Evidence of crystallization in residual, Cl–F-rich, agpaitic, trachyphonolitic magmas and primitive Mg-rich basalt–trachyphonolite interaction in the lava domes of the Phlegrean Fields (Italy)

Abstract

Abstract. The lava domes in the northwestern (Cuma), northern (Punta Marmolite) and central (Accademia) parts of the Phlegrean Fields are the subject of this study. The Cuma and Punta Marmolite trachyphonolitic lava domes are among the oldest Phlegrean products cropping out. The Cuma rocks have an agpaitic groundmass, with early alkali feldspar, Fe-rich clinopyroxene, Fe-edenite and sodalite and late rosenbuschite, fluorite, baddeleyite, pyrochlore, britholite, monazite, aegirine (often Zr-rich) and exceptionally Fe–Mn-rich olivine. The bulk-rock compositions at Cuma have some of the highest concentrations of Zn, Mn, Zr, Nb, Th, U and lanthanides among the Phlegrean Fields rocks, and some of the lowest MgO, P2O5, Sr, Eu and Ba. The Punta Marmolite dome is chemically less evolved, and lacks characteristic agpaitic minerals, but features alkali feldspar, sodalite, nepheline and relatively Na-poor, Fe-rich hedenbergite, with rare Ca-rich plagioclase xenocryst cores. The Accademia dome, belonging to the recent activity, is latitic to trachytic in composition, has highly forsteritic olivine (with chromiferous spinel inclusions), calcic plagioclase and Mg-rich diopside (± phlogopite) xenocrysts in an evolved host rock (with phenocrysts and microlites of alkali feldspar, Fe-rich clinopyroxene, Fe-rich amphibole, magnetite, Fe-rich olivine and accessory baddeleyite, zirconolite and fluorite). There is clear evidence of open-system magma crystallization in the form of interaction between a crystallizing, primitive shoshonitic basalt in a reservoir already filled by rather evolved trachytic magma. The magmatic evolution towards the evolved compositions is dominated by crystallization of more and more Na-rich alkali feldspar in a Cl-, F-rich and relatively H2O-poor environment. Input of mafic magma is evident in many trachytic eruptions of the Phlegrean Fields and even in the products of the Campanian Ignimbrite, but eruptions having mineral assemblages rich in xenocryst phases as well as eruptions virtually free of mafic magma input are also frequently observed throughout the history. This suggests a variable pattern of open- and closed-system crystallization, which may or may not be linked to explosive activity, and that can be caused by intermittent supply of basaltic magma from depth

The Neapolitan Yellow Tuff caldera offshore the Campi Flegrei: Stratal architecture and kinematic reconstruction during the last 15ky

“…The compositions of the analysed tephras are typical of the slightly undersaturated potassic products (PB1 and PB3) erupted at Campi Flegrei during the last 15 kyr Melluso et al, 2012 and references therein) and the highly undersaturated potassic products (PB2) erupted at Somma-Vesuvius during the last 3 kyr (Santacroce et al, 2008 and references therein), (Fig. 5).…”

Halogen Contents of Mineralized Versus Unmineralized Potassic Igneous Rocks

“…Although most of the preliminary studies did not focus on mica compositions of potassic igneous rocks, mica phenocrysts of unmineralized suites of these rocks are known to have intrinsically higher halogen contents than those of K-poor intrusions (Kesler et al 1975), in agreement with studies showing that the Cl solubility in volatile-rich melts increases with the alkalinity of the melt (Metrich and Rutherford 1992;Dixon et al 1997;Zajacz et al 2012). Numerous studies have shown that alkaline magmas such as potassic melts are characteristically enriched in halogens (Bailey and Hampton 1990;Yang and Bodnar 1994;Foley 1994;Zhang et al 1995;Müller et al 2001;Kroll et al 2002;Chevychelov et al 2008;Melluso et al 2012), suggesting that the strongest Cl enrichments of magmatic-hydrothermal fluids, and ore metals complexed with Cl, occur in fluids exsolved from magmas that are relatively enriched in K 2 O (Webster 1992;Webster et al 2014;Chen et al 2015). This is consistent with experiments by Gammons and Williams-Jones (1997), which indicate that, at 500°C, gold solubility as AuCl 2 − is highest for fluids that are oxidized (SO 2 / H 2 S > 1), highly saline, and potassium-rich.…”

Magma Mixing: History and Dynamics of an Eruption Trigger

“…For compositionally homogenous magmas such as those extruded at the Campi Flegrei caldera (trachytes and phonolites being by far the most abundant rocks), major oxide and trace element variations cannot be used to unequivocally establish which magma evolution processes operated. Thus, together with petrographic, mineral chemistry and chemical data, isotopic investigations on volcanic rocks spanning the history of the volcano have been performed in recent decades in order to define the role of variable magmatic processes in the evolution of its feeding system up to eruption (e.g., Civetta et al 1997;D'Antonio et al 1999D'Antonio et al , 2013de Vita et al 1999;Pappalardo et al 2002;Fedele et al 2008Fedele et al , 2009Tonarini et al 2004Tonarini et al , 2009Arienzo et al 2010Perugini et al 2010;Melluso et al 2012;Arienzo et al 2015).…”