Multiple high- and ultrahigh-pressure orogenies in the Qinling Mountains: boundary conditions permitting their formation and exhumation

In the last years, numerous occurrences of ultrahigh-pressure (UHP) metamorphic rocks have been found all over the world. In most cases, these rocks experienced a subduction due to a continental collision and were rapidly transported to and exchumed from a depth of more than 100 km, which means that they reached the stability field of coesite and sometimes diamond. Understanding how ultrahigh-pressure metamorphic rocks form and exhume is an outstanding tectonometamorphic question, as it highlights processes dealing with the exchange of material between the crust and mantle, the formation and destruction of mountain belts, the composition of deep continental crust, and tectonic plate motions. The huge and well-exposed UHP terranes of the Qinling mountains in central eastern China allow case studies in natural laboratories, i.e. orogen-scale geologic work, yielding crucial parameters like HP/UHP phase petrology and fluid studies within continental crust in mantle depth. Furthermore, information is supplied on plate geometry, plate-tectonic framework, and detailed structural research that provides the geometry and the kinematics of exhumation (the subduction history has remained mostly enigmatic) and the rheology of deformation. This project, building on earlier work in the archetype of UHP orogens, the Dabie-Sulu belt, will (i) establish the value of four recently discovered and/or enigmatic eclogite facies (HP and UHP) events in the Qinling mountain belt for research on HP-UHP orogeny; (ii) set up the physico-chemical and structural boundary conditions that permitted formation and exhumation of these rocks; and (iii) make first-order contributions to the understanding of the centrepiece of Chinese geology, the Qinling orogenic collage. Recent meetings and thematic volumes on UHP research, to which we contributed, highlighted hypotheses that need long-term study by the tectonics community; these are among others: - How do UHP rocks form and exhume? - Which plate-tectonic processes and boundary conditions permit/prohibit formation and exhumation of UHP rocks? - What are the rates that transfer continental crust into mantle and vice versa? - Do UHP terranes constitute a significant portion of the continental lower crust? To solve such a complex problem, an inter-disciplinary project with international scientific partners has been enforced. A first application on this topic was already granted to Lothar Ratschbacher (Freiberg University of Mining and Technology/Saxony) by the German Research Community (Deutsche Forschungsgemeinschaft DFG) in December of 2006. The working group in Freiberg will mainly concentrate on structural geology and - in co-operation with our U.S. American partners Bradley Hacker (UC Santa Barbara) and Mike McWilliams (Stanford University) - on geochronology. The Swiss working consist of Leander Franz (field petrology, PTtd-paths), Christian de Capitani (theoretical petrology, thermodynamics) and Josef Mullis (fluid inclusions). Two PhD students (Thomas Bader from Basel and Carsten Weise from Freiberg) will unravel the tectonometamorphic evolution of the Qinling mountains combining state of the art methods of petrology, fluid inclusion studies, structural geology and geochronology.