Early Neoproterozoic Magmatism Research Articles

Caledonian reactivation and reworking of Timanian thrust systems and implications for latest Mesoproterozoic to mid-Paleozoic tectonics and magmatism in northern Baltica

Background The Trollfjorden–Komagelva Fault Zone is the southernmost thrust fault of the Timanian Orogen and extends for thousands of kilometers from northwestern Russia to northern Norway. Though there is little about its location onshore northeastern Norway, where it is mapped as a major fault system dominantly comprised of NNE-dipping thrust faults, its continuation to the west below Caledonian nappes and offshore post-Caledonian sedimentary basins remains a matter of debate. Methods The present study provides a more definitive answer about the continuation of Trollfjorden–Komagelva Fault Zone west of the Varanger Peninsula by using seismic reflection, bathymetric, topographic, and magnetic data onshore Finnmark and offshore on the Finnmark Platform. Results The NNE-dipping Trollfjorden–Komagelva Fault Zone merges with a recently identified northwest-dipping brittle–ductile thrust, the Sørøya–Ingøya shear zone, which was previously thought to have formed during the Caledonian Orogeny. The present study demonstrates that the Sørøya–Ingøya shear zone represents a portion of the Trollfjorden–Komagelva Fault Zone that was folded into a NE–SW orientation and reactivated as a top-southeast thrust during the Caledonian Orogeny, while other portions of the Trollfjorden–Komagelva Fault Zone (e.g., on the Varanger Peninsula) were reactivated as strike-slip faults. The study also documents the presence of another major, NNE-dipping Timanian shear zone with a similar geometry to the Trollfjorden–Komagelva Fault Zone north of the Varanger Peninsula. Conclusions The present study suggests that (1) the Seiland Igneous Province formed in a backarc setting, (2) metasedimentary rocks of the Kalak Nappe Complex deposited along the Baltican margin of the Iapetus Ocean, possibly in a late–post-Grenvillian collapse basin, (3) the Iapetus Ocean was much narrower than the several thousands of kilometers width commonly proposed, and (4) early Neoproterozoic magmatism in northern Norway is related to the initial breakup of Rodinia.

Early Neoproterozoic magmatism in the Central Tianshan Block, China: Implications for its tectono-magmatic evolution and relationship to Rodinia

As an important ancient continental constituent of the Central Asian Orogenic Belt (CAOB), the Central Tianshan Block (CTB) is indispensable to improving our understanding of the early evolution and architectural framework of this orogenic belt. Here, we present new zircon U–Pb ages and Hf isotopes, as well as bulk-rock geochemistry, for amphibolite, gneissic granite, and augen granite from several localities in the CTB. Zircon U–Pb ages of ten samples demonstrate that the protolith of the amphibolite, gneissic granite, and augen granite formed in the early Neoproterozoic (from 905.9 ± 4.8 Ma to 940.0 ± 4.5 Ma). Geochemically, the early Neoproterozoic amphibolite is depleted in Nb, Ta, P, and Ti, but enriched in light REE (LREE) and large ion lithophile elements (LILE), similar to arc-related basalt. The gneissic granites from the Shardland area and the Alatage area, and gneissic granites and augen granites from the Xingxingxia area are enriched in SiO2, Na2O + K2O, LREE, and LILE, and depleted in heavy REE (HREE) and high field strength elements (HFSE), exhibiting an I-type granite affinity. Zircons from the amphibolite in the Shardland area exhibit positive εHf(t) values of +1.7 to +4.9 and TDM1 model ages of 1.27–1.39 Ga, indicating that the protolith crystallized from a mantle-derived magma. Zircons from the gneissic granites in the Shardland area, the Alatage area and the Xingxingxia area have εHf(t) values that range from –5.9 to +7.3 and TDM2 model ages of 1.33–2.17 Ga, indicating that the granites may have been derived from partial melting of ancient continental crust that contained compositionally variable juvenile material. These new results, combined with previous studies, indicate that early Neoproterozoic magmatic rocks are well-developed throughout the CTB and formed along a subduction-related continental arc. The CTB and other microcontinents of the southwestern CAOB may have been located proximal to the northern margin of the Tarim Craton during the early Neoproterozoic, thereby representing the active continental margin of the circum-Rodinia subduction–accretion system.

Early Neoproterozoic magmatism and Caledonian metamorphism recorded by the Mårma terrane, Seve Nappe Complex, northern Swedish Caledonides

Petrology, geochronology and bulk-rock chemistry are combined to investigate the early Neoproterozoic magmatism and Cambrian–Ordovician metamorphism in the northern Swedish Caledonides. This work includes several lithologies of the Mårma terrane in the Seve Nappe Complex exposed in the Kebnekaise region. U–Pb zircon geochronology yielded crystallization ages of 835 ± 8 Ma for a mylonitic orthogneiss, 864 ± 3 Ma for the Vistas Granite and 840 ± 7 Ma for an intruded granitic dyke, whereas a gabbro and a granodiorite intrusion gave U–Pb zircon crystallization ages of 856 ± 3 Ma and 850 ± 1 Ma, respectively. U–Pb monazite dating of the mylonitic orthogneiss gave an upper intercept age of 841 ± 7 Ma and a lower intercept age of 443 ± 20 Ma. Pressure–temperature estimates derived for the mylonitic orthogneiss reveal metamorphic peak-pressure and peak-temperature of 10.5–11.75 kbar and 550–610°C and 7.4–8.1 kbar at 615–675°C, respectively. Metamorphic pressure–temperature estimates for the Vistas Granite yield 6.5–7.5 kbar at 600–625°C. Whole-rock chemistry coupled with U–Pb geochronology indicates that bimodal magmatism was related to attempted rifting of Rodinia between 870 and 840 Ma. Supplementary material: Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6675075 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle

