LREE Depletion Research Articles

Chemical composition of garnet from the Xintianling skarn W deposit in southern Hunan and its geological significance

新田岭矿床是南岭钨锡成矿带中的一个大型矽卡岩型钨矿床，产于骑田岭岩体东北部与石炭系碳酸盐地层的接触带位置。本文运用LA-ICP-MS技术对该矿床矽卡岩中的石榴子石进行了系统的成分分析，获得了其主量、微量和稀土元素含量。结果显示，新田岭矿床中的石榴子石属于钙铁榴石-钙铝榴石固溶体系列（And₂₄Gro₆₆-And₇₁Gro₂₂），石榴子石的端元成分在富钙铝榴石和富钙铁榴石之间变化。稀土元素的配分模式也同时出现了左倾、Eu负异常和右倾、Eu正异常两种类型，暗示新田岭矿床石榴子石结晶过程中热液流体存在不同的氧化还原环境和水/岩比条件，这也与其晶体中是否出现振荡环带相对应。将不同矽卡岩型矿床中石榴子石的W、Sn含量进行对比显示，含W矿化的矽卡岩型矿床中石榴子石的W、Sn含量整体上显著高于不含W矿化的矿床，指示石榴子石中的W、Sn含量在一定程度上具有预测矽卡岩型矿床成W矿潜力的作用。此外，石榴子石中Fe、Eu、U等元素的含量还可以进一步区分矽卡岩W矿床中的伴生金属元素类型（包括W-Mo、W-Sn、W-Cu-Fe和W-Mo-Cu-Fe等）。本文研究表明，石榴子石的成分特征不仅可以指示矽卡岩的成矿环境，还可用于评估矽卡岩中金属（尤其是W）的成矿潜力，具有一定的理论意义和应用价值。;Xintianling is a large skarn W deposit in the Nanling W-Sn metallogenic belt, and it is located in the contact zone between the northeastern Qitianling pluton and the Carboniferous carbonate rocks. In this study, the typical skarn mineral, garnet, has been analyzed by LA-ICP-MS to quantify the concentrations of major elements, trace elements, as well as rare earth elements (REEs). The results reveal that the garnet from the Xintianling deposit mainly belongs to the andradite-grossular solid solution (And₂₄Gro₆₆ to And₇₁Gro₂₂), which shows a transitional trend dominated by either andradite or grossular. The REEs of the garnet show two distinct patterns, one is characterized by HREE enrichment and LREE depletion with negative Eu anomaly, and the other is featured with LREE enrichment and HREE depletion with positive Eu anomaly. These differences suggest that the garnet crystals grow under variable redox conditions and water/rock ratios in the hydrothermal system, which has also been reflected in the oscillatory zones. A compilation of available data from other skarn deposits combined with the results from this study indicates that W and Sn concentrations of garnet from skarn deposits with economic W mineralization are significantly higher than those without W mineralization, implying that the W and Sn concentrations could probably predict the potential economic metals (especially W) in a skarn system. Furthermore, the results also indicate that the Fe, Eu and U concentrations of garnet can provide useful information for distinguishing the associated metal(s) of a W skarn system, including W-Mo, W-Sn, W-Cu-(Fe) and W-Mo-Cu-(Fe). In conclusion, the current study shows that the compositional characteristics of garnet have certain guiding significance in understanding the ore-formation conditions and evaluating mineralization potentials of metals in a skarn system, especially for these W skarns like Xintianling.

Genesis and tectonic setting of Late Jurassic-Early Cretaceous granites in Nachatang area, Central Lhasa Terrane: Constraints from geochemistry, chronology and Hf isotopes

西藏中拉萨地块东段大规模侏罗纪-白垩纪花岗岩类的成因类型及构造背景尚未得到有效约束，该时期岩浆作用的时空分布、岩石成因以及深部动力学机制等问题亟需新的深入研究。本文对中拉萨地块东段南缘那茶淌地区的花岗岩类进行了系统的岩相学、元素地球化学、年代学和锆石Hf同位素研究。LA-MC-ICPMS锆石U-Pb定年结果显示，那茶淌地区黑云母二长花岗岩成岩年龄为147±1.4Ma，花岗闪长岩年龄为140.6±1.3Ma，系晚侏罗世-早白垩世中酸性岩浆活动的产物。在元素地球化学组成上，黑云母二长花岗岩和花岗闪长岩的主量元素组成具有富SiO₂（分别为71.02%~71.81%和65.17%~66.73%）、Al₂O₃（分别为13.45%~13.57%和14.43%~15.20%）、碱金属（Na₂O+K₂O）（分别为6.79%~7.48%和6.55%~7.37%），贫TiO₂（分别为0.15%~0.21%和0.10%~0.13%）等特征，显示Ⅰ型准铝-弱过铝质（A/CNK=0.98~1.14）的高钾钙碱性系列特征。微量元素以富集K、Rb、Ba、Th等大离子亲石元素，亏损Ta、Nb、P、Ti等高场强元素为特征。花岗闪长岩稀土元素总量（ΣREE=263.9×10^-6~313.8×10^-6）高于黑云母二长花岗岩（ΣREE=116.3×10^-6~168.8×10^-6），均表现为轻稀土元素富集，重稀土元素相对亏损，具弱的负Eu异常。两种岩石锆石Hf同位素均以负的ε_Hf（t）值和古老的地壳模式年龄为特征，黑云母二长花岗岩ε_Hf（t）值为-21.64~-7.66，地壳模式年龄（t_DM^C）为1.68~2.56Ga，花岗闪长岩锆石ε_Hf（t）值为-11.95~-8.15，地壳模式年龄（t_DM^C）为1.71~1.95Ga。结合前人研究资料，认为中拉萨地块东段那茶淌地区晚侏罗世-早白垩世花岗岩类形成的构造背景与班公湖-怒江洋南向俯冲有关，岩浆来源于俯冲作用诱发古老下地壳物质的重熔。;The genetic types and tectonic settings of the large-scale Jurassic-Cretaceous granites in the eastern part of the Central Lhasa Terrane in Tibet have not been well constrained, and the temporal and spatial distribution of their magmatism, petrogenesis and deep dynamic mechanism in this period are in urgent need of further research. In this paper, the lithography, chronology, geochemistry and zircon Hf isotope of the biotite monzogranite and granodiorite from the Nachatang area in the southern margin of the eastern segment of the Central Lhasa Terrane are systematically studied. LA-MC-ICPMS zircon U-Pb dating results show that the biotite monzogranite and granodiorite are 147±1.4Ma and 140.6±1.3Ma, respectively, showing they are the products of magmatism in Late Jurassic-Early Cretaceous. In terms of geochemical compositions, the biotite monzogranite and granodiorite have low TiO₂(0.15%~0.21% and 0.10%~0.13%), high SiO₂ (71.02%~71.81% and 65.17%~66.73%), Al₂O₃ (13.45%~13.57% and 14.43%~15.20%) and alkali (Na₂O+K₂O) (6.79%~7.48% and 6.55%~7.37%) contents, showing the characteristics of Ⅰ-type peraluminum to weak peraluminum calc-alkaline series (A/CNK=0.98~1.14). Trace element compositions of the measured samples are characterized by enrichment of large ion lithophile elements such as K, Rb, Ba and Th, and depletion of high field strength elements such as Ta, Nb, P and Ti. The total amount of REE in the granodiorite (ΣREE=263.9×10^-6~313.8×10^-6) is higher than that in the biotite monzonite (Σ REE=116.3×10^-6~168.8×10^-6), which shows the enrichment of LREE and relative depletion of HREE, with weak negative Eu anomaly. The zircon Hf isotopes are characterized by negative ε_Hf(t) values and ancient crustal model ages. The ε_Hf(t) values of the biotite monzogranite range from -21.64 to -7.66, and the crustal model ages (t_DM^C) range from 1.68Ga to 2.56Ga. Zircon ε_Hf(t) values of the granodiorite vary from -11.95 to -8.15, and the crustal model ages (t_DM^C) from 1.71Ga to 1.95Ga. Finally, we conclude that the tectonic setting of Late Jurassic to Early Cretaceous granites in the Nachatang area is related to the southward subduction of the Bangong-Nujiang Ocean seafloor, and the magma is derived from the remelting of ancient lower crust material induced by subduction.

