Chondrite-normalized Rare Earth Element Research Articles

Elemental geochemistry and organic facies of selected cretaceous coals from the Benue Trough basin in Nigeria: Implication for paleodepositional environments

Benue Trough basin is geologically subdivided into three segments. It consists of marine and continental sediments that are affected by folding movements. In the upper section, the continental coal-bearing Campano-Maastrichtian-age is found within Gombe Sandstone and the Cenozoic Kerri-Kerri Formation of the Tertiary. In the middle segment, the late Turonian/early Santonian coal-bearing Awgu Formation lies conformably on the Ezeaku Formation. In the lower part, shales at Nkporo mark marine incursion and deposition of the Maastrichtian Mamu Formation coal measures. Cretaceous coal samples taken from the three segments of Benue Trough basin were examined using ultimate and proximate, organic petrographic, x-ray fluorescence (XRF) and laser ablation-induced coupled plasma spectroscopy (LA-ICPMS) analytical techniques. Results of trace element ratios, Ce/Ce* values (“where Ce* is the theoretical Ce value derived from a chondrite-normalized rare earth element (REE) pattern”), authigenic uranium values, and low hydrogen content indicate deposition under oxic to suboxic conditions. However, the V/Ni and V/Sc ratios indicate under oxic, suboxic to reducing conditions. The Y/Ho values for selected coals are lower than the range for seawater, indicating terrigenous influence. The Sr/Ba values indicate low influence of saline water during deposition and a strong continental rather than marine influence. High value of vitrinite and low proportion of liptinite/intertinite suggest that peat in the study areas were situated on the irregular basin floor where sediment thicknesses vary from place to place thus the depositional environments shifted causing mineral matter filling in coal. Based on the tissue preservation index (TPI) and gelification index (GI), LFB, CHK, OKB, LMZ, IMG; and ENG (high GI and high TPI) were deposited in telmatic facies; wet-forest swamps with rapid organic matter accumulation. This trend indicates mild humification and strong gelification of plant tissues which ascribed to high rate of subsidence with wood- and bark-derived telovitrinite. The MGA peats were deposited in terrestrial facies in a dry-forest swamp with wood-derived telo-inertinite. The coal samples code-named IMG and ENG were deposited in planar margin mires. However, coal samples code-named LFB, CHK and LMZ were deposited in planar central mires. Lastly, OKB and MGA were deposited in transitional-wet and transitional-dry mire. The low V and Ni concentrations, low H/C, high O/C, high C/N, and very low N/C ratios <0.05 implies Type III land-plant organic matter that is not oil-prone. The depositional environments interpretations of investigated coal seams indicate that the coals were deposited within a telmatic, limnic, limno-telmatic and open march zone. Generally, a combination of results from inorganic geochemistry and organic petrology is required for precise and all-inclusive determination of coal depositional environments.

Geochemistry of serpentinized and multiphase altered Atlantis Massif peridotites (IODP Expedition 357): Petrogenesis and discrimination of melt-rock vs. fluid-rock processes

International Ocean Discovery Program (IODP) Expedition 357 drilled 17 shallow sites distributed ~10 km in the spreading direction (from west to east) across the Atlantis Massif oceanic core complex (Mid-Atlantic Ridge, 30°N). Mantle exposed in the footwall of the Atlantis Massif oceanic core complex is predominantly nearly wholly serpentinized harzburgite with subordinate dunite. Altered peridotites are subdivided into three types: (I) serpentinites, (II) melt-impregnated serpentinites, and (III) metasomatic serpentinites. Type I serpentinites show no evidence of melt-impregnation or metasomatism apart from serpentinization and local oxidation. Type II serpentinites have been intruded by gabbroic melts and are distinguishable in some cases on the basis of macroscopic and microscopic observations, e.g., mm-cm scale mafic-melt veinlets, rare plagioclase (˂0.5 modal % in one sample) or by the local presence of secondary (replacive) olivine after orthopyroxene; in other cases, ‘cryptic’ melt-impregnation is inferred on the basis of incompatible element enrichments. Type III serpentinites are characterized by silica metasomatism manifest by alteration of orthopyroxene to talc and amphibole, and by anomalously high anhydrous SiO2 concentrations (59–61 wt%) and low MgO/SiO2 values (0.48–0.52). Although many chondrite-normalized rare earth element (REE) and primitive mantle-normalized incompatible trace element anomalies, e.g., negative Ce-anomalies, are attributable to serpentinization, other compositional heterogeneities are due to melt-impregnation. On the basis of whole rock incompatible trace elements, a dominant mechanism of melt-impregnation is distinguished in the central and eastern serpentinites from fluid-rock alteration (mostly serpentinization) in the western serpentinites, with increasing melt-impregnation manifest as a west to east increase in enrichment in high-field strength elements and light REE. High degrees of melt extraction are evident in low whole-rock Al2O3/SiO2 values and low concentrations of Al2O3, CaO and incompatible elements. Estimates of the degree of melt extraction based on whole rock REE patterns suggest a maximum of ~20% non-modal fractional melting, with little variation between sites. As some serpentinite samples are ex situ rubble, the magmatic histories observed at each site are consistent with a local source (from the fault zone) rather than rafted rubble that would be expected to show more heterogeneity and no spatial pattern. In this case, the studied sites may provide a record of enhanced melt-rock interactions with time, consistent with proposed geological models. Alternatively, sites may signify heterogeneities in these processes at spatial scales of a few km.

