Low Heavy Rare Earth Research Articles

Geochemistry of the Serra das Melancias Pluton in the Serra da Aldeia Suite: a classic post-collisional high Ba-Sr granite in The Riacho do Pontal Fold Belt, NE Brazil

ABSTRACT: The Serra da Aldeia Suite is composed by circular or oval-shaped plutons, intrusive in meta-sedimentary and meta-volcanosedimentary rocks in the Riacho do Pontal Fold Belt, NE Brazil. The Serra das Melancias Pluton, belonging to Serra da Aldeia Suite, is located southeastern of Piaui state, near Paulistana city. These plutons represent a major magmatic expression in this area and contain important information about the late magmatic/collisional geologic evolution of the Brasiliano Orogeny. Based on petrographic and geochemical data, three facies were defined in the Serra das Melancias Pluton: granites, syenites and quartz monzonites. The rocks display high-K and alkaline to shoshonitic affinities, are metaluminous and show ferrous character. They are enriched in Light Rare Earth Elements and Large Ion Lithophile Elements, with negative anomalies in Nb, Ta and Ti. Their high Ba, Sr, K/Rb, low Rb, relatively low U, Th, Nb to very low Heavy Rare Earth Elements and Y resemble those of typical high Ba-Sr granitoids. The geochemical data suggest the emplacement of Serra das Melancias Pluton in a transitional, late to post-orogenic setting in the Riacho do Pontal Fold Belt during the late Brasiliano-Pan African Orogeny.

Petrography, geochemistry, and provenance of the Chalki rocks in Kurdistan region, North Iraq

Outcrops of the Paleozoic Chalki volcanic rocks are restricted to part of the northern Thrust Zone of Iraq close to Iraqi-Turkish border. Petrographically, the volcanic rocks from the Chalki area are mainly layered, appearing fresh in the field and exhibiting some basaltic lava flows. Porphyritic, amygdaloidal, and microlite-porphyric are the main observed textures. Phenocrysts of primary phases (i.e., olivine, iron oxides) are in a groundmass of feldspars and clinopyroxene. Chalki Formation is intercalated with Pirispiki Formation which consists of thin to medium bedded, greenish gray marl, red mudstone, and veins of calcite. The Chalki rocks are mafic of theolitic basalt type. Geochemically, they have high chromium and nickel concentrations in most samples. Rare earth element (REE) patterns illustrate parallel to sub-parallel, moderately fractionated REE patterns. The low heavy REE (HREE) contents in the studied samples appear to be due to partial melting of metamorphosed oceanic crust leaving HREE-rich accessory minerals (i.e., garnet) as a residual phase in the source. No Eu anomalies were observed in the Chalki samples which may indicate a back-arc basin pattern. The non-subduction signature of the Chalki rocks is confirmed by the Nb/Yb versus Th/Yb diagram, which shows that most of the studied rocks fall in the compositional field of non-arc-related rocks—well within the field of the mid-ocean ridge basalt (MORB)-ocean island basalt (OIB) mantle array.

Factors controlling the geochemical variations of lateritic bauxites formed upon different parent rocks in Peninsular Malaysia

Three lateritic bauxites representing felsic (Rompin area), intermediate (Johor area), and mafic (Kuantan area) parent rocks from Malaysia were subjected to detailed mineralogical and geochemical investigations to examine the factors controlling their geochemical variations. The bauxitic mineral gibbsite occurs as in all areas. Non-bauxitic minerals are represented by kaolinite, quartz, and goethite in bauxites from granites and intermediate tuffs and by goethite only in bauxite from basalts. Bauxites from basalts and intermediate tuffs have higher TiO2, Fe2O3, Ba, V, Cu, Pb, and Zn but lower SiO2, Th, Rb, and rare earth elements (REE) when compared with bauxite from granites. Bauxite from basalts has higher concentrations of Cr, Nb, Cu, and Zn and lower Ba, As, and REE when compared with bauxite from intermediate tuffs. Chondrite-normalized REE patterns of bauxite from granites showed heavy rare earth elements (HREE) enrichments over light rare earth elements (LREE), pronounced negative EuN/EuN* anomalies and strong positive CeN/CeN* anomalies. In contrast, bauxites from intermediate tuffs and basalts exhibited LREE enrichment relative to HREE, slight to strong negative CeN/CeN* anomalies, and absence or slight negative EuN/EuN* anomalies. Compositional variations among the studied bauxites are controlled not only by the parent rock compositions but also by the degree of lateritization and mineralogy of its products. The effect of each of these factors can be traced by a number of compositional parameters in these bauxites. The influence of parent rock compositions is indicated by absence of kaolinite and quartz, high TiO2 and Fe2O3 and low SiO2 in bauxites from basalts and intermediate tuffs as well as negative EuN/EuN* anomalies in bauxite from granites and absence of EuN/EuN* anomalies in bauxites from basalts and intermediate tuffs. The impact of lateritization degree was evident by abundance of quartz and kaolinite in bauxite from granites as well as relative LREE enrichments over HREE in bauxites from basalts and intermediate tuffs. The effect of lateritic products mineralogy was observed in the variations of REE patterns between bauxite from granites that are characterized by HREE enrichment and Ce anomaly due to occurrence of REE as residual zircon and bauxites from intermediate tuffs and basalts that are characterized by low HREE and high LREE due to occurrence of REE as authigenic florencite and/or cerianite. The distinct geochemical composition of Fe concretions such as lower TiO2 and most of the trace elements, enrichment of HREE relative to LREE and negative EuN/EuN* anomaly compared to the host bauxite at Johor area also indicates the effect of mineralogy of lateritic products on their geochemistry.

