Geochemistry of orthogneisses and metasedimentary rocks across a proposed terrane boundary in the Central Domain of Borborema Province,NE Brazil: Geodynamic implications

The Borborema Province, northeastern Brazil, occupies a central position in pre-drift reconstructions of western Gondwana, making an understanding of its geological evolution crucial for Neoproterozoic reconstructions. In recent years, it has been proposed that the Borborema Province grew by accretion of distinct tectonic terranes. In order to test this hypothesis, we compare here the geochemistry of orthogneisses and metasedimentary rocks across a proposed terrane boundary in the Central Domain of the province. Orthogneiss samples show smooth trends in Harker diagrams and similar rare earth element (REE) patterns, characterized by sharp decreases from La to Sm (chondrite-normalized La/Sm = 3–6) and flat heavy REE profiles (chondrite-normalized Tb/Yb = 1.5–2.5), with small or no Eu anomalies. In primitive mantle-normalized multi-element diagrams, all samples show parallel patterns characterized by sharp negative anomalies of U, Ta, Nb, P and Ti. The metasedimentary samples show little scatter of the major elements in Harker diagrams, suggesting that their chemistry was little affected by post-depositional diagenesis and metamorphism. They have indistinguishable chondrite-normalized REE patterns, characterized by light REE enrichment, flat heavy REE (normalized Tb/Y = 1–2) and small or no negative Eu anomalies, and similar ratios of immobile trace elements (e.g., Th/Sc, Zr/Sc). The geochemistry of the metasedimentary samples is comparable in many ways to those of the orthogneisses, suggesting that these may have been an important source of the precursor sedimentary rocks. These data do not support the terrane accretion hypothesis, rather suggesting the existence of a continuous basement that became available for erosion during intraplate continental extension in the late Neoproterozoic. Comparisons of the studied sequences with those present in the Northern Domain suggest that most, if not all of the Neoproterozoic geodynamic evolution of Borborema Province, occurred in an intracontinental setting.     

Geochemistry of black shales from the Lower Cretaceous Paja Formation,Eastern Cordillera,Colombia: Source weathering,provenance, and tectonic setting

The major and trace element characteristics of black shales from the Lower Cretaceous Paja Formation of Colombia are broadly comparable with those of the average upper continental crust. Among the exceptions are marked enrichments in V, Cr, and Ni. These enrichments are associated with high organic carbon contents. CaO and Na₂O are strongly depleted, leading to high values for both the Chemical Index of Alteration (77–96) and the Plagioclase Index of Alteration (86–99), which indicates derivation from a stable, intensely weathered felsic source terrane. The REE abundances and patterns vary considerably but can be divided into three main groups according to their characteristics and stratigraphic position. Four samples from the lower part of the Paja Formation (Group 1) are characterized by LREE-enriched chondrite-normalized patterns (average La_N/Yb_N = 8.41) and significant negative Eu anomalies (average Eu/Eu¹ = 0.63). A second group of five samples (Group 2), also from the lower part, have relatively flat REE patterns (average La_N/Yb_N = 1.84) and only slightly smaller Eu anomalies (average Eu/Eu¹ = 0.69). Six samples from the middle and upper parts (Group 3) have highly fractionated patterns (average La_N/Yb_N = 15.35), resembling those of Group 1, and an identical average Eu/Eu¹ of 0.63. The fractionated REE patterns and significant negative Eu anomalies in Groups 1 and 3 are consistent with derivation from an evolved felsic source. The flatter patterns of Group 2 shale and strongly concave MREE-depleted patterns in two additional shales likely were produced during diagenesis, rather than reflecting more mafic detrital inputs. An analysis of a single sandstone suggests diagenetic modification of the REE, because its REE pattern is identical to that of the upper continental crust except for the presence of a significant positive Eu anomaly (Eu/Eu¹ = 1.15). Felsic provenance for all samples is suggested by the clustering on the Th/Sc–Zr/Sc and Gd_N/Yb_N–Eu/Eu¹ diagrams. Averages of unmodified Groups 1 and 3 REE patterns compare well with cratonic sediments from the Roraima Formation in the Guyana Shield, suggesting derivation from a continental source of similar composition. In comparison with modern sediments, the geochemical parameters (K₂O/Na₂O, La_N/Yb_N, La_N/Sm_N, Eu/Eu¹, La/Sc, La/Y, Ce/Sc) suggest the Paja Formation was deposited at a passive margin. The Paja shales thus represent highly mature sediments recycled from deeply weathered, older, sedimentary/metasedimentary rocks, possibly in the Guyana Shield, though Na-rich volcanic/granitic rocks may have contributed to some extent.     

