Production Of Juvenile Crust Research Articles

U-Pb ages and Hf isotopic composition of detrital zircon in western Algarve Triassic sandstone (SW Iberia): Implications for crustal evolution, provenance, and paleogeography

The Triassic paleogeographic configuration before the Central Atlantic Ocean's opening and the Pangea's breakup establishes that Iberia was close to North America and North Africa. We present UPb and Hf isotope data of Triassic siliciclastic rocks of the Western Algarve basin (Southwest Iberia, Portugal) to draw inferences on their provenance to define the crustal evolution of the potential sources. Detrital zircon age populations of five samples of Triassic sandstones (68–80 % Neoproterozoic; 5–23 % Paleoproterozoic; 5–14 % Mesoproterozoic; <5 % Archaen and < 4 % Paleozoic) differ significantly from those of the underlying Carboniferous turbidites of the South Portuguese Zone suggesting provenance from outside present-day SW Iberia. Hf isotope signatures of the zircon grains from the western Algarve Triassic sandstones unravel a complex crustal evolution characterized by successive cycles of input of juvenile magmas and magmas derived from the reworking of older crust. Archean detrital zircon grains with positive εHf(t) values and model ages of 3.1–3 Ga indicate juvenile crust formation (Leonian event), while a grain dated at ca. 2.6 Ga showing negative εHf(t) values and model ages of 3.1 Ga suggests that the Archean crust became reworked later (Liberian event). Zircon grains dated between 2.4 and 2.5 Ga having positive to slightly negative εHf(t) values and model ages of 3.3–2.6 Ga model ages correspond to magmatism older than the Eburnean event known in the Reguibat shield, involving a juvenile component mixed with variable proportions of reworked Archean crust. Younger Paleoproterozoic grains (ca. 2.10 to 2.05 Ga; Eburnean-Birimian event) having positive εHf(t) values and model ages of 2.3–2.1 Ga and negative εHf(t) values and model ages of 3.6–2.6 Ga, also indicate interaction of juvenile sources with mixing of old crust. Mesoproterozoic grains show positive εHf(t) values and model ages of 2.3–2.1 Ga, suggesting juvenile crust formation and reworking of the Eburnean crust. Neoproterozoic (ca. 695–555 Ma) detrital zircon with positive εHf(t) values and model ages of 0.76–0.58 Ga point to juvenile crust formation (i.e. Cadomian/Pan-African and Avalonian arcs). They also have negative εHf(t) values in the range − 21.6 to −11.7 and model ages of 3.2–2.1 Ga, suggesting reworking of older crust. In summary, the εHf(t) values and UPb age pattern of the zircon grains from the Triassic siliciclastic rocks are similar to NW Africa and North America potential sources. Still, there is a notable gap in juvenile crust production during the Siderian in the peri-Eastern Laurentia, Avalonia, and Ganderia potential sources unknown in NW Africa. The lack of ca. 525–380 Ma and ca. 380–330 Ma detrital zircon in the western Algarve Triassic sandstones makes it challenging to admit an Atlantic margin of North America and SW Iberia provenance.

Late Mesoproterozoic–early Neoproterozoic quartzite–schist sequences of the Aktau–Mointy terrane (Central Kazakhstan): Provenance, crustal evolution, and implications for paleotectonic reconstruction

Metasedimentary complexes represented by quartzite–schist Kiik Group quartzite–schists play a significant role in the Aktau–Mointy terrane structure, which is one of the largest Precambrian blocks in the western Central Asian Orogenic Belt (CAOB). U-Pb LA-ICP-MS dating of detrital zircons from quartzites in different Kiik Group areas and related magmatic age data establish that accumulation occurred in the late Mesoproterozoic to early Neoproterozoic (from 1150 to 920 Ma). Concordant age estimates of detrital zircons from Kiik Group quartzites predominately fell within intervals of 1149–2105 and 2272–2850 Ma with the prominent peaks at 1217, 1332, 1466, 1582, 1621, 1748, 1849, 2517 and 2689 Ma, and minor peaks at 1992, 2282, 2791 and 2823 Ma. Hf-in-zircon and Nd whole rock data suggest that the primary stages of continental crustal growth in the Aktau–Mointy terrane were associated with the tectono-magmatic processes of the Neoarchean, Paleoproterozoic, and Mesoproterozoic. Juvenile continental crust production occurred during the Neoarchean and Paleoproterozoic, with older complex tectono-magmatic reworking, and juvenile crust production finally prevailed in the mid-Mesoproterozoic. Based on the age patterns and Hf isotopic compositions of detrital zircons from the Aktau–Mointy terrane and other terranes in the western CAOB, the crustal basement of the Aktau–Mointy terrane exhibits certain similarities with those of the Northern Kazakhstan and Issyk–Kul terranes. They all exhibit a close tectonic affinity with the Chinese Central Tian Shan terranes, Yili Block, North and China Craton, while differing from the Tarim Craton. In the Rodinia supercontinent structure, the Aktau–Mointy terrane was probably located near the Sveconorwegian Orogen in the western Baltica with other Kazakhstan and Northern Tian Shan Precambrian blocks.

