Ultraslow cooling of an ultrahot orogen Open Access

The rate of cooling of metamorphic rocks provides a first-order constraint on the tectonic processes controlling heat flow and exhumation. Rates of cooling are commonly fast (generally ∼20–30 °C/m.y.) during exhumation of metamorphic core complexes or transpression (Brown and Dallmeyer, 1996; Scibiorski et al., 2015). By contrast, cooling in some granulite terranes was slow (<5 °C/m.y.) and close to isobaric (Ashwal et al., 1999; Clark et al., 2018; Korhonen et al., 2013b; Mezger et al., 1991), implying they were close to isostatic equilibrium as a result of thin lithosphere, sustained high mantle heat flow, and conductive cooling that limited exhumation by erosion (Oxburgh, 1990). However, constraining initial cooling rates in granulite terranes can be difficult, particularly where the rocks reached ultrahigh temperatures (UHTs; >900 °C), exceeding the closure temperature of many geochronometers (Cherniak, 2010; Harley, 2016).

Despite the high closure temperature of zircon to diffusion of Pb and Ti (Cherniak, 2010), results of U-Pb zircon geochronology and Ti-in-zircon thermometry may be difficult to reconcile due to chemical decoupling (Kunz et al., 2018) or tectonothermal overprinting (Harley, 2016). Here, we combined U-Pb zircon geochronology with Ti-in-zircon thermometry to investigate the thermal history of metapelitic granulites from the Eastern Ghats Province (EGP), eastern India (Fig. 1). The EGP underwent extensive crustal melting at UHT conditions (Korhonen et al., 2013a, 2014; Mitchell et al., 2019) that may have been sustained regionally for up to 200 m.y. after peak temperatures were attained (Korhonen et al., 2013b). Our results support the proposal that the lower crust in some orogens remained melt-bearing for several hundred million years (Taylor et al., 2020).

As part of the Eastern Ghats mobile belt (Fig. 1), the EGP is a crustal block with distinct isotopic, metamorphic, and lithological characteristics (Rickers et al., 2001) and evidence for multiple tectono-magmatic episodes from the late Neoarchean to early Paleozoic (Dasgupta et al., 2013; Gupta, 2012; Rickers et al., 2001). A late Mesoproterozoic to early Neoproterozoic UHT regional metamorphic event records peak metamorphic conditions of >950 °C and 7–8 kbar followed by near-isobaric cooling (Korhonen et al., 2013a, 2013b, 2014; Mitchell et al., 2019).

Based on U-Pb zircon and monazite geochronology, Korhonen et al. (2013b) estimated the duration of UHT conditions to have been at least 50 m.y. and perhaps as long as 200 m.y., from ca. 1130 to 930 Ma, during a single tectono-metamorphic event with a counterclockwise pressure-temperature (P-T) evolution. On a per sample basis, discrete clusters of concordant zircon and monazite age data suggest crystallization at different temperatures due to loss of variable quantities of hydrous melt.

A compilation of concordant U-Pb zircon and monazite ages from multiple localities within the EGP reveals a continuum extending from 1130 Ma to 800 Ma (Fig. 2A; Table S1 in the Supplemental Material¹). Near-concordant zircon ages of 613 Ma and 566 Ma (Korhonen et al., 2013b) and 511 Ma (Mitchell et al., 2019) may correspond to new zircon growth or be related to Pb loss during Gondwana assembly (Korhonen et al., 2013b). Estimates of time-averaged cooling rates from the thermal peak of UHT metamorphism range from ∼1 °C/m.y. (Korhonen et al., 2013b) to 0.13 ± 0.08 °C/m.y. (Mitchell et al., 2019). To investigate the range of crystallization temperatures of metasedimentary granulites in the EGP, zircon was separated from six garnet–sillimanite gneisses collected along a broadly orogen-parallel NE-SW transect (Fig. 1), but further from the craton margin than those dated previously (Korhonen et al., 2013b).

Zircon grains separated from six samples using standard methods were mounted in 2.5 cm (1 in.) round epoxy blocks, polished, and analyzed for U, Th, Pb, Ti, and rare earth elements (REEs) by laser-ablation split stream–inductively coupled plasma–mass spectrometry (LASS–ICPMS) at the GeoHistory Facility in the John de Laeter Centre, Curtin University, Perth, Western Australia. For analytical methods, descriptions, and cathodoluminescence (CL) images of zircon indicating analytical spot localities, data tables, and supporting plots, see the Supplemental Material.

For Ti-in-zircon thermometry, we used the calibration of Crisp et al. (2023). All six samples were quartz-bearing (aSiO₂ = 1). All samples that contained ilmenite lacked rutile, so we applied a range of aTiO₂ values from 0.5 to 1.0 (Clark et al., 2019). Although the uncertainty in aTiO₂ introduces small uncertainties (up to tens of degrees Celsius) in calculated T, this has little effect on the relative difference (ΔT) between individual Ti-in-zircon temperatures within the same sample, demonstrating that our estimated cooling rates are robust (see the Supplemental Material).

