Multiple ages of rutile from a single sample of granulite

Durgalakshmi1, Ian S. Williams1, K. Sajeev2

1 Research School of Earth Sciences, Australian National University, Canberra, Australia, 2 Centre for Earth Sciences, Indian Institute of Science, Bengaluru, India

Rutile (TiO2) is a common accessory mineral in hydrothermal and metamorphic rocks that is stable across a wide range of P-T conditions. It can incorporate up to 200 ppm of U, and has a lower closure temperature than zircon, making it a reliable mineral with which to date retrograde metamorphism and low- to medium-grade metamorphic events by the U-Pb technique. Further, Pb and Th are incompatible in rutile, so corrections for its small initial Pb can be made accurately using its 208Pb content.

During prograde metamorphism, rutile forms following the breakdown of biotite or ilmenite as part of a continuous reaction. During retrogression, rutile can be replaced by ilmenite and titanite. Secondary rutile can be formed by hydrothermal alteration, oxidation or exsolution. In high-grade metamorphic rocks, rutile occurs as single crystals in the matrix and/or as inclusions in other minerals such as garnet, pyroxene and amphibole. In low- to medium-grade rocks it usually occurs as needles or polycrystalline aggregates. Using an ion microprobe (SIMS), rutile can be dated in its textural context in thin section, providing age information directly linked to metamorphic reactions. Dating and trace element analysis of separated rutile grains have a wide range of applications from sedimentary provenance studies to dating vein mineralisation and granitic pegmatites that host mineral deposits.

In the Neoarchæan Southern Granulite Terrane, India, rutile preserves a record of the late thermal history that is not provided by other datable minerals such as zircon or monazite. One studied sample of granulite grade felsic gneiss contains at least three distinct generations of rutile that preserve a range of Neoproterozoic to early Palæozoic U-Pb ages. The zircon from the same sample is Neoarchæan, with no evidence of a younger component. The rutile occurs as single ~ 0.5–0.8 mm crystals in the matrix, some of which are rimmed by titanite. Rutile-forming reactions, which can be linked to the metamorphic conditions, have been dated, contributing to unravelling the polymetamorphic and tectonic history of this complex terrane.


Biography

Miss Durgalakshmi is a PhD student at Research School of Earth Sciences, ANU. She works on the Archaean rocks of Southern Granulite Terrane, India.

Way out west – does the Arunta Orogen continue westward beneath the Canning Basin?

Kelsey, David E.1,3, Spaggiari, Catherine V.1, Wingate, Michael T.D.1, Lu, Yongjun1, Fielding, Imogen O.H.1, Finch, Emily G.1,2,3

1Geological Survey of Western Australia, 100 Plain St, East Perth, WA 6004, Australia, 2University of South Australia, 101 Currie St, Adelaide, SA 5001, Australia, 3MinEx CRC

Crystalline basement lies beneath the southeastern margin of the Canning Basin and immediately west of the exposed Arunta Orogen, although whether that basement is the continuation of the Arunta Orogen is unknown. The major bounding fault of the Canning Basin coincides with the inferred trace of the Lasseter Shear Zone, which truncates dominant east-trending structures of the orogen and potentially also terminates it. The Top Up Rise prospect is located above a distinct northeast-trending gravity anomaly bound by northeast-trending shear zones coincident with the Lasseter Shear Zone. Five co-funded Exploration Incentive Scheme diamond drillcores from Top Up Rise contain partially melted or melt-injected upper-amphibolite to low-granulite facies basement rocks. These are currently the only drillcores that intersect the Canning Basin basement in this region and provide a means to test its tectonic affinity, and the significance of the Lasseter Shear Zone.

Petrography of granitic gneiss and paragneiss indicates distinctly higher metamorphic grade than is observed in exposed rocks of the west Arunta Orogen, consistent with their separation by a significant shear zone. SHRIMP U–Pb zircon geochronology of several granitic gneiss samples has so far revealed a single magmatic protolith crystallization age of c. 1875 Ma, which is distinctly older than known magmatic rocks in the Arunta Orogen. Maximum deposition ages of 1877–1822 Ma for the metasedimentary rocks are similar to or younger than the magmatic protolith ages of the granitic gneiss, suggesting that emplacement of granitic rocks predated deposition of at least some of the sedimentary rocks. Zircon rims in both granitic gneiss and paragneiss samples record high-grade metamorphism at 1622–1604 Ma. Although metamorphism of this age is unknown in the west Arunta Orogen, thermal events of similar age may have occurred in the central Arunta Orogen (Alessio et al., 2020, Lithos, https://doi.org/10.1016/j.lithos.2019.105280), and granitic magmatism occurred at this time in the Haast Bluff Domain of the Warumpi Province (NTGS Special Publication 5, 2013).

