Heavy REE Depletion Research Articles

The Subduction-Related Metavolcanic Rocks of Maroua, Northern Cameroon: New Insights into a Neoproterozoic Continental Arc Along the Northern Margin of the Central African Fold Belt

The metavolcanic rocks around Maroua in the Far North Region of Cameroon are located at the northern margin of the Central African Fold Belt (CAFB) and have not been studied to date. The petrographic and whole-rock geochemical data presented in this paper highlight their magma genesis and geodynamic evolution. The lavas are characterized by basaltic, andesitic, and dacitic compositions and belong to the calc-alkaline medium-K and low-K tholeiite series. The mafic samples are essentially magnesian, while the felsic samples are ferroan. On a chondrite-normalized REE diagram, mafic and felsic rocks display fractionated patterns, with light REE enrichment and heavy REE depletion (LaN/YbN = 1.41–5.38). The felsic samples display a negative Eu anomaly (Eu/Eu* = 0.59–0.87), while the mafic lavas are characterized by a positive Eu anomaly (Eu/Eu* = 1.03–1.35) or an absence thereof. On a primitive mantle-normalized trace element diagram, the majority of the samples exhibit negative Ti and Nb–Ta anomalies (0.08–0.9 and 0.54–0.74, respectively). These characteristic features exhibited by the metavolcanic rocks of Maroua are similar to those of subduction-zone melts. This subduction would have taken place after the convergence between the Congo craton (Adamawa-Yadé domain) and the Saharan craton (Western Cameroonian domain). Petrological modelling using major and trace elements suggests a derivation of the Maroua volcanics from primitive parental melts generated by the 5–10% partial melting of a source containing garnet peridotite, probably generated during the interaction between the subducted continental crust and the lithospheric mantle and evolved chemically through fractional crystallization and assimilation.

Fractional crystallization of garnet in alkali basalts at >1.8 GPa and implications for geochemical diversity of Large Igneous Provinces

A suite of lavas from northwest Mull, British Palaeocene Igneous Province, exhibit major and trace element characteristics that are best explained by very high pressure (>1.8 GPa) crystallization of an assemblage comprising aluminous clinopyroxene (Al-Cpx) plus garnet. The resulting series of consanguineous magmas are mildly Si-undersaturated. The trace element effects of this crystallizing assemblage are manifested in increasing light rare earth element (LREE) enrichment and heavy REE (HREE) depletion with decreasing whole-rock MgO, a predictable consequence of the bulk distribution coefficient (D) of this assemblage being > > 1 for HREE but <<1 for the LREE. Early crystallization of Al-Cpx and replacement of plagioclase feldspar by garnet in the crystallizing assemblage also results in increasing or near constant Al2O3 abundances in cotectic compositions with MgO < ∼7 wt%. Garnet as opposed to plagioclase crystallization is also reflected in the incompatibility of Sr and compatibility of Y resulting in very high Sr/Y lavas (up to 100) with high Sr (∼1200 ppm) and low Y contents (∼12 ppm). Lavas also have high Zr/Y (up to 30). Major element constraints suggest the crystallizing assemblage comprised ∼40% garnet and 60% Al-Cpx.Magmas that fractionated at ∼1.8 GPa rose to the surface without interacting with the continental crust. Immediately underlying basalts and picrites show evidence of crustal contamination and assimilated fusible portions of the crust, therefore effectively lining the plumbing system and allowing later magmas to rise to the surface without crustal contamination. A local change in tectonic regime from extension to passive appears to be linked to a change from low pressure to very high-pressure crystallization of magmas. Whilst evidence for garnet fractionation in continental flood basalts is very rare, this paper provides a characterization of its geochemical consenquences.

Origin of Erzgebirge ultrahigh‐pressure garnetite: Formation from a basaltic protolith by serpentinization‐assisted metasomatism?

AbstractErzgebirge ultrahigh‐pressure (UHP) garnet peridotite includes scarce layers of garnet pyroxenite, nodules of garnetite and, very rarely, of eclogite. Peridotite‐hosted eclogite shows the same subalkali‐basaltic bulk rock composition, mineral assemblage and peak conditions as gneiss‐hosted eclogite present in the same UHP unit. Garnetite has considerably more Mg, moderately enhanced Ca and Fe and significantly lower contents of Na, Ti, P, K and Si than eclogite, whereas Al is very similar. In addition, the compatible trace elements (Ni, Co, Cr, V) are elevated and most incompatible elements (Zr, Hf, Y, Sr, Rb and rare Earth elements [REE]) are depleted in garnetite relative to eclogite. In contrast to other large ion lithophile elements (LILEs), Pb (+121%) and Ba (+83%) are strongly enriched. The REE patterns of garnetite are characterized by depletion of light and heavy REE and a medium REE hump indicative of metasomatism, features being absent in eclogite. An exceptional garnetite sample shows an REE distribution similar to that of eclogite. Garnetite is interpreted to have formed from the same, but metasomatically altered, igneous protolith as eclogite. Except for Ba and Pb, the chemical signature of garnetite is explained best by metasomatic changes of its basaltic protolith caused by serpentinization of the host peridotite. Garnetite is chemically similar to basaltic rodingite/metarodingite. Although rodingite is commonly more enriched in Ca, there are also examples with moderately enhanced Ca matching the composition of Erzgebirge garnetite. Limited Ca metasomatism is attributed to the preservation of Ca in peridotite during hydrous alteration. This can be explained by incomplete serpentinization favouring metastable survival of the original clinopyroxene. In this case, most Ca is retained in peridotite and not available for infiltration and metasomatism of the garnetite protolith. This inescapable consequence is supported by the fact that clinopyroxene is part of the garnet peridotite UHP assemblage, which would not be the case if Ca had been removed from the protolith prior to high‐pressure metamorphism. The enrichment of compatible elements in garnetite is attributed to decomposition of peridotitic olivine (Ni, Co) and spinel (Cr, V) during serpentinization. Enrichment of Ba and Pb contrasts the behaviour of other LILEs and is ascribed to dehydration of the serpentinized peridotite (deserpentinization). This requires two separate stages of metasomatism: (1) intense chemical alteration of the basaltic garnetite precursor, together with serpentinization of peridotite at the ocean floor or during incipient subduction; and (2) prograde metamorphism and dehydration of serpentinite during continued subduction, thereby releasing Pb–Ba‐rich fluids that reacted with associated metabasalt. Finally, subduction to &gt;100 km and UHP metamorphism of all lithologies led to formation of garnetite, eclogite and garnet pyroxenite hosted by co‐facial garnet peridotite as observed in the Erzgebirge.

