Enrichment In Th Research Articles

Mineral chemistry of the Neoproterozoic Alaskan-type Akarem Intrusion with special emphasis on amphibole: Implications for the pluton origin and evolution of subduction-related magma

Akarem Intrusion in the southern part of the Eastern Desert of Egypt is one of Alaskan-type ultramafic-mafic complexes of the Neoproterozoic age. The olivine and pyroxene-rich ultramafic core is surrounded by hornblende gabbro and hornblendite margin with gradational contacts. The pyroxene chemistry indicates that the intrusion was crystallized from tholeiitic arc magmas. The calculated liquids in equilibrium with clinopyroxene show enrichment in Th, U and LREE over the HREE, similar to arc magma formed from the mantle metasomatized by fluids from subducted sediments. Both olivine and pyroxene compositions indicate fractional crystallization from the ultramafic core to the mafic margin. However, amphiboles show systematic increase in concentrations of compatible elements, such as V, and shift in REE patterns from a bowed-up shape to bowed-down shape from the mafic margin to the ultramafic core. Such variation in trace elements of amphiboles indicates progressive crystallization of amphiboles from the mafic margin to the ultramafic core.The variation in amphibole chemistry has implication on both development of Alaskan-type intrusion and subduction zone geochemical signature of the evolved magma. The formation of the Akarem Intrusion started with an early fractional crystallization stage which led to the formation of a stratified magma chamber. The circulation in the magma chamber resulted in the solidification of the mafic magma along the margin where the early-stage amphiboles crystallized along with plagioclase. Subsequently, an evolved liquid from the crystallizing mafic margin percolated into the liquid-rich ultramafic core to form the later generation of interstitial amphiboles. The calculated trace element composition of the liquids in equilibrium with amphibole shows a progressive development of Nb and Ti negative anomalies and increase in La/Yb ratios from the margin to the core of the intrusion. Such variations manifest the role of amphibole fractionation in the evolution of the geochemical signature of arc magma.

Geochemistry and Tectonic Significance of Mafic Volcanic Rocks of the Hindoli Belt, Southeastern Rajasthan: Implications for Continent Assembly

Abstract Mafic volcanic rocks that occur within the sedimentary pile of the Hindoli Group were analyzed for major and trace elements (including REE) to establish tectonic setting of volcanism during the early Proterozoic history of the North Indian Craton. The mafic volcanics are sub-alkaline showing compositional variation from picrobasalt to basalt. They are LREE enriched with (La/Yb)N ratio ranging from 4.67-6.19 (avg.5.27) and exhibit slightly concave REE patterns relative to chondrite. The multi-element patterns of these mafic volcanic rocks display relative enrichment in Th and LREE and negative anomalies of Nb and P. These geochemical characteristics are consistent with a subduction related origin. Various variation diagrams, involving immobile trace elements, distinguish the Hindoli lavas as arc basalt. However, their Ti and Nb contents are higher than those of subduction related magmas. Probably the wedge melting, along with mixing of rising asthenosphere might have produced these characteristics. It is suggested that the Hindoli basin originated by rifting of island-arc lithosphere, caused by rising plume in an extensional back arc region. Based on the results of the present geochemical study, it is proposed that in the early Proterozoic the Mewar block had an active-type continental margin on its present eastern side. The continental magmatic arcs and intra-arc basins developed on this margin were subsequently incorporated into the Mewar protocontinent. Possibly, the plate carrying the Bundelkhand block subducted beneath the eastern margin of the Mewar block, resulting in the final amalgamation of the two blocks along Great Boundary Fault zone or Banas Dislocation Zone. The arc related volcanism of north Indian shield at about 1850-1832 Ma, appears to represent the global subduction event, which resulted in the amalgamation and formation of Columbia supercontinent.

Metabasic rocks in the Varied Group of the Moldanubian Zone, southern Bohemia - their petrology, geochemical character and possible petrogenesis

Metabasic rocks form an important constituent of the Chýnov and Ceský Krumlov units belonging to the Varied Group (Moldanubian Zone, south Bohemia). The amphibolites are dominated by amphibolite-facies mineral assemblages of mainly tschermakitic amphibole and plagioclase. Hornblendes show compositional variation with Si ~ 6.5 apfu, Mg/(Mg + Fe) ~ 0.5 and (Na + K)A ~ 0.5 apfu. Garnet with clinopyroxene are subordinate and occur in a few samples only. No relics of previous greenschistor granulite-facies assemblages have been observed, most likely due to the relatively simple metamorphic history. The petrology indicates rather close correlation of the Chýnov and Ceský Krumlov units. The similarities include presence of dolomite in carbonate bodies, graphite schists, rocks with marialitic scapolite, locally also Ti-andradite (± magnetite, epidote) oxidic assemblages and thin layers of Mn-rich garnet-quartz rocks. However, there is a major difference in the oxidation state. Most Chýnov amphibolites have Fe2O3/FeO = 0.70–1.00 and their protolith probably experienced an early incipient oxidation. Great deal of the parental basalts thus could have been effusive. The Ceský Krumlov amphibolites have Fe2O3/FeO ≤ 0.4, perhaps because they show much closer association with graphite schists that could have been responsible for the reduction of the adjacent rock units. The dataset is dominated by EMORB-like tholeiite basalts interpreted as having been derived by Early Palaeozoic melting of a strongly depleted mantle source (e50 Nd 0 = +8.6 to +9.4; TD Nd M = 0.43–0.50 Ga). This argues stoutly against Precambrian age of the Varied Group in south Bohemia. The composition of the remaining samples reflects contamination by upper continental crust (e50 Nd 0 = +3.1 to +1.3, progressive enrichment in Th, development of a significant negative Nb, and lesser P and Ti anomalies on the NMORB-normalized spiderplots). A much smaller group of amphibolites is characterised by steep REE patterns (LaN/YbN = 5.5–11) and high contents of HFSE (Nb, Ta, Zr and P). It is of a clear OIB affinity, with parental alkali basalt (Nb/Y = 0.7–1.6) generated by a low degree of partial melting of a deep, garnet-bearing asthenospheric mantle source (e50 Nd 0 = +4.5 to +6.1; TD Nd M = 0.75–0.83 Ga). Metamorphosed doleritic/gabbroic dykes cutting the Palaeoproterozoic Světlik orthogneiss show rather unradiogenic Nd isotopic composition (e50 Nd 0 = +0.1 and –3.6; TD Nd M = 1.34 and 2.03 Ga). This precludes closed-system crystallization from depleted mantle derived melts in Phanerozoic times. The exact age and nature of their parental magma remain enigmatic but any genetic link with the amphibolites in the structurally overlying Ceský Krumlov Varied Unit seems ruled out. Overall, the most likely tectonic setting of the magmatism was attenuated lithosphere, subjected to an Early Palaeozoic extension, leading eventually to fragmentation of the northern Gondwana margin. The minor OIB component preserved as alkali basalts as well as some contribution to the EMOR-like basaltic magmas was probably added by a rising mantle plume.

