Emplacement and Paleozoic and Cretaceous recrystallisation of the Broughton Arm Peridotite in Western Fiordland, New Zealand

摘要

Ultramafic rocks are rare on Earth's surface but can provide important insights into crust and mantle evolution. The recently discovered Broughton Arm Peridotite in the mountains of central Fiordland is exposed as two 75-150m wide by several hundred metres-long harzburgite-dunite lobes encased sequentially in hornblendite, amphibolite and then quartzofeldspathic gneiss. On the basis of the refractory harzburgitic-dunitic composition and the bulk rock Mg# (86-87) lower than most mantle peridotites, the peridotite is interpreted to have an igneous cumulate rather than an exhumed mantle peridotite origin. The marginal hornblendite and amphibolite and internal pods of edenite-diopside rock may represent metasomatised peridotite. The Broughton Arm Peridotite has been metamorphosed twice. The first event generated an anhydrous assemblage comprising olivine (Mg#=79.4-92.8), enstatite and Cr-magnetite. This assemblage has been overprinted by an anastomosing weak to pervasive mylonitic foliation of Mg-chlorite, tremolite +/- serpentinite +/- talc. The age of the anhydrous assemblage is unknown but suspected from regional data to be Early Carboniferous. A titanite Pb-208/U-238 lower intercept age of 105.6 +/- 6.8Ma in the host Deep Cove Gneiss is correlated with the peridotite hydration-deformation event. The proximity of the Early Cretaceous ( 114Ma) Western Fiordland Orthogneiss to the east, west and north means that the Broughton Arm Peridotite and surrounding rocks may have recrystallised within the extensional Doubtful Sound Shear Zone, which elsewhere is an up to several hundred meter-wide shear zone active at c. 110-100Ma separating Western Fiordland Orthogneiss from Deep Cove Gneiss. The Broughton Arm Peridotite therefore preserves a history that involves emplacement in the Early Paleozoic, penetrative anhydrous metamorphism in the Early Carboniferous, and hydration and recrystallisation during regional extensional tectonism in the Early Cretaceous.