Lower Light Rare Earth Elements Research Articles

Geochronology and geochemistry of zircon and columbite–tantalite group minerals from the Weilasituo Sn–polymetallic deposit, northeastern China: Implications for the relationship between mineralization and the magmatic–hydrothermal transition

The Weilasituo Sn–polymetallic deposit, located in the southern portion of the Great Xing'an Range of Inner Mongolia, China, is a super-large Sn deposit associated with abundant Li, Rb, Nb, Ta, Zn, and Cu resources. Despite that previous studies have characterized the age, hydrothermal evolution, and genetic relationship between the alkali feldspar granite porphyry and associated Sn–polymetallic mineralization, the magmatic–hydrothermal evolution and ore–forming processes remain ambiguous. To address these ambiguities and bridge this knowledge gap, this contribution characterizes the texture, geochemistry, and U–Pb isotope composition of zircon and columbite–tantalite group minerals (CGMs) from amazonitized alkali feldspar granite (AMG) and albitized alkali feldspar granite (ABG) in the Weilasituo deposit. Type-I zircon (Zr1) is inclusion-poor, translucent, and is characterized by low light rare earth element (LREE) contents, high (Sm/La)N ratios (mean = 655.7), and low Th/U (mean = 0.21 < 0.3), Zr/Hf (mean = 5.46), and Y/Ho ratios (mean = 4.42), indicating that it formed during the magmatic–hydrothermal transition. Type-II zircon (Zr2) is inclusion-rich, porous, has a low CL response, and is characterized by low Th/U ratios (0.08–0.32, mean = 0.17), high LREE contents, weakly positive Ce anomalies, and low (Sm/La)N ratios (mean = 20.7), indicating that it formed via hydrothermal alteration of Zr1 (i.e., magmatic–hydrothermal zircon). Type-I CGM (CGM1) generally lacks oscillatory zoning, and has similar and positively correlated Ta/(Nb + Ta) and Mn/(Mn + Fe) values (AMG: R2 = 0.93; ABG: R2 = 0.75), indicative of a magmatic origin. Type-II CGMs (CGM2) occur as rims on CGM1 and are characterized by higher Ta/(Nb + Ta) and lower Nb/Ta ratios than CGM1, indicative of precipitation from a hydrosilicate liquid during the magmatic–hydrothermal transition. Based on the characteristics of CGMs, two stages are evident in the evolution of the Weilasituo granite porphyry: an earlier magmatic differentiation stage and a later magmatic–hydrothermal transition stage. Furthermore, the magmatic–hydrothermal transition stage can be subdivided into the AMG and ABG substages within the porphyry system based on petrological and mineralogical characteristics. The U–Pb ages for zircons and CGMs from the alkali feldspar granite porphyry range from 137.2 ± 2.0 Ma to 129.8 ± 1.4 Ma, without a notable age gap. The similarity in U–Pb ages between CGM1–CGM2 and Zr1–Zr2 indicates that the magmatic stage and the magmatic–hydrothermal transition at Weilasituo were continuous processes that occurred during the Early Cretaceous.

Fluid-rock interactions recorded by bulk-rock and mica compositions in the Xiuwacu granite-related quartz-vein W–Mo deposit, western Yunnan Province, China

The Xiuwacu deposit is a typical granite-related quartz-scheelite-molybdenite vein type deposit hosted within sandstones and sandy slates in the southern Yidun Terrane, western Yunnan Province, China. The Xiuwacu complex comprises the Late Triassic biotite granite and Late Cretaceous monzogranite porphyry (MGP) and aplite granite. Extensive and intensive alteration including potassic, propylitic, phyllic, silicic and argillic alteration developed within the Late Cretaceous monzogranite porphyry. Greisenization and pegmatization occurred in the roof of the Xiuwacu complex, suggesting large-scale fluid-rock interactions. However, the effects of fluid-rock interactions on the elemental exchange and ore-forming processes at Xiuwacu still remained unclear. This study firstly reported whole-rock major and trace elements of representative altered rocks and in situ major and trace elements of micas from fresh and altered rocks, and quantitively constrained the mobilization of elements by normalized isocon diagrams. The altered MGP are characterized by higher loss on ignition (LOI) values (>2), lower light rare earth elements (LREEs) and variable large ion lithophile elements (LILEs) contents, with higher Cs, K, Rb and Ba contents and lower Sr content than fresh MGP. The classification diagram based on mica composition shows that magmatic biotites from the Late Triassic biotite granite and Late Cretaceous monzogranite porphyry belong to Fe biotites. Hydrothermal micas from pegmatoid and greisen belong to Li-phengites and zinnwaldites, and hydrothermal micas from quartz-molybdenite vein are classified as zinnwaldites. Li-phengites and zinnwaldites have relatively lower F contents, higher W, Sn, Li, Cs contents than biotites, intergrown with calcite and ankerite, suggesting CO2-rich, F-poor and rare metal-rich nature of hydrothermal fluids. Based on a compilation of fluid inclusions, H–O–S-Sr isotopes, scheelite geochemistry from previously published literatures and compositions of altered rocks and micas from this study, the tungsten ore-forming processes at Xiuwacu could be concluded as follows: (1) the initial moderately-oxidized, F-rich fluids exsolved from parental magmas of MGP form fluorite-quartz assemblages (pegmatoid; 400–300 °C); (2) the F-poor, W-rich, moderately acid fluids cause the breakdown of Ca-rich plagioclases, which consumes H+ and provides Ca2+ for the precipitation of early-stage scheelites (greisen; approximately 300 °C); the evolved nearly neutral and medium- to low-temperature fluids (300–150 °C) drive the fluid-rock interactions to supply Ca2+ for the deposition of main-stage scheelites. The deposition of Mo was not determined by the fluid-rock interactions, but by the ore-forming fluids transformed from oxidized to reduced nature.

