Petrology and Petrogenesis of the Latera Caldera, Central Italy |
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| Authors: | TURBEVILLE BRUCE N |
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| Institution: | Department of Geological Sciences, The University of Texas at Austin Austin, Texas 78713-7909 |
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| Abstract: | At least 8 km3 of felsic ignimbrites and high-K silica-undersaturatedlavas and tephra were erupted from the Latera caldera between250 and 150 ka. Four distinct periods of explosive eruptions(at about 232, 206, 195, and 156 ka) produced ignimbrite sequencesthat show an upward compositional progression from trachyteand differentiated phonolite to less evolved phonolite. Duringthe last two of these periods, the tuffs grade upward from phonoliteto tephriphonolite. The stratigraphy indicates that eruptionssampled magmas that were stratified downward from trachyte andphonolite to tephriphonolite, and the compositional cyclicitysuggested by the timing of the eruptions implies an unusualcontemporaneity of silica-saturated and -undersaturated compositions. At Latera, pumice fragments in the same deposit can exhibitup to 10-fold differences in vesicularity and crystal content(from <5 to >50 vol.% phenocrysts). These clasts, in conjunctionwith glassbearing syenite and skarn xenoliths, represent a rangeof progressively crystallized magmas that were quenched at theinstant of their eruptive entrainment. The syenites compriseeutectic mineral assemblages with high percentages of titanite,apatite, and melanite garnet as accessory minerals. Least-squaresmodels based on major element and compatible trace element (e.g.,Ba and Sr) abundances of the pumices and syenite indicate thatthe fractionation of plagioclase and sanidine largely controlledthe liquid lines of descent for phonolite and trachyte, respectively.Additional mineral phases that may have contributed to magmaticdifferentiation include fassaitic diopside, leucite, biotite,apatite, and alkali amphibole. Models further imply that tephriphonoliticliquids required roughly 70% crystallization of tephritic orbasanitic parent magmas, whereas the evolved phonolitic liquidswere obtained after the removal of >85% of the above mineralassemblage. The commonly aphyric trachytic tuffs represent themost evolved derivatives. Despite the limited range in major element contents, trace elementsvary considerably among the different pumice types and syenites.Large ranges in Rb/Sr, Nb/Ta, Zr/Hf, La/Yb, and Ba/Th reflectthe selective partitioning of some elements into accessory phases.However, the variations of B, Sc, Rb, Nb, Hf, Y, and Yb cannotbe explained completely by crystal fractionation. Syenite compositions,for example, bracket the range of most elements in all pumicetypes, and chemical models demonstrate that processes operatingalong the chamber margins could have greatly influenced thebehavior of trace elements in the evolved liquids. Plausiblemechanisms that might have accompanied crystal fractionationin these magmas include the mixing of several magma batches,and the possible dilution of central reservoir magmas by back-mixingwith fractionated liquids or with CO2-rich fluids released fromskarns.
*Present Address: Bureau of Economic Geology, Mineral Studies Laboratory, University of Texas, Austin, Texas 78713-7508 |
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