The ca. 1.13–0.92 Ga magmatism in the western Yangtze Block, South China: Implications for tectonic evolution and paleogeographic reconstruction

The tectonic regime of the Yangtze Block in the South China and its palaeogeographic location in the Rodinia supercontinent during the late Mesoproterozoic to early Neoproterozoic remain enigmatic due to limited geological records. In this study, we report late Mesoproterozoic basalts and granodiorites and early Neoproterozoic plagioclase amphibolites in the western Yangtze Block. Zircon LA-ICP-MS U-Pb dating yield concordant ages of 1130–1128 Ma for basalts, 1026 ± 1.6 Ma for granodiorites, and 917 ± 1.4 Ma for plagioclase amphibolites. The basalts are tholeiitic in chemical composition and are slightly enriched in LREEs with (La/Yb)N ratios of 2.87–3.99 and exhibit E-MORB-like trace element patterns. They are depleted in whole-rock Nd (+3.3 to +5.9) and zircon Hf isotopes (+11.1 to +12.8). Geochemical compositions showed that the basalts were derived from high-degree (7%–20%) partial melting of spinel lherzolite mantle. The granodiorites belong to the A-type granites and have negative whole rock εNd(t) (−5.5 to −7.8) and zircon εHf(t) (−15.3 to +0.6) values. We suggested that these A-type granites were mainly generated by the partial melting of felsic crustal rocks under low-pressure conditions. The plagioclase amphibolites are tholeiitic and exhibit arc-like geochemical affinities. They show depleted isotopic compositions (zircon εHf(t) = +11.0 to +13.6; whole rock εNd(t) = +0.6 to +2.5) and were interpreted as derived from a lithospheric mantle source that was metasomatized by subduction-related fluids. Geochemical compositions show that basalts were formed in an intracontinental rift setting, whereas granodiorites and plagioclase amphibolites were emplaced into a subduction background. Integrated with published work, we suggest that the late Mesoproterozoic and early Neoproterozoic magmatism in the western Yangtze Block record a tectonic switch from a passive margin to a convergence setting. In addition, we also proposed that the South China may be located at the western periphery of the Rodinia supercontinent.

Early Neoproterozoic magmatism in the Central Qilian block, NW China: Geochronological and petrogenetic constraints for Rodinia assembly

Abstract The supercontinent Rodinia existed as a coherent large landmass from 900 to 750 Ma and is now dispersed over all current major continents. Controversy has long surrounded the reconstructions of the East Asian blocks in Rodinia, especially the South China craton and nearby microcontinents. The Central Qilian block is a Precambrian microcontinent in the early Paleozoic Qilian orogenic belt, which is located in the northeastern part of the Qinghai-Xizang (Tibet) Plateau and marks the junction of the North China, South China and Tarim cratons. The formation and tectonic affinity of the Precambrian basement in the Central Qilian block is unclear, which affects our understanding of the assembly of Rodinia. The Huangyuan Group and the Maxianshan Group crop out in the eastern part of the block and represent the lower part of the basement. In this paper, we present a systematic study of the petrology, whole-rock geochemistry, and geochronology of amphibolites and orthogneisses from the Huangyuan and Maxianshan Groups. The protolith of the amphibolites was tholeiitic and calc-alkaline gabbro or gabbroic diorite formed in a continental arc environment, with laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) zircon U-Pb ages of 967–957 Ma, a wide range of εHf(t) values of –3.74 to +5.06 and TDM1 model ages peaking at 1470 Ma and 1607 Ma. Minor inherited zircon grains with older ages of 1207–1515 Ma were collected from the amphibolites. The primitive magma was derived from partial melting of a spinel-facies fertile (lherzolite) lithospheric mantle that was modified by fluids and melts from a subducted slab. Fractionation of olivine, Fe-Ti oxides and plagioclase played a dominant role in the magma differentiation for gabbroic rocks in the Huangyuan Group, while fractionation of olivine and clinopyroxene controlled differentiation to form Maxianshan Group gabbros. The protolith of orthogneisses includes weakly peraluminous I-type and A2-type granites with consistent LA-ICPMS zircon U-Pb ages of 963–936 Ma, a wide range of εHf(t) values of –3.86 to +6.15 and TDM2 model age peaks at 2001 Ma and 1772 Ma. A few inherited zircon grains yield ages of 1033–2558 Ma. The peraluminous I-type granites resulted from a low-pressure partial melting process and the peraluminous A-type granites were derived from a charnockite source heated by large-scale magmatic underplating. Fractionation of plagioclase, biotite, and K-feldspar from the magma played the main role during the generation of the granitoids. The intrusion of these granites is related to a back-arc extension. It is inferred that the lower part of Precambrian basement of the Central Qilian block is composed mainly of early Neoproterozoic rock assemblages formed in a trench-arc-basin system during the assembly of the Rodinia supercontinent, with probable existence of late Paleoproterozoic to Mesoproterozoic continental nuclei. Combining our results with existing data, we identify a sequence of initial intra-oceanic subduction (ca. 1121–967 Ma) in front of a continental nucleus, continuous subduction of oceanic crust beneath the continental mass with formation of a mature continental arc and a back-arc basin (ca. 967–896 Ma) and continental rifting (<ca. 882 Ma) during the formation of the Central Qilian block. As a mature continental arc after ca. 967 Ma, the Central Qilian block was located at the margin of Rodinia and faced the Neoproterozoic Mirovoi Ocean. The breakup of the supercontinent left the Central Qilian block as a late Neoproterozoic isolated arc terrane.

