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Zircon U-Pb geochronology is vital for providing age constraints for the calibration of the geologic timescale. The presence of zircon grains that yield concordant ages younger than the host sedimentary rock is often ignored, attributed to young outliers and possible contaminants. Using in situ U-Pb dating techniques, which analyze zircon crystals drilled directly from polished thin sections, two tuff beds from the Brockman Iron Formation, Pilbara Craton, Australia, yielded weighted mean 207Pb/206Pb ages of 2466 ± 6 Ma and 2469 ± 5 Ma (95% confidence level), consistent with independent age constraints. The tuffs also contain zircons that gave younger, yet concordant, U-Pb dates (2450–2270 Ma) as well as some with highly discordant dates. The "young" zircons have low or negligible common Pb, and display magmatic-type euhedral outlines, oscillatory zonation and Th/U ratios, indistinguishable from syn-depositional zircons, and cannot be readily dismissed on the grounds of high discordance or common-Pb contents. The "young" zircon dates are statistical outliers that overlap with the age of hydrothermal xenotime in deformed, bedding-parallel veins in shale that underlies the iron formation, which yields an age of 2383 ± 4 Ma (95% confidence level). The xenotime in overpressure veins provides independent evidence for hydrothermal fluid flow broadly coeval with isotopic resetting of "young" zircon crystals. Although SEM-CL imaging found no evidence for alteration, element mapping shows that most zircon grains have been altered by fluid infiltration, which proceeded along fractures or radiation-damaged growth zones leading to elevated Fe, Ca and Y contents in the altered domains. Total and partial Pb loss caused by hydrothermal alteration can explain the young concordant and discordant ages, respectively. These results have implications for the use of U-Pb zircon geochronology, particularly in detrital zircon studies, which routinely use the age of the youngest zircon grains to constrain the age of deposition. The erosion and re-deposition or in situ growth of young zircons in low-grade metasedimentary rocks may lead to erroneously young depositional ages. [ABSTRACT FROM AUTHOR]

Mafic-ultramafic rocks of the Kabanga- Musongati alignment in the East African nickel belt occur as Bushveld-type layered intrusions emplaced in metasedimentary sequences. The age of the mafic-ultramafic intrusions remains poorly constrained, though they are regarded to be part of ca. 1375 Ma bimodal magmatism dominated by voluminous S-type granites. In this study, we investigated igneous monazite and zircon from a differentiated layered intrusion and metamorphic monazite from the contact aureole. The monazite shows contrasting crystal morphology, chemical composition, and U-Pb ages. Monazite that formed by contact metamorphism in response to emplacement of mafic-ultramafic melts is characterized by extremely high Th and U and yielded a weighted mean 207Pb/206Pb age of 1402 ± 9 Ma, which is in agreement with dates from the igneous monazite and zircon. The ages indicate that the intrusion of ultramafic melts was substantially earlier (by ~25 m.y., 95% confidence) than the prevailing S-type granites, calling for a reappraisal of the previously suggested model of coeval, bimodal magmatism. Monazite in the metapelitic rocks also records two younger growth events at ca. 1375 Ma and ca. 990 Ma, coeval with metamorphism during emplacement of S-type granites and tin-bearing granites, respectively. In conjunction with available geologic evidence, we propose that the Kabanga-Musongati maficultramafic intrusions likely heralded a structurally controlled thermal anomaly related to Nuna breakup, which culminated during the ca. 1375 Ma Kibaran event, manifested as extensive intracrustal melting in the adjoining Karagwe-Ankole belt, producing voluminous S-type granites. The Grenvillian-aged (ca. 990 Ma) tin-bearing granite and related Sn mineralization appear to be the far-field record of tectonothermal events associated with collision along the Irumide belt during Rodinia assembly. Since monazite is a ubiquitous trace phase in pelitic sedimentary rocks, in contact aureoles of mafic-ultramafic intrusions, and in regional metamorphic belts, our study highlights the potential of using metamorphic monazite to determine ages of maficultramafic intrusions, and to reconstruct postemplacement metamorphic history of the host terranes. [ABSTRACT FROM AUTHOR]

