BACKGROUNDThe zircon U-Pb dating and analysis of major, trace and rare earth elements of rocks are the main research methods used to study the genesis of intrusive rocks.

OBJECTIVESTo constrain the genesis of neoproterozoic gneissic moyite of the Yangsanzhai, Xiaotian volcanic basin.

METHODSChemical composition of rocks was analyzed by X-ray fluorescence spectrometry (XRF), inductively coupled plasma-mass spectrometry (ICP-MS), and the zircon U-Pb age was determined by laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS).

RESULTSResults show that the rock was characterized by high silicon, kalium and sodium, and low iron, magnesium, calcium and phosphorus. The content of SiO₂, Al₂O₃, MgO, K₂O and Na₂O were 65.86%-78.29%, 10.89%-16.02%, 0.17%-1.17%, 1.37%-6.44% and 0.53%-6.50%, respectively. The rock belonged to the high potassic calc-alkaline series. The samples were characterized by depletion of Nb, Sr, P and Ti, and enrichment of La, Ce, Nd and Zr. The samples had high content of rare earth elements (ΣREE=152.70-650.88μg/g) with light rare earth enrichment (LREE/HREE=6.77-20.64). Most of the samples had weakly negative europium anomalies (δEu=0.29-1.15) without cerium anomaly. The normalized curve of rare earth elements was characterized by right-incline. The ²⁰⁶Pb/²³⁸U ages of the 30 zircon points ranged from 720 to 828Ma, with a weighted average age of 776±11Ma (MSWD=2.1).

CONCLUSIONSThe discovery of Neoproterozoic gneissic moyite in the Xiaotian Basin indicates that there is uplift in the center of this basin, not the 'bucket' shape of thick in the middle and thin at the sides, as previously recognized. This has important guiding significance for the understanding of the tectonic morphology of the basin and further prospecting.