Paleoproterozoic Components Research Articles

Petrology and geochemistry of Late Triassic rapakivi-type granites in the Dupangling area, South China: Role of fluid-rock interaction in the origin of rapakivi texture

Rapakivi-type granites not only document magmatic processes but also archive subsolidus interactions between granitic rock and fluid, and thereby provide crucial information about fluid infiltration in the Earth's crust. However, the key process to generate rapakivi texture is still debated. Here, we report comprehensive mineralogical, geochemical and Hf isotopic compositions of the Dupangling rapakivi granites (210.4–214.9 Ma) in the South China Block to investigate the effect of fluid-rock interaction on the origin of rapakivi feldspar. Dupangling rapakivi granites have high SiO2 (72.63–78.21 wt%) and alkali contents (Na2O + K2O = 6.14–9.06 wt%) with high K2O/Na2O ratios (1.37–2.64), characteristic of weakly peraluminous (A/CNK = 1.04–1.10) high-K calc-alkaline granites. Samples have weakly fractionated REE patterns with negative Eu anomalies (δEu = 0.05–0.38). They have negative whole-rock εNd(t) (−8.45 to −7.91), variable zircon εHf(t) (−9.7 to −1.0) and two-stage Hf isotope model ages (1.32–1.84 Ga), due to their derivation from a major early Mesoproterozoic crustal source with a subordinate late Paleoproterozoic component. Rapakivi feldspars in Dupangling granites are composed of an alkali-feldspar core and a plagioclase mantle. Fractures, pores and secondary inclusions (biotite, sericite, epidote and hematite) are closely associated with the mantles and oligoclase patches within alkali-feldspar cores. These pervasive microtextural features and variations of Be, Rb, Ba, Sr and LREE within rapakivi feldspars are interpreted to have formed through subsolidus fluid-induced dissolution-reprecipitation replacement processes. This study highlights that fluid-rock interaction plays an important role in generating rapakivi texture.

Evolution of deep crustal hot zones constrained by the diversity of Late Mesozoic magmatic rocks in SE China

The formation of widespread Late Mesozoic magmatic rocks (dominantly felsic rocks) in SE China needs a physical and chemical balance between crustal and mantle melting, which could be reconciled by complicated transcrustal magmatic systems in deep crustal hot zones with time. After the relative magmatic quiescence at ca. 210–180 Ma, magmatic activities were mainly developed in the interior of SE China with an areal distribution and subsequently concentrated in the present coastal area with an overall linear pattern after ca. 140 Ma except the Lower Yangtze region. The spatiotemporal change requires a unique crust-mantle geodynamic coupling with the evolution of deep crustal hot zones from intracontinental to continent marginal setting. Correspondingly, the magmatic rocks show a compositional change from bimodal distribution in Early and Middle Jurassic to unimodal and weakly bimodal pattern in Late Jurassic and Early Cretaceous. In general, most of the Late Mesozoic intermediate to felsic magmatic rocks show higher Mg# than typical rifting-related rocks at given SiO2, which requires the addition of water and possible mass transfer and mixing of underplated hydrous basaltic magma in the deep crustal hot zones. The occurrence of high-Sr/Y and La/Yb rocks and possibly thickened crust (ca. 40 km) before ca. 140 Ma, combined with the current thin crust thickness (ca. 31-33 km) as revealed by seismic data, indicates a widespread crustal thinning after ca. 140 Ma. The relatively thickened crust before ca. 140 Ma is beneficial for melting of underlying old crust that is composed of dominantly Neoproterozoic and Paleoproterozoic components, the formation of porphyritic Cu deposits, the subsequent magmatic differentiation and associated giant W, Sn, Nb, Ta and Li ore deposits. In contrast, the deep crustal hot zones progressively migrated to the present coastal area after ca. 140 Ma, driving the increasing amounts of intermediate rocks and intermediate-felsic volcanic rocks. In general, the diversity and compositional change of Late Mesozoic magmatic rocks and associated ore deposits in SE China may be controlled by the evolution of deep crustal hot zones in different tectonic settings in response to the subduction of Paleo-Pacific Plate.

Mesozoic magmatism in the eastern North China Craton: Insights on tectonic cycles associated with progressive craton destruction

