Heavy stable isotope constraints on high Ba-Sr granite petrogenesis and potential for sanukitoid research.

Sanukitoids first became abundant during the Meso- to Neoarchaean, and represent interaction of upper mantle peridotites with a wide range of fluid and melt additions, for example from TTG, mafic crust and/or sediment. Some of them may be physical samples of Earth’s early subduction systems, about which very little is known in comparison with modern counterparts. Phanerozoic equivalents of (some) sanukitoids are “high Ba-Sr” granites and related rocks. Both rock suites have Hf-Nd-Sr-O isotope characteristics of continental crust yet are accompanied by petrological and geochemical evidence for mantle derivation. Consequently, crustal growth models that rely on Hf-O isotopes incorrectly exclude them from the juvenile budget. It is therefore critical to understand the “enriched” (continental) isotope signature and constrain its volumetric proportion. The geochemical toolkit for petrogenetic investigations is always expanding, and now includes “non-traditional” stable isotope systems such as Ba, Mg, Ti and Zn. These have been used as effective tracers of recycled materials in subduction-related settings, especially to illuminate mass transfer and distinguish mélange and metasomatism models. Strong limits on the involvement of pelagic sediment are provided by Ba isotopes, while carbonate contributions have been recognized through the use of Mg and Zn.  In metabasite or metasedimentary melts, Ti isotopes are dependable markers of residual rutile and/or amphibole. 

High Ba-Sr plutons from the Caledonian type area show a range of enrichment severity, offering an ideal test for these stable isotope methods. Within these rocks, Ba isotopes overlap normal mantle values but extend to lighter compositions, consistent with addition of a little pelagic sediment that has incorporated biogenic barite. The majority of Mg isotopes fall within the typical mantle range, with light outliers possibly indicative of carbonate involvement but for which fractional crystallization cannot be excluded.  Associated syenites have relatively light Mg and marginally heavier Ba, suggesting a larger component of carbonate origin. Mafic samples have Ti isotopes heavier than typical mantle, indicating the presence of metabasite or metasedimentary melt component(s). 

Thus, a stable isotope approach shows considerable promise for further constraining the petrogenesis of such magmas. Preliminary Ba isotope data for sanukitoids from 3.4 Ga and 2.9 Ga (Pilbara), 2.65 Ga (Yilgarn), 2.5 Ga (Bundelkhand) to 2.1 Ga (Sao Francisco craton) show intriguing variation from rather lighter to considerably heavier than the assumed mantle range, but further work is required to define the responsible fractionation mechanism. However, sanukitoid Ti isotopes show strong similarity to the high Ba-Sr granitoid suite, strongly indicating metabasite melt involvement.