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The crystallization of shoshonitic to peralkaline trachyphonolitic magmas in a H2O-Cl-F- rich environment at Ischia (Italy), with implications for the feeder system of the Campania Plain volcanoes
 
 
Melluso * L., Morra* V., Guarino* V., de’ Gennaro** R., Franciosi* L., Grifa*** C.
 
 
* Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Università degli Studi di Napoli Federico II, via Mezzocannone 8, 80134, Napoli, Italy
** Centro Interdipartimentale Strumentazioni per Analisi Geomineralogiche (C.I.S.A.G.), Università degli Studi di Napoli Federico II, via Mezzocannone 8, 80134, Napoli, Italy
*** Dipartimento di Scienze e Tecnologie, Università del Sannio, Via dei Mulini 59/A, 82100 Benevento, Italy
 
Lithos 201-211 (2014), 242-259
 
Abstract
Bulk-rock and mineralogical characterization of massive samples (lava flows, lava domes, scoria, spatter), chosen to be representative of different activity periods and of the whole known compositional range of Ischia island (Roman Magmatic Province, Campanian district) is reported in this paper. The rocks vary in composition from shoshonites to peralkaline trachyphonolites. Crystallization started with Al-rich chromite inclusions and their host magnesian olivine in shoshonites and latites, and ended with låvenite, rinkite, kochite, hiortdahlite, hainite, Mn-aenigmatite, britholite and Ti-Zr-rich aegirine in the groundmass of the peralkaline trachyphonolites. Removal of feldspar-bearing assemblages (with calcic-to-sodic plagioclase in shoshonites and latites, sodic sanidine and anorthoclase in trachytes and trachyphonolites) is established throughout the compositional range, and is accompanied by interaction between variably evolved magmas (with their differing phenocryst assemblage), mostly in mafic and intermediate compositions. This led to enrichment in Mn, alkalis, Zr, Nb, REE, Rb, Th, U, Cl and F, and depletion in Mg, Fe, V, Ca, Ba, Sr and Eu in the most evolved magmas. The Ischian rocks have a tendency to sodic affinity and by peculiar mineral compositions and compositional trends, which do not indicate extremely oxidizing conditions. The Ischian rocks thus differ from the Phlegrean Fields analogues and the ultrapotassic, more silica undersaturated rocks of Somma-Vesuvius. Overall, variations in the chemical compositions of the rocks and their trends imply unrelated feeding systems and thus are inconsistent with the hypothesis of a common magma reservoir beneath the main volcanic areas of the Campanian Plain.
 
Keywords: shoshonites, trachyphonolites, peralkaline, magmatic mineralogy, geochemistry, Ischia Island, Italy