Eclogitization of glaucophanites by fluid infiltration

The characteristic textures of eclogitic veins infilling the interstitial spaces among zoisite-glaucophanite pillows, the fractures crosscutting massive zoisite-glaucophanites and the relic conduits within the eclogite boudins demonstrate that the eclogitization of glaucophanites was accompanied by the presence of a free fluid phase in the Western Tianshan high-pressure metamorphic belt. Non-planar populations of fluid inclusions, parallel toC-axis, have been preserved within the omphacites in eclogitic veins. The microthermometric results indicate that fluids are low-salinity (1–4 wt%NaCl) aqueous fluids. The chemical compositions of major, trace and rare earth elements of eclogitic veins and related host rocks indicate that the eclogitization of glaucophanites required the addition of external Si and Na elements, and was accompanied by the loss of large-ion lithophile elements, rare earth elements and water. The record of eclogitization of glaucophanites along fractures and zones of high permeability by fluid infiltration has been preserved in the Wetsern Tianshan high-pressure metamorphic belt.     

Temporal evolution of the Mariana arc during rifting of the Mariana Trough traced through the volcaniclastic record

The sedimentary sequences that accumulate around volcanic arcs may be used to reconstruct the history of volcanism provided the degree of along-margin sediment transport is modest, and that reworking of old sedimentary or volcanic sequences does not contribute substantially to the sediment record. In the Mariana arc, the rare earth and trace element compositions of ash layers sampled by Deep Sea Drilling Project (DSDP) site 451 on the West Mariana Ridge, and sites 458 and 459 on the Mariana Forearc, were used to reconstruct the evolution of the arc volcanic front during rifting of the Mariana Trough. Ion microprobe analysis of individual glass shards from the sediments shows that the glasses have slightly light rare earth element (LREE)-enriched compositions, and trace element compositions typical of arc tholeiites. The B/Be ratio is a measure of the involvement of subducted sediment in petrogenesis, and is unaffected by fractional crystallization. This ratio is variable over the period of rifting, increasing up-section at site 451 and reaching a maximum in sediments dated at 3–4 Ma, ∼ 3–4 million years after rifting began. This may reflect increased sediment subduction during early rifting and roll-back of the Pacific lithosphere. Parallel trends are not seen in the enrichment of incompatible high field strength (HFSE), large ion lithophile (LILE) or rare earth elements (REE), suggesting that flux from the subducting slab alone does not control the degree of melting. Re-establishment of arc volcanism on the trench side of the basin at ca 3 Ma resulted in volcanism with relative enrichment in incompatible REE, HFSE and LILE, although these became more depleted with time, possibly due to melt extraction from the mantle source as it passed under the developing back-arc spreading axis, prior to melting under the volcanic front.     

Origin of Baotoudong syenites in North China Craton: Petrological,mineralogical and geochemical evidence

Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Triassic alkaline magmatic belt along the northern margin of the North China Craton(NCC). Zircon U-Pb age, petrological, mineralogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteristics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside(cores)-aegirine-augite(rims), biotite, orthoclase and nepheline are the major minerals. The Baotoudong syenites have high contents of rare earth elements(REE), and are characterized by enrichment in light rare earth elements(LREE) and large ion lithophile elements(LILE; e.g., Rb, Ba, Sr), depletion in heavy rare earth elements(HREE) and high field strength elements(HFSE). They show enriched Sr-Nd isotopic compositions with initial ~87Sr/~86Sr ranging from 0.7061 to 0.7067 and ε_Nd(t) values from –9.0 to –11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO_2-undersaturated potassic-ultrapotassic, and is characterized by high contents of Ca O, Fe_2O_3, K_2O, Na_2O and fluid compositions(H_2O), and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area(80 km). The occurrence of the Baotoudong syenites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic.     

