Geochemistry of the Siberia Hill Volcanics and the Ultramafic Inclusions, Siberia Hill, East Otago, New Zealand


Rae, AJ


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Siberia Hill is located approximately 35km southwest of Oamaru, in the Kakanui Ranges. The study area includes four Miocene volcanic centres, Siberia Hill, Mt. Dasher, Kattothyrst, and Camelback. All, with the exception of Siberia Hill, are plug remnants resting on schist basement. Siberia Hill consists of a series of flows, of varying rock types, that rest upon Tertiary sediments.
Rock types in the area range from olivine nephelinite through basanites and olivine basalt to hawaiite, nepheline hawaiites and nepheline mugearite. Geochemical trends point to the importance of clinopyroxene and kaersutite fractionation in the early stages of crystallization. Plagioclase and titanomagnetite have an influence in the later stages of crystallization of the Mt. Dasher suite, and the Siberia Hill olivine basalt to hawaiite suite.
Four types of clinopyroxene phases are recognized: 1) Groundmass titanaugites; 2) Titanaugite phenocrysts; 3) Titanaugite xenocrysts, derived from Group II (Frey & Prinz, 1978) xenoliths; 4) Alkali-rich acmitic augite xenocrysts that are thought to either represent wall rock debris or are products of the crystallization of evolved melts. The presence of nepheline and alkali feldspar xenocrysts, and the alkali-rich megacryst suite, suggests that crystallization of more evolved melts did occur at depth.
Mantle derived inclusions are found in seven of the ten rock types. They include spinel lherzolites, clinopyroxenites and peridotites, megacrysts, and composite xenoliths. The clinopyroxenites and peridotites are typical of the Group II series xenoliths and represent dikes of alkali basaltic magma injected into the host peridotite. The cumulate textures and the euhedral habits of the megacrysts is consistent with crystallization from a magma at depth. Two composite xenoliths contain Group II lithologies in contact with Group I (Frey & Prinz, 1978) lherzolite. These represent pieces of the peridotite wall rock with intruding basaltic dike rock still intact. Geochemical studies reveal that the Group II lithology has undergone metasomatism and trends towards Group I compositions.
The spinel lherzolites from the Otago region are interpreted to be fragments of the upper mantle peridotite. Equilibration temperatures ranging from 991°- 1206°C were obtained, using coexisting enstatite and diopside, and are consistent with an upper mantle source. Petrographically the lherzolites are very homogeneous, with a four pha~e assemblage of olivine, enstatite, Cr-diopside, and Cr-spinel. The exception is the presence of a pargasitic amphibole in a lherzolite from Kaitangata. This represents the product of a reaction between Crdiopside, spinel, and an infiltrating hydrous melt.
It is concluded that the rock types of the Siberia Hill area are derived from partial melts of upper mantle peridotite. The most primitive ankaramitic basanites, nepheline basanites, and sanidine basanites have undergone the least amount of fractionation. The Siberia Hill nepheline hawaiite has undergone high pressure fractionation, evidenced by the presence of ubiquitous Group II inclusions and megacrysts.
The presence of megacrysts and Group II. xenoliths in relatively primitive magmas suggests that the upper mantle and deep crust beneath the East Otago region consists of a series of complex intersecting dikes and magma chambers that are scoured and eroded by uprising magmas. These magmas are either fresh pulses of primary liquid or are differentiating magmas held in mantle or deep crustal regions prior to eruption.

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x. 210 p. ill (some colour)., map folded in pocket., 30 cm


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Rae, AJ, “Geochemistry of the Siberia Hill Volcanics and the Ultramafic Inclusions, Siberia Hill, East Otago, New Zealand,” Otago Geology Theses, accessed May 22, 2024,

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