Hafnium Isotope Data Research Articles

Devonian plutons in the eastern Meguma terrane, Nova Scotia, Canada: zircon U–Pb, Lu–Hf, and O isotopic compositions, age, and petrogenetic implications

Abundant granitic plutons intruded the eastern Meguma terrane of Nova Scotia in the middle- to late Devonian. Less voluminous diorite–tonalite and gabbro intrusions are associated with the granitic plutons along the northern margin of the terrane adjacent to the Cobequid–Chedabucto fault zone. All plutons contain metasedimentary xenoliths, and the mafic plutons show magma mingling textures with their adjacent granitic plutons. New U–Pb zircon data from autocrystic zircon in 13 samples indicate coeval emplacement of mafic and granitic plutons between ca. 382 and 368 Ma. However, the zircon grains contain numerous inherited domains that range in age from Palaeoproterozoic to Devonian. These inherited ages correspond to detrital zircon U–Pb dates from the Cambrian to Ordovician metasedimentary host rocks. Zircon oxygen isotopic data (δ18O) are between +7.4 ± 0.2‰ and +9.3 ± 0.3‰ indicating significant involvement of the crust as the magma source or contaminant. If the high δ18Ozrn values are a result of contamination, the contaminant was likely the metasedimentary rocks of the Meguma terrane. Hafnium isotopic data from autocrystic zircon have εHf( t) between −6.0 ± 1.5 and +2.1 ± 2.5. The new zircon U–Pb, O, and Hf isotopic data from plutons in the eastern Meguma terrane are indistinguishable from published data from the South Mountain Batholith. The data suggest that Devonian magmatism in the Meguma terrane post-dated the main orogenic event that caused folding and regional metamorphism and involved the same magma source and/or contaminants throughout the terrane.

Formation of juvenile continental crust in northern Nubian Shield: New evidence from granitic zircon U-Pb-Hf-O isotopes

The Neoproterozoic Arabian-Nubian Shield (ANS) consists of continental crust formed prior to and during the collision between East and West Gondwana. The Eastern Desert of Egypt (i.e., northern Nubian Shield) constitutes the north-western part of the ANS. Neoproterozoic magmatism in the Eastern Desert region occurred between ∼ 800 Ma and ∼ 550 Ma. Although there is a broad consensus that ∼ 800–650 Ma pre-collisional igneous rocks comprise the main magmatic stage contributing to Eastern Desert crustal evolution, whether this crust is purely of a juvenile composition, or was generated by recycling/reworking of much older continental crust, remains debated. Here, we present new whole-rock geochemical and Sr-Nd isotopic data and an integrated zircon U-Pb-Hf-O-trace element dataset for two granitoids (Um Khariga and Genina Gharbia) from the Eastern Desert. Samples from the Um Khariga granitoids yield zircon U-Pb ages of 746 ± 4 and 753 ± 4 Ma that overlap within error, and the Genina Gharbia granitoids have an age of ∼ 690 Ma, indicating that both granitoids belong to different phases of pre-collisional magmatism. Zircon hafnium isotopic data of both granitoids yield weighted mean εHf(t) values ranging from + 8.01 ± 0.23 to + 10.52 ± 0.13, indicating that both granitoid suites were derived by melting of a juvenile source. SIMS oxygen isotope data for zircon show that the magmatic zircon has mantle-like δ18O values with weighted means ranging from 4.73 ± 0.02 to 5.04 ± 0.08 ‰. Based on previously published zircon U-Pb-Hf-O-trace element data for granitoids and volcanic rocks of the Eastern Desert, plus our new isotopic datasets obtained from the Um Khariga and Genina Gharbia granitoids, we show that the ∼ 800–650 Ma magmatism is characterized by high zircon εHf(t) (+8 to + 15), and mantle-like zircon δ18O values (∼+5‰), implying that the continental crust of the Eastern Desert was either extracted directly from a depleted mantle and/or reworked from accreted juvenile oceanic crust or oceanic arc with no evidence for the input of old continental material. The overall zircon εHf(t) and δ18O evolution trends for the northern Nubian Shield show that juvenile mantle input dominated the early phases (ca. 800–670 Ma) of crustal growth and evolution in this part of the ANS, coinciding with the break-up stage of the supercontinent Rodinia. The involvement of continental crustal recycling only started to play a significant role since the ca. 670 Ma regional collisional event during the early stage of Gondwana assembly.

