High Field Strength Elements Nb Research Articles

Zircon U-Pb chronology and geochemistry of porphyritic granites: A case study of the Dalancun granites in Zhaoyuan, Shandong Province

The granite in Dalancun, Zhaoyuan, Shandong Province is mainly porphyry granite. In this paper, detailed field geology, petrology, isotopic chronology and whole-rock principal and microgeochemistry have been studied. LA-ICP-MS zircon U-Pb chronology shows that it was formed in the Early Cretaceous (129.7 ± 0.8 Ma), later than Linglong granite (160-150 Ma) and earlier than Laoshan granite (120-114 Ma). It is consistent with the main ore-forming age of Jiaodong large gold deposit and the diagenetic age of Guojialing granite (130-126 Ma). K2O+Na2O=7.41%~9.37%, K2O/Na2O=0.78~1.24; A/CNK=0.89~1.06, MgO=0.63%~1.01%, with high K, Al characteristics, belongs to the quasi-aluminum-peralumin high potassium calc-alkaline series. The strong differentiation of rare earth elements (La/Yb) N=68.24~93.96, weak positive Eu anomaly (δEu=1.26~1.769), enrichment of large ion lithophile elements Ba, Sr, high field strength elements Nb, Ta, Ti deficit, indicating the geochemical characteristics of continental lithosphere. In addition, high Sr (1817×10-6~2017×10-6), low Y (7.55×10-6~9.36×10-6), Sr/Y ratio between 199.89~262.29, Nb/Ta ratio between 14.67~16.73, consistent with continental crust ratio, It shows the characteristics of type I granite and Adakite. Combined with other geochemical indexes and regional geological studies, this paper suggests that the Dellancun porphyritic granite belongs to crust-mantle granite, which may have been formed in the arc environment of continental margin during the transition from compressive to extensional tectonic system in Jiaodong area, and is closely related to the subduction of the paleo-Pacific plate. This study not only helps to understand the Mesozoic tectonic evolution in eastern China, but also provides a basis for revealing the formation mechanism of Jiaodong gold deposit.

Spatial Relationship between Eclogite and Copper-Nickel Mineralization in East Kunlun, China

In recent years, Cu-Ni deposits have been discovered at different localities in the Eastern part of the Kunlun orogenic belt such as Xiarihamu, Langmuri, Shitoukengde, and Wenquan. Eclogites are usually exposed in the areas associated with these deposits, thereby implying a certain coupling relationship between the Cu-Ni deposits and eclogite distribution. In this study, eclogite samples from the Xiarihamu and Langmuri areas were analyzed using petrogeochemistry, U-Pb zircon geochronology, and electron probe microanalysis (EPMA). Further, eclogite protolith properties, the formation environment, and the metallogenic mechanism were also investigated. Geochemically, eclogite is rich in MgO and FeO and low in alkali and SiO2. Its m/f ratios are 0.72 to 1.53 and Mg# values of 42 to 61. Overall, the chondrite-normalized rare-earth elements (REE) patterns showed characteristics of weak enrichment with LREE, weak negative Eu anomalies, relative enrichment of large-ion lithophile elements such as K and Rb, active incompatible element Th, the depletion of high-field strength elements Nb, Ta, Zr, and Hf, and V-shaped valleys caused by depletion in Sr, P, and Ti. These geochemical characteristics indicated that the protolith is highly differentiated Fe gabbro that formed in a continental margin type of rift environment. The EPMA analyses showed that the composition of garnet consists of almandite and grossularite, and omphacite often contains augite. Geochronological investigations showed that the peak metamorphic age of eclogite in Xiarihamu and Langmuri is 415.6 ± 2.7 Ma (MSWD = 0.43, n = 16) and 449.1 ± 8.5 Ma (MSWD = 0.88, n = 19), which are related to the early Paleozoic orogenic cycle and formed slightly earlier than the formation of the magmatic liquation type of Cu-Ni deposits in this area. On the basis of spatial coupling, formation age approximation, and geochemical correlation between eclogite and mafic rock masses, in combination with the previous research results of earlier work, it has been considered that the Cu-Ni ore deposits in the East Kunlun Range were formed in the post-collisional extension environment after the deep subduction of the continental crust. The ultra-high-pressure metamorphic melange formed by continental deep subduction or the enriched mantle formed by crust-mantle metasomatism was partially melted to form sulfur-rich mafic–ultramafic magmas in the post-collision extension environment. During the deep subduction of the continental crust, a large amount of crust-derived sulfur was brought into the mantle, which is the key factor for the mineralization of Cu-Ni ore in the region.

