The Mesoproterozoic Guaporé suture in the SW Amazonian Craton: Geotectonic implications based on field geology,zircon geochronology and Nd–Sr isotope geochemistry

A major Mesoproterozoic paleo-plate boundary in the southwestern Amazonian Craton, the Guaporé Suture Zone, is investigated by U–Pb zircon geochronology, Sr–Nd isotope geochemistry and aeromagnetic data. This suture zone is constituted dominantly by ophiolitic mafic–ultramafic rocks of the Trincheira Complex, and minor proportion of tonalites of the Rio Galera and São Felipe complexes, Colorado Complex, amphibolites of the Rio Alegre Terrane and syn- to late-kinematic mafic to felsic plutonic rocks. The ophiolitic Trincheira Complex formed during an accretionary phase from 1470 to 1430 Ma and was overprinted by upper amphibolite–granulite facies metamorphism during the collisional phase of the Ectasian followed by syntectonic emplacement of gabbro and granite plutons (1350–1340 Ma). The ophiolites were intruded by syntectonic tonalitic–plagiogranitic plutons ca. 1435 Ma. Mafic–ultramafic rocks of the Trincheira ophiolites show moderate to highly positive initial epsilon Nd (t = 1.46 Ga) values (+2.6 to +8.8) and very low initial ⁸⁷Sr/⁸⁶Sr ratio (0.7013–0.7033). It is suggested that these magmas originated from a depleted mantle source in an island-arc–back-arc setting. The identification of a fossil ophiolite in the Guaporé Suture Zone early as 1470–1435 Ma and later collisional phase, as late as 1350 Ma, marks the impingement of the proto-Amazonian Craton against the Paragua Block, before the formation of the Rodinia supercontinent. The results provide important insights into the geodynamic history of the SW Amazonian Craton, with evidence for both accretionary orogen and subduction of oceanic lithosphere in the Mesoproterozoic, and provide information that allows other workers to evaluate the configuration of supercontinents.     

Archaean to Palaeoproterozoic high-grade evolution of the Belomorian eclogite province in the Gridino area,Fennoscandian Shield: Geochronological evidence

The Belomorian eclogite province was repeatedly affected by multiple deformation episodes and metamorphism under moderate to high pressure. Within the Gridino area, high pressure processes developed in a continental crust of tonalite–trondhjemite–granodiorite (TTG) affinity that contains mafic pods and dykes, in which products of these processes are most clearly evident. New petrological, geochemical and geochronological data on mafic and felsic rocks, including PT-estimates, mineral chemistry, bulk rock chemistries, REE composition of the rocks and zircons and U–Pb and Lu–Hf geochronology presented in the paper make it possible to reproduce the magmatic and high-grade metamorphic evolution in the study area. In the framework of the extremely long-lasting geologic history recorded in the Belomorian province (3–1.7 Ga), new geochronological data enabled us to define the succession of events that includes mafic dyke emplacement between 2.87 and 2.82 Ga and eclogite facies metamorphism of the mafic dykes between ~ 2.82 and ~ 2.72 Ga (most probably in the time span of 2.79–2.73 Ga). The clockwise PT path of the Gridino association crosses the granulite- and amphibolite-facies PT fields during the time period of 2.72 Ga to 2.64 Ga. A special aspect of this work concerns the superposed subisobaric heating (thermal impact) with an increase in the temperature to granulite facies conditions at 2.4 Ga. Later amphibolite facies metamorphism occurred at 2.0–1.9 Ga. Our detailed geochronological and petrological studies reveal a complicated Mesoarchaean–Palaeoproterozoic history that involved deep subduction of the continental crust and a succession of plume-related events.     

High-temperature,high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian,NW Scotland: Crustal-scale subduction in the Neoarchaean

Eclogites and associated high-pressure (HP) rocks in collisional and accretionary orogenic belts preserve a record of subduction and exhumation, and provide a key constraint on the tectonic evolution of the continents. Most eclogites that formed at high pressures but low temperatures at > 10–11 kbar and 450–650 °C can be interpreted as a result of subduction of cold oceanic lithosphere. A new class of high-temperature (HT) eclogites that formed above 900 °C and at 14 to 30 kbar occurs in the deep continental crust, but their geodynamic significance and processes of formation are poorly understood. Here we show that Neoarchaean mafic–ultramafic complexes in the central granulite facies region of the Lewisian in NW Scotland contain HP/HT garnet-bearing granulites (retrogressed eclogites), gabbros, lherzolites, and websterites, and that the HP granulites have garnets that contain inclusions of omphacite. From thermodynamic modeling and compositional isopleths we calculate that peak eclogite-facies metamorphism took place at 24–22 kbar and 1060–1040 °C. The geochemical signature of one (G-21) of the samples shows a strong depletion of Eu indicating magma fractionation at a crustal level. The Sm–Nd isochron ages of HP phases record different cooling ages of ca. 2480 and 2330 Ma. We suggest that the layered mafic–ultramafic complexes, which may have formed in an oceanic environment, were subducted to eclogite depths, and exhumed as HP garnet-bearing orogenic peridotites. The layered complexes were engulfed by widespread orthogneisses of tonalite–trondhjemite–granodiorite (TTG) composition with granulite facies assemblages. We propose two possible tectonic models: (1) the fact that the relicts of eclogitic complexes are so widespread in the Scourian can be taken as evidence that a > 90 km × 40 km-size slab of continental crust containing mafic–ultramafic complexes was subducted to at least 70 km depth in the late Archaean. During exhumation the gneiss protoliths were retrogressed to granulite facies assemblages, but the mafic–ultramafic rocks resisted retrogression. (2) The layered complexes of mafic and ultramafic rocks were subducted to eclogite-facies depths and during exhumation under crustal conditions they were intruded by the orthogneiss protoliths (TTG) that were metamorphosed in the granulite facies. Apart from poorly defined UHP metamorphic rocks in Norway, the retrogressed eclogites in the central granulite/retrogressed eclogite facies Lewisian region, NW Scotland have the highest crustal pressures so far reported for Archaean rocks, and demonstrate that lithospheric subduction was transporting crustal rocks to HP depths in the Neoarchaean.     

