Hugh Rollinson

Eclogite xenoliths in west African kimberlites as residues from Archaean granitoid crust formation

Eclogites are a comparatively rare but petrologically important member of kimberlite xenolith suites. Their broadly basaltic chemistry has led many authors to propose that they represent ancient, subducted ocean crust. Recent studies, however, have suggested an alternative origin and propose that kimberlitic eclogites are residues from the process of Archaean granitoid crust formation. Geochemical arguments in support of this new model were previously based on the trace-element chemistry of eclogitic minerals. Here I report that the major-element chemistry of eclogite xenoliths also supports a crustal residue model. I examine eclogite xenoliths from kimberlite pipes at Koidu, Sierra Leone, which sample the lithospheric mantle underlying the Archaean (2.8 Gyr) granitoid crust of the West African craton. Geochemical plots of major elements measured in unaltered, whole-rock samples of low-silica eclogite demonstrate that they are complementary to the granitoids of the West African craton and have compositions which indicate that both were derived from a common basaltic parent rock.

Coupled evolution of Archean continental crust and subcontinental lithospheric mantle

The observations that Archean continental crust and the subcontinental lithospheric mantle (SCLM) have different compositions from their Phanerozoic counterparts, that komatiite extraction models for the origin of the Archean SCLM do not work, and that non-arc Archean basalts are not necessarily formed in a plume setting are used to challenge the mantle plume model for the formation of the Archean SCLM. Petrological modeling suggests that, instead, the SCLM formed at a hot ocean ridge giving rise to dense, Fe-rich basaltic ocean crust and highly depleted thick oceanic lithosphere. Typically this lithosphere would subduct, but where slab melting and tonalite-trondhjemite-granodiorite (TTG) production took place, the SCLM coupled to felsic crust would be sufficiently buoyant to be conserved. Thus Archean SCLM is transposed normal Archean oceanic lithosphere created at a hot ridge.

The metamorphic history of the Isua Greenstone Belt, West Greenland

Abstract New geological investigations in the c. 3.7–3.8 Ga Isua Greenstone Belt in West Greenland have revealed that the belt comprises a number of separate structural domains. Five such domains have been identified on the basis of lithological and structural differences. This study uses the morphology and compositional zoning of garnet porphyroblasts in pelites to investigate the extent to which the various domains within the greenstone belt preserve contrasting deformational and metamorphic histories. Up to three episodes of garnet growth have been identified in a single domain and significant differences in garnet growth history are noted between domains. A distinction is drawn between the relatively simple metamorphic history of a low-strain zone in the NE of the greenstone belt and other domains where more complex histories are preserved. Combining this result with existing geochronology suggests that in the south and west, the greenstone belt was metamorphosed twice, at c. 3.74 Ga and at c. 2.8 Ga, whereas in the NE there was a single event at 3.69 Ga. Preliminary garnet-rim thermometry indicates that some rocks experienced an early metamorphism in which temperatures exceeded 610°C. Kyanite is thought to have been in equilibrium with these assemblages, indicating pressures of at least 6 kbar. A later prograde metamorphic event shows a temperature rise from 480 to 550°C. The high pressures indicate a crustal thickness of at least 20 km at 3.7 Ga.

Chromitites from the Fiskenæsset anorthositic complex, West Greenland: clues to late Archaean mantle processes

Abstract Chromitites in the late Archaean Fiskenæsset anorthosite complex are characterized by a most unusual mineral assemblage: highly calcic plagioclase, iron-rich aluminous chromites and primary amphibole. In particular, the chromite compositions are atypical of chromitites in layered igneous intrusions. However, rare occurrences of this mineral assemblage are found in modern arcs and it is proposed here that the late Archaean calcic anorthositic chromitites formed by the partial melting of unusually aluminous harzburgite in a mantle wedge above a subduction zone. This melting process produced a hydrous, aluminous basalt, which fractionated at depth in the crust to produce a variety of high-alumina basalt compositions, from which the anorthosite complex with its chromitite horizons formed as a cumulate within the continental crust. The principal trigger for the late precipitation of chromite is thought to have been the removal of Al from the basaltic melt through plagioclase crystallization, and the build-up of Cr through an absence of clinopyroxene. It is proposed that the aluminous mantle source of the parent magma was produced by the melting of a harzburgitic mantle refertilized by small-volume, aluminous slab melts. This process ceased at the end of the Archaean because the dominant mechanism of crust generation changed such that melt production shifted from the slab into the mantle wedge, thus explaining why highly calcic anorthosites are almost totally restricted to the Archaean.