Depleted mantle beneath an oceanic island arc: New insights into peridotites of the RasKoh ophiolite, western Pakistan

The RasKoh ophiolite of the Sorap Massif, western Pakistan, is a relatively little known body in the vast Alpine–Himalayan orogenic belt (AHOB). This paper focuses on the RasKoh peridotites to better understand the origin and evolution of the ophiolite. Field observations, confirmed by thin-section study, identified three types of peridotite; lherzolite, harzburgite, and dunite, which commonly have gradational boundaries. The mineral compositions of the peridotites (olivine, Cr-spinel, and pyroxene) suggest that these ultramafic rocks mostly represent refractory residues of mantle peridotites after ~12 to 25% partial melting and various degrees of melt-rock reaction. This interpretation is supported by variable contents of major elements and the chondrite-normalized rare earth element (REE) patterns. Enrichment of water-soluble elements, such as Rb, Ba, Sr, and light rare earth elements (LREE) also indicate infiltration of fluids into the depleted peridotites. Synthesizing available geological information with our new geochemical data, we suggest that the RasKoh ophiolite is a fragment of a former Triassic to Jurassic oceanic plate that became a mantle wedge above an intra-ocean subduction zone during the Cretaceous. Fluids from the subducted plate re-enriched the depleted peridotites of the RasKoh island arc. The ophiolite was emplaced during collision between the RasKoh island arc to the south and the Chagai continental arc to the north leading to exhumation of the re-enriched peridotites. This new model is much simpler than most previous ones and better explains the tectonic evolution of western Pakistan.

Elemental and Sr–Nd isotopic compositions of surface clay-size sediments in the front end of major ice shelves around Antarctica and indications for provenance

Geochemical observations of continental outcrops and marine sediments provide insights into the nature of bedrock and erosion in Antarctica. The accelerated melt of ice shelves has increased the flux of terrigenous materials to the marginal seas and has changed the sedimentological environment. We characterized the provenance of the surficial fine-grained detrital sediments in the front ends of major ice shelves in the Amundsen Sea, Ross Sea, and Prydz Bay sections through comparisons to proximal rocks using the elemental compositions, as well as the Sr and Nd isotopic compositions, of the clay-size (<2 μm) fraction, rock samples, and bulk sediments and some residual fractions. The results show that the 87Sr/86Sr ratios of clay-size sediments range from 0.70962 to 0.71662, 0.70936 to 0.71960, and 0.72569 to 0.73492, and that the εNd values range from −2.1 to +0.8, −10.9 to −1.43, and −22.3 to −17. 3 in the Amundsen Sea, Ross Sea, and Prydz Bay, respectively, while those of rocks range from 0.70350 to 0.77080 and from 5.1 to −22.4 in all three areas. Elemental compositions of both bulk sediments and the clay-size fraction in the Western Ross Sea have similar patterns to those in Prydz Bay, showing that most major elements (e.g., Mg, Ca, and Fe) are lower than in the upper continental crust (UCC). Chondrite-normalized rare earth elements (REEs) are highest in the bulk sediments of the Amundsen Sea and lowest in the clay-size fraction of all three sections. We concluded that clay-size sediments in the Western Amundsen Sea shelf are largely from the Eastern Amundsen Sea coastal area, while some sediments reflect contributions from the Hobbs Coast due to gravitational currents. The Western Ross Sea can receive sediments from both East Antarctica and West Antarctica, while the clay-size fractions of western sites in Transect R (a latitudinal transect near Inexpressible Island, East Antarctica) in this study are mainly from southeastern Victoria Land, transported northward by bottom currents. The contribution from coarse ice-rafted debris (IRD) is minor, possibly because of blocking by the Drygalski Ice Tongue. The clay-size sediments of Eastern Prydz Bay are largely contributed by Princess Elisabeth Land and western Wilhelm II Land, while those in Western Prydz Bay are possibly derived from the Lambert-Amery Ice Shelf system and the western coast. These very fine-grained sediments are generally affected by oceanic currents (e.g., Antarctic Coastal Currents and Antarctic Bottom Currents). Most residual fraction show very similar εNd values to the bulk of clay-size sediments. It indicates that authigenic phases are generally sourced from similar regions to the silicate component of clay-size sediments.