Mineralogical, Geochronological, and Geochemical Characteristics of Early Cretaceous Granite in South China: Implications for Tectonic Evolution and REE Mineralization

One of the most important geological features of South China are the widespread Mesozoic igneous rocks that play a key role in revealing the tectonic evolution of South China. Due to the thick covering of vegetation and Quaternary sediments, the early Cretaceous magmatism in southwestern South China is still not well constrained. In this paper, we report newly identified early Cretaceous granites in Guangxi, South China. Zircon U–Pb dating results showed that representative fine-grained and coarse-grained granites in northeastern Guangxi indicate the early Cretaceous ages of 141 ± 3 Ma and 141 ± 4 Ma, respectively. Geochemically, both fine-grained and coarse-grained granites had high 10,000 × Ga/Al ratios and belonged to A-type granite. They had undergone high degrees of magma differentiation, as evidenced by extremely negative Sr, Ba, and Eu anomalies. They had high REE (rare earth elements) contents (&gt;451 ppm). The fine-grained granites were characterized by higher HREE (heavy rare earth elements) contents, lower LREE (light rare earth elements) contents, and lower LREE/HREE ratios than the coarse-grained granites. Integrated with regional geological data, the early Cretaceous granites were likely formed in a back-arc extensional environment in response to the increased subduction angle of the Paleo-Pacific plate. Different REE contents in the fine- and coarse-grained granites may be a result of fractional crystallization. Magma differentiation and hydrothermal alteration might have played an important role in REE mineralization of the early Cretaceous granites in Guangxi.

Composition of Garnet from the Xianghualing Skarn Sn Deposit, South China: Its Petrogenetic Significance and Exploration Potential

The Xianghualing skarn Sn deposit in the southwestern part of the southern Hunan Metallogenic Belt is a large Sn deposit in the Nanling area. In this paper, the garnet has been analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to obtain the concentrations of the major and trace elements. The results reveal that the garnets from the Xianghualing deposit mainly belong to andradite-grossular (grandite) solid solution and are typically richer in Al than in Fe. They show enrichment in heavy rare earth elements (HREEs) and notably lower light rare earth elements (LREEs), and commonly negative Eu anomalies, indicative of a relatively reduced formation environment. The garnets have high Sn concentrations between 2313 ppm and 5766 ppm. It is also evident that there is a positive correlation between Sn and Fe, suggesting that Sn4+ substitutes into the garnets through substituting for Fe3+ in the octahedral position. Combined with previous studies, it can be recognized that the Sn concentrations of garnet in skarn Sn deposits are generally high, whereas the W concentrations are relatively low. This is just the opposite in garnets from skarn W deposits that typically have high W, but low Sn concentrations. In polymetallic skarn deposits with both economic Sn and W, the concentrations of both metals in garnets are relatively high, although varying greatly. Therefore, the Sn and W concentrations in garnets can be used to evaluate a skarn deposit’s potential to produce Sn and (or) W mineralization, which is helpful in exploration.

Cretaceous to Mid-Eocene pelagic sediment budget in Puerto Rico and the Virgin Islands (northeast A.

Island arc basalts (IAB) in the Greater Antilles, dating between Albian and mid-Eocene time (~112 to 45 Ma), consist of an early low-K, primitive island arc (PIA) basalt series and a later, predominantly intermediate calcalkaline (CA) series. The rocks resemble modern sediment-poor, low-light rare earth element (LREE)/heavy rare earth element (HREE) arc basalts from intra-oceanic tectonic settings and sediment-rich, high-LREE/HREE types from continental margin arcs, respectively. Isotope and incompatible trace element distribution along a 450 km segment of the arc in the northeast Antilles demonstrates that low-LREE/HREE basalts predominate in Albian to Santonian (~85 Ma) stratigraphic sequences in the Virgin Islands (VI) and northeast Puerto Rico (NEPR), while there is a gradual but spectacular increase in both LREE/HREE and absolute abundances of incompatible elements in central Puerto Rico (CPR). Northeastern Antilles basalts have consistently elevated La/Nb and relatively low Nb/Zr, both inconsistent with the presence of a significant ocean island basalt component. Hence, observed differences are interpreted to reflect variation in proportions of pelagic sediment subducted by the south-dipping Antilles arc system as it swept north-eastward across the Caribbean region and eventually approached the Bahama Banks along the south-eastern fringes of the North American Plate. Trace element mixing models indicate sediment proportions in VI and NEPR were limited, averaging considerably below 1.0%. In comparison sediment content in CPR increased from an average slightly above 1.0% in Albian (~112 Ma) basalts to as high as 8% in Cenomanian (100-94 Ma) types. Hypothetical pre-arc pelagic sedimentary facies in the subducted proto-Atlantic (or proto-Caribbean) basin, included 1) a young, centrally located longitudinal ridge-crest facies, with a thin sediment cover, eventually subducted by VI and NEPR, 2) a slightly older basin-margin facies of variable width and moderate sediment thickness, subducted by CPR during Albian time, and 3) a thick, pre-arc continental margin facies in the vicinity of Central America, subducted by CPR during Cenomanian time. Following collision of neighboring Hispaniola with the Bahamas sediment budgets in the northeast Antilles stabilized at moderate levels from 2 to 3%, reflecting widespread subduction of North Atlantic Cretaceous pelagic sediment (AKPS).