Late Paleozoic subduction system in the southern Central Asian Orogenic Belt: Evidences from geochronology and geochemistry of the Xiaohuangshan ophiolite in the Beishan orogenic belt

The Xiaohuangshan ophiolite of the Beishan (Inner Mongolia) is located in the southern margin of the Central Asian Orogenic Belt. It consists of several blocks composed dominantly of serpentinized ultramafic rocks, cumulative gabbros and basalts. The geochemical data of gabbros and basalts obtained from the Xiaohuangshan ophiolite are similar to tholeiitic rocks. They all have low TiO₂ and high Al₂O₃ contents. Their light rare earth elements are slightly enriched, (La/Yb)_N = 3.62–6.80, similar to the typical enriched mid-ocean ridge basalts. The mafic rocks display enrichments in large ion lithophile elements and depletions in high field strength elements, as well as significant Nb–Ta–Ti negative anomalies, similar to subduction-derived rocks. All these geochemical characteritics indicate that the Xiaohuangshan ophiolite would form in a subduction zone from a slightly enriched mantle source. Ion microprobes (SHRIMP) U–Pb dating were conducted on zircons from the basalt and gabbro. The weighted mean ages are 336.4 ± 4.1 Ma and 345 ± 14 Ma, which are considered as the crystallization ages of the basalt and gabbro, respectively. Together with other two units, the Dongqiyishan arc belt and the Yueyashan–Xichangjing ophiolite, the Xiaohuangshan ophiolite forms a Late Paleozoic arc-basin system, indicating that the Paleo-Asian Ocean did not close in the early Carboniferous. Based on the geochemical characteristics of adjacent geological bodies and their settings, the Xiaohuangshan ophiolite is considered as an indicator of a suture zone between the different epicontinental belts in the Beishan region.     

Likely “mantle plume” activity in the Skellefte district,Northern Sweden. A reexamination of mafic/ultramafic magmatic activity: Its possible association with VMS and gold mineralization

Most attention has been given to the geology of the extensive VMS and subordinate precious metals mineralization in the Skellefte district. Less attention has been given to indications of deep-seated origins of felsic and mafic/ultramafic volcanic rocks; of VMS and precious metals mineralizing fluids; and the primary origins of these metals. A holistic view of the significance of mafic/ultramafic volcanic rocks to both the geotectonic evolution of the area and the existence of its important base and precious metals deposits has never been presented. These subjects are discussed in this investigation.Primitive mantle normalized spider diagrams of rare-earth-elements (REE) distinguish two groups of mafic/ultramafic volcanic rocks, each with distinct geochemical characteristics: a mid-ocean-ridge “MORB”-type, and a geochemically unusual and problematic calc–alkaline–basalt “CAB”-type which is the main subject of this investigation. The “MORB”-type mafic volcanic rocks are mostly older than the Skellefte Group felsic volcanic rocks hosting the VMS deposits, whereas the more primitive “CAB”-type mafic/ultramafic volcanic rocks are mostly younger.A common source for these “CAB”-type, mafic-(MgO wt.% < 14%) and ultramafic-(MgO wt.% > 14%) volcanic rocks is suggested by their similar and distinctive geochemical features. These are near-chondritic (Al-undepleted) Al₂O₃/TiO₂ ratios; moderate to strong high-field-strength-element (HFSE) depletion; light-rare-earth-element (LREE) enrichment and moderate heavy-rare-earth-element (HREE) depletion. They outcrop throughout an area of at least 100 × 100 km. Gold mineralization is spatially associated with ultramafic volcanic rocks.Zr and Hf depletion has been shown to be associated with Al-depletion in mafic/ultramafic volcanic rocks elsewhere, and has been attributed to deep-seated partial melting in ascending mantle plumes. Zr and Hf depletion in “CAB”-type Al-undepleted mafic/ultramafic volcanic rocks is therefore unusual. The solution to this dilemma is suggested to be contamination of an Al-depleted mantle plume by felsic crustal rocks whereby Al-depleted ultramafic magmas become Al-undepleted. It will be argued that this model has the potential to explain previous observations of deep-seated origins; the spatial association of ultramafic volcanic rocks with occurrences of gold mineralization; and even the primary origin of metals in VMS deposits.     

Sm–Nd isotope geochemistry of metamorphic volcano-sedimentary successions in the São Gabriel Block,southernmost Brazil: evidence for the existence of juvenile Neoproterozoic oceanic crust to the east of the Rio de la Plata craton

