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The Boyongan and Bayugo porphyry copper-gold deposits are part of a belt of gold-rich copper deposits in the Surigao district of northeast Mindanao, Philippines. The detailed age relationships described in this study provide insight into the geologically

White micas from the Penninic units of the Urtier Valley, in the Western Alps, have been dated by the 40Ar/39Ar step-heating method. A strong correlation is observed between measured ages, mica composition and deformation fabrics. Samples characterized by a single mineral reservoir yielded relatively flat age spectra, allowing to estimate an age of 44.4 ± 0.4 Ma for the formation of phengite-bearing fabrics in eclogite facies conditions (SHP) and an age of 42.0 ± 0.4 Ma for the microstructurally younger muscovite-bearing shear fabrics, which formed in greenschist facies conditions (SZgr1). Several micaschist samples are characterized by the overprint of the phengite-bearing SHP fabric by a younger muscovite-bearing fabric (FU) that is axial planar to tight upright folds that post-date the SZgr1 shearing event. BSE images reveal that the different generations of syn-kinematic micas are not exclusively tied to their respective fabric, but muscovite is commonly found also in the SHP microlithons, while relict phengite is often transposed into the new FU axial planar fabric. All the samples containing these two generations of white micas yielded convex-upward spectra, which, following considerations based on the 36Ar/40Ar Vs 40Ar/39Ar plots, have been interpreted to indicate mixing of two argon reservoirs of different ages, related to the older phengite and the younger muscovite visible with optical and electron microscopy. This method allowed to estimate a maximum age of ca. 36.5 ± 0.5 Ma for the folding of the shear-related structures. The ages estimated for SHP and SZgr1 in the study area are consistent with those obtained in other parts of the Penninic units with other geochronological techniques. Therefore, the post-crystallization loss of argon from the phengite associated with SHP and the muscovite defining SZgr1 is negligible, despite evidence indicating that conditions of T = 480-510 °C were reached during the later upright folding event.

The Centralian Superbasin in central Australia is one of the most extensive intracratonic basins known from a stable continental setting, but the factors controlling its formation and subsequent structural dismemberment continue to be debated. Argon thermochronology of K-feldspar, sensitive to a broad range of temperatures (∼150 to 350°C), provides evidence for the former extent and thickness of the superbasin and points toward thickening of the superbasin succession over the now exhumed Arunta Region basement. These data suggest that before Palaeozoic tectonism, there was around 5-6km of sediment present over what is now the northern margin of the Amadeus Basin, and, if the Centralian superbasin was continuous, between 6 and 8km over the now exhumed basement. 40Ar/39Ar data from neoformed fine-grained muscovite suggests that Palaeozoic deformation and new mineral growth occurred during the earliest compressional phase of the Alice Springs Orogeny (ASO) (440-375Ma) and was restricted to shear zones. Significantly, several shear zones active during the late Mesoproterozoic Teapot Orogeny were not reactivated at this time, suggesting that the presence of pre-existing structures was not the only controlling factor in localizing Palaeozoic deformation. A range of Palaeozoic ages of 440-300Ma from samples within and external to shear zones points to thermal disturbance from at least the early Silurian through until the late Carboniferous and suggests final cooling and exhumation of the terrane in this interval. The absence of evidence for active deformation and/or new mineral growth in the late stages of the ASO (350-300Ma) is consistent with a change in orogenic dynamics from thick-skinned regionally extensive deformation to a more restricted localized high-geothermal gradient event.

