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

Abstract

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.

Tectonothermal transition from continental collision to post‐collision: Insights from eclogites overprinted in the ultrahigh‐temperature granulite facies (Yadong region, central Himalaya)

“…Given the convergence rate is still 1–2 mm/year in the central Himalaya (Ader et al, 2012), the Himalayan orogen shows that the initiation of post‐collisional extension can overlap the decaying collisional convergence. UHT granulites have been widely reported from continent–continent collisional orogens and attributed to variable post‐collisional geodynamic process, including slab break‐off (e.g., B. C. Lee et al, 2016), lower crustal delamination (e.g., Q. Zhang et al, 2020), orogen‐based rifting (Y. F. Zheng & Chen, 2017), and magmatic underplating in thickened crust (Harley, 1989). Essentially, UHT metamorphism in continental orogens is likely related to the post‐collisional extension (Lei & Xu, 2018), which could facilitate the exhumation and bring the asthenospheric mantle to the Moho depth.…”

“…Wang et al, 2014). The resulting upwelling of asthenospheric heat profoundly affects the thermal regime, which could be recorded by high-and ultrahightemperature (HT/UHT) granulites (e.g., Engvik et al, 2018;Usuki et al, 2017;Q. Zhang et al, 2020).…”

“…Meanwhile, the post‐collisional collapse and the related magmatism are linked to the destruction of deep orogenic structure (e.g., Song et al, 2015; M. Wang et al, 2014). The resulting upwelling of asthenospheric heat profoundly affects the thermal regime, which could be recorded by high‐ and ultrahigh‐temperature (HT/UHT) granulites (e.g., Engvik et al, 2018; Usuki et al, 2017; Q. Zhang et al, 2020). Hence, the metamorphic (U)HP/(U)HT rocks are fossil records of orogenic tectonothermal evolution.…”

Significance of selective crystal entrainment and differential crystal‐melt separation in petrogenesis of granites from the Tongbai orogen

“…The composite P–T path and long‐lived HT metamorphism suggest that these granulites were produced in a post‐collisional extensional setting (e.g. Zhang, Gao, Zhang, et al, 2020).…”

Extreme metamorphism and metamorphic facies series at convergent plate boundaries: Implications for supercontinent dynamics

“…In either case, the HP granulite-facies superimposition would be subsequent to the eclogite-facies UHP metamorphic event. On the other hand, there are collisional orogens where HP granulite-facies superimposition significantly postdates HP to UHP eclogite-facies metamorphism, such as in the Dabie orogen (Gao et al, 2017) and the Tongbai orogen (Zhang et al, 2020). This points to continental rifting as the mechanism responsible for metamorphic superimposition in continental interiors (Zheng and Chen, 2017).…”