Source Of Detrital Zircons Research Articles

Early-stage intracontinental rifting in the Neoproterozoic Centralian Superbasin: Systematic U–Pb and Lu–Hf isotopes in detrital and inherited zircons from the Yeneena Basin, northwest Australia

Detrital zircons from different lithostratigraphic units of Supersequence 1 across the Neoproterozoic northwest Officer and Yeneena basins were investigated using LA-ICPMS U−Pb and Lu − Hf isotope analyses to understand the source of detritus and potential links to regional tectono-magmatic events, as well as the crustal evolution of the sedimentary sources terranes. All data show that Proterozoic detritus dominate the age spectra of samples from the Yeneena Basin and northwest Officer Basin, with signatures varying in the heights of peaks rather than the presence or absence of age groups in different formations. The Archean and Proterozoic ages overlap with known magmatic events in the Pilbara Craton, Fortescue Basin, Rudall Province, Capricorn Orogen, Musgrave Province, Madura Province, and the Coompana Province suggesting these terranes are possible sources of the detritus. The detrital zircon population of the samples have main age peaks in the range 1300 to 1000 Ma, suggesting magmatic rocks of the Rudall and Musgrave provinces are the main sources of detrital zircons, which can be differentiated by Hf signature. The distal Musgrave Province persistently dominates the provenance of detritus in the northwestern Officer and Yeneena basins, while detritus from the proximal Rudall Province contributes to a lesser extent. This suggests that the distal Musgrave Province remained emergent during the evolution of the northwestern Officer Basin and the Yeneena Basin, whereas the proximal Rudall Province potentially became buried by the accumulating sedimentary rocks and contributed less detrital zircons. The 833.6 ± 1.4 Ma maximum depositional age obtained in this study for the sedimentary rocks of the northwestern Officer and Yeneena basins point to the approximate time of the start of basins development. The extensional event in this age period is related to emplacement and extrusion of the ca. 830 Ma Willouran Large Igneous Province (LIP). The Duke gabbro sills observed in the Yeneena Basin are part of this LIP, which extends from the Adelaide Rift Complex, through the Musgrave Province, central–northwest Officer Basin and into the Yeneena Basin. The sediment source terranes suggest a southeast–northwest oriented depositional system between central and Western Australia, possibly because of intracontinental rifting related to widespread mafic–ultramafic magmatic events that could include the earlier Ngaanyatjarra Rift formed by the 1090–1040 Ma Warakurna LIP as well as the ca. 830 Ma Willouran LIP, synchronous with the breakup of Rodinia in the mid-Neoproterozoic.

BOUNDARY BETWEEN THE TIMANIAN AND URALIAN TECTONIC CYCLES: RESULTS OF DATING OF DETRITAL ZIRCONS FROM BASAL HORIZONS OF THE LOWER PALEOZOIC SYNRIFT COMPLEX OF THE POLAR URALS, AND IGNEOUS ROCK AGE DATA REPORT

