SHRIMP U-Pb age, geochemical characteristics and its introcontinental extension of Fenghuangshan syenogranite in the east Liaoning

This paper is the result of the geological survey engineering.

Objective Aperiod of A-type granite is developed in Dandong, Liaoning Province, which has been widely concerned because of its special genetic evolution, specific tectonic setting and important geodynamic significance.

Methods In this paper, the SHRIMP zircon U-Pb chronology and petrogeochemical characteristics of the Fenghuangshan pluton in Dandong, Liaoning Province, were analyzed to explore the formation age, petrogenesis and geodynamic background of the pluton.

Results The zircon U-Pb ages of the biotite syenogranite in Fenghuangshan, Liaodong are (122.3±1.7) Ma, (125.0±1.7) Ma and (122±2) Ma, representing that the magma of the biotite syenogranite in Fenghuangshan crystallized and intruded in the Early Cretaceous. Geochemical analysis shows that the SiO₂ content is 65.65%-73.62%, whereas that of K₂O and Na₂O is 3.52%-5.76% and 3.64%-4.26%, respectively. They also contain 13.4%~15.49% of Al₂O₃, with A/CNK ratio of 1.02-1.46, indicative of their peraluminous nature. The alkalinity rate (AR) ranges from 2.71 to 5.13, and this range is within the AR of aluminous A-type granites. Its FeO_t/MgO ranges from 4.69 to 18.05, which shows the A1 and A2 transition types of A-type granites. The Rb/Nb ranges from 6.02 to 8.64, which display a higher level than that of the continental crust of 2.2-4.7. This implies that continental crust components, which lead to an increase in Rb content, had a significant influence on its formation. The biotite syenogranites are relatively enriched in large ion lithophileelements Rb, Th, Zr, Hf, and depleted in Sr, P, Ba, Ti, Nb. The total content of rare earth elements is high, the fractionation between light and heavy rare earth elements is not obvious, and Eu has obvious negative anomalies.

Conclusions In summary, Fenghuangshan pluton might have been produced in stretching environment related to intracontinental shearing, which ought to be a response to the tectonization of the East Asia continental margins caused by the subduction of plates starting from the Middle Jurassic.