Petrology and geochemistry of Sangeang Api and recent volcanism in the Sumbawa-Flores sector of the Sunda Arc: the response of along-arc geochemistry to subduction processes.

Abstract

This thesis documents data from an active volcano (Sangeang Api) and previously unstudied, extinct volcanoes (Wai Sano and Doro Kota and Doro Kuta) from adjacent sectors of the eastern Sunda Arc, Indonesia. Sangeang Api erupts co-magmatic suites of lavas and cumulate xenoliths. Lavas are ne-normative, silica-undersaturated, volatile-rich, shoshonitic basalts to basaltic-trachyandesites. They have trace-element compositions typical of arc magmas; enrichment in LREE, alkali-earth elements and Sr, depletion in Nb, Ta, Zr and Ti and high U/Th. They are also enriched in fluid-mobile elements (Cl, Ba, Cs, etc.). The cumulate xenoliths are separated into two distinct groups; the cpx+mgt and cpx+ol pyroxenites and the cpx+mgt+plag±amph gabbros. These groups are compositionally distinct, with their chemistry reflecting their cumulate mineralogy and are shown to drive magma evolution in the system by fractional crystallisation. Oxidation state, water contents and pressures of crystallisation are the primary controls on the primary cumulate mineral assemblages of the xenoliths. Sangeang Api magmas degassed as they ascended through the crust, with degassing driving oxidation at depth and reduction more shallowly. Many of the cumulate xenoliths are variably contaminated by melts indicating percolation and incomplete compaction or post crystallisation intrusion. Fe-isotope studies of the Sangeang Api products shows that the lavas record δ⁵⁷Fe compositions (average = 0.099‰ ±0.051) typical of arc settings. Cumulate xenoliths record heavy Fe-isotope compositions compared to the lavas (mean δ⁵⁷Fegabbro = 0.166‰ ±0.051; mean δ⁵⁷Fepyroxenite = 0.109‰ ±0.066). Using published fractionation factors, it is shown that whilst magnetite mineral separates records equilibrium compositions (mean δ⁵⁷Fe = 0.142‰ ±0.072), Fe-Mg silicate mineral separates display significant disequilibrium in their compositions (mean δ⁵⁷Feoliv = -0.313‰ ±0.284, mean δ⁵⁷Feamph = 0.125‰ ±0.081 and δ⁵⁷Fecpx = 0.109‰ ±0.090). Iron isotope disequilibria highlights the pervasiveness of post-crystallisation contaminative processes. Lavas from the Quaternary (~2Ma) D. Kota and D. Kuta, E. Sumbawa, are geochemically and petrologically similar to those from nearby Sangeang Api. However, they are less potassic (high-K calc-alkaline) with smaller enrichments in Ba and Sr, higher Rb/Sr and lower U/Th, highlighting a role for residual amphibole in the mantle source. Wai Sano at the far western end of Flores, is unique in the Sunda Arc, erupting adakites with characteristically low MgO, high Al₂O₃, high SiO₂, low Y and high Sr/Y. However, the Sr-Nd isotope systematics of these lavas suggests that they cannot have been produced by slab melts as is often suggested for adakites. Trace element characteristics and thermodynamic modelling suggest that a possible source of these magmas is through partial melting (10-13%) of basaltic underplates. Ultrapotassic magmatism in the Sunda Arc is confined to the rear-arc and characterised by trace-element and isotopic evidence of mantle metasomatism and the influence of slab tear windows. Slab window development has changed the loci and composition of volcanism on Sumbawa. Thus, highlighting the effects of local tectonic setting on arc volcanoes.