Ultrahigh-pressure metamorphic rocks in the Dabie–Sulu orogenic belt: compositional inheritance and metamorphic modification

A combined study of petrology and geochemistry was carried out for granulites from the Tongbai orogen in central China. The results reveal the tectonic evolution from collisional thickening to extensional thinning of the lithosphere at the convergent plate boundary. Petrographic observations, zircon U–Pb dating, and pseudosection calculations indicate that the granulites underwent four metamorphic stages, which are categorized into two cycles. The first cycle occurred at 490–450 Ma and involves high‐P (HP) metamorphism (M1) at 785–815°C and 10–14 kbar followed by decompressional heating to 840–880°C and 8–9 kbar for medium‐pressure granulite facies metamorphism (M2), defining a clockwise P–T path. The high pressure is indicated by the occurrence of inclusions of rutile+kyanite+K‐feldspar in the garnet mantle. The second cycle occurred at c. 440 Ma and shows an anticlockwise P–T path with continuous heating to ultrahigh‐temperature (UHT) metamorphism (M3) at 890–980°C and 9–11 kbar, followed by decompressional cooling to 740–880°C and 7–9 kbar (M4) till 405 Ma. The HP metamorphism is synchronous with the ultrahigh‐pressure eclogite facies metamorphism in the Qinling orogen, indicating its relevance to the continental collision in the Cambrian. The UHT metamorphism took place at reduced pressures, indicating thinning of the collision‐thickened orogenic lithosphere. Therefore, the Tongbai orogen was initially thickened by the collisional orogeny and then thinned, possibly as a result of foundering of the orogenic root. Such tectonic evolution may be common in collisional orogens where compression during continental collision switched to extension during continental rifting.

Section: Introductionmentioning

confidence: 99%

Granulites record the tectonic evolution from collisional thickening to extensional thinning of the Tongbai orogen in central China

Zhang

Gao

Chen

et al. 2020

“…The post‐collisional anatexis in this area was probably caused by the extensional heating of the collisional orogen with some possible contribution from the foundering of collision‐thickened lower crustal material within the Sulu UHP zone (e.g. Liu et al, ; Zheng, Zhao, & Chen, ). This interpretation is consistent with the presence of 174–141 Ma Jurassic granitoids within the northeastern Sulu orogen (e.g.…”

Section: Discussionmentioning

confidence: 95%

“…These differences in the Jurassic reworking may be related to the post‐Jurassic subduction of the Palaeo‐Pacific slab beneath eastern China (Zheng, Xiao, & Zhao, ; Zheng, Xu, Zhao, & Dai, ). The Dabie and Sulu orogens formed part of a coherent orogenic belt during the late Triassic (Zheng et al, , ). However, the Late Triassic partial melting of UHP metamorphic rocks is much more widespread in the Sulu orogen than in the Dabie orogen (Zhao, Liu, et al, ; Zhao, Zheng, et al, ).…”

Section: Implications For the Tectonic Evolution Of The Dabie–sulu Ormentioning

confidence: 99%

Migmatites record multiple episodes of crustal anatexis and geochemical differentiation in the Sulu ultrahigh‐pressure metamorphic zone, eastern China

