Geochemical and Strontium-Isotope Studies of Reactions between Basic Magma, Chalk, and Flint, and the Role of Groundwater, in the Carneal Plug, Co. Antrim, Northern Ireland

The Carneal plug, Co. Antrim, Northern Ireland, is a Tertiarydolerite intrusion, 150 m in diameter, which incorporates blocks(up to several metres across) of the underlying Cretaceous Chalkand the flints originally contained in the Chalk. It is oneof the rare natural occurrences showing reaction between basalticmagma and limestone under conditions of very high temperatureand low pressure. The pressure was controlled by lithostaticload at about 0.2 kb; the upper temperature limit for the progradereactions was close to the liquidus temperature of basalticmagma around 1100 °C and retrograde reactions with secondaryhydrous fluids have produced a series of hydrated calc-silicatesover the temperature range 700 °C to less than 100 °C.The rocks form complex assemblages with two complementary suitesof rocks developed by the metamorphic and metasomatic reactions.An exomorphic suite, derived mainly from the chalk, consistsof assemblages of larnite, spurrite, bredigite, merwinite, melilite(gehlenite), calcite, or spinel; and wollastonite, melilite,hibschite and quartz derived from flint. The endomorphic suiteis derived by desilication reactions between the basic magmaand the incorporated chalk and flint. The minerals are thoseof the dolerite, together with fayalitic olivine, sahlite, nepheline,aegirine, wollastonite, melilite (humboldtilite), merwinite,scawtite, and hydrogrossular. Retrograde processes provide afurther range of scarce to rare mineral suites. The Carnealplug thus provides an ideal opportunity to investigate the applicationof strontium isotope-ratio measurements to crustal contaminationof basic magmas, since the three rock end-members dolerite,chalk, and flint, are readily identifiable and relatively homogeneous,and the conditions of the formation of the resulting metamorphicand metasomatic rocks are well understood. Although the petrology, albeit complicated, is well established,few geochemical and no isotopic studies have hitherto been made.Major- and trace-element data for a total of twenty-one whole-rocksamples (thirteen new analyses from the plug) are now reported.Strontium isotope ratios for these samples (with two mineralseparates), forty-two samples of Cretaceous Chalk and five samplesof unmetamorphosed flints from other localities or regions arealso presented and discussed. Strontium isotope ratios are alsogiven for pore-water from the Chalk, and drill-stem test watersamples from Irish Triassic strata. The main conclusions which may be drawn are as follows: The petrological identity of the two complementary suites ofmineral assemblages is reflected by the new geochemical data.Initial ⁸⁷Sr/⁸⁶Sr ratios presented for the end-members, chalk,flint, and dolerite show that the initial ratios in the metamorphicassemblages cannot be produced by simple mixtures of these end-members. A major component of the strontium budget in the metamorphicassemblages in the Carneal plug appears to have been transportedinto the plug probably in a secondary, low-temperature fluidphase containing Sr with higher ⁸⁷Sr/⁸⁶Sr ratios. Strontiumand thus probably calcium were mobile on a relatively largescale during the prograde metamorphism as well as during theretrograde stages. Metasomatism was an important process during metamorphism andmany elements were mobile on the relatively small scale of thehand specimens. The new studies show for example that the exomorphicassemblages have contents of Mg, Ni, and Cu which were mostprobably derived from the enclosing dolerite magma. The contentsof major elements in many of the assemblages differ from concentrationsto be expected as a result of simple mixtures of the three end-membersbut may be explained in terms of phase equilibria during contamination. The new high precision ⁸⁷Sr/⁸⁶Sr ratios determined for Chalksamples of known stratigraphic position suggest that there maybe sufficient variation between stratigraphical Stages to usesuch measurements as a method of correlating or dating Mesozoicand Cenozoic limestones.