Metamorphic processes at high temperature and low pressure: the petrogenesis of the metasomatized and assimilated rocks of Carneal, Co. Antrim

The Tertiary dolerite plug at Carneal cuts basalt lavas and incorporates blocks of chalk and flint from underlying Cretaceous rocks. Assimilation by the dolerite of the pure limestone and flint took place under the very rare highest-temperature, low-pressure conditions. Only about twenty-five examples of the resulting metamorphosed and metasomatized rocks are known in the world, few fully described. The rock suite enables the conditions and mechanisms of assimilation to be deduced. The pressure, about 200 x 10 5 Pa (200 bar), and the temperature, estimated as 1050-1100 °C, produced an exomorphic suite of larnite, spinel, merwinite, spurrite, scawtite and related assemblages, with wollastonite, quartz, plagioclase, hydrogrossular, xonotlite and related minerals representing flint. Complementary desilication of the igneous rocks gives the endomorphic suite of pyroxene-rich dolerite, pyroxenite, titanaugitemelilite-rock and aegirine- and nepheline-bearing types. Chemical analyses o titanaugite, sahlite, melilite, wollastonite and the main rock types are provided and optical and other properties of the minerals. Two related mechanisms of limestone assimilation occurred. Most of the rocks resulted from the incorporation of the chalk in the olivine-dolerite magma, paradoxically, the addition of 18-26 % CaO to the dolerite magma so lowers the silica ratio that 17-18% additional S i0 2 is required to produce the endomorphic hybrids, with the complementary exomorphic suite. The second mechanism, a metasomatic replacement, preserves existing mineral (and fossil) textures. Mineral textures and the preservation of a cyclostome bryozoan now composed of wollastonite (by silicification of calcite) show the metasomatism to have been a tranquil process despite the high temperature. To produce the vein assemblage of merwinite, hydrogrossular and melilite, this mechanism required 63 % CaO and 23 % H 2 0 , an addition which is only slightly more hydrous than calcium hydroxide. These mechanisms are evidence for the production of peralkaline rocks by limestone assimilation but only on a very small scale. The retrograde phase of metamorphism produced minerals in order of approximately increasing water content, including xonotlite, bicchulite (a new mineral), thomsonite, tobermorite, tacharanite and plombierite.