Early Cretaceous A-type Granites in the Dandong Area, NE China and its Geodynamical Implications

A-type granites, as an important petrologic sign to build up environmental recognition, were mainly formed in extensional tectonic settings. A biotite syenogranite from the Fenghuangshan pluton in Dandong, Liaoning Province gave SHRIMP zircon U-Pb ages of 122.5 +/- 1.6 Ma, 124.9 +/- 1.7 Ma and 126.9 +/- 1.1 Ma. The monzogranites have SHRIMP zircon U-Pb ages of 118.2 +/- 1.6 Ma, 128.1 +/- 1.7 Ma and 131.6 +/- 1.9 Ma, giving an emplacement age for the pluton in the Early Cretaceous. SiO2 is 65.48-74.49 wt%, whereas that of K2O is 4.16-6.44 wt%, and that of Na2O is 2.99-4.70 wt%. They also contain 13.24-15.76 wt% of Al2O3, with an A/CNK ratio of 0.92-1.10, averaging 1.02. The alkalinity rate (AR) ranges from 2.68 to 5.12, and this range is within the AR of peraluminous type rocks. The granites of Fenghuangshan pluton are characterized by high contents of Na and K and low contents of Ca (thermophile element) and Mg, which are features of A-type granites. The (Na2O + K2O) - Fe2O3* x 5 - (CaO + MgO) x 5 discrimination diagram also shows that Fenghuangshan pluton is an A2-type granite. The above granite has zircon SMALL ELEMENT OFHf(t) values ranging from -17.06 to -9.09, with single-stage Hf model ages (T-DM) of 1141 Ma to 1498 Ma and two-stage Hf crustal model ages (T-DM(C)) of 1762 Ma to 2263 Ma. A comprehensive analysis suggests that the Fenghuangshan pluton might have been formed from the lithospheric-plate sliding during the late stages of evolution of hot rift structures and might have been closely associated with the tectonic settings of Mesozoic Eurasia and the ancient Pacific Plate.