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

Stretching for > 2000 km between the Sino-Korean craton in the north and the Yangtze craton in the South, the Qinling-Tongbai-Hong’an-Xinxian-Dabie-Sulu-Imjingang orogen is the centrepiece of Chinese geology. From north to south, it comprises the Kuanping, the Erlangping, the Qinling unit Liuling, the Douling and the Wudang, i.e. tectono-metamorphic units with complex evolutions.
In Cambrian times, deep subduction of the Qinling microcontinent below an intra-oceanic Erlangping arc created ultra-high pressure metamorphic eclogites and gneisses. The coesite-eclogite facies stage was constrained at 550°C and 3.1 GPa. During uplift, a quartz-eclogite facies recrystallization occured at 2.0-2.3 GPa and ~660 °C. Further uplift was characterized by nearly isothermal decompression and a penetrative overprint at 630-640 °C and 1.1-1.5 GPa. Ar/Ar phengite and U/Pb titanite ages of ~470 Ma highlight exhumation into the crust together with cooling in the Middle Ordovician.
The southern margin of the Qinling microcontinent faced a north-trending subduction zone and a magmatic arc set up that was active from the Middle Cambrian till the Early Devonian (~110 Ma). Due to the high heat flow, the central-southern part of the Qinling unit underwent ultra-metamorphism at peak metamorphic conditions of 680-775 °C at 0.5-0.75 GPa.
Metamorphics of the Liuling indicate an at least a two-stage burial-exhumation history during Late Carboniferous-Permian. Thermodynamic modelling of zoned garnet reveals a first clockwise Barrovian metamorphism, which took place under medium pressure amphibolite facies conditions (560-590 °C at 0.4-0.6≈GPa) and was followed by a second stage of high pressure amphibolite facies metamorphism (590 °C at 0.9 GPa). Lithology and geochronology classify the Liuling as a Devonian to Early Carboniferous forearc basin, which received its metamorphic overprint at 250-320 Ma. The Douling forms a basement wedge intercalated between the Liuling and the Wudang. Medium to high-grade metamorphic conditions (560-710 °C at 0.8-1.2 GPa) likely reflect a Neoproterozoic event. A thorough LT/HP metamorphic overprint (280-340 °C at 0.5-0.9 GPa) of probable Triassic age affected the Douling Complex as well as its cover units. Similar HP/LT metamorphic conditions (~300 °C at 0.4-0.9 GPa) are recorded in blueschists of the neighbouring northern Wudang Complex. In the centre of the Wudang Complex, HP/MT metamorphic conditions (500-550 °C at 1.0-1.2 GPa) recorded by garnet gneisses and amphibolites are followed by a HP/LT overprint of 300 °C and 0.6-0.7 GPa. Thermobarometry and geochronology indicate that the metamorphism in the Wudang Complex occurred due to subduction of the Yangtze craton underneath the amalgamated Qinling-Erlangping-Sino-Korean continent in the Triassic, which is also true for the Douling complex and its cover.
In the western part of the orogen, along a north-south profile, petrological invesigations reveal amphibolite facies PT conditions of 590 °C at 0.6 GPa in the northern part and upper amphibolite facies conditions of 760 °C at ~0.7 GPa along with the migmatisation of felsic gneisses in the centre of the profile. Further south, Theria_g modeling applied to garnet from a garnet-staurolite gneiss point to a rather fast prograde clockwise evolution with pronounced heating along with minor burial and nearly isothermal exhumation. The southermost sample gives evidence for medium to upper greenschist-facies conditions.
The age of this metamorphic overprint is constrained by Th/Pb monazite ages of 205.8 ± 2.8 Ma and 194.1 ± 2.1 Ma as well as Ar/Ar ages of 192-207 Ma while Ar/Ar ages from adjacent magmatics give a somewhat larger timespan of 186-227 Ma as the younger ages originate from pegmatites and reflect late magmatic activities. Published and new U/Pb zircon arges highlight Triassic – Early Jurassic magmatism spanning ~50 Ma with a major cluster at ~225-205 Ma.
Alltogether, these findings highlight a Triassic tectono-metamorphic event in the middle to lower crust of the western Qinling orogen which was strongly influenced by the intensive syn- and post-tectonic magmatic activity.
Triassic-Early Jurassic Ar/Ar ages from the Mianlüe mylonite zone reflect a Mesozoic metamorphic overprint. However, new and recently published U/Pb zircon ages from mafic rocks prove a Neoproterozoic origen of ophiolite blocks. Thus, there are no indications for the existence of a Late Palaeozoic Myanlüe ocean and the hypothesis of the existence of a “Mianlüe suture” stretching all through the northern Yangtze craton is falsified.