Basement geology, northern Franklin Mountains, northern Fiordland, New Zealand : with emphasis on the origin and evolution of Fiordland granulites
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This report integrates results of field, petrographic, mineral-chemical, geochemical, and geochronological studies of amphibolite and granulite facies metamorphites and plutonic rocks in a large area (>1000 km2) of northern Fiordland. The area is dominated by dioritic orthogneisses belonging to a number of mapably distinct metaplutonic complexes, chief among them being the Early Cretaceous Western Fiordland Orthogneiss, the Early Cretaceous Indecision Creek Complex, and the at least in part Paleozoic Arthur River Complex.
Regionally extensive granulite facies rocks are restricted to the Western Fiordland Orthogneiss (WFO). The bulk of this unit consists of a distinctive suite of two-pyroxene dioritic to minor monzonitic rocks which are concordantly intrusive into a varied country rock consisting of amphibolite facies metasediments and older orthoqneisses. Relict igneous textures in weakly recrystallized rooks indicate that the two-Pyroxene granulite tacies assemblage is inherited from the magmatic assemblage: it does not result from prograde dehydration of initially hydrous assemblages. These rocks have similar mineral compositions and whole rock chemical trends to mangeritic-jotunltic suites characteristic of many Precambrian granulite terranes.
The two-pyroxene bearing granulitic rocks are locally criss-crossed by networks of veins and fracture systems in which hornblende is replaced in patchy and diffuse mesoscopic reaction zones by garnet granulite (garnet ± clinopyroxene + quartz). This replacement reflects local lowering of aH2O in response to influx of mantle-denved CO2 -rich fluidS along channelways formed by fractures. Local isofacial development of eclogitic rocks is attributed to dehydration of hornblendite.
The granulitic rocks have experienced two major phases of deformation: i) an early synmagmatic phase which accompanied emplacement of the WFO and resulted in development of pervasive planar and linear fabrics: and 2) a later phase associated with development of locally extensive ductile shear zones and retrogression to amphibolite facies. The two phases were punctuated by a static, interkinematic period of crystallization, at which time the garnet granulite reaction zones formed.
Metapelitic rafts within the WFO are migmatitic and have locally undergone extensive partial melting at sillimanite + K-feldspar grade to form granite gneiss. Some of these rocks preserve a Proterozoic Sm-Nd and Rb-Sr isotopic signature interpreted as the average age of the sediment source terrane. Some rafts of granitic to tonalitic composition have been extensively mobilized and now show complex intrusive relationships with the enclosing WFO.
Two distinct phases of regional metamorphism are recognized. The first, a synkinematic high-T low-P event (M(1)), is responsible tor andalusite - sillimanite facies series metamorphism in the countrv rock: it is attributed to magmatic heat input into the "middle" crust by emplacement of the WFO and other voluminous Early Cretaceous intrusions. This was followed by a high pressure, largely interkinematic phase of metamorphism (M(2)) recorded in both the country rock and Western Fiordland Orthogneiss. Influx of CO2 and related development of garnet granulite and eclogite occurred during this event. Physical conditions of crystallization are estimated from mineral thermometry and barometry to be -650-700°C at 10-13 kb.
Strongly zoned garnets coexisting with Al2SiO5 minerals in metapelitic rafts, country rock gneisses and contamination zones overprinting WFO, show a continuous core to rim increase in grossular content. Zoned clinopyroxenes coexisting with plagioclase and quartz show a core to rim increase in jadeite content. Computed phase equilibria in both systems are compatible with a pressure increase of >6 kb occurring during mineral crystallization.
The increase in pressure is equivalent to onloading of material of ~20 km thickness over rocks already buried at depths of 15-20 km. Crustal thickening is attributed to overthrusting during a hitherto unrecognized major plate boundary collision event. Isotopic age constraints bracket collision, high pressure metamorphism, and final Uplift and cooling of the WFO to -300-400°C in the interval between -120 and 100 Ma. Uplift rates following high pressure metamorphism may have been as rapid as 3.5 mm/yr.
In eastern Fiordland, essentially unmetamorphosed gabbrohorites of the Early Cretaceous Darran Complex are separated from rocks of the high pressure metamorphic terrane by a profound structural break (the Surprise Creek Fault). This boundary is represented by a narrow, steeply dipping zone of intense ductile deformation. Although some previous investigators have suggested that the gabbronorites represent the protolith of the granulite facies rocks exposed further west, it is shown here that the rocks not only have different ages, but also different isotopic and major and trace element characteristics suggesting derivation from separate mantle sources.
The Early Cretaceous magmatic and tectono-metamorphic history of Fiordland has important implications for the evolution of relative plate motions and Mesozoic terrane, accretion along the New Zealand segment of the Gondwana margin.
Regionally extensive granulite facies rocks are restricted to the Western Fiordland Orthogneiss (WFO). The bulk of this unit consists of a distinctive suite of two-pyroxene dioritic to minor monzonitic rocks which are concordantly intrusive into a varied country rock consisting of amphibolite facies metasediments and older orthoqneisses. Relict igneous textures in weakly recrystallized rooks indicate that the two-Pyroxene granulite tacies assemblage is inherited from the magmatic assemblage: it does not result from prograde dehydration of initially hydrous assemblages. These rocks have similar mineral compositions and whole rock chemical trends to mangeritic-jotunltic suites characteristic of many Precambrian granulite terranes.
The two-pyroxene bearing granulitic rocks are locally criss-crossed by networks of veins and fracture systems in which hornblende is replaced in patchy and diffuse mesoscopic reaction zones by garnet granulite (garnet ± clinopyroxene + quartz). This replacement reflects local lowering of aH2O in response to influx of mantle-denved CO2 -rich fluidS along channelways formed by fractures. Local isofacial development of eclogitic rocks is attributed to dehydration of hornblendite.
The granulitic rocks have experienced two major phases of deformation: i) an early synmagmatic phase which accompanied emplacement of the WFO and resulted in development of pervasive planar and linear fabrics: and 2) a later phase associated with development of locally extensive ductile shear zones and retrogression to amphibolite facies. The two phases were punctuated by a static, interkinematic period of crystallization, at which time the garnet granulite reaction zones formed.
Metapelitic rafts within the WFO are migmatitic and have locally undergone extensive partial melting at sillimanite + K-feldspar grade to form granite gneiss. Some of these rocks preserve a Proterozoic Sm-Nd and Rb-Sr isotopic signature interpreted as the average age of the sediment source terrane. Some rafts of granitic to tonalitic composition have been extensively mobilized and now show complex intrusive relationships with the enclosing WFO.
Two distinct phases of regional metamorphism are recognized. The first, a synkinematic high-T low-P event (M(1)), is responsible tor andalusite - sillimanite facies series metamorphism in the countrv rock: it is attributed to magmatic heat input into the "middle" crust by emplacement of the WFO and other voluminous Early Cretaceous intrusions. This was followed by a high pressure, largely interkinematic phase of metamorphism (M(2)) recorded in both the country rock and Western Fiordland Orthogneiss. Influx of CO2 and related development of garnet granulite and eclogite occurred during this event. Physical conditions of crystallization are estimated from mineral thermometry and barometry to be -650-700°C at 10-13 kb.
Strongly zoned garnets coexisting with Al2SiO5 minerals in metapelitic rafts, country rock gneisses and contamination zones overprinting WFO, show a continuous core to rim increase in grossular content. Zoned clinopyroxenes coexisting with plagioclase and quartz show a core to rim increase in jadeite content. Computed phase equilibria in both systems are compatible with a pressure increase of >6 kb occurring during mineral crystallization.
The increase in pressure is equivalent to onloading of material of ~20 km thickness over rocks already buried at depths of 15-20 km. Crustal thickening is attributed to overthrusting during a hitherto unrecognized major plate boundary collision event. Isotopic age constraints bracket collision, high pressure metamorphism, and final Uplift and cooling of the WFO to -300-400°C in the interval between -120 and 100 Ma. Uplift rates following high pressure metamorphism may have been as rapid as 3.5 mm/yr.
In eastern Fiordland, essentially unmetamorphosed gabbrohorites of the Early Cretaceous Darran Complex are separated from rocks of the high pressure metamorphic terrane by a profound structural break (the Surprise Creek Fault). This boundary is represented by a narrow, steeply dipping zone of intense ductile deformation. Although some previous investigators have suggested that the gabbronorites represent the protolith of the granulite facies rocks exposed further west, it is shown here that the rocks not only have different ages, but also different isotopic and major and trace element characteristics suggesting derivation from separate mantle sources.
The Early Cretaceous magmatic and tectono-metamorphic history of Fiordland has important implications for the evolution of relative plate motions and Mesozoic terrane, accretion along the New Zealand segment of the Gondwana margin.
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xvii, 379 pages, plates in back pocket
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OU geology Identifier:
1986Bradshaw
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Bradshaw, John Yates, “Basement geology, northern Franklin Mountains, northern Fiordland, New Zealand : with emphasis on the origin and evolution of Fiordland granulites,” Otago Geology Theses, accessed February 10, 2026, https://theses.otagogeology.org.nz/items/show/196.