Addition Of Juvenile Crust Research Articles

Orogen-scale inverted metamorphism during Cretaceous–Paleogene terminal suturing along the North American Cordillera, Alaska, USA

Abstract The northern North American Cordilleran margin has been active for >200 million years, as recorded by punctuated phases of crustal growth and deformation. Accretion of the exotic Wrangellia Composite Terrane (Insular Belt) is considered the largest addition of juvenile crust to the Cordilleran margin, though margin-parallel translation during the Cenozoic has obscured much of the accretionary history. Three zones of inverted metamorphism spatially correspond to the Insular–North American suture zone from north to south: (1) Clearwater Mountains; (2) Kluane Lake; and (3) Coast Mountains, each preserving kinematics indicative of thrusting of North American–derived rocks over Insular-derived assemblages. We performed in situ monazite petrochronology on samples collected across strike in both the Clearwater and Coast Mountain regions. New and recently published data from these three metamorphic belts indicate that thrust-sense deformation accompanied the formation of inverted metamorphic isograds from 72 to 56 Ma. We leverage recent estimates of Denali fault offset to reconstruct a >1000-km-long zone of inverted metamorphism and interpret it as the Insular–North America terminal suture.

The onset of Paleo-Pacific subduction: Key evidence from the Kaishantun Accretionary Complex, northeastern Eurasia

Subduction of the Paleo-Pacific oceanic lithosphere has dominated the tectonic evolution of northeastern Eurasia since the Mesozoic. We document the time of subduction initiation based on the age, character, and paleogeographic record of the Jilin-Yanji Suture that separates the Jiamusi-Khanka Block (of Northeast China) from the northeastern North China Craton. The suture contains a series of accretionary complexes that provide abundant information for elucidating the evolution of the oceanic plates. Zircon U-Pb and Lu-Hf isotope, as well as zircon trace element data, from nine sedimentary rock samples from the Kaishantun Accretionary Complex in the easternmost segment of the Jilin-Yanji Suture document a volcanic arc setting in the end-Permian to Middle Triassic (255−244 Ma) involving the addition of juvenile crust. Based on our new data and previous studies, we propose that southwestern-directed subduction of the Jilin-Heilongjiang Ocean dominated the evolution of regional tectonics between 260 Ma and 230 Ma, which resulted in the formation of arc-related volcano-sedimentary rocks and the generation of accretionary complexes within the Jilin-Yanji Suture. The Paleo-Pacific Ocean started to subduct beneath northeastern Eurasia at ca. 235 Ma, which accelerated the closure of the Jilin-Heilongjiang Ocean and provided the major driving force for the final amalgamation of the northeastern North China Craton and the Jiamusi-Khanka Block.

Zircon U-Pb-Hf isotopes constraints on the petrogenesis of Neoarchean granitic rocks from the Northeastern part of the Eastern Dharwar Craton, Southern India

In the present study, we integrate bulk rock chemistry and in-situ zircon U-Pb-Hf isotopes of high-K Neoarchean granitoids from the northeastern part of the Eastern Dharwar Craton (EDC) to investigate their source, petrogenesis, and plausible tectonic setting. Based on the mineral assemblage, in coherence with their geochemical characteristics, these granitoids are classified as Hornblende-biotite granite (HBG) with Microgranular enclaves (ME), Biotite granite (BTG) and Monzogranite (MG). Field observations and zircon U-Pb ages reveal coeval emplacement of these granitoids between 2.53 and 2.50 Ga. The HBG, including ME, has low silica, metaluminous affinity, and high ferromagnesian element content, consistent with their derivation from a mafic source. Some of the HBG and ME samples are strongly enriched in incompatible elements, similar to the sanukitoids. The geochemical attributes and strongly evolved zircon Hf isotopic compositions (ɛHf(t) = −4.3 to +1.8 and −7.3 to +0.2) of the HBG and ME suggest their derivation from a mantle source metasomatized by subducted sediments. The BTG and MG are compositionally similar, with high silica, low ferromagnesian element content, and moderate peraluminous affinity, suggesting the involvement of felsic crustal sources. The sub-chondritic to chondritic zircon Hf isotopic compositions (ɛHf(t) = −2.7 to +1.4 and −6.9 to +1.9) of these granitoids support the involvement of heterogeneous ancient crustal sources. While all granitoids exhibit similar zircon Hf isotopic compositions, their geochemical attributes suggest distinct sources. The HBG reflects juvenile crustal additions, whereas BTG and MG are products of the crustal reworking. We propose that these granitoids formed during the continent-continent collision between Western and Eastern Dharwar cratons, that took place after the break-off of the eastward subducting slab. The asthenospheric upwelling induced by slab break-off and/or lithospheric delamination of thickened crust led to the genesis of metasomatized mantle derived magma, which drove the crustal reworking. The observed zircon Hf isotopic composition of the EDC granitoids are similar to the present day Phanerozoic Alpine-Himalayan type orogenies supporting subduction followed by continental collision model for the stabilisation of Archean cratons. Evidence for the existence of Paleo- Mesoarchean felsic crust from this part of the EDC warrants further studies to test the three-terrane model for the Dharwar Craton.

