Zircon petrochronology in large igneous provinces reveals upper crustal contamination processes: new U\u2013Pb ages, Hf and O isotopes, and trace elements from the Central Atlantic magmatic province (CAMP)

J H F L Davies,H Bertrand,G Simpson,M Ackerson,A Marzoli,N Youbi,F Farina,M Ernesto,T Pettke,U Schaltegger,N D Greber,A.-S Bouvier,H V Ahrenstedt,L Baumgartner

doi:10.1007/s00410-020-01765-2

Abstract

Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon crystallizes in these magmas is not clearly understood, since their mafic compositions should limit zircon saturation. Here, we investigate the occurrence of zircon (and baddeleyite) in intrusive and extrusive mafic rocks from Central Atlantic Magmatic Province (CAMP) using petrography, trace-element analysis, Ti temperatures, Hf and oxygen isotopes, and high-precision U–Pb geochronology, along with petrological and thermal modeling. We provide new ages for CAMP sills that intruded into Paleozoic sediments in Brazil, indicating that the high and low Ti magmatism in this area occurred synchronously over 264 ± 57 ka. We show that upper crustal assimilation, especially of shales, during the emplacement of the CAMP likely led to zircon saturation. Assimilation of upper crustal sediments is also supported by high δ18O values and some rare negative εHf values in the zircon crystals. The only extrusive sample analyzed was the North Mountain basalt in Nova Scotia, Canada. This sample contains a large age variation in its zircon crystals (up to 4 Ma), and the older crystals have slightly more negative εHf values suggesting the presence of small (micron scale) xenocrystic cores associated with very late-stage sediment assimilation. However, the CAMP dataset as a whole suggests that the presence of xenocrystic cores is rare. Assuming no xenocrystic cores, and considering the zircon undersaturated nature of LIP mafic melts, the oldest zircon age clusters in a population should record the magma emplacement (or time when assimilation occurred), and the younger ages in a population are more likely to reflect Pb loss, especially given the high U concentrations of LIP zircon. Our identification of heterogeneous isotopic and elemental compositions in LIP zircon indicates that zircon in these magmas saturate in isolated minute melt pockets just before the system cools below its solidus.

Highlights

Zircon and other Zr-bearing phases may be found in rocks from mafic large igneous provinces (LIPs) and are frequently used to provide highprecision age control of these events (Svensen et al 2009; Schoene et al 2010a; Svensen et al 2012; 2017; Blackburn et al 2013; Burgess et al 2015; Davies et al 2017; Greber et al 2020)
Zircon crystallizes in late-stage enriched melt pockets in LIP magmas
It is likely that crustal contamination of the melts is required for zircon to crystallize in many samples of the Central Atlantic Magmatic Province (CAMP) and for LIP magmas in general

Summary

Introduction

Zircon and other Zr-bearing phases (e.g., baddeleyite and zirconolite) may be found in rocks from mafic large igneous provinces (LIPs) and are frequently used to provide highprecision age control of these events (Svensen et al 2009; Schoene et al 2010a; Svensen et al 2012; 2017; Blackburn et al 2013; Burgess et al 2015; Davies et al 2017; Greber et al 2020). There is growing evidence that the intrusion of LIP sills into sedimentary basins that contain abundant organic-rich or evaporitic sediments induce degassing of volatiles, a process that can lead to mass extinctions (Svensen et al 2007, 2009, 2010, 2017; Ganino and Arndt 2009; Davies et al 2017; Burgess et al 2017; Heimdal et al 2018, 2019, 2020) This model is the preferred explanation for some of the Phanerozoic mass extinction events, in part due to the observation that the ages of sills intruding the sedimentary basins coincide with mass extinction events. Advances in geochemical methods, for example measuring Hf isotopes from very low concentration solutions (D’Abzac et al 2016; Bauer and Horstwood 2018), and measuring in situ oxygen isotopes by SIMS in zircon and baddeleyite (see Linge et al 2017; Davies et al 2018), allow for a detailed investigation of the chemistry of these minerals, helping to constrain their origin and LIP petrogenesis

Results

Discussion

Conclusion