Granulites record the tectonic evolution from collisional thickening to extensional thinning of the Tongbai orogen in central China

A combined study of petrology and geochemistry was carried out for granulites from the Tongbai orogen in central China. The results reveal the tectonic evolution from collisional thickening to extensional thinning of the lithosphere at the convergent plate boundary. Petrographic observations, zircon U-Pb dating, and pseudosection calculations indicate that the granulites underwent four metamorphic stages, which are categorized into two cycles. The first cycle occurred at 490-450 Ma and involves high-P (HP) metamorphism (M1) at 785-815 degrees C and 10-14 kbar followed by decompressional heating to 840-880 degrees C and 8-9 kbar for medium-pressure granulite facies metamorphism (M2), defining a clockwise P-T path. The high pressure is indicated by the occurrence of inclusions of rutile+kyanite+K-feldspar in the garnet mantle. The second cycle occurred at c. 440 Ma and shows an anticlockwise P-T path with continuous heating to ultrahigh-temperature (UHT) metamorphism (M3) at 890-980 degrees C and 9-11 kbar, followed by decompressional cooling to 740-880 degrees C and 7-9 kbar (M4) till 405 Ma. The HP metamorphism is synchronous with the ultrahigh-pressure eclogite facies metamorphism in the Qinling orogen, indicating its relevance to the continental collision in the Cambrian. The UHT metamorphism took place at reduced pressures, indicating thinning of the collision-thickened orogenic lithosphere. Therefore, the Tongbai orogen was initially thickened by the collisional orogeny and then thinned, possibly as a result of foundering of the orogenic root. Such tectonic evolution may be common in collisional orogens where compression during continental collision switched to extension during continental rifting.