Early Neoproterozoic (ca. 913–895 Ma) arc magmatism along the central–western Korean Peninsula: Implications for the amalgamation of Rodinia supercontinent

Neoproterozoic arc magmatism in or around the Korean Peninsula provides important insights into the history of the Rodinia supercontinent. Here we report the petrology, zircon U-Pb ages, geochemistry, and Hf isotope data from the Early Neoproterozoic (ca. 913–895 Ma) igneous complexes (gabbro, diorite and granitic dyke) in the northern part of the Gogunsan Islands along the southwestern margin of the Hongseong–Imjingang Belt in the central–western Korean Peninsula. The gabbro and diorite- yielded zircon U–Pb ages of ca. 913 Ma, and ca. 909–899 Ma, respectively. The granitic dykes within this complex show ages of ca. 903–895 Ma. The gabbro-diorite, and granite belong to the subalkaline series, and are characterized by large ion lithophile element (LILE) enrichments, Nb–Ta troughs, and depletion in P and Ti anomalies similar to magmatism in the continental arc setting. The initial 176Hf/177Hf ratios and εHf(t) values obtained of zircon grains are in the ranges of 0.282132–0.282268 and –2.7 to 2.3 for gabbro, 0.282010–0.282363 and –6.4 to 6.0 for diorite, and 0.282071–0.282285 and –5.0 to 2.6 for granitic dykes, suggesting juvenile crust production with mixing of older crust materials. In combination with previously published U-Pb ages and geochemical features of Late Paleoproterozoic, Mesoproterozoic, and Early Neoproterozoic rift-related rocks in and around the central–western Korean Peninsula, the Early Neoproterozoic arc magmatism reported in this study provides clear evidence for a tectonic transition from the disruption of the Columbia supercontinent to the amalgamation and disruption of Rodinia supercontinent.

Old Continental Crust Underlying Juvenile Oceanic Arc: Evidence From Northern Arabian‐Nubian Shield, Egypt

AbstractThe Neoproterozoic Arabian‐Nubian Shield (ANS) is the best preserved and the largest exposed Neoproterozoic juvenile crust on Earth. While the lithology and early Sr and Nd isotopic data demonstrate that the ANS crust is overwhelmingly juvenile, pre‐ANS old zircon crystals have been increasingly recognized in the ANS igneous and sedimentary rocks, casting doubt on the “juvenility” of the ANS crust. In order to understand the origin of the old continental materials in the ANS and its roles in generation of juvenile oceanic arcs, we carry out for the first time an integrated in situ analysis of zircon U‐Pb age and Hf‐O isotopes for greywacke and felsic volcanic cobble samples from the Atud Formation in the Eastern Desert of northwestern part of the ANS. Our data indicate that the Atud Formation was deposited between ca. 720 and 700 Ma, concurrent with the production of oceanic arcs in the ANS. The Atud greywacke was derived from the erosion of a proximal arc terrane that contains numerous old continental crust materials. We identify for the first time a 755‐Ma felsic volcanic cobble from the Atud Formation that is derived from old continental materials during juvenile crust production, suggesting presence of an old continental crust substrate that underlies the ANS. Our work demonstrates that reworking of old continental crust played important roles in generation of oceanic arcs in the northwestern ANS that is likely much less juvenile than previously thought. Thus, the crustal growth rates calculated based on estimates of temporal island arc development need to be revised.

Constraining a Precambrian Wilson Cycle lifespan: An example from the ca. 1.8 Ga Nagssugtoqidian Orogen, Southeastern Greenland

In the Phanerozoic, plate tectonic processes involve the fragmentation of the continental mass, extension and spreading of oceanic domains, subduction of the oceanic lithosphere and lateral shortening that culminate with continental collision (i.e. Wilson cycle). Unlike modern orogenic settings and despite the collection of evidence in the geological record, we lack information to identify such a sequence of events in the Precambrian. This is why it is particularly difficult to track plate tectonics back to 2.0Ga and beyond. In this study, we aim to show that a multidisciplinary approach on a selected set of samples from a given orogeny can be used to place constraints on crustal evolution within a P-T-t-d-X space. We combine field geology, petrological observations, thermodynamic modelling (Theriak-Domino) and radiogenic (U-Pb, Lu-Hf) and stable isotopes (δ18O) to quantify the duration of the different steps of a Wilson cycle. For the purpose of this study, we focus on the Proterozoic Nagssugtoqidian Orogenic Belt (NOB), in the Tasiilaq area, South-East Greenland.Our study reveals that the Nagssugtoqidian Orogen was the result of a complete three stages juvenile crust production (Xjuv) – recycling/reworking sequence: (I) During the 2.60–2.95Ga period, the Neoarchean Skjoldungen Orogen remobilised basement lithologies formed at TDM 2.91Ga with progressive increase of the discharge of reworked material (Xjuv from 75% to 50%; δ18O: 4–8.5‰). (II) After a period of crustal stabilization (2.35–2.60Ga), discrete juvenile material inputs (δ18O: 5–6‰) at TDM 2.35Ga argue for the formation of an oceanic lithosphere and seafloor spreading over a period of ~0.2Ga (Xjuv from <25% to 70%). Lateral shortening is set to have started at ca. 2.05Ga with the accretion of volcanic/magmatic arcs (i.e. Ammassalik Intrusive Complex) and by subduction of small oceanic domains (M1: 520±60°C at 6.6±1.4kbar). (III) Continental collision between the North Atlantic Craton and the Rae Craton occurred at 1.84–1.89Ga. Crustal thickening of ~25km was accompanied by regional metamorphism M2 (690±20°C at 6.25±0.25kbar) and remobilization of pre-existing supracrustal lithologies (Xjuv~40%; δ18O: 5–10.5‰).Rates and durations obtained for seafloor spreading (175±25Ma), subduction (125±75Ma) and continental collision (ca. 60Ma) are similar to those observed in Phanerozoic Wilson Cycle but differ from what was estimated for Archean terrains. Therefore, timespans of the different steps of a Wilson cycle might have progressively changed over time as a response to the progressive cratonization of the lithosphere.

