Depleted Mantle Model Ages Research Articles

Origin of Triassic granites in central Hunan Province, South China: constraints from zircon U–Pb ages and Hf and O isotopes

Triassic granites crop out extensively in central Hunan Province, South China. Representative granites include the Baimashan, Weishan, and Ziyunshan plutons. Lithologically, these granites mainly comprise biotite monzogranite, two-mica granites, hornblende-biotite granite, hornblende-biotite monzogranite, and garnet-muscovite granite. These granites have in situ zircon secondary ion mass spectrum U–Pb ages between 223.2 ± 3.3 and 209.3 ± 4.0 Ma, indicating that they likely formed predominantly in the Late Triassic. These granitic plutons have similar zircon Hf and O isotopic compositions, with εHf(t) values of −0.8 to −9.0, two-stage depleted mantle model ages (TDM2) of 1.81–1.31 Ga, and weighted mean of δ18OZrc values of 8.53 ± 0.58‰ to 9.12 ± 0.28‰. Combined with U–Pb dating and Hf isotopic data, the elevated and variable δ18O and εHf(t) values of the individual granites indicate that these Triassic granites were likely produced by partial melting of upper Paleoproterozoic to lower Mesoproterozoic metasedimentary rocks and are S-type granites. The variable proportions of inherited zircons in certain samples with U–Pb ages of 627–992 Ma indicate the involvement of lower–middle Neoproterozoic crustal materials during magma crystallization through wall-rock contamination, which resulted in the wide range of isotopic compositions. Underplating of mantle-derived magma may have provided the thermal energy for partial melting of the upper Paleoproterozoic and lower Mesoproterozoic basements, thereby generating these late Triassic granites. The lack of positive εHf(t) values and high δ18OZrc values indicate that the contribution of mantle-derived magmas to these granites may be insignificant.

Mixing between enriched lithospheric mantle and crustal components in a short-lived subduction-related magma system, Dry Valleys area, Antarctica: Insights from U-Pb geochronology, Hf isotopes, and whole-rock geochemistry

Granitoids associated with the Neoproterozoic–early Paleozoic Ross orogeny are extensively exposed in the Dry Valleys region of southern Victoria Land, Antarctica, affording an exceptional opportunity to gain insight into the temporal and spatial scales of continental arc magmatism. Samples spanning 150 km along strike and 50 km across strike were selected for isotopic and geochemical analysis. Zircon U-Pb geochronology and the first Hf isotope data for Dry Valleys granitoids, coupled with whole-rock elemental data, reveal mixing between enriched lithospheric mantle and Precambrian crustal components and indicate that the principal phase of magmatism in the Dry Valleys area was restricted to a period of 23 m.y., from ca. 515 to 492 Ma. This relatively short period of magmatism contrasts with other segments of the Ross orogen, in which magmatism spanned greater than 100 m.y. Most calc-alkaline intrusions spanned 515–500 Ma, while postkinematic granitoids with alkali-calcic geochemical signatures spanned 505–492 Ma, indicating a transitional shift to an overall extensional tectonic regime. Zircon εHf(i) values range between −0.3 and –7.2, with two-stage depleted-mantle model ages ranging from 1.5 to 1.9 Ga. Low εHf(i) values in mafic samples are consistent with derivation from an enriched subcontinental lithospheric mantle source, while the large-volume granitic intrusions show evidence for increasing assimilation of old crust over time. A broadening of the εHf(i) range to more negative values in the younger intrusions may reflect crustal thickening or underplating of fertile continental material into the source region of the arc.

Geochemistry and isotopic evolution of the central African Domes, Bangweulu and Irumide regions: Evidence for cryptic Archean sources and a Paleoproterozoic continental arc

The interregional cratonic relations between the Paleo- and Mesoproterozoic basement units surrounding the Neoproterozoic Central African Copperbelt are still largely unresolved, although they are regarded as major potential metal sources. This study focuses on the Domes region basement at depth below the Copperbelt and its relationship to the neighboring Bangweulu Block and its destabilized margin, the Irumide Belt. We applied an integrated whole rock petrochemical and Sm–Nd isotopic approach to major lithological units to assess the proposed mid-Proterozoic arc setting for the Domes basement inliers along with their relationship to the neighboring areas. The available petrochemical and isotopic data for the Paleoproterozoic eastern Domes granitoids and magmatic units in the SW Bangweulu Block is consistent with a continental arc setting. Moreover, the mid-Paleoproterozoic Nd isotope ratios preclude an island arc because they are significantly less radiogenic than the depleted mantle. Predominantly Archean and Early Paleoproterozoic depleted mantle model ages in all terranes indicate limited juvenile input during Paleo- and Mesoproterozoic magmatic phases. Finally, broadly similar model ages in the Domes inliers and the Bangweulu–Irumide region suggest a relationship between these terranes.