Titanite coexists with zircon in a felsic tuff in the Paleoproterozoic Timeball Hill Formation, southern Africa. Two generations of titanite are preserved: euhedral, brown crystals with apatite inclusions, and colorless, matrix-filling cement. The brown titanite has elevated U, Th and Fe, and low Al, consistent with a magmatic origin, whereas the colorless titanite has higher Al and F contents, suggestive of a metamorphic paragenesis. In situ SHRIMP geochronology of zircons extracted from a polished thin section of the tuff bed suffer from inheritance and Pb-loss and yield an imprecise date (2259±35Ma), whereas coexisting brown titanites give a weighted mean 207Pb/206Pb age of 2278±7Ma. This date is consistent with U Pb zircon ages for tuffs in the same stratigraphic unit from a nearby drill-hole. The brown titanite is interpreted to be magmatic and to provide a reliable estimate for the age of deposition. In situ U Pb dating of matrix-filling titanite cement from the same stratigraphic interval yields an age of 2145±12Ma, which corresponds with previous estimates for a low-grade tectonothermal event in southern Africa. Our results demonstrate that titanite is a versatile chronometer that can be used to constrain depositional ages, and those of low-grade metamorphism, thus increasing the number of sedimentary rocks that can be dated and elucidating the low-temperature geological history of depositional basins. [ABSTRACT FROM AUTHOR]

The Hadean (before 4.0 Ga) crust has long been considered to comprise mainly primitive mafic and ultramafic rocks. However, mineral inclusions in detrital zircons as old as 4.4 Ga from Jack Hills, Australia, have been interpreted to be magmatic and to provide evidence for extensive granitic crust. In situ U-Pb dating of monazite and xenotime inclusions in 4.25-3.35 Ga detrital zircons from Jack Hills shows that these inclusions are not magmatic, but formed during metamorphism at either 2.68 Ga or 0.8 Ga. Monazite-xenotime thermometry of intergrowths in the inclusions and the quartz-muscovite rock matrix constrain temperatures to between 420-475 °C, corresponding with conditions during peak regional metamorphism. Petrography and U-Pb geochronology of zircon inclusions from other localities show that the replacement of primary inclusions may commence in the igneous host rock and continue through weathering, sedimentation, and diagenesis. With increasing metamorphic grade, the inclusion assemblage increasingly reflects the composition of the rock matrix. In Jack Hills, most of the inclusions have the same composition and abundances as the metamorphic matrix, consistent with their formation during metamorphism. The titanium content of quartz inclusions indicates formation temperatures of 350-490 °C, supporting a metamorphic origin. Several lines of evidence indicate that at least some of the muscovite inclusions are also secondary. The lack of apatite inclusions in zircons from Jack Hills, relative to zircon in common granitic rocks, suggests that secondary minerals may have replaced primary apatite. Thus, detrital zircon may not be impermeable to post-depositional fluids, raising doubts about the use of the mineral inclusions they contain to infer initial magma chemistry. These results call for a reassessment of the source melts of the Hadean zircons and the composition of the earliest crust. [ABSTRACT FROM AUTHOR]

Abstract: Quartz-pebble conglomerate and quartzite at Eranondoo Hill in the Jack Hills belt in the northwestern Yilgarn Craton, Australia, contain monazite and xenotime grains in heavy mineral layers. In situ SHRIMP U–Th–Pb geochronology of detrital monazite and xenotime, as well as monazite and xenotime inclusions in detrital quartz, gives ages between 3.3 and 3.1Ga. The conglomerates, which host abundant Hadean zircon grains, contain metamorphic monazite in the muscovite matrix that yields an age of 2653±5Ma. This date is interpreted to represent the time of low-grade metamorphism and therefore is a direct minimum age for sediment deposition. The conglomerates also contain authigenic xenotime that gives an age of 800±25Ma, which suggests that part of the succession was affected by a previously unrecognised Neoproterozoic event. Authigenic xenotime in conglomerate immediately to the east of Eranondoo Hill gives an age of 2660±10Ma. Metamorphic monazite in garnet-bearing quartz-mica schist from the middle of the belt records an age of 1858±6Ma, which likely reflects an episode of deformation and low-grade metamorphism associated with major shearing. Several monazite grains from the schist indicate that the centre of the belt also experienced metamorphism at ∼2.65Ga. Massive magnetite–hematite ore from banded iron formation in the northeastern part of the belt contains authigenic xenotime that formed at 3080±20Ma. Co-existing monazite gives a similar age (3080±35Ma), indicating that at least part of the belt was deposited before ∼3.08Ga. A second population of xenotime was identified, indicating a later episode of heating and fluid flow at 1820±25Ma, possibly related to reactivation during the 1.83–1.78Ga Capricorn orogeny. The sedimentary rocks dated in this study were deposited before ∼2.65Ga, and in some cases, before ∼3.08Ga, indicating that much of the Jack Hills belt is Neoarchean or older. The metasedimentary rocks, and the Hadean zircons they preserve, experienced a protracted history of deformation and heated fluid flow spanning more than two billion years (3080-800Ma). [Copyright &y& Elsevier]

