Ga Grains Research Articles

Reconstruction of microcontinents during the assembly of Rodinia: A case study from the Central Tianshan Block

There are multiple Precambrian microcontinents within the Central Asian Orogenic Belt (CAOB), and the paleogeographic locations of those in the southwest CAOB (e.g. Central Tianshan Block, Yili Block, Aktau-Mointy terrane, Erementau-Niyaz terrane and Kokchetav terrane) remain controversial. The Central Tianshan Block is one of the representative Precambrian microcontinents in the southwest CAOB. It can be divided into the western Central Tianshan Block and eastern Central Tianshan Block. To investigate their relationships and paleogeographic locations, we collected metasedimentary rocks from the Kekesu Group in western Central Tianshan Block. The results of zircon LA–ICP–MS U-Pb dating, combining with previously obtained magmatic ages, suggest that the Kekesu Group was deposited during the latest Mesoproterozoic to earliest Neoproterozoic (ca. 1010–969 Ma). Detrital zircon populations of Kekesu Group were dominated by 2.0–0.9 Ga grains with an age peak at 1474 Ma. The Archean to end-Paleoproterozoic zircon grains had variable εHf(t) values from +9.7 to −8.7, and most of the Mesoproterozoic zircons had positive εHf(t) values (+13.9 to +2.4). The Kekesu Group in western Central Tianshan Block has similar zircon age populations and εHf(t) values to those in eastern Central Tianshan Block. In combination with magmatism, these data suggest that western and eastern Central Tianshan Block had similar tectonic affinities and locations during the earliest Neoproterozoic. We compiled 4566 concordant detrital zircon U-Pb age data of Mesoproterozoic to early Neoproterozoic strata from microcontinents in the southwest CAOB and potential sources. The zircon age probability density plots, kernel density estimates and multidimensional scaling plot suggest that the Central Tianshan and Yili blocks had similar sources mainly derived from the Musgrave Province in central Australia in the latest Mesoproterozoic to early Neoproterozoic, and were possibly located near Australia during assembly of Rodinia. The sources for the Erementau-Niyaz and Kokchetav terranes were probably mainly within the northern Laurentia in the latest Mesoproterozoic to earliest Neoproterozoic.

Zircon dating of the basalt and felsic dyke in Haveri, SW Finland

The E-MORB type Haveri basalt differs from the volcanic arc type rocks in the Tampere belt showing no subduction signature. It is considered to have formed in a marginal basin prior to the volcanic arc. We present here zircon U-Pb dating on two samples. The basalt and the felsic dyke yielded 207Pb/206Pb ages of 1902 ± 5 Ma and 1891 ± 2 Ma, respectively, interpreted as crystallisation ages. The basalt also contains older 1.98 Ga grains while the felsic dyke contains older 1.92 Ga, 1.94 Ga, 1.98 Ga and 2.0 Ga grains, which are inferred as inherited. The age dating of E-MORB type basalts can be used to identify the extensional episodes of the accretionary Svecofennian orogeny.

Provenance of Devonian–Carboniferous strata of Colorado: The influence of the Cambrian and the Proterozoic

ABSTRACTWe report new LA-ICP-MS U–Pb detrital zircon ages and sedimentary petrology of silty to sandy limestones and dolostones, as well as calcareous to dolomitic sandstones of the Devonian–Carboniferous (Mississippian) Chaffee Group. We also report new detrital zircon ages from the late Cambrian Sawatch Quartzite, and a U–Pb zircon crystallization age on a late Mesoproterozoic (1087.9 ± 13.5 Ma) granitoid of underlying basement from the Eagle Basin of northwest Colorado. Grain populations in the Chaffee Group are mostly bimodal. More than 84% of zircons centered around a Paleoproterozoic (ca. 1.78 Ga) mode typical of the Yavapai province that forms much of the basement of Colorado and an early Mesoproterozoic (ca. 1.42 Ga) mode typical of A-type granites that intrude this region. A notable late Mesoproterozoic (ca. 1.08 Ga) mode exists in some Chaffee samples, giving those samples a trimodal detrital zircon age distribution. These bipartite or tripartite detrital zircon age modes exist in Cambrian, Devonian, and Carboniferous strata from paleogeographically adjacent successions, but the correlation between the Chaffee zircons is highest with the region’s basal Cambrian sandstones of the Sawatch Quartzite, Flathead Sandstone, and Ignacio Quartzite, which have similar (ca. 1.08 Ga, 1.43 Ga, 1.70 Ga, respectively) zircon populations, and a paucity of > 1.8 Ga grains. This similarity suggests that most grains in the Chaffee Group derive from recycling of these basal sandstones, and that little sediment was derived directly from thenexposed Precambrian basement highs, from the Wyoming craton to the north, or from Paleoproterozoic arcs and orogens to the west and northeast. Minor Mesoarchean to early Paleoproterozoic (ca. 3.00 to 2.40 Ga) grains exist in the Chaffee Group, an attribute shared by the Late Ordovician Harding Sandstone of Colorado’s Front Range, but that is absent from the region’s underlying Cambrian sandstones—suggesting some recycled mixture of Cambrian and Ordovician sedimentary rocks. No near-depositional age grains are present in the Chaffee Group. The youngest grain is Early Devonian (~417 Ma), > 45 million years (m.y.) older than these strata. Additionally, Paleozoic grains are extremely uncommon (< 0.1%; n = 2,927 grains).

