Element Geochemical Characteristics Research Articles

贵州省黔西县二叠系龙潭组主采煤层微量元素分布特征及制约因素探讨

黔西县是贵州主要的产煤县之一，煤炭资源丰富，煤层主要产于二叠系上统龙潭组(P31)地层中，属于织纳煤田。通过对主要矿区主采煤层系统取样分析，研究了不同煤层、不同矿区微量元素空间分布特征；与相邻的大方、赫章等区域煤层中微量元素含量对比发现，在陆源供给多的三角洲平原环境形成的煤层中微量元素含量相对高，在受海水影响较大的潮坪环境煤层中微量元素相对含量低，揭示了研究区陆源物质供给母岩——玄武岩具有较高的微量元素地球化学背景是其主要制约因素。个别煤层中有害元素Pb、Tl等含量高，是后期热液活动和断裂构造影响的结果。 Guizhou has abundant coal resources, but little research has been done on the element geochemi-stry of coal measure strata in Guizhou Province, and the effective utilization of coal resources in Guizhou is a serious defect. Based on element geochemical studies of the main Permian exploitable coal measure strata in western Guizhou, the element geochemical distribution characteristics of the different coal seam and different mining area have been revealed, contrasting with the trace element content in Dafang, Hezhang, etc. The element contents are mainly affected by terrestrial material supply. Coal measures formed in the delta plain environment where sufficient terrestrial materials are supplied contain relatively abundant trace elements, whereas those formed in the tidal-flat environment influenced greatly by seawater have relatively low contents of trace elements, mainly controlled by the geological fact that basalts—the parent rocks of source regions contain high trace elements. The content of Tl and Pb in coal seam is higher, and it is the result of the late hydrothermal activity and the effect of the fracture structure.

Element Geochemical Characteristics of Lower Jurassic Ritang Formation from the Zhaxikang Sb‐Pb‐Zn‐Ag Polymetallic Ore‐Concentrated Area in South Tibet

Element Geochemical Characteristics of Lower Jurassic Ritang Formation from the Zhaxikang Sb‐Pb‐Zn‐Ag Polymetallic Ore‐Concentrated Area in South Tibet

Sr enrichment in mantle pyroxenes as a result of plagioclase alteration in lherzolite

Melt impregnated plagioclase lherzolites from the Nain mélange, central Iran, contain pyroxenes enriched and chemically zoned with Sr. Pyroxenes from the lherzolite and the clinopyroxenite seams, which have been precipitated from the impregnating melt, show similar trace element geochemical characteristics, including a similar Sr anomaly. The associated plagioclase, precipitated from the impregnating melt, has been selectively altered to isotropic saussurite. Strontium concentration increases in the pyroxenes from the core to the rim and toward crosscutting saussurite trails in orthopyroxene porphyroclast cores. The highest Sr content (up to 10.8ppm in clinopyroxene and 3.8ppm in orthopyroxene) is found in the finer pyroxenes surrounded by thicker saussurite layers.The Sr enrichment within pyroxenes is neither caused by metasomatism nor modified by fluids involved in hydrothermal alteration because pyroxenes are extremely depleted in fluid-mobile and light rare earth elements. Also, Sr enrichment cannot be related to the melt impregnation, since the Sr supply from the impregnating melt was consumed by plagioclase crystallization. The Sr enrichment in the pyroxene postdates the melt impregnation and is due to the relatively high-temperature (375°C–850°C) of saussuritization, that is, the breakdown of plagioclase. Plagioclase decomposition has released appreciable amounts of Sr to enrich adjacent pyroxenes. Saussurite shows significantly lower Sr contents than the plagioclase. Sr enrichment in peridotite pyroxenes, which is ascribed primarily to the metasomatism of slab-derived fluids, should be treated carefully, particularly when altered plagioclase is present.