Crustal source of the Late Cretaceous Satansarı monzonite stock (central Anatolia – Turkey) and its significance for the Alpine geodynamic evolution

The Late Cretaceous granitic rocks within central Anatolia (Turkey) not only date and show the magmatic aspects of the Alpine realm, but also give clues about its geodynamic character. Among them, the Satansarı monzonite stock (SMS), part of the Terlemez pluton (Aksaray), characterizing the inception of an extensional tectonic regime in central Anatolia, has a subalkaline, metaluminous and magnesian geochemical nature with depletion in Ba, Nb, P and Ti, and with enrichment of Th, U, K and Pb relative to primitive mantle. The SMS has LREE enriched patterns ([La/Yb]N=18.45–21.21) with moderately negative Eu-anomalies ([Eu/Eu*]N=0.65–0.73). The geochemical data infer a crustal source with an inherited subduction-related component, and fractionation of plagioclase and amphibole. A crustal signature for the SMS is also inferred from high 87Sr/86Sr(t) ratios (0.70826–0.70917), and low ɛNd(t) values (−6.9 to −7.6).Zircon crystals from the SMS typically have magmatic rims overgrowing inherited cores that are reworked, resorbed and overgrown. Completely new zircon crystals grown in a single magmatic episode have also been identified. Laser ablation ICP-MS U–Pb zircon analyses yield a mean 206Pb/238U age of 74.4±0.6Ma (2σ) for the intrusion of the SMS. Rare discordant analyses range from the Devonian to the Proterozoic (i.e., 207Pb/206Pb ages between 364Ma and 1263Ma). In situ zircon Hf isotope analyses reveal low 176Lu/177Hf ratios and negative ɛHf(t) values, which is consistent with a predominantly crustal source of the SMS.We suggest that the water-rich magmas were generated in a hot zone within the crust produced by residual melts from basalt crystallization and partial melts of pre-existing metamorphic and igneous rocks within the lower crust of central Anatolia. The SMS likely formed by episodic injections of these hybrid monzonite melts by adiabatic ascent to shallow crust where they crystallized. This interpretation may be useful in interpreting the involvement of crustal sources for other monzonitic rocks in central Anatolia and granitic magmatism in other similar tectonic environments.

Tracing the chemical evolution of primary pyrochlore from plutonic to volcanic carbonatites: the role of fluorine

AbstractThree Angolan carbonatites were selected to evaluate the change in composition of pyrochlores during magmatic evolution: the Tchivira carbonatites occur in a plutonic complex, the Bonga carbonatites represent hypabyssal carbonatites and the Catanda carbonatites are volcanic in origin. In Tchivira pyrochlore, zoning is poorly developed; fluorine is dominant at the Y site; chemical zoning may arise as a result of substitutions for Nb in the B site; and the rare earth element (REE), U, Th and large-ion lithophile element (LILE) contents are very low. Pyrochlores from Bonga show oscillatory zonation; the F and Na contents are lower than those in the pyrochlores from Tchivira; and as substitution of Na at the A site increases, the Th, U, REE contents and inferred vacancies also increase. Pyrochlores from Catanda display complex textures. They generally have a rounded corroded core, which is mantled by two or three later generations. The core composition is similar to the Bonga pyrochlores. The rims are enriched in Zr, Ta, Th, Ce and U, but depleted in F and Na. In pyrochlores from the Angolan carbonatites, the F and Na contents decrease from plutonic to volcanic settings and there is enrichment of Th, U and REE in the A site and Ta and Zr in the B site. Zoning may be explained by changes in the activity of F, due to the crystallization of fluorite or apatite in the plutonic and hypabyssal carbonatites, or to volatile exsolution in the volcanic carbonatites.

