Home About us Contact
|
Home About us Contact | ||
|
|
||
Light Rare Earth Elements (light + rare_earth_element)
Selected AbstractsPetrogenesis and tectonic setting of bimodal volcanism in the Sakoli Mobile Belt, Central Indian shieldISLAND ARC, Issue 1 2009Talat Ahmad Abstract The Sakoli Mobile Belt comprises bimodal volcanic rocks that include metabasalt, rhyolite, tuffs, and epiclastic rocks with metapelites, quartzite, arkose, conglomerate, and banded iron formation (BIF). Mafic volcanic rocks are tholeiitic to quartz-tholeiitic with normative quartz and hypersthene. SiO2 shows a large compositional gap between the basic and acidic volcanics, depicting their bimodal nature. Both the volcanics have distinct geochemical trends but display some similarity in terms of enriched light rare earth element,large ion lithophile element characteristics with positive anomalies for U, Pb, and Th and distinct negative anomalies for Nb, P, and Ti. These characteristics are typical of continental rift volcanism. Both the volcanic rocks show strong negative Sr and Eu anomalies indicating fractionation of plagioclases and K-feldspars, respectively. The high Fe/Mg ratios for the basic rocks indicate their evolved nature. Whole rock Sm,Nd isochrons for the acidic volcanic rocks indicate an age of crystallization for these volcanic rocks at about 1675 ± 180 Ma (initial 143Nd/144Nd = 0.51017 ± 0.00017, mean square weighted deviate [MSWD] = 1.6). The ,Ndt (t = 2000 Ma) varies between ,0.19 and +2.22 for the basic volcanic rock and between ,2.85 and ,4.29 for the acidic volcanic rocks. Depleted mantle model ages vary from 2000 to 2275 Ma for the basic and from 2426 to 2777 Ma for the acidic volcanic rocks, respectively. These model ages indicate that protoliths for the acidic volcanic rocks probably had a much longer crustal residence time. Predominantly basaltic magma erupted during the deposition of the Dhabetekri Formation and part of it pooled at crustal or shallower subcrustal levels that probably triggered partial melting to generate the acidic magma. The influence of basic magma on the genesis of acidic magma is indicated by the higher Ni and Cr abundance at the observed silica levels of the acidic magma. A subsequent pulse of basic magma, which became crustally contaminated, erupted as minor component along with the dominantly acidic volcanics during the deposition of the Bhiwapur Formation. [source] Geochemistry of peridotite xenoliths in alkali basalts from Jeju Island, KoreaISLAND ARC, Issue 4 2002SEONG HEE CHOI Abstract Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB,oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history. [source] High oxidation state during formation of Martian nakhlitesMETEORITICS & PLANETARY SCIENCE, Issue 1 2010Anja SZYMANSKI Oxygen fugacities obtained cluster closely around the FMQ (Fayalite,Magnetite,Quartz) buffer (NWA998 = FMQ , 0.8; Y-000593 = FMQ , 0.7; Nakhla = FMQ; Lafayette = FMQ + 0.1). The corresponding equilibration temperatures are 810 °C for Nakhla and Y-000593, 780 °C for Lafayette and 710 °C for NWA998. All nakhlites record oxygen fugacities significantly higher and with a tighter range than those determined for Martian basalts, i.e., shergottites whose oxygen fugacities vary from FMQ , 1 to FMQ , 4. It has been known for some time that nakhlites are different from other Martian meteorites in chemistry, mineralogy, and crystallization age. The present study adds oxygen fugacity to this list of differences. The comparatively large variation in fO2 recorded by shergottites was interpreted by Herd et al. (2002) as reflecting variable degrees of contamination with crustal fluids that would also carry a light rare earth element (REE)-enriched component. The high oxygen fugacities and the large light REE enrichment of nakhlites fit qualitatively in this model. In detail, however, it is found that the inferred contaminating phase in nakhlites must have been different from those in shergottites. This is supported by unique 182W/184W and 142Nd/144Nd ratios in nakhlites, which are distinct from other Martian meteorites. It is likely that the differences in fO2 between nakhlites and other Martian meteorites were established