Linking continental deep subduction with destruction of a cratonic margin: strongly reworked North China SCLM intruded in the Triassic Sulu UHP belt

The collision between the North and South China cratons in Middle Triassic time (240–225 Ma) created the world’s largest belt of ultrahigh-pressure (UHP) metamorphism. U–Pb ages, Hf isotope systematics and trace element compositions of zircons from the Xugou, Yangkou and Hujialing peridotites in the Sulu UHP terrane mainly record a ~470 Ma tectonothermal event, coeval with the Early Paleozoic kimberlite eruptions within the North China craton. This event is interpreted as the result of metasomatism by fluids/melts derived from multiple sources including a subducting continental slab. The peridotites also contain zircons with ages of ~3.1 Ga, and Hf isotope data imply a component ≥3.2 Ga old. Most zircon Hf depleted mantle model ages are ~1.3 Ga, suggesting that the deep subcontinental lithospheric mantle beneath the southeastern margin of the North China craton experienced a intense mid-Mesoproterozoic metasomatism by asthenospheric components, similar to the case for the eastern part of this craton. Integrating data from peridotites along the southern margin of the craton, we argue that the deep lithosphere of the cratonic margin (≥3.2 Ga old), from which the Xugou, Yangkou and Hujialing peridotites were derived, experienced Proterozoic metasomatic modification, followed by a strong Early Paleozoic (~470 Ma) tectonothermal event and the Early Mesozoic (~230 Ma) collision and northward subduction of the Yangtze craton. The Phanerozoic decratonization of the eastern North China craton, especially along its southern margin, was not earlier than the Triassic continental collision. This work also demonstrates that although zircons are rare in peridotitic rocks, they can be used to unravel the history of specific lithospheric domains and thus contribute to our understanding of the evolution of continental cratons and their margins.

Reprint of “Depositional environment and tectonic implications of the Paleoproterozoic BIF in Changyi area, eastern North China Craton: Evidence from geochronology and geochemistry of the metamorphic wallrocks”

The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. Three types of metamorphic wallrocks interbedded with the BIF bands are identified, including plagioclase gneisses and leptynites, garnet-bearing gneisses and amphibolites. Protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials, the garnet-bearing gneisses are Fe-rich pelites contaminated by clastics, and the amphibolites are tholeiitic rocks. Trace elements of La, Th, Sc and Zr of the plagioclase gneisses and leptynites and the garnet-bearing gneisses support that these meta-sedimentary rocks were probably derived from recycling of Archean rocks with felsic and mafic materials differentiated into different rock types. 207Pb/206Pb ages of detrital zircons from the meta-sedimentary rocks concentrate at 2.7–3.0Ga, confirming their derivation from the Archean rocks. The presence of several Paleoproterozoic detrital zircons (2240 to 2246Ma), however, also suggests minor involvement of Paleoproterozoic materials. The Archean detrital zircons have εHf(t) values varying from −0.7 to 7.6, which mainly fall between the 3.0Ga and 3.3Ga average crustal evolution lines on the age vs. εHf(t) diagram, further illustrating that the rocks providing materials for the meta-sedimentary rocks mainly originated from partial melting of a Mesoarchean crust. This is strongly supported by their crust-like trace element distribution patterns (such as Nb, Ta, P and Ti depletion) and ancient Nd depleted mantle model ages (TDM=2.9–3.4Ga). In addition, the remarkably high εHf(t) values (7.5 to 9.3) of the Paleoproterozoic detrital zircons constrain the Paleoproterozoic materials to originate from a depleted mantle. The amphibolites show low SiO2 (46.5 to 52.8wt.%) and high MgO (5.68 to 10.9wt.%) contents, crust-like trace element features and low εNd(t) values (−4.5 to −0.3), suggesting that these ortho-metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by Archean crustal materials. Since an intra-continental environment was required for the formation of the above metamorphic rocks, these rocks not only confine the depositional environment of the Changyi BIF to be an intra-continental rift, but also support the rifting processes of the eastern NCC during Paleoproterozoic.

Discovery of mafic impact melt in the center of the Vredefort dome: Archetype for continental residua of early Earth cratering?

Melting by impact heating is thought to have been a significant process in the modification of early planetary crusts; however, crustally derived melt bodies in ancient terrestrial crust are frequently presumed to be absent due to erosion. Here we demonstrate that in the central basement uplift of the 2.020 Ga Vredefort impact basin (South Africa), components of mafic impact melt have survived amid Archean gneiss as decimeter-scale dikes and lenses of variably foliated gabbronorite. Zircon microstructural, trace element, and isotopic analyses (U-Pb, Lu-Hf) of the gabbronorite reveal a dominant population of 2.02 Ga unshocked igneous zircon with apparent Ti-in-zircon temperatures of 800–900 °C, similar to those from the mafic sublayer of the Sudbury impact melt sheet. Highly negative subchondritic e Hf values of −1.4 ± 1.1 to −7.9 ± 1.4 are consistent with a depleted mantle model age of ca. 3 Ga and gabbronorite derivation from the once superjacent Witwatersrand basin lithologies. The recrystallized igneous mineral textures and Archean felsic gneiss inclusions in the gabbronorite are attributable to the effects of emplacement and crater modification following ∼20 km elevation of the central uplift. Long mistaken as preimpact basement, the setting and characteristics of the Vredefort gabbronorite may provide new benchmarks in the search for remnants of large cratering events and melt residua on Earth’s cratons.