The Upper Cambrian-Lower Ordovician terrigenous strata, unconformably overlying the Upper Riphean-Vendian rocks of the Timanide orogen in the north of the Urals, and contemporaneous bimodal volcanics and intrusive rocks are considered complexes marking the beginning of rifting which led to the opening of the Paleo-Ural Ocean later on. The article presents the results of U-Pb (LA-ICP-MS) dating of detrital zircons from sandstones lying at the base of the section of the rift complex in the Malaya Usa River basin in the Polar Urals. It was found that the clastic sequence (identified as the Khoydyshor formation) began to accumulate no earlier than the Cambrian–Ordovician transition. Zircon ages fall within the continuous Vendian to Early Ordovician (575–478 Ma) interval with peak at 512 Ma. This age range overlaps with the age of rhyolites interlayered conformably with sandstones and rhyolite porphyry dikes intruding the Khoydyshor formation, thus indicating a possible admixture of products of synsedimentary volcanism. A narrow zircon age pattern allows us to conclude that the main sources of detrital zircons in sandstones were the Early-Middle Cambrian igneous rocks formed at the stage of pre-rift uplift, and, to a lesser extent, the Late Cambrian-Early Ordovician riftogenic magmatic complexes, marking the beginning of the Uralian tectonic cycle, as well as the Late Vendian igneous rocks of the underlying Timanide orogen. Judging by the Th/U ratio, most of the detrital zircons within the sandstones were derived from the Early Paleozoic silicic volcanic and hypabyssal rocks and the Vendian granitoids and diorites. The almost complete absence of older grains, which are typical of coeval sandstones of the northern part of the Urals, may indicate the accumulation of the considered sandstones in a local trough with local clastic material sources. Probability density estimation of U-Pb ages for zircon from igneous and metamorphic rocks of the Polar Urals indicates that there were no gaps in the Late Riphean to Early Ordovician endogenous activity in this region. The main peaks occur at 552, 521 and 500 Ma, and an additional peak – at 665 Ma. The results of dating of detrital zircons from sandstones of the Khoydyshor formation together with the database of U-Pb isotope ages of igneous and metamorphic rocks of the Polar Urals (119 items), compiled by the authors, indicate that the change in geodynamic regime from collisional orogenesis in the Late Vendian to the Early-Middle Cambrian pre-rift uplift and following Late Cambrian rifting was not accompanied by a longterm discontinuity in magmatic activity.

Age of Detrital Zircons and Sources of Terrigenous Deposits of the Olokit Zone (Northern Baikal Region)

This work presents the results of U–Th–Pb (LA–ICP–MS) geochronological study of detrital zircons from terrigenous deposits of the Olokit zone of the Baikal–Patom fold–thrust belt of the Siberian Craton. It was established that the age of deposits of the Olokit Formation is in the range of 0.86–0.72 Ga. Archean and Early Proterozoic rocks of the southern part of the Siberian Craton and the Neoproterozoic complexes of the Baikal–Muya belt were the sources of detrital zircons of the terrigenous rocks of the Olokit zone. There is no evidence of rocks that could have been the sources of Mesoproterozoic (1.08–1.46 Ga) and Late Paleoproterozoic (1.65 Ga) detrital zircons for the deposits of the Ondok paleo-uplift in the Olokit zone, in the Baikal mountain region, and in the Siberian Craton. The data obtained and the results of paleomagnetic reconstructions indicate that Siberia and Laurentia in the Meso- and Early Neoproterozoic were in a fixed position relative to each other; an unknown continental block composed of Mesoproterozoic rocks could have occupied the space between them.

Sources of Zircons from Clastic Rocks of the Riphean Sequences of the Urals

New dates of detrital zircons from sandstones expand the possibilities to interpret their source areas. These interpretations are often constrained by a formal comparison of the geochronological and compositional features of detrital crystals with any very remote complexes. However, there is another situation when the local complexes pretend to be a source of detrital material. Analyses of isotope–geochronological (SHRIMP and TIMS) dates of zircons, the U and Th concentrations, and the comparison of age histograms of primary zircons from Riphean volcanics and rocks of the Taratash complex and detrital zircons from the Vendian (Asha Group) and Lower Riphean (Ai Formation) sandstones have shown that zircon age variations and source areas of zircons are comparable in many aspects. It follows that the geochronological features of primary zircons from the Riphean volcanics and rocks of the Taratash complex as sources of detrital zircons for the Riphean and Vendian sandstones in the Southern Urals are controlled by redeposition processes, though the influence of distant source areas remains possible.

Ages and sources of detrital zircons from the Early Mesozoic metasedimentary rocks of the Un’ya-Bom terrane of the Mongol-Okhotsk fold belt: results of U-Th-Pb and Lu-Hf isotope studies

