Continental growth histories revealed by detrital zircon trace elements: A case study from India

Simultaneous acquisition of detrital zircon Pb-Pb ages and trace element abundances from grains collected across the Indian craton, spanning ~3 b.y., reveals prominent shifts in Eu/ Eu* and light and middle to heavy rare earth element ratios. These shifts correspond to a ca. 3.0–2.2 Ga interval of crustal thickening during Indian craton formation, followed by a period wherein arc magmatism occurred along thinner craton margins from ca. 1.9 to 1.0 Ga, with arc magmatism concentrated along attenuated continental margins after ca. 1.0 Ga. Similar temporal shifts in trace element concentrations are recognized in global whole-rock compilations. We propose that the post–1.0 Ga increase in juvenile magmatism reflects a switch to lateral arc terrane accretion as the primary style of continental growth over the past billion years. INTRODUCTION In contrast to thin dense mafic oceanic crust, thick continental crust is composed of buoyant, intermediate to felsic rock. These compositional distinctions govern how modern plate tectonics operate: dense oceanic crust is consumed along subduction zones, whereas long-lived continents resist subduction (Rudnick and Gao, 2003; Korenaga, 2013). Because tectonic processes regulate conditions on Earth’s surface environment, primarily through volcanism and chemical weathering, and the presence of exposed continental crust is critical for maintaining habitability, investigating the chemical and structural evolution of continental lithosphere is essential for understanding Earth system evolution. Various geochemical proxies have been used to track crustal growth through time (e.g., Taylor and McLennan, 1995; Rudnick and Gao, 2003), including zircon U-Pb age compilations (Campbell and Allen, 2008; Condie and Aster, 2010). Potential preservational biases in the U-Pb zircon record directed attention to zircon Hf isotopic compositions under the assumptions that these data record extraction of melts from the upper mantle (Kemp et al., 2006; Belousova et al., 2010; Voice et al., 2011). While zircons with mantle-like δ18O values avoid signals from crustal recycling (Dhuime et al., 2012; Kemp et al., 2006), thus serving as a viable means to track volumetric extraction of continental crust from the mantle, these data lack information on the compositional evolution of continents via contributions from mixed, highly fractionated melts. Here we explore the potential for detrital zircon trace element (ZrTE) compositions as a proxy for crustal evolution. Detrital ZrTE data can be used as a provenance tool to distinguish between igneous source rocks (e.g., Belousova et al., 2002; Barth et al., 2013; Grimes et al., 2015); however, these data have not been systematically evaluated with respect to crystallization age over time scales relevant to continent formation. Zircon saturation is elevated in silicic melts, which are often generated along continental arcs (Lee and Bachmann, 2014). The detrital zircon record is therefore strongly influenced by regional arc magmatism (Lee et al., 2016; McKenzie et al., 2016), an important mechanism for crustal addition via water-induced melting of the upper mantle (Rudnick and Gao, 2003). Before the onset of lateral plate tectonics, tonalite-trondhjemitegranodiorites (TTGs) may have been prominent sources of detrital zircon (Moyen and Martin, 2012). However, the detrital zircon record is dominated by zircons younger than 3 Ga (Lee et al., 2016), and so largely reflects secular changes in arc magmatism since plate tectonic initiation (Dhuime et al., 2012, 2015). METHODS AND RATIONALE Three factors suggest that igneous zircon rare earth element (REE) abundances faithfully preserve a record of the composition of silicic parental melts. (1) REE diffusion in zircon is exceedingly slow (Cherniak et al., 1997), precluding diffusive equilibration after crystallization. (2) Zr is an incompatible trace element of moderately high abundance (~101–102 ppm) in silica-rich and intermediate composition magmas, meaning that fractional crystallization elevates melt Zr concentrations to the point of zircon saturation for a large range of melt compositions, H2O contents, and, critically, temperatures (Watson and Harrison, 1983); typical arc magmas are expected to reach zircon saturation at temperatures in excess of ~750 °C (Lee and Bachmann, 2014). (3) Zircon has a propensity to sequester heavy (H) REEs relative to light (L) REEs and middle (M) REEs from the host melt. This is likely driven by the xenotime coupled substitution mechanism (Y + REE)3+ + P5+ = Zr4+ + Si4+ in which the capacity of zircon to accommodate LREE3+ is limited by the large ionic radii of LREEs and attendant lattice strain at the Zr site (Speer and Cooper, 1982). Conversely, the smaller mismatch between ionic radii of Zr4+ and the HREEs accounts for partition coefficients >101 for the REEs Gd and Lu (Hanchar and van Westrenen, 2007). These crystalchemical controls on partitioning mean that REE abundances in zircon are sensitive to the presence of cogenetic HREE-compatible minerals phases, notably garnet or amphibole, in the parental melt. Application of combined detrital zircon U-Pb geochronology and TE abundances has been limited by collection of ages and elemental data from different portions of the same grain (Hoskin and Ireland, 2000). Laser ablation split stream– inductively coupled plasma–mass spectrometry (LASS-ICP-MS) circumvents this shortfall by simultaneous collection of U-Pb isotopic and TE abundance data from the same analytical volume, enabling zircon crystallization ages to be linked to melt composition. Provided that ZrTE abundances reflect the primary composition of the parental melt, this technique is ideally suited to assess secular trends in TE chemistry of detrital accessory phases. Our study focuses on a detrital zircon data set derived exclusively from the Indian subcontinent. Samples are from southern India (Kaldagi Basin), central India (Vindhyan, Aravalli-Delhi, and Marwar sectors), and northern *E-mails: [email protected]; [email protected] GEOLOGY, March 2018; v. 46; no. 3; p. 275–278 | GSA Data Repository item 2018077 | https://doi.org/10.1130/G39973.1 | Published online 24 January 2018 © 2018 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. 276 www.gsapubs.org | Volume 46 | Number 3 | GEOLOGY India (Himalaya) (Fig. 1) (see the GSA Data Repository1). LASS analyses, following the methodology presented by Kylander-Clark et al. (2013), were undertaken at the University of Texas (Austin, Texas, USA) using two ThermoFisher Element 2 high-resolution ICP-MS instruments, coupled to a Photon Machines Analyte G.2 ArF 192 nm excimer laser ablation system (www.teledynecetac.com/; see the Data Repository for analytical details). Our ZrTE and U-Pb data set comprises 574 singlegrain analyses with Pb-Pb ages between ca. 0.4 and 3.4 G—an ~3 b.y. record. To assess variations of mean ZrTE concentration with time, we used Monte Carlo bootstrap resampling (see the Data Repository) (Fig. 2). The bootstrap analysis yields an estimate of the average Indian ZrTE composition through time. To avoid inclusions and metamict grains, ZrTE analyses with Ti > 50 ppm and REE + Y > 1 wt% (Hoskin and Schaltegger, 2003) were discarded (n = 82) from the bootstrap analysis.