Detrital zircon in an active sedimentary recycling system: Challenging the ‘source‐to‐sink’ approach to zircon‐based provenance analysis

ABSTRACTThe age and properties of detrital zircon in a sediment reflect properties of the rocks in which the zircon crystallized, and not necessarily the immediate precursor of the host sediment. Where clastic sediments are recycled, these properties are preserved so that U‐Pb ages and Lu‐Hf data on zircon grains will no longer give information on the routing of detritus. Because the crustal evolution and sedimentary recycling history of Southern Africa is well‐established, Holocene sediments in the region provide a good test example for detrital zircon geochronology applied to sands or sandstones with a complex recycling history. Data from 16 samples of unconsolidated sand, including dunes from the southern part of the Kalahari Basin, beach sands at the Atlantic coast, and isolated inland dune occurrences, yield ranges of early Mesozoic to late Archaean ages. Uranium–lead and lutetium–hafnium isotope data provide no evidence of direct derivation of zircon from protosources in crystalline bedrock. The complex distribution patterns can be decomposed into seven ‘provenance components’ that have previously been encountered in Palaeoproterozoic to Jurassic sedimentary rocks in the region. These components occur in non‐random combinations, in proportions that suggest mixing of recycled material from different sedimentary precursors. Taking pre‐Cenozoic geology, Cretaceous to Pleistocene drainage history of southern Africa and Holocene wind and current patterns into account, it is possible to work out a consistent model for transport of detritus from sedimentary precursors into the basin, followed by aeolian remobilization within the basin itself. The detrital zircon distributions can thus be interpreted from prior knowledge of the basin filling history, regional geology, geomorphology, drainage evolution and wind pattern. This amounts to a line of reasoning from ‘sink back to (intermediate) ‘source’, which is the opposite of the ‘(proto)source‐to‐sink’ paradigm commonly invoked in detrital zircon studies.

New clues for magma-mixing processes using petrological and geochronological evidence from the Castelo Intrusive Complex, Araçuaí Orogen (SE Brazil)

Bimodal magmatism was recorded during the post-collisional stage that occurred during the Brasiliano/Pan-African cycle in the Araçuaí Orogen (southeastern Brazil) at ∼500 Ma. The Castelo Intrusive Complex (CIC) is a zoned post-collisional pluton in the orogen and is composed of monzogranites, granodiorites, quartz monzodiorites and diorites. CIC (∼100 km2) was emplaced along a regional lineament during the post-collisional stage of the orogeny and cut pre-collisional granitoids and metasedimentary sequences. In this work, a detailed petrographic study associated with data on U–Pb ages and Hf isotopic compositions of zircons from monzogranites, quartz monzodiorites and leucocratic dikes is presented to detail the geochronology, composition and context of CIC emplacement. Macroscale and microscale records show that several CIC lithotypes demonstrate the direct interaction of contemporaneous felsic and mafic magmas. U–Pb age dating of zircons (by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)) reveal crystallization ages of 486 ± 6 Ma for quartz monzodiorites and 524 ± 5 Ma to 499 ± 4 Ma for monzogranites. The analyzed leucocratic dike at 426 ± 15 Ma represents a late magmatic pulse. Hafnium isotopic data show variable ƐHf(t) values (from −24.05 to +12.7) and depleted mantle model ages (TDM) of 2.71–0.64 Ga, indicating the involvement of distinctly different mantle and crustal sources. At least two crustal sources with different Hf isotope signatures were distinguished (TDM of 2.6–2.4 Ga and ƐHf(t) of −26 to −22; and TDM of 2.2–1.7 Ga and ƐHf(t) of −17 to −8). These results suggest that the rocks in this study were partially derived from Archean and Paleoproterozoic crust. The isotopic data also show TDM model ages ranging from 1.28 to 0.64 Ga, with highly positive ƐHf(t) values (from +1.6 to +12.7). These data suggest the existence of a mantle reservoir source for magma generation and for heat that induced crustal melting during the post-collisional stage of the Araçuaí Orogen.

Zircon megacrysts from Devonian kimberlites of the Azov Domain, Eastern part of the Ukrainian Shield: Implications for the origin and evolution of kimberlite melts

