The isotopic evolution of the British lithosphere

Davies, Gareth Rees (1983). The isotopic evolution of the British lithosphere. PhD thesis The Open University.



A Sm-Nd mineral isochron obtained from an olivine gabbro yields an age of 375 +/- 34 Ma (M.S.W.D. = 0.006) and represents the formation age of the Lizard Igneous Complex. Two magmatic suites are recorded in the complex, an early trace element enriched (ENd375 = + 8.9 - + 10.1) and a late MORB-like( ENd375 = + 9.2 - + 11.7). The early magmatic suite records a decoupling between low Sm/Nd and high (143Nd/144Nd)375 ratios and indicates that the low SW/Nd ratios were generated immediately prior to the petrogenesis of the early magmatic suite, due to either i) L.REE enrichment of the source or ii) enrichment processes during magma genesis.

The marked L.REE depletion of the spinel lherzolites, which comprise the majority of the Lizard peridotite, reflects multiple, small percentage (<10%), melt extraction from the rocks while in the garnet stability field. The petrology and Sm-Nd isotope systematics of the plagioclase and pargasite peridotites record the local infiltration of a melt. Significantly the Sm-Nd isotope systematics of the "metasomatised" peridotites are the same as the calculated sources of the dolerite dyke suites and are probably equivalent to these, at depth, sources.

Two suites of metabasaltic rocks occur in the Mona Complex (Anglesey), the Gwna Group ((Ce/Yb)N = 0.6 - 2.1) and New Harbour Group ((Ce/Yb)N = 0.3 - 0.5). The Gwna Group yield a whole-rock Sm-Nd isochron of 595 +/- 86 Ma (M.S.W.D. = 1.5) with an initial ratio equivalent to ENd of 7.1 +/- 1.0. This isochronous relationship and large variation in the degree of L.REE enrichment indicates, as in the case of the early magmatic suite of the Lizard Complex, that the low Sm/Nd ratios were generated immediately prior to the petrogenesis of the metabasalts.

Nd model ages of the Mona gneisses, a sediment and the Coedana Granite, in conjunction with the age of the Gwna Group, argue for an origin of the entire Mona Complex during the late Precambrian.

The Lower Palaeozoic subduction related volcanism of southeast Ireland occurs in 2 distinct suites, north and south of the Leinster Granite. The southern suite has trace element, Sr and Nd isotope systematics equivalent to the present day calc alkaline volcanism of Java. It is estimated that 80% of the Sr and 50% of the Nd in these rocks are slab derived. The northern tholeiitic suite is markedly trace element enriched, and the rocks show a correlation between their Sm/Nd and (143Nd/144Nd)I ratios, and the % of slab derived Nd. The calculated Sr/Nd ratio of the slab component is 19, such that mixing lines between slab and mantle wedge components are straight lines on an ESr vs ENd diagram. It is not possible to distinguish between an ancient LILE and L.REE enrichment event, circa 1100 Ma, or sediment subduction to account for the isotopic systematics of the slab component.

The dioritic primary magma to the Leinster Granite is predominantly derived from juvenile lower crust. The maximum mantle component involved in the diorites' petrogenesis is 30% and the maximum TCHUR age of the lower crust is 720 Ma. The primary dioritic magma assimilated Lower Palaeozoic sediments and fractionated a plagioclase, K feldspar and biotite assemblage, resulting in the production of granitic rocks with more and less radiogenic Sr and Nd isotope ratios respectively.

Lower curstal xenoliths from central Ireland and the Midland Valley of Scotland indicate that crustal accretion occurred during both Grenvillian and Late Precambrian ti~es. The N.E. Ox Mount, lins inlier also provides evidence of a Late Precambrian crust forming event (605 +/- 39 Ma). (No evidence of Archaean crust is found in these regions.) The lower crustal garnet granulite xenoliths from Ireland and the Midland Valley represent possible crustal components involved in the petrogenesis of both the pre and post-tectonic Caledonian granites of Central Scotland.

The majority of basement rocks of southern Britain formed during the Late Precambrian with only the Rushton Schists and Rosslare Complex having Nd and Sr isotope systematics characteristics of old, >1300 Ma, crust. Late Precambrian and Cambro-Ordovician sediments have Nd isotope systematics equivalent to the majority of the Late Precambrian basement rocks. Subsequent to the closure of the Iapetus Ocean the sedimentary pile within southern Britain records the gradual introduction of an "ancient" crustal component, presumably from the mixed Archean/Proterozoic terrains of northern Europe. A steady state is reached by Jurassic times such that present day coastal sediments have equivalent Nd isotope systematics to those of Jurassic sediments.

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