Record of Early Tonian mafic magmatism in the central Espinhaço (Brazil): New insights for break-up of the Neoproterozoic landmass ancestor of São Francisco-Congo paleocontinent

Petrological characterization, U–Pb geochronology, Lu–Hf analyses and major and trace element data from mafic intrusions in the Central Espinhaço (central portion of the Brazilian shield) are used here to investigate the geological significance of the Early Neoproterozoic magmatism in the context of the São Francisco-Congo paleocontinent. These mafic bodies are represented by medium to coarse-grained metagabbros with plagioclase, amphibole and clinopyroxene. Zircon U–Pb isotopic data from two samples yielded weighted mean 206Pb/238U ages of 895 ​± ​3.4 ​Ma (MSWD ​= ​1.7) and 896 ​± ​2.4 ​Ma (MSWD ​= ​0.64), regarded as the best estimates for the crystallization age of these mafic rocks. Major and trace element data (including REEs) show that the gabbros originated from a subalkaline tholeiitic magma, typical of intraplate magmatism. Such rocks are slightly enriched in LREEs and LILEs and depleted in HFSEs. Our new isotope and geochemical data, along with regional knowledge, indicate that these metagabbros mark the beginning of an important Tonian-age extensional tectonic event of the landmass of which the São Francisco-Congo paleocontinent was part (Rodinia supercontinent or Central African block?). We furthermore suggest that these rocks belong to a prominent suite of Tonian-age mafic rocks that mark a diachronic breakup attempt of this landmass which may have occurred from south to north along the Espinhaço mountain range.

Ca. 1.04 Ga hot Grenville granites in the western Yangtze Block, southwest China

The late Mesoproterozoic to early Neoproterozoic magmatism in the western Yangtze Block, though minor in volume, has offered a rare opportunity for understanding the role of the Yangtze Block in the Grenville orogeny. The Huidong K-feldspar granites have SIMS zircon U-Pb ages of 1048.5 ± 4.9 Ma and 1043.1 ± 5.1 Ma, whereas the Yuanmou biotite granites were dated at 1041 ± 12 Ma by LA-ICP-MS. Their ages are indistinguishable within error, suggesting that they were emplaced contemporaneously. Both Huidong and Yuanmou rocks show geochemical features typical of A-type granites, including high FeO*/MgO, Ga/Al ratios and elevated contents of high-field-strength elements (HFSE). The Huidong granites exhibit positive whole-rock εNd(t) (+0.58 to +4.4) and zircon εHf(t) (+6.0 to +8.3) values close to those of regional coeval mafic rocks. They have a narrow range of zircon δ18O from 6.2‰ to 7.2‰, which is only slightly higher than the ‘mantle zircon’ value of 5.3 ± 0.6‰. High Nb/La and Nb/Th ratios in these rocks overlap with those of regional coeval mantle-derived rocks. Along with high zircon saturation temperatures of >1000 °C, we suggest that the Huidong granites could have formed mainly by extreme fractionation from basaltic parental magmas, or alternatively by partial melting of newly emplaced tholeiite at high temperatures. By contrast, the Yuanmou granites have lower whole-rock εNd(t) (−2.0 to +0.59) and zircon εHf(t) (−1.5 to +5.1) values and a Hf isotope crustal model age of ∼1.56–1.97 Ga, suggesting that its parental magmas may be generated by partial melting of old crust during intrusion of mantle-derived magmas. Ca. 1.04 Ga granites in the region show geochemical features similar to A-type granites in orogenic belts, e.g., the aluminous A-type granites in the Lachlan Fold Belt, Southeast Australia. They are also characterized by high-extremely high (>1000 °C) temperature in their genesis, similar to coeval hot Grenville granites globally. Temporal correlation of metamorphic and magmatic records between South China and the Grenville Province leads us to conclude that the ∼1.04 Ga A-type granites in the western Yangtze Block form parts of the hot Grenville granites emplaced during post-orogenic crustal extension.