Zircon megacrysts are commonly found in kimberlites and, together with olivine, low-Cr garnet, pyroxene, phlogopite, and ilmenite megacrysts, they constitute a mineral assemblage known as the “low-Cr suite”. The generally close similarity of ages and similar isotope geochemical characteristics of megacrysts and matrix minerals in the host kimberlites support a cognate origin. However, alteration rims commonly develop on zircon and ilmenite megacrysts, providing evidence for a lack of chemical equilibrium between the megacrysts and kimberlitic melts. Here, we report results of a detailed geochronological and geochemical study of zircon megacrysts found in the Middle Devonian Novolaspa kimberlite pipe and dyke located in the Azov Domain of the Ukrainian Shield. The concordia age of zircons is 397.0 ± 2.0 Ma, and it is 14 m.y. older than the age of kimberlite emplacement as defined by a Rb-Sr isochron on phlogopite. The average εHf(397) value for unaltered zircon megacrysts is 6.8 ± 0.14, with the alteration rims having similar Hf isotope systematics. These hafnium isotope data indicate a moderately depleted mantle source for zircon. Unaltered megacrystic zircons have low abundances of trace elements and fractionated REE, with pronounced positive Ce/Ce* anomalies and almost no Eu/Eu* anomalies. In contrast, alteration rims have very high and variable concentrations of trace elements, indicating a reaction between zircon and kimberlite melt. The melt or fluid responsible for zircon and ilmenite megacryst formation, in contrast to kimberlitic melt, was poor in incompatible trace elements, except for the HFSE (Zr, Hf, Nb, Ta, and Ti). The oxygen fugacity during crystallization of the megacryst suite was close to the FMQ buffer.Azov zircon megacrysts do not demonstrate close geochronological and isotope-geochemical similarities with their host kimberlites. They are cognate in the broad sense of being related to the same plume event, but their direct affinity is not clearly defined. The megacryst suite may have crystallized from the earliest melts/fluids that separated from the ascending mantle plume, whereas kimberlite magmas were emplaced 14 m.y. after this event.

Provenance of upper Permian–Triassic sediments in the south of North China: Implications for the Qinling orogeny and basin evolution

The Triassic Qinling Orogenic Belt was formed during the closure of the Paleo-Tethys Ocean, and its evolution can be traced by provenance analysis of sediments in the south of North China. We present detrital zircon ages, sandstone petrographic observations, paleocurrent data, and paleogeographic reconstructions that allow the evolution of this basin–mountain system to be reconstructed. Detrital zircons from upper Permian sedimentary rocks yield two major age peaks at 1876 and 259 Ma, and two minor age peaks at 2526 and 363 Ma. These zircons were likely sourced from recycled sediments in the Southern North China Block. Detrital zircons from Lower Triassic sedimentary rocks have multiple age peaks at ca. 2500, 1850, 950, 745, 450–430, and 310–250 Ma, reflecting a mixed provenance from the Qinling Orogenic Belt and Southern North China Block. Zircons from a Middle Triassic sandstone have a concentrated age peak at ca. 250 Ma and two subordinate age peaks at ca. 1800 and 2500 Ma. Hafnium isotope data for the late Paleozoic detrital zircons are characterized by negative εHf(t) values of −23.11 to −0.11, and only two grains have positive εHf(t) values of +1.45 and +2.10. We infer that these zircons were derived from recycled sediments in the Southern North China Block. The provenance data show that the North Qinling Belt and Southern North China Block had been uplifted prior to the Early Triassic. The uplift was mainly related to the continental collision between the South China and North China blocks. These and previously published data indicate the Southern North China Block experienced exhumation during the Middle−Late Triassic, firstly involving Permian cover rocks, followed by Cambrian and Neoproterozoic strata, and finally Mesoproterozoic basement rocks. The fold-and-thrust belt in the Southern North China Block probably formed during the Middle Triassic. Uplift and denudation of the Qinling Orogenic Belt controlled Triassic basin evolution in the south of North China.

Crustal rejuvenation stabilised Earth\u2019s first cratons

The formation of stable, evolved (silica-rich) crust was essential in constructing Earth’s first cratons, the ancient nuclei of continents. Eoarchaean (4000–3600 million years ago, Ma) evolved crust occurs on most continents, yet evidence for older, Hadean evolved crust is mostly limited to rare Hadean zircons recycled into younger rocks. Resolving why the preserved volume of evolved crust increased in the Eoarchaean is key to understanding how the first cratons stabilised. Here we report new zircon uranium-lead and hafnium isotope data from the Yilgarn Craton, Australia, which provides an extensive record of Hadean–Eoarchaean evolved magmatism. These data reveal that the first stable, evolved rocks in the Yilgarn Craton formed during an influx of juvenile (recently extracted from the mantle) magmatic source material into the craton. The concurrent shift to juvenile sources and onset of crustal preservation links craton stabilisation to the accumulation of enduring rafts of buoyant, melt-depleted mantle.

