Tectonic affinity and evolution of the Foziling Group: A window into the Dabie orogenic belt's shallow subduction process

Abstract

The Dabie orogenic belt (DOB) has been a classical hot spot for the study of deep continental subduction. However, research on the characteristics of the shallow subduction of the belt is lacking, which limits the full interpretation of the subduction and exhumation processes in the DOB. The Foziling Group, located at the northernmost edge of the DOB, has been considered the best carrier for studying the shallow subduction. Unfortunately, there has been a considerable amount of controversy regarding the metamorphic grade and tectonic affinity of the Foziling Group. For this reason, detailed studies involving geological surveys, petrography, mineral chemistry, geothermobarometry, and detrital zircon geochronology were conducted on the group. The Foziling Group is mainly composed of mica schist, garnet-mica schist, mica-quartz schist, and a small amount of quartzite and marble. Based on the evaluation of the peak metamorphic P-T conditions for 45 samples and previously reported six peak metamorphic P-T values, the Foziling Group can be divided into Zone I and Zone II, whose peak metamorphic conditions are T = 467–519 °C and P = 0.39–0.57 GPa and T = 535–645 °C and P = 0.59–0.96 GPa, respectively. Both P-T estimates fall within the amphibolite facies, and a ∼ 26 °C/km geothermal gradient line was constructed in the field, indicating that the Foziling Group experienced Barrovian metamorphism. Moreover, UPb dating of 465 detrital zircon grains from 5 samples of the group revealed five age peaks at 2.48, 0.94, 0.82, 0.63, and 0.44 Ga and two sub-peaks around ∼2.0–1.85 and ∼ 1.4 Ga. Among them, the youngest six detrital zircon ages range from 412 ± 12 Ma to 389 ± 14 Ma, which indicates that the Foziling Group formed during the Late Devonian. Combining the provenance analysis of detrital zircons, all zircons from the Foziling Group are derived from the DOB, suggesting that the Foziling Group belongs to the Yangtze Block. Compared with the peak metamorphic conditions, the timing of protolith formation, and the ∼ 17 °C/km geothermal gradient line of the Susong Complex at the southern margin of the DOB, both units are interpreted as Late Paleozoic passive continental margin sediments at the northern edge of the Yangtze Block, having both experienced shallow crustal-level subduction during the Indosinian period of the Dabie orogeny. This subduction was of the warm type, terminating near the Moho discontinuity. In contrast, the deep continental subduction of the belt reached the lithospheric mantle and asthenospheric depths, representing a cold subduction type. The transition between these subduction types is speculated to be due to the change in the subduction angle of the tectonic plate.