HF-ND ISOTOPE CONSTRAINTS ON THE MANTLE SOURCE CHARACTERISTICS FOR THE TENGCHONG VOLCANICS FROM SE TIBETAN PLATEAU

Hafnium and neodymium isotope systematics in mafic volcanic rocks provide useful information about mantle source characteristics and processes. Hf and Nd isotope array is useful for identifying the types of subducted sediments involved in enriched mantles. The Tengchong volcanic field is located in Yunnan Province, SE Tibetan Plateau. It is one of two active volcanic fields on Tibetan Plateau. Although a great deal of geochemical and petrological research has been conducted to investigate the origin of these volcanoes, application of combined Hf and Nd isotopes to study their mantle source characteristics is relatively lacking.

In this study, Hf and Nd isotopic compositions together with major and trace element concentrations are reported for the four active volcanoes in the Tengchong volcanic field, including Heikongshan, Dayingshan, Maanshan, and Laoguipo volcanoes. All studied samples have LREE enriched patterns. They are depleted in high field strength elements Ti, Nb, and Ta in primitive mantle-normalized trace element spider diagram. Preliminary Hf and Nd isotope results indicate that the Tengchong volcanics plot between mantle igneous rock array and seawater array, around the zircon-free sediments arrays, in the εHf versus εNd diagram. This implies that the sediment component in the mantle source for Tengchong volcanic rocks was mature clay sediments or mudstones rather than other sediments such as sandstones. This interpretation from Hf-Nd isotope systematics is consistent with the conclusion on mantle source characteristics based on ²³⁸U-²³⁰Th isotope disequilibrium data of the Tengchong volcanics. Finally, new constraints on the mantle source rocks from Hf-Nd isotope systematics in Tengchong might exemplify the characteristics of the source rocks for older widespread post-collisional volcanism in other areas of the Tibetan Plateau.