Combined thermodynamic and rare earth element modelling of garnet growth during subduction: Examples from ultrahigh-pressure eclogite of the Western Gneiss Region, Norway

Major and trace element zonation patterns were determined in ultrahigh-pressure eclogite garnets from the Western Gneiss Region (Norway). All investigated garnets show multiple growth zones and preserve complex growth zonation patterns with respect to both major and rare earth elements (REE). Due to chemical differences of the host rocks two types of major element compositional zonation patterns occur: (1) abrupt, step-like compositional changes corresponding with the growth zones and (2) compositionally homogeneous interiors, independent of growth zones, followed by abrupt chemical changes towards the rims. Despite differences in major element zonation, the REE patterns are almost identical in all garnets and can be divided into four distinct zones with characteristic patterns.In order to interpret the major and trace element distribution and zoning patterns in terms of the subduction history of the rocks, we combined thermodynamic forward models for appropriate bulk rock compositions to yield molar proportions and major element compositions of stable phases along the inferred pressure-temperature path with a mass balance distribution of REEs among the calculated stable phases during high pressure metamorphism. Our thermodynamic forward models reproduce the complex major element zonation patterns and growth zones in the natural garnets, with garnet growth predicted during four different reaction stages: (1) chlorite breakdown, (2) epidote breakdown, (3) amphibole breakdown and (4) reduction in molar clinopyroxene at ultrahigh-pressure conditions.Mass-balance of the rare earth element distribution among the modelled stable phases yielded characteristic zonation patterns in garnet that closely resemble those in the natural samples. Garnet growth and trace element incorporation occurred in near thermodynamic equilibrium with matrix phases during subduction. The rare earth element patterns in garnet exhibit distinct enrichment zones that fingerprint the minerals involved in the garnet-forming reactions as well as local peaks that can be explained by fractionation effects and changes in the mineral assemblage.     

The hydrogeochemistry of thermal springs on Mutnovskii Volcano, southern Kamchatka

This paper reports results from a hydrogeochemical study of thermal springs on Mutnovskii Volcano. The solutions of boiling pots and the pore water of thermal localities in the Donnoe Field were found to have an anomalous composition over a wide range of chemical elements, including rare earth elements (REE) and platinum-group elements (PGEs). The solutions of different springs exhibit considerable variations in their major and minor element compositions, indicating differences in the generation and upward transport of solutions. Physicochemical modeling showed that the mud pots studied could have been formed when the separate was ascending along an open fissure channel from the secondary zone of mixed boiling fluids. The high concentrations of titanium, vanadium, chromium, nickel, cobalt, REEs, and PGMs suggest a deep-seated source for these. The chemical forms of the elements in the solutions (primarily free aquaions) and the acidic environment favor high mobility of these elements. Deposition and concentration of these elements under near-surface conditions is unlikely.     

Petrogenesis of diabase from accretionary prism in the southern Qiangtang terrane,central Tibet: Evidence from U–Pb geochronology,petrochemistry and Sr–Nd–Hf–O isotope characteristics

The Bangong–Nujiang suture (BNS) between the Lhasa and Qiangtang terranes is an important boundary and its petrogenesis is controversial. Diabase from the accretionary prism in the southern Qiangtang terrane yields a zircon U–Pb age of 181.3 ± 1.4 Ma. All the diabases show tholeiitic basalt compositions, gentle enrichment patters of light rare earth elements (REE), variable enrichment in incompatible element concentrations (e.g. Th and Rb), and no anomaly in high field strength elements (e.g. Nb and Ta), similar to that of enriched mid‐ocean ridge basalt (E‐MORB). They have relatively homogeneous whole rock Nd (εNd(t) = 7.3–9.1) and zircon Hf–O isotopic compositions (εHf(t) = 14.8–16.1, and δ¹⁸O = 4.57–6.12‰), possibly indicating melting of the depleted mantle and no significant crustal contamination during the petrogenesis. The element variations suggest that the diabases were formed by plume–ridge interaction at a mid‐ocean ridge within the Bangong–Nujiang ocean.     

Fine-grained aggregates in L3 chondrites

The textures and chemical compositions of the constituent minerals of the fine-grained aggregates (FGA's) of L3 chondrites were studied by the backscattered electron image technique, electron probe microanalysis, and transmission electron microscopy. Plagioclase and glass in the interstices between fine grains of olivine and pyroxene indicate that the FGA's once partly melted. Compositional zoning and decomposition texture of pyroxenes are similar to those observed in chondrules, indicating a common cooling history of the FGA's and chondrules. Therefore, the mechanism that caused melting of the FGA's is considered to be the same as for chondrules. Bulk compositions of the FGA's are within the range of those of chondrules, so some chondrules probably were produced by complete melting of the same precursor materials as those of the FGA's. The precursor materials must have included fine olivine and other grains that probably are condensates.     