Early Neoproterozoic magmatism and the associated metamorphism in the Songnen Massif, NE China: Petrogenesis and tectonic implications

Zircon U–Pb dating and geochemistry data were collected for Neoproterozoic rocks in the Songnen Massif of NE China, with the aim of understanding the extent and nature of the Precambrian basement as well as the early stages of tectonic evolution history of the massif. The newly discovered Neoproterozoic magmatism can be subdivided into three stages: (1) 929–927 Ma quartz-syenitic and syenogranitic gneisses, (2) ∼895 Ma granitic gneisses, and (3) 871 Ma granitic pegmatites. Associated metamorphisms can also be subdivided into two stages, i.e., the 492 Ma and 546–530 Ma, suggesting that the study area underwent two stages of metamorphism during the late Pan-African. The petrographical and geochemical data indicate that the 929–927 Ma syenitic rocks most likely originated from enriched mantle sources, which were metasomatized by subducted sediments and underwent fractional crystallization during ascension, whereas the ca. 895 Ma granitic gneisses were generated by the partial melting of ancient mafic lower crustal material. Combined with our previous study, it is suggested that the ∼929–927 and 895 Ma magmatism were formed in a convergent margin related to compressive orogenesis, consistent with the evolution of the Rodinia supercontinent. Similar tectono-magmatic thermal events occurred within the Songnen and Jiamusi massifs, suggesting that they could be one coherent continental unit in Precambrian.

Nature of Late Mesoproterozoic to Early Neoproterozoic magmatism in the western Gyeonggi massif, Korean Peninsula and its tectonic significance

The western margin of the Gyeonggi massif, southern Korean Peninsula, has preserved N–S trending Neoproterozoic and sporadic Late Mesoproterozoic metaigneous rocks. Here we present the results from systematic field mapping, sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb dating, and whole-rock geochemical analyses of the Mesoproterozoic and Early Neoproterozoic metaplutonic rocks in the Hongseong area, together with previously published data from the western Gyeonggi massif. The SHRIMP ages of these rocks are categorized into three groups: (1) Late Mesoproterozoic (ca. 1.25–1.15Ga), (2) Early Neoproterozoic (ca. 900–770Ma), and (3) late Early Neoproterozoic (ca. 762–730Ma). The geochronological and geochemical features of the Late Mesoproterozoic rocks suggest that they were possibly formed in association with convergent plate motion. The Early and late Early Neoproterozoic rocks are interpreted to arc-related orogenic and rift-related post-orogenic environments, respectively. These age results and the tectonic signatures provide insight into the convergence process along the margins of the Rodinia supercontinent.

An Early Neoproterozoic gabbro–granite association in the Bureya Continental Massif (Central Asian fold belt): First geochemical and geochronological data

The fact that gneissose granites and gabbros of the Nyatygran Complex in the Bureya Continental Massif are not Palaeoproterozoic in age, as previously thought, but Neoproterozoic, ~933 Ma is proved. New data with the first direct evidence of Early Neoproterozoic magmatism in continental massifs composing the Bureya–Jiamusi Superterrane are given. At the moment, the obtained age estimates are the oldest for the magmatic rocks of this superterrane.

Zircon U–Pb ages and geochemistry of newly discovered Neoproterozoic orthogneisses in the Mishan region, NE China: Constraints on the high-grade metamorphism and tectonic affinity of the Jiamusi–Khanka Block

The Jiamusi–Khanka Block, located in the easternmost segment of the Central Asian Orogenic Belt (CAOB), is one of the least understood blocks in northeastern China. The age of its basement, the timing of the high-grade metamorphism, and the tectonic affinity of this block have all been subjects of controversy. Here we describe newly discovered Neoproterozoic orthogneisses from the Mishan region in the central Jiamusi–Khanka Block. SIMS and LA–ICP–MS U–Pb dating of the magmatic cores of zircons from three of these high-grade orthogneisses yielded weighted mean ages that range from 898±4 to 891±13Ma, indicating that early Neoproterozoic magmatism did occur in the Jiamusi–Khanka Block. Twenty-two analyses of metamorphic zircons (rims as well as some grains) gave two groups of concordant ages, one at ca. 563Ma, interpreted as the timing of the high-grade metamorphism, and the other around 518–496Ma, interpreted as representing a subsequent retrograde metamorphism. The orthogneisses represent metamorphosed peraluminous syn-collisional syenogranites with SiO2=71.29–78.08wt%, A/CNK=1.06–1.99, and Na2O/K2O=0.03–1.49, and they belong chemically to the high-potassium calc-alkaline series. They have low Sr (32–134ppm) contents and Sr/Y ratios (1.6–16.9), and display strongly fractionated to nearly flat REE patterns with negative Eu anomalies (δEu=0.22–0.95) and depletions in Nb–Ta. These geochemical characteristics suggest that the syenogranitic rocks were derived under low pressures from a crustal source with a high proportion of sedimentary rocks, perhaps in a subduction-related setting that was undergoing transition from compression to extension. In the context of the reconstruction of Gondwana, the ca. 898–891Ma magmatic activity and the ca. 563Ma high-grade metamorphism in the Mishan region provide constraints on the possible linkages between the Jiamusi–Khanka Block and East Gondwana during the Neoproterozoic to early Paleozoic.