Petrogenesis of the Neoproterozoic low-δ18O granitoids at the western margin of the Yangtze Block in South China

The western margin of the Yangtze Block in South China preserves voluminous arc-like intrusive and extrusive igneous rocks which were formed in an active continental margin during the Neoproterozoic time. Rocks from the ca. 800 Ma Moutuo pluton are calc-alkaline I-type granitoids which have high SiO2 (67.13–77.96 wt%), K2O (2.08–4.32 wt%) and low MgO (0.16–1.81 wt%). They show concave chondrite-normalized rare earth element patterns, and are rich in large ion lithophile elements (e.g. Rb, Ba, Th, U and Pb), but depleted in high field strength elements (e.g. Nb, Ta and Ti). The magmatic zircons have low δ18O (3.29–4.80‰) and high εHf(t) values (+6.10 to +13.16) with two-stage Hf model ages (TDM2) of 0.87 to 1.33 Ga. These lines of evidence suggest that the juvenile mafic crust, protolith of the low-δ18O granitoids, was derived from the sub-arc mantle that was modified by slab-derived materials (~90% altered oceanic crust and ~10% oceanic sediments). Partial melting of the newly formed mafic crust followed by fractional crystallization played an important role in the petrogenesis of the Moutuo granitoids. Based on the available oxygen and hafnium isotopic data for the Neoproterozoic magmatic zircons in this region, the low-δ18O character of the I-type granitoids in active continental margins was probably inherited from the sub-arc mantle that was mixed with the altered oceanic crust and oceanic sedimentary materials.

Shallow reworking of magmatic zircon grains of latest Neoproterozoic (Timanian) age in serpentinite of the Voykar Massif, Polar Urals: new constraints from U-Pb isotopic data, and first trace elements and Lu-Hf isotopic data

ABSTRACTThe Voykar Massif of the Polar Urals in Russia consists of an ultramafic complex (mantle section) in the northwest, followed by a late Cambrian to Silurian mafic complex (intra-oceanic primitive island arc) and early Devonian intrusive rocks of an evolved island arc to the southeast. These complexes represent tectonic nappes thrust over the East European continental margin during the late Palaeozoic Uralian Orogeny. LA-ICP-MS U-Pb dating of zircon grains (n = 42) from an antigorite-serpentinite lens within the mafic complex yielded a Concordia age of 542 ± 2 Ma with an age range of 549–527 Ma. Additionally, few grains contain inherited domains with ages between ~990 and 3277 Ma. Hafnium isotopic data of the main age group show 176Hf/177Hft from 0.28242 to 028249 and εHft ranging from +1.9 to −6.5. The evolved Hf isotope data and the trace-element composition of the zircon grains point to an involvement of a continental crustal component in the parental magma. The zircon grains originate from igneous rocks formed during the Timanian Orogeny that affected the East European margin in the latest Neoproterozoic. During the Timanian or Uralian Orogeny, the magmatic zircons were eroded and shallowly recycled into the serpentinised mantle above the subduction zone. Finally, Uralian thrusting led to juxtaposition and imbrication of the zircon-bearing serpentinite and intra-oceanic volcanic rocks of the mafic complex.

Hf-Pb isotope and trace element constraints on the origin of the Jacupiranga Complex (Brazil): Insights into carbonatite genesis and multi-stage metasomatism of the lithospheric mantle

The Lower Cretaceous Jacupiranga complex, in the central-southeastern portion of the South American Platform, includes carbonatites in close association with silicate rocks (i.e. strongly and mildly silica-undersaturated series). Here we document the first hafnium isotope data on the Jacupiranga complex, together with new trace element and Pb isotope compositions. Even though liquid immiscibility from a carbonated silicate melt has been proposed for the genesis of several Brazilian carbonatites, isotopic and geochemical (e.g., Ba/La ratios, lack of pronounced Zr-Hf and Nb-Ta decoupling) information argues against a petrogenetic relationship between Jacupiranga carbonatites and their associated silicate rocks. Thus, an origin by direct partial melting of the mantle is considered. The isotopic compositions of the investigated silicate samples are coherent with a heterogeneously enriched subcontinental lithospheric mantle (SCLM) source of rather complex evolution. At least two metasomatic processes are constrained: (1) a first enrichment event, presumably derived from slab-related fluids introduced into the SCLM during Neoproterozoic times, as indicated by consistently old TDM ages and lamprophyre trace signatures, and (2) a Mesozoic carbonatite metasomatism episode of sub-lithospheric origin, as suggested by εNd-εHf values inside the width of the terrestrial array. The Jacupiranga parental magmas might thus derive by partial melting of distinct generations of metasomatic vein assemblages that were hybridized with garnet peridotite wall-rocks.