Whole Rock, Mineral Chemistry during Skarn Mineralization-Case Study from Tongshan Cu-Mo Skarn Profile

Studying the activation, migration and precipitation processes of ore-forming elements is essential for understanding the genesis and mechanisms of skarn deposits. A typical skarn profile formed by the intrusion of Yanshanian granodiorite into the Carboniferous carbonate strata was studied. The profile is highly consistent with the classic skarn profile, ranging from the intrusion, weak alteration belt, skarn belt (inner and outer skarn belt) and mineralization belt (mainly characterized by Cu mineralization) to the surrounding marble without being affected by late-stage low-temperature or supergene weathering alteration. Whole-rock data show that the major and trace elements exhibit relatively small changes in the granodiorite and inner skarn, but huge variation in the boundary between the inner and outer skarn; Na, Al, Ti and Sr show significant decreases, while Fe, Mg, Zn, V and Ni show significant increases. The elemental content in the outer skarn is 10–100 times or more higher than that in the marble, but the elements such as Ca, Sr and Cs diluted from the marble. During the migration process from the inner skarn to the outer skarn, some elements (such as K, Rb and Ba) were depleted in the inner, but not enriched in the outer, indicating that they may migrate to farther locations. Grossularite developed in the inner skarn, with light rare earth element (LREE) depletion and heavy REE enrichment, as well as positive and negative anomalies of Eu (δEu = 0.42–3.95). Andradite developed in the outer skarn, with zonation development, light REE enrichment, and heavy REE depletion and a positive Eu anomaly (δEu = 0.36–46.83). Some negative Eu anomalies appear at the edges of garnets in the outer skarn, indicating fluctuations in fO2 during the late skarn process. A positive correlation between Fe3+ and REE3+ in the garnets from the inner skarn, as well as between Al3+ and REE3+ from the outer skarn indicated that there are different YAG substitution mechanisms of REE between the inner and outer skarn. Low garnet REE contents and highly variable Y/Ho ratios in outer skarn suggest that the significant fluctuations in REEs may be primarily controlled by water-rock interactions. Considering the whole-rock major and trace element contents, as well as the trace element features of garnet, we found that whole-rock Na, Al, Ti and Sr elements, garnet Ti, Zr and Nb elements exhibit significant differences between the inner and outer skarn. These characteristics can be used to distinguish the boundary between the rock body and carbonate during the skarnification process.

Geochemical, mineralogical, and petrological analyses for the interpretation of the sedimentary environment of the Middle-Late Ordovician Majiagou Formation (northern China) as a tool for more effective gas exploration

Core samples from the deeply buried Ordovician Majiagou Formation below the Huainan Coalfield (E China) have been investigated for their carbonate types, major and trace elements (including rare earth elements) and C and O isotopes. The objective was to get a better insight into the possible occurrences of gas (and possibly oil) derived from Carboniferous coals. It was found that the carbonates are dolomites with strongly varying amounts of CaO and MgO. The low concentrations of SiO 2 , Al 2 O 3 , and Fe 2 O 3 indicate deposition in a normal marine environment with little terrigenous input. The Na 2 O/K 2 O, Fe/Mn and Sr/Ba ratios, as well as the Ga values indicate mainly a marine salinity and a hot and humid climate. The slight depletion of Ce and Eu, the depletion of heavy REE and the enrichment of light REE indicate deposition in a reducing environment. It thus appears that the Majiagou Formation below the Huainan Coalfield closely resembles that in the eastern part of the Ordos Basin, where several gas reservoirs are present, so that the Majiagou Formation under the Huainan Coalfield represents a promising target for hydrocarbon exploration.

Sheared and Granular Peridotites from the Udachnaya–East Kimberlite (Yakutia): Mineralogy, Chemistry, and PGE Patterns

Abstract —Mantle xenoliths of fresh sheared and granular peridotites from the Udachnaya kimberlite pipe (Yakutian kimberlite province, Siberia, Russia) are studied in terms of mineralogy, chemistry, and behavior of platinum-group elements (PGE), with a focus on difference between the two types of peridotites and on the role of PGEs in their origin. The analyzed samples of sheared and granular peridotites differ in mineral-liquid equilibrium temperatures and pressures: 1230–1350 °C, 56–67 kbar inferred for the former and 750–1300 °C, 30–67 kbar for the latter. The contents of major oxides, trace elements, and PGEs differ both between sheared and granular peridotites and within each group. Some sheared and all granular peridotites are enriched in incompatible elements, have low percentages of clinopyroxene and contain garnet rich in middle rare-earth elements (MREE). These features result from interaction of the peridotites with melts and related fluids of presumably carbonate compositions containing incompatible elements in high abundances. Sheared peridotites were more strongly enriched and gained more Ca, Al, and Ti than the granular variety. On the other hand, granular peridotites have higher light REE (LREE) and lower heavy REE (HREE) enrichment, which may record compositional difference of metasomatic agents. Metasomatism acted in two stages: first silicate agents caused LREE enrichment and HREE depletion of melts and then the fractionated melts percolated through the mantle and affected the compositions of granular peridotites and, partly, sheared peridotites. The two types of peridotites differ also in PGE patterns, with largely variable Os–Pt concentrations in granular peridotite. This difference may be due to uneven distribution of the Os–Ir and Pt–Os–Ir intermetallic compounds, which formed as sulfide melting residue in the lithospheric mantle. Sheared peridotites show almost uniform Os–Pt distribution and are enriched in Pd, Re, and partly Pt. The enrichment possibly occurred during metasomatism when the percentages of garnet and clinopyroxene increased while sulfide phases precipitated into interstitials.