Petrology, geochemistry and zircon U-Pb geochronology of a layered igneous complex from Akarui Point in the Lützow-Holm Complex, East Antarctica: Implications for Antarctica-Sri Lanka correlation

The Lützow-Holm Complex (LHC) of East Antarctica forms part of a complex subduction-collision orogen related to the amalgamation of the Neoproterozoic supercontinent Gondwana. Here we report new petrological, geochemical, and geochronological data from a metamorphosed and disrupted layered igneous complex from Akarui Point in the LHC which provide new insights into the evolution of the complex. The complex is composed of mafic orthogneiss (edenite/pargasite+plagioclase±clinopyroxene±orthopyroxene±spinel±sapphirine±K-feldspar), meta-ultramafic rock (pargasite+olivine+spinel+orthopyroxene), and felsic orthogneiss (plagioclase+quartz+pargasite+biotite±garnet). The rocks show obvious compositional layering reflecting the chemical variation possibly through magmatic differentiation. The metamorphic conditions of the rocks were estimated using hornblende-plagioclase geothermometry which yielded temperatures of 720–840°C. The geochemical data of the orthogneisses indicate fractional crystallization possibly related to differentiation within a magma chamber. Most of the mafic-ultramafic samples show enrichment of LILE, negative Nb, Ta, P and Ti anomalies, and constant HFSE contents in primitive-mantle normalized trace element plots suggesting volcanic arc affinity probably related to subduction. The enrichment of LREE and flat HREE patterns in chondrite-normalized REE plot, with the Nb-Zr-Y, Y-La-Nb, and Th/Yb-Nb/Yb plots also suggest volcanic arc affinity. The felsic orthogneiss plotted on Nb/Zr-Zr diagram (low Nb/Zr ratio) and spider diagrams (enrichment of LILE, negative Nb, Ta, P and Ti anomalies) also show magmatic arc origin. The morphology, internal structure, and high Th/U ratio of zircon grains in felsic orthogneiss are consistent with magmatic origin for most of these grains. Zircon U-Pb analyses suggest Early Neoproterozoic (847.4±8.0Ma) magmatism and protolith formation. Some older grains (1026–882Ma) are regarded as xenocrysts from basement entrained in the magma through limited crustal reworking. The younger ages (807–667Ma) might represent subsequent thermal events. The results of this study suggest that the ca. 850Ma layered igneous complex in Akarui Point was derived from a magma chamber constructed through arc-related magmatism which included components from ca. 1.0Ga felsic continental crustal basement. The geochemical characteristics and the timing of protolith emplacement from this complex are broadly identical to those of similar orthogneisses from Kasumi Rock and Tama Point in the LHC and the Kadugannawa Complex in Sri Lanka, which record Early Neoproterozoic (ca. 1.0Ga) arc magmatism. Although the magmatic event in Akarui Point is slightly younger, the thermal event probably continued from ca. 1.0Ga to ca. 850Ma or even to ca. 670Ma. We therefore correlate the Akarui Point igneous complex with those in the LHC and Kadugannawa Complex formed under similar Early Neoproterozoic arc magmatic events during the convergent margin processes prior to the assembly of the Gondwana supercontinent.

Neoproterozoic crustal evolution in Sri Lanka: Insights from petrologic, geochemical and zircon U–Pb and Lu–Hf isotopic data and implications for Gondwana assembly