Building Mesoarchaean crust upon Eoarchaean roots: the Akia Terrane, West Greenland

Constraining the source, genesis, and evolution of Archaean felsic crust is key to understanding the growth and stabilization of cratons. The Akia Terrane, part of the North Atlantic Craton, West Greenland, is comprised of Meso-to-Neoarchaean orthogneiss, with associated supracrustal rocks. We report zircon U–Pb and Lu–Hf isotope data, and whole-rock geochemistry, from samples of gneiss and supracrustals from the northern Akia Terrane, including from the Finnefjeld Orthogneiss Complex, which has recently been interpreted as an impact structure. Isotope data record two major episodes of continental crust production at ca. 3.2 and 3.0 Ga. Minor ca. 2.7 and 2.5 Ga magmatic events have more evolved εHf, interpreted as reworking of existing crust perhaps linked to terrane assembly. Felsic rocks from the Finnefjeld Orthogneiss Complex were derived from the same source at the same time as the surrounding tonalites, but from shallower melting, requiring any bolide-driven melting event to have occurred almost simultaneously alongside the production of the surrounding crust. A simpler alternative has the Finnefjeld Complex and surrounding tonalite representing the coeval genesis of evolved crust over a substantial lithospheric depth. Hafnium isotope data from the two major Mesoarchaean crust-forming episodes record a contribution from older mafic Eoarchaean crust. Invoking the involvement of an Eoarchaean root in the growth of younger Mesoarchaean crust puts important constraints on geodynamic models of the formation of the discrete terranes that ultimately assembled to form Earth’s cratons.

Geochemical, isotopic, and zircon (U-Pb, O, Hf isotopes) evidence for the magmatic sources of the volcano-plutonic Ollo de Sapo Formation, Central Iberia

The Ollo de Sapo Formation comprises variably metamorphosed felsic peraluminous volcanic rocks and highlevel granites that crop out over some 600km from the Cantabrian coast to central Spain in the northern part of the Central Iberian Zone. The Ollo de Sapo magmatism is not obviously connected with any major tectonic or metamorphic event so its origin is controversial. Some authors, based on trace-elements, have proposed that the Ollo de Sapo magmas originated in a supra-subduction setting but others, based on abnormally high zircon inheritance and field and structural data, favored a rifting environment. Here we present new oxygen and hafnium isotope data from the very characteristic Ollo the Sapo zircons, which in most cases, consist of ca. 485Ma rims and ca. 590-615Ma cores. We found that the Cambrian-Ordovician rims yielded unimodal distributions that cluster around ∂18O = 10, typical of S-type magmas formed from melting of altered crust. The Ediacaran cores, in contrast, cluster around ∂18O = 6.5, consistent with being arc-magmas. Rims and cores have the same average Hf isotope composition, but the rims are considerably more uniform. These data, coupled with existing wholerock element and Sr and Nd isotopic data, indicate that the Ollo de Sapo were S-type magmas that resulted from anatexis of younger-than-600Ma immature sediments mostly derived from different Ediacaran igneous rocks with a wide range of Hf isotope composition.

U–Pb ages, geochemistry, C–O–Nd–Sr–Hf isotopes and petrogenesis of the Catalão II carbonatitic complex (Alto Paranaíba Igneous Province, Brazil): implications for regional-scale heterogeneities in the Brazilian carbonatite associations

The Catalao II carbonatitic complex is part of the Alto Paranaiba Igneous Province (APIP), central Brazil, close to the Catalao I complex. Drill-hole sampling and detailed mineralogical and geochemical study point out the existence of ultramafic lamprophyres (phlogopite-picrites), calciocarbonatites, ferrocarbonatites, magnetitites, apatitites, phlogopitites and fenites, most of them of cumulitic origin. U–Pb data have constrained the age of Catalao I carbonatitic complex between 78 ± 1 and 81 ± 4 Ma. The initial strontium, neodymium and hafnium isotopic data of Catalao II (87Sr/86Sri = 0.70503–0.70599; eNdi = −6.8 to −4.7; 176Hf/177Hf = 0.28248–0.28249; eHfi = −10.33 to −10.8) are similar to the isotopic composition of the Catalao I complex and fall within the field of APIP kimberlites, kamafugites and phlogopite-picrites, indicating the provenance from an old lithospheric mantle source. Carbon isotopic data for Catalao II carbonatites (δ13C = −6.35 to −5.68 ‰) confirm the mantle origin of the carbon for these rocks. The origin of Catalao II cumulitic rocks is thought to be caused by differential settling of the heavy phases (magnetite, apatite, pyrochlore and sulphides) in a magma chamber repeatedly filled by carbonatitic/ferrocarbonatitic liquids (s.l.). The Sr–Nd isotopic composition of the Catalao II rocks matches those of APIP rocks and is markedly different from the isotopic features of alkaline-carbonatitic complexes in the southernmost Brazil. The differences are also observed in the lithologies and the magmatic affinity of the igneous rocks found in the two areas, thus demonstrating the existence of regional-scale heterogeneity in the mantle sources underneath the Brazilian platform.