Geochemical systematics and precious metal content of the sedimentary horizons of Lower Gondwanas from the Sattupalli coal field, Godavari Valley, India

Abstract Major, trace, rare earth (REE) and platinum group elements (PGE) have been analysed from the carbonaceous horizons of Permian Barakar Formation of Sattupalli coal field of Godavari Valley to understand the provenance and depositional environment. Barakar Formation of coal field consists of sandstone, siltstone, carbonaceous shale and coal seams. Coal is enriched in trace elements compared with the Clarke's background values. Ferromagnesian trace elements (Ni, Cr and Co) are similar in range in both coal and carbonaceous shales along with Large ion lithophile elements (LILE; Rb, Sr, Cs and Ba), High field strength elements (HFSE; Nb, Ta, Zr and Ti), Light REE (LREE; La-Sm) and Ag. Archean Upper Crust (AUC) normalized REE and multielement variation patterns of the coal show a flat REE with a slight depletion in Ce and Eu contents in most of the samples. Their multielement patterns exhibit enrichment of Th and U; negative Ta, Zr, Hf and positive Ti anomalies. Pronounced positive anomaly occurs at Zr, Hf and Ti that are combined with both positive and negative Nb and Ta anomalies in carbonaceous shales and clay. Sandstones have a high SiO 2 (50–80 wt.%), low Fe 2 O 3 , Al 2 O 3 and moderately high K 2 O contents. AUC normalized REE patterns are almost flat with slight negative to no Eu anomalies followed by small negative Ce anomalies (Ce/Ce* = 0.87–0.95 with one exception). Their multielement patterns exhibit positive U, small negative Nb-Ta, a pronounced positive Zr-Hf anomaly followed by negative Ti and V anomalies. Enrichment of both Zr and TiO 2 in some samples reflects on the mafic and felsic sources at the source region. The fractionated REE patterns, HREE enrichment and negative Eu anomalies are consistent with contribution of zircon through felsic source. Nb/Ta troughs endorse their inheritance from schistose rocks of Eastern Ghats complex. The high Al 2 O 3 /Yb and Th/Yb ratios along with negative Eu anomalies reflect on the TTG/felsic rocks of provenance. Based on the geochemical, available petrological and paleocurrent analysis indicate the source rocks of the Barakar Formation were granite, granitic gneiss, high grade metamorphic rocks from Eastern Ghats, and basic rocks. The provenance is located west and south east (i.e. partly Peninsular gneiss and partly Eastern Ghat Complex). The Au-PGE contents of Sattupalli coalfield have been deposited through NE–SW trending post depositional transverse faults that have affected all the Gondwana Formations during the early Cretaceous. The mafic rocks present within the basement could have been the source of these elements.

Petrography, geochemistry and regional significance of crystalline klippen in the Garhwal Lesser Himalaya, India

Uphalda gneisses (UG) is a crystalline klippe located near Srinagar in Garhwal Himalaya. These gneisses are compared with Debguru porphyroids (DP) (≈Ramgarh group) of Garhwal–Kumaun Himalaya and Baragaon mylonitic gneisses (BMG) of Himachal Himalaya. Petrographic study reveals that the deformation of UG was initiated at higher temperature (above 350°C) and continued till lowering of temperature and deformation led to the mylonitization. Geochemically, these granitic gneisses (UG, DP and BMG) exhibit similar composition. Features such as high molecular A/CNK value (>1), presence of normative corundum and absence of normative diopside, enhanced Rb/Sr, Rb/Zr ratios, enrichment of Th and containing rounded zircons support their crustally-derived S-type granitic nature. The linear plot in major oxides is interpreted in terms of fractional crystallization processes. Mantle normalized multi-element spider diagram of UG illustrates depletion of Ba, Nb, Sr, P and Ti and enrichment of Th and show similarities with DP and BMG. Similarities were observed in lithology, petrographic characters and chemical composition of UG, DP, BMG and Ulleri augen gneisses (Nepal). Comparison with the rocks of Higher Himalayan crystallines (≈Vaikrita), suggests that these rocks (UG) are not transported from Higher Himalaya as understood earlier. This study however proposes that, these gneissic bodies represent an older basement occurring as a tectonic sliver which emplaced within the cover sequence as wedges at different structural levels. This is a regional phenomena observed throughout the Lesser Himalayan region.

Permian bimodal volcanism in the Zhangguangcai Range of eastern Heilongjiang Province, NE China: Zircon U–Pb–Hf isotopes and geochemical evidence

In this paper, we report on zircon U–Pb dating, Hf isotopes, major and trace elements, and Sr–Nd isotope data, with the aim of constraining the petrogenesis and regional tectonic evolution of late Paleozoic volcanic rocks in the Zhangguangcai Range of eastern Heilongjiang Province, NE China. Located in the eastern segment of the Central Asian Orogenic Belt (CAOB), between the Siberian and North China cratons, the late Paleozoic volcanic rocks in the Zhangguangcai Range are composed mainly of basalt, basaltic andesite, rhyolite, and minor dacite. The results of LA–ICP–MS zircon U–Pb dating for two basaltic rocks, three rhyolites, and one dacite indicate that they formed in the early Permian (ca. 292 Ma). The mafic rocks have SiO 2 = 50.13–53.80 wt.%, K 2O = 0.98–2.28 wt.%, Mg# = 0.51–0.71, Cr = 144–541 ppm, Ni = 74–260 ppm, ( 87Sr/ 86Sr) i = 0.7044, and ε Nd ( t) = +4.28, whereas the felsic rocks have SiO 2 = 69.12–77.98 wt.%, K 2O = 3.09–5.33 wt.%, Mg# = 0.17–0.36, ( 87Sr/ 86Sr) i = 0.7032, and ε Nd ( t) = +4.32. These data are typical of bimodal volcanism. The mafic volcanic rocks are characterized by a strong enrichment in large ion lithophile elements (LILEs) such as Rb, Ba, Sr, and Pb, depletion in high field-strength elements (HFSEs) such as Nb, Ta, and Ti, depletion in heavy rare-earth elements (HREEs), and weak negative Eu anomalies (Eu/Eu * = 0.88–0.94). On the other hand, the felsic rocks show a strong depletion in Nb, Ta, Sr, P, and Ti, enrichment in Th, U, and K, and relatively large negative Eu anomalies (Eu/Eu * = 0.28–0.95). Taken together, these data suggest that the mafic magma was derived from the partial melting of a depleted lithospheric mantle, modified by subducted slab-derived fluids, and that the felsic magma originated by partial melting of newly accreted crust. The early Permian bimodal volcanic rocks, together with the coeval A-type granites, indicate an extensional environment, similar to a back-arc basin. Such a setting was possibly related to subduction of the Paleo-Asian oceanic plate beneath the Jiamusi and Songnen–Zhangguangcai Range massifs.

