Olivines as probes into assimilation of silicate rocks by carbonatite magmas: Unraveling the genesis of reaction rocks from the Jacupiranga alkaline‑carbonatite complex, southern Brazil

Abstract

Assimilation of clinopyroxenite xenoliths by carbonatite magma plays an important role in the Jacupiranga complex, a classical mafic-ultramafic alkaline‑carbonatite occurrence outcropping in Southeastern Brazil. In this complex, different batches of dolomite-bearing carbonatites and calcite carbonatites are found, all of them intrusive in the main clinopyroxenite body. Particularly for the calcite-rich carbonatites (with accessory olivine), the contacts with wall-rock clinopyroxenite are marked by xenolith-rich zones, formed by clinopyroxenite blocks embedded in the carbonatite. Banded reaction rocks occur enveloping such xenoliths, presenting conspicuous textures and compositions, with mineral assemblages changing from clinopyroxenite towards the carbonatite from (i) amphibole + phlogopite ± titanite (amphibole-domain) via (ii) phlogopite (phlogopite domain) to (iii) olivine + phlogopite (olivine domain). The contents of Ni, Co and Sc in olivines from reaction rocks suggest that their trace element budget was strongly influenced by clinopyroxenite assimilation. Schematic reactions between clinopyroxenite and carbonatite melt are presented and these suggest that the observed features can be explained by a single magmatic high-T event. The required carbonatite component increases from amphibole- through phlogopite- to olivine-domains, mirroring their distance to the carbonatite front, reinforcing that carbonatite melt activity is a major control for the formation of the reaction rocks. The proposed reactions also imply that initially Mg-rich carbonatite melts consume diopside in the clinopyroxenite to produce the silicate phases (olivine, phlogopite, amphibole, and titanite), while shifting to Ca-rich melt compositions. The lack of xenoliths zones related to the olivine-free dolomite-bearing carbonatites, the absence of dolomite in the carbonatites proximal to the reaction rocks, and the presence of olivine seams in the carbonatite-reaction rock interface reinforce the link between olivine crystallization in the carbonatite and the consumption of dolomite component of the melt. Unlike previous models, we propose that the investigated reaction rocks formed in a magmatic environment, where carbonatite magmas have both assimilated and modified clinopyroxenite xenoliths. These observations may have general implications and further underline the capability of carbonatitic magmas for reactive bulk assimilation of diverse silicate rocks.