Igneous Rock Associations 28. Construction of a Venusian Greenstone Belt: A Petrological Perspective

The crustal evolution of Venus appears to be principally driven by intraplate processes that may be related to mantle upwelling as there is no physiographic (i.e. mid-ocean ridge, volcanic arc) evidence of Earth-like plate tectonics. Rocks with basaltic composition were identified at the Venera 9, 10, 13, and 14, and Vega 1 and 2 landing sites whereas the rock encountered at the Venera 8 landing site may be silicic. The Venera 14 rock is chemically indistinguishable from terrestrial olivine tholeiite but bears a strong resemblance to basalt from terrestrial Archean greenstone belts. Forward petrological modeling (i.e. fractional crystallization and partial melting) and primary melt composition calculations using the rock compositions of Venus can yield results indistinguishable from many volcanic (ultramafic, intermediate, silicic) and plutonic (tonalite, trondhjemite, granodiorite, anorthosite) rocks that typify Archean greenstone belts. Evidence of chemically precipitated (carbonate, evaporite, chert, banded-iron formation) and clastic (sandstone, shale) sedimentary rocks is scarce to absent, but their existence is dependent upon an ancient Venusian hydrosphere. Nevertheless, it appears that the volcanic–volcaniclastic–plutonic portion of terrestrial greenstone belts can be constructed from the known surface compositions of Venusian rocks and suggests that it is possible that Venus and Early Earth had parallel evolutionary tracks in the growth of proto-continental crust.

Cryogenian glaciostatic and eustatic fluctuations and massive Marinoan-related deposition of Fe and Mn in the Urucum District, Brazil

Global Neoproterozoic glaciations are related to extreme environmental changes and the reprise of iron formation in the rock record. However, the lack of narrow age constraints on Cryogenian successions bearing iron-formation deposits prevents correlation and understanding of these deposits on a global scale. Our new multiproxy data reveal a long Cryogenian record for the Jacadigo Group (Urucum District, Brazil) spanning the Sturtian and Marinoan ice ages. Deposition of the basal sequence of the Urucum Formation was influenced by Sturtian continental glaciation and was followed by a transgressive interglacial record of >600 m of carbonates that terminates in a glacioeustatic unconformity. Overlying this, there are up to 500 m of shale and sandstone interpreted as coeval to global Marinoan glacial advance. Glacial outwash delta deposits at the top of the formation correlate with diamictite-filled paleovalleys and are covered by massive Fe and Mn deposits of the Santa Cruz Formation and local carbonate. This second transgression is related to Marinoan deglaciation. Detrital zircon provenance supports glaciostatic control on Cryogenian sedimentary yield at the margins of the Amazon craton. These findings reveal the sedimentary response to two marked events of glacioeustatic incision and transgression, culminating in massive banded iron deposition during the Marinoan cryochron.

A reinterpretation of the Archaean stratigraphy south of Nkandla, southern Kaapvaal Craton, South Africa: Geophysical and stratigraphic constraints on a sheared granitoid-greenstone remnant

A new lithostratigraphic framework based upon a review of historic data, field mapping and remote sensing, including aerial photography, high-resolution airborne aeromagnetic and radiometric data, is proposed for the Archaean geology along the southeastern margin of the Kaapvaal Craton, South Africa. A synthesis of new and existing data reveals that previously accepted lithostratigraphic schemes require complete revision, with reinterpretations identifying multiple major shear zones and previously unidentified granitoid successions along the margin of the craton. In this new lithostratigraphic framework, lithologies of the Southern Syncline previously correlated with the Nsuze Group of the Pongola Supergroup, are redefined as greenstone lithologies associated with the Ilangwe Greenstone Belt. The geology of the Nkandla region can be subdivided into five distinct geophysical domains including: (i) an extension of the Ilangwe Greenstone Belt, (Domain 1) which is subdivided into; a lower volcanic succession, the Thathe Formation, comprising pillow and amygdaloidal volcanics; the adjoining Sabiza Formation, comprising pillow volcanics exposed in the southeast of the study area; the volcano-sedimentary Mtshwili Formation, which overlies the Thathe and Sabiza formations, consisting of quartz (sericite) schist, phyllite, metavolcanics and iron formation; the Nomangci Formation, which occurs as a region of highly deformed quartz-kyanite-sericite schists, and the Simbagwezi Formation, which comprises maroon to green phyllites and schists in the north of the study area. (ii) granitoids of the Impisi Granitoid Suite (Domain 2) which border the greenstone succession to the north, intruding the Nomangci and Simbagwezi formations. (iii) a southern complex of sheared granitoids termed the Umgabhi Granitoid Suite (Domain 3), which intrudes the Thathe, Sabiza and Mtshwili formations. (iv) The two remaining domains, comprise the Mesoproterozoic Mfongosi and Ntingwe Groups (Domain 4) and Mesoarchaean volcano-sedimentary sequences of the Pongola Supergroup (Domain 5).