Juvenile Neoproterozoic dioritic, tonalitic, trondhjemitic and granodioritic gneisses in the São Gabriel block, southern Brazil, have been identified by geochronologic studies. Age proposals for associated (ultra-)mafic metavolcanic and metasedimentary rocks, however, range from Archean to Neoproterozoic. Whole rock Sm–Nd analyses presented here support a Neoproterozoic age for these rocks. TDM model ages of the (ultra-)mafic metavolcanic rocks range between 0.65 and 1.35 Ga with ɛNd(t) positive values between 3.16 and 6.87; TDM model ages of metasedimentary and metavolcanoclastic rocks vary between 0.77 and 1.19 Ga with ɛNd(t) values between 1.2 and 6.23; tonalitic calc-alkaline gneisses show ɛNd(t) values of 4.34 and 6.3 and TDM model ages of 0.89 and 0.72 Ga, respectively. A late-kinematic granite (Santa Zélia granite) display slightly negative ɛNd(t) values (−1.6) and a higher TDM model age of about 1.4 Ga. These data support the existence of Meso/Neoproterozoic juvenile oceanic crust and island arc rocks during the Brasiliano orogenic events. The main source rocks of the metasedimentary units are previously formed juvenile rocks. The data also indicate minor assimilation of older crustal material and/or contamination of the melts by radiogenic Nd released from older rocks on the subducting slab. Existence of widespread old sialic crust in the subduction zone environment, however, can be ruled out indicating important orogenic accretion between 0.9 and 0.7 Ga. A geotectonic model for the São Gabriel block and the eastern margin of the Rio de la Plata craton comprises eastward subduction and following accretion of an intra-oceanic island arc between 0.9 and 0.8 Ga and a subsequent westward subduction with formation of an active continental margin at the eastern margin of the Rio de la Plata craton between 0.8 and 0.7 Ga. We postulate that the juvenile rocks of São Gabriel block represent relics of a Neoproterozoic ocean between the Rio de la Plata craton and a continental block (Encantadas block) possibly derived from the Kalahari craton. Subduction and arc accretion began roughly coeval with the initial stages of the break-up of Rodinia (0.9 Ga) and indicate a peripheric Rio de la Plata craton in relation to the Rodinia supercontinent with evolution from a passive margin to an active margin in the beginning of the Neoproterozoic Brasiliano orogenic events.     

Petrogenesis of Dongguashan skarn-porphyry Cu-Au deposit related intrusion in the Tongling district,eastern China: Geochronological,mineralogical, geochemical and Hf isotopic evidence

The Dongguashan skarn-porphyry Cu-Au deposit, located in the Tongling district of the Middle-Lower Yangtze River Valley metallogenic belt (MLYB), consists of skarn ore bodies in the upper part and porphyry ore bodies in the lower part, both of which are hosted in quartz diorite and quartz monzodiorite. Zircon U-Pb age and geochemical studies show that the quartz diorite of the Dongguashan intrusion formed at 140.3 ± 2.0 Ma (MSWD = 0.19) and belongs to the high potassium calc-alkaline series. It is enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE), depleted in high field-strength elements (HFSE) and heavy rare earth elements (HREE), and has a slightly negative Eu anomaly. ¹⁷⁶Hf/¹⁷⁷Hf values of the rims of zircons show a variable range (0.282087–0.282391), corresponding with calculated ε_Hf(t) values of − 10.72 to − 21.46. Plagioclases in the quartz diorite have unbalanced structure characterized by bright andesine and labradorite (An = 37.0–65.5) cores with higher contents of Fe and Sr and are corroded by dark oligoclase (An = 13.8–27.6) rim. Major elements, trace elements, Hf isotope, and the composition of plagioclases indicate that the parental magma of the Dongguashan intrusion was produced by the mixing of underplating mafic magma and felsic magma formed by remelting of Paleoproterozoic and Neoarchean crustal rocks, Neoproterozoic crust may also provide some material to the felsic magma. Mafic magma played a key role and made the parental magma rich in water, sulfur, metals (Cu, Au) and gave it a high oxygen fugacity. During its magmatic evolution, the parental magma underwent fractional crystallization of hornblende, apatite, sphene and other mafic minerals. Some quartz diorite and quartz monzodiorite samples that show adakitic signatures, may result from injection of mafic magma. Some inherited zircons of the quartz diorite in the Dongguashan intrusion gave ages of 2.40–2.50 Ga, 1.95–2.05 Ga and 0.74–0.81 Ga, coming from ultramafic, mafic and andesitic igneous rocks, and this indicates that there may have been three periods (2.4, 2.0, and 0.8 Ga) of magmatic activity in the Tongling district.     

Petrology,geochemistry and geochronology of the magmatic suite from the Jianzha Complex,central China: Petrogenesis and geodynamic implications

The intermediate–mafic–ultramafic rocks in the Jianzha Complex (JZC) at the northern margin of the West Qinling Orogenic Belt have been interpreted to be a part of an ophiolite suite. In this study, we present new geochronological, petrological, geochemical and Sr–Nd–Hf isotopic data and provide a different interpretation. The JZC is composed of dunite, wehrlite, olivine clinopyroxenite, olivine gabbro, gabbro, and pyroxene diorite. The suite shows characteristics of Alaskan-type complexes, including (1) the low CaO concentrations in olivine; (2) evidence of crystal accumulation; (3) high calcic composition of clinopyroxene; and (4) negative correlation between FeO^tot and Cr₂O₃ of spinels. Hornblende and phlogopite are ubiquitous in the wehrlites, but minor orthopyroxene is also present. Hornblende and biotite are abundant late crystallized phases in the gabbros and diorites. The two pyroxene-bearing diorite samples from JZC yield zircon U–Pb ages of 245.7 ± 1.3 Ma and 241.8 ± 1.3 Ma. The mafic and ultramafic rocks display slightly enriched LREE patterns. The wehrlites display moderate to weak negative Eu anomalies (0.74–0.94), whereas the olivine gabbros and gabbros have pronounced positive Eu anomalies. Diorites show slight LREE enrichment, with (La/Yb)_N ratios ranging from 4.42 to 7.79, and moderate to weak negative Eu anomalies (Eu/Eu¹ = 0.64–0.86). The mafic and ultramafic rocks from this suite are characterized by negative Nb–Ta–Zr anomalies as well as positive Pb anomalies. Diorites show pronounced negative Ba, Nb–Ta and Ti spikes, and typical Th–U, K and Pb peaks. Combined with petrographic observations and chemical variations, we suggest that the magmatism was dominantly controlled by fractional crystallization and crystal accumulation, with limited crustal contamination. The arc-affinity signature and weekly negative to moderately positive ε_Nd(t) values (−2.3 to 1.2) suggest that these rocks may have been generated by partial melting of the juvenile sub-continental lithospheric mantle that was metasomatized previously by slab-derived fluids. The lithologies in the JZC are related in space and time and originated from a common parental magma. Geochemical modeling suggests that their primitive parental magma had a basaltic composition. The ultramafic rocks were generated through olivine accumulation, and variable degrees of fractional crystallization with minor crustal contamination produced the diorites. The data presented here suggest that the subduction in West Qinling did not cease before the early stage of the Middle Triassic (∼242 Ma), a back-arc developed in the northern part of West Qinling during this period, and the JZC formed within the incipient back-arc.     

