Metamorphism and deformation of gaolan group, Hochangchuan - Juchiayao section, eastern Qilian fold belt

The stratigraphic sequence along the route Hochangchuan-Juchiayao is from bottom upward, mica-quartz schists, biotite-quartz schists, and amphibole-biotite-quartz schists. Their respective protoliths are pelitic rocks, siltstones, and tuffaceous rocks with increasing tuffaceous component upward. Mineral assemblages of the mica-quartz schists (samples 3 through 9A), biotite-quartz schists (samples 9B through 13B), and amphibole-biotite-quartz schists (samples 13C through 31) are quartz + biotite + muscovite ± garnet ± chlorite ± calcite, quartz + biotite ± amphibole ± muscovite ± chlorite ± garnet ± calcite and quartz + biotite + amphibole + plagioclase + calcite ± chlorite ± sphene ± muscovite ± garnet, respectively. Combined application of GAPQ geobarometer and HP geothermometer yields an estimation of pressure in the range 3.81- 6.07kb. The b₀ value of muscovite falls in the interval of intermediate P-T metamorphic facies series. Garnet porphyroblasts have growth zoning typical in greenschist facies and amphibolite facies rocks. The Na(M4)-Al(IV) plot of Ca-amphibole indicates that the upper limit of pressure is 6kb. The temperature estimation yields 609°C on the average. These data indicate that the rocks in the study route were subjected to metamorphism of the lower amphibolite facies under the continental orogenic tectonic setting. Some garnet porphyroblasts have recorded profiles of increasing temperature from core to rim, with the rim temperature at about 600°C. The Gaolan Group has further recorded a retrogressive metamorphism of greenschist facies and the prehnite-pumpellyite facies.