The North China Craton (NCC) provides a classic example for lithospheric thinning associated with craton destruction (or decratonization) through multiple tectonic events. The peak of craton destruction is considered to have occurred during Mesozoic, especially in Early Cretaceous, linked to the subduction/collision between the NCC and Yangtze Craton during the Triassic in the south and the Paleo-Pacific plate subduction during Early Cretaceous in the east. Here we investigate a suite of intrusive and volcanic suites including granite, gabbro, diorite, basalt, andesite, volcanic tuff, agglomerate, siliceous tuff and trachyte from the southern Yishui and Juxian domains to gain insights into the Mesozoic tectonic evolution of the eastern NCC in relation to decratonization. Zircon U-Pb data from basalt show multiple populations with xenocrystic grains ranging in age from Paleoproterozoic (2457–1940 Ma), through Neoproterozoic (896–559 Ma), to Paleozoic (442–380 Ma) and Mesozoic (204–122 Ma). Inherited grains from the granite and gabbro yield weight mean 206Pb/238U ages of 650.0 ± 13 to 773.4 ± 2.6 Ma suggesting the involvement of components from the Yangtze Craton. Magmatic zircons from diorite show weight mean 206Pb/238U ages of 121.5 ± 1.2 Ma representing the emplacement age of this rock. Magmatic zircon grains from the volcanic suite (andesite, volcanic tuff, agglomerate, siliceous tuff and trachyte), show weighted mean 206Pb/238U ages of 132.4 ± 1.4 Ma, 123.02 ± 0.71 Ma, 122.97 ± 0.69 Ma, 121.96 ± 0.49 Ma, and 124.6 ± 1.1 Ma respectively, corresponding to the timings of their eruption. The Paleoproterozoic zircon grains correspond to xenocrysts captured from the NCC basement rocks, whereas the Neoproterozoic and Paleozoic grains were inherited from the Yangtze Craton. The Late Triassic to Jurassic zircon grains can be correlated to delamination of the thickened continental crust during deep subduction of Yangtze continental crust and the continent-continent collision between NCC and the Yangtze Craton. The Early Cretaceous population can be linked to the subduction of the Paleo-Pacific plate from the east and related extensional setting. The geochemical data display significant enrichment in LREE along with negative Eu anomalies, with high Th/Ta and La/Nb ratios, negative Nb–Ta anomalies, and high concentration of LILE (K, Rb, Ba and Pb) than HFSE (Nb, Ta and Ti), suggesting subduction-related arc magmatic setting in an active continental margin. Zircon Lu-Hf data from these rocks display dominantly negative εHf(t) values ranging from −36.8 to −3.1, TDM in the range of 1105–2572 Ma, and TDMC between 1785 Ma and 3776 Ma, indicating that the magma sources involved Paleoproterozoic components with minor input of reworked Archean crust through different degrees of enrichment with crustal material and minor input from extension-related mantle melting. In conjunction with the results from previous studies in the eastern NCC, we propose that the late Paleozoic southward subduction of the Paleo-Asian Ocean, the Triassic northward subduction and collision of the Yangtze Craton with the NCC, and the Early Cretaceous westward subduction of the Paleo-Pacific plate jointly contributed to the cratonic destruction of NCC. The Paleo-Pacific plate subduction beneath the NCC (major contributing mechanism) with the slab rollback induced lithospheric extension likely facilitated back-arc spreading, asthenospheric upwelling resulting in lithospheric thinning including delamination, melting of lithosphere mantle, and underplating of the modified mantle beneath the lower crust of the NCC during the Early Cretaceous.

Neoproterozoic tectonic transition in the South Qinling Belt: New constraints from geochemistry and zircon U–Pb–Hf isotopes of diorites from the Douling Complex

The Douling Complex, which represents the oldest rock unit found in the South Qinling Belt (SQB), was intruded by voluminous Neoproterozoic magmatic rocks. These Neoproterozoic magmatic rocks can provide important constraints on the tectonic evolution and crust growth of the SQB, but are still poorly studied. In this contribution, we present geochemical, in-situ zircon U–Pb–Hf isotope and trace element data of Neoproterozoic granodiorite and quartz diorite from three (i.e., Gangou, Sanpinggou and Fenzishan) plutons of the Douling Complex. Zircon U-Pb isotopic dating shows that these granodiorite and quartz diorite have ages in a small range of 693–719 Ma and zircon εHf(t) values from −12.3 to 12.6. The granodiorite and quartz diorite samples have intermediate SiO2 (55.05–66.50 wt%), K2O (1.23–3.00 wt%) and high Mg-number (Mg# = 46–67). They display enrichment in large ion lithophile elements (LILEs) and depletion in high-field strength elements (HFSEs), indicating that their mantle source was modified by subducted materials. Our new data, integrated with previous results, indicate that the Neoproterozoic magmatism in the SQB had three major episodes at ca. 630, 750 and 860 Ma, respectively. Zircon Hf isotopic data of these rocks reveal that the magmatic sources involved both Meso- to Neoproterozoic juvenile crust and reworked Neoarchean to Paleoproterozoic components. An abrupt increase in the contribution of juvenile crust and depleted mantle relative to old crust occurred in 750 to 630 Ma. In combination with the occurrence of synchronous A-type granite in the region, this geochemical shift indicates the tectonic shift from the convergence to divergence in the SQB area.

