The Colorado River has flowed across the dextral strike-slip plate boundary between the North American and Pacific plates since the latest Miocene or earliest Pliocene. The Fish Creek-Vallecito Basin ...(FCVB) lies on the Pacific Plate in southern California, dextrally offset from the point where the modern Colorado river enters the Salton Trough; it contains a record of ancestral Colorado River sedimentation from 5.3–2.5 Ma. The basin stratigraphy exhibits a changing balance between locally derived (L-Suite) and Colorado River (C-Suite) sediments. This paper focuses on the Palm Springs Group (PSG), a thick fluvial and alluvial sequence deposited on the upper delta plain (between 4.2–2.5 Ma) when the Colorado was active in the area, allowing the detailed examination of the processes of sediment mixing from two distinct provenance areas. The PSG consists of three coeval formations: 1) Canebrake Conglomerate, a basin margin that has coarse alluvial fan deposits derived from surrounding igneous basement; 2) Olla Formation, fan-fringe sandstones containing L-Suite, C-Suite, and mixed units; and 3) Arroyo Diablo Formation, mineralogically mature C-Suite sandstones. Stratigraphic analysis demonstrates that the river flowed through a landscape with relief up to 2000 m. Satellite mapping and detailed logging reveal a variable balance between the two suites in the Olla Formation with an apparent upward increase in L-Suite units before abrupt cessation of Colorado sedimentation in the basin. Stable heavy mineral indices differentiate L-Suite (high rutile:zircon index: RZi 40–95) from C-Suite (RZi: 0–20). Both suites have garnet:zircon index (GZi) and apatite:tourmaline index (ATi) mostly above 50, although many L-suite and mixed Olla samples have much lower ATi (20–50), suggesting that the distal floodplain was wet and the local sediment had a longer residence time there, or went through several cycles of erosion and redeposition. Heavy mineral analysis, garnet geochemical analysis, and detrital zircon U-Pb age spectra allow us to quantify the amount of mixing from different sediment sources. These data show that about 30% of the mixed units are derived from the Colorado River and that up to 20% of the L-Suite is also derived from the Colorado River, suggesting that there was mutual cannibalisation of older deposits by fluvial channels in a transitional area at the basin margin. Although this study is local in scope, it provides an insight into the extent and nature of sediment mixing in a two-source system. We conclude that most ‘mixing’ is actually interbedding from separate sources; true mixing is facilitated by low subsidence rates and the rapid migration of fluvial channels.
The geodynamic history of the Precambrian basement in central North Africa as well as the age and provenance of its sedimentary cover sequence are still poorly constrained. Here we present first ...detrital zircon ages (obtained by LA-SF-ICP–MS and SHRIMP) from Paleozoic and Mesozoic sandstones of the eastern Murzuq Basin, southern Libya, which unconformably overlie the Saharan Metacraton. Establishing the age and provenance of these sandstones has important implications for our understanding of the evolution of northern Gondwana during the Paleozoic, especially for reconstructions of paleo-source areas and transport paths.
Detrital zircons from the sandstones show mainly early Paleozoic to Neoarchean ages with four main age populations, at 2750–2500
Ma (8%), 2200–1750
Ma (16%), 1060–920
Ma (18%), and 720–530
Ma (39%). About 13% of all concordant grains yield ages of 1600–1000
Ma. In addition, there are 9 zircon grains (0.7% of all concordant grains) with ages of 3600–2800
Ma. The presence of a high number of ca. 1
Ga zircons is enigmatic and their origin is controversial. Besides direct sourcing from ca. 1
Ga igneous rocks in eastern Chad and ca. 1
Ga igneous rocks along the southeastern margins of the Congo and Tanzania cratons, recycling of Neoproterozoic sediments containing ca. 1
Ga zircons is another alternative hypothesis to explain the presence of ca. 1
Ga zircons in the Paleozoic sedimentary sequence of central North Africa. The ubiquitous occurrence of ca. 1
Ga zircons in Paleozoic sediments of southern Libya provides insights into the correlation and paleotectonic arrangement of Gondwana-derived terranes, present, for example, in the eastern Mediterranean and in southwestern Europe. Current paleotectonic models of dextral terrane transport along the northern Gondwana margin during the early Paleozoic may need to be revised.
