Volcanology and petrology of Tama Lakes area, Tongariro volcanic centre, New Zealand

Abstract

Tama Lakes area is located in the saddle between Mt. Ngauruhoe and Mt. Ruapehu, Tongariro Volcanic Centre, Taupo Volcanic Zone. The TVZ is situated about 80 km above the westward-dipping Wadati-Benioff zone where the Pacific plate is being subducted beneath TVZ crust. The structural geology of the study area is highlighted by normal faults which have NNE-SSW trends and are parallel to the faults of the central Taupo Volcanic Zone. Six craters have been recognized as eruption centres of lava flows and pyroclastic deposits, younger than 15 ka. Geologic mapping and stratigraphic studies have led to recognition that Tama Lakes volcanic rocks consist of old (Tama Lakes) lava flows (~ 260 ka), young (Saddle Cone) lava flows and pyroclastic deposits ( < 15 ka) and secondary deposits. Old lava flows are acid andesites and dacites and consist of at least three units of lava flows. Young lavas consist of two flows and are mostly acid andesite block lavas. Eight units of pyroclastic deposits are recognized at the Tama Lakes area. Most of the pyroclastic deposits are lapilli-ash fall deposits which have grain-size characteristics of plinian-subplinian, strombolian, vulcanian and phreatomagmatic eruption types. The source of the deposits are mostly from Tongariro Volcanic Centre with a few layers are from the Taupo Volcanic Centre. Some of the products of explosive eruptions and to a lesser of extrusion, are redeposited slope wash deposits or have formed secondary deposits such as lahars and colluvium. Four possible forms of renewed volcanism of this area are; magmatic (sub-plinian and vulcanian) explosion eruption, formation of new parasitic vents and extrusion of lava flows from the vents, phreatomagmatic explosion through Lower and Upper Tama Lakes, and lahars. Lahars may pose a serious hazard and risk in the Tama Lakes area. The rocks from Tama Lakes area are calc-alkaline, range from 56.5 to 63.4 % .SiO2 and are mostly acid andesites with minor basic andesites and dacites. Nearly all rocks are porphyritic with 21 to 47 % phenocrysts in microcrystalline or glassy groundmass. The principal phenocryst phases are plagioclase (sodic andesine-sodic bytownite), clinopyroxene (mainly augite), orthopyroxene (enstatite) and Fe-Ti oxide (mostly titanomagnetite), and in some rock samples, olivine (Fo70 - Fo84; mainly in basic andesite bombs) and amphibole (essentially ferroan pargasite hornblende and edenitic hornblende). Apatite is found as inclusions in plagioclase and clinopyroxene phenocrysts. Gneissic and quartzo-feldspathic xenoliths are ubiquitous throughout the lavas, and are thought to be of lower crustal origin. Major and trace elements variations are not consistent with time which is may reflect a complexity of a secondary processes. Petrography, mineral chemistry and bulk rock chemistry indicate dominance of combined process of assimilation and fractional crystallization (AFC), and other processes such as crystal fractionation and magma mixing. The Tama Lakes magmas are not primary as they have low Mg-numbers (46-59), low Ni (7-72 ppm) and low Cr (16-178 ppm) contents compared to primitive magmas from other parts of Taupo Volcanic Zone. The rocks from Tama Lakes area are characterized by enrichment of LILE and depletion in HFS elements which is similar to the general incompatible trace element pattern of many arc derived volcanics. The enrichment of LIL-elements can be attributed to enrichment and metasomatism of the mantle source by fluids released from the subduction slab. The depletion of HFS elements is attributed to higher degrees of partial melting or depletion of the arc mantle wedge due to previous melting events in the mantle wedge.