Counterclockwise tectonometamorphic evolution of the Pringles Metamorphic Complex,Sierras Pampeanas of San Luis (Argentina)

The crystalline basement of the Sierra de San Luis, which belongs to the Eastern Sierras Pampeanas in central Argentina, consists of three main units: (1) Conlara, (2) Pringles, and (3) Nogolí metamorphic complexes. In the Pringles Metamorphic Complex, mafic–ultramafic bodies occur as discontinuous lenses along a narrow central belt concordant with the general NNE–SSW structural trend. A metamorphic gradient from granulite to greenschist facies is apparent on both sides of the mafic–ultramafic bodies. This work focuses on the characteristics of the mylonitization overprinted on the mafic–ultramafic intrusives in the Pringles Metamorphic Complex and their gneissic–migmatitic surroundings, both previously metamorphosed within the granulite facies. Petrogenetic grid and geothermobarometry applied to the paragenesis equilibrated during the mylonitic event, together with mineral deformation mechanisms, indicate that mafic and adjacent basement mylonites developed under upper amphibolite transitional to granulite facies metamorphic conditions at intermediate pressures (668–764 °C, 6.3–6.9 kbar, 0.3 < XCO₂ < 0.7). However, the following mylonitic assemblages can be distinguished from the external limits of the Pringles Metamorphic Complex to its center: lower amphibolite facies ⇒ middle amphibolite facies ⇒ upper amphibolite transitional to granulite facies. Geothermobarometry applied to mylonitic assemblages indicate a temperature gradient from 555 °C to 764 °C and pressures of 6–7 kbar for the mylonitic event. This event is considered to have developed on a preexisting temperature gradient attributed to the intrusion of mafic–ultramafic bodies. The concentration of sulfides in mylonitic bands and textural relationships provide evidence of remobilization of primary magmatic sulfides of the mafic–ultramafic rocks (+PGM) during the mylonitic event. A lower-temperature final overprint produced brittle fracturing and localized retrogression on mafic–ultramafic minerals and ores by means of a water-rich fluid phase, which gave rise to a serpentine + magnetite ± actinolite association. Concordantly in the adjacent country rocks, fluids channeled along preexisting mylonitic foliation planes produced local obliteration of the mylonitic texture by a randomly oriented replacement of the mylonite mineralogy by a chlorite + sericite/muscovite + magnetite assemblage. Observed mineral reactions combined with structural data and geothermobarometry suggest a succession of tectonometamorphic events for the evolution of the Pringles Metamorphic Complex of Sierra de San Luis, developed in association with a counterclockwise P–T–d path. The most likely geological setting for this type of evolution is a backarc basin, associated with east-directed Famatinian subduction initiated in Mid-Cambrian times and closed during the collision of the allochthonous Precordillera terrane in Mid-Ordovician times.     

Early Permian East-Ujimqin mafic–ultramafic and granitic rocks from the Xing’an–Mongolian Orogenic Belt,North China: Origin,chronology, and tectonic implications

The East-Ujimqin complex, located north of the Erenhot–Hegenshan fault, North China, is composed of mafic–ultramafic and granitic rocks including peridotite, gabbro, alkali granite, and syenite. We investigated the tectonic setting, age, and anorogenic characteristics of the Xing’an–Mongolian Orogenic Belt (XMOB) through field investigation and microscopic and geochemical analyses of samples from the East-Ujimqin complex and LA-MC-ICP-MS zircon U–Pb dating of gabbro and alkali granite. Petrographic and geochemical studies of the complex indicate that this multiphase plutonic suite developed through a combination of fractional crystallization, assimilation processes, and magma mixing. The mafic–ultramafic rocks are alkaline and have within-plate geochemical characteristics, indicating anorogenic magmatism in an extensional setting and derivation from a mantle source. The mafic–ultramafic magmas triggered partial melting of the crust and generated the granitic rocks. The granitic rocks are alkali and metaluminous and have high Fe/(Fe + Mg) characteristics, all of which are common features of within-plate plutons. Zircon U–Pb geochronological dating of two samples of gabbro and alkali granite yielded ages of 280.8 ± 1.5 and 276.4 ± 0.7 Ma, placing them within the Early Permian. The zircon Hf isotopic data give inhomogeneous ε_Hf(t) values of 8.2–14.7 for gabbroic zircons and extraordinary high ε_Hf(t) values (8.9–12.5) for the alkali granite in magmatic zircons. Thus, we consider the East-Ujimqin mafic–ultramafic and granitic rocks to have been formed in an extensional tectonic setting caused by asthenospheric upwelling and lithospheric thinning. The sources of mafic–ultramafic and granitic rocks could be depleted garnet lherzolite mantle and juvenile continental lower crust, respectively. All the above indicate that an anorogenic magma event may have occurred in part of the XMOB during 280–276 Ma.     

