R. J. Muir

The Cretaceous Separation Point batholith, New Zealand: granitoid magmas formed by melting of mafic lithosphere

The Early Cretaceous Separation Point batholith of the South Island, New Zealand, represents the final magmatic stage of an extensive arc system located on the SW Pacific margin of Gondwana during the Mesozoic. The batholith consists of Na-rich, alkali-calcic diorite to biotite-hornblende monzogranite. The rocks are distinct from calc-alkaline subduction-related granitoids, but comparable with those of adakite and Archaean trondhjemite-tonalite-dacite suites. Primitive Sr and Nd isotopic ratios and the absence of inherited zircon, indicate that the granitoids experienced little, if any, interaction with felsic crust. Their geochemistry is consistent with melting of a basaltic protolith of amphibolite mineralogy, either young, hot, subducted oceanic crust or newly underplated material beneath a thickened continental arc. The latter model is preferred because Separation Point rocks do not posess MORB isotopic characteristics, and cannot be explained as mixtures of MORB-melt and continental crust. Most likely it involves melting of basal arc material in response to the collision and thrusting of the arc beneath the continental margin following subduction of a back-arc basin. On the basis of strong geochemical similarities, the Early Cretaceous Western Fiordland Orthogneiss of SW New Zealand is considered to be the lower crustal equivalent of the Separation Point batholith.

Geochronology and geochemistry of a Mesozoic magmatic arc system, Fiordland, New Zealand

The Median Tectonic Zone in Eastern Fiordland, SW New Zealand, comprises a tectonically disrupted belt of Mesozoic magmatic arc rocks related to subduction along the palaeo-Pacific margin of Gondwana. New ion microprobe (SHRIMP) U–Pb zircon ages confirm that the bulk of the plutonic rocks in eastern Fiordland range from Mid-Jurassic to Early Cretaceous (168–137 Ma) in age. Carboniferous age granitoids occur in SW Fiordland, along the western side of, and within the zone. Triassic plutonic rocks appear to be restricted to the eastern side of the zone. The Mid-Jurassic–Early Cretaceous igneous rocks (collectively referred to as the Darran Suite) are cut by several plutons of Na-rich granitoid (Separation Point Suite) that give ages of c. 124 Ma, slightly older than equivalent rocks in the NW part of the South Island. Early Cretaceous granulite facies orthogneisses (126–119 Ma) in western Fiordland (Western Fiordland Orthogneiss) are considered to be the lower crustal equivalent of the Separation Point plutons. The majority of the Darran Suite rocks are I-type, hornblende-bearing calc-alkaline igneous rocks, most likely derived from melting in the mantle wedge above a subducting slab of oceanic lithosphere. In contrast, the Separation Point-type plutons are Na-rich, alkali-calcic granitoids with high concentrations of Sr (typically >500 ppm and up to 1000 ppm) and low concentrations of Y (≤5 ppm) and heavy REE (<10 times chondritic). Isotopic compositions are primitive, with 87Sr/86Sr initial ratios of c. 0.7038, and åNd values of c. +3 at 120 Ma. Their geochemistry is consistent with melting of a mafic protolith of garnet amphibolite mineralogy. Mafic Darran Suite rocks have the appropriate chemical and isotopic compositions to generate the Western Fiordland Orthogneiss and the higher level Separation Point type plutons. We suggest that the sudden appearance of large volumes of Na-rich magma during the Early Cretaceous was triggered tectonically, perhaps by thrusting of the Median Tectonic Zone arc beneath western New Zealand. Melting of basal arc underplate at depths of >40 km would then have generated Na-rich granitoids, leaving residues of garnet + clinopyroxene + amphibole.

SHRIMP U-Pb geochronology of Cretaceous magmatism in northwest Nelson-Westland, South Island, New Zealand

Abstract Ion microprobe U‐Pb zircon ages have been obtained from four samples of Cretaceous granitoid and two samples of volcanogenic sediment from the northwest Nelson‐Westland region of the South Island of New Zealand. Crow Granite, which intrudes lower Paleozoic metasedi‐mentary rocks in the Buller Terrane on the eastern side of the Karamea Batholith, has given a crystallisation age of 137 ± 3 Ma (2a). This age is typical of the Jurassic‐Early Cretaceous plutonic rocks that dominate the Median Tectonic Zone, and raises the possibility that the Western Province and the Median Tectonic Zone were linked some 20 m.y. earlier than previously proposed. The “Gouland granod‐iorite”, which forms a large pluton at the northeastern margin of the Karamea Batholith, has a crystallisation age of 119 ± 2 Ma (2a). This age is similar to the Separation Point Batholith (118 Ma), and the distinctive chemistry of the batholith (high Na, Al, Sr, and low Y) is also displayed by the Gouland granodiorite. The “Big Deep granit...