Petrogenesis of two Cretaceous granitoid episodes in the Luk Ulo region, Central Java, Indonesia, and its implication for the tectonic evolution of the easternmost Neo-Tethys

The central part of Java, Indonesia, is located on the southern margin of Sundaland, and records the tectonic-magmatic history of the easternmost Neo-Tethys. Elucidating the petrogenesis and its tectonic setting of the Cretaceous Luk Ulo granites in Central Java is key to reconstructing the late subduction-accretion history of the easternmost Neo-Tethys. This study reports for the first time the zircon U-Pb geochronology, Hf-O isotope, whole-rock major and trace elements, and Sr-Nd isotope geochemistry of two granitoid episodes from Central Java. Zircon U-Pb dating results show that the Luk Ulo granites formed in two Cretaceous episodes at ca. 115 Ma and ca. 70 Ma. These two Cretaceous granitoid episodes contain both calc-alkaline with enriched large-ion lithophile elements and light rare earth elements and depleted high field strength elements, exhibiting arc geochemical signatures. The older (ca. 115 Ma) granites show slightly higher (87Sr/86Sr)i values of 0.70772 to 0.70784 and weakly negative εNd(t) values of −2.37 to −1.67, indicating the contribution of ancient crust. However, the scattered zircon εHf(t) values of +0.23 to +11.39 suggest a probable mixture of ancient crustal components and juvenile crustal additions in the magma source. The high zircon δ18O values (+8.86‰ to +14.24‰) show the possible incorporation of supracrustal components (sediments and fluids) in the magma formation. In contrast, the younger (ca. 70 Ma) granites show low (87Sr/86Sr)i values of 0.70427−0.70442, strong positive εNd(t) values of +3.89 to +4.03, and zircon εHf(t) values of +14.04 to +17.38 and slightly higher zircon δ18O values of +5.01‰ to +6.49‰ than the depleted mantle, indicating that the magma was predominantly derived from the juvenile crust with negligible supracrustal involvement. In general, zircon εHf(t) values and whole-rock εNd(t) values of the Late Cretaceous (after ca. 90 Ma) granitic rocks in the southwest margin of Sundaland were higher than those of the Early Cretaceous granitoids, suggesting a change in the source region from ancient to the juvenile in response to the rollback of the Neo-Tethys ocean. Our findings from the two Cretaceous granitoid episodes in Central Java provide new insights into the tectono-magmatic evolution along the southwestern margin of Sundaland and the two different episodes of crustal growth in the easternmost Tethys during the Cretaceous.

Geochronological and geochemical constraints for the metavolcanosedimentary succession of the Nyong Complex, northwestern margin of the Congo craton: Implications for depositional age and tectonic setting of associated banded iron formations

The Mewongo area of the Nyong Complex is located in the northwestern part of the Congo craton in southwestern Cameroon. The metavolcanosedimentary succession in this area comprises banded iron formations (BIFs), interlayered with amphibolites and gneisses and intruded by metamorphosed dolerite dykes. Within the framework of detailed field investigation, we report whole-rock geochemical data for the BIFs, metadolerites, and gneisses to constrain their provenance and tectonic setting, and present LA-ICP-MS U-Pb-Hf isotopic data for zircons from amphibolite and gneiss to assess the depositional age of BIFs from the Nyong Complex and understand the crustal evolution at the northwestern margin of the Congo craton. The geochemical composition of the Mewongo BIFs indicates a contribution of hydrothermal fluids, derived via submarine alteration of volcanics, and seawater. Geochemical and LA-ICP-MS U-Pb detrital zircon data for the gneisses reveal felsic protolith with ages between 2.97 and 2.65 Ga. Our zircon U-Pb-Hf isotope data suggest that a significant growth of juvenile crust occurred at ca. 2.7–2.2 Ga, followed by a reworking event at ca. 2.1 Ga with only minor juvenile crust addition. The metadolerites, cutting the metavolcanosedimentary succession, range in composition between tholeiitic and calc-alkaline basalts. They can be subdivided into two groups based on their trace element signature. The group I metadolerites have OIB-like geochemical composition (e.g., enriched LILE, HFSE, and LREE), while the group II metadolerites exhibit MORB-like geochemical signature (e.g., flat HFSE and HREE patterns). Metadolerites belong to two different so far undated magmatic events on the Congo craton that were previously recognized on the adjacent Ntem Complex.LA-ICP-MS U-Pb zircon dating of amphibolites, combined with chemical composition of metadolerites and gneisses, indicate deposition of the metavolcanosedimentary sequence at ca. 2.1 Ga in a pro-foreland basin sandwiched between the West and East Gabonian blocks, at the time when the Congo and São Francisco cratons collided during the Eburnean-Transamazonian orogeny. The Nyong Complex BIFs were therefore deposited in a deep, open-marine setting during the ca. 2.1–2.0 Ga time interval.