Geochronology and Geochemistry of Late Paleozoic Volcanic Rocks and Their Relationship With Iron and Molybdenum Deposits in Xilekuduk Area, Northern Margin of Junggar

A large number of intermediate basic volcanic rocks and porphyry Cu-Mo deposits as well as volcanic-hosted magnetite deposit have been recently discovered in the Xilekuduk area. However, no reports concerning petrogenesis and age or their relationship with mineralization have been published to date. The purpose of this study was to make up for the absence of previous studies on Devonian volcanic activities in the area and to confirm the relationship between two stages of volcanic activities and mineralization so as to provide important theoretical basis for mineral exploration. Based on research results of zircon U-Pb geochronology and element geochemistry of volcanic rocks in the area, the ages of dacite, andesite, and stomatal andesite are considered as 375.2 ± 2.9 Ma, 386.5 ± 3.0 Ma, and 317.9 ± 2.9 Ma, respectively, corresponding to the Middle Devonian and Late Carboniferous Period. The Devonian volcanic rocks belong to the high-K calc-alkaline series and island arc volcanic rocks, which are enriched in LREE, strongly enriched in large ion lithophile elements Th, Rb, Ba, and K and relatively depleted in high-field strength elements (HFSEs) Nb, Ta, and Ti. The Carboniferous volcanic rocks are enriched in LREE, as well as the large ion lithophile elements Th, Rb, Ba, and K are strongly enriched, while depleted in the HFSEs Nb, Ta, and Ti; moreover, the contents of TiO2and V are 0.94–0.97% and 178–183×10–6, which are higher than those of island arc basalts. According to mineralogical typomorphic characteristics and geochemical analysis, magnetite mineralization is divided into two phases. The early stratiform magnetite ore MT1 has magmatic characteristics, forming a volcanic rock type magnetite deposit related to Devonian volcanic eruption and sedimentation (375–386 Ma). The magnetite MT2 in the magnetite-quartz vein is considered as hydrothermal genesis, which is a metal mineral in the early metallogenic stage of Carboniferous (317.1 ± 2.9 Ma) volcanic eruption and subvolcanism, and may be related to porphyry molybdenum mineralization. Therefore, the volcanism and Fe-Cu-Mo mineralization in this area is characterized by multistage superimposed mineralization.

Geochemical constraints on the middle Triassic Kani Zarrineh karst bauxite deposit, Irano–Himalayan belt, NW Iran: Implications for elemental fractionation and parental affinity

The middle Triassic Kani Zarrineh bauxite deposit, in the Irano–Himalayan belt, located 20 km northwest of Saqqez city in northwestern Iran, occurs in depressions of the early Triassic bedrock carbonates of the Elika Formation. The ores formed during the Carnian Pluvial Event under high atmospheric pCO2 conditions and humid episodes. Powder-XRD analyses revealed that the Kani Zarrineh bauxite is mainly composed of diaspore, hematite, and kaolinite. There is a broad inverse relationship between hematite abundance and diaspore abundance in the profile with a downward increase of hematite and an upward increase of diaspore. Among minor constituents, single crystals rich in high field strength elements, such as florencite, monazite-(Ce), xenotime-(Y), and zircon were detected by SEM-EDS analyses. R-mode factor analysis indicates that most of the variance of the dataset is explained by a factor accounting for the competition between the Al2O3–TiO2–Ga–HFSE pool and the SiO2–Fe2O3–U–first row transition metals pool, and related to the coupled effect of a change of redox conditions and climatic regime. The other two factors are related to the importance of phosphate minerals (e.g., florencite and monazite) in hosting LREE, and to the influence that the chemistry of the soil solutions, especially pH, exerted in promoting the preferential mobility of HREE. Both these factors contribute to explaining LREE/HREE fractionation in the Kani Zarrineh bauxite ores. A negative correlation between the Ce/Ce* and the TOC (r = –0.91) corroborates the fact that bauxitization occurred in an environment characterized by a progressive increase in Eh upward. The coupled Ce/Ce* increase and TOC decrease upward are likely related to soil aeration, a progressive increase in soil temperature, and a reduction in soil moisture. An upward decrease of soil moisture is also consistent with an increase in the abundance of Al hydroxides that require drier conditions than those necessary for the formation of Fe oxyhydroxides. Finally, the evaluation of elemental mobility and results of the R-mode factor analysis indicate that, in addition to Al, high field strength elements Nb and Ta can be used confidently for assessing parental affinity of the Kani Zarrineh bauxite ores. The Al/Ta and Nb/Ta ratios and the Sm/Nd and Eu/Eu* provenance proxies reveal that mafic rocks interbedded in the early Triassic bedrock carbonates of the Elika Formation are likely protoliths of the Kani Zarrineh bauxite ores. This finding is consistent with what is observed for some bauxite deposits from the Mediterranean-type karst bauxite belt, especially from northwestern Iran of different ages that basalt rocks were the parent rock of these deposits.

