Origin and evolution of mid-Cretaceous, garnet-bearing, intermediate and silicic volcanics from Canterbury, New Zealand

摘要: Abstract The Mt Somers Volcanics are part of a suite mid-Cretaceous (89 ± 2 Ma) intermediate to silicic volcanics, erupted onto an eroded surface Torlesse sediments. Rock types vary from basaltic andesite high-silica rhyolite. Andesites medium- high-K with phenocrysts plagioclase, orthopyroxene and pigeonite. Dacites peraluminous commonly contain granulite facies xenoliths garnet xenocrysts. Equilibrium mineral assemblages indicate metamorphic pressures close 6 kbar at 800°C. Rhyolites quartz, sanidine, biotite, orthopyroxene. ferromagnesian phases show textural evidence magmatic crystallization chemically distinct xenocryst in dacites. that early occurred 7 (⩾20 km depth) above 850°C, melt-water contents less than 3.5%. Major-element contents, trace-element initial 87 Sr/ 86 Sr ratio 0.7085 the rhyolites formed by partial melting dominantly quartzo-feldspathic sediments, leaving granulite-facies residue. chemical variation displayed is best explained fractional observed high-pressure phenocryst assemblage. Most elements compositional gap between rhyolite dacite. major-element, isotope compositions lavas assimilation lower crustal material combined mantle-derived tholeiitic magmas. Magmatism was result heat magma flux mantle, during change compressive extensional tectonics after culmination Rangitata Orogeny.