TECTONIC SETTING OF EARLY-MIDDLE JURASSIC VOLCANIC EVENTS IN LISHU FAULT DEPRESSION OF SONGLIAO BASIN: CONSTRAINTS FROM ZIRCON U-PB DATING, GEOCHEMISTRY, AND HF ISOTOPES

摘要

The Mesozoic volcanic rocks in Songliao Basin arc very well developed, but most of that formed from Late Jurassic to Early Cretaceous. There are few reports of volcanic events in the Early-Middle Jurassic. In this paper, the LA-MC-ICP-MS zircon U-Pb dating of volcanic rocks were carried out, which were collected at the bottom of Huoshiling Formation in SuJia2 well of Lishu fault depression in Songliao Basin. The ages of volcanic rocks arc 172.2±0.56Ma and 163.7±1.4Ma, which show that volcanic event had happened in Songliao Basin in the Early-Middle Jurassic. Through the study of petrology, geochemistry and Hf isotopic characteristics, it is shown that the Early-Middle Jurassic volcanic rocks belong to the metaluminous calc-alkaline series, which have the characteristics of enriching light rare earth element(LREH) relative to heavy rare earth element(HREE),enriching large ion lithophile clcments(LILE) and depletion of high field strength elements(HFSE).The ￡nKt) of the Early-Middle Jurassic volcanic rocks arc between -2.6 and 5.6,with the two-stage Hf model ages (Tdmc) between 862 and 1384Ma.Combincd with the previous studies, we conclude that the Early-Middle Jurassic volcanic rocks in Songliao Basin were formed in the island arc tectonic environment, which was related to sub-duction of the Paleo-Pacifific oceanic plate.