Sri Lanka, the ‘pendant’ of Gondwana, is a collage of distinct crustal blocks that preserve important records of major Neoproterozoic tectonothermal events. Here, we present the petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes on a suite of meta-igneous rocks including granodiorite, diorite and garnet amphibolite from the Kadugannawa Complex (KC), granodiorite from the Wanni Complex (WC) and mafic granulites, gabbros and garnet-bearing charnockite from the Highland Complex (HC) along a NW–SE transect. The regional metamorphic peak P–T conditions were estimated from garnet-clinopyroxene-plagioclase-quartz assemblage in the metagabbro as 830–860°C and 9.4–9.8kbar. Slightly lower temperature ranges of 700–780°C were obtained from garnet amphibolite, metagranodiorite and metadiorite, corresponding retrograde conditions. Trace element and rare earth element patterns as well as Rb-Y-Nb and Rb-Yb-Ta discrimination plots show volcanic arc affinity for the granodiorite, diorite and garnet charnockite suggesting that the protoliths of the rocks were formed from felsic to intermediate arc magmas. The mafic granulites and magnesian metagabbro also suggest volcanic arc affinity and indicate subduction-related mafic magmatism and magma underplating. The garnet-bearing metagabbro shows N-MORB signature, whereas the garnet amphibolite displays oceanic island alkali basalt affinity. These rocks therefore represent accretion of the remnants of oceanic lithosphere during the subduction-collisional event.Zircons in a metadiorite and the surrounding metagranodiorite from the KC yield ages of 980±16Ma to 916±57Ma marking early Neoproterozoic magmatism followed by metamorphism at 532±18Ma. Zircons in the garnet amphibolite from this complex show extensive metamorphic recrystallization yielding a weighted mean 206Pb/238U age of 520.7±6.6Ma. From the WC, zircons in a metagranodiorite define three groups of weighted mean 206Pb/238U ages at 805±12Ma (emplacement of the magmatic protolith), 734.0±4.6Ma (Cryogenian thermal event) and 546.0±5.7Ma (latest Neoproterozoic-Cambrian metamorphism). Zircons from the HC record multiple late Neoproterozoic-Cambrian thermal events with weighted mean 206Pb/238U ages of 576.8±9.3Ma and 523.0±7.1Ma (metagabbro) and 579±10Ma, 540.4±6.0Ma and 511.1±5.9Ma (garnet charnockite), 553.0±3.2Ma (mafic granulite), and 539.1±4.4Ma (mafic granulite sill). Lu–Hf data reveal dominantly positive ɛHf(t) values for zircons in the metadiorite and metagranodiorite from the KC (−1.1 to 7.2) and Hf crustal model ages (TDMC) in the range of 1206–1733Ma suggesting a mixed source from both juvenile and Paleo-Mesoproterozoic components. However, zircons in the garnet amphibolite from this complex show dominantly negative ɛHf(t) values (mean −16.5) with TDMC in the range of 2356–2828Ma suggesting reworked Neoarchean-Paleoproterozoic crustal source. Zircons in metagranodiorite of the WC also possess negative ɛHf(t) values (mean −6.2) with TDMC in the range of 1799–2498Ma suggesting reworked Paleoproterozoic crust as the magma source. The HC rocks also preserve distinct imprints of reworking of older crust. The zircon ɛHf(t) values in mafic granulite show a tight cluster from −2.2 to 0.1 with TDMC in the range of 1501–1651Ma suggesting a mixed source from both juvenile Neoproterozoic and reworked Mesoproterozoic components. Zircons in the metagabbro from this complex show negative ɛHf(t) values (mean −6.3) and TDMC of 1847–1978Ma. Zircons in the garnet charnockite also display highly negative ɛHf(t) values (mean −17.7) and older TDMC) (mean 2614Ma) suggesting reworked Paleoproterozoic crust. Zircons in the mafic granulite sample show negative ɛHf(t) values (mean −14.1) and TDMC between 2263 and 2790Ma indicating that the source material for the magma evolved from the Neoarchean-Paleoproterozoic crust. In summary, the 916–980Ma ages from the KC represent arc magmatism during early Neoproterozoic, followed by the 805Ma granodioritic magma emplacement in the WC. Repeated thermal events during mid and late Neoproterozoic are also recorded from the Wanni and Highland Complexes, culminating in Cambrian high-grade metamorphism that reached ultrahigh-temperature conditions. We propose a model of double-sided subduction during the Neoproterozoic, where the Wanni Complex to the west and the Vijayan Complex to the east represent continental arcs, culminating in collision along the HC during late Neoproterozoic-Cambrian.

Zircon U–Pb and Hf isotopic study of Neoproterozoic granitic gneisses from the Alatage area, Xinjiang: constraints on the Precambrian crustal evolution in the Central Tianshan Block

The architecture and growth history of Precambrian crustal basements in the Central Tianshan Block play a key role in understanding the tectonic evolution of the Chinese Tianshan Orogenic Belt. In this study, we present precise LA-ICP-MS zircon U–Pb dating and LA-MC-ICPMS zircon Hf isotopic data for two granitic gneisses from Alatage area in the Central Tianshan Block. The magmatic zircons from both samples yield similar protolith ages of 945 ± 6 and 942 ± 6 Ma, indicating that the early Neoproterozoic magmatism is prevailed in the Alatage area. These zircons have crustal Hf model ages of 1.82–2.22 and 1.70–2.03 Ga, respectively, which are significantly older than their crystallization ages. It indicates that their parental magmas were derived from the reworking of ancient crust. However, we suggest that these Paleoproterozoic Hf model ages might result from mixing of continental materials with different ages in the Neoproterozoic crust. The inherited (detrital) zircon cores not only yield a wide age range of ca. 989–1617 Ma, but also exhibit large Hf-isotope variations with Hf model ages of 1.54–2.30 Ga. In particular, some 1.4–1.6 Ga zircons show high initial 176Hf/177Hf ratios, consistent with those of depleted mantle, which indicates that the Mesoproterozoic event involved both reworking of older crust and generation of juvenile crust. The Central Tianshan Block has different Precambrian crustal growth history from the Tarim Craton. Therefore, it would not be a fragment of the Precambrian basement of the Tarim Craton.

Hf isotope study of Palaeozoic metaigneous rocks of La pampa province and implications for the occurrence of juvenile early Neoproterozoic (Tonian) magmatism in south-central Argentina

Abstract On a global scale, juvenile Tonian (Early Neoproterozoic) magmatic rocks are associated with the extensional events that lead to the breakup of the Rodinia supercontinent. In Argentina, no geological record is available for this time interval, lasting from 1000 to 850 Ma. We present indirect evidence for the existence of Tonian extension in Argentina, as supported by Hf and Nd isotope determinations on Phanerozoic magmatic and sedimentary rocks. We mainly focus on our own Hf isotope determinations carried out on U–Pb SHRIMP dated zircons from Palaeozoic metaigneous rocks of La Pampa province, south-central Argentina, i.e. metagabbros of Valle Daza, dioritic orthogneiss of Estancia Lote 8, and metadiorite of Estancia El Carancho, having found that these rocks were derived from sources of ca. 920 to ca 880 Ma, with eHf values between +6.83 and + 9.59. Inherited zircons of this age and character identified in these rocks also point to the same source. We also compile additional Hf and Nd studies from previous work on Phanerozoic magmatic and sedimentary rocks. We preliminarily compare the age of the juvenile Tonian sources referred to in our work with that of two extensional events identified in the Sao Francisco craton, Brazil.