Reappraising the Provenance of Early Neoproterozoic Strata in the Southern–Southeastern North China Craton and Its Implication for Paleogeographic Reconstruction

The early Neoproterozoic sediments in the southern–southeastern (S-SE) North China Craton (NCC) are critical in paleogeographic reconstruction. We present new detrital zircon U–Pb–Hf data of five sandstone samples from the Sangwon Supergroup in SE-NCC and the Wufoshan Group in S-NCC. We integrate published zircon U–Pb data to appraise their provenance. The new dataset constrains the maximum depositional age of the Sangwon Supergroup to be ca. 1.0 Ga. The similar provenance transition and the comparable sequence stratigraphy imply that the Wufoshan Group could be an extension of the Xuhuai–Dalian–Pyongnam basins in the SE NCC with a maximum depositional age of ca. 1.0 Ga. The zircon age spectra of the successions show four major populations at ca. 2.5 Ga, ca. 2.0–1.8 Ga, ca. 1.6–1.4 Ga, ca. 1.3–1.0 Ga, with rare >2.5 Ga grains. The Archean–Paleoproterozoic grains could be derived from the NCC, which is confirmed by their εHf(t) values. After a review of the possible paleocontinental reconstructions, we suggest that the ca. 1.6–1.0 Ga grains with different εHf(t) values (mostly positive) were from the southwestern Congo craton, supporting a NCC–SW Congo/SE NCC-S São Francisco connection at ca. 0.9 Ga.

Growth of the Archean sialic crust as revealed by zircon in the TTGs in eastern Finland

U–Pb age determinations on zircon from granitoids in the Archean of eastern Finland were conducted using the SIMS, LA-ICP-MS and TIMS techniques, with an emphasis on low-HREE granitoids. The oldest rocks in the Fennoscandian Shield are 3.4–3.5 Ga. Several samples were collected close to these rocks, but none of the samples were as old, indicating that the oldest rocks are only small, possibly allochthonous fragments in the Neoarchean bedrock. Some tonalite–trondhjemite–granodiorite (TTG) samples yielded homogeneous 2.72–2.73 Ga zircon populations, and in these samples, the initial εNd was also close to the depleted mantle (DM) values. However, several granitoid samples with a main zircon population of 2.7–2.8 Ga had 2.9–3.2 Ga grains or inherited cores, and in some samples, all grains were of 2.9–3.0 Ga. In these samples, the εNd value was also close to zero or slightly negative. These features suggest that apart from the juvenile Neoarchean magmas, the abundance of reworked 2.9 Ga material is considerable in the Archean crust, which developed during successive juvenile magmatic inputs that melted and assimilated the older sialic crust. The low- HREE geochemical character of granitoids has no correlation with their age, with the low-HREE granitoids yielding an age span of 2.72–2.98 Ga.

