ARC VOLCANIC ROCKS IN AN INTRACONTINENTAL SETTING, TONGCHONG, SW CHINA: GENESIS AND TECTONIC IMPLICATIONS

Pliocene to Holocene volcanoes in the Tenchung region, Yunnan Province, SW China, crop out parallel to the Nujiang suture zone about 200 km southwest of Dali. They form small cinder cones and lava flows resting directly on granites and metamorphic rocks of Mesozoic age. Based on ²³⁰Th-²³⁸U, K/Ar and thermoluminescence dating the volcanic rocks range in age from 5.5 Ma to 13 Ka. They comprise a weakly alkaline series composed of trachybasalt, basaltic trachyandesite and trachyandesite ranging from 48-61 wt % SiO₂ and 5-8 wt% Na₂O+K₂O. Associated hornblende dacite flows are all subalkaline and have a narrow range of SiO₂ from 65-68 wt%.. All of the rocks show clear evidence of crustal contamination with common corroded xenocrysts of quartz and feldspar. The rocks all have similar chondrite-normalized REE patterns with strong LREE enrichment and marked negative Eu anomalies. In primitive mantle-normalized spider diagrams they all show negative Ta, Nb and Ti anomalies and positive Th, U, La and Pb anomalies. Initial ⁸⁷Sr/⁸⁶Sr ratios range from 0.705790 to 0.710019) and εNd values from -3.13 to -11.81. The least radiogenic samples plot in the field of EM1 and the more radiogenic samples plot along a crustal contamination trend. The parental magmas for the Tengchong mafic rocks were therefore likely derived from an EMI-type mantle source, which was modified by subduction of Indian continental crust during the India-Asia collision. The dacitic rocks have chemical and isotopic compositions significantly different from those of the mafic rocks. Their high initial ⁸⁷Sr/⁸⁶Sr ratios, low εNd values and radiogenic ²⁰⁷Pb and ²⁰⁸Pb isotopic compositions suggest that they formed by melting of felsic continental crust, however, many of these rocks have been intensely hydrothermally altered, which strongly modified their whole-rock chemical compositions. The Tengchong volcanic field lies ~320 km northeast of the Burma arc and seismic data indicate the presence of an eastward dipping zone of earthquake foci beneath the back arc region. However, this zone does not extend under the Tengchong area, suggesting that melting was triggered by delamination of a previously subducted lithospheric slab that dipped beneath Asia.