Geochronology and geochemistry of meta-mafic dykes in the Quanji Massif, NW China: Paleoproterozoic evolution of the Tarim Craton and implications for the assembly of the Columbia supercontinent

The Quanji Massif, located in the northeastern margin of the Tibet Plateau, is interpreted as a fragment of the Tarim Craton. The massif exposes a suite of metamorphosed mafic dykes. LA-ICP-MS U–Pb dating constrains the timing of intrusion of these dykes at ca. 1834±23Ma. The dykes display minor variations in major elements, with SiO2=46.8–53.4wt%, MgO=5.19–8.10wt%, FeOt=8.83–15.6wt%, TiO2=0.58–1.78wt% and Mg#=46.3–69.7. Their immobile trace element compositions show a sub-alkali basalt affinity. The positive correlation of TiO2 with FeOt/MgO in these rocks shows an arc tholeiite evolutionary trend. The enrichment of LILE and LREE and depletion in HFSE suggest that the precursor magma was generated in a back-arc environment. Furthermore, these rocks possess (Nb/La)N of 0.31–0.52 and (Th/La)N of 0.61–1.39, whole rock (87Sr/86Sr)i values of 0.707598–0.724141, ɛNd(t) of −2.9 to +0.3 and (206Pb/204Pb)t of 16.8148–24.1513, (207Pb/204Pb)t of 15.3422–16.3630 and (208Pb/204Pb)t of 36.8415–39.7926. The magmatic zircons yielded ɛHf(t) of −3.4 to +6.1 and depleted mantle model ages (TDM) of 2.01–2.37Ga. The geochemical and isotopic characteristics of these rocks suggest that their precursor magma was derived from a subduction-related fluid-metasomatized subcontinental lithospheric mantle mixed with a depleted mantle component. We suggest that a prolonged subduction-accretion-collision process along the southeastern margin of the Tarim Craton prevailed at ∼2.1–1.80Ga, with local consumption of the Southeast Tarim Archipelagic Ocean during the collision of the Southeast Tarim Block with the Quanji Massif (microcontinent) at around 2.1–1.9Ga. The final closure of the whole Southeast Tarim Archipelagic Ocean occurred at ∼1.85–1.82Ga and collision between the Southeast margin of the Tarim Craton and other unknown continental blocks at 1.82–1.80Ga, broadly coeval with the amalgamation of the Western and Eastern Blocks of the North China Craton and their assembly in the Columbia supercontinent.

Origin and evolution of granitoids associated with the Kadiri greenstone belt, eastern Dharwar craton: A history of orogenic to anorogenic magmatism

Different tectonic models have been proposed to explain the Neoarchaean evolution of the eastern Dharwar craton (EDC). Here we present whole-rock elemental and Nd isotopic compositions coupled with zircon U–Pb age and trace elemental data on granitoids associated with the Neoarchaean Kadiri greenstone belt, EDC to track the tectonic evolutionary history of the terrain. Earlier works on volcanic rocks of the Kadiri belt (tholeiitic oceanic plateau basalt, high Mg# andesite and potassic adakite) envisaged continental margin arc–oceanic plateau collision. In this work the oldest recognized granitoid is a 2.56Ga syntectonic, sanukitoid-like hornblende granodiorite enriched in large-ion lithophile as well as ferromagnesian elements. It was probably derived from a metasomatized mantle wedge above a subduction zone. This is followed by syntectonic emplacement of 2.54Ga transitional TTGs and potassic adakitic volcanic rocks. Intrusion of 2.51–2.52Ga crustally derived granites (mostly K-rich; ɛNd=+0.9 to −3.7) marks the end of orogenesis in the area. A convergent orogenic setting characterized by assembly of diverse terranes, repeated slab-break-off and crustal reworking is proposed to interpret the association of various granitoids and volcanic rocks. Finally, intrusion of crustally derived, highly silicic, K-rich A-type granite (ɛNd=−0.6 to −4.8) and mantle-derived syenite occurred in a post-subduction, non-compressive environment during Palaeoproterozoic. Lithospheric delamination and attendant mantle melting and crustal reworking are suggested as possible mechanism for generation of these rocks. The negative ɛNd values and Mesoarchaean Nd depleted mantle model ages of some Neoarchaean to Palaeoproterozoic Kadiri granitoids attest to the presence of older crust in the eastern part of the EDC. The final cratonization of the EDC (2.52–2.50Ga) occurred later than in most of the other cratons.

