Abstract
The Triassic eclogite-bearing central Qiangtang metamorphic belt (CQMB) in the northern Tibetan Plateau has been debated whether it is a metamorphic core complex underthrust from the Jinsha Paleo-Tethys or an in-situ Shuanghu suture. The CQMB is thus a key issue to elucidate the crustal architecture of the northern Tibetan Plateau, the tectonics of the eastern Tethys, and the petrogenesis of Cenozoic high-K magmatism. We here report the newly discovered Baqing eclogite along the eastern extension of the CQMB near the Baqing town, central Tibet. These eclogites are characterized by the garnet + omphacite + rutile + phengite + quartz assemblages. Primary eclogite-facies metamorphic pressure–temperature estimates yield consistent minimum pressure of 25 ± 1 kbar at 730 ± 60 °C. U–Pb dating on zircons that contain inclusions (garnet + omphacite + rutile + phengite) gave eclogite-facies metamorphic ages of 223 Ma. The geochemical continental crustal signature and the presence of Paleozoic cores in the zircons indicate that the Baqing eclogite formed by continental subduction and marks an eastward-younging anticlockwise West–East Qiangtang collision along the Shuanghu suture from the Middle to Late Triassic.
Highlights
The Qiangtang terrane in the northern Tibetan Plateau represents Gondwana-derived continental fragments and is bounded to the north by the Jinsha Paleo-Tethyan suture and to the south by the Bangong Meso-Tethyan suture, respectively (Fig. 1a,b)
The western central Qiangtang metamorphic belt (CQMB) eclogite and blueschist occur as blocks in garnet-mica-quartz schist, marble or metampelite, with eclogite-facies mineral assemblage of garnet + omphacite + rutile + quartz consistent with pressure of 20–25 kbar at 410–460 °C4,9
The relatively moderate Mg contents of garnet (XMg = Mg/(Mg + Fe2+)) are from 0.09 to 0.43, on average, 0.34 (Table S2), compared to mantle eclogite (XMg = 0.78–0.93)[4,32], imply that the Baqing eclogite is unlikely of mantle origin, consist with the orogenic origin of the C-type eclogite field[18] (Fig. 3c)
Summary
The Qiangtang terrane in the northern Tibetan Plateau represents Gondwana-derived continental fragments and is bounded to the north by the Jinsha Paleo-Tethyan suture and to the south by the Bangong Meso-Tethyan suture, respectively (Fig. 1a,b). The western CQMB eclogite and blueschist occur as blocks in garnet-mica-quartz schist, marble or metampelite, with eclogite-facies mineral assemblage of garnet + omphacite + rutile + quartz consistent with pressure of 20–25 kbar at 410–460 °C4,9. The eclogite pods are generally massive at outcrop scale (Figs S1, S2) They occur as variably retrogressed elongate lens-like boudins, a few centimeters to tens of meters in size, with long axis sub-parallel to the foliation strike of the host schists (Figs S1, S2). These Baqing eclogite boudins juxtapose the host garnet-mica-quartz schist with ductile fault contact (Figs S1, S2a). The Baqing eclogite shows coarse-grained texture with mineral assemblages of garnet, omphacite, rutile, phengite, and minor quartz, epidote and titanite (Fig. S2b; Table S1)
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