Trace element partitioning between amphibole and hydrous silicate glasses at 0.6–2.6 GPa

Partitioning behavior between amphibole and silicate glass of thirty-three minor and trace elements(Sc,Ti, V, Cr, Co, Rb, Sr, P, Y, Zr, Nb, Cs, Ba, K, La, Ce, Pr,Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb,Th, and U) have been determined experimentally. Products of crystallization of hydrous basalt melts from 0.6 GPa/860 °C up to 2.6 GPa/970 °C were obtained in a multianvil apparatus. Major and trace element compositions of amphibole and glass were determined with a combination of electron microprobe and laser ablation inductively coupled plasma mass spectrometry. The main mineral phase is calcic amphibole, and the coexisting glass compositions are tonalite, granodiorite, and granite. The compatibility of rare earth elements increase at 915 °C and then decrease at 970 °C, but the compatibility of most of these elements shows a continued, significant increase with increasing pressure. For high-field strength elements, large ion lithophile elements, actinide compatibility decrease with increasing temperature or pressure, but transition metals show a continued increase in compatibility within the temperature–pressure conditions. From mathematical and graphical fitting, we determined best-fit values for the ideal ionic radius(r_0, 1.01–1.04 ?), the strain-free partitioncoefficient(D_0, 1.18–1.58), and apparent Young's modulus(E, 142–370 GPa) for the M4 site in amphibole according to the lattice strain model. The D_0^M4 for rare earth elements rises at 915 °C and then drops at 970 °C at 0.6 GPa.However, the D_0^M4 values are positively proportional to the pressure for rare earth elements in the amphibole-glass pairs at 0.6–2.6 GPa and 970 °C. Furthermore, the derived best-fit values for r_0^M4 and E^M4 are almost constant and trend to increase with rising temperature and pressure,respectively. The partition coefficient is distinctly different for different melt compositions. The rare earth elements become more enriched in amphibole if the quenched glass is granodiorite or granite compared to the tonalitic glasses.     

Trace element characteristics of the fluid liberated from amphibolite-facies slab: Inference from the metamorphic sole beneath the Oman ophiolite and implication for boninite genesis

Major and trace element compositions of amphibolites and quartzose rocks in the 230-m-thick metamorphic sole underlying the mantle section of the Oman ophiolite in Wadi Tayin area were determined to investigate the chemical characteristics of the hydrous fluid released from subducted amphiboltie-facies slab. The fluid-immobile element compositions indicate that protoliths of these rocks are mid-ocean ridge basalt-like tholeiite and deep-sea chert, which is consistent with the idea that these rocks represent Tethyan oceanic crust overridden during the early, intraoceanic thrusting stage of the Oman ophiolite emplacement. The rare-earth element (REE) and high field-strength element concentrations of the amphibolites show limited variations, within a factor of two except for a few evolved samples, throughout transect of the sole. On the other hand, concentrations of fluid-mobile elements, especially B, Rb, K and Ba, in amphibolites are highly elevated in upper 30 m of the sole (> 600 °C in peak metamorphic temperature), suggesting the equilibration with evolved, B-Rb-K-Ba-rich fluids during prograde metamorphism. The comparison with amphibolites in the lower 150 m (500 to 550 °C) demonstrates that the trace element spectra of the fluids equilibrated with the high-level amphibolites may vary as a function of metamorphic temperature. The fluids are characterized by striking enrichments of B, Rb, K and Ba and moderate to minor enrichments of Sr, Li, Be and Pb. At higher temperature (up to 700 °C), the fluids become considerably enriched in light REE and Nb in addition to the above elements. The estimated trace element spectra of the fluids do not coincide with the compositions of basalts from matured intra-oceanic arcs, but satisfactorily explain the characteristics of the low-Pb andesites and boninites found in the Oman ophiolite. Compositional similarity between the boninites of Oman and other localities suggests that the fluids estimated here well represent the amphibolite-derived fluids involved in the magmatism of immatured, hot, shallow subduction zones.     