Geochemistry and SHRIMP U–Pb zircon geochronology of the Korla mafic dykes: Constrains on the Neoproterozoic continental breakup in the Tarim Block, northwest China

The Kuruktag uplift is located to the north of the Tarim Block, NW China. At the western end of the Kuruktag uplift, mafic dykes crop out in the Korla area, which were rarely subjected to deformation and metamorphism in contrast to their metamorphic wall-rocks that were strongly deformed and metamorphosed to amphibolite facies. A SHRIMP U–Pb zircon age of 634 Ma was obtained for a spessartite dyke, documenting the youngest known igneous activity associated with rifting in the Tarim Block during the Neoproterozoic. Most samples of the Korla mafic dykes show clear enrichments in Th, La, and variable depletions in Nb, Ta and Ti, except for samples 08T-14 and 08T-19 that display incompatible element distribution patterns similar to those of OIB without obvious depletions in Nb, Ta and Ti. The geochemical signatures suggest that the Korla mafic dykes were formed in an intra-plate setting and their primary magama was possibly produced by partial melting of a sub-continental lithospheric mantle that has been metasomatized by previous subduction processes, and then heated by a rising mantle plume. In combination with previous geochronological data of Neoproterozoic igneous rocks throughout the Tarim Block, at least three pulses of magmatic activity, from ca. 830 to 800 Ma, from ca. 790 to 740 Ma and from ca. 650 to 615 Ma, are recognized, which reveal that multiple episodes of rifting occurred within the Tarim Block, implying that the breakup of the Rodinia supercontinent in the Tarim Block may have been a long-lasting process.

Lithogeochemistry as a tracer of the tectonic setting, lateral integrity and mineralization of a highly metamorphosed Mesoproterozoic volcanic arc sequence on the eastern margin of the Namaqua Province, South Africa

The Areachap Group represents the medium- to high-grade metamorphic and deformed remnants of a Mesoproterozoic (ca. 1.24–1.30 Ga) volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the early stages of the Namaqua-Natal Orogeny. Few regional geochemical studies of the succession have been undertaken and despite recent geochronological work from throughout the outcrop belt there is still uncertainty as to the lateral integrity and correlation of the succession from one area to another. This uncertainty has been aided by minimal outcrop and extensive deformation and dislocation of one area from another. This study uses trace element geochemistry combined with Sm–Nd-isotopic data from the northern and southern ends of the succession to test its lateral integrity. Minor differences, including the abundance of certain lithologies, varying influences of a crustal component and differences in age between different areas are apparent, but the succession, as a whole, displays lateral integrity as a continental island arc. All lithologies are characterised by enrichment in Th, U, Pb and the large ion lithophile elements relative to primitive mantle, along with Nb–Ta depletion relative to La, and strong light rare earth element (LREE) enrichment relative to heavy REE depletion, which, along with positive ε Nd(t) values of mafic gneisses from both ends of the succession, indicates mantle-derivation within a subduction-related environment with a substantial amount of crustal assimilation. Despite some minor geochemical and isotopic differences in the lithological architecture on a local scale the Areachap Group exhibits coherent geochemical characteristics along its entire strike length. Localised remobilization of elements, particularly in areas characterised by granulite grade metamorphism, as well as in the altered footwall zones of volcanogenic massive sulphide lenses, is present. These alteration trends, along with primary geochemical signatures are remarkably well preserved despite the high grade of metamorphism, as illustrated in clearly defined hydrothermal alteration trends on the alteration box plot, and serves as an efficient tool to delineate areas that have undergone hydrothermal alteration spatially associated with massive sulphide ore bodies. The systematic association of hydrothermal alteration and sulphide ore bodies provides support for the isochemical character of rock compositions during high-grade metamorphism and the lack of extensive post-metamorphic alteration processes.

Interaction between ultrapotassic magmas and carbonate rocks: Evidence from geochemical and isotopic (Sr, Nd, O) compositions of granular lithic clasts from the Alban Hills Volcano, Central Italy