سن سنجی U-Pb زیرکن، ژئوشیمی ایزوتوپ های Rb-Sr و Sm-Nd و پتروژنز توده های گرانیتوئیدی منطقه اکتشافی کجه، شمال باختر فردوس: شاهدی بر ماگماتیسم کرتاسه پسین در بلوک لوت

Granitoid rocks of Kaje area, northwest Ferdows, with the composition of diorite, monzodiorite, monzonite, monzogranite, syenogranite and granite, with calk-alkaline and high potassium affinities, have trace and rare earth element geochemical characteristics similar to those from subduction zones, belonging to I-type granitoid rocks. Most of these rocks are oxidized (magnetite series), while one suite is reduced (ilmenite series) showing S-type characteristics. Three samples of granitoid rocks were dated using zircon U-Pb method. Granitoid rocks belong to upper Cretaceous epoch. A granite porphyry with the age of 84.2±1.3 Ma (Santonian stage) is the oldest body followed by a biotite hornblende monzogranite of 70.8±1.4 Ma (Campanian stage) and the youngest body is a hornblende quartz diorite with the age of 67.9±1 Ma (Maastrichtian stage). Initial 87Sr/86Sr and 143Nd/144Nd ratios as well as єNdi for the granite porphyry are 0.708080, 0.512129 and -7.81, respectively. Those ratios for the biotite hornblende monzogranite are 0.706125, 0.512416 and -2.55, respectively; and for the hornblende quartz diorite are 0.707491, 0.512221 and -6.43, respectively. Based on geochemical and isotopic data, three types of granitoid can be distinguished having three different sources and characteristics. The first and oldest one is the granite porphyry of continental crust source, the second one is the monzonite and monzodiorite and the third group is the diorite of mantle source which originated and emplaced in different depths with various rates of contamination. These rocks have higher (87Sr/86Sr)i values compared with the other upper Cretaceous rocks in Bazman and Gazu areas, and lower (87Sr/86Sr)i values in comparison with the Bejestan rocks. Petrochemistry of the granite group is similar to the Bejestan rocks, but these are 10 m.y. younger and the other two groups have similar geochemistry and source to Bazman and Gazu with more contamination by the crust.

Sources and trace element geochemical characteristics of the coarse sediment in the Ningxia-Inner Mongolia reaches of the Yellow River

Siltation of coarse sediment in the river channel is the main cause of flooding and dike burst on China’s Yellow River, and the loess are thought as the coarse sediment sources of the lower reach of the Yellow River. The Ningxia-Inner Mongolia reaches of the Yellow River flow through an extensive area of aeolian desert, with flood events occurring frequently in this region. In this paper, we investigate the trace element contents of the coarse sediment in the Ningxia-Inner Mongolia reaches of the Yellow River. The main chemical components of the coarse sediments consist of 28 trace elements. Correlation analysis and factor analysis of 21 of these trace elements indicates that Ti and Zr are stable indicators of the coarse sediment sources. Comparing the spatial distribution of some of the main trace elements to major sources and riverbed sediment of the main reaches of the Yellow River suggests that the coarse sediments deposited in the Ningxia-Inner Mongolia reaches are mostly controlled by the local sediment sources. The results of R-factor analysis further proved that desert sand, riverbank material, and the Ten Tributaries are the primary sources of the coarse sediment in the Ningxia-Inner Mongolia reaches of the Yellow River.

Platinum‐Group Element Geochemical Characteristics of the Picrites and High‐Ti Basalts in the Binchuan Area, Yunnan Province

AbstractThe Binchuan area of Yunnan is located in the western part of the Emeishan large igneous province in the western margin of the Yangtze Block. In the present study, the Wuguiqing profile in thickness of about 1440 m is mainly composed of high‐Ti basalts, with minor picrites in the lower part and andesites, trachytes, and rhyolites in the upper part The picrites have relatively higher platinum‐group element (PGE) contents (ΣPGE=16.3–28.2 ppb), with high Cu/Zr and Pd/Zr ratios, and low S contents (5.03–16.9 ppm), indicating the parental magma is S‐unsaturated and generated by high degree of partial melting of the Emeishan large igneous province (ELIP) mantle source. The slightly high Cu/Pd ratios (11 000–24 000) relative to that of the primitive mantle suggest that 0.007% sulfides have been retained in the mantle source. The PGE contents of the high‐Ti basalts exhibit a wider range (ΣPGE=0.517–30.8 ppb). The samples in the middle and upper parts are depleted in PGE and have εNd (260 Ma) ratios ranging from −2.8 to −2.2, suggesting that crustal contamination of the parental magma during ascent triggered sulfur saturation and segregation of about 0.446%–0.554% sulfides, and the sulfide segregation process may also provide the ore‐forming material for the magmatic Cu‐Ni‐PGE sulfide deposits close to the studied basalts. The samples in this area show Pt‐Pd type primitive mantle‐normalized PGE patterns, and the Pd/Ir ratios are higher than that of the primitive mantle (Pd/Ir=1), indicating that the obvious differentiation between Ir‐group platinum‐group elements (IPGE) and Pd‐group platinum‐group elements (PPGE) are mainly controlled by olivine or chromites fractionation during magma evolution. The Pd/Pt ratios of most samples are higher than the average ratio of mantle (Pd/Pt=0.55), showing that the differentiation happened between Pt and Pd. The differentiation in picrites may be relevant to Pt hosted in discrete refractory Pt‐alloy phase in the mantle; whereas the differentiation in the high‐Ti basalts is probably associated with the fractionation of Fe‐Pt alloys, coprecipitating with Ir‐Ru‐Os alloys. Some high‐Ti basalt samples exhibit negative Ru anomalies, possibly due to removal of laurite collected by the early crystallized chromites.

