Abstract
Lavas and pyroclastic products of Nisyros volcano (Aegean arc, Greece) host a wide variety of phenocryst and cumulate assemblages that offer a unique window into the earliest stages of magma differentiation. This study presents a detailed petrographic study of lavas, enclaves and cumulates spanning the entire volcanic history of Nisyros to elucidate at which levels in the crust magmas stall and differentiate. We present a new division for the volcanic products into two suites based on field occurrence and petrographic features: a low-porphyricity andesite and a high-porphyricity (rhyo)dacite (HPRD) suite. Cumulate fragments are exclusively found in the HPRD suite and are predominantly derived from upper crustal reservoirs where they crystallised under hydrous conditions from melts that underwent prior differentiation. Rarer cumulate fragments range from (amphibole-)wehrlites to plagioclase-hornblendites and these appear to be derived from the lower crust (0.5–0.8 GPa). The suppressed stability of plagioclase and early saturation of amphibole in these cumulates are indicative of high-pressure crystallisation from primitive hydrous melts (≥ 3 wt% H2O). Clinopyroxene in these cumulates has Al2O3 contents up to 9 wt% due to the absence of crystallising plagioclase, and is subsequently consumed in a peritectic reaction to form primitive, Al-rich amphibole (Mg# > 73, 12–15 wt% Al2O3). The composition of these peritectic amphiboles is distinct from trace element-enriched interstitial amphibole in shallower cumulates. Phenocryst compositions and assemblages in both suites differ markedly from the cumulates. Phenocrysts, therefore, reflect shallow crystallisation and do not record magma differentiation in the deep arc crust.
Highlights
Arc magmas typically represent mixtures of one or more distinct volatile, melt and mineral components that are not necessarily in equilibrium with each other
We prefer to classify the two suites based on their petrography and field occurrence in a lowporphyricity andesite (LPA) suite and high-porphyricitydacite (HPRD) suite (Fig. 1)
high-porphyricity (rhyo)dacite (HPRD) units occur as domes (EMB, Profitis Ilias domes (PFI)), pyroclastic deposits related to explosive eruptions (LP and UP) and viscous block lava flows (LF7, NLF)
Summary
Arc magmas typically represent mixtures of one or more distinct volatile, melt and mineral components that are not necessarily in equilibrium with each other. In addition to textural observations, in situ geochemical analyses of minerals and melt inclusions provide abundant evidence for open-system differentiation of arc rocks (e.g., Davidson et al 2005; Ginibre et al 2007). The composition and textural relationships of mineral phases in arc magmas provide a wealth of information that can be used to investigate petrogenetic processes (e.g., Davidson et al 2005). The important role of amphibole in the evolution of arc magmas, as deduced from lava trace element compositions, is at odds with the common absence of amphibole phenocrysts (e.g., Davidson et al 2007). Arc cumulates have the potential to record the first stages of magma differentiation that are rarely preserved in the erupted lavas
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