Mineralogy of high-field-strength elements (Y,Nb, REE) in the world-class Vergenoeg fluorite deposit,South Africa

The Vergenoeg fluorite deposit in the Bushveld Complex in South Africa is hosted by a volcanic pipe-like body. The distribution characteristics, composition and formation conditions of high-field-strength element (HFSE)-rich minerals in different lithological units of the deposit were investigated by optical and cathodoluminescence microscopy, scanning electron microscopy, X-ray fluorescence, inductively-coupled plasma mass-spectrometry and electron-probe microanalysis. The Vergenoeg host rocks comprise a diverse silica-undersaturated assemblage of fayalite–magnetite–fluorite with variably subordinate apatite and mineral phases enriched in rare-earth elements (REEs). The Sm–Nd isotope systematics of the fluorite from the various lithological units of the pipe support the model that the HFSE budget of the Vergenoeg pipe was likely derived from a Lebowa-type granitic magma. Isotopically, there is no evidence for other REE sources. Formation of the pipe, including development of the fluorite mineralization, occurred within the same time frame as the emplacement of other magmatic rock units of the Bushveld Complex (Sm–Nd isochron age for fluorite separates: 2040 ± 46 Ma). Hydrothermal alteration is manifested in strongly disturbed Rb–Sr isotope systematics of the Vergenoeg deposit, but did not affect its HFSE and REE budget. Whole-rock chondrite-normalized REE + Y distribution patterns of two types were observed: (i) flat patterns characteristic of magnetite–fluorite unit, gossan, metallurgical-grade fluorite (“metspar”) plugs and siderite lenses, and (ii) U-shaped patterns showing enrichment towards the heaviest REE (Tm–Lu) observed in the fayalite-rich units. Common HFSE minerals are complex Nb-rich oxides (samarskite, fergusonite), REE phosphates and fluorocarbonates. Additionally, fluocerite and REE silicates, whose identification requires further work, were found. Most of the HFSE-rich minerals are spatially associated with Fe-rich phases (e.g., pyrite, magnetite, greenalite and hematite). To a smaller extent, they are found finely disseminated or healing micro-fractures in fluorite. The whole-rock REE + Y distribution patterns of the individual lithological units are mainly controlled by the distribution of Yb-rich and Y-rich xenotime in these rocks. The common occurrence of bastnäsite-(Ce) in the gossan, “metspar” plugs and especially in the rhyolitic carapace at the pipe–wall-rock contact, controls the REE + Y distribution patterns of these rocks. HFSE minerals in the Vergenoeg pipe rocks have formed in several stages. Samarskite and coarse fluorapatite belong to the primary mineral assemblage. Fergusonite and Yb-rich xenotime formed during high- to moderate-temperature hydrothermal activity. Significant remobilization of the HFSE from the early-crystallized minerals (breakdown of fluorapatite and possibly allanite with release of REE + Y) and subsequent partial redistribution of these elements into near surface rocks are inferred. The late-stage assemblages are characterized by the presence of fine-grained REE fluorocarbonates, monazite-(Ce), monazite-(La) and xenotime-(Y).     

Geochronological,geochemical and mineralogical constraints on the petrogenesis of appinites from the Laoniushan complex,eastern Qinling,central China

Appinites are commonly derived from a mantle source and are potentially significant in constraining the tectonic nature and evolution of ancient orogens, yet they have received little attention because of their limited outcrop. Here we investigate the newly identified appinitic rocks from the Laoniushan complex in the eastern Qinling Orogen. The appinites are composed of coarse-grained hornblendite, medium- and fine-grained hornblende-gabbro, and diorite porphyrite in the field occurrence. Winthin the appinitic rocks, the hornblendite displays features of cumulates. This study presents LA-ICP-MS zircon U-Pb data, mineral chemistry and whole rock geochemistry of the appinites. Zircons in the mafic to ultramafic rocks yield a U-Pb age of 152 ± 1Ma. The geochemistry of the rocks displays: lower SiO₂, higher Fe₂O₃^T and MgO contents, relatively flat chondrite normalized REE patterns with slight enrichment in light REE and a minor negative Eu anomaly; enrichment in large-ion lithophile elements(LILE, e.g. Rb, Ba, Sr and P), and depletion in high field strength elements(HFSE, e.g. Nb, Zr, Hf and Ti). Such geochemical features, together with crust-like bulk Sr-Nd isotopic compositions(initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7057–0.7072, ε_Nd(t) = −17.2 to −9), suggest that the Laoniushan appinites likely originated from an ancient metasomatised mantle, followed by fractional crystallization in the petrogenetic process. The studied appinites were most likely generated in an intracontinental extensional environment in the Late Mesozoic.     

Remnants of arc-related Mesoarchaean oceanic crust in the Tartoq Group of SW Greenland