Depositional environment and tectonic implications of the Paleoproterozoic BIF in Changyi area, eastern North China Craton: Evidence from geochronology and geochemistry of the metamorphic wallrocks

The Changyi banded iron formation (BIF) in the eastern North China Craton (NCC) occurs within the Paleoproterozoic Fenzishan Group. Three types of metamorphic wallrocks interbedded with the BIF bands are identified, including plagioclase gneisses and leptynites, garnet-bearing gneisses and amphibolites. Protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials, the garnet-bearing gneisses are Fe-rich pelites contaminated by clastics, and the amphibolites are tholeiitic rocks. Trace elements of La, Th, Sc and Zr of the plagioclase gneisses and leptynites and the garnet-bearing gneisses support that these meta-sedimentary rocks were probably derived from recycling of Archean rocks with felsic and mafic materials differentiated into different rock types. 207Pb/206Pb ages of detrital zircons from the meta-sedimentary rocks concentrate at 2.7–3.0Ga, confirming their derivation from the Archean rocks. The presence of several Paleoproterozoic detrital zircons (2240 to 2246Ma), however, also suggests minor involvement of Paleoproterozoic materials. The Archean detrital zircons have εHf(t) values varying from −0.7 to 7.6, which mainly fall between the 3.0Ga and 3.3Ga average crustal evolution lines on the age vs. εHf(t) diagram, further illustrating that the rocks providing materials for the meta-sedimentary rocks mainly originated from partial melting of a Mesoarchean crust. This is strongly supported by their crust-like trace element distribution patterns (such as Nb, Ta, P and Ti depletion) and ancient Nd depleted mantle model ages (TDM=2.9–3.4Ga). In addition, the remarkably high εHf(t) values (7.5 to 9.3) of the Paleoproterozoic detrital zircons constrain the Paleoproterozoic materials to originate from a depleted mantle. The amphibolites show low SiO2 (46.5 to 52.8wt.%) and high MgO (5.68 to 10.9wt.%) contents, crust-like trace element features and low εNd(t) values (−4.5 to −0.3), suggesting that these ortho-metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by Archean crustal materials. Since an intra-continental environment was required for the formation of the above metamorphic rocks, these rocks not only confine the depositional environment of the Changyi BIF to be an intra-continental rift, but also support the rifting processes of the eastern NCC during Paleoproterozoic.

Geochemistry and Zircon U–Pb–Hf Isotopic Systematics of the Sanchahe Quartz Monzonite Intrusion in the North Qinling Tectonic Zone, Central China: Implications for its Petrogenesis and Tectonic Setting

AbstractThe Sanchahe quartz monzonite intrusion is situated in the middle segment of the North Qinling tectonic belt, Central China mainland, and consists chiefly of sanukitoid–like and granodioritic‐monzogranitic rocks. The sanukitoid–like rocks are characterized by quartz monzonites, which display higher Mg# (55.0–59.0), and enrichments in Na2O+K2O (7.28–8.94%), Ni (21–2312 ppm), Cr (56–4167 ppm), Sr (553–923 ppm), Ba (912–1355 ppm) and LREE ((La/Yb)N=9.47–15.3), from negative to slightly positive Eu anomalies (δEu=+0.61 to +1.10), but also depletion in Nb, Ta and Ti. The granodioritic‐monzogranitic rocks diaplay various Mg# of 6.00–53.0, high Na2O+K2O (7.20–8.30%), Sr (455–1081 ppm) and (La/Yb)N (27.6–47.8), with positive Eu anomalies (δEu=1.03–1.57) and depleted Nb, Ta and TL Laser ablation inductively coupled plasma mass spectrometry (LA‐ICPMS) zircon U‐Pb isotopic dating reveals that the sanukitoid‐like rocks were emplaced at two episodes of magmatism at 457±3 Ma and 431±2 Ma, respectively. The monzogranites were emplaced at 445±7Ma. Sanukitoid‐like rocks have their ∊Hf(r) values ranging from +0.3 to +15.1 with Hf–depleted mantle model ages of 445 to 1056 Ma, and the monzogranite shows its ∊Hf(t) values ranging from ‐21.6 to +10.8 with Hf–depleted mantle model ages of 635 to 3183 Ma. Petrological, geochemical and zircon Lu–Hf isotopic features indicate that the magmatic precursor of sanukitoid–like rocks was derived from partial melting of the depleted mantle wedge materials that were metasomatized by fluids and melts related to subduction of oceanic slab, subsequently the sanukitoid magma ascended to crust level. This emplaced mantle magma caused partial melting of crustally metamorphosed sedimentary rocks, and mixing with the crustal magma, and suffered fractional crystallization, which lead to formations of quartz monzonites. However, the magmatic precursor of the granodioritic‐monzogranitic rocks were derived from partial melting of subducted oceanic slab basalts. Integrated previous investigation for the adackitic rocks in the south of the intrusion, the Sanchahe intrusion signed that the North Qinling tectonic zone was developed in an early Paleozoic transitionally tectonic background from an island arc to back–arc.

