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

摘要

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 (530-493 Ma), Permian (299-254 Ma) and Late Triassic (223-212 Ma), with two possible subordinate magmatic events that occurred in the Silurian (434-416 Ma) and Carboniferous (331-317 Ma). The Permian diorite and granodiorite contain high Al2O3 (15.81-19.07 wt.%), Na2O (K2O/Na2O = 0.25-0.44) and Sr (748-949 ppm) contents and low Y (4-18 ppm) and Yb (037-1.94 ppm) 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 (75.90-77.40 wt.%) and K2O (4.17-4.87 wt.%), 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 Hf-176/Hf-177 ratios (0.282518-0.282842) with Hf two-stage model ages of 2005-809 Ma, and most of the magmatic zircons have positive epsilon(Hf)(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. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.