ALBERT

Description: Apatite can provide geologists with an exceptionally wide range of ages and temperatures to investigate processes that operate from Earth's surface right down to the lower crust. Apatite is a widespread accessory mineral in igneous, metamorphic, and clastic sedimentary rocks and can be dated using four radioactive decay schemes, each with a different temperature window for isotopic closure: Lu–Hf (675–750 °C); U–Pb (350–550 °C); apatite fission track (60–110 °C); (U–Th)/He (40–80 °C). The fission-track and (U–Th)/He methods are popular for studying upper-crustal and near-surface processes, whereas the U–Pb and Lu–Hf systems are used to investigate the thermal, tectonic, and magmatic histories of the deeper crust.

Description: U–Pb detrital zircon data from the Baixo Alentejo Flysch Group in the South Portuguese Zone show significant age differences between formations. The Visean Mértola Formation and Serpukhovian to early Bashkirian Mira Formation are dominated by zircons in the 316–388 Ma age range, whereas the late Bashkirian to late Moscovian Brejeira Formation is dominated by zircons with an age range of 498–687 Ma. Detrital zircons spanning an age range of 0.9–1.1 Ga are present in the Brejeira Formation but are absent in the Mértola and Mira formations. Detrital zircon ages of the Mértola and Mira formations indicate provenance from an extra-basinal source (Ossa–Morena Zone) with a minor intra-basinal contribution (South Portuguese Zone). The abundant presence of detrital zircon with age ranges of 500–750 and 0.9–1.1 Ga in the Brejeira Formation suggests a sediment source from the Avalon–Meguma terranes with limited recycling from the SW Portugal Domain. The different inferred source areas for the Baixo Alentejo Flysch Group formations are attributed to the presence of a forebulge that was formed in Mid-Visean times during the foreland phase of the South Portuguese Zone. The forebulge acted as a physical barrier separating sub-basins that accumulated the Mértola–Mira and Brejeira sediments respectively.

Description: Foreland basin sedimentary strata are key records of orogenic development, but identifying detrital thermochronometers of appropriate temperature sensitivity to detect provenance shifts can be challenging. Here, we utilize the medium-temperature apatite U-Pb system as a detrital thermochronometer. With a closure temperature range of ~375–550 °C, it is a system sensitive to the typical peak temperatures attained by the amphibolite-eclogite cores of many magma-poor Phanerozoic orogens, while apatite itself is a common accessory mineral that is virtually ubiquitous in clastic rocks. We present apatite U-Pb data from the Oligocene–Miocene Barrême and Valensole basins of the Alpine foreland in SE France, together with data from the rutile U-Pb, apatite fission-track (AFT), and zircon U-Pb systems. At ca. 26–25 Ma, an abrupt cessation of detrital apatites yielding Alpine U-Pb ages derived from the eclogite-facies core of the Western Alps records a major westward migration of the primary drainage divide of the orogen. This shift, not visible in AFT and zircon U-Pb data, was synchronous with uplift and exhumation of the Argentera external basement massif. The drainage connection from the foreland to the eclogite-facies core of the Western Alps was restored by ca. 13 Ma and likely as early as ca. 16 Ma, and the primary drainage divide location has likely remained stable since this time. The synchronicity of late Oligocene–early Miocene uplift and initial exhumation of the external basement massifs around the western Alpine arc suggests that a major westward drainage divide migration occurred throughout the Western Alps at that time.

Description: 〈span〉〈div〉Abstract〈/div〉The Paleo-Yangtze Basin is hypothesized to have connected the Sichuan, Xichang, and Chuxiong basins during the Late Mesozoic to Early Cenozoic. The Paleo-Yangtze Basin subsequently fragmented later in the Cenozoic, which makes it challenging to decipher its provenance shifts and the exhumation of the surrounding hinterland. Heavy mineral analyses combined with detrital apatite, rutile, and zircon U-Pb dating were employed to elucidate the stratigraphic signatures and exhumation patterns along the eastern margin of the Tibetan Plateau during the Late Cretaceous to Paleogene. Two key horizons have been identified across these basins based on their detrital mineralogy: a garnet-rich horizon within the southern Sichuan Basin, and a garnet-poor horizon in the Xichang and Chuxiong basins. Furthermore, there is a distinct increase in garnet-zircon (GZi) and apatite-tourmaline (ATi) indices in the Upper Cretaceous Gaokan Formation in the Sichuan Basin, indicating increased sediment input from metamorphic and granitic sources as a result of enhanced uplift and exhumation during the Late Cretaceous. A subsequent decrease in these indices in the Sichuan Basin during the Early Paleogene is attributed to the tectonic quiescence of the western Yangtze region during the Early Cenozoic. It should be noted that a contemporaneous decrease in ATi and RZi indices in both the Xichang and Chuxiong basins sections is observed from the Upper Cretaceous to the Paleogene. Detrital apatite, rutile and zircon U-Pb ages from the Sichuan, Xichang and Chuxiong basins share similar characteristics, and the zircon U-Pb data are dominated by age peaks of ca. 2600–2200 Ma, ca. 1900–1700 Ma, ca. 900–600 Ma, ca. 500–380 Ma, and ca. 260–200 Ma. These data indicate a dominant source from the northwestern and northern Yangtze areas and the region around the Kangdian rift, consistent with predominant southeast- and southwest-directed palaeocurrents in these basins. We thus argue that detritus derived from the northwestern and northern Yangtze areas and Kangdian rift region fed the basins located on the western Yangtze Block throughout Late Cretaceous to Early Paleogene times. The Lanping-Simao and Khorat basins on the Indo-China Block were also fed by these sedimentary source regions, indicating a major transcontinental drainage system across the western Yangtze Block.〈/span〉