Middle Jurassic–Lower Cretaceous glendonites from the eastern Barents Shelf as a tool for paleoenvironmental and paleoclimatic reconstructions

Glendonites are carbonate pseudomorphs after ikaite which use as indicators of near-freezing temperatures. We describe the first glendonites of Middle–Late Jurassic to Early Cretaceous age from the Barents Shelf, including the first glendonites of Late Barremian–Early Aptian age ever discovered on Earth and provide the detailed mineralogical-geochemical and isotopic studies. Mesozoic glendonites are well-known across the Arctic realm, however the ikaite-calcite transformation and implication of glendonite findings are still debatable. Pseudomorphs are mainly composed of calcite. Cathodoluminescence and scanning electron microscopy studies reveal three types (I-III) of calcite: type I - elongated, rounded and irregular calcite; type II - blocky, fibrous and needle-like calcite/siderite; type III - blocky calcite in pores. Elemental analyses distinguish geochemical differences between the calcite types: type I comprises pure CaCO3; type II calcite contains Mg and P (in all studied samples), Fe (Bajocian–Callovian), S (Middle Volgian) or Sr (Upper Barremian–Lower Aptian); type III calcite is depleted in Mg, P and Fe. Concentrations of Mn, Fe, Sr and calculated Mn/Ca, Fe/Ca and Sr/Ca ratios are suggestive of diagenetic alteration. Bulk δ18O values range from −5.39 to −1.71‰ VPDB, supporting overprinting of primary values during burial diagenesis, while bulk δ13C values range from −33.3 to −22.6‰ VPDB, providing the influence of organic matter on ikaite-glendonite formation. However, 87Sr/86Sr values range from 0.7070 to 0.7110, mainly corresponding to Jurassic–Cretaceous seawater. Rare-earth elements characterize porewater REE-patterns, with slight middle REE-enrichment and depletion in light and heavy REE. Weak positive and negative Ce anomalies indicate fluctiating from anoxic to oxic conditions during ikaite-glendonite formation. Our new biostratigraphic data refine the ages of glendonite-bearing horizons in the Barents Shelf region, allowing a more definitive correlation with coeval glendonite occurrences across the Arctic realm and with global climatic changes during the Jurassic–Cretaceous.

Tracking geochemical signatures of rare earth and trace elements in spring waters and outcropping rocks from the hidden geothermal system of Acoculco, Puebla (Mexico)

A geochemical mapping of rare earth (REE) and other trace elements in water and rock samples of the hidden geothermal system of the Acoculco caldera was carried out. REE and other trace elements were measured at low concentrations in waters (ΣREE <50 μg/L) and rocks (ΣREE <773 μg/g), and used, for the first time, to understand the water-rock interaction processes of this geothermal system. A bimodal frequency distribution of pH was found among acidic waters (pH <3.5) that emerge from cold springs located inside the caldera, and slightly-acid to hot alkaline spring waters (pH 5 to 9.2) from outside the caldera. The geochemical modelling of REE showed that minerals containing Al3+, Fe, and F− were the main phases that control their concentrations in acidic waters. Different degrees of REE complexation in waters located inside the Acoculco caldera (Alcaparrosa and Los Azufres) clearly showed new signatures of geothermal activity. LnSO4+ complexes and free ions of Ln3+ were identified as major species of the acidic waters of Alcaparrosa, whereas in less acidic waters of Los Azufres, LnF2+ show an increase in their complexation percentages. The REE chondrite-normalized patterns observed in acid waters display a depletion from light-REE (LREE) to heavy-REE (HREE), and an inverse correlation between high REE concentrations and acid pH's. The REE concentration in felsic and intermediate rocks shows “gull-wing” chondrite-normalized patterns characterised by an enrichment of LREE, a negative anomaly of Eu, and depletion of HREE with a flat tendency. The present study of water-rock interaction contributed to the identification of new geochemical signatures of major and trace elements that provide new evidences of deep formation temperatures between 200 °C and 300 °C, becoming in an effective tool for finding out convincing anomalies in hidden geothermal systems.

>1.8 Ga cold subduction of lithospheric mantle: Evidences from the Fengzhen olivine-bearing garnet pyroxenite xenoliths in Trans-North China Orogen

Whether modern-style subduction exists in Paleoproterozoic has been hotly debated because of the scarcity of robust petrological evidence. Here, we present a comprehensive study of olivine-bearing garnet pyroxenite xenoliths hosted in the Paleoproterozoic Fengzhen carbonatite. The carbonatite is located in the conjugate area between the Paleoproterozoic Trans-North China Orogen (TNCO) and the Khondalite Belt in North China Craton (NCC) with a dated age of 1810 ± 3 Ma. Petrographically, four-stages of evolution have been identified in the studied olivine-bearing garnet pyroxenite: 1) the formation of the protolith spinel websterite (S1), 2) the prograde metamorphism of spinel-lherzolite facies to garnet-lherzolite facies (M1), 3) retrograde metamorphism to Ariegite subfacies (M2) with formation of symplectite-I, and 4) Seiland subfacies (M3) forming symplectite-II. The clinopyroxene in the xenoliths display high Mg# (Mg2+/(Mg2++Fe2+)*100 = 89–94), strongly depleted HREE (heavy REE) and HFSE (high field strength elements; e.g., Nb, Zr, Ti) and enriched LREE (light REE) and LILE (large ion lithophile elements; e.g., Th and U). Similarly, the whole-rock chemistry exhibits high Mg# (85–88) and high Al2O3 + CaO (20.2–21.9 wt%) with enrichment in LREE and LILE (Th, U) and depletion in HREE and HFSE (e.g., Nb, Ta, Zr, Hf, Ti). The geochemical signatures imply that they might originate from the refractory mantle wedge in subduction zone and have been metasomatized by crust-derived melts. Major mineral thermobarometry yields the peak PT conditions of 26–33 kbar and 890–962 °C corresponding to a cold subduction geothermal gradient (307 ± 35 °C/GPa) with a depth up to 110 km for olivine-bearing garnet pyroxenites, which is also consistent with the results by zirconium-in-rutile and REE thermobarometers. This studies suggest that that the modern-style subduction of the lithospheric mantle was initiated at least since 1.8 Ga.