Abstract Magma–carbonate rock interaction is investigated through a geochemical and Sr–Nd–O isotope study of granular lithic clasts (ejecta) from the Alban Hills ultrapotassic volcano, Central Italy. Some samples (Group-1) basically represent intrusive equivalents of Alban Hills magmas. A few samples (Group-2) are ultramafic, have high MgO (∼30 to 40 wt%) and δ18O‰, and originated by accumulation of mafic phases that crystallised from ultrapotassic melts during assimilation of dolomitic rocks. Group-3 ejecta consist of dominant K-feldspar, and show major element compositions similar to phonolites, which, however, are absent among the Alban Hills volcanics. Finally, another group (Group-4) contains corroded K-feldspars, surrounded by a microgranular to porphyritic matrix, made of igneous minerals (K-feldspar, foids, clinopyroxene, phlogopite) plus wollastonite, garnet, and some cuspidine. Group-4 ejecta are depleted in SiO2 and enriched in CaO with respect to Group-3. The analysed ejecta have similar 143Nd/144Nd (0.51204–0.51217) as the Alban Hills lavas, whereas 87Sr/86Sr (0.70900–0.71067) is similar to lower. Whole rocks δ18O‰ ranges from +7.0 to +13.2, reaching maximum values in ultramafic samples. A positive correlation with CaO is observed in single rock groups. Large Ion Lithophile Element (LILE) abundances and REE fractionation are generally high, and extreme values of Th, U and LREE are found in some Group-3 and Group-4 rocks. Mineralogical, petrological and geochemical data reveal extensive interaction between magma and carbonate wall rocks, involving both dolostones and limestones. These processes had dramatic effects on magma compositions, especially on phonolites, which were transformed to foidites. Evidence of such a process is found in Group-4 samples, in which K-feldspar is observed to react with a matrix that represents strongly undersaturated melts formed by interaction between silicate magma and carbonates. Trace element data also testify to a very important role for F–CO2–H2O–S fluids during magma–wall rock interaction. Fluid transfer was responsible for extreme enrichments in Th, U, and LREE especially observed in Group-3 and Group-4 rocks. Implications of these processes for potassic magma evolution in Central Italy are discussed.

Crystallization of REE minerals and redistribution of U, Th, and REE at contact boundary between granite and gabbro during hydrothermal alteration

In this study, redistribution behaviors of U, Th, and rare earth elements (REE) in association with igneous activity in the Unazuki region of southwest Japan were investigated. At the contact boundary between granite and gabbro, REE minerals such as monazite, xenotime, and REE oxides formed with Fe–Ti oxides in K-feldspar assimilated by gabbro. U was enriched in xenotime (UO 2 > 0.72 wt.%) and REE oxides (UO 2 > 0.10 wt.%), whereas Th was concentrated in REE oxides (ThO 2 > 1.32 wt.%). Considering the similarity between the ionic radii of U 4+ and Y 3+, U was incorporated into xenotime and REE oxides together with heavy REE. Th incorporation into xenotime (eightfold coordinated site) and monazite (ninefold coordinated site) was constrained by the lower contents of Gd and Dy, respectively. Saussuritization of plagioclase in gabbro and chloritization of biotite in granite at the contact boundary suggest low-temperature hydrothermal alteration at late stages of gabbro intrusion that occurred at 253 ± 1 Ma. In contrast, zircon in the chloritized granite contained altered domains characterized by a dark response between back-scattered electron and cathodoluminescence images; this was caused by a low average atomic number due to the release of Zr and Hf; the incorporation of non-formula elements such as Ca, Fe, and Al; and low crystallinity because of radiation damage. The altered domains showed enrichments of Th (ThO 2 > 1.50 wt.%) and light REE (LREE) to middle REE (MREE) as well as positive Eu anomalies; this suggests the preferential incorporation of Th and LREE–MREE (particularly Eu 2+) with Ca occurring during the hydrothermal alteration. The redistribution behaviors of U, Th, and REE during low-temperature hydrothermal alteration strongly depend on their ionic radii. Thorite in zircon was probably formed by the exsolution of high-Th altered domains.

Amalgamation between the Yangtze and Cathaysia Blocks in South China: Constraints from SHRIMP U–Pb zircon ages, geochemistry and Nd–Hf isotopes of the Shuangxiwu volcanic rocks

South China was formed through the amalgamation of the Yangtze Block with the Cathaysia Block, but the timing of this amalgamation is controversial, ranging from Mesoproterozoic to Mesozoic. We report here SHRIMP U–Pb zircon ages, geochemistry and Nd–Hf isotopes of the Shuangxiwu Group volcanic rocks from the southeastern Yangtze Block. These rocks were strongly deformed, metamorphosed to greenschist-facies, intruded by 849 ± 7 Ma dolerites, and unconformably overlain by Neoproterozoic rift successions of no older than ca. 820 Ma. The Beiwu and Zhangcun volcanic rocks from the middle and uppermost Shuangxiwu Group were dated at 926 ± 15 Ma and 891 ± 12 Ma, respectively. All the studied rocks are characterized by highly positive ɛNd(T) (5.4–8.7) and ɛHf(T) (11.0–15.3) values. The Pingshui basaltic and andesitic rocks from the lower Shuangxiwu Group, which were previously dated at ca. 970 Ma, are high in Al 2O 3 (15–20%) but low in MgO (<8%), and are characterized by enrichments in Th and LREE but depletions in Nb, Ta, Zr, Hf and Ti, broadly similar to high-Al basaltic rocks in many volcanic arcs. The Beiwu andesitic to rhyolitic rocks have higher MgO than the experimental melts of basaltic rocks, and their Al 2O 3 content decreases with increasing SiO 2, similar to the regional coeval tonalites and granodiorites, suggesting their formation by crystal fractionation of basaltic parent magma. The Zhangcun volcanic rocks are high in SiO 2 (mostly >69%), low in MgO (0.35–1.2%), and have nearly constant Al 2O 3 contents of 14–15% and relatively uniform trace element concentrations. They were generated by remelting of juvenile mafic to intermediate arc rocks. Overall, the Shuangxiwu Group volcanic rocks and associated intrusive tonalites and granodiorites constitute a typical calc-alkaline magmatic assemblage of a 970–890 Ma active continental margin. These results and the 849 ± 7 Ma zircon U–Pb age for the undeformed doleritic dikes intruding the Shuangxiwu Group suggest that the tectonic regime of the study region transformed from plate convergence to intracontinental rifting in the time period between ca. 890 Ma and ca. 850 Ma. Previously reported 1.04–0.94 Ga metamorphic and deformation ages from the nearby Tianli Schists and evidence for the final closure of the back-arc basin at ca. 880 Ma (ophilitic obduction at Xiwan), further suggest that the amalgamation between the Yangtze and Cathaysia Blocks, likely through “soft docking” at the eastern segment of the Sibao orogen, was completed at ca. 880 Ma or soon after.