Tectonic Evolution of the Western Yarlung Zangbo Ophiolitic Belt, Tibet: Implications from the Petrology, Mineralogy, and Geochemistry of the Peridotites

The northern subbelt in the western segment of the Yarlung Zangbo suture zone, Tibet, China, includes the Dajiweng, Kazhan, Baer, Cuobuzha, Jianabeng, and Zhalai ophiolitic massifs. These ophiolites are strongly dismembered, typically 1–2 km wide and 10–20 km long, and composed chiefly of peridotites with minor volcanic and siliceous sedimentary rocks. No cumulates have been observed in the northern ophiolitic belt. Harzburgites of the Dajiweng and Zhalai ophiolites have prominent light rare earth element (LREE)–enriched (U-shaped or spoon-shaped) chondrite-normalized rare earth element (REE) patterns. Such patterns have generally been interpreted as the result of modification by suprasubduction zone (SSZ) melts/fluids. However, the abundance of peridotites sampled from mid-ocean ridge with similar LREE-enriched REE patterns suggest that this feature is not unique to SSZ peridotites. The U-shaped REE patterns of the Dajiweng harzburgites, combined with their low heavy rare earth element (HREE) contents and their mineral chemistry, indicate that these rocks most likely have been modified by SSZ melts (e.g., boninitic melts) in a forearc setting. In contrast, the Zhalai harzburgites, which also have U-shaped REE patterns but are characterized by high HREE contents, high Al2O3/SiO2 ratios, low MgO/SiO2 ratios, and relatively fertile mineral compositions, most likely have been refertilized in a mid-ocean ridge setting. The Zhalai, Kazhan, Baer, and Cuobuzha peridotites are similar to abyssal and back-arc peridotites in mineral chemistry and whole-rock geochemistry. Combining the mafic intrusions from Jianabeng, Baer, and Cuobuzha massifs, we propose that the ophiolites in the northern belt of the western segments have been trapped in an intraoceanic forearc–arc–back-arc system. According to the zircon U-Pb age of mafic intrusions, the geochemical characteristics of both mafic and ultramafic rocks, a detrital zircon study of Zhongba terrane, and the klippen structure of ophiolitic massif in the southern belt, we conclude that the northern and southern ophiolitic belts were developed in the same intraoceanic subduction system.

The crust‐mantle interaction in continental subduction channels: Zircon evidence from orogenic peridotite in the Sulu orogen

AbstractA combined secondary ion mass spectrometer and laser ablation‐(multicollector)‐inductively coupled plasma mass spectrometer study of zircon U‐Pb ages, trace elements, and O and Hf isotopes was carried out for orogenic peridotite and its host gneiss in the Sulu orogen. Newly grown zircon domains exhibit weak zoning or no zoning, relatively low Th/U ratios (<0.1), low heavy rare earth element (HREE) contents, steep middle rare earth element‐HREE patterns, negative Eu anomalies, and negative to low δ18O values of −11.3 to 0.9‰ and U‐Pb ages of 220 ± 2 to 231 ± 4 Ma. Thus, these zircons would have grown from metasomatic fluids during the early exhumation of deeply subducted continental crust. The infiltration of metasomatic fluids into the peridotite is also indicated by the occurrence of hydrous minerals such as amphibole, serpentine, and chlorite. In contrast, relict zircon domains exhibit magmatic zircon characteristics. Their U‐Pb ages and trace element and Hf‐O isotope compositions are similar to those for protolith zircons from ultrahigh‐pressure metamorphic rocks in the Dabie‐Sulu orogenic belt. Thus, these relict magmatic zircons would be physically transported into the peridotite by metasomatic fluids originated from the deeply subducted continental crust. Therefore, the peridotite underwent metasomatism by aqueous solutions derived from dehydration of the deeply subducted continental crust during the early exhumation. It is these crustally derived fluids that would have brought not only such chemical components as Zr and Si but also tiny zircon grains from the deeply subducted crustal rocks into the peridotite at the slab‐mantle interface in continental subduction channels. As such, the orogenic peridotite records the crust‐mantle interaction at the deep continental subduction zone.