Petrology and geochemistry of Carboniferous volcanic rocks from the Awulale Iron Metallogenetic Belt in the West Tianshan Orogen (NW China): Constraints on petrogenesis and tectonic setting

The Awulale Iron Metallogenic Belt (AIMB) is the most largest submarine volcanic‐hosted iron metallogenic belts in Tianshan Orogen (NW‐China). The volcanic rocks from the Dahalajunshan Formation, extensively occurred in the western Tianshan Mountains, are mainly hosted‐iron rocks in the AIMB. In combination with published data, our new major and trace elements and isotopic data of volcanic rocks from the AIMB help define their petrogenesis and the tectonic evolution of the western Tianshan. The results indicate that these volcanic rocks belong to shoshonite, high‐K calc‐alkaline, and med‐K series with a small amount of tholeiite series, consisting mainly of basalt, trachybasalt, basaltic andesite, basaltic trachyte andesite, andesite, trachyte andesite, rhyolite, and dacite. They display similar geochemical features such as enrichment in large‐ion lithophile elements (LILE) and light REE (LREE), depletion in heavy REE and high‐field strength elements (HFSE), and prominent Nb‐Ta‐Ti negative anomalies, associated with the subduction‐related magmas. The basaltic and andesitic rocks show a more scattered range of initial 87Sr/86Sr ratios and higher positive εNd(t) values, whereas the acid volcanic rocks exhibit lower positive εNd(t) values with limited initial 87Sr/86Sr ratios. The basaltic rocks were derived from partial melting of phlogopite‐bearing mantle sources in the spinel stability field triggered by the slab‐released fluids of the subducted plate. The andesitic rocks originated by partial melting of mafic migmatites in response to metasomatism formed by early magmas derived from the mantle intruded into the crustal rocks. The acid volcanic rocks are likely derived from the partial melting of components in the lower crust. Therefore, the generation of the volcanic rocks in the AIMB is genetically linked to the geodynamic evolution of the western Tianshan from convergent subduction to collision during the Carboniferous. Both magmatism and hydrothermal processes control the mineralization, and in general, the parental magma of iron mineralization is basaltic and andesitic magma.

Late Cenozoic intra-plate basalts of the Greater Khingan Range in NE China and Khangai Province in Central Mongolia

This paper presents new major and trace element data, Sr-Nd-Pb isotopic ratios and K-Ar ages from volcanic rocks of the Greater Khingan Range of NE China (Nuomin–Keluo and Arxan–Chaihe volcanic fields) and the Khangai volcanic province of Central Mongolia. These data are discussed in correlation with available in literature geological, geochronological, geochemical, isotopic, petrologic and geophysical data from intra-plate volcanic fields of Central and East Asia. All volcanic rocks possess geochemical affinities to intra-plate oceanic and continental basalts. The Nuomin-Keluo samples (2.75 and 0.41 Ma) show enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE; La/YbN = 43.6–91.5), as well as depletion in heavy REE (HREE), and Nb peaks relative to La and Th (Nb/Lapm = 0.61–1.24; Nb/Thpm = 0.96–1.70) in the multi-element spectra. The Arxan-Chaihe basalts (0.91 and 0.53–0.27 Ma) have lower K, less fractionated REE (La/YbN = 15.7–28.1), but higher Nb peaks (Nb/Lapm = 1.56–1.74; Nb/Thpm = 1.27–1.79), as well as higher epsilon Nd and 206Pb/204Pb, but lower Sr isotopic ratios. The Khangai volcanic province consists of central and peripheral areas, which are dominated by low–alkali and high–alkali varieties, respectively. The central Khangai have lower LREEs and LILEs, and lower Gd/YbN ratios compared to the peripheral Khangai. In the 143Nd/144Nd vs. 87Sr/86Sr diagram, the Nuomin-Keluo and Khangai samples plot close to EM1, whereas the Arxan-Chaihe samples possess less enriched compositions. In Pb isotope diagrams, the Nuomin-Keluo and peripheral Khangai samples plot between DM and EM1, whereas the Arxan-Chaihe and central Khangai samples plot near PREMA. Geochemistry-based melting modeling indicates that all the volcanic rocks were derived from garnet-bearing, probably enriched, mantle sources melted at variable degrees Based on the whole set of new and previous data we suggest that the Great Khingan and Khangai volcanic rocks were probably produced by hydrous mantle plumes generated in the MTZ and ascending from the subducted Pacific slab stored therein. In general, the late Cenozoic intra-plate volcanism in Central and East Asia can be triggered by the material, which has accumulated beneath the Asian continent and metasomatized the mantle underneath since the amalgamation of Laurasia and formation of the Central Asian Orogenic Belt at ca. 300–250 Ma.

Large Paleozoic and Mesozoic porphyry deposits in the Central Asian Orogenic Belt: Geodynamic settings, magmatic sources, and genetic models