Formation processes and tectonic implications of mantle peridotites of the Yushigou ophiolite in the North Qilian Orogenic Belt, NW China

The upper-mantle section of the Yushigou ophiolite in the North Qilian Orogenic Belt consists mainly of moderately depleted harzburgite. New mineral and whole-rock major- and trace-element geochemical data show that these rocks could not have formed by simple partial melting of more enriched peridotites. The harzburgites contain high-Mg olivine (Fo = 91.0–92.2) and orthopyroxene (Mg# 91.4–92.6) accompanied by sparse high-Cr magnesium chromite (Cr# = 54.1–67.6) and clinopyroxene with low Al2O3 (1.19–2.11 wt%) and TiO2 (<0.02 wt%) contents. Most of the analysed samples exhibit depleted, U-shaped, chondrite-normalized rare earth element patterns, whereas primitive mantle-normalized PGE display flat patterns from Os to Pt with significant depleted in Pd. We propose that these peridotites underwent a multi-stage melting process beginning with 20%–25% partial melting of a mantle source beneath a mid-ocean ridge environment followed by second-stage partial melting and related refertilization in a supra-subduction zone tectonic setting, experience an additional 5%–10% melting, yielding a total of 25%–35% melting in the two episodes. TiO2 enrichment (up to 0.09 wt%) of spinels in some of the harzburgites point to interaction between an already depleted mantle and various proportions of silicate melts with LREE-enriched, boninitic compositions in a forearc setting. Both the ReOs isotope and the PGE characteristics of the Yushigou harzburgites were controlled mainly by ancient depletion events. All the samples have low Re concentrations (0.006–0.228 ppb) and subchondritic Os isotopic compositions (0.11216–0.11979). The Re-depletion model ages (1.5–1.2 Ga) of the Yushigou harzburgites may record a period of Mesoproterozoic intracontinental rifting and formation of the original ocean basin in the North Qilian Orogenic Belt.

Assessment of selected rare earth elements, HF, Th, and U in the Donetsk region using moss bags technique

To assess the accumulation of rare earth elements, Hf, Zr, Th, and U in moss species the active moss biomonitoring technique was applied in the Donetsk region. Moss bags with the Ceratodon purpureus and Brachythecium campestre species were exposed for six months in the surroundings of two parks, two steelworks, and a power station. The concentrations of 12 elements (Sc, La, Ce, Nd, Sm, Tb, Yb, Eu, Hf, Zr, Th and U) were determined in the moss transplants by neutron activation analysis. The degree of ambient contamination has been studied using the relative accumulation factor, contamination factor, pollution load index and enrichment factor. Geochemical indices – ternary plot of Sc–La–Th, ratios of normalized concentrations of light rare earth elements and heavy rare earth elements, possible anomalies of Eu, and chondrite normalized rare earth element curves were used to prove the geochemical origin of rare earth elements. The chondrite-normalized rare earth element patterns of both moss species showed light rare earth elements enrichment and a negative Eu anomaly.

Geochemical record of the sediments in the continental shelf of the northwestern South China Sea: Implications for the provenance and sedimentary evolution

The marine stratigraphic record in the northwestern South China Sea provides an excellent case for understanding the provenance and sedimentary evolution of the marginal sea. However, considerable uncertainty exists in unraveling the geochemical record since the Oligocene due, in part, to a lack of systematic measurements of the sediments. In this study, the major and trace elements of sedimentary rocks from the northern Yinggehai-Song Hong Basin were analyzed to ascertain the provenance and reconstruct the sedimentary evolution in the northwestern South China Sea. The chondrite-normalized rare earth element (REE) patterns show fractionation between light REEs (LREEs) and heavy REEs (HREEs) with significant negative Eu anomalies (Eu/Eu* = 0.59–1.10), combining with discriminant function analysis, implies that the sediments were predominantly eroded from granitic rocks, which are mostly felsic igneous materials. The relatively high index of chemical variability (ICV) values (1.30–3.54) and low chemical index of alteration (CIA) values (36.2–61.5) indicate that these sediments are immature and might have experienced weak to intermediate chemical weathering during transportation and deposition. The geochemical data also show that the Red River catchment was the primary source region for the northern Yinggehai-Song Hong Basin since the late Oligocene. The variations in the element contents and ratios are consistent with the multi-stage tectonic activities of the basin and the provenance shift caused by the recombination of the Red River system in different sedimentary periods.

Provenance of sediments from Barra del Tordo and Tesoro beaches, Tamaulipas State, northwestern Gulf of Mexico