Tectonic implications of Early Cretaceous low-Mg adakitic rocks generated by partial melting of thickened lower continental crust at the southern margin of the central North China Craton

This paper reports new whole-rock geochemical, Sr–Nd–Pb isotopic, and zircon U–Pb and Hf isotopic data for Early Cretaceous intrusive rocks in the Sanmenxia–Houma area of central China, and uses these data to constrain the petrogenesis of low-Mg adakitic rocks (LMAR) and the spatial extent of the influence of the deeply subducted Yangtze slab during the Triassic evolution of this region. New zircon laser-ablation inductivity coupled plasma mass spectrometry (LA-ICP-MS) U–Pb data indicate that the early- and late-stage southern Quli, Qiligou, and Gaomiao porphyritic quartz diorites, the Canfang granodiorite, and the northern Wangmao porphyritic quartz monzodiorite were emplaced during the Early Cretaceous (~130Ma) and the late Early Cretaceous (~116Ma). These rocks are characterized by high Na2O/K2O, Sr/Y, and (La/Yb)n ratios as well as high Sr concentrations, low Mg# [molar 100×Mg/(Mg+Fe2+tot)] values, and low heavy rare earth element and Y concentrations, all of which indicate an LMAR affinity. The samples have relatively high initial 87Sr/86Sr ratios (0.7054–0.7095), and low εNd(t) (−11.90 to −22.20) and εHf(t) (−16.7 to −32.7) values, indicative of a lower continental crust origin. The presence of Neoproterozoic (754–542Ma) and inherited Late Triassic (220Ma) metamorphic zircons within the late Early Cretaceous LMAR and the relatively high 206Pb/204Pb ratios of these rocks suggest that they formed from primary magmas derived from partial melting of Yangtze Craton (YC) basement material that had undergone ultrahigh-pressure metamorphism. In contrast, the presence of Paleoproterozoic and Archean inherited zircons within early Early Cretaceous LMAR in this area and the relatively low 206Pb/204Pb ratios of these rocks are indicative of derivation from primary magmas generated by partial melting of the thickened lower continental crust of the North China Craton (NCC). These rocks may have formed in an extensional environment associated with the upwelling of asthenospheric mantle material. The presence of YC basement material within the NCC in the Sanmenxia–Houma area suggests that the deeply subducted Yangtze slab influenced an area of ~100km in lateral extent within the southern margin of the central NCC during the Triassic.

Eocene magmatic processes and crustal thickening in southern Tibet: Insights from strongly fractionated ca. 43 Ma granites in the western Gangdese Batholith

This study reports zircon U–Pb age and Hf isotope, whole-rock major and trace element, and Sr–Nd–Pb–Hf isotope data for the Dajia pluton, western Gangdese Batholith, in southern Tibet. These data indicate that the pluton consists of moderately (Group 1) and strongly (Group 2) fractionated granites that were emplaced synchronously at ca. 43Ma. Group 1 samples have SiO2 contents of 69–72wt.% and vary in terms of the differentiation index (DI=84–93). These rocks are depleted in Ba, Nb, Sr, P, and Ti, with moderate negative Eu anomalies, and display low heavy rare earth elements (HREEs) and Y abundances. Group 2 samples are characterized by high SiO2 (75–78wt.%) and DI (95–97); significantly negative Eu anomalies; marked concave-upward middle REE (Gd–Ho) patterns; and Ba, Sr, P, and Ti anomalies that are significantly more negative than those of Group 1 samples. Group 1 samples have whole-rock εNd(t) (−5.9 to −6.0), εHf(t) (−4.0 to −4.5), and zircon εHf(t) (−6.0 to +5.8) values identical to those of Group 2 samples [εNd(t)=−5.7 to −6.7, εHf(t)=−3.5 to −2.9, and zircon εHf(t)=−2.0 to +4.2], as well as similar initial Pb isotopic compositions. These data indicate that the two groups were derived from a common source region with garnet as a residual mineral phase. Group 1 samples were most likely derived from partial melting of garnet-bearing amphibolite (rather than eclogite) within the juvenile southern Lhasa crust and mixed with the enriched components from the subducting ancient Indian continental crust and/or the ancient central Lhasa basement. Group 2 samples are interpreted as the products of extensive fractional crystallization (plagioclase, K-feldspar, biotite, apatite, allanite, titanite, monazite, and ilmenite) of the melts represented by Group 1 samples. Low HREEs and Y abundances of the Dajia pluton, together with the presence of strongly fractionated granites (Group 2) identified for the first time in the Gangdese Batholith, indicate that the crust beneath the Dajia region had already been thickened by ca. 43Ma. High whole-rock zircon saturation temperatures (815°C–869°C) of Group 1 samples and the other ca. 43Ma coeval magmatism documented both in the Gangdese Batholith and in the Tethyan Himalaya can be best interpreted as the final consequences of the magmatic responses to the Neo-Tethyan oceanic slab breakoff.