The authors reply to a comment on their paper which discussed the origin of mineral inclusions in detrital zircons from Jack Hills, Australia. They find that the critics failed to address the preponderance of muscovite and quartz inclusions or paucity of apatite, not only in Hadean zircons but also in younger Archean zircons from Jack Hills. The authors also suggest an alternative explanation to the heteregenous silicon/aluminum (Si/Al) ratios in muscovite inclusions.

The Earaheedy Basin is one of several Paleoproterozoic siliciclastic basins in the eastern part of the Capricorn Orogen in Western Australia. In the absence of an adequate chronostratigraphic framework, multiple conflicting models have been proposed for the tectonic history of the basin. We present in situ SHRIMP U–Pb zircon dates of 1971 ± 11 Ma, 1954 ± 5 Ma and 1949 ± 10 Ma from three thin tuff horizons (“micro-tuffs”) in the Yelma Formation at the base of the basin succession. These micro-tuffs are only slightly younger than the 1990 ± 6 Ma Imbin porphyry, which has been interpreted as a basement inlier. Our new dates, combined with field mapping, suggest that the Imbin porphyry is a volcanic–subvolcanic complex within the basin. Micro-tuffs in the Yelma Formation are more than 50 m.y. older than c. 1890 Ma micro-tuffs within the overlying Frere Formation. Deposition of the Yelma Formation was coeval with continent–continent collision farther west in the Orogen marked by the 2005–1950 Ma Glenburgh Orogeny, and the formation is possibly correlative with peripheral foreland deposits of the Padbury Group. The Frere Formation was deposited in shallow marine conditions within an intracratonic basin about 60 m.y. after the Glenburgh Orogeny. The Frere Formation is unconformably or disconformably overlain by the Miningarra Group, the youngest unit of which is the Mulgarra Sandstone. This sandstone was deposited after c. 1865 Ma, and derived either from exposed Frere Formation or from a now-buried source within the Capricorn Orogen that was uplifted by inception of the 1820–1770 Ma Capricorn Orogeny. Therefore, the Earaheedy Basin comprises at least three distinct, unconformity-bound successions deposited in contrasting tectonic settings. [ABSTRACT FROM AUTHOR]

Abstract: A population of oscillatory zoned, igneous zircon grains in a Javanese andesite contains fluid and mineral inclusions (up to 10 μm across) trapped during zircon growth. Orientation contrast imaging and orientation mapping by electron backscatter diffraction reveal that crystal-plastic deformation overprints growth zoning and has localized around 1–10 μm pores and inclusions. Cumulative crystallographic misorientation of up to 25° around pores and inclusions in zircon is predominantly accommodated by low-angle (<5°) orientation boundaries, with few free dislocations in subgrain interiors. Low-angle boundaries are curved, with multiple orientation segments at the sub-micrometer scale. Misorientation axes associated with the most common boundaries align with the zircon c-axis and are consistent with dislocation creep dominated by <100>(010) slip. A distinctly different population of sub-micron pores is present along subgrain boundaries and their triple junctions. These are interpreted to have formed as a geometric consequence of dislocation interaction during crystal-plasticity. Dislocation creep microstructures are spatially related to differences in cathodoluminescence spectra that indicate variations in the abundance of CL-active rare earth elements. The extent of the modification suggests deformation-related fast-pathway diffusion distances that are over five orders of magnitude greater than expected for volume diffusion. This enhanced diffusion is interpreted to represent a combination of fast-diffusion pathways associated with creep cavitation, dislocations and along low-angle boundaries. These new data indicate that ductile deformation localised around inclusions can provide fast pathways for geochemical exchange. These pathways may provide links to the zircon grain boundary, thus negating the widely held assumption that inclusions in fracture-free zircon are geochemically armoured once they are physically enclosed. [Copyright &y& Elsevier]