Geochemistry of the trace elements and rare-earth elements at the boundary between Cambrian Series 2 and Series 3 at Jianshan, South China: Paleoenvironmental and stratigraphic implications

The Jianshan section in Jianhe County, Guizhou Province, is a very important auxiliary section to the candidate stratotype at the boundary between Cambrian Series 2 and Series 3. Twenty four samples collected from the Jianshan section of the Kaili Formation were analyzed for the concentrations of trace element and rare-earth elements (REEs) by inductively coupled plasma mass spectrometry. The trace element and REE geochemical characteristics of the Kaili Formation at the Jianshan section show that trace element composition and REE distribution patterns across the section are covariant. Notably, at 44 m above the base of the Kaili Formation, most trace element contents and REEs concentrations are lower than those values observed below 44 m. Above 44 m, an increasing trend in the concentration of both trace elements and REEs is observed. These geochemical data indicate important, but subtle, changes at the 44 m horizon. If this is, in fact, verified by work in progress, geochemical criteria can provide additional supports for defining the top surface of Cambrian Series 2, especially in sections where Oryctocephalus indicus is absent. Trace element ratios, specifically Ni/Co, V/Cr, V/(Ni+V) and Th/U as well as Ce and Eu anomalies indicate that the sedimentary environment of the Kaili Formation at the Jianshan section was oxidizing, that benthic redox conditions were less oxic, and may have been influenced by freshwater as compared to the Balang area. Moreover, the Eu anomalies observed in the Kaili Formation are similar to the negative Eu anomalies observed in post-Archean sedimentary rocks, and reveal fluctuations in oxygen content from the bottom to the top of the Kaili Formation.

Trace element and REE geochemistry of fine- and coarse-grained sands in the Ordos deserts and links with sediments in surrounding areas

In addition to mineral analyses, REE and trace element geochemical characteristics of fine- and coarse-grained sands in the Ordos deserts and other sediments in surrounding areas are investigated. Commonly the samples consist of quartz, feldspar and muscovite and less clinochlore, dolomite and ankerite. In few samples muscovite is absent. REE and trace compositions are spatially uniform for the same grain-size sands, suggesting that they could have the same sources or/and were well homogenized. However, fine- and coarse-grained sands in the Ordos deserts show different REE and trace element compositions. Fine-grained sands show higher contents of REE and trace elements than those of coarse-grained sands. They differ in Eu anomalies and (La/Yb) N ratios although both fractions are characterized by the steep LREE and smooth HREE patterns. The fine- and coarse-grained sands are also distinct in some characteristic element ratios (e.g., Th/Co, La/Sc, Th/Sc and Y/Ni). REE and trace element patterns of the two different grain-size fractions are closely associated with geological properties of individual sources rather than the mineralogical differentiation induced by wind sorting. The coarse-grained sands mainly resulted from sandstone weathering in the Ordos deserts and movements of coarse particles by wind. REE and trace element patterns of fine-grained sands in the Ordos deserts differ from those of sandstones in the Ordos deserts, the alluvial sands in the surrounding mountains and the coarse fluvial sands in the Yellow River. They resemble the fine fluvial sands in the Yellow River. In addition, arid areas of Northwest China such as the Tarim Basin and the Alxa Plateau should not be ruled out as the source of the fine-grained sands in the Ordos deserts because these arid areas reserve plenty of fine-grained sediments and also located in the upwind directions of the Ordos deserts.