A geochronological and geochemical study on the granodiorite porphyry and its implication for the mineralization in the Dayaoshan metallogenic belt, Southeastern China

A systematic study combining U-Pb zircon dating, lithogeochemical and Sr–Nd isotopic analyses was carried out upon the Xinping granodiorite porphyry in the Dayaoshan metallogenic belt to understand its petrogenesis and tectonic significance. LA-ICP-MS U–Pb zircon dating yielded a 442.7 ± 5.8 Ma age, indicating that the granodiorite porphyry was emplaced during the Llandovery Silurian of the Early Paleozoic. The granodiorite porphyry shares the same geochemical characteristics such as Eu negative anomaly as other syn-tectonic granite plutons in the region, including the granodiorite porphyry in Dawangding and granite porphyries in the Dali Cu–Mo deposit and Longtoushang old deposit, indicating a similar magma evolution process. The Xinping granodiorite porphyry has high contents of SiO2 (67.871.8%) and K2O (1.78–3.42%) and is metaluminous–peraluminous with A/CNK ratios ranging from 0.97 to 1.06, indicative of high-potassium calc-alkaline to calc-alkaline affinity. It is a I-type granite enriched in large ion lithophile elements Rb, Sr, while depleted in Ba and high field-strength element Nb. Tectonically, a collision between the Yunkai Block from the south and the Guangxi Yunnan-North Vietnam Block from the north during the Early Paleozoic was followed by uplifting of the Dayaoshan terrane. The Xinping granodiorite porphyry was likely emplaced during the collision. Sr–Nd isotopic analyses show that the granodiorite porphyry has initial 87Sr/86Sr ratios (Isr) of 0.7080–0.7104, eNd(t) range from − 0.08 to − 4.09, and t2DM between 1.19 and 1.51 Ga, well within the north-east low-value zone of the Cathaysia block, indicating a Paleoproterozoic Cathaysia basement source and an involvement of under plating mantle magma. Field observations, geochronological data, and 3D spatial distribution all lead to the conclusion that the Early Paleozoic Xinping granodiorite porphyry does not have any metallogenic and temporal relationships with the Xinping gold deposit (which has a Jurassic-Early Cretaceous age based on previous studies) but a close metallogenic relation to W–Mo mineralization.

Partitioning of V and 19 other trace elements between rutile and silicate melt as a function of oxygen fugacity and melt composition: Implications for subduction zones

AbstractVanadium is a multivalent element that can speciate as V2+, V3+, V4+, and V5+ over a range of geologically relevant oxygen fugacities (fO2). The abundance of V in planetary materials can be exploited as a proxy for fO2 when its partitioning behavior is known. The mineral rutile (TiO2) is an important carrier of the high field strength elements Nb and Ta in the solid Earth, but it can also incorporate substantial quantities of vanadium (up to ~2000 ppm; e.g., Zack et al. 2002). However, little work has been done to systematically investigate how the partitioning of V in rutile-bearing systems changes as a function of both fO2 and composition. We measured the partitioning of V and 19 other trace elements (Sc, Cr, Y, Zr, Nb, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Yb, Lu, Hf, and Ta) between rutile and three silicate melt compositions equilibrated at 1 atm pressure, 1300 °C and fO2 values from two log units below the quartz-fayalite-magnetite oxygen buffer (QFM-2) to air (QFM+6.5). Rutile/melt partition coefficients (DVrt/melt) change dynamically over an eight-log unit range of fO2 and are greatest at fO2 = QFM-2 in all compositions. Vanadium solubility in rutile declines continuously as fO2 increases from QFM-2 and approaches unity in air. Trace-element partitioning between rutile and melt is also correlated with melt composition, with the greatest values of Drt/melt measured in the most polymerized melt systems containing the least TiO2. We do not find any circumstances where V becomes incompatible in rutile. Our results indicate that rutile is a considerable sink for V at terrestrial fO2 values and will contribute to the retention of V in refractory slab residues in subduction zones. In agreement with previous work, we find that DTart/melt>DNbrt/melt under all conditions investigated, suggesting that rutile fractionation does not lead to low Nb/Ta ratios in Earth's continental crust.

Geochemistry, Zircon U–Pb Geochronology, and Lu–Hf Isotopes of the Chishan Alkaline Complex, Western Shandong, China

Mass alkaline magmatic activities in Western Shandong during the late Mesozoic controlled the mineralization processes of gold and rare earth element (REE) polymetallic deposits in the region. The Chishan alkaline complex is closely associated with the mineralization of the Chishan REE deposit, which, as the third largest light REE deposit in China following the Baiyenebo (Inner Mongolia) and Mianning (Sichuan) deposits, is considered a typical example of alkaline rock mineralization throughout the North China Craton. To determine how the Chishan alkaline complex and REE deposit interact with each other, a systematic study was conducted on the petrology, rock geochemistry, zircon U–Pb geochronology, Lu–Hf isotopes of the quartz syenite, and alkali granite contained in the Chishan alkaline complex. The results reveal that the deposits feature similar geochemical characteristics typical of an alkaline rock series—both are rich in alkali, high in potassium, metaluminous, and poor in Ti, Fe, Mg, and Mn. In terms of REEs, the deposits are strongly rich in light REEs but poor in heavy REEs, with weak negative Eu anomalies. In terms of trace elements, they are rich in large ion lithophile elements Ba, Sr, and Rb but poor in high field-strength elements Nb, Ta, and Hf. Zircon LA-ICP-MS U–Pb dating indicated that the quartz syenite and alkali granite formed in Early Cretaceous at 125.8 ± 1.2 Ma and 127.3 ± 1.0 Ma, respectively; their εHf(t) values are −22.67 to −13.19, with depleted model ages (TDM) ranging from 1296 Ma to 1675 Ma and crustal model ages (TDMC) of 2036–2617 Ma. The Chishan alkaline complex originated from partial of the EM I-type (enriched mantle I) lithospheric mantle with assimilation of ancient crustal materials. The complex is of the same origin as the REE deposit, and developed in an extensional setting that resulted from plate subduction and lithospheric thinning and upwelling in the eastern area of the North China Craton.