Paleoproterozoic granitoids of the Losevo terrane, East European Craton: Age, magma source and tectonic implications

The Losevo terrane lies between the Sarmatia and Volgo-Uralia segments of the East European Craton and is considered to be part of the East-Sarmatian Orogen (ESO). Here we report the results from field, petrologic, geochemical, Sm–Nd isotopic and zircon U–Pb geochronological studies on a suite of granitoids from the Losevo terrane (LT). The LT granitoids are divided into four types: (1) migmatite leucosome (from quartz diorite to granodiorite); (2) tonalite, trondhjemite and granodiorite (TTG) with high Si, Na, Sr contents, high Sr/Y, La/Yb ratios and low Mg, K, Y, Yb contents; (3) trondhjemite, granodiorite, granite (TGG) spatially related to the TTG suite, but showing higher K, negative Eu anomalies and flat REE patterns; and (4) high-K calc-alkaline monzogranite and granodiorite of I-type. The early (2115Ma) migmatites occur as lensoid and stromatic layers. The TTG and TGG (2100–2075Ma) constitute about 20% of the LT and form batholiths (150–540km2) and stocks. Small massifs of the I-type granites (2081±12Ma) intrude the migmatites. The migmatites, TTG and TGG contain inherited zircon cores that yield ages between 2130 and 2172Ma. The granitic suite shows positive εNd values ranging from +2.1 to +5.7 for the TTG and TGG and +2.1 and +2.2 for the migmatites, suggesting magma derivation from juvenile Paleoproterozoic sources (model ages=2255–2450Ma). In contrast, the I-type granite shows slightly negative εNd value of −0.5, suggesting that the magma sources involved a mixture of Archean and Paleoproterozoic components. All the granitoids from LT record varying degrees of fractional crystallization with or without crustal contamination. The temperature of parental melts of the granites are estimated to be in the range of 793±33°C to 878±16°C. The migmatites were derived mainly from metaigneous-sedimentary protoliths under P∼5–6kbar; whereas the TTGs were derived from lower crustal metabasites under P∼15kbar. The TGG were derived from mid crustal metagreywackes under P<10kbar, and the I-type granites were mainly derived from ancient lower crustal components involving amphibolites, gneisses and metagreywackes under P⩾12kbar. Our new data suggest that the anatexis and formation of migmatites occurred during the peak collisional event, and the other members of the granitoid suite including the TTGs formed during the post-collisional orogenic collapse of the ESO. The estimated average exhumation rate of the granitic suite during collision to post-collisional collapse is about 235m/Ma.

Contrasting SHRIMP U–Pb zircon ages of two carbonatite complexes from the peri-cratonic terranes of the Reguibat Shield: Implications for the lateral extension of the West African Craton

The Oulad Dlim Massif of the Western Reguibat Shield contains several carbonatite complexes of previously unknown age. The largest and best studied are Gleibat Lafhouda, composed of magnesiocarbonatites, and Twihinate, composed of calciocarbonatites. Gleibat Lafhouda is hosted by Archean gneisses and schists. It has a SHRIMP U–Th–Pb zircon crystallization age of 1.85±0.03Ga, a Nd model age of TCR=1.89±0.03Ga, and a Sm–Nd age of 1.85±0.39Ga. It forms part of the West Reguibat Alkaline province. Twihinate, on the other hand, is much younger. It is hosted by Late Silurian to Early Devonian deformed granites and has a zircon crystallization age of 104±4Ma, which is within error of the age of the carbonatites of the famous Richat Structure in the southwest Reguibat Shield. Like these, the Twihinate carbonatites are part of the Mid-Cretaceous Peri-Atlantic Alkaline Pulse. The Twihinate carbonatites contain abundant inherited zircons with ages that peak at ca. 420Ma, 620Ma, 2050Ma, 2466Ma, and 2830Ma. This indicates that their substratum has West African rather than, as previously suggested, Avalonian affinities. It has, however, a Paleoproterozoic component that is not found in the neighboring western Reguibat Shield. The 421Ma to 410Ma gneissic granites hosting Twihinate are epidote+biotite+Ca-rich garnet deformed I-type to A-type granites derived from magmas of deep origin compatible, therefore, with being generated in a subduction environment. These granites form a body of unknown dimensions and petrogenesis, the study of which will be of key importance for understanding the geology and crustal architecture of this region.