Geochemical characteristics of Guizhou Permian coal measure strata and analysis of the control factors

Based on element geochemical studies of the main Permian exploitable coal measure strata in Western Guizhou, the element geochemical distribution characteristics of the main exploitable coal measures were revealed in the regions of Dafang, Qianxi, Weining, Hezhang, Zhijin, etc., of Guizhou Province, and the results show that their element contents are mainly affected by terrestrial material supply. Coal measures formed in the delta plain environment where sufficient terrestrial materials are supplied contain relatively abundant trace elements and rare-earth elements, whereas those formed in the tidal-flat environment influenced greatly by seawater have relatively low contents of trace elements and rare-earth elements, mainly controlled by the geological fact that basalts the parent rocks from source regions contain high trace elements and rare-earth elements. In addition, coal measures affected by later hydrothermal activities and fault tectonics contain a large amount of harmful elements. According to the rules of distribution of elements in coal measures, a new idea was put forward to classify coal-forming environments by using the geochemical composition characteristics, which is of great significance in dissolving the problem of whether coal measures were formed either in delta environments or in tidal-flat environments in Western Guizhou. At the same time, the rules of distribution of elements in the main exploitable coal measures in Western Guizhou were fully understood, which is of direct significance in utilizing coal resources on the basis of classification of coals, as well as in developing the coal chemical industry.

Eclogite xenoliths from Kimberley, South Africa — A case study of mantle metasomatism in eclogites

Eclogite xenoliths from the Kimberley diamond mine dumps were studied for major, trace elements and oxygen isotopes. The suite consists of typical garnet and clinopyroxene rocks as well as kyanite-eclogites and olivine- or orthopyroxene-bearing samples. All samples have Al[6]/Al[4] ratios > 2 which indicate high equilibration pressures, i.e. in the eclogite stability field as opposed to pyroxenites. Most samples are modally and cryptically metasomatized and contain up to 10 vol.% phlogopite plus ilmenite and rutile. Trace element geochemical characteristics of phlogopite and Ti-oxides are similar to those described for metasomatized peridotite xenoliths from the Kaapvaal craton, and show that the eclogite xenoliths experienced the same metasomatic event. Oxygen isotopic values of the phlogopites plot with those for pristine mantle, and are taken as evidence that the metasomatic agent did not have an exotic oxygen isotopic composition. In contrast, the eclogitic garnets are significantly lighter in oxygen isotopes than pristine mantle. These values cannot have been caused by the metasomatism that produced phlogopite, but are a preserved feature of the eclogite protoliths. The occurrence of kyanite is a strong indication for a low pressure origin of the protoliths, This, together with low oxygen isotopic values, a range of Mg-numbers of 59.5 to 88.8 and overall high trace element abundances compared to other eclogite xenolith suites, is taken as evidence that the protoliths of the Kimberley eclogites were seawater-altered oceanic cumulates which were metamorphosed in eclogite facies conditions and later overprinted by a mantle-derived metasomatic event.

Research on Element Geochemical Characteristics of Coal-Ge Deposit in Shengli Coalfield, Inner Mongolia, China

The newly found coal-germanium deposit in Shengli Coalfield in Inner Mongolia is a super-scale germanium deposit. It is the largest coal-germanium in the world with 1805 ton proven resource of germanium. In this deposit, the elemental geochemical characteristics show that the alkaline elements, such as Ca, K, Na, Mg, Al, and so on are enriched, and positively correlated to germanium; that with increasing germanium content, LREE enrichment is gradually weakened and HREE is gradually enriched; that the germanium content is negatively correlated to ∑REE and LREE, while positively to LREE/HREE and δEu, and negatively to La/Yb, La/Sm, (La/Yb) n, (La/Sm) n and δCe; that the content of germanium is negatively correlated to ash index of coal. The coal-forming swamp with sluggish water favored the sufficient absorption of germanium from solution by organic matters. The coal-germanium deposit of Shengli Coalfield is formed in the weaker alkaline and stronger reductive environment. REE geochemical characteristics in coal-germanium deposit have inheritance, and REE mainly comes from inorganic materials in the source area. And, the rocks from the southwest margin of the basin are rich in germanium, and probably become the original source of germanium of this coal-germanium deposit. The supply of germanium from the source area and the hydrologic geology conditions for germanium coming into coal forming swamps are the controlling factors for germanium enrichment.

