West Waihola basanites and associated segregated liquids

Walls, Damian John J., 1963-

The area of study is located 32km south-west of Dunedin City, centred on the western shore of Lake Waihola. Within the field area alkaline volcanics, outlying members of the Dunedin Volcanic Group (DVG) (Coombs et al., 1986), are observed to rest unconformably on quartzofeldspathic basement schists, and on a sequence of regionally tilted, Late Cretaceous to Late Tertiary, non-marine to marine sedimentary units that were deposited on a Cretaceous peneplain surface cut into the metamorphosed basement. The sedimentary sequence is composed of; Taratu Formation carbonaceous sandstones to quartz pebble conglomerates, Wangaloa Formation bioturbated sandstones, and mudstones, greensand, Oarendon phosphatic sandstone, and the locally restricted East Otago Waihola Chert.
Alkaline volcanics crop out over an area extending approximately 5km westward from the lakeshore, and are composed of mantle and crustal xenolith bearing, fine grained basanites and the coarser grained, more evolved Waihola Olivine Theralite (Benson, 1942). The fine grained xenolith bearing basanites are the major topic in this project, and have been informally grouped into two suites; the Waihola Hill Suite (WHS), and the Lakeside Suite (LSS) based on geochemical and petrological data.
WHS is the oldest (21.3 - 22.5 Ma) and the most extensive volcanic unit mapped, croping out over much of the field area westward of the lakeshore. It is composed of a suite of geochemically very similar basanites of a derivative nature (Mg# ~60) which contain numerous small crustal (quartzose) xenoliths and displays evidence for forceful emplacement from at least three eruption centres.
The LSS is considered to be coeval or slightly younger than the WHS, and is restricted in outcrop to the lakeshore area. Unlike the WHS, the LSS is considered to represent primary melts derived from the mantle (Mg# 66 -73; Cr 380 to 600 ppm, Ni 272 to 500 ppm) and is subdivided geochemically (Mg#, P2O5, SiO2, MgO and Al2O3) into three groups; an 'Early' primitive group near the lakeshore, a 'Younger' group with late-stage emplacement features, and a 'Middle' group representing the main mass of the LSS. Samples from the 'Early' LSS group appear to be the most 'primitive', in terms of their Ni and Cr contents, yet reported for members of the DVG.
Mineralogically, the LSS is the more complex of the two, containing megacrysts of orthopyroxene, kaersutite, Mg-garnet lherzolite composition clinopyroxene, plus Group I and 11 (Frey and Prinz, 1978) mantle inclusions as well as low pressure phases which suggest an involved crystallization history. Geochemical modelling would suggest fractionation controlled by clinopyroxene and olivine for the LSS.
The clinopyroxene phase within both suites have be classified into 5 Types. Type la, Type lb, Type lc are mantle derived xenocrysts and megacrysts; Type 2 are strongly zoned xenocrystic microphenocrysts; Type 3 euhedral microphenocrysts, Type 4 groundmass phases, and Type 5 segregated liquid clinopyroxenes.
Both the LSS and WHS contain, regardless of primary or derivative nature, evolved melt systems in the form of segregated liquids associated with relatively late stage entrained quartzose xenolith material derived from the basement schists. The 'segregated liquid blebs' (SLB) and a larger mass of 'segregated alkaline liquid' (SAL) not directly associated with a quartzose xenolith, generally possess a mafic outer mantle defined by diopsidic/augitic pyroxene surrounding a felsic inner core dominated by alkali feldspars. Another segregated liquid, assumed to be produced by fractional crystallisation, is observed in the upper levels of the LSS within elongated vesicles (a peralkaline vapour phase?).
Within these late stage liquids all major mineral species display progressive mineralogical 'evolution' trends from the outer mafic mantle inwards towards the inner evolved regions of the felsic core. This evolution has been described in three steps;
Step 1: Entrainment of the xenolith and the formation of a buchite glass
Step 2: The formation of the diopsidic/augitic pyroxene mafic mantle and inner alkali feldspar felsic core.
Step 3: Within the inner felsic core, the formation of Ti-enriched aegirines and amphiboles, as well as Fe-enriched alkali feldspars (?)
The unusual mineralogy of these evolved inner core region liquids has been studied in some detail. It has been found that there is significant enrichment in Ti within the late-stage aegirines and amphiboles, and Fe within the late-stage alkali feldspars. The feldspars with the highest Fe contents, up to 4.53 wt% FeO*, are intrafasiculate textured, pseudo-uniaxial (-ve) "Ferro-Kspars" grains. Problems with the allocation of Fe within the feldspar structure for these grains may indicate that they are a new mineral phase. More work is needed to resolve this question.
Late-stage, Ti-enriched pleochroic amphibole forms a 'solid solution series' between titanian richterite and titanian ferro eckermannite. The most Ti-enriched titanian ferro eckermannite contains 8.14 wt% TiO2, a new compositional high for Ti in an amphibole of non-kaersutite composition.
A total of 17 analyses obtained from these late-stage amphiboles are higher in Ti content than the previous compositional high (6.39 wt% TiO2- Czamanske and Atkin, 1985) and may indicate the occurrence of a new amphibole end member composition, NaNa2(Fe,Mg)4.5Ti0.5Si8O22(OH,F)2. The mechanism for the formation of these segregated liquids in which the evolved phases occur, has been studied. Initial evidence supported their formation as immiscible silicate liquids (ocelli), but on more detailed examination, a mechanism involving the siliceous metasomatic dilution of host basanite due to the partial (to total) melting of an entrained quartzose xenolith, with partial component input from micaceous laminae observed within some xenolith, is considered more likely. Once formed, the evolution of liquids appear strongly control by fractional crystallization and mirco-environment (-domain) equilibria.

239, [25], A124, B21, C6, D2, E16 p., [1] folded leaf : ill. (some col.), maps ; 30 cm.

1991Walls

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