Ar/Ar thermochronologic constraints on deformation, metamorphism and cooling/exhumation of a Mesozoic accretionary wedge, Otago Schist, New Zealand

Structural thickening of the Torlesse accretionary wedge via juxtaposition of arc-derived greywackes (Caples Terrane) and quartzo-feldspathic greywackes (Torlesse Terrane) at 120 Ma formed a belt of schist (Otago Schist) with distinct mica fabrics defining (i) schistosity, (ii) transposition layering and (iii) crenulation cleavage. Thirty-five ⁴⁰Ar/³⁹Ar step-heating experiments on these micas and whole rock micaceous fabrics from the Otago Schist have shown that the main metamorphism and deformation occurred between 160 and 140 Ma (recorded in the low grade flanks) through 120 Ma (shear zone deformation). This was followed either by very gradual cooling or no cooling until about 110 Ma, with some form of extensional (tectonic) exhumation and cooling of the high-grade metamorphic core between 109 and 100 Ma. Major shear zones separating the low-grade and high-grade parts of the schist define regions of separate and distinct apparent age groupings that underwent different thermo-tectonic histories. Apparent ages on the low-grade north flank (hanging wall to the Hyde-Macraes and Rise and Shine Shear Zones) range from 145 to 159 Ma (n=8), whereas on the low-grade south flank (hanging wall to the Remarkables Shear Zone or Caples Terrane) range from 144 to 156 Ma (n=5). Most of these samples show complex age spectra caused by mixing between radiogenic argon released from neocrystalline metamorphic mica and lesser detrital mica. Several of the hanging wall samples with ages of 144–147 Ma show no evidence for detrital contamination in thin section or in the form of the age spectra. Apparent ages from the high-grade metamorphic core (garnet–biotite–albite zone) range from 131 to 106 Ma (n=13) with a strong grouping 113–109 Ma (n=7) in the immediate footwall to the major Remarkables Shear Zone. Most of the age spectra from within the core of the schist belt yield complex age spectra that we interpret to be the result of prolonged residence within the argon partial retention interval for white mica (430–330 °C). Samples with apparent ages of about 110–109 Ma tend to give concordant plateaux suggesting more rapid cooling. The youngest and most disturbed age spectra come from within the ‘Alpine chlorite overprint’ zone where samples with strong development of crenulation cleavage gave ages 85–107 and 101 Ma, due to partial resetting during retrogression. The bounding Remarkables Shear zone shows resetting effects due to dynamic recrystallization with apparent ages of 127–122 Ma, whereas overprinting shear zones within the core of the schist show apparent ages of 112–109 and 106 Ma. These data when linked with extensional exhumation of high-grade rocks in other parts of New Zealand indicate that the East Gondwana margin underwent significant extension in the 110–90 Ma period.     

Metamorphic discontinuities in orogenic belts: example of the garnet–biotite–albite zone in the Otago Schist, New Zealand

A regional petrographic reconnaissance of psammitic and pelitic rocks in the Otago Schist, New Zealand, has revealed the presence of garnet (“grossalspite” with typical rim composition almandine₄₁, spessartine₂₅, grossular₃₃, pyrope₁) and biotite in 37 new samples, more than doubling the previously known number. A new garnet–biotite–albite zone can now be defined in the greenschist facies Otago Schist that is distinct from the better-known biotite, garnet and oligoclase zones in the along-strike Alpine Schist. The garnet–biotite–albite zone is in part metamorphically discontinuous with adjacent schists and does not support models of simple, continuous, progressive Jurassic regional metamorphism in Otago. The structurally higher (lower grade) boundary of the zone coincides in at least three places with previously mapped regional shear zones. The structurally lower (expected higher grade) boundary of the zone appears to be obliterated by a chlorite zone overprint which can be spatially related to Alpine Schist recrystallisation of ?Cretaceous age. The Otago situation serves as an example of the subtle metamorphic discontinuities that probably pervade many orogenic belts.     