The Tartoq Group, located in SW Greenland, consists of supracrustal rocks of mainly tholeiitic basaltic composition, including pillow lavas, sills/dykes and gabbros, as well as ultramafic rocks. Metamorphic grade ranges from greenschist facies to granulite facies. The Tartoq Group crops out as a series of blocks and slivers that are imbricated with originally intrusive Mesoarchaean TTG orthogneisses. The supracrustal rocks form part of a SE vergent fold and thrust belt consistent with the imbrication of TTG gneisses and supracrustal rocks along a convergent margin. LA-ICP-MS U–Pb zircon dating of an intrusive TTG sheet yields a minimum age of 2986 ± 4 Ma for the Tartoq Group. This age is consistent with MC-ICP-MS Lu–Hf and Sm–Nd isotopic whole-rock data for mafic samples from different blocks of the Tartoq Group, which yield errorchron ages of 3189 ± 65 Ma and 3068 ± 220 Ma, respectively. The mafic supracrustal rocks of the Tartoq Group have chondrite-normalized REE patterns with La_CN/Sm_CN of 0.67–1.96 and rather flat primitive mantle-normalized multi-element patterns, except for scattered LILE contents, and generally negative Nb-anomalies with Nb/Nb* of 0.26–1.31. Th/Yb varies between 0.06 and 0.47 and Nb/Yb between 0.45 and 4.4 indicative of an arc affinity when compared to rocks from modern settings. The similar geochemistry of the different lithological units, together with their coeval formation, as evident from trace element geochemical trends, supports a co-magmatic origin for the rock assemblage and their formation as imbricated relics of oceanic crust. Accordingly, we propose that the Tartoq Group represents remnants of Mesoarchaean oceanic crust, which formed in a suprasubduction zone geodynamic environment.     

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K₂O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K₂O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO₂-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.     

Petrology and geochemistry of the banded iron-formations from Ntem complex greenstones belt,Elom area,Southern Cameroon: Implications for the origin and depositional environment

Banded iron-formations (BIFs) form an important part of the Archaean to Proterozoic greenstone belts in the Southern Cameroon. In this study, major, trace and REE chemistry of the banded iron-formation are utilized to explore the source of metals and to constraint the origin and depositional environment of these BIFs. The studied BIF belongs to the oxide facies iron formations composed mainly of iron oxide (mainly magnetite) mesobands alternating with quartz mesobands. The mineralogy of the BIF sample consists of magnetite and quartz with lesser amount of secondary martite, goethite and trace of gibbsite and smectite. The major element chemistry of these iron-formations is remarkably simple with the main constituents being SiO₂ and Fe₂O₃ which constitute 95.6–99.5% of the bulk rock. Low Al₂O₃, TiO₂, and HFSE concentrations show that they are relatively detritus-free chemical sediments. The Pearson’s correlation matrix of major element reveals that there is a strong positive correlation (r = 0.99) of Al with Ti and no to weak negative correlation of Ti with Mn, Ca and weak positive correlation of Si with Ca, suggesting the null to very minor contribution of detrital material to chemical sediment. The trace elements with minor enrichments are transition metals such as Zn, Cr, Sr, V and Pb. This is an indicator of direct volcanogenic hydrothermal input in chemical precipitates. The studied BIF have a low ΣREE content, ranging between 0.41 and 3.22 ppm with an average of 0.87 ppm, similar to that of pure chemical sediments. The shale-normalized patterns show depletion in light REE, slightly enrichment in heavy REE and exhibit weak positive europium anomalies. These geochemical characteristics indicate that the source of Fe and Si was the result of deep ocean hydrothermal activity admixed with sea water. The absence of a large positive Eu anomaly in the studied BIF indicates an important role of low-temperature hydrothermal solutions. The chondrite-normalized REE patterns are characterized by LREE-enriched (Mean La_CN/Yb_CN = 8.01) and HREE depletion (Mean Tb_CN/Yb_CN = 1.61) patterns and show positive Ce anomalies. With the exception of one sample (LBR133), all of the BIF samples analyzed during this study have positive Ce anomalies on both chondrite- and PASS-normalized plots. This may indicate that the BIFs within the Elom area were formed within a redox stratified ocean. The positive Ce anomalies in the studied samples likely suggest that the basin in which Fe formations were deposited was reducing with respect to Ce, probably in the suboxic or anoxic seawaters.     

The petrogenesis and tectonic implications of the granitoid gneisses from Xingxingxia in the eastern segment of Central Tianshan

As part of Central Asian Orogenic Belt (CAOB), the Central Tianshan zone plays a crucial role in the reconstruction of the tectonic evolution of the CAOB. Furthermore, it is bordered by the Tarim Craton to the south, and the comparable evolutionary history between them enables the Central Tianshan zone to provide essential information on the crustal evolution of the Tarim Craton. The eastern segment of the Central Tianshan tectonic zone is characterized by the presence of numerous Precambrian metamorphic rocks, among which the Xingxingxia Group is the most representative one. The granitoids gneisses, intruded into the Xingxingxia Group, consist of two major lithological assemblages: (1) biotite-monzonitic gneisses and (2) biotite-plagioclase gneisses. These metamorphosed granitoid rocks are characterized by enrichment in SiO₂, Al₂O₃ and K₂O and depletion in MgO and FeO_T. The Rittmann index (σ) spreads between 1.44 and 2.21 and ACNK (Al₂O₃/(CaO + Na₂O + K₂O)) ranges from 1.03 to 1.08, indicating that these granitoid gneisses are high-K calc-alkaline and peraluminous. Trace element data indicate that the studied samples are enriched in LREE with moderate REE fractionated patterns ((La/Yb)_N = 10.5–75.3). The concentrations of HREE of the garnet-bearing gneisses are significantly higher than those of garnet-free gneisses. The former show pronounced negative Eu anomalies (Eu/Eu^* = 0.32–0.57), while the latter are characterized by negligible negative Eu anomalies to moderate positive Eu anomalies (Eu/Eu^* = 0.80–1.35). In addition, the enrichment of LILE (Rb, Th, K, Pb) and depletion of HFSE (Ta, Nb, P, Ti) of the examined granitoid gneisses are similar to typical volcanic-arc granites. Zircons U–Pb dating on the biotite monzonitic gneiss yields a weighted mean ²⁰⁶Pb/²³⁸U age of 942.4 ± 5.1 Ma, suggesting their protoliths were formed in the early Neoproterozoic, which is compatible with the time of the assembly of supercontinent Rodinia. The zircons have a large ε_Hf(t) variation from −5.6 to +3.2, suggesting that both old crust-derived magmas and mantle-derived juvenile materials contributed to the formation of their protoliths. Based on field observation, and petrological, geochemical and geochronological investigations, we infer that the granitoid gneisses from Xingxingxia were probably formed on a continental arc that resulted from the interaction of Australia and the Tarim Craton during the assembly of the Rodinia supercontinent, and that the Central Tianshan zone was a part of the Tarim Craton during that time. Besides, the Grenvillian orogenic events may have developed better in the Tarim Craton than previously expected.     

