High‐P granulites of the Songshugou area (Qinling Orogen, east‐central China): Petrography, phase relations, and U/Pb zircon geochronology

Abstract

High-pressure (HP) granulites provide telling records of mineral reactions at upper mantle to lower crustal levels and key information on the fate of material in subduction systems. The latter especially applies when they abut eclogite and mantle dunite because such rock associations are crucial for understanding the incompletely known processes at the interface of converging plates. A continental arc, active c. 520-395 Ma ago, formed an enigmatic example of such a rock association in the Songshugou area, Qinling Orogen. To unravel the juxtaposition of the distinct rocks, this study combines petrography, phase equilibria modelling, conventional thermobarometry, and zircon U-Th-Pb-Ti-REE analysis. Two mafic HP granulites, which contain the mineral assemblages garnet-clinopyroxene-plagioclase-rutile-mesoperthite-quartz and garnet-clinopyroxene-plagioclase-rutile, experienced peak metamorphic conditions of ≤1.4 GPa, 860°C and ~1.3 GPa, ≥910°C, respectively. During decompression and cooling, at 489 ± 4 Ma, amphibole lamellae unmixed from a clinopyroxene solid solution and orthopyroxene in part replaced garnet. A felsic HP granulite shows equilibration of garnet, perthite, antiperthite, kyanite, quartz, and rutile at 810-860°C, ~1.2 GPa, sillimanite growth during decompression, and upper amphibolite facies cooling at 510 ± 4 Ma. Though the thermobarometric data are just within the methodological errors, the U/Pb zircon ages imply the HP granulites did not evolve coherently. The HP granulites either represent foundered lower arc crust or originated from subduction erosion because their geochemistry is indistinguishable from that of the hanging-wall plate. Published and new pressure-temperature-time-deformation paths converge at ~710°C, ~0.9 GPa, and ≲470 Ma, implying exhumation tectonics juxtaposed the HP granulites with a mélange of eclogite and mantle dunite at lower crustal levels. This study highlights that lower arc crust can comprise material of diverse evolution. K E Y W O R D S high-P granulite, phase equilibria modelling, Qinling Orogen, zircon petrochronology 422 |

Thermal Structure of the Paleo‐Continental Subduction Zone: Insights From Quantitatively Constrained Prograde P–T Paths of Exhumed LT/UHP Eclogites in the Dabie Orogen

“…In addition, it could also be possible that the estimated peak metamorphic conditions of exhumated metamorphic rocks are deviated from the true values. On the one hand, as high/ultrahigh pressure (HP/UHP) rocks usually underwent significant heat overprinting during the exhumation stage, especially for continental subduction zones such as those in the North Dabie UHP belts, China (e.g., Gao et al., 2017), the North Qinling orogen, China (e.g., Bader et al., 2020), and the Western Gneiss Region, Norway (e.g., Butler et al., 2018), it is reasonable to assume that the re‐equilibration of mineral compositions in the exhumation stage could lead to the calculated temperature ( T ) at P max to a higher value. In comparison to modeling results, temperatures recorded by exhumed oceanic rocks show smaller discrepancies than continental rocks (Agard et al., 2018; Penniston‐Dorland et al., 2015), supporting that part of the recovered peak P – T conditions of exhumed continental rocks with high temperatures may be influenced by decompressional heating.…”

Paleozoic Tectonothermal Evolution in the West Qinling Orogen, Central China: Petrological and Chronological Evidence from Garnet Amphibolites

“…In the East Qinling Orogen, the Qinling Complex is mainly comprised of amphibolite-facies to granulite-facies gneisses with subordinate amounts of UHP-HP and ultrahigh-temperature (UHT) rocks [14][15][16][17][18]. Most studies suggest that the UHP-HP rock in the East Qinling Orogen formed in 500-485 Ma, representing the Cambrian continental deep subduction, whereas the widely distributed amphibolite-facies to granulite-facies rocks with clockwise P-T paths formed in 450-400 Ma, reflecting the collision and post-collision (overprint) event [14][15][16][17][18][19][20][21][22]. However, some authors suggested the Silurian high-temperature metamorphic event in the East Qinling Orogen formed in arc or arc-related tectonic environments which were related to the subduction of the Shangdan Ocean [23][24][25][26][27][28][29].…”