Boron isotopic variations in NW USA rhyolites: Yellowstone, Snake River Plain, Eastern Oregon

The geochemistry of NW USA rhyolites correlates strongly with geography and the nature of the underlying basement terranes. Rhyolites from the Snake River Plain-Yellowstone (SRPY) province have higher 87Sr/ 86Sr, 207Pb/ 206Pb, and lower 143Nd/ 144Nd than those from the Oregon High Lava Plains (HLP) province, reflecting a dominant influence of Precambrian cratonic crust east of the western Idaho suture zone versus accreted oceanic terranes of Phanerozoic age to the west. Rhyolites from the cratonic domain show significant enrichments of Th, U, and LREE/HREE, whereas B concentration and especially B/Nb and B/Rb are systematically higher west of the tectonic boundary. This decoupling of B from the other incompatible elements is best explained in terms of distinctive magmatic sources east and west of the suture zone. B isotopic composition [ δ 11B] was measured for natural and synthetic glasses via multiple multiplier laser ablation-ICP-MS. δ 11B values are systematically lighter in SRPY rhyolites (− 5.6 to − 8.9 ‰) compared to those from the HLP (− 0.8 to − 3.1 ‰). These data are consistent with strongly fluid-depleted and/or metamorphosed sources for SRPY rhyolites, whereas HLP sources resemble those of typical oceanic basalts, and could reflect melting of juvenile basalt-derived protoliths in the crust. B isotope ratios of low- δ 18O rhyolites are indistinct from those with normal δ 8O, suggesting that δ 11B values are not strongly affected by hydrothermal processes that alter source materials with meteoric water. Considering all data, it is likely that B compositions of the rhyolites are inherited from their sources in the crust. Although low δ 11B (< 0 ‰) is also observed in many mantle-derived basalts (i.e., OIB, MORB), in SRPY rhyolites it is associated with enrichments of elements (e.g., U, Th, Rb, LREE) typically concentrated in continental crust, and thus old high-grade metamorphic continental crust is inferred to be 11B-depelted. If rhyolite protoliths were originally metasediments, it is likely that bulk B and δ 11B were selectively removed by metamorphic dehydration reactions and transported to the surface via 11B-enriched fluids.

High and low temperature alteration of uranium and thorium minerals, Um Ara granites, south Eastern Desert, Egypt

The Um Ara area, in the south Eastern Desert of Egypt contains a number of uranium occurrences related to granitic rocks. U-rich thorite, thorite and zircon are the main primary uranium- and thorium-bearing minerals found in mineralized zones of the Um Ara alkali-feldspar granites; uranophane is the most common secondary uranium mineral. U-rich thorite contains blebs of galena, has rims of uranophane and contains inclusions of Zr-rich thorite. Electron probe microanalysis (EPMA) provides an indication of a range of solid solution between thorite and zircon, in which intermediate phases, such as Th-rich zircon and Zr-rich thorite, were formed. These phases have higher sum of all cations per formula (2.05 to 2.06 apfu, for 4 oxygen atoms) than that of ideal thorite and zircon. This is attributed to the presence of substantial amount of interstitial cations such as Ca, U and Al in these phases. Some zircon grains are stoichiometric in composition, other altered grains display lower SiO 2 and ZrO 2 contents. Enrichment of Th and U in altered zircon preferentially involves coupled substitution (Ca 2+ + (Th,U) 4+ ↔ 2Zr 4+ + 2Si 4+), implying that significant U and Th may enter the Zr and Si position in zircon. Negative correlation of Zr vs. Hf and Al may indicate that Hf and Al have been introduced to the zircon during later fluid alteration rather than during the primary magmatic event. A two-stage metallogenetic model is proposed for the alteration processes and origin of U- and Th-bearing minerals in the Um Ara alkali-feldspar granite: 1) the first stage was dominated by hydrothermal alteration and accompanied by albitization, k-feldspathization, desilicification, chloritization, hematitization, silicification, argillization, fluoritization and corrosion of primary U-bearing minerals. Solid-solution between thorite and zircon occurred during this stage. The second stage occurred at the near-surface profile where circulating meteoric water played an important role in mobilizing the early formed primary U-bearing minerals. Uranium was likely transported as a calcium uranyl carbonate complexes. When these complexes lost their stabilities by precipitation of calcite, they decomposed in the presence of silica to form uranophane.

