Heterogeneous mantle source and magma differentiation of quaternary arc-like volcanic rocks from Tengchong, SE margin of the Tibetan Plateau

摘要

The Tengchong volcanic field north of the Burma arc comprises numerous Quaternary volcanoes in the southeastern margin of the Tibetan Plateau. The volcanic rocks are grouped into four units (1-4) from the oldest to youngest. Units 1, 3 and 4 are composed of olivine trachybasalt, basaltic trachyandesite and trachy-andesite, and Unit 2 consists of hornblende dacite. The rocks of Units 1,3, and 4 form a generally alkaline suite in which the rocks plot along generally linear trends on Harker diagrams with only slight offset from unit to unit. They contain olivine phenocrysts with Fo values ranging from 65 to 85 mol% and have Cr-spinel with Cr# ranging from 23 to 35. All the rocks have chondrite-normalized REE patterns enriched in LREE and primitive mantle-normalized trace element patterns depleted in Ti, Nb and Ta, but they are rich in Th, Ti and P relative to typical arc volcanics. Despite minor crustal contamination, ~(87)Sr/~(86)Sr ratios (0.706-0.709), eNd values (-3.2 to -8.7), and eHf values (+4.8 to -6.4) indicate a highly heterogeneous mantle source. The Pb isotopic ratios of the lavas (~(206)Pb/~(204)Pb = 18.02-18.30) clearly show an EMI-type mantle source. The underlying mantle source was previously modified by subduction of the Neo-Tethyan oceanic and Indian continental lithosphere. The present heterogeneous mantle source is interpreted to have formed by variable additions of fluids and sediments derived from the subducted Indian Oceanic lithosphere, probably the Ninety East Ridge. Magma generation and emplacement was facilitated by transtensional NS-trending strike-slip faulting.