Geochemical investigations of the alkalic rocks of the Dunedin volcano, East Otago, New Zealand

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Price, Richard Charles.

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Abstract:

The Dunedin Volcano, of Mid - Late Miocene age, is a complex alkalic volcano which was active over a period of about 3 m.y. (13-10 m.y.). Activity commenced with the eruption of basaltic flows and tuffs in the central Otago Harbour depression. The earliest tuffs are interbedded with marine sands and calcareous sands indicating that the initial activity was submarine. Pillow structures and 'bomb sags' in the lower units of the volcanic massif are additional evidence that the initial activity was submarine. The initial basaltic activity was followed by extensive emplacement of quartz normative trachyte flows and tuffs in the central area, and a small emergent cone of trachytic composition was built. The formation of the early trachyte cone was followed by extensive, subaerial, shield-building eruptions of flows ranging in composition from basalt through intermediate rock types to phonolite and feldspathoidal trachyte. These were erupted from vents within the central depression, and the last significant activity in the central area resulted in the emplacement of coarse breccias which occupy vents on an axis through Port Chalmers, Portobello and Sandymount.
The central activity was followed by a period of eruption controlled largely by activity from non-central fissures probably related to basement fractures. The conspicuous topographic highs of the Otago Peninsula and the Flagstaff-Mihiwaka ridges were built. Activity in the Dunedin Volcano was terminated about 10 m.y. ago with the emplacement of nepheline benmoreite lava domes at Mt. Cargill.
The volcano has a complicated history. Activity from various vents (of the order of 40 have been recognized) over a period of 3 million years, during which the volcanics were being actively eroded, has resulted in stratigraphic complexities which are irresolvable on a regional scale. Correlation of units over distances in ex6ess of a few km are invalid, and the regional stratigraphy established by Benson (MS, 1968) is rejected.
Although syn- and post-volcanic faults can be recognized in the Dunedin Volcano, structural complexities are very rarely observed. Relationships between the various volcanic units result from the ·interaction of flows, coulées, and intrusions erupted onto an actively eroding volcanic topography. Most folding observed in the volcano occurs around small domical phonolite intrusions which have arched the overburden during intrusion into quartz trachytes and the underlying sediments in the central depression.
Volcanism in East Otago is intimately associated with extensive block faulting. Although the latest movements appear reverse there is strong evidence that faults in East Otago were normal during Cretaceous and, possibly, Early Tertiary time.
A number of endogenous lava domes have been identified in the Dunedin Volcano, especially amongst the products of the later phases of the volcano's history. Most are composed of phonolite, but other examples consist of mugearite, hawaiite, benmoreite, trachyandesite, and nepheline benmoreite. Commonly the domes have been emplaced in cinder cones developed on the flanks of the volcano.
The isotopic composition of strontium has been determined for samples from the Dunedin Volcano covering the range basalt, basanite, intermediate compositions, phonolite and quartz normative trachyte. The basaltic, intermediate and phonolitic rocks appear to be comagmatic and have similar low initial 87Sr/86Sr ratios around 0.7030, comparable with those of other alkalic provinces. The quartz normative trachytes have initial ratios significantly higher than those of the other rocks (0.7040) although their age is comparable. Rb-Sr ages obtained are comparable with published K-Ar dates. The Rb-Sr age for the trachytes is 14. 4±7 m.y. and the other alkali-enriched rocks give ages ranging within the limits of 14.4 to 12.0 m.y.
An electron microprobe study of the mineralogy of representative samples from the Dunedin Volcano has established fractionation trends for clinopyroxenes, olivines, and feldspars. The mineral fractionation trends indicate that the host rocks are representative of magmas related by a crystal fractionation process. The clinopyroxene fractionation trend, from diopsidic to intermediate acmitic clinopyroxene is similar to the Nandewar trend (Abbott, 1968) and indicates low oxygen fugacity throughout crystallization of the host magmas. Low oxygen fugacity conditions are indicated for all the lavas of the Dunedin Volcano examined during this study by:
