The volcanology, petrology and petrochemistry of Mt Edgecumbe volcano, Bay of Plenty, New Zealand

Abstract

Mt. Edgecumbe volcano is located on the active volcanic front that defines the eastern boundary of the Taupo Volcanic Zone, North Island, New Zealand. The Mt. Edgecumbe complex consists of main cone andesite lavas, dacite lavas, which form basal lava flows, and a series of western lavas. Tephrochronology constrains the activity at Mt. Edgecumbe as occurring between 8530 yrs B.P. and 1850 yrs B.P. A new radiocarbon date of 3115 ± 35 yrs B.P. was obtained from charred wood remains within a Mt. Edgecumbe block-and ash-flow deposit. Localised pyroclastic fall deposits occur toward the summit and overlie altered summit lavas. Pyroclastic block-and ash-flow deposits are present around the northern and eastern slopes of the main cone are may have formed by the collapse of summit lavas and are probably coeval with the pyroclastic fall deposits. All Mt Edgecumbe lavas are porphyritic, comprising plagioclase (An92 - 39), orthopyroxene (En51 - 76), clinopyroxene (Mg#65 - 78) and titanomagnetite, with additional quartz, biotite and rare hornblende more abundant in the andesite lavas. The Mt. Edgecumbe suite also contains igneous glomerocrysts, mid-upper crustal xenoliths and amphibole bearing cumulates. Amphibole is present in varying degrees of decrepitition, probably retlated to ascent rates of the magmas. Lavas carrying unaltered amphibole probably ascended from depths greater than 7 km over a period of 4 - 5 days. Major and trace element data indicate that the lavas and their enclaves are geochemically related, and are similar to ither lavas from the Bay of Plenty region. The lavas show enriched LILE and flat HFSE multi-element patterns typical of subduction related magmas. Rare-earth element diagrams show enriched LREE and flat HREE patterns. 87Sr/86Sr and 143Nd/144Nd isotope ratios from Mt. Edgecumbe are more radiogenic than most of the TVZ andesite and dacite lavas. The Mt. Edgecumbe suite has undergone a complex crystallisation history, involving AFC (Torlesse) and possible magma mixing. The proposed model for Mt. Edgecumbe involves rising basaltic-andesite magma interacting with shallow felsic magmas to produce the early dacite lavas, progressing into a period producing the younger andesite main cone lavas, pyroclastic fall and block-and ash-flow deposits.