Zhou

Chen

Zheng

et al. 2019

Partial melting of ultrahigh‐pressure (UHP) metamorphic rocks is common during collisional orogenesis and post‐collisional reworking, indicating that determining the timing and processes involved in this partial melting can provide insights into the tectonic evolution of collisional orogens. This study presents the results of a combined whole‐rock geochemical and zirconological study of migmatites from the Sulu orogen in eastern China. These data provide evidence of multiple episodes of crustal anatexis and geochemical differentiation within the UHP metamorphic rocks. The leucosomes contain higher concentrations of Ba and K and lower concentrations of the rare earth elements (REE), Th and Y, than associated melanosomes and granitic gneisses. The leucosomes also have homogenous Sr–Nd–O isotopic compositions that are similar to proximal (i.e. within the same outcrop) melanosomes, suggesting that the anatectic melts were generated by the partial melting of source rocks that are located within individual outcrops. The migmatites contain zircons with six different types of domains that can be categorized using differences in structures, trace element compositions, and U–Pb ages. Group I domains are relict magmatic zircons that yield middle Neoproterozoic U–Pb ages and contain high REE concentrations. Group II domains represent newly grown metamorphic zircons that formed at 230 ± 1 Ma during the collisional orogenesis. Groups III, IV, V, and VI zircons are newly grown anatectic zircons that formed at 222 ± 2 Ma, 215 ± 1 Ma, 177 ± 2 Ma, and 152 ± 2 Ma, respectively. The metamorphic zircons have higher Th/U and lower (Yb/Gd)N values, flat heavy REE (HREE) patterns with no significantly negative Eu anomalies relative to the anatectic zircons, which are characterized by low Th/U ratios, steep HREE patterns, and negative Eu anomalies. The first two episodes of crustal anatexis occurred during the Late Triassic at c. 222 Ma and c. 215 Ma as a result of phengite breakdown. The other two episodes of anatexis occurred during the Jurassic period at c. 177 Ma and c. 152 Ma and were associated with extensional collapse of the collision‐thickened orogen. The majority of Triassic anatectic zircons and all of the Jurassic zircons are located within the leucosomes, whereas the melanosomes are dominated by Triassic metamorphic zircons, suggesting that the leucosomes within the migmatites record more episodes of crustal anatexis. Both metamorphic and anatectic zircons have elevated εHf(t) values compared with relict magmatic zircon cores, suggesting that these zircons contain non‐zircon Hf derived from material with more radiogenic Hf isotope compositions. Therefore, the Sulu and Dabie orogens experienced different episodes of reworking during the exhumation and post‐collisional stages.

“…220 Ma, Schmidt et al., ), and were interpreted to record the timing of the final stage of UHP metamorphism or the early post‐peak stage (Schmidt et al., , ). These garnet ages suggest that eclogites from the CCSD borehole may be distinct from those in the northern Sulu, and potentially represent a distinct exhumed crustal slice (Zheng et al., ). The interpretation of the c .…”

Section: Discussionmentioning

confidence: 99%

“…Zheng et al. () presented a subduction channel model for the Dabie–Sulu UHP belt, in which subducted continental crust is ‘sliced' into blocks which may move independently of one another. According to this model, the west–east sub‐blocks in the Dabie–Sulu belt may correlate with different crustal slices in the subduction channel.…”

Section: Discussionmentioning

confidence: 99%

Integrated garnet and zircon–titanite geochronology constrains the evolution of ultra‐high–pressure terranes: An example from the Sulu orogen

Vervoort

Fisher

et al. 2019

Dating ultra‐high–pressure (UHP) metamorphic rocks provides important timing constraints on deep subduction zone processes. Eclogites, deeply subducted rocks now exposed at the surface, undergo a wide range of metamorphic conditions (i.e. deep subduction and exhumation) and their mineralogy can preserve a detailed record of chronologic information of these dynamic processes. Here, we present an approach that integrates multiple radiogenic isotope systems in the same sample to provide a more complete timeline for the subduction–collision–exhumation processes, based on eclogites from the Dabie–Sulu orogenic belt in eastern China, one of the largest UHP terranes on Earth. In this study, we integrate garnet Lu–Hf and Sm–Nd ages with zircon and titanite U–Pb ages for three eclogite samples from the Sulu UHP terrane. We combine this age information with Zr‐in‐rutile temperature estimates, and relate these multiple chronometers to different P–T conditions. Two types of rutile, one present as inclusions in garnet and the other in the matrix, record the temperatures of UHP conditions and a hotter stage, subsequent to the peak pressure (‘hot exhumation') respectively. Garnet Lu–Hf ages (c. 238–235 Ma) record the initial prograde growth of garnet, while coupled Sm–Nd ages (c. 219–213 Ma) reflect cooling following hot exhumation. The maximum duration of UHP conditions is constrained by the age difference of these two systems in garnet (c. 235–220 Ma). Complementary zircon and titanite U–Pb ages of c. 235–230 Ma and c. 216–206 Ma provide further constraints on the timing of prograde metamorphism and the ‘cold exhumation' respectively. We demonstrate that timing of various metamorphic stages can thus be determined by employing complementary chronometers from the same samples. These age results, combined with published data from adjacent areas, show lateral diachroneity in the Dabie–Sulu orogeny. Three sub‐blocks are thus defined by progressively younger garnet ages: western Dabie (243–238 Ma), eastern Dabie–northern Sulu (238–235 Ma) and southern Sulu terranes (225–220 Ma), which possibly correlate to different crustal slices in the recently proposed subduction channel model. These observed lateral chronologic variations in a large UHP terrane can possibly be extended to other suture zones.