Subduction-related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India: Constraints from petrology and geochemistry

Major and trace elements as well as the first oxygen isotopes are reported on eclogite xenoliths from the Mesoproterozoic KL2 and P3 kimberlite pipes of the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India. Garnets in kyanite-bearing samples are rich in grossular, whereas they are predominantly pyrope-almandines in the bimineralic (kyanite-free) samples. The kyanite eclogite from the P3 pipe is more Mg-rich than those from KL2 pipe. Equilibration temperatures indicate derivation from 4.5 to 5.3GPa and 1060 to 1220°C for the KL2 samples and 3.6GPa, 918°C for the P3 sample.Garnet rare earth element patterns show two characteristic types, one with relatively low and flat heavy rare earth element patterns: Wajrakarur Group 1 and a second with lower light to heavy rare earth element ratios: Wajrakarur Group 2. Most samples in Wajrakarur Group 1 show pronounced positive Eu anomalies in garnet and positive Eu and Sr anomalies in the reconstructed whole rock trace element patterns; these are among the strongest anomalies in eclogite xenoliths worldwide. In contrast, Wajrakarur Group 2 samples show only subtle positive Eu anomalies.Oxygen isotopic ratios of garnets range between +5.3‰ and +7.8‰ δ18O. This range extends significantly beyond the range for unchanged mantle. Similar to many other eclogite suites worldwide, the Wajrakarur Group 1 and Group 2 eclogite suites shows evidence for an origin as crustal gabbroic material, likely once part of the oceanic crust, which was subducted and imbricated under the Eastern Dharwar craton. Their surface origin therefore lends support to geodynamic models that favor amalgamation of the Dharwar craton by subduction.

Serpentinization and Deserpentinization Reactions in the Upper Mantle beneath Fuerteventura Revealed by Peridotite Xenoliths with Fibrous Orthopyroxene and Mottled Olivine

Mantle xenoliths collected from Fuerteventura, one of the easternmost Canary Islands, exhibit a complex evolutionary history comprising events of depletion, serpentinization, dehydration and melt metasomatism. Each of these events left imprints on both the texture and chemistry of the xenoliths. Extensive partial melting is shown by complete lack of primary clinopyroxene, the ultra-refractory trace element composition of orthopyroxene porphyroclasts, and low heavy rare earth element contents as compared with abyssal peridotites sampled along mid-ocean ridges and oceanic fracture zones, in the xenoliths least affected by later metasomatism. In many xenoliths the original orthopyroxene porphyroclasts and some olivines are replaced by fibrous aggregates of orthopyroxene and/or large, deformed olivine porphyroclasts with mottled rims with stringy glass and fluid inclusions. Such features are very rare in ocean island xenoliths. Unusually high H2O and Cl concentrations, together with very high H2O/Ce and Cl/K ratios in interstitial glasses, suggest that the fibrous orthopyroxene formed by local serpentinization by hot seawater. The volume increase accompanying the serpentinization caused extensive fracturing of adjacent olivine porphyroclasts. The most likely scenario for local mantle invasion by hydrous fluids is along deep faults and fractures caused by tectonic movements along the continent‐ocean transition during the early phases of the opening of the Atlantic Ocean. The peridotites were later (probably during the Canary Islands magmatism) dehydrated, causing the serpentine minerals to be replaced by porous domains of fibrous orthopyroxene. Hydrous fluids released by the deserpentinization escaped into neighbouring and overlying rocks leaving trails of fluid inclusions along fractures and grain boundaries causing mottled rims and zones in olivine porphyroclasts. During the Canary Islands magmatism the upper mantle beneath Fuerteventura was also infiltrated by enriched silicate magmas that caused different degrees of Fe‐Ti-metasomatism. A higher degree of melt metasomatism in rocks with fibrous orthopyroxene and mottled olivine than in the massive harzburgites strongly suggests that the sublithospheric Canarian magmas reused serpentinized extensional faults during their rise to the surface. The strongest degree of melt metasomatism appears to have resulted in the formation of lherzolites, wehrlites, and dunites.

A subduction-related metasomatically enriched mantle origin for the Luoboling and Zhongliao Cretaceous granitoids from South China: implications for magma evolution and Cu–Mo mineralization

The Luoboling granodiorite porphyry and Zhongliao porphyritic biotite-granodiorite occur within the Zijinshan ore field in Fujian Province, southeast China. LA–ICP–MS zircon U–Pb dating yields ages of 103.1 ± 1.1 Ma (2σ, MSWD = 3.1) and 95.9 ± 0.6 Ma (2σ, MSWD = 1.2) for the Luoboling and Zhongliao intrusions, respectively. All rocks show high SiO2, K2O, and light rare earth element levels, variable CaO and Fe2O3 T, but low heavy rare earth element and high field strength element (Nb, Ta, Ti) concentrations. They also exhibit uniform initial 87Sr/86Sr ratios (0.7064–0.7068) and εNd(t) values (−4.0 to −2.6), falling within the compositional field of Cretaceous basalts and mafic dikes in the Cathaysia Block. Together with their relatively high Hf isotopic ratios (εHf(t) = −5.8 to +0.7), these data suggest that the Luoboling and Zhongliao intrusions were derived from a subduction-enriched mantle source. We infer fractional crystallization processes involving early fractionation of clinopyroxene and olivine, and subsequent fractionation of garnet forming the Luoboling granodiorite porphyry, eventually followed by amphibole- and biotite-dominated crystallization, with minor accessory mineral contribution, producing the (younger) Zhongliao porphyritic biotite-granodiorite. Asthenospheric mantle sources may have also contributed to generating the melts as indicated by initial Pb isotopic compositions and Hf isotopic compositions. The generation of these intrusions was associated with the Pacific subduction in an extensional setting during the Cretaceous.

