Fluid-induced Dehydration of the Paleoarchean Sand River Biotite-Hornblende Gneiss, Central Zone, Limpopo Complex, South Africa

Journal article

Authors/Editors

Rajesh, Hariharan

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Publication Details

Author list: Rajesh HM, Belyanin GA, Safonov OG, Kovaleva EI, Golunova MA, Van Reenen DD

Publisher: Oxford University Press

Place: OXFORD

Publication year: 2013

Journal: Journal of Petrology (0022-3530)

Journal acronym: J PETROL

Volume number: 54

Issue number: 1

Start page: 41

End page: 74

Number of pages: 34

ISSN: 0022-3530

eISSN: 1460-2415

Languages: English-Great Britain (EN-GB)

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Abstract

A clear case study of local-scale, fluid-induced dehydration of the Paleoarchean Sand River biotite-hornblende gneiss from the Central Zone of the Limpopo Complex is presented here. Field and petrographic examination of three adjacent zones-darker Sand River orthogneiss with local occurrence of orthopyroxene and clinopyroxene, a lighter intermediate gneissic zone with more orthopyroxene than the Sand River orthogneiss, and tonalitic veins containing large orthopyroxene-bearing patches-indicates the local transformation of a light grey, fine- to medium-grained, hornblende-biotite gneiss into a greenish brown, medium- to coarse-grained orthopyroxene-bearing dehydration zone. Field evidence indicates that the tonalitic veins were emplaced in discrete ductile shear zones, with development of large orthopyroxene-bearing patches in a sigmoidally transposed foliation bounded by shear planes. Orthopyroxene-forming reaction textures after biotite and amphibole together with the occurrence of microveins of K-feldspar along quartz-plagioclase grain boundaries in the three adjacent zones, and the higher modal abundance of orthopyroxene and K-feldspar with lesser biotite and amphibole from the Sand River orthogneiss to the intermediate gneissic zone to the orthopyroxene-bearing patches, indicate that the three adjacent zones represent progressive stages of the dehydration process. Such K-feldspar microveins along quartz-plagioclase grain boundaries have been proposed as evidence for the presence and passage of a low H2O activity fluid. Further, the occurrence of monazite inclusions in fluorapatite in orthopyroxene-bearing zones suggests dissolution and reprecipitation involving a free fluid phase. Fluid inclusion studies indicate the presence of a fluid with CO2, NaCl and H2O components, with higher salinity of the fluid (up to 29% NaCl) in the orthopyroxene-bearing patches relative to the intermediate gneissic zone. The increase in Cl content in amphibole, biotite and fluorapatite from the Sand River orthogneiss to the orthopyroxene-bearing patches supports the presence of a Cl-rich brine fraction in the fluid responsible for the dehydration process. Further, the increase in An content of plagioclase at the contact with the K-feldspar rims on quartz reflects an increase in potassium activity in the fluid. The whole-rock major, trace and rare-earth element enrichment or depletion patterns of the orthopyroxene-bearing zones relative to the precursor support the dehydration process. The diffuse contact relationship of a granite pegmatite occurring in the vicinity of the dehydration zones, together with fluid inclusion and whole-rock major, trace and rare element characteristics of samples collected along a traverse from the granite pegmatite to the Sand River orthogneiss, suggests a scenario in which the dehydrating fluids derived from an external source utilized lithological contrasts, such as the gneiss-pegmatite boundaries, as fluid conduits. Dehydration of the gneissic wall-rock occurred where permeability was sufficient for fluid penetration. The occurrence of orthopyroxene-bearing tonalitic veins along deformation-transposed foliation planes further attests to a structural control to the channeling of the dehydrating fluids.

Keywords

CO2-rich fluid with additional Cl-rich brine and H2O, fluid-induced dehydration, fluid infiltration along deformation-transposed foliation planes, Limpopo Complex, orthopyroxene-bearing patches, Paleoarchean Sand River biotite-hornblende gneiss

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