Geology of the crystalline basement between Milford Sound and the Hollyford Valley, New Zealand

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Abstract
The crystalline basement between Milford Sound and the Upper Hollyford Valley, east of the Alpine Fault, New Zealand, consists largely of intrusive, layered to massive gabbroic rocks, which grade autometamorphically into hornblende-diorites both on a local scale along joints and dikes of the Darran Mts, and regionally from the Central Darran Mts westwards towards Milford. West of the locality Milford, which lies at the head of Milford Sound, the still predominant basic intrusives, and possible former sediments and volcanics, have been subject to dynamothermal amphibolite facies metamorphism with westward increasing deformation. Garnet and muscovite are abundant metamorphic minerals of this belt and do not occur further east. Pronounced mesoscopic structures west of Milford indicate two stages of folding, the last one characterised by the predominant subvertical foliation and axial planes striking N 20° E. The term Darran Basic Complex is introduced for the basic intrusives, which are essentially unmetamorphic east of, and metamorphic west of Milford. K-Ar dating of well preserved unmetamorphic hornblendes suggests a Jurassic minimum age of consolidation. The metamorphic event in the western part of the area is associated with the Rangitata Orogeny (⩽; 140 m.y.) and early formation of the Alpine Fault. Enclaves of older granulite facies hypersthene-metadiorites (hornblende-granolite) are preserved among the amphibolite facies gneisses; they contain planar zones of gamet-pyroxene-granolite whose distribution imitates a joint pattern. The granulite facies metamorphism has taken place after intrusion of the Darran Basic Complex, but before or early during the Rangitata Orogeny. At their eastern margin, the gabbro and diorite of the Basic Complex give way to a belt of foliated N-S-striking leucoquartzdiorites. Both rock-suites contain significant magnetite, but their mutual genetic relation is uncertain. Unusual depletion in 18O of some of the leucoquartzdiorites may be due to high temperature interaction with meteoric ground water. The Milford-Hollyford terrain may contain parts of an early Mesozoic continental margin that became isolated through the north-eastward removal of the former western mainland by strike-slip faulting. The terrain now lies over an east-facing incipient subduction zone.