Petrogenesis of the Permian granitoids in the western Gonghe basin, NE Tibetan Plateau (China): Implications for the Late Paleozoic tectonic evolution of the Paleo-Tethys Ocean

The western Gonghe basin in the NE Tibetan Plateau, witnessed extensive magmatism during the Triassic, whereas only few magmatic flare-up events occurred during the Permian. Understanding the geodynamic setting of these magmatic pulses is important in the reconstruction of the Paleo-Tethys tectonic history. In this contribution, we present in-situ zircon U Pb geochronological and Lu Hf isotopic data, as well as whole-rock major and trace elements data from monzogranite and syenogranite dyke in the western Gonghe region to constrain their petrogenesis and tectonic implications. Zircon U Pb data show that the monzogranite (253–260 Ma) and syenogranite dyke (247-261 Ma) were formed during the Late Permian, rather than the Jurassic as presumed in previous studies. Mineralogical characteristics and geochemical affinities reveal that these rocks are high-K calc-alkaline, metaluminous to weakly peraluminous (ASI = 0.95–1.02), moderately-highly fractionated (DI = 81–95) I-type granites. The major and trace element characteristics (including Mg#, Ni, Cr, Zr/Hf, Nb/Ta, and Th/U contents), together with the zircon ε Hf ( t ) values (−5.90 to 1.22, T DMC = 845–1250 Ma) and 176 Hf / 177 Hf ratios (0.282481 to 0.282674) suggest that the magmas were derived through partial melting of the Early Mesoproterozoic metagreywacke in the lower crust with minor juvenile crust addition. Extensive fractional crystallization of plagioclase and subordinate K-feldspar as well as apatite is inferred. Our findings in this study, in conjunction with those from previous studies, suggest that the magmatic flare-up event during the Late Permian in the western Gonghe area is related to local extension concomitant with the break-off of the subducting slab associated with the northward subduction of the Paleo-Tethys oceanic lithosphere. • Monzogranite and syenogranite dyke were emplaced at 253–260 Ma and 247–261 Ma. • Highly to moderately fractionated, arc-related I-type granites. • Magma derived from partial melting of Mesoproterozoic metagreywacke. • Magmatism correlated to slab break off of the Paleo-Tethys Ocean.

Jurassic Arc: Reconstructing the Lost World of eastern Gondwana

AbstractThe tectonic setting of the Australian sector of the eastern Gondwanan margin during the Jurassic and Cretaceous is enigmatic. Whether this involved convergent tectonism and a long-lived continental magmatic arc or rift-related extension unrelated to subduction is debated. The paucity of Australian Jurassic–Cretaceous igneous outcrops makes resolving these competing models difficult. We used the detrital zircon record of the Jurassic–Cretaceous Great Australian Superbasin (GAS) as a proxy for igneous activity. We attribute the persistent magmatism recorded in GAS sedimentary fill throughout the Mesozoic to ca. 95 Ma to continuation of the established Paleozoic continental arc system. The detrital zircon record signals short (~10 m.y.) pulses of elevated Jurassic and Cretaceous magmatic activity and strongly positive εHf values, indicating juvenile crust or mantle-derived magmatism. Margin reconstruction indicates sustained continental growth at rates of at least ~55 km3 km–1 m.y.–1, mainly to the tract now represented by submerged northern Zealandia, due to the retreat of this arc system. We posit that arc retreat was a key factor in rapid crust generation and preservation, and that continental sedimentary systems globally may host cryptic records of juvenile crustal addition that must be considered in estimating crustal growth rates along convergent plate margins.

Detrital chromites reveal Slave craton's missing komatiite

Abstract Komatiitic magmatism is a characteristic feature of Archean cratons, diagnostic of the addition of juvenile crust, and a clue to the thermal evolution of early Earth lithosphere. The Slave craton in northwest Canada contains >20 greenstone belts but no identified komatiite. The reason for this dearth of komatiite, when compared to other Archean cratons, remains enigmatic. The Central Slave Cover Group (ca. 2.85 Ga) includes fuchsitic quartzite with relict detrital chromite grains in heavy-mineral laminations. Major and platinum group element systematics indicate that the chromites were derived from Al-undepleted komatiitic dunites. The chromites have low 187Os/188Os ratios relative to chondrite with a narrow range of rhenium depletion ages at 3.19 ± 0.12 Ga. While these ages overlap a documented crust formation event, they identify an unrecognized addition of juvenile crust that is not preserved in the bedrock exposures or the zircon isotopic data. The documentation of komatiitic magmatism via detrital chromites indicates a region of thin lithospheric mantle at ca. 3.2 Ga, either within or at the edge of the protocratonic nucleus. This study demonstrates the applicability of detrital chromites in provenance studies, augmenting the record supplied by detrital zircons.

