SHRIMP zircon U-Pb dating and geochemical analysis of the Pliocene volcanic rocks from Longchuanjiang valley, Tengchong area, western Yunnan Province

In this paper, Pliocene ages (ca. 2.8 Ma) of trachyandesite and trachyte from Longchuanjiang valley in Tengchong area were yielded using SHRIMP Ⅱ in Beijing SHRIMP Laboratory. The ages of trachyandesite and trachyte correspond with each other within uncertainties and are taken to represent the time of eruption (2.81 Ma±0.11 Ma). The volcanic rocks are rich in alkalis (Na2O=3.53-3.66 wt%, K2O=3.33-3.64 wt%) and calcium (CaO=4.04-5.05 wt%). They contain high Ba (1057×10-6~1145×10-6), Sr (502.0×10-6~650.7×10-6), REE (ΣREE=315.53×10-6~365.16×10-6), and Zr(308.8×10-6~382.2×10-6). These geochemical features are different from those of typical island arc volcanic rocks but show an affinity with contaminated asthenosphere-generated continental basaltic rocks. The Pliocene volcanic rocks from Longchuanjiang valley in Tengchong area were derived from partial melting of an enriched mantle source, contaminated by large crustal materials in an intraplate extensional environment.