Igneous rock area and age in continental crust

Abstract Rock quantity and age are fundamental features of Earth's crust that pertain to many problems in geoscience. Here we combine new estimates of igneous rock area in continental crust from the Macrostrat database (https://macrostrat.org/) with a compilation of detrital zircon ages in order to investigate rock cycling and crustal growth. We find that there is little or no decrease in igneous rock area with increasing rock age. Instead, igneous rock area in North America exhibits four distinct Precambrian peaks, remains low through the Neoproterozoic, and then increases only modestly toward the recent. Peaks in Precambrian detrital zircon age frequency distributions align broadly with peaks in igneous rock area, regardless of grain depositional age. However, detrital zircon ages do underrepresent a Neoarchean peak in igneous rock area; young grains and ca. 1.1 Ga grains are also overrepresented relative to igneous area. Together, these results suggest that detrital zircon age distributions contain signatures of continental denudation and sedimentary cycling that are decoupled from the cycling of igneous source rocks. Models of continental crustal evolution that incorporate significant early increase in volume and increased sedimentation in the Phanerozoic are well supported by these data.

Zircon and apatite U-Pb age constraints from the Bundelkhand craton and Proterozoic strata of central India: Insights into craton stabilization and subsequent basin evolution

The geologic processes involved in attaining strengthened cratonic lithosphere remain debated despite their importance for stabilization and long-term preservation. In central India, stabilization of the Bundelkhand craton has conventionally been attributed to the youngest magmatic event impacting the craton at ~ 2.5 Ga, though the post-amalgamation evolution of the craton prior to Proterozoic basin development is poorly understood. This study presents new basement zircon and apatite U-Pb age data along with new detrital zircon U-Pb age data from Proterozoic marginal sedimentary basin deposits to explore the post-magmatic and burial evolution of the Bundelkhand craton. Apatite from ~ 3.4–2.5 Ga granitoids and gneisses collected across the ~ 250 km wide craton yielded near uniform U-Pb ages between ~ 2.4–2.3 Ga, indicating broad-scale exhumation of the Bundelkhand craton through mid-crustal depths following amalgamation and felsic magmatism. Unroofing of the Bundelkhand craton at this time is corroborated by ~ 2.7–2.5 Ga detrital zircon U-Pb age peaks from basal sandstones of the Bijawar and Gwalior groups, which lie in direct nonconformable contact with the craton along both its southeastern and northwestern margins, respectively. These age populations reveal an abundance of zircon sourced directly from the Bundelkhand craton, and a sub-population of ~ 2.2–2.3 Ga grains provide a maximum depositional age for the oldest strata deposited on the craton. We speculate that the redistribution of heat producing elements associated with shallow emplacement of Bundelkhand granitoids and subsequent erosion may have enhanced lithospheric strengthening and facilitated a long-term thermal regime that promoted craton stability by ~ 2.2 Ga. Following stabilization, far-field marginal tectonism likely influenced the vertical motions within the Bundelkhand craton, inducing stages of broad subsidence and erosion recorded within the strata of the Lower and Upper Vindhyan successions.

Detrital zircon U-Pb and Hf constraints on provenance and timing of deposition of the Mesoproterozoic to Cambrian sedimentary cover of the East European Craton, part II: Ukraine

We present the U-Pb geochronology and Hf isotope analysis of detrital zircons from the Ediacaran/Cambrian sediments of Podillya and south Volyn in western Ukraine, supplemented by the bulk rock XRD mineralogy of the host rocks. Such a combined analytical approach allows for identifying the source areas supplying detritus to sediments and for constraining an age of deposition. Our provenance analysis is based on fourteen samples collected from six exposures, mostly in the valley of the Dniester river. 84 mudstone samples were also examined by the XRD method. U-Pb dating of detrital zircons yielded two sets of maximum depositional ages: 578–546 Ma and 547–523 Ma, for the Mohyliv-Podilsky and Kanyliv Series, respectively. This suggests that the Ediacaran-Cambrian boundary in Podillya coincides with a major erosional gap, with a major change in provenance, and the disappearance of the Ediacaran fauna at the base of the Kanyliv Series, with implications for the stratigraphy and paleogeography of the entire East European Platform. Zircon U-Pb age spectra from the lower part of the Mohyliv-Podilsky Series include a large quantity of 2.2 to 1.9 Ga grains that reveal predominantly negative to nearly chondritic ɛHf values, jointly suggesting detritus supply from the crystalline basement of Sarmatia. Both U-Pb and mineralogical data also indicate a major contribution of volcanic detritus from the Volyn flood basalts. The younger Nagoryany rocks yielded zircon age spectra with peaks at c. 1.80 and 1.49 Ga, implying a shift of the catchment area to Fennoscandia. Above an erosional gap, the zircon age spectra in the Kanyliv and Baltic Series are dominated by peaks at 560–535 Ma. These data and ɛHf values ranging from negative to chondritic and juvenile suggest, in line with the mineralogical data, detritus supply from a continental magmatic arc and collisional orogen. Thus, we interpret the Kanyliv Series as infill of an early Cambrian foreland basin that was established in front of the Scythides and Santacrucides orogens, overriding the SW margin of Baltica.