The Central Asian Orogenic Belt (CAOB) or Altaids contains several large porphyry Cu(-Au-Mo) and Au(-Cu) deposits. These deposits are genetically associated with equigranular and porphyritic tonalite, granodiorite and diorite. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that these hydrous, mineralized calc-alkaline intrusions were emplaced from the Cambrian to the Triassic.Trace element geochemistry and Sr–Nd–Hf–O isotopic compositions show two different signatures of the mineralizing intrusions. Mineralizing intrusions at Bozshakol, Aktogai, Nurkazghan, and Tuwu-Yandong have depleted Sr–Nd–Hf–O isotopic compositions (Isr = 0.7033 to 0.7047, εNd(t) = +3.6 to +6.6, εHf(t) = +9.6 to +16.4, δ18O = 6.8 to 4.7‰) and variable high Nb/Ta (11.6 to 16.4) ratios, implying a dominantly juvenile lower crustal source with mantle involvement at Bozshakol and Aktogai and sedimentary rocks contamination at Nurkazghan. Mineralizing intrusions at Erdenet, Koksai and Yubileinoe have slightly depleted Sr–Nd–Hf isotopic compositions (Isr = 0.70403 to 0.7043, εNd(t) = +2.9 to +3.8, εHf(t) = +6.6 to +11.6) and low δ18O (3.1 to 2.5‰), slightly enriched Sr–Nd–Hf–O isotopic compositions (Isr = 0.7044 to 0.7049, εNd(t) = +0.04 to +1.9, εHf(t) = 3.2 to 12.0, δ18O = 5.2 to 6.4‰) and enriched Sr–Nd isotopic compositions (Isr = 0.7041 to 0.7081, εNd(t) = −2.6 to −2.9), respectively, as well as intermediate to low Nb/Ta ratios (13.4–8.1), implying a juvenile lower crustal source with wall rocks contamination at Erdenet and ancient crust contamination at Koksai and Yubileinoe. Therefore, Juvenile sources played a significant role in the generation of fertile magmas for porphyry-Cu and -Au style mineralization in the CAOB.Mineralizing intrusions include the adakitic, transitional, and normal arc rocks. Mineralizing intrusions in some porphyry Cu(-Mo) deposits (e.g., Aktogai, Erdenet) have adakitic affinities based on their high Sr/Y (30–193) and La/Yb (17–49) ratios coupled with low Y (1.5–10.3 ppm) and Yb (0.2–1.0 ppm) concentrations. Variable high Sr/Y, Sm/Yb and La/Yb ratios suggest that the lower crustal source is probably a garnet-bearing amphibolite. These adakitic rocks were formed by partial melting of thickened lower crust at depths of >40 km. Mineralizing intrusions at most porphyry Cu(-Au) and Au deposits, including Bozshakol, Nurkazghan, Koksai, Tuwu-Yandong, and Yubileinoe, have variable intermediate Sr/Y (13–84) and La/Yb (7–23) ratios, intermediate to low Y (4.7–16.2 ppm) and Yb (0.5–1.3 ppm) concentrations, showing a transitional signature between adakitic and normal arc rocks. Variable moderate Sr/Y, Sm/Yb and La/Yb ratios suggest that the lower crustal source is probably a hydrous amphibole. These transitional rocks were formed by the MASH (melting, assimilation, storage, and homogenization) processes at depths of <40 km and AFC (assimilation and fractional crystallization) processes in a mid-crustal magma chamber. One large porphyry Cu-Au deposit (e.g., Nurkazghan) is associated with intrusions that have calc-alkaline geochemical signatures, with enrichment of light rare earth elements (LREE), depletion of heavy REE, Ba, Nb, Sr and Ti and low Sr/Y, Sm/Yb and La/Yb ratios, suggesting that the lower crustal source could be a clinopyroxenite. They are formed by partial melting of lower crust at depths of 30–35 km with AFC important to magma evolution. Here, three genetical models are suggested for these mineralizing magmas in the CAOB: (1) thick (> 40 km) lower crust melting for adakitic magmas; (2) MASH and AFC for transitional magmas; and (3) thin (30–35 km) lower crust melting followed by AFC for normal arc magmas. A majority of transitional and normal arc magmas suggest that thinner crust was important to the formation of porphyry Cu(-Au) and Au deposits that dominate in the CAOB.Our geochronology and geochemistry results indicate that most of the mineralizing intrusions from the porphyry deposits in the CAOB formed in island arc settings from the Cambrian to Triassic. The intrusions at Erdenet, Koksai and Yubileinoe are exceptions, forming in continental arcs in the Permian, Silurian and Devonian, respectively. Paleozoic and minor Mesozoic porphyry deposits in the CAOB formed during six periods. The greatest metal endowment is associated with the Devonian porphyry deposits (374 to 372 Ma, e.g. Oyu Tolgoi). The largest number of porphyry deposits were emplaced in the Carboniferous (334 to 317 Ma, e.g., Aktogai, Kounrad, Tuwu-Yandong, Almalyk). The CAOB is a complex collage of Paleozoic and Mesozoic tectonic elements and has considerable potential for further discoveries of large porphyry Cu(-Au-Mo) and Au deposits.

Zircon LA-ICP-MS Dating and Geochemical Characteristics of I-type Granitoids from the Yanhu Area, West Segment of the Bangongco-Nujiang Suture (Western Tibet): Petrogenesis and Implications for the Southward Subduction of the Tethyan Ocean

ABSTRACT The Yanhu granitoids are located in the west segment of the Bangongco-Nujiang suture in the western Tibetan Plateau. The main rock types of the granitoids are diorite porphyry, quartz diorite, granodiorite, granite and granite porphyry. Here, their zircon LA-ICP-MS U-Pb ages and petrogeochemical data are reported. Three groups of magmatic events can be distinguished from the Yanhu area: group 1 includes samples AK01 and ZK01 of diorite porphyry, and sample D3658 of quartz diorite that yield mean zircon U-Pb ages of 121.0 ± 2.7 Ma, 116.6 ± 2.0 Ma and 116.0 ± 3.9 Ma, respectively; group 2 includes sample D0050 of diorite porphyry, samples D1393 and D3660 of granodiorite and sample D3065 of granite porphyry that yield mean zircon U-Pb ages of 104.9 ± 2.0 Ma, 105.4 ± 3.8 Ma, 104.2 ± 1.9 Ma and 104.2 ± 1.9 Ma, respectively; group 3 includes sample D3093 of granite that yields mean zircon U-Pb ages of 93.6 ± 1.5 Ma. The zircon LA-ICP-MS U-Pb ages suggest that the Yanhu granitoids were emplaced at 121.0 – 93.6 Ma, representing Cretaceous magmatism in the west segment of the Bangongco-Nujiang suture. The granitoids are composed of SiO2 (56.57 to 76.98 wt.%), Al2O3 (12.20 to 17.90 wt.%), Na2O (3.61 to 4.98 wt.%), K2O (2.06 to 4.71 wt.%) and CaO (0.27 to 5.74 wt.%). The Yanhu granitoids exhibit enrichment in LREE (light REE) and LILE (large ion lithophile elements) such as Rb, Th, U, Pb and K and depletion of HREE (heavy REE), P, Ti, Nb, Ta and Zr. Their A/CNK ratios of 0.85-1.06 are &amp;lt;1.1, implying that they are high-K, metaluminous-weakly peraluminous I-type granites. The Yanhu granitoids were generated mainly by partial melts of the meta-igneous lower crust and some arc-related materials. The Yanhu granitoids probably formed in VAG and syn-COLG tectonic settings related to the southward subduction of the Tethyan Ocean. Diorite porphyry and quartz diorite magmatism from 121.0 Ma to 116.0 Ma may be associated with the southward Bangongco–Nujiang Tethys oceanic crust subduction. Diorite porphyry, granodiorite, and granite porphyry magmatism from 105.4 Ma to 104.2 Ma may be associated with the rising asthenosphere induced by the slab breakoff. Granite magmatism from 93.6 Ma may be related to the crustal thickening induced by the final amalgamation of the Lhasa Terrane and the Qiangtang Terrane.

