Intermittent subduction of the Paleo-Tethys Ocean in the middle–late Permian: evidence from the mafic–intermediate intrusive rocks in the East Kunlun Orogenic Belt

Abstract

In this study, we investigate the geochronology, geochemistry, zircon Lu–Hf isotopes and Sr–Nd isotopes of mafic–intermediate intrusive rocks in the Naxiguole area of the East Kunlun Orogenic Belt (EKOB), northwest China. Zircon U–Pb dating yields formation ages of 254.8 ± 1.2 Ma for gabbros and 251.9 ± 1.3 Ma for gabbrodiorites, suggesting that the mafic–intermediate intrusive rocks were formed in the late Permian. The gabbrodiorites show the geochemical characteristics of arc magmatic rocks, while gabbros were similar to those of backarc basin basalts. The gabbros yielded (87Sr/86Sr)i ratios of 0.7068–0.7097, low ε Nd(t) values of –4.40 to +0.16, and a limited range of ε Hf(t) values of –1.09 to +0.04. The gabbrodiorites display (87Sr/86Sr)i ratios of 0.7090–0.7098, ε Nd(t) values ranging between –3.99 and –3.55, and ε Hf(t) values of –0.81 to +0.34. The Sr–Nd and Lu–Hf isotopes from both types of samples resemble an enriched mantle but distinctly different from the crust. Based on their geochemical, and some trace elemental ratios (e.g. Ba/Th, La/Sm, Th/Yb and Sm/Yb), we suggest that the parental magmas of the gabbros and gabbrodiorites were formed by 1–5% partial melting of spinel lherzolite and garnet–spinel lherzolite, respectively. The gabbros originated from a mantle source modified by subducted slab fluids, while the gabbrodiorites were derived from a mantle source altered by subducted slab melts. By synthesising all the data, we propose that the gabbros were formed in a backarc extension setting and the gabbrodiorites in a continental arc setting. This suggests a geodynamic setting transition from backarc extension to subduction during the late Permian. In combination with previous research, we propose that the subduction mode in the Paleo-Tethys Ocean in the EKOB is intermittent. KEY POINTS Zircon U–Pb dating indicates an age of ca 254.8 ± 1.2 Ma for the gabbros and ca 251.9 ± 1.3 Ma for the gabbrodiorites (i.e. late Permian). The gabbros were derived from an enriched mantle source modified by slab-derived fluids whereas the gabbrodiorites had an enriched mantle source modified by subducted slab melts. The gabbros were in a backarc extension setting while the gabbrodiorites formed in a continental arc setting. This suggests that there was a transition in the geodynamic setting (from backarc extension to subduction).