Boninites in the ~3.3 Ga Holenarsipur Greenstone Belt, Western Dharwar Craton, India

In this contribution, we present detailed field, petrography, mineral chemistry, and geochemistry of newly identified high-Si high-Mg metavolcanic rocks from the southern part of the ~3.3 Ga Holenarsipur greenstone belt in the western Dharwar craton, India. The rocks occur as conformable bands that were interleaved with the mafic-ultramafic units. The entire volcanic package exhibits uniform foliation pattern, and metamorphosed under greenschist to low grade amphibolite facies conditions. The rocks are extremely fine grained and exhibit relict primary igneous textures. They are composed of orthopyroxene and clinopyroxene phenocrysts with serpentine, talc, and amphibole (altered clinopyroxene). Cr-spinel, rutile, ilmenite, and apatite occur as disseminated minute grains in the groundmass. The mineralogical composition and the geochemical signatures comprising of high SiO2 (~53 wt. %), Mg# (~83), low TiO2 (~0.18 wt. %), and higher than chondritic Al2O3/TiO2 ratio (~26), reversely fractionated heavy rare earth elements (REE) (GdN/YbN ~ 0.8), resulting in concave-up patterns, and positive Zr anomaly, typically resembled with the Phanerozoic boninites. Depletion in the high field strength elements Nb, and Ti relative to Th and the REE in a primitive mantle normalized trace element variation diagram, cannot account for contamination by pre-existing Mesoarchean continental crust present in the study area. The trace element attributes instead suggest an intraoceanic subduction-related tectonic setting for the genesis of these rocks. Accordingly, the Holenarsipur high-Si high-Mg metavolcanic rocks have been identified as boninites. It importantly indicates that the geodynamic process involved in the generation of Archean boninites, was perhaps not significantly different from the widely recognized two-stage melt generation process that produced the Phanerozoic boninites, and hence provides compelling evidence for the onset of Phanerozoic type plate tectonic processes by at least ~3.3 Ga, in the Earth’s evolutionary history.

Neoproterozoic granitic gneiss offshore the Shandong Peninsula of Eastern China: the eastward extension of the Sulu Orogenic Belt

ABSTRACTThe Sulu Orogenic Belt in eastern China has experienced a multistage tectonic evolutionary history. However, its geological evolution has not yet been corroborated by sufficient direct evidence from basement rocks. Chaolian Island on the Qianliyan Uplift provides an opportunity to study the formation and evolution of the Sulu Orogenic Belt using direct geochronological and geochemical evidence. We determined that the characteristic mineral assemblage in the study region is quartz + K-feldspar + perthite + biotite + muscovite. The samples are silica- (SiO2 = 72.8%–75.8%) and alkali-rich (ALK(Na2O+K2O) = 8.7%–9.3%), with high iron-magnesium ratios (FeO*/(FeO*+MgO) = 0.92–0.96) and low CaO and MgO concentrations. Furthermore, they are rich in large-ion lithophile elements K, Rb, Ba, and U, but depleted in high field strength elements Nb, Ta, and Zr. They exhibit high Ga/Al values (Ga × 104/Al = 3.33–3.74) and significant fractionation between light and heavy rare earth elements. The samples are A-type granites. In the discrimination diagrams for granite genesis types, the samples plotted in the post-orogenic A2-type granite region. Secondary ion mass spectrometer (SIMS) zircon U–Pb dating results indicated that the granitic gneiss formed ~782.6–802.3 Ma (Middle Neoproterozoic), consistent with the timing of the breakup of the Rodinia supercontinent on the northeastern margin of the Yangtze Plate. Comparing geochemical characteristics and zircon U–Pb ages of the A-type granitic gneisses of the Sulu Orogenic Belt, the Qianliyan Uplift appears to be an extension of the belt across the ocean and is affiliated with the Yangtze Plate. The granitic gneiss on Chaolian Island is related to the formation of a mantle superplume during the breakup of Rodinia, and the northeastern margin of the Yangtze Plate during the Middle Neoproterozoic was located in a back-arc extension setting induced by the subduction of oceanic plates.