Unraveling a hidden Rhyacian magmatic arc through provenance of metasedimentary rocks of the Crixás greenstone belt, Central Brazil

The Crixás-Goiás Domain, Central Brazil, comprises an association of Archean TTG (tonalite-trondhjemite-granodiorite) and Archean-Paleoproterozoic greenstone belts. The Crixás greenstone belt, located in the northern part of the domain, is constituted of Mesoarchean metakomatiites and metabasalts overlaid by Paleoproterozoic carbonaceous phyllites and metagraywackes that host important gold deposits. In this work, we present new integrated petrographical, geochemical and isotopic data to investigate the provenance and tectonic setting of the metasedimentary rocks of the Crixás greenstone belt. Mineral and textural immaturities, particularly of the metagraywackes, are consistent with a proximal source area; which is also evidenced by the chemical aspects that denote absence of significant sediment sorting and recycling. Geochemical data indicate provenance from mixed mafic and felsic sources. U-Pb detrital zircon geochronology shows unimodal age spectra, with a dominant peak at ca. 2.2–2.1 Ga and minor contribution of ca. 2.7 and 2.4 Ga grains. Concordia diagrams yielded upper intercepts between 2213.4 ± 5.6 and 2162.2 ± 9.2 Ma, interpreted as the age range of the main source area. The rocks present relatively homogeneous Sm-Nd model ages (2.44 to 2.34 Ga) and slightly negative to positive ƐNd, ranging from −0.27 to +1.25. We argue that the original sedimentary protoliths were deposited in a trench or forearc basin related to a ca. 2.2–2.1 Ga island arc formed adjacent to an Archean paleocontinent constituted of TTG-greenstone terranes. Rapid erosion of the rising intraoceanic arc resulted in syn-orogenic sedimentation that started with deep ocean black shale deposition in an anoxic environment followed by episodic turbidite currents of poorly-sorted immature sands. Sedimentary rock layers were tectonically transported to the continent and preserved in the elongated keels of Mesoarchean mafic–ultramafic volcanic rocks. This orogeny can be correlated with the amalgamation of the Goiás Massif during the Siderian-Rhyacian crustal growth related to successive collage of arc-related terranes in the formation of the western margin of the São Francisco paleocontinent. Sediment provenance of the Crixás greenstone belt proved to be crucial to the recognition and characterization of Paleoproterozoic crust that are apparently no longer preserved in outcrops.

U-Th-Pb Geochronology and Lu-Hf Isotope Geochemistry of Detrital Zircons in Metasedimentary Rocks of the Southern Coast Mountains Batholith

Abstract Most metasedimentary rocks in the southern Coast Mountains batholith are of uncertain tectonic affinity because they occur in discontinuous pendants surrounded by large intrusive bodies, and many protolith features are obscured by regional deformation and metamorphism. This study uses U-Th-Pb ages and Lu-Hf isotope signatures of detrital zircons in metasedimentary rocks in Bute, Loughborough, and Knight Inlets in an effort to test possible correlations with the adjacent Wrangellia, Alexander, Taku, Yukon-Tanana, and Stikine terranes. Detrital zircons from metasedimentary samples yield ages that belong to age groups of 590-528 Ma (peak age of 560 Ma), 485-432 Ma (peak age of 452 Ma), 356-286 Ma (peak age of 307 Ma), and 228-185 Ma (peak ages of 215 and 198 Ma). A small number of ~1.1-1.9 Ga grains are also present. εHft values of the 590-185 Ma grains yield a progression from intermediate (0 to +5) values to more juvenile (mostly +4 to +15) values from Neoproterozoic through early Mesozoic time. The Comparison of these results with similar data sets from adjacent terranes demonstrates that primary connections with the Yukon-Tanana and Taku terranes are unlikely but are consistent with primary connections with the Wrangellia, Stikine, and/or Alexander terranes. Unfortunately, the available constraints are not sufficient to eliminate any of these options or the possibility that the pendants are a unique tectonic fragment. Zircons from the metasedimentary samples also yield U-Th-Pb ages of 165-128 Ma (peak age of 152 Ma) and 114-88 Ma (peak age of 102 Ma). εHft values of these zircon domains are mostly juvenile (+7 to +13). Comparison of U concentrations, U/Th values, and CL textures of zircons from the metasedimentary samples, leucocratic sills that intrude the pendants, and surrounding plutonic bodies suggests that most of the young grains, as well as widespread younger rims on older grains, grew during metamorphism associated with emplacement of the adjacent plutonic bodies. Some young grains were derived from thin felsic sills or veins that were unintentionally included in the sampled material.