► Detrital zircon U–Pb ages of Paleozoic and Mesozoic sandstones from southern Libya. ► Ubiquitous occurrence of Mesoproterozoic and Neoproterozoic detrital zircons. ► Likely sediment sourcing from central Africa. ► Insights into the paleotectonic arrangement of Gondwana-derived terranes.
Rare-element granites and pegmatites represent important sources of raw materials for "clean, green and high technologies", such as lithium and tantalum, for example. However, mechanisms of ...rare-element granites and pegmatite's origin are still far from being fully understood. Several rare-element pegmatite fields and a rare-element granite are known in the Variscan realms located in Iberia (Spain and Portugal), enhancing the interest of this area for studying the formation of these extremely fractionated melts. In situ U-Pb dating by LA-SF-ICP-MS of columbite-group minerals from rare-element granites and pegmatites of the Iberian Variscan belt provides new constraints on the generation of rare-element melts. Three events have been recognized: (i) Emplacement of the Argemela rare-element granite, in the Central Iberian Zone (CIZ), with an age of 326±3 Ma; (ii) Emplacement of rare-element pegmatites from the Galicia-Tras-os-Montes Zone (GTOMZ), at an average age of 310±5 Ma; (iii) Emplacement of rare-element pegmatites in the CIZ and in the southern GTOMZ at about 301±3 Ma. These two last events are coeval with the two peaks of ages for the late orogenic magmatism at ca. 308 Ma and 299 Ma, and all dated rare-element pegmatites clearly emplaced during the late-orogenic evolution of the Variscan belt. Contemporaneous fields of rare-element pegmatites are arranged in belts following those formed by similar granitoid suites. Pegmatite fields from both the GTOMZ and the CIZ reveal a southward propagation of ages of emplacement, which matches the observed propagation of deformation, metamorphism and magmatism in the two different geotectonic zones. Existence of three successive rare-element events in the Iberian Massif argues against the involvement of lower crustal HP-HT metamorphism in the generation of rare-element melts. Possible sources of rare-element-enriched melts are more likely located in the middle to upper crust, as are the major components of granitic magmatism. Analyses of U and Pb isotopes from columbite-group minerals are very robust and reproducible, making them good candidates for dating ore deposits related to peraluminous magmatism as well as REE- and Nb-bearing deposits.
We introduce and propose zircon M257 as a future reference material for the determination of zircon U‐Pb ages by means of secondary ion mass spectrometry. This light brownish, flawless, cut gemstone ...specimen from Sri Lanka weighed 5.14 g (25.7 carats). Zircon M257 has TIMS‐determined, mean isotopic ratios (2s uncertainties) of 0.09100 ± 0.00003 for 206pb/238U and 0.7392 ± 0.0003 for 207pb/235U. Its 206pb/238U age is 561.3 ± 0.3 Ma (unweighted mean, uncertainty quoted at the 95% confidence level); the U‐Pb system is concordant within uncertainty of decay constants. Zircon M257 contains ∼ 840 μg g−1 U (Th/U ∼ 0.27). The material exhibits remarkably low heterogeneity, with a virtual absence of any internal textures even in cathodoluminescence images. The uniform, moderate degree of radiation damage (estimated from the expansion of unit‐cell parameters, broadening of Raman spectral parameters and density) corresponds well, within the “Sri Lankan trends”, with actinide concentrations, U‐Pb age, and the calculated alpha fluence of 1.66 × 1018 g−1. This, and a (U+Th)/He age of 419 ± 9 Ma (2s), enables us to exclude any unusual thermal history or heat treatment, which could potentially have affected the retention of radiogenic Pb. The oxygen isotope ratio of this zircon is 13.9%o VSMOW suggesting a metamorphic genesis in a marble or calc‐silicate skarn.