The mineralogy, bulk sediment geochemical composition, and U–Pb ages of detrital zircons retrieved from the Barra del Tordo (Tordo) and Tesoro beach sediments in the northwestern Gulf of Mexico were analyzed to determine their provenance. The beach sediments are mainly composed of quartz, ilmenite, magnetite, titanite, zircon, and anorthite. The weathering proxies such as the Chemical Index of Alteration (CIA), Chemical Index of Weathering (CIW), and Plagioclase Index of Alteration (PIA), reveal a moderate-to-high intensity of weathering in the source area. The chondrite-normalized rare earth element (REE) patterns are similar to felsic igneous rocks, with large negative europium anomaly (Eu/Eu* = ~ 0.47–0.80 and ~ 0.57–0.67 in the Tordo and Tesoro beach sediments, respectively).Three major zircon U–Pb age groups are identified in the Tordo and Tesoro beach sediments, i.e., Proterozoic (~ 2039–595 Ma), Mesozoic (~ 244–70.3 Ma), and Cenozoic (~ 65.9–1.2 Ma). The differences of the zircon age spectrum between the Tordo and Tesoro beach sediments are not significant. The comparison of zircon U–Pb ages in this study with ages of potential source terranes suggests that the Mesozoic and Cenozoic zircons of the studied Tordo and Tesoro beach sediments were derived from the Eastern Alkaline Province (EAP) and Mesa Central Province (MCP). Similarly, the likely sources for the Proterozoic zircons were the Sierra Madre Oriental (SMOr) and Oaxaquia in the northwestern Gulf of Mexico. The results of this study further indicate that the sediments delivered to the beaches by rivers and redistributed by longshore currents were crucial in determining the sediment provenance.

Quantifying the Tetrad Effect, Shape Components, and Ce\u2013Eu\u2013Gd Anomalies in Rare Earth Element Patterns

Plots of chondrite-normalised rare earth element (REE) patterns often appear as smooth curves. These curves can be decomposed into orthogonal polynomial functions (shape components), each of which captures a feature of the total pattern. The coefficients of these components (known as the lambda coefficients—lambda ) can be derived using least-squares fitting, allowing quantitative description of REE patterns and dimension reduction of parameters required for this. The tetrad effect is similarly quantified using least-squares fitting of shape components to data, resulting in the tetrad coefficients (tau ). Our method allows fitting of all four tetrad coefficients together with tetrad-independent lambda curvature. We describe the mathematical derivation of the method and two tools to apply the method: the online interactive application BLambdaR, and the Python package pyrolite. We show several case studies that explore aspects of the method, its treatment of redox-anomalous REE, and possible pitfalls and considerations in its use.

Mineral Geochemistry of Basaltic Rocks from IODP Expeditions 334 and 344: Implications for Magmatic Processes of Cocos Ridge Segment Being Subducted Beneath the Middle America Trench

The Cocos Ridge, which is subducted beneath the Central American Volcanic Arc, has a complex tectonic evolution history due to plume-ridge interaction between the Galápagos plume and the Cocos—Nazca spreading center. This study presents major and trace element analyses of plagioclase and clinopyroxenes hosted by Cocos Ridge basaltic rocks that were drilled in three holes (U1381A, U1381C and U1414A) of Sites U1381 and U1414 on the Cocos Ridge close to the Middle America Trench during the Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344. The results show that (1) plagioclases are mainly bytownite and labradorite with subordinate andesine, which are enriched in light rare earth elements (LREE) and some large-ion lithophile elements (LILE) and exhibit marked positive Eu anomalies; and (2) that clinopyroxenes are augites, which are depleted in highly incompatible elements such as LREE and LILE, have nearly flat heavy rare earth elements patterns (HREE) and lack Eu anomalies in chondrite-normalized rare earth element (REE) diagrams. During the ascent to the surface, the primary magmas experienced fractional crystallization of plagioclase, clinopyroxene, Ti-Fe oxides and possibly olivine (complete replacement of olivine by secondary minerals). The crystallization temperatures of plagioclase phenocrysts and microlites are 1050 to 1269 °C, and 866 to 1038 °C, respectively, and the pressures of plagioclase phenocrysts are 0.3–0.7 GPa. The crystallization temperatures of clinopyroxene phenocrysts/micro-phenocrysts is 1174–1268 °C, similar to those of plagioclase phenocrysts, suggesting some of clinopyroxene and plagioclase phenocrysts cotectic crystallized during early stage of magmatic evolution. In addition, the equilibrium pressures of clinopyroxene phenocrysts/micro phenocrysts are 0.02–0.97 GPa, implying that the clinopyroxene started to crystallize within the mantle, and magma evolution has undergone an early crystallization stage with clinopyroxene and no plagioclase.

Mafic-ultramafic intrusion formed by multi-stage evolution of hydrous basaltic melts

