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

摘要

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 =51-55 wt%; Na_2O/K_2O =3.7-8.8 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 =21-35 wt%); homblendite (MgO = 15-16 wt%); and pyroxenite (MgO =21 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) =0.77-0.82); (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) =0.54-0.57. The olivine grains therefore represent a plausible early crystallising phase of the adjacent Western Fiordland Orthogneiss (Mg/(Mg+Fe) = 0.51-0.55).