Geochemistry,provenance, and tectonic setting of Neoproterozoic metavolcanic and metasedimentary units,Werri area,Northern Ethiopia

Major, trace and rare earth element (REE) compositions of upper Proterozoic metavolcanic and metasedimentary rocks from the Tsaliet and Tembien Groups in the Werri district of northern Ethiopia were determined to examine their tectonic setting of eruption, provenance and source area weathering conditions. Tsaliet Group metavolcanic rocks in the Werri area have sub-alkaline chemistry characterized by low to intermediate SiO₂ contents, high Al₂O₃, low MgO and very low Cr and Ni. High field strength element (HFSE) abundances are highly variable. ∑REE abundances vary from 66.7 to 161.3 ppm, and chondrite-normalized REE patterns are moderately fractionated, with La_N/Yb_N values of between 3.1 and 9.0. Europium anomalies are variable (Eu/Eu* 0.80–1.21) but are generally positive (average Eu/Eu* 1.06). On tectonic discrimination diagrams, most samples have either volcanic-arc chemistry or fall in the overlap field with mid-oceanic ridge basalt (MORB). However, primitive mantle-normalized trace element abundances are comparable with sub-alkaline basalts from developed island arcs. ¹⁴⁷Sm/¹⁴⁴Nd ratios range from 0.1167 to 0.1269 (n = 3), yielding initial εNd_(800 Ma) of +3.8 to +4.9 and mean T_DM model age of 0.96 Ga, indicative of derivation from juvenile Neoproterozoic mantle. Metasediments from three locations (Werri1, Werri2 and Tsedia) in the Werri and Tsedia Slates have similar Al₂O₃, TiO₂ and HFSE contents but variable and low Na₂O, CaO and K₂O. Cr and Ni are slightly enriched in the Werri2 and Tsedia suites. SiO₂ is very variable, with average values of 70.75, 72.2 and 66.4 wt.% in the Werri1, Werri2 and Tsedia suites, respectively. ∑REE abundances in the metasediments (14.74–108.1) are lower than in the metavolcanics, and are slightly less fractionated, with La_N/Yb_N ratios of 0.8–5.9. Europium anomalies vary (Eu/Eu* 0.80–1.21) but are insignificant on average (Eu/Eu* 0.96). High values for the Chemical Index of Alteration (generally 70–90), and Plagioclase Index of Alteration (>75) in the Werri metasediments indicate moderate to severe chemical weathering in their source. Average major and trace element compositions of the metasediments and their REE patterns are comparable with the metavolcanics. ¹⁴⁷Sm/¹⁴⁴Nd ratios of the metasediments range from 0.1056 to 0.1398 (n = 4), with initial εNd_(800 Ma) of +3.4 to +5.0 and mean T_DM model age of 0.97 Ga, indicating derivation from juvenile Neoproterozoic crust similar to the underlying metavolcanics, with minimal (4–10%) contribution from older crust. The most sensitive tectonic setting discriminators indicate the Werri metasediments represent developed oceanic island arc sediments. The chemical similarity of the Werri metavolcanics to the nearby Adwa metavolcanics, Nakfa terrane in Eritrea, and volcanic units in central Saudi Arabia imply that juvenile Neoproterozoic Arabian Nubian Shield crust extended south at least as far as the Werri area of northern Ethiopia. The comparable geochemistry of the metasediments and their underlying lithologies attests to their derivation from this juvenile crustal material.     