very early in the history of Mars. Parental trace element rich and trace element poor regions (reservoirs) of Mars mantle (Brandon et al. 2000) must have been kept isolated throughout Martian history. Our results further show significant differences in closure temperature among the different nakhlites. The observed range in equilibration temperatures together with similar fO2 values is attributable to crystallization of nakhlites in the same cumulate pile or lava layer at different burial depths from 0.5 to 30 m below the Martian surface in agreement with Mikouchi et al. (2003) and is further confirmed by similar crystallization ages of about 1.3 Ga ago (e.g., Misawa et al. 2003). [source] Authenticating the recovery location of meteorites: The case of CastenasoMETEORITICS & PLANETARY SCIENCE, Issue 3 2007Luigi Folco Using the hypothesis that Castenaso was instead a hot-desert meteorite, we conducted a comparative mineralogical and geochemical study of major weathering effects on European and Saharan ordinary chondrites as potential markers of the environment where Castenaso resided during its terrestrial lifetime. Inductively coupled plasma-mass spectrometry (ICP-MS) data reveals that Castenaso is significantly enriched in Sr, Ba, Tl, and U, and suggests geochemical alteration in a hot-desert environment. The alteration is minor: Castenaso is not coated by desert varnish and does not show significant light rare earth element (LREE) enrichment or loss of Ni and Co. The apparent contrast in size, morphology, and composition between the soil particles filling the external fractures of Castenaso and those from the bank of the Idice Stream observed under the scanning electron microscope (SEM) suggests that Castenaso did not reside at the reported find location. Abraded quartz grains (up to 1 mm in size) in Castenaso are undoubtedly from a hot-desert eolian environment: they are well-rounded and show external surfaces characterized by the presence of dish-shaped concavities and upturned silica plates that have been subject to solution-precipitation and subsequent smoothing. We therefore conclude that Castenaso is one of the many hot-desert ordinary chondrite finds, probably from the Sahara, that is currently available on the market. This forensic work provides the scientific grounds for changing the name of this meteorite. [source] A petrologic and trace element study of Dar al Gani 476 and Dar al Gani 489: Twin meteorites with affinities to basaltic and lherzolitic shergottitesMETEORITICS & PLANETARY SCIENCE, Issue 2 2001M. WADHWA Having resided in a hot desert environment for an extended time, DaG 476 and DaG 489 were subjected to terrestrial weathering that significantly altered their chemical composition. In particular, analyses of some of the silicates show light rare earth element (LREE)-enrichment resulting from terrestrial alteration. In situ measurement of trace element abundances in minerals allows us to identify areas unaffected by this contamination and, thereby, to infer the petrogenesis of these meteorites. No significant compositional differences between DaG 476 and DaG 489 were found, supporting the hypothesis that they belong to the same fall. These meteorites have characteristics in common with both basaltic and lherzolitic shergottites, possibly suggesting spatial and petrogenetic associations of these two types of lithologies on Mars. However, the compositions of Fe-Ti oxides and the size of Eu anomalies in the earliest-formed pyroxenes indicate that the two Saharan meteorites probably experienced more reducing crystallization conditions than other shergottites (with the exception of Queen Alexandra Range (QUE) 94201). As is the case for other shergottites, trace element microdistributions in minerals of the DaG martian meteorites indicate that closed-system crystal fractionation from a LREE-depleted parent magma dominated their crystallization history. Furthermore, rare earth element abundances in the orthopyroxene megacrysts are consistent with their origin as xenocrysts rather than phenocrysts. [source] Melt,wall rock interaction in the mantle shown by silicate melt inclusions in peridotite xenoliths from the central Pannonian Basin (western Hungary)ISLAND ARC, Issue 2 2009Csaba Szabó Abstract In this paper we present a detailed textural and geochemical study of two equigranular textured amphibole-bearing spinel lherzolite xenoliths from Szigliget, Bakony,Balaton Highland Volcanic Field (BBHVF, western Hungary) containing abundant primary silicate melt inclusions (SMIs) in clinopyroxene rims and secondary SMIs in orthopyroxene (and rarely spinel) along healed fractures. The SMIs are dominantly composed of silicate glass and CO2 -rich bubbles. Clinopyroxene and orthopyroxene are zoned in both studied xenoliths, especially with respect to Fe, Mg, Na, and Al contents. Cores of clinopyroxenes in both xenoliths show trace element distribution close to primitive mantle. Rims of clinopyroxenes are enriched in Th, U, light rare earth elements (LREEs) and medium REEs (MREEs). Amphiboles in the Szg08 xenolith exhibit elevated Rb, Ba, Nb, Ta, LREE, and MREE contents. The composition of silicate glass in the SMIs covers a wide range from the basaltic trachyandesite and andesite to phonolitic compositions. The glasses are particularly rich in P2O5. Both primary and secondary SMIs are strongly enriched in incompatible trace elements (mostly U, Th, La, Zr) and display a slight negative Hf anomaly. The development of zoned pyroxenes, as well as the entrapment of primary SMIs in the clinopyroxene rims, happened after partial melting and subsequent crystallization of clinopyroxenes, most probably due to an interaction between hot volatile-bearing evolved melt and mantle wall-rocks. This silicate melt filled microfractures in orthopyroxenes (and rarely spinels) resulting in secondary SMIs. [source] The formation of foliated (garnet-bearing) granites in the Tongbai-Dabie orogenic belt: partial melting of subducted continental crust during exhumationJOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2009L. ZHANG Abstract Foliated (garnet-bearing) (FGB) granites are associated closely with and are usually the major wall rocks of the high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks in the Tongbai-Dabie region, the mid segment of the Qinling-Dabie-Sulu orogenic belt in central China. These granites appear either as small plutons or as veins, which commonly intrude into or surround the HP and UHP metamorphic eclogites or gneisses. The veins of FGB granites usually penetrate into the retrograded eclogites or gneisses along the foliations. Condensation rims can occasionally be found along the margins of granite veins. These granites are rich in Si and alkali with high Ga/Al ratios, and depleted in Ca, Mg, Al, Ti, Sc, V, Ni, Co, Cr and Sr, which are similar to A-type granites. In a chondrite normalized diagram, the samples are light rare earth elements enriched with different extent of negative Eu anomaly. Moreover, Rb, Nb, Ta, Sr, P and Ti show different degrees of negative anomalies, whereas Ba, K, La, Zr and Hf show positive anomalies in the primitive mantle normalized diagram. Negative anomalies of Eu and Sr indicate strong influence of plagioclase. In conventional discrimination diagrams, these FGB granites belong to the A-type granite, with geochemical characteristics affinitive to post-collisional granites. The ,Nd (230 Ma) values (,15.80 to ,2.52) and TDM values (1.02,2.07 Ga) suggest that magma for the FGB granites were derived from a heterogeneous crustal source. Therefore, the FGB granites may provide clues for deciphering the formation of post-collisional granites. It is proposed that the magma of the FGB granites both in the HP and UHP units was formed in an extensional tectonic setting slightly post-dating the HP and UHP metamorphism, most likely as a result of decompressional partial melting of UHP retrograded eclogites during exhumation. [source] Trace element studies of silicate-rich inclusions in the Guin (UNGR) and Kodaikanal (IIE) iron meteoritesMETEORITICS & PLANETARY SCIENCE, Issue 7-8 2007Gero Kurat It contains a rutile grain and is in contact with a large Cl apatite. The latter is very rich in rare earth elements (REEs) (,80 × CI), which display a flat abundance pattern, except for Eu and Yb, which are underabundant. The devitrified glass is very poor in REEs (<0.1 × CI), except for Eu and Yb, which have positive abundance anomalies. Devitrified glass and Cl apatite are out of chemical equilibrium and their complementary REE patterns indicate a genesis via condensation under reducing conditions. Inclusion 1 in the Kodaikanal (IIE) iron consists of glass only, whereas inclusion 2 consists of clinopyroxene, which is partly overgrown by low-Ca pyroxene, and apatite embedded in devitrified glass. All minerals are euhedral or have skeletal habits indicating crystallization from the liquid precursor of the glass. Pyroxenes and the apatite are rich in trace elements, indicating crystallization from a liquid that had 10,50 × CI abundances of REEs and refractory lithophile elements (RLEs). The co-existing glass is poor in REEs (,0.1,1 × CI) and, consequently, a liquid of such chemical composition cannot have crystallized the phenocrysts. Glasses have variable chemical compositions but are rich in SiO2, Al2O3, Na2O, and K2O as well as in HFSEs, Be, B, and Rb. The REE abundance patterns are mostly flat, except for the glass-only inclusion, which has heavy rare earth elements (HREEs) > light rare earth elements (LREEs) and deficits in Eu and Yb,an ultrarefractory pattern. The genetic models suggested so far cannot explain what is observed and, consequently, we offer a new model for silicate inclusion formation in IIE and related irons. Nebular processes and a relationship with E meteorites (Guin) or Ca-Al-rich inclusions (CAIs) (Kodaikanal) are indicated. A sequence of condensation (CaS, TiN or refractory pyroxene-rich liquids) and vapor-solid elemental exchange can be identified that took place beginning under reducing and ending at oxidizing conditions (phosphate, rutile formation, alkali and Fe2+ metasomatism, metasomatic loss of REEs from glass). [source] A petrogenetic model for the origin and compositional variation of the martian basaltic meteoritesMETEORITICS & PLANETARY SCIENCE, Issue 12 2003Lars E. BORG The models are based on low to high pressure phase relationships estimated from experimental runs and estimates of the composition of silicate Mars from the literature. These models attempt to constrain the mechanisms by which the martian meteorites obtained their superchondritic CaO/Al2O3 ratios and their source regions obtained their parent/daughter (87Rb/86Sr, 147Sm/144Nd, and 176Lu/177Hf) ratios calculated from the initial Sr, Nd, and Hf isotopic compositions of the meteorites. High pressure experiments suggest that majoritic garnet is the liquidus phase for Mars relevant compositions at or above 12 GPa. Early crystallization of this phase from a martian magma ocean yields a liquid characterized by an elevated CaO/Al2O3 ratio and a high Mg#. Olivine-pyroxene-garnet-dominated cumulates that crystallize subsequently will also be characterized by superchondritic CaO/Al2O3 ratios. Melting of these cumulates yields liquids with major element compositions that are similar to calculated parental melts of the martian meteorites. Furthermore, crystallization models demonstrate that some of these cumulates have parent/daughter ratios that are similar to those calculated for the most incompatible-element-depleted source region (i.e., that of the meteorite Queen Alexandra [QUE] 94201). The incompatible-element abundances of the most depleted (QUE 94201-like) source region have also been calculated and provide an estimate of the composition of depleted martian mantle. The incompatible-element pattern of depleted martian mantle calculated here is very similar to the pattern estimated for depleted Earth's mantle. Melting the depleted martian mantle composition reproduces the abundances of many incompatible elements in the parental melt of QUE 94201 (e.g., Ba, Th, K, P, Hf, Zr, and heavy rare earth elements) fairly well but does not reproduce the abundances of Rb, U, Ta and light rare earth elements. The source regions for meteorites such as Shergotty are successfully modeled as mixtures of depleted martian mantle and a late stage liquid trapped in the magma ocean cumulate pile. Melting of this hybrid source yields liquids with major element abundances and incompatible-element patterns that are very similar to the Shergotty bulk