Geochemistry and geochronology of Mesoproterozoic basement rocks from the Eastern Amery Ice Shelf and southwestern Prydz Bay, East Antarctica: Implications for a long-lived magmatic accretion in a continental arc

The high-grade metamorphic rocks from the eastern Amery Ice Shelf and southwestern Prydz Bay of East Antarctica represent reworked Rayner Complex during the late Neoproterozoic/Cambrian metamorphism. These Mesoproterozoic basement rocks can provide important information for the earlier tectonic evolution of the Rayner orogen. Chemical compositions of the mafic granulites from the Søstrene Island and Munro Kerr Mountains area suggest that their protoliths resemble Nb-enriched arc basalts, whereas those from the McKaskle-Mistichelli Hills, Reinbolt Hills and Manning Nunataks show typical characteristics of island arc basalts. Nd isotopic analyses yielded initial Nd isotope compositions, expressed as ε~Nd~(T) values, ranging from +4.1 to −0.4 for the former, and mostly from −3.2 to −4.7 for the latter. The felsic orthogneisses have the characteristics of volcanic arc granites, and one fifth of the samples belong to high Sr/Y types. The felsic orthogneisses yielded ε~Nd~(T) values of −2.4 to −7.6 and Nd depleted-mantle model ages of 2.2 to 1.9 Ga, implying an important episode of crustal formation in the Paleoproterozoic. The high Sr/Y orthogneisses have high K~2~O/Na~2~O ratios (\>1), positive Eu anomalies, significant HREE depletion and negative ε~Nd~(T) values, suggesting their derivation from partial melting of garnet-bearing K-rich mafic sources at the lower crust of the continental arc. Geochronological data give protolith ages ranging from 1380 to 1020 Ma for these arc-related rocks, indicating a long-lived magmatic accretion in the Rayner continental arc for ∼360 Ma. In addition, a younger island arc (\<1080 Ma) was probably formed in the south of the Fisher oceanic arc. Coupled with the available data for the adjacent areas, we propose that the tectonic process of the Rayner orogen involved the collision of several island arcs with East Antarctica (the Lambert Terrane or the Ruker craton), followed by the closure of ocean and final collision of the Indian craton with the newly accreted Antarctic margin.

Geochemistry and geochronology of the 1.3 Ga metatonalites from the Central Metasedimentary Belt boundary thrust zone in southern Ontario, Grenville Province, Canada

Studies of pre-Grenvillian (1.4–1.3 Ga) plutons offer insight into the dynamics of arc amalgamation and backarc rifting prior to continental collision during the Ottawan orogeny. The Central Metasedimentary Belt boundary thrust zone (CMBbtz) is a northeast-southwest–trending thrust zone consisting of metaplutonic thrust sheets enveloped in gneissic tectonites and calcitic-dolomitic marble. Tonalitic CMBbtz thrust sheets (Dysart and Redstone), located in the southern Ontario Grenville Province (Canada) are made up of upper amphibolite facies, foliation-concordant metatonalite (+ amphibole ± biotite ± accessory zircon and titanite) and amphibolite (± biotite ± clinopyroxene). These thrust sheets are thought to have formed and amalgamated onto the Laurentian margin prior to Ottawan orogeny. Major and trace element analyses show that the metatonalite rocks have calc-alkaline affinity and amphibolite rocks have both calc-alkaline and tholeiitic affinities, suggesting an arc environment. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) geochronology of zircon from the two thrust sheets yield igneous ages of ca. 1350–1300 Ma for diorite and granodiorite, ca. 1150–1100 Ma ages from Dysart tonalite interpreted to record metamorphic zircon growth, and a ca. 1086 Ma Ottawan metamorphic age from a Dysart amphibolite. The ca. 1150–1100 Ma metamorphic event has not been previously documented within these thrust sheets of the CMBbtz, but correlates well with thermal events in the allochthonous Parry Sound domain to the west, as do ca. 1350 Ma igneous ages of tonalite in both areas. These data support the hypothesis that the CMBbtz and Parry Sound domain may have been initially linked. Widespread ca. 1350 Ma crust along with distinct 1460–1400 Ma depleted mantle model ages (TDM) are also consistent with a shared genesis with the Dysart–Mount Holly suite in New York and Vermont, and support the correlation between the CMBbtz thrust sheets and the Adirondack Highlands–Mount Holly belt as a rifted arc. Alternatively, the CMBbtz thrust sheets and the Adirondack Highlands–Mount Holly belt may represent contemporaneous arc development at different parts of the convergent margin; however, we support the correlation between the CMBbtz thrust sheets and the Adirondack-Highlands–Mount Holly belt.

