Petrogenesis and tectonic setting of the peralkaline Pine Canyon caldera,Trans-Pecos Texas,USA

摘要

The Pine Canyon caldera is a small (6-7 km diameter)ash-flow caldera that erupted peralkaline quartz trachyte,rhyolite,and high-silica rhyolite lavas and ash-flow tuffs about 33-32 Ma.The Pine Canyon caldera is located in Big Bend National Park,Texas,USA,in the southern part of the Trans-Pecos Magmatic Province (TPMP).The eruptive products of the Pine Canyon caldera are assigned to the South Rim Formation,which represents the silicic end member of a bimodal suite (with a "Daly Gap"between 57 and 62 wt.% SiO_2);the mafic end member consists primarily of alkali basalt to mugearite lavas of the 34-30 Ma Bee Mountain Basalt.Approximately 60-70% crystallization of plagioclase,clinopyroxene,olivine,magnetite,and apatite from alkali basalt coupled with assimilation of shale wall rock (M_a/M_c=0.3-0.4)produced the quartz trachyte magma.Variation within the quartz trachyte-rhyolite suite was the result of approx 70% fractional crystallization of an assemblage dominated by alkali feldspar with subordinate clinopyroxene,fayalite,ilmenite,and apatite.High-silica rhyolite is not cogenetic with the quartz trachyte-rhyolite suite,and can be best explained as the result of approx 5% partial melting of a mafic granulite in the deep crust under the fluxing influence of fluorine.Variation within the high-silica rhyolite is most likely due to fractional crystallization of alkali feldspar,quartz,magnetite,biotite,and monazite.Lavas and tuffs of the South Rim Formation form A-type rhyolite suites,and are broadly similar to rock series described in anorogenic settings both in terms of petrology and petrogenesis.The Pine Canyon caldera is interpreted to have developed in a post-orogenic tectonic setting,or an early stage of continental rifting,and represents the earliest evidence for continental extension in the TPMP.