Oligocene crustal xenolith‐bearing alkaline basalt from Jandaq area (Central Iran): implications for magma genesis and crustal nature

The Oligocene alkaline basalts of Toveireh area (southwest of Jandaq, Central Iran) exhibit northwest–southeast to west–east exposure in northwest of the central‐east Iranian microcontinent (CEIM). These basalts are composed of olivine (Fo_70–90), clinopyroxene (diopside, augite), plagioclase (labradorite), spinel, and titanomagnetite as primary minerals and serpentine and zeolite as secondary ones. They are enriched in alkalis, TiO₂ and light rare earth elements (La/Yb = 9.64–12.68) and are characterized by enrichment in large ion lithophile elements (Cs, Rb, Ba) and high field strength elements (Nb, Ta). The geochemical features of the rocks suggest that the Toveireh alkaline basalts are derived from a moderate degree partial melting (10–20%) of a previously enriched garnet lherzolite of asthenospheric mantle. Subduction of the CEIM confining oceanic crust from the Triassic to Eocene is the reason of mantle enrichment. The studied basalts contain mafic‐ultramafic and aluminous granulitic xenoliths. The rock‐forming minerals of the mafic‐ultramafic xenoliths are Cr‐free/poor spinel, olivine, Al‐rich pyroxene, and feldspar. The aluminous granulitic xenoliths consist of an assemblage of hercynitic spinel + plagioclase (andesine–labradorite) ± corundum ± sillimanite. They show interstitial texture, which is consistent with granulite facies. They are enriched in high field strength elements (Ti, Nb and Ta), light rare earth elements (La/Yb = 37–193) and exhibit a positive Eu anomaly. These granulitic xenoliths may be Al‐saturated but Si‐undersaturated feldspar bearing restitic materials of the lower crust. The Oligocene Toveireh basaltic magma passed and entrained these xenoliths from the lower crust to the surface.     

High precision in-situ Pb isotopic analysis of sulfide minerals by femtosecond laser ablation multi-collector inductively coupled plasma mass spectrometry

An in-situ microanalysis of Pb isotopic compositions in sulfide minerals is carried out by using femtosecond laser-ablation multi-collector inductively coupled plasma mass spectrometry(fs LA-MC-ICP-MS).High-temperature-activated carbon was used to filter Hg contained in the carrier gas,which reduced the Hg background signal by 48%and also lowered the detection limit of the analysis.Fractionation and mass discrimination effects existing in the ICP-MS analytical processes were corrected using an internal reference Tl in conjunction with an external reference NIST SRM 610.The proposed method was used to analyze the Pb isotopic compositions of chalcopyrite,pyrite,and sphalerite from the Dulong Sn-Zn-In polymetallic ore district.The results showed that in this ore district,the sulfide minerals and different grains of the same sulfide mineral show a large variation in Pb content up to 1000-fold.The studied pyrites show relatively higher Pb contents and homogeneous Pb isotopic compositions,whereas the sphalerites have low Pb contents but most variable Pb isotopic compositions.It is suggested that the large variation of Pb isotopic composition may reflect a late hydrothermal superimposition on the primary sulfide formation.In addition,radiogenic Pb accumulated by radioactive decay of trace amounts of U over time in the host minerals may also be one of the causes for the large variation range of Pb content and Pb isotopic composition of those low-Pb sphalerites.Chalcopyrite and sphalerite grains with Pb content greater than 10 ppm presented a consistent Pb isotopic distribution,whereas all the sulfide grains with Pb content greater than 100 ppm had consistent Pb isotopic composition within 2s measurement uncertainties.The in-situ analysis of Pb isotopic composition agreed well with the results obtained by conventional chemical methods within2s measurement uncertainties,indicating that the data obtained by fs LA-MC-ICP-MS are reliable.Additionally,this study indicates that the Pb isotopic composition could truthfully record the source of ore-forming minerals only for sulfide minerals with high Pb content.On the contrary,the Pb isotopic composition of low-Pb sulfide minerals may be affected by trace amounts of U in the host minerals that may lead to a highly radiogenic Pb isotope ratio.Alternatively,it is also possible that late fluid metasomatic overprinting may alter the Pb isotopic compositions.     

