Geochemical, petrographic, and stratigraphic analyses of the Portage Lake Volcanics of the Keweenawan CFBP: implications for the evolution of main stage volcanism in continental flood basalt provinces

Abstract

Continental Flood Basalt Provinces (CFBPs) are large igneous features formed by the extrusion of massive amounts of lavas that require significant evolution within the lithosphere. Although sequential lava flows are effective probes of magmatic systems, CFBPs are typically poorly preserved. We focus on lava flows from the well-preserved 1.1 Ga Keweenawan CFBP that erupted within the Midcontinent Rift System. We present a new geochemical, petrographic, and stratigraphic synthesis from the Main stage Portage Lake Volcanics (PLV). Flow-by-flow analysis of the PLV reveals that major element behavior is decoupled from trace element behavior; MgO exhibits limited variability, while compatible and incompatible trace elements deviate from high to low concentrations throughout the sequence. The concentrations of incompatible trace elements slightly decrease from the base of the sequence to the top. We investigate these observations by applying a recharge, evacuation, assimilation, and fractional crystallization model to geochemical and petrographic data. Our modelling demonstrates a magmatic system experiencing increased evacuation rates while fractionation and assimilation rates decrease, indicating an increase in magmatic flux. The outcome of this modelling is a progressively more efficient magma system within the PLV. This study highlights the utility of joint petrographic and geochemical interpretation in constraining CFBP magma evolution.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5424758

Magma Evolution During Main‐Phase Continental Flood Basalt Volcanism: A Case for Recharge‐Evacuation‐Assimilation‐Fractional Crystallization in the Ethiopian Low‐Ti Province

“…The significance of REAFC on the chemical evolution of a magma chamber have been illustrated in numerous theoretical and numerical frameworks (Albarede, 1985; Bohrson et al., 2014; DePaolo, 1981; Lee et al., 2014; O'Hara, 1977; O'Hara & Mathews, 1981; O'Neill & Jennar, 2012). To date, very few studies have applied REAFC models to CFB provinces (Davis et al., 2021; Grant et al., 2020; Steiner et al., 2021; Yu et al., 2015), despite our understanding that LIPs have large magmatic plumbing systems that continually undergo recharge, evacuation, and FC. This is in part because the refining of such models into a user‐friendly interface is a relatively recent advancement (Bohrson et al., 2020; Heinonen et al., 2020; Lee et al., 2014).…”

“…These large outpourings of lava are the result of the most significant mass transfers from the mantle to the crust on the planet and yet a significant amount of the magma generated remains trapped in the crust as crystal cumulates (Bryan et al, 2010). While there has been a wealth of studies aimed at understanding the source of these voluminous magmas (Beccaluva et al, 2009;Heinonen et al, 2016;Ivanov et al, 2008;Melluso et al, 2006;Rooney et al, 2022;Xu et al, 2007), a growing number of studies have begun to focus on the differentiation of the magmas as they interact with the lithosphere (Davis et al, 2021;Grant et al, 2020;Heinonen et al, 2019;Jean, 2013;Steiner et al, 2021;Yu et al, 2015). Many CFB provinces preserve a temporal sequence of outputs of the magma plumbing system (lava flows) and thus provide a unique opportunity to evaluate the temporal chemical evolution of these mass transfer systems as they traverse and become modified by the lithosphere.…”

“…A. Baker, 2000; Ewart et al., 1998; Larsen & Pedersen, 2009; Molzahn et al., 1996). More recently, energy‐constrained models (EC‐AFC) and Recharge‐Evacuation‐Assimilation‐Fractional Crystallization (REAFC) models, which can evaluate multiple differentiation parameters simultaneously, have decreased the amount of crustal assimilation required to explain the geochemical trends observed in CFB provinces (Davis et al., 2021; <20%; Fowler et al., 2004; Heinonen et al., 2016; Jourdan et al., 2007; Steiner et al., 2021) and it is reasonable to assume these latter modeling approaches are a better representation on the magmatic system feeding CFB provinces (Bryan et al., 2010; Heinonen et al., 2019; O'Hara & Herzberg, 2002). Furthermore, REAFC modeling has shown that different elements in a magma chamber undergoing simultaneous recharge and evacuation will achieve steady‐state at different intervals based on their bulk partition coefficient (Lee et al., 2014); this not only helps explain how such large volumes of differentiated, largely homogeneous magmas can erupt at the surface, but also the decoupling in incompatible trace elements observed over differentiation.…”

Joint Inversion of SPREE Receiver Functions and Surface Wave Dispersion Curves for 3‐D Crustal and Upper Mantle Structure Beneath the U.S. Midcontinent Rift

“…A large volume of basalt syntectonically filled parts of the MCR, though its origin is unknown. A mantle plume hypothesis has been used to explain the existence of the flood basalts in the rift valley on both sides of the MCR (e.g., W. R. Davis et al., 2022; Hutchinson et al., 1990; Nicholson et al., 1997; Prasanna et al., 2022), although the lack of significant topographic relief has been used to argue against this (Campbell, 2001).…”

“…(e.g., W. R. Davis et al, 2022;Hutchinson et al, 1990;Nicholson et al, 1997;Prasanna et al, 2022), although the lack of significant topographic relief has been used to argue against this (Campbell, 2001). S. Stein et al (2018) explain the development of the MCR as a result of deformational processes during a reorganization of plate motions and the resulting evolution of plate margins, suggesting that deformation caused decompression melting and leaky transform faults that resulted in a large volume (up to a million cubic kilometers) of volcanic basalt.…”