Late Paleozoic subduction system in the northern margin of the Alxa block,Altaids: Geochronological and geochemical evidences from ophiolites

The northern margin of the Alxa block (NMAB), located in the southernmost part of the Altaids, is important for understanding the tectonic processes associated with the closure of the Paleo-Asian ocean. In this study, we report results from our studies on two ophiolitic belts (the Enger Us and Quagan Qulu ophiolitic belts) to constrain the tectonic evolution of the Altaids. The tectonic blocks in the Enger Us ophiolite are mainly composed of ultramafic and mafic rocks, with a matrix comprising highly deformed Carboniferous clastic rocks and tuffs. Zircons from a pillow lava sample yielded SHRIMP zircon U–Pb age of 302 ± 14 Ma. Massive and pillow basalts in the Enger Us ophiolite exhibit N-MORB geochemical affinities, displaying high TiO₂ and low K₂O contents with tholeiitic signatures. They are characterized by depletion of light rare earth elements (LREEs) without fractionation of high field strength elements (HFSEs) and negative Nb–Ta anomalies. It is inferred that the magmas of these rocks were derived from a depleted mantle source in a mid-ocean ridge setting. The Quagan Qulu ophiolite is composed of tectonic blocks, including ultramafic, gabbros and siliceous rocks, and matrix, including deformed clastic rocks and limestones. Zircons in a gabbro sample from the Quagan Qulu ophiolite yielded SHRIMP zircon U–Pb age of 275 ± 3 Ma. The gabbros show high MgO contents, compatible elements (Ni, Co, Sc, and V), and Al₂O₃/TiO₂ ratios, but low TiO₂ and SiO₂ contents. They are enriched in large-ion lithophile elements (LILEs) and depleted in LREEs and HFSEs, indicating that they were derived from an extremely depleted mantle source which was infiltrated by a subduction-derived fluid or melt. Our geochemical data suggest that gabbros in the Quagan Qulu ophiolite were formed in a back-arc basin setting. A synthesis of evidence from geochemistry, regional geology, and paleobiogeography support the notion that the Enger Us ophiolitic belt represents the major suture of the Paleo-Asian Ocean in the NMAB and the Quagan Qulu ophiolitic belt represents a back-arc basin. These two ophiolitic belts, together with the Zongnaishan–Shalazhashan arc have been suggested to be a late Paleozoic ocean-arc–back-arc basin system in the southernmost part of the Altaids. The geochronological data suggest that the subduction process occurred even in the early Permian, indicating that the final closure of the Paleo-Asian Ocean might have taken place later than the early Permian.     

The Guarguaraz Complex and the Neoproterozoic–Cambrian evolution of southwestern Gondwana: Geochemical signatures and geochronological constraints

The Guarguaraz Complex, in western Argentina, comprises a metasedimentary assemblage, associated with mafic sills and ultramafic bodies intruded by basaltic dikes, which are interpreted as Ordovician dismembered ophiolites. Two kinds of dikes are recognized, a group associated with the metasediments and the other ophiolite-related. Both have N-MORB signatures, with ε_Nd between +3.5 and +8.2, indicating a depleted source, and Grenville model ages between 0.99 and 1.62 Ga. A whole-rock Sm–Nd isochron yielded an age of 655 ± 76 Ma for these mafic rocks, which is compatible with cianobacteria and acritarchae recognized in the clastic metasedimentary platform sequences, that indicate a Neoproterozoic (Vendian)–Cambrian age of deposition.The Guarguaraz metasedimentary–ophiolitic complex represents, therefore, a remnant of an oceanic basin developed to the west of the Grenville-aged Cuyania terrane during the Neoproterozoic. The southernmost extension of these metasedimentary sequences in Cordón del Portillo might represent part of this platform and not fragments of the Chilenia terrane. An extensional event related to the fragmentation of Rodinia is represented by the mafic and ultramafic rocks. The Devonian docking of Chilenia emplaced remnants of ocean floor and slices of the Cuyania terrane (Las Yaretas Gneisses) in tectonic contact with the Neoproterozoic metasediments, marking the Devonian western border of Gondwana.     

Composition and geochronology of the deep-seated xenoliths from the southeastern margin of the North China Craton

A variety of deep-seated xenoliths occur within the Mesozoic Jiagou dioritic porphyry in the southeastern margin of the North China Craton (NCC). In this study we present a combined petrologic, geochronological, Hf isotope and geochemical study on the different types of xenoliths and use these data to better constrain the composition and age of the deep crust beneath the area. Most of the xenoliths are mafic meta-igneous rocks, among which garnet-bearing lithologies are common. The xenoliths can be classified into three broad petrographic groups: spinel-bearing garnet clinopyroxenite/phlogopite clinopyroxenite/spinel pyroxenite (Group 1), garnet amphibolite or hornblendite/garnet granulite/mafic gneiss lacking pyroxene (Group 2), and garnet-bearing felsic (intermediate-acid) gneiss (Group 3). Among these, the mafic–ultramafic rocks constitute the dominant category. The protoliths of the studied xenoliths range from basalt through andesite to dacite. Geochemical and Hf-isotope data indicate that most xenoliths belonging to Groups 2 and 3 resemble magmatic rocks formed at convergent continental margin arc setting. A few of them (mostly belonging to Group 1) represent mantle-derived products. Multiple metasomatic imprints, with contribution from subduction-related or mantle-derived fluids or melts have been recognized from the multistage mineral assemblages and ages.SHRIMP zircon U–Pb dating, Hf isotope and geochemical data offer evidence for subduction-related adakite-like and arc-related rocks in the southeastern margin of the NCC at ca. 2.5 Ga and 2.1 Ga, and confirm the occurrence of high-pressure granulite-facies metamorphism at ca. 1.8 Ga. These data suggest an episodic growth of Precambrian lower crust beneath this region in response to two stages of subduction–accretion and one vertical accretion of mantle-derived basaltic magma at the base of the lower crust. Additionally, a previously unknown late mantle-derived basaltic magmatism at 393 ± 7 Ma has also been recognized. The data presented in this paper demonstrate that the deep crust beneath the southeastern margin of the NCC is composed of hybrid protoliths derived from Paleozoic, Paleoproterozoic and late Neoarchean sources.     