Arc magmatism in the Delhi Fold Belt: SHRIMP U–Pb zircon ages of granitoids and implications for Neoproterozoic convergent margin tectonics in NW India

The northwestern region of Peninsular India preserves important records of Precambrian plate tectonics and the role of Indian continent within Proterozoic supercontinents. In this study, we report precise SHRIMP zircon U–Pb ages from granitoids from the Sirohi terrane located along the western fringe of the Delhi Fold Belt in Rajasthan, NW India. The data reveal a range of Neoproterozoic ages from plagiogranite of Peshua, foliated granite of Devala, and porphyritic granite of Sai with zircon crystallization from magmas at 1015 ± 4.4 Ma, 966.5 ± 3.5 and 808 ± 3.1 respectively. The plagiogranite shows high SiO₂, Na₂O and extremely low K₂O, Rb, Ba, comparable with typical oceanic plagiogranites. These rocks possess low LREE and HREE concentrations and a relatively flat LREE–HREE slope, a well-developed negative Eu-anomaly and conspicuous Nb and Ti anomalies. Compared to the plagiogranite, the foliated Devala granite shows higher SiO₂ and moderate Na₂O, together with high K₂O and comparatively higher Rb, Ba, Sr and REE, with steep REE profiles and a weak positive Eu anomaly. In contrast to the plagiogranite and foliated granite, the porphrytic Sai granite has comparatively lower SiO₂ moderately higher Na₂O, extremely high Y, Zr, Nb and elevated REE. The geochemical features of the granitoids [HFSE depletion and LILE enrichment, Nb- and Ta-negative anomalies], and their plots in the fields of Volcanic Arc Granites and those from active continental margins in tectonic discrimination diagrams suggest widespread Neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm. Our results offer important insights into a long-lived active continental margin in NW India during early and mid Neoproterozoic, consistent with recent similar observations on Cryogenian magmatic arcs widely distributed along the margins of the East African Orogen, and challenge some of the alternate models which link the magmatism to extensional tectonics associated with Rodinia supercontinent breakup.     

REE and HFS element behaviour in the alteration facies of the Erenler Dağı Volcanics (Konya,Turkey) and kaolinite occurrence

Mainly high-K, calc-alkaline, Late Miocene to Pliocene volcanic rocks cropped out of the Konya area in Central Anatolia, Turkey. The volcanic rocks are predominantly andesitic to dacitic in composition and rarely basalt, basaltic andesite, basaltic trachyandesite and pyroclastics. Kaolinite, illite, Ca-montmorillonite, alunite, jarosite, minamiite and silica polymorphs were formed by widespread and intense hydrothermal alteration in or around the volcanic products. To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, the whole rock chemical composition (major and trace elements, including rare-earth elements (REE) was analysed. The results of the study demonstrate notable differences in the REE behaviour in the different sample groups. REE trends of fresh parent rocks to weakly-, moderately-altered, kaolinitic and alunitic rocks are characterised by strong LREE enrichment ((La/Lu)_cn = 14.57, 11,8 to 15.20, 4.54 to 13.30, 12.5 to 24.2 and 34.6 to 47.26, respectively). Most of the samples have pronounced negative and/or weakly-negative Eu anomalies ranging from 0.75 to 0.98 while three samples have weakly-positive Eu anomalies. LRE element contents are higher than those of HREE in the samples. The LRE elements were strongly enriched in the kaolinitic and alunitic alteration; in weakly- and moderately-altered rocks. LREE are nearly immobile whereas HRE elements show different behaviour in different rock groups. The HFS and TRT elements are slightly mobilised in weakly-altered rocks, but enriched in other alteration types. Elements commonly assumed to be immobile (e.g. Y, Zr, Nb, Hf, TiO₂, Al₂O₃, REE) show variation in mass calculation. LIL elements showed enrichment over LREE and MREE, and similar behaviour, in contrast with HFSE. A clear increment of trans-transition elements (TRTE) was found mainly in alunitic and partly in kaolinitic samples.     

REE and isotope (Sr,S, and Pb) geochemistry to constrain the genesis and timing of the F–(Ba–Pb–Zn) ores of the Zaghouan District (NE Tunisia)

The F–(Ba–Pb–Zn) ore deposits of the Zaghouan District, located in NE Tunisia, occur as open space fillings or stratabound orebodies, hosted in Jurassic, Cretaceous and Tertiary layers. The chondrite-normalized rare earth element (REE) patterns may be split into three groups: (i) “Normal marine” patterns characterizing the wallrock carbonates; (ii) light REE (LREE) enriched (slide-shaped) patterns with respect to heavy REE (HREE), with small negative Ce and Eu anomalies, characteristic of the early ore stages; (iii) Bell-shaped REE patterns displaying LREE depletion, as well as weak negative Ce and Eu anomalies, characterizing residual fluids of subsequent stages. The ⁸⁷Sr/⁸⁶Sr ratios (0.707654–0.708127 ± 8), show that the Sr of the epigenetic carbonates (dolomite, calcite) and ore minerals (fluorite, celestite) are more radiogenic than those of the country (Triassic, Jurassic, Cretaceous, lower Miocene) sedimentary rocks. The uniformity of this ratio, throughout the District, provides evidence for the isotopic homogeneity and, consequently, the identity of the source of the mineralizing fluids. This signature strongly suggests that the radiogenic Sr is carried by Upper Paleozoic basinal fluids.The δ³⁴S values of barite, associated to mineralizations, are close to those of the Triassic sea water (17‰). The δ³⁴S values of sulfide minerals range from − 13.6‰ to + 11.4‰, suggesting two sulfur-reduced end members (BSR/TSR) with a dominant BSR process.Taking account of the homogeneity in the Pb-isotope composition of galenas (18.833–18.954 ± 0.001, 15.679–15.700 ± 0.001 and 38.690–38.880 ± 0.004, for the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios respectively), a single upper crustal source for base-metals is accepted. The Late Paleozoic basement seems to be the more plausible source for F–Pb–Zn concentrated in the deposits. The genesis of the Zaghouan District ore deposits is considered as the result of the Zaghouan Fault reactivation during the Late Miocene period.     