Geochemistry of Late Mesozoic mafic dykes in western Fujian Province of China: Sr-Nd isotope and trace element constraints

The Bancun diabase dyke and the Bali hornblende gabbro dyke in western Fujian Province were emplaced in the Early and Late Cretaceous periods, respectively; the former is designated to calc-alkaline series and the latter to K-high-calc-alkaline rock series. Both the dykes are characterized by such geochemical characteristics as high Al and Na2O>K2O. As for the Bancun dyke, Al2O3=16.32%−17.54% and K2O/Na2O=0.65−0.77; as for the Bali dyke, Al2O3=16.89%−17.81% and K2O/Na2O=0.93−0.99. Both the Bancun and Bali mafic dykes are relatively enriched in LILE and LREE, but depleted in HSFE, displaying the geochemical characteristics of continental marginal arc, with high initial Sr isotopic ratios and low ɛNd(t) values. The (87Sr/86Sr)i ratios of the Bancun diabase dyke are within the range of 0.708556–0.70903 and their ɛNd(t) values vary between −6.8 and −6.3; those of the Bali hornblende dyke are within the range of 0.710726–0.710746 and their ɛNd(t) values are −4.7–−4.9, showing the characteristics of enriched mantle EM II. The isotope and trace element data showed that the mafic dykes have not experienced obvious crustal contamination, and metasomatism caused by subduction fluids is the main factor leading to LILE and LREE enrichments. The enriched mantle is the source region for the mafic dykes, and mixing of subduction fluid metasomatized enriched mantle and EM II-type mantle constituted the mantle source region of both the Bancun and Bali mafic dykes. Upwelling of the asthenosphere mantle provided sufficient heat energy for the generation of magmas. In accordance with the discrimination diagram of their tectonic settings as well as their trace element geochemical characteristics, it is considered that the dykes both at Bancun and Bali possess the characteristics of continental marginal arc, revealing the tectonic environment of formation of the mafic dykes, the continental dynamic background as an intraplate tensional belt in which the mafic dykes were emplaced. Meanwhile, it is also indicated that the tensional tectonic stress mechanism is responsible for the formation of the mafic dykes in western Fujian Province.

Comparison of element abundance between the exposed crust of the continent of China and the global averaged upper continental crust: Constraints on crustal evolution and some speculations

Based on the results of a study of regional element abundance in eastern China and the 1:200 000 geochemical surveys in northern Xinjiang, the element geochemical characteristics of the exposed crust in 23 tectonic units of the continent of China are summarized. Compared with the global average abundance of the upper continental crust, the exposed crust of the continent of China is compositionally more evolved than the upper crust of the island arc, but less evolved than the mature Precambrian Canadian shield. The exposed crust of the North China and Yangtze platforms has a lower SiO2 content, but markedly higher CaO and MgO contents due to the presence of widespread carbonate strata, which suggests that we should not neglect the contribution of carbonate rocks in the study of the exposed crust and the element abundance of the upper crust. In comparison with two recently published average compositional models of the global upper continental crust, the exposed crust of the continent of China is depleted in Au, Hg, Mo, Sn, and W, which suggests that their abundance in the present global models is overestimated. The exposed crust of the North China platform and the Qinling-Dabieshan fold belt to its south has lower μ(238U/204Pb) values (<8), but other regions of the continent of China exhibit much higher μ values, which implies that the low μ feature of the North China platform and its adjacent regions does not have global significance. Considering the apparent lateral variation in composition of the exposed crust for the tectonic units of the continent of China, there is no adequate reason to take the average upper crust compositional model of the North China platform and its adjacent regions as a reliable composition representative for Chinese and global upper continental crust composition.

Elemental Geochemistry of the Interlayer Oxidation Zone in the Shihongtan Sandstone Type Uranium Deposit, Xinjiang

Abstract According to the oxidation intensity of ore‐hosting sandstone, the interlayer oxidation zone of the Shihongtan sandstone‐type uranium deposit in the Turpan‐Hami basin can be divided into 4 geochemical subzones, namely, intensely‐oxidized, weakly‐oxidized, redox and unoxidized primary subzones. The elemental geochemical characteristics of the four subzones have been studied in detail, and the results show that U, together with other elements such as Re, Mo, Se, Sr, S, REE, Corganic etc., is enriched in the redox subzone. Re and U have similar geochemical properties in the reduction‐oxidation process. The geochemical properties of Mo and Se are similar to those of U in the reduction condition, but different from those of U in the oxidation condition. It is proposed that the ore‐hosting layers can provide a curtain mount of uranium for uranium mineralization.