The U-Pb and Lu-Hf isotope studies have been performed to investigate detrital zircons from metaterrigenous deposits (Nel, Kurnal and Amkan f ormations) of the Un’ya-Bom terrane of the eastern part of the Mongol-Okhotsk fold belt. T he concordant age s of the youngest zircon in the metasiltstone of the studied formations are as follows: 213±3.6 Ma – N el , 194±4.4 Ma – K urnal , and 202±2.5 Ma – A mkan. The new data suggest that a previously assumed (Middle Jurassic) age of the A mkan f ormation is uncertain . Most likely, the Kurnal and Amkan formations are of the same ( E arly Jurassic) age. In any case, a relationship between these formation s needs to be clarified . I n co mbination with the data from previous studies of the Tukuringra t errane, our study results show that the E arly Mesozoic sedimentary rocks in the structure of the eastern part of the Mongol-Okhotsk fold belt are more abundant than it is currently assumed . Our study results suggest that during the accumulation of the Nel, Kurnal and Amkan formation s , the materials were supplied from different provinces, especially from the Amur s uperterrane ( i.e. from the south , in modern coordinates) , as well as from the fram e of the North Asian craton ( i.e from the north , in modern coordinates), although the contribution of the craton source was insignificant.

Age and sources of detrital zircons from Jurassic conglomerates of the Strelka Depression (northern frame of the Mongol-Okhotsk fold belt)

The article presents the results of U-Th-Pb geochronological studies of clastic zircons from cement of the Jurassic conglomerates of the Strelka Depression extended in a sublatitudinal direction along the border between the southern margin of the Selengino-Stanovoy Superterrane and the Mongol-Okhotsk fold belt. It has been shown that Paleoproterozoic and Neoarchean zircons dominate in the conglomerate cement. This indicates that the main demolition of the material to the sedimentation basin was carried out from the southeastern frame of the North Asian Craton (from the north in modern coordinates). In our view, the Strelka Depression formed after the completion of orogenic processes associated with the formation of the Mongolo-Okhotsk structure.

Age and Sources of Detrital Zircons from Jurassic Conglomerates of Strelka Depression (Northern Framing of the Mongol–Okhotsk Fold Belt)

This paper presents the results of U–Th–Pb geochronological study of detrital zircons from cement of the Jurassic conglomerates of the Strelka Depression extending in the sublatitudinal direction along the border between the southern margin of the Selenga-Stanovoy superterrane and the Mongol–Okhotsk fold belt. It has been shown that Paleoproterozoic and Neoarchean zircons overwhelmingly dominate in the conglomerate cement. This indicates that the main input of the material to the sedimentation basin was carried from the Southeast border of the North Asian craton (from the north in modern coordinates). In our opinion, the Strelka Depression formed after the completion of orogenic processes associated with the formation of the Mongol–Okhotsk structure.

The Barreiro suite in the central Ribeira Belt (SE-Brazil): a late Tonian tholeiitic intraplate magmatic event in the distal passive margin of the São Francisco Paleocontinent

New geochemical, U-Pb, Lu-Hf and Sr-Nd data from the Barreiro Suite metabasites in comparison with enclaves within the distal Andrelândia Group and the orthogranulites of the Juiz de Fora Complex are presented. Geochemical data suggest intraplate setting, with high and low- TiO 2 , TDM Nd ages between 1.80 and 1.41 Ga, negative ENd t and ( 87 Sr/ 86 Sr) i between 0.714 and 0.747. Results contrast with part of the Juiz de Fora Complex enclaves, with island arc tholeiites- calcalkaline basalts (IAT-CAB) geochemical signatures, TDM Nd ages between 2.58 and 2.16 Ga, positive ENd t values and ( 87 Sr/ 86 Sr) i between 0.700 and 0.712. U-Pb data for the Barreiro Suite yielded a crystallization age of 766 ± 13 Ma and a metamorphic overprint of 619 ± 6 Ma. The results indicate three episodes of mafic magmatism in the Occidental terrane of the Ribeira Belt. The two older episodes are related to Rhyacian arc evolution (ca. 2.2 to 2.1 Ga) and to the Statherian (ca. 1.7 Ga) tectonics, and occur only within the Juiz de Fora Complex, while the younger ca. 766 Ma episode constrains the timing of distal passive margin evolution. An important implication is that these late Tonian metabasic rocks could have been a source of detrital zircons for the sedimentation along the distal Andrelândia basin.