Petrochemistry and tectonic setting of the Davarzan-Abbasabad Eocene Volcanic (DAEV) rocks, NE Iran

The Davarzan–Abbasabad Eocene Volcanics (DAEV) is located at the northeastern edge of the volcanic–plutonic belt of the Central Iran structural zone. DAEV start with continental to shallow marine sediments of the Paleocene–Eocene and then continue extensively with shallow submarine to sub-aerial basaltic and andesitic volcanics, related volcaniclastics and sedimentary rocks during the Middle–Upper Eocene. The volcanics are olivine basalts, basaltic andesites and andesites in composition and show porphyric, hyaloporphyritic, glomeroporphyric, fluidal, and seriate textures. The geochemistry of DAEV points to a fractional series from olivine basalt to andesite with transitional and medium to high K calc–alkaline nature. Their primitive mantle and chondrite-normalized trace element patterns show that these rocks have high light- and low heavy rare earth elements and are enriched in large-ion lithophile elements and depleted in high field strength elements (Nb, Ti, P, and Zr). Integration of geochemical and petrological data with regional studies indicates that the magma(s) forming the DAEV rocks were derived from 14 to 16 % partial melting of an enriched mantle source in 70–100 km depths during Middle–Upper Eocene time in an intra-arc extensional setting. This mantle source had been previously metasomatized by fluids derived from Sabzevar Neo–Tethyan-subducted slab during the Upper Cretaceous–Paleocene.

Coercivity enhancement of sintered Nd-Fe-B magnets by chemical bath deposition of TbCl3

The chemical bath deposition (CBD) and the grain boundary diffusion method were combined to diffuse the heavy rare earth for obtain the thick magnets with high coercivity and low heavy rare earth. The jet mill powders were soaked into the alcohol solution of 0.2 wt. % TbCl3. A thin layer of TbCl3 was wrapped to the surface of (PrNd)2Fe14B powder particles. The coercivity of magnet is increased from 11.89 kOe to 14.72 kOe without significant reduction of remanence after grain boundary diffusion in the sintering and the annealing processes. The temperature coefficients of the remanence and the coercivity are improved by the substitution of PrNd by Tb in the surface of grains. The highly accelerated temperature/humidity stress test (HAST) results indicate that the CBD magnet has poor corrosion resistance, attributing to the present of Cl atoms in the grain boundaries.

The Demir Kapija Ophiolite, Macedonia (FYROM): a Snapshot of Subduction Initiation within a Back-arc

The Demir Kapija ophiolitic complex in southern Macedonia–FYROM (Former Yugoslav Republic of Macedonia) represents the southernmost exposure of the Tethyan Eastern Vardar ophiolitic unit in the Eastern Mediterranean. It consists of a mafic volcanic sequence (pillow basalts, sheeted dyke diabases and gabbros) that was subsequently intruded by island arc magmas with and without adakitic affinity. The mafic volcanic sequence is characterized by slightly increased ratios of large ion lithophile elements to high field strength elements (LILE/HFSE), flat rare earth element (REE) patterns, radiogenic 143Nd/144Nd (up to 0·51272) and high TiO2 contents (which reflect Pl + Ol + Cpx fractionation). The relationship between TiO2 and MgO contents indicates that Ti saturation was eventually reached and that Ti-magnetite fractionated. The mafic volcanic sequence of the ophiolite complex formed around 166·4 Ma in a short-lived intra-oceanic back-arc basin by slab roll-back of the Western Vardar Ocean. The rocks with and without adakitic affinity are spatially and temporally closely related. Their crystallization age is around 164 Ma. Our data suggest that two subgroups of arc lavas evolved as discrete volcanic lineages that are not related by fractional crystallization of a common parental magma, and that two different parental magmas are required. The arc lavas with adakitic affinity show some of the typical features of adakites; that is, low heavy REE, elevated Sr/Y, high LILE and high light REE. Major and trace element compositions of clinopyroxene phenocrysts resemble those of typical adakite-derived clinopyroxene. The very high Th/La, Th/Yb and Ba/Yb ratios and the reduced 143Nd/144Nd values (around 0·51245) of the rocks with adakitic affinity are considered to reflect contributions of sedimentary material to their mantle source. By analogy with adakites, these rocks are interpreted as the product of slab + sediment melting in an unusually hot subduction zone (subduction of young oceanic crust). In contrast, the arc lavas without adakitic affinity are related to a different parental melt, similar to common arc magmas. The Demir Kapija ophiolite formed in a short-lived intra-oceanic back-arc basin during subduction initiation within a back-arc. The arc intrusions are related to the change from an extensional to a compressional regime.

