Suzanne L. Baldwin

Plutonism and the origin of metamorphic core complexes

The formation of metamorphic core complexes may be triggered by plutonic activity during episodes of continental extension. Pulses of ductile deformation have taken place during short-lived thermal events initiated by the heat input from intruded plutons, sills, or dikes. Such intrusions may be the underlying cause for differential uplift of the footwall during tectonic denudation of metamorphic core complexes. Fast cooling inferred from 40 Ar/ 39 Ar apparent ages may have taken place after periods of magma arrival, and need not be the result of rapid erosional or tectonic denudation. Heterogeneity of 40 Ar/ 39 Ar apparent ages can be explained by rapid cooling of deforming mylonites formed at shallow crustal levels in the thermal aureoles of intruded sills (or sill swarms). Under such circumstances, apparently isothermal decompression paths may actually link pressure-temperature points set during periods of transient mineral growth during and after periods of igneous activity.

Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea

As lithospheric plates are subducted, rocks are metamorphosed under high-pressure and ultrahigh-pressure conditions to produce eclogites and eclogite facies metamorphic rocks. Because chemical equilibrium is rarely fully achieved, eclogites may preserve in their distinctive mineral assemblages and textures a record of the pressures, temperatures and deformation the rock was subjected to during subduction and subsequent exhumation. Radioactive parent–daughter isotopic variations within minerals reveal the timing of these events. Here we present in situ zircon U/Pb ion microprobe data that dates the timing of eclogite facies metamorphism in eastern Papua New Guinea at 4.3 ± 0.4 Myr ago, making this the youngest documented eclogite exposed at the Earths surface. Eclogite exhumation from depths of ∼75 km was extremely rapid and occurred at plate tectonic rates (cm yr-1). The eclogite was exhumed within a portion of the obliquely convergent Australian–Pacific plate boundary zone, in an extending region located west of the Woodlark basin sea floor spreading centre. Such rapid exhumation (> 1 cm yr-1) of high-pressure and, we infer, ultrahigh-pressure rocks is facilitated by extension within transient plate boundary zones associated with rapid oblique plate convergence.

ASYMMETRIC EXHUMATION ACROSS THE PYRENEAN OROGEN : IMPLICATIONS FOR THE TECTONIC EVOLUTION OF A COLLISIONAL OROGEN

Abstract The Pyrenees are a collisional mountain belt formed by convergence between the Afro–Iberian and European plates. Apatite fission track thermochronology from three vertical profiles along the ECORS seismic line constrain the exhumation history of the Pyrenean orogen and hence tectonic models for its formation. In the Eocene there is relatively uniform exhumation across the Pyrenees, but significantly more exhumation occurs on the southern flank of the axial zone in the Oligocene. The variation in exhumation patterns is controlled by a change in how convergence is accommodated within the Pyrenean double-wedge. Accommodation of thrusting on relict extensional features that leads to inversion dominated thrust stacking resulted in relatively slow exhumation in the Eocene. However, subsequent crustal wedging and internal deformation in the upper crust under the stacked duplex of antiformal nappes resulted in extremely rapid exhumation on the southern flank in the Oligocene. The Maladeta profile in the southern axial zone records extremely rapid Early Oligocene exhumation followed by dramatic slowing or cessation of exhumation in the middle Oligocene and the formation of an apatite partial annealing zone (PAZ). This PAZ has subsequently been exhumed 2–3 km since the Middle Miocene, supporting the observations of Coney et al. [J. Geol. Soc. London 153 (1996) 9–16] that the southern flank of the range was buried by ≤2–3 km of syntectonic conglomerates in the Oligocene and subsequently re-excavated from Late Miocene to Recent. The present-day topographic form of the Pyrenees is largely a relict of topography that formed in the Eocene and the Oligocene. Comparison with paleoclimatic records indicates that the Eocene–Oligocene exhumation patterns are controlled by tectonic forces rather than resulting from an orographic effect due to uplift of the Pyrenees.

Modelling the effect of arbitrary P-T-t histories on argon diffusion in minerals using the MacArgon program for the Apple Macintosh

Argon diffusion in mineral grains has been numerically modelled using P-T-t histories that may be relevant to multiply metamorphosed orogenic terranes and for rocks that have resided at high ambient temperatures in the Earths crust for long durations. The MacArgon program1 generates argon concentration profiles in minerals assuming argon loss occurs via volume diffusion. It can be run on an Apple Macintosh computer, with arbitrary P-T-t histories used as input. Finite-difference equations are used in the calculation of 40Ar∗ concentration profiles across individual diffusion domains. The associated MacSpectrometer generates model spectra after a P-T-t history has been specified. The form of model 40Ar39Ar apparent age spectra suggests that considerable caution needs to be exercised in the use of the closure temperature concept and in the interpretation of the significance of plateaux observed in many 40Ar39Ar apparent age spectra, particularly in cases involving metamorphic rocks, where complex P-T-t histories might apply. Although modelled spectra cannot be directly compared to experimentally determined 40Ar39Ar age spectra, especially when hydrous phases are involved or in cases where loss of argon has not occurred via volume diffusion, they do provide insight into theoretically expected age spectra for samples that have experienced complex P-T-t histories.