REE and trace element geochemistry of Yinachang Fe-Cu-REE deposit, Yunnan Province, China

REE and other trace elements in ores, wall rocks, alkaline volcanic rocks and diabase dikes have been determined in the Yinachang Fe-Cu-REE deposit. Comparative studies of REE and trace element geochemical characteristics of these geological bodies indicate that the ores and alkaline volcanic rocks contain abundant REE and Y (65.9−4633) × 10−6, with higher abundances of As, Mo, Nb, Co and U than those of the crust. Their chondrite-normalized patterns show a strong enrichment of LREE and a positive Eu anomaly, in contrast to those of the dolomites which are characterized by slight LREE enrichment and moderate negative Eu anomaly. The REE patterns of ores are similar to those of hydrothermal sediment cores in the East Pacific Rise, whereas the REE patterns of dolomites are similar to those of PAAS (Post-Archean Average Shale). In combination with the geological setting of the deposit, the primary ore-forming fluids might have higher REE and volatile elements, and might have been derived from mantle degassing, or the alkaline volcanic magmas. The Yinachang Fe-Cu-REE ore deposit is considered to be of volcanic exhalation-hydrothermal sedimentary origin.

Trace element geochemistry of the Lower Cambrian black rock series from northwestern Hunan, South China *

Abstract Samples of black rock series collected from the Lower Cambrian Niutitang Formation, northwestern Hunan Province, China, were analyzed for trace elements. The trace element geochemical characteristics of the ore layer (high enrichments of Ni, Mo, V, U, Cu, Pb, Zn, Cd, Tl, and Ba; high U/Th ratios and LREE-enriched patterns with positive Eu and Y anomalies) support the assumption that the Ni-Mo polymetallic layer is of submarine hydrothermal origin. High field strength elements (HFSE) and lithophile elements such as Nb, Ta, Zr, Hf, Li, Be, Rb, Cs, Th and Sc are remarkably higher in the upper unit than those in the lower unit of the Niutitang Formation. These features may reflect that hydrothermal contributions were diminished, and terrigenous constituents were increased in the sediments of the upper unit of the Niutitang Formation.

Petrogenesis of the Mesozoic intrusive rocks in the Tongling area, Anhui Province, China and their constraint on geodynamic process

Petrology, element and isotopic geochemistry of the Mesozoic intrusive rocks in the Tongling area were systematically investigated in this study. The intrusive rocks can be divided into two groups, one contains shoshonitic rocks with SiO 2 ≤55%, the other consists mainly of high-potassic calc-alkaline rocks with SiO 2 >55%. The shoshonitic rocks (SiO 2 ≤55%) were generated by the fractional crystallization of the primary basaltic magma sourced from an enriched mantle, then the evolved basaltic magma likely experienced low-degree contamination with the lower crust materials when they ascended. On the other hand, although the intrusive rocks with SiO 2 >55% show most elemental geochemical characteristics similar to an adakite, such as high Na 2 O, Al 2 O 3 , Sr contents, high Sr/Y and La/Yb ratios, they have isotopic compositions much different from an adakite, such as low e Nd(t) (-9.16—16.55) and high ( 87 Sr/ 86 Sr) i (0.7068—0.7105), and some of them show relatively high Y and Yb contents than those of an adakite. We propose that the intrusive rocks with SiO 2 >55% were most probably produced by mixing of the mantle-derived basaltic magma and adakite-like magma derived from the melting of basaltic lower crust that was heated by the underplating mantle-derived shoshonitic magmas. The delamination of lower crust likely took place after or during the formation of these adakite-like rocks in the Tongling area.

Geochemical characteristics of volcanic rocks of the Laochang Ag polymetallic deposit, Lancang, Yunnan Province, China

In terms of major element, trace element and REE geochemical characteristics of volcanic rocks in the area studied and by making use of the TiO2-K2O-P2O5, Th-Hf-Ta and Zr-Nb-Y trianglar diagrams and the environmental discrimination diagrams of incompatible element distribution patterns, the authors have drawn some conclusions that are different from those by previous workers. It is concluded that volcanic rocks in the Laochang Ag polymetallic deposit at Lancang, Yunnan belong to continental within-plate alkali basalts, and that their geotectonic setting seems to be at the northeastern margin of East Gondwana Land.

