U-Pb ages and Hf isotopic composition of zircons and whole rock geochemistry of volcanic rocks from the Fangniugou area: Implications for early-middle Paleozoic tectonic evolution in Jilin Province, NE China

摘要

The Changchun-Yanji suture in Jilin Province, NE China is generally interpreted to mark the closure position of the Paleo-Asian Ocean along the north margin of the North China Craton. However, the issue about the early-middle Paleozoic tectonic evolution in this region has long been debated. In order to provide evidence for resolving this issue, we carried out a geochemical and zircon U-Pb-Hf study on the Fangniugou and Taoshan volcanic rocks from the Fangniugou area of Jilin Province, in the Changchun-Yanji suture zone. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating indicates that the Fangniugou volcanic rocks are products of at least two volcanic events (~ 425 and 390 Ma) and that episodes of magmatism existed in Jilin Province during the early-middle Paleozoic period. These rocks, in conjunction with relevant data from the literature, indicate that early-middle Paleozoic magmatism occurred along the Changchun-Yanji suture. The volcanic rocks belong to medium- to high-K calc-alkaline series and display features of I-type granites of subduction-related igneous rocks. During the late Silurian-middle Devonian, the Y and Yb concentrations progressively decreased with time, accompanied by an increase in K_2O concentrations and (La/Yb)_N values, indicating a process of crustal thickening. This interpretation is supported by the presence of early Paleozoic arc igneous rocks and the formation of early Devonian molasse in central Jilin Province. Zircon grains separated from the rocks have high e_(Hf)(t) values and two-stage Hf model (T_(DM2)) ages varying from +4.92-8.76 and 0.84-1.09 Ga, respectively. All these characteristics, in conjunction with trace element features, suggest that the late Silurian rocks were generated by partial melting of a depleted mafic lower crust in a subduction zone setting, whereas the middle Devonian rocks were derived from partial melting of a thickened lower crust the most probably related to a collisional event.