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The Hohonu Batholith of North Westland, New Zealand: granitoid compositions controlled by source H2O contents and generated during tectonic transition
Authors:Tod E Waight  Stephen D Weaver  Roderick J Muir  Roland Maas  G Nelson Eby
Institution:(1) Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand, NZ;(2) School of Earth Sciences, La Trobe University, Bundoora, Victoria 3083, Australia, AU;(3) Department of Environmental, Earth and Atmospheric Sciences, University of Massachusetts, Lowell, Massachusetts 01854, USA, US
Abstract:Geochemical studies on the Hohonu Batholith, of the West Coast, South Island, New Zealand, have recognised two distinct but chemically related suites of mid-Cretaceous granitoids. The suites are characterised by restricted radiogenic isotopic compositions (Sr(i) = 0.7062 to 0.7085; ɛNd(i) = −4.4 to −6.1), and represent melting of a mafic lithosphere source followed by interaction with Ordovician metasediments. The two suites (Te Kinga Suite and Deutgam Suite) are distinguished by contrasting contents of Al2O3, Na2O, Sr, Ba, Eu and HREE, attributable to different residual asssemblages controlled by differing H2O contents during melting of a metabasaltic source. The relatively mafic, metaluminous, I-type Deutgam Suite represents magmas derived by dehydration melting in equilibrium with an amphibolitic (plagioclase + amphibole) residue. In contrast, the peraluminous, high silica compositions of the Te Kinga Suite were produced by melting at higher H2O contents, reducing the stability of plagioclase and resulting in a melt in equilibrium with a plagioclase-free eclogitic (garnet + amphibole) residue. Residual plagioclase during generation of the Deutgam Suite resulted in lower Al2O3, Na2O, Sr, Ba and Eu contents, whereas residual garnet during generation of the Te Kinga suite resulted in depleted HREE contents. The mid-Cretaceous granitoids of the Hohonu Batholith were generated during a period of rapid tectonic transition from crustal thickening during collision to crustal thinning and core complex formation during extension. Received: 23 July 1996 / Accepted: 21 August 1997
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