Major, trace and rare earth element (REE) geochemistry of different colored fluorites in the Bobrynets region, Ukraine

The Bobrynets fluorite mineralization in the exo-contact zone of Krivograd-Novoukrainian granite intrusion is found in the southern part of the Kirovograd-Cherkassy fault zone between the Novoukrainian anticlinoria and the Ingulo-Ingulets synclinoria. The fluorite mineralization appears as vein and disseminat, as well as in the form of a cement in the tectonic breccia among the metasomatically altered granites. Fluorite mineralization is observed for 5 km along this fault zone, down to a depth of 300 m with the thickness of the individual mineralized zones ranging from a few meters to 75 m, with average fluorite contents ranging from 1–2% to 28%. The estimated reserve of Bobrynets fluorites is approximately 1.56 million tonnes. Fluid inclusion data in the studied fluorites indicate that the study area has two types of fluorites: high temperature fluorites (240–280 °C) and low temperature fluorites (100–150 °C). The total rare earth element (REE) contents of the fluorites range from 62.4 ppm to 527.9 ppm, with a mean of 218 ppm. The mean ∑REE concentrations of different colored massive fluorites range from 80.2 ppm for yellow fluorites to 258 ppm for green fluorites, 97.4 ppm for violet fluorites and 526.5 ppm for granite samples. The mean Y concentrations of the studied fluorites are 86.2 ppm for violet fluorites, 104.5 ppm for yellow fluorites, 242.5 ppm for green fluorites and 39.9 ppm for granite samples. The chondrite-normalized REE patterns of the Bobrynets fluorites exhibit trends similar to each other and show a decrease from light REEs (LREE) to heavy REEs (HREEs). All fluorites in the study area have positive Ce anomalies between 1.06 and 1.16 (with mean of 1.12), negative Eu anomalies (with a range of 0.35 to 0.58; mean: 0.46), and positive Y anomalies between 1.38 and 2.67 (with mean of 2.12). The Y/Ho values change from 48.3 to 92.2 with mean of 71.1. The Tb/Ca and Tb/La ratios of the Bobrynets fluorites are between 0.00000249 and 0.0000404 and between 0.098 and 0.30, respectively, and these ratios also indicate a formation from fractionated ore-bearing fluids at an early stage of mineralization, mainly a hydrothermal and less pegmatitic stage. The (Tb/Yb)n–(La/Yb)n ratios are low because these fluorites are not LREE rich when compared to other fluorite deposits. The (La/Yb)n–(Eu/Eu∗)n values also indicate low negative Eu anomalies and low LREE enrichments for the Bobrynets fluorites. The Sc/Eu vs Sr diagram and Sc vs ∑REE diagram show that the Bobrynets fluorites have low Sr contents and high Sc/Eu values and that they cover the same areas as the Rift Deposits of New Mexico and the skarn fluorite deposits of Akdagmadeni. The 208Pb/204Pb isotope values of fluorites vary between 53.5 and 62.6 and the 206Pb/204Pb isotope values of fluorites vary between 23.3 and 26.8. The 208Pb/204Pb vs 206Pb/204Pb ratios indicate that fluorites originated from upper crust materials. All these findings indicate that REE contents of the Bobrynets fluorites have different physicochemical parameters such as decrease in temperature or increases in ƒO2 and pH, high oxygen fugacity in the source of the hydrothermal fluids, and low Sr and high Sc values similar to rift and skarn deposits.

Can magmatic zircon be distinguished from hydrothermal zircon by trace element composition? The effect of mineral inclusions on zircon trace element composition

Mineral inclusions, e.g., apatite, titanite, monazite, K-feldspar, are common in magmatic zircons. Although many studies mention that light rare earth element (LREE) contents of zircons could be compromised by an analytical artefact of the accidental sampling of mineral inclusions, how and to what degree these inclusions influence analysis of zircon composition is still not well constrained. Here we report U–Pb ages and trace element abundances for zircon crystals, where apatite and K-feldspar inclusions are observed, from diorite porphyry in the Weibao deposit, East Kunlun Mountains, Northern Tibetan Plateau. Although zircon morphological and chronological evidence consistently advocates a magmatic origin without undergoing significant hydrothermal alteration, 7 of 15 analytical spots show LREE-enriched patterns and low Ce/Ce* ratios which are comparable to those for published “hydrothermal” zircon. Quantitative modelling in this study manifests that these LREE-enriched patterns and low Ce/Ce* ratios can be achieved with only 0.1 to 2 vol% contamination from sub-micrometer apatite inclusions, which in practice are hard to monitor under the LA–ICP–MS (normally with large pit diameter and depth) and conventional microscopes. Titanite, monazite, xenotime, and allanite have similar roles to apatite, and LREE contents of zircon can be significantly elevated with only 0.05 vol% contamination from these inclusions. We therefore suggest that the widely used geochemical discrimination criteria for magmatic and hydrothermal zircon, e.g., (Sm/La)N vs. La and Ce/Ce* vs. (Sm/La)N diagrams and the degree of Ce anomalies, are ambiguous since they are extremely susceptible to contamination by mineral inclusions, and that within single samples only Ce4+/Ce3+ values calculated from zircons of low LREE values probably represent the oxidation state of magmas.

