NERC Isotope Geoscience Laboratories

Keyworth, United Kingdom

NERC Isotope Geoscience Laboratories

Keyworth, United Kingdom
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Hastie A.R.,University of Birmingham | Hastie A.R.,University of Edinburgh | Fitton J.G.,University of Edinburgh | Mitchell S.F.,University of the West Indies | And 4 more authors.
Journal of Petrology | Year: 2014

Understanding how the Earth's first continental land masses were generated is important because the processes responsible directly affected the evolution of the planet's primordial silicate interior, and also its atmosphere and hydrosphere. Archaean continental crust is dominated by rocks of the trondhjemite-tonalite-granodiorite (TTG) suite. These can be divided into (1) a mid- to late Archaean (~3-5-2-5 Ga) suite with low SiO2 and high MgO, Sr and transition element contents, and (2) an Eoarchaean (>3-5 Ga) suite with higher SiO2 and lower MgO, Sr and transition element concentrations. Cenozoic adakites are considered to be compositionally similar to mid- to late Archaean (~3-5-2-5 Ga) TTGs, but not the oldest TTG rocks. Conversely, a suite of Early Eocene adakite-like rhyodacites (Jamaican-type adakites: JTA) from Jamaica are shown to be geochemically similar to the Eoarchaean TTGs. In contrast to newly discovered JTA-like rocks (Ryozen low Sr/Y) in Japan, new trace element and Nd-Hf radiogenic isotope data in this study confirm that the Jamaican JTA cannot be formed by complex mixing, assimilation and fractional crystallization processes. New partial melt models here explore several different source compositions (mid-ocean ridge basalt, ocean island basalt and oceanic plateau), mineral modes, melt modes and partition coefficients. The results of these models clearly demonstrate that the JTA and the Eoarchaean TTG can be generated by partial melting of plagioclase- and garnet-bearing amphibolite source regions with oceanic plateau-like compositions. Further modelling shows that the JTA and Eoarchaean TTG low MgO and transition element abundances can be derived from two dominant processes: (1) relatively shallow partial melting of subducting oceanic crust (compositionally similar to Mesozoic oceanic plateau basalt) whereby the slab melts ascend without interacting with a mantle wedge (2) partial melting of oceanic plateau-like subducting oceanic crust followed by interaction of the slab melts with a thin and/or discontinuous (boudinage-like?) mantle wedge whereby the expected increase of MgO, Ni, and Cr in the slab melts is obliterated by fractional crystallization of ferromagnesian minerals (mostly amphibole). Consequently, using the JTA as a modern analogue for Eoarchaean TTG production, we propose the existence of subduction zones consuming oceanic plateau-like oceanic crust in Eoarchaean times. © The Author 2015.

Goodenough K.M.,British Geological Survey | Styles M.T.,British Geological Survey | Schofield D.,British Geological Survey | Thomas R.J.,British Geological Survey | And 6 more authors.
Frontiers in Earth Sciences | Year: 2013

The Oman-United Arab Emirates ophiolite is the world's largest ophiolite. It is divided into 12 separate fault-bounded blocks, of which the northern three lie wholly or partly in the United Arab Emirates. Extensive mapping has shown that the United Arab Emirates blocks contain mantle and crustal sections which correspond to the classic 'Penrose conference' ophiolite definition but which are cut by a voluminous later magmatic sequence including ultramafic, mafic and felsic components. Samples from the later magmatic sequence are dated at 96.4±0.3, 95.74±0.3 and 95.2±0.3 Ma; the early crustal section, which has not been dated directly, is thus constrained to be older than c. 96.4 Ma. Petrological evidence shows that the early crustal section formed at a spreading ridge, but the later magmatic sequence was formed from hydrous magmas that produced different mineral crystallisation sequences to normal midocean ridge basalt (MORB). Mineral and whole-rock geochemical analyses show that the early crustal rocks are chemically similar to MORB, but the later magmatic sequence has chemical features typically found in supra-subduction zone (SSZ) settings. The ophiolite in the United Arab Emirates thus preserves clear evidence for two stages of magmatism, an early episode formed at a spreading centre and a later episode associated with the onset of subduction. Similar two-stage magmatism has been recognised in the Oman sector, but the United Arab Emirates contains the most voluminous SSZ magmatism yet described from this ophiolite. © Springer-Verlag Berlin Heidelberg 2013.

Hastie A.R.,Kingston University | Kerr A.C.,University of Cardiff | McDonald I.,University of Cardiff | Mitchell S.F.,University of the West Indies | And 4 more authors.
Chemical Geology | Year: 2010

