Geochemical and zircon isotopic evidence for extensive high level crustal contamination in Miocene to mid-Pleistocene intra-plate volcanic rocks from the Tengchong field, western Yunnan, China

Abstract

SHRIMP zircon Pb/U dating of Cenozoic volcanic rocks in the Tengchong area, western Yunnan Province, China, shows that the dadte and andesitic breccia lavas from Qushi village were intruded at 480 10 ka and 800 40 ka, respectively. Moreover, Pb/U dating of trachyandesite from Tuantian village and olivine basalt from Wuhe village give weighted mean (PbID)-Pb-206/U-238 ages of 2.82 +/- 0.08 Ma and 12.28 +/- 030 Ma. Corrections for initial Th-230 disequilibrium of zircon were used for the former two younger ages.

The Tengchong volcanic rocks have a large range of SiO2 (48.6-66.9 wt.%) and mostly belong to a high-K calcalkaline series. The lavas originated from heterogeneous sources and were modified by subsequent fractional crystallization. The REE and other trace element patterns of the Tengchong volcanic rocks resemble magmas having a large component of continental crust All have similar degrees of LREE and HREE fractionation and are enriched in LILE, La, Ce and Pb, with depletions in Nb, Ta, Ti, Sr and P relative to primitive mantle. Zircon 6180 values of 6.96 +/- 0.17 and 7.01 +/- 0.24 parts per thousand and highly varied negative epsilon(Hf)(t) values of -1.5 to -11.0 and -103 to -13.7, as well as the presence of inherited zircon grains in the studied samples, indicate that the magmas contain crustal material on a large scale.

The Tengchong volcanic rocks have HFSE ratios (e.g., Nb/Ta, La/Nb, Zr/Y) similar to continental flood basalts, indicative of an intra-plate extensional tectonic setting. Widespread distributed faults might have facilitated upwelling of mantle-derived melts and eruptions from shallow crustal magma chambers to form the large volcanic field. (C) 2017 Elsevier B.V. All rights reserved. Show more