AbstractThe magmatic processes beneath the active continental margins are very complicated and affect structures and compositions of the arc roots. Neoproterozoic igneous rocks are widely distributed around the margins of the Tarim Block in NW China. The Xingdier mafic-ultramafic intrusion is a composite body, located at the northern margin of the Tarim Block, and consists of gabbro, pyroxenite, and peridotite units. The gabbro unit has a secondary ion mass spectrometry zircon U-Pb age of 727 ± 5 Ma. Rocks from the Xingdier intrusion have a large range of MgO (12.9–32.8 wt%) and SiO2 (43.0–57.9 wt%), and low K2O + Na2O (0.11–2.25 wt%) contents. They have right inclined chondrite-normalized rare earth element patterns with (La/Yb)N ratios of 2.2–8.6. Their primitive mantle normalized trace element patterns show arc-affinity geochemical features characterized by enrichment in Rb, Ba, Th, U, and Pb and depletion in Nb, Ta, and Ti. They have variable initial 87Sr/86Sr ratios (0.7063–0.7093), εNd(t) values (−2.9 to −7.8), 206Pb/204Pb (17.08–17.80), 207Pb/204Pb (15.42–15.49), and 208Pb/204Pb ratios (37.48–38.05), forming an evolution trend from the peridotite unit to the gabbro and pyroxenite units. Clinopyroxene in the three units is chemically similar to those formed in hydrous magmas. The spinel inclusions in olivine from the peridotite unit show unmixing texture and have high Al contents and oxygen fugacity of ~FMQ+1. Therefore, the parental magma was probably derived from a lithospheric mantle enriched by slab-derived fluids. Rocks from the gabbro and peridotite units are proposed to have been derived from olivine-normative melts, whereas rocks from the pyroxenite unit are cumulates from the quartz-normative melts. Such contrasting parental magmas resulted from variable degrees of crustal contamination and fractional crystallization in the arc root.

Petrogenesis and tectonic implications of TTG granitoids from the Daqingshan Complex of the Khondalite Belt, North China Craton

Located in the Western Block of the North China Craton, the Khondalite Belt is one of the three Paleoproterozoic tectonic belts that were linked to the final assembly of the craton. At present, a popular model is that the Khondalite Belt was formed by the collision between the Yinshan and Ordos blocks at ∼1.95 Ga. However, the initiation of oceanic subduction and its related arc magmatism and accretionary process before the collisional event were poorly constrained. The Daqingshan Complex is located in the middle East part of the Khondalite Belt, and contains highly deformed and metamorphosed rock assemblages, and thus represents a key area to decipher the above issue. In this study, we carried out petrological, geochemical and geochronological analysis on the TTG granitoids of the Daqingshan Complex. Zircon U-Pb results from three typical TTG samples yielded upper intercept ages of 2545 ± 50 Ma, 2484 ± 68 Ma and 2452 ± 32 Ma, indicating that the TTG granitoids were emplaced in the late Neoarchean. Metamorphic zircons from two samples gave ²⁰⁷Pb/²⁰⁶Pb weighted mean ages of 1892 ± 53 Ma and 1906 ± 27 Ma, respectively, recording the timing of a continent-to-continent collisional event. Thirteen TTG granitoid samples are geochemically low-, medium- and high-K calc-alkaline, with metaluminous to peraluminous trends and are enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, La, Ce, Nd, and depleted in high field strength elements (HFSEs) such as Nb and Ta. Chondrite-normalized rare earth element (REE) patterns show fractionation with (La/Yb) _N ratios ranging from 8.20 to 27.47, with weak Eu negative anomalies (δEu = 0.50 – 0.98). In addition, TTG granitoids of the Daqingshan Complex belong to I-type granitoids, and their igneous protoliths were intimately related to a subduction-related magmatic arc environment. New results of this study reveal that the initial oceanic lithosphere subduction operated since ∼2.55 Ga along the southern margin of the Yinshan Block, and generated the coeval arc-related TTG granitoids. Closure of the ocean led to the final collision between the Yinshan and Ordos blocks and the amalgamation of the Western Block at 1.95 to 1.85 Ga.

New evidence for upper Permian crustal growth below Eifel, Germany, from mafic granulite xenoliths

Abstract. Granulite xenoliths from the Quaternary West Eifel Volcanic Field in Germany record evidence of magmatism in the lower crust at the end of the Permian. The xenoliths sampled two distinct bodies: an older intrusion (ca. 264 Myr old) that contains clinopyroxene with flat, chondrite-normalised rare earth element (REE) profiles and a younger (ca. 253 Myr old) intrusion that crystallised middle-REE-rich clinopyroxene. The younger body is also distinguished based on the negative Sr, Zr and Ti anomalies in primitive mantle-normalised multi-element plots. REE-in-plagioclase–clinopyroxene thermometry records the magmatic temperature of the xenoliths (1100–1300 ∘C), whereas Mg-in-plagioclase and Zr-in-titanite thermometry preserve an equilibration temperature of ca. 800 ∘C. These temperatures, together with a model of the mineral assemblages predicted from the composition of one of the xenoliths, define the pressure of crystallisation as ∼1 GPa. The xenoliths also preserve a long history of reheating events whose age ranges from 220 to 6 Myr. The last of these events presumably led to breakdown of garnet; formation of symplectites of orthopyroxene, plagioclase and hercynite; and redistribution of heavy rare earth elements into clinopyroxene. The data from the West Eifel granulite xenoliths, when combined with the existing data from granulites sampled in the East Eifel, indicate that the lower crust has a long a complex history stretching from at least 1.6 Ga with intrusive events at ca. 410 and 260 Ma and reheating from the Triassic to late Miocene.