Late Cretaceous to Paleocene magmatic record of the transition between collision and subduction in the Western and Central Cordillera of northern Colombia

Deformational field relations, compositional and U–Pb geochronological data of magmatic rocks of the northwestern Colombian Andes provide insights to recognize the different pre to post-collisional stages in an arc-arc collisional scenario until the re-initiation of a new subduction zone. A western belt of undeformed to mylonitized volcanic rocks with flat REE patterns and a weak LREE depletion present similar geochemical features of adjacent oceanic plateau basalts that were intruded by arc-related dykes at ca. 82 Ma, before their accretion to the continental margin. A sequence of greenschist and quartz + muscovite + graphite schist, and their associated mylonitized dikes, are part of the eastern continental arc, and present ages between 69 and 71 Ma, show tholeiitic affinity, flat rare-earth patterns, and Nb–Ti anomalies. These characteristics mark a decrease in the mantle wedge and older crustal signatures during magma genesis, as the oceanic-continent collision was advancing. Both suites were metamorphosed in a high geothermal gradient with temperatures between 490 and 400 °C and pressures between 2 and 4 Kbar. After this metamorphic event associated to the end of the collision a series of dykes intrude the different units at ca. 58 Ma, including basalts and andesites with Nb–Ti anomalies, adakite-like, and high niobium compositional signatures, suggesting that they were probably formed associated with slab melting in a young subduction environment that evolved after 62 Ma, following a ≤7 Ma magmatic hiatus. This magmatic and metamorphic record can be related to an arc-arc collision scenario in which mantle involvement in the late stages of collision influence both the magmatic and thermal regimes.

Petrology and geochemistry of the Archean Huangling greenstone belt in the Yangtze Craton, South China

The Archean Huangling greenstone belt (HGB) is preserved as amphibolites in the Huangling granite–greenstone terrane, Yangtze Craton. Geologically and geochemically, the amphibolites are metavolcanic rocks and can be subdivided into amphibole schist, massive amphibolite and banded amphibolite, with komatiite, komatiitic basalt and tholeiitic basalt protoliths, respectively. The komatiites have high MgO (20–30%), Cr, Co and Ni contents; low Rb, Sr, Ba and ΣREE contents; and slight LREE depletion and HREE enrichment, characterizing them as the Al-undepleted type according to the criteria (CaO/Al2O3)adj ˃ 20, (Al 2O3/TiO2)adj and (Gd/Yb)PM < 1. The komatiitic basalts and tholeiitic basalts have lower MgO (5–16%), Cr, Co and Ni contents; higher Rb, Sr, Ba and ΣREE contents; and slight LREE enrichment and HREE depletion. The geochemical plots (Zr/Y vs. Nb/Y, Nb/Yb vs. Th/Yb, La/Ta vs. La/Sm and Y vs. Zr/Y), negative Nb anomalies, wide ƐNd values (−8.0 to +6.2), negative zircon ƐHf values (−11.59 to −3.98), and old TNd (DM) (3.9–2.9 Ga) and zircon THf (DM) ages (3.9–3.3 Ga) of the amphibolites reflect a mantle source with slight crustal contamination and show that the komatiitic basalts and tholeiitic basalts are more closely related to each other than to the komatiites. The tholeiitic basalts originated from moderate-degree melting of a mantle plume head, the komatiitic basalts originated from the fractional crystallization of tholeiitic basalt melts as cumulates, and the komatiites originated from high-degree melting of the mantle plume tail with garnet entering the melt phase. These high-Mg lavas erupted in a continental rift when the mantle plume impinged on the continental lithosphere, generating rifting and subsequent volcanic eruptions. These ultramafic–mafic rocks (3.2–3.0 Ga) formed the HGB, which was subsequently intruded by TTG (3.0–2.9 Ga), together forming the Archean Huangling granite–greenstone terrane in the Yangtze region.