rock. [source] A Special Orogenic-type Rare Earth Element Deposit in Maoniuping, Sichuan, China: Geology and GeochemistryRESOURCE GEOLOGY, Issue 3 2001Denghong WANG Abstract: The Maoniuping REE deposit is the second largest light rare earth elements deposit in China, explored recently in the northern Jinpingshan Mountains, a Cenozoic intracontinental orogenic belt in southwestern China. It is a vein-type deposit hosted within, and genetically related to, carbonatite-alkalic complex. Field investigation and new geochemical data of the carbonatites from the carbonatite-alkalic complex support an igneous origin for the Maoniuping carbonatites and related REE mineralization. Carbonatite itself carries rare earth elements which were enriched by hydrothermal solution. It is known that most of the REE deposits related to carbonatite-alkalic complexes were formed in relatively stable tectonic setting such as cratonic or rifting environment. The Maoniuping deposit, however, was formed during the processes of Cenozoic orogeny. Although the Maoniuping deposit is located in the north sector of the Panxi paleo-rift zone, the rift had been closed before early Cenozoic and evolved into an intracontinental orogenic belt, i.e., the Jinpingshan Orogen, which was formed since later Mesozoic to early Cenozoic. Geochronological and geochemical data also prove that the Maoniuping REE deposit was formed in an intracontinental orogenic belt instead of rift system or stationary block. The Maoniuping REE deposit is similar to the Mountain Pass REE deposit in many respects such as the high contents of bastnaesite and barite, the low content of niobium, and the common occurrence of sulfides. The discovery of the Maoniuping deposit and other REE deposits during the past two decades suggest a good potential for prospecting REE deposits along the alkalic complex belt located on the eastern side of the Qinghai,Xizang,West Sichuan Plateau. [source] Mineralogical and Petrological Characteristics of the Neoproterozoic Orthoamphibolite and Orthogneisses in the Mutki Area, the Bitlis Massif, Southeast TurkeyACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010Kerim KOCAK Abstract: The rocks form as amphibolite ± garnet ± epidote and orthogneisses in the Pan-African basement of the Bitlis Massif. The petrochemical data of the studied metamorphic rocks suggest different igneous protoliths ranging from calcalkaline basalt to andesite in composition. Petrochemically, the rocks can be classified as group 1 (low Zr and La) and group 2 (high Zr and La), all showing various enrichments in large ion lithophiles and light rare earth elements, and a depletion in high-field strength elements, suggestive of a destructive plate margin setting. The protoliths of the all samples might have formed mostly by the partial melting of an enriched source, possibly coupled with the fractional crystallization of plagioclase, apatite, and titaniferous magnetite ± olivine ± clinopyroxene ± amphibole in relation with subduction-related magmatism neighboring the Andean-type active margins of Gondwana. The group 2 samples could, however, be generated by a relatively lower degree of the partial melting of an inhomogeneous source with a preponderance of a high-level, fractional crystallization process in comparison to group 1. The protoliths of the samples were metamorphosed up to amphibolite facies conditions, which destroys original igneous texture and mineral assemblages. Geothermobarometric calculations show that the metamorphic rocks are finally equilibrated between 540 and 610°C and ,5 kbars, following a clockwise P-T-t path. [source] Rare Earth Element and Trace Element Features of Gold-bearing Pyrite in the Jinshan Gold Deposit, Jiangxi ProvinceACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2010Guangzhou MAO Abstract: Jinshan gold deposit is located in northeastern Jiangxi, South China, which is related to the ductile shear zone. It has a gold reserve of more than 200 tons, with 80% of gold occurring in pyrite. The ,REE of gold-bearing pyrite is as higher as 171.664 ppm on average, with relatively higher light rare earth elements (LREE; 159.556 ppm) and lower HREE (12.108 ppm). The ,LREE/,HREE ratio is 