Linking the Qinling Orogeny with the Chang 7 shale (Triassic Yanchang Formation) deposition: Evidence from major, trace, and rare earth element geochemistry

The lacustrine organic‐rich shale of Triassic Chang 7 Member is an unconventional shale oil reservoir that is a major hydrocarbon target in the Ordos Basin, northern China. This study uses major, trace, and rare earth element (REE) abundance documented through a 115‐m‐thick core obtained from the western part of the Ordos Basin to assess dominant controls on temporal variations in total organic carbon in the Chang 7 shale. Our results suggest that total organic carbon trends in the Chang 7 can be directly linked with the tectonic and magmatic evolution of Qinling orogen. The Chang 73 and Chang 72 submembers of the lower part of the Chang 7 were likely deposited in association with heighted tectonic and magmatic activity of the Qinling Orogeny. Sediments derived from these submembers display light REE and trace element compositions similar to the Tianshui rhyolites suggesting deposition contemporaneous with magmatic and/or volcanic activity of the Qinling Orogeny. Higher nutrient concentrations introduced by volcanic eruptions would have stimulated primary productivity in the surface water enhancing the organic carbon flux to the lake bottom. In addition, higher rates of mountain building in the Qinling Orogeny may have been responsible for increasing subsidence rate of the southern Ordos Basin, also favourable for the establishment of anoxic bottom‐water conditions. In contrast, lower light REE and trace element abundances of the Chang 71 submember at the top of the Chang 7 suggest deposition during a tectonically and magmatic quiescent period of the Qinling Orogeny. The diminished volcanic flux could have lowered the rate of primary productivity. Moreover, a reduced rate of mountain building would have decreased the rate of foredeep subsidence favouring shallower water and oxic/suboxic bottom‐water conditions. Thus, the hydrography of the lake during accumulation of the Chang 71 was not conducive for organic carbon accumulation. Together, the correlation of the Qinling mountain building and magmatism and deposition of the Chang 7 shale provide evidence for the role of tectonism in the accumulation of organic carbon enriched sediment.

Rare-earth-element geochemistry in soils developed in different geological settings of Cuba

The increased exploitation of rare-earth elements (REEs) over the past few decades and the disposal of various products containing these elements have prompted numerous environmental studies and the need to establish regulatory guidelines. We determined the concentration of REEs in topsoil samples of 33 soil profiles collected from areas of different geological settings in Cuba. The average REE concentrations revealed the following chemical abundances in decreasing order: Ce>Nd>La>Pr>Sm>Er>Yb>Dy>Tb>Lu>Ho>Gd>Eu>Tm. The significant variation in REE levels reflects the geological diversity of Cuba. The highest REE concentrations were observed in Typic Rhodudalf and Rhodic Eutrodox, which are derived from hard limestone. In contrast, the lowest REE concentrations were detected in the Typic Kandiustalf, which originated from mica schist. The lower light REE (LREE) contents in Cuban soils compared with soils from other countries is related to the low abundance of felsic rocks on the island. We also observed a strong positive correlation between REEs and some major elements in soils. This finding implies that the distribution of REEs in soils is partly governed by phosphates and aluminosilicates. The low fractionation between LREEs and heavy REEs is likely related to the predominance of soils developed from carbonates. This finding was also supported by the low LaN/SmN and GdN/YbN ratios and principal component analysis. The spatial distribution of REEs normalized to Upper Continental Crust coupled with univariate and multivariate statistical techniques indicated that parent materials control the prevalence of REEs in Cuban soils. The data presented here represent the first national-wide survey of REEs in Cuban soils.

Evaluation of late Permian mafic magmatism in the central Tibetan Plateau as a response to plume-subduction interaction

An integrated study of the geochronology, mineralogy, geochemistry and Sr–Nd isotope compositions of the upper Permian Yushu mafic rocks in the central Tibetan Plateau (CTP) was conducted to evaluate the interaction between the Emeishan mantle plume and the Paleo-Tethyan subduction system. These mafic rocks can be geochemically subdivided into three groups. Group 1 rocks yielded a weighted mean 206Pb/238U age of 258±2Ma. They have relatively high TiO2, FeOt, Nb/Y and high εNd(t) values and display oceanic island basalts (OIB)-like rare earth and trace element patterns, with obvious enrichment of Nb and Ta. Group 2 and 3 rocks yield weighted mean 206Pb/238U ages of 258±1Ma and 257±1Ma, respectively. Both have relatively low TiO2, FeOt, Nb/Y and εNd(t) values and island arc tholeiites (IAT)-like rare earth and trace element patterns with obvious depletion of Nb and Ta. However, the Group 2 rocks have relatively low FeOt/MgO ratios, but high MgO, Mg#, Cr and Ni contents, resembling primitive magmas. They have lower light rare earth element (LREE), Nb, Ti and Zr contents and higher εNd(t) values than the Group 1 rocks. The geochemical and isotopic variations suggest that the Group 1 rocks might be derived from a plume-related mantle source, whereas Group 2 and Group 3 rocks originated from subduction-modified asthenospheric mantle and lithospheric mantle. Moderate degrees of olivine and clinopyroxene fractionation under low oxygen fugacities (fO2) appear to be responsible for the Fe-Ti enrichment in the Group 1 rocks. We propose a model involving plume-subduction interaction to explain the geodynamics and generation of the late Permian mafic magmatism in the CTP. The study region was rifted by the Emeishan mantle plume activity in association with rollback of Longmuco-Shuanghu oceanic lithosphere in late Permian times. Deep melting of the plume source led to the generation of the Group 1 plume-related magmas, whereas shallower melting of the subduction-metasomatized asthenospheric and lithospheric mantle resulted in the generation of the Group 2 and Group 3 subduction-related magmas.

