MAGMATIC PULSES IN THE WESTERN MOURNE TERTIARY CENTER, NE IRELAND

The Mourne Mountains Tertiary Center is one of a number of classic A-type granite ring-dike complexes associated with the British Tertiary Igneous Province. It comprises two discrete centers (Richey, 1928: Emeleus, 1955). The Eastern center is composed of three consecutive granitic intrusions (G1, G2 and G3). At the younger, contiguous Western Mourne Center (WMC) two granites, G4 and G5, were emplaced. Rb-Sr whole-rock ages suggest G5 is younger than the main Mourne series (G1 – G4). However, a new Canberra SHRIMP U-Pb zircon age (57.7 ± 0.7 Ma) confirms it is contemporaneous at ~ 56.5 Ma.

Textural/mineralogical variations are present in the WMC granites and a number of textural units can be mapped out. G4 is subdivided into coarse-grained (CG) and finer-grained (FrG) units. Variations in texture and CI enable two facies to be recognized in G5; a relatively mafic-rich (MR) unit with microgranitic and granophyric textures and a mafic-poor (MP) variant with granitic textures and miariolitic cavities. Field relations suggest that gradational rather than intrusive contacts exist between the textural units. Geochemical data show that each unit is compositionally distinct plotting in well-defined fields on bivariate diagrams. TiO₂, Ba and Zr contents and Rb/Sr and K/Rb ratios distinguish the G4 units. The MR G5 unit, the least evolved granite of the WMC, can be distinguished from its MP counterpart on the basis of TiO₂, Fe₂O₃, P₂O₅, Sr and Ba and K/Rb and Rb/Sr ratios. REE analyses also differ with the MR and CG units having smaller negative Eu anomalies compared to the MP and FrG units.

The combined textural and geochemical characteristics of, and field relations between textural units mapped within the WMC granites point to their origin by magmatic pulsing (Harry and Richey, 1963). G4 was emplaced as two magmatic pulses: one crystallized to produce the CG unit and the other the FrG one. Similarly G5 crystallized from two pulses to form the MR and MP units. The magmatic pulses, now recognized as textural units, succeeded each other temporally to form gradational rather than intrusive contacts. The distinctive geochemistry of each unit along with the overall Mourne compositional trends suggest that the successive pulses originated in a fractionally crystallizing zoned magma chamber(s).