Submarine intraplate volcanism in the South Pacific: Geological setting and petrology of the society and the austral regions

The southeastern prolongations of the Society and Austral islands volcanic chains are terminated by several recent submarine volcanoes (300–3800 m in height) built on irregularly shaped crustal swells or bulges (3600–3950 m in depth). The crustal swells (about 100 km in width) is bounded by deeper abyssal hill regions (>4000 m in depth) where old volcanoes with thick Fe-Mn coatings are encountered. The rocks sampled on this ancient oceanic crust are depleted mid-ocean ridge basalts (MORBs) similar to modem volcanics encountered on the East Pacific Rise. The volcanics of the Society hot spot consist of ankaramites, picrites, alkali-basalts, basanites, tephrites, and trachytes. Highly vesiculated pillow lavas show a wide compositional range in their large ion lithophile elements (LILE) (K2O=1–4%, Nb=25–80 ppm, Zr=200–400 ppm, Ba=300–840 ppm) and compatible element variations (Mg #=40–70, Ni=80–500 ppm), and low Zr/Nb ratios (5–8). Low-K tholeiites (K2O<0.15%, Nb=3–6 ppm, Zr=56–74 ppm, Zr/Nb=10–25), also found associated with alkali-basalts and basanites from recent volcanoes, are believed to have been exposed during rejuvenation of ancient edifices. The Macdonald volcano (Austral island chain), with a base diameter of 45 km at a depth of 3850 m, was sampled from its summital area (<100 m in depth) down to 2000 m in depth. Highly vesiculated pillow lavas, volcanic ejecta, and accidental rock debris recovered differ from those of the Society hot spot volcanoes by their lower Zr/Nb (3–5), Rb/Sr (<0.045), and (Ce/Yb)N (5–10) ratios and their lower Ba (350–400 ppm) and light rare eath elements (LREE) content. The accidental rock debris (metabasalt, metadolerite, isotropic gabbros, and metagabbros) were scattered during phreatic explosive events on the flanks of the Macdonald volcano. Volcanic ejecta of basanite lapilli partially covering the accidental rock debris were formed during more recent phreatomagmatic explosions. The most undersaturated glassy pillow lava encountered in the Austral hot spot is a normative nepheline-rich rock with incipient amphibole recovered from an ancient seamount (Ra seamount) located at about 100 km northwest of the Macdonald volcano. Based on their high LREE content and low Zr/Nb (3–8) and on their high (Ce/Yb)N (5–20) (5–20), (La/Sm)N (1–5), and (La/Lu)N (7–40) ratios with respect to the MORBs, it is speculated that the intraplate volcanics (Austral and Society volcanoes) have originated from the partial melting of mixed mantle sources made up of lherzolite (MORB mantle source) and another complementary source enriched in LREEs and Nb. A recycling of continental crust or pelagic sediments in subduction zones represents a possible supply of LREEs but not of Nb. Continental metasomatized peridotites and oceanic amphibole-peridotites such as Zabargad and St. Paul's Rocks samples were proposed as potential mantle sources for intraplate magmas. Using modal and chemical (Zr, Nb, Ce, and Yb) data available in the literature, a model of partial melting of a composite mantle, obtained by mixing homblendite (Queensland's xenoliths) and spinel-lherzolites, was tested as being a possible origin for the studied rocks. This model provides adequate Zr/Nb ratios and suggests that the Society mantle source contained smaller amount of homblendite (<5 wt %) and underwent lower degree of partial melting (about 5 %) than that of the Austral hot spot (10 wt % and 10–20% respectively). However, the complementary mantle source in the Society hot spot requires larger amount of LREEs than it is necessary for the Austral volcanoes. While the Zabargad amphibole-peridotites are reasonable candidates, the metasomatized peridotite xenoliths from continental alkali-basalts and the amphibole-peridotite mylonites from St. Paul's Rocks are more suitable as a potential source for the Society parental melts enriched in Ce and La. Hence the intraplate lava compositions could be the result of recycling and remelting of an ancient oceanic upper mantle which is partially metasomatized during its lateral transfer off the ridge axis. Local mantle heterogeneities and variable amounts of melting account for the occurrence of several different magmatic lineages through crystal fractionation processes in shallow magmatic reservoirs underlying individual volcanic centers of the Society and Austral hot spots.