Rare earth elements in surface soils of the Davarzan area, NE of Iran

Rare earth element (REE) abundances, Ni–Cr content and physico-chemical characteristics in different topsoils and bedrocks of Davarzan area, NE of Iran, were determined to understand the relationships of theses parameters in an ophiolitic region. Resulted data show that REE geochemistry could be a suitable proxy for parental materials of surface soils. The soils derived by serpentinite and peridotite rocks display REE properties similar to these ultramafic rocks. They have low light REE (LREE), negative Ce anomaly (δCe=0.51–0.82) and positive Eu anomaly (Eu/Eu⁎=1.17–1.35). The REE concentrations of ultramafic-derived surface soils are significantly higher than parental rocks because of clay minerals. There is positive correlation between REE content and clay% and organic matter of these topsoils indicative of direct effect of such factors in REE abundances.The surface soils of low elevation samples in the studied area have been formed by weathering and transportation of different lithologies. Therefore, REE geochemistry is more sporadic and differs in various land uses. Agricultural surface soils have the highest organic matter and REE content (∑REE>100ppm). Chondrite-normalized REE patterns of agricultural samples depict fractionated appearance with steep LREE and flat high REE (HREE) [(La/Yb)N=15.31–16.41]; while other samples have lower REE content (∑REE=31.89–58.3ppm) and lower LREE/HREE. P-fertilizers are the major factor that produces REE enrichment in agricultural soils. Since these rocks are composed of mixed sources, there is no regular relation between REE content and other parameters such as organic matter and clay percentage.The concentrations of heavy metal elements such as Ni and Cr in surface soils of Davarzan area (693–1513ppm) are higher than all threshold standards defined for environmental assessments. The Ni–Cr concentration increases with REE content in ultramafic-related topsoils; but the relation of REE content and heavy metal composition is more complicated and influenced by other factors in the low elevation soils with mixed parental materials.

통가열도 해저화산 표층 퇴적물 내 열수변질의 화학적 특성

통가열도 내 우리나라 해저열수광상 독점 탐사지역에 위치하는 5개 해저화산(TA12, TA19, TA22, TA25, TA26) 주변 표층 퇴적물의 열수변질 여부를 규명하고, 열수변질 특성을 확인하기 위하여 각 해저화산에서 채취된 29개의 표층 퇴적물에 대해 미량 원소와 희토류 원소 분석을 실시하였다. 분석 결과 TA12, TA19, TA22 해저화산에서는 열수에 의한 퇴적물의 변질작용이 없거나 매우 적은 것으로 나타났으며, TA25와 TA26 해저화산의 일부 지역은 현재에도 열수의 영향을 받고 있는 것으로 나타났다. 열수의 영향이 있는 해저화산 사이에도 부화된 미량 원소의 종류에 차이가 있는데, TA25 해저화산의 경우 주로 Ni, Cu, Sn, Zn, Pb, Cr, Cd, Sb, W, Ba, Ta, Rb, Sr, As가, TA26 해저화산에서는 주로 Cu, Sn, Zn, Pb, Cd, Sb, Ba, Rb, Sr이 부화되어 있다. 미량 원소의 부화가 확인된 지역은 희토류 원소의 부화도 관찰된다. 상부지각 희토류 원소 농도로 표준화된 희토류 원소의 분포형태는 가벼운 희토류 원소(LREE)가 매우 낮고 중간 희토류 원소(MREE)에서 무거운 희토류 원소(HREE)로 갈수록 증가하는 형태를 보이며, TA25와 TA26 해저화산의 일부 정점에서 Eu의 부화가 확인된다. 또한 TA26 해저화산의 일부 정점에서 Ce이 부화가 관찰되었는데, 이는 주변지역에서 채취된 열수 내 희토류 원소의 높은 Ce분포형태와 매우 유사하다. 또한 TA25와 TA26 해저화산은 부화된 원소의 특징이 다르게 나타나는데, TA25 해저화산에서는 Cu를 포함한 미량 원소가, TA26 해저화산에서는 Ce와 Eu를 포함한 희토류 원소가 우세하게 부화되어 있는 것으로 나타났다. 조사결과 나타난 각 해저화산 표층 퇴적물 내 부화된 미량 원소 및 희토류 원소의 종류와 농도는 차후 통가열도 지역의 열수광상 탐사시에 지역적인 열수변질의 지시자로써 유용하게 활용될 것으로 판단된다. We analyzed 29 surface sediment samples in five submarine volcanoes (TA12, TA19, TA22, TA25, and TA26) located in the southern part of the Tonga arc for trace elements and rare earth elements to investigate characteristics of the hydrothermal alteration of surface sediments. Based on analytical results of trace element and rare earth element (REE), surface sediments of TA12, TA19, and TA22 submarine volcanoes, which are located in the northern part of the study area, were very little or not influenced by hydrothermal fluids. In contrast, some stations of TA25 and TA26 submarine volcanoes were strongly affected by hydrothermal fluids. However, these two submarine volcanoes showed different features in element concentration in the sediments. Some stations of TA25 submarine volcano showed enrichment of Ni, Cu, Sn, Zn, Pb, Cr, Cd, Sb, W, Ba, Ta, Rb, Sr, and As, however, those of TA26 submarine volcano showed enrichment of Sn, Zn, Pb, Cd, Sb, Ba, Rb, and Sr. Stations which enriched trace elements were observed, enriched REEs were also observed. Average upper continental crust (UCC)-normalized REE patterns of the surface sediments generally showed low light REE (LREE) abundances and increased heavy REE (HREE) abundances. Eu enrichment was identified at several stations of TA25 and TA26 submarine volcanoes. In addition, enrichment of Ce was found at some stations of TA26 submarine volcano and these enrichment patterns were similar with hydrothermal fluid of near stations. Furthermore, TA25 and TA26 submarine volcanoes showed different enrichment characteristics of trace elements and REE. Trace elements were concentrated at TA25 submarine volcano. TA26 submarine volcano, on the other hand, observed highly enrichment of REE especially, Eu and Ce. As a result of the investigation, the characteristics and concentrations of REEs and trace elements in the surface sediments of each submarine volcano can be applied to identify hydrothermal alteration of sediments during exploration for hydrothermal deposits.

