Aeromagnetometry and aerogammaspectrometry integrated with U-Pb zircon geochronology of northern Bossoroca ophiolite, Brasiliano Orogen

Age delimitation integrated with aeromagnetometric and aerogammaspectrometric survey advances the understanding of ophiolite evolution in the Brasiliano Orogen. We focused on the Bossoroca ophiolite, because oceanic crustal and mantle rocks contain zircon in metasomatic chloritite. A metadiorite and a metavolcanoclastic rock were also studied to delimit relationship between ophiolite and island-arc infrastructure and superstructure. Zircon crystals were dated by laser ablation inductively coupled plasma emission spectroscopy. Ages of zircon from Campestre metavolcanoclastic rock are 920-840 (peak 842) Ma, Bossoroca chloritite 900-800 (peak 868 Ma) and Capivaras metadiorite 698 Ma. Ages 920-800 Ma correspond to processes in the oceanic crust, whereas 698 Ma was a late magmatic intrusion (Capivaras metadiorite) into the island-arc infrastructure. Aeromagnetometric and aerogammaspectrometric data delimit the occurrence and structure of the ophiolite. These are major multiproxy markers of geotectonic processes early in the Brasiliano Orogen

Late Neoarchean subduction-related crustal growth in the Northern Liaoning region of the North China Craton : evidence from ∼2.55-2.50 Ga granitoid gneisses

This study is financially supported by the National Natural Science Foundation of China (Grant Nos. 41272209, 41472165, 41502179, 41530207) and the Central University Basic Scientific Research Business Expenses of China University of Geosciences (Beijing) (Grant No. 2652015038).The North China Craton (NCC), dominated by ∼2.6-2.5 Ga tectonothermal events, provides a natural laboratory to study Neoarchean crustal growth and geodynamic evolution. Late Neoarchean granitoid gneisses are well exposed in the Northern Liaoning Province, located north of the ancient Anshan-Benxi terrane along the northeastern margin of the Eastern Block (EB) of the NCC. LA-ICPMS zircon U-Pb isotopic dating reveal that granitoid gneisses in the Qingyuan area can be grouped into two major episodes, i.e., ∼2559-2534 Ma strongly gneissic quartz dioritic and tonalitic to trondhjemitic gneisses; and ∼2529-2495 Ma weakly gneissic to massive quartz monzodioritic and monzogranitic gneisses, with subordinate tonalitic to trondhjemitic gneisses. The late magmatic episode was accompanied by regionally high-grade metamorphism (∼2510-2495 Ma). Most granitoid gneisses display highly depleted zircon εHf(t2) values (+4.2-+8.1), whereas one monzogranitic gneiss shows negative values of -4.7 to -1.0, indicating late Neoarchean crustal growth with minor involvement of ancient continental materials probably sourced from the Anshan-Benxi terrane. Geochemical and petrogenetic studies reveal that the quartz dioritic magmas were derived from partial melting of plagioclase-poor garnet amphibolites or eclogites metamorphosed from oceanic slab materials, with slab melts contaminated by mantle wedge peridotites during ascent. The tonalitic to trondhjemitic magmas stemmed from partial melting of mainly juvenile metabasaltic rocks with minor metagreywackes of lower arc crust. In comparison, the quartz monzodioritic and monzogranitic magmas were derived respectively from partial melting of depleted mantle sources metasomatized by slab-derived fluids and metagreywackes with different crustal resident ages at middle to lower crustal levels. Combined with previous studies of metavolcanic rocks, the Northern Liaoning Province records late Neoarchean crustal growth, evolving from mid-ocean ridge, through initiation and maturation of an intra-oceanic arc, to arc-continent collision. Arc-continent accretion and possibly slab rollback processes may have triggered reworking of both juvenile arc crust and minor ancient continental margin materials, generating the magmatic precursors for the monzogranitic gneisses. Overall, the intense late Neoarchean crustal growth of the EB was controlled mainly by arc-continent accretion, possibly linked to global assembly of cratonic fragments.PostprintPeer reviewe

Late Neoarchean crust-mantle geodynamics : evidence from Pingquan Complex of the Northern Hebei Province, North China Craton