Mid-Cretaceous granitic magmatism during the transition from subduction to extension in southern New Zealand: a chemical and tectonic synthesis

Abstract Regional geochronological studies indicate that mid-Cretaceous plutonism (the Hohonu Suite at ∼110 Ma) in the Hohonu Batholith, Western Province of New Zealand, occurred during a period of rapid tectonic change in the SW Pacific portion of Gondwana. The 30–40 m.y. preceding Hohonu Suite magmatism were dominated by the subduction-related plutonism of the Median Tectonic Zone volcanic arc. Between 125–118 Ma there was a major collisional event, inferred to be the result of collision between the Median Tectonic Zone and the Western Province. This collision resulted in melting of the Median Tectonic Zone arc underplate and generation of a distinctive suite of alkali-calcic granitoids, termed the Separation Point Suite. At ∼110 Ma there was another pulse of magmatism, restricted to the Buller terrane of the Western Province, and including the Hohonu Suite granitoids. This was followed almost immediately by extension, culminating in the opening of the Tasman Sea some 30 m.y. later. The Hohonu Suite granitoids overlap temporally with the last vestiges of collisional Separation Point magmas and the onset of crustal extension in the Western Province, and thus represent magmatism in a post-collisional setting. Hohonu Suite magmas are typically calc-alkaline, but retain a chemical signature which suggests that the earlier Separation Point Suite magmas and/or sources were involved in Hohonu Suite petrogenesis. A model is proposed in which rapid isothermal uplift, resulting from the post-collisional collapse of continental crust previously thickened during the Median Tectonic Zone collision, caused melting of lower continental crust to generate the Hohonu Suite granitoids. In this example, granitoid composition is a consequence of the composition of the source rocks and the conditions present during melting, and no geochemical signature indicative of the tectonic setting during magmatism is present.

Field characteristics, petrography, and geochronology of the Hohonu Batholith and the adjacent Granite Hill Complex, North Westland, New Zealand

Abstract Detailed geological mapping, petrography, geochemistry, and geochronological studies in the Hohonu Batholith, North Westland, have identified 10 granitoid plutons emplaced during three intrusive episodes. The earliest episode is represented by a single dated Paleozoic pluton, Summit Granite (new) (381.2 ± 7.3 Ma), which is correlated with a discrete pulse of Mid—Late Devonian plutonism recognised in the Karamea Batholith. The undated Mount Graham Granite (new) is also likely to be Paleozoic, based on chemical and petrographic characteristics. The bulk of the batholith (seven plutons) was emplaced in the mid Cretaceous (114–109 Ma) and comprises two related, yet distinct, geochemical suites, which correlate with the previously defined Rahu Suite. The plutons identified are (from north to south): Pah Point Granite; Jays Creek Granodiorite (new); Uncle Bay Tonalite; Te Kinga Monzogranite; Deutgam Granodiorite; Turiwhate Granodiorite (new); and Arahura Granite (new). Mid‐Cretaceous plutonism in the W...

Mid‐Cretaceous oroclinal bending of New Zealand terranes

Abstract Recently published results and new data suggest that the Jurassic‐Cretaceous magmatic rocks of the Median Tectonic Zone of New Zealand, and the Cretaceous Separation Point Batholith that locally intrudes it, were emplaced subparallel to the Mesozoic Gondwana margin. Together, they provide a valuable piercing point on the Alpine Fault for 118 Ma. They also lie almost exactly parallel to mean extension lineations in exhumed metamorphic core complexes and extension directions indicated by fault‐bounded Cretaceous sedimentary basins and dike swarms in the overlying cover. Continental extension and subsequent breakup in the Tasman Sea and the eastern Bounty Trough was towards the northeast, almost perpendicular to the overall trend of the Gondwana margin but parallel to the margin‐related rocks in the central sector. Together, these relationships suggest almost 90° of rotation and major dextral shear. New geochronology now constrains the rotation to the period between the intrusion of the Separation P...