Generation of Arc and Within-plate Chemical Signatures in Collision Zone Magmatism: Quaternary Lavas from Kurdistan Province, Iran

New whole-rock and Sr–Nd isotopic analyses of Quaternary lavas from Kurdistan Province, western Iran, shed light on the nature of orogenic plateau magmatism during continental collision and the possible role of accessory minerals during mantle partial melting in this tectonic setting. The sampled lavas are from the Turkish–Iranian plateau within the Arabia–Eurasia collision zone. Compositions are typically basanite, hawaiite and alkali basalt, with minor rhyolite. Most of the basic samples from the Qorveh–Bijar region have elevated abundances of large ion lithophile elements (LILE) and light rare earth elements (LREE) (e.g. 76 ppm < La < 165 ppm), with steep REE patterns: 45 < La/Yb < 101. Two flows from Takab in the north of the study area have higher K2O/Na2O and Rb than the Qorveh–Bijar samples, but lower LREE and Ba. Sr–Nd isotope values for all the basic samples plot close to Bulk Silicate Earth, with 143Nd/144Nd between 0·51263 and 0·51276, and 87Sr/86Sr between 0·70467 and 0·70600; this is similar to Quaternary alkali basalts from the Iran–Turkey borderlands to the NW, but distinct from a more depleted source melting elsewhere in the same collision zone; for example, at Mount Ararat and the Kars plateau (Turkey). Crustal contamination does not appear to be an important factor affecting magma composition. The range of chemical signatures suggests variable melting of at least two distinct sources. One inferred source produced melts with La/Nb ranging from ∼3·5 to ∼1·2, which is unusual for volcanic rocks that are otherwise coeval. We interpret this variation as the result of depletion of a K-richterite- and rutile-bearing source during melting in the garnet stability field. The importance of this result is that the size of the negative Nb anomaly and fractionation of LILE may depend on K-richterite and rutile in the source mineralogy, rather than simply the bulk abundances of incompatible trace elements in the source region. We infer the presence of phlogopite in a second mantle source, the melting of which produced the more potassic lavas from Takab. Lithosphere delamination or slab break-off mechanisms for triggering melting are problematic in the study area, as the lithosphere is reportedly ∼150–200 km thick. Late Cenozoic extension has not been recognized, and so extension is unlikely as a cause of melting. The NW–SE alignment of volcanic centres, parallel to the regional structural grain, does imply a structural control, at least on the final eruption site. Eurasian mantle lithosphere was probably metasomatized by fluids derived from Mesozoic–early Cenozoic subduction and the early stages of collision. More recent convergence has had the potential to cause further fluxing and melting of the re-fertilized lithosphere, with rapid magma ascent assisted by transcurrent motion along the orogen. It is also possible that the negative dT/dP section of the amphibole peridotite solidus was crossed as a result of lithospheric thickening in the collision zone (i.e. compression melting rather than decompression melting). Exhaustion of accessory phases may also help explain larger-scale transitions from arc to within-plate chemistry in orogens, such as the Paleogene Arabia–Eurasia system, and the Mesozoic–Cenozoic magmatism of eastern China.

Ages and geochemistry of granites in the Pingtan–Dongshan Metamorphic Belt, Coastal South China: New constraints on Late Mesozoic magmatic evolution

Zircon U–Pb dating and Hf-isotope compositions and bulk geochemical analyses of granites and metasedimentary rocks from the NE-striking Pingtan–Dongshan Metamorphic Belt (PDMB), coastal South China, are used to constrain their formation ages and tectonic settings. LA-ICPMS zircon U–Pb dating results reveal four periods of magmatism/metamorphism along the PDMB, including ~187Ma, 133–130Ma and 108–100Ma granitic magmatism in the northern PDMB and 147–140Ma granites and metamorphic rocks in the southern belt. A low P/T metamorphic event at 144–140Ma and two deformation phases at 130–125Ma and 108–100Ma are also recognized. The Early Yanshanian high-K calc-alkaline granites (187–140Ma) are characterized by high contents of SiO2, alkali and low light rare earth element (LREE)/ heavy rare earth element (HREE) fractionation (LREE/HREE=4.82–11.54, (La/Yb)N=3.99–13.53) and weak Eu depletion (0.54–0.61), whereas the Late Yanshanian calc-alkaline granites (140–100Ma) have low A/CNK, middle LREE/HREE ratios (10.13–19.18) and positive to inconspicuous Eu anomalies (0.79–1.95). Granites in both periods are depleted in high field strength elements (HFSE, e.g. Nb, Ta, Ti) but enriched in LREE and large ionic lithophile elements (LILE), with low Rb/Ba and Rb/Sr ratios and A/CNK values, indicating that they are typical I-type granites in the arc-related tectonic setting due to subduction of the Paleo-Pacific plate. However, both the Early and Late Yanshanian granites have distinct Sr, Nb and K2O contents and La/Yb, Sr/Y and Eu/Eu* ratios, suggesting different P–T conditions. Zircon grains from the granites in the northern PDMB have higher Hf-isotope compositions with εHf(t)=+0.01 to +6.47 and Hf model ages (TCDM)=0.82–1.16Ga, whereas those from the granites in the southern PDMB show slightly lower Hf-isotope compositions with εHf(t)=−6.63 to +0.07 and older Hf model ages (TCDM)=1.19–1.61Ga. Compared with the metamorphic basement near the study area, the granites in the northern PDMB were probably derived from a Neoproterozoic crust, whereas those in the southern PDMB may have originated from a mixed source of Neoproterozoic and minor older crustal materials. We propose that the Early Yanshanian granites in the PDMB are the eastern extension of the E–W-striking granitoid–volcanic belt in the Nanling Range, whereas the Late Yanshanian igneous rocks mostly distribute in coastal Southeast China and strike NE. The distinct distribution and geochemical features between the Early and Late Yanshanian magmatic events can be related to the change of direction and dip angle of the Paleo-Pacific plate that was subducted underneath the Eastern Eurasian continent in the two episodes.