Although CL images showed rare zircon grains with cores and metamorphic overgrowths, most had homogeneous or faded complex patterns of ambiguous origin. Therefore, we distinguished analyses of unmodified metamorphic zircon from those of detrital (igneous) zircon or zircon affected by postcrystallization modification (e.g., Pb-loss, metamictization, fluid alteration) based on concordance and trace-element composition. Of the 735 zircon LASS analyses from the six samples, we first discarded grains >10% discordant (group X in Table S2), followed by grains with chondrite-normalized Yb/Gd ratios (Yb_N/Gd_N) >2, indicative of zircon growth in the absence of garnet (group I in Table S2; Taylor et al., 2016). Last, we eliminated analyses with elevated light REEs (LREEs; e.g., La >5 ppm) and spuriously high Ti (>100 ppm) contents, indicative of postcrystallization modification of zircon or submicroscopic inclusions (e.g., Hoskin, 2005; group F in Table S2). The remaining 163 analyses were interpreted to be from “pristine” granulite-facies metamorphic zircon.

Metamorphic zircon grains are commonly ∼50–200 μm, but they can be up to ∼350 μm in diameter and may exhibit sector zoning or “soccer ball” morphologies typical of zircon grown during anatexis (Taylor et al., 2016). The 163 zircon U-Pb dates (with 2σ uncertainties) from all six samples are plotted in Figure 2B (for individual concordia diagrams, see Fig. S2). This combined data set defines a continuous spectrum of ²³⁸U/²⁰⁶Pb dates (within 2σ uncertainty) that spreads over a duration of ∼500 m.y. (Fig. 2B). Within this age spectrum, four groups of zircon grains are distinguished. Group 1 grains yield dates >1000 Ma that are discordant and are associated with relatively large uncertainties. Group 2 grains yield concordant dates and range in age from 974 to 793 Ma. Group 3 grains yield dates that are either discordant or have large errors and range in age from 790 to 550 Ma. Group 4 grains yield concordant dates that form a single population at 522 ± 1 Ma (n = 14, mean square of weighted deviates [MSWD] = 0.16).

Group 2 zircon grains are interpreted to be unmodified metamorphic zircon that crystallized during cooling of the Eastern Ghats following the early Neoproterozoic UHT peak. Only these grains were used for calculation of the cooling rates for individual samples. The relative decrease in temperature (ΔT) over time (Δt) for each of the six samples is plotted in Figures 3A–3F, along with best-fit lines (with 2σ uncertainty envelopes) assuming a linear cooling rate. Five of the six samples yielded statistically significant (p <0.05) cooling rates. Samples 17-37 and 17-72 (Figs. 3A and 3D) returned time-averaged cooling rates that are within 2σ uncertainty (0.24 ± 0.09 °C/m.y. and 0.28 ± 0.07 °C/m.y., respectively), whereas samples 17-59, 17-91, and 17-78 (Figs. 3B, 3E, and 3F) returned faster time-averaged cooling rates (0.52 ± 0.05 °C/m.y., 0.73 ± 0.11 °C/m.y., and 1.44 ± 0.60 °C/m.y.). The remaining sample, 17-66 (Fig. 3C), also yielded a slow cooling rate of 0.42 ± 0.63 °C/m.y., which is not statistically significant and is not considered further.

Given the morphology of the metamorphic zircon grains and the peak P-T conditions recorded by the EGP rocks (Korhonen et al., 2013a, 2013b, 2014; Mitchell et al., 2019), we interpret the observed spectrum of near-concordant zircon dates from 950 to 800 Ma (Fig. 2, group 2) to have formed during cooling of melt-bearing ultrahot lower crust. In this case, crystallization of zircon would have occurred when the anatectic melt became saturated in zirconium, either in situ (Kelsey et al., 2008; Yakymchuk and Brown, 2014) or during melt migration (Yakymchuk et al., 2017). Given the correlation between age and temperature for each sample, and for the data set as a whole, the simplest interpretation of the data is that they record at least 150 m.y. of melt crystallization during ultraslow cooling at rates of (much) less than 1 °C/m.y. Emplacement of the Chilka Lake anorthosite at ca. 790 Ma in the northern part of the EGP provides support for the idea that the lithosphere remained anomalously hot until ca. 800 Ma (Krause et al., 2001). A potential alternative interpretation is that the spread of data is related to one or more Pb-loss events, leading to reset dates that plot within 2σ uncertainty of concordia, but that are geologically meaningless. However, this interpretation requires that the Ti content also varies in a systematic way with the Pb.