Detrital zircon U‐Pb ages and Hf isotopic constraints on the terrigenous sediments of the Western Alps and their paleogeographic implications

AbstractDetrital zircons from Cretaceous micaschist, late Eocene‐earliest Oligocene sandstone and early Oligocene siltstone of the Western Alps fall into three main separable age clusters at 610–540 Ma, 490–430 Ma, and 340–280 Ma that correspond to the Cadomian (Neoproterozoic), Ordovician, and Variscan (Carboniferous) events widespread in western and central Europe. Hf isotopic results indicate that these three magmatic and tectonic episodes did not give rise to significant production of juvenile crust. A distinguishable group of Triassic zircons, around 250–200 Ma which is considered to derive from the Southern Alps, has been detected in the early Oligocene “Schistes à Blocs” formation and the Briançonnais “Flysch Noir”. In contrast, this age group is absent in late Eocene to earliest Oligocene sandstones. In agreement with sedimentological studies, our results show that the main source areas of the Eocene sandstone were probably located in the European continent. The arrival of detritus from the Internal Zone occurred in early Oligocene, coeval with the tectonic rotation from northwestward to westward in the propagation of allochthonous units. Based on previous studies and our new data, we argue that the Briançonnais Zone was likely a paleorelief since the middle Eocene that accounts for the lack of detritus from the Adriatic units. Contemporary sediments were accumulated in the foredeep of the Adriatic plate. From Oligocene time onward, the blockage was cut through after a regional uplifting, and thus, the Internal Zone started to provide detritus into the western flexural basins.

Osmium isotope evidence for Early to Middle Proterozoic mantle lithosphere stabilization and concomitant production of juvenile crust in Dish Hill, CA peridotite xenoliths

The 187Os/188Os compositions in peridotite samples from the sub-continental lithospheric mantle (SCLM) can be used to constrain the timing of melt extraction and potentially test the link between large-scale mantle melting and juvenile crust production. The SCLM has often experienced a complex history such that some lithophile elements such as REEs (rare earth elements) in these rocks typically record overprinting during metasomatism. New 187Os/188Os, major and trace element compositional data were obtained on sixteen Dish Hill peridotite xenoliths (California, USA) and are used to examine these issues. The samples show strong correlations between 187Os/188Os and indicators of melt depletion such as Lu abundance in clinopyroxene, modal abundance of clinopyroxene, bulk rock Al2O3 and the Cr# (Cr/(Cr+Al)) in spinel. These relationships indicate that metasomatism did not compromise the 187Os/188Os systematics. The data appear to form two melt depletion trends consistent with Re depletion model ages (TRD) obtained from the two Al2O3 versus 187Os/188Os trends are 2.1±0.5Ga and 1.3±0.3Ga (±95% conf.).It has been suggested that the SCLM under Dish Hill may be fragments of oceanic lithosphere emplaced as the result of Farallon plate subduction during the Late Cretaceous (Luffi et al., 2009). However, the strong melt depletion trends, major element compositions and Re-depletion ages are not consistent with the interpretation of this suite of xenoliths having an oceanic lithospheric origin. Rather, the 2.1Ga age coincides with Nd model ages of 2–2.3Ga (Bennett and DePaolo, 1987; Rämö and Calzia, 1998) for the overlying Mojavia crustal province. The 1.3Ga age is consistent with large-scale A-type magmatism in the nearby region at this time that is purported to be the result of mantle plume melting processes. Therefore, data from this study point to the SCLM under Dish Hill being formed by two ancient mantle-melting events, which could be the result of interleaving SCLM at depth. These interpretations indicate that the primary mechanism of SCLM formation under Dish Hill was through stabilization following partial melting in the convecting mantle that also results in contemporaneous juvenile crust production.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America

