Enrichment In LILE Research Articles

Magma Genesis and Arc Evolution at the Indochina Terrane Subduction: Petrological and Geochemical Constraints From the Volcanic Rocks in Wang Nam Khiao Area, Nakhon Ratchasima, Thailand

Volcanic rocks and associated dikes have been exposed in Wang Nam Khiao area, Nakhon Ratchasima Province, northeastern Thailand where complex tectonic setting was reported. These volcanic rocks are classified as rhyolite, dacite, and andesite while dikes are also characterized by andesitic composition. These dikes clearly cut into the volcanic rocks and Late Permian hornblende granite in the adjacent area. Rhyolite and dacite are composed of abundant plagioclase and quartz whereas andesite and andesitic dike contain mainly plagioclase and hornblende with minor quartz. The volcanic rocks typically show plagioclase and hornblende phenocrysts embedded in fine-grained quartz and glass groundmass whereas dike rocks contain less glass matrix with more albitic laths. P-T conditions of crystallization are estimated, on the basis of Al-in-hornblende geobarometry and hornblende geothermometry, at about 4.5 – 5.5 kbar, 861 – 927 oC and 4.8 – 5.5 kbar, 873 – 890 oC for the magma intrusions that fed volcanic rocks and andesitic dikes, respectively. Whole-rock geochemistry indicates that these rock suites are related to calc-alkaline hydrous magma. The enriched LILE (e.g. Rb, Sr) and depleted HFSE (e.g. Nb, Ce, Ti), with similar REE patterns indicate arc magmatism. The results of this study are comparable to the continental arc magmatism along the Loei Fold Belt as a consequence of the Late Permian Palaeo-Tethys subducted beneath Indochina Terrane.

Tracing arclogites in the Paleoproterozoic Era – A shift from 1.88 Ga calc-alkaline to 1.86 Ga high-Nb and adakite-like magmatism in central Fennoscandian Shield

Arclogites, i.e., lower crustal garnet-pyroxenite cumulates, are suggested to play an important role in controlling magma differentiation in modern continental arcs. Until now, arclogite-related magmatism has only been described from the Phanerozoic Era. The Svecofennian orogen in the central Fennoscandian Shield hosts a rare association of 1.86 Ga igneous rocks geochemically distinct from the surrounding and much more abundant 1.90–1.87 Ga subduction-related calc-alkaline magmatism. The 1.86 Ga magmatic rocks are divided into three groups: 1) high-Nb gabbros (HNB) which are enriched in Fe2O3T, TiO2, P2O5, F, LILE, and HFSE (especially Nb; 18.9–44 ppm), show positive initial εNd value, and near-chondritic but variable initial zircon εHf values; 2) high-Mg gabbros (HMG) which are characterised by high MgO, CaO, Cr and Ni contents, slight enrichment in LILE, positive εNd, and positive but variable zircon εHf values; 3) adakite-like rocks showing high Al2O3 and Na2O contents, slight enrichment in LILE, relative depletion in some HFSE, positive εNd value, and chondritic to negative zircon εHf values. The three groups yield zircon U-Pb ages of ~1.86 Ga and exhibit undeformed textures in contrast to the surrounding supracrustal rocks metamorphosed at ~1.88 Ga. The ages and compositions are clearly different from the adjacent 1.90–1.87 Ga arc-related igneous rocks suggesting a distinct origin. Despite similar ages and close spatial relationship, separate sources are required for each of the different 1.86 Ga rock groups. Trace element modelling of partial melting suggests that arclogites, with compositions similar to pyroxenite xenoliths found in the kimberlite pipes of eastern Finland, are the source for the HNB rocks. In contrast, subduction-modified mantle peridotite is the source for the HMG rocks, and a mafic lower crustal source is suggested for the adakite-like rocks. The following geodynamic model is suggested: (rutile-bearing) arclogite formation at 1.90–1.87 Ga followed by arclogite delamination and partial melting during extension of the thickened Svecofennian crust at 1.86 Ga.

The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment

The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petro-geochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al2O3: 0.12–1.53 wt%, Ti: 3.5–64.2 ppm and Nb: 0.004–0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr# (0.44–0.81) and low to intermediate Mg# (0.25–0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite.

Incompatible element-enriched mantle lithosphere beneath kimberlitic pipes in Priazovie, Ukrainian shield: volatile-enriched focused melt flow and connection to mature crust?

ABSTRACT Major, minor and trace element compositions of mantle xenocrysts from Devonian kimberlite pipes in the Priazovie give an insight into the mantle structure beneath the SE Ukranian Shield and its evolution. Garnets yield low temperature conditions as determined by monomineral thermobarometry. The mantle lithosphere is sharply divided at 4.2 GPa, marked by a high temperature Cpx-Ilm-Phl trend, eclogites and changes in pyrope geochemistry. Seven layers are detected: Ist layer at 2.5–1 GPa is enriched mantle (Fe#Ol ~ 0.11 − 0.14) with Gar- pyroxenites and Sp peridotites; IInd at 2.5–3.2 GPa – Gar-Sp (Fe#Ol 0.08 − 0.10) peridotite. IIId at 4.3–3.2 GPa is formed of Archaean- Proterozoic peridotites with Fe#Ol ~0.07 − 0.095. IVth at 3.2–5 GPa- contains pyroxenitic Gar with higher Ca, eclogites, Chr and Cpx (Fe#Ol ~0.10 − 0.125); Vth at 5.8 − 5 GPa is marked by sub-Ca garnets, Cr-rich chromites and Mg-Cr ilmenites; VIth layer at 5.8–6.8 GPa contains Fe-enriched pyropes, almandines and Cr-Mg ilmenites near the lithosphere base; VIIth layer > 6.8 GPa consists of ‘hot’ Fe-rich garnets. Garnets show increasing enrichment in LREE, LILE, Hf, Zr with decreasing pressure. Primitive garnets have round REE patterns; depleted ones have S-type patterns inflected at Nd. Garnets from 6.5 to 3 GPa show increasing La/Ybn, Zr-Hf, LILE. Peridotitic clinopyroxenes have inclined linear trace element patterns rounded from La to Pr with high LILE and HFSE levels. The Fe-rich group (reacted with eclogites) shows bell-shaped irregular patterns with LILE close to the LREE levels. A possible reason for LILE (HFSE and) enrichment of the upper part of the mantle is subduction metasomatsm in Archaean times (with participation of mature continental sediments) activated by plumes at 1.8 Ga and earlier which produced pervasive focused melt flow with remelting of mica-amphibole metasomatites giving continuous REE and LILE enrichment in mantle lithologies from 5.8 to 2.5 GPa.

Geochemistry, geochronology, and geodynamic implications of mafic‐felsic dykes from the Hadamengou area, north‐western North China Craton

Occurrences of multistage dykes provide a critical opportunity to constrain the nature of geological setting in the Daqingshan region during the Late Neoarchean to Middle Palaeoproterozoic. In this article, we present the results of zircon U–Pb ages and whole‐rock geochemical data from mafic‐felsic dykes in the Hadamengou area