Geochemistry and origin of alkaline granites at Wadi Umm Adawi-Yahmid area, south Sinai- Egypt

The alkaline granites of the study area are characterized by the predominance of alkali feldspars, quartz minor amounts of riebeckite, plagioclase and biotite. Accessories are represented by zircon, apatite, titanite, allanite, chevkinite with secondary chlorite. They have peralkaline to slightly metalumminous character and were emplaced during post orogenic stages. The chondrite-normalized REE patterns display enrichment of light REE against depletion of heavy REE with significant Eu anomaly indicating extreme feldspar fractionation. They belong to A-type granites and have mantle derived source characterized by high K2O, extremely Low MgO and CaO contents, the pronounced Eu anomaly and depleted in Ba, Sr and Ta against enrichment of (HFSE) Zr, Y, Rb,Nb and ƩREE. They have formed under low oxygen fugacity (fO2) conditions with high magmatic temperature up to 896 °C with fairly low water pressures. Intrusions of alkali feldspar granite followed by riebeckite granite (alkaline granite) are consistent with fractional crystallization model. This was supported by the presence of syenitic microgranular enclaves with felsic alkaline granite. The most plausible source of these alkaline granites is mantle derived potassium rich source, that may accompanied by further magma mixing or assimilations.

Petrogenesis of the mineralized granitoids from the Kounrad and Borly porphyry Cu deposits and the East Kounrad porphyry Mo deposit in Kazakhstan: Implication for tectonic evolution and mineralization of the western part of the Central Asian Orogenic Belt

The Kounrad region, located in the western part of the Central Asian Orogenic Belt, hosts the Kounrad porphyry Cu, the Borly porphyry Cu–Mo, and the East Kounrad porphyry Mo deposits. Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that the mineralized granitoids from the Kounrad, the Borly and the East Kounrad deposits emplaced at 331.7±2.2Ma, 311.6±2.6Ma, and 295.4±2.9Ma, respectively. The mineralized granodiorite porphyries at Kounrad show a geochemical affinity to adakitic rocks with high Sr (357–670ppm), Sr/Y (40–68) and Mg numbers (Mg#=molar Mg/(Mg+Fe2+)) from 0.43 to 0.51, low Yb (0.97–1.1ppm) and Y (8.3–11.1ppm). They have variable Sr–Nd–Hf–O isotopic compositions ((87Sr/86Sr)i=0.7046 to 0.7051, εNd(t)=−0.1 to +1.1, εHf(t)=+5.2 to +9.0, δ18O=+5.7 to +6.8). These features indicate that the Kounrad adakitic magmas derived from the MASH (melting, assimilation, storage, homogenization) zone at depth of ~40km with 5–15% ancient basement rocks contamination. The mineralized granodiorite porphyries at Borly have a normal arc magma geochemical signature (e.g., enrichment of light rare earth elements (LREE) and depletion of heavy REE, Nb and Ti) and experienced fractional crystallization. They also have variable Sr–Nd–Hf–O isotopic compositions ((87Sr/86Sr)i=0.7047 to 0.7053, εNd(t)=0 to −1.3, εHf(t)=−0.6 to +7.4, δ18O=+5.5 to +6.7) and Mg# (0.45 to 0.51), indicating that they were generated by melting of juvenile basaltic lower crust with normal thickness, followed by 10–30% ancient crustal contamination. The East Kounrad mineralized intrusions, consisting of granite and leucogranite, have experienced advanced degrees of fractional crystallization and have similar Sr–Nd–Hf–O isotopic compositions ((87Sr/86Sr)i=0.7048, ɛNd (t)=+1.0 to +2.8, εHf(t)=+4.8 to +8.9, δ18O=+5.6 to +6.4) and low Mg# (0.18–0.37), indicating a juvenile lower crust source at depths of <27km with 10–20% ancient crustal contamination.The link between geochemistry and mineralization indicate that the subduction of the Junggar-Balkhash Ocean at 332Ma resulted in the formation of the Kounrad adakitic rocks and assocatied Cu deposit, and continuing subduction at around 311Ma resulted in the formation of the Borly granodiorite porphyry and assocatied Cu–Mo deposit, but entered a collisional stage by the Early-Permian (~295Ma) and resulted in the formation of the East Kounrad granite and leucogranite and assocatied Mo deposit. It is likely that evolution from porphyry Cu to Mo mineralization in the Kounrad region is owing to tectonic evolution from subduction to collision settings, crust depth variation from thickness to thinness, and magma evolution from adaktic and normal arc magma to highly fractionated I-type granitic magma.