Detrital zircon U–Pb geochronology of early Cretaceous sedimentary rocks in Dingzi Bay and Taolin area from the Sulu Orogen: Provenances and tectonic implications

Early Cretaceous sedimentary rocks in the Sulu Orogen (SO) have been a popular topic in recent years. The provenances of sedimentary rocks can help elucidate the tectonic history of the SO, China. The Dingzi Bay and Taolin areas in the SO have always been poorly known. Thus, we date detrital zircons from four sedimentary rock samples including sandstone, siltstone, and greywacke collected in both areas. Age distributions from Dingzi Bay exhibit three major age populations including Palaeoproterozoic (1900–1800 Ma), Neoproterozoic (800–700 Ma), and Early Cretaceous (136–106 Ma) ages and small age populations including early Mesozoic, Palaeozoic, and Palaeoproterozoic–Archean ages, while age distributions in the Taolin area show the largest age population of Early Cretaceous (132–102 Ma) with minor early Mesozoic, Palaeozoic, and Neoproterozoic populations. The youngest detrital zircon populations yield weighted mean ages of ca. 116 Ma in Dingzi Bay and ca. 120 Ma in the Taolin area as the maximum depositional ages, which correspond to the Early Cretaceous Lower Qingshan Group. Age distributions in both areas indicate that the local SO served as the primary source area, whereas the Yangtze Block might have been a lesser source of detrital zircons. The comparison of geochronological, geochemical, and sedimentary data among Dingzi Bay, Taolin, Laoshan, and Lingshan Island suggests an Early Cretaceous marine basin formed over the SO. Further, considering previous regional tectonic history, we propose a two‐stage (Laiyang and Qingshan stages) evolutionary model for this marine basin. We suggest that transpression between the Laiyang and Qingshan stages due to subduction of the Pacific Plate might have caused rapid uplift of the south‐eastern SO and closing of the marine basin. Correspondingly, this process influenced the input of the source material from the Yangtze Block to the marine basin during the Qingshan stage.

Nature and Age of Detrital Zircons from Rocks of the Shear Zone: The Problem of Occurrence of the Archean Basement in the Transangarian Yenisei Ridge

Petrogeochemical and geochronological study of boudined quartzite sandstones and tonalites, as well as host amphibolites, in the shear zone showed that potential sources of detrital zircons were Neoarchean–Paleoproterozoic rocks of the Angara–Kan block and products of their metamorphism. Interpretation of the available data does not confirm the inferred presence of the Early Precambrian basement of the Siberian Craton in the Transangara region.

U-Pb age of zircon from paragneisses in granulite terrane of the Sharyzhalgai uplift (southwest of the Siberian craton): Evidence for the Archean sedimentation and evolution of continental crust from Eoarchean to Mesoarchean

The Archean stage of sedimentation has been first substantiated for the granulite-gneiss terranes of the Sharyzhalgai uplift (southwest of the Siberian craton). High-alumina paragneisses contain detrital zircons varying in age from 3.7 to 2.74 Ga and corresponding in REE patterns to magmatic zircons. The Paleoproterozoic (~1.86 Ga) metamorphic zircons are strongly depleted in HREE and Y as a result of their formation in equilibrium with garnet. Zircons with an age of ~2.5 Ga also show geochemical signs of alteration during metamorphism. The formation of terrigenous sedimentary rocks preceded the Neoarchean stage of magmatism (~2.7-2.6 Ga), and their metamorphism occurred at the Neoarchean-Paleoproterozoic boundary and in the Late Paleoproterozoic. The sources of detrital zircons were mainly Mesoarchean rocks, such as magmatic protoliths of granulites and intermediate-felsic magmatic rocks. The single Eoarchean and Paleoarchean detrital zircon grains in the paragneisses are the first direct evidence for the oldest crust (up to 3.7 Ga) in the granulite terranes of the Sharyzhalgai uplift. The set of geochronological data for granulites and paragneisses suggests the following sequence of geologic events for the granulite-gneiss terranes: ~3.7—the beginning of crustal formation, 3.4-3.2 Ga—intermediate-felsic magmatism, including the recycling of the more ancient crust, and ~3.0 Ga—coeval magmatic and metamorphic processes and differentiation of the continental crust. Thus, the whole cycle from the beginning of crustal growth to the crustal differentiation and turn into continental crust proceeded from 3.7 to 3.0 Ga.

