Petrology of quartz syenite and hauyne syenite clasts from the Pitigliano Formation, Latera caldera, Vulsini District, Central Italy
Renzulli, Alberto; Upton, Brian G.J.; Boyce, Adrian; Ellam, Rob M.
European Journal of Mineralogy Volume 10 Number 2 (1998), p. 333 - 354
published: Mar 31, 1998
manuscript accepted: Nov 19, 1997
manuscript received: Apr 2, 1997
ArtNo. ESP147051002014, Price: 29.00 €
Abstract The textures and mineralogy of quartz syenite and hauyne syenite clasts within the phonolitic (pyroclastics) to tephriphonolitic (lavas) Pitigliano Formation indicate different origins and separate cooling histories. Whole-rock geochemistry of both quartz syenites and hauyne syenites indicates they are slowly cooled equivalents of differentiaded potassic magmas, approximately retaining a liquid composition. Clinopyroxene and amphibole compositions reflect the higher and lower silica activity of the melts from which the two suites crystallized. The foiditic syenite clasts, cognates with the Pitigliano Formation host-rocks, are inferred to have grown at shallow depths, as side-wall or roof cumulates (ideal orthocumulates) near the top of a chamber containing a HKS phonolitic-tephriphonolitic magma. By contrast, the quartz syenite clasts are inferred to be xenoliths of older rocks from deeper parts of the magmatic system crystallized from KS-trachitic magmas. There is little variation with respect to 143Nd/144Nd and 87Sr/86Sr ratios in the two syenitic suites. These ratios lie within the ranges shown by the mafic lavas of Vulsini, implying that no significant crustal contamination by siliceous material occurred beyond the mafic stage of evolution. The hauyne syenites, however, have relatively high δ34S values (between +10.4 and +12.9 ‰), which are believed to reflect assimilation of underlying Triassic evaporites with δ34S values of ca. +14.3 ‰. Evidence from SO4-rich apatites and the mineral chemistry of some pegmatite clasts appears to confirm that sulphur-rich fluids emanating from the Triassic carbonate-evaporite sedimentary wall-rock sequences contaminated the crystallizing HKS magma.