Rare earth element characteristics of the carboniferous Huanglong Formation dolomites in eastern Sichuan Basin, southwest China: Implications for origins of dolomitizing and diagenetic fluids

Marine dolostones of Carboniferous Huanglong Formation constitute major gas reservoir rocks in eastern Sichuan Basin. However, the investigation with respect to sources of dolomitizing and diagenetic fluids is relatively underexplored. The current study attempts to investigate the REE characteristics of dolomites using seawater normalization standard, and therefore discusses the origins of dolomitizing and diagenetic fluids, on the basis of continuous 47.33-m-long core samples from the second member of Huanglong Formation (C2h2) in eastern Sichuan Basin. Low Th, Sc, and Hf concentrations (0.791 × 10−6, 4.751 × 10−6, and 0.214 × 10−6, respectively), random correlation between total REE concentration (ΣREE) and Fe or Mn abundance, and seawater-like Y/Ho ratios (mean value of 45.612) indicate that the carbonate samples are valid for REE analysis. Based on petrographic characteristics, four dolomite types are identified, including micritic-sized dolomite (type Dol-1), fine-to medium-sized dolomite (type Dol-2), medium-to coarse-sized dolomite (type Dol-3), and coarse-to giant-sized saddle dolomite (type Dol-4). Dol-1 dolomites, characterized by positive Ce anomaly (mean value of 6.398), light REE (LREE) enrichment, and heavy REE (HREE) depletion with mean LREE/HREE ratio of 12.657, show micritic calcite-like REE patterns, indicating seawater origin of their dolomitizing fluids. Dol-1 dolomites were formed in sabkha environment whereas the dolomitizing fluids originated from evaporative brine water due to their micritic crystal sizes and tight lithology. Dol-2 dolomites, particularly subtype Dol-2a barely developing vuggy porosity, also show micritic calcite-like REE patterns, suggesting their dolomitizing fluids were seawater or seawater-derived fluids. This inference is confirmed by low Fe and Mn concentrations, which range from 651 μg/g to 1018 μg/g (mean value of 863 μg/g) and 65 μg/g to 167 μg/g (mean value of 105 μg/g), respectively, whereas homogenization temperatures (Th, mean value of 103 °C) indicate that Dol-2 dolomites were formed under burial environment. Dol-3 dolomites, in form of cements of Dol-2 dolomites, show similar REE patterns to their host minerals (i.e., Dol-2 dolomites), indicating their parent source was possibly derived from Dol-2 dolomites. Dol-3 dolomites have high Fe and Mn concentrations with mean values of 3346 μg/g (ranging from 2897 μg/g to 3856 μg/g) and 236 μg/g (ranging from 178 μg/g to 287 μg/g), respectively, indicating the involvement of meteoric water. Meanwhile, it confirms that the dissolution in Dol-2 dolomites was caused by meteoric water leaching. Positive Eu anomalies (mean value of 1.406) in Dol-4 dolomites, coupled with high homogenization temperatures (mean value of 314 °C), suggest that Dol-4 dolomites precipitated from hydrothermal fluids. High Fe and Mn concentrations (mean values of 2521 μg/g and 193 μg/g, respectively) in Dol-4 dolomites likely results from interactions of hydrothermal fluids with deep burial clastic rocks.

Mineralogy, geochemistry, and radiocarbon ages of deep sea sediments from the Gulf of Mexico, Mexico

The mineralogy, geochemistry, and radiocarbon ages of two sediment cores (GMX1 and GMX2) collected from the deep sea area of the Southwestern Gulf of Mexico (∼876–1752 m water depth) were studied to infer the sedimentation rate, provenance, heavy metal contamination, and depositional environment. The sediments are dominated by silt and clay fractions. The mineralogy determined by X-Ray diffractometry for the sediment cores reveals that montmorillonite and muscovite are the dominant clay minerals. The sections between 100 and 210 cm of the sediment cores GMX1 and GMX2, respectively, are characterized by the G. menardii group and G. Inflata planktonic foraminiferal species, which represent the Holocene and Pleistocene, respectively. The radiocarbon-age measurements of mixed planktonic foraminifera varied from ∼268 to 45,738 cal. years B.P and ∼104 to 25,705 cal. years B.P, for the sediment cores GMX1 and GMX2, respectively. The variation in age between the two sediment cores is due to a change in sediment accumulation rate, which was lowest at the location GMX1 (0.006 cm/yr) and highest at the location GMX2 (0.017 cm/yr).The chemical index of alteration (CIA), chemical index of weathering (CIW), and index of chemical maturity (ICV) values indicated a moderate intensity of weathering in the source area. The total rare earth element concentrations (∑REE) in the cores GMX1 and GMX2 vary from ∼94 to 171 and ∼78 to 151, respectively. The North American Shale Composite (NASC) normalized REE patterns showed flat low REE (LREE), heavy REE (HREE) depletion with low negative to positive Eu anomalies, which suggested that the sediments were likely derived from intermediate source rocks.The enrichment factor of heavy metals indicated that the Cd and Zn concentrations in the sediment cores were impacted by an anthropogenic source. The redox-proxy trace element ratios such as V/Cr, Ni/Co, Cu/Zn, (Cu + Mo)/Zn, and Ce/Ce* indicated that the sediments were deposited under an oxic depositional environment. The similarity in major element concentrations, REE content, and the NASC normalised REE patterns between the cores GMX1 and GMX2 revealed that the provenance of sediments remained relatively uniform or constant during deposition for ∼4.5 Ma. The major and trace element based multidimensional discrimination diagrams showed a rift setting for the core sediments, which is consistent with the geology of the Gulf of Mexico.