U-Pb geochronologic and Hf isotopic analyses have been conducted on detrital zircons extracted from 36 samples of Neoproterozoic through Triassic passive margin strata from western North America. The data serve as an improved reference for comparison with inboard strata that accumulated on the North American craton and outboard strata belonging to potentially displaced Cordilleran terranes. As expected, this reference documents significant variations in ages and Hf isotope compositions both north-south and also through time. The data also provide insights into the provenance of Cordilleran passive margin strata. During Neoproterozoic, Cambrian, and Early–Middle Devonian time, most grains were shed from relatively local basement rocks and from Mesoproterozoic clastic strata containing 1.2–1.0 Ga grains that originated in the Grenville orogen. This pattern was interrupted during Ordovician time, when much of the Cordilleran margin was blanketed by detritus shed from the northern Canadian Shield. Beginning in Late Devonian time, and continuing through late Paleozoic and Triassic time, most regions were dominated by locally derived detritus (largely recycled from underlying strata), but also received 0.7–0.4 Ga grains that were shed from the Franklinian, Caledonian, Appalachian, and Ouachita-Marathon orogens. This pattern is complicated in southern transects as a result of mid-Paleozoic emplacement of off-shelf assemblages onto the continental margin (e.g., Antler orogeny) and construction of Permo-Triassic magmatic arcs along the margin. Our data also provide a robust record of the crustal evolution of western North America, with significant production of juvenile crust during late Archean (3.0–2.5 Ga) and Paleoproterozoic (1.78–1.6 Ga) time and phases of mainly crustal reworking at 2.0–1.78, 1.5–1.3, 1.2–1.0, and 0.6–0.2 Ga. This history is somewhat different from that of other continents, with western Laurentia comprising a greater overall proportion of juvenile crust, punctuated by greater degrees of crustal reworking between 2.2 and 1.78 Ga and 0.3–0.2 Ga.

The U–Pb and Hf isotope evidence of detrital zircons of the Ordovician Ollantaytambo Formation, southern Peru, and the Ordovician provenance and paleogeography of southern Peru and northern Bolivia

Abstract The Ordovician Ollantaytambo Formation represents one of only two known occurrences of Lower Paleozoic volcanic rocks in southern Peru and northern Bolivia. Its lower part consists of mafic lapilli tuffs, shales and mature sandstones form the upper part. We present LA-ICP-MS U–Pb ages and Hf isotope data of detrital zircons from one of the upper member sandstones in order to determine both the duration of volcanism and the provenance of the mature detritus, and to use the data to further define the paleogeography of the Ordovician basin in the northern Central Andes. The detrital zircon ages of the Ollantaytambo Formation range from 2013 Ma to 445 Ma. They are grouped mainly between 1400 and 1100 Ma (35%), 1100 and 900 Ma (14%), 770 and 650 Ma (14%), and from 500 Ma to 440 Ma (30%). Within these groups the main peaks are at 1249 Ma, 1052 Ma, 741 Ma and 459 Ma. The older groups correspond to major orogenic cycles recorded on the southwestern Amazonia craton, the Rondonia-San Ignacio, Sunsas, and Brasiliano orogenies. The younger one reflects the activity of the Early Paleozoic Famatinian magmatic arc known mainly from the southern Central Andes, but also recognized on the Arequipa Massif and in northern Peru. The provenance of the grains with ages between 770 Ma and 650 Ma is enigmatic as there are no known sources in southwestern Amazonia or the Central Andes. The ɛHf(t) values of selected Ollantaytambo Formation zircons are between −22 and +3 and considered to be moderately juvenile to evolved. Truely juvenile zircons with a composition similar to the depleted mantle were not identified. Together with additional literature data from Ordovician formations in southern Peru, the Hf-isotope data indicate production of juvenile crust mainly in the Mesoproterozoic, and increasing recycling of this crust during the Neoproterozoic and Early Paleozoic orogenic events.

An estimate for the bulk composition of juvenile upper continental crust derived from glacial till in the North American Cordillera

We use a compilation of bulk chemical analyses for 2771 rocks and 3487 till samples (silt + clay fraction) from British Columbia, Canada to construct an estimate of juvenile upper crust composition in the North American Cordillera for comparison with upper crust estimates for older continental crust. Principle component analyses demonstrate that glacial tills are a robust natural average of their surface rock provenance. The chemical weathering during the generation and deposition of tills is small, yet they preserve the chemical weathering history of their source regions. Our results show that the upper continental crust of the Cordillera is broadly similar to other estimates in terms of the abundance of Si, Al, Mg, Ca, Na, Zr, Y, Sr, Ba, Th and U. The till-based upper crustal estimate, however, shows consistent and marked enrichment in Fe, Ti, Mn, P and HREE and depletion in LREE and K. These attributes are not obviously attributed to any analytical or sampling bias; surface rock averages and modern river sediments from British Columbia show identical patterns of enrichment and depletion as the tills. We attribute the unique character of our Cordilleran juvenile upper crust estimate to a significant component of basaltic soils in till generation, as compared to upper crustal estimates from more mature denuded terrains. Nonetheless, our crustal estimate remains overall ‘andesitic’, similar to those from Precambrian shields, supporting the notion that reworking of old crust is volumetrically far more important than production of juvenile crust in the growth of the North American continent.