Possible early Neoproterozoic magmatism associated with slab window in the Pingshui segment of the Jiangshan-Shaoxing suture zone: Evidence from zircon LA-ICP-MS U-Pb geochronology and geochemistry

We report here geochemical data, U-Pb zircon ages, and Hf isotopes for the high-Mg diorites (HMDs), Nb-enriched basaltic porphyrys (NEBPs) and plagiogranites (PLAGs) in the Pingshui segment of the Jiangshan-Shaoxing suture zone. The HMDs are characterized by high Mg# (>60), high Na and LREE contents, depletion of HREE and HFSE, and pronounced positive eNd(t) values of 7.0 to 7.7, similar to some adakitic high-Mg andesites. The NEBPs are relatively Na-rich (Na2O/K2O>6) and display high abundances of P2O5 (∼1.00%), TiO2 (∼3.08%) and HFSE (e.g., Nb=9.53–10.27 ppm). Their Nd isotopic compositions (eNd(t)=6.8–8.0) are comparable to those of the HMDs. The PLAGs are metaluminous (A/CNK=0.84–0.89) and sodic (Na2O/K2O>10). Their depletion in HFSE (e.g., Nb, Ta) is consistent with “SSZ-type” plagiogranite. Zircon LA-ICP-MS U-Pb dating yields an age of 932±7 Ma for the HMD, 916±6 Ma for the NEBP, and 902±5 Ma for the PLAG, respectively, indicating that they were products of early Neoproterozoic magmatism. The PLAGs exhibit relatively high zircon Hf isotopes and positive eHf(t) values of 11.0 to 16.2, consistent with their Nd isotopic data (eNd(t)=7.5–8.4). Such features are similar to those of oceanic plagiogranites in ophiolites and distinct from those of crust-derived granites. The PLAGs were most likely derived from partial melting of subducted oceanic crust in an active continental margin. Considering these results in the context of the regional geology, we suggest that a slab window in the subducting oceanic crust between the Yangtze Block and Cathaysia Block was possibly the principal cause for the unique arc magmatism in the area. The upwelling asthenosphere below the slab window may have provided significant thermodynamic conditions.

Polyphase tectonothermal history recorded in granulitized gneisses from the north Qaidam HP/UHP metamorphic terrane, western China: Evidence from zircon U-Pb geochronology

High-grade gneisses of the north Qaidam high-pressure/ultrahigh-pressure metamorphic terrane enclose minor eclogites and ultramafic rocks. In combination with petrological data, sensitive high-resolution ion microprobe U-Pb geochronology on the granulitized gneisses in the Luliangshan and Xitieshan, western north Qaidam Mountains, reveals a polyphase tectonothermal history including Early Neoproterozoic and Early Paleozoic events. The rocks investigated are two granulite-facies paragneisses and one orthogneiss that both surround garnet peridotite in the Luliangshan, and two paragneisses that enclose eclogite in the Xitieshan. The inherited zircon cores from paragneiss and orthogneiss yield ages between ca. 1000 Ma and ca. 2500 Ma, representing Mesoproterozoic to Archaean source material for these gneisses. A ca. 900 Ma age obtained from one orthogneiss and one paragneiss is interpreted as the age of simultaneous Early Neoproterozoic magmatism and metamorphism. This Early Neoproterozoic tectonothermal event is similar in age to the Jinning orogeny, which is commonly recognized in the metamorphic basement of the south China block, and suggests that the north Qaidam Mountains have an affinity to the south China block. Early Paleozoic metamorphism is recorded in all gneiss samples. In conjunction with cathodoluminescence imagery and mineral inclusions, U-Pb dating of zircons reveals two Early Paleozoic age groups: ca. 450 Ma represents the age of high-pressure granulite metamorphism (Grt + Rt inclusions in zircon; kyanite porphyroblasts), whereas ca. 425 Ma reflects the time of medium-pressure, granulite-facies metamorphism (Pl + Sil inclusions in zircon) and associated anatexis during decompression. The ages obtained suggest that the granulite-facies metamorphism lasted for ~25 m.y. and was related to the Early Paleozoic continental collision between the Qilian and Qaidam blocks, and to subsequent thermal relaxation and exhumation.

