Abstract
Alkaline volcanic rocks from explosive monogenetic centers often carry an unusual cargo of crystals and rock fragments, which may provide valuable constraints on magma source, ascent and eruption. One of such examples is the Cenozoic Calatrava Volcanic Field in central Spain, a still poorly explored area to address these issues. Clinopyroxene, amphibole and phlogopite appear either as megacryst/phenocrysts or forming fine-grained cumulates (pyroxenite enclaves s.l.) in some eruptive centers of this volcanic field. They have previously been interpreted as cogenetic high-P minerals formed within the upper lithospheric mantle. The presence of Fe-Na-rich green and Mg-Cr-rich colorless clinopyroxene types as phenocryst cores or as oscillatory zoned crystals in pyroxenite enclaves points to a complex evolution of mineral fractionates from petrogenetically related magmas. In trace element chemistry all studied clinopyroxene types show parallel rare earth element patterns irrespective of whether they are megacrysts, colorless or green core phenocrysts, or zoned crystals within pyroxenite cumulates. This similarity indicates a genetic relationship between all the fractionated minerals. This is in agreement with the overlapping of initial 143Nd/144Nd and 87Sr/86Sr ratios of pyroxenite enclaves (0.512793–0.512885 and 0.703268–0.703778) that is within the chemical field of the host magmas and the Calatrava volcanics. The initial 143Nd/144Nd and 87Sr/86Sr ratios of megacrystic clinopyroxene, amphibole and phlogopite show a more restricted range (0.512832–0.512890 and 0.703217–0.703466), also falling within the isotopic composition of the Calatrava volcanic rocks. Deep magmatic systems beneath monogenetic volcanic fields involve several stages of melt accumulation, fractionation and contamination at variable depths. Trace element and isotope mineral chemistry are powerful tools to understand the history of ascent and stagnation of alkaline basaltic magmas and discriminate between magma mixing, wall-rock contamination and closed magmatic system evolution. In our study, we establish a cogenetic origin for green and colorless clinopyroxene as high-pressure precipitates from liquids of different fractionation degrees (up to 80%, for the highly evolved melts equilibrated with the green clinopyroxene), originated from a highly solidified front of silica-undersaturated alkaline magmas at mantle reservoirs.
Highlights
The study of monogenetic volcanic fields provide important information regarding primary magma genesis, deep-seated differentiation processes and the architecture of the subvolcanic plumbing systems (e.g. Mattsson et al, 2013; Re et al, 2017)
Many monogenetic volcanoes are formed by alkaline rocks which transport mafic megacrysts and ultramafic xenoliths (e.g. Irving and Frey, 1984; Praegel, 1981; Jankovics et al, 2016), whereas pyroxenite enclaves are not so common (Downes, 2007)
Green and colorless anhedral to subhedral phenocryst/megacryst cores can be found in alkaline volcanic rocks and have been studied in order to unravel their enigmatic origin (e.g. Duda and Schmincke, 1985; Ubide et al, 2014; Jankovics et al, 2016)
Summary
The study of monogenetic volcanic fields provide important information regarding primary magma genesis, deep-seated differentiation processes and the architecture of the subvolcanic plumbing systems (e.g. Mattsson et al, 2013; Re et al, 2017). Mattsson et al, 2013; Re et al, 2017) These magmas commonly carry a heterogeneous crystal cargo with complex compositional and textural features, relevant to interpreting the depths and rates of magma storage, transport, fractionation and mixing Irving and Frey, 1984; Praegel, 1981; Jankovics et al, 2016), whereas pyroxenite enclaves are not so common (Downes, 2007) These enclaves have been interpreted as recycled subducted lithosphere (Allègre and Turcotte, 1986), metasomatic products (Garrido and Bodinier, 1999) or high pressure cumulates In situ trace element data from such a variety of relic clinopyroxene cores are still sparse and completely lacking in most of the Cenozoic volcanic fields of the Iberian Peninsula, where mafic megacrysts and phenocrysts are common within the pyroclastic deposits of these mafic volcanic centers
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