Discovery of Ordovician Plagiogranite from East Junggar, Xinjiang

A set of Ordovician plagiogranites, of which the SHRIMP U-Pb zircon dating showed that the crystallization age was 468.1±5.5 Ma, was found intruding the Pre-paleoordovician metamorphic rocks in the northeastern Laojunmiao of East Junggar. The Ordovician plagiogranite age was first reported in the East Junggar. The content of SiO₂ is 70.09%; MgO (1.82%) and Na₂O (6.12%) content are high while K₂O (1%)content is low. The content of standard mineral corundum molecules reached 1.96. The total REE is low(72.97×10^-6), and negative Eu anomaly is not obvious, while the fractionation between LREE and HREE ((La/Yb)_N=16.05) is evident. There is lower Sr, Y, and Yb content, but higher K, Rb, Ba, Th content; there are also Nb and Ta negative anomalies and Ce positive anomaly, reflecting that the magma of plagiogranites was mantle-derived probably formed in ocean environment, and contaminated by the crust. The discovery of the Ordovician granites confirmed the existence of an extensive magma-metamorphic thermal event in the east Junggar through Ordovician period.