Paleoproterozoic crustal growth in the North China Craton: Evidence from the Lüliang Complex

The building of continental blocks and their assembly into supercontinents are linked to episodic crustal growth through Earth history, where both vertical and horizontal accretions in convergent plate margins play a significant role. Here we investigate the Paleoproterozoic crustal growth in the North China Craton (NCC) from a traverse covering continent-trench-ocean section in the Lüliang Complex within the western margin of the Trans-North China Orogen, a major accretionary belt that sutures the Eastern and Western Blocks in the NCC. We present petrological, geochemical and zircon U–Pb and Lu–Hf data from a suite of samples including meta granitoids and gabbroic diorite that built the continental arc, and metabasalts, felsic tuff, mafic and pelitic schists, and garnet amphibolites that form part of an imbricated package of oceanic and continental material that was accreted onto the eastern margin of the Western Block in the NCC. The salient geochemical features of the magmatic suite such as high Th/Ta and La/Nb ratios, negative Nb–Ta anomalies, and high concentration of LILE as compared to HFSE suggest magma derivation from sub-continental mantle source which has undergone different degree of enrichment with crustal components or material derived from subduction-related older components. In tectonic discrimination diagrams, the granitoids show VAG affinity and all the rocks straddle the field between IAB and MORB reflecting subduction-related arc signature. The majority of zircon grains in all the rock types from the Lüliang Complex show typical magmatic crystallization textures and high Th/U values. Zircons from the meta-granitoids and diorite enclave yield upper intercept or 207Pb/206Pb mean ages in the range of 2250±20 to 2084±69Ma. Those in the mafic schist show a mean age of 2234±9Ma. In the felsic schist group, the mean ages are in the range of 2298±15 to 1933±11Ma. A minor older population of zircons with ages of 2992±28Ma and 2610±80Ma also occurs in these rocks. In the metabasalt, zircon 207Pb/206Pb mean ages are between 2391±40 and 2022±39Ma with rare magmatic xenocrysts showing Mesoarchean age of 3294±21Ma. Zircons in the felsic tuff show mean ages of 2155±25 to 1897±24Ma, with xenocryst populations yielding mean ages of 2528±31Ma and 2710±20Ma. The youngest zircons with 207Pb/206Pb mean age of ca. 1.9Ga from the felsic schist (metasediment) and felsic tuff (volcanic suite) suggest deposition within the trench of an active convergent margin at this time. In the garnet-bearing amphibolite, the zircons yield a limited age range of 1996±17Ma to 1984±12Ma. The Lu–Hf data on zircons from all samples show dominantly positive ɛHf(t) values. In the granitoid–diorite suite, the ɛHf(t) values range up to 7.6 with crustal residence ages (TDMC) of 2371–2729Ma. Zircons in the mafic and felsic schists also possess positive ɛHf(t) values (up to 9.6) with TDMC in the range of 2366–2798Ma. In the metabasalt, both positive and negative ɛHf(t) values are displayed (3.8 to −3.2) with TDMC in the range of 2495–2830Ma. Zircons in the felsic tuff also show prominent positive ɛHf(t) value (up to 7.4) and crustal residence ages range from 2274 to 2607Ma. In the garnet-bearing amphibolite, the ɛHf(t) values lie between 3.3 and 4.8 and TDMC are in the range of 2321–2413Ma. The majority of ɛHf(t) values from all the rock types in the present study are compatible with juvenile magma sources, with limited input from crustal components. Their TDMC values indicate that both Neoarchean and Paleoproterozoic components were involved in the source, attesting to a continental arc setting. The Hf data also suggest the possible presence of Mesoarchean vestiges among the dominantly Neoarchean–Paleoproterozoic basement. Our data demonstrate active plate tectonics and major crustal growth during the Paleoproterozoic history of the NCC, broadly coeval with the making of the global supercontinent Columbia, and provide insights into continent building through subduction–accretion–collision tectonics analogous to those in Phanerozoic orogenic belts.

Early to late Neoproterozoic magmatism and magma mixing–mingling in Sri Lanka: Implications for convergent margin processes during Gondwana assembly