Field relationship of high-grade Neo- to Mesoarchaean rocks of South-East Greenland: Tectonometamorphic and magmatic evolution

South-East Greenland forms part of the North Atlantic Craton and is characterized by migmatitic orthogneisses, narrow bands of mafic granulite, ultramafic and possible meta-sedimentary rocks, and alkaline-carbonatitic intrusive rocks. Mafic granulite, meta-sedimentary and ultramafic rocks form the basement for the emplacement of granitic intrusions at ca. 2865 Ma that lasted episodically until ca. 2790 Ma and continuously during 2750–2700 Ma. The area is structurally complex with evidence of at least seven deformation events including reclined and mushroom-like fold interference patterns. An older (> 2790 Ma) foliation formed in granitic rocks and the basement during the Timmiarmiut Orogeny (D_T). Deformation associated with the ca. 2790–2700 Ma Skjoldungen Orogeny folded this early foliation, and is associated with a penetrative foliation that is refolded progressively in a northeast–southwest oriented stress field. The orientation of the stress field progressively rotated into a northnorthwest–southsoutheast orientation during the last stages of the orogeny. The orogeny is also characterized by syn-deformational anatexis at granulite-facies (at approximately 800 °C and 5–8 kbar, ca. 2790–2740 Ma), which decreased to the amphibolite-facies at ca. 2730 Ma.The late- to post-tectonic granite and alkaline rocks assigned to the Skjoldungen Alkaline Province intruded the central-northern part around 2710 Ma. This was followed by north–south extensional deformation during the Singertat Stage forming discrete shear-zones at greenschist-facies grades, which is coeval with the emplacement of pegmatite, ijolite, and carbonatite emplacement during ca. 2680–2650 Ma.Similar lithology and tectonic processes in the Tasiusarsuaq Terrane of southern West Greenland and the Lewisian Complex in Scotland suggest a possibly large Archaean terrane at that time, which, taking the present size, at least covered around 500–600 km in an east–west direction and approximately 200 km in a north–south direction.     

Cu–(Ni–Co–Au)-bearing massive sulfide deposits associated with mafic–ultramafic rocks of the Main Urals Fault,South Urals: Geological structures,ore textural and mineralogical features,comparison with modern analogs

Cu-rich massive sulfide deposits associated with mafic–ultramafic rocks in the southern portion of the Main Urals Fault (MUF) are characterized by variable enrichments in Ni (up to 0.45 wt.%), Co (up to 10 wt.%) and Au (up to 16 ppm in individual hand-specimens). The Cu (Ni–Co)-rich composition of MUF deposits, as opposed to the Cu (Zn)-rich composition of more eastward massive sulfide deposits of broadly similar age along the western flank of the Magnitogorsk arc, reflects the abundance of seafloor-exposed, Ni–Co-rich ultramafic rocks in the most external portion of the Early-Devonian Magnitogorsk forearc. Morphological, textural, and compositional differences between individual deposits are interpreted to be the result of the sulfide deposition style and, in part, of the original subseafloor lithology. One deposit produced by dominantly on-seafloor hydrothermal processes is characterized by pyrite–marcasite ≫ pyrrhotite, not so low Zn grades (occasionally up to 2 wt.%), abundant clastic facies and periodical superficial oxidation. Deposits produced by dominantly subseafloor hydrothermal processes are characterized by pyrrhotite > pyrite, very low Zn (generally < to ≪ 0.1 wt.%), volumetrically minor clastic facies, and multi-layer deposit morphology. Very low Ni/Co ratios in the on-seafloor deposit may indicate a dominant metal contribution from a mafic rather than ultramafic source. The sulfide mineralization was associated with extensive hydrothermal alteration of the host ultramafic and mafic rocks, leading to formation of abundant talc, talc–carbonate and chlorite rocks. Occurrence of large volumes of such altered lithotypes in ophiolitic belts may be considered as a potential searching criteria for MUF-type (Cu, Co, Ni)-deposits. In spite of the contrasting geodynamic environment, geological, geochemical, textural and mineralogical peculiarities of the MUF deposits in many respects are similar to those of ultramafic-hosted massive sulfide deposits along the Mid-Atlantic Ridge. In geological time, supra subduction-zone settings appear to have been more effective than mid-ocean ridge settings for preservation of ultramafic-hosted massive sulfide deposits.     