Constraints on formation and evolution of the lunar crust from feldspathic granulitic breccias NWA 3163 and 4881

Lunar granulitic meteorites provide new constraints on the composition and evolution of the lunar crust as they are potentially derived from outside the Apollo and Luna landing sites. Northwest Africa (NWA) 3163, the focus of this study, and its paired stones NWA 4881 and NWA 4483, are shocked granulitic noritic anorthosites. They are petrographically and compositionally distinct from the Apollo granulites and noritic anorthosites. Northwest Africa 3163 is REE-depleted by an order of magnitude compared to Apollo granulites and is one of the most trace element depleted lunar samples studied to date. New in-situ mineral compositional data and Rb–Sr, Ar–Ar isotopic systematics are used to evaluate the petrogenetic history of NWA 3163 (and its paired stones) within the context of early lunar evolution and the bulk composition of the lunar highlands crust. The NWA 3163 protolith was the likely product of reworked lunar crust with a previous history of heavy REE depletion. The bulk feldspathic and pyroxene-rich fragments have 87Sr/86Sr that are indistinguishable and average 0.699282±0.000007(2σ). A calculated source model Sr TRD age of 4.340±0.057Ga is consistent with (1) the recently determined young FAS (Ferroan Anorthosite) age of 4.360±0.003Ga for FAS 60025, (2) 142Nd model ages for the closure of the Sm–Nd system for the mantle source reservoirs of the Apollo mare basalts (4.355–4.314Ga) and (3) a prominent age peak in the Apollo lunar zircon record (c. 4.345Ga). These ages are ∼100Myr younger than predicted timescales for complete LMO crystallization (∼10Myrs after Moon formation, Elkins-Tanton et al., 2011). This supports a later, major event during lunar evolution associated with crustal reworking due to magma ocean cumulate overturn, serial magmatism, or a large impact event leading to localized or global crustal melting and/or exhumation. The Ar–Ar isotopic systematics on aliquots of paired stone NWA 4881 are consistent with an impact event at ⩾3.5Ga. This is inferred to record the event that induced granularization of NWA 3163 (and paired rocks). A later event is also recorded at ∼2Ga by Ar–Ar isotopes is consistent with an increase in the number of impacts on the lunar surface at this time (Fernandes et al., 2013). Northwest Africa 3163 and its paired stones therefore record a c. 2.4Gyr record of lunar crustal production, metamorphism, brecciation, impacts and eventual ejection from the lunar surface.

Heterogeneidades do Manto Litosférico Subcontinental ao extremo Sul da Placa Sul-Americana: influência da subducção atual e interações litosfera-astenosfera sob o Campo Vulcânico de Pali Aike

O Campo Vulcânico de Pali Aike (CVPA) está localizado no extremo sul da América do Sul em uma área complexa devido à interação das placas oceânicas de Nazca, Antártica e Scotia com a placa continental Sul-Americana, e a proximidade com a zona de subducção da dorsal do Chile (Junção Tríplice do Chile). É formado por basanitos, basaltos alcalinos e rochas piroclásticas que hospedam xenólitos mantélicos compostos essencialmente por granada-espinélio peridotitos e espinélio peridotitos, com presença de pargasita e flogopita. Os xenólitos mantélicos estudados neste trabalho pertencem ao maar Laguna Timone, em Pali Aike, e são predominantemente granada-espinélio lherzolitos, espinélio lherzolitos, granada-espinélio hazburgitos e um glimerito. A paragênese mineralógica granada+espinélio+Ti-flogopita+K-pargasita, presente nas amostras, indica que as rochas provêm da zona de transição granada-espinélio peridotito no manto superior. Eventos de fusão parcial são indicados pela correlação negativa definida entre os elementos maiores contra o #Mg, bem como pelo padrão com enriquecimento das ETR leves com relação às pesados dos granada-espinélio harzburgitos, e pelo empobrecimento dos ETR pesados nos espinélio lherzolitos, submetidos a modelamento que sugere até 16% de fusão parcial (fusão em equilíbrio não modal). Os granada-espinélio lherzolitos possuem valores similares ao do Manto Primitivo, caracterizando-se como um manto “fértil” comparado às demais amostras. Todos os xenólitos possuem expressivo enriquecimento nos elementos calcófilos (W, Pb, Mo, Sn), devido ao metassomatismo pela subducção da placa de Antártica sob a placa Sul-Americana. A presença de um glimerito, de peridotitos com minerais hidratados (flogopita e pargasita) e as similaridades com peridotitos metassomatisados por líquidos astenosféricos (e.g. peridotitos do distrito de Manzaz, Argélia e peridotitos do campo vulcânico Vitim, Baikal, Sibéria) com baixas razões Ba/Nb, Ba/La e U/Nb, indicam metassomatismo por fluidos astenosféricos devido ao soerguimento da astenosfera durante a passagem da Junção Tríplice do Chile pela região de Pali Aike.

Contrasting rift and subduction‐related plagiogranites in the Jinshajiang ophiolitic mélange, southwest China, and implications for the Paleo‐Tethys

The Jinshajiang ophiolitic mélange zone in southwest China represents a remnant of the eastern Paleo‐Tethys Ocean. Field, geochronological and geochemical studies have identified two distinct suites of plagiogranites within the mélange, the Dongzhulin trondhjemite and Jiyidu tonalite, which represent rift and subduction settings, respectively, related to opening and closing of the ocean. SHRIMP U‐Pb analysis on zircons extracted from the Dongzhulin trondhjemite yields a mean 206Pb/238U age of 347 ± 7 Ma. REE and isotopic characteristics suggest an origin from low pressure partial melting of an amphibolitic protolith. Highly variable Hf isotopic compositions for zircons from this body may indicate a heterogenous source involving both depleted mantle and enriched continental components. This, together with geologic relations, suggests formation near an embryonic spreading center in a continent‐ocean transition setting. The Jiyidu tonalite has a U‐Pb zircon age of 283 ± 3 Ma, and geochemical data indicates high Sr/Y, (La/Yb)N, Nb/Ta and low Y, and marked heavy REE depletion. These signatures suggest derivation from low degree partial melting of subducted slab at pressure high enough to stabilize garnet and rutile. A slab‐melt origin is also supported by in situ Hf and O data for zircon that show isotopic compositions comparable with typical altered oceanic crust. Thus, the crystallization age of the Jiyidu high Sr/Y tonalite provides a constraint for the subduction of the Jinshajiang ocean floor. The rift‐related Dongzhulin trondhjemite and subduction‐related Jiyidu high‐Sr/Y tonalite constrain the timing and setting of opening and closing of this segment of the Paleo‐Tethys Ocean.