Geochemistry of the Murdering Beach Flow, East Otago, New Zealand

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McIntosh, P

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The Murdering Beach flow (MBF) is an inclusion-rich lava flow located within a small bay 16km northeast of Dunedin and was erupted during the First Main Eruptive Phase (Benson, 1941) of the Dunedin volcano. It contains a large variety of lithic inclusions and megacrysts many of which are commonly associated with calcite thought to be of primary magmatic origin.
The MBF contains phenocrysts of clinopyroxene, rounded nepheline, and sodalite within a microphenocrystic aggregate of clinopyroxene, feldspathoids, alkali feldspar and titanomagnetite and an aphanitic matrix of feldspathoids and glass.
Nepheline is commonly rounded in shape, possibly due to resorption, and displays highly birefringent micaceous overgrowths considered to be polymorphic nepheline. Sodalite is invariably associated with carbonate.
A large and diverse suite of clinopyroxene phenocrysts are also present within the MBF and exhibit a variety of zoning, rimming and resorption textures. Three petrographically distinct groups of clinopyroxene are recognised; these groups are also distinguishable geochemically. Fe-depleted clinopyroxene rims, not previously described from rocks of the Dunedin volcano, are thought to have developed from low pressure re-equilibration of the clinopyroxene phenocrysts with the MBF. The presence of strongly resorbed and euhedral clinopyroxene phenocrysts within the MBF may have resulted from "delayed fractionation" and rapid cooling of the MBF.
The heterogeneous appearance of the MBF is not reflected in its major element chemistry. The composition of the MBF is enigmatic in that its high alkali content together with its relatively high Mg-number indicate that the MBF exhibits characteristics of both primitive and more evolved lavas. The alkali-rich nature of the MBF, together with the presence of mantle derived inclusions within this flow suggests an origin for the MBF of fractionation of mantle derived liquids. The similarity of the MBFs trace element pattern to that of crustally fractionated rocks from the Dunedin volcano suggests however, that on the basis of trace element patterns alone it is difficult to distinguish between rocks derived by low as opposed to high pressure fractionation processes.
The MBF contains a diverse suite of upper mantle- and lower crustal-derived inclusions. The spinel lherzolite and clinopyroxenite inclusions are typical of the Cr-diopside group (Wilshire and Shervais, 1975}, while the feldspathic clinopyroxenite and poikilitic-amphibole dominated inclusions are typical of the more Fe-rich Al-augite group (Wilshire and Shervais, 1975). The spinel lherzolite inclusions display metamorphic textures and are interpreted to be fragments of upper mantle wallrock. The clinopyroxenite and feldspathic clinopyroxenite inclusions commonly display modified igneous textures, and may have formed either from dikes of alkali basaltic magma injected into the host peridotite which were subsequently deformed, or as magma chamber cumulates which were also similarly deformed.
Equilibration temperatures ranging from 996- 1069°C were obtained for coexisting enstatite and diopside within the spinel lherzolite and clinopyroxenite inclusions, and are consistent with an upper mantle origin for these inclusions.
The poikilitic amphibole-dominated inclusions most likely represent high pressure precipitates of alkaline magmas, formed within the upper mantle or lower crust. The gabbroic inclusions are independent of the rest of the inclusion suite, and trace element data presented in this study shows that they are unrelated to the origin of the MBF, and are derived from the upper crustal plutonic roots of the Dunedin volcano.
Textural evidence suggests that glass present within many of the inclusions was formed by partial melting. However there is also evidence that host rock infiltration may be responsible for some of the glass present in the inclusions.
The MBF also contains a large megacryst suite, dominated by aluminous clinopyroxene, with subordinate amounts olivine, kaersutite and titanomagnetite. The euhedral shape and large size of the megacrysts is consistent with them forming as high pressure precipitates of the MBF or related magmas.
Major and trace element modelling carried out in this study suggests the composition of the MBF has resulted from fractionation of magma of basanitic composition, by the removal of aluminous clinopyroxene and kaersutite, and minor olivine and Ti-spinel of similar composition to the megacrysts present within the MBF.
Evidence presented in this study suggests that the range of compositions within the Dunedin volcano produced by low pressure fractionation, are equally capable of being produced by high pressure fractionation at upper mantle depths.
A model for the origin of the MBF is proposed, involving the generation of alkali basaltic magmas by partial melting of enriched, metasomatised spinel lherzolite within the upper mantle, followed by significant high pressure fractionation to the derivative liquid prior to xenolith entrainment. The presence of xenoliths within mantle-derived alkali basaltic lavas therefore does not place limitations upon the degree of fractionation of these lavas that may have occurred.

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x. 206 p. ill. 30cm.

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1989McIntosh

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http://download.otagogeology.org.nz/temp/Abstracts/1989McIntosh.pdf

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McIntosh, P, “Geochemistry of the Murdering Beach Flow, East Otago, New Zealand,” Otago Geology Theses, accessed April 16, 2021, http://theses.otagogeology.org.nz/items/show/232.

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