Trace elements in ocean ridge basalts

A correlary of sea floor spreading is that the production rate of ocean ridge basalts exceeds that of all other volcanic rocks on the earth combined. Basalts of the ocean ridges bring with them a continuous record in space and time of the chemical characteristics of the underlying mantle. The chemical record is once removed, due to chemical fractionation during partial melting. Chemical fractionations can be evaluated by assuming that peridotite melting has proceeded to an olivine-orthopyroxene stage, in which case the ratios of a number of magmaphile elements in the extracted melt closely match the ratios in the mantle. Comparison of ocean ridge basalts and chondritic meteorites reveals systematic patterns of element fractionation, and what is probably a double depletion in some elements. The first depletion is in volatile elements and is due to high accretion temperatures of a large percentage of the earth from the solar nebula. The second depletion is in the largest, most highly charged lithophile elements (“incompatible elements”), probably because the mantle source of the basalts was melted previously, and the melt, enriched in these elements, was removed. Migration of melt relative to solid under ocean ridges and oceanic plates, element fractionation at subduction zones, and fractional melting of amphibolite in the Precambrian are possible mechanisms for depleting the mantle in incompatible elements. Ratios of transition metals in the mantle source of ocean ridge basalts are close to chondritic, and contrast to the extreme depletion of refractory siderophile elements, the reason for which remains uncertain. Variation of ocean ridge basalt chemistry along the length of the ridge has been correlated with ridge elevation. Thus chemically anomalous ridge segments up to 1000 km long appear to broadly coincide with regions of high magma production (plumes, hot spots). Basalt heterogeneity at a single location indicates mantle heterogeneity on a smaller scale. Variation of ocean ridge basalt chemistry with time has not been established, in fact, criteria for recognizing old oceanic crust in ophiolite terrains are currently under debate. The similarity of rare earth element patterns in basalt from ocean ridges, back-arc basins, some young island arcs, and some continental flood basalts illustrates the dangers of tectonic labeling by rare earth element pattern.     

The Matsue Formation: Evidence for gross mantle heterogeneity beneath Southwest Japan at 11 Ma

Abstract Middle Miocene basalts and basaltic andesites of the Matsue Formation outcrop within a 5 km radius of Matsue city in eastern Shimane Prefecture. Despite their limited outcrop and age (11.0 ± 1.5 Ma), they show a wide range in ⁸⁷Sr-⁸⁶Sr(0.70370–0.70593), ¹⁴³Nd-¹⁴⁴Nd(0.512904–0.512471) and large ion lithophile element (LILE) contents, but a relatively narrow range for some high field strength elements (HFSE) such as Nb and Ti. These basalts and andesites can be divided into three groups based on petrography, major element, trace element and isotope chemistry. Although one group has undergone some fractional crystallization, isotope chemistry precludes linkage of the groups by a closed-system process. Crustal contamination can explain isotope chemistry, but is not consistent with trace element variations. The most satisfactory model is eruption of two compositionally distinct magmas, with limited magma mixing and fractional crystallization. Published experimental work shows that one end-member resulted from shallow melting of upwelling mantle at ∼25 km. The simultaneous eruption of the other end member magma in the same area points towards a heterogeneous mantle. The isotopic composition of Matsue Formation basalts and andesites covers the entire range of Late Miocene mafic volcanic rocks of southwest Japan. Such gross heterogeneity developed on a local scale has implications for models that deal with regional chemical variations of mafic volcanic rocks in southwest Honshu.     

Intergranular trace elements in mantle xenoliths from Russian Far East: Example for mantle metasomatism by hydrous melt

Based on both major and trace element chemistry, the occurrence of the intergranular component in mantle-derived xenoliths from far eastern Russia has been constrained. Whole-rock trace element measurements of one xenolith show apparent negative anomalies in Ce, Th, and high field strength elements on normalized trace element patterns. The trace element pattern of the whole rock differs from those of constituent minerals, indicating that the anomalies in the whole rock are attributable to the presence of an intergranular component. That assumption was confirmed using in situ analysis of trace elements in the intergranular substance and melt inclusion using laser ablation inductively coupled plasma–mass spectrometry. Both the intergranular component and the melt inclusions have identical trace element patterns, which mean that these materials are a cognate metasomatizing agent. The anomalies are regarded as mantle metasomatism related to an aqueous fluid. Hydrous minerals were observed on the wall of the melt inclusions using micro-Raman spectroscopy, indicating that the melt inclusions contained a large amount of water. Thus, this study reveals a trace element composition of a hydrous metasomatizing agent in the mantle.     