Birthplace of the São Francisco Craton, Brazil: Evidence from 3.60 to 3.64 Ga Gneisses of the Mairi Gneiss Complex

AbstractRecords of Earth's primitive crust are scarce. Eoarchean (older than 3.6 Ga) banded mafic to felsic gneisses have been discovered in the São Francisco Craton, Brazil, pushing back by over 100 million years the oldest gneisses known to date in South America (3.5 Ga). Zircon U‐Pb data yield rock ages from 3,598 to 3,642 Ma with a few ca. 3.65–3.69 Ga grains suggesting even older rocks in the area. Zircon grains show significantly negative to nearly chondritic initial εHf values and two‐stage model ages from 3.82 to 4.33 Ga, which may indicate the existence of a recycled Hadean to early Eoarchean crust in the region. The felsic gneisses are chemically similar to the low‐pressure Tonalite‐Trondhjemite‐Granodiorite association whereas the mafic gneisses have geochemical signatures that resemble within‐plate basaltic andesite to andesite of Iceland (icelandites). The results are relevant to constrain the composition of Earth's first continental crust.

Provenance of the Newfoundland Appalachian foreland basins

The tectonic history of the Appalachian orogen is recorded in the adjacent foreland. Offshore seismic data suggest that an initial, Middle Ordovician foreland basin was filled by sources in Newfoundland, whereas the Late Ordovician foreland basin was loaded and filled by sources to the SW, in the Quebec segment of the orogen, where NW-vergent Taconian arc-continent collision may have continued later than in Newfoundland. We test this hypothesis using U/Pb ages of detrital zircon within foreland basin successions in western Newfoundland. Previously published results from the oldest foreland succession, the Middle Ordovician Goose Tickle Group, demonstrate ages similar to units preserved with the Humber Arm Allochthon, emplaced during Middle Ordovician Taconic orogenesis, including a predominant Paleoproterozoic peak at 1.85 Ga. In this study we investigated U/Pb geochronology of detrital zircon from younger foreland successions from Upper Ordovician to Devonian. The largest proportion of analyses within all foreland successions fall between 0.95 and 1.3 Ga, with largest peaks occurring between 1.0 and 1.1 Ga, typical of zircon derived from the Grenville Orogen. Earlier Mesoproterozoic and Paleoproterozoic ages range from 1.3 to 2.0. The abundance of Mesoproterozoic grains and conspicuous lack of 1.85 Ga Paleoproterozoic zircon in the overlying Upper Ordovician Long Point Group and latest Silurian to Early Devonian Clam Bank Formation indicates that these sediments were not derived from units within the Humber Arm Allochthon. Probability density plots of continental margin units in the Quebec/New England segment of the orogen demonstrate a similar strong Mesoproterozoic and weak Paleoproterozoic signature, suggesting derivation of the Long Point Group from the Quebec segment of the orogen. Within the mid-Paleozoic Clam Bank - Red Island Road succession, typical Gondwanan ages are absent and 1.0 Ga grains derived from the Grenville Orogen are abundant. This is consistent with underthrusting of the Gondwanan microcontinents Ganderia and Avalonia during Salinic and Acadian orogenesis. Only Mesoproterozoic zircon grains were found in the Early Devonian Red Island Road Formation, consistent with derivation from Mesoproterozoic Grenville massifs in western Newfoundland or Cape Breton Island which were exhumed during Devonian Acadian inversion.

