About Ga Impact Cluster

By 3.5 Ga - a period characterized by extensive subaqueous mafic-ultramafic volcanic activity, episodic bombardment by large asteroids (Lowe et al., 2003; Byerly et al., 2002; Glikson et al., 2004) has been closely associated with intermittent felsic volcanic activity in the 3.47-Ga Warrawoona Group (Pilbara Craton) and the Hoogenoeg Formation (Barberton Mountain Land, Transvaal) (Fig. 8.3.A-F). In the Pilbara at least two and possibly a larger number of impacts are represented, including (1) a 0.6-0.8-m-thick microkrys-tite spherule-bearing diamictite (ACM-S2) with pebble to cobble-size chert-intraclasts; (2) 14-m-thick chert-hosted lenses of impact spherules interlayered with spherule-bearing arenites (ACM-S3). The microkrystite spherules are discriminated from detrital volcanic fragments by their high sphericities, inward-radiating fans of sericite pseudomorphs after K-feldspar, relic quench textures and Ni-Cr-Co anomalies. The Presence of spherule-bearing chert fragments in ACM-S3 may hint at an older spherule-bearing chert (?ACM-S1). The near-perfect sphericities of chert-hosted spherules and arenite-hosted spherules militates against wave corrosion and spherule breakage in high-energy shallow water environment, suggesting the spherules were either protected by rapid burial or, alternatively, disturbance was limited to a short-term high-energy perturbation induced by a deep-amplitude impact-triggered tsunami wave.

Possible equivalents of the —3.47-Ga bombardment and volcanic events are suggested from —3.5-Ga Ar-Ar ages of lunar impact spherules (Fig. 8.2.A-C). During this period stromatolite-like structures appear in association with hydrothermal barite-chert zones (—3.49-Ga Dresser Formation) and above barite-bearing submerged or lagunal felsic volcanic shoals and other unconformably underlying units (—3.34-3.426-Ga Panorama Formation) (Dunlop and Buick, 1981; Van Kranednonk et al., 2003). A prime example for the development of stromatolites is in the Strelley Pool Chert, a 3426-3350 Ma quartz arenite, carbonate, evaporate (?), and conglomerate-bearing succession deposited across a regional unconformity (Hoffman et al., 2001; Van Kranendonk, 2000, Van Kranendonk et al., 2003). Here conical structures show a resemblance, at least in gross morphology, to certain forms of stromatolites known from younger stratigraphic units, and to living stromatolites growing in hot springs, such as at Yellowstone or in New Zealand. Superficially they are similar to laterally linked forms such as Thyssagetes Vlasov 1977, and a few cones have lateral branches, a feature common in

Fig. 8.3. 3.47-Ga impact cluster, Antarctic Chert Member, Apex Basalt, upper Warrawoona Group, central Pilbara Block, Western Australia. (A) Intercalated mi-crokrystite spherule-bearing arenite (ACM-S3) (ss - light colored) and chert (ch - dark colored)(lens cap - 6 cm); (B) impact spherule-bearing chert fragment to boulder-size diamictite (ACM-S2); (C) spherule-bearing intraclast microconglomer-ate containing a pebble of black chert with an embedded microkrystite spherule, possibly derived from an early microkrystite spherule-bearing unit ACM-S1. Spherules are indicated by arrows; (D) lenses of microkrystite spherules within chert (ACM-S3); (E) microkrystite spherule showing inward-radiating sericite pseudo-morphs after K-feldspar [F] devitrification textures, and a centrally offset vesicle; (F) microkrystite spherule showing quench textured pseudomorphs, probably after ferromagnesian phases (olivine, pyroxene).

Fig. 8.3. 3.47-Ga impact cluster, Antarctic Chert Member, Apex Basalt, upper Warrawoona Group, central Pilbara Block, Western Australia. (A) Intercalated mi-crokrystite spherule-bearing arenite (ACM-S3) (ss - light colored) and chert (ch - dark colored)(lens cap - 6 cm); (B) impact spherule-bearing chert fragment to boulder-size diamictite (ACM-S2); (C) spherule-bearing intraclast microconglomer-ate containing a pebble of black chert with an embedded microkrystite spherule, possibly derived from an early microkrystite spherule-bearing unit ACM-S1. Spherules are indicated by arrows; (D) lenses of microkrystite spherules within chert (ACM-S3); (E) microkrystite spherule showing inward-radiating sericite pseudo-morphs after K-feldspar [F] devitrification textures, and a centrally offset vesicle; (F) microkrystite spherule showing quench textured pseudomorphs, probably after ferromagnesian phases (olivine, pyroxene).

Jacutophyton Schapovalova 1965. However, details of laminar structures have been lost through recrystallization, and the structures lack an axial zone, a character typically present in most fossil taxa including both Thyssagetes and Jacutophyton. Moreover, the branches in Jacutophyton are much more frequent and reguar in shape. Conical stromatolites also occur in the

2.67-Ga-old Kanowna district, Eastern Goldfields, in the Abitibi greenstone belt, Superior Province, Canada, and as intercalations in the 515 Ma Cambrian, Antrim Plateau Volcanics. The stromatolite units have been deformed by tectonic movements, uplift, and erosion, represented by erosional surfaces preserved within the sequences. In some instances, granitic bodies have reached the surface as small islands or continental nuclei due to local uplift and erosion (Buick et al., 1995).

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