THE CALEDONIAN NEWRY COMPLEX, NE IRELAND: NEW U-PB AGES, A SUBSURFACE EXTENSION AND MAGMATIC EPIDOTE

Emplaced into the paratectonic Caledonides, the post-collisional (? slab breakoff-related) Newry complex comprises 3 contiguous, zoned, I-type granodiorite plutons (d ¹⁸ O < +9.4 per mil) and a small, multi-pulsed, intermediate/ ultramafic cumulate body at its NE end. Until now only Rb-Sr ages have been published, all approximately 400 Ma. Preliminary ID – TIMS results on zircons from the central (reversely zoned) granodiorite pluton yield a provisional ²⁰⁷Pb/²⁰⁶Pb age of 426 ± 7 Ma, indistinguishable from a 423 ± 7 Ma U-Pb zircon age for the same body determined by SHRIMP methods (ANU). Titanites from the geologically older (normally zoned) NE granodiorite pluton yield a concordant ²⁰⁶Pb/²³⁸U result of 410.4 ±1.3 Ma (2s, MSWD=1.0; TIMS), interpreted as a cooling age. Thus the emplacement of at least the earlier parts of the Newry complex was significantly earlier than previously understood, an important conclusion re the timing of Iapetus Ocean closure in Ireland.

High-K, microcline-bearing granite enclaves have been reported from Tertiary basic dykes to the E and SE of the Newry complex, some over 20 km away from its outer margin. Microcline typifies the Caledonian, as opposed to the higher level Tertiary granitoids of this region, and the Rb-Sr systematics of an enclave indicate ⁸⁷Sr/⁸⁶Sr which is compatible with a Caledonian as opposed to a Lower Tertiary age. Importantly, this suggests a major, subsurface extension (including, significantly, the area of the 56 Ma Mourne Mountain granites), possibly elevating the Newry complex to batholithic status.

Recently magmatic epidote has been found in the unfoliated core of the NE Newry granodiorite pluton, to which it seems to be confined. This suggests a separate (later) pulse might have ascended rapidly from the lower crust, where large-scale fractional crystallisation/AFC may have generated the granodiorite magmas.