U–Pb zircon geochronology of Proterozoic and Paleozoic rocks, North Islesboro, coastal Maine (USA): links to West Africa and Penobscottian orogenesis in southeastern Ganderia?

Within the Ganderian inlier of Penobscot Bay, coastal Maine, the Islesboro fault block occupies a central position between the St. Croix terrane of continental affinity and, to the east, the Ellsworth terrane of oceanic affinity. New field, petrographic, geochemical, and U–Pb LA-ICP-MS geochronological data on detrital and magmatic zircon grains constrain the provenance and transfer history of these terranes from Gondwana to the Appalachian margin of Laurentia. On North Islesboro, the Coombs Limestone and Hutchins Island Quartzite (new name), intruded by E-MORB amphibolite, constitute a newly recognized local inlier of Proterozoic basement. Together with the nearby Seven Hundred Acre Island Formation, these mature, carbonate-rich strata record deposition on a low-latitude passive margin. Abundant detrital zircon grains in the Hutchins Island Quartzite, all older than ca. 1.8 Ga, have a predominant population at ca. 2.0 Ga and a small peak between ca. 2.8 Ga and 2.4 Ga, an age spectrum strikingly similar to those of both the Paleoproterozoic Taghdout Quartzite in Morocco, on the West African craton, and basement rocks from Georges Bank, offshore Massachusetts. The overlying Neoproterozoic–Cambrian Islesboro Formation records a second period of extension (interstratified EMORB greenstone) synchronous with accumulation of interbedded siliciclastic and carbonate sediment, prior to recumbent folding. At the base of the moderately deformed Turtle Head Cove (new name) cover sequence, immature greywacke has a youngest zircon population of ca. 515 Ma, large late Neoproterozoic populations (ca. 624 Ma and 678 Ma), a small peak at 1.2 Ga, a moderate number of ca. 1.5 Ga to 2.0 Ga grains, and a few Late Archean grains. Compared with many similar Ganderian age spectra reported from Vermont to New Brunswick, which are all consistent with a source in either the Amazonian or West African cratons, this new age spectrum most closely resembles those from quartzites in the Grand Manaan and Brookville terranes of coastal New Brunswick. Significantly, exotic blocks lithologically indistinguishable from Proterozoic strata on Islesboro occur in the St. Croix terrane within a Lower Ordovician black shale mélange at the base of the Penobscot Formation, suggesting that the St. Croix terrane, Islesboro block, and Ellsworth terrane were initially juxtaposed by Penobscottian thrusting prior to the Middle Ordovician. Subsequently, the Islesboro block was isolated between the bounding post-Silurian, pre-Late Devonian Turtle Head and Penobscot Bay dextral strike-slip faults. Along the North Islesboro fault, a fault-bounded lens of foliated pyritic felsic volcanic and volcaniclastic rock, dated at ca. 372 Ma and containing Devonian to Archean detrital zircons, records late Paleozoic deformation recognized previously in coastal New Brunswick but not in Penobscot Bay.

The Paleoproterozoic Vishnu basin in southwestern Laurentia: Implications for supercontinent reconstructions, crustal growth, and the origin of the Mojave crustal province

The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8–1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ∼1.87 and 2.4–2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ∼1.8 to 1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ∼1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791 ± 15 to 1691 ± 15 Ma. Plutons show a systematic change in Hf-isotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8–2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7–1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.

Juvenile crust formation in the Zimbabwe Craton deduced from the O-Hf isotopic record of 3.8–3.1 Ga detrital zircons

Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9–2.8Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207Pb/206Pb age spectra (concordance >90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35Ga, corresponding to documented geological events, both globally and within the Zimbabwe Craton. Zircon δ18O values from +4 to +10‰ point to both derivation from magmas in equilibrium with mantle oxygen and the incorporation of material that had previously interacted with water in near-surface environments. In εHf-time space, 3.8–3.6Ga grains define an array consistent with reworking of a mafic reservoir (176Lu/177Hf ∼0.015) that separated from chondritic mantle at ∼3.9Ga. Crustal domains formed after 3.6Ga depict a more complex evolution, involving contribution from chondritic mantle sources and, to a lesser extent, reworking of pre-existing crust. Protracted remelting was not accompanied by significant mantle depletion prior to 3.35Ga. This implies that early crust production in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs that were tapped by later magmas, possibly because the volume of crust extracted and stabilised was too small to influence (asthenospheric) mantle isotopic evolution. Growth of continental crust through pulsed emplacement of juvenile (chondritic mantle-derived) melts, into and onto the existing cratonic nucleus, however, involved formation of complementary depleted subcontinental lithospheric mantle since the early Archaean, indicative of strongly coupled evolutionary histories of both reservoirs, with limited evidence for recycling and lateral accretion of arc-related crustal blocks until 3.35Ga.

