Pb-Nd-Sr isotopic compositions and trace element characteristics of young volcanic rocks from Egmont Volcano and comparisons with basalts and andesites from the Taupo Volcanic Zone, New Zealand

Abstract

Egmont Volcano (Mt. Taranaki) is the most westerly expression of volcanic activity in New Zealand's North Island and is 140 km west of the Taupo Volcanic Zone (TVZ), the principal locus of volcanism in the North Island. It is the youngest of the Taranaki volcanoes and is located approximately 180 km above the Wadati-Benioff Zone. Egmont Volcano is largely composed of basalts, basaltic andesites and andesites, and, in contrast to the equivalent eruptives in the TVZ, amphibole is common and orthopyroxene rare in the phenocryst assemblages. Relative to their TVZ equivalents, Egmont lavas are substantially more potassic and show significantly higher abundances of elements such as Cs, Rb, Ba, Th and the light rare earth elements. However, like their counterparts to the east, they show a characteristic “arc” signature on mantle normalised trace element plots with strong depletions of Nb relative to La and enrichments in Pb relative to Ce. On 87Sr 86Sr vs. 143Nd 144Nd , 207Pb 206Pb vs. 206Pb 204Pb and 208Pb 206Pb vs. 206Pb 204Pb diagrams young Egmont eruptives plot within the fields defined by TVZ basalts. The isotopic homogeneity of basalts from both Egmont and the TVZ and arc-like trace element signatures suggest that basaltic magmas in both areas were generated by subduction related processes from isotopically similar sources; most probably in the mantle wedge above the subducting slab. Higher abundances of LILE in Egmont basalts compared to TVZ basalts may indicate that lower degrees of partial melting were involved in the generation of Egmont basalts at the mantle source. The “arc” trace element signatures of Egmont and Taupo basaltic and andesitic rocks are attributed to the influence on the overlying mantle wedge of fluids generated by dehydration reactions in the descending slab. These fluids metasomatise the overlying mantle and provide a source from which later melting events will produce magmas with convergent margin trace element characteristics. Below Egmont, relatively limited melting takes place in the mantle wedge and parental basaltic magmas tend to be relatively undersaturated with respect to SiO 2, whereas beneath the TVZ, melting is more extensive and occurs at shallower depths so that parental melts tend to be sub-alkaline. TVZ basalts have substantially lower Ba/La ratios than Egmont equivalents and this may indicate a more significant slab input at Egmont than is the case in the TVZ. Egmont lavas evolved primarily through crystal fractionation accompanied by assimilation of crustal material, but there is also petrographic evidence for magma mixing.