Identification of ~1.3 Ga hydrothermal zircon from the giant Bayan Obo REE deposit (China): Implication for dating geologically-complicated REE ore system

The Bayan Obo deposit in China hosts the largest REE resource in the world. However, the genesis of this deposit is still highly debated, largely due to a lack of consensus on the mineralization age. The Banyan Obo deposit has experienced intensive post-ore metamorphic and metasomatic overprints, so it is crucial to choose robust chronometers to determine the primary mineralization age. In this study, zircon is identified in the REE ore of the Bayan Obo deposit. The zircon grains are in contact with or contain inclusions of hydrothermal minerals (e.g., magnetite, biotite, monazite, allanite, apatite and ilmenorutile). They have variable Th contents (14 to 1139 ppm) and are poor in U (<0.8 ppm), consistent with the Th-rich and U-poor feature of the Bayan Obo REE ore. Moreover, they have substantially low contents of light REE (La to Nd; LREE), which may be due to zircon coprecipitation with LREE-rich monazite and allanite. The combined textural and compositional features imply a hydrothermal origin of the zircon grains. The hydrothermal zircon grains have a weighted mean 208Pb-232Th age of 1297 ± 13 Ma, which places a direct constraint on the timing of REE mineralization at Bayan Obo. Notably, substantial LREE depletion and Th/U fractionation have not been documented before for hydrothermal zircon from other environments, and thus they could be taken as discriminative features for hydrothermal zircon from carbonatite-related REE deposits.Thorium-lead dating was also conducted on monazite from the zircon-bearing sample, yielding 208Pb-232Th ages (993 to 319 Ma) variably younger than that of zircon. The younger monazite Th-Pb ages could have resulted from disturbance of the Th-Pb isotopic system during post-ore modification process. Similarly, monazite has younger Th-U-Pb dates than zircon in many other geologically-complicated carbonatites or REE deposits. Such a phenomenon implies that the zircon Th-U-Pb chronometer is robust even if the host-rocks have experienced intensive modification, whereas monazite Th-U-Pb chronometer is more susceptible to modification. Therefore, younger Th-U-Pb dates of monazite should be interpreted with caution as they may record the timing of post-ore modification event or represent partial resetting, and hence geologically meaningless results.

Geochemistry and Tectonic History of Seamount Remnants in the Xingshuwa Subduction Accretionary Complex of the Xar Moron Area, Eastern Margin of the Central Asian Orogenic Belt

AbstractThis study focuses on the geology, geochemistry, zircon U‐Pb geochronology and tectonic settings of the three types of seamount basalts from the Xingshuwa subduction accretionary complex in the Xar Moron area, eastern margin of the Central Asian Orogenic Belt (CAOB). The seamount remnants are composed of carbonate ‘cap’ sediments, large volumes of pillow and massive basalts, carbonate breccia slope facies and radiolarian cherts. Group 1 basalts are characterized by high contents of P2O5 and TiO2 with alkaline affinity and LREE enrichment, indicating that they are derived from intraplate magma. Group 2 basalts display N‐MORB LREE depletion patterns, indicating that they were formed at a mid‐ocean ridge. Group 3 basalts have shown distinct Nb depletion and high Th/Yb ratios, indicating that they were generated in an island arc tectonic setting. The zircon U‐Pb age of Group 1 basalt sample XWT18‐131 is 576.4 ± 9.4 Ma, suggesting that the oceanic island seamount was the product of intraplate magmatism related to a mantle plume or ‘hot spot’ in the late Neoproterozoic. The zircon U‐Pb age of Group 2 basalt sample XWT18‐132 is 483 ± 22 Ma, indicating that the Paleo‐Asian Ocean (PAO) was continuously expanding in the Early Ordovician. The zircon U‐Pb age of Group 3 basalt sample XWT18‐101 is 240.5 ± 8.2 Ma, suggesting that this area underwent the evolutionary path of ocean‐continent transition, developing towards continentalization during the Middle Triassic. Thus, we believe that there was both mantle plume‐related intraplate magmatism and intraoceanic subduction during the evolution of the PAO, the CAOB possibly being an evolutionary model of an intraoceanic subduction and mantle plume magmatism complex.

Zircon U–Pb Dating and Lu-Hf Isotope of the Retrograded Eclogite from Chicheng, Northern Hebei Province, China

We report zircon U–Pb ages and Lu-Hf isotopic data from two sample of the retrograded eclogite in the Chicheng area. Two groups of the metamorphic zircons from the Chicheng retrograded eclogite were identified: group one shows characteristics of depletion in LREE and flat in HREE curves and exhibit no significant Eu anomaly, and this may imply that they may form under eclogite facies metamorphic condition; group two is rich in HREE and shows slight negative Eu anomaly indicated that they may form under amphibolite facies metamorphic condition. Zircon Lu-Hf isotopic of εHf from the Chicheng eclogite has larger span range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may be mixed with partial crustal components. The peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5–344.2 Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0 Ma. The Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494–2448 Ma), Late Paleoproterozoic (1900–1734 Ma, peak age = 1824.6 Ma), and Phanerozoic (495–234 Ma, peak age = 323.7 Ma). Thus, the metamorphic event (348.5–325 Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. The eclogite facies metamorphic age of the eclogite is in accordance with the metamorphism (granulite facies or amphibolite facies) of its surrounding rocks, which implied that the tectonic subduction and exhumation of the retrograded eclogite may cause the regional metamorphism of garnet biotite plagioclase gneiss.