Isotope Lu–Hf composition of detrital zircon from paragneisses of the Sharyzhalgai uplift: evidence for the Paleoproterozoic crustal growth

We present results of study of the trace-element and Lu–Hf isotope compositions of zircons from Paleoproterozoic high-grade metasedimentary rocks (paragneisses) of the southwestern margin of the Siberian craton (Irkut terrane of the Sharyzhalgai uplift). Metamorphic zircons are represented by rims and multifaceted crystals dated at ~1.85 Ga. They are depleted in either LREE or HREE as a result of subsolidus recrystallization and/or synchronous formation with REE-concentrating garnet or monazite. In contrast to the metamorphic zircons, the detrital cores are enriched in HREE and have high (Lu/Gd)n ratios, which is typical of igneous zircon. The weak positive correlation between 176Lu/177Hf and 176Hf/177Hf in the zircon cores evidences that their Hf isotope composition evolved through radioactive decay in Hf = the closed system. Therefore, the isotope parameters of these zircons can give an insight into the provenance of metasedimentary rocks. The Paleoproterozoic detrital zircon cores from paragneisses, dated at ~2.3–2.4 and 2.0–1.95 Ga, are characterized by a wide range of εHf values (from +9.8 to –3.3) and model age TC 2.8–2.0 Ga. The provenance of these detrital zircons included both rocks with juvenile isotope Hf parameters and rocks resulted from the recycling of the Archean crust with a varying contribution of juvenile material. Zircons with high positive εHf values were derived from the juvenile Paleoproterozoic crustal sources, whereas the lower εHf and higher TC values for zircons suggest the contribution of the Archean crustal source to the formation of their magmatic precursors. Thus, at the Paleoproterozoic stage of evolution of the southwestern margin of the Siberian craton, both crustal recycling and crustal growth through the contribution of juvenile material took place. On the southwestern margin of the Siberian craton, detrital zircons with ages of ~2.3–2.4 and 1.95–2.0 Ga are widespread in Paleoproterozoic paragneisses of the Irkut and Angara–Kan terranes and in terrigenous rocks of the Urik–Iya graben, which argues for their common and, most likely, proximal provenances. In the time of metamorphism (1.88–1.85 Ga), the age of Paleoproterozoic detrital zircons (2.4–2.0 Ga), and their Lu–Hf isotope composition (εHf values ranging from positive to negative values) the paragneisses of the southwestern margin of the Siberian craton are similar to the metasedimentary rocks of the Paleoproterozoic orogenic belts of the North China Craton. In the above two regions, the sources of detrital zircons formed by both the reworking of the Archean crust and the contribution of juvenile material, which is evidence for the crustal growth in the period 2.4–2.0 Ga.

Sources of detrital zircons from terrigenous deposits in the Yankan terrane of the Mongolian-Okhotsk mobile belt

This report presents the results of U-Pb-geochronological studies (LA-ICP-MS) of detrital zircons from metasedimentary rocks of the west of the Yankan terrane (one of the largest within the structure of the Mongolian-Okhotsk mobile belt). The performed studies ascertain sufficiently the current notions of the geological development of the terrane. It is shown that the rocks are different in age from the main populations of detrital zircons and in the Sm-Nd isotope-geochemical characteristics. In view of this, one may suppose that these rocks do not belong to the unified stratigraphic series but are included into the structures of different-aged accretionary complexes. The terrigenous deposits of the Baldizhak series, as well as those of the Krestovka and Dzhalinda suites, probably belong to the accretionary complexes formed in front of the southeastern margin of the North-Asian craton. On the contrary, the sedimentary formations of the Preobrazhenovka suite are most likely constituents of the accretionary complex formed near the southern margin of the Amur superterrane. The absence of detrital zircons of Mesozoic age in the treated rocks allows one to assume that the considered accretionary complexes were formed during the Paleozoic.

