Petrogenesis of the St. David's Head layered intrusion, Wales; a complex history of multiple magma injection and in situ crystallisation

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Authors:Bevins, R. E.; Lees, G. J.; Roach, R. A.; Rowbotham, G.; Floyd, P. A.
Author Affiliations:Primary:
National Museum of Wales, Department of Geology, Cardiff, United Kingdom
Other:
University of Keele, United Kingdom
Volume Title:Transactions of the Royal Society of Edinburgh: Earth Sciences
Source:Transactions of the Royal Society of Edinburgh: Earth Sciences, Vol.85 (Part 2), p.91-121. Publisher: Royal Society of Edinburgh, Edinburgh, United Kingdom. ISSN: 0263-5933
Publication Date:1994
Note:In English. 49 refs.; illus., incl. 8 tables, geol. sketch map
Summary:This intrusion on the coast of N Pembrokeshire is a tholeiitic sill up to 570 m thick comprising principally mafic gabbros, which in part show layering from the centimetre to the metre scale. A range of gabbroic compositions defines seven major petrological types, which are cut by thin silicic aplitic veins. Representative EPMA results are given for clinopyroxenes, orthopyroxenes, amphiboles, biotites, plagioclases and ilmenites, together with whole-rock analyses for major-, trace- and REE. Log-log plots of incompatible elements from the various lithological units indicate that all the rocks in the intrusion are petrogenetically linked. The least-evolved compositions are those of the xenolithic laminated olivine gabbros, although this is due to the abundant mafic, cognate xenoliths. The various laminated gabbros reflect crystal accumulation in situ after magma emplacement, leading in certain layers to extreme enrichment in Fe, Ti and V, related to high modal proportions of cumulate ilmenite. Further in situ crystallization led to differentiation of the residual liquid, producing more silicic gabbros with well-developed granophyric textures; extreme differentiation gave the cross-cutting aplite veins. Three different types of layering occur: preferred oriention of tabular minerals in the laminated gabbro, macrorhythmic modal layering with alternate layers rich and poor in ilmenite, and a cm-scale felsic-mafic microrhythmic layering in the quartz gabbros of the envelope. Emplacement is envisaged to have occurred as a series of magma pulses, of contrasting composition but petrogenetically linked. [R.A.H.]
Subjects:Amphibole group; Basalts; Biotite; Chain silicates; Clinopyroxene; Crystallization; Feldspar group; Framework silicates; Gabbros; Genesis; Igneous rocks; Ilmenite; Intrusions; Mafic composition; Magmas; Mica group; Orthopyroxene; Oxides; Petrology; Plutonic rocks; Protoliths; Pyroxene group; Sheet silicates; Silicates; Sills; Tholeiite; Volcanic rocks; Europe; Great Britain; Pembrokeshire Wales; United Kingdom; Wales; Western Europe; Carn Llidi; Dyfed Wales; Quartz leucogabbro; Saint David's Head Intrusion
Abstract Numbers:95M/3445
Record ID:1996025059
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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