Origins of the meta‐mafic rocks in the southern Dunhuang Block (NW China): Implication for tectonic framework of the southernmost Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogenic collages in the world with prominent juvenile crust addition, where the southernmost margin of the CAOB located is one hot topic in the international studies. The Dunhuang Block located to the south of the Beishan Orogen is a key position for outlining the tectonic framework of the south margin of the CAOB. However, at present, the tectonic attribution of the Dunhuang Block is still controversial. In this paper, we focus on the origins and protoliths of the meta‐mafic rocks from the Hongliuxia complex in the southern Dunhuang Block for determining the tectonic attribution of the Dunhuang Block. In the field, the meta‐mafic rocks are mainly exposed as tectonic blocks within the metasedimentary rocks displaying the “block‐in‐matrix” fabrics or as tectonic slices juxtaposed by faults. Geochemically, the meta‐mafic rocks could be divided into three groups based on the rare earth elements (REE) and trace elements: the group 1 samples are depleted in the LREE relative to HREE and show flat HREE patterns, similar to the mid‐oceanic ridge basalt; the group 2 samples show flat REE patterns without obvious differentiation between the LREE and HREE, like the oceanic plateau basalt; the group 3 samples are enriched in LREE relative to LREE, and have negative Nb, Ta, and Ti anomalies, similar to the island arc basalt. Conclusively, the protoliths of the meta‐mafic rocks dominantly belong to the oceanic plate components. They were metamorphosed during Early Silurian to Middle Devonian in different depths of the subduction zone documented in previous studies. So, we suggest the Hongliuxia complex is an exhumed Palaeozoic subduction–accretion complex (SAC) formed during the oceanic plate subduction. This result confirms the Palaeozoic subduction event occurred in the Dunhuang region and indicates that the Dunhuang Block is a Palaeozoic Orogen rather than a part of Precambrian continent block. Considering the regional geology context, we speculate the Dunhuang Orogen belongs to the southernmost margin of the CAOB.

Geologic-tectonic setting of the gold-endowed Kilo Terrane in the eastern Central Kibalian Superterrane, northeastern Democratic Republic of Congo

Solid geological mapping, granite geochemistry and robust U-Pb SHRIMP geochronology confirm our subdivision of the eastern Central Kibalian Superterrane (ECKS) into two terranes: the ca 2570–2552 Ma Western Ituri Terrane and the ca 2680–2570 Ma Kilo Terrane. The ECKS is part of the Central, Northern and Southern Kibalian Superterranes, first-order interpretative crustal blocks, which constitutes the Kibalian Greenstone Region in the Bomu-Kibalian craton. The Kilo Terrane comprises: 1) the lower Kibalian lithostratigraphic units of deep-sea facies mafic–ultramafic volcanic rocks, interlayers of pelitic-sedimentary rocks and mafic granite and diorite-tonalite complexes in the Galaya-Kilo Sub-Terrane; and 2) the upper Kibalian lithostratigraphic units of intermediate-felsic volcanic-sedimentary, mafic–ultramafic intrusive rocks and widespread quartz-dioritic, tonalitic to granodioritic granites in the Balazana-Bunia Sub-Terrane.Mafic granite and diorite-to-tonalite magmatism in the Kilo Terrane is demonstrated by: 1) the ca 2680–2640 Ma syn-volcanic mafic-dioritic granite magmatism, including the ca 2650–2640 Ma albitic granites extensive in the Nizi-Tsi Belt; 2) the ca 2640–2600 Ma syn-volcanic hornblende tonalite-to-granodiorite magmatism in the Kilo and Moto-Zani Terranes; and 3) a ca 2600–2570 Ma syenite, quartz-monzonite and felsic granite intrusions along extensional zones and reactivated major terrane boundaries. The coincidence between the ca 2603 Ma tectonic uplifting and ca 2605–2590 Ma hydrothermal activity in the Kilo Terrane is associated with the gold mineralization.The 2711–2790 Ma xenocryst ages point to potential existence of the > 2790 Ma basement rocks extending from underneath the Western Ituri Terrane into underneath the Bunia West Domain. It’s absence in the gold-hosting domains of the Kilo Terrane does not exclude the existence of > 2.8 Ga basement, but instead it invokes on the existence of an unconformity separating the basement rocks from rootless, ca 2680–2570 Ma complexly folded and faulted juvenile granite-greenstone domains, sutured together by first- to second-order fault/shear zones. Protracted periods of Archean to post-Archean tectonic uplifting and inversion were responsible for preservations and erosion of upper crustal-level granite greenstone belts, including orogenic gold depositsThe ca 2680–2570 Ma juvenile crustal addition in the Kilo Terrane occurred some 30 m.y. after the ca 2850–2600 Ma juvenile crustal addition events in well-researched Archean cratons, worldwide.