Nous présentons et proposons à la communauté le zircon M257 qui pourrait devenir un matériau de référence pour la détermination des âges U‐Pb sur zircon par spectrométrie de masse à ions secondaires (SIMS). Ce spécimen scié d'une gemme brun clair, sans defauts, provenant du Sri Lanka, pèse 5.14 g (25.7 carats). Les moyennes des rapports isotopiques (et les incertitudes associées 2s) mesurées par TIMS sur ce zircon M257 sont 0.09100 ± 0.00003 en 206pb/238U et 0.7392 ± 0.0003 en 207pb/235U. Son âge 206pb/238U est de 561.3 ± 0.3 Ma (moyenne non pondérée, incertitude: 95% intervalle de confiance); le système U‐Pb y est concordant, aux erreurs sur les constantes de désintégration près. Le zircon M257 contient ∼ 840 μg g−1 U (Th/U ∼ 0.27). Ce matériel montre de très faibles hétérogénéités, avec une quasi absence de textures internes, même a I'examen par cathodoluminescence. Le degrê de dommage lié aux radiations (estimé par I'expansion des paramètres de la cellule unitaire, I'élargissement des paramètres des spectres Raman et la densité) est modéré et uniforme. II se corrèle bien, (sur I'alignement “Sri Lanka”) avec les concentrations en actinides, I'âge U‐Pb et la fluence calculée de 1.66 × 1018 g−1 Ceci, coupléà son âge (U+Th)/He de 419 ± 9 Ma (2s), nous permet d'exclure I'existence d'événement thermique passé, ou de traitement par la chaleur, qui aurait potentiellement pu affecter le processus de rétention de Pb radiogénique. Le rapport isotopique de I'oxygène de ce zircon est 13.9%o VSMOW, suggérant une génèse par métamorphisme, au sein d'un marbre ou d'un skarn calco‐silicaté.
•Hf data for the magmatic rocks of the Rehoboth Basement Inlier show juvenile input.•Elim Formation and Kangas Metamorphic Complex originated from low-δ18O crustal source.•Two magmatic events within ...the Rehoboth Province occurred at 1.87 and 2.05Ga.•Crustal reworking within the Rehoboth Province involved mainly Archaean crust.
The Rehoboth Province of southern Africa is defined by its smooth magnetic character by contrast to the adjoining mobile belts. It has been slow to yield information about its crustal evolution because most of it is covered by Kalahari sands. A combined microbeam study of U-Pb, Lu-Hf in zircon from metasedimentary and magmatic rocks of the well-exposed Rehoboth Basement Inlier (RBI) provides new constraints on the crustal evolution of the Rehoboth Province. The detrital zircons provide evidence for five U-Pb age groups corresponding to magmatic events within the crust, whereas Lu-Hf zircon data allows estimates of crustal residence age. Zircon oxygen isotope data for the magmatic rocks implies diversity in their genesis.
The sparse 3.41–2.45Ga U-Pb zircon age group I (6 zircons) strengthens the concept of an Archaean foundation to the Rehoboth Province, previously suggested from kimberlite xenoliths and granitic cobbles transported by the Dwyka icesheet from further south in the Rehoboth Province. Detrital age group II from 2.15 to 1.92Ga has not been identified in outcropping magmatic rocks. However several Dwyka cobbles have 2.05 ages, similar to the Bushveld Complex and thermal event seen in the adjoining Kaapvaal Craton. This suggests that the Rehoboth Province may already have been attached to the Kaapvaal Craton at this time. Age group II has Lu-Hf isotope character requiring mixing between Archaean crustal source rocks and juvenile mantle material.
Age group III (1.92–1.83Ma) corresponds in age to the largely metabasaltic 1870±6Ma Elim Formation, now the oldest dated unit in the RBI. The detrital Lu-Hf data again requires an Archaean crustal source mixing with mantle-derived material. However the magmatic Elim Formation zircons originated from a distinctly more juvenile source than the detrital zircons of the same age (group III), and they have a low mantle δ18Ozrc value of 4.83 (assuming mantle 5.3±0.6). This suggests a mantle or lower oceanic crustal affinity for the Elim Formation, whereas the detrital zircons may have come from a more evolved crustal source located in the continental hinterland. Detrital age group IV (1.83–1.61Ga) corresponds to eight precisely dated volcanic and plutonic rocks which have similar Lu-Hf character and probably provided the detrital zircons. Zircon oxygen isotope values for the magmatic rocks show a considerable range. The sub-mantle values of the 1826±5Ma Kalkbrak Gneiss and 1753±6Ma Marienhof meta-rhyolites (δ18Ozrc 4.30 and 4.31 respectively) are thought to have originated from the same low-δ18O crustal source as the Elim Formation. Three other samples in this group show a trend to higher δ18O values, culminating in a high value of δ18Ozrc 9.35±0.53 for the 1769±6Ma Gaub Valley andesitic, volcanoclastic sample. This trend is thought to reflect addition of low-T altered supracrustal material or high δ18O metasomatic fluids to the source rocks in a geochemically documented subduction setting.