Voluminous Paleogene volcanism in the southern Mesa Central, Mexico: Unravelling the fissure-fed origin of rhyolitic ignimbrites of the Villa Garcia-Loreto Volcanic Complex

The southern portion of the Mesa Central (MC) physiographic province is covered by numerous Oligocene to Miocene volcanic successions that are part of the Sierra Madre Occidental silicic large igneous province. The MC merges with the Sierra Madre Occidental and the southern Basin and Range extensional province. The voluminous Villa Garcia-Loreto Volcanic Complex (VGLVC) is located in the southern portion of the MC and consists of predominantly rhyolitic lava flows, and voluminous products of explosive rhyolitic volcanism, mainly as lava-like to rheomorphic ignimbrite sheets. The volcanism in the area appears in two events, first, at 60.4 ± 0.3 Ma (K–Ar whole-rock age) with the fissure-fed eruption of an andesitic lava and second, with the fissure-fed eruption of two densely-welded, crystal-poor ignimbrites at 31.59 ± 0.14 Ma and 28.66 ± 0.07 Ma (40Ar–39Ar sanidine ages). Chondrite-normalized rare earth element patterns and primitive mantle normalized multi-element patterns show enrichment in light rare earth elements (LREE) and high field strength elements (HFSE). Geochemical analysis and mineral chemistry show that the andesitic magmas were generated by partial melting of the uppermost mantle in a subduction-related setting and ascended directly through the brittle crust and erupted along N-S trending faults. Rhyolitic magmas were generated by partial melting of the lower crust in an intra-plate and extensional setting, associated with an intense phase of the southern Basin and Range extension. During the ascent and storage in a shallow magma chamber fractional crystallization of mainly feldspars and pyroxene was the major process taking place in the evolution of the magma. Geophysical results suggest that the volcanic rocks of the VGLVC were originated by fissure-fed eruptions associated with the Loreto Graben fault. The major Basin and Range faults locally reach a depth of 7 km and trending in NNW-SSE direction. The combined results show that the Basin and Range extension favored the formation, ascent, and eruption of rhyolitic and andesitic magmas in the southern MC.

Paleoproterozoic metaluminous syenites synchronous with the c. 2.21 Ga mafic dyke swarms from the Eastern Dharwar Craton, India: implications for alkaline magmatism associated with the breakup of supercraton Superia

Abstract The Eastern Dharwar Craton (EDC) hosts numerous Late Neoarchean to Paleoproterozoic syenites whose genesis remains unclear. Here we present a petrological and geochemical study on the syenites from Peddavaduguru, Danduvaripalle and Vannedoddi, towards the western margin of the Paleo-Mesoproterozoic Cuddapah Basin in the EDC. These syenite bodies constitute a part of the Paleoproterozoic (2210 ± 110 Ma) Dancherla alkaline complex and are contemporaneous with a mafic dyke swarm emplacement in the EDC. The Danduvaripalle and Vannedoddi syenites display geochemical affinities to adakites. The Peddavaduguru syenite shows a negative Eu anomaly and relatively gentle chondrite-normalized rare earth element (REE) pattern, suggesting an origin by a different process. The ɛ Nd values range between 0.5 and 0.8 for the Peddavaduguru syenite, − 5.2 and − 4.2 for the Danduvaripalle syenite, and − 6.0 and − 1.0 for the Vannedoddi syenite. In tectonic discrimination diagrams, the Peddavaduguru syenite shows affinities to within-plate granitoids similar to syenites from the Deccan large igneous province whereas the others show geochemical similarities to arc-related alkaline rocks and volcanic arc granitoids. The adakitic syenites show deficiency of MgO, Ni, Cr and Sc, highly fractionated REE patterns and negative Nb–Ta–Ti anomalies along with low Nb/U and high Th/U ratios. These geochemical traits are compatible with their origin by the partial melting of a mafic crustal source that subsequently underwent fractionation of amphibole and garnet responsible to impart an adakitic character. We suggest that the crustal source of these syenites underwent partial melting by heat from the rising plume during the Paleoproterozoic extension of the Superia supercraton. The non-adakitic Peddavaduguru syenite, on the other hand, is suggested to have originated from direct fractionation of mafic magma.

In situ LA-ICPMS U[sbnd]Pb dating and geochemical characterization of fault-zone calcite in the central Tarim Basin, northwest China: Implications for fluid circulation and fault reactivation