Paleoproterozoic sources for Cordilleran-type Neoproterozoic granitoids from the Araçuaí orogen (SE Brazil): Constraints from Hf isotope zircon composition

The Araçuaí orogen of SE Brazil exposes numerous amounts of magnesian, metaluminous, calc-alkaline, and medium- to high-K granitoid batholiths that were emplaced during early convergence between the São Francisco and Congo cratons during West Gondwana assembly in the late Neoproterozoic. Previously proposed sources for these pre-collisional granitoids include the Paleoproterozoic basement and mantle-derived magma based on isotopic signatures, abundant microgranular enclaves and contemporaneous mafic intrusions. Zircons from migmatitic basement gneiss, a noritic intrusion and five granitoid intrusions were analysed for U–Pb–Hf isotopes to evaluate their magma source. Paleoproterozoic migmatites represent addition of juvenile crust in an accretionary orogenic setting at ca. 2.2–2.0 Ga evidenced by their positive zircon εHf values. The Hf isotopic composition of zircon from pre-collisional granitoids has a large range of negative εHf values between −28.8 and −1.7 indicating crustal protoliths. This range of unradiogenic Hf isotopic compositions is explained by inefficient mixing of magmas derived from felsic and mafic Paleoproterozoic sources that hybridised during incremental growth of the plutons. Zircon grains of contemporaneous mafic intrusions display slightly higher, yet enriched, Hf isotopic compositions consistent with crustal contamination of mantle magmas. The new Hf isotope data, integrated with other chemical and isotopic evidence, suggest derivation of the pre-collisional granitoids from lower crustal sources of the Paleoproterozoic basement, with only minor contributions from mantle-derived magmas. Thus, magmatism of the Araçuaí orogen related to the assembly of West Gondwana was dominantly a crustal-reworking event.

Formation of early Archean Granite-Greenstone Terranes from a globally chondritic mantle: Insights from igneous rocks of the Pilbara Craton, Western Australia

The continental crust grows via juvenile additions from the mantle. However, the timing of initial continent stabilisation and the rate of subsequent continental growth during the first billion years of Earth history is widely debated, in part due to uncertainty over the composition of the mantle source of new crust. Well-preserved Archean granite-greenstone terranes, as present within the Pilbara Craton (Western Australia), provide insights into the sources of felsic magmas and the processes of continental growth and evolution in the distant geological past at the regional scale. Here, we present zircon U-Pb, O and Hf isotope data from ancient gneissic and granitic rocks of the Pilbara Craton, to decipher magma sources and the timing and processes of craton growth. There is no evidence for depleted mantle compositions, and the simplest interpretation is that the crust of the Pilbara Craton was generated from mantle with a chondritic Hf isotope composition. Our results indicate crustal addition at ~3.59 Ga, represented by emplacement of gabbroic to anorthositic rocks. We suggest that the formation of these igneous rocks, and the foundering of the complementary residues, triggered extensive melting of hot, upwelling mantle, leading to the subsequent accumulation of the >12 km thick greenstone belt eruptive sequences from 3.53 Ga, with emplacement of coeval felsic magmas at depth. This process shaped the initial crustal configuration of the proto-craton, which subsequently underwent gravitationally driven overturn and reworking to generate stable, cratonic continental crust with the distinctive dome and keel architecture. The zircon Hf and O isotope signatures of the Pilbara igneous rocks from ~3.59–3.4 Ga do not support remelting of an ancient (> 3.8 Ga) basement, and reinforce the overwhelmingly chondritic to near-chondritic zircon Hf isotope composition of Eoarchean meta-igneous rocks from a number of different Archean cratons. A corollary of this remarkable global consistency is that a significant volume of the mantle maintained a chondritic composition for the Lu-Hf system from the formation of the Earth into the Paleoarchean (up to 3.6–3.5 Ga), as would be the case if stabilised volumes of felsic continental crust prior to 3.5 Ga were relatively small. One implication is that the common assumption of a linear evolution of depleted mantle from 4.5 Ga to the present day is inappropriate for determining the timing and volume of continental crust extraction in the Archean. The nearly identical early evolution of the Pilbara and Kaapvaal cratons suggests a common process to generate Archean granite-greenstone terranes that does not require extensive reworking of ancient crust, but rather involves juvenile crustal addition above persistent zones of upwelling, chondritic mantle.