The broad detrital age group V (1.33–1.09Ga), found in the one ≤1.1Ga Langberg Formation metasediment investigated, reflects the entire Namaqua Wilson Cycle including rifting and drifting, arc processes, 1.2Ga collision and the post-tectonic 1.1Ga Umkondo plume event. This group shows a large spread of Lu-Hf crustal residence ages from 2.3 to 1.45Ga, reflecting a mixture of sources from Archaean to juvenile Namaqua crust. A 1221±6Ma Gamsberg Granite sample showed 2.2–2.0Ga crustal residence ages and mantle-like δ18Ozrc of 4.99, suggesting that its source was melted Palaeoproterozoic lower crust. Part of the group V detrital zircon population is not known as magmatic rocks in the RBI and was probably derived from the collisional mountain belt in the Namaqua Province to the west of the Rehoboth Province.
The Rehoboth Province is thus revealed as an ancient crustal block with possible Archaean foundations, major Palaeoproterozoic events involving mantle additions mixed with reworked Archaean crust, and involving a number of different tectonic settings, culminating with the entire Mesoproterozoic Namaqua Wilson Cycle.
In situ laser ablation inductively coupled plasma mass spectrometry (LA–ICP-MS) and electron probe microanalysis (EPMA) are used to characterize magnetite and ilmenite of the phoscorite–carbonatite ...association at Phalaborwa. We trace the behavior of the compatible elements for two different generations of magnetite related to (1) a magmatic stage, with variable Ti–V content, which pre-dates the copper mineralization, and (2) a late hydrothermal, low-Ti, low-temperature event, mostly post-dating sulfide formation. Magnetite is shown to be a robust petrogenetic indicator; no influence on its chemical composition is detected from the intergrowth with the accompanying phases, including the interaction with coexisting sulfides. High spatial resolution EPMA characterize the tiny late-stage hydrothermal magnetite veins, as well as the ilmenite granular and lamellar exsolutions mostly developed in the magnetite from the phoscorite. By combining geochemical data with geothermo-oxybarometry calculations for magnetite–ilmenite pairs, we infer that the most primitive magnetite probably formed at oxygen fugacity above the nickel nickel oxide (NNO) buffer, revealing an evolutionary trend of decreasing temperature and oxygen fugacity. Geochemical similarity exists between magnetite from phoscorite and carbonatite, thus supporting a common mantle source for the phoscorite–carbonatite association.
We present an automated method for U–Pb age dating of zircon by single collector laser ablation-magnetic sectorfield-inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS). The high sensitivity ...of SF-ICP-MS allows routine analysis with spot diameter of 20 to 30 μm and ablation time of 30 s, resulting in an ablation crater depth of ∼
15–20 μm (∼
35 to ∼
65 ng of zircon). Zircon consumption is therefore limited to <
3% of a typical crystal and only by a factor of 6–10 times larger compared to secondary ion mass spectrometry (SIMS) based techniques.
Precision and accuracy has been assessed using a number of well-characterised zircons as secondary quality control standards and are comparable to those of laser ablation- and ion-microprobe based techniques that use more complex and time-consuming approaches. Average measurement uncertainties (2σ, mean analytical uncertainty) based on 402 analyses of the Plešovice zircon standard were 2.2%, 3.1% and 2.1% for the
206Pb/
238U,
207Pb/
235U and
207Pb/
206Pb ratios, respectively, comparable with those attainable by SIMS. The weighted mean of all 402 analyses yielded a
206Pb/
238U age of 338
±
1 Ma, which is in excellent agreement with the ID-TIMS
206Pb/
238U age of 337.1
±
0.4 Ma reported for the Plešovice zircon.
Data acquisition is done in automated mode for up to 16 h/day with analytical points pre-set with only minimal operator presence during the data acquisition. Individual U–Pb zircon analysis last ≤
75 s, and sample throughput is more than an order of magnitude higher (∼
1000 U–Pb ages/day) compared to SIMS techniques and 3–5 fold higher compared to conventional LA-ICP-MS techniques. The methods presented here are therefore expected to have significant impact on many aspects of zircon U–Pb geochronology, with particular benefits for studies on the Archean crustal evolution and the provenance of detrital zircon crystals from clastic sediments where a large number of high-quality stand-alone
in situ zircon U–Pb age determinations are needed.