The granitic protolith ages of c. 1875 Ma from Top Up Rise are different to those of granitic suites in the Lamboo Province and Granites–Tanami Orogen; however, felsic volcanic rocks of the lower Halls Creek Group have been dated at c. 1880 Ma (Phillips et al., 2016, GSWA Report 164) and c. 1880 Ma granitic rocks are known in the Arnhem Province (NTGS Record 2017-008). In contrast, detrital zircon age components at c. 1875 Ma are widespread and form significant age components in the Lander Rock Formation of the Arunta Orogen, the Marboo Formation and Tickalara Metamorphics of the Halls Creek Orogen, as well as in Top Up Rise paragneisses. Hence, the newly identified granitic basement in the Top Up Rise drillcores may be representative of a major source component that fed detritus into turbidite fan systems that included the Lander Rock Formation. This basement could represent part of the ‘missing’ Arunta basement, which would support interpretations that the Arunta does continue westwards, or it could be part of a previously unrecognized Proterozoic crustal element that underlies the Canning Basin.


Biography

The author team is made up of geologists and geochronologists from the GSWA. Emily Finch is a MinEx CRC Embedded Researcher at the GSWA.

Neodymium and oxygen isotope maps of Western Australia

Lu, Yongjun1, Smithies, RH1, Champion, DC2, Wingate, MTD1, Johnson, SP1, Martin, L3, Jeon, H4, Poujol, M5, Zhao, J6, Maas, R7, Creaser, RA8

1Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, 2Geoscience Australia, GPO Box 378, Canberra ACT 2601, 3Centre for Microscopy, Characterisation, and Analysis, University of Western Australia, Perth, WA 6009, 4Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden, 5Univ Rennes, CNRS, Géosciences Rennes – UMR 6118, 35000 Rennes, France, 6Radiogenic Isotope Facility, School of Earth Sciences, The University of Queensland, Brisbane, QLD 4072, Australia, 7School of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia, 8Dept. Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada

Multi-isotope maps can characterise lithospheric architecture through time, play an increasingly important role in predictive exploration targeting, and are consequently sought-after datasets by industry. We present the first zircon oxygen isotope map and an updated whole-rock Sm–Nd isotope map of Western Australia. These shed new light on crustal evolution and mineral system distributions. The isotope maps were generated from datasets that are subject to ongoing updates as new data are generated and compiled.

Median zircon δ18O values for about 125 igneous rocks have been spatially visualized so far, and coverage currently extends across the Pilbara and Yilgarn Cratons, the Capricorn, Paterson, and Albany-Fraser Orogens and the Eucla basement (Madura and Coompana Provinces). The Pilbara and Yilgarn Cratons are dominated by mantle-like δ18O values (4.7 – 5.9‰), consistent with reworking of igneous material that had not been exposed at the surface. A c. 3.47 Ga diorite, four c. 3.3 Ga hornblende-bearing granitic rocks, and a c. 2.95 Ga hornblende monzogranite in the Pilbara Craton exhibit weakly elevated zircon δ18O values (5.9 – 6.5‰), which together with trace element enrichment were attributed to hydrous sanukitoids or to derivation from a sanukitoid-enriched source. The Capricorn, Paterson, and Albany–Fraser Orogens and the Eucla basement also contain rocks with elevated δ18O values (6.6 – 9.9‰), suggesting significant reworking of upper crustal material during magma genesis. Zircons with sub-mantle δ18O values (<4.7‰) were found for granitic rocks of c. 3.55 Ga in the Sylvania Inlier, of c. 3.44 Ga in the northern Pilbara Craton, and of c. 3.0 and 2.67 Ga in the South West Terrane, suggesting recycling of crustal material subjected to high-temperature hydrothermal alteration, such as observed in post-Archean rift systems or calderas.

Sm–Nd isotopes for about 1120 felsic igneous rocks provide regionally extensive images of crustal architecture. The map of two-stage depleted mantle model ages (TDM2) highlights the distinction between Archean cratons (TDM2 >2.6 Ga) and Proterozoic orogens (TDM2 <2.2 Ga), and isotopic boundaries correlate well with most existing proposed terrane boundaries. However, the isotopic boundary between the South West Terrane and the Youanmi Terrane appears to be about 100 km west of the previously proposed boundary, but correlates well with magnetic and gravity anomaly zones and the distribution of gold mineralization. The crustal residence map highlights predominantly short residence times (<0.5 Ga) for the Pilbara and Yilgarn Cratons, and much longer crustal residence times (>0.8 Ga) in the Paterson, Albany–Fraser, Pinjarra and Capricorn Orogens, suggesting decreased juvenile crust generation in these orogens. 

These maps are directly applicable to metallogeny. For example, most giant gold deposits in WA are located on or near significant isotopic boundaries and tectonic structures. Interestingly, Telfer, Plutonic and giant gold deposits in the Murchison are aligned along a northeast-trending isotopic boundary. Similar boundaries occur between the eastern and western parts of the Pilbara Craton and between the Yilgarn Craton and the Albany–Fraser Orogen. These isotopically defined discontinuities may be important clues to the earliest architectural elements in Western Australia.