Late Neoarchean granites in the Qixingtai region, western Shandong: Further evidence for the recycling of early Neoarchean juvenile crust in the North China Craton

The Neoarchean basement rocks of the Qixingtai region in the northwest of western Shandong, eastern North China Craton are divided into western and eastern domains, separated by a ductile shear zone. The western domain is mainly composed of early Neoarchean tonalitic and supracrustal rocks, and the eastern domain is composed of late Neoarchean crustally‐derived granite with diverse early Neoarchean enclaves up to several kilometres long of supracrustal and gneissic TTG‐diorite rocks. This study reports an integrated field, whole‐rock geochemistry and zircon U–Pb‐Hf study on the granites and their enclaves in the eastern domain. The supracrustal enclaves are mainly composed of meta‐basaltic and meta‐ultramafic rocks and are the same in lithological association and formation time as the 2,750–2,700 Ma Liuhang “Formation” in the western area. The TTG‐diorite enclaves can be divided into early and late groups. Magmatic zircons from early TTG‐diorite samples have U–Pb ages of 2,731 ± 17 to 2,748 ± 13 Ma and εHf(t) values of +2.7 to +9.5, whereas trondhjemite samples of the late group have U–Pb zircon ages of 2,588 ± 18 to 2,606 ± 12 Ma and εHf(t) values of +0.6 to +6.5. The Hf isotopic signatures on magmatic zircon from the >2,700 Ma diorite‐tonalites indicate that they are predominantly juvenile crustal additions, probably with minor involvement of older crust in their genesis, as given by tDM (Hf) ages ranging back to 2.92 Ga. The eastern granites range from monzogranite to syenogranite in composition and commonly have negative Eu anomalies, indicating residual plagioclase in the source region. Magmatic zircons from two granite samples have U–Pb ages of 2,516 ± 24 and 2,527 ± 18 Ma and εHf(t) values of +1.0 to +7.0. A fine‐grained syenogranite dyke contains inherited early Neoarchean zircons with εHf(t) values of +1.6 to +7.4. The isotopic signatures of the late Neoarchean granites that dominate the eastern domain show that they were derived from partial melting recycling of older continental crust recognized in the region – early Neoarchean rocks formed as a result of juvenile crustal additions. This is a common signature for late Archean crustal evolution throughout much of the North China Craton, and is the hallmark of some other Archean terranes, such as the Dharwar Craton of south India. The geodynamic explanations for the formation globally of vast amounts of crustally‐derived late Neoarchean granite are discussed.

Five hundred million years of punctuated addition of juvenile crust during extension in the Goochland Terrane, central Appalachian Piedmont Province

ABSTRACTThe Goochland Terrane is an enigmatic crustal block in the Appalachian Piedmont Province of central Virginia, USA. Sparse exposures of terminal Mesoproterozoic and late Neoproterozoic igneous rocks in the central Goochland Terrane offer the opportunity to investigate both the continental affinity of the terrane during the Proterozoic Eon and the timing and mechanisms of crustal growth. We apply multiple geochemical tools to these rocks: tectonic discrimination using whole-rock major and trace element abundances; whole-rock Sm-Nd isotopes; O, U-Pb, and Lu-Hf isotope analyses of spots in zircon; and measurement of O isotopes in multi-grain quartz separates. Eruption of the Sabot Amphibolite protolith is difficult to date, but we tentatively assign an age of 552 ± 11 Ma. Goochland Terrane continental crust first separated from the mantle prior to ca. 1050–1010 Ma intrusion of the Montpelier Anorthosite and the State Farm Gneiss protolith. The granitic magma that became the State Farm Gneiss protolith could have been derived entirely from partial melting of this initial Goochland Terrane crust. In contrast, the magmas that became the Montpelier Anorthosite, Neoproterozoic granitoid, and the Sabot Amphibolite were mixtures of mantle melt and preexisting Goochland Terrane crust. This production of juvenile continental crust occurred during continental extension and, eventually, rifting. The timing and compositions of terminal Mesoproterozoic magmatism in the Goochland Terrane closely match those in the nearby Blue Ridge Province. Although the compositions of the Neoproterozoic magmas in the two regions are similar, intrusion and possibly eruption occurred about 10 M.y. later in the Goochland Terrane.

