Abstract
The most recent volcanism on the island of Viti Levu, Fiji, is represented by Pliocene and younger mafic-intermediate rocks found in the areas of Sabeto, Tavua, Rakiraki, and Vatia, including Cainozoic shoshonites in the Vatukoula area. Some post-subduction (Pleistocene?) rocks are best exposed in the Rakiraki area, particularly along the Nakauvadra River and in the cones of the Narewa Plug complex. The rocks along the river are mainly andesites and basaltic-andesites with sporadic breccia horizons and dykes. In contrast, the rocks exposed in the Plug are Hbl-andesites. These rocks belong to the Mba Basaltic Group. Porphyritic basaltic andesites from the River are predominantly composed of microlites of Pl, Cpx, Fe-Ti oxides, Opx, and Ol with phenocrysts of altered and serpentinized Ol and subhedral to euhedral lath-shaped to tabular Pl (sometimes showing resorbed cores with mantled overgrowth) and sporadic Cpx. Rarely, Opx and Pl are found as inclusions in Cpx. Fine-grained basalts are composed mainly of microlites of Pl and Cpx with groundmass rich in Pl and Cpx.
Phenocrysts in the porphyritic Hbl-andesites from the plug complex are represented mostly by euhedral, zoned, and twinned Pl and Hbl with lesser amounts of euhedral Cpx. In contrast, Pl, Hbl, Cpx, and Fe-Ti oxides are found in the groundmass. There are zeolites (wairakites) that seem to have originated at the expense of Pl, indicating a hydrothermal activity.
Field studies at both of these sites show preliminary evidence for mixing/mingling between magmas, i.e.: a) chilled margins around MME and FME in the basaltic-andesites in the River b) cm-scale quenched enclaves of basaltic composition, finer in grain size (made up of microlites of Pl and Cpx) and coarser-grained mafic enclaves with chilled margins in andesites of the Plug. In addition, in the River section, coeval masses of basalts and andesites indicate a genetic link and fragments of sedimentary rocks are present in the basaltic andesites, confirming phreatomagmatic eruption. Although previous studies indicated that andesites from the Plug were the products of fractional crystallization of gabbroic magma, the present study suggests stronger support for magma mingling. However, a supporting geochemical investigation is needed.
Phenocrysts in the porphyritic Hbl-andesites from the plug complex are represented mostly by euhedral, zoned, and twinned Pl and Hbl with lesser amounts of euhedral Cpx. In contrast, Pl, Hbl, Cpx, and Fe-Ti oxides are found in the groundmass. There are zeolites (wairakites) that seem to have originated at the expense of Pl, indicating a hydrothermal activity.
Field studies at both of these sites show preliminary evidence for mixing/mingling between magmas, i.e.: a) chilled margins around MME and FME in the basaltic-andesites in the River b) cm-scale quenched enclaves of basaltic composition, finer in grain size (made up of microlites of Pl and Cpx) and coarser-grained mafic enclaves with chilled margins in andesites of the Plug. In addition, in the River section, coeval masses of basalts and andesites indicate a genetic link and fragments of sedimentary rocks are present in the basaltic andesites, confirming phreatomagmatic eruption. Although previous studies indicated that andesites from the Plug were the products of fractional crystallization of gabbroic magma, the present study suggests stronger support for magma mingling. However, a supporting geochemical investigation is needed.
| Original language | English |
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| Publication status | Published - 2024 |
Funding
SMG, Nazarbayev University