Description: 〈p〉The development of laser ablation techniques using inductively coupled plasma mass spectrometry has enabled the routine and fast acquisition of 〈i〉in situ〈/i〉 U–Pb and Pb–Pb isotope ratio data from single detrital grains or parts of grains. Detrital zircon dating is a technique that is increasingly applied to sedimentary provenance studies. However, sand routing information using zircon analysis alone may be obscured by repeated sedimentary reworking cycles and mineral fertility variations. These biases are illustrated by two clear case studies from the Triassic–Jurassic of the Barents Shelf where the use of U–Pb geochronology on apatite and rutile and Pb–Pb isotopic data from K-feldspar is highly beneficial for provenance interpretations. In the first case study, U–Pb apatite ages from the (Induan – Norian) Havert, Kobbe and Snadd formations indicate an evolving provenance and identify possible episodes of storage within foreland basins prior to delivery onto the Barents Shelf. In the second case study, U–Pb rutile and Pb isotopic analyses of K-feldspar from the Norian–Pliensbachian Realgrunnen Subgroup provide a clear distinction between north Norwegian Caledonides and Fennoscandian Shield sources and suggest that a similar approach may be used to test competing models for sand dispersal for this Subgroup in regions farther north than this study.〈/p〉 〈p〉〈b〉Supplementary material:〈/b〉 U–Pb and trace element data for detrital apatite and detrital rutile, Pb isotopic data for detrital K-feldspar, U–Pb data for detrital zircon and a detailed analytical method is available at 〈a href="https://doi.org/10.6084/m9.figshare.c.4363838"〉https://doi.org/10.6084/m9.figshare.c.4363838〈/a〉〈/p〉

Description: The clastic record is commonly interrogated by analysis of detrital heavy mineral assemblages, with the bulk of modern detrital geochronological studies employing U-Pb dating of detrital zircon. However, the bias of zircon toward felsic igneous sources, and the limited ability of the U-Pb system in zircon to record low- to medium-grade metamorphic events, means that the U-Pb detrital zircon record is largely insensitive to magma-poor orogens. In this study, U-Pb ages were obtained by laser ablation–inductively coupled plasma–mass spectrometry for apatite and rutile extracted from alluvium of the French Broad River (FBR) in the southern Appalachians (southeastern United States). In contrast to previously published FBR U-Pb zircon data sets, which yield essentially no record of the most recent Appalachian metamorphic events (ca. 320 Ma) associated with assembly of Pangea, the U-Pb detrital rutile and especially the U-Pb apatite systems together provide a complete record of complex polyphase Appalachian orogenesis. It is unexpected that the apatite and rutile U-Pb Appalachian age populations differ significantly, with probable low-temperature breakdown of rutile biasing the rutile data set toward the most recent (Alleghanian) metamorphic event. These data make the FBR one of the most intensely studied river systems globally for multiproxy single-grain U-Pb analysis, clearly demonstrate dependence of provenance information on mineral proxy choice, and emphasize the resolving power of multiproxy provenance studies.

Description: The clastic record is commonly interrogated by analysis of detrital heavy mineral assemblages, with the bulk of modern detrital geochronological studies employing U-Pb dating of detrital zircon. However, the bias of zircon toward felsic igneous sources, and the limited ability of the U-Pb system in zircon to record low- to medium-grade metamorphic events, means that the U-Pb detrital zircon record is largely insensitive to magma-poor orogens. In this study, U-Pb ages were obtained by laser ablation–inductively coupled plasma–mass spectrometry for apatite and rutile extracted from alluvium of the French Broad River (FBR) in the southern Appalachians (southeastern United States). In contrast to previously published FBR U-Pb zircon data sets, which yield essentially no record of the most recent Appalachian metamorphic events (ca. 320 Ma) associated with assembly of Pangea, the U-Pb detrital rutile and especially the U-Pb apatite systems together provide a complete record of complex polyphase Appalachian orogenesis. It is unexpected that the apatite and rutile U-Pb Appalachian age populations differ significantly, with probable low-temperature breakdown of rutile biasing the rutile data set toward the most recent (Alleghanian) metamorphic event. These data make the FBR one of the most intensely studied river systems globally for multiproxy single-grain U-Pb analysis, clearly demonstrate dependence of provenance information on mineral proxy choice, and emphasize the resolving power of multiproxy provenance studies.

Description: Sediment provenance studies utilizing detrital geochronology are often limited by source-rock non-uniqueness with respect to mineral age. Thus, the development of integrated techniques which permit identification of source rock age and lithology are highly desirable. In this case study from the southern Massif Central, France, we target modern-river sediment from the river Tarn to assess the utility of combined U-Pb and multiple trace-element geochemical analysis of detrital apatite as a provenance tool. The study area was chosen because the sediment source areas chiefly comprise a relatively simple mix of medium- to low-grade Variscan metasediments and late Variscan granitoids, which should yield detrital apatite readily distinguishable by age, trace-element chemistry, or both. Based on comparison with previously published apatite trace-element data from metasedimentary rocks and granitoids, pelitic apatite in the river Tarn detritus is primarily distinguished by high Sr/Mn, light rare-earth element depletion (LREE) and low actinide contents, whereas granitic apatite is characterized by much lower Sr/Mn, and high LREE and actinide abundances. These source rock determinations are highly consistent with apatite trace-element data from Tarn tributaries that drain either predominantly metapelitic or granitoid catchments. U-Pb analysis of detrital rutile was also undertaken on those catchments for comparative purposes. As pelitic and granitic apatite can be readily distinguished, samples that have experienced downstream sediment mixing can then be comprehensively characterized. Using this method we identify a source lithology for nearly every analyzed apatite grain in the river sediment, even though 59% of the analyzed grains do not yield reliable U-Pb ages. © 2018. American Geophysical Union. All Rights Reserved.