Diverse subduction and exhumation of tectono-metamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt, northwestern China

High-pressure mafic granulite and garnet amphibolite are identified as small-scale tectonic slices within pelitic or semi-pelitic gneiss in the Kalatashitage area, which is located in the western Paleozoic Dunhuang Orogenic Belt, northwestern China. These rocks retain three generations of metamorphic mineral assemblages: prograde assemblage (M1) preserved as inclusions within garnet porphyroblasts, metamorphic peak assemblage (M2) consisting of matrix minerals and garnet porphyroblasts, and retrograde assemblage (M3) mainly represented by the symplectic minerals surrounding the embayed garnet and the retrograded hornblende rimming matrixtype clinopyroxene. Metamorphic pressure and temperature (P-T) paths of high-pressure mafic granulite, amphibolite, and metapelite retrieved by thermobarometry are all clockwise, passing from 640 to 720 degrees C/6.2-12.6 kbar (M1) through 840-920 C/14.6-16.2 kbar (M2) to 750-815 degrees C/5.5-7.9 kbar (M3) for high-pressure mafic granulite, from similar to 650 degrees C/5.7 kbar (M1) through -750 degrees C/9.2 kbar (M2) to -780 degrees C/8.1 kbar (M3) for amphibolite, and from similar to 615 degrees C/7.9 kbar (M1) through 730-820 degrees C/8.6-11.7 kbar (M2) to 675-740 degrees C/5.4-8.7 kbar (M3) for pelitic and semi-pelitic gneiss. Furthermore, pseudosection modeling of high-pressure mafic granulite indicates that the growth zonation of garnet porphyroblast exhibits pro grade metamorphism in a P-T range of 510-800 degrees C/8.5-13 kbar and demonstrates peak metamorphic P-T conditions of similar to 850 degrees C/16 kbar, which are consistent with the the rmobarometric estimates. The significant pressure differences in peak metamorphism observed in different rocks indicate that the rocks initially subducted to remarkably different depths and were subsequently juxtaposed at shallower crustal levels during exhumation. Sensitive high-resolution ion microprobe (SHRIMP) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U Pb dating of metamorphic zircon indicates that the metamorphic events occurred at ca. 430-420 Ma (M2) and ca. 400-390 Ma (M3), respectively. Metamorphism was followed by the intrusion of granitic dykes at ca. 244 Ma. Moreover, the metamorphic evolution indicates that the Kalatashitage area was involved in the subduction, collision and subsequent tectonic exhumation in the Paleozoic. Combined with previous literature, it is inferred that the discrepant subduction and exhumation of high-grade metamorphic rocks is a universal phenomenon in the Paleozoic Dunhuang Orogenic Belt, supporting the ubiquitous existence of subductioncollision complexes in this orogenic belt. (C) 2020 Elsevier B.V. All rights reserved.