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Palaeoarchaean sea-floor alteration and metamorphism of pillow basalts from the Nondweni greenstone belt, southeastern Kaapvaal Craton, South Africa

L. Saha, A. Hofmann, N. Jele and C. Harris
Palaeoarchaean sea-floor alteration and metamorphism of pillow basalts from the Nondweni greenstone belt, southeastern Kaapvaal Craton, South Africa
South African Journal of Geology (September 2012) 115 (3): 259-282

Abstract

Pillow basalts from the Witkop Formation of the Nondweni granitoid-greenstone terrain preserve evidence of Palaeoarchaean sea-floor alteration and high-grade metamorphism. Post-depositional sea-floor alteration preferentially affected volcanic glass and led to a marked contrast in compositions and mineral assemblages of the cores and rims of the pillows. The pillow rims have elevated Al (sub 2) O (sub 3) , TiO (sub 2) , Fe (sub 2) O (sub 3) , Zr, Nb, Nd, Th concentrations and lower SiO (sub 2) , Na (sub 2) O, Sr contents, relative to pillow cores. Igneous clinopyroxene and plagioclase is preserved exclusively in pillow cores, whereas the pillow rims consist of a metamorphic mineral assemblage of garnet-hornblende-plagioclase-chlorite-quartz. The whole rock delta (super 18) O values as high as 9.6 per mil suggest that hydrothermal alteration of the pillow basalts initially occurred under low-temperature conditions. delta (super 18) O values as low as 2.8 per mil may indicate water/rock interaction at a later stage under high-temperature conditions during metamorphism. Following sea-floor alteration, the pillow basalts were deformed and metamorphosed under amphibolite facies conditions that led to formation of hornblende-defined foliation and syntectonic garnet growth in pillow rims. Garnet-hornblende-plagioclase-quartz geothermo-barometry from pillow rims reveal peak metamorphic conditions of approximately 6.5 kbar, 600 to 650 degrees C. P-T pseudosection analysis also confirms that contrasting mineral assemblages in pillow cores and rims developed during metamorphism principally due to the difference in major element concentrations in these domains. P-T pseudosection analysis also predicts an isobaric cooling of the basalts along an anticlockwise P-T path. The P-T estimates indicate burial to a depth of approximately 17 to 20 km post-tectonic to hydrothermal alterations. The occurrence of high-grade mineral assemblages in the basalts was observed only in the vicinity of the intrusive Mvunyana granodiorite. The presence of migmatitic gneiss with steeply dipping foliations at their contact indicate that metamorphism of basalts was concomitant with intrusion of the granodiorite. Hence the time-frame of metamorphism of the Witkop Formation pillow basalts can be constrained from the crystallization age of the Mvunyana granodiorite, which is approximately 3.29 Ga. These observations along with the high geothermal gradient (30 degrees C/km) recorded during peak metamorphism, and the post-peak near-isobaric cooling along an anticlockwise P-T path, are consistent with a magmatic arc setting.


ISSN: 1012-0750
EISSN: 1996-8590
Serial Title: South African Journal of Geology
Serial Volume: 115
Serial Issue: 3
Title: Palaeoarchaean sea-floor alteration and metamorphism of pillow basalts from the Nondweni greenstone belt, southeastern Kaapvaal Craton, South Africa
Affiliation: University of KwaZulu Nata, School of Agricultural, Earth and Environmental Sciences, Durban, South Africa
Pages: 259-282
Published: 201209
Text Language: English
Publisher: Bureau for Scientific Publications, Pretoria, South Africa
References: 78
Accession Number: 2013-044399
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. geol. sketch maps
S27°00'00" - S18°00'00", E18°00'00" - E30°00'00"
Secondary Affiliation: University of Johannesburg, ZAF, South AfricaUniversity of Cape Town, ZAF, South Africa
Country of Publication: South Africa
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, Copyright, Geological Society of South Africa. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201328
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