Plutonic petrogenesis and mineralisation in southwest Fiordland

Gollan, Malcolm Russell, 1972-

Southwest Fiordland contains an important record of early Cretaceous plutonism, which is not complicated by a polyphase metamorphic overprint as in central and northern Fiordland. Recent mapping as part of the GNS Science Qmap Fiordland programme has identified multiple Paleozoic through Mesozoic plutons composing the previously undivided plutonic units in southwest Fiordland. Field, petrographic, geochemical, and geochronologic data reveal a transition in time and composition between early calc-alkaline Darran Suite and later ‘adakitic-like’ Separation Point Suite. The Revolver Pluton (Rp) of c.200 km2 is the largest unit. It is a calc-alkaline I-type, coarse grained biotite granite with distinctive pink alkali feldspar megacrysts. LA-ICP-MS zircon UPb dating of Rp from Revolver Bay yields an age of 132.4 ± 1.0 Ma. Granodiorite from the northeast sector of Rp, here named the Long Scarp Granodiorite (Lsg), gives an age of 133.1 ± 0.8 Ma and has characteristics requiring high fH2O during melting similar to the Separation Point Suite. Treble Mountain Granite (Tmg) is medium to coarse grained biotite granite that intrudes Rp on the west side of Isthmus Sound, and yields an age of 130.4 ± 0.9 Ma. Tmg locally exhibits severe hydrothermal alteration and hosts epithermal base-metal vein mineralisation which was worked in the historic Tarawera Mine. Trevaccoon Diorite (Tdi) is a hornblende gabbro that outcrops along the western shore of Long Sound north of Lady Bay and has an age of 128.7 ± 1.0 Ma. A younger granite, here named Upper Blacklock Granite (Ubg), occurs between Rp and Lsg. Ubg is petrographically similar to Rp, but can be distinguished on the basis of its geochemistry and age of 124.7 ± 1.0 Ma. Hornblende-rich HiSY diorite to gabbro, here named Only Island Diorite (Oid), outcrops south from Only Island where it intrudes Ubg. The composition and age of 122.1 ± 0.9 Ma of Oid suggest it is a higher level, unmetamorphosed equivalent of the Western Fiordland Orthogneiss. Nd isotopes (εNd = 0 - +1), oxygen isotopes (whole rock δ18O = 6-9, quartz δ18O = 8-10) for all units other than Tmg, are consistent with a isotopically depleted uniform mafic source, while a rarity of older inherited zircons, limit contamination by the surrounding Ordovician metasediments (δ18O whole rock = 12-18). The presence of 15% inherited 149-136 Ma zircons in all units except Oid suggests a shared contamination by a component comparable in age to Darran Suite rocks in eastern Fiordland. Tmg quartz and feldspar δ18O values are strongly depleted indicating that hydrothermal alteration was caused by circulation of meteoric water. This supports the interpretation that plutons in the region were intruded at shallow crustal levels (< 10 km). Three Paleozoic ages were obtained; one from Big Pluton (Bp, formally Kakapo Granite) which has to date not yielded a well resolved age. The sample located in the eastern portion of Bp yields an age of 367 ± 2 Ma. This sample is geochemically distinct from western portions of Bp where previous dating has been attempted, which is attributed to different melting conditions of a shared source. Two samples of the Tine Peak Tonalite (Tpt) yielded ages of 347 ± 3 Ma and 352 ± 2 Ma. Tpt shows extreme depletion in alkalis, while showing often extreme enrichment in HFS elements. Tpt most likely represents a previously unrecognised extremely high temperature A-type melt of a dominantly restitic metasedimentary source, which has previously yielded an S-type melt. Initial Nd (εNd = -4.6 and -1.9) and Sr (Sri = 0.707537 and 0.707136) isotopes and oxygen isotopes (whole rock δ18O = 12.3) are consistent with such a source.

210 p. : ill., maps ; 30 cm. + 1 map..

2006Gollan

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