The Munali Ni sulfide deposit,southern Zambia: A multi-stage,mafic-ultramafic,magmatic sulfide-magnetite-apatite-carbonate megabreccia

The Munali Intrusive Complex (MIC) is a flattened tube-shaped, mafic-ultramafic intrusion located close to the southern Congo Craton margin in the Zambezi belt of southern Zambia. It is made up of a Central Gabbro Unit (CGU) core, surrounded by a Marginal Ultramafic-mafic Breccia Unit (MUBU), which contains magmatic Ni sulfide mineralisation. The MIC was emplaced into a sequence of metamorphosed Neoproterozoic rift sediments and is entirely hosted within a unit of marble. Munali has many of the characteristics of craton-margin, conduit-style, dyke-sill complex-hosted magmatic sulfide deposits. Three-dimensional modelling of the MUBU on the southern side of the MIC, where the Munali Nickel Mine is located, reveals a laterally discontinuous body located at the boundary between footwall CGU and hangingwall metasediments. Mapping of underground faces demonstrates the MUBU to have intruded after the CGU and be a highly complex, multi stage megabreccia made up of atypical ultramafic rocks (olivinites, olivine-magnetite rocks, and phoscorites), poikilitic gabbro and olivine basalt/dolerite dykes, brecciated on a millimetre to metre scale by magmatic sulfide. The breccia matrix is largely made up of a sulfide assemblage of pyrrhotite-pentlandite-chalcopyrite-pyrite with varying amounts of magnetite, apatite and carbonate. The sulfides become more massive towards the footwall contact. Late stage, high temperature sulfide-carbonate-magnetite veins cut the rest of the MUBU. The strong carbonate signature is likely due, in part, to contamination from the surrounding marbles, but may also be linked to a carbonatite melt related to the phoscorites. Ductile deformation and shear fabrics are displayed by talc-carbonate altered ultramafic clasts that may represent gas streaming textures by CO₂-rich fluids. High precision U-Pb geochronology on zircons give ages of 862.39 ± 0.84 Ma for the poikilitic gabbro and 857.9 ± 1.9 Ma for the ultramafics, highlighting the multi-stage emplacement but placing both mafic and later ultramafic magma emplacement within the Neoproterozoic rifting of the Zambezi Ocean, most likely as sills or sheet-like bodies. Sulfide mineralisation is associated with brecciation of the ultramafics and so is constrained to a maximum age of 858 Ma. The Ni- and Fe-rich nature of the sulfides reflect either early stage sulfide saturation by contamination, or the presence of a fractionated sulfide body with Cu-rich sulfide elsewhere in the system. Munali is an example of a complex conduit-style Ni sulfide deposit affected by multiple stages and sources of magmatism during rifting at a craton margin, subsequent deformation; and where mafic and carbonatitic melts have interacted along deep seated crustal fault systems to produce a mineralogically unusual deposit.     

Field geology,geochronology and geochemistry of mafic–ultramafic rocks from Alxa,China: Implications for Late Permian accretionary tectonics in the southern Altaids

The time of termination of orogenesis for the southern Altaids has been controversial. Systematic investigations of field geology, geochronology and geochemistry on newly discriminated mafic–ultramafic rocks from northern Alxa in the southern Altaids were conducted to address the termination problem. The mafic–ultramafic rocks are located in the Bijiertai, Honggueryulin, and Qinggele areas, stretching from west to east for about 100 km. All rocks occur high-grade gneisses as tectonic lenses that are composed of peridotite, pyroxenite, gabbro, and serpentinite, most of which have undergone pronounced alteration, i.e., serpentinization and chloritization. Geochemically, the rocks are characterized by uniform compositional trends, i.e., with low SiO₂-contents (42.51–52.21 wt.%) and alkalinity (Na₂O + K₂O) (0.01–5.45 wt.%, mostly less than 0.8 wt.%), and enrichments in MgO (7.37–43.36 wt.%), with Mg# = 52.75–91.87. As the rocks have been strongly altered and have a wide range of loss-on-ignition (LOI: 0.44–14.07 wt.%) values, they may have been subjected to considerable alteration by either seawater or metamorphic fluids. The REE and trace element patterns show a relatively fractionated trend with LILE enrichment and HFSE depletion, similar to that of T-MORB between N-MORB and E-MORB, indicating that the parental melt resulted from the partial melting of oceanic lithospheric mantle overprinted by fluid alteration of island-arc origin. The ultramafic rocks are relics derived from the magma after a large degree of partial melting of oceanic lithospheric mantle with superposed island arc processes under the influence of mid-ocean-ridge magmatism. LA-ICP MS U–Pb zircon ages of gabbros from three spots are 274 ± 3 Ma (MSWD = 0.35), 306 ± 3 Ma (MSWD = 0.49), 262 ± 5 Ma (MSWD = 1.2), respectively, representing the formation ages of the mafic–ultramafic rocks. Therefore, considering other previously published data, we suggest that the mafic–ultramafic rocks were products of south-dipping subduction, most probably with a slab window caused by ridge subduction, of the Paleo-Asian Ocean plate beneath the Alxa block in the Late Carboniferous to Late Permian before the Ocean completely closed. This sheds light on the controversial tectonic history of the southern Altaids and supports the concept that the termination of orogenesis was in the end-Permian to Triassic.     