Temporal and genetic link between incremental pluton assembly and pulsed porphyry Cu-Mo formation in accretionary orogens

Abstract Economically important porphyry Cu-Mo deposits (PCDs) are generally hosted by upper-crustal plutons of variable chemical compositions related to distinct geodynamic settings. The absolute timing and duration of pluton assembly and PCD formation are critical to understanding the genetic relationship between these interrelated processes. Here, we present new comprehensive zircon U-Pb and molybdenite Re-Os ages that tightly constrain the timing and duration of pluton assembly and the age of mineralization in one of the largest ore-bearing plutons of the central Tethyan metallogenic belt, the Meghri-Ordubad pluton, southern Armenia and Nakhitchevan, Lesser Caucasus. This composite pluton was incrementally assembled during three compositionally distinct magmatic episodes over ∼30 m.y., comprising Middle Eocene (48.9–43.1 Ma) calc-alkaline subduction-related magmatism lasting 5.8 ± 0.8 m.y., followed by postsubduction Late Eocene–Middle Oligocene (37.8–28.1 Ma) shoshonitic magmatism over 9.7 ± 0.9 m.y., and Late Oligocene–Early Miocene (26.6–21.2 Ma) adakitic magmatism consisting of shoshonitic dikes and high-K calc-alkaline granodioritic magmas emplaced over 5.4 ± 0.4 m.y. Despite the distinct geodynamic settings and magma compositions, each intrusive suite culminated in the formation of variably sized PCDs, including the giant Oligocene Kadjaran porphyry Cu-Mo deposit associated with high-Sr/Y shoshonitic magmas. Complementary in situ zircon hafnium (εHfzircon = +8 to +11.3) and oxygen (δ18Ozircon = +4.6‰ to +6.0‰) isotope data support a mantle-dominated magma source with limited crustal contribution and/or cannibalization of young and juvenile lower-crustal cumulates. We conclude that, independent of geodynamic setting and magma composition, long-lived (5–10 m.y.) incremental mantle-derived magmatism is a prerequisite to form fertile magmatic-hydrothermal systems, and especially giant PCDs.

Dual provenance signatures of the Triassic northern Laurentian margin from detrital-zircon U-Pb and Hf-isotope analysis of Triassic–Jurassic strata in the Sverdrup Basin

The tectonic setting of northern Laurentia prior to the opening of the Arctic Ocean is the subject of numerous tectonic models. By better understanding the provenance of detrital zircon in the Canadian Arctic prior to rifting, both the prerift tectonic setting and timing of rifting can be better elucidated. In the Sverdrup Basin, two distinct provenance assemblages are identified from new detrital-zircon U-Pb data from Lower Triassic to Lower Jurassic strata in combination with previously published detrital-zircon data. The first assemblage comprises an age spectrum identical to that of the Devonian clastic wedge in the Canadian Arctic and is termed the recycled source. In contrast, the second assemblage is dominated by a broad spectrum of near syndepositional Permian–Triassic ages derived from north of the basin and is termed the active margin source. Triassic strata of Yukon and Arctic Alaska exhibit a similar dual provenance signature, whereas in northeastern Russia, Chukotka contains only the active margin source. Complementary hafnium isotopic data on Permian–Triassic zircon have eHf values that are consistent with the common evolved crustal signature of the Devonian clastic wedge detrital-zircon grains and Neoproterozoic–Paleozoic basement rocks in the Arctic Alaska–Chukotka microcontinent. Furthermore, newly identified volcanic ash beds throughout the Triassic section from the northern part of the Sverdrup Basin, along with abundant Permian–Triassic detrital zircon, suggest a protracted history of magmatism to the north of the basin. We interpret that these zircons were sourced from a magmatically active region to the north of the Sverdrup Basin, and in the context of a rotational model for opening of Amerasia Basin, this was probably part of a convergent margin fringing northern Laurentia from the northern Cordillera along the outboard edge of Arctic Alaska and Chukotka terranes. In Early Jurassic strata, Permian–Triassic zircons decrease substantially, implying the diminution of the active margin as a sediment source as initial rifting isolated the Permian–Triassic source from the Sverdrup Basin.