The Massabesic Gneiss Complex, New Hampshire: a study of a portion of the Avalon Terrane

Geochemical data from the 625 Ma Massabesic Gneiss Complex of southern New Hampshire show strong affinities for other Avalonian rocks of southern New England and suggest continental rifting in the Late Proterozoic. Migmatized paragneiss, the dominant rock type in the complex, has major and trace element compositions that are compatible with graywackes from continental arcs. The paragneiss also has strong lithologic, metamorphic, and isotopic similarities to the rocks of the Hope Valley zone of Connecticut Avalon, suggesting a possible Hope Valley— Massabesic correlation. At 625 Ma, the paragneiss ε_Nd values are similar to Avalonian crust in other locations of the orogen. Two types of amphibolite are present in minor amounts in the paragneiss of the Massabesic Gneiss Complex. The first type is a paramphibolite and consists of calc-silicate layers in the Massabesic paragneiss, the second type is metaigneous. Major and trace element abundances reveal that the protoliths of the orthoamphibolites range from continental rift alkaline basalts and tholeiites to N-type MORBs. Orthoamphibolite ε_Nd (625 Ma) values range from 2.4 to 4 as expected of rift-related magmas derived from partial melting of a depleted mantle source and have the same values as Iapetus ocean floor rocks of similar age. Orthoamphibolite major and trace element geochemical characteristics overlap those of the Middlesex Fells amphibolites of the Esmond-Dedham zone of eastern Massachusetts Avalon, which range from alkaline to transitional basalts erupted in a continental rift setting. The compositions of orthoamphibolites define a potential magmatic continuum produced by batch partial melting of the mantle initiated during continental rifting and proceeded to ocean basin formation. The inferred continuity of mafic magmatism from the Esmond-Dedham (Middlesex Fells Formation) to the Massabesic Gneiss Complex (and Hope Valley zone) suggests that these zones are not distinct lithotectonic zones but are parts of a single landmass. Massachusetts Avalon (Esmond-Dedham) represents the continental section of Avalon where the alkaline to transitional magmas of the early rifting stages are preserved. According to our tectonic reconstruction, the Massabesic Gneiss Complex is the oceanward, continental margin represented by volcanoclastic sediments with the MORBs representing the initiation of ocean basin development. The leading edge of this landmass, of which the Massabesic Gneiss Complex is the only observable remnant, collided with Laurentia during the Acadian Orogeny. The inboard, thicker, more continental trailing-edge, that is, platform Avalon (Esmond-Dedham) collided later during the Alleghanian Orogeny.

Parental basaltic melts and fluids in eastern Manus backarc Basin: implications for hydrothermal mineralisation

The eastern Manus Basin is an actively forming backarc extensional zone behind the New Britain Island arc, which hosts a number of submarine volcanic edifices and hydrothermal fields. Isotopic and trace element geochemical characteristics of the edifices are comparable with those of the adjacent subaerial New Britain arc, and differ significantly from those of MORB-like lavas on and near the Manus Spreading Ridge in the central part of the basin. Fractional crystallisation dominates magma evolution from primitive basalts to andesites, dacites and rhyodacites in the eastern Manus Basin, but several lineages with differing trace element enrichment have been delineated. Melt inclusions within olivine phenocrysts (Fo82–92) of two representative east Manus basalts, respectively, with modest (0.2 wt%) and high (0.8 wt%) potassium contents, host ubiquitous CO2-bearing vapour bubbles, denoting presence of an immiscible fluid phase at early stages of crystallisation. Bubbles often carry precipitate phases whose abundance is broadly proportional to the bubble size reaching a maximum in fluid bubbles with little or no melt. Among the precipitates, detected by laser Raman spectroscopy and EDS-scanning electron microscopy, carbonates are common and include magnesite, calcite, ankerite, rhodochrosite and nahcolite (NaHCO3). Gypsum, anhydrite, barite, anglesite, pyrite, and chalcopyrite have also been found. Some amorphous precipitates recrystallise after bubbles are opened to Na–Ca carbonates, halite and Na–K–Ca alumino-silicates. Copper abundances decrease from basalt to dacite across the eastern Manus fractionation spectrum, whereas Pb behaves as an incompatible element, increasing to highest values in the dacites. Zinc abundance reaches maximum concentrations in andesite, and decreases during further fractionation. Loss of Cu especially from the fractionating magmas, in the absence of immiscible sulphide liquid, strongly implies metal partitioning into CO2–H2O fluid, which is degassed significantly during magma fractionation. Hydrothermal fluids in the PACMANUS system may carry a direct contribution of the magmatic metal-bearing fluid, exsolved from the crystallising arc-like magmas at this immature backarc basin, and are able to transport and concentrate major amounts of ore metals, particularly Cu.