Multiple sources for mineralizing fluids in the Charmitan gold(-tungsten) mineralization (Uzbekistan)

Mineral assemblages present within the Charmitan gold(-tungsten) quartz-vein mineralization have been investigated for their cathodoluminescence behaviour, chemical composition and noble gas isotope systematics. This inventory of methods allows for the first time a systematic reconstruction of the paragenetic relationships of quartz, scheelite, sulphides and native gold within the gold mineralization at Charmitan and provides the basis to utilise noble gas data in the discussion of sources and evolution of ore-forming fluids. The vein quartz is classified into four generations based on microscopic and cathodoluminescence investigations. Quartz I shows intense brittle deformation as associated scheelite I. Undeformed scheelite II overgrows scheelite I and has lower light rare earth element and higher intermediate rare earth element contents as well as higher strontium concentrations. Scheelite II is associated with the economic gold mineralization and formed during re-crystallisation and re-precipitation of material which was partly re-mobilised from early scheelite I during infiltration of gold-bearing fluids. Early stage native gold inclusions are often associated with stage 2 sulphides, scheelite II and bismuth tellurides and contain Ag (3.6–24.4 wt.%), Hg (≤1.0 wt.%) and Bi (≤0.2 wt.%). Later stage electrum grains occur in association with stage 3 sulphides and sulphosalts and contain Hg (<0.8 wt.%) and elevated Sb concentrations (up to 3.0 wt.%). Noble gas isotope data (3He/4He: 0.2-0.4 Ra) for hydrothermal ore fluids trapped in the gold-related sulphides and sulphosalts (stage 2 pyrite and arsenopyrite; stage 3 pyrite, sphalerite, galena and lead sulphosalts) suggest that diverse fluid sources were involved in the formation of the Charmitan gold deposit. These data are indicative of a small, but significant input of fluids from external, deep-seated (mantle and possibly lower crust) sources. A decrease in the input of mantle helium and an increasing role of crustal helium from early to later stages of the mineralization is suggested by the measured 3He/4He and 40Ar*/4He ratios. Sulphides from ore veins in meta-sedimentary rocks contain higher portions of meteoric fluids than those in intrusive rock types as indicated by their lower 3He/36Ar ratios. The 3He/36Ar ratios in the meta-sedimentary rocks agree well with ratios typical of gold mineralizations in the Tien Shan gold province completely hosted by meta-sedimentary sequences, indicating intense fluid-wall rock interaction.

北海道日高町の濃鮮緑色角閃石斑状変晶を含む角閃岩転石の岩石学

A peculiar amphibolite rich in deep vivid green-colored amphibole porphyroblast was found as a boulder in the Chiroro River of Hidaka Town, Hokkaido, Japan. The amphibolite mainly consists of deep vivid green-colored amphibole (porphyroblast), colorless amphibole (matrix) and plagioclase with a small amount of chlorite, epidote and chromian spinel. Deep vivid green-colored amphibole porphyroblast is more than 20 modal % in the rock. Chromian spinel rarely occurs as inclusions in deep vivid green-colored amphibole porphyroblast. The Cr2O3 content of the deep vivid green-colored domain is usually ∼1 wt% (up to 2.5 wt% nearby chromian spinel) whereas that of colorless amphibole in the matrix is low, 10 wt%) green-colored domain, (2) relatively high-Cr2O3 (∼1 wt%) and low- Al2O3 (< 10 w%) green-colored domain and (3) low-Cr2O3 (< 0.1wt%) and Al2O3 (< 10 wt%) colorless domain. Chondrite-normalized rare earth element (REE) patterns of both deep vivid green-colored amphibole porphyroblast and colorless amphibole in the matrix are characterized by low light REE with positive Eu anomaly. The Cr#[= Cr/(Cr + Al) atomic ratio] and TiO2 content of chromian spinel in the amphibolite are similar to those in a serpentinite complex of the Kamuikotan Zone. The amphibolite was formed by either amphibolitization of a rock containing chromian spinel derived from the surrounding serpentinite complex or extensively metasomatized serpentinite due to interaction with surrounding metamorphic rocks.

Podiform chromitites in the lherzolite‐dominant mantle section of the Isabela ophiolite, the Philippines

Abstract The Isabela ophiolite, the Philippines, is characterized by a lherzolite‐dominant mantle section, which was probably formed beneath a slow‐spreading mid‐ocean ridge. Several podiform chromitites occur in the mantle section and grade into harzburgite to lherzolite. The chromitites show massive, nodular, layered and disseminated textures. Clinopyroxene (±orthopyroxene/amphibole) inclusions within chromian spinel (chromite hereafter) are commonly found in the massive‐type chromitites. Large chromitites are found in relatively depleted harzburgite hosts having high‐Cr♯ (Cr/(Cr + Al) atomic ratio = ∼0.5) chromite. Light rare earth element (LREE) contents of clinopyroxenes in harzburgites near the chromitites are higher than those in lherzolite with low‐Cr♯ chromite, whereas heavy REE (HREE) contents of clinopyroxenes are lower in harzburgite than in lherzolite. The harzburgite near the chromitites is not a residual peridotite after simple melt extraction from lherzolite but is formed by open‐system melting (partial melting associated with influx of primitive basaltic melt of deeper origin). Clinopyroxene inclusions within chromite in chromitites exhibit convex‐shaped REE patterns with low HREE and high LREE (+Sr) abundances compared to the host peridotites. The chromitites were formed from a hybridized melt enriched with Cr, Si and incompatible elements (Na, LREE, Sr and H2O). The melt was produced by mixing of secondary melts after melt–rock interaction and the primitive basaltic melts in large melt conduits, probably coupled with a zone‐refining effect. The Cr♯ of chromites in the chromitites ranges from 0.65 to 0.75 and is similar to those of arc‐related magmas. The upper mantle section of the Isabela ophiolite was initially formed beneath a slow‐spreading mid‐ocean ridge, later introduced by arc‐related magmatisms in response to a switch in tectonic setting during its obduction at a convergent margin.