Coupled Precambrian crustal evolution and supercontinent cycles: Insights from in-situ U-Pb, O- and Hf-isotopes in detrital zircon, NW india

Oxygen and hafnium isotopic compositions, measured in-situ on U-Pb dated zircon grains from Paleoproterozoic to early Cambrian successions in NW India have implication for regional crustal evolution and supercontinent cycles. Analyzed zircon grains have high Th/U ratios (>0.1), display strongly fractionated REE patterns, metamorphic overprint, and evidence of interaction with low temperature fluids. Their positive Ce and negative Eu anomalies preclude any Pb loss after zircon crystallization. The U-Pb age spectra (concordance between 90 and 110%) indicate prominent peaks at 2.6 to 2.4 Ga, 1.9 to 1.7 Ga, 1.6 to 1.5 Ga, 1.2 to 1.0 Ga and 0.9 to 0.7 Ga that coincide with the assembly and breakup of Precambrian supercontinents. The Hf model ages of zircon grains with mantle like δ18O values reveal continuous generation of the continental crust from 3.3 to 1.3 Ga in NW India with major episodes during 3.3 to 2.7 Ga and 1.7 to 1.5 Ga. These ages correspond well with the 3.4 to 2.9 Ga and 2.2 to 1.6 Ga age peaks recognized in detrital zircon populations from eastern Australia and North America, underlining the significance of these time brackets in continental crust generation during the global continental evolution. Magmatic episodes at 1.9 to 1.7, 1.2 to 1.0 and 0.9 to 0.7 Ga are considered to represent crustal reworking rather than juvenile addition and the former two phases correspond with periods of supercontinent assembly. However, a progressive depletion in 18O from supra-mantle to mantle values in the 1.7 to 1.5 Ga zircons, coupled with their mantle-like eHf(t) values, indicate at least some juvenile input. Moreover, the 1344 to 1120 Ma zircon grains with low δ18O (3.7–1.5‰) but high eHf(t) (+8.1− +1.9 with one exception of −2.5) values signify rapid reworking of mantle derived materials in an extensional setting during this period. The 0.9 to 0.7 Ga peak, corresponding to the fragmentation of Rodinia supercontinent, documents crustal reworking that is in contradiction to the generally considered juvenile crustal addition in extensional setting associated with supercontinent breakup.

Tectonometamorphic Evolution of Jutulsessen, Gjelsvikfjella, cDML, East Antarctica

The Jutulsessen area, can provide a vital clue to the supercontinent assembly of Gondwana Land as it is situated within the Circum East Antarctic Mobile Belt just east of the Penksockett rift marking the divide between the central Dronning Maud Land from the Western Dronning Maud Land. This landmass is dominated by migmatitic quartzo-feldspathic rocks intruded by syn to post-tectonic granites. The work highlights the data from western part cDML area with a view to arrive at a more comprehensive model for the cDML and subsequently to the super continent assembly. Granitic and migmatitic gneisses comprising of amphibolitic and biotite rich enclaves. The gneisses show variations from quartzo-felspathic gneiss to amphibolitic gneiss. The area has witnessed complex geological history involving at different deformational episodes with concomitant metamorphism. The pervasive dominant foliation trends NW-SE with shallow to medium dips towards SW. In the Stabben area, a nonfoliated intrusive syenite-gabbro pluton limits the gneissic exposures. Compositionally, the orthogneisses plot in the monzogranitegranodiorite field where as the mafic dykes/enclaves plot in the basalt-andesite-rhyodacite field. The bulk geochemical characteristics suggest significant crustal contamination. Garnet-biotite Fe-Mg exchange thermometry gives peak metamorphic temperature of 483° C for the gneisses and 628° C for the dioritic enclave within gneisses. A peak metamorphic grade of upper amphibolite to granulite facies is deduced from the mineral assemblages. Widespread anatexis has led to extensive occurrence of migmatites in the area. Recent geochronological studies assign an age of 1170 Ma to 970 Ma for the migmatites/gneisses and an emplacement age of 501 Ma for the Stabben gabbro and syenite. The discriminant plots of the Jutulsessen rocks indicate diverse origin ranging from pre-plate collision to post-collision orogenic tectonic setting. The mafic enclaves/dykes show ocean island arc to MORB affinities. Voluminous addition of juvenile crust during the Pan-African orogeny strongly overprints earlier structures.

Episodic crustal growth in the Tanzania Craton: evidence from Nd isotope compositions

ABSTRACT The analysis of available Nd isotope data from the Tanzania Craton places important constraints on the crust-mantle separation ages, and events marking juvenile crustal addition and crustal recycling. Nd model ages date the oldest crust extraction to 3.16 Ga in the Tanzania Craton, although a rock record of such antiquity is yet to be found there. The most significant period of juvenile crustal addition as well as crustal recycling is 2.7–2.6 Ga. The Nd isotopes of mafic samples show that chemical heterogeneity existed in the mantle beneath the Tanzania Craton, with some samples originating from significantly depleted mantle, and most samples originating from the mixture of primitive mantle and depleted mantle. The Nd isotope section reveals significant differences in Nd isotopes between the north craton and central craton; compared to the north craton, the central craton yields a Nd model age that is approximately 100 Ma older, and its eNd(t) values are more negative, indicating that the two parts of the craton have different mantle source regions. Different types of granitoids are distributed in the Tanzania Craton, such as high-K and low-Al granite, calc-alkaline granite, peraluminous granite and transitional types of tonalite-trondhjemite-granodiorites (TTGs). Most of the granitoids formed later than the mafic rocks in syn-collision and post-collision events.