Granite provenance and intrusion in arcs: Evidence from diverse zircon types in Big Bear Lake Intrusive Suite, USA

Textural, geochemical and hafnium isotopic data from diverse zircon domains allow discrimination between source and emplacement-level processes in the formation of a large-volume calc-alkalic intrusion. The Big Bear Lake Intrusive Suite is composed of satellite plutons and a main intrusive mass zoned from mafic granodiorites at its margins to central biotite±muscovite granites, and is estimated to be 7–10km thick and have a volume of 3500–5100km3. Zircons in the main intrusive mass and in the satellite plutons are composed of one or more of four domain types: (a) Archean to Proterozoic premagmatic domains and (b) Mesozoic premagmatic domains, both occurring as cores, which are overgrown by (c) luminescent early magmatic domains with low U+Th and relatively high estimated crystallization temperatures and (d) high U+Th main phase magmatic domains. U-Pb zircon geochronology indicates the main intrusive mass was emplaced 78–77Ma, preceded by satellite plutons intruded 85–81Ma. Zircon hafnium isotope ratios span 54 epsilon units, recording age and compositional diversity in magma sources and magma batches. We propose a model for assembly of the intrusive suite involving mixing between lithospheric mantle-derived magma and a hybrid lower crustal source, followed by incremental emplacement of magmas in the upper crust at ~0.003–0.005km3 my−1. This flux rate was sufficiently rapid to generate a large volume of mobile magma that underwent differentiation by limited and imperfect fractional crystallization to form the granodioritic margins and central granites. The estimated flux rate is several times higher than that estimated for other Cretaceous, incrementally emplaced intrusive suites in the California arc, indicating that both source-level and emplacement-level processes played roles in forming these intrusions.

Implications of U–Pb and Lu–Hf isotopic analysis of detrital zircons for the depositional age, provenance and tectonic setting of the Permian–Triassic Palaeotethyan Karakaya Complex, NW Turkey

New zircon U–Pb age data, combined with Lu–Hf isotopic data, are presented here for sandstones of mainly arkosic composition from the Permian–Triassic Karakaya Complex. Predominantly, Carboniferous, Triassic and Devonian zircon age groups are recognised, most of which have a Late Triassic (Carnian–Norian) maximum depositional age. Carboniferous- and Devonian-aged zircon populations exhibit intermediate e Hf(t) values (−11 to +2), consistent with formation in a continental margin arc setting where juvenile mantle-derived magma mixed with (recycled) old crust of Palaeoproterozoic Hf model age. In contrast, the Triassic-aged zircon population exhibits higher e Hf(t) values (−5 to +4), consistent with mixing of juvenile mantle-derived melts with (recycled) old crust of Neoproterozoic Hf model age. Potential igneous source rocks for the sandstones of the Karakaya Complex exist in the Devonian and Carboniferous granitic rocks of the Sakarya continental basement to the north. Their e Hf(t) and corresponding model ages are nearly identical to the age-equivalent zircon populations within the Karakaya Complex sandstones. However, the Triassic granitic rocks of the Sakarya continental crust differ significantly in e Hf(t) and corresponding model age from the sandstones of the Karakaya Complex. Late Triassic sandstones from the Tauride continental unit to the south lack the dominant Late Palaeozoic and Triassic zircon populations of the Karakaya Complex sandstones. Triassic granitic bodies and intermediate-composition extrusive rocks in the Tauride continental unit also differ in e Hf(t) and corresponding Hf model ages from the Karakaya Complex sandstones. In addition, Late Triassic sandstones of the Kocaeli Triassic unit (Istanbul Terrane) in the north differ strongly from the Karakaya Complex sandstones in zircon population ages and e Hf(t). In the regional context, the new zircon age and lutetium–hafnium isotopic data are consistent with derivation of the Late Triassic Karakaya Complex sandstones from a Late Palaeozoic–Triassic continental margin arc located somewhere along the southern margin of Eurasia, although its exact position cannot be pinpointed at present owing to lack of suitable outcrop and comparable isotopic data.

Towards unravelling the Mozambique Ocean conundrum using a triumvirate of zircon isotopic proxies on the Ambatolampy Group, central Madagascar

