Emplacement and deformation ages of the Wyangala Granite, Cowra, NSW

<div><p>New geochronological data from the Wyangala Granite in the Hill End Trough, Eastern Lachlan Fold Belt, constrain the timing of granite emplacement and subsequent deformation. SHRIMP U–Pb dating of zircon indicates that the granite crystallised in the late Silurian (425.2 ± 3.5 Ma). Subsequent west-over-east thrusting resulted in the development of mylonite along its eastern margin and cataclasis of megacrystic and groundmass feldspars, especially in pervasive shear zones within the granite. ⁴⁰Ar/³⁹Ar dating of feldspar from two deformed granite samples yielded ‘plateau’ ages in the range 375–365 Ma that are interpreted to reflect the timing of deformation associated with major shearing. These ages broadly overlap with the proposed age range of the late Middle Devonian Tabberabberan event and some biotite K–Ar and Rb–Sr ages in the nearby Sunset Hills Granite.</p></div

Neogene rock uplift and erosion in northern Borneo: evidence from the Kinabalu granite, Mount Kinabalu

<p>Thermochronological data from the Kinabalu granite, emplaced between <em>c</em>. 7.2 and 7.8 Ma, provide a unique record of northern Borneo’s exhumation during the Neogene. Biotite ⁴⁰Ar/³⁹Ar ages (<em>c</em>. 7.32–7.63 Ma) record rapid cooling of the granite in the Late Miocene as it equilibrated with ambient crustal temperatures. Zircon fission-track ages (<em>c</em>. 6.6–5.8 Ma) and apatite (U–Th–Sm)/He ages (central age <em>c</em>. 5.5 Ma) indicate rapid cooling during the Late Miocene–Early Pliocene. This cooling reflects exhumation of the granite, uplift and erosion bringing it closer to the Earth’s surface. Thermochronological age versus elevation relationships suggest exhumation rates of more than 7 mm a⁻¹ during the latest Miocene and Early Pliocene. Neither the emplacement of the Kinabalu granite nor its exhumation is related to the Sabah orogeny, which terminated in the Early Miocene. Instead, granite magmatism was caused by extension related to subduction rollback of the Sulu Arc, and Mio-Pliocene exhumation of the Kinabalu granite was driven either by lithospheric delamination or break-off of a subducted slab beneath Sabah. Plio-Pleistocene tectonism offshore and onshore northern Borneo reflects continuing large-scale gravity-driven tectonics in the region. </p

Highly retentive core domains in K-feldspar and their implications for ⁴⁰Ar/³⁹Ar thermochronology illustrated by determining the cooling curve for the Capoas Granite, Palawan, The Philippines

<div><p>K-feldspar from the late Miocene Capoas Granite on Palawan in The Philippines appears to contain highly retentive diffusion domains that are closed to argon diffusion at near-solidus temperatures during cooling of this ∼7 km-diameter pluton. This is an important result, for K-feldspar is commonly considered not retentive in terms of its ability to retain argon. Closure temperatures for argon diffusion in K-feldspars are routinely claimed to be in the range ∼150–400°C but the release of ³⁹Ar from irradiated K-feldspar during furnace step-heating experiments <i>in vacuo</i> yields Arrhenius data that imply the existence of highly retentive core domains, with inferred closure temperatures that can exceed ∼500–700°C. These high closure temperatures from the Capoas Granite K-feldspar are consistent with the coincidence of ⁴⁰Ar/³⁹Ar ages with U–Pb zircon ages at <i>ca</i> 13.5 ± 0.2 Ma. The cooling rate then accelerated, but the rate of change had considerably slowed by <i>ca</i> 12 Ma. Low-temperature (U–Th)/He thermochronology shows that the cooling rate once again accelerated at <i>ca</i> 11 Ma, perhaps owing to renewed tectonic activity.</p></div