Geochemistry and petrogenesis of an adakitic quartz-monzonitic porphyry stock and related cross-cutting dike suites, Kighal, northwest Iran

A late Oligocene–early Miocene quartz-monzonitic porphyry stock intrudes upper Eocene andesitic-latitic lava flows, producing Cu–Mo mineralization and hydrothermal alteration zones in the Kighal area, northwest Iran. Numerous cross-cutting dikes of various compositions, ranging from quartz-diorite to granodiorite and microdiorite, branch off from three later-stage barren sub-volcanic bodies, and intrude the porphyry stock. Both the stock and comagmatic dikes display I-type, magnesian, calc-alkaline to high-K calc-alkaline signatures and metaluminous to peraluminous geochemical characters, being emplaced within an active continental margin setting. Analysed samples display the typical geochemical characteristics of adakites. Moreover, rare earth elements exhibit a fractionated pattern with low heavy rare earth element concentrations. The general trend in an La/Yb–Yb diagram shows that partial melting, rather than fractional crystallization, was the dominant process of magma generation. Based on the compositional classification of modern adakites, the studied rocks are of high-SiO2 adakite type. They were generated through low degrees of high-pressure partial melting of subducted rutile- and amphibole-bearing eclogitic Neo-Tethyan oceanic slab. The descending slab underwent break-off, leaving behind a residue rich in garnet + amphibole ± rutile, lacking significant plagioclase. Interaction of the slab-derived magma with the mantle-wedge peridotite is also evident.

Juvenile subcontinental lithospheric mantle beneath the eastern part of the Central Asian Orogenic Belt

Mineral compositions, clinopyroxene trace-element and Sr–Nd–Hf isotopic compositions, whole‐rock major‐element and Platinum Group Element (PGE) contents, and Re–Os isotopes of peridotite xenoliths in the Cenozoic alkali basalts from Abaga, Inner Mongolia (China) help to constrain the nature of the subcontinental lithospheric mantle beneath the eastern part of the Central Asian Orogenic Belt. The mantle xenoliths are mainly spinel lherzolites, with minor olivine websterite and spinel harzburgite. Clinopyroxene–orthopyroxene equilibration temperatures are higher than 840°C. An olivine websterite (sample A2-2) has the lowest MgO and highest Al2O3, CaO and TiO2 contents, whereas a harzburgite (sample A2-1) has the highest MgO and low Al2O3, CaO and TiO2 contents. The spinel lherzolites have major-element concentrations transitional between the harzburgites and websterites. According to the chemical characteristics and Sr–Nd–Hf isotopic compositions of clinopyroxenes, two groups of spinel lherzolites can be identified. Group 1 spinel lherzolites are moderately depleted in Light Rare Earth Elements (LREEs) and with flat Heavy Rare Earth Element (HREE) patterns, indicating a low degree of melt extraction and minimal metasomatism. They have clinopyroxenes with homogeneous 87Sr/86Sr (0.7030−0.7033), 143Nd/144Nd (0.5131–0.5132) and 176Hf/177Hf (0.2834–0.2835) ratios. They have flat chondrite-normalized PGE patterns and radiogenic 187Os/188Os ratios (0.1232–0.1256), within the range of modern convective mantle. Group 2 spinel lherzolites have low HREE contents with variable LREE concentrations, suggesting complex melt extraction and mantle metasomatism processes. They have relatively heterogeneous Sr–Nd–Hf isotopic compositions with 87Sr/86Sr of 0.7040–0.7054, εNd values of +6.1 to +12.1 and εHf values of +19.2 to +474. The Group 2 spinel lherzolites have flat chondrite-normalized PGE patterns and radiogenic 187Os/188Os ratios (0.1231–0.1252). The websterite has a magmatic origin with weakly decoupled Nd–Hf isotopes and the lowest 187Os/188Os ratio of 0.1219. The harzburgite has a 187Os/188Os ratio of 0.1280 similar to that of the modern convective mantle. All these characteristics indicate that the Group 2 spinel lherzolites represent ancient lithospheric mantle that experienced high‐degree melt extraction and subsequent extensive silicate metasomatism. Thus, it is proposed that the SCLM beneath the Abaga region of the eastern Central Asian Orogenic Belt (CAOB) is mostly juvenile with minor relicts of old depleted lithospheric mantle, which cooled from near-adiabatic to near-lithospheric temperatures to form new lithospheric mantle during the formation of the eastern CAOB.

Zircon U–Pb ages and geochemistry of the Huai’an TTG gneisses terrane: Petrogenesis and implications for ∼2.5 Ga crustal growth in the North China Craton

The Huai’an gneiss terrane is a typical Archean terrane within the Trans-North China Orogen of the North China Craton (NCC) and is composed of Tonalite–Trondhjemite–Granodiorite (TTG) suite and dioritic granulite facies gneisses. Precise SHRIMPII and Cameca IMS1280 zircon U–Pb ages demonstrate that the Huai’an gneisses formed around 2.55–2.50Ga and underwent granulite facies metamorphism at 1.85–1.80Ga. The TTG gneisses are compositionally equivalent to high-SiO2 adakites, with high SiO2 and Al2O3 concentrations, strongly fractionated rare earth element (REE) patterns ((La/Yb)N=10–54) with positive or negligible Eu anomalies (Eu/Eu*=0.9–3.4), low heavy REE (HREE) and Y concentrations, high Sr (435–938ppm) concentrations, and high Sr/Y ratios (48–261). High Mg numbers (33–60) and high compatible element concentrations (e.g., Cr=11–183ppm and Ni=11–82ppm) indicate that the magmas that formed these gneisses were derived from interaction between adakitic melts and mantle material. The dioritic gneisses have compositions similar to high-Mg diorites or sanukitoids, with high Al2O3 (14.46–17.61wt%), MgO (3.0–6.4wt%), Cr (100–304ppm), and Ni (57–109ppm) concentrations, and fractionated REE patterns with weak negative or no Eu anomalies (Eu/Eu*=0.8–1.0). All the TTG and dioritic gneiss samples analysed during this study have negative Nb, Ta, and Ti anomalies, compared to neighbouring elements in spider-diagrams, with the majority having high positive ɛNd(t) and ɛHf(t) values, close to values of the depleted mantle. These geochemical characteristics strongly suggest that protoliths of the Huai’an TTG and dioritic gneisses originated in a juvenile crust-forming event at 2.55–2.50Ga through partial melting of subducted oceanic crustal material; these magmas also assimilated mantle peridotite material during their ascent. This indicates that ∼2.5Ga mantle-derived magmatism led to significant crustal growth in the North China Craton, and was probably associated with subduction.