Voluminous granite gneisses and granites straddle the boundary between the Kakamas and Areachap Terranes in the eastern Namaqua Sector (NS) of the Mesoproterozoic Namaqua-Natal Metamorphic Province ...(NNMP). These rocks have been previously poorly defined and loosely grouped into the Keimoes Suite, but a recent U-Pb age study has suggested the suite be subdivided into syn-tectonic and post-tectonic groups relative to the main phase of the Namaqua Orogeny. This study adds new whole rock geochemical, isotopic and age data for these granites that confirms the subdivision is appropriate. The older group of syn-tectonic granite gneisses, dated between 1175 and 1146 Ma, have penetrative foliations and are largely derived from fractionated, leucogranitic metaluminous to peraluminous magmas with low maficity, low Ti, Mn and Ca. They were derived from mildly depleted sources (εNd(t): −1.47 to 1.78), with Meso-to Paleoproterozoic Nd model ages (1.57–1.91 Ga), and high initial Sr ratios (0.71970–0.75567) suggesting mixing between younger depleted and older, arc-like sources imparting an arc-like signature to the magmas. High initial Sr ratios appear to be an intrinsic character of these granites reflecting those of granites in the region and the highly radiogenic nature of the NS.
The weakly to unfoliated late-to post-tectonic megacrystic granodiorites and monzogranites, including charnockites, intruded between 1110 and 1078 Ma and constitute the Keimoes Suite proper. They have I-type characteristics, being strongly metaluminous and locally hornblende- and orthopyroxene-bearing with moderate SiO2 and with arc-type affinities (LILE enrichment relative to the HFSE, Ta-Nb, Ti and P anomalies). However, the granitoids also have high Fe/Mg ratios, along with high HFSE, LILE and REE contents more indicative of A-type granites. They show an increasing maficity, metaluminous character, and general decreasing degree of fractionation with decreasing age. They are similar to the syn-tectonic granites in having εNd(t) values close to zero (−2.95 to 2.83) and Meso-to Paleoproterozoic model ages (TDM: 1.38–1.99 Ga) but lower initial Sr ratios (<0.723 in general) suggesting derivation from relatively depleted sources with a variable enriched and/or crustal component.
The timing of emplacement of the post-tectonic granites places peak D2 deformation in the eastern NS predominantly at ∼1.13–1.10 Ga, with peak D2 varying from ∼1.18–1.13 Ga. There was more voluminous granitic magmatism in the Areachap Terrane to the east during the late-to post-tectonic magmatic episode, whereas the earlier, ∼1.18–1.14 Ga syn-tectonic magmatic episode is more concentrated to the west in the Kakamas Terrane. The broad, protracted period of magmatism in the eastern NS attests to a long-lived duration of high-heat flow in this portion of the southern African crust at this time. Nd model ages of Meso-to Paleoproterozoic age reflect those in other granites throughout the NS suggesting extensive reworking of Paleoproterozoic crust during the 1.2–1.0 Ga Namaquan Orogeny.
•Voluminous syn- and late- to post-tectonic granitic magmatism in eastern Namaqua Sector.•Older syn-tectonic granite gneisses, emplaced 1175–1146 Ma, are fractionated metaluminous to peraluminous leucogranites.•Keimoes Suite comprises weakly foliated 1110–1078 Ma late- to post-tectonic megacrystic, ferroan metaluminous granitoids.•Model ages of all granites reflect mixing of Meso- and Paleoproterozoic sources and reworking of Paleoproterozoic arc crust.•Timing of emplacement of granites constrains peak D2 Namaquan deformation in region to ∼1.13–1.11 Ga.
