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The Tengchong volcanic field east of the Burma arc in SW China comprises numerous Quaternary volcanoes. The volcanic rocks can be grouped into four units, numbered 1–4 from oldest to youngest. Units 1, 3 and 4 are composed of trachybasalt, basaltic trachyandesite and trachyandesite, respectively, whereas Unit 2 consists of hornblende dacite. Primary minerals in the dacite include plagioclase, clinopyroxene, amphibole and magnetite, all set in a glassy groundmass. Some of the dacites are moderately to highly weathered and show effects of low temperature alteration, with loss on ignition (LOI) up to 9.6 wt% and positive correlations of LOI with Al 2 O 3 , but negative correlations with SiO 2 , CaO and Na 2 O. Fresh dacites (LOI <3 wt%) belong to the calk-alkaline series and have pronounced negative Ti, Nb and Ta anomalies. They have whole-rock 87 Sr/ 86 Sr ratios ranging from 0.7090 to 0.7101, εNd values from −10.8 to −11.8, and εHf values from −5.1 to −6.8. Many of the large plagioclase crystals in the fresh dacites have reverse zoning, ranging from An 55 in the cores to An 72 in the rims. The smaller phenocrysts have relatively uniform An values between 56 and 50. One plagioclase crystal has an An value of 33.5. Plagioclase in the fresh dacite has relatively high Sr contents and 87 Sr/ 86 Sr ratios ranging from 0.7050 to 07125 except for one value of 0.7516, whereas plagioclase in the altered dacite has low Sr contents and highly variable Sr isotopic compositions (0.7039–0.7138), indistinguishable from those of the fresh rocks. Both An values and Sr isotopic compositions of the plagioclase indicate mixing of mantle- and crustally-derived magmas. We therefore propose that mantle-derived basaltic magma caused partial melting of the lower–middle crust and that mixing of the mafic and felsic magmas produced the dacite in staging magma chambers. The dacites contain minerals crystallized from both the mafic and felsic magmas, as well as a few xenocrysts plucked from the country rocks. [ABSTRACT FROM AUTHOR]

The rare occurrences of active volcanoes on the Tibetan Plateau provide a unique opportunity to investigate time scales of magma chamber processes for post-collisional potassic volcanism. This research utilizes high-spatial resolution U–Th disequilibrium dating methods to date zircons from the active Dayingshan volcano, Tengchong, SE Tibetan Plateau, which erupted last during the Holocene. Zircon crystal ages can be used to provide constraints on the magma chamber storage time. Measured zircon interiors yielded an isochron age of 87.5±6.5ka, while zircon outermost rims yielded an isochron age of 58±13ka. The calculated zircon crystal storage time for the most recent eruption is 48ka. At least two separate late Pleistocene zircon crystallization events have occurred where the older age population represents antecrysts remobilized from an earlier magmatic episode, but protracted crystallization cannot be ruled out. Regardless, the similarity in zircon age populations between Dayingshan volcano and Maanshan volcano 13km apart in the Tengchong volcanic field suggests a synchronous thermal history of the magma reservoir, possibly in an extensive interconnected magma chamber beneath the Tengchong volcanic field. [ABSTRACT FROM AUTHOR]

There are wide spread Cenozoic volcanic rocks in Tengchong ( CVRT), Yunnan province, SW China. These rocks comprise three rock types: basalt, andesite (dominant type) and dacite. Most samples are sub-alkaline, and among the sub-alkaline rocks, most are high- K calc-alkaline. These rocks have a SiO2 range of 49.1 wt.% to 66.9 wt.%. TiO2 contents are not high and have a variation of 0.7 wt.%-1.6 wt.%. Trace element concentrations and element ratios (such as Nb/ U, Ce/ Pb, Nb/ La, etc.) of these rocks have a large variation. 87Sr/ 86Sr values fall in the range of 0.7057-0.7093 and 143Nd/ 144Nd values change from 0.5120 to 0.5125. 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios are in the range of 17.936-19.039, 15.614-15.810, and 38.894-39.735, respectively. These geochemical characteristics of CVRT make them resemble island-arc volcanic rocks. We suggest that the magmas were generated in the lithospheric mantle that had already been metasomatized by previous subduction processes. By the study of the uplift history of the Tibetan Plateau, we found that the beginning of the geotectonic processes to the eruption of CVRT was coeval with one uplift event. Therefore, we propose that the uplift of the Tibetan Plateau caused collapse of the collisional orogeny in Tengchong, which further triggered the generation and eruption of the CVRT magmas. [ABSTRACT FROM AUTHOR]

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 trachyandesite, 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, Sr/Sr ratios (0.706-0.709), εNd values (−3.2 to −8.7), and εHf values (+4.8 to −6.4) indicate a highly heterogeneous mantle source. The Pb isotopic ratios of the lavas (Pb/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. [ABSTRACT FROM AUTHOR]