Petrological and geochronological constraints on the genesis of the Foping gneiss dome, South Qinling Belt, central China

Abstract

The Foping dome in the South Qinling Belt is a typical gneiss dome that records a multi-stage tectonic evolution from syn-collisional orogenesis to post-orogenic extension. We conducted a detailed petrological and geochronological investigation to constrain the genesis of the dome and its tectonic implications. Field occurrence, mineral assemblage, and metamorphic grade were used to divide the rocks of the Foping dome into three tectonic units. Conventional thermobarometry and pseudosection modelling were used to estimate the P-T conditions of the three units, yielding 640–680 °C and 3.4–5.0 kbar for the upper unit, 690–740 °C and 4.8–7.8 kbar for the middle unit, and 770–800 °C and 5.1–6.8 kbar for the lower unit, corresponding to amphibolite to granulite facies metamorphism under a geothermal gradient of ∼35 °C/km. The metamorphic patterns of three units reveal a metamorphic zonation characterized by decreasing metamorphic temperature from the centre to the periphery of the dome. Monazite SHRIMP U-Pb dating of metapelites yielded two major age clusters at 214–210 Ma and 207–197 Ma. The former cluster represents the timing of early regional progressive metamorphism induced by crustal thickening in a compressional setting, and the latter represents the timing of subsequent thermal metamorphism induced by magmatic diapirism in an extensional setting. The metamorphic zonation characteristics and ages of the Foping dome match with the pattern, nature, and ages of magmatism in this area, suggesting that the dome was formed by magmatic diapirism in a post-collisional extensional setting that was superimposed on the preceding progressive metamorphism associated with crustal thickening.