日本列島の地体構造区分再訪―太平洋型（都城型）造山帯構成単元および境界の分類・定義―

The geotectonic subdivision and relevant definitions of geotectonic units in the Japanese Islands are revised on the basis of new data, particularly with detrital zircon dating of U-Pb ages and seismic profiling of the deep arc crust across the islands. In addition to the final confirmation of the subhorizontal structures of the Paleozoic to Cenozoic accretionary complexes and their high-P/T metamorphosed equivalents, several new aspects were recognized; i.e., detection of the eastern extension of the collisional suture between the Sino-Korean and Yangtze cratons in the Higo belt with medium-pressure-type metamorphism in SW Japan, and separation of the traditional Sanbagawa belt into two distinct metamorphic belts characterized by mutually different ages of protolith AC-formation and peak metamorphism. The occurrence and consumption of 4 Paleozoic to Mesozoic granite batholiths, as major provenances for the ancient Japanese Islands, are documented by detrital zircon dating of Paleozoic–Mesozoic sandstones. With respect to these new findings, the definitions of unit boundaries were thoroughly revised in terms of chronological spectrum in “ocean plate stratigraphy–metamorphism”. The geological significance of 5 major tectonic lines (faults) of the Pacific-type (or Miyashiro-type) orogen in Japan, i.e., the Nagato–Hida marginal TL, Osayama–Omi TL, Ishigaki–Kuga TL, Paleo–Median TL, and Butsuzo TL, is discussed. The current revision of the geotectonic subdivision and definitions of component units and their mutual boundaries leads to the following conclusions, which challenge the conventional understanding of the orogenic history of the Japanese Islands. (1) Proto-Japan in the Early Paleozoic was located closer to the South China (Yangtze) craton rather than the North China (Sino–Korean) craton. (2) Ever since 520 Ma, subduction of past Pacific ocean floors formed mature arc-trench systems with a full set of granite batholith, fore-arc basin, accretionary complex, and high-P/T metamorphosed equivalents at least 5 times; however, the former 4 sets were almost completely destroyed, with the exception of smaller tectonic blocks that currently occur within serpentinite melange. (3) Tectonic erosion played a significant role in consuming ancient fore-arc crusts including 4 granite batholiths of the Paleozoic to mid-Mesozoic. (4) Serpentine melange represents the former Wadati–Benioff plane along which tectonic erosion took place. (5) The Japanese Islands, which basically developed along the Yangtze continental margin, have experienced multiple episodes of oceanward growth and continentward retreat due to alternating subduction-accretion and tectonic erosion. (6) Net production of juvenile crust occurred on a large scale along the Japan margin during the 500 million year-long oceanic subduction regime since the Cambrian; however, intensive tectonic erosion effectively erased the older crusts from the surface and enriched the underlying sub-arc mantle with heat-generating continental material.

Rift melting of juvenile arc-derived crust: Geochemical evidence from Neoproterozoic volcanic and granitic rocks in the Jiangnan Orogen, South China

A combined study of zircon U–Pb and Lu–Hf isotopes, mineral O isotopes, whole-rock elements and Sr–Nd isotopes was carried out for Neoproterozoic volcanics and granites from the eastern part of the Jiangnan Orogen in South China. The results are used to test controversial models of petrogenesis (plume-rift, slab-arc and plate-rift) for similar ages of magmatic rocks in South China. Zircon U–Pb dating yields two groups of ages at ∼780 and ∼825 Ma, respectively, corresponding to syn-rift and pre-rift magmatic events in response to supercontinental rifting. Both volcanic and granitic rocks show trace element features similar to those of arc-derived igneous rocks, but with more significant enrichment in large ion lithospheric elements relative to oceanic arc basalts. They have positive ɛ Hf( t) values of 3.6–6.3 for zircons, with Hf model ages of 1.12–1.21 Ga. This indicates reworking of late Mesoproterozoic juvenile crust for the origin of the Neoproterozoic magmatic rocks. Thus, oceanic arc magmatism would occur in the late Mesoproterozoic, with remarkable production of juvenile crust at the southeastern margin of the Yangtze Block. Because of the tectonic collapse of arc–continent collision orogen in the pre-rift stage, S-type magmatic rocks were generated by burial and anatexis of juvenile arc-derived crust to form the pre-rift episode of granodiorites and volcanics. In the syn-rift stage, the volcanics formed by reworking of arc-derived sedimentary rocks whereas the granite was generated by melting of the pre-rift igneous rocks. Emplacement of evolved felsic magmas along the rift tectonic zone would cause subsolidus high- T meteoric-hydrothermal alteration, resulting in varying δ 18O values for minerals from the volcanic and granitic rocks. Therefore, the plate-rift model is advanced to account for petrogenesis of all Neoproterozoic magmatic rocks in South China, with lithospheric extsension as the driving force of supercontinental rifting. Partial melting due to arc-continent collision, orogenic collapse and supercontinental rifting is also proposed as a mechanism for the chemical differentiation of continental crust towards the felsic composition.

