https://ogma.newcastle.edu.au/vital/access/ /manager/Index en-au 5 https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:47528 Wed 24 May 2023 09:56:51 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:41948 Tue 16 Aug 2022 14:31:41 AEST ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:36164 Thu 20 Feb 2020 14:45:21 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:31472 Hf(t)-in-zircon values (to −40) require an Archean source, which is not proximal to the Terra Australis active margin. Based on similar ɛ_Hf(t) arrays defined by Neoproterozoic granites in Western Australia and detrital zircon populations from the surrounding basins, we suggest that PG zircon grains were derived from the >2000-km-long, late Neoproterozoic Paterson-Petermann orogen, which sutured northern and southern Australia at 550–530 Ma. This Himalayan-style orogen was responsible for amalgamating Southeast Asian terranes into northeast Gondwana, thereby constraining the paleogeography of the northern Gondwanan margin at the Precambrian-Cambrian boundary. Remarkable isotopic similarity of zircon grains with the Lhasa terrane of Tibet suggests that the Paterson-Petermann orogen was the eastern sector of the developing circum-Gondwana subduction system from ca. 700 Ma.]]> Sat 24 Mar 2018 08:44:55 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:31295 Sat 24 Mar 2018 08:44:14 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:13658 Sat 24 Mar 2018 08:25:19 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:17135 Sat 24 Mar 2018 08:02:29 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:20992 Sat 24 Mar 2018 07:50:40 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:29759 Sat 24 Mar 2018 07:32:17 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:30080 900 °C). The values contrast with Tzr from alkaline rocks from the Cenozoic U.S. Cordillera, which are typically >800 °C for 65–70 wt% SiO2. Case studies of titanium-in-zircon thermometry from the U.S. Cordillera also suggest that alkaline magma injections into granitic magma chambers are hot, but calc-alkaline magma injections are usually cooler. A model is presented suggesting that silicic Cordilleran magmas form in magmatic arcs where hydrous basaltic magmas solidify in the arc root, producing mafic underplates that exsolve aqueous fluids, which transfer to the crust and promote water-fluxed partial melting at ambient pressure-temperature (∼750–800 °C at 8 kbar) conditions. Subsequent rock-buffered melting reactions modulate the water content of arc magmas. The granitic partial melts are water undersaturated, rise adiabatically as increments, but stall in the middle to upper crust, building cool and hydrous, crystal-rich magma chambers (batholiths). However, injections of hotter magmas are required to drive volcanic eruption. In the backarc, granitic magma chambers are intermittently recharged with hotter, drier alkaline magmas, which are produced mostly by decompression melting during lithospheric extension, not hydrous fluxing. This highlights the control of subduction dynamics on water content and consequently magmatic temperatures in silicic magma systems.]]> Sat 24 Mar 2018 07:31:17 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:28212 15 kbar). Zircons show four main age groupings: 1.0–0.5 Ga, 50–45, 35–20, and 16–13 Ma. The oldest group is similar to common inherited zircons in the Gangdese belt, whereas the 50–45 Ma zircons match the crystallization age and juvenile character (εHf_i +0.5 to +6.5) of Eocene Gangdese arc magmas. Together these two age groups indicate that a component of the xenolith was sourced from Gangdese arc rocks. The 35–20 Ma Miocene ages are derived from zircons with similar Hf–O isotopic composition as the Eocene Gangdese magmatic zircons. They also have similar steep REE curves, suggesting they grew in the absence of garnet. These zircons mark a period of early Miocene remelting of the Eocene Gangdese arc. By contrast, the youngest zircons (13.0 ± 4.9 Ma, MSWD = 1.3) are not zoned, have much lower HREE contents than the previous group, and flat HREE patterns. They also have distinctive high Th/U ratios, high zircon δ¹⁸O (+8.73–8.97 ‰) values, and extremely low εHf_i (−12.7 to −9.4) values. Such evolved Hf–O isotopic compositions are similar to values of zircons from the UPV lavas that host the xenolith, and the flat REE pattern suggests that the 13 Ma zircons formed in equilibrium with garnet. Garnets from a strongly peraluminous meta-tonalite xenolith are weakly zoned or unzoned and fall into four groups, three of which are almandine-pyrope solid solutions and have low δ¹⁸O (+6 to 7.5 ‰), intermediate (δ¹⁸O +8.5 to 9.0 ‰), and high δ¹⁸O (+11.0 to 12.0 ‰). The fourth is almost pure andradite with δ¹⁸O 10–12 ‰. Both the low and intermediate δ¹⁸O groups show significant variation in Fe content, whereas the two high δ¹⁸O groups are compositionally homogeneous. We interpret these features to indicate that the low and intermediate δ¹⁸O group garnets grew in separate fractionating magmas that were brought together through magma mixing, whereas the high δ¹⁸O groups formed under high-grade metamorphic conditions accompanied by metasomatic exchange. The garnets record complex, open-system magmatic and metamorphic processes in a single rock. Based on these features, we consider that ultrapotassic magmas interacted with juvenile 35–20 Ma crust after they intruded in the deep crust (>50 km) at ~13 Ma to form hybridized Miocene granitoid magmas, leaving a refractory residue. The ~13 Ma zircons retain the original, evolved isotopic character of the ultrapotassic magmas, and the garnets record successive stages of the melting and mixing process, along with subsequent high-grade metamorphism followed by low-temperature alteration and brecciation during entrainment and ascent in a late UPV dyke. This is an excellent example of in situ crust–mantle hybridization in the deep Tibetan crust.]]> Sat 24 Mar 2018 07:23:51 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:3380 Sat 24 Mar 2018 07:18:58 AEDT ]]> https://ogma.newcastle.edu.au/vital/access/ /manager/Repository/uon:54946 Fri 22 Mar 2024 15:22:30 AEDT ]]>