S. Nirmal Charan

A long-lived enriched mantle source for two Proterozoic carbonatite complexes from Tamil Nadu, southern India

We report new neodymium and strontium isotopic data for two Proterozoic carbonatites and related alkalic rocks, at Hogenakal and Sevathur in southern India. These complexes were emplaced into the crust at 2.4 Ga (Hogenakal) and 0.77 Ga (Sevathur). Their initial strontium and neodymium isotopic compositions, together with oxygen isotope data, suggest the involvement of a single long-lived enriched mantle source in their origin. The isotopic evolution of this source indicates that it formed approximately contemporaneously with the accretion and metamorphism of the overlying crust at the southern margin of the Dharwar craton and survived convective disruption in the mantle from early Proterozoic until at least 770 Ma ago. The older of the two carbonatites was intruded into young crust that was not older than about 150 Ma at the time of emplacement. The isotopic data contrast with those from carbonatites of the Canadian Shield, for which isotopic evidence also suggests origin from a long-lived lithospheric source, but one with a depleted chemical signature. They, therefore, indicate that there is no geochemically unique lithospheric source for carbonatites.

Evidence for recent hydrothermal activity in the Central Indian Basin

Abstract This study documents the first actual proof of recent intraplate volcanic-hydrothermal activity in the Central Indian Basin (CIB). Twenty-six surface sediments and a spade core (37 cm long) from the CIB were examined for the presence of volcanogenic-hydrothermal materials (vhm). High concentrations of vhm were discovered in a grab and the core top—both located at the base of an intraplate seamount. The vhm consist of ochrous metalliferous sediments, volcanic spherules and glass shards. The radiolaria associated with the vhm suggest a ∼ 10 ka age for the hydrothermal episode. The metalliferous sediments are semi-indurated, yellow to orange colored FeSi oxyhydroxides with FeO and Si02 contents between 54–73% and 16–30%, respectively, and have been derived as a result of hydrothermal precipitation. Incipient formation of nontronite is noted to co-occur with these sediments. The CIB metalliferous sediments have close similarities to those reported from the intraplate regions of the Pacific Ocean. The volcanic spherules occur in various shapes and sizes and are dominantly composed of magnetite and lesser amounts of ilmenite, hematite and maghemite. Electron microscopy shows the arrangement of magnetite crystals in various textural forms. Inclusions within the spherules are of olivine, pyroxene and feldspar. The spherules have formed by a process of liquid immiscibility of a silicic-basic magma, dependent on oxygen fugacity. Rhyolitic glass shards are ubiquitous at 1–2 cm depth in the core and constitute 55% of the coarse fraction. Microprobe analyses of the CIB shards show clear differences in Ti- and SiAl ratios that, together with the vast differences in age of eruption, preclude their derivation from Toba (Indonesia).

Geology and geochemistry of arenite–quartzwacke from the Late Archaean Sandur schist belt—implications for provenance and accretion processes

Abstract Detailed geological, petrological and geochemical studies have been carried out on an arenite–quartzwacke suite of rocks constituting a part of the Late Archaean Sandur schist belt in Dharwar craton, southern India for understanding the nature of provenance for these sedimentary rocks. The arenite–quartzwacke consists of rounded to sub-rounded and angular fragments of monocrystalline–polycrystalline quartz, quartzite and chert embedded in a fine-grained matrix of quartz and sericite. While arenites are more siliceous (SiO 2 , 80–92 wt.%), the quartzwacke have relatively lower silica content (ca. 69–78 wt.%). The arenites and quartzwackes have CIA values ranging from 76 to 96 which suggest intense chemical weathering. This is further corroborated by the positive correlation between Al 2 O 3 and TiO 2 in both these rock types. The ACNK modeling of arenites and quartzwackes show evidence for addition of K 2 O during later metasomatic alteration. In the ACNKFM ternary diagram all the samples plot along a mixing line between chlorite and sericite indicating alteration during K-metasomatism and the presence of mafic rocks in the source. The high concentration of HFSE such as Zr, Hf, Nb and Ta and the trace element ratios Th/Sc, La/Sc, Th/U and Ce/Th in the arenite–quartzwacke indicate a mixed provenance. The rare earth element modeling of quartzwackes considering tonalite, granite and amphibolite end members in the provenance suggests equal proportions of mafic and felsic end members. A composition comprising of 25% tonalite+25% granite+50% amphibolite in the provenance appears to match with the observed range of REE patterns of quartzwackes. The presence of higher proportions of granite in the provenance is evidenced by the large negative Eu anomalies in these sediments. Field evidence and structural discordance suggest that the arenite–quartzwacke suite is an allochthonous part of the Sandur schist belt.