U–Pb and Hf-isotope analyses of zircon from the Kundelungu Kimberlites, D.R. Congo: Implications for crustal evolution

The Gungwania and Talala kimberlitic pipes on the Kundelungu Plateau (Katanga, SE Congo) have been used as drillholes, to obtain crustal zircons for a study of crustal evolution in the region and to constrain the age of the basement and the sedimentary provenance of the Katangan Supergroup. Two hundred and twenty-nine zircon grains were analysed for U–Pb ages and Hf-isotope compositions. Juvenile Mesoarchean (∼3.4–3.3 Ga) crust in the region underwent recycling during Neoarchean and Paleoproterozoic time. The Paleoproterozoic event involved little production of juvenile crust, and this Paleoproterozoic crust was recycled during Mesoproterozoic and Neoproterozoic time. Both the Mesoproterozoic and Neoproterozoic were characterised by bimodal magmatism reflecting extension, linked with the break up of Rodinia and the opening of the Katangan basin, and the development of juvenile crust. Detailed studies of the external morphology of the zircons in different age groups suggest that the basement beneath the Kundelungu region is predominantly Paleoproterozoic in age; other zircon populations may be derived from the Katangan sediments. The absence of any zircons younger than 560 Ma supports deposition of the Biano Subgroup, which hosts the kimberlites, during the Pan-African Lufilian orogeny. The Archean Congo and Zimbabwe cratons, the Paleoproterozoic Ubendian Belt (Bangweulu Block), the Paleo-to-Mesoproterozoic Irumide Belt, the Mesoproterozoic Kibaran Belt and the Choma Kalomo Block may all have contributed material to the sediments of the Katangan Supergroup.

U–Pb isotopic ages and Hf isotopic composition of single zircons: The search for juvenile Precambrian continental crust

U–Pb isotopic ages and Hf isotope compositions by laser microprobe ICPMS and MC-ICPMS can be useful in identifying detrital zircons derived from juvenile continental sources. Hf isotopes in detrital zircons from modern river deposits in Brazil, Australia, India and the Ukraine show evidence for production of juvenile crust at about 2.5 Ga. However, if detrital zircon populations in the 2.4–2.2 Ga time window are representative of the proportion of juvenile crust in their primary sources, they yield little evidence for significant volumes of juvenile crust of this age. Hf isotopic compositions of detrital zircons from the Ukraine and eastern Australia also record production of juvenile continental crust between 1.65 and 1.40 Ga. Zircons from granitoids in south-central Laurentia and in western Brazil have E Hf( T) values that fall near the depleted mantle growth curve recording production of juvenile continental crust in these regions between 1.5 and 1.3 Ga. Although Hf isotope compositions of detrital zircons can be useful in identifying detrital zircons derived from juvenile continental sources, the results do not necessarily equate to the volume of juvenile continental crust produced during specific time intervals.

Frontiers in large igneous province research

Earth history is punctuated by events during which large volumes of mafic magmas were generated and emplaced by processes distinct from “normal” seafloor spreading and subduction-related magmatism. Large Igneous Provinces (LIPs) of Mesozoic and Cenozoic age are the best preserved, and comprise continental flood basalts, volcanic rifted margins, oceanic plateaus, ocean basin flood basalts, submarine ridges, ocean islands and seamount chains. Paleozoic and Proterozoic LIPs are typically more deeply eroded and are recognized by their exposed plumbing system of giant dyke swarms, sill provinces and layered intrusions. The most promising Archean LIP candidates (apart from the Fortescue and Ventersdorp platformal flood basalts) are those greenstone belts containing tholeiites with minor komatiites. Some LIPs have a substantial component of felsic rocks. Many LIPs can be linked to regional-scale uplift, continental rifting and breakup, climatic shifts that may result in extinction events, and Ni–Cu–PGE (platinum group element) ore deposits. Some current frontiers in LIP research include: (1) Testing various mantle plume and alternative hypotheses for the origin for LIPs. (2) Characterizing individual LIPs in terms of (a) original volume and areal extent of their combined extrusive and intrusive components, (b) melt production rates, (c) plumbing system geometry, (d) nature of the mantle source region, and (e) links with ore deposits. (3) Determining the distribution of LIPs in time (from Archean to Present) and in space (after continental reconstruction). This will allow assessment of proposed links between LIPs and supercontinent breakup, juvenile crust production, climatic excursions, and mass extinctions. It will also allow an evaluation of periodicity in the LIP record, the identification of clusters of LIPs, and postulated links with the reversal frequency of the Earth's magnetic field. (4) Comparing the characteristics, origin and distribution of LIPs on Earth with planets lacking plate tectonics, such as Venus and Mars. Interplanetary comparison may also provide a better understanding of convective processes in the mantles of the inner planets. In order to achieve rapid progress in these frontier areas, a global campaign is proposed, which would focus on high-precision geochronology, integrated with paleomagnetism and geochemistry. Most fundamentally, such a campaign could help hasten the determination of continental configurations in the Precambrian back to 2.5 Ga or greater. Such reconstructions are vital for the proper assessment of the LIP record, as well as providing first-order information related to all geodynamic processes.

