Paleogeographic implications Directory UMM :Data Elmu:jurnal:P:Precambrian Research:Vol101.Issue1.2000:

to the deposit as it comes to rest. Caution is warranted in extending this interpretation too far because of the intensity of deformation in these units. 4 . 3 . Composition and source Framework clasts are set in a matrix of bi- otite + quartz + garnet 9 chlorite 9 pyrite schist. These represent replacement and metamorphic minerals, not detrital mineralogy. The biotite and chlorite are considered coarsely recrystallized and metasomatized clay mud that formed the matrix to the meta-conglomerate. Dimroth 1982 noted a restricted framework clast assemblage of chert and biotite schist, while Nutman et al. 1984 considered the deposit as ‘oligomictic’ with quartz nodules and biotite-rich clasts. My analysis reveals more overall clast di- versity than originally reported, which helps to identify the unit as truely of detrital origin. In all examined exposures, clasts of quartzite dominate the gravel-sized composition Fig. 3A – H. The quartzite is equigranular, very pure, and coarse grained crystal units 1 – 2 mm across are com- mon, and interpreted as coarsely recrystallized chert. This is consistent with the texture of ‘chert’ interlayers in BIF deposits elsewhere in the study area Bridgwater et al., 1981. A number of differ- ent colors ornament the quartzite clasts, but I suggest the coloration represents stains related to later fluid infiltration events, not primary varia- tion. Rare clasts of oxide-facies BIF have coarsely recrystallized chert bands and display internal lay- ering inclined to foliation. Clasts dominated by biotite are less abundant than homogenous quartzite, appear to blend in with the surrounding matrix, and show an array of textural variation. Common clast variants in- clude: 1 pure biotite flattened into cm-wide lenses Fig. 3F; 2 light gray clasts of biotite + quartz schist with mm-scale streaks of pure biotite Fig. 3F; 3 even gray clasts of quartz + biotite schist Fig. 3G and 4 biotite + quartzite schist with clusters of quartz rodded in the direction of stretching Fig. 3G. I interpret the collection of biotite-rich clasts to represent original mafic vol- canic source rocks, which is consistent with the mineralogy of basaltic pillow breccia deposits B 1 km away Appel et al., 1998. Current mineralogy and geochemistry represent the cumulative effects of multiple metamorphic and metasomatic events that are known to have occurred Rosing et al., 1996. Textural variants from above are thought to mimic primary eruptive diversity, especially the quartz-rodded lithology, which looks identical to deformed and recrystallized amygdaloidal pillow basalt fragments seen elsewhere Appel et al., 1998. Such a polymict assortment of lithotypes would be difficult to generate from tectonic pro- cesses alone, and so a detrital origin is favored. Surrounding rocks of the central domain contain all the lithologies observed as clasts in the con- glomerate, suggesting the meta-conglomerate rep- resents reworking of the adjacent stratigraphy.

5. Paleogeographic implications

I consider the presence of the meta-conglomer- ate in the central domain of the Isua Greenstone Belt enigmatic. It represents a brief episode of very coarse clastic sedimentation in a section oth- erwise comprised of pillow basalt, pillow breccia, chert, and BIF, that resembles other ‘mafic-ultra- mafic volcanic association’ greenstone belt succes- sions Eriksson et al., 1994, and that is most simply interpreted as having formed under below wave-base conditions Rosing et al., 1996. How- ever, the rounded and polymict nature of frame- work clasts is more consistent with formation in a shallow subaqueous or emergent setting Fisher and Schmincke, 1984, pp. 265 – 276. Deciding on a potential environment of deposi- tion for the meta-conglomerate proceeds with ex- treme care because of its limited lateral extent, thickness, and state of deformation. Dimroth 1982 favored a tidal channel origin for the meta- conglomerate, although that can be rejected now that the associated carbonate is recognized as of replacement origin. The juxtapositioning of depo- sitional units formed by tractive processes that contain cobbles and by debris flows requires fast- moving shallow water and a slope, respectively. Erosion and rounding during transport were im- portant processes in the development of the meta- conglomerate. The overwhelming dominance of basalt in the central domain would indicate that as an important source lithology, which is consis- tent with the biotite + garnet schist composition of the matrix and the difficulty in recognizing basaltic clasts. Weathering of an exposed volcanic source is consistent with the relative abundance of chert clasts now quartzite over mafic clasts, which would be far more susceptible to degrada- tion during weathering processes. Using these indirect lines of evidence, I hypoth- esize that the meta-conglomerate was derived from a subaerially exposed volcanic edifice that underwent chemical weathering processes, which concentrated more chemically resistant chert clasts. It is likely that the production of mud for matrix material was enhanced by early sea-water alteration of the lavas. The weathered material was then transported and deposited, probably near the source Fisher and Schmincke, 1984, pp. 275, in subaerial or shallow subaqueous environ- ments by a combination of processes. The pres- ence of chert interpreted as below wave base in origin as clasts implies that sea-floor material was deposited, lithified, exposed, and eroded. A tec- tonic origin for uplifting of the source would be consistent with early Earth processes Eriksson et al., 1994, although other mechanisms to expose the source, such as dramatic boiling off of ocean water resulting from a large asteroid impact Sleep et al., 1989 or primary volcano growth must be considered as possible. An absence of other coarse sedimentary units in the central do- main, or anywhere else in the IGB, implies that exposure of the volcanic-dominated source was a temporally limited, anomalous event in the strati- graphic development of the succession.

6. Conclusions