The Suwannee suture: Significance for Gondwana-Laurentia terrane transfer and formation of Pangaea

In southeastern Laurentia the Suwannee suture separates the undeformed part of the Gondwanan Suwannee terrane (SWT) from crust of Laurentia and previously accreted Gondwanan terranes that were deformed during the Alleghanian orogeny. Undeformed early Paleozoic sedimentary rocks of the SWT are considered of Gondwanan-African derivation on the basis of various considerations, including a lack of Mesoproterozoic (Grenville) detrital zircons. The cratonic heritage of Suwannee lithosphere is complicated, however, because Sm–Nd depleted mantle model ages from both Neoproterozoic and Alleghanian (Carboniferous–Permian) granites north and south of the suture zone are Mesoproterozoic. Mesoproterozoic lithosphere has not been reported in conjugate West Africa, but is present in Amazonia, suggesting that the SWT: 1) may be a composite terrane, containing both African upper crust and Amazonian lower crust/lithosphere, 2) was separated from Gondwana during Atlantic Ocean rifting along a Mesoproterozoic–Neoproterozoic suture, and/or 3) contains Laurentian Grenville-age lower crust and lithosphere that must also underlie other accreted proximal Gondwanan terranes (e.g., Uchee terrane). In addition, the ~300Ma, syn- to post-tectonic granitoids emplaced across the Suwannee suture zone are the product of crustal anatexis, not supra-subduction magmatism, on the basis of Mesoproterozoic Sm–Nd Tdm and zircon xenocrysts. The lack of documented ~300Ma, supra-subduction magmatism in both Laurentian and Gondwanan components in southern Laurentia (e.g., southernmost Appalachian and Ouachita orogens) strongly suggests that accretion of the SWT and related Gondwanan terranes occurred during dextral transpressional convergence between Laurentia and a combined African-Amazonian craton, a characteristic shared with conjugate crust in southwestern Europe. In this interpretation, intervening oceanic lithosphere and contemporary volcanic arcs (if any) were largely removed by disaggregation and dispersion along the transpressional boundary possibly combined with highly oblique overthrusting and/or partly subducted at the leading edge(s) of other blocks of Gondwanan lithosphere that were accreted to southern Laurentia in the late Paleozoic (e.g., Wiggins, Sabine, Coahuila, Maya, etc.). Mesozoic opening of the Gulf of Mexico has subsequently and variably altered the original configuration of these terranes.

Permian–Carboniferous arc magmatism in southern Mexico: U–Pb dating, trace element and Hf isotopic evidence on zircons of earliest subduction beneath the western margin of Gondwana

Undeformed felsic to mafic igneous rocks, dated by U–Pb zircon geochronology between 311 and 255 Ma, intrude different units of the Oaxacan and Acatlan metamorphic complexes in southwestern Mexico. Rare earth element concentrations on zircons from most of these magmatic rocks have a typical igneous character, with fractionated heavy rare earths and negative Eu anomalies. Only inherited Precambrian zircons are depleted in heavy rare earth elements, which suggest contemporaneous crystallization in equilibrium with metamorphic garnet during granulite facies metamorphism. Hf isotopic signatures are, however, different among these magmatic units. For example, zircons from two of these magmatic units (Cuanana pluton and Honduras batholith) have positive eHf values (+3.8–+8.5) and depleted mantle model ages (using a mean crustal value of 176Lu/177Hf = 0.015) (TDMC) ranging between 756 and 1,057 Ma, whereas zircons from the rest of the magmatic units (Etla granite, Zaniza batholith, Carbonera stock and Sosola rhyolite) have negative eHf values (−1 to −14) and model ages between 1,330 and 2,160 Ma. This suggests either recycling of different crustal sources or, more likely, different extents of crustal contamination of arc-related mafic magmas in which the Oaxacan Complex acted as the main contaminant. These plutons thus represent the magmatic expression of the initial stages of eastward subduction of the Pacific plate beneath the western margin of Gondwana, and confirm the existence of a Late Carboniferous–Permian magmatic arc that extended from southern North America to Central America.