Evidence of early Archean crust in northwest Gondwana, from U–Pb and Hf isotope analysis of detrital zircon, in Ediacaran surpacrustal rocks of northern Spain

The Mora Formation (Narcea Group) is one of the oldest Precambrian supracrustal successions in northern Spain. Here, we use U–Pb and in situ Hf isotope analysis on detrital zircon to determine its age and provenance. The youngest U–Pb dates constrain the maximum depositional age of the Mora Formation at 565 ± 11 Ma. Results indicate: (1) a dominant Ediacaran zircon population (33%; 565–633 Ma, Cadomian) within a spectrum of Neoproterozoic ages (40%; 636–996 Ma); and (2) smaller Mesoproterozoic (5%; 1004–1240 Ma), Palaeoproterozoic (11%; 1890–2476 Ma) and Archean (11%; 2519–3550 Ma) populations. Results here do not point to one specific cratonic source area; instead, detritus may have been derived from the West African craton and Amazonia, or even the concealed Iberian basement. The lack of 1.3–1.8 Ga grains suggests exclusion of the Sahara Craton as a major source, but this is not certain. This mixed composition favours a complex source history with reworking of detritus across terrane/craton boundaries. Hafnium isotope compositions indicate a range of crustal and juvenile sources, with initial eHf values between −15.8 and 11.1, and Hf model ages from 0.8 to 3.7 Ga. For Neoproterozoic zircons (80%), juvenile components (eHf(i) ~+10) may be related to Rodinia fragmentation and the onset of an active margin setting leading to the Cadomian orogeny. Palaeoproterozoic to Paleoarchean grains (20%) all have negative eHf values and Meso- to Eoarchean Hf model ages. This indicates an early (Archean) sialic crustal component for northwestern Gondwana.

Detrital zircon geochronology and provenance of the Middle Cambrian Flathead Sandstone, Park County, Wyoming

We report the results of analyses of detrital zircon from the middle Cambrian Flathead Sandstone from four locations in Park County, WY. The Flathead U-Pb zircon age spectra (n=355) includes one peak at 2702–3345 Ma (95%) and two small age peaks at 1784 Ma (4%) and ∼1830 Ma (1%). Regional paleocurrent data for the Flat-head indicate sediment transport from east to west but the dominant Archean detrital zircons in our sample suite indicates a proximal source in structurally and topographically high Wyoming Province crust. The Archean ages are consistent with the >3.0 Ga fraction being derived from basement rocks present in the northern Beartooth Range and areas further to the west. The 2.8–3.0 Ga grains were derived from the Beartooth-Bighorn magmatic zone, which directly underlies and extends to the south and east of these Flathead sampling localities. The 1.7–1.8 Ga grains were derived from 100s of km to the south and east, in the Cheyenne Belt or more distal Yavapai Province rocks. This 1.7–1.8 Ga crust was uplifted along the Transcontinental Arch during late Precambrian time. The Transcontinental Arch, and perhaps the Midcontinent Rift further to the east as well, prevented easterly-sourced (i.e. Grenville) zircons from reaching the Wyoming Sauk shoreline.

Age and depositional setting of the Paleoproterozoic Gantaohe Group in Zanhuang Complex: Constraints from zircon U–Pb ages and Hf isotopes of sandstones and dacite