Depositional Environment and Genesis of the Nabeba Banded Iron Formation (BIF) in the Ivindo Basement Complex, Republic of the Congo: Perspective from Whole-Rock and Magnetite Geochemistry

The Nabeba high-grade iron deposit (Republic of the Congo) is hosted by banded iron formation (BIF) in the Ivindo Basement Complex, which lies in the northwestern part of the Congo Craton. The Nabeba BIF is intercalated with chlorite-sericite-quartz schist and comprises two facies (oxide and a carbonate-oxide). In this study, whole-rock and LA-ICP-MS magnetite geochemistry of the BIF was reported. Magnetite samples from both BIF facies had fairly similar trace element compositions except for the rare earth element plus yttrium (REE + Y) distribution patterns. The high V, Ni, Cr, and Mg contents of the magnetite in the Nabeba BIF could be ascribed to the involvement of external medium-high temperature hydrothermal fluids during their deposition in relatively reduced environment. The Post-Archean Australian Shale (PAAS)-normalized REY patterns of the Nabeba BIF magnetite were characterized by LREE depletion coupled with varying La and positive Eu anomalies. Processing of the information gathered from the geochemical signatures of magnetite and the whole-rock BIF suggested that the Nabeba BIF was formed by the mixing of predominantly anoxic seawater (99.9%) with 0.1% of high-temperature (&gt;250 °C) hydrothermal vent fluids, similar to the formation mechanism of many Archean Algoma-type BIFs reported elsewhere in the world.

Geochronology, geochemistry, and isotope compositions of “Grenvillian” S-type granites in the North Qinling unit, central China: Petrogenesis and tectonic significance

An integrated study of whole-rock elements, Sr–Nd isotopes, and zircon U–Pb ages and Hf-O isotopes was carried out for the “Grenvillian” granites from the North Qinling unit, central China. The results not only place clear constraints on the petrogenesis of these granites, but also give insights into the geodynamic evolution of the Qinling orogen during the assemblage of supercontinent Rodinia. LA-ICPMS zircon U–Pb dating of three representative granites from the Dehe, Fangzhuang, and Zhaigen outcrops yielded formation ages of ca. 940–900 Ma. These muscovite-bearing granites have high silica contents (72–79 wt%) and are weak to strong peraluminous (ASI = 1.01–1.15). They exhibit arc-like trace element distribution patterns with enrichment of LREE and LILE (e.g., Rb, K, Pb) but depletion in HFSE (e.g., Nb, Ta, Ti). Besides, they show variable initial 87Sr/86Sr ratios of 0.704 to 0.729 and relatively low εNd(t) values of −3.2 to −6.1, as well as variable zircon εHf(t) and δ18O values (−8.1–+2.6, 7.9‰–12.0‰). These results suggest their derivation from partial melting of a heterogeneous source that dominated by supracrustal components. Furthermore, a few inherited grains give negative εHf(t) values of −9.4 to −18.4 with two-stage Hf model ages up to 2.9 Ga, suggesting the involvement of minor Archean crustal components. Along with their peraluminous features, high zircon δ18O values, and enriched Sr–Nd isotope compositions, these granites were unambiguously derived from partial melting of metasedimentary rocks and thus are of S-type affinity. Geothermometric and compositional approaches suggest these S-type granites were likely generated by dehydration melting of metagraywacke- and/or metapelite-dominated sources. They might be formed in an accretionary orogen associated with the Rodinia-assembly during the early Neoproterozoic, through which the outboard North Qinling terrane was eventually adjacent to the South China Block.

In situ LA ICP-MS analysis of trace elements in scheelite from the Xuefeng Uplift Belt, South China and its metallogenic implications

Abstract The Xuefeng Uplift Belt (XUB), in the western part of the Jiannan Orogenic Belt (JOB) South China, hosts numerous low-temperature W-Sb-Au deposits, including the Woxi W-Sb-Au deposit, the Zhazixi Sb-W deposit and the Darongxi W deposit. In order to understand the origin of these low-temperature W deposits and the difference from other moderate-high temperature tungsten deposits in the South China Block, Cathodoluminescene (CL) and LA-ICP-MS are employed to investigate the detailed texture of scheelite , and to identify trace element composition and distribution in scheelite, especially rare earth elements (REEs). LA-ICP-MS results show that the most abundant trace elements in scheelite in this study were Sr, Y, Nb, Mo, Zr, Hf, Ta and REEs. Six types of scheelite can be classified according to variations of REEs pattern. Type I and II scheelites from the Zhazixi Sb-W deposit and the Woxi W-Sb-Au deposit have elevated concentration of Sr, low concentrations of Nb, Mo and variable Nb/Ta ratio, reflecting low temperature ( skarn deposit, Type IV scheelite from the Dabaoshan polymetallic deposit, Type V scheelite from the Niutangjie skarn W deposit and Type VI scheelite from the Daping W deposit have relatively higher Nb, Mo and relatively constant Nb/Ta ratios, and lower Sr concentration, consistent with the feature of moderate to high temperature oxidizing environments. Type I scheelite from the Zhazixi Sb-W deposit can be subdivided into Ia, Ib and Ic, which cover a wide range from MREE-rich to MREE-depleted, indicating a complete dynamic evolution process. Type II scheelite from the Woxi W-Sb-Au deposit has uniform patterns with significant depletion of LREE and enrichment of MREE, revealing that the substitutions of MREE into scheelite lattices are dominated by crystallographic factors, especially the Ca-site size in scheelite crystals rather than the REE concentrations and REE complex stability in hydrothermal fluids . Type III scheelite from the Darongxi W deposit shows two distinctive REE patterns, types IIIa and IIIb, which are likely to be two end members, suggesting that successive precipitation results in changes in REE concentrations and Eu anomalies. Type IV scheelite from the Dabaoshan polymetallic deposit inherited the signature of the granitic porphyry . Type IV b and IVc are similar to those Type V scheelites from Niutangjie, both showing the characteristics of skarn type scheelite mineralization. Trace element variations suggest the predominant control factors differ according to temperature. Trace element concentrations and distribution of scheelite formed at low temperature were mainly controlled by crystallographic factors, which contrasted with fluid-controlled feature of the medium–high temperature scheelite mineralization.