We have dated 1060 detrital zircon grains from the Neoproterozoic to lower Palaeozoic Volta basin, Ghana, and from sandstones in the adjoining thrust sheets belonging to the Pan-African Dahomeyide ...belt. All dated zircons in the lower Voltaian Bombouaka Group are older than ~
1000 Ma, while samples from the middle Voltaian Oti Group and the upper Voltaian Obosum Group contain numerous zircons of 600–1000 Ma. The samples we have studied from the Dahomeyide thrust sheets (Buem and Togo structural units) have zircon age spectra similar to those from the Bombouaka Group, confirming a correlation of the investigated sandstones with lower Voltaian strata. The Bombouaka Group was deposited between ~
1000 and 600 Ma, perhaps shortly after 1000 Ma, as suggested by earlier Rb–Sr data on clay minerals. Deposition of the Oti and Obosum Groups took place shortly after 600 Ma, perhaps continuing into the lower Palaeozoic. Most samples contain Palaeoproterozoic zircons with ages of 2000–2200 Ma that probably have been derived from the surrounding crystalline (Birimian) basement. Archaean zircons, present in smaller proportions, may have come from Archaean rocks in the West-African craton. Most zircons of 1000–1900 Ma were probably derived from sources outside the West-African craton, the Amazonian craton being a plausible source region. Zircons with ages around 1200 Ma are believed to have been derived from Grenvillian orogenic belts, perhaps those that fringe the Amazonian craton. If this is so, no major seaways could have been present between these belts and the West-African craton during the early Neoproterozoic. Zircons younger than 1000 Ma were probably derived from an eastern continental block that collided with the West-African craton during the Pan-African orogeny ~
600 Ma ago.
The Torud–Ahmad Abad magmatic belt is located in the south-southeast of Shahrood (East of Semnan Province, NE Iran) and lies in the northern part of the Central Iran Structural Zone (CISZ), where a ...thick sequence of Paleocene to middle Eocene volcanic and volcanosedimentary rocks cropped out. This sequence was intruded by numerous dikes, hypabyssal igneous domes and one small gabbrodioritic intrusion, with compositions ranging from trachybasaltic andesite, trachyandesite, dacite, trachyte, gabbro, diorite and syenite. Various enclaves (cogentic and noncogenetic) with different composition, size and shape have been found in these domes and dikes. These enclaves are evidence of magma mixing and crustal contamination. Geochemically, the studied rocks exhibit a calc-alkaline to high potassium calc-alkaline affinity, and are enriched in LREE and LILE and depleted in HREE and HSFE. Other geochemical characteristics, such as a silica content varying between 59–63wt% and 51–59wt%, a Na2O content >3wt%, Al2O3 content>16wt%, Yb<1.8ppm, and Y<18ppm, make it possible to classify these rocks as high silica adakite in the Ahmad Abad region and low silica adakite in the Sahl-Razzeh region or at least, adakitic like rocks. Also, depletion of Nb and Ti, and high enrichment in Rb, Ba, K and Th, imply crustal contamination of the mentioned adakitic domes. The petrographical and geochemical evidence show that the magma forming of the high silica adakites has been originated from partial melting of the subducted oceanic slab of Neo-Tethys (Sabzevar–Darouneh branch) in amphibolite to eclogite facies and the low silica adakites formed by partial melting of the metasomatized or modified mantle wedge, above the subduction zone. Gabbroic to syenitic rocks are the products of fractional crystallization of basic magma which originated from a nearly non-modified mantle wedge above the subducted oceanic slab. U-Pb dating of the dacitic and andesitic rocks belong to hypabyssal rocks yielded age of 41.4±0.3Ma, and 35.5±0.2Ma respectively and consistent to Middle to Late Eocene.
•The geochemistry indicates that all samples represent adakitic and adakitic like features.•Adakitic rocks can be divided into high-silica adakite and low-silica adakite.•Low-silica adakites are evident of partial melting of a metasomatized mantle.•High-silica adakite originated from melting of subducted metamorphosed Neotethian oceanic slab.•The andesite rocks crystallized around 41.4±0.3Ma and the dacite rocks crystallized near 35.5±0.2Ma.