Petrogenesis and geodynamic significance of the Ganhe Formation lavas,eastern Great Xing'an Range,China: Evidence from geochemistry and geochronology

Mesozoic volcanic rocks are widespread throughout the Great Xing'an Range of northeastern China. However, there has been limited investigation into the age and petrogenesis of the Mesozoic volcanics in the eastern Great Xing'an Range. According to our research, the volcanic rocks of the Dayangshu Basin, eastern Great Xing'an Range are composed mainly of trachybasalt, basaltic andesite, and basaltic trachyandesite, with minor intermediate–basic pyroclastic rocks. In this study, the geochemistry and geochronology of the Mesozoic volcanic rocks are presented in order to discuss the petrogenesis and tectonic setting of the Ganhe Formation in the Dayangshu Basin. Zircon U–Pb dating by laser ablation inductively coupled plasma–mass spectrometry indicates that the Mesozoic lavas formed during the late Early Cretaceous (114.3–108.8 Ma). This suite of rocks exhibits a range of geochemical signatures indicating subduction‐related genesis, including: (i) calc‐alkaline to high‐K calc‐alkaline major element compositions; (ii) enrichment of large ion lithophile elements (e.g. Rb, Ba, K) and light rare earth elements (LREEs/HREEs =7.33–9.85); and (iii) weak depletion in high field strength elements (e.g. Nb, Ta, Ti). Furthermore, Sr–Nd–Pb isotopic data yield initial ⁸⁷Sr/⁸⁶Sr values of 0.70450–0.70463, positive ε_Nd(t) values of +1.8 to +3.3, and a mantle‐derived lead isotope composition. Combined with the regional tectonic evolution, the results of this study suggest that the Ganhe Group lavas are derived from decompression melting of a metasomatized (enriched) lithospheric mantle, related to asthenospheric upwelling, which resulted from lithospheric mantle delamination and produced extension of the continental margin following the subduction of the Paleo‐Pacific Plate.     

Possible sub-arc origin of podiform chromitites

Abstract The sub-arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr#) of their chromian spinel frequently is higher than 0.7, which is comparable with the range for arc-related primitive magmas. This almost excludes the possibility of their sub-oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr# < 0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of high-field strength elements (HFSE) to large-ion lithophile elements (LILE), one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc-type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has chromitites.     

Transition from platemargin to intraplate environment: Geochemistry of basalts in Paleogene Liaohe basin,northeastern China

Mesozoic and Cenozoic volcanic rocks are widely distributed in the circum-Pacific area of eastern China. These rocks have long been genetically linked to westward subduction of the paleo-Pacific oceanic plate to the eastern Asia continent[1,2]. Research in re-cent years[3―6] has attained conclusions that a simple paleo-Pacific subduction model does not work well in interpreting all the volcanic rocks in eastern China, although some of them could be attributed to circum-Pacific interaction …     

The Permian seamount stratigraphic sequence in Chiang Mai,North Thailand and its tectogeographic significance

The widespread Permian carbonate strata outcropped in northwestern Thailand are considered as the evidence for the Late Paleozoic shallow Tethys. Our investigation, however, shows that basalt can be discovered usually under the Permian carbonate sequence in Chiang Mai-Fang area, northwestern Thailand. The basalt belongs to subalkalic basalt and potassic trachybasalt. They are characterized by high P and Ti in major elements, by high content, enriching LREE, lacking δEu anomaly in rare earth elements, and by enrichment of the large ion lithophile element (LILE) (K, Rb, Ba) and high field strength element (HFSE) (Nb, Ta, Zr) in trace elements, which can be compared with the characters of the oceanic island basalt in Three Rivers (Jinsha River, Lancang River, and Nujiang River) area, southwestern China. Therefore, the Permian carbonate in the studied area was deposited on a sea-mount, rather than on a stable carbonate platform. The oceanic basin is correlated to the Late Paleozoic ocean represented by the Changning-Menglian Belt in southwestern China and they are a major basin of the Paleo-Tethyan Archipelagoes Ocean. The result indicates nonexistence of a Shan-Thai Block in the Late Paleozoic.     