Geochemical Appraisal on History and Evolution of Barail Sandstones of Zote-Ngur, Champhai District, Mizoram, India

Mizoram is part of Surma basin which later evolved into the present state of geological terrain due to Indo- Myanmar tectonic collision during the Oligocene period. The present work deals with geochemical characteristics of Barail sandstones exposed in Champhai area of eastern region in Mizoram. The major/minor oxides, trace elements and rare earth elements data are used to infer the geological history and evolution of the sandstone in the study area of Champhai. The petrographic study shows the presence of various detrital grains like quartz, lithic fragments, feldspar, chertz, mica, etc., which are cemented by siliceous and ferruginous materials. Geochemically, the sandstones indicate high wt% of SiO2, Al2O3 and MgO compared to Upper Continental Crust (UCC) while rest of the major oxides indicate low concentrations. The geochemical classification indicated the sandstones as litharenite and wacke. The chondrite normalised REE pattern shows the enrichment of HREE and depletion of LREE with negative Eu anomaly. The value of Eu/Eu*, La/Lu, La/Co, Th/Sc, Th/Co, Cr/Th and high ratio of LREE/HREE of Barail sandstone suggest felsic source rock. The analysis of paleoweathering history indicated moderate to intensive weathering in the provenance. Various tectonic discriminant function diagrams suggested Active Continental Margin settings.

Constraints on Archean crust recycling and the origin of mantle redox variability from the δ44/40Ca – δ18O – fO2 signatures of cratonic eclogites

The nature of the deep calcium geochemical cycle through time is unresolved, in part due to the dearth of information about the calcium isotope composition of Archean recycled oceanic crust. Remnants of such ancient oceanic crust are preserved in the form of cratonic mantle eclogites, brought to surface as xenoliths in kimberlite magma eruptions. The δ44/40Ca of fresh mantle-derived eclogite xenoliths (i.e., garnet and omphacite mineral separates) from the Bellsbank kimberlite on the Kaapvaal craton in South Africa are presented here in combination with their trace element compositions, garnet Fe3+ contents and δ18O values.The studied Bellsbank eclogite xenoliths have geochemical compositions that indicate oceanic crustal protoliths, with bulk Al2O3 from 15 to 27 wt.%, Eu anomalies from 0.8 to 2.6 and, significantly, garnet δ18O values from +2.7 to +6.2‰. Garnet Fe3+/ΣFe contents yield logfO2(ΔFMQ) values between −4.0 and −1.2 for a depth range of 110–180 km, recording strong redox heterogeneity of the eclogite component within the Archean Kaapvaal mantle lithosphere. Reconstructed bulk eclogite MgO contents correlate negatively with fO2, suggesting that the redox compositions are related to magmatic differentiation during oceanic crust formation, excluding secondary metasomatic overprints. These data may thus emphasize that Archean basaltic oceanic crust had a similarly variable redox composition to modern MORB-type crust.Reconstructed bulk δ44/40Ca values for the Bellsbank eclogites range from +0.28 to +1.56‰. Although some of the xenoliths have δ44/40Ca values that overlap with the average mantle composition and modern MORB (+0.94 ± 0.1 and +0.83 ± 0.05‰), half of our dataset shows excursions to more extreme Ca isotopic compositions. Both higher and lower δ44/40Ca relative to mantle compositions are recorded by the eclogites, with a general negative correlation with δ18O suggestive of seawater-alteration of oceanic crust. The combined low δ44/40Ca (+0.28‰) and δ18O (+3.4‰) measured for one eclogite xenolith may record a subtle imprint by carbonate-rich mantle melts, which are known to contain isotopically light calcium contributed by recycled sediments. In contrast, the high δ44/40Ca of up to +1.56‰ for some eclogite xenoliths, coupled with strong LREE depletion, can be explained by calcium isotope fractionation during partial melting.The protracted history of recycled oceanic crust as probed by cratonic mantle eclogites is recorded by their highly variable δ44/40Ca–δ18O–fO2 signatures. Whereas some of this heterogeneity can be linked to processes that operated on the Archean ocean floor such as seawater-alteration of basaltic crust, other sources of compositional variability are introduced by loss and addition of melts during subduction recycling and mantle residence. The observed δ44/40Ca complexity of ancient recycled oceanic crust components at the scale of a single mantle-derived eclogite xenolith suite implies that mantle plume sourced intraplate magmas should reveal similarly strong calcium isotope variations contributed by apparently essential recycled crust components – as observed in the global oceanic island basalt record.

Source of dolomitizing fluids and dolomitization model of the upper Permian Changxing and Lower Triassic Feixianguan formations, NE Sichuan Basin, China

Dolostone reservoirs have been found in the Changxing (P2c) and Feixianguan (T1f) formations in both the east and west platform edges of the Kaijiang-Liangping trough; they are the main natural gas exploration targets in the NE Sichuan Basin. In this study, the sources of the dolomitizing fluids were determined, and a dolomitization model was established based on petrography, rare earth elements (REEs), major and trace element data, and 87Sr/86Sr ratios. The dolostone and limestone samples with ΣREE values of >5 ppm exhibit relatively flat PAAS-normalized REE patterns; their Zr, Sc, and Th concentrations increase with increasing ΣREE; and their Rb/Sr ratios are generally greater than 0.01, suggesting that these samples were likely contaminated by terrigenous components. Therefore, only the REEs and 87Sr/86Sr ratios of samples with ΣREE values of <5 ppm were selected to constrain the dolomitizing fluids. The PAAS-normalized REE patterns of the P2c–T1f dolostones, which are characterized by LREE depletion, negative Ce anomalies (Ce/Ce* of 0.70–0.85), and relatively high Y/Ho ratios (49.0–152.4), are similar to those of the unaltered coeval limestones, suggesting that the dolomitizing fluid was sourced from fluids similar to coeval seawater. The Ce/Ce* values of the dolostones are lower than those of the unaltered limestone, suggesting that the source of the dolomitizing fluids had more oxidizing conditions and most probably contained evaporated seawater from an evaporite platform. The Eu/Eu* values of the P2c dolostone in the west platform (means of 4.95 and 2.76 for the YB and XLC gas fields, respectively) are notably higher than the Eu/Eu* values of the P2c and T1f dolostones in the east platform (mean 1.29 and 1.39 in the PG gas field, respectively), suggesting that the P2c dolostone in the west platform was formed under more reducing conditions and at a deeper burial depth. The 87Sr/86Sr ratios of the T1f and P2c dolostones in the east platform (such as in the Puguang gas field) are similar to those of the co-existing early T1f limestone, while the 87Sr/86Sr ratios of the P2c dolostone in the west platform (such as in the Yuanba gas field) are higher than those of the coexisting P2c limestone but are close to those of the late T1f dolostone in the east platform. This indicates that the dolomitizing fluids were mainly from the early T1f and late T1f evaporated seawater on the east and west platforms, respectively. Driven by the density difference between the evaporated seawater and normal seawater, the evaporated seawater flowed downward and laterally into the limestone in the high-energy reefs and shoals through the original pores and the early selective dissolution pore system and/or through synsedimentary fractures. The continual seepage of evaporated seawater and the relatively high temperatures (50–60 °C) resulted in the large-scale shallow burial dolomitization in the NE Sichuan Basin.