Madagascar occupies an important location within the East African Orogen (EAO), which involves a collection of Neoproterozoic microcontinents and arc terranes lodged between older cratonic units during the final assembly of the supercontinent Gondwana. The detrital zircon record of Proterozoic metasedimentary rock packages within the Antananarivo Domain (the Ambatolampy, Manampotsy, Vondrozo, Itremo and Ikalamavony Groups) is used to identify pre-orogenic tectonic affiliations of the region—affiliations that control interpretations of the evolution of the Mozambique Ocean and the formation of this part of central Gondwana. Here we focus on the Ambatolampy Group, a previously poorly known group of high-grade siliciclastic metasedimentary rocks, and compare these data to similar data from the other sedimentary packages in central Madagascar (the Vondrozo, Sofia, Manampotsy, Itremo, Molo, Ikalamavony, Androyen, Ambodiriana, Iakora and Maha Groups). New U–Pb (SHRIMP) zircon data for the Ambatolampy Group yield detrital age maxima of ~3000Ma, ~2800–2700Ma, ~2500Ma, ~2200–2100Ma and ~1800Ma. The youngest near-concordant detrital zircon age is 1836±25Ma, which we suggest represents the maximum depositional age of the Ambatolampy Group, in contrast to younger ages reported elsewhere. The detrital spectra are very similar to those from the Itremo Group and we suggest that they correlate with each other. Metamorphic zircons and zircon rims constrain the minimum depositional age to be ~540Ma; the interpreted age of metamorphism. New δ18O data (SHRIMP-SI) and hafnium (MC-LA–ICP–MS) isotopic data complement the U–Pb data and provide new constraints on the age, geochemistry and provenance of the metasedimentary rocks. We suggest that central Madagascar contained a Mesoproterozoic (to possibly Tonian) siliciclastic sedimentary basin, within which the Ambatolampy Group, and the Itremo and Maha Groups (and possibly part of the Iakora Groups) were deposited. Later sedimentary systems are represented by the Ikalamavony, Ambodiriana, Manampotsy and Molo Groups.

Growth of continental crust: a balance between preservation and recycling

AbstractOne of the major obstacles to our understanding of the growth of continental crust is that of estimating the balance between extraction rate of continental crust from the mantle and its recycling rate back into the mantle. As a first step it is important to learn more about how and when juvenile crust is preserved in orogens. The most abundant petrotectonic assemblage preserved in orogens (both collisional and accretionary) is the continental arc, whereas oceanic terranes (arcs, crust, mélange, Large Igneous Provinces, etc.) comprise <10%; the remainder comprises older, reworked crust. Most of the juvenile crust in orogens is found in continental arc assemblages. Our studies indicate that most juvenile crust preserved in orogens was produced during the ocean-basin closing stage and not during the collision. However, the duration of ocean-basin closing is not a major control on the fraction of juvenile crust preserved in orogens; regardless of the duration of subduction, the fraction of juvenile crust preserved reaches a maximum of ∼50%. Hafnium and Nd isotopic data indicate that reworking dominates in external orogens during supercontinent breakup, whereas during supercontinent assembly, external orogens change to retreating modes where greater amounts of juvenile crust are produced. The most remarkable feature of εNd (sedimentary rocks and granitoids) and εHf (detrital zircons) distributions through time is how well they agree with each other. The ratio of positive to negative εNd and eHf does not increase during supercontinent assembly (coincident with zircon age peaks), which suggests that supercontinent assembly is not accompanied by enhanced crustal production. Rather, the zircon age peaks probably result from enhanced preservation of juvenile crust. Valleys between zircon age peaks probably reflect recycling of continental crust into the mantle during supercontinent breakup. Hafnium isotopic data from zircons that have mantle sources, Nd isotopic data from detrital sedimentary rocks and granitoids and whole-rock Re depletion ages of mantle xenoliths collectively suggest that ≥70% of the continental crust was extracted from the mantle between 3500 and 2500 Ma.

Constraining the process of Eoarchean TTG formation in the Itsaq Gneiss Complex, southern West Greenland

We present new major and trace element, high-precision high-field-strength-element, hafnium and neodymium isotope data for well preserved Eoarchean TTGs within the Itsaq Gneiss Complex (IGC) of southern West Greenland. These data are combined with thermodynamic model predictions of partial melting and fractional crystallization to gain new insights into continental crust formation in the Archean. Our results show that the observed compositional range of Eoarchean TTGs can be explained by a combination two processes: (1) 5–25% partial melting of amphibolite within thickened mafic crust and (2) subsequent fractional crystallization processes. The Eoarchean TTG suite of SW Greenland probably formed through mixing of melt batches that originally formed at different source depths between 10 and 14 kbar and ponded as plutons at mid-crustal levels. The trace element compositions of some TTGs point to subsequent fractional crystallization processes involving plagioclase, clinopyroxene, amphibole and garnet. Our model is consistent with recent studies proposing that the Eoarchean Itsaq Gneiss Complex TTGs from the IGC formed by re-working of mafic protocrust that stabilized as accreted juvenile crustal terranes in the Eoarchean. The model is also in good agreement with field observations from the area.