The Sri Lankan fragment of Gondwana preserves the records of Neoproterozoic tectonothermal events associated with the final assembly of the supercontinent. Here we investigate a suite of magmatic rocks from the Wanni, Kadugannawa and Highland Complexes through geological, petrological, geochemical and zircon U–Pb and Lu–Hf isotopic techniques. The hornblende biotite gneiss, charnockites, metagabbro and metadiorites investigated in this study show geochemical features consistent with calc-alkaline affinity and subduction-related signature including LILE enrichment relative to HFSE coupled with distinct Nb–Ta depletion and weak negative Zr–Hf anomalies. The felsic suite falls in the volcanic arc granites (VAGs) field and the mafic suite shows island arc basalt affinity in tectonic discrimination plots, suggesting that the protoliths of the rocks were derived from arc-related magmas in a convergent margin setting. LA-ICPMS zircon U–Pb analyses show crystallization of charnockite and dioritic mafic magmatic enclave from the Highland Complex during ca. 565 and 576Ma corresponding to bimodal magmatism. The diorite also contains metamorphic zircons of ca. 525Ma. Hornblende–biotite gneiss from the Kadugannawa Complex shows protolith emplacement age at 973–980Ma, followed by new zircon growth during repeated thermal events through late Neoproterozoic. The dioritic enclaves in these rocks are much younger, and form part of a deformed and metamorphosed dyke suite with emplacement ages of 559Ma, broadly coeval with the bimodal magmatism in the Highland Complex at that time. The youngest group of zircons in this rock shows ages of 508Ma, corresponding to the latest thermal event. A charnockite from this locality shows oldest group of zircons at 962Ma, corresponding to emplacement age similar to that of the magmatic protolith of the hornblende biotite gneiss. This rock also shows zircon growth during repeated thermal events at 832Ma, 780Ma, 721Ma and 661–605Ma. The lower intercept age of 543Ma marks the timing of collisional metamorphism. Charnockite from the Wanni Complex shows emplacement age at 1000Ma, followed by thermal event at 570Ma, the latter correlating with the bimodal magmatic event in the Highland Complex. The dioritic enclave within this charnockite shows an age of ca. 980Ma, suggesting intrusion of mafic magma into the felsic magma chamber. Zircons in the diorite also record multiple zircon events during 950 to 750Ma. Zircons in the Highland Complex charnockite possess negative εHf(t) values in the range −6.7 to −12.6 with TDMC of 2039–2306Ma suggesting magma derivation through melting of Paleoproterozoic source. In contrast, the εHf(t) range of −11.1 to 1.6 suggests a mixed source of both of older crustal and juvenile material. The εHf(t) values of −4.5 to 4.5 and TDMC of 1546–1962Ma for the hornblende biotite gneiss also shows magma derivation from mixed sources that included Paleoproterozoic components. The younger dioritic intrusive, however, has a more juvenile magma source as indicated by the mean εHf(t) value of 1.3. The associated charnockite shows a tight positive cluster of εHf(t) from 0.6 to 5.1, suggesting juvenile input. Charnockite from the Wanni Complex shows clearly positive εHf(t) values of up to 13.1, and TDMC in the range 937–1458Ma suggesting much younger and depleted mantle source. The diorite enclave also has positive εHf(t) values with an average value of 8.5 and TDMC in the range of 709–1443Ma clearly suggesting younger juvenile sources. The early and late Neoproterozoic bimodal suites are correlated to convergent margin magmatism associated with the assembly of Sri Lanka within the Gondwana supercontinent.

Zircon U–Pb and Hf isotopic studies of the Xingxingxia Complex from Eastern Tianshan (NW China): Significance to the reconstruction and tectonics of the southern Central Asian Orogenic Belt

The Chinese Tianshan occupies the southernmost part of the Central Asian Orogenic Belt (CAOB). High-grade metamorphic rocks are widely distributed in its central uplift zones, composing the crustal basement of the Central Tianshan Block of the Chinese Tianshan. However, the origin of the crustal basement and whether the high-grade metamorphism was associated with the Paleozoic orogeny are still unresolved. Here, we present precise LA-ICP-(MC)-MS zircon U–Pb dating and Hf isotopic data for three meta-sedimentary rocks and one orthogneiss from the Xingxingxia Complex, which represents the Precambrian basement of the Central Tianshan Block. Zircon U–Pb dating results show that the protolith age of the orthogneiss is ca. 880Ma and that the meta-sedimentary rocks consist dominantly of 0.8–1.0Ga and 1.3–2.0Ga materials with minor early Paleoproterozoic components. In combination with the zircon Hf isotopic data, it is suggested that the crustal basement of the Central Tianshan Block was formed in the early Mesoproterozoic, which is evidently different from the Archean basement formation of the Tarim Craton. We suggest that the Central Tianshan Block may have formed through a magmatic arc accretionary orogen along the continental margin of Baltica during the Mesoproterozoic. Moreover, both the meta-sedimentary rocks and the orthogneiss of the Xingxingxia Complex simultaneously suffered a metamorphic overprint at ca. 380Ma. Zircon REE and Hf isotopic data indicate that the metamorphic zircon rims were formed by new zircon overgrowths in partial melts. Geothermobarometry and average P–T calculations using THERMOCALC on the Grt–Bt gneiss yielded ca. 720–730°C and 4–6kbar, suggesting amphibolite- to granulite-facies conditions. The Late Devonian metamorphism of the Xingxingxia Complex is possibly related to orogenic low-pressure/high-temperature metamorphism in the middle or upper crust during the closure of the eastern segment of South Tianshan Ocean. Our new data, together with the occurrences of Ordovician–Silurian eclogites and HP mafic granulites along the northeastern margin of the Tarim Craton, indicate that progressive continental collisions occurred in the southern CAOB during the Paleozoic.