Geochemistry of the Neoproterozoic Mbondo-Ngazi Tina Metasediments, Adamawa Area, Central Cameroon: Source Provenance and Tectonic Setting

The Mbondo-Ngazi Tina area belongs to the Adamawa-Yade domain within the Pan-African Central Africa Fold Belt in Cameroon (CAFB). The basement of this area is dominated by metasedimentary rocks composed of sericite schist, chlorite schist and muscovite schist. Whole-rock geochemical compositions of these rocks were investigated in order to determine their provenance and tectonic setting. The studied metasedimentary rocks have SiO2 and Al2O3 contents comparable to the average composition of the Neoproterozoic upper continental crust (UCC). These rocks are strongly depleted in CaO, MgO, and enriched in K2O, Ba and Rb with respect to UCC, reflecting K addition during diagenesis. The CIA, CIW, PIA and the SiO2/Al2O3 and Th/U ratios indicated that these rocks had suffered varying degrees of weathering as the source rocks underwent mild to moderate chemical weathering. The PAAS-normalized REE patterns are almost flat with slightly LREE depletion with respect to HREE and null to weakly positive Eu anomalies. Their chondrite-normalized REE patterns are parallel to sub-parallel, LREE-enriched, and display distinct negative Eu anomalies and weakly fractionated HREE segments. Overall, they are geochemically mature and have suffered sedimentary recycling. They derived mainy from felsic to intermediate rocks with minor contamination of mafic rocks. The Mbondo-Ngazi Tina metasedimentary rocks show REE and trace element compositions similar to those of Archean sediments, suggesting that the continental crust of the study area during the early Proterozoic had chemical compositions similar to those of the Archean crust and were probably deposited in active to passive continental margin settings.

Elemental and isotopic geochemistry of carbonate rocks from the Pila Spi Formation (Middle–Late Eocene), Kurdistan Region, Northern Iraq: implication for depositional environment

Major, trace, rare earth elements (REE), and stable isotopic compositions in carbonate rocks of Pila Spi Formation were measured to constrain the source of rare earth elements and depositional environment. The low ΣREE content (average = 6.53 ppm, n = 14) indicates that the carbonates seawater-like REE patterns with (1) LREE depletion (average (Nd/Yb)n = 0.81), (2) HREE enrichment, (3) negative Ce anomaly (average = 0.91), and (4) superchondritic Y/Ho ratio (average = 30.07); slightly lower average value of Y/Ho ratio than the typical seawater value (~ 44–74) suggests modification of the seawater by input of fresh water. The (Nd/Yb)n and (Dy/Yb)n ratios of the Pila Spi carbonates have similarity with that of the Arabian sea carbonate. The low concentrations of U (0.35–1.0 ppm) suggest the deposition under oxygen-rich environment. This study indicates that the carbonates still preserve their original seawater-like REE pattern, provided that the contamination with terrigenous materials was small, and they serve as a seawater proxy. The Pila Spi carbonates have δ13C values ranged from − 6.17 to 0.76‰ PDB, and the δ18O values range between − 3.85 and 0.46‰ PDB. The positive correlation between δ13C and δ18O (r = 0.949, n = 14) values indicates the alteration of original isotopic compositions due to diagenesis. Finally, it is conceived that the Pila Spi carbonate was deposited in the lagoon environment with seawater invading and mixing of the continental (terrigenous) materials to the basin is viable.

Geochemistry of high-pressure to ultrahigh-pressure granitic melts produced by decompressional melting of deeply subducted continental crust in the Sulu orogen, east-central China

