Zircon U-Pb age and Sr-Nd-Hf isotope geochemistry of the Ganluogou dioritic complex in the northern Triassic Yidun arc belt, Eastern Tibetan Plateau: Implications for the closure of the Garzê-Litang Ocean

Abstract

The Triassic Yidun arc belt (YAB) lies between the Jinshajiang suture zone to the west and the Garzê-Litang suture zone to the east, Eastern Tibetan Plateau. To study the YAB can not only help us to better understand the evolutionary history of the Garzê-Litang Ocean but can also provide some important information to constrain the evolution of the eastern Paleo-Tethys. In this paper, the geochronological and geochemical data of the Ganluogou dioritic complex were systematically investigated in order to decipher the geodynamic setting of the complex and to further determine the final closure time of the Garzê-Litang Ocean. The Ganluogou dioritic complex is located in the northern part of the YAB. It consists of ferrodiorite, diorite and a mixing zone between them and is the largest intermediate-mafic pluton in the YAB. The ferrodiorites were emplaced at 213 ± 2 Ma have low SiO2 and high Fe2O3* contents, whereas the diorites formed at 209 ± 2 Ma and have relatively higher SiO2, Na2O + K2O, Th, U, Zr, and Hf contents, but lower Al2O3, MgO, CaO, Co, and Sr contents than the ferrodiorites. Relative to the primitive mantle both the ferrodiorites and diorites are depleted in Nb and Ta. However, the ferrodiorites exhibit strong depletion in Zr and Hf, whereas the diorites contain relatively higher Th and U contents without negative Zr and Hf anomalies. Both rock-types have similar chondrite-normalized rare earth element patterns with (La/Yb)N ratios = 4.4 to 18.2, and show weak Eu anomalies, with Eu/Eu* of 0.47 to 1. They both show narrow ranges in Sr-Nd-Hf isotopic compositions. However, the ferrodiorites contain lower initial 87Sr/86Sr ratios (0.7052-0.7057) and relatively higher eNd(t) values (-3.8 to -2.4) than the diorites, which record values of 0.7062-0.7066 and -5.5 to -5.7, respectively. For the zircon Hf isotopic composition, the ferrodiorites also exhibit higher 176Hf/177Hf ratios (0.282738-0.282804) and more depleted eHf(t) values (3.4-5.6) than those of the diorites (176Hf/177Hf ratios of 0.282690-0.282728 and eHf(t) values of 1.7-2.8). These geochemical and isotopic features suggest that the ferrodiorites and diorites were derived from the different magma sources. We propose that the ferrodiorites were produced by differentiation of mantle-derived tholeiitic arc magma, whereas the diorites were derived either from dehydration melting of meta-basaltic rocks from the Kangding Complex (the basement of the west Yangtze Craton) or young arc volcanic rocks but mixed with a small amount of sediments. Both ferrodiorites and diorites were emplaced at relatively low pressure (3.3-3.9 kbar) but with quite different magma temperature (ferrodiorites: 821-960 °C; diorites: 692-788 °C). The Ganluogou dioritic complex was therefore formed under an extension setting that was associated with the closure of the Garzê-Litang Ocean. Previous studies on Late Triassic magmatism in the area have shown that there was slab-break off event at ~216 Ma. Combined with regional geological background, we conclude that the final closure of the Garzê-Litang segment of the Paleo-Tethys Ocean occurred prior to 216 Ma.