REVISITING THE STRATIGRAPHIC SETTING OF THE ~3 GA BUHWA GREENSTONE BELT, ZIMBABWE

The ~3 Ga Buhwa Greenstone Belt (BGB), Zimbabwe preserves a km-thick accumulation of greenschist-facies supracrustal rocks that was likely deposited upon the Tokwe Segment, a Paleoarchean crustal block that defines the oldest part of the Zimbabwe Craton. Along the southern boundary of the BGB, uplift of granulite-facies intrusive rocks of the Limpopo Belt accompanied folding of the supracrustal rocks into an east-plunging syncline, which has a long northern limb and a short southern limb. The supracrustal succession shows a dramatic change in sedimentary association along the northern limb of the fold from a more continental shelf (west) to a more offshore or basinal association (east). The shelf association is several kilometers thick and divided into three interbedded units arranged in a deepening upward order: a quartz arenite (Q1), overlain by shale (S1), overlain by a thick Superior-type BIF (IF1) and additional shale (S2). Overlying IF1 and S2 is a greenstone unit (G1) whose contact relationship with underlying units is uncertain. Abundant sedimentary structures in Q1 include trough cross bedding, symmetrical ripple marks, and wavy/flaser bedding, all of which supports deposition in a marine-shelf setting floored by continental crust. Along strike from Q1 of shelf association, the basinal association is dominated by greenstone, chert, and Algoma-type BIF. In between is a succession of shale with thin stringers of chert and BIF, suggestive of a progressive basin deepening below wave base from west to east. Consequently, at least part of the basinal association could have formed in relatively deeper water. Exposures are not sufficient to map key stratal surfaces that connect the shelf and basinal associations. Given the compositional difference between the shelf association and overlying G1 greenstones, it is possible that G1 and lateral equivalents belong to a separate cycle of supracrustal deposition. Overall, the stratigraphic successions comprising the BGB afford the opportunity to examine both continentally and hydrothermally influenced deposition, including Superior- and Algoma-type BIF, presumably deposited in the same basin.