This study is financially supported by the National Natural Science Foundation of China (Grant Nos. 41530207, 41502179, 41472165, 41602198) and Central University Basic Scientific Research Business Expenses (Grant No. 2652015038). PAC acknowledges support from Australian Research Council grant FL160100168.A late Neoarchean intra-oceanic arc along the northwestern margin of Eastern Block (EB), North China Craton, provides important insights into the nature of Archean mantle sources and crust-mantle geodynamics. The Pingquan Complex and the entire Northern Hebei Province (NHB) are located in the middle part of the arc, and overlap the northern extent of the Trans-North China Orogen. Zircon U-Pb isotopic age data reveal that the Pingquan Complex consists of ∼2537-2515 Ma dioritic gneisses, ∼2506-2503 Ma amphibolites, and ∼2491 Ma quartz monzodioritic to monzogranitic gneisses, and they show dominantly positive zircon εHf(t) (-0.6-+5.4) that are lower than coeval model depleted mantle values. Geochemical data for the Pingquan rocks and synchronous metabasalts and granitoid gneisses of Huai’an-Xuanhua and Dantazi complexes in the NHB are integrated. Except for the monzogranitic gneisses that were derived from partial melting of juvenile metagreywackes, the other rocks of the Pingquan Complex were derived from a metasomatized lithospheric mantle, and subjected to variable fractionation of clinopyroxene, hornblende and plagioclase, without significant crustal contamination. Moderately depleted zircon εHf(t), and high Sm/Hf and Nb/Ta (mostly of 1.34-3.96 and 15.50-32.58) suggest that the lithospheric mantle was enriched by subducted pelagic sediments metamorphosed to rutile-bearing eclogites before melting. Late Neoarchean crust-mantle geodynamic processes in the NHB are reconstructed. Intra-oceanic subduction initiated offshore of the northwestern margin of the EB at ∼2.55 Ga or earlier. Partial melting of slab basalts occurred at ∼2542-2499 Ma, with the melts contaminated by mantle wedge materials forming TTGs. Meanwhile, the sub-arc lithospheric mantle was enriched by fluids and melts released from slab basalts and pelagic sediments, and partial melting of this moderately depleted mantle generated ∼2537-2503 Ma diorites and basalts. Following final accretion of the arc onto the continental margin of the EB, the slab rollback/breakoff and asthenospheric mantle upwelling triggered partial melting of the metasomatized lithospheric mantle and crustal anatexis, generating ∼2491 Ma quartz monzodioritic and monzogranitic rocks. Accordingly, the NHB records Neoarchean crustal growth linked to oceanic subduction and arc-continent accretion, and highlights the importance of resolving the nature of mantle sources and crust-mantle interactions in understanding Archean crustal growth and evolution.PostprintPeer reviewe

Neoarchean and Paleoproterozoic K-rich granites in the Phan Si Pan Complex, north Vietnam : constraints on the early crustal evolution of the Yangtze Block

This study was financially supported by projects from the China Natural Science Foundation (41672222) and State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Wuhan (MSFGPMR201802). PAC acknowledges support from Australian Research Council grant FL160100168. This study was also supported by the Ministry of Natural Resources and Environment of Vietnam, Project BĐKH.29/16-20 to Dung My Tran.Precambrian igneous and metamorphic rocks of the Phan Si Pan Complex, North Vietnam, constitute the southern extension of the Yangtze Block, and provide a valuable record of the early evolution of the continental crust. We present results of U-Pb zircon geochronology and geochemistry for Precambrian granites in this complex to constrain their emplacement age and genesis. Granites from three plutonic bodies yielded ages of 2848 ± 15 Ma, 2768 ± 19 Ma and 1869 ± 30 Ma, which represent newly-recognized late Archean to Paleoproterozoic potassic granite plutonism in the southern Yangtze Block. The average εHf(t) values range from −6.2 to 0.1 for the 2.85–2.77 Ga granitic rocks and -13.1 to -9.2 for the ca. 1.86 Ga granitic rocks, with two-stage model ages of 3.64 to 3.20 Ga and 3.31 to 3.07 Ga, respectively, suggesting derivation from partial melting of Paleoarchean and Mesoarchean crust. The late Archean potassic granites exhibit high K2O, and high Sr/Y and (La/Yb)N ratios with negligible Eu anomalies, indicating derivation from melting of the thickened lower crust, which is inferred to have occurred in an active margin setting. The late Paleoproterozoic alkali feldspar granites are characterized by high FeOT/(FeOT + MgO)(0.96–0.99) and 10000∗Ga/Al (2.75–2.94) ratios, showing an affinity of A-type granite. These A-type granites exhibit flat chondrite-normalized HREE patterns and strong negative Eu anomalies, and low Sr/Y and (La/Yb)N ratios, corresponding to melting at a shallow depth, probably in a post-collisional extension setting. Comparison of the rock units and events recorded by the Phan Si Pan complex with other Archean to Paleoproterozoic complexes (Houhe, Dongchuan, Yudongzi, Douling, Zhongxiang and Kongling complexes) in the Yangtze Block indicate spatially distinct histories of crustal growth, and thus may reflect independent terranes. The ca. 1.86 Ga post-collisional magmatism, which succeeds a 2.0–1.9 Ga metamorphic event, is distributed throughout the Yangtze Block, including the Phan Si Pan Complex, suggesting assembly of the disparate terranes and final cratonization of the Yangtze Block overlaps with, and may be related to, assembly of the Nuna supercontinent.PostprintPeer reviewe