(i) Homogeneous titanomagnetites (Oxidation Index I - II, of Watkins & Haggerty, 1967).
(ii} Aenigmatite and riebeckite in evolved lavas.
(iii} Indirect calculations using mineral chemistry and thermodynamic relationships.
Oxygen fugacities calculated for basanite and trachyandesite were 10-10, 10-12 atm. respectively.
The electron microprobe study indicated the presence of xenocrysts or megacrysts of olivine, aluminous clinopyroxene, and calcic plagioclase in basanite and nepheline benmoreite lavas.
Using a combination of mineral geothermometers and the results of direct melting experiments such as those summarized by Thompson {1972) it was possible to establish ·some crystallization temperature limits for alkalic lavas. The results are consistent with a crystal fractionation model for the evolution of the Dunedin lavas. Temperature estimates for the commencement of crystallization range from 1170°C for basanite to 975°C for phonolite.
New major and trace element analyses are presented for 9 kaersutites from basic alkalic rocks. K/Rb ratios lie between 1209 and 4276. Rb is low, averaging 6 ppm. Sr ranges from 532 to 1060 ppm and Ba from 181 to 701 ppm. Zr averages 109 ppm, Nb 44 ppm and V 390 ppm. There is a moderate enrichment in light and intermediate REE. Zn correlates with FeO + Fe2O3 but the concentrations of Ni, Co, Cr, Cu and Pb are variable. Large variations in trace element concentration in kaersutites reflect only small variations in the melt when the distribution coefficient for a given element strongly favours the amphibole.
Kaersutite is significant in the petrogenesis of alkalic rocks as a possible accessory phase in the upper mantle source regions, and as an important phase in the fractionation of basic alkalic liquids over a wide range of pressures.
Sixty-six new major and trace element whole rock analyses are presented along with additional published and unpublished major element analyses from the Dunedin Volcano. Trace elements analyzed include Cs, Ba, Rb, Sr, Pb, Th, U, Zr, Hf, Nb, Sn, Y, V, Cr, Ni, Cu, Zn, Ga.
The following is a summary of the geochemical trends observed:
(a) Basalt-hawaiite-mugearite-benmoreite
(i) There is a continuous depletion through the series in Ti, Mg, total iron, Ca, Ni, V, Cr Cu.
(ii) The following elements are continuously enriched from basalt to benmoreite Si, Al, Na, K, Cs, Rb, Ba, Pb, Th, U, Zr, Hf, Sn, Nb, Ga.
(iii) Sr and P show maximum concentration in the mugearites and are depleted in benmoreites.
(iv) Zn and Mn show erratic behaviour but abundances are rather low.
(b) Phonolites
(i) Phonolites are strongly enriched in Na, K, Cs, Rb, Pb, Th, U, Zr, Hf, Sn, Nb, Ga, Zn, Y, Cl.
(ii) Phonolites show strong depletion in Ti, Mg, total iron, Ca, Ba, Sr, Ni, V, Cr, Cu, P.
(c) Basanite-nepheline hawaiite-nepheline trachyandesite-nepheline benmoreite These rocks show behaviour similar to the basalt-hawaiite-mugearite group, but there are differences between the benmoreites and nepheline benmoreites:
(i) Both Ba and Sr are strongly enriched in the nepheline benmoreites.
(ii) P is higher in the nepheline benmoreites than in the benmoreites.
(iii) The nepheline benmoreites are more mafic than the benmoreites.
(d) Quartz-normative trachytes The quartz normative trachytes show features which set them apart as a distinct geochemical group:
(i) Although high in Si content the quartz normative trachyte is surprisingly depleted in Rb, Th, U, Zr, Hf, Sn, Nb, Ga, Zn, Y.
(ii) K and Ba are enriched in the quartz normative trachyte while Cs is slightly depleted, compared with the other rocks.
(iii) Ti, Mg, total iron, Ca, P, the ferromagnesium trace elements are all very strongly depleted.
Twelve alkalic lavas from the Dunedin Volcano have been analyzed for the rare earth elements (REE) La-Yb. The compositions analyzed were: basalt-hawaiite-mugearite-benmoreite; basanite, nepheline hawaiite, nepheline trachyandesite and nepheline benmoreite; trachyte; phonolite. The series from basalt to mugearite shows continuous enrichment in the REE, consistent with a crystal fractionation m6del involving removal of olivine and clinopyroxene. From mugearite to benmoreite there is a depletion in the REE which is explained by the appearance of apatite as a liquidus 6 phase. The chondrite normalized REE patterns for the phonolites are characterized by strong enrichment and fractionation coupled with a sharp depletion in Eu. Removal of feldspar from a more basic magma is suggested for the derivation of the phonolites. The series basanite-nepheline hawaiite, and basanite-nepheline hawaiite-nepheline benmoreite appear to be high PH 0 analogues of the series basalt-benmoreite, with enrichment of the REE being achieved by removal of clinopyroxene, kaersutite and olivine. Compared with other lavas the trachyte has low REE abundances and is characterized by a striking positive Eu anomaly.