Direct evidence for fault reactivation is crucial for understanding long-term fault behavior and reconstruction of fluid circulation and hydrocarbon migration and accumulation during active tectonics. However, absolute dating of fault reactivations and fluid circulation is still challenging due to dissolution and recrystallization of fault zone materials and obliteration of previous events by the latest deformation/hydrothermal event. The lower Ordovician limestone strata at 3.8 km depth in the Central Uplift of the Tarim Basin, northwest China experienced multi-stage regional tectonic activities that developed a series of NW–SE striking reverse faults and NE–SW oriented strike-slip faults. In this study, we performed in situ laser-ablation ICP-MS UPb dating and trace element characterization of calcites collected from borehole cores within one of the main fault zones in the Central Uplift of the Tarim Basin. A new in-house calcite standard AHX-1A (209.8 ± 1.3 Ma), collected from the Tarim Basin, calibrated against well-calibrated WC-1 and ASH-15 calcite standards, is used for mass-bias correction. We obtained in situ UPb ages of 9 calcite samples from two wells within a Paleozoic fault zone in the Tarim Basin. They are 456 ± 11 Ma, 454.7 ± 7.2 Ma, 450.4 ± 6.2 Ma, 435.2 ± 9.7 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma from well TZ2, and 371 ± 18 Ma, 390.6 ± 6.0 Ma and 395 ± 14 Ma from well TZ4 (all reported uncertainties are 2σ). Combined with petrographic evidence, these results define at least six generations of secondary calcite precipitations in the fault zone, that occurred during 456 ± 11 to 450.4 ± 6.2 Ma (weighted mean 452.8 ± 4.3 Ma), 435.2 ± 9.7 Ma, 395 ± 14 to 390.6 ± 6.0 Ma (weighted mean 391.3 ± 5.5 Ma), 371 ± 18 Ma, 328.0 ± 9.2 Ma and 307.6 ± 7.1 Ma. Chondrite-normalized rare earth element (REE) + yttrium (Y) patterns of these calcite samples show prominent positive Gd and Y anomalies, lack of negative Ce anomaly, and enrichment in light REE but depletion in heavy REE, similar to the shallow marine platform host-rock limestone. The results suggest multi-stage fluid circulation under shallow burial, low temperature, and non-oxidizing environments along the fault zone, with the secondary calcite precipitating from a similar fluid source that originated from dissolution of host-rock carbonate or early-stage diagenesis. The detail petrographical and mineralogical analysis of this study, combined with the new interpretation of high-resolution seismic data, constrains two main episodes of faulting and related uplift during 456–435 Ma and 395–371 Ma, as well as a younger faulting episode with weaker intensity during 328–308 Ma. This study demonstrates that the central fault zone of the Tarim Basin experienced ~150 Ma of episodical reactivation during the Paleozoic, which has important implications for hydrocarbon exploration.

Petrological and geochemical characterization of the arc-related Suru–Thasgam ophiolitic slice along the Indus Suture Zone, Ladakh Himalaya

AbstractThe Ladakh Himalayan ophiolites preserve remnants of the eastern part of the Neo-Tethyan Ocean, in the form of Dras, Suru Valley, Shergol, Spongtang and Nidar ophiolitic sequences. In Kohistan region of Pakistan, Muslim Bagh, Zhob and Bela ophiolites are considered to be equivalents of Ladakh ophiolites. In western Ladakh, the Suru–Thasgam ophiolitic slice is highly dismembered and consists of peridotites, pyroxenites and gabbros, emplaced as imbricate blocks thrust over the Mesozoic Dras arc complex along the Indus Suture Zone. The Thasgam peridotites are partially serpentinized with relict olivine, orthopyroxene and minor clinopyroxene, as well as serpentine and iron oxide as secondary mineral assemblage. The pyroxenites are dominated by clinopyroxene followed by orthopyroxene with subordinate olivine and spinel. Gabbros are composed of plagioclase and pyroxene (mostly replaced by amphiboles), describing an ophitic to sub-ophitic textural relationship. Geochemically, the studied rock types show sub-alkaline tholeiitic characteristics. The peridotites display nearly flat chondrite-normalized rare earth element (REE) patterns ((La/Yb)N = 0.6–1.5), while fractionated patterns were observed for pyroxenites and gabbros. Multi-element spidergrams for peridotites, pyroxenites and gabbros display subduction-related geochemical characteristics such as enriched large-ion lithophile element (LILE) and depleted high-field-strength element (HFSE) concentrations. In peridotites and pyroxenites, highly magnesian olivine (Fo88.5-89.3 and Fo87.8-89.9, respectively) and clinopyroxene (Mg no. of 93–98 and 90–97, respectively) indicate supra-subduction zone (SSZ) tectonic affinity. Our study suggests that the peridotites epitomize the refractory nature of their protoliths and were later evolved in a subduction environment. Pyroxenites and gabbros appear to be related to the base of the modern intra-oceanic island-arc tholeiitic sequence.