Primary Silica-rich Picrite and High-Ca Boninite Melt Inclusions in Pyroxenite Veins from the Kamchatka Sub-arc Mantle

Island arc picrites and boninites are magnesian magmatic rocks believed to be generated by high degrees of melting of depleted mantle sources fluxed by subduction-derived, volatile-rich components. These magmas can be probes of both the mantle wedge protoliths and subduction components, but are rare among other, usually more evolved, types of arc lavas. Furthermore, many arc picrites and boninites show evidence for late-stage differentiation prior to or during eruption, masking their primary, mantle-derived geochemical signatures. We report textural and chemical data on spinel-hosted melt inclusions of mantle origin in amphibole-bearing websterite veins cross-cutting spinel harzburgite xenoliths from the active andesitic Avacha volcano (south Kamchatka, Russia). The data are used to constrain the composition and origin of melts that formed the websterite veins in the sub-arc lithospheric mantle. The melt inclusions typically contain euhedral orthopyroxene and clinopyroxene and occasionally minor amphibole in silicate glass. The melt inclusions were homogenized using heating stages and gas-mixing furnaces. The homogenized glasses range from subalkaline primitive silica-rich picrite and high-Ca boninite (>15 wt % MgO, 48–54 wt % SiO2) to rhyolite. High-Ca boninite glasses have moderate volatile and low heavy rare earth element contents and elevated Cs, Rb, Ba, U, Sr, and Li abundances, with extremely high U/Th. In turn, the glasses display no negative spikes in the high field strength elements Nb, Ta, Zr, Hf, and Ti. We show that the silica-rich picrite and high-Ca boninite liquids in this study formed by high degrees of melting (>25%), at volatile under-saturation, of hybrid melt-depleted but silica-rich mantle sources at ≥1·5 GPa. The hybrid sources formed in two stages: first, by extraction of ∼15% melt from the convecting mantle to form a refractory protolith, which was subsequently enriched in silica via interaction with subduction-derived components prior to or during remelting in the mantle wedge. The subduction-derived components were enriched in fluid-mobile elements and probably oxidized. Overall, our results suggest that silica-rich picrites and high-Ca boninites can be primary melts in mature subduction zones and differentiate within the mantle wedge and the deep arc crust to form more evolved andesite magmas.

Petrogenesis of metaluminous A-type granitoids in the Tengchong–Lianghe tin belt of southwestern China: Evidences from zircon U–Pb ages and Hf–O isotopes, and whole-rock Sr–Nd isotopes

Abstract The Tengchong–Lianghe tin belt is the most importantpart of the tin metallogenic belt in southwest China’s Sanjiang Metallogenic province. In this district, two A-type granite plutons that are spatially associated with the tin deposits have recently been discovered. These granitoids are dominated by biotite granites with weakly peraluminous to metaluminous compositions, high SiO2 contents (73.3–76.2 wt.%), and high alkali contents (K2O + Na2O = 8.3–9.17 wt.%). Trace element spider diagrams show that these granitoids are also clearly enriched with the large-ion lithophile elements Rb, Th, U, and K, but are markedly depleted in Ba and Sr. They are significantly depleted in the high field-strength elements Nb and Ti, but are enriched with Zr and Hf. These rocks have relatively high zircon saturation temperatures (774–833 °C), high Zr + Nb + Ce + Y contents (272–416 ppm), and 10,000 × Ga/Al ratios (almost > 2.7) that are typical of A-type granites. They also have high total rare earth element (REE) contents (174–404 ppm) and relatively flat chondrite-normalized REE patterns with significantly negative Eu anomalies. LA–ICP–MS zircon U–Pb dating results indicate that the emplacement of these two granite plutons occurred during the early Tertiary (52.7 ± 0.3–53.0 ± 0.4 Ma) and the Late Cretaceous (73.3 ± 0.5–73.3 ± 0.5 Ma) periods, respectively. Isotopic compositions show that the granitoids have highly variable (87Sr/86Sr)i ratios (0.7182–0.7457), relatively constant low eNd(t) values (− 11.2 to − 12.4), and ancient Nd and Hf model ages (1.56–1.88 Ga), suggesting that they were derived from the partial melting of the Paleoproterozoic continental crust. Their zircon δ18O values (6.6–8.5‰) and eHf values (from − 8.6 to − 11.3) also suggest that these granitoids mainly originated from the middle-lower continental crust consisting of mafic and metasedimentary rocks. Such geochemical characteristics indicate that these two A-type granite plutons were generated by the partial melting of Paleoproterozoic mafic and the metasedimentary basement of the Tengchong block in a post-collisional or syn-collisional extensional setting of the Neo-Tethy anorogenic zone during the Late Cretaceous–Early Cenozoic. The Late Cretaceous granitoids were probably associated with the subduction of the Neo-Tethyan plate under the post-collisional extension of the western Burma and Tengchong microplates’ collision during the Jurassic–Early Cretaceous. The Early Tertiary granitoids might be the syn-collisional product of stretching relaxation during the middle and late main-collisional period (55–41 Ma) between India and Asia.