Petrogenesis and tectonic implication of Paleoproterozoic granites and granulites in the Fengzhen area of North China craton

The North China Craton (NCC) is one of the oldest cratons in the world that preserves the records of many important geological events in the early evolution of the Earth. However, the early Precambrian tectonic evolution history of the NCC is still controversial. Here we report Paleoproterozoic leucogranites, garnet granites and granulites in the Fengzhen area that constitutes a junction between the Khondalite Belt and the Trans-North China Orogen (TNCO). The leucogranites have high SiO2 content (75–85wt%), and magmatic zircon U-Pb age of 2199±3 Ma. Their zircons have slightly various εHf(t) values (−0.45 to 1.41) with TDM model ages of 2.5–2.6Ga. The granites show strong LREE enrichment with La/Ybcn ratio of 12–176, and have high initial Nd isotope (εNd(t)=3.5–4.5). Most of them display positive Eu anomaly, excluding fractional crystallization contribution on the high Si feature. The leucogranite was produced by melting of Late Archean felsic TTGs (tonalite–trondhjemite–granodiorite) and addition of juvenile crust. Magmatic zircons yield the garnet granite age of 2002±3Ma. The zircons show various εHf(t) values (−5.8 to 7.1) and TDM model ages (2.1–2.6Ga). Low initial εNd(t) isotope value (−4.2 to −8.3) was determined in the granites. The granulites are high pressure metamorphic rocks with a peak pressure of 12.0±0.8 kbar, and have similar metamorphic age of 1967±10Ma to the garnet granites. This indicates that the Fengzhen area underwent 2-Ga inter-continental orogeny, in which the melting of sedimentary rock formed the garnet granites. Similarly, sodic TTG melting to form K-rich leucogranites also needs high pressure. However, the leucogranite age is obviously older than the peak metamorphic timing of continent–continent collision. This implies that the Fengzhen area may have underwent arc-continental collision at 2.2Ga.

Widespread Neoarchean (~ 2.7–2.6 Ga) magmatism of the Yangtze craton, South China, as revealed by modern river detrital zircons

The Kongling Terrane, which is the Archean nucleus of the Yangtze craton, preserves Paleoarchean-Proterozoic rocks as old as 3.45Ga. However, the dominant stage of formation of this Archean terrane remains unclear. In this paper, U-Pb and Lu-Hf isotopes of detrital zircons from two rivers and one stream in the northern part of the Kongling Terrane were studied by LA-ICP-MS and LA-MC-ICP-MS, respectively. These zircons show complicated internal structures in cathodoluminescence images, but the majority of them have linear or oscillatory zoning patterns, indicating magmatic origins. In general, the detrital zircons from these three local rivers show similar U-Pb age distributions. Together, they yield age peaks at 3.3–3.1Ga (5%), 3.0–2.8Ga (18%), 2.7–2.6Ga (30%), 2.6–2.2Ga (15%), 2.0–1.9Ga (27%), 1.9–1.7Ga (1%), 1.7–1.5Ga (2%), and 1.0–0.8Ga (1%). This age distribution implies that the North Kongling Terrane formed primarily during the Meso- to Neoarchean (3.0–2.6Ga). Lu-Hf isotopic data reveal predominant subchondritic to chondritic εHf(t) values (−11.3 to 0) for the Mesoarchean zircons, indicating the dominant role of crustal reworking during this period. In comparison, most Neoarchean zircons exhibit near chondritic to suprachondritic εHf(t) values (up to +7.4), suggesting juvenile crustal additions. The 2.0–1.9Ga zircons have similar initial Hf isotopic compositions as those of the 2.7–2.6Ga zircons, suggesting a metamorphic recrystallization origin. This observation further implies that the proportion of Neoarchean ages might be underestimated. Previous studies have reported several 2.7–2.6Ga Archean outcrops, minor detrital zircons in Precambrian sedimentary rocks and xenocrystal zircons in volcanic rocks from the Yangtze craton. Combined with our data, we propose that the 2.7–2.6Ga magmatism may have played an important role in forming the Kongling Terrane and have widely affected other parts of the Yangtze craton as well, similar to many other Archean cratons worldwide. The ~2.7–2.6Ga may correspond to the initial stabilization of the Archean nucleus of the Yangtze craton. Thereafter, this craton shows comparable 2.5–2.2Ga age records to the recently proposed Nunavutia supercraton, which could provide further clues to the early tectonic process of the Yangtze craton.

Lamprophyres from Southern Karnataka, Dharwar Craton, India: Insight on the Rodinia Break‐up and Addition of Juvenile Crust

Lamprophyres from Southern Karnataka, Dharwar Craton, India: Insight on the Rodinia Break‐up and Addition of Juvenile Crust