Clinopyroxene and Garnet Mantle Cargo in Kimberlites as Probes of Dharwar Craton Architecture and Geotherms, with Implications for Post-1·1 Ga Lithosphere Thinning Events Beneath Southern India

Abstract

The Wajrakarur Kimberlite Field (WKF) on the Eastern Dharwar Craton in southern India hosts several occurrences of Mesoproterozoic kimberlites, lamproites, and ultramafic lamprophyres, for which mantle-derived xenoliths are rare and only poorly preserved. The general paucity of mantle cargo has hampered the investigation of the nature and evolution of the continental lithospheric mantle (CLM) beneath cratonic southern India. We present a comprehensive study of the major and trace element compositions of clinopyroxene and garnet xenocrysts recovered from heavy mineral concentrates for three ca. 1.1 Ga old WKF kimberlite pipes (P7, P9, P10), with the goal to improve our understanding of the cratonic mantle architecture and its evolution beneath southern India. The pressure-temperature conditions recorded by peridotitic clinopyroxene xenocrysts, estimated using single-pyroxene thermobarometry, suggest a relatively moderate cratonic mantle geotherm of 40 mW/m2 at 1.1 Ga. Reconstruction of the vertical distribution of clinopyroxene and garnet xenocrysts, combined with some rare mantle xenoliths data, reveals a compositionally layered CLM structure. Two main lithological horizons are identified and denoted as layer A (∼80–145 km depth) and layer B (∼160–190 km depth). Layer A is dominated by depleted lherzolite with subordinate amounts of pyroxenite, whereas layer B comprises mainly refertilised and Ti-metasomatised peridotite. Harzburgite occurs as a minor lithology in both layers. Eclogite stringers occur within the lower portion of layer A and at the bottom of layer B near the lithosphere–asthenosphere boundary at 1.1 Ga. Refertilisation of layer B is marked by garnet compositions with enrichment in Ca, Ti, Fe, Zr and LREE, although Y is depleted compared to garnet in layer A. Garnet trace element systematics such as Zr/Hf and Ti/Eu indicate that both kimberlitic and carbonatitic melts have interacted with and compositionally overprinted layer B. Progressive changes in the REE systematics of garnet grains with depth record an upward percolation of a continuously evolving metasomatic agent. The intervening zone between layers A and B at ∼145–160 km depth is characterised by a general paucity of garnet. This ‘garnet-paucity’ zone and an overlying type II clinopyroxene-bearing zone (∼115–145 km) appear to be rich in hydrous mineral assemblages of the MARID- or PIC kind. The composite horizon between ∼115–160 km depth may represent the product of intensive melt/rock interaction by which former garnet was largely reacted out and new metasomatic phases such as type II clinopyroxene and phlogopite plus amphibole were introduced. By analogy with better-studied cratons, this ‘metasomatic horizon’ may be a petrological manifestation of a former mid-lithospheric discontinuity at 1.1 Ga. Importantly, the depth interval of the present-day lithosphere–asthenosphere boundary beneath Peninsular India as detected in seismic surveys coincides with this heavily overprinted metasomatic horizon, which suggests that post-1.1 Ga delamination of cratonic mantle lithosphere progressed all the way to mid-lithospheric depth. This finding implies that strongly overprinted metasomatic layers, such as the ‘garnet-paucity’ zone beneath the Dharwar craton, present structural zones of weakness that aid lithosphere detachment and foundering in response to plate tectonic stresses.