Mineralogy, Geochemistry, and Genesis of Kaolinitic Claystone Deposits in the Datong Coalfield, Northern China

AbstractGray-black kaolinitic claystones of industrial value are abundant in Upper Carboniferous–Lower Permian coal-bearing strata of the Datong Coalfield of northern China. The main types are tonsteins and cryptocrystalline kaolinitic claystones, distinguished by the thinness and greater crystallinity of kaolinite in the former and by the presence of detrital illite and authigenic pyrite in the latter. In order to determine the formation history of these two types of kaolinitic claystone, the petrological, mineralogical, and geochemical characteristics of borehole samples from the Upper Carboniferous Taiyuan Formation which comprises siliciclastics and coal seams deposited in a coastal environment, were analyzed. In addition to kaolinite, the claystones contain subordinate illite, quartz, pyrite, anatase, feldspar, siderite, and calcite. The tonsteins and cryptocrystalline kaolinitic claystones have different sources, as shown by petrographic data, elemental ratios, and chondrite-normalized rare earth element (REE) patterns. The volcanic origin of the tonsteins is revealed by an abundance of volcanic quartz and vitric fragments as well as Al2O3/TiO2, Zr/Hf, and Nb/Ta ratios consistent with a felsic igneous source. Their REE fraction was derived from feldspars or micas of the parent rocks, and the fraction decreased with alteration of these minerals to kaolinite. The sedimentary origin of the cryptocrystalline kaolinitic claystones is revealed by an abundance of detrital quartz and illite grains derived from either granite or sedimentary upper crust, and by the total REE contents (ΣREE) and (La/Yb)N values which are consistent with granitic material. Their depositional environment was in a transitional (coastal) setting (as shown by intermediate Sr/Ba ratios) hosting an open acidic hydrologic system (as shown by high chemical index of alteration (CIA) values indicative of intensive chemical weathering) that was suboxic to anoxic (as shown by high U/Th ratios and trace-metal enrichment factors). The present chemistry of these claystones was thus controlled by a combination of parent rock type and diagenetic alteration.

Formation of Ti-rich bauxite from alkali basalt in continental margin carbonates, Payas region, SE Turkey: implications for sea level change in the Upper Cretaceous

The Payas region bauxite deposits occur as a sandwiched layer that is a few kilometers long and an average of 10 m thick between the lower and upper Cretaceous carbonates of the Arabian Platform. The bauxites occur as 2 types, comprising blanket and pocket, are chemically and texturally homogeneous, and have a thrust structure with ophiolitic mélange formations. The bauxite varies in color, from reddish-brown to grayish-green to black, and has a massive, patchy, and very rare oolitic-pisolitic texture. The bauxite mainly consists of diaspore, hematite, rutile, anatase, rare kaolinite, boehmite, and pyrite minerals.The prismatic and lath-shaped euhedral rutile within the bauxites indicated in situ formation of the bauxites from Ti-rich basaltic pyroclastics. The chondrite-normalized rare earth element pattern of the bauxite was similar to the basalt pattern and had a very weak Eu anomaly (0.9). There are 2 groups of elements have been enriched in the Payas region bauxite: the first group contains TiO2 (9.67wt%), Cr (752 ppm), V (617 ppm) and Ni (72 ppm), and the second group comprises Zr (993 ppm), Nb (86 ppm) and Sn (7 ppm). These 2 groups of element enrichment indicated that the parental material of bauxite had an alkali basalt character. The carbon and oxygen isotope stratigraphy throughout the carbonate section from the bottom to the top indicated that the climate was warm during bauxitization (mean δ18OVPDB: -6.15‰) and relatively cold after bauxite deposition (mean δ18OVPDB: -4.71‰). A marine regression during the warm climate could have been related to the uplifting of the coastal zone, which was linked to the vertical fault movements of the normal faults. A rapid transgression after bauxite formation during the cold climate period can be explained by subsidence of the continental margin, which was associated with episodic ophiolitic nappe loading during the closure of the Neotethyan Ocean in the region.

Petrogenesis of Silurian ultramafic–mafic plutons in southern Jiangxi: implications for the Wuyi–Yunkai orogen, South China

AbstractThe South China Block is one of the largest continental blocks located on the East Asian continent. The early Palaeozoic Wuyi–Yunkai orogen of the South China Block (known as the Caledonian orogen in Europe) is a major orogenic belt in East Asia and represents the first episode of extensive crustal reworking since Neoproterozoic time. Although this orogen is key to deciphering the formation and evolution of the South China Block, details about the orogen remain poorly defined. The Songshutang and Wushitou ultramafic–mafic units in southern Jiangxi Province, South China, have206Pb–238U ages ofc. 437 Ma, suggesting a Silurian formation age. All the Songshutang and Wushitou ultramafic–mafic rocks show relatively flat chondrite-normalized rare earth element patterns, depletions in Nb, Ta, Zr, Hf and Ti, and low ϵNd(t) values from −9.12 to −5.49 with negative zircon ϵHf(t) values from −10.84 to −2.58, resembling a typical arc magma affinity. Geochemical and isotopic data indicate that the newly identified ultramafic–mafic rocks, along with the reported Silurian mafic rocks in South China, possibly originated from the similar partial melting of an ancient subducted slab, fluid/sediment and metasomatized lithospheric mantle with varying degrees of fractional crystallization. In conjunction with other records of magmatism and metamorphism in South China, a late-orogenic extensional event led to the melting of the sub-continental lithospheric mantle in Silurian time and generated ultramafic–mafic rocks with a limited distribution along the Wuyi–Yunkai orogen and widespread late-orogenic granitic plutons in the South China Block.