Basement–cover relationships of the Kalak Nappe Complex, Arctic Norwegian Caledonides and constraints on Neoproterozoic terrane assembly in the North Atlantic region

The Kalak Nappe Complex (KNC) has been regarded as Baltica passive margin metasediments telescoped eastwards onto the Baltic (Fennoscandian) Shield during the Caledonian Orogeny. Recent studies have questioned this interpretation, instead pointing to a Neoproterozoic exotic origin. In an effort to resolve this controversy we present a Sm–Nd and U–Th–Pb study of gnessic units, traditionally considered as the depositional basement, along with cover rock sediments and intrusives. Late Palaeoproterozoic gneisses now beneath the KNC were deposited after 1948 ± 33 Ma, before intrusion of the Tjukkfjellet Granite at 1796 ± 3 Ma, and were affected by later melting events at 1765 ± 9 and 1727 ± 9 Ma. These gneisses are interpreted as part of the Baltic Shield and underlie the KNC across a tectonic contact. An unconformity between psammites of the KNC and other paragneisses previously considered as its Precambrian basement is reinterpreted as a modified sedimentary contact between Neoproterozoic metasediments. These metasediments have statistically very similar detrital zircon populations with grains as young as 1034 ± 22, 1025 ± 32 and 1014 ± 14 Ma. The results indicate that the KNC sediments were deposited during the Neoproterozoic in basins along the Laurentian margin of eastern Rodinia and were not connected to Baltica via a depositional basement. Dating of the 851 ± 5 Ma Eidvågvatnet and 853 ± 4 Ma Nordneset granites shows that intrusive material associated with the Porsanger Orogeny (c. 850 Ma) affected a considerable region of the upper KNC terrane. Later Neoproterozoic events at 711 ± 6, 687 ± 12 and 617 ± 6 Ma are also recognised the latest of which may be an expression of rifting. Since early Neoproterozoic magmatism (c. 840–690 Ma) is unknown in Baltica, these results support an exotic origin for the KNC terranes.

Early Neoproterozoic magmatism (1000–910 Ma) of the Zadinian and Mayumbian Groups (Bas-Congo): onset of Rodinia rifting at the western edge of the Congo craton

New ion microprobe U–Pb zircon ages, as well as some geochemical and isotopic analyses, for key igneous units within the central part of the West Congo belt are integrated with geological information to provide an updated geological map (1:1 000 000 scale) and a synthetic type cross-section of the belt, as well as an updated lithostratigraphic chart of the ‘West Congo Supergroup’. Three Neoproterozoic units are recognised, from oldest to youngest, the Zadinian, Mayumbian and West Congolian ‘Groups’. Emplacement of early Zadinian peralkaline granites (Noqui massif, 999±7 Ma) and rhyolites (Palabala) was accompanied by incipient rift sedimentation, corresponding to the onset of transtensional rifting, preferentially in a transverse mega-shear setting along the margin of the Congo craton. Subsequent upper Zadinian magmatism produced a thick (1600–2400 m) basaltic sequence (Gangila), which has geochemical characteristics typical of continental flood basalts (CFBs). The Gangila basalts, associated with major pull-apart rifting, were followed rapidly by the 3000–4000 m thick Mayumbian rhyolitic lavas, dated at 920±8 Ma at the base and 912±7 Ma at the top. The felsic lavas are intruded by coeval high-level (micro)granites, whose emplacement is dated at 924±25 Ma (Mativa body) and at 917±14 Ma (Bata Kimenga body) in the Lufu massif. This voluminous bimodal magmatic province is similar to the Paraná and Deccan provinces, and shares similar lithospheric sources. It corresponds to the initial, transtensional rifting stage along the western edge of the Congo craton before Rodinia breakup. The early Neoproterozoic rocks of the West Congo Supergroup rest unconformably on a ca. 2.1 Ga Palaeoproterozoic polycyclic basement (Kimezian Supergroup). No Mesoproterozoic events are recorded in the area. Following the initial, transtensional early Neoproterozoic (ca. 1000–910 Ma) rifting stage, Bas-Congo behaved as a passive margin of the Congo craton, as indicated by deposition of ca. 4000 m of Neoproterozoic (pre-Pan-African) platform sediments (lower part of West Congolian Group) preceding ca. 2000 m of Pan-African molasse-type sediments (upper part of West Congolian Group). In the late Neoproterozoic, during Pan-African assembly of Gondwanaland, the Bas-Congo passive margin, which was largely protected by thick lithosphere of the Congo craton, collided with a western active margin to form the Brasiliano-Araçuaı́ belt, now preserved adjacent to the São Francisco craton of Brazil. This collision, which ended in Bas-Congo at ca. 566 Ma, induced relatively limited effects in the West Congo belt, which experienced no late Neoproterozoic magmatic activity.

Grenville-age basement provinces in East Antarctica: Evidence for three separate collisional orogens

Three Grenville-age provinces can be distinguished in East Antarctica with U-Pb zircon data. The Maud, Rayner, and Wilkes provinces each have a distinctive age signature for late Mesoproterozoic–early Neoproterozoic magmatism and high-grade metamorphism and are correlated with similar rocks in the Namaqua-Natal (Africa), Eastern Ghats (India), and Albany-Fraser (Australia) provinces, respectively. These crustal segments represent three separate collisional orogens. They are separated by regions of intense late Neoproterozoic–Early Cambrian tectonism, consistent with their juxtaposition during the final assembly of Gondwana and indicating that previous models for a single, continuous, Grenville-age mobile belt around the East Antarctic coastline should be discarded.