Detrital zircon U–Pb age constraints on Cretaceous sedimentary rocks of Lingshan Island and implications for tectonic evolution of Eastern Shandong, North China

In order to clarify sedimentary environment and tectonic history of the Sulu orogenic belt, we date zircons from four sandstones and one interbedded rhyolite collected on Lingshan Island in East Shandong, North China. U–Pb age distributions of detrital zircon populations from the newly named Lingshandao Formation resemble that of overlying fluvial sedimentary rocks. Age distributions exhibit four main populations of Paleoproterozoic (2.5–2.3Ga), Mesoproterozoic (2.1–1.9Ga), Neoproterozoic (850–700Ma) and Early Cretaceous (138–121Ma) age, accompanied by sporadic occurrences of Archean, Paleozoic and early Mesozoic aged grains. Data indicate a depositional age of ∼121Ma for the Lingshandao Formation, which constrains the maximum depositional age of overlying fluvial sedimentary rocks. The Lingshandao Formation thus more likely correlates to the uppermost Laiyang Group of the Jiaolai basin. Proterozoic age populations indicate a mixture of material derived from the North China (NCB) and Yangtze Block (YZB) basement along with protolith material from the HP–UHP metamorphic units of the Sulu orogenic belt. Proterozoic age populations differ from those of the roughly coeval Jiaolai basin, which represents a terrestrial depositional environment to the north of the Sulu terrane. The YZB appears to be a significant source of detrital zircons, consistent with field observations.

Architecture of the Chugach accretionary complex as revealed by detrital zircon ages and lithologic variations: Evidence for Mesozoic subduction erosion in south-central Alaska

Subduction erosion is an important process at convergent margins but evidence in the geologic record is scarce because the involved materials are typically lost into the mantle. Detrital zircon dating of accretionary complex sediment allows us to document episodic growth that can be linked to possible triggers. Detrital zircons (n = 2508) from the Mesozoic Chugach accretionary complex, southern Alaska, have U-Pb ages that record progressive subduction accretion punctuated by two periods of tectonic erosion. Lithology and maximum depositional ages permit division of the Chugach accretionary complex into four main units. The oldest, the blueschist-greenschist unit, represents partially subducted sediment associated with a Jurassic oceanic arc, with subduction erosion from 180 to 170 Ma. The end of this erosion period is dated by the oldest maximum depositional age of the Potter Creek assemblage, a <156–169 Ma unit consisting of chert, argillite, and volcanic rocks. A trondhjemite pluton that intrudes the forearc was caused by ridge subduction at 125 Ma and resulted in a second period of subduction erosion lasting until 104 Ma. The end of Aptian–Albian erosion was marked by deposition of massive sandstone and conglomerate of the 101–91 Ma McHugh Creek assemblage. This influx of clastic sediment is interpreted to have occurred in response to the collision of Wrangellia with North America. This event and the erosional event preceding it steepened the forearc region, allowing mass wasting of forearc crust into the trench, filling it by 89 Ma, the oldest maximum depositional age of the Valdez Group flysch. Clasts in conglomerate include granodiorite from the basement of the Talkeetna arc dated from 199 ± 3 Ma to 179 ± 3 Ma, and sandstone clasts with maximum depositional ages of 100 Ma. From 89 Ma to at least 72 Ma, the Valdez Group flysch was deposited via turbidite fans onto the oceanic crust beyond the trench and accreted as imbricate thrust slices that retained coherent bedding. The source for detrital zircons in the Potter Creek assemblage is likely a Middle–Late Jurassic oceanic arc, possibly the Talkeetna arc. The abundance of zircons from ash fall tuffs is consistent with easterly winds and suggests the Chugach accretionary complex was south of latitude 25°N in the Late Jurassic. The dominant source for the Albian McHugh Creek assemblage and the Upper Cretaceous Valdez Group flysch was likely the arc associated with the Coast Mountains batholith. Jurassic (ca. 165 Ma) zircons in the McHugh Creek assemblage could have been derived from exhumed plutons, or may be second-cycle zircons derived from the Potter Creek assemblage. The first appearance of Proterozoic and Archean zircons in the Valdez Group records the breakdown of topographic barriers formed by the accreted arc terranes as rivers encroached into continental North America by the middle Late Cretaceous.