Geochemical characterization and isotopic age of Caradocian magmatism in the northeastern Iberian Peninsula: Insights into the Late Ordovician evolution of the northern Gondwana margin

This paper presents new geochronological, geochemical and isotopic data for Late Ordovician magmatism (455 ± 2 Ma) from the Variscan massifs in the northeastern Iberian Peninsula. This magmatism is mainly composed of aluminous and metaluminous plutonic rocks and a sequence of calc-alkaline ignimbrites and epiclastic volcanic rocks, which were deformed and metamorphosed during the Variscan orogeny. The metaplutonic rocks are located in the lower part of a Neoproterozoic/Early Cambrian metasedimentary sequence in the Canigó massif (Eastern Pyrenees). Aluminous orthogneisses have isotopic signatures that indicate a crustal origin, whereas metaluminous orthogneisses originate from a mixture of juvenile and crustal melts. The metavolcanic rocks define a calc-alkaline ignimbritic sequence of crustal origin located in the lower part of the Late Ordovician sequence in the Les Gavarres massif (Catalan Coastal Ranges). T DM ages for all the studied Late Ordovician rocks range from Mesoproterozoic to Paleoproterozoic (1.3 Ga, 1.5 Ga, 1.7 Ga and 2.2 Ga), suggesting a Neoproterozoic crust with Paleoproterozoic components and an arc signature, as the most probable source for the crustal melts. The isotopic similarities between the studied samples and other Early Ordovician magmatic rocks of the Central and Western Iberian Massif suggest repeated melt extractions from a common basement as the source for the Early and Late Ordovician magmatism in the Iberian realm. However, the studied massifs are more closely linked to other massifs from the Mediterranean realm. The most probable tectonic setting for the emplacement of the Late Ordovician volcanic and plutonic bodies is that of an extensional regime, which postdates a previous Ordovician contractional event and the opening of the Rheic Ocean.

In situ isotopic analyses of U and Pb in zircon by remotely operated SHRIMP II, and Hf by LA-ICP-MS: an example of dating and genetic evolution of zircon by 176Hf/177Hf from the Ita quarry in the Atuba Complex, SE Brazil

Operar o espectrômetro de massa SHRIMP à distância, remotamente via Internet, é sem dúvida uma técnica alternativa de grande interesse para a datação de cristais de zircão. Embora não represente avanço no método geocronológico U-Pb em zircão, a técnica de operação remota traz, além de facilidade, grande economia nesse tipo de análise. Foi executada pela primeira vez em análises espectrométicas envolvendo dois laboratoratórios internacionais (São Paulo, Brasil - Beijing, China) possibilitando a obtenção de resultados em tempo real. O procedimento foi aplicado em três amostras de rochas gnáissico-migmatíticas da pedreira Ita (próxima à cidade de Curitiba - Paraná - Brasil) pertencentes ao Complexo Atuba. Tais rochas, quando analisadas através do método U-Pb (TIMS) demonstraram uma evolução complexa, com idades bastante imprecisas. A presença de importantes heranças arqueanas e paleoproterozoicas, neste complexo, foram confirmadas nas zonas internas de cristais de zircão obtidas em leucossomas neoproterozoicos. Análises adicionais realizadas em rochas dioríticas indicaram ser intrusivas, e não encaixantes, e apresentaram idades relacionadas às colisões continentais (0.6 Ga) envolvidas durante a assembléia de Gondwana, no Neoproterozoico. A determinação das razões de isótopos de Hf em cristais de zircão, por meio de LA-ICP-MS, representa uma nova opção para checar a importância relativa da contribuição de material do manto (&#949;Hf >; 0) e crosta (&#949;Hf < 0). No Complexo Atuba, o componente arqueano sugere derivação de material oriundo do manto (&#949;Hf +1,5 a +8,7), enquanto o componente paleoproterozoico indica contribuição crustal (&#949;Hf -9,1 a -10,1).

Age, provenance and tectonic setting of the Canastra and Ibiá Groups (Brasília Belt, Brazil): Implications for the age of a Neoproterozoic glacial event in central Brazil