Petrogenesis and geodynamic evolution of the Kajan Neogene subvolcanic rocks,Nain, Central Iran

Kajan subvolcanic rocks in the Urumieh–Dokhtar magmatic arc (UDMA), Central Iran, form a Late Miocene-Pliocene shallow-level intrusion. These subvolcanics correspond to a variety of intermediate and felsic rocks, comprising quartz diorite, quartz monzodiorite, tonalite and granite. These lithologies are medium-K calc-alkaline, with SiO₂ (wt.%) varying from 52% (wt.%) to 75 (wt.%). The major element chemical data also show that MgO, CaO, TiO₂, P₂O₅, MnO, Al₂O₃ and Fe₂O₃ define linear trends with negative slopes against SiO₂, whilst Na₂O and K₂O are positively correlated with silica. Contents of incompatible trace elements (e.g. Ba, Rb, Nb, La and Zr) become higher with increasing SiO₂, whereas Sr shows an opposite behaviour. Chondrite-normalized multi-element patterns show enrichment in LILE relative to HFSE and troughs in Nb, P and Ti. These observations are typical of subduction related magmas that formed in an active continental margin. The Kajan rocks show a strong affinity with calc-alkaline arc magmas, confirmed by REE fractionation (La_N/Yb_N = 4.5–6.4) with moderate HREE fractionation (Sm_N/Yb_N = 1.08–1.57). The negative Eu anomaly (Eu/Eu* <1), the low to moderate Sr content (< 400 ppm) and the Dy/Yb values reflect plagioclase and hornblende (+- clinopyroxene) fractionation from a calc-alkaline melt Whole–rock Sr and Nd isotope analyses show that the ⁸⁷Sr/⁸⁶Sr initial ratios vary from 0.704432 to 0.705989, and the ¹⁴³Nd/¹⁴⁴Nd initial ratios go from 0.512722 to 0.512813. All the studied samples have similar Sr-Nd isotopes, indicating an origin from a similar source, with granite samples that has more radiogenic Sr and low radiogenic Nd isotopes, suggesting a minor interaction with upper crust during magma ascent. The Kajan subvolcanic rocks plot within the depleted mantle quadrant of the conventional Sr-Nd isotope diagram, a compositional region corresponding to mantle-derived igneous rocks.     

Alteration mineralogy,lithochemistry and stable isotope geochemistry of the Murgul (Artvin,NE Turkey) volcanic hosted massive sulfide deposit: Implications for the alteration age and ore forming fluids

The Murgul (Artvin, NE Turkey) massive sulfide deposit is hosted dominantly by Late Cretaceous calc-alkaline to transitional felsic volcanics. The footwall rocks are represented by dacitic flows and pyroclastics, whereas the hanging wall rocks consist of epiclastic rocks, chemical exhalative rocks, gypsum-bearing vitric tuff, purple vitric tuff and dacitic flows. Multi-element variation diagrams of the hanging wall and footwall rocks exhibit similar patterns with considerable enrichment in K, Rb and Ba and depletion in Nb, Sr, Ti and P. The chondrite-normalized rare earth element (REEs) patterns of all the rocks are characterized by pronounced positive/negative Eu anomalies as a result of different degrees of hydrothermal alteration and the semi-protected effects of plagioclase fractionation.Mineralogical results suggest illite, illite/smectite + chlorite ± kaolinite and chlorite in the footwall rocks and illite ± smectite ± kaolinite and chlorite ± illite in the hanging wall rocks. Overall, the alteration pattern is represented by silica, sericite, chlorite and chlorite–carbonate–epidote–sericite and quartz/albite zones. Increments of Ishikawa alteration indexes, resulting from gains in K₂O and losses in Na₂O and the chlorite–carbonate–pyrite index towards to the center of the stringer zone, indicate the inner parts of the alteration zones. Calculations of the changes in the chemical mass imply a general volume increase in the footwall rocks. Abnormal volume increases are explained by silica and iron enrichments and a total depletion of alkalis in silica zone. Relative K increments are linked to the sericitization of plagioclase and glass shards and the formation of illite/smectite in the sericite zone. In addition, Fe enrichment is always met by pyrite formation accompanied by quartz and chlorite. Illite is favored over chlorite, smectite and kaolinite in the central part of the ore body due to the increase in the (Al + K)/(Na + Ca) ratio. Although the REEs were enriched in the silicification zone, light REEs show depletion in the silicification zone and enrichment in the other zones in contrast to the heavy REEs' behavior. Hydrothermal alteration within the hanging wall rocks, apart from the gypsum-bearing vitric tuffs, is primarily controlled by chloritization with proportional Fe and Mg enrichments and sericitization.The δ¹⁸O and δD values of clay minerals systematically change with increasing formation temperature from 6.6 to 8.7‰ and − 42 to − 50‰ for illites, and 8.6 and − 52‰ for chlorite, respectively. The O- and H-stable isotopic data imply that hydrothermal-alteration processes occurred at 253–332 °C for illites and 136 °C for chlorite with a temperature decrease outward from the center of the deposit. The positive δ³⁴S values (20.3 to 20.4‰) for gypsum suggest contributions from seawater sulfate reduced by Fe-oxide/-hydroxide phases within altered volcanic units. Thus, the hydrothermal alteration possibly formed via a dissolution–precipitation mechanism that operated under acidic conditions. The K–Ar dating (73–62 Ma) of the illites indicates an illitization process from the Maastrichtian to Early Danian period.     