The production of granitic melts by decompressional melting of deeply subducted crustal rocks under high-pressure (HP) to ultrahigh-pressure (UHP) conditions is important for the crust-mantle interaction at convergent plate boundaries. However, little is known on the geochemical composition of natural HP to UHP granitic melts from collisional orogens. This problem is partially resolved here through an integrated study of the Yangkou granitic veins in the Sulu orogen, which provide the excellent records of HP to UHP granitic melts from the deeply subducted continental crust. In trace element composition, these granitic veins show enrichment in LREE and LILE but depletion in HFSE with remarkable negative anomalies of Nb, Ta, P and Ti. They have low Sr/Y (5.92–26.3) and (La/Yb)N ratios (12.7–45.6), which are primarily controlled by the solubility of accessory minerals such as apatite and epidote/allanite in anatectic melts. Phengite in the granitic veins have high Si (3.29–3.42 p.f.u.) and Mg (0.16–0.35 p.f.u.) with Mg# of 37.7–68.1 and calculated pressures of 1.7–3.1 GPa, indicating their crystallization under HP to UHP conditions. Geochemical variables such as K2O/Na2O, A/CNK and LREE for whole-rock exhibit a positive correlation with pressure, indicating the significant pressure effect on the mineral stability that in turn controls the composition of anatectic melts. Furthermore, the granitic veins can be divided into two groups based on their formation pressures and geochemical compositions. Group I granitic veins formed at low pressures of 1.7–2.4 GPa and exhibit higher SiO2 and Na2O contents, but lower contents of K2O, MgO, TiO2, P2O5 and most trace elements (e.g., Rb, Ba, LREE) with lower K2O/Na2O ratios and A/CNK values compared to Group II granitic veins that formed at high pressure of 2.6–3.1 GPa. Zircon from the granitic veins give concordant U–Pb ages of 755 ± 10 to 769 ± 9 Ma with steep HREE patterns for relict magmatic cores and 216 ± 4 to 222 ± 4 Ma with steep to flattened HREE patterns for anatectic rims. In comparison to concordant U–Pb ages of 220 ± 3 to 227 ± 3 Ma for newly grown zircon in the host eclogites, it appears that these granitic veins were crystallized from anatectic melts produced by crustal anataxis during the exhumation of the deeply subducted continental crust. The anatectic rims are characterized by the absence of not only oscillatory zoning in CL images but also Eu anomalies and low U contents, indicating their growth from anatectic melts under HP to UHP conditions. The granitic veins show different Sr–Nd–O isotope compositions from the host UHP eclogites, indicating that the former did not originate from partial melting of the latter. This is also supported by the distinct δ18O and εHf(t) values for the Triassic zircon from the granitic veins and eclogites. Instead, there are comparable isotopic compositions between the granitic veins and regional UHP granitic gneisses, indicating that the granitic veins would be derived from partial melting of the underlying UHP granitic gneisses. Group II granitic veins have higher εNd(t) values than Group I ones and the granitic gneisses by 3–5 εNd unit, indicating that the anatectic melts forming Group II granitic veins are in Nd isotope disequilibrium with their source rocks. This can be caused by the behavior of accessory minerals like apatite and epidote/allanite during crustal anataxis at different pressures. The contributions of apatite together with garnet can also account for the increased Hf isotopes in the anatectic zircon. Therefore, the accessory minerals play a key role in constraining the geochemical features of incompatible-element enriched granitic melts during the crustal anatexis under HP to UHP conditions.

Permian post-collisional basic magmatism from Corsica to the Southeastern Alps

Post-Variscan Early Permian magmatism is widespread from Corsica to the Eastern Alps. After the formation of the Variscan orogenic belt between Laurussia and Gondwana, felsic and mafic bodies were emplaced during an extensional tectonic phase. This study focuses on a mafic dyke swarm that intruded in the region of Ajaccio (Corsica, France) and on a gabbroic intrusive complex outcropping in the Eastern Alps (Bressanone/Brixen, South Tyrol, Italy) both of which were emplaced during this extensional event. New U-Pb data from zircon show that both of these intrusions were emplaced at ca. 282 Ma. Most Ajaccio dykes display a calc-alkaline affinity and are characterized by enrichment of LILE (Large Ion Lithophile Elements) and LREE over HREE (Light and Heavy Rare Earth Elements) and HFSE (Nb, Ta, Hf, Zr; High Field Strength Elements). Two dykes show tholeiitic affinity and are characterized by marked LREE and Th-U depletion and by minor negative Nb-Ta anomalies on multi-element diagrams, which is typical for within-plate basalts. Bressanone/Brixen gabbros display similar geochemical features to the Ajaccio calc-alkaline samples with LREE enrichment over MREE (Medium Rare Earth Elements) and HREE, and HFSE depletion. Such characteristics are also confirmed by trace element data obtained by LA-ICP-MS on clinopyroxene and plagioclase crystals.The calc-alkaline to tholeiitic dykes are characterized by enriched to depleted Sr-Nd-Pb isotopic compositions, respectively. Some Ajaccio dykes and the Bressanone/Brixen gabbros are clearly affected by crustal contamination. However, the combination of trace element and isotopic variations suggests that mantle source heterogeneities were also involved. Isotopic compositions of the Ajaccio dykes reveal the presence of two distinct mantle source signatures: an enriched mantle (EM) component, which dominates the composition of the calc-alkaline-like suite, while a DMM (Depleted MORB Mantle) component dominates the source of the tholeiitic suite. The EM mantle source likely formed during Variscan subduction, however the composition of the tholeiitic suite requires a source composed dominantly of depleted mantle, with a minor contribution from both the subducting slab and supra-subduction mantle wedge. The mantle source of the tholeiitic magmas possibly originated from greater depths (compared to the calc-alkaline-like suite) during the post-orogenic extensional phase potentially due to break-off and/or roll-back of the Variscan slab or mantle flow from the front or margins of the subducted slab.Notably, coeval Permian mafic intrusive bodies from throughout Corsica (Bocca di Tenda, Porto, Pila Canale) and from the Australpine and Southalpine Alps intruded at a similar time and have geochemical features that are very similar to the Ajaccio dyke swarm and Brixen gabbro. This indicates that a widespread Permian magmatic province developed in a post-orogenic extensional tectonic setting at the margin of the former Variscan belt. During the Middle-Late Triassic magmatism with similar enriched geochemical signatures occurred in the Southern Alps, Dinarides, Australpoine domains and at the end of the Triassic over central Pangea (Central Atlantic Magmatic Province). This would highlight the importance that the Variscan orogeny had in the development of a more fertile mantle.