U–Pb ages of metamorphic monazite and detrital zircon from the Northampton Complex: evidence of two orogenic cycles in Western Australia

The Pinjarra Orogen, stretching along the western coast of Western Australia, is one of the least understood orogens on the continent. Exposure is limited to fault-bound basement blocks within the Phanerozoic Perth Basin, which have been interpreted as either remnants of a Mesoproterozoic orogen or as allochthonous blocks within a Neoproterozoic orogen. Thus even the timing of orogenesis is controversial. However, since the orogen is in a critical position for reconstructions of both Rodinia and Gondwana, a better understanding is highly desirable. New U–Pb ages of detrital zircon cores, metamorphic zircon rims and metamorphic monazites from the Northampton Complex, the northernmost and largest of the exposed basement blocks, provide better insights into the sedimentary provenance and metamorphic history of the metasediments exposed in this part of the orogen. Both the monazite ages and the youngest metamorphic zircon rims constrain metamorphism in the Northampton Complex to 1090–1020Ma. Additionally, a large number of older metamorphic zircon rims pre-date this event and were inherited from the Albany-Fraser Orogen in southwestern Australia, which is also identified as the main source region for the detrital zircon cores. Additional sources of detrital zircons are the Mawson Continent (the South Australian Craton and parts of East Antarctica) and possibly the North Australian Craton, as well as synsedimentary volcanic rocks. However, the combination of ages from the cores and inherited metamorphic rims shows that zircons from the Mawson Continent/North Australian Craton were not directly transported to the Northampton Complex but arrived there after being incorporated into the Albany-Fraser Orogen and then eroded from there. The dataset records therefore two orogenic cycles: (1) erosion of the Mawson Continent/North Australian Craton, deposition along the craton margins, collision between the Mawson Continent and North-West Australian Craton and metamorphism in the Albany-Fraser Orogen and (2) erosion of the Albany-Fraser Orogen, deposition along Australia's western margin and metamorphism during the Pinjarra Orogeny. This highlights the care that must be taken in detrital zircon studies, since many of the detrital cores record a second-order provenance. It also explains how zircons from the Mawson Continent could be transported across the Albany-Fraser Orogen that should normally have been a barrier to northwest-directed sediment transport. Finally, in light of these new and previously published data, we support the interpretation that the Pinjarra Orogen is a Mesoproterozoic orogen formed along Australia's western margin.

Isotopic studies on detrital zircons of Silurian–Devonian siliciclastic sequences from Argentinean North Patagonia and Sierra de la Ventana regions: comparative provenance

The Silurian–Devonian siliciclastic sedimentary units known as Sierra Grande Formation and the upper part of the Ventana Group crop out in the eastern area of the North Patagonian Massif and in the Ventania system, toward the Atlantic border of Argentina. Both sequences show similar stratigraphical characteristics and were deposited in a shallow marine platform paleoenvironment. Previous contributions have provided evidence of an allochthonous Patagonia terrane that amalgamate to Gondwana during the Permian–Triassic. However, other lines of research support a crustal continuity southward, where the Pampean and Famatinian events extend into the northern Patagonia. In either case, the detrital input to the Eo–Mesopaleozoic basins generated along the passive margin tectonic setting should reflect the sedimentary sources. In this contribution, new age data on the sedimentary provenance of these units is provided by U–Pb and Lu–Hf isotopic studies on detrital zircons, using LA-ICP-MS and SHRIMP methodologies. The main sedimentary sources of detrital zircons for both regions are of Cambrian–Ordovician and Neoproterozoic age, while a secondary mode is Mesoproterozoic. Zircons from older cratonic sources (Mesoarchean–Paleoproterozoic ages) are scarcely recorded. The sample from the upper section of the Devonian Lolen Formation (Ventana Group) shows an important change in the sedimentary provenance, with a main mode of Mesoproterozoic detrital zircons. Detrital source areas considering the orogenic cycles known for southwest South America (Famatinian, Pampean–Brasiliano, Mesoproterozoic–‘Grenvillian’ and Paleoproterozoic–‘Transamazonian’) are proposed.