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

Abstract 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 820±50° 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 1150±100° C and 15±3 kb .

Phanerozoic sanukitoids from Caledonian Scotland: Implications for Archean subduction

Sanukitoids represent a volumetrically minor but important series of Neoarchean granitoids enriched in large-ion lithophile elements (e.g., Ba, Sr, and light rare earth elements), and with relatively high compatible elements (e.g., Mg, Ni, and Cr). Petrogenetic models have favored an origin in the suprasubduction mantle wedge, so their sudden appearance ca. 2.95–2.68 Ga has made them central to the ongoing debate about the beginnings of modern plate tectonics. Caledonian high Ba-Sr granites from the Northern Highlands of Scotland are petrological and compositional equivalents. The Caledonian examples have not undergone subsequent reworking, and their petrogenetic setting can be used to inform genetic models for sanukitoids. Prolonged subduction preceded emplacement of the high Ba-Sr granites in a short interval (∼10 Ma) at the end of the Caledonian orogeny. Sediment subduction was responsible for elemental and isotopic enrichments and slab breakoff triggered melting in the subcontinental lithospheric mantle. This implies that during the Neoarchean the evolving tectonothermal regime moved the locus of melting from the basaltic slab to the sediment-infused mantle wedge, creating the first subcontinental lithospheric mantle. Sanukitoids may then have resulted from widespread, closely subsequent slab-breakoff events, producing evolved magmas by familiar fractionation mechanisms. Careful study of secular variations in high Ba-Sr magmas could constrain the evolution of the subcontinental lithosphere.

The Lewisian Complex: insights into deep crustal evolution

Abstract The Lewisian Complex is an Archaean/Proterozoic craton fragment found in NW Scotland and throughout the Outer Hebrides. The 1907 memoir recognized, simply from field relationships and petrographic observation, key features of Lewisian evolution. The bulk of the Lewisian is an old, deformed complex consisting mainly of acid igneous rocks, with some basics, ultrabasics and metasediments. In the Central District of the mainland these are pyroxene bearing (now recognized as granulite facies). The Lewisian Complex was intruded by a suite of basic and ultrabasic dykes which show variable states of later deformation, the intensity of strain being correlated with the development of hornblende schist in the dykes and amphibolite facies assemblages in the country rocks. In the Northern and Southern Districts, this deformation is pervasive and the dykes become concordant hornblende schist sheets. The new foliation with transposed dykes and metasediment sheets is then folded around NW–SE axes. Today there is no single agreed model for the evolution of the complex but an outline is as follows. In the pre-dyke (Scourian) history, subduction led to melting of oceanic crust which provided vast volumes of tonalite-trondhjemite-granodiorite in the period 3100–2700 Ma. Ages show geographic variations but it is not proven whether that implies large displacements between pieces of crust or whether it represents intrusions into other intrusions. The subcontinental lithospheric mantle dates from c. 3000 Ma. K, U and other large ion lithophile elements are depleted in the Central District of the mainland; this is due to depletion in the downgoing oceanic slab which in turn is a result of dehydration prior to melting. Other areas are not depleted in such elements, so various tectonic settings were involved. Remnants of metabasic material in the Lewisian may be relics of oceanic crust. Granulite facies metamorphism with, in places, P>10 kb and T>1000 °C occurred a considerable time after intrusion so is not necessarily linked to igneous events. This ‘Badcallian’ episode affected mainly the Central District and a part of the southern Outer Hebrides; other areas show only amphibolite facies. Zircon dating indicates two high-grade events at 2500 and 2700 Ma. During the ‘Inverian’ episode a series of wide amphibolite-facies shear zones affected the granulite-facies Scourian gneiss prior to the intrusion of the Scourie dykes. The Scourie dykes were intruded from 2400–2000 Ma and are largely quartz tholeiites derived from enriched subcontinental lithospheric mantle; there are some picrites which yield the oldest ages but are also seen to crosscut basic dykes. The dykes trend NW–SE and are steep where not affected by later deformation except where they intrude along, and are controlled by, Inverian fabrics. Post-dyke (Laxfordian) history involves the development of calc-alkaline igneous rocks in the Outer Hebrides and mainland (c. 1900 Ma). Volcanics associated with sediments younger than 2000 Ma comprise an accretionary complex formed in a subduction setting; they are now intercalated between slabs of Archaean basement indicating that the complex was involved in collision with continental crust. Huge strains transposing dykes and country rocks affected almost all of the Outer Hebrides and the mainland except for the Central District. The NW–SE trending lineation indicates the collision direction; the metasediments on the mainland and the South Harris Igneous Complex may mark a folded suture between two continents. Metamorphism was amphibolite facies almost everywhere; in South Harris it was granulite facies at c. 1880 Ma. At 1750–1675 Ma, a distinct event, called late Laxfordian but much younger than earlier Laxfordian metamorphism and with a distinct tectonic setting, caused folding of the previous structures along NW–SE axes, migmatization and renewed amphibolite facies metamorphism.