Geochemical signatures of uranium oxides in the Lufilian belt: From unconformity-related to syn-metamorphic uranium deposits during the Pan-African orogenic cycle

The Pan-African Lufilian belt (Zambia and Democratic Republic of Congo) is known for its world-class copper and cobalt deposits. In addition, the Lufilian Copperbelt hosts several uranium occurrences concentrated within deformed siliciclastic rocks of the basal Neoproterozoic Katanga Supergroup. We report LA-ICPMS and EMP analyses of the rare earth element (REE) and yttrium (Y) abundances (designated as the REY signatures) of uranium oxides from two uranium mineralizing events of the Lufilian belt previously dated at 652 ± 8 Ma and 530 ± 6 Ma by the U–Pb method on uraninite. Uranium oxides dated at ca. 650 Ma from the External fold-and-thrust belt are characterized by (i) bell shape REE patterns centered on middle REE (MREE), (ii) positive europium (Eu) anomalies and (iii) relatively low Y contents. In contrast, uranium oxides dated at ca. 530 Ma from the Domes region are characterized by (i) REE patterns but with a less pronounced light REE (LREE) fractionation, (ii) negative Eu anomalies and (iii) higher Y contents. Moreover, the External fold-and-thrust belt also contains uranium mineralization dated at ca. 530 Ma having the same characteristics as the ca. 530 Ma uranium oxides from the Domes region (a moderately fractionated REE pattern and a negative Eu anomaly).As REY signatures are known to reflect mineralizing processes, the distinct geochemical signatures of the two uranium oxide generations (ca. 650 Ma and ca. 530 Ma) provide meaningful information about the uranium cycle during the Pan-African orogeny. Compared to the REY signatures of the known worldwide uranium deposit types, the REY signature of uranium oxides dated at ca. 650 Ma of the External fold-and-thrust belt is similar to the REE patterns from unconformity-related U deposits (Athabasca in Canada and Kombolgie in Australia). Uranium oxides of the Domes region and some of the External fold-and-thrust belt display similar characteristics to syn-metamorphic U deposit (Mistamisk in Canada). Accordingly, we propose that the two stages of uranium oxide crystallizations within the Lufilian belt, at ca. 650 and ca. 530 Ma, occurred under distinct physico-chemical conditions. The first stage, at ca. 650 Ma, may be related to late diagenesis hydrothermal processes, at the basement/cover interface, with the circulation of highly saline basinal brines linked to evaporites of the Roan Group. This Pan-African unconformity-related uranium deposit is the youngest of this type described to date. The second stage may be connected to metamorphic fluid circulations, at about 530 Ma, during the Lufilian orogeny in the Domes region and also in the External fold-and-thrust belt.     

Petrogenesis and geochemical characteristics of plagiogranites from Naga Ophiolite Belt,northeast India: Fractional crystallization of MORB-type magma

The Naga Ophiolite Belt is a part of the Naga-Arakan-Yoma flysch trough that occurs along the Indo-Myanmar border. It is represented by peridotites, mafic-ultramafic cumulates, mafic volcanics, mafic dykes, plagiogranites, pelagic sediments and minor felsic to intermediate intrusives. Minor plagiogranites, gabbros and thin serpentinite bands occur juxtaposed near Luthur, with the slate-phyllite-metagreywacke sequence (Phokpur Formation) adjacent to the contact. The development of tonalites, trondhjemites and diorites in the oceanic crust, which is grouped as plagiogranites, offers an opportunity to study the process of formation of silicic melts from mafic crust. Plagiogranites from Naga Ophiolite Belt contains moderate SiO₂ (51.81–56.71 wt.%), low K₂O (0.08–1.65 wt.%) and high Na₂O (4.3–5.03 wt.%). The Naga Ophiolite Belt plagiogranites like ocean-ridge granites contain low K₂O, high Na₂O and CaO. The rocks investigated from Naga Ophiolite Belt contain TiO₂ concentrations above the lower limit for fractionated Mid Oceanic Ridge Basalt which is above 1 wt% of TiO₂ and the ternary plots of A (Na₂O + K₂O) F(FeO^T) M(MgO) and TiO₂-K₂O-SiO₂/50 indicate that the plagiogranite are tholeiitic in character and gabbro samples are calc-alkaline in nature. The plagiogranites are enriched in Rb, Ba, Th, U, Nb and Sm against chondrite with negative anomalies on Sr and Zr whereas Y and Yb are depleted to Mid Oceanic Ridge Basalt. The chondrite normalized REE patterns of the plagiogranite display enrichments in LREE (La_N/Sm_N: 2.37–3.62) and flat HREE (Eu/Eu*: 0.90–1.06). The Mid Oceanic Ridge Basalt normalization of gabbro is characterized by strong enrichment of LILE like Ba and Th. The REE pattern is about 50–100 times chondrite with slight enrichment of LREE (La_N/Sm_N = 2.21–3.13) and flat HREE (Eu/Eu*: 0.94–1.19). The major-element and trace element data of the NOB plagiogranites and their intrusive nature with host gabbroic rock suggest that the plagiogranites were produced by fractional crystallization of basaltic parental magmas at Mid Oceanic Ridge.     