Abstract The Brasilia Belt is one of the best preserved Neoproterozoic orogens in Brazil. It comprises a thick Meso–Neoproterozoic sedimentary/metasedimentary pile including the Canastra and Ibia Groups, which are the object of this study. The Canastra Group constitutes a regressive sedimentary sequence made mainly of greenschist-facies metapelitic and metapsammitic rocks, including phyllite, sandy metarhythmite and quartzite, with minor intercalations of limestone, as well as carbonaceous and carbonatic phyllite. The Ibia Group is formed of a basal diamictite followed upwards by phyllites and calc-schists. It rests on an erosional unconformity on top of the Canastra Group. A provenance study based on U–Pb zircon geochronology on a selection of seven samples helped to establish the various source areas and maximum depositional ages of the original sediments. In addition, seven new Sm–Nd analyses are presented and discussed together with previously published data. LAM-ICP-MS U–Pb dating of detrital zircon grains indicates a maximum depositional age of the Canastra and Ibia Groups of ca. 1030 and 640 Ma, respectively. The provenance signature of the Canastra Group comprises a wide range of detrital zircon ages with a significant Paleoproterozoic component (∼1.8 and ∼2.1 Ga) and an important Mesoproterozoic source (1.1–1.2 Ga), especially for the Paracatu Formation, indicating the Sao Francisco–Congo Craton as main source. These provenance data, in particular the absence of Neoproterozoic zircon grains, typical of the active margin of the Brasilia Belt, allied with the homogeneous Paleoproterozoic TDM values are consistent with the previous interpretation that the Canastra Group represents a sedimentary sequence deposited on a passive margin setting. Zircon grains from the diamictite of the Ibia Group yielded ages ranging from 936 to 2500 Ma. In contrast, the overlying calc-phyllite of the Rio Verde Formation reveals a dominant Neoproterozoic provenance pattern with important peaks at 665, 740 and 850 Ma. The Sao Francisco-Congo Craton and Goias Magmatic Arc are, most probably, the two main source regions for the Ibia Group which may represent, therefore, a former fore- or back-arc sedimentary sequence. Tectonically, therefore, the Ibia Group is equivalent to the Araxa Group exposed in central Goias and both represent syn-orogenic sedimentary sequences formed with important detrital contributions derived from the Neoproterozoic Goias Arc. The provenance data presented here indicate that the Cubatao Formation is most possibly representative of a Marinoan or younger glacial event.

Geochronological constraints on the evolution of high-pressure felsic granulites from an integrated electron microprobe and ID-TIMS geochemical study

A combined geochronological, geochemical, and Nd isotopic study of felsic high-pressure granulites from the Snowbird Tectonic Zone, northern Saskatchewan, Canada, has been carried out through the application of integrated electron microprobe and isotope dilution thermal ionization mass spectrometry (ID-TIMS) techniques. The terrane investigated is a 400 km 2 domain of garnet–kyanite–K–feldspar-bearing quartzofeldspathic gneisses. Monazite in these granulites preserves a complex growth history from 2.6 to 1.9 Ga, with well-armored, high Y and Th grains included in garnet yielding the oldest U–Pb dates at 2.62 to 2.59 Ga. In contrast, matrix grains and inclusions in garnet rims that are not well-armored are depleted in Y and Th, and display more complicated U–Pb systematics with multiple age domains ranging from 2.5 to 2.0 Ga. 1.9 Ga monazite occurs exclusively as matrix grains. Zircon is typically younger (2.58 to 2.55 Ga) than the oldest monazite. Sm–Nd isotope analysis of single monazite grains and whole rock samples indicate that inclusions of Archean monazite in garnet are similar in isotopic composition to the whole rock signature with a limited range of slightly negative initial ɛ Nd. In contrast, grains that contain a Paleoproterozoic component show more positive initial ɛ Nd, most simply interpreted as reflecting derivation from a source involving consumption of garnet and general depletion of HREE's. Our preferred interpretation is that the oldest monazite dates record igneous crystallization of the protolith. The ca. 2.55 Ga dates in zircon and monazite record an extensive melting event during which garnet and ternary feldspar formed. Very high-pressure (> 1.5 GPa) metamorphism during the Paleoproterozoic at 1.9 Ga produced kyanite from garnet breakdown, and resulted in limited growth of new monazite and zircon. In the case of monazite, this is likely due to the armoring and sequestration of early-formed monazite such that it could not participate in metamorphic reactions during the high-pressure event, as well as the depletion of the REE's due to melt loss following the early melting event.

Lu–Hf and U–Pb isotope systematics of zircons from the Storgangen intrusion, Rogaland Intrusive Complex, SW Norway: implications for the composition and evolution of Precambrian lower crust in the Baltic Shield

The Storgangen orebody is a concordantly layered, sill-like body of ilmenite-rich norite, intruding anorthosites of the Rogaland Intrusive Complex (RIC), SW Norway. 17 zircon grains were separated from ca. 5 kg of sand-size flotation waste collected from the on-site repository from ilmenite mining. These zircons were analysed for major and trace elements by electron microprobe, and for U–Pb and Lu–Hf isotopes by laser ablation microprobe plasma source mass spectrometry. Eight of the zircons define a well-constrained (MSWD=0.37) concordant population with an age of 949±7 Ma, which is significantly older than the 920–930 Ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the RIC. The remaining zircons, interpreted as inherited grains, show a range of 207Pb/ 206Pb ages up to 1407±14 Ma, with an upper intercept age at ca. 1520 Ma. The concordant zircons have similar trace element patterns, and a mean initial Hf isotope composition of 176Hf/ 177Hf 949 Ma=0.28223±5 ( ε Hf=+2±2). This is similar to the Hf-isotope composition of zircons in a range of post-tectonic Sveconorwegian granites from South Norway, and slightly more radiogenic than expected for mid-Proterozoic juvenile crust. The older, inherited zircons show Lu–Hf crustal residence ages in the range 1.85–2.04 Ga. One (undated) zircon plots well within the field of Hf isotope evolution of Paleoproterozoic rocks of the Baltic Shield. These findings indicate the presence of Paleoproterozoic components in the deep crust of the Rogaland area, but do not demonstrate that such rocks, or a Sveconorwegian mantle-derived component, contributed significantly to the petrogenesis of the RIC. If the parent magma was derived from a homogeneous, lower crustal mafic granulite source, the lower crustal protolith must be at least 1.5 Ga old, and it must have an elevated Rb/Sr ratio. This component would be indistinguishable in Sr, Nd and Hf isotopes from some intermediate mixtures between Sveconorwegian mantle and Paleoprotoerzoic felsic crust, but it cannot account for the initial 143Nd/ 144Nd of the most primitive, late Sveconorwegian granite in the region, without the addition of mantle-derived material.

