Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust

Characterization of spinel peridotites by olivine-spinel compositional relationships: Review and interpretation

Iron formations are economically important sedimentary rocks that are most common in Precambrian sedimentary successions. Although many aspects of their origin remain unresolved, it is widely accepted that secular changes in the style of their deposition are linked to environmental and geochemical evolution of Earth. Two types of Precambrian iron formations have been recognized with respect to their depositional setting. Algoma-type iron formations are interlayered with or stratigraphically linked to submarine-emplaced volcanic rocks in greenstone belts and, in some cases, with volcanogenic massive sulfide (VMS) deposits. In contrast, larger Superior-type iron formations are developed in passive-margin sedimentary rock successions and generally lack direct relationships with volcanic rocks. The early distinction made between these two iron-formation types, although mimimized by later studies, remains a valid first approximation. Texturally, iron formations were also divided into two groups. Banded iron formation (BIF) is dominant in Archean to earliest Paleoproterozoic successions, whereas granular iron formation (GIF) is much more common in Paleoproterozoic successions. Secular changes in the style of iron-formation deposition, identified more than 20 years ago, have been linked to diverse environmental changes. Geochronologic studies emphasize the episodic nature of the deposition of giant iron formations, as they are coeval with, and genetically linked to, time periods when large igneous provinces (LIPs) were emplaced. Superior-type iron formation first appeared at ca. 2.6 Ga, when construction of large continents changed the heat flux at the core-mantle boundary. From ca. 2.6 to ca. 2.4 Ga, global mafic magmatism culminated in the deposition of giant Superior-type BIF in South Africa, Australia, Brazil, Russia, and Ukraine. The younger BIFs in this age range were deposited during the early stage of a shift from reducing to oxidizing conditions in the ocean-atmosphere system. Counterintuitively, enhanced magmatism at 2.50 to 2.45 Ga may have triggered atmospheric oxidation. After the rise of atmospheric oxygen during the GOE at ca. 2.4 Ga, GIF became abundant in the rock record, compared to the predominance of BIF prior to the Great Oxidation Event (GOE). Iron formations generally disappeared at ca. 1.85 Ga, reappearing at the end of the Neoproterozoic, again tied to periods of intense magmatic activity and also, in this case, to global glaciations, the so-called Snowball Earth events. By the Phanerozoic, marine iron deposition was restricted to local areas of closed to semiclosed basins, where volcanic and hydrothermal activity was extensive (e.g., back-arc basins), with ironstones additionally being linked to periods of intense magmatic activity and ocean anoxia.

Late Paleoproterozoic iron formations and Paleozoic ironstones were deposited at the redoxcline where biological and nonbiological oxidation occurred. In contrast, older iron formations were deposited in anoxic oceans, where ferrous iron oxidation by anoxygenic photosynthetic bacteria was likely an important process. Endogenic and exogenic factors contributed to produce the conditions necessary for deposition of iron formation. Mantle plume events that led to the formation of LIPs also enhanced spreading rates of midocean ridges and produced higher growth rates of oceanic plateaus, both processes thus having contributed to a higher hydrothermal flux to the ocean. Oceanic and atmospheric redox states determined the fate of this flux. When the hydrothermal flux overwhelmed the oceanic oxidation state, iron was transported and deposited distally from hydrothermal vents. Where the hydrothermal flux was insufficient to overwhelm the oceanic redox state, iron was deposited only proximally, generally as oxides or sulfides. Manganese, in contrast, was more mobile. We conclude that occurrences of BIF, GIF, Phanerozoic ironstones, and exhalites surrounding VMS systems record a complex interplay involving mantle heat, tectonics, and surface redox conditions throughout Earth history, in which mantle heat unidirectionally declined and the surface oxidation state mainly unidirectionally increased, accompanied by superimposed shorter term fluctuations.

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Iron Formation: The Sedimentary Product of a Complex Interplay among Mantle, Tectonic, Oceanic, and Biospheric Processes

About 30% of the Earth is covered by continents, but only about 10 small kernels of these continentsae—known as Archaean cratonsae—are continental fragments formed before 2.5 Gyr ago. The Kaapvaal craton of South Africa, which formed and stabilized between 3.7 and 2.7 Gyr ago, is one of the oldest reasonably sized examples of these continental fragments. It consists of a mosaic of subdomains that have been welded together by processes similar to those of modern-day plate tectonics. The earliest subdomains may have owed their origin to the onset of efficient recycling from the Earth's hydrosphere into the mantle.

Formation of an Archaean continent

http://or.nsfc.gov.cn/bitstream/00001903-5/261436/1/1000014253607.pdf

Tectonic architecture and multiple orogeny of the Qinling Orogenic Belt, Central China

The Geology of the Great 'Dyke', Zimbabwe: The Ultramafic Rocks

The Bushveld Complex was emplaced and cooled in less than one million years – results of zirconology, and geotectonic implications

