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PALAEOARCHAEAN SEA-FLOOR ALTERATION AND METAMORPHISM OF PILLOW BASALTS FROM THE NONDWENI GREENSTONE BELT, SOUTHEASTERN KAAPVAAL CRATON, SOUTH AFRICA

2012; Geological Society of South Africa; Volume: 115; Issue: 3 Linguagem: Inglês

10.2113/gssajg.115.3.259

1996-8590

Laxman Saha, Axel Hofmann, Nkosinathi Luvuno. Jele, Chris Harris,

Paleontology and Stratigraphy of Fossils

Research Article| September 01, 2012 PALAEOARCHAEAN SEA-FLOOR ALTERATION AND METAMORPHISM OF PILLOW BASALTS FROM THE NONDWENI GREENSTONE BELT, SOUTHEASTERN KAAPVAAL CRATON, SOUTH AFRICA L. SAHA; L. SAHA School of Agricultural, Earth and Environmental Sciences (Discipline: Geology), University of KwaZulu Natal, Private Bag X54001, 4000 Durban, South Africa, e-mail: saha.lopamudra@gmail.com Search for other works by this author on: GSW Google Scholar A. HOFMANN; A. HOFMANN Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg, South Africa, e-mail: ahofmann@uj.ac.za Search for other works by this author on: GSW Google Scholar N. JELE; N. JELE School of Agricultural, Earth and Environmental Sciences (Discipline: Geology), University of KwaZulu Natal, Private Bag X54001, 4000 Durban, South Africa, e-mail: jele.nkosinathi@gmail.com Search for other works by this author on: GSW Google Scholar C. HARRIS C. HARRIS Department of Geological Sciences, University of Cape Town, Rondebosch 7701, Cape Town, South Africa, e-mail: chris.harris@uct.ac.za Search for other works by this author on: GSW Google Scholar Author and Article Information L. SAHA School of Agricultural, Earth and Environmental Sciences (Discipline: Geology), University of KwaZulu Natal, Private Bag X54001, 4000 Durban, South Africa, e-mail: saha.lopamudra@gmail.com A. HOFMANN Department of Geology, University of Johannesburg, PO Box 524, Auckland Park 2006, Johannesburg, South Africa, e-mail: ahofmann@uj.ac.za N. JELE School of Agricultural, Earth and Environmental Sciences (Discipline: Geology), University of KwaZulu Natal, Private Bag X54001, 4000 Durban, South Africa, e-mail: jele.nkosinathi@gmail.com C. HARRIS Department of Geological Sciences, University of Cape Town, Rondebosch 7701, Cape Town, South Africa, e-mail: chris.harris@uct.ac.za Publisher: Geological Society of South Africa First Online: 09 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2012 Geological Society of South Africa South African Journal of Geology (2012) 115 (3): 259–282. https://doi.org/10.2113/gssajg.115.3.259 Article history First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation L. SAHA, A. HOFMANN, N. JELE, 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 2012;; 115 (3): 259–282. doi: https://doi.org/10.2113/gssajg.115.3.259 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search 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 Al2O3, TiO2, Fe2O3, Zr, Nb, Nd, Th concentrations and lower SiO2, Na2O, 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 δ18O values as high as 9.6‰ suggest that hydrothermal alteration of the pillow basalts initially occurred under low-temperature conditions. δ18O values as low as 2.8‰ 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 ~6.5 kbar, 600 to 650°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 ~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 ~3.29 Ga. These observations along with the high geothermal gradient (30°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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.