Dynamic melting of the Precambrian mantle: evidence from rare earth elements of the amphibolites from the Nellore–Khammam Schist Belt, South India

Trace and Rare Earth Elements (REEs) data are used to constrain the geochemical evolution of the amphibolites from Ifewara in the Ife-Ilesha schist belt of southwestern Nigeria. The amphibolites can be grouped into banded and sheared amphibolites. Major element data show SiO2 (48.34%), Fe2O3 (11.03-17.88%), MgO (5.76-9.90%), CaO (7.76-18.6%) and TiO2 (0.44-1.77%) contents which are similar to amphibolites in other schist belts in Nigeria. The Al2O3 (2.85-15.55%) content is varied, with the higher values suggesting alkali basalt protolith. Trace and rare earth elements composition reveal Sr (160-1077ppm), Rb (0.5-22.9ppm), Ni (4.7-10.2ppm), Co (12.2-50.9 ppm) and Cr (2-7ppm). Chondrite-normalized REE patterns show that the banded amphibolites have HREE depletion and both negative and positive Eu anomalies while the sheared variety showed slight LREE enrichment with no apparent Eu anomaly. The study amphibolites plot in the Mid Oceanic Ridge Basalts (MORB) and within plate basalt fields on the Zr/Y vs Zr discriminatory diagrams. They are further classified as volcanic arc basalt and E-type MORB on the Th- Hf/3- Ta and the Zr-Nb-Y diagrams. The amphibolites precursor is considered a tholeiitic suite that suffered crustal contamination, during emplacement in a rifted crust.

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Deformation-driven emplacement-differentiation in the Closepet pluton, Dharwar Craton, South India: an alternate view

Geological Society London Special Publications ◽

10.1144/sp489-2019-315 ◽

2019 ◽

Vol 489 (1) ◽

pp. 261-274 ◽

Cited By ~ 1

Author(s):

Abhijit Bhattacharya

Keyword(s):

Rare Earth ◽

Rare Earth Elements ◽

South India ◽

Major Element ◽

Volume Fraction ◽

Dharwar Craton ◽

The North ◽

Magmatic Fabric ◽

Melt Composition ◽

Crystal Content

AbstractIn the Late Archean north-trending Closepet pluton, trains of euhedral K-feldspar phenocrysts and matrix-supported idiomorphic K-feldspar crystals in the central part of the pluton define oblique-to-pluton margin steep-dipping east/ENE-trending magmatic fabrics. The magmatic fabric is defined by phenocryst-rich and phenocryst-poor layers, with the euhedral porphyries continuous across the layers. The fabrics are near-orthogonal to the gently-dipping gneissic layers in the host gneisses. The fabrics curve adjacent to locally-developed north/NNE-trending melt-hosted dislocations parallel to the axial planes of horizontal/gently-plunging north-trending upright folds in the host gneisses. In the pluton interior, both fabrics in the intrusives formed at supra-solidus conditions, although the volume fraction of melts diminished drastically due to cooling/melt expulsion. At the pluton margin, the north-trending fabric is penetrative and post-dates magma solidification. Within the pluton, the major element oxides, rare earth elements, anorthite contents in plagioclase, and (Mg/Fe + Mg) ratios in biotite decrease with increasing SiO2 from phenocryst-rich (up to 75% by volume) granodiorite to phenocryst-poor (<15 vol%) granite that broadly correspond to minimum melt composition. The chemical-mineralogical variations in the pluton is attributed to deformation-driven ascent of magma with heterogeneous crystal content, ascending at variable velocities (highest in crystal-poor magma) along oblique-to-pluton margin east/ENE-trending extensional fractures induced by dextral shearing.

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