Lead isotopic evidence for the origin of Paleo- and Mesoproterozoic rocks of the Colorado Province, U.S.A.

Lead isotopic ratios of K-feldspars and whole-rocks from 1.7- and 1.4-Ga plutonic rocks of the Colorado Province are relatively non-radiogenic for 207 Pb 204 Pb , plotting below the average crust model curve of Stacey and Kramers (1975), indicating that the terrane was derived primarily from juvenile, mantle material. Slightly more radiogenic ratios in the northern part of the terrane, near the Archean Wyoming Province, suggest minor inclusion of an older component. The data from 1.7-Ga plutons plot in a broad field suggesting two episodes of re-equilibration with whole-rock Pb, probably related to heating events in the Mesoproterozoic (1.4 Ga) and Cretaceous (70 Ma). Possible differences in calculated whole-rock Th U , coupled with slight Pb isotopic variations, along the north-south transect suggest either a terrane boundary through central Colorado (near Salida and Gunnison), or fundamental differences in source rocks (metasedimentary vs. metavolcanic). UPb analyses of multigrain splits of detrital zircons from quartzites throughout the Colorado Province have failed to identify Archean detritus. The oldest 207Pb 206 Pb ages found (in two samples of quartzite from northern Colorado) are about 2.0 Ga (perhaps derived from rocks of the Trans-Hudson orogen), in contrast to 2.75-Ga detrital zircon in a Paleoproterozoic quartzite from the southern part of the Wyoming Province. While we are not yet able to discern if these ages are true provenance ages or mixtures of Archean and Paleoproterozoic components, the absence of easily recognizable Archean zircons supports other isotopic data and a conclusion that most of the Paleoproterozoic crust of the Colorado Province was ultimately derived from a juvenile (at 1.8 Ga) mantle reservoir.

Vol. 27, nos. 1–8 author index

The Chinese Tianshan occupies the southernmost part of the Central Asian Orogenic Belt (CAOB). High-grade metamorphic rocks are widely distributed in its central uplift zones, composing the crustal basement of the Central Tianshan Block of the Chinese Tianshan. However, the origin of the crustal basement and whether the high-grade metamorphism was associated with the Paleozoic orogeny are still unresolved. Here, we present precise LA-ICP-(MC)-MS zircon U–Pb dating and Hf isotopic data for three meta-sedimentary rocks and one orthogneiss from the Xingxingxia Complex, which represents the Precambrian basement of the Central Tianshan Block. Zircon U–Pb dating results show that the protolith age of the orthogneiss is ca. 880 Ma and that the meta-sedimentary rocks consist dominantly of 0.8–1.0 Ga and 1.3–2.0 Ga materials with minor early Paleoproterozoic components. In combination with the zircon Hf isotopic data, it is suggested that the crustal basement of the Central Tianshan Block was formed in the early Mesoproterozoic, which is evidently different from the Archean basement formation of the Tarim Craton. We suggest that the Central Tianshan Block may have formed through a magmatic arc accretionary orogen along the continental margin of Baltica during the Mesoproterozoic. Moreover, both the meta-sedimentary rocks and the orthogneiss of the Xingxingxia Complex simultaneously suffered a metamorphic overprint at ca. 380 Ma. Zircon REE and Hf isotopic data indicate that the metamorphic zircon rims were formed by new zircon overgrowths in partial melts. Geothermobarometry and average P–T calculations using THERMOCALC on the Grt–Bt gneiss yielded ca. 720–730 °C and 4–6 kbar, suggesting amphibolite- to granulite-facies conditions. The Late Devonian metamorphism of the Xingxingxia Complex is possibly related to orogenic low-pressure/high-temperature metamorphism in the middle or upper crust during the closure of the eastern segment of South Tianshan Ocean. Our new data, together with the occurrences of Ordovician–Silurian eclogites and HP mafic granulites along the northeastern margin of the Tarim Craton, indicate that progressive continental collisions occurred in the southern CAOB during the Paleozoic.