Contrasting Group I and Group II Eclogite Xenolith Petrogenesis: Petrological, Trace Element and Isotopic Evidence from Eclogite, Garnet-Websterite and Alkremite Xenoliths in the Kaalvallei Kimberlite, South Africa

Eclogite xenolith varieties at Kaalvallei include accessory-free bimineralic xenoliths, diamond-bearing eclogite, corundum-bearing eclogite, garnet-websterite, alkremite and spinel-bearing eclogite. The xenoliths can be accurately classified into previously defined Group I and Group II varieties on both petrographic and geochemical principles. Kaalvallei Group I eclogites (including diamond-bearing eclogite) are considered to derive from a heterogeneous protolith. Eclogite genesis might have been by residue formation associated with the dehydration and partial melting of a protolith consisting of variably mixed, subducted ocean floor basalt and sediment. Kaalvallei Group II eclogite xenoliths are likely to have formed through crystallization of small-volume melts within conduits in old, enriched subcontinental lithosphere. Kaalvallei websterite xenoliths might be petrogenetically related to Group II xenoliths. Isotopic data for Kaalvallei corundum-bearing eclogite and alkremite xenoliths do not provide constraints on petrogenesis. Spinel-bearing eclogite xenoliths are ultradepleted in virtually all trace elements, with very low light rare earth element contents, relatively high heavy rare earth element concentrations, extreme 87Sr/86Sr (∼0·915) and extreme 143Nd/144Nd (0·517) isotopic compositions. These xenoliths are considered to be the residues of a partial melting event.

かんらん岩捕獲岩の記載岩石化学的性質：日本列島下のリソスフェリック・マントルの特徴

Recent developments in instrumentation for in situ trace element analysis of peridotite minerals give us valuable data set of petrological and geochemical insights of the upper mantle. Here, we summarize petrographical, petrological and geochemical characteristics of mantle xenoliths from the Northeastern and Southwestern Japan arcs. They have clear correlations between microtexture and mineral compositions in terms of both major and trace elements. That suggests that influx-assisted melt extraction occurs simultaneously with deformation/recrystalization in the upper mantle. Their characteristics of trace-elements in clinopyroxene are distinguished from those of abyssal peridotite and peridotite xenoliths from continental regions. The geochemistry of Japan arcs’ clinopyroxenes have characteristics of arc-type mantle source; i.e., low light rare earth element (LREE) and high field strength element (HFSE; Ti, Zr and so on), rather high heavy rare earth (HREE) relative to LREE elements and Sr concentrations. They also have a rather constant Ti/Zr ratio, variable REE patterns, and relatively low LREE/HREE ratios. The simple melt extraction model can not explain these characteristics. These features are due to metasomatism, which is different from carbonatite metasomatism.

Tracing Lithosphere Evolution through the Analysis of Heterogeneous G9-G10 Garnets in Peridotite Xenoliths, II: REE Chemistry

Following previous publication of major---minor element data, this paper presents rare earth element (REE) data for heterogeneous (chemically zoned) garnets belonging to the peridotite suite of mantle xenoliths from the Jagersfontein kimberlite pipe, South Africa. The rim compositions of the garnets in the highest temperature---pressure (deepest) deformed peridotites show a typical megacryst-like pattern, of very low light REE (LREE) increasing through the middle REE (MREE) to a plateau of heavy REE (HREE) at c. 20 times chondrite; these compositions would be in equilibrium with small-volume melts of the mid-ocean ridge basalt (MORB) source (asthenosphere). With decreasing depth the garnet rims show increasing LREE and decreasing HREE, eventually resulting in humped relative abundance patterns. A set of compositions is calculated for melts that would be in equilibrium with the garnet rims at different depths. These show decreasing relative abundance of each REE from La to Lu, and the La/Lu ratio of the melts increases with decreasing depth of formation. Modelling of the effects of crystal fractionation shows that this process could largely generate the sequence of garnet rim and melt compositions found with decreasing depth, including the humped REE patterns in high-level garnets. Considering the behaviour of major---minor elements as well as REE, a process of percolative fractional crystallization is advocated in which megacryst source melts percolate upwards through peridotites and undergo fractionation in conjunction with exchange with the peridotite minerals. The initial megacryst melt probably includes melt of lithospheric origin as well as melt from the MORB source, and it is suggested that the process of percolative fractional crystallization may form a variety of metasomatic and kimberlitic melts from initial megacryst melts. Repeated metasomatism of the lower lithosphere by such differentiating melts is suggested by consideration of garnet core compositions. Such metasomatism would progressively convert harzburgites to lherzolites by increasing their CaO content, and this may account for the fact that the Cr-rich diamond---garnet harzburgite paragenesis is commonly preserved only where it has been encapsulated in diamonds.