Detrital zircon analysis from the Neoproterozoic–Cambrian sedimentary cover (Cuyania terrane), Sierra de Pie de Palo, Argentina: Evidence of a rift and passive margin system?

Fil: Naipauer, Maximiliano. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

SHRIMP U–Pb in zircon geochronology of the Chor granitoid: evidence for Neoproterozoic magmatism in the Lesser Himalayan granite belt of NW India

Porphyritic and peraluminous granitoids of the Lesser Himalayan granite belt fringe the High Himalaya from Nepal to Pakistan. Magmatic zircons in the Chor granitoid in Himchal Pradesh define a SHRIMP U–Pb age of 823±5 Ma. This Neoproterozoic age contrasts with Cambro-Ordovician ages for other dated Lesser Himalayan granites, but is coeval with granite magmatism in the South China craton. Neoproterozoic magmatism within the northern margin of Gondwanaland may provide a more local source for detrital zircons in High Himalayan metasediments during the Neoproterozoic and early Paleozoic than the recently postulated East African orogen.

Evaluation of the Baja controversy using paleomagnetic and faunal data, plume magmatism, and piercing points

The controversy over Late Cretaceous locations of Baja Alaska, Baja BC, Stikinia and western Quesnellia centres around whether (a) they were within 1000 km of their present positions, or (b) located 1000–5000 km south along the western coast of Laurentia to be transported northwards by the Kula plate during the Late Cretaceous and Cenozoic. The latter is supported by paleomagnetic data (including data that pass a fold, contact or reversal test) whereas the former is based upon correlation of strata across faults. Although various critical tests have been proposed and tried, interpretations are equivocal, e.g. alternative interpretations for the source of detrital zircons in Baja BC. We propose that the presence of plume magmatism provides unique vertical piercing points. Thus, correlation of the 70 Ma plume magmatism in northern Stikinia with the Yellowstone hotspot locates Stikinia ∼2000 km to the south and ∼1000 km off the western coast of Laurentia. On the other hand, the absence of 90–70 Ma Yellowstone plume magmatism in the presently adjacent Baja BC, but the presence of 60–47 Ma Yellowstone plume/spreading ridge magmatism in southern Baja BC (southern Vancouver Island and Coast Ranges of Washington and Oregon) implies that it moved at least 1200 km northwards between 70 and 60 Ma (i.e. a rate of ≥12 cm/year): the spreading ridge may be correlated with the Kula–Farallon ridge. Furthermore, the presence of middle–late Triassic plume magmatism in Wrangellia and the Peninsula and Alexander terranes suggests ∼2000 km of relative dextral movement between Baja Alaska and Baja BC. Applying these conclusions to pre-90 Ma paths of Baja Alaska/BC across the Pacific ocean suggests that it had three components: (1) northeasterly between 230 and 180 Ma; (2) eastsoutheasterly between 180 and 140 Ma, and (3) northeasterly between 140 and 90 Ma. Whereas the latter component is consistent with published terrane tracks, the 180–140 Ma component is at odds suggesting that this latter segment be reassessed. The weight of evidence in favour of correlating geological piercing points across the Gulf of California appears to outweigh paleomagnetic evidence for 1000–2500 km of apparent northward displacement of Baja California in the Cretaceous and early Tertiary, which may be explained by ∼15–20° tilting of fault blocks recorded by 40Ar/ 39Ar cooling ages. Thus, we conclude that whereas Baja Alaska, Baja BC, Stikina and western Quesnellia were transported between 1000 and 5000 km northwards along the western margin of Laurentia during the Late Cretaceous and Cenozoic, Baja California has not moved more than that required to open the Gulf of California.