Early Permian–Late Triassic granitic magmatism in the Jiamusi–Khanka Massif, eastern segment of the Central Asian Orogenic Belt and its implications

Abstract

Whole-rock major and trace elements, together with zircon U–Pb and Hf-isotope data, are reported for the granitic intrusions in the central part of the Jiamusi–Khanka Massif in the eastern segment of the Central Asian Orogenic Belt (CAOB), in order to investigate their precise geochronological framework, petrogenesis, sources and tectonic implications. LA-ICP-MS zircon U–Pb data, combined with previous ages, indicate that the Jiamusi–Khanka Massif is dominated by three stages of granitic magmatism, i.e., Cambrian (530–493Ma), Permian (299–254Ma) and Late Triassic (223–212Ma), with two possible subordinate magmatic events that occurred in the Silurian (434–416Ma) and Carboniferous (331–317Ma). The Permian diorite and granodiorite contain high Al2O3 (15.81–19.07wt.%), Na2O (K2O/Na2O=0.25–0.44) and Sr (748–949ppm) contents and low Y (4–18ppm) and Yb (0.37–1.94ppm) contents, with high Sr/Y ratios of 54–207, indicating that they probably originated from partial melting of a mafic lower crust at high-pressure conditions. However, the coeval monzogranite and granite porphyry have higher SiO2, Y, Yb and HREE contents and lower Al2O3, Na2O/K2O ratio and Sr contents, together with the features of LILE-enriched, HFSE-depleted and Nb–Ta–Ti troughs, reflecting that they were likely produced by partial melting of arc-type crustal materials at a relatively shallow crustal level. Meanwhile, the Late Triassic granites contain high SiO2 (75.90–77.40wt.%) and K2O (4.17–4.87wt.%), combined with low Sr and high Y and HREE, suggesting that they were possibly generated from crustal materials with involvement of slab-derived fluids at low pressure. All the granitoids have high 176Hf/177Hf ratios (0.282518–0.282842) with Hf two-stage model ages of 2005–809Ma, and most of the magmatic zircons have positive εHf(t) values, indicating that the primary magmas of these rocks were derived from partial melting of a dominantly “old” source with a possible significant addition of juvenile materials. Based on the geochemical data and the regional geological investigations, we propose that the Jiamusi–Khanka Massif was an exotic block probably derived from Gondwana, which might be located in a relatively southerly place compared with the current position and faced the Paleo-Pacific Ocean during the Early Permian–Late Triassic time. The Permian granitoids would form in an active continental margin setting, whereas the Late Triassic granites were most likely generated within a local extensional environment, both of which were closely related to the westward subduction of the Farallon plate of the Paleo-Pacific Ocean beneath the Jiamusi–Khanka Massif.