DISCOVERY OF THE EARLY JURASSIC VOLCANIC ROCKS IN XINLIN AREA OF NORTHERN DAXINGANLING MOUNTAINS: Geological Implication

The Early Jurassic acid volcanic rocks are newly discovered in Zhanbeicun Formation of Xinlin area in northern Daxinganling Mountains, from which two samples of zircon LA-ICP-MS U-Pb ages are determined. The first is sericitized rhyolite, with the testing point deviating to the right side of concordant curve on the concordia diagram-a typical discordant age, yielding the intersecting age with concordant curve of 192.0±1 Ma(MSWD=1.2), which is the cooling crystallization age of rhyolites after extrusion. The second one is weakly silicified rhyolite with a concordant age of 178.0±1 Ma, which is the result of the loss of radioactive trace element Pb, and the rock-forming age should be close to that of the sericitized rhyolite. The volcanic rocks are petrochemically and geochemically characterized by high Ti rhyolite and I-type rhyolite. The Mg^# value of the rhyolite(0.43-0.73) and content of hypersthene in CIWP norm minerals (0.25%-0.96%) support that the rhyolite is derived from crystallization-differentiation of basic magma.