12.612 and (La/Yb)N is 11.765. These indicate that pyrite is rich in LREE. The (La/Sm)N ratio is 3.758 and that of (Gd/Yb)N is 1.695. These are obvious LREE fractionations. The rare earth element (REE) distribution patterns show obvious Eu anomaly with average ,Eu values of 0.664, and ,Ce anomalies of 1.044. REE characteristics are similar to those of wall rocks (regional metamorphic rocks), but different from those of the Dexing granodiorite porphyry and Damaoshan biotite granite. These features indicate that the ore-forming materials in the Jinshan gold deposit derived from the wall rocks, and the ore-forming fluids derived from metamorphic water. The Co/Ni ratio (average value 0.38) of pyrite suggests that the Jinshan gold deposit formed under a medium,low temperature. It is inferred from the values of high-field strength elements, LREE, Hf/Sm, Nb/La, and Th/La of the pyrite that the ore-forming fluids of the Jinshan gold deposit derived from metamorphic water with Cl>F. [source] Geochronology and Geochemistry of Mafic Dikes from Hainan Island and Tectonic ImplicationsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 6 2009CAO Jianjin Abstract: In the present study, the major and trace element compositions, as well as Sr, Nd isotopic compositions and K-Ar age data in mafic dikes from Hainan Island, China, have been analyzed. Whole-rock K-Ar dating yielded a magmatic duration of 61,98 Ma for mafic dikes. Mafic dikes have a very high concentration of incompatible elements, for example, Ba, Rb, Sr, K, rare earth elements, and especially light rare earth elements (LREE), and negative anomalies of Nb, Ta, and Ti in the normalized trace element patterns. The initial 87Sr/86Sr ratios and ,Sr(t) of the mafic dikes are 0.70634,0.71193 and +27.7 to +112.2, respectively. In the 87Sr/86Sr versus ,Nd(t) diagram, the Hainan Island mafic dikes plot between fields for depleted mantle and enriched mantle type 2. All these characteristics show that the mantle (source region) of mafic dikes in this area experienced metasomatism by fluids relatively enriched in LREE and large ion lithophile elements. The genesis of Hainan Island mafic dikes is explained as a result of the mixing of asthenospheric mantle with lithospheric mantle that experienced metasomatism by the subduction of the Pacific Plate. This is different from the Hainan Island Cenozoic basalts mainly derived from depleted asthenospheric mantle, and possibly, minor metasomatised lithospheric mantle. This study suggests that the Mesozoic and Cenozoic lithospheric revolutions in Hainan Island can be divided into three stages: (1) the compression orogenesis stage before 98 Ma. The dominant factor during this stage is the subduction of the ancient Pacific Plate beneath this area. The lithospheric mantle changed into enriched mantle type 2 by metasomatism; (2) the thinning and extension stage during 61,98 Ma. The dominant factor during this stage is that the asthenospheric mantle invaded and corroded the lithospheric mantle; and (3) the large-scale thinning and extension stage after 61 Ma. The large-scale asthenospheric upwelling results in the strong erupting of Cenozoic basalts, large-scale thinning of the lithosphere, the southward translating and counterclockwise rotating of Hainan Island, and the opening of the South China Sea. [source] Geochemistry of Platinum Group and Rare Earth Elements of the Polymetallic Layer in the Lower Cambrian, Weng'an, Guizhou ProvinceACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2009Yong FU Abstract: The black shales of the Lower Cambrian Niutitang Formation in Weng'an, on the Yangtze platform of south China, contain voluminous polymetallic sulfide deposits. A comprehensive geochemical investigation of trace, rare earth, and platinum group elements (PGE) has been undertaken in order to discuss its ore genesis and correlation with the tectono-depositional setting. The ore-bearing layers enrich molybdenum (Mo), nickel (Ni), vanadium (V), lead (Pb), strontium (Sr), barium (Ba), uranium (U), arsenic (As), and rare earth elements (REE) in abundance. High uranium/thorium (U/Th) ratios (U/Th>1) indicated that mineralization was mainly influenced by the hydrothermal process. The ,U value was above 1.9, showing a reducing sedimentary condition. The REE patterns showed high enrichment in light rare earth elements (LREE) (heavy rare earth elements (HREE) (LREE/HREE=5,17), slightly negative europium (Eu) and cerium (Ce) anomalies (,Eu=0.81,0.93), and positive Ce anomalies (,Ce=0.76,1.12). PGE abundance was characterized by the PGE-type distribution patterns, enriching platinum (Pt), palladium (Pd), ruthenium (Ru) and osmium (Os). The Pt/Pd ratio was 0.8, which is close to the ratios of seawater and ultramafic rocks. All of these geochemical features suggest that the mineralization was triggered by hydrothermal activity in an extensional setting in the context of break-up of the Rodinian supercontinent. [source] Discovery of a Miocene Mafic Dyke from the Western Hills of Beijing and its Geological ImplicationsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2009Zhicheng ZHANG Abstract: The present study is the first report of a Miocene mafic dyke from the Dahuichang, in the Western Hills of Beijing. The dyke cuts the fossil-dated Changxindian Formation of Eocene sequences and yields K-Ar ages of 14,15 Ma. The dyke is fine-grained diabase and has 49.84%,50.81% SiO2 and 3.56,3.97% Na2O+K2O, high TiO2 (1.65%,1.93%) and MgO (7.36%,9.85%), and low K2O (<1.22%) contents, with Na2O>K2O and slightly varied magnesium numbers (Mg#=55.54,62.74). In trace elements geochemistry, the dyke is very similar to the Miocene basalts from Jining and Hanuoba. The enrichment of light rare earth elements ([La/Yb]N=5.03,6.12) and large ion lithophile elements (LILEs), no negative Eu anomalies, relatively high Cr (265,326 ppm) and Ni (155,262 ppm), and almost constant V concentrations (194,213 ppm) reveal that the composition close to the primary basaltic magma from an enriched-mantle source, with little crustal contamination and fractional crystallization. The basaltic magma was possibly derived from the upwelling asthenosphere mantle beneath eastern China during the Miocene lithospheric thinning. [source] Chronology and Geochemistry of Mesozoic Volcanic Rocks in the Linjiang Area, Jilin Province and their Tectonic ImplicationsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2009Yang YU Abstract: Zircon U-Pb ages and geochemical analytical results are presented for the volcanic rocks of the Naozhigou, Ergulazi, and Sidaogou Formations in the Linjiang area, southeastern Jilin Province to constrain the nature of magma source and their tectonic settings. The Naozhigou Formation is composed mainly of andesite and rhyolite and its weighted mean 206Pb/238U age for 13 zircon grains is 222±1 Ma. The Ergulazi Formation consists of basaltic andesite, basaltic trachyandesite, and andesite, and six grains give a weighted mean 206Pb/Z38U age of 131±4 Ma. The Sidaogou Formation consists mainly of trachyandesite and rhyolite, and six zircon grains yield a weighted mean 206Pb/238U age of 113±4 Ma. The volcanic rocks have SiO2= 60.24%,77.46%, MgO = 0.36%,1.29% (Mg#= 0.32,0.40) for the Naozhigou Formation, SiO2= 51.60%,59.32%, MgO = 3.70%,5.54% (Mg#= 0.50,0.60) for the Ergulazi Formation, and SiO2= 58.28%,76.32%, MgO = 0.07%,1.20% (Mg#= 0.14,0.46) for the Sidaogou Formation. The trace element analytical results indicate that these volcanic rocks are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), relative depletion in heavy rare earth elements (HREEs) and high field strength elements (HFSEs, Nb, Ta, and Ti), and negative Eu anomalies. Compared with the primitive mantle, the Mesozoic volcanic rocks in the Linjiang area have relatively high initial 87Sr/86Sr ratios (0.7053,0.7083) and low ,Nd(t) values (,8.38 to ,2.43), and display an EMU trend. The late Triassic magma for the Naozhigou Formation could be derived from partial melting of a newly accretional crust with the minor involvement of the North China Craton basement and formed under an extensional environment after the collision of the Yangtze Craton and the North China Craton. The Early Cretaceous volcanic rocks for the Ergulazi and Sidaogou Formations could be formed under the tectonic setting of an active continental margin related to the westward subduction of the Izanagi plate. [source] | ||||||||||