New Zealand's Geological Foundations

New Zealand is a fragment of Gondwana that, before Late Cretaceous sea floor spreading, was contiguous with Australia and Antarctica. Only about 10% of the area of continental crust in the wider New Zealand region (Zealandia) is emergent above sea level as the North and South Islands. No Precambrian cratonic core is exposed in onland New Zealand. The Cambrian to Early Cretaceous basement can be described in terms of nine major volcano-sedimentary terranes, three composite regional batholiths, and three regional metamorphic-tectonic belts that overprint the terranes and batholiths.The terranes (from west to east) are: Buller, Takaka, Brook Street, Murihiku, Maitai, Caples, Bay of Islands (part of former Waipapa), Rakaia (older Torlesse) and Pahau (younger Torlesse). The western terranes are intruded by three composite batholith (>100 km²) sized belts of plutons: Karamea-Paparoa, Hohonu and Median, as well as by numerous smaller plutons. Median Batholith (including the Median Tectonic Zone) is a recently-recognised Cordilleran batholith that represents the site of subduction-related magmatism from ca. 375–110 Ma. Parts of the terranes and batholiths are variably metamorphosed and deformed: Devonian and Cretaceous amphibolite-granulite facies gneisses are present in Buller, Takaka, Median and Karamea-Paparoa units; Jurassic-Cretaceous subgreenschist-amphibolite facies Haast Schist overprints the Caples, Bay of Islands and Rakaia Terranes; Cretaceous subgreenschist facies Esk Head and Whakatane Mélanges bound the Pahau Terrane. In the South Island, small areas (<5 km² total) of Devonian, Permian, Triassic and Jurassic Gondwana sequences have been identified. In the North Island a widespread Late Jurassic overlap sequence, Waipa Supergroup (part of former Waipapa Terrane), has recently been proposed.     

石榴子石Lu-Hf年代学研究进展

高Lu/Hf比的石榴子石可以用于构筑高精度的Lu-Hf等时线,因而石榴子石Lu-Hf法被广泛地应用于各类岩石的定年研究中。特别是在造山带研究中,石榴子石Lu-Hf法揭示了许多其他定年体系所不能揭示的信息。但是由于石榴子石生长历史复杂,影响石榴子石 Lu-Hf 定年结果的因素多样,合理解释石榴子石Lu-Hf年龄指示的地质意义成为石榴子石Lu-Hf年代学研究的难点。这些因素包括石榴子石的生长模式、石榴子石的成核过程、封闭温度、包裹体、石榴子石生长历史、矿物间元素平衡、几何效应以及样品的预处理方法等。在进行石榴子石 Lu-Hf 定年研究时需要对这些因素进行综合的判定,才能对年龄做出合理的解释。本文初步探讨了这些因素对石榴子石 Lu-Hf 定年结果的影响,并以苏鲁-大别造山带为例介绍了石榴子石Lu-Hf年代学在碰撞造山带研究中的进展,揭示了石榴子石Lu-Hf体系在造山带及超高压变质研究中的巨大潜力。     

Stable isotopic and fluid inclusion evidence for meteoric fluid penetration into an active mountain belt; Alpine Schist, New Zealand

Calcite and quartz veins have formed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt of New Zealand. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle-ductile transition, which has been raised to 5-6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H₂O + NaCl-CO₂ mixtures at 200-350 C. Bulk fluid composition is 15-20 mol% CO₂ and <4.3 total mol CH₄+ N₂+ Ar/100mol H₂O. Water hydrogen isotopic ratio δD_H2O in the fissure veins spans -29 to -68‰, δ¹⁸O_H2O -0.7 to 8.5‰, and bulk carbon isotopic ratio δ¹³C ranges from -3.7 to -11.7‰. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be generated by oxidation of graphite.     

The lithospheric geodynamics of plate boundary transpression in New Zealand: Initiating and emplacing subduction along the Hikurangi margin, and the tectonic evolution of the Alpine Fault system

In contrast to the normal ‘Wilson cycle’ sequence of subduction leading to continental collision and associated mountain building, the evolution of the New Zealand plate boundary in the Neogene reflects the converse—initially a period of continental convergence that is followed by the emplacement of subduction. Plate reconstructions allow us to place limits on the location and timing of the continental convergence and subduction zones and the migration of the transition between the two plate boundary regimes. Relative plate motions and reconstructions since the Early to Mid-Miocene require significant continental convergence in advance of the emplacement of the southward migrating Hikurangi subduction—a sequence of tectonism seen in the present plate boundary geography of Hikurangi subduction beneath North Island and convergence in the Southern Alps along the Alpine Fault. In contrast to a transition from subduction to continental convergence where the leading edge of the upper plate is relatively thin and deformable, the transition from a continental convergent regime, with its associated crustal and lithospheric thickening, to subduction of oceanic lithosphere requires substantial thinning (removal) of upper plate continental lithosphere to make room for the slab. The simple structure of the Wadati–Benioff zone seen in the present-day geometry of the subducting Pacific plate beneath North Island indicates that this lithospheric adjustment occurs quickly. Associated with this rapid lithospheric thinning is the development of a series of ephemeral basins, younging to the south, that straddle the migrating slab edge. Based on this association between localized vertical tectonics and slab emplacement, the tectonic history of these basins records the effects of lithospheric delamination driven by the southward migrating leading edge of the subducting Pacific slab. Although the New Zealand plate boundary is often described as simply two subduction zones linked by the transpressive Alpine Fault, in actuality the present is merely a snapshot view of an ongoing and complex evolution from convergence to subduction.     

