Tectonic Evolution of the Southern Dabie Orogenic Belt, China: Insights from Peak P–T Conditions and U–Pb Zircon Dating of the Susong Metamorphic Complex

Abstract

The Susong metamorphic complex (SSC) in the southern margin of the Dabie orogenic belt (DOB) in central-eastern China is a key metamorphic unit for understanding subduction and exhumation processes in the DOB. However, the formation age and metamorphic grade of the SSC remain uncertain, hampering our understanding of the mechanism of the formation of the DOB. An integrated study of field survey, regional metamorphic petrology, geothermobarometry, and U–Pb dating of zircon was carried out in this study. Our results reveal that the SSC was metamorphosed under epidote amphibolite- to amphibolite-facies conditions with average metamorphic P–T values of 0.98 ± 0.07 GPa and 531 ± 35 °C. The smooth spatial variation in peak P–T conditions and an apparent geothermal gradient of ~17 °C/km indicate that the SSC as a whole fall into Barrovian-type metamorphic environments. Zircon U–Pb dating for garnet–mica schists of sample ZT003, ZT005 and ZT006 yield five (Groups I to V), six (Groups I to VI) and five (Groups I to V) age groups, respectively, concentrating on the Meso-Neoarchean, early-middle Paleoproterozoic, middle Mesoproterozoic, early Neoproterozoic, Palaeozoic and Triassic-lower Jurassic. Therein, a 259–190 Ma (Group V) from zircons with Th/U ratios of <0.1 in sample ZT006 record the timing of both peak and retrograde metamorphism for the SSC. All other ages are detrital zircon ages, and from age provenances in the DOB or the Yangtze Block (YZB), indicating the YZB affinity of the SSC. The two youngest age populations of 427–415 Ma (Group VI) and 475–418 Ma (Group V) from samples ZT005 and ZT006, respectively, suggest that the formation age of the SSC could be Middle Devonian. The similarity of formation age and peak P-T conditions of the SSC to Foziling Group, located in the northernmost DOB, implies that both units formed the sedimentary cover on the passive continental margin of the YZB during the late Palaeozoic, and subducted into the middle-lower crust of 20–40 km depth as a whole, corresponding to the shallow subduction. Compared to the deep subduction defined by high-pressure (HP) and ultrahigh-pressure (UHP) units, larger differences in peak P–T conditions, age and geothermal gradient between two different tectonic environments happen. Accordingly, it is speculated that a transitional subduction from shallow to deep levels occurred at Moho depths during the Early Triassic, and is due to a change in subduction dip angle.

Early Jurassic initiation of the modern drainage pattern of the Dabie orogen (East China) revealed by a multi‐proxy provenance approach

“…In contrast, the sedimentary basins on the northeastern, eastern, and southern sides of the Dabie orogen generally lack Triassic metamorphic zircons (Figure 8) but are characterised by Triassic phengite 40 Ar/ 39 Ar ages (with a main peak at 213-217 Ma; Grimmer et al, 2003) and Triassic rutile U-Pb ages (Figure 6). Most importantly, the main rutile peaks of the Hefei basin (northernmost basin, 216 Ma), Nanjing basin (223 Ma), Yueshan basin (251 Ma), and Huangshi basin (southernmost basin, 256 Ma; Figure 6) mirror the trend of younger metamorphic ages from north to south in the present-day Dabie orogen (Shi et al, 2022). The main rutile U-Pb peak of the Huangshi and Yueshan basins (251-256 Ma) matches with metamorphic zircon ages of the SuSong Complex (259 Ma; Shi et al, 2022), the earliest exhumed HP unit in the Dabie orogen.…”

Sedimentological Evidence for Pre‐Early Permian Continental Subduction in the Dabie Orogen, Central‐East China

