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Middle to Late Triassic granitic magmatism in the East Kunlun Orogenic Belt, NW China: Petrogenesis and implications for a transition from subduction to post‐collision setting of the Palaeo‐Tethys Ocean
Author(s) -
Gao Hongchang,
Sun Fengyue
Publication year - 2021
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.4104
Subject(s) - porphyritic , zircon , geology , petrogenesis , geochemistry , crust , magmatism , adakite , partial melting , petrology , early triassic , subduction , underplating , mantle (geology) , oceanic crust , tectonics , paleontology , permian , quartz , structural basin
In this study, we present new geochronological and petrogenetic data for the Triassic granitoids of the East Kunlun Orogenic Belt (EKOB), in order to constrain their precise ages, petrogenesis, and tectonic settings. LA‐ICP‐MS zircon U–Pb data indicate that the Triassic granitoids were emplaced in two stages: (a) Middle Triassic (247–240 Ma), represented by a suite of porphyritic granites and granodiorites; and (b) Late Triassic (234–227 Ma), forming an intrusive rock association of K‐feldspar granites, granodiorites, and porphyritic granites. Geochemical analyses and mineral associations suggest that all the Triassic granitoids belong to I‐type granites but have different origins. The Middle Triassic granitoids have high SiO 2 , low to moderate Mg # values (25–37 for XSG porphyritic granite; 45–47 for DB granodiorite), low Sr/Y ratios (2.2–4.6 for XSG porphyritic granite; 17.8–20.8 for DB granodiorite), and relatively restricted zircon εHf(t) values (+2.4 − +4.6 for the ca. 247 Ma porphyritic granite, and − 8.0 to −1.5 for the ca. 240 Ma granodiorite), indicating that they were dominantly generated from partial melting of different crust sources (either juvenile or ancient) in a normal lower crust level. In contrast, the Late Triassic granitoids have high SiO 2 , K 2 O, and Y contents, low MgO and HREE contents, and variable zircon εHf(t) values (from negative to positive, −4.9 to +3.3), implying a strong crustal–mantle interaction that occurred during the Late Triassic, and this stage of granitoids were derived from a complex magma source possibly a mixture of mantle‐derived and ancient crustal‐derived materials. By combining these new data with the previous data, we conclude that the two stages of Triassic granitoids were emplaced in an active continental margin setting and a post‐collisional extension setting, respectively. Moreover, this study suggests a tectonic shift of the Palaeo‐Tethys Ocean in the EKOB from subduction during the Middle Triassic to a post‐collision during the Late Triassic.