deep Well is near to the Zoige County in the northeast of Songpan-Aba area. The absence of geothermometer and geobarometer based on experiments under very low grade metamorphic condition, it is difficult to set up a P-T path for very low grade metamorphic rocks. Table 1 shows that XRD-based b0 values can be used to infer geothermal gradient, after interpolation, we got the average geothermal gradient during Middle-Late Triassic in the Well is 29.94, therefore, we can draw a field P-T curve (G1). Five Chlorite geothermometers were used and evaluated (see Fig.2), the result shows T5 given by Stefano in 1999 is more accord with the true metamorphic temperature in the study area. At last, we marked the peak temperatures from calculated T5 in the G1, and then got the values of peak pressure. The current geothermal gradient in the Well is measured of 33.22, and then we can also draw another P-T curve (G2) in the P-T space. Since the location of each sample in the well is known, we projected all the known pressures in the G2, and got the current temperatures of those samples. Finally, we connected all the same samples from the two P-T curves (G1 and G2) and made the retrograde P-T path for the Middle-Late Triassic very low grade metapelite from the Hongcan Deep Well in Songpan-Aba area (Fig 3). The P-T path is clockwise, and shows that after the Paleo-Tethys Ocean closed during the Late Triassic, the study area experienced a strong uplift and erosion.