Lithospheric Structure of the South India Precambrian Terrains From Surface Wave Tomography

“…Using the above definition of LAB in the shear wave velocity model, we interpret a 150–200 km thick lithosphere in the northern part of the Dharwar craton (WDC‐N and EDC‐N) (Figure 8a), correlating well with the 200 km thick lithosphere computed from joint inversion of receiver function and surface wave data (Bodin et al., 2014) and 160–190 km thick lithosphere derived from xenoliths of the diamond‐bearing kimberlite field (WKF, Figure 1a) (Ganguly & Bhattacharya, 1987; Griffin, Kobussen, 2009; Shaikh et al., 2020). Our result is also in accord with global observations of 150–200 km lithospheric thickness over the cratons based on heat flow and surface wave studies (Nataf & Ricard, 1996) and similar to the 130–190 km thickness based on SS precursors and the deepest origin depths of diamonds (Tharimena et al., 2017).…”

“…Out of these various definitions that exist, we follow the depth to the shear wave velocity of 4.5 km/s below the mantle high-velocity lid as the base of the continental lithosphere. We choose the velocity of 4.5 km/s as (a) it is the average of global Earth models in the depth of 100-200 km, (b) the minimum of low velocity for the global craton model (Lebedev et al, 2009) and (c) the mantle at 150 km depth with peridotite composition (Lee et al, 2011) is characterized by shear wave velocity of 4.56 km/s (Bruneton Using the above definition of LAB in the shear wave velocity model, we interpret a 150-200 km thick lithosphere in the northern part of the Dharwar craton (WDC-N and EDC-N) (Figure 8a), correlating well with the 200 km thick lithosphere computed from joint inversion of receiver function and surface wave data (Bodin et al, 2014) and 160-190 km thick lithosphere derived from xenoliths of the diamond-bearing kimberlite field (WKF, Figure 1a) (Ganguly & Bhattacharya, 1987;Griffin, Kobussen, 2009;Shaikh et al, 2020). Our result is also in accord with global observations of 150-200 km lithospheric thickness over the cratons based on heat flow and surface wave studies (Nataf & Ricard, 1996) and similar to the 130-190 km thickness based on SS precursors and the deepest origin depths of diamonds (Tharimena et al, 2017).…”

Low hydrogen concentrations in Dharwar cratonic lithosphere inferred from peridotites, Wajrakarur kimberlites field: Implications for mantle viscosity and carbonated silicate melt metasomatism

“…We from this study, small red symbols are from previous studies for Dharwar craton (Ganguly andBhattacharya, 1987 andNehru andReddy, 1989;and Shaikh et al, 2020) and small grey symbols are from previous studies for peridotite xenoliths from others cratons: Canada (Kopylova et al, 1999;MacKenzie and Canil, 1999;Menzies et al, 2004); Kaapvaal (Grégoire et al, 2005;Gibson et al, 2008;Lazarov et al, 2009;Konzett et al, 2013;Hanger et al, 2015;Bell et al, 2005); Siberia (Ionov et al, 2010;Doucet et al, 2012; (Dongre et al, 2015) and grey squares are for garnets from other eclogites transported by kimberlites from Koidu (Hills andHaggerty, 1989), Roberts Victor (MacGregor andManton, 1986), Yakutia (Snyder et. al, 1997;Jerde et al, 1993;Sobolev et al, 1994).…”

“…Kimberlites from the Wajrakarur field are poorly diamondiferous, while the other fields are diamond-free (Ravi et al, 2013). Ages based on 40 Ar/ 39 Ar and U-Pb isotope systems on separate grains of phlogopite and crustal perovskite indicate that kimberlite emplacement occurred around 1.1 Ga (Gopalan and Kumar, 2008;Osborne et al, 2011;Chalapathi Rao et al, 2014; see also study on major and trace element compositions of clinopyroxenes and garnets in Shaikh et al, 2020).…”

Mechanism for epeirogenic uplift of the Archean Dharwar craton, southern India as evidenced by orthogonal seismic reflection profiles

“…Tappe et al 38 and Shaikh et al 39 from the kimberlite studies on the Dharwar craton provided convincing evidence for the existence of a relatively thick lithosphere (~ 190 km) till 1.1 Ga. Subsequently, the mantle lithosphere was delaminated, leading to a thinner lithosphere (~ 120 km).…”