Sources of Phanerozoic granitoids in the transect Bayanhongor–Ulaan Baatar, Mongolia: geochemical and Nd isotopic evidence, and implications for Phanerozoic crustal growth

The Central Asian Orogenic Belt (CAOB) is renowned for massive generation of juvenile crust in the Phanerozoic. Mongolia is the heartland of the CAOB and it has been subject to numerous investigations, particularly in metallogenesis and tectonic evolution. We present new petrographic, geochemical and Sr–Nd isotopic analyses on Phanerozoic granitoids emplaced in west-central Mongolia. The data are used to delineate their source characteristics and to discuss implications for the Phanerozoic crustal growth in Central Asia. Our samples come from a transect from Bayanhongor to Ulaan Baatar, including three tectonic units: the Baydrag cratonic block (late Archean to middle Proterozoic), the Eo-Cambrian Bayanhongor ophiolite complex and the Hangay–Hentey Basin of controversial origin. The intrusive granitoids have ages ranging from ca. 540 to 120 Ma. The majority of the samples are slightly peraluminous and can be classified as granite (s.s.), including monzogranite, syenogranite and alkali feldspar granite. Most of the rocks have initial 87Sr/ 86Sr ratios between 0.705 and 0.707. Late Paleozoic to Mesozoic granitoids (≤250 Ma) are characterized by near-zero ε Nd( T) values (0 to −2), whereas older granitoids show lower ε Nd( T) values (−1.5 to −7). The data confirm the earlier observation of Kovalenko et al. [Geochemistry International 34 (1996) 628] who showed that granitoids emplaced outside of the Pre-Riphean basement rocks are characterized by juvenile positive ε Nd( T) values, whereas those within the Pre-Riphean domain and the Baydrag cratonic block, as for the present case, show a significant effect of ‘contamination’ by Precambrian basement rocks. Nevertheless, mass balance calculation suggests that the granitoids were derived from sources composed of at least 80% juvenile mantle-derived component. Despite our small set of new data, the present study reinforces the general scenario of massive juvenile crust production in the CAOB with limited influence of old microcontinents in the genesis of Phanerozoic granitoids.

Recognizing Mantle Plumes in the Geological Record

▪ Abstract Mantle plumes are recognized by domal uplift, triple junction rifting, and especially the presence of a large igneous province (LIP), dominated in the Phanerozoic by flood basalts, and in the Proterozoic by the exposed plumbing system of dykes, sills, and layered intrusions. In the Archean, greenstone belts that contain komatiites have been linked to plumes. In addition, some carbonatites and kimberlites may originate from plumes that have stalled beneath thick lithosphere. Geochemistry and isotopes can be used to test and characterize the plume origin of LIPs. Seismic tomography and geochemistry of crustal and subcrustal xenoliths in kimberlites can identify fossil plumes. More speculatively, plumes (or clusters of plumes) have been linked with variation in the isotopic composition of marine carbonates, sea-level rise, iron formations, anoxia events, extinctions, continental breakup, juvenile crust production, magnetic superchrons, and meteorite impacts. The central region of a plume is located using the focus of a radiating dyke swarm, the distribution of komatiites and picrites, etc. The outer boundary of a plume head circumscribes the main flood basalt distribution and approximately coincides with the edge of domal uplift that causes shoaling and offlap in regional sedimentation.

Highly fractionated I-type granites in NE China (I): geochronology and petrogenesis

Northeastern (NE) China is the easternmost part of the Central Asian Orogenic Belt (CAOB), which is celebrated for its accretionary tectonics and the world's most important juvenile crust production in the Phanerozoic era. Abundant granitoids occur in the Great Xing'an, Lesser Xing'an and Zhangguangcai Ranges in NE China. This paper presents partial results of a series of studies on the granitoids from this region, aiming to understand their role in the building of new continental crust in eastern Asia. Three composite granite plutons (Xinhuatun, Lamashan and Yiershi) were chosen for geochemical and isotopic study in order to determine their emplacement ages and petrogenesis. Petrographically, they range from granodiorite (minor), monzogranite, syenogranite to alkali-feldspar granite. Quartz and perthitic feldspar are principal phases, accompanied by minor amounts of plagioclase, biotite (<5%) and other accessory minerals. In addition, many contain abundant miarolitic cavities which suggest that they were emplaced at shallow levels with extensive fractional crystallization. Geochemically, the granites are silica-rich, peraluminous and have high contents of alkalis. They invariably show enrichment in light rare earth elements (LREE) and significant negative Eu anomalies. All the granitic rocks demonstrate the characteristic negative anomalies in Ba, Nb, Sr, P, Eu, and Ti, and a positive anomaly in Pb in the spidergram. The emplacement of the Xinhuatun pluton took place at 184±4 Ma as revealed by zircon SHRIMP U–Pb data. This is also supported by the slightly younger Rb–Sr whole-rock (WR) isochron age of 173±3 Ma. A whole-rock (WR) Rb–Sr isochron age of 154±3 Ma was obtained for the Lamashan pluton, which is interpreted as close to the time of emplacement. The Yiershi pluton was intruded at about 140 Ma as evidenced by a zircon U–Pb age of 137±2 Ma and WR Rb–Sr isochron age of 143±5 Ma. Biotite-WR Rb–Sr isochrons and 40Ar/ 39Ar ages of feldspars allow us to estimate the cooling rate of each pluton. Geochemical data suggest that the rocks are highly fractionated I-type granites. Fractionation of biotite and feldspars was the principal process of magmatic differentiation and responsible for major element variation. Rb, Sr and Ba concentrations were controlled by feldspar separation, whereas REE elements were fractionated by accessory minerals, such as apatite, allanite and monazite.