P-T conditions in coeval greenstone belts and granulites from the Archaean of Sierra Leone

Supracrustal rocks in the Archaean of Sierra Leone can be divided into two distinct geographical provinces which are characterised by high and low grades of metamorphism. In the west of the craton greenstone belts up to 130 km long with thick successions of mafic lavas and clastic sediments are metamorphosed to amphibolite facies. In the east of the craton there are smaller schist relics metamorphosed to granulite facies. The dominant lithologies are banded iron formation and mafic granulites. Mineral equilibria indicate that the metamorphic climax in the greenstone beltswas 595 ± 50°C, 5.5 ± 0.5 kbar (Nimini Hills), 565 ± 50°C, 4.9 ± 2.5 kbar (Gori Hills) and in the granulites was 770 ± 50°C, 7.5 ± 1.5 kbar. The difference in P-T conditions for the metamorphic maximum in the greenstone belt and granulites can be explained by four possible models for crustal thickening and subsequent uplift and erosion. It is possible that the crustwas thickened by tectonic processes, so that the greenstone belts and granulites formed part of the same vertical pile.In this case the difference in recorded P-T conditions results from variable erosion rates or from a variably thickened crust. On the other hand the crust may have been thickened by the addition of magmas and similarly, the recorded difference in P-T conditions results from variable erosion rates or non-uniform thickening. Geological evidence favours, although does not prove, a magmatic model for crustal thickening. The recorded P-T conditions reflect a difference in erosion level between the greenstone belts and granulites, but it is not possible to show whether this is the product of variable erosion rates or a variably thickened crust.

New Mössbauer measurements of Fe 3+/ΣFe in chromites from the mantle section of the Oman ophiolite: Evidence for the oxidation of the sub-oceanic mantle

Abstract Room temperature Mössbauer and electron-probe measurements of Fe3+/ΣFe in chromite from the mantle section of the Oman ophiolite define two groups of samples: a low Fe3+/ΣFe group (with Fe3+/ΣFe = 0.21-0.36) have cr# = Cr/(Cr + Al) in the range 0.49-0.75, whereas a smaller more geographically localized high Fe3+/ΣFe group (with Fe3+/ΣFe = 0.71-0.78) have a more restricted range of cr# ratios of 0.72-0.75. The low Fe3+/ΣFe chromitites have very variable Fe3+/ΣFe ratios. They are thought to have crystallized from melts that have interacted with depleted mantle and thereby acquired their variable Fe3+/ΣFe ratio. The high Fe3+/ΣFe chromitites are restricted to one small area of the mantle and their high oxidation state is thought to be post magmatic. They are either the product of later heating, related to melt flux or interaction with a later oxidising melt. A difference in oxygen fugacity between the MORB-depleted harzburgite host, which is at the quartz-fayalite-magnetite (QFM) buffer and the later chromite-bearing melts (QFM + 2) implies that there is a real difference in the oxidation state of the MORB and arc-magma sources.

Tectonic Evolution of the Oman Mountains

The Oman Mountains contain one of the worlds best- exposed and best-understood fold–thrust belts and the largest, best-exposed and most intensively studied ophiolite complex on Earth. This volume presents new international research from authors currently active in the field focusing on the geology of the Oman Mountains, the foreland region, the carbonate platforms of Northern and Central Oman and the underlying basement complex. In addition there is a particular focus on geoconservation in the region. The volume is divided into three main sections that discuss the tectonics of the Arabian plate using insights from geophysics, petrology, structural geology, geochronology and palaeontology; the petrology and geochemistry of the Oman Ophiolite and the sedimentary and hydrocarbon systems of Oman, drawing on the geophysics, structure and sedimentology of these systems. The volume is enhanced by numerous colour images provided courtesy of Petroleum Development Oman.