An Australian provenance for the eastern Otago Schist protolith,South Island,New Zealand: evidence from detrital zircon age patterns and implications for the origin of its gold

Uranium–lead age patterns of detrital zircons in Otago Schist meta-sandstones from eastern Otago, including areas of orogenic gold mineralisation, are mostly consistent with a Rakaia Terrane (Torlesse Composite Terrane) accretionary wedge protolith. Southwest of the Hyde-Macraes and Rise & Shine shear zones the depositional age is regarded as Middle–Late Triassic. At the south and west margins, there are two areas in the Late Triassic Waipapa Terrane protolith. Northeast of the Hyde-Macraes Shear Zone, the schist protolith has Middle to Late Triassic and middle to late Permian depositional ages of Rakaia Terrane affinity. At the northeastern margin of the Hyde-Macraes Shear Zone, there is a narrow strip with a mid-Carboniferous protolith, which may be a counterpart of the Carboniferous accretionary wedge in the New England Orogen, eastern Australia. Ordovician–Silurian zircons are a minor but distinctive feature in many of the protolith age patterns and form significant age components at hard-rock gold locations. These constrain the provenance of Rakaia Terrane protolith sediments to Late Triassic time and within the Permian–Triassic magmatic arcs at the northeastern Australian continental margin and partly within the Ordovician–Silurian granitoids of the Charters Towers Province hinterland and environs. The latter have extensive gold mineralisation and thus upon exhumation might be the origin of Otago gold.     

Shallow-level metamorphic fluids in a high uplift rat metamorphic belt; Alpine Schist, New Zealand

Abstract Fluids, some of which are CO₂-rich (up to 40 mol.% CO₂) and some of which are highly saline (up to 18 wt% NaCl equivalent), are trapped as fluid inclusions in quartz-calcite (∼ metallic minerals) veins which cross-cut the pumpellyite-actinolite to amphibolite facies rocks of the Alpine Schist. Fluids were commonly trapped as immiscible liquid-vapour mixes in quartz and calcite showing open-space growth textures. Fluid entrapment occurred at fluid pressures near 500 bars (possibly as low as 150 bars) at temperatures ranging from 260 to 330° C. Saline fluids may have formed by partitioning of dissolved salts into an aqueous phase on segregation of immiscible fluids from a low-density CO₂-rich fluid. Calcite deposited by these fluids has δ¹³C ranging from – 8.4 to – 11.5 and δ¹⁸O from + 4 to + 13. Isotopic data, fluid compositions and mode of occurrence suggest that the fluids are derived from high-grade metamorphic rocks. Fluid interaction with wall-rock has caused biotite crystallization and/or recrystallization in some rocks and retrogression of biotite to chlorite in other rocks.
Fluid penetration through the rock is almost pervasive in many areas where permeability, probably related to Alpine Fault activity, has focussed fluids on a regional scale into fractured rocks. The fluid flow process is made possible by high uplift-rates (in excess of 10 mm/year) bringing hot rocks near to the surface.     

Detrital zircon U–Pb geochronology of Permian strata in the Galilee Basin,Queensland, Australia

The late Carboniferous to Triassic tectonic history of eastern Australia includes important periods of regional-scale crustal extension and contraction. Evidence for these periods of tectonism is recorded by the extensive Pennsylvanian (late Carboniferous) to Triassic basin system of eastern Australia. In this study, we investigate the use of U–Pb dating of detrital zircons in reconstructing the tectonic development of one of these basins, the eastern Galilee Basin of Queensland. U–Pb detrital zircon ages were obtained from samples of stratigraphically well-constrained Cisuralian and Lopingian (early and late Permian, respectively) sandstone in the Galilee Basin. Detrital zircons in these sandstones are dominated by a population with ages in the range of 300–250 Ma, and ages from the youngest detrital zircons closely approximate depositional ages. We attribute these two fundamental findings to (1) appreciable derivation of detrital zircons in the Galilee Basin from the New England Orogen of easternmost Australia and (2) syndepositional magmatism. Furthermore, Cisuralian sandstone of the Galilee Basin contains significantly more >300 Ma detrital zircons than Lopingian sandstone. The transition in detrital zircon population, which is bracketed between 296 and 252 Ma based on previous high-precision U–Pb zircon ages from Permian ash beds in the Galilee Basin, corresponds with the Hunter–Bowen Orogeny and reflects a change in the Galilee Basin from an earlier extensional setting to a later foreland basin environment. During the Lopingian foreland basin phase, the individual depocentres of the Galilee and Bowen basins were linked to form a single and enormous foreland basin that covered >300 000 km² in central and eastern Queensland.