Paleozoic alkaline volcanism: geochemistry and petrogenesis of Um Khors and Um Shaghir trachytes of the central Eastern Desert, Egypt

The Um Khors and Um Shaghir trachyte (UKT and UST) plugs and sheets represent two conspicuous outcrops of Paleozoic alkaline volcanism in the central Eastern Desert of Egypt. The trachyte magmatisms erupted along the Pan-African NW-trending shear zone (302 ± 15 and 273 ± 15 Ma, respectively) and intruded the Late Proterozoic rocks of the studied area. The trachyte rocks consist mainly of sanidine, anorthoclase, albite, and quartz with a noticeable amount of aegirine–augite, aegirine, hedenbergite, and arvedsonite. The studied trachytes are moderately evolved in composition (with 62–67.5 wt.% SiO2) and exhibit a limited compositional range in most of the major elements. They are alkaline in nature and considered as silica-oversaturated rocks. The rare earth elements (REE) patterns are somewhat uniform and highly fractionated, being enriched in light REE over heavy REE and show prominent negative Eu anomalies. The UKT and UST are enriched in high field strength elements Nb, Zr, and Y, consistent with typical within-plate alkaline magmatisms of extensional tectonic regimes. They were generated through the fractional crystallization of mantle-derived magmas. Although the UST is younger than the UKT, they show approximately similar chemical compositional ranges of the most major and trace elements, with somewhat higher MgO, Cr, Ni, and Ba contents in the former. This may argue against the evolution of the UST via a continuous fractional crystallization of the residual magmatic melt of the UKT. Thus, the UKT and UST are genetically related but could be emplaced through two various magmatic pulses of the same parent source (i.e., asthenospheric mantle source) at different times. The ascending magmatisms were subjected to variable significant degrees of crustal contamination during their generation.

Magnesian andesites in north Xinjiang, China

Springer-Verlag 2008 Abstract Middle Devonian magnesian andesites (MAs) are widely distributed in south Altay and Carboniferous MAs are present in Alataoshan and west- and east-Tian- shan in the north Xinjiang region. These MAs are andesitic rocks with 53-65% SiO2,\1% (0.21-1.08%; average of 0.72%) TiO2, and C50 Mg # . Magnesian dacites and dior- ites, with 52.38-66.91% SiO2,\0.30% TiO2 and C42 Mg # commonly occur with these MAs. Relative to boninites, MAs have lower MgO contents (average 6.39%) but higher Ti, K and Na. They have characteristic flat chondrite- normalized REE patterns with weak to no Eu anomalies (Eu depletion, or Eu/Eu* = 0.65-1.15), low (La/Yb)N (0.98-6.4, mostly 4±) and low total REE contents (15- 95 ppm). They also have high contents of compatible elements Cr and Ni (72-790 and 29-276 ppm, respec- tively). Their relative depletion in high field strength elements Nb, Ta and Ti, and relative enrichment in mobile large-ion lithophile elements Rb, K and Pb are evident on primitive mantle-normalized trace element spidergrams. If magnesian andesites are melts coming from the subducted oceanic crust, as proposed elsewhere, then the relatively high Y contents ((15 ppm), low Sr/Y ratios (4.4-6.2), low (La/Yb)N, and high Mg # of the MAs in north Xinjiang provide evidence of interaction of such melts with mantle

A petrological and geochemical study of the volcanic rocks of the Crowsnest Formation, southwestern Alberta, and of the Howell Creek suite, British Columbia

Alkaline igneous rocks of the Crowsnest Formation in southwestern Alberta and in the Howell Creek area in southeastern British Columbia have been suggested previously to be cogenetic. To test this hypothesis, samples of both suites were characterized petrographically and their major and trace element geochemistry was determined. A subset of the samples was analyzed for whole-rock Sr and Nd isotope geochemistry. The samples of the two suites are latites, trachytes, and phonolites based on the International Union of Geological Sciences (IUGS) total alkalis versus silica (TAS) diagram. Samples from both suites show similar patterns on mantle-normalized trace element diagrams, being enriched relative to mantle values but depleted in the high field-strength elements Nb, Ta, and Ti relative to the large-ion lithophile elements. The chondrite-normalized rare-earth element (REE) patterns for both suites are light REE enriched, with no Eu anomaly and flat heavy REE. The isotope geochemistry of both suites is characterized by low initial 87Sr/86Sr (SrT = 0.704 to 0.706) and low εNdT (–7 to –16). The Howell Creek samples have lower εNdT and higher SrT than do the Crowsnest samples. Based on the intra- and intersuite differences in the isotope geochemistry, we conclude that these samples are not cogenetic, but rather represent samples that have experienced similar evolutionary histories from a heterogeneous source region in the subcontinental lithospheric mantle.

On the formation of an inverted weathering profile on Mount Kilimanjaro, Tanzania: Buried paleosol or groundwater weathering?

