The geochemistry and petrogenesis of Rarotonga, an ocean island in the South Pacific

Thompson, Gary M. (1998) The geochemistry and petrogenesis of Rarotonga, an ocean island in the South Pacific. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Rarotonga is the emergent summit of a Plio-Pliestocene volcanic complex built by effusive and pyroclastic eruptions of mainly mafic magma. Petrographically the mafic rock types are ankaramite and basalt. Phenocryst assemblages provide the basis for recognizing two types of ankaramites and basalt, one with a relatively simple equilibrium assemblage of olivine, titanaugite and magnetite (type I), and one with olivine, diopside-augite and titanaugite in which phenocrysts show dis-equilibrium textures (type II) The final stages of volcanism on Rarotonga witnessed the pyroclastic eruption of phonolites and the effusive eruption of foidal phonolites. -- Chemically the two mafic types are divided on silica content with, the type I rocks generally have lower silica content than the type n rocks. The remaining elements (both major and trace) having similar concentrations in both types. Although some of the mafic rocks have primitive compositions they probably do not represent primary compositions due to the presence of accumulated minerals. Both type J and type II rocks are enriched in incompatible trace elements compared to primitive mantle. On Bowen diagrams the foidal phonolites and phonolites plot as extensions of the trends defined by the mafic rocks. There is, however, a compositional gap between the mafic rocks and the felsic rocks. -- Distribution coefficients determined on mafic rocks from Rarotonga show high concentrations of most of the incompatible trace elements (e.g., Sr, Th, Y, REE etc.) in the pyroxenes relative to the other phases suggesting that fractionation of pyroxenes dominated trace element distributions during crystal fractionation. In such cases, the highly incompatible elements (Kd < 0.001) are the LFSE Ba and Cs, and the HFSE. Elements that could also be classified as strongly incompatible are Nb and Ta (Kd < 0.1). The remaining trace elements have Kd values that range from ~0.1 (La) up to ~0.8 (Yb). Magnetite incorporates higher amounts of Nb and Ta compared to the titanaugites, and any significant fractionation of magnetite will effect the bulk distribution of Nb and Ta. The LREE have lower Kd values than MREE and HREE with the HREE having Kd values close to unity. Consequently, the separation of titanaugite produces an overall enrichment of REE in the residual liquid with an increase in the La/Yb ratio. -- Most of the bulk compositional variation observed in the Rarotonga lavas is due to crystal fractionation processes. Rayleigh fractional crystallization is preferred to equilibrium crystallization on account of the abundance of zoned phenocrysts in many of the Rarotonga rocks. Olivine and clinopyroxene were the two phases controlling crystal fractionation of the mafic rocks. In the type II magmas, the first clinopyroxenes to crystallize had diopside-augite compositions while later clinopyroxenes had titanaugite compositions. As the magma fractionated towards more intermediate compositions kaersutite began to crystallize. Due to slight differences in silica saturation the type I magmas fractionated toward a foidal phonolite end-member forming a low silica suite. The type II magmas fractionated towards phonolite forming a high silica suite. -- Viscosity increased during fractionation of the magma chambers inhibiting eruption of the intermediate compositions. A late stage fluid (probably formed during pneumatolysis) was concentrated in the fractionating magma chambers (Na₂O, Th and Y, etc.). This fluid along with continued increase of alkalis decreased the overall viscosity of the magma to a point where eruptions could start again. The phonolite magmas, which only formed pyroclastic eruptions, were slightly more viscous than the foidal phonolites which erupted as flows. -- Whereas much of the compositional variability observed amongst Rarotonga lavas can be attributed to fractional crystallization, important differences in incompatible element ratios and bulk composition of primitive Rarotonga lavas must have been inherited from processes occurring in their mantle source regions. Quantitative modeling shows that the primary magmas which produced the lavas on Rarotonga were produced by low degrees of partial melting (<5%) of a (garnet-free) spinel lherzolite source. The mantle source also contained either an amphibole or a mica phase and was enriched relative to primitive mantle in the highly incompatible elements. The type I magmas were probably produced slightly deeper in the mantle than the type II magmas.

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