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Concomitant evolution of tectonic environment and magma geochemistry, Ambrym volcano (Vanuatu, New Hebrides arc)

Abstract:

Ambrym volcano lies in the central part of the New Hebrides island arc near the latitude where the D'Entrecasteaux Zone is colliding with the arc. It is a 35 km × 50 km, mainly basaltic stratovolcano constructed in four distinct parts: (i) well-preserved remnants of an old edifice in the north; (ii) a N100°-aligned, oval-shaped basal Hawaiian-type shield volcano; (iii) a pyroclastic cone, cut by a concentric 12 km wide caldera; and (iv) post-caldera basaltic suites, both intra- and extra-caldera. Considering K2O, La and Zr variations, three major trends characterize the geochemistry of Ambrym; one medium-K (MK) to high-K (HK) tholeiitic to calc-alkaline basaltic trend, and two more evolved trends, respectively MK and HK, from basaltic to dacitic and rhyodacitic compositions. These magmatic suites correspond to several volcanic phases: (1) older MK basalts forming the Tuvio-Vetlam-Dalahum edifice; (2) MK to HK basalts forming the basal shield volcano; (3) MK andesites and rhyodacites forming the first pyroclastic sequence of the Ambrym Pyroclastic Series (APS), which probably initiated the formation of the caldera; (4) MK to HK basalts and andesites forming Surtseyan then Strombolian pyroclastic sequences 2, 3 and 4 of the APS; and (5) post-caldera MK to HK basalts forming the recent olivine porphyritic- and plagioclase-porphyritic suites, locally associated with more evolved HK andesitic volcanic rocks in the eastern part of the caldera. (La/Yb)n and (La/K)n ratios (2-5.9 and 1.3-2.2, respectively) and Zr contents indicate that the parental magmas originated from incremental batch melting or fractional melting (21-25%) during uplift of a single spinel lherzolite source (from at least 60 to 45 km depth) in response to modifications in the tectonic environment due to ongoing collision with the D'Entrecasteaux Zone. However, K and La contents require prior enrichments by possible mantle metasomatism associated with subduction. Fractionation of olivine, plagioclase, clinopyroxene and Fe-Ti oxides explain most of the major and trace element variations in the resulting liquids and interaction between magmas rising from two distinct chambers combined with massive introduction of seawater into the edifice appear to be the major causes of the giant eruption leading to caldera formation. More recent activity of the volcano is principally related to shallow incremental batch melting or fractional melting and to fractionation in magma chambers associated with N100° rifting. © 1994 The Geological Society.