Unroofing of active metamorphic core complexes in the D'Entrecasteaux Islands, Papua New Guinea

Metamorphic core complexes in the D9Entrecasteaux Islands are forming as a result of active extension at the western end of the Woodlark Basin spreading center. High-grade metamorphic rocks exist in the cores of gneissic domes, including eclogites formed at minimum depths of 45-75 km (i.e., 13-21 kbar). High temperatures were maintained during uplift of basement rocks from 13-21 kbar ( T ≈ 730-900 °C) through 7-11 kbar ( T ≈ 570-730 °C) to ∼5-6 kbar ( T ≈ 730 °C). Subsequent rapid cooling (>>100 °C/m.y.) of the basement occurred between 1 and 2 Ma (on the basis of 40 Ar/ 39 Ar geochronology). The core-zone rocks were transected by kilometre-scale, amphibolite-facies ductile shear zones active at depths of 25-35 km. Extensive intrusion of granodiorite plutons occurred, and the age of granodiorite intrusions constrains late-stage shear-zone activity to ∼2 Ma and younger. Unroofing was accomplished by faults and shear zones bounding the gneiss domes, juxtaposing them against ultramafic rocks and sedimentary deposits. Surface uplift continues today and has led to topographic elevations of up to 2.5 km.

Diffusion of 40Ar in metamorphic hornblende

Isothermal, hydrothermal experiments were performed on two compositionally contrasting hornblendes from amphibolites in order to examine Ar diffusion behavior in metamorphic hornblendes. Ten experiments on sample RF were performed at temperatures of 750°C, 800°C, and 850°C and pressures of 1 kbar using measured grain radii of 158, 101, and 34 μm. Eight experiments on sample 118576 were performed under the same conditions using measured grain radii of 145, 77, and 25 μm. Minor (<5%) alteration was observed in high temperature runs. Diffusion coefficients were calculated from measured radiogenic 40Ar loss following treatment assuming a spherical geometry for the mineral aggregate. Diffusivities calculated for different grain sizes vary by up to an order of magnitude for a given temperature indicating that the effective diffusion radius was less than the measured grain radius. Diffusivities for RF and 118576 calculated for grain radii of 101 and 145 μm, respectively, form a linear array on an Arrhenius diagram with slopes indicating activation energies of ∼ 60 kcal/mol. No correlation between Mg number (100 Mg/(Mg+Fe)) and activation energy was observed. Diffusivities calculated for these experiments are higher than previously reported results from similar experiments performed on hornblendes. A comparison of results for 34 μm splits from these two studies indicates higher apparent diffusivities (by a factor of 5), which probably result from observed phyllosilicate inter-growths (chlorite) and/or exsolution lamellae that partition the metamorphic hornblendes into smaller subdomains. Diffusivities calculated for experiments performed on 65 μm and 34 μm splits of 40Ar/39Ar standard MMhb-1 at 800°C and 1 kbar are consistent with a previously reported activation energy of 65 kcal/mol. Arrhenius parameters which emerge from the empirical model of Fortier and Giletti (1989) agree with experimental results to within analytical uncertainty. Although results of these experiments support previously reported estimates of the activation energy of 40Ar in hornblende (∼60 kcal/mol), phyllosilicate intergrowths and/or microstructures such as exsolution lamellae within the two metamorphic hornblendes result in extremely small diffusion domains, which may lead to lower Ar retentivities and lower closure temperatures. The effective diffusion dimension for 40Ar in hornblende is not likely to be defined by dislocations but rather by some larger structure within the crystal. TEM and SEM studies may provide some insight into the effective diffusion dimension for 40Ar in amphiboles, thereby enabling better estimates of closure temperatures and more precise temperature-time reconstructions.

Ages of deformation from K/Ar and 40Ar/39Ar dating of white micas

A structural and isotopic dating study of white micas from mylonites in the Ruby Gap duplex off central Australia shows that mylonitic deformation occurred during the Paleozoic Alice Springs orogeny. Deformation of the white micas took place under greenschist facies conditions, in the approximate temperature range 250-350 °C. Two distinct populations of white mica have been found: (1) porphyroclasts of original muscovite and (2) neocrystallized phengites that define the fabric associated with deformation. 40Ar/39Ar age spectra of neocrystallized phengites yield plateau-like spectra of Paleozoic age. In contrast, 40Ar/39Ar age spectra of muscovite porphyroclasts exhibit Proterozoic apparent ages, indicating that white micas were not open to significant argon diffusion during formation of the Ruby Gap duplex. We conclude that the ages measured on neocrystallized phengites record the cessation of ductile deformation and migration of tectonic activity toward the internal zones of the thrust system.