Early to Middle Devonian granitic and volcanic rocks from the central Gulf of Maine

Cashes Ledge igneous suite in the central Gulf of Maine is represented by 10 granitic and two felsic tuff samples collected from bedrock outcrops using the submersible Alvin in 1971–1972 and archived at the Woods Hole Oceanographic Institute. Laser ablation ICP-MS analyses of zircon grains yielded crystallization ages of 414.9 ± 1.1 Ma and 399.7 ± 1.5 Ma for two alkali feldspar granite samples, 407.0 ± 1.9 Ma for a syenogranite sample, and 384.4 ± 2.3 Ma and 383.9 ± 1.6 Ma for two felsic tuff samples. The samples contain iron-rich mafic minerals, including aegirine-augite, grunerite/ferroedenite, and annite. Most of the samples are alkaline to slightly peralkaline, with high concentrations of SiO₂, Y, Zr, Nb, and REE, strong negative Eu anomalies, and positive epsilon Nd values (1.8 to 3.7). The suite resembles part of a belt of similar Silurian–Devonian rocks with ages between 426 and 370 Ma now recognized in the central part of Avalonia in southeastern New England. They formed in a long-lived, likely extensional regime linked to subduction and subsequent complex transcurrent motions among Ganderia, Avalonia, and Meguma, culminating in the closure of the Rheic Ocean.     

Sampling oxygenated Archean hydrosphere: Implications from fractionations of Th/U and Ce/Ce* in hydrothermally altered volcanic sequences

Hydrothermally altered Archean igneous suites erupted in the submarine environment record variable excursions of Ce/Ce* and Th/U from primary magmatic values of 1 and ~ 4 respectively. Rhyolites of the 2.96 Ga bimodal basalt–rhyolite sequence of the Murchison Domain, Yilgarn Craton, Western Australia, hosting the Golden Grove VMS deposit, are enriched in MnO up to ten fold over primary values. Th/U ratios span 2.6–4.7, Ce/Ce* = 2.5–16, and Eu/Eu* = 1.3–3. The 2.8 Ga Lady Alma ultramafic–mafic subvolcanic complex of the same domain features highly dispersed MREE and LREE due to intense hydrothermal alteration. Th/U ratios span 0.005–0.16 from preferential addition of U, with Ce/Ce* = 0.6–2.2, and Eu/Eu* = 1–1.4. The eastern Dharwar Craton, India, includes greenstone terranes dominantly 2.7–2.6 Ga. Adakites of the Gadwal terrane preserve near primary magmatic Th/U, Ce/Ce*, and Eu/Eu*. In contrast, igneous lithologies of the Hutti greenstone terrane are characterized by total ranges of Th/U = 2–5.8, Ce/Ce* = 1.01–1.28, and Eu/Eu* = 0.82–1.26, and counterparts of the Sandur terrane have Th/U = 0.4–6.0, Ce/Ce* = 0.9–1.25, and Eu/Eu* = 0.8–1.8. Coexistence of Ce and Eu anomalies may reflect a two-stage process: low-temperature hydrothermal alteration at high water–rock ratios by oxidizing fluids, with evolution of the hydrothermal systems to high temperature, low water–rock ratios, under reducing conditions. Uranium is dominantly added to these lithologies over Th in common with Recent altered ocean crust. Iron-rich shales in the Sandur terrane record U-enrichment where Th/U = 2–4. Three shales record true negative Ce anomalies and Eu/Eu* = 0.8–2.4: true negative Ce anomalies, present in some other Archean iron formations, are interpreted as a signature of precipitates from the ocean water column whereas Eu anomalies are hydrothermal in origin. Volcanic flows of the 2.7 Ga Blake River Group, Abitibi greenstone terrane, Canada, preserve Th/U = 1.5–8.5, the conjunction of low Th/U values with Ce/Ce* = 1.4 in two samples, and Eu/Eu* = 0.15–1.3. Mobility of U and Ce in these hydrothermally altered Archean lithologies is in common with their mobility in Phanerozoic counterparts by oxygenated fluids.