This paper presents an investigation into the degree and nature of chemical weathering during soil formation on a volcanic (phonolite) substrate on the southern slopes of Mt. Kilimanjaro in northern Tanzania. The high field strength elements Nb and Ta were used to estimate enrichments and depletions relative to the bedrock. The degree of weathering was found to increase with depth in the soil profile. At depths greater than 200 cm, Si, Na, K, Ca, and Mg have been depleted by nearly 100% while Al has been enriched, resulting in a highly aluminous soil residue (40–50 wt.% Al 2O 3). At depths shallower than 200 cm, the soil is also depleted in Si, Na, K, Ca and Mg though not to the extents seen at depths greater than 200 cm. The lower degrees of weathering in the upper 200 cm are also evidenced by the fact that the layer above 200 cm is characterized by slight positive Eu anomalies relative to other rare earth elements whereas the deeper layer exhibits no Eu anomalies. The rare earth element systematics are consistent with preferential weathering of the glassy matrix in the upper 200 cm, leaving behind plagioclase phenocrysts, which are enriched in Eu. In the deeper layer, weathering appears to be so extensive that both Eu-rich plagioclase phenocrysts and Eu-poor glass/ash have largely weathered away. These observations collectively show that the upper and lower layers of the weathering profile have undergone different weathering histories. Four scenarios may explain the apparent inverted weathering profile: re-precipitation followed by erosion, Aeolian deposition, a buried paleosol, and enhanced weathering due to lateral subsurface water flow. The first hypothesis fails to explain the massive losses of Si, Na, K, Ca, and Mg below 200 cm. The Aeolian deposition hypothesis is also untenable because it contradicts the trace element and REE's behaviors. The latter two hypotheses are both reasonable; however, the buried paleosol model is inconsistent with some physical and geochemical observations and the subsurface flow model requires the influence of hydraulic conditions not tested in this study. It is concluded here that either the buried paleosol model or the subsurface flow model can explain the formation of the Machame soils with the latter having novel implications for the transport of dissolved cations to the ocean.

The evolution of the Eagle Peak volcano — a distinctive phase of middle miocene volcanism in the western Mogollon-Datil volcanic field, New Mexico

The andesitic to dacitic Eagle Peak volcano represents a distinctive phase of Middle Miocene, post-caldera volcanism in the western part of the Mogollon-Datil volcanic field in southwestern New Mexico. Erupted during Basin and Range extensional tectonism, rocks of the Eagle Peak volcano are chemically and isotopically distinct from the bimodal suite, extension-related basalt and rhyolite that also erupted in this area from the Early Miocene to the Pleistocene. Instead, they have close petrogenetic affinities to the early post-caldera (~ 27–23 Ma) calc-alkaline, Bearwallow Mountain Andesite erupted from shield volcanos aligned along prominent Basin and Range fault structures. Geologic mapping and detailed petrographic and chemical studies of the Eagle Peak volcano has enabled the distinction of five different flow units, a central plug and a feeder dike. The flows were erupted from a central vent and two subsidiary “satellitic” centers on the western and southwestern flanks of the volcano. 40Ar 39Ar age-spectrum and paleomagnetic studies indicate that the Eagle Peak volcano was active between 12.1 and 11.4 Ma; its activity spanned at least one magnetic polarity reversal. With exception of late satellitic eruptions on the northwestern margin of the volcano, central vent and satellitic flows were erupted in rapid succession and have an average age of 11.7 Ma. The central plug yielded a plateau age of 11.4 Ma, which is a minimum of 90,000 years (2σ) younger than the 11.7 Ma average age of the central vent and satellitic flows. Major-oxide, trace-element and isotope geochemistry define two distinct magmatic series: a central vent and a satellitic series. Rocks of the satellitic series, although similar in modal mineralogy and rare earth element patterns, are slightly more alkaline and relatively enriched in the high field strength elements Nb, Ta, P and Ti compared to the central vent eruptives. Sr and Nd isotopes further demonstrate these differences; a sample of the satellitic flows exhibits lower ( 87Sr 86Sr ) i (0.7084) and higher ϵ Nd values ( ϵ Nd = −4.8) relative to an upper flow of the central vent series [ ( 87Sr 86Sr ) i = 0.7096), ϵ Nd = −8.5 ]. Major- and trace-element data support petrogenetic models based on periodic tapping of the central vent and satellitic series magmas, which both evolved by crystal fractionation. Central vent magmas evolved mainly by a modified process of filter pressing that accompanied the transfer of magma from a deep into a higher-level reservoir. Thus, a portion of the melt with some of the original crystals was extracted during this transer changing the resulting bulk magma chemistry but not affecting the major phenocryst compositions. In contrast, crystal fractionation within the satellitic magmas accompanied a progressive evolution in both phenocryst composition and bulk magma chemistry. Although temporally associated with bimodal basalt-rhyolite volcanism (< 21 Ma to Holocene), Eagle Peak rocks differ markedly from the suite of mafic bimodal rocks in trace-element and Sr and Nd isotope chemistry. Mafic rocks of the bimodal suite have Sr and Nd isotopic initial ratios that range from depleted mantle to bulk earth values [ ( 87Sr 86Sr ) i = 0.7030-0.7057, ϵ Nd = 0 to +9.12 ], and are substantially more primitive than Eagle Peak rocks [ ( 87Sr 86Sr ) i = 0.70839-0.70958, ϵ Nd = − 8.4 to − 4.9 ]. In contrast, Eagle Peak volcanics are geochemically more similar to the 27-23 Ma post-caldera Bearwallow Mountain Andesite, which is characterized by ( 87Sr 86Sr ) i = 0.7070–0.7102 and ϵ Nd = −8.15 to −5.95. The Eagle Peak volcanics, like the Bearwallow Mountain Andesite, assimilated a significant component of crustal material and were both likely derived from a similar lithospheric mantle source beneath the western Mogollon-Datil volcanic field.

