TY - JOUR
T1 - Silica precipitation triggered by clastic sedimentation in the Archean
T2 - New petrographic evidence from cherts of the Kromberg type section, South Africa
AU - Ledevin, Morgane
AU - Arndt, Nicholas
AU - Simionovici, Alexandre
AU - Jaillard, Etienne
AU - Ulrich, Marc
N1 - Funding Information:
We acknowledge the EPOV program (CNRS) and the ANR project BEGDy (BLAN-0109) for their financial contribution to the project. The manuscript has been improved following discussions with Donald Lowe, Frances Westall and Christoph Heubeck. We thank Gordon Chunnett and Axel Hofmann for their contribution on the field, Anne-Marie Boullier for thin section interpretations, and also for the help in the lab at ISTerre we thank Sarah Bureau, Christelle Poggi and Catherine Chauvel for geochemical analyses, Nathaniel Findling for X-ray diffraction, Manuel Munoz for X-ray microfluorescence and Valentina Sobolev for microprobe analyses. Appendix A
Publisher Copyright:
© 2014 Elsevier B.V.
PY - 2014/12/1
Y1 - 2014/12/1
N2 - The Kromberg Formation (ca. 3432Ma) in the Barberton Greenstone Belt, South Africa, contains well-preserved chert beds at the tops of turbidite deposits. At the interface, siltstone, which consist of K-feldspar, K-mica, microquartz with minor lithic fragments and heavy minerals, grades into chert, which consists of microquartz and minor K-mica (<15%). K-feldspars show preserved twins typical of microcline, orthoclase and sanidine. Based on the heterogeneity of the clastic fraction (i.e. shape, size, nature), the lack of in situ metasomatic features (i.e. crystal overgrowths, silica replacement) and the continuity of the siliceous matrix through the siltstone-to-chert transition, we argue that (1) the clastic particles are detrital, (2) some were altered and metasomatized at their source, (3) in situ metasomatism was limited to minor seritization of K-feldspars, and (4) the silica is of primary origin and precipitated from ambient marine fluids. Our petrographic observations reinforce the model advocated by Rouchon and Orberger (2008) and Rouchon et al. (2009) for chert deposited in clastic-rich setting and we favor a formation of both the siltstones and cherts as chemico-clastic sediments. We argue for the contemporaneous deposition of clastic grains from turbiditity currents and precipitation of silica on phyllosilicate reactive surfaces, both in the water column and at the sediment-water interface. As the rate of clastic sedimentation declined, the accumulation of silica flocs on suspended phyllosilicates first accompanied, then replaced the deposition of detrital grains, to form a siliceous ooze at the seafloor. Contrary to current interpretations for detritus-rich cherts, which invoke a secondary origin via Si- and K-metasomatism, we propose that the present model prevailed in a variety of Archean settings where fine-grained sediments were deposited. The composition of both the siltstone and chert reflects mainly the environment in which they formed. They are interpreted as mixtures of two main components: (1) silica, which contains extremely low concentrations of trace elements and contributes only SiO2 to the bulk composition, and (2) another phase that dominates the trace element composition. Here, K-mica and K-feldspar control the chemical signal and reflect a felsic source to the turbidites (i.e. Hooggenoeg dacites and volcaniclastics).
AB - The Kromberg Formation (ca. 3432Ma) in the Barberton Greenstone Belt, South Africa, contains well-preserved chert beds at the tops of turbidite deposits. At the interface, siltstone, which consist of K-feldspar, K-mica, microquartz with minor lithic fragments and heavy minerals, grades into chert, which consists of microquartz and minor K-mica (<15%). K-feldspars show preserved twins typical of microcline, orthoclase and sanidine. Based on the heterogeneity of the clastic fraction (i.e. shape, size, nature), the lack of in situ metasomatic features (i.e. crystal overgrowths, silica replacement) and the continuity of the siliceous matrix through the siltstone-to-chert transition, we argue that (1) the clastic particles are detrital, (2) some were altered and metasomatized at their source, (3) in situ metasomatism was limited to minor seritization of K-feldspars, and (4) the silica is of primary origin and precipitated from ambient marine fluids. Our petrographic observations reinforce the model advocated by Rouchon and Orberger (2008) and Rouchon et al. (2009) for chert deposited in clastic-rich setting and we favor a formation of both the siltstones and cherts as chemico-clastic sediments. We argue for the contemporaneous deposition of clastic grains from turbiditity currents and precipitation of silica on phyllosilicate reactive surfaces, both in the water column and at the sediment-water interface. As the rate of clastic sedimentation declined, the accumulation of silica flocs on suspended phyllosilicates first accompanied, then replaced the deposition of detrital grains, to form a siliceous ooze at the seafloor. Contrary to current interpretations for detritus-rich cherts, which invoke a secondary origin via Si- and K-metasomatism, we propose that the present model prevailed in a variety of Archean settings where fine-grained sediments were deposited. The composition of both the siltstone and chert reflects mainly the environment in which they formed. They are interpreted as mixtures of two main components: (1) silica, which contains extremely low concentrations of trace elements and contributes only SiO2 to the bulk composition, and (2) another phase that dominates the trace element composition. Here, K-mica and K-feldspar control the chemical signal and reflect a felsic source to the turbidites (i.e. Hooggenoeg dacites and volcaniclastics).
KW - Archean
KW - Barberton Greenstone Belt
KW - Chert
KW - Kromberg formation
KW - Metasomatism
KW - Siliceous ooze
KW - Turbidite
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U2 - 10.1016/j.precamres.2014.10.009
DO - 10.1016/j.precamres.2014.10.009
M3 - Article
AN - SCOPUS:84912096879
VL - 255
SP - 316
EP - 334
JO - Precambrian Research
JF - Precambrian Research
SN - 0301-9268
IS - P1
ER -