Geochemistry and petrogenesis of the Huashan granites and their implications for the Mesozoic tectonic settings in the Xiaoqinling gold mineralization belt, NW China

The Huashan complex granitic batholith, occurred in the core of the Xiaoqinling orogen and gold mineralization belt along the south margin of the North China Craton, consists of amphibole monzogranite and biotite monzogranite in the Wengyu and Fangshanyu valleys, respectively. Zircon LA-ICP-MS U–Pb dating yields ages of 205±2Ma and 132±1Ma for the Wengyu and Fangshanyu granites, respectively, which represent the two major episodes of Mesozoic magmatism in the region. These rocks are characterized by metaluminous, high silica and total alkalinity, and have high FeOtotal/(FeOtotal+MgO) ratios, high large-ion lithophile elements (LILEs, especially Sr and Ba) and light rare earth elements (LREEs), and low heavy rare earth elements (HREEs) and Y concentrations and insignificant negative Eu anomalies. The Wengyu granites have higher Ba and Sr contents, which can be classified as high Ba–Sr granites, whereas the Fangshanyu granites show much lower Ba and Sr contents and display an adakitic affinity. Elemental and isotopic compositions suggest that the main sources of both the Wengyu and Fangshanyu granites are likely the ancient basement materials of Taihua Group. However, the high Ba and Sr contents of the Wengyu granites require addition of small amounts of enriched lithospheric mantle metasomatised by fluid/melt derived from pelagic sediments-bearing subducted slab in the sources. The Wengyu granites were most likely generated earlier by partial melting of the lowest part of the crust due to the subducted slab break-off under the post-collision extensional stage of the continental collision orogeny. The Fangshanyu granites might be mainly derived later from the partial melting of thickened lower crustal materials, representing by the Taihua Group basement rocks, corresponding to the tectono-magmatism of the post-orogeny to rift extensional environment during the tectonic transition from the Paleo-Tethys subduction-collision system to the Paleo-Pacific regime.

Two-Stage Exhumation of Ultrahigh-Pressure Metamorphic Rocks from the Western Dabie Orogen, Central China

Zircon trace element, U-Pb age, and Lu-Hf isotope composition were determined for two eclogite samples from the Xinxian ultrahigh-pressure (UHP) unit in the western Dabie orogen to constrain its exhumation processes. Cathodoluminescence imaging reveals that most zircons are metamorphic, while some have inherited magmatic cores in one sample. The magmatic cores have high Th/U and Lu/Hf ratios, high trace element contents, and a clear negative Eu anomaly, consistent with their igneous genesis. They yield an upper intercept age of Ma and a positive εHf(t) value of 8.0, arguing for reworking of early Mesoproterozoic crust during the middle Neoproterozoic in the western Dabie orogen. The metamorphic zircons contain mineral inclusions of garnet, omphacite, rutile, and quartz. They are characterized by low Nb, Ta, Y, and heavy rare earth elements (HREE) contents, a nearly flat HREE pattern, and an insignificant negative Eu anomaly. These indicate that the metamorphic zircons formed under high-pressure (HP) quartz eclogite-facies metamorphic conditions. The metamorphic zircons in the two samples have Ti-in-zircon temperatures of and C, respectively, suggesting that they formed in different HP eclogite-facies metamorphic stages. The metamorphic zircons in the two samples yield weighted mean U-Pb ages of and Ma, which may date the initial exhumation to ∼90 km and ∼700°C and a subsequent HP quartz eclogite-facies retrogression at ∼50 km and ∼600°C for ultrahigh-pressure (UHP) rocks in the western Dabie orogen. This yields a maximum exhumation and average cooling rates of ∼0.33 cm/yr and 8°C/Ma, respectively. Based on the published Rb-Sr and Ar-Ar ages of ∼212 Ma for the UHP rocks in the area, the subsequent stage of exhumation of UHP rocks has a maximum exhumation and average cooling rates of 0.67 cm/yr and 65°C/Ma, respectively. This two-stage cooling process may be common for HP-UHP metamorphic terranes in continental collision zones.