Geochemistry and spectrometric prospection of younger granites and granitic pegmatites bearing uranium mineralization at G.Kab El Rakeb area,Central Eastern Desert,Egypt

The present study deals with the detailed field geology,mineralogy,geochemistry,and spectrometric prospection of rare metals and radioactive minerals associated with granitic pegmatites in the Gabal Um LassafAbu Dob-Um Atelia-Kadabora-Kab El Rakeb area(LDAKK),which are situated in the central Eastern Desert of Egypt.The study area is composed of granodiorites and monzogranites.The latter is dissected by granitic pegmatites with a discriminatory distribution of economically important mineralization(rare-metals and U-Th minerals).The granitic pegmatites display a zoning structure composed of feldspar-rich zones enveloping pockets of mica(mostly muscovite) and all zoning internal quartz core.Microscopically,the monzogranite and granitic pegmatites are composed of potash-feldspar,albite,and quartz with a small amount of biotite and muscovite.The mineralogical investigation of the highly radioactive zones in the granitic pegmatite shows enrichment in kasolite,uranothorite,thorite,xenotime,columbite,zircon,bismuth,and galena.Geochemically,the granitic pegmatites are enriched with high field strength elements(HFSE;Zr,Nb,Th,and U) and ion lithophilic elements(LILE;Rb,and Pb) and depleted in Ba,Sr,and Eu.They are strongly peralkaline to metalumainous except for two samples,which are peraluminous.They are formed by partial melting of metagreywackes at depth.The studied granitic pegmatites could belong to the NYF family(Niobium-Yttrium-Fluorine family) according to the geochemistry of the trace elements.The studied granitic pegmatites show deep negative Eu anomaly and display a strong M-type tetrad effect of rare earth elements.The field spectroscopy measurements showed the localization of the radiometric anomalous associated with the granitic pegmatites,which are intruded in the granitic host rocks.Their eU and eTh contents reached up to 316 and 1367 ppm,respectively.Uranium and thorium occurrences in the investigated granitic pegmatites are related to magmatic(syngenetic)origin with hydrothermal(epigenetic) input.The magmatic mineralization is evidenced by the occurrence of thorite and zircon,whilst the hydrothermal activity is represented by alteration of feldspars and formation of pyrite and iron oxides associated with the radioactive minerals.The high concentrations of uranium and rare metals mineralization in the investigated granitic pegmatites make them a target to exploration and to enlarge the potentiality of the highly mineralized zones.     

Alkaline lamprophyric province of Central Iran

Paleozoic lamprophyres exhibit good exposures in the western part of the Central–East Iranian microcontinent. These rocks crop out as volcanoes, dykes, and plugs. The constituent minerals are amphibole, clinopyroxene, plagioclase, K‐feldspar, olivine, Cr‐spinel, titanite, biotite, and ilmenite. The main textures in volcanic lamprophyres are porphyritic, trachytic, microlithic, and variolitic, whereas in dykes and plugs, intergranular texture is common. These lamprophyres are regionally metamorphosed in some areas. Petrographical and geochemical characteristics of the studied rocks suggest that they are classified as alkaline lamprophyres and camptonites. They are enriched in alkalis (Na₂O + K₂O), large ion lithophile elements, and light rare earth elements, and the features of trace element concentrations are similar to those of within‐plate basalts. This study suggests that the lamprophyres were derived from different degrees of partial melting of metasomatized amphibole‐bearing spinel lherzolite. Subduction of Paleo‐Tethys oceanic crust from the Early to late Paleozoic resulted in enrichment in fluids in the mantle, and lamprophyric magmatism occurred along the minor and major faults. This limited but typical lamprophyric magmatism in a broad area of Central Iran suggests that, in spite of the long length of the Paleozoic (～250 my), it was a relatively calm era from the viewpoint of magmatism in Central Iran.