Using the composition of the carbonate phase to investigate the geochemical evolution of subvolcanic intrusions

Brady, A.E., Moore, K.R.

Department of Earth and Ocean Sciences, National University of Ireland Galway, Galway, Ireland

Carbonatite is increasingly being associated with a broad spectrum of silicate magmas that vary widely in composition and include such rocks types as wholly different as kimberlite and trachyte.Á To investigate the role of the carbonate phase and its paragenesis in such rock associations, three diverse localities containing carbonated igneous rocks ranging from bona fide carbonatites to carbohydrothermal residua are now examined in order to understand better the carbonate component.Á They include a carbonated diatreme and subvolcanic dykes and sills from the Beara peninsula along the south coast of Ireland, a carbonatite diatreme and related silicocarbonatite dykes from the Chagatai region in Uzbekistan and a selection of ultramafic lamprophyres, carbonatite dykes and calcite kimberlites from southern West Greenland.

The subvolcanic dykes, sills and diatreme on the southwest coast of the Beara Peninsula, Co. Cork contain a significant carbonate component. The diatreme is interpreted as an alkaline ultramafic lamprophyre on the basis that its assemblage of kaersutite megacrysts, Ti-phlogopite and Ti-magnetite supports an alkaline lamprophyric affinity, while the absence of feldspars/feldspathoids and the presence of abundant Sr-rich groundmass calcite (2.00 wt % SrO) is evidence of an ultramafic affinity. A small carbohydrothermal calcite carbonatite (0.80 wt % SrO) with fenite cross-cuts the lamprophyre. Multiple populations of carbonate have been identified as minor or dominant minerals in the assemblages of the mainly trachyte sills and lamprophyric tuffisite dykes, and include dolomite (FeOt <7.27 wt %), ferroan dolomite (FeOt <14.58 wt %) and ankerite (FeOt <18.64 wt %).Á The carbonate textures (overgrowths) observed in the trachyte indicates a secondary, metasomatic origin, consistent with the fact that the trachyte sills pre-date both the diatreme and lamprophyric dykes.Á The abundance and intimate textural association of the carbonate with the mineral assemblage in the lamprophyric dykes points towards a primary origin.Á This is reflected in the SrO concentration in the carbonate minerals.Á The SrO content of the Fe-Mg carbonates in trachyte sills is less than 0.1 wt % in keeping with secondary/late stage carbonate.Á Conversely, the SrO content of the Fe-Mg carbonates in tuffisite dykes ranges from 0.2-0.5 wt % which is appreciably high especially for Fe-rich carbonates strongly indicating a magmatic originÁ The Sr-poor Fe-Mg carbonates represent metasomatism of the earlier trachyte sills by residual carbonates whereas the Sr-rich Fe-Mg carbonates form part of the primary magmatic assemblage of the later cross-cutting lamprophyric dykes. ÁÁNew stable isotope data indicates that all the carbonate phases on the Beara Peninsula have an ultimate mantle source.Á The occurrence of such carbonatitic rocks with lamprophyres is not uncommon and is recognised as one of the seven main carbonatite-silicate rock associations with 4% of carbonatites recorded globally occurring in association with (mostly ultramafic) lamprophyres (Woolley & Kjarsgaard, 2008).Á

Contrasting with the lamprophyre composition of the Irish diatreme containing abundant mantle xenocrysts, the Uzbekistan diatremes have silicocarbonatite composition and contain abundant lapilli and xenoliths of crustal rock.Á The carbonate component of the diatreme host magmas has been identified as calcite and is more iron-rich than in the Irish case (FeOt < 1.46 wt %) Interestingly, minor zoned dolomite rhombs have been found dispersed throughout the calcite possibly indicating a more Fe-Mg rich carbonatite magma at depth.Á The related dykes under investigation are composed of trachyte, with albite-chlorite- and albite-calcite-dominated assemblages comparable to the trachyte sills in southwest Ireland.Á While the main carbonate component associated with the dykes is calcite, there are a number of chemically distinct calcite compositions including Fe-rich calcite (up to 0.9 wt % FeO), Fe poor calcite (< 0.1 wt % FeO), Sr-rich (up to 0.87 wt % SrO), and Sr poor calcite (< 0.1 wt % SrO).Á Unlike the Irish trachyte, the carbonate appears to be part of the primary assemblage of these dykes.Á Carbonatite and trachytes occurrences are another of the main silicate-carbonatite associations (Woolley & Kjarsgaard, 2008).

The calcite-rich kimberlites and carbonate-bearing lamprophyres (aillikites of the ultramafic lamprophyre branch (UML)) from southern West Greenland contain mantle carbonate of deep origin.Á These kimberlite and ultramafic magmas have been transported rapidly and directly from the mantle and thus the carbonate has not been subject to much fractionation in contrast to the Irish and Uzbekistan samples.Á Carbonate is extremely abundant in the Greenland dykes and constitutes virtually the entire groundmass.Á Two types of carbonate are identified within almost every sample examined of kimberlite and UML:Á calcite (kimberlite: up to 56.27 wt % CaO, UML: up to 57.16 wt % CaO) and minor dolomite (kimberlite up to 19.91 MgO wt %; UML up to 18.59 wt % MgO).Á There is no obvious difference between these mantle calcite compositions and the calcite compositions of their subvolcanic equivalents at the Irish and Uzbekistan localities and the compositions are not unique in any way.Á It is noticeable though that SrO content is in the relatively low end of the range expected for carbonatites and is somewhat variable ranging from < 0.1 to 0.9 wt % SrO.Á There are no marked distinctions observed between Sr-poor calcite and relatively Sr-rich calcite in the kimberlite and UML dykes.Á It appears that Sr and other such trace elements are preferentially hosted in the co-existing silicate minerals of the kimberlite and UML magmas, which are comparable to extrusive carbonatites that also have low concentrations of trace elements.Á Current opinion is mixed on the carbonatite-kimberlite association with Mitchell (2005) regarding calcite kimberlites as small volume late-forming differentiates unrelated to carbonatites or their parental magmas, whereas Rock (1991) and Dalton and Presnall (1998) propose that ultramafic lamprophyres provide the link between carbonatite and kimberlite magmatism.

In summary, the carbonated locality magmatism represented by the rocks on the Beara Peninsula, Co. Cork, Ireland can be compared to, and provides a crucial link between, the very compositionally different carbonated rocks of south West Greenland and Chagatai, Uzbekistan. Furthermore, the idea that ultramafic lamprophyres provide the link between kimberlite and carbonatite magmas can be explored further from the viewpoints of bona-fide primary carbonatite from Greenland to late-stage carbohydrothermal carbonatite from Ireland.Á ÁÁ

ÁÁ This study was financially supported by INTAS project 05-100008-7983.

References:

Dalton, J.Á A. and Presnall, C. D. 1998.Á The Continuum of Primary Carbonatitic - Kimberlitic Melt Compositions in Equilibrium with Lherzolite: Data from the system CaO-MgO-Al2O₃-SiO₂-CO₂ at 6 GPa.Á Journal of Petrology.Á 39, 1953-1964.

Mitchell, R. H. 2005.Á Carbonatites and carbonatites andÁ carbonatites.Á Can. Min. 43, 2049-2068.

Rock, N. M. S. 1991.Á Lamprophyres.Á Blackie, London, 285 pp.

Woolley, A. R. and Kjarsgaard, B. A., 2008.Á Paragenetic types of carbonatite as indicated by the diversity and relative abundances of associated silicate rocks: evidence from a global database.Á Can. Min. 46, 741-752.