The petrology, geochemistry and petrogenesis of the Riwaka Complex, South Island, New Zealand

Saunders, Jeffrey Keith (1997) The petrology, geochemistry and petrogenesis of the Riwaka Complex, South Island, New Zealand. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Riwaka Complex is a tholeiitic igneous intrusion (40km long, up to several km wide) located in the northwestern part of the South Island of New Zealand. It intrudes the Takaka Terrane which represents an assemblage of sedimentary rocks that are part of New Zealand's geologic Western Province. The Takaka Terrane is thought to have formed in a fore-arc environment associated with either one or two westward dipping Benioff zones. -- In this subduction zone environment the tholeiitic magma crystallized in a manner similar to that, that form Alaskan - type ultramafic complexes. Its distinctive petrological features include: cumulate olivine and clinopyroxene and interstitial amphibole; absence of orthopyroxene and plagioclase; and little or no Fe - enrichment, but a Ca - enrichment in the clinopyroxenes. The Riwaka Complex's geochemical evolution was dominated by olivine and clinopyroxene fractionation from as many as five pulses of magma, which gave rise to an intrusion made up almost entirely of ultramafic rocks ranging from clinopyroxene - bearing dunites to olivine - hornblende clinopyroxenites. Some minor gabbros and diorites are also present. -- The small amount of gabbro present represents an important part of the Riwaka Complex as these rocks contain significant amounts of primary sulphides which are rich in nickel, copper and platinum group elements. The nickel and copper ratios indicate that the composition of the parental magma was similar to that of a komatiite. This magma evolved to a basaltic type composition which gave rise to the gabbroic rocks. The high temperature platinum group elements (Os, Ir and Ru) are not found in the same abundances within the gabbroic rocks as their lower temperature counterparts (Rh, Pt and Pd) indicating that they have undergone an earlier fractionation, possibly within the early crystallized ultramafic rocks. These rocks may therefore represent an important exploration target.

Item Type: Thesis (Masters)
URI: http://research.library.mun.ca/id/eprint/6561
Item ID: 6561
Additional Information: Bibliography: leaves 138-147.
Department(s): Science, Faculty of > Earth Sciences
Date: 1997
Date Type: Submission
Geographic Location: New Zealand--South Island
Library of Congress Subject Heading: Petrology--New Zealand--South Island; Geochemistry--New Zealand--South Island; Petrogenesis--New Zealand--South Island

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