The Merensky Reef is the consequence of several major influxes of magma into the Bushveld magma chamber that gave rise to at least two cyclic units, including the Pre-Merensky and Merensky cyclic units. Analysis of the S, Ni, Cu, platinum group element (PGE) and Au contents of 1140 samples from 24 profiles through the rocks forming these units and two profiles extending downward from the Reef to close to the UG-2 horizon of the Western Bushveld Complex provide important petrogenetic constraints on the origin of the Reef. Particular attention is paid to variations in the calculated PGE tenors and Cu/Pd ratios of sulfides, both vertically and laterally. All of the profiles show a decrease in Pt tenor and concomitant increase in Cu/Pd ratio upward across the pyroxenite of the Merensky unit and laterally away from certain centres that are interpreted as feeder zones. The data also document that more mafic horizons within the Merensky to UG-2 interval of the southwestern Bushveld are characterized by much lower Cu/Pd (<100–400) than the ambient ratio (∼4000). This is interpreted as demonstrating that these horizons are the consequence of incursions of magma whose composition has been modified in a staging chamber by reaction with sulfide resident there. The favoured interpretation for the Merensky Reef is that the Bushveld chamber(s) was occupied by a resident magma crystallizing orthopyroxene and plagioclase. Pre-Merensky and, subsequently, Merensky magmas entered the chamber(s) intermittently from localized feeder structures, mixing or mingling with the resident magma and giving rise to the more mafic cumulates. At specific horizons, including the Merensky Reef, the mixing or mingling caused sulfide immiscibility. In these cases, as the new magma pulses flowed laterally away from the entry zones they deposited sulfide, becoming depleted in PGE. On coming to rest, the magma continued to segregate sulfide and crystallize pyroxene, so that the sulfides became progressively depleted in PGE and acquired a higher Cu/Pd upward through the pyroxenite. This model requires that the Merensky magmas could dissolve up to two orders of magnitude more PGE than current measurements on dry diopside–anorthite melts have indicated. It is suggested that very PGE-rich magmas can develop as a result of sulfide-unsaturated magma reacting with early formed sulfide in a staging chamber, dissolving FeS and enriching the remaining sulfide in PGE. Later magmas equilibrating with this enriched sulfide will themselves become highly enriched in PGE.

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PGE Tenor and Metal Ratios within and below the Merensky Reef, Bushveld Complex: Implications for its Genesis

This article reports a study of chromitites from the LG-1 to the UG-2/3 from the Bushveld Complex. Chromite from massive chromitite follows two compositional trends on the basis of cation ratios: trend A—decreasing Mg/(Mg + Fe) with increasing Cr/(Cr + Al); trend B—decreasing Mg/(Mg + Fe) with decreasing Cr/(Cr + Al). The chromitites are divided into five stages on the basis of which trend they follow and the data of Eales et al. (Chemical Geology 88:261–278, 1990) on the behaviour of the Mg/Fe ratio of the pyroxene and whole rock Sr isotope composition of the environment in which they occur. Following Eales et al. (Chemical Geology 88:261–278, 1990), the different characteristics of the stages are attributed to the rate at which new magma entered the chamber and the effect of this on aAl2O3 and, in the case of stage 5, the appearance of cumulus plagioclase buffering the aAl2O3. The similarity of PGE profiles across the MG-3 and MG-4 chromitites that are separated laterally by up to 300 km and the variation in V in the UG-2 argue that the chromitites have largely developed in situ. Modelling using the programme MELTS shows that increase in pressure, mixing of primitive and fractionated magma, felsic contamination of primitive magma or addition of H2O do not promote crystallization of spinel before orthopyroxene (in general they hinder it) and that the Cr2O3 content of the magma was of the order of 0.25 wt.%. Less than 20% of the chromite in the magma is removed before orthopyroxene joins chromite, which implies a >13-km thickness of magma for the Critical Zone. It is suggested that the large excess of magma has escaped up marginal structures such as the Platreef. The PGE profile of chromitites depends on whether sulphide accumulated or not along with chromite. Modelling shows that contamination of Critical Zone magma with a felsic melt will induce sulphide immiscibility, although not chromite precipitation. The LG-1 to LG-4 chromitites developed without sulphide, whilst those from the LG-5 upwards had associated liquid sulphide. Much of the sulphide originally in the LG-5 and above has been destroyed as a result of reaction with the chromite.

The origin of chromitites and related PGE mineralization in the Bushveld Complex: new mineralogical and petrological constraints

Felsic igneous rocks within the circa 3.5-Ga ophiolitic Onverwacht Group of the Barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession. These felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession. Our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic-ultramafic rocks of the Onverwacht Group. Rather, the data indicate that these felsic units are high-level equivalents of the widespread gneissic to isotropic trondhjemite-tonalite plutons which surround and intrude the greenstone belt. The felsic igneous rocks are predominantly shallow-level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the Onverwacht Group by 40–50 Ma. The felsic intrusions were preferentially emplaced along thrust zones, cutting the ophiolite basement and its overlying sedimentary cover. Structural and chemical data imply that the hydrated ophiolitic rocks of the Barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting. Such structural imbrication of simatic crust can be best incorporated in a model advocating Archean intraoceanic obduction associated with shallow-angle subduction, similar to that documented for the neotectonics of the Western Aleutians.

Felsic igneous rocks within the 3.3- to 3.5-Ga Barberton Greenstone Belt: High crustal level equivalents of the surrounding Tonalite-Trondhjemite Terrain, emplaced during thrusting

Allan Wilson

Papers

The Geology of the Great 'Dyke', Zimbabwe: The Ultramafic Rocks

The Bushveld Complex was emplaced and cooled in less than one million years – results of zirconology, and geotectonic implications

PGE Tenor and Metal Ratios within and below the Merensky Reef, Bushveld Complex: Implications for its Genesis

The origin of chromitites and related PGE mineralization in the Bushveld Complex: new mineralogical and petrological constraints

Felsic igneous rocks within the 3.3- to 3.5-Ga Barberton Greenstone Belt: High crustal level equivalents of the surrounding Tonalite-Trondhjemite Terrain, emplaced during thrusting