REE Characteristics of Lower Cretaceous Limestone Succession in Gümüşhane, NE Turkey: Implications for Ocean Paleoredox Conditions and Diagenetic Alteration

Trace and rare earth elements (REEs) are considered to be reliable indicators of chemical processes for the evolution of carbonate systems. One of the best examples of ancient carbonate successions (Berdiga Formation) is widely exposed in NE Turkey. The Lower Cretaceous limestone succession of Berdiga Formation may provide a case study that reveals the effect of ocean paleoredox conditions on diagenetic alteration. Measurement of major, trace and REEs was carried out on the Lower Cretaceous limestones of the Berdiga Formation, to reveal proxies for paleoredox conditions and early diagenetic controls on their geochemistry. Studied micritic limestone microfacies (MF-1 to MF-3) indicate deposition in the inner platform to a deep shelf or continental slope paleoenvironment during the Hauterivian-Albian. The studied limestone samples mainly exhibit low Mg-calcite characteristics with the general chemical formula of Ca98.35–99.34Mg0.66–1.65(CO3). They are mostly represented by a diagnostic REE seawater signature including (1) slight LREE depletion relative to the HREEs (ave. 0.72 of Nd/YbN and ave. 0.73 of Pr/YbN), (2) negative Ce anomalies (Ce/Ce* = 0.38–0.81; ave. 0.57), (3) positive La anomaly (La/La* = 0.21–3.02; ave. 1.75) and (4) superchondritic Y/Ho (ave. 46.26). Studied micritic limestones have predominantly low Hf (bdl to 0.5 µg/g), Sc (bdl to 2 µg/g) Th (bdl to 0.9 µg/g) contents suggesting negligible to minor shale contamination. These findings imply that micritic limestones faithfully record chemical signals of their parental and diagenetic fluids. The succession also exhibits high ratios of Eu/Eu* (1.01–1.65; ave. 1.29 corresponding to the positive Eu anomalies), Sm/Yb (1.26–2.74; ave. 1.68) and La/Yb ratios (0.68–1.35; ave. 0.9) compared to modern seawater and wide range of Y/Ho ratios (29.33–70.00; ave. 46.26) which are between seawater and hydrogenetic Fe-Mn crusts. Several lines of geochemical evidence suggest water-rock interaction between parental seawater and basaltic rocks at elevated temperatures triggered by hydrothermal activity associated with Early Cretaceous basaltic magma generation. The range of Ce/Ce* values is suggestive of mostly oxic to dysoxic paleoceanographic conditions, with a sudden change to dysoxic conditions (Ce/Ce* = 0.71–0.81), in the uppermost part of the MF-1. This is followed by an abrupt deepening paleoenvironment with a relative increase in the oxic state of the seawater and deposition of deeper water sediments (MF-2 and MF-3) above a sharp transition. The differences in microfacies characteristics and foraminifera assemblage between MF-1 and overlying facies (MF-2 and MF-3) may also confirm the change in paleoceanographic conditions. Therefore, REEs data obtained from studied limestones have the potential to contribute important information as to regional paleoceanographic conditions of Tethys during an important period in Earth history.

Temporal variation of titanite morphology and chemistry in a long-lived shear zone: The Clarke Head syenite in the Minas Fault Zone, Nova Scotia

The mineral chemistry of titanite has been studied in hydrothermally altered syenite preserved as blocks in a salt wall within a regional intracontinental shear zone. The study focusses on the controls on temporal variation in REEs, Zr, Nb, U and Th. Several generations of titanite are recognised on the basis of morphology, chemistry, and relationship to other secondary minerals. Magmatic euhedral to subhedral titanite has Ti and Zr contents characteristic of magmatic titanite in other studies, but does not have a magmatic abundance of REE. Rather, it is 102–104 times depleted in LREE and only slightly depleted in HREE. The leaching of LREE began during a regionally recognised phase of potassic alteration, as amphibole and biotite in gabbro and diorite recrystallized along shear zones and released K, Ca, and F. Later, some 15 Ma after emplacement, the alkali feldspar in the syenite was largely altered to scapolite and analcime by interaction with adjacent halite and gypsum deposits. Titanite formed at that time, from dissolution-reprecipitation reactions in large rutile crystals, is less depleted in LREE, as a result of the dominance of Cl− in hydrothermal waters. Following that event, F−-dominated hydrothermal waters prevailed again, resulting in more LREE depletion in previously formed titanite and in neoformed hydrothermal titanite. At that time, there was also depletion in Zr and U. The trace elements in hydrothermal titanite are thus very informative on the hydrothermal history of the rocks. A combination of morphology and trace-element signature allows different hydrothermal stages to be distinguished.

Geochemistry of a lateritic mantle developed on basalt in the Cameroon Western Highlands (Cameroon Volcanic Line)

Although lateritic mantles have been strongly documented, there is need to continuously characterise and update information at site-scale so as to optimize exploration and exploitation operations in various industrial domains. In this paper, four soil profiles developed on basalt are described in detail to appreciate their morphology, mineralogy, geochemistry, pedogenesis and ore potential. The basalt is composed of olivine, plagioclase feldspars and pyroxenes and accessory magnetite, sphene and apatite. The basalt shows high levels of Ba, Cr, V, Sr, Ni, Zn and Zr; meanwhile total REEs range from 185.03 to 199.45 mg kg−1. The weathering indices reveal an extreme weathering intensity of the parent rock. Newly formed minerals are kaolinite (15–48%), haematite (13–40%), Goethite (10.5–19%) and gibbsite (10–12%), magnetite (4–9.56%), feldspars (4.67–6.10), apatite (2.8–3.9) and sphene (1.77–2.46%). The weathered materials show high levels of Cr (40–850 mg kg−1), V (145–582 mg kg−1), Zr (300–583 mg kg−1) and Ce (24.2–510 mg kg−1). The total REEs of the weathered materials range from 126.52 to 1267.69 mg kg−1, with the predominance of LREE. The REE basalt-normalized patterns revealed slight LREE depletion, more pronounced for the B horizons relative to the underlying C horizons as well as HREE enrichment that increase with depth and altitude. In these soils, mass balance calculations reveal depletion of Si, Ca, Mg, Ba, K, Na, Mn, P, Rb, Sr, La and Lu, enrichment of Al, Fe, Ti, Ga, U, Zr, Dy, Hf, U, Tm and Yb meanwhile Cr, Cs, Sn, Ta, Th, Tl, V, W, Y, Nb, Ce, Pr, Nd, Sm, Eu, Gd, Tb and Ho are either depleted or enriched from one profile to the other. Apart from Cr, V, Zr and Ce whose concentrations respectively attain 850 mg kg−1, 582 mg kg−1, 487 and 510 mg kg−1, the mining potential of the studied lateritic mantle is low.