Assessing the potential involvement of an early magma staging chamber in the generation of the Platreef Ni–Cu–PGE deposit in the northern limb of the Bushveld Complex: a pilot study of the Lower Zone Complex at Zwartfontein

The Platreef of the northern limb of the Bushveld Complex is one of the world's most significant deposits of platinum-group elements (PGE) with associated Ni and Cu. The origin of the Platreef is controversial. Some workers suggest that it is a northern facies of the Merensky Reef or part of the Upper Critical Zone, while others have suggested that the Platreef formed by processes entirely contained within the northern limb, unrelated to mineralisation events elsewhere in the complex. The northern limb is separated from the rest of the complex by the Thabazimbi–Murchison Lineament (TML) and the effect that this structure had on the intrusion of Bushveld magmas is debated. The presence of chilled rocks and cross-cutting relationships between the Platreef and its hangingwall gabbronorites would seem to preclude the magma that formed the hangingwall also acting as a source of PGE to the Platreef. The base metal sulphides in the Platreef carry very high PGE tenors (comparable with the Merensky Reef) indicating that the PGE must have been concentrated from a large volume of magma, but the source of that magma has not been established. In order to solve this PGE mass balance paradox, McDonald and Holwell have suggested that the magmas that formed the (pre-Platreef) Lower Zone may have been the source of PGE. At present, other models do not involve any significant role for the Lower Zone magmas in forming the Platreef. The data presented in this pilot study of the Lower Zone intrusion at Zwartfontein test some of the predictions arising from the McDonald and Holwell model. They highlight some important first order differences between Lower Zone intrusions in the northern limb compared with the rest of the Bushveld Complex. The enrichment in Th and LREE that characterizes the Lower Zone rocks in the eastern and western Bushveld appears to be missing in the northern limb. The lithophile element signatures of the different types of Lower Zone are suggested to result from mafic magmas intruding north and south of the TML and being contaminated by these different types of crust. Most significantly, the study has also revealed strong depletion of chalcophile elements (Ni, Cu and PGE) in the Lower Zone intrusion at Zwartfontein. The results are consistent with the depleted products expected from the processing of pre-Platreef magma(s) by interactions with sulphides at a deeper level within the magmatic plumbing system. The results provide a positive first test of one of the predictions arising from the McDonald and Holwell Platreef model. The existence of a system capable of removing PGE and producing a large volume of depleted ultramafic cumulates, in close proximity to the most highly mineralised sector of the Platreef, is suggested to be highly significant.

Petrogenetic and Tectonic Significance of Rift-Related Pre-Early Cambrian Mafic Dikes, Central Taurides, Turkey

Widespread swarms of tholeiitic dikes of pre-Early Cambrian age intrude Cadomian post-collisional Late Neoproterozoic granitoid rocks of the Sandikli Basement Complex in the Central Taurides, and the succession is disconformably overlain by Early Cambrian rift-controlled clastic and mafic rocks of the Gögebakan Formation. The studied dikes show distinct negative anomalies for Nb, Zr, Hf, and Ti, and enrichment of Th and LREE. These data indicate a MORB signature with subduction-modified mantle chemistry. The dikes are less intensely enriched in LREE with rather unfractionated spider diagrams ([Nb/Yb]N = 1.10-4.28) than N-MORB, and lack conclusive chemical evidence of crustal contamination. All analyzed rocks possess slightly negative Eu anomalies (Eu/Eu* = 0.77-1.02). The dikes show sharp negative Nb and Ta anomalies relative to LREE and low Ti/Y, Zr/Y, and Nb/U ratios, suggesting that they were derived through incorporating subduction-released fluids at the source during the partial melting of a spinel lherzolitic source. Petrogenetic modeling suggests that the dikes were produced by low-degree partial melting (<5%) of spinel lherzolite subsequent to subduction enrichment during magma ascent. The mafic dikes apparently erupted in a marginal basin formed during an initial stage of pre-Early Cambrian rifting of peri-Gondwanan continental margin. Emplacement was controlled by an extensional regime that followed after calc-alkaline granitoidic intrusion of the Cadomian orogeny.

NEW GEOCHEMICAL AND AGE DATA ON THE OPHIOLITES FROM THE OTHRYS AREA (GREECE): IMPLICATION FOR THE TRIASSIC EVOLUTION OF THE VARDAR OCEAN

This paper presents new radiolarian biostratigraphic and volcanic geochemical data for the Agoriani Melange Unit and Fourka Unit, which represent the basal part of the Othrys ophiolitic nappe (central Greece). The Agoriani Melange Unit is the base of the Othrys ophiolitic nappe, consists of a tectono-sedimentary melange that is, in turn, tectonically overthrust by the Fourka Unit, which consists of basalt-radiolarian chert sequences. Geochemical data show that, in the study area, the Agoriani Melange includes alkaline within-plate basaltic (WPB) and normal-type mid-ocean ridge (N-MORB) sequences in the pillowfacies, while the Fourka Unit mainly consists of normal-type mid-ocean ridge basaltic (N-MORB) sequences, as well as low-K tholeiitic (L-KT) sequences, both in occurring in the pillow-facies. Alkaline WPB have a clear enriched nature, with high concentration of high field strength elements (HFSE), relative enrichment in light (L-) rare earth elements (REE) with respect to heavy (H-) REE, and high Nb/Y (> 0.7), Ce/Y (> 1.5) Ta/Hf (> 0.35), Th/Yb (> 0.7) and Ta/Yb (> 0.3) ratios. N-MORB rocks have a more depleted nature with flat N-MORB normalized HFSE patterns, flat or slightly LREE depleted chondrite normalized REE patterns, and low Nb/Y (< 0.3), Ce/Y (< 0.5) Ta/Hf (< 0.1), Th/Yb (< 0.08) and Ta/Yb (< 0.05) ratios. L-KT basalts show an evident subduction- related geochemical signature exemplified by depletion of Ta, Nb, Zr, and Ti relative to other HFSE, slight LREE/HREE enrichment, and relative enrichment of Th with respect to Ta. Biostratigraphic data for the Fourka Unit show that radiolarites associated with N-MORBs were deposited, on the whole, in Middle and Late Triassic time. In particular the late Anisian - early Ladinian age found in one stratigraphic section represents the oldest age ever reported for a MORB sequence in the Hellenides. Radiolarites deposited over the L-KT basaltic sequence are early Carnian - middle Norian/Late Norian in age. The radiolarites associated with alkaline WPB in the Agoriani Melange did not yield determinable radiolarians, while a radiolarite sequence in this melange not associated with basalts is middle Bathonian to late Bathonian/early Callovian in age. The data presented in this paper provide new constraints on the early (Middle- Late Triassic) phase of evolution of the Vardar Ocean in the Hellenides. The MORBs are interpreted to have erupted during Middle-Late Triassic time, related to the initial opening of the Vardar oceanic basin. The L-KT basalts are interpreted to have erupted during Middle-Late Triassic time (i.e., in part contemporaneously with MORBs), in the ocean-continent transition zone adjacent to the rifted continental margin. Moreover, we speculate that the alkaline WPB have formed in oceanic seamounts or in the ocean-continent transition zone adjacent to the rifted continental margin.

