Stepwise exhumation of the Triassic Lanling high‐pressure metamorphic belt in Central Qiangtang, Tibet: Insights from a coupled study of metamorphism, deformation, and geochronology

Abstract

The E‐W trending Central Qiangtang metamorphic belt (CQMB) is correlated to the Triassic orogeny of the Paleo‐Tethys Ocean prior to Cenozoic growth of the Tibetan Plateau. The well‐exposed Lanling high‐pressure, low‐temperature (HP‐LT) metamorphic complex was chosen to decipher the process by which it was exhumed, which thereby provides insights into the origin of the CQMB and Qiangtang terrane. After a detailed petrological and structural mapping, three distinct N‐S‐trending metamorphic domains were distinguished. Microscopic observations show that core domain garnet (Grt)‐bearing blueschist was exhumed in a heating plus depressurization trajectory after peak eclogitic conditions, which is more evident in syntectonic vein form porphyroblastic garnets with zoning typical of a prograde path. Grt‐free blueschist of the mantle domain probably underwent an exhumation path of temperature increasing and dehydration, as evidenced by pervasive epidote veins. The compilation of radiometric results of high‐pressure mineral separates in Lanling and Central Qiantang, and reassessments on the published phengite data sets of Lanling using Arrhenius plots allow a two‐step exhumation model to be formulated. It is suggested that core domain eclogitic rocks were brought onto mantle domain blueschist facies level starting at 244–230 Ma, with exhumation continuing to 227–223.4 Ma, and subsequently were exhumed together starting at 223–220 Ma, reaching lower greenschist facies conditions generally after 222–217 Ma. These new observations indicate that the CQMB formed as a Triassic autochthonous accretionary complex resulting from the northward subdcution of the Paleo‐Tethys Ocean and that HP‐LT rocks therein were very probably exhumed in an extensional regime.

Petrology, geochemistry and P–T–t path of lawsonite‐bearing retrograded eclogites in the Changning–Menglian orogenic belt, southeast Tibetan Plateau

“…Furthermore, the HP metamorphic rocks in the LCSS and CMOB commonly occur as blocks or lenses within micaschists (Dong & Li, ; Fan et al., ; Liang et al., ; Wang et al., ; Zhai, Jahn, et al., ; Zhai et al., ; Zhai, Zhang, et al., ; this study). Blueschists in the two belts also exhibit consistent lithological features in terms of their mineral association of glaucophane+actinolite+phengite+epidote+albite±garnet±lawsonite (Fan et al., ; Liang et al., ; Liu, Lu, Chen, Nan, & Xie, ; Wang et al., ; Zhai et al., ), metamorphic ages of 242–223 Ma mainly derived from glaucophane 39 Ar– 40 Ar dating (Table ; Bao et al., ; Fan et al., ; Liang et al., ; Tang & Zhang, ; Zhai et al., ) and OIB‐like protolith signature with formation age of 304–260 Ma (Deng et al., ; Liang et al., ; Liu, Lu, et al., ). In conclusion, the consistent peak metamorphic assemblages, metamorphic P–T condition, P–T path, metamorphic ages, and the similar protolith geochemical features for the LCSS and CMOB eclogites as well as the blueschists provide robust evidence for Triassic oceanic subduction event in the two belts as well as in the main segment of the Palaeo‐Tethys.…”

“…However, eclogites have so far not been reported from the CMOB. In contrast, both Triassic blueschists and eclogites were extensively reported in the Longmu Co–Shuanghu suture (LCSS) in the Qiangtang area, Tibetan Plateau (Bao, Xiao, Wang, Li, & Hu, ; Dong & Li, ; Li et al., ; Liang et al., ; Liu, Santosh, Zhao, Niu, & Wang, ; Zhai, Jahn, Zhang, Wang, & Su, ; Zhai, Zhang, et al., ). Thus, the apparent contrast in metamorphic sequences between the CMOB and LCSS, and the lower P – T conditions registered by the CMOB blueschists with respect to the LCSS eclogites have hampered, so far, the complete understanding of the evolution of the major suture of the Palaeo‐Tethys from a regional geodynamic perspective.…”

Sedimentary Evolution and Provenance of the late Permian‐middle Triassic Raggyorcaka Deposits in North Qiangtang (Tibet, Western China): Evidence for a Forearc Basin of the Longmu Co‐Shuanghu Tethys Ocean

“…This inference is based on the Paleozoic‐early Triassic ophiolite suites discovered within the hanging wall of the detachment fault (Li et al, ; Li, Chen, et al, ; Li, Huang, et al, , ; Zhu et al, ; Wang, Pan, et al, ; Wu et al, ; Zhai, Jahn, Wang, et al, , ; Zhang et al, ; Fan et al, ) and the distinct stratigraphy and biota in the two blocks (Li, ; Li et al, ;Li, Chen, et al, ; Li, Huang, et al, , ; Liu et al, ; Metcalfe, ). The in situ suture model is also supported by the observation that the CQMB features multistage penetrative deformations associated with oceanic subduction and the ensuing late Triassic‐early Jurassic continental collision between the NQB and SQB (Liang et al, , ; Li et al, ; Wang et al, ; Zhao et al, ). In the past, the NQB was thought to be derived from the Cathaysian realm (Li et al, ; Li, Chen, et al, ; Li, Huang, et al, ; ).…”

“…On the other hand, the in situ suture model with northward oceanic subduction under the NQB (Li et al, , Li, Chen, et al, , Li, Huang, et al, , ; Zhai, Zhang, et al, , Zhai, Jahn, et al, ; Liang et al, , ; Li et al, ; Wang et al, ) could potentially better explain the late Devonian‐middle Triassic geological evolution in the southern NQB (Figure ), primarily based on the following evidence. First, the Longmu Co‐Shuanghu Tethys Ocean, which once separated the NQB and SQB (Li, ; Li et al, ), was demonstrated to be a diachronous ocean as supported by the continuous Cambrian‐early Triassic (501–242 Ma) ophiolite suites (Fan et al, ; Hu et al, ; Li et al, , Li, Chen, et al, , Li, Huang, et al, , ; Wang, Pan, et al, ; Wu et al, ; Zhai, Jahn, Wang, et al, , ; Zhang et al, ; Zhu et al, ).…”

“…Numerous studies have investigated the CQMB, especially the Permian (283–259 Ma) and Triassic (244–211 Ma) HP‐LT metamorphic rocks (Figure ; Deng et al, ; Kapp et al, ; Li et al, ; Li, Zhai, Dong, et al, ; Li, Zhai, Chen, et al, ; Liang et al, , ; Yin & Harrison, ; Zhao et al, , ; Zhang et al, ) and Paleozoic‐early Triassic ophiolites (Li et al, ; Li, Chen, et al, ; Li, Huang, et al, , ; Zhu et al, ; Wang, Pan, et al, ; Wu et al, ; Zhai, Jahn, Wang, et al, , ; Zhang et al, ). In contrast, the southern margin of the NQB, especially the late Paleozoic‐Triassic sedimentary evolution and its tectonic relationship with the CQMB, remains poorly studied.…”