Chemical and isotopic trends and tectonics of the Milford Gneiss Complex and Darran Batholith of northern Fiordland, New Zealand

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Blattner, P. 2006 Chemical and isotopic trends and tectonics of the Milford Gneiss Complex and Darran Batholith of northern Fiordland, New Zealand. Lower Hutt, N.Z.: GNS Science. GNS Science report 2006/36 39 p.

Abstract: Northern Fiordland contains a variety of igneous and meta-igneous rocks, emplaced along the Median Tectonic Zone (MTZ). Near the Alpine Fault plutons and host rocks are overprinted by Cretaceous to Tertiary dynamothermal metamorphism. The tectonite fabrics define the Milford Gneiss Complex (MGC) like a screen overlay that fades eastwards into the more brittly deformed Darran batholith (D) and Mackay Intrusives. The MGC gneiss structure affects the Pembroke Granulite (PHG), of which a low-strain window remains in a sea of Milford Orthogneiss (MO), Thurso Gneiss, and other tectonised plutons and terrane shards, some more intensely deformed in the Anita Shear Zone. The MGC structure extends well into the Darran margin (Western Darran transition zone D*), and southward into the Indecision Creek Complex (ICC), and the 120 Ma old Western Fiordland Orthogneiss (WFO), turning their northernmost parts into hornblende gneiss similar to MO. Before the imprint of the gneiss-structure, and as confirmed by geochemical data, much of the Milford Orthogneiss was part of a larger mass, closely related to Pembroke Granulite. The meta-igneous units D, MO/PHG and WFO show distinctive major and trace element profiles, and widespread trondhjemite dikes (TRO) resemble parts of the ICC. Dehydration joints and chromatographic garnet reaction zones in the Pembroke Granulite indicate the small-scale migration of volatiles towards undersaturated partial melts. The granulitic WFO shares this phenomenon (which must then be less than 120 Ma old), but has much higher Sr and could be a high-pressure partial melt of material similar to MO/PHG at depth. However, although WFO is much lower than MO/PHG in Al, it also has a higher Mg/Fe ratio than MO/PHG or D. The chemical data on plutonic rocks complement isotopic age interpretations. Samples of MO/PHG indicate Paleozoic and Cretaceous ages, and both whole rock Rb-Sr and zircon U-Pb dating point to a loss of meaning of simplistic chronologies in magmatogenic conditions. The batholiths on the margins of and within the MGC have very similar, mantle related, oxygen isotope ratios, and their initial Sr ratios (Sri) correlate in some detail with accepted ages, except that MO/PHG may have had its likely Paleozoic Sri reset by exchange with the Darran batholith or a similar body. (auth)