NEW EVIDENCE FOR SUPRA-SUBDUCTION ZONE OPHIOLITES IN THE VARDAR ZONE OF NORTHERN GREECE: IMPLICATIONS FOR THE TECTONO-MAGMATIC EVOLUTION OF THE VARDAR OCEANIC BASIN

The Vermion Massif (northern Greece) is located across the boundary between the Pelagonian and Vardar Zones and includes several tectonic units bearing ophiolitic rocks, which represent remnants of the oceanic lithosphere formed in the Neotethyan Vardar Ocean, between the Pelagonian and the Serbo- Macedonian continental realms. This massif consists of tectonic units belonging to the Pelagonian Domain, which are tectonically overlain by units associated with the Almopias sub-Zone (Vardar Zone). Ophiolitic rocks consist of mantle harzburgites and ophiolitic melanges. The ophiolitic melanges incorporate rocks exhibiting a wide range of composition, including various intrusive rocks and volcanic rocks ranging from basalts, basaltic andesites, andesites, dacites, to rhyolites. Incompatible elements and rare earth elements analyses indicate that a number of different rock-types formed in distinct tectonic settings can be identified. Mantle harzburgites have a very depleted nature and represent portions of the supra-subduction (SSZ) mantle developed in an intra-oceanic arc setting. The melanges units include six rock types variably distributed in the Pelagonian and Almopias Units. They are: (1) calc-alkaline rocks with marked depletion in Nb, Ta Ti and enrichment in LREE, Th; (2) LREE-depleted N-MORB; (3) LREE-enriched E-MORB; (4) alkaline within-plate basalts showing marked enrichments in Th, Ta, Nb, LREE; (5) low-Ti island arc tholeiites featuring depletion in HFSE; and (6) very-low-Ti boninites characterized by strong depletion in HSFE and REE. Previous interpretations have referred the Vardar ophiolites to a MORB-type oceanic setting and to a MORB-type backarc setting; however, the widespread occurrence of SSZ ophiolites has never been documented in the Vardar Zone before and is particularly important as it testifies for the existence of an intra-oceanic arc basin in the Vardar oceanic domain. The results presented in this paper compared with literature data on other magmatic rocks within the Vardar Zone suggest that the opening and closure of the Vardar Ocean record several distinct accretion events in this basin, that is oceanic crust generation at mid-ocean ridge, alkaline seamounts in the oceanic domain and SSZ setting, as well as two accretion events in the western realm of the Serbo-Macedonian continent, that is volcanic arc and backarc settings. Based on the comparison between the modern west Pacific subduction system and the results from this study a new model for a multistage tectono-magmatic evolution of the Vardar Ocean is proposed.

Chronological and geochemical studies of granite and enclave in Baimashan pluton, Hunan, South China

Zircon LA-ICP-MS U-Pb dating reveals that the Baimashan Pluton is composed mainly of late Indosinian (204.5±2.8 Ma-209.2±3.8 Ma) biotite granodiorites/monzonitic granites (LIGs) and early Yanshanian (176.7±1.7 Ma) two-micas monzonitic granites (EYGs), and the coeval (203.2±4.5 Ma-205.1±3.9 Ma) mafic microgranular enclaves (MMEs) are generally found in the former. In addition, the ages of cores within zircons from LIGs and MMEs ranging from 221.4±4.0 Ma to 226.5±4.1Ma provide evidence of multistage magma intrusion during Indosinian in the study area. Measured 3010±20.6 Ma of inherited zircon age suggests that there may be recycling Archaean curstal material in existence in this area. LIGs and EYGs share some similar geochemical features: subalkaline and peraluminous granites, enrichment of Th, U, K, Ta, Zr, Hf and LREE but depletion of Ba, Nb, P, Ti and Eu, low ɛ Nd(t) values but high (87Sr/86Sr)i ratios, and old T 2DM (ca. 1.9–2.0 Ga). The behaviors of incompatible elements and REE are mainly dominated by fractional crystallization of plagioclase, K-feldspar, ilmenite and apatite, but that of Sr isotope mainly controlled by EC-AFC. They are crust-sourced and derived from partial melting of paleo-Proterozoic metagreywackes and related to biotite dehydration melting. LIGs are formed in post-collisional tectonic setting as crustal local extension and thinning during late Indosinian. But EYGs may be evolved products of congeneric granitic magma with LIGs formed in late Indoinian, which were emplaced again when crust underwent extensive thinning and extension in post-orogenic tectonic setting during Yanshanian in SC after undergoing EC-AFC. MMEs should be cognate enclaves and derived from liquid immiscibility of host magma.