Linking Tarim with North India in northern Gondwana due to final closure of the Proto-Tethys Ocean: Insights from provenance of early Paleozoic sedimentary rocks in the Altyn Tagh orogen

The 11th Symposium on Polar Science/Ordinary sessions: [OG] Polar Geosciences, Wed. 2 Dec

Proto-Adamastor ocean crust (920 Ma) described in Brasiliano Orogen from coetaneous zircon and tourmaline.

Proto-Adamastor ocean bathed Rodinia and successor continental fragments from 1.0e0.9 Ga up to 0.75 Ga, and evolved into world Adamastor Ocean at 0.75e0.60 Ga. Mesoproterozoic oceanic crust is poorly preserved on continents, only indirect evidence registered in Brasiliano Orogen. We report first evidence of ophiolite originated in proto-Adamastor. We use multi-technique U-Pb-Hf zircon and d11B tourmaline isotopic and elemental compositions. The host tourmalinite is enclosed in metaserpentinite, both belonging to the Bossoroca ophiolite. Zircon is 920 Ma-old, 3 Hf(920 Ma)??12, HfTDM ? 1.0 Ga and has ?oceanic? composition (e.g., U/Yb < 0.1). Tourmaline is dravite with d11B ? ?1.8& (Tur 1), 0& (Tur 2), 8.5& (Tur 3). These characteristics are a novel contribution to Rodinia and associated world ocean, because a fragment of proto-Adamastor oceanic crust and mantle evolved at the beginning of the Brasiliano Orogen

Evolução mineralógica da porção norte do ofiolito Bossoroca, terreno São Gabriel

O Ofiolito Bossoroca pertence ao Complexo Bossoroca e é limitado pelo Complexo Cambaí, ao oeste, e pela Formação Campestre, ao leste. A obducção deste corpo ocorreu durante o Neoproterozoico, de oeste para leste. Este evento causou o metamorfismo das rochas em fácies anfibolito inferior, predominantemente. A Formação Arroio Lajeadinho, que abrange as rochas ofiolíticas, é composta de serpentinito, anfibolito, tremolita xisto, formação ferrífera bandada, metachert e rochas metassomáticas. Com o objetivo de interpretar a evolução mineralógica do metamorfismo de rochas representativas do ofiolito, foram utilizadas análises químicas por microssonda eletrônica e geoquímica de rocha total. Hornblenda, turmalina e cromita são os principais minerais indicadores da evolução tectônica e pós-tectônica das rochas da região. Hornblenda é encontrada em anfibolito, Formação Arroio Lajeadinho, em duas fases metamórficas: M1 como tschermakita, e M2 como magnesiohornblenda. O mineral também é encontrado na rocha metavulcanoclástica, Formação Campestre, como magnesiohornblenda, com tendência indicando aumento de álcalis (site A) e Al, Fe e Ti (site C) entre a primeira e segunda geração. Outra ocorrência da magnesiohornblenda é no Diorito Capivaras, Complexo Cambaí, com diminuição de álcalis no site A e aumento de Al, Fe e Ti no site C entre a primeira e segunda geração. Turmalina é classificada como dravita e está relacionada ao cloritito de blackwall. Cromita ocorre em cromita-talco-magnesita granofels e possui textura corona de magnetita. Todas essas características dos minerais citados indicam duas fases de metamorfismo em fácies anfibolito inferior e a ocorrência de metassomatitos no ofiolito. Por meio de aerogeofísica, foi possível delimitar as unidades estratigráficas com características químicas e magnéticas significativamente distintas.The Bossoroca Ophiolite belongs to the Bossoroca Complex and is limited by the Cambaí Complex, to the west, and the Campestre Formation, to the east. The obduction of this body occurred during the Neoproterozoic, from west to east. This event caused the metamorphism of rocks in lower amphibolite facies, predominantly. The Arroio Lajeadinho Formation, which includes ophiolitic rocks, is composed of serpentinite, amphibolite, tremolite, banded iron formation, metachert and metasomatic rocks. To interpret the mineralogical evolution of the metamorphism of representative rocks of the ophiolite, chemical analyses by electronic microprobe and geochemistry of total rock were used. Hornblende, tourmaline and chromite are the main minerals that indicate the tectonic and post-tectonic evolution of the rocks. Hornblende is found in amphibolite, Arroio Lajeadinho Formation, in two metamorphic phases: M1 as tschermakite, and M2 as magnesiohornblende. The mineral is also found in the metavolcanoclastic rock, Campestre Formation, as magnesiohornblende, with a trend indicating an increase in alkalis (site A) and Al, Fe and Ti (site C) between the first and second generations. Another occurrence of magnesiohornblende is in Capivaras Diorite, Cambaí Complex, with a decrease in alkalis at site A and an increase in Al, Fe and Ti at site C between the first and second generations. Tourmaline is classified as dravite and is related to blackwall chloritite. Chromite occurs in chromite-talc-magnesite granofels and has a corona magnetite texture. All these characteristics indicate two phases of metamorphism in lower amphibolite facies and the occurrence of metasomatites in the ophiolite. Through aerogeophysics, it was possible to delimit stratigraphic units with significantly different chemical and magnetic characteristics