The Gantaohe Group, located in the Zanhuang area, contains typical North China Craton Paleoproterozoic strata, and is one of the best-preserved examples of these rocks. It is therefore a significant area for deciphering evolutionary process in the trans-North China Orogen. In this paper, we present petrologic, detrital and volcanic zircon U–Pb ages and Lu–Hf isotopes from three formations of the Gantaohe Group. The zircons from a conglomerate at the base of the Gantaohe Group yielded a young population of ca. 2.1Ga. Dacite from the upper section of the Nansizhang Formation yielded zircon 207Pb/206Pb ages of 2090±14Ma. Thus, the initial deposition of the Gantaohe Group occurred at ∼2.1Ga. The Group underwent greenschist to low amphibolite facies metamorphism, identical to regional 1.87–1.85Ga metamorphism in Zanhuang Complex. Therefore, we propose that the Gantaohe Group was deposited in the period between 2.1Ga and 1.9Ga. Sandstones in the Gantaohe Group are dominated by Q (monocrystalline and polycrystalline quartz) and F (K-feldspar and plagioclase) with only minor lithic fragments, suggesting that detrital components were mainly derived from a cratonic source. Detrital zircons from sandstones exhibit a dominant population of 2.55–2.5Ga grains with minor contributions of ∼2.7Ga and ∼2.1Ga grains, which further indicate they were dominantly sourced from ∼2.5Ga TTG gneisses, late Archean Zanhuang and late Archean to Paleoproterozoic Guandu Groups, with subordinately input from ∼2.7Ga TTG gneisses and the Paleoproterozoic Xuting granite. Given that the sandstone clastic components chiefly comprise quartz and feldspar, there is a lack of volcanic lithics, and the major detrital zircons are 400Ma older than the initial deposition age, we propose that the Gantaohe Group was deposited in a rift-related setting far from any back-arc basin at ∼2.1Ga or 1880–1810Ma syn-collisional processes. A few 2.5–2.7Ga zircons have positive εHf(t) values identical with the contemporaneous depleted mantle and the 207Pb/206Pb ages are similar to their single stage model ages (TDM1), which suggests some degree of 2.5–2.7Ga crustal growth. Most ∼2.5 zircons, however, show 207Pb/206Pb ages that are much younger than the Hf single stage model ages, which indicates intensive crustal reworking in the late Neoarchean. The zircons collectively present two stage model ages ranging from 2.6 to 2.9Ga. Therefore, we suggest that the 2.6–2.9Ga periods represents a time of intensive crustal growth in the Zanhuang Complex.

Microstructural characteristics and in situ reinforcement in NbC-doped Fe81Ga19 magnetostrictive alloys

Abstract The microstructural phase distribution and the origin of mechanical enhancement of NbC-doped Fe–Ga magnetostrictive alloys are not very clear. In this study, a detailed investigation into the microstructural characteristics and their effects on the mechanical properties has been performed on NbC-doped Fe81Ga19 alloys. Increasing NbC causes remarkable grain refinement of the Fe-Ga alloy matrix. Rod-like NbC particles precipitating in Fe–Ga grain matrix and NbC dendrite clusters segregating at Fe–Ga grain boundaries are observed. As a result, NbCparticle / Fe81Ga19 and (NbCparticle + NbCdendrite) / Fe81Ga19 magnetostrictive composites are obtained. The mechanical properties are found to be significantly improved by combined effects of grain-refinement strengthening and precipitate reinforcement, generating a maximal tensile strength of 620 MPa and an elongation ratio of 0.9%.

Age and implications of the phosphatic Birmania Formation, Rajasthan, India

The Birmania inlier in western Rajasthan, India, contains important phosphate deposits, the depositional age of which is poorly constrained. Here we provide the first direct age constraints for the phosphate-bearing Birmania Formation using a combination of paleontological and detrital zircon U–Pb data. The occurrence of the multicellular algal fossil Wengania exquisita in phosphatic chert of the Birmania Formation suggest that it was deposited during the Ediacaran. Detrital zircon age distributions contain prominent populations of 1.7–1.9Ga grains, with subordinate younger grains that range from 650 to 980Ma. The distribution broadly resembles those from Neoproterozoic strata from both cratonic and Himalayan India but, like zircon age distributions from the Marwar Group, lacks 1.0–1.2Ga grains, which may suggest that both areas shared similar local sources. The lack of zircon grains younger than ∼650Ma is consistent with an Ediacaran depositional age because almost all Cambrian or younger strata from India have yielded Cambrian or latest Neoproterozoic age grains. These findings raise the possibility of a previously unrecognized late Neoproterozoic episode of phosphogenesis on the India craton. The presence of Wengania exquisita further supports strong palaeogeographic affinity between the India and South China during the Neoproterozoic and Cambrian.