To test the various scenarios that could have led to the development of the observed age and temperature spectra, we modeled thermal and growth histories of zircon using a newly developed code that simulates diffusion of Ti and the products of radiogenic ingrowth along a temperature-time (T-t) path (Fig. 4A; see Supplemental Material for details). The T-t profiles that we consider to be the most geologically plausible are shown in Figure 4B. In the first three scenarios, zircon is grown in a single pulse at 1000 Ma and then experiences a thermal pulse at 525 Ma, assuming either initial slow cooling to 800 Ma (profile 1) or rapid cooling with or without a thermal pulse at 800 Ma (profiles 2 and 3). A fourth scenario, in which zircon is grown in six discrete stages along T-t profile 1 from 1000 Ma to 820 Ma, each separated by 30 m.y., was modeled as a proxy for progressive crystallization (growth) during slow cooling.

In the first three scenarios, although Pb-loss age arrays are generated (Fig. S8), there is no change in Ti content and no cooling trend. As such, a single stage of zircon growth followed by overprinting thermal pulses cannot replicate the observed age-temperature relationship. In our simulation of progressive crystallization, the age trend (Fig. 4C), the concordia plot (Fig. 4D), and the regression results (Fig. 4E) closely match those observed in the natural data (Figs. 2A, 2B, and 3). These observations support protracted (duration) crystallization of melt due to ultraslow cooling (<1 °C/m.y.) of the lower levels of (EGP) orogenic crust as the simplest explanation. Furthermore, the consistency among the data in Figure 2A with the data in Figure 2B indicates that melt migration did not substantially disturb the thermal structure of the terrane.

Partial melting and redistribution of melt affect the thermal and mechanical properties of the deep crust. Melt-producing reactions buffer heat flow (Schorn et al., 2018), the presence of melt reduces the shear strength of the crust (Rosenberg and Handy, 2005), and drainage of melt may remove a significant proportion of heat-producing elements (HPEs) from metasedimentary protoliths (Yakymchuk and Brown, 2014). However, recent field studies (Alessio et al., 2018) and thermodynamic modeling (Yakymchuk and Brown, 2019) favored the retention of HPEs under certain conditions. Granulite-facies metasedimentary rocks in the EGP have nominally high HPE concentrations (2.4–3.9 μW·m^–3; Kumar et al., 2007), as do granites postulated to have formed through their partial melting (Korhonen et al., 2015). In addition, the latent heat of crystallization buffers temperature (and ΔT) and, together with low erosion rates and high mantle heat flux, may sustain suprasolidus temperatures in the lower crust on time scales >100 m.y. (Clark et al., 2011; Sizova et al., 2014).

The rocks of the EGP followed a counterclockwise P-T evolution, with initial heating followed by moderate thickening, during which the lowermost crust was at ∼7–8 kbar (Korhonen et al., 2013a), such that significant topography is unlikely (Sizova et al., 2014). With no gravitational driving force for exhumation, the crust likely remained close to isostatic equilibrium, leading to low rates of erosion. A decrease in mantle heat flow would have led to a colder lithospheric root, crustal subsidence, and development of a sedimentary basin (Oxburgh, 1990). However, if mantle heat flow remained sustained, for example, by small-scale convection (Watanabe et al., 1977), both erosion and subsidence would have been minimized (Oxburgh, 1990), allowing a terrane to retain heat and remain melt-bearing for >100 m.y. Although we considered only the high-T (suprasolidus to crystallization) part of the cooling path, previous studies have documented the onset of metamorphism at ca. 1130 Ma, meaning that the duration of metamorphism in the EGP likely exceeded 300 m.y. (Korhonen et al., 2013b).

In a wider context, paleogeographic reconstructions place the EGP against the Rayner Province (RP) during the early Neoproterozoic and the composite EGP-RP terrane on the margin of the supercontinent Rodinia (Merdith et al., 2017). Significant compressive forces were likely not applied to the outboard margin of the EGP-RP composite terrane, driving its exhumation over the Bastar craton, until the major plate reorganization that accompanied the formation of Gondwana.

1. In four metasedimentary samples from the Eastern Ghats Province, eastern India, zircon crystallization occurred over an ∼150 m.y. interval during which rocks remained at T >800 °C and melt-bearing conditions, corresponding to time-averaged cooling rates of < or <<1 °C/m.y.

2. The limited development of topography and minimal erosion permitted retention of HPEs in the lower crust, which, with the latent heat of crystallization, buffered the cooling rate.

3. The position of the Eastern Ghats Province on the margin of a supercontinent resulted in a setting where there were no clear lateral or vertical drivers to exhume the crust, allowing ultraslow cooling over ∼150 m.y.

We received support through Australian Research Council (ARC) project FT220100566. We thank Simon Harley, Andrew Kylander-Clark, Roberto Weinberg, and an anonymous reviewer for their reviews.