DETRITAL ZIRCON U-Pb GEOCHRONOLOGY OF THE MARICÁ GROUP: SOURCES AREAS AND MAXIMUM DEPOSITIONAL AGE: Geocronologia U-Pb de Zircão Detritíco do Grupo Maricá: Áreas Fontes e Idade Máxima de Deposição

The Maricá Group is the basal unit of the Camaquã Basin and comprises mainly fluvial deposits and shallow marine beds. Provenance studies of the Maricá Group are crucial to understanding the initial development of this basin. This paper aims to present new LA-ICP-MS U-Pb provenance data for the evaluation of the source areas and the maximum depositional age of the Maricá Group. This work includes fieldwork recognition, petrographic analysis, and U-Pb dating of one sandstone and two clasts samples, from the Arroio América Formation, the youngest unit from Maricá Group. The Arroio América Formation was deposited on a braided fluvial system and is composed of sandstones, conglomeratic sandstones, and conglomerates rocks. The sandstone and the sandstone clast display a similar zircon age range and pattern. They are derived from mixed sources (599 – 3025 Ma) with a major contribution of Syderian (2422 Ma), Ediacaran (621 Ma), and Statherian (1749 Ma) sources. Minor contributions from Ryacian (2212-2040 Ma) and Archean (2823-2503) are also present. The Ediacaran sources are related to Ediacaran granites from Bagé Region as the Santo Afonso Suite. The maximum depositional age of the sandstone was constrained at 599.8 ± 11.5 Ma and the sandstone clast at 619.4 ± 11 Ma. The similarity between the sandstone and the clast ages shows that portions of this basin and probably the Maricá Group itself served as a source of sediments, indicating a dynamic setting with deposition, burial, uplift, and rework in a short time.O Grupo Maricá é a unidade basal da Bacia do Camaquã e é composto principalmente por depósitos fluviais e marinhos rasos. Estudos de proveniência do Grupo Maricá são fundamentais para o entendimento das fases iniciais dessa bacia. Este trabalho objetiva apresentar novos dados de proveniência do LA-ICP-MS U-Pb para o reconhecimento das áreas fontes e determinação da idade máxima de deposição do Grupo Maricá. Este trabalho inclui reconhecimento de campo, análises petrográficas e a datação U-Pb de um arenito e dois clastos, da Formação Arroio América, a unidade mais jovem do Grupo Maricá. A Formação Arroio América registra depósitos de um sistema fluvial entrelaçado e é composta por arenitos, arenitos conglomeráticos e conglomerados. Dados de proveniniência de uma amostra de arenito e de um clasto de arenito exibem intervalos de idades e padrões de distribuição semelhantes. São, ambos derivados de fontes mistas (599 - 3025 Ma), com grande contribuição de fontes Siderianas (2422 Ma), Ediacaranas (621 Ma) e Estaterianas (1749 Ma). Contribuições menores Riacianas (2212-2040 Ma) e Arqueanas (2823-2503) também estão presentes. As fontes ediacaranas estão relacionadas aos granitos ediacaranos da região de Bagé como a suíte Santo Afonso. A idade máxima de deposição do arenito é de 599,8 ± 11,5 Ma e do clasto de arenito de 619,4 ± 11 Ma. A semelhança de idades máximas e áreas fontes indicam que porções dessa bacia e provavelmente do próprio Grupo Maricá serviram como fonte de sedimentos, indicando um contexto dinâmico com deposição, soterramento, soerguimento e retrabalhamento em um curto espaço de tempo