“…Our conclusions are supported by other geochronological data from the scattered medium‐high grade Permian metamorphic bedrock, and limited paleomagnetic and provenance data, including: (a) c. 260–300 Ma 40 Ar/ 39 Ar ages from the Dabie‐Sulu orogen which are interpreted to record tectonothermal events associated with early continental collision and subduction (c. 265 Ma muscovite 40 Ar/ 39 Ar ages from low‐grade schists in the Sulu orogen, J.‐B. Zhou et al., 2008; c. 261–279 Ma muscovite 40 Ar/ 39 Ar ages from the Xinyang–Beihuaiyang unit on the northern margin of the Dabie orogen, Hacker et al., 2000; Ratschbacher et al., 2003, 2006; c. 300 Ma illite K‐Ar and illite‐whole rock Rb‐Sr isochron ages from low greenschist facies (epizonal) slates, Zhu et al., 1994); (b) Early‐Middle Permian (c. 258–297 Ma) garnet Lu‐Hf, apatite and zircon U‐Pb dates from (U)HP eclogite‐facies rocks in the Dabie orogen (Brouwer et al., 2011; H. Cheng & Cao, 2015; H. Cheng et al., 2009, 2016, 2018; Shi et al., 2022; K. Sun et al., 2022) that may record prograde metamorphism associated with continental subduction; (c) a Permian (c. 250–270 Ma) medium‐pressure, amphibolite‐facies metamorphic belt that supposedly extended for over 500 km in the Qinling‐Tongbai‐Hong'an‐Dabie orogen and interpreted to be related to Permian initial continental collision between the SCB‐NCC (L. Chen et al., 2014, 2020); (d) detrital metamorphic zircon of Permian age (280 Ma, n = 12) in the Lower Cretaceous of the Jiaolai Basin to the north of the Sulu Orogen (L. X. Zhang et al., 2021); (e) comparable regional medium‐to high‐pressure metamorphism in the eastern continuation of the Dabie‐Sulu orogen in South Korea (the Ogcheon metamorphic belt), which initiated as early as the Early Permian (281–291 Ma) and peaked in the Triassic (c. 225 to 255 Ma) (Cheong et al., 2003; D.‐L. Cho et al., 2008; M. Cho & Kim, 2005); (f) paleomagnetic results that show that the easternmost NCC and SCB were juxtaposed during the Middle Permian (c. 270 Ma) (X. Zhao & Coe, 1987); (g) various disparate lines of evidence that indicate initial continental subduction could have initiated in the Early Permian including c. 258–300 Ma geochronological ages recording metamorphism and rapid cooling, the Permian sedimentary facies distribution in the SCB, and Permian thrusting events on the southeastern NCC (Li, 1998).…”

“…Cheng et al, , 2016H. Cheng et al, , 2018Shi et al, 2022;K. Sun et al, 2022) that may record prograde metamorphism associated with continental subduction; (c) a Permian (c. 250-270 Ma) medium-pressure, amphibolite-facies metamorphic belt that supposedly extended for over 500 km in the Qinling-Tongbai-Hong'an-Dabie orogen and interpreted to be related to Permian initial continental collision between the SCB-NCC (L. Chen et al, 2014Chen et al, , 2020; (d) detrital metamorphic zircon of Permian age (280 Ma, n = 12) in the Lower Cretaceous of the Jiaolai Basin to the north of the Sulu Orogen (L. X.…”

“…These rocks comprise the leading edge of the subducted SCB and the depth‐profiling U‐Pb age data record old (264 ± 6 Ma to 283 ± 5 Ma) metamorphic events that currently represent the best estimates for the onset of initial continental subduction (H. Tang, 2022). Similarly, recent geochronological data on rare HP eclogites of continental affinity suggest that the Dabie orogen experienced Late Carboniferous‐Permian (c. 310 Ma and c. 256–297 Ma) HP metamorphism, also suggesting it experienced older phase(s) of continental subduction (Brouwer et al., 2011; H. Cheng & Cao, 2015; H. Cheng et al., 2009, 2011, 2018; X. Liu et al., 2011; Shi et al., 2022). Hence the onset of continental subduction associated with the Dabie orogen, during which the continental margin of the SCB was underthrust and subducted to great depths, is poorly constrained.…”

“…Cheng et al, , 2011H. Cheng et al, , 2018Shi et al, 2022). Hence the onset of continental subduction associated with the Dabie orogen, during which the continental margin of the SCB was underthrust and subducted to great depths, is poorly constrained.…”