The Okanogan gneiss dome, Washington, is located in the hinterland of the North American Cordillera and is part of a chain of metamorphic core complexes containing gneiss and migmatite domes exhumed during Eocene extension of thickened crust. U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses of zircon, monazite, and titanite, and 40Ar-39Ar analyses of biotite from migmatites exposed in the footwall of the Okanogan detachment, coupled with a detailed structural analysis, document the timing and duration of migmatite crystallization and indicate coeval crystallization, extensional deformation, and exhumation of the dome. Okanogan migmatites are folded and deformed, and preserve successive generations of leucosomes; generated by synkinematic anatexis. Analyses of migmatite samples from a highmelt fraction subdome near Stowe Mountain suggest that the Okanogan dome records a history of migmatite crystallization spanning at least 12 m.y., as indicated by 206Pb/238U ages ranging from ca. 61 to 49 Ma for new zircon growth and rim overgrowths attributed to migmatite crystallization. Zircons from a granodiorite in a domain of diatexite near Stowe Mountain preserve rims that have a mean 206Pb/238U age of 51.1 ± 1.0 Ma for the youngest population attributed to migmatite crystallization. Zircon from folded and discordant granitic leucosome in the diatexite domain yields a calculated 206Pb/238U age of 53.5 ± 0.5 Ma for migmatite crystallization. Zircon from discordant leucosome of the metatexite domain has a mean 206Pb/238U age of 59.8 ± 0.5 Ma, with ages as young as ca. 53 Ma attributed to final crystallization of the leucosome. Core domains of zircon samples have 206Pb/238U ages that range from ca. 85 to 70 Ma and are interpreted to be related to an earlier phase of the orogeny. Monazite from two samples gives 206Pb/238U crystallization ages of 52.9 ± 0.6 Ma for the granodiorite diatexite and 52.0 ± 0.6 Ma for nearby boudinaged and foliated layers of biotite grano

Application of multiple chronometers (including U-Pb and 40Ar/ 39Ar geochronology and zircon and apatite (U-Th)/He thermochronology) to porphyry intrusions at the Bajo de la Alumbrera porphyry copper-gold deposit, Argentina, reveals a complex history of reheating that spans millions of years. Previous U-Pb geochronology, combined with our new 40Ar/39Ar data, shows that the multiple porphyritic intrusions at Bajo de la Alumbrera were emplaced during two episodes, the first at about 8.0 Ma (P2 and associated porphyries) and the second about a million years later (Early and Late P3 porphyries). Complex overprinting alteration events have obscured the earliest hydrothermal history of the deposit. By contrast, 40Ar/39Ar data reveal the close temporal relationship of ore-bearing potassic alteration assemblages (7.12±0.13 Ma; biotite) to the emplacement of the P3 intrusions. Consistent with low closure temperatures, younger ages have been determined for associated hydrothermal alkali feldspar (6.82±0.05 Ma and 6.64±0.09 Ma). The temperature-sensitive Ar data also record an unexpected prolonged cooling history (to below 200°C) extending to 5.9 Ma. Our data suggest that the Bajo de la Alumbrera system underwent protracted cooling, after the collapse of the main hydrothermal system, or that one or more low-temperature (∼100-200°C) reheating events occurred after emplacement of the porphyritic intrusions at Bajo de la Alumbrera. These have been constrained in part by our new 40Ar/39Ar data (including multidomain diffusion modeling) and (U-Th)/He ages. Single-grain (U-Th)/He ages (n = 5) for phenocrystic zircon from P2 and P3 intrusive phases bracket these thermal events to between 6.9 (youngest crystallization of intrusion) and 5.1 Ma. Multidomain modeling of alkali feldspar data (from both igneous and hydrothermal crystals) is consistent with the deposit cooling rapidly from magmatic temperatures to below about 300°C, with a more protracted history down to 150°C. We conclude

Several descriptions of argon diffusion in K-feldspar have been developed, including the quantitative multiple-diffusion-domain (MDD) and multipath models and the qualitative 'recrystallization' model. These models remain contentious and the natural range in behaviour remains poorly understood. Feldspars from the Klokken syenite intrusion show a systematic relationship between 40Ar/39Ar age and microtexture suggesting that the loss of argon from different feldspars operates via different mechanisms such that the existing models represent only end-member scenarios. The MDD model can yield reliable thermal history information only for feldspars with pristine microtexture, with this model best viewed as a cipher for the real (but unknown) domain structure in such samples.

Outcrops of leucitite lavas occur as scattered remnants up to 40 m thick in the El Capitan area, northeast of Cobar in western New South Wales. Two eruption sites have been located for these lavas. Preserved volcanic features indicate that the lavas were