Brenna, M., Ubide, T., Nichols, A. R. L., Mollo, S., and Pontesilli, A. (2021). “Anatomy of Intraplate Monogenetic Alkaline Basaltic Magmatism,” in Crustal Magmatic System Evolution: Anatomy, Architecture, and Physico-Chemical Processes. AGU Geophysical Monograph. Editors M. Masotta, C. Beier, and S. Mollo, 264, 79–103. doi:10.1002/9781119564485.ch4 Anatomy of intraplate monogenetic alkaline basaltic magmatism: clues from magma, crystals and glass Marco Brenna1,*, Teresa Ubide2, Alexander R. L. Nichols3, Silvio Mollo4, Alessio Pontesilli1 1 Geology Department, University of Otago, Dunedin, New Zealand 2 School of Earth and Environmental Sciences, The University of Queensland, Brisbane, Australia 3 School of Earth and Environment, University of Canterbury, Christchurch, New Zealand 4 Dipartimento di Scienze della Terra, Sapienza Università di Roma, Rome, Italy * corresponding author: [email protected] Abstract Intraplate basaltic systems, often occurring as fields of small monogenetic volcanoes, are dominated by eruption of alkaline basaltic rocks, ranging from nephelinite/basanite to transitional/subalkaline. Their generally primitive erupted compositions imply limited crustal modification, and hence they provide an important probe into deep, lithospheric mantle and partial melting processes. Partial melting and magmatic ascent processes can be investigated using the composition of crystals, glass and whole- rock, although a combination of these is preferable. The whole-rock chemical variability within single eruptions or over the temporal and spatial extent of a volcanic field is controlled by the characteristics of the primary melting source, as well as near source percolative/reaction processes. Coupled crystal- and -whole-rock detailed investigations are most promising to constrain the processes that modify primary melts into the primitive magmas that accumulate before ascent. Complex crystal textures and chemistry have so far demonstrated that basaltic magmas are principally processed and modified within the lithospheric mantle with minor modification en-route through the crust. Fractional crystallization and magma mixing modify melts throughout ascent, and can imprint secondary chemical intra-eruptive variability. Quantifiable temperature and pressure parameters based on crystal- melt compositions constrain the depth of formation, and hence provide information about the role of different mineral phases in deep versus shallow chemical evolution. Volatile components in the melt (e.g. H2O and CO2) can be quantified on glass and melt inclusions. These analyses, coupled with solubility models, may help to reconstruct initial dissolved volatile content to further constrain the source characteristics and magmatic ascent dynamics. Integrated studies of crystals and melt paint a picture of extended lithospheric mantle to minor crustal processing resulting from the complex deep plumbing of monogenetic basaltic systems. This highlights the need for improved resolution to characterize true primary signatures and hence elucidate the formation of intraplate alkaline basalts. Keywords Intraplate basalt, alkaline, whole-rock composition, crystal composition, glass composition, magmatic volatiles 1. Introduction fundamental for developing our understanding of processes of magma formation and Basalt is the most common rock in the Earth’s evolution. Mid-Oceanic Ridge Basalts crust, although it is admittedly mostly (MORBs) are generated where tectonic plates submerged as the oceanic floor. Basalts (sensu diverge and new crust is formed. Melting in lato) also represent the compositions of partial these settings occurs by adiabatic melts forming in the mantle (Green, 1973), and decompression (Langmuir et al., 1992; hence primary magmas from which many Oxburgh and Turcotte, 1968), and despite local intermediate and silicic igneous rocks are and regional variability (Shimizu et al., 2016; derived. Basaltic rocks are therefore Wood, 1979), MORBs have a restricted compositional range compared to intraplate monogenetic magma batches with OIB-like Oceanic Island Basalts (OIBs) (e.g. Pilet, alkaline basalt/basanite associations. We will 2015). Despite the misleading appellation of extend the discussion to subalkaline basalts these latter magma types, rocks with OIB where they occur in the context of their alkaline characteristics occur in continental as well as correlatives. The rationale for focusing our oceanic settings. Our understanding of the attention on monogenetic alkaline basaltic generation of these intraplate rock suites, and rocks is that they generally provide a direct the associated mantle plume theory (Morgan, probe into the processes involved with magma 1971), was originally developed using intra- generation and