Magmatic diversity of western Mexico as a function of metamorphic transformations in the subducted oceanic plate

This geochemical study of the Mexican subduction zone elucidates how metamorphic and dehydration reactions affecting the subducted oceanic plate at different depths can influence magmatic diversity. In the western Trans-Mexican Volcanic Belt, there is a narrow potassic volcanic front running in parallel to the Middle American Trench that becomes replaced by intraplate-like high-Nb rocks to the north, and by more typical calc-alkaline products to the southeast. Potassic rocks have high MgO and are enriched in incompatible trace elements, but have lower heavy rare earth element contents than more evolved calc-alkaline and high-Nb magmas, and slightly more enriched Sr, Nd and Pb isotopes. Potassic magmas also have higher Rb/Cs and Ba/Cs ratios than the calc-alkaline and high-Nb suites, and extend to unusually high Nb/Ta ratios that correlate positively with Rb/Ta, Zr/Ta, La/Ta and Gd/Yb. These chemical variations are inconsistent with different extents of melting of a peridotitic source, but are also incompatible with melting of a phlogopite-rich mantle (vein-plus-wall-rock relationship), unless mica is totally consumed during melting, and a titaniferous phase such as rutile remains in the residue together with garnet. This assemblage is unlikely in the source region of primitive hydrous magmas, but it is what would be expected during dissolution of phengite and monazite/allanite in the subducted slab, with the concurrent formation of an anhydrous rutile-bearing eclogite. The magmatic diversity of western Mexico can thus be explained by invoking contributions of chemically different subduction agents as a function of slab depth and residual mineralogy: a low-pressure/temperature aqueous fluid would induce melting of the peridotitic mantle wedge and form typical calc-alkaline volcanoes, whereas a deeper and hotter slab-derived melt (or supercritical liquid) would contribute to the formation of potassic magmas due to phengite/monazite/allanite disintegration. In this context, intraplate-like magmas derive from decompression melting of the upper mantle as a natural consequence of subduction geodynamics.

Composition of the Lithospheric Mantle in the Siberian Craton: New Constraints from Fresh Peridotites in the Udachnaya-East Kimberlite

We present petrographic, major element and trace element data for bulk-rocks and minerals for a suite of 34 remarkably fresh peridotite xenoliths in the Udachnaya kimberlite recovered from deep horizons of the open pit mine. The xenoliths are spinel peridotites, granular and deformed garnet peridotites (as well as a megacrystalline dunite) in proportions similar to those reported by previous work on altered xenoliths from less deep levels in the mine. Equilibration temperatures (T) range from 760–965°C for the spinel peridotites to 1200–1320°C for sheared garnet peridotites; the latter yield pressures (P) of 5·4–6·6 GPa. The majority of the granular garnet peridotites equilibrated at 860–1000°C and 2·6–5·5 GPa, but two samples yield much higher T and P values (1176°C, 6·1 GPa; 1340°C, 6·8 GPa) indicating the presence of undeformed rocks near the base of the lithosphere. Strong enrichments in silica and opx relative to dry melting residues are not common in our granular peridotites (e.g. 11 samples out of 18 analyzed plot on Boyd’s ‘oceanic trend’); they have distinctly lower modal opx than low-T peridotites from the Kaapvaal craton and could be formed by ∼30–45% of melt extraction at 1–5 GPa. Minor to moderate enrichments in silica relative to experimental melting trends seen in some xenoliths may indicate that subduction-related settings were involved in their origin. Garnets are usually unzoned but some are enriched in Ti at the rims. Their contents of CaO (4–7 wt %) and Cr2O3 (2–12 wt %) show inverse power-law correlations with whole-rock CaO, Al2O3 and Al/Cr ratios; the latter can be estimated from garnet analyses. Garnets from the dominant cpx-free and cpx-bearing harzburgites and less common low-cpx (5–6%) lherzolites fall into the G9 ‘lherzolite’ field; garnets from three cpx-rich (9–15%) lherzolites are in the G5 ‘pyroxenite’ field. Rare earth element (REE) patterns in cpx and garnet range from sinusoidal to bell-shaped and light REE (LREE)-depleted; the latter are common in sheared rocks and may be close to equilibrium with metasomatic liquids. Garnets from some granular rocks show low heavy REE (HREE) and continuous depletion in LREE, as in melting residues. Trace element patterns and ratios in many bulk peridotites are distinct from those in the host kimberlites and may not be much affected by contamination. Bulk-rock abundances of moderately incompatible elements are lowest in spinel harzburgites (20–50 times lower than in primitive mantle for HREE) and highest in sheared rocks. Some bulk-rocks show depletion from Lu to Er attributed to 30–40% of melt extraction first in the presence of garnet, then in the spinel field. Overall, the observations on fresh Udachnaya peridotites show less evidence for silica and opx enrichments and smaller compositional effects of syn- and post-eruption alteration than earlier work. The abundances of cpx and garnet and bulk-rock Al2O3 do not vary systematically with depth except that rare sheared xenoliths anomalously enriched in cpx and garnet occur near the base of the lithosphere. The remarkable preservation of olivine and opx shows in detail the transformation of granular microstructures into different types of porphyroclastic fabrics and their relations to chemical compositions.