Assimilation and metamorphism at a basalt-limestone contact, Tokatoka, New Zealand

Abstract

SummarySmall-scale assimilation of limestone during the intrusion of an olivine basalt feeder dyke into an Eocene argillaceous, siliceous biomicrite in the Tokatoka area has resulted in the incorporation of large amounts of calcium into the parent magma. Initial effects of assimilation have caused partial resorption of the early-formed igneous mineralogy and the precipitation of calcic, iron-rich clinopyroxenes (ferrosahlite to hedenbergite), wollastonite, schorlomite, and pyrrhotine. Pyroxene compositions show a trend of strong enrichment in Catschermak and ferrosilite components. Derivative hydrothermal solutions, rich in Ca, Si, Al and alkalis have precipitated and altered anhydrous phases to tobermorite, thomsonite, prehnite, pectolite, cebollite, hydrogrossular, gismondine, analcime, Sr- and Ba-bearing zeolites, and calcite.Modelling of the basalt-limestone assimilation process by least-squares mixing methods has shown that the observed chemical variation can largely be accounted for by the dilution of the basalt with up to 30 wt. % decarbonated limestone. Desilication of the liquid, a result of this dilution effect, has been accommodated in the chemistry of the early-formed mineralogy rather than by the crystallization of minerals characteristic of an undersaturated rock type.Contemporaneous with the intrusion of the basalt was the high-temperature contact metamorphism of the limestone. This produced assemblages of rankinite, kilchoanite, larnite, spurrite, grossular, and tobermorite. Subsequent injection of the basalt and hybrid phases into fractures has resulted in the alteration of the primary metamorphic assemblage to wollastonite, scawtite, foshagite, hydrogrossular, calcite, and vaterite.