evolution, and they are oceanic islands as type localities, notably ubiquitous in most continental intraplate Hawaii (Wilson, 1963), and hence the name. settings. Intraplate monogenetic alkaline OIBs are predominantly understood to be basalts often host inclusions of lithospheric produced by adiabatic decompression peridotite, indicative of their relatively rapid (Cawthorn, 1975; Green and Ringwood, 1967). ascent from the mantle (O’Reilly and Griffin, If exotic (metasomatic) amphibole and/or 2011; Spera, 1984). The lack of complex phlogopite are present, they may breakdown plumbing and storage has implications in terms and cause melting upon compression, such as of minimising the potential processes of during lithospheric delamination (Allen et al., magma modification en-route to the surface 2013). OIBs may also not be derived directly (McGee and Smith, 2016). The relatively from the partially molten portion of an limited crustal interaction makes monogenetic upwelling mantle diapir, with post-melting alkaline basaltic rocks a useful tool to processes such as chromatographic percolation, investigate deep processes of partial melting reactions with overlying lithosphere and re- and melt modification. Their generally melting due to thermal perturbation elementary crystal cargo, often consisting of contributing to final basalt magma formation only olivine and clinopyroxene is another (Godard et al., 1995; Harte et al., 1993; Mallik beneficial aspect to help the reconstruction of and Dasgupta, 2012; Menzies and Murthy, primitive and parental melt characteristics. 1980; Pilet, 2015). The wide compositional Occasional complex zonation patterns of spectrum (major and trace elements and crystals can help resolve deep to shallow isotopes) of OIBs implies that the process of plumbing processes (Coote and Shane, 2018; melting and melt modification through Duda and Schmincke, 1985; Jankovics et al., crystallization involve a greater number of 2013; Jankovics et al., 2016) and hence variables than those governing MORB improve the resolution of petrogenetic models. generation. As a consequence, the tectonic and Here we summarize current geochemical and petrogenetic processes of intraplate rock suites petrological tools employed in the are still hotly debated (Foulger and Jurdy, interpretation of continental alkaline basaltic 2007; Foulger et al., 2005; Herzberg, 2010; suites to elucidate the magmatic processes Pilet et al., 2008). within the plumbing system of intraplate monogenetic volcanoes. Systems with OIB-like characteristics are associated with magma spanning a broad 2. Origin of intraplate monogenetic basaltic compositional spectrum from subalkaline systems (tholeiitic) basalt to basanite/nephelinite (and Within the framework of the plate tectonic some lamprophyre), with rhyolite to phonolite theory, intraplate volcanoes erupt away from being their evolved derivatives. They also the direct influence of either convergent or occur in a variety of tectonic settings from divergent plate boundaries. In this context, oceanic and continental intraplate to some subduction-related and intraplate volcanoes back-arc environments. In this contribution we (Arc versus OIB magmas) bear distinct will focus particularly on continental intraplate chemical signatures (Pearce and Peate, 1995). basaltic suites consisting of multiple Due to the input of fluids and residual mineralogy from subducting slabs, the former Magmatic intraplate systems generally occur as are enriched in large ion lithophile elements fields of small monogenetic volcanoes on both (LILEs), depleted in high field strength continental and oceanic crust (Smith and elements (HFSEs) and have generally flat Németh, 2017). Because of the spatially and patterns of rare earth elements (REEs) (Elliott temporally dispersed nature of eruptions and et al., 1997; Hawkesworth et al., 1994; Kessel their relatively short duration (weeks to few et al., 2005; Tatsumi et al., 1986). In contrast, years), such systems do not generally construct magmas erupted in intraplate settings tend to be large volcanic edifices. The lack of burial of enriched in HFSEs and LILEs, also showing early eruptions means that field-scale eruptive relatively high light/heavy REE ratios histories can be investigated comprehensively (LREEs/HREEs), frequently interpreted to from the onset of activity (Condit and Connor, indicate residual garnet in the source of the 1996; Leonard et al., 2017), revealing changes melts (Clague and Frey, 1982; Floyd, 1991; occurring within the magma source area over Sun and McDonough, 1989). Where the spatial the lifetime of the volcanic field (Brenna et al., distinction between these different rock suites 2012a; Valentine and Perry, 2006). Individual is not obvious, the chemical