Petrogenesis of Early to Middle Jurassic granitoid rocks from the Gangdese belt, Southern Tibet: Implications for early history of the Neo-Tethys

The Gangdese belt, Tibet, records the opening and closure of the Neo-Tethyan ocean and the resultant collision between the Indian and Eurasian plates. Mesozoic magmatic rocks generated through subduction of the Tethyan oceanic slab constitute the main component of the Gangdese belt, and play a crucial role in understanding the formation and evolution of the Neo-Tethyan tectonic realm. U–Pb and Lu–Hf isotopic data for tonalite and granodiorite from the Xietongmen–Nymo segment of the Gangdese belt indicate a significant pulse of Jurassic magmatism from 184Ma to 168Ma. The magmatic rocks belong to metaluminous medium-K calc-alkaline series, characterized by regular variation in major element compositions with SiO2 of 61.35%–73.59%, low to moderate MgO (0.31%–2.59%) with Mg# of 37–45. These magmatic rocks are also characterized by LREE enrichment with concave upward trend in MREE on the chondrite-normalized REE patterns, and also LILE enrichment and depletion in Nb, Ta and Ti in the primitive mantle normalized spidergrams. These rocks have high zircon εHf(t) values of +10.94 to +15.91 and young two-stage depleted mantle model ages (TDM2) of 192Ma to 670Ma. The low MgO contents and relatively depleted Hf isotope compositions suggest that the granitoid rocks were derived from the partial melting of the juvenile basaltic lower crust with minor mantle materials injected. In combination with the published data, it is suggested that northward subduction of the Neo-Tethyan slab beneath the Lhasa terrane began by the Late-Triassic, which formed a major belt of arc-related magmatism.

Not all supercontinents are created equal: Gondwana-Rodinia case study

The geologic records associated with the formation of the supercontinents Rodinia and Gondwana have markedly different seawater Sr and zircon Hf isotopic signatures. Rodinia-related (Grenville-Sveconorwegian-Sunsas) orogens display significantly less enriched crustal signatures than Gondwana-related (Pan-African) orogens. Seawater Sr isotope ratios also exhibit a more pronounced crustal signal during the span of the Gondwana supercontinent than at the time of Rodinia. Such isotopic differences are attributed to the age and nature of the continental margins involved in the collisional assembly, and specifically to the depleted mantle model ages, and hence the isotope ratios of the material weathered into the oceans. In our preferred model the isotopic signatures of Rodinia-suturing orogens reflect the closure of ocean basins with dual subduction zones verging in opposite directions, analogous to the modern Pacific basin. This would have resulted in the juxtaposition of juvenile continental and island arc terrains on both margins of the colliding plates, thus further reworking juvenile crust. Conversely, the assembly of Gondwana was accomplished primarily via a number of single-sided subduction zones that involved greater reworking of ancient cratonic lithologies within the collisional sutures. The proposed geodynamic models of the assembly of Rodinia and Gondwana provide a connection between the geodynamic configuration of supercontinent assembly and its resulting isotopic signature.

A U–Pb age for the Late Caledonian Sperrin Mountains minor intrusions suite in the north of Ireland: timing of slab break-off in the Grampian terrane and the significance of deep-seated, crustal lineaments

An intrusion of trachy-andesite, representative of a newly discovered suite of high-K–Ba–Sr, calc-alkaline minor intrusions (termed herein the Sperrin Mountains suite), hosted within the Grampian terrane in the north of Ireland, has been dated by U–Pb zircon at 426.69 ± 0.85 Ma (mid-Silurian; Wenlock–Ludlow boundary). Geochemistry reveals a close association with the Fanad, Ardara and Thorr plutons of the Donegal Batholith and the Argyll and Northern Highlands Suite of Scotland. The deep-seated Omagh Lineament appears to have limited eastward propagation of the Sperrin Mountains suite from beneath the main centre of granitic magmatism in Donegal. A Hf depleted mantle model age (T DM Hf) of c . 800 Ma for trachy-andesite zircons indicates partial melting from a source previously separated from the mantle. Whole-rock geochemistry of the suite is consistent with a model of partial melting, triggered by slab break-off, following thrusting of Ganderia–Avalonia under the Southern Uplands–Down–Longford accretionary prism (i.e. Laurentian margin). The new age constrains the timing of this event in the north of Ireland and is consistent with the petrogenesis of Late Caledonian high-K granites, appinites and minor intrusions across the Caledonides of northern Britain and Ireland. Supplementary material: Geochemical data are available at www.geolsoc.org.uk/SUP18600 .

Multiple mineralization events at the Jiru porphyry copper deposit, southern Tibet: Implications for Eocene and Miocene magma sources and resource potential

