Shoshonitic volcanism of the Bodrum caldera (SW Turkey): Hybridization of enriched mantle-derived and crustal melts

Highlights

• Miocene Bodrum volcanism is represented by shoshonitic series magmatic rocks.

• Magma mixing/mingling was a frequent process during magmatic evolution.

• Partial melting of enriched mantle and subducting crustal rocks produced Bodrum magmas.

• AFC/FCA processes probably have produced relatively evolved compositions.

Abstract

Bodrum Caldera, located at the southwestern tip of the Anatolian plate, comprises volcanic rocks formed by intrusive, effusive and explosive volcanic activity during Miocene. Volcanic rocks chemically belong to K-rich shoshonitic series and comprised of a range of compositions from absarokites to rhyolites with intrusive micromonzogabbros and micromonzonites. The least evolved magmatic rocks are the post-caldera micromonzogabbros. Blebs/droplets with different mineralogical/petrographical characteristics and resorbed xenocrystic glomerocrysts/cumulates comprised of feldspar and clinopyroxene in disequilibrium with the host rocks suggest that mixing between compositionally different magmas was an important process for the evolution of Bodrum volcanism. Especially shoshonites contain mixing and mingling textures probably occurred between monzogabbroic and more evolved monzonitic magmas. Fractional crystallization was limited to felsic (>60% SiO₂) rocks where feldspar, pyroxene, biotite, apatite and zircon were the main fractionating phases. Trace element abundances indicate a garnet-bearing enriched mantle peridotite, resembling EM-1 with a modal assemblage of garnet, rutile, titanite and phlogopite, as the common mantle source rock. Non-modal fractional melting models exhibit that micromonzonites, absarokites and micromonzogabbros were derived from lower (1–3%), moderate (10–15%) and higher (>20%) degrees partial melting of an enriched mantle, respectively. Besides, monzonitic compositions can be produced by partial fusion of a subducting slab with hypothetical composition of 80% GLOSS and 20 % N-MORB. Basic-intermediate compositions can be produced by the mixing/mingling of the micromonzogabbroic and monzonitic magmas. Crustal assimilation and fractional crystallization of hybrid absarokitic melts can yield intermediate-to-felsic compositions of Bodrum shoshonitic series.

Introduction

Potassic volcanic rocks are characteristic of collisional settings where potassium-rich partial melts and/or fluids derived from subducted continental material initiate and/or mix with mantle-derived melts (Palmer et al., 2019). Potassium-rich volcanic rocks belonging to the shoshonite suite are common features of post-orogenic extensional settings inboard from subduction zones (Pe-Piper et al., 2009). Despite having some common geochemical characteristics, shoshonitic series of rocks display a diversity of compositions as a result of the tectonic setting in which they occur, particularly in a) rifts and back-arc basins in intra-oceanic island arcs b) rifts in continental magmatic arcs and c) post-collisional orogenic settings (Gill et al., 2004). Due to the variety of their formation settings, partial melting of metasomatized (lithospheric) mantle and delaminated or thickened crust have been proposed for the genesis of shoshonitic magmas (Wang et al., 2017 and references therein). Moreover, experimental decompression melting of metasomatized phlogopite + pargasite lherzolite produced magmas with shoshonitic affinity (Conceiçao and Green, 2004).

Widespread magmatism was active in the Aegean and the Eastern Mediterranean regions during Neogene (Robert et al., 1992), producing igneous rocks most of which are recognized as resulting from extension and derived mainly from the sub-continental lithospheric mantle (Pe-Piper and Piper, 2001). According to the compilation of radiometric data (Akal et al., 2013) on Western Turkey, the potassic and ultrapotassic magmatism have occurred between 20 and 4 My. Especially, lamprophyric potassic and ultrapotassic volcanism occur around Afyon and Isparta regions (Aydar et al., 2003, Akal et al., 2013, Ersoy and Palmer, 2013, Prelević et al., 2010, Prelević et al., 2012, Prelevic et al., 2015, Elitok, 2019). Prelević et al. (2010) propose to use the lamproitic rocks emplaced between 20 and 4 My as an indicator of accretion and/or shallow subduction event in the assembly of southwestern Turkey. Moreover, Palmer et al. (2019) suggest that continental material was subducted to depths of > 75 km, where phengite breaks down to yield potassic melts that initiate melting of the overlying lithospheric mantle to produce the ultrapotassic volcanism in western Turkey.

Bodrum peninsula, where potassic volcanism is observed in western Turkey, is located in southwestern Anatolia and northeast of Hellenic Arc (Fig. 1a). The volcanism of Bodrum is related to an 18x13 km resurgent caldera with outcropped monzonitic body (magma chamber) and highly altered volcanics at its center (Ulusoy et al., 2004). Lava flows, domes and associated block-and-ash flows, ignimbrite deposits as well as subvolcanic intrusions decorate the volcanic landscape. In this study, we present new petrological data from the Miocene shoshonitic Bodrum volcanism, co-genetic with back-arc volcanic rocks of Cyclades (Robert et al., 1992). In order to better understand the petrogenesis, we discussed the petrological characteristics in terms of magma mixing, different degrees of partial fusion of various sources and assimilation – fractional crystallization.