Geochemistry, zircon SHRIMP U-Pb dating and origin of alkali-rich porphyries in Beiya area, north Erhai Lake, western Yunnan, China

In this paper SHRIMP U-Pb ages,TY-1(31.5±1.1Ma)、TY-3(31.34±0.73 Ma) of Tanyao quartz syenite porphyries in Beiya are published for the first time, which suggest that the intrusions are formed in Cenozoic. The results show that alkali-rich porphyries in Beiya are characterized by enrichment in alkaline and potassium. They are high in LREE and LILE(such as Rb、Ba etc.), with depletion of HFSE(such as Nd、Ta etc.), high ∑LREE/∑HREE and (La/Yb)N ratios, and lack of apparent Eu anomalies. Based on the characteristics of major elements, trace elements, REE, and isotope geochemistry of alkali-rich intrusion in Beiya area, we've concluded that they have many similar petrological and geochemical characteristics to adakites, but are quite different from those of typical features of adakite defined by Defant and Drummond (1990), which are more close to those adakites in Hoh Xil Area. Systematical studies on Sr、Nd、Pb isotopic characteristics in this paper found out that the source region of the alkali-rich porphyries have the typical geochemical characteristics of EMⅡ enriched mantle.