Petrogenesis and geochemical characterisation of ultramafic cumulate rocks from Hawes Head, Fiordland, New Zealand

Abstract

Early Cretaceous parts of the western Median Batholith (Western Fiordland Orthogneiss) represent the exposed root of a magmatic arc of dioritic to monzodioritic composition (SiO 2 051Á55 wt%; Na 2 O/K 2 O 03.7Á8.8 in this study). We characterise for the first time the field relationships, petrography, mineralogy and geochemistry of ultramafic and mafic cumulates at Hawes Head, the largest exposure of ultramafic rocks in western Fiordland. We distinguish three related rock types at Hawes Head: hornblende peridotite (MgO 021Á35 wt%); hornblendite (MgO 015Á16 wt%); and pyroxenite (MgO 021 wt%). Petrogenetic relationships between the ultramafic rocks and the surrounding Misty Pluton of the Western Fiordland Orthogneiss are demonstrated by: (i) mutually cross-cutting relationships; (ii) similar mafic phases (e.g. pyroxene and amphibole) with elevated Mg-numbers (e.g. olivine Mg/(Mg'Fe) 00.77Á0.82); (iii) fractionation trends in mineral geochemistry; and (iv) shared depleted heavy rare earth element patterns. In addition, the application of solid/liquid partition coefficients indicates that olivine in the ultramafic rocks at Hawes Head crystallised from a magma with Mg/(Mg'Fe) 00.54Á0.57. The olivine grains therefore represent a plausible early crystallising phase of the adjacent Western Fiordland Orthogneiss (Mg/(Mg'Fe) 0 0.51Á0.55).

Local partial melting of the lower crust triggered by hydration through melt–rock interaction: an example from Fiordland, New Zealand

“…A model of melt flux, rather than injection and freezing, may have important ramifications for the chemistry of arc magmas at higher crustal levels. The geochemistry of arc lavas indicates cryptic fractionation of amphibole in arc magma petrogenesis via the formation of amphibole cumulates (Davidson et al ., ; Daczko et al ., ). In addition, the reaction–replacement of cumulate clinopyroxene by amphibole in the lower arc crust is also proposed (Smith, ).…”

Origin of the pleonaste-bearing mafic-ultramafic rocks from the Armutlu peninsula, NW Turkey

“…For example, the presence of Al-rich spinels in pelitic and basic-ultrabasic rocks is commonly taken as evidence for amphibolite or granulite facies metamorphism (e.g., Evans and Frost, 1975;Bucher and Frey, 2002;Rodriguez et al, 2012;Gargiulo et al, 2013). On the other hand, it is stated that the Al-rich spinels can primarily crystallize from basaltic melts (e.g., Della-pasqua et al, 1995;Babu et al, 1997;Ho et al, 2000;Melcher et al, 2002;Amortegui et al, 2011;Daczko et al, 2012), or formed by metasomatic processes (e.g., Claeson, 1998;Franz and Wirth, 2000).…”

“…In the present study Al-rich spinel is used as a general name for spinel, hercynite, and pleonaste. The Al-rich spinel solid solution series have been observed in the amphibolite-granulite facies basic-ultrabasic and pelitic rocks, the basic-ultrabasic magmatic rocks, and the mantle xenoliths in volcanic rocks (e.g., Evans and Frost, 1975;Babu et al, 1997;Ho et al, 2000;Bucher and Frey, 2002;Topuz et al, 2004;Amortegui et al, 2011;Daczko et al, 2012;Rodriguez et al, 2012;Gargiulo et al, 2013). Diverse origins, such as magmatic, metamorphic, and metasomatic, have been proposed for the Al-rich spinel occurrences in these rocks (e.g., Evans and Frost, 1975;Della-Pasqua et al, 1995;Claeson, 1998;Franz and Wirth, 2000;Berger et al, 2010;Daczko et al, 2012;Gargiulo et al, 2013).…”

The Neoarchean ultramafic–mafic complex in the Yinshan Block, North China Craton: Magmatic monitor of development of Archean lithospheric mantle

“…They can occur as enclaves in dioritic, sanukitic or alkaline basaltic magma, which are considered to be the products of mineral fractionation and accumulation from a water-rich magma (e.g. McPherson et al, 1996;Daczko et al, 2012;Berger et al, 2013). Hornblendites can also be a mafic component of appinite suites, which are spatially and temporally associated with coeval amphibole-bearing granitoids (e.g.…”