Biography

Dr. Yongjun Lu is Senior Geochronologist Isotope Geologist at GSWA. Yongjun has over 15 years’ geological experience, extensive collaboration with industry, government and academia. He has made important contributions to mineral systems science, highlighted by being the 52nd recipient of the Waldemar Lindgren Award of Society of Economic Geologists (SEG).

Records of the Earth’s early crust from apatite inclusions in zircon – development and applications of in situ 87Sr/86Sr analysis by SIMS

Gillespie, Jack1, Kinny, Pete1, Martin, Laure2, Kirkland, Christopher1, Nemchin, Alexander1, Cavosie, Aaron1

1The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, Perth, WA 6845, Australia, 2Center for Microscopy, Characterisation and Analysis (CMCA), University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia

Rb-Sr isotopes in geological materials provide a system for tracing crustal differentiation processes and insights into the evolution of planetary bodies. The ingrowth of radiogenic 87Sr from the decay of 87Rb leads to increased 87Sr/86Sr over time, and due to the strong relationship between the Rb/Sr and SiO2 contents of igneous rocks, this provides a time-integrated window into the evolution of geochemical reservoirs. However, the high geological mobility of both Rb and Sr in whole rocks during metamorphism and fluid alteration means that this record becomes progressively less reliable in older rocks that have experienced post-crystallization geological events.

Strontium is easily substituted into the crystal lattice of apatite, occurring as a trace element in concentrations ranging from less than a hundred parts per million to several weight percent. In contrast, apatite nearly entirely excludes Rb (<1 ppm) resulting in negligible radiogenic ingrowth of 87Sr, and consequently the initial 87Sr/86Sr ratio of the melt from which an apatite crystallizes is faithfully recorded by the mineral. Inclusions of apatite within magmatic zircons are particularly valuable as they are armoured by the more robust host mineral, allowing them to survive subsequent events that might otherwise cause isotopic reset or recrystallization. However, the typically very small size of apatite inclusions in zircon and the complex isobaric interferences on the isotopes of Sr during in-situ mass spectrometry have previously limited the information that can be obtained from this archive.

Our recent development of a method to measure the 87Sr/86Sr ratio in apatite by SIMS with a spot size appropriate for accessing typical mineral inclusions in zircon (<15 µm) makes it possible to routinely analyse the commonly occurring inclusions of apatite in zircon. We have applied this method to determine the initial 87Sr/86Sr ratios of various Eo-Meso Archean igneous rocks by analysing the Sr isotope composition of apatite inclusions. High resolution SEM imaging and EPMA analysis illustrate the primary nature of these inclusions. Combining the measured 87Sr/86Sr of apatite inclusions with the U-Pb age and Hf isotopic composition of the co-genetic zircon host allows for the ‘triangulation’ of the Rb/Sr necessary for the ingrowth of radiogenic strontium over the crustal residence interval calculated from the crystallization and Hf model ages. Examples from SW Greenland and the Narryer Gneiss Terrane of Western Australia suggest that these rocks were derived from the melting of ancient crustal material that was on average of intermediate-felsic rather than mafic composition.


Biography

Jack Gillespie is a post-doctoral researcher at Curtin University working on developing new methods for understanding the evolution of the early earth

Contrasting growth of the Pilbara and Yilgarn cratons from hafnium and neodymium isotopes

Kemp, Dr Tony1

1University Of Western Australia, , Australia

Long-lived radiogenic isotope systems such as 147Sm-143Nd and 176Lu-176Hf suggest that large volumes of the Earth’s continental crust formed in the Archean Eon (> 2.5 Ga). The onset of substantial continent stabilization in the geological record is marked by the distinctive ‘granite-greenstone’ terranes that are the hallmarks of Archean crustal blocks. Yet, to what extent generation of the buoyant, silica-rich (i.e. continental) components in these terranes involved the re-melting of pre-existing, primordial crust as opposed to rapid differentiation of new mantle additions, remains uncertain. Establishing the composition of the mantle source from which early crust was extracted, and comparing this with the compositions of felsic crust, is key to this question. The geochemical signatures of ancient, unambiguously mantle-derived rocks are, however, susceptible to modification by later metamorphism. Here, hafnium and neodymium isotope data are reported for well preserved mafic-ultramafic and felsic igneous rocks of the Pilbara and Yilgarn Cratons, Western Australia. Comparing the mantle and crustal records of Archean continent formation in these cratons reveals a striking isotopic link that endured over 500 million years. In the Pilbara Craton, this linkage is interpreted to reflect the efficient transformation of new mafic inputs from the mantle into felsic continental crust throughout the history of the craton. In contrast, broadly coeval rocks in the Yilgarn Craton formed by remelting older rocks, although the crustal evolutionary records of both cratons converge in the Neoarchean. The possible reasons for the cratonic contrasts are considered.


Biography

Currently in the School of Earth Sciences at the University of Western Australia

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