The effects of assimilation of country rocks by magmas on ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr systematics in igneous rocks

Abstract

Examples of positive correlations between initial ⁸⁷Sr/⁸⁶Sr and δ¹⁸O have now been shown to be very common in igneous rock series. These data in general require some type of mixing of mantle-derived igneous rocks with high-¹⁸O, high-⁸⁷Sr crustal metamorphic rocks that once resided on or near the Earth's surface, such as sedimentary rocks or hydrothermally altered volcanic rocks. Mixing that involves assimilation of country rocks by magmas, however, is not a simple two-end-member process; heat balance requires appreciable crystallization of cumulates. In such cases, the isotopic compositions may strongly reflect this open-system behavior and indicate the process of assimilation, whereas the major element chemical compositions of the contaminated magmas will be largely controlled by crystal-melt equilibria and crystallization paths fixed by multicomponent cotectics. A variety of oxygen and strontium isotope "mixing" curves were therefore calculated for this process of combined assimilation-fractional crystallization. The positions and characteristics of the resultant curves on δ¹⁸O-⁸⁷Sr/⁸⁶Sr diagrams markedly diverge from simple two end-member mixing relationships. Based on the above, model calculations can be crudely fitted to two igneous rock suites (Adamello and Roccamonfina in Italy), but the shapes of the calculated curves appear to rule out magmatic assimilation as an explanation for most δ¹⁸O-⁸⁷Sr/⁸⁶Sr correlations discovered so far, including all of those involving calc-alkaline granitic batholiths and andesitic volcanic rocks. The isotopic relationships in such magma types must be inherited from their source regions, presumably reflecting patterns that existed in the parent rocks (or magmas) prior to or during melting.

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