The Jiru porphyry copper deposit in the Gangdese Porphyry Copper Belt (GPCB) is hosted by monzogranite and monzogranite porphyry with SHRIMP U–Pb ages of 48.6±0.8Ma and 16.0±0.4Ma, respectively. Rhenium–Os ages of molybdenite from the monzogranite and monzogranite porphyry are 44.9±2.6Ma and 15.2±0.4Ma, slightly younger than ages of the host rocks, respectively. These geochronological data indicate that there are two mineralization events at the Jiru deposit, in contrast to other porphyry deposits in the eastern part of the GPCB that are only Miocene in age. The Eocene monzogranite is characterized by high SiO2 (63.0–71.4%) and K2O (3.7–5.9%), enrichment in LILEs, depletion in Nb, Ta, and Ti, moderate negative Eu anomalies (δEu=0.55–0.94), and relatively low Sr/Y (14–39) and (La/Yb)n (9–20) ratios. It also has young εNd(t) values (−0.43 to −0.25), low initial 87Sr/86Sr ratios (0.7044–0.7048), and young depleted-mantle model ages TDM (742–821Ma), compared to Eocene melts derived from mature continental crust in the central Lhasa subterrane. These geochemical features suggest that the Jiru monzogranite was most likely derived from the hydrated asthenospheric mantle wedge with involvement of subducted sediments related to the Neo-Tethyan oceanic slab breakoff. The Miocene monzogranite porphyry contains hydrous phenocryst phases (hornblende and biotite) and displays LREE-enrichment patterns, with high Sr/Y (131–183) and (La/Yb)n (22–72) ratios, and weak or absent Eu anomalies. The porphyry has slightly negative εNd(t) values (−3.8 to −3.5), low initial 87Sr/86Sr ratios (0.7057–0.7058), and young TDM (952–974Ma). The Miocene porphyry is likely the product of remelting of the stalled Neo-Tethyan oceanic slab, with input from the lower crust during the convective removal of thickened lithosphere below southern Tibet. Recognition of the Eocene magmatic-hydrothermal ore-forming event indicates a newly recognized potential for copper resources of that age in the eastern GPCB.

Zircon U[sbnd]Pb geochronology and Hf isotopes of major lithologies from the Yishui Terrane: Implications for the crustal evolution of the Eastern Block, North China Craton

As an important component of the Western Shandong Complex in the Eastern Block of the North China Craton, the Yishui Terrane consists of Neoarchean high-grade supracrustal rocks, granitoid gneisses and charnockites. LA-ICP-MS zircon UPb dating and Hf isotopic analyses on these lithologies have been carried out and the results provide new insights into the Neoarchean crustal evolution of the Eastern Block of the North China Craton. New zircon dating results reveal that the magmatic precursors of the supracrustal metapelites and granitoid gneisses were generated at 2.54–2.53Ga and 2.57–2.55Ga, respectively, and the charnockites were emplaced contemporaneously around 2.56–2.53Ga. Single metamorphic zircon grains and overgrowth rims from these rocks document consistent metamorphic ages at ~2.50Ga, suggesting that the Yishui Terrane experienced a high-grade metamorphic event at the end of the Neoarchean. Detrital zircons from pelitic gneisses yield 207Pb/206Pb ages of 2.89–2.65Ga, with a major age peak at ~2.53Ga and a subordinate age peak at ~2.70Ga, implying that the sedimentary protoliths of the pelitic gneisses may have been sourced from ~2.53Ga rocks in the Yishui Terrane and ~2.70Ga rocks in the adjacent Luxi Granite-Greenstone Terrane. Hf isotopic compositions show that the Neoarchean magmatic zircons have positive εHf(t) values ranging from +1.4 to +7.8 and depleted mantle model ages of 2.92–2.60Ga with a prominent peak at 2.8–2.7Ga, suggesting that the Neoarchean crust was derived mainly from juvenile sources and partly from the recycling of old continental crust in the Yishui area. Combined with previous data from the adjacent Luxi Granite–Greenstone Terrane and other complexes in the Eastern Block, it is concluded that the Neoarchean was an important period of crust accretion in the Eastern Block of the North China Craton.

Sr–Nd–Pb–Hf isotope systematics of the Hugo Dummett Cu–Au porphyry deposit (Oyu Tolgoi, Mongolia)

Major and trace element geochemistry including Sr–Nd–Pb–Hf isotopic data are presented for a representative sample suite of Late Devonian to Early Carboniferous plutonic and volcanic rocks from the Hugo Dummett deposit of the giant Oyu Tolgoi porphyry Cu–Au district in South Gobi, Mongolia. Sr and Nd isotopes (whole-rock) show restricted ranges of initial compositions, with positive εNdt mainly between +3.4 and +7.4 and (87Sr/86Sr)t predominantly between 0.7037 and 0.7045 reflecting magma generation from a relatively uniform juvenile lithophile-element depleted source. Previously dated zircons from the plutonic rocks exhibit a sample-averaged range of εHft values of +11.6 to +14.5. Depleted-mantle model ages of 420–830 (Nd) and 320–730Ma (zircon Hf) limit the involvement of pre-Neoproterozoic crust in the petrogenesis of the intermediate to felsic calc-alkaline magmas to, at most, a minor role. Pb isotopes (whole-rock) show a narrow range of unradiogenic initial compositions: 206Pb/204Pb 17.40–17.94, 207Pb/204Pb 15.43–15.49 and 208Pb/204Pb 37.25–37.64, in agreement with Sr–Nd–Hf isotopes indicating the dominance of a mantle component. All four isotopic systems suggest that the magmas from which the large Oyu Tolgoi porphyry system was generated originated predominantly from juvenile material within the subduction-related setting of the Gurvansayhan terrane.