A Comparative Study Of Metamorphosed Supracrustal Rocks From The Western Nahaqualand Metamorphic Complex

ABSTRACT

A regional study of highly metamorphosed supracrustal rocks was

undertaken in the western portions of the Namaqualand Metamorphic Complex. The

study area was essentially restricted to a north - south section some 50

kilometres wide and 220 kilometres long . Eight east-west-trending belts of

supracrustal rocks were examined, together with several smaller paragneiss

remnants , in an area dominated by quartzo - feldspathic gneisses of granitic

composition.

The supracrustal rocks were classified into seven major lithological

groups: quartzitic rocks, metapelitic and metapsammitic rocks , quartzo feldspathic

rocks, metabasites, metacarbonate rocks, magnesium-rich cordierite

rocks and iron formations. Further subdivision, based on variations in mineral

constituents within each group, also occurred, as well as the presence of

lithologies with compositions transitional between certain groups.

The various supracrustal sequences were subdivided into formations

containing minor distinctive members on an informal lithostratigraphic basis.

Correlation between the major supracrustal belts was then undertaken. Four

subgroups were identified across the study area, comprising a quartzo feldspathic

gneiss subgroup and an overlying feldspathic quartzite/garnetcordierite

gneiss subgroup that both predominate in the southern and central

part of the area, a glassy quartzite/mica- sillimanite schist subgroup that

predominates in the northern part , and a cordierite gneiss/metacarbonate

subgroup that is restricted to the Geselskapbank synform. The supracrustal

rocks appear to have been emplaced on a basement of augen gneisses . This

relationship is , however , complicated by the intrusion of granit i c rocks

within the contact zone.

The bulk compositions of the metapelitic rocks were used to examine

potential modifying effects on compositi on caused by granulite - and

amphibolite- facies metamorphism in the light of previous studies that proposed

major amounts of partial melting. Trends displayed by incompatible elements

across the amphibolite/granulite facies boundary indicate significant

depletion in Rb (20%) and lesser K (5%) in the granulite facies. Residual

enrichment in Ba and Sr is also possible . Estimations of potential partial

melt loss were undertaken by least-squares approximation using mean

compositions of amphibolite- and granulite-facies metapelitic rocks and

various minimum melt compositions . Partial melting resulting in the formation

of up to 20% minimum-melt granite and 80% restite was estimated for the

granulite facies, which is compatible with observed in-situ leucosome

formation.

Metapelitic rocks have bulk compositions similar to unmetamorphosed

Proterozoic platform shales. Two distinctive groups of metapelitic rocks were

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APA

MOORE, J (2021). A Comparative Study Of Metamorphosed Supracrustal Rocks From The Western Nahaqualand Metamorphic Complex. Afribary. Retrieved from https://afribary.com/works/a-comparative-study-of-metamorphosed-supracrustal-rocks-from-the-western-nahaqualand-metamorphic-complex

MLA 8th

MOORE, JOHN "A Comparative Study Of Metamorphosed Supracrustal Rocks From The Western Nahaqualand Metamorphic Complex" Afribary. Afribary, 15 May. 2021, https://afribary.com/works/a-comparative-study-of-metamorphosed-supracrustal-rocks-from-the-western-nahaqualand-metamorphic-complex. Accessed 25 Jul. 2024.

MLA7

MOORE, JOHN . "A Comparative Study Of Metamorphosed Supracrustal Rocks From The Western Nahaqualand Metamorphic Complex". Afribary, Afribary, 15 May. 2021. Web. 25 Jul. 2024. < https://afribary.com/works/a-comparative-study-of-metamorphosed-supracrustal-rocks-from-the-western-nahaqualand-metamorphic-complex >.

Chicago

MOORE, JOHN . "A Comparative Study Of Metamorphosed Supracrustal Rocks From The Western Nahaqualand Metamorphic Complex" Afribary (2021). Accessed July 25, 2024. https://afribary.com/works/a-comparative-study-of-metamorphosed-supracrustal-rocks-from-the-western-nahaqualand-metamorphic-complex