Chromitites from an Archean layered intrusion in the Western Dharwar Craton, southern India

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Chromite deposits hosted in a layered ultramafic-mafic
intrusion in the Archean Chithradurga greenstone belt are part of a
suture that divides the Western and Central Dharwar craton blocks in
southern India. Serpentinised ultramafic rocks including dunite,
peridotite, and phlogopite-rich clinopyroxenite occur with massive
chromitite. The olivine compositions show two distinct groups, one with
Fo ~91-92 olivines, in the massive chromitite and another group with
olivine Fo values of ~95-96, in the serpentinised dunite. The chromite
occurs as massive chromitite and as an accessory phase in dunite. The
former are characterized by high Cr# (Cr/(Cr+Al)×100=67-70) and moderate
to high Mg# (Mg/(Mg+Fe)=52-66); The latter have a larger variation in Cr
which is likely related to sub-solidus re-equilibration of the chromite
during the serpentinization event. In the massive chromitites, subsolidus
re-equilibration of chromite is less significant or absent. Bulkrock
major and trace element analyses of the peridotites reveal a
composition less magnesian than depleted mantle rocks, with low TiO2,
Na2O, CaO and high Cr, Ni content, a general depletion in Sr and
enrichment in Th, U. The chromitites contain varied PGE abundances (ΣPGE
= 14-167 ppb) and IPGEs (Ir, Os, Ru) as well as enriched primitive-mantle
normalized PGE patterns (PdN/IrN= 0.004-0.241). These features suggest
the chromitites are cumulates formed from a highly magnesian parental
magma with komatiitic affinity, derived by large degrees of partial
melting of a refractory mantle protolith, which underwent crustal
contamination. The Re-Os isotope data demonstrate a minimum Re-depletion
model age (TRD) of 2895 ±24 Ma for the serpentinized dunite (MY11/3), and
a more robust age of 3120 ±12 Ma for two chromitite samples, which we
interpret as the best estimate of the magmatic age of this ultramafic
intrusion. Abundant zircon grains also occur within these ultramafic
rocks with U-Pb ages spanning from ca. 2.9 Ga to ca. 2.4 Ga, with a
dominant age population in the range 2.8-2.6 Ga. We interpret the upper
intercept age at ca. 2.9 Ga to represent the initial crystallization of
zircon in the ultramafic rocks, potentially as a result of melt
infiltration and metasomatism associated with regional granitoid crust
formation and related metasomatic activity. The 2.5-2.4 Ga zircon ages
reflect a late-stage high-grade metamorphic overprint.
Antal sider15
StatusUdgivet - 1 dec. 2020

ID: 248146219