New major and trace element data are presented for examples in alkalic provinces of differentiated lavas containing evidence of high pressure origin, along with published data on similar localities. The examples analyzed are:
(i) 'Mafic phonolite' - Pigroot, East Otago.
(ii) Phonolite from Bokkos, Jos Plateau, Nigeria.
Both bodies contain lherzolite inclusions which are evidence for rapid rise from lower crustal or upper mantle regions. Additional published data for hawaiites, nepheline mugearites, and a phonolite from Heldberg in East Germany, all showing evidence of a high pressure origin are included.
The function MgX100/Mg+Fe2+ is used to distinguish 'derivative' from 'primary' magmas, and it is shown that all the differentiated magmas of high pressure origin are in fact 'derivative' and have evolved by crystal fractionation processes from basalt or basanite 'primary' magmas~ A high pressure (>25 km) crystal fractionation series from basanite through nepheline hawaiite, nepheline mugearite, and nepheline benmoreite to phonolite is proposed. The following features can be used to distinguish magmas of high pressure (sub-crustal) origin from those of low pressure (crustal) derivation:
(i) The occurrence of high pressure inclusions such as lherzolite xenoliths or high pressure megacrysts (e.g. aluminous clinopyroxene).
(ii) The involvement of feldspar implies fractionation at relatively low pressures. In this respect Ba, Sr, Pb, REE (in particular Eu) are important trace elements.
The Dunedin lavas are the result of crystal fractionation processes acting upon mantle derived basaltic magmas at various levels in the crust. The broad processes and lines of descent established by this study are in essential agreement with the suggestions made by Coombs & Wilkinson (1969). A diversity among parental materials and operation of the fractionation process at varying levels in the crust and mantle, under varying conditions of PH 0 have resulted in a diverse series of overlapping fractionation trends. 'End member' series are as follows:
(i) Basalt-hawaiite-mugearite-benmoreite; controlled largely by removal of olivine, clinopyroxene, titanomagnetite and, in the step from intermediate compositions to benmoreite, apatite.
(ii) A high PH2O analogue of this series probably developing at higher Ptotal: basanite-nepheline hawaiite-nepheline mugearite-nepheline benmoreite; controlled by removal of olivine, clinopyroxene, kaersutite, and titanomagnetite.
(iii) Moderately potassic variations on both these trends, including the rock types trachyandesite (in the mildly alkalic trend), nepheline trachyandesite (in the ultra-alkalic trend) and K-benmoreite (tristanite) and K-nepheline benmoreite (nepheline tristanite).
All phonolites appear to have arisen by a similar process operating on a variety of parental magmas. They are low pressure (crustal) differentiates derived by fractional crystallization processes involving feldspar. Phonolites are probably derived principally as end products in the crystal fractionation series outlined above, but small quantities of phonolitic liquid may derive directly from quite basic parents.
The strontium isotopic chemistry and in particular the REE chemistry of the quartz normative trachytes require that special circumstances be invoked for the origin of these oversaturated melts. Two suggestions are made here. Either the trachytes derive at the lower crust by partial melting of a quartz free parent, or they represent the end product in a long process of crystal fractionation in an isolated crustal ·magma chamber. The former model raises problems regarding the nature of the parental material, and the latter requires some form of preferential feldspar contamination to account for the REE pattern and other peculiarities of the chemistry.

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1 v. (various pagings) : illus., plates ; 30 cm. 2 copies.

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1973Price

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Open

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POLYGON ((170.474922459129061 -45.918244930019398,170.407741138802692 -45.840532734427775,170.609471757480776 -45.724668630518849,170.681231260366502 -45.798188589716503,170.474922459129061 -45.918244930019398))

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http://download.otagogeology.org.nz/temp/Abstracts/1973Price.pdf

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Price, Richard Charles., “Geochemical investigations of the alkalic rocks of the Dunedin volcano, East Otago, New Zealand,” Otago Geology Theses, accessed May 22, 2024, https://theses.otagogeology.org.nz/items/show/73.

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