Masirah—the other Oman ophiolite:A better analogue for mid-ocean ridge processes?

Oman has two ophiolites – the better known late Cretaceous northern Oman (or Semail) ophiolite and the lesser known and smaller, Jurassic Masirah ophiolite located on the eastern coast of the country adjacent to the Indian Ocean. A number of geological, geochronological and geochemical lines of evidence strongly suggest that the northern Oman ophiolite did not form at a mid-ocean ridge but rather in a supra-subduction zone setting by fast spreading during subduction initiation. In contrast the Masirah ophiolite is structurally part of a series of ophiolite nappes which are rooted in the Indian Ocean floor. There are significant geochemical differences between the Masirah and northern Oman ophiolites and none of the supra-subduction features typical of the northern Oman ophiolite are found at Masirah. Geochemically Masirah is MORB, although in detail it contains both enriched and depleted MORB reflecting a complex source for the lavas and dykes. The enrichment of this source predates the formation of the ophiolite. The condensed crustal section on Masirah (ca. 2 km) contains a very thin gabbro sequence and is thought to reflect its genesis from a cool mantle source associated with the early stages of sea-floor spreading during the early separation of eastern and western Gondwana. These data suggest that the Masirah ophiolite is a suitable analogue for an ophiolite created at a mid-ocean ridge, whereas the northern Oman ophiolite is not. The stratigraphic history of the Masirah ophiolite shows that it remained a part of the oceanic crust for ca. 80 Ma. The chemical variability and enrichment of the Masirah lavas is similar to that found elsewhere in Indian Ocean basalts and may simply reflect a similar provenance rather than a feature fundamental to the formation of the ophiolite.     

The eastern French Pyrenees: from mountain belt to foreland basin

The Pyrenees is a young mountain belt formed as part of the larger Alpine collision zone. This excursion explores the development of the Pyrenean Mountain Belt in southern France, from its early extensional phase in the mid‐Cretaceous and subsequent collisional phase, through its uplift and erosion in the Late Cretaceous and again in the Eocene, which led to the development of the Aquitaine‐Languedoc foreland basin. One of the complexities of the Pyrenean Belt is that thrusting, uplift and erosion during the Pyrenean orogeny exposed older Variscan basement rocks in the central core of the mountains, rocks which were metamorphosed during an earlier event in the late Carboniferous. Thus, this orogenic belt also tells the story of an earlier collision between Laurussia in the north and Gondwana in the south at c. 300 Ma, prior to the onset of the Pyrenean events at c. 100 Ma. Here we seek to unravel these two separate orogenic stories.     

The Triangle Shearzone, Zimbabwe, revisited: new data document an important event at 2.0 Ga in the Limpopo Belt

The Limpopo Belt in Southern Africa has been used to demonstrate that modern-style continent-continent collision operated during the Late Archaean (2.6–2.7 Ga). We have studied the age and PT conditions of strike-slip tectonism along the important right-lateral Triangle Shearzone. Our results substantiate existing Proterozoic metamorphic mineral age data of prior uncertain significance. Using the PbPb and SmNd garnet chronometers and the ArAr step heating technique for amphibole, we have dated pre- and syn-tectonic metamorphic minerals at 2.2 and 2.0 Ga. Thus the Triangle Shearzone can now be regarded as an important Proterozoic suture. Examination of corresponding high-grade PT conditions, reaching 800°C at 9 kbar, indicates a clockwise metamorphic evolution with pronounced isothermal uplift. Although the evidence that thrusting of the Marginal Zones of the Limpopo Belt over the adjoining cratons occurred during the Late Archaean clearly remains, it is now very uncertain to which event the various PT paths obtained in the Limpopo Belt may be assigned. Therefore the question of whether the 2.6–2.7 Ga tectonism fits on its own a modern-style continental collision model remains open and has to be reassessed.     

Garnet-pyroxene thermometry and barometry in the Scourie granulites,NW Scotland

Garnets and pyroxenes from granulites ranging in composition from trondhjemitic to ultramafic were analysed with the electron probe in order to test current geothermometric and geobarometric models. A consistent pressure and temperature estimate based on garnet-pyroxene equilibria shows that the peak of metamorphism was at $\text{820±50°}\text{C}$ and 11 kb and implies a minimum crustal thickness of 30 km and a maximum geothermal gradient of 25–28°C, km^?1, at 2700 Ma in the Scourie area. These results are in contrast to earlier more extreme P-T estimates of $\text{1150±100°}\text{C and}\text{15±3}\text{kb}$.     

Contrasting high and intermediate pressures of metamorphism in the Archaean Sargur Schists of southern India

The Archaean Karnataka craton of southern India contains Eastern and Western crustal blocks (separated by a major thurst) in which Sargur Schists occur as lenses within tonalitic Peninsular Gneisses. The Schist complex comprises pelites, quartzitic psammites, carbonates and calc-silicates, iron formations, and basic rocks, and thus provides many mineral assemblages ideal for the calculation of PT conditions. With their gneisses the Sargur rocks are unconformably overlain by the Dharwar greenstone belts, and are generally thought to be older than 3,000 my.In the Western block maximum metamorphic conditions are given by meta-basic rocks as 790±50° C and 13±2 kb, but adjacent meta-sediments give a pressure of 9 kb, suggesting that the differences in P and T recorded in this block mark a polychronic metamorphic geotherm related to the exhumation of the terrain by uplift and erosion. In the eastern block maximum temperatures were in the range 750°-850° C and maximum pressures were 7 kb. The rocks of the two blocks were sampled 100 km apart, and thus there was probably a regional pressure difference between the two blocks caused by differentiated crustal thickening prior to or during metamorphism.The shape of the geotherm from the Western block shows near-isothermal decompression over 20 km. Our data suggest that during Sargur metamorphism maximum crustal thicknesses were in excess of 45 km and that there was a minimum difference of 20 km in crustal thickness between the Eastern and Western blocks.     

Recognising early Archaean mantle: a reappraisal

This paper examines 3.8 Ga peridotites from Greenland and Labrador to test claims that these samples are unmodified early Archaean mantle. Geochemical criteria were applied in which samples were compared to the mantle array in Mg/Si versus Al/Si (wt%) space, their REE patterns were compared to those of different mantle types and their chromite compositions were compared to mantle chromite compositions as expressed by their cr# and fe#. Geochemical data were used from the previously published works of Friend et al. (2002) and Bennett et al. (2002). Only two samples, from the region south of Isua satisfied all criteria, indicating that the area south of the Isua Greenstone Belt in west Greenland is a suitable place to search for early Archaean mantle. This study also confirms the observation by Friend et al. (2002) that early Archaean mantle from south of Isua is of a different character from Archaean mantle from the subcontinental lithosphere. Calculations presented here show that some mantle fragments from south of Isua experienced a lower degree of melt extraction and were probably more oxidising than early Archaean mantle preserved in the subcontinental lithosphere. Elemental concentrations of Os in early Archaean mantle are lower than the new estimate for the primitive upper mantle of Becker et al. (2006). Peridotites from the Isua greenstone belt are not mantle, but have an affinity with the layered intrusions found south of Isua.     

Searching for the earliest record of life on Earth

Are the remains of the Earth's earliest life now to be found in the hostile wilderness of Greenland?