Rhyodacite lavas (Newcastle Volcanic Formation) from the Wagwater Basin in eastern Jamaica dated at 52.74±0.34Ma (2σ) have adakitic-like major element compositions, low Y and heavy rare Earth element (REE) concentrations and negative Nb and Ta anomalies on a normal mid-ocean ridge basalt normalised multi-element diagram. They also have lower Sr (<400ppm), MgO (≤2.0wt.%), Ni (mostly≤30ppm) and Cr (mostly≤40ppm) concentrations compared to other modern adakites and middle-late Archaean (3.5-2.5Ga) trondhjemite, tonalite and granodiorite/dacites (TTG/Ds) εNd(i) and εHf(i) values indicate that the Newcastle adakite-like lavas cannot be formed by assimilation and fractional crystallisation processes involving any other igneous rock in the area and so the composition of the lavas is largely the result of the residual mineralogy in the source region. Low Sr and Al2O3 contents indicate a fluid/vapour-absent source with residual plagioclase and REE systematics point to residual amphibole and garnet in the source region. Similarly, high silica values and constant Zr and P2O5 concentrations suggest residual quartz and accessory zircon and apatite. The plagioclase and garnet residue implies that the Newcastle magmas were derived from partially melting a metabasic protolith at 1.0-1.6GPa, which would intersect the amphibole dehydration partial melt solidus at ~850-900°C. Radiogenic isotopes along with the low MgO, Ni and Cr concentrations in the Newcastle lavas demonstrate that the garnet amphibolite source region cannot be part of (1) the lower Jamaican arc crust, (2) delaminated lower crust or (3) subducted Proto-Caribbean "normal" oceanic crust that may, or may not, have detached. This data, in addition to partial melting models involving a theoretical garnet-amphibolite source region for the Newcastle lavas, shows that the adakite-like rocks are derived from metamorphosed Caribbean oceanic plateau crust that underthrust Jamaica in the early Tertiary. The underplated oceanic plateau crust partially melted by either (1) influx of basaltic magma during lithospheric extension in the early Tertiary or (2) direct partial melting of the underthrusting (subducting) plateau crust. The Newcastle magmas ascended and erupted without coming into contact with a mantle wedge thus forming the low MgO, Ni and Cr contents.Most Cenozoic adakites have compositions similar to the middle-late Archaean TTG/D suite of igneous rocks. In contrast, early (> 3.5. Ga) Archaean TTG/D crustal rocks have lower Sr, MgO, Ni and Cr concentrations and prior to this study had no modern adakite analogue. However, the Newcastle adakites have similar compositions to the, early Archaean TTG/D. The discovery of these rocks has important implications for our understanding of the formation of the Earth's earliest continental crust and so it is proposed that the Newcastle lavas be classified as a unique subgroup of adakites: Jamaican-type adakite. © 2010 Elsevier B.V.

Hastie A.R.,Kingston University | Mitchell S.F.,University of the West Indies | Kerr A.C.,University of Cardiff | Minifie M.J.,University of Cardiff | Millar I.L.,NERC Isotope Geoscience Laboratories
Geochimica et Cosmochimica Acta | Year: 2011

Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb. >. 20. ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas. © 2011 Elsevier Ltd.

Hastie A.R.,University of Cardiff | Kerr A.C.,University of Cardiff | McDonald I.,University of Cardiff | Mitchell S.,University of the West Indies | And 3 more authors.
Geology | Year: 2010

Archean continental crust largely comprises the trondhjemite, tonalite, and granodiorite/dacite (TTG/D) suite of igneous rocks. Formation of the earliest Archean (>3.5 Ga) TTG/Ds is controversial, being attributed to either subduction zone processes with active plate tectonics or thermochemical mantle convection with no plate tectonic processes. A suite of Cenozoic adakite-like lavas in Jamaica has geochemical compositions comparable to early Archean TTG/D. The data indicate that the adakites were generated by underthrusting (or subducting) and partial melting of oceanic plateau crust beneath Jamaica. This setting is analogous to proposed plate tectonic processes in the early Archean where hot, thick, and more buoyant Archean oceanic crust underthrusts adjacent plates. The new adakite data imply that earliest Archean TTG/D continental crust could have formed above primitive subduction zones. © 2010 Geological Society of America.

Storey C.D.,University of Portsmouth | Storey C.D.,University of Leicester | Brewer T.S.,University of Leicester | Anczkiewicz R.,Polish Academy of Sciences | And 3 more authors.
Journal of the Geological Society | Year: 2010

The Glenelg-Attadale Inlier is the largest basement inlier within the Caledonian orogen in NW Scotland. A Western Unit consists of trondhjemite-tonalite-granodiorite (TTG) gneisses and subordinate mafic intrusions with locally preserved mafic high-pressure granulite and eclogite assemblages. An Eastern Unit comprises TTG gneisses, Grenville-age (c. 1.1 Ga) eclogite and metasediments. U-Pb zircon ages from the Western Unit TTG gneisses are highly disturbed with Neoarchaean upper intercepts and Palaeo- to Mesoproterozoic lower intercepts, suggesting strong reworking at these times. U-Pb zircon, Sm-Nd and Lu- Hf garnet-clinopyroxene dates from the Western Unit high-pressure granulite and eclogite yield Neoarchaean (c. 2.6-2.8 Ga) and Palaeoproterozoic (c. 1.75 Ga) ages, respectively. These ages correspond to the ages of partial resetting of the TTG gneisses. The eclogite in the Western Unit may represent the high-pressure convergent margin to the lower-pressure Laxfordian events within the Lewisian Gneiss Complex, whereas the Mesoproterozoic eclogites in the Eastern Unit may represent a farther eastward Grenville-age margin. Further east, Ordovician high-pressure granulites within the Moine Supergroup may represent another, later, Grampian convergent margin. These high-pressure belts were developed sequentially towards the east and telescoped westwards from Palaeoproterozoic to Early Palaeozoic times. © 2010 Geological Society of London.

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