PETROGENESIS OF THE ST-DAVIDS HEAD LAYERED INTRUSION, WALES - A COMPLEX HISTORY OF MULTIPLE MAGMA INJECTION AND IN-SITU CRYSTALLIZATION

The St David's Head Intrusion, exposed in North Pembrokeshire, is a th oleiitic sill up to 570 m thick, comprising principally mafic gabbros, which are in part layered. Layering varies from the centimetre to the metre scale. A range of gabbroic compositions is present, defining se ven major petrological types, which are cut by thin silicic (aplitic) veins. Log-log plots of incompatible elements from the various litholo gical units indicate that all of the rocks in the intrusion are petrog enetically linked, although a variety of processes has been operative. Roach (1969) considered the quartz gabbros and dolerites, which form an envelope around the other units, to relate most closely to the pare ntal composition. However, the least evolved compositions in the intru sion are from the xenolithic laminated olivine gabbros, although these mafic compositions are due primarily to the presence of abundant, maf ic, cognate xenoliths. These xenoliths are thought to relate to an ear lier episode of crystal accumulation in a high-level magma chamber. Th e various laminated gabbros reflect crystal accumulation in situ after magma emplacement, leading in certain layers to extreme enrichments i n Fe, Ti and V, related to high modal proportions of cumulus ilmenite. Further in situ crystallisation led to differentiation of the residua l liquid, producing more silicic gabbros with well-developed granophyr ic textures, the granophyre reflecting the silicic residuum. Extreme d ifferentiation, possibly combined with expulsion of silicic residual l iquid during crystal accumulation and compaction, resulted in the cros s-cutting aplite veins. Three different types of layering are present in the intrusion. Firstly, preferred orientation of tabular minerals i n the laminated gabbro units is thought to result from discrete sedime ntation episodes from a convecting magma chamber. Secondly, macrorhyth mic modal layering up to Im thick consists of an alternation of relati vely ilmenite-rich and ilmenite-poor layers in the laminated gabbro un its, although the reason for the modal variation is not certain. Third ly, a centimetre-scale felsic-mafic microrhythmic layering is present in the envelope quartz gabbros and dolerites, which is similar to the inch-scale layering in the Stillwater Igneous Complex. This layering i s thought to relate to metasomatic reaction in the gabbro in the prese nce of water at a late magmatic stage. Overall, these various litholog ical units themselves define a large-scale layering in the intrusion. Combined, the petrological and geochemical data suggest that the St Da vid's Head Intrusion was not emplaced in a single event. Rather, a ser ies of magma pulses, of contrasting compositions but petrogenetically linked, was intruded. Some of the chemical variations now seen existed prior to emplacement, indicating the former presence of high-level cr ustal magma chambers, while other variations developed as a result of in situ crystallisation processes and related chemical differentiation .