Subalkaline andesite from Valu Fa Ridge, a back-arc spreading center in southern Lau Basin: petrogenesis, comparative chemistry, and tectonic implications

Tholeiitic andesite was dredged from two sites on Valu Fa Ridge (VFR), a back-arc spreading center in Lau Basin. Valu Fa Ridge, at least 200 km long, is located 40–50 km west of the active Tofua Volcanic Arc (TVA) axis and lies about 150 km above the subducted oceanic plate. One or more magma chambers, traced discontinuously for about 100 km along the ridge axis, lie 3–4 km beneath the ridge. The mostly aphyric and glassy lavas had high volatile contents, as shown by the abundance and large sizes of vesicles. An extensive fractionation history is inferred from the high SiO 2 contents and FeO∗ MgO ratios. Chemical data show that the VFR lavas have both volcanic arc and back-arc basin affinities. The volcanic arc characteristics are: (1) relatively high abundances of most alkali and alkaline earth elements; (2) low abundances of high field strength elements Nb and Ta; (3) high U/Th ratios; (4) similar radiogenic isotope ratios in VFR and TVA lavas, in particular the enrichment of 87 Sr 86 Sr relative to 206 Pb 204 Pb ; (5) high 238 U 230 Th , 230 Th 232 Th , and 226 Ra 230 Th activity ratios; and (6) high ratios of Rb/Cs, Ba/Nb, and Ba/La. Other chemical characteristics suggest that the VFR lavas are related to MORB-type back-arc basin lavas. For example, VFR lavas have (1) lower 87 Sr 86 Sr ratios and higher 143 Nd 144 Nd ratios than most lavas from the TVA, except samples from Ata Island, and are similar to many Lau Basin lavas; (2) lower Sr/REE, Rb/Zr, and Ba/Zr ratios than in arc lavas; and (3) higher Ti, Fe, and V, and higher Ti/V ratios than arc lavas generally and TVA lavas specifically. Most characteristics of VFR lavas can be explained by mixing depleted mantle with either small amounts of sediment and fluids from the subducting slab and/or an older fragment of volcanic arc lithosphere. The eruption of subalkaline andesite with some arc affinities along a back-arc spreading ridge is not unique. Collision of the Louisville and Tonga ridges probably activated back-arc extension that ultimately led to the creation and growth of Valu Fa Ridge. Some ophiolitic fragments in circum-Pacific and circum-Tethyan allochthonous terranes, presently interpreted to have originated in volcanic arcs, may instead be fragments of lithosphere that formed during early stages of seafloor spreading in a back-arc basin.

A reconnaissance study of the distribution of Ba, Nb, Y, and Zr in some Jotun kindred gneisses from Central Jotunheimen, southern Norway

Abstract The Bergen‐Jotun kindred rocks of this study, the Storådalen Complex (SCX), Svartdalen Gneiss (SG) and Mjølkedøla Purple Gabbro (MPG), have been shown to be a co‐magmatic series with calc‐alkaline affinities. The analyses of Ba, Nb, Y, and Zr presented here show no variation in these elements between the three rock units and are consistent with the calc‐alkaline character of the rocks. The lithophile elements Ba, K, and Sr are enriched relative to MORB and the high field strength elements Nb, Y, and Zr are depleted relative to MORB, Zr especially so.The SCX contains rocks with low (&gt;30) differentiation indices which are interpreted as plagioclase + pyroxene ± olivine ± amphibole cumulates. The remainder of the SCX, together with the MPG and SG, is regarded as the congealed liquid in equilibrium with these cumulates. The distribution of trace elements between these two components of the SCX can be adequately modelled using a Rayleigh fractionation process, measured ‘liquid’compositions, and calculated bulk distribution coefficients. It is thus concluded that the trace element geochemistry of the rocks of this study is consistent with subduction‐related, mantle‐derived magmas that fractionate within a continental or mature island arc environment. Subsequent high‐grade metamorphism and deformation of Sveconorwegian age have been essentially isochemical.