Fluid-induced Dehydration of the Paleoarchean Sand River Biotite–Hornblende Gneiss, Central Zone, Limpopo Complex, South Africa (original) (raw)

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1Department of Geology, University of Johannesburg, Auckland Park, Johannesburg 2006, South Africa

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1Department of Geology, University of Johannesburg, Auckland Park, Johannesburg 2006, South Africa

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1Department of Geology, University of Johannesburg, Auckland Park, Johannesburg 2006, South Africa

2Institute of Experimental Mineralogy RAA, Chernogolovka, Moscow Region, Russia

3Department of Petrology, Moscow State University, Moscow, Russia

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3Department of Petrology, Moscow State University, Moscow, Russia

4Labor Für Geochronologie, Department Für Lithosphärenforschung, Universität Wien, Austria

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2Institute of Experimental Mineralogy RAA, Chernogolovka, Moscow Region, Russia

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1Department of Geology, University of Johannesburg, Auckland Park, Johannesburg 2006, South Africa

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Received:

18 November 2011

Published:

29 September 2012

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H. M. Rajesh, G. A. Belyanin, O. G. Safonov, E. I. Kovaleva, M. A. Golunova, D. D. Van Reenen, Fluid-induced Dehydration of the Paleoarchean Sand River Biotite–Hornblende Gneiss, Central Zone, Limpopo Complex, South Africa, Journal of Petrology, Volume 54, Issue 1, January 2013, Pages 41–74, https://doi.org/10.1093/petrology/egs062
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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.

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