Abstract
ABSTRACTPalaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al2O3 and (Gd/Yb)N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga.
Highlights
Komatiites are MgO-rich (>18 wt.%) ultra-high-temperature, low-viscosity lava flows mostly restricted to Archaean greenstone belts (Arndt et al 2008)
The main purpose of this contribution is to present field relationships, petrographic characteristics, and whole-rock geochemical data for the komatiite flows from the Sargur Group Jayachamaraja Pura (J.C.Pura) and Banasandra greenstone belts in the western Dharwar craton to constrain their physical volcanology and petrogenesis, including mantle evolution and the geodynamic context of komatiite volcanism
The komatiite flows from the present study show characteristics akin to undifferentiated massive lava flows, with polyhedral joints and fully differentiated lava flows with a spinifex-textured upper layer and olivine cumulate lower layers (Pyke et al 1973; Arndt et al 1977)
Summary
Komatiites are MgO-rich (>18 wt.%) ultra-high-temperature, low-viscosity lava flows mostly restricted to Archaean greenstone belts (Arndt et al 2008). They are often considered to be windows to secular development of dynamic mantle (Mole et al 2014) and are fundamental to our understanding of the thermal, chemical, and tectonic evolution of the early Earth. The main purpose of this contribution is to present field relationships, petrographic characteristics, and whole-rock geochemical data for the komatiite flows from the Sargur Group Jayachamaraja Pura (J.C.Pura) and Banasandra greenstone belts in the western Dharwar craton to constrain their physical volcanology and petrogenesis, including mantle evolution and the geodynamic context of komatiite volcanism
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