Rapid Growth of Continental Crust Between 2.2 to 1.8 Ga in the South American Platform: Integrated Australian, European, North American and SW USA Crustal Evolution Study

The production of juvenile continental crust in the South American Platform (SAP) and other continents was larger in the Proterozoic than in the Archean. In the SAP the juvenile crust production was about 65% during the Proterozoic, while during the Archean it was around 34% of the total volume. Similar proportions are found in the Australian continent. The largest accretion from mantle to crust happened between 2.2 to 1.8 Ga and corresponded to 35% of the total volume of the present SAP continental crust. During the Archean an intense recycling between mantle and continental crust took place, while in the Paleoproterozoic the rate of mantle accretion to continental crust was larger than the assimilation. For the other continents (Europe and North America) the Paleoproterozoic was also the main accretion event. In the SW USA, rapid continental growth (about 45% of actual volume) occurred in a short time interval between 1.9-1.7 Ga. During Meso and Neoproterozoic little accretion of juvenile material occurred in the SAP, where crustal reworking predominated, but in Eurasia much juvenile continental crust was generated during the Phanerozoic in Central Asia.

Strontium isotopic variations of Neoproterozoic seawater: Implications for crustal evolution

We report high precision Sr isotopic data on carbonates from the Neoproterozoic Shaler Group, Victoria Island, Northwest Territories, Canada. Lithostratigraphic correlations with the relatively welldated Mackenzie Mountains Supergroup constrain Shaler deposition to ~770–880 Ma, a range corroborated by 723 ± 3 Ma lavas that disconformably overlie Shaler carbonates and by Late Riphean microfossils within the section. Samples with low 87Rb 86Sr ratios (< 0.01) were selected for Sr isotopic analysis. δ 18 O, Mn, Ca, Mg, and Sr data were used to recognize altered samples. The altered samples are characterized by high Mn Sr (≥2) and variable δ 18 O; most are dolomites. The data indicate that between ca. 790–850 Ma the 87Sr 86Sr ratio of seawater varied between 0.70676 and 0.70561. The samples show smooth and systematic variation, with the lowest 87Sr 86Sr value of 0.70561 at ca. 830 Ma. The low 87Sr 86Sr ratio of carbonates from the lower parts of our section is similar to a value reported for one sample from the Adrar of Mauritania (≈900 Ma), West African Craton. Isotopic ratios from the upper part of the Shaler section are identical to values from the lower part of the Neoproterozoic Akademikerbreen Group, Spitsbergen. Although a paucity of absolute age determinations hinders attempts at the precise correlation of Neoproterozoic successions, it is possible to draw a broad outline of the Sr isotopic composition of seawater for this period. Indeed, the Sr isotope data themselves provide a stratigraphic tool of considerable potential. Data from this study and the literature are used to construct a curve of the 87Sr 86Sr ratio of Neoproterozoic seawater. The new data reported in this study substantially improve the isotopic record of Sr in seawater for the period 790–850 Ma. The Sr isotope composition of seawater reflects primarily the balance between continental Sr input through river input and mantle input via hydrothermal circulation of seawater through mid-ocean ridges. Coupling of Nd and Sr isotopic systems allows us to model changes in seafloor spreading rates (or hydrothermal flux) and continental erosion. The Sr hydrothermal flux and the erosion rate (relative to present-day value) are modeled for the period 500–900 Ma. The results indicate that the hydrothermal flux reached a maximum value at ca. 830 Ma. In contrast, a large peak in erosion rate is indicated at ca. 570 Ma. The peaks in hydrothermal flux and erosion rate are most likely related to developments in the Pan-African and related orogenic events, whose initial development is characterized by production of juvenile crust during supercontinental break up and rifting. The time ca. 570 Ma is characterized by continent-continent collision and production of recycled crust. Sr isotope data from Proterozoic carbonates offer a valuable resource for understanding large-scale crust dynamics.