Quartzose–mafic spectral feature space model: A methodology for extracting felsic rocks with ASTER thermal infrared radiance data

The original spectral features of felsic rocks are often intermingled with other surface objects, which results in difficulty of detecting felsic rocks using remote sensing techniques. Few felsic rock indices were proposed and visual interpretation with RGB false color composition is widely used to detect felsic rocks. This paper aims to construct a two-dimensional spectral feature space model to extract felsic rocks using ASTER thermal infrared radiance data. The study area is located in northern Qinghai Province, western China with average altitude of approximately 4200 m. A large number of training pixels of mafic–ultramafic rock, quartz-rich rock, felsic rock, carbonate rock and vegetation were selected from the ASTER images as samples of these surface objects. Then we used a quartz-rich rock index (QI, QI = band14 − 0.844 × band12 − 1.897) and a mafic–ultramafic rock index (MI, MI = 0.915 × band10 − band13 + 1.437) to generate a two-dimensional scatter plot. The plot was named after quartzose–mafic spectral feature space (QMFS). The samples show an approximate triangular shape in the QMFS. Mafic–ultramafic rock, quartz-rich rock and carbonate rock are located in separate locations in the three vertex regions, respectively, while felsic rock is located in the central region of the triangle. Next, we calculated a linear belt of silicate rocks in which silicate rocks vary regularly by using a linear regression analysis in the QMFS. Statistical characteristics of the felsic rock samples are analyzed. Afterwards, a polygon which delineates the distribution of felsic rock samples was constructed from the linear belt of silicate rocks. Then we generated a system of inequalities based on the equations of the edges of the polygon. The application of the inequalities to the ASER images shows a good performance of the QMFS for extracting felsic rocks.     

A review of the tectonic evolution of the Sunsás belt,SW Amazonian Craton

The Sunsás–Aguapeí province (1.20–0.95 Ga), SW Amazonian Craton, is a key area to study the heterogeneous effects of collisional events with Laurentia, which shows evidence of the Grenvillian and Sunsás orogens. The Sunsás orogen, characterized by an allochthonous collisional-type belt (1.11–1.00 Ga), is the youngest and southwesternmost of the events recorded along the cratonic fringe. Its evolution occurred after a period of long quiescence and erosion of the already cratonized provinces (>1.30 Ga), that led to sedimentation of the Sunsás and Vibosi groups in a passive margin setting. The passive margin stage was roughly contemporary with intraplate tectonics that produced the Nova Brasilândia proto-oceanic basin (<1.21 Ga), the reactivation of the Ji-Paraná shear zone network (1.18–1.12 Ga) and a system of aborted rifts that evolved to the Huanchaca–Aguapeí basin (1.17–1.15 Ga). The Sunsás belt is comprised by the metamorphosed Sunsás and Vibosi sequences, the Rincón del Tigre mafic–ultramafic sill and granitic intrusive suites. The latter rocks yield ε_Nd(t) signatures (?0.5 to ?4.5) and geochemistry (S, I, A-types) suggesting their origin associated with a continental arc setting. The Sunsás belt evolution is marked by “tectonic fronts” with sinistral offsets that was active from c. 1.08 to 1.05 Ga, along the southern edge of the Paraguá microcontinent where K/Ar ages (1.27–1.34 Ga) and the Huanchaca–Aguapeí flat-lying cover attest to the earliest tectonic stability at the time of the orogen. The Sunsás dynamics is coeval with inboard crustal shortening, transpression and magmatism in the Nova Brasilândia belt (1.13–1.00 Ga). Conversely, the Aguapeí aulacogen (0.96–0.91 Ga) and nearby shear zones (0.93–0.91 Ga) are the late tectonic offshoots over the cratonic margin. The post-tectonic to anorogenic stages took place after ca. 1.00 Ga, evidenced by the occurrences of intra-plate A-type granites, pegmatites, mafic dikes and sills, as well as of graben basins. Integrated interpretation of the available data related to the Sunsás orogen supports the idea that the main nucleus of Rodinia incorporated the terrains forming the SW corner of Amazonia and most of the Grenvillian margin, as a result of two independent collisional events, as indicated in the Amazon region by the Ji-Paraná shear zone event and the Sunsás belt, respectively.     

Geochemistry,petrogenesis and tectonic setting of Neoproterozoic mafic–ultramafic rocks from the western Jiangnan orogen,South China

The Jiangnan orogenic belt (JOB) has been interpreted as a suture zone between the Yangtze craton and Cathaysian terranes in South China. The Neoproterozoic mafic–ultramafic rocks are extensively exposed in the western JOB, providing an ideal opportunity to study the Neoproterozoic assembly and tectonic evolution of South China. We present integrated field and geochemical studies including LA-ICP-MS zircon U–Pb dating, and whole-rock major and trace element and Sm–Nd isotope analyses of the Neoproterozoic mafic–ultramafic rocks exposed in the northern Guangxi Province, South China. Geochronological results show that the magmatic events took place in two distinct periods: the early Neoproterozoic (861–834 Ma) and the late Neoproterozoic (770–750 Ma). Early Neoproterozoic ultramafic rocks of the Sibao Group have positive ε_Nd(t) values (+ 2.7 to + 6.6) whereas mafic rocks exhibit negative ε_Nd(t) values (− 5.8 to − 0.9). The basaltic rocks show TiO₂ contents of 0.62–0.69 wt.% and Mg-number of 59–65, and also display an enrichment of light rare earth elements (LREEs) and pronounced negative Nb, Ta and Ti anomalies on chondrite- and primitive mantle-normalized diagrams, consistent with subduction-related geochemical signatures. Late Neoproterozoic rocks of the Danzhou Group show ε_Nd(t) values (− 1.23 to + 3.19) for both ultramafic and mafic rocks. The basaltic rocks have TiO₂ contents of 1.01–1.33 wt.% and Mg-number of 57–60, and have a mixture of MORB- and arc-like geochemical affinities, inferred to have formed in an extensional arc environment. Geochemical signatures suggest that all rock types in this study were derived from subarc mantle wedge sources and underwent various degrees of crustal contamination. Thus, we suggest that subduction may have continued to ca. 750 Ma in the western JOB, implying that the amalgamation event between the Yangtze craton and Cathaysian terranes was later than 750 Ma.