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Petrogenesis of a Late Jurassic Peraluminous Volcanic Complex and its High-Mg, Potassic, Quenched Enclaves at Xiangshan, Southeast China
Authors:JIANG  YAO-HUI; LING  HONG-FEI; JIANG  SHAO-YONG; FAN  HONG-HAI; SHEN  WEI-ZHOU; NI  PEI
Institution:STATE KEY LABORATORY FOR MINERAL DEPOSITS RESEARCH, DEPARTMENT OF EARTH SCIENCES, NANJING UNIVERSITY, NANJING 210093, P.R. CHINA
Abstract:A late Mesozoic belt of volcanic–intrusive complexes occursin SE China. Volcanic activity at Xiangshan in the NW of thebelt took place mainly in the Late Jurassic (158–135 Ma).The volcanic rocks from the Xiangshan volcanic complex includerhyolitic crystal tuffs, welded tuffs, rhyolite lavas, porphyriticlavas, and associated subvolcanic rocks. Mineral assemblagesin these magmatic rocks include K-feldspar, plagioclase, quartz,Fe-rich biotite and minor amphibole, orthopyroxene and almandine.Mineral geothermometry indicates a high crystallization temperature(>850°C) for the Xiangshan magmas. The volcanic rocksare generally peraluminous; SiO2 contents are between 65·4%and 76·8% and the samples have high alkalis, rare earthelements (REE), high field strength elements and Ga contentsand high Ga/Al ratios, but are depleted in Ba, Sr and transitionmetals. Trace element geochemistry and Sr–Nd–O isotopesystematics imply that the Xiangshan magmas were probably derivedfrom partial melting of Middle Proterozoic metamorphic lower-crustalrocks that had been dehydrated during an earlier thermal event.These features suggest an A-type affinity. Quenched mafic enclaves,hosted by the subvolcanic rocks, consist mainly of alkali feldspar,plagioclase, clinopyroxene, phlogopite and amphibole. Geothermometrycalculations indicate that the primary magmas that chilled toform the quenched enclaves had anomalously high temperatures(>1200°C). The quenched enclaves have boninitic affinities;for example, intermediate SiO2 contents, high MgO and low TiO2contents, high Mg-numbers and high concentrations of Sc, Ni,Co and V. However, they also have shoshonitic characteristics,e.g. enrichment in alkalis, high K2O contents with high K2O/Na2Oratios, high light REE and large ion lithophile element contents,low initial {varepsilon}Nd values (–4·2) and high initial 87Sr/86Srratios (0·7081). We suggest a phlogopite-bearing spinelharzburgitic lithospheric mantle source for these high-Mg potassicmagmas. Underplating of such anomalously high-temperature magmascould have induced granulite-facies lower-crustal rocks to partiallymelt and generate the Xiangshan A-type volcanic suite. A back-arcextensional setting, related to subduction of the Palaeo-Pacificplate, is favoured to explain the petrogenesis of the Xiangshanvolcanic complex and quenched enclaves. KEY WORDS: volcanic complex; quenched enclaves; petrology; geochemistry; back-arc extension setting; Xiangshan; SE China
Keywords:: volcanic complex  quenched enclaves  petrology  geochemistry  back-arc extension setting  Xiangshan  SE China
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