Thermochronology of the South Cyclades Shear Zone, Ios, Greece: Effects of ductile shear in the argon partial retention zone

Micas and potassium feldspars from the South Cyclades Shear Zone, Ios, Cyclades, Greece, yield varied and complex 40Ar/39Ar apparent age spectra. A correlation exists between 40Ar/39Ar apparent ages and the relative timing of different episodes of recrystallization and grain growth, as indicated by fabric and microstructural analysis. The 40Ar/39Ar apparent age spectra record the effects of variation in the degree of recrystallization and grain growth, and partial to complete resetting of argon systematics in potassium-bearing minerals during Hercynian (M0), Alpine (M1) and late Oligocene - early Miocene (M2) metamorphism. Deformation was strongly partitioned within the shear zone and this led to localized recrystallization and heterogeneous resetting of argon systematics within preexisting minerals. Modeling suggests the Oligo-Miocene thermal events were of insufficient magnitude and/or duration to completely reset the isotopic systematics in these samples. Our data lead to the concept of the argon partial retention and resetting zone (PRZ), defined as that portion of the crust where temperatures are insufficient to completely reset argon systematics within preexisting potassium-bearing minerals. Within the PRZ, some radiogenic argon in preexisting potassium bearing minerals will be outgassed and only partially retained. Tectonic exhumation of the PRZ involves movement on crustal-scale ductile shear zones, accompanied by strongly partitioned deformation and localized recrystallization. Recrystallization leads to resetting of argon systematics, and thus will result in heterogeneous 40Ar/39Ar age distributions within these ductile shear zones (e.g., in the South Cyclades Shear Zone).

A tale of two eras: Pliocene-Pleistocene unroofing of Cenozoic and late Archean zircons from active metamorphic core complexes, Solomon Sea, Papua New Guinea

U/Pb ion microprobe analyses of zircons from gneisses and granodiorites exposed in the D9Entrecasteaux Islands, and from conglomerate sections of the Goodenough No. 1 well in the adjacent Trobriand Basin, provide constraints on the age of magmatism, peak metamorphism, and nature of rocks unroofed during initial stages of metamorphic core complex formation in the Solomon Sea. The youngest populations of zircons from felsic gneisses and granodiorites indicate late Pliocene 206 Pb*/ 238 U ages. No inherited zircons were identified in the granodiorites, and the 206 Pb*/ 238 U ages (1.65 ± 0.18 Ma; 1.98 ± 0.08 Ma [2σ]) are interpreted as crystallization ages. These synkinematically emplaced granodiorites, intruded into actively extending continental crust, are some of the youngest known granitoids currently exposed at the Earth9 surface. Zircon ages from felsic gneisses (2.63 ± 0.16 Ma; 2.72 ± 0.28 Ma [2σ]) are interpreted to date zircon growth subsequent to eclogite facies metamorphism. Felsic gneiss samples also contained zircon xenocrysts from Cretaceous-Miocene protoliths. In striking contrast, zircons from igneous and metamorphic clasts from the Goodenough No. 1 well indicate a single population with a 207 Pb*/ 206 /Pb* age of 2781 ± 9 Ma (2σ). We speculate that they are derived from basement rocks unroofed during initial stages of development of the D&Entrecasteaux metamorphic core complexes. These results provide the first direct evidence for the existence of Archean protoliths in the basement rocks of southeastern Papua New Guinea.

Ross Sea mylonites and the timing of intracontinental extension within the West Antarctic rift system

There are few direct constraints on the timing and style of faulting in the Ross Sea sector of the West Antarctic rift system, although Cretaceous plate reconstructions indicate that Ross Sea extension between East and West Antarctica occurred prior to breakup of the Gondwana margin ca. 80 Ma. Mylonitic gneisses dredged from the eastern Ross Sea indicate shear-zone deformation considerably earlier, at 98-95 Ma. Strain analysis of fab- rics indicates 85%-100% extension. Overprinting brittle structures record translation of shear-zone gneisses into the upper crust. Samples yield sensitive high-resolution ion- microprobe U-Pb zircon ages of 102-97 Ma, correlated to Byrd Coast Granite onshore, and concordant 40 Ar/ 39 Ar biotite and K-feldspar ages of 98-95 Ma, indicating that granites were mylonitized soon after emplacement and cooled rapidly. Apatite fission-track data corroborate this rapid cooling event, and reveal a second rapid cooling event ca. 80 Ma. Evidence for contemporaneous deformation and a similar thermal evolution at Deep Sea Drilling Project Site 270 on the Ross Sea central high and for a migmatite dome on land attests to the regional extent of intracontinental extension. Extension occurred at a time of complex microplate interactions along the Cretaceous active Gondwana margin, sug- gesting that distributed deformation in the overriding Antarctic plate may be related to plate boundary dynamics.