A Caledonian age for the Kiloran Bay appinite intrusion on Colonsay, Inner Hebrides

Synopsis A sample of hornbendite from the Kiloran Bay intrusion on the island of Colonsay in the Inner Hebrides, previously of uncertain age, has yielded a U-Pb zircon SHRIMP age of 439.0 ± 8.8 Ma (2σ). This age is the same, within error, as the Caledonian Appinite Suite on the Scottish mainland. The distinctive geochemical characteristics of the Appinite Suite, high Mg, K, Cr, Ni, Sr, Ba and the light REE, are faithfully reproduced in the Kiloran Bay intrusion. There are no published Sr and Nd isotopic data for the Appinite Suite, but the Rb-Sr and Sm-Nd isotope systematics of the Kiloran Bay rocks are comparable with the late Caledonian Granites. Proterozoic (1.7 Ga) Nd model ages and inherited zircons indicate the involvement of old crusta material during magma genesis. Correlation of the Kiloran Bay intrusion with the Appinite Suite supports a Colonsay Group–Dalradian Supergroup correlation. However, difficutlies remain in correlating the structural history of the two successions and a stratigraphic link cannot be proven.

The Colonsay Group and basement–cover relationships on the Rhinns of Islay, Inner Hebrides

Synopsis Detailed mapping on the Rhinns of Islay has prompted a revision of the stratigraphy of the lower part of the Colonsay Group (Neoproterozoic). Previously unrecorded metasedimentary rocks exposed along the SE coast rest upon and are intersliced with the Rhinns Complex (Palaeo-proterozoic basement). These metasedimentary rocks, here termed the Octofad Sandstone Formation, are considered to represent the lowest preserved part of the Colonsay Group. On the central-west side of the Rhinns, the Colonsay Group comprises four formations (Eilean Liath Grit, Kilchiaran Phyllite, Rubha Gaidhealach Grit and Rubha na h-Airde Moire Phyllite) with a total stratigraphical thickness of c. 550 m. The stratigraphy is repeated by large-scale NE–SW-trending upright folds. The basement-cover contact in central-west Islay is a major tectonic dislocation, here termed the Kilchiaran Shear Zone. An unknown thickness of stratigraphy has been excised along this contact. There has also been extensive basement–cover interslicing along the SE coast of the Rhinns.

The influence of basement structure on the evolution of the Taranaki Basin, New Zealand

The Taranaki Basin, situated offshore western New Zealand, is one of several large sedimentary basins formed during the Late Cretaceous in response to break-up of the palaeo-Pacific margin of Gondwana. A review of published structural, stratigraphic and geochronological data indicates that NE to NNE striking basement faults, generated during Palaeozoic to Mesozoic terrane accretion along the Gondwana margin, have strongly influenced the development of the basin. The main basin-bounding faults, the Cape Egmont Fault Zone and the Taranaki Fault, correspond to the boundaries of a narrow belt of plutonic rocks known as the Median Tectonic Zone. Geological data from onshore South Island suggests that right-lateral movement occurred along the boundaries of the Median Tectonic Zone during the Early Cretaceous. From the Late Cretaceous to Early Tertiary, NE to NNE striking normal faults within the Taranaki Basin controlled deposition in a series of en-echelon half-grabens and sub-basins. Many of the normal faults were later reactivated during a phase of compressional deformation associated with the development of the Australian-Pacific plate boundary through New Zealand. Reverse movement on the Taranaki Fault began in the early Miocene and deformation propagated westward reaching the Cape Egmont Fault Zone in the late Miocene to early Pliocene.

Mafic dykes within the Lewisian Complex on Tiree and Coll, Inner Hebrides

Synopsis Mafic dykes within the Lewisian Complex on Tiree and Coll were emplaced after a granulite-facies metamorphic event in the host gneisses. They were subsequently deformed and metamorphosed under amphibolite-facies conditions and cut by quartzofeldspathic pegmatites. The dykes are quartz-tholeiites and have a typical ‘continental’ trace element signature; showing enrichment in large ion lithophile and light rare earth elements and depletion in the high field strength elements. The intrusions are indistinguishable from Proterozoic (2.4–2.0 Ga) quartz-dolerite Scourie dykes of mainland Scotland, in terms of their field relationships, petrography, geochemistry and Sm-Nd isotope systematics. The occurrence of Scourie dyke-like bodies enables pre-and post-dyke events within the Lewisian on Tiree and Coll to be tentatively assigned to Scourian (2.9–2.5 Ga) and Laxfordian (1.9–1.5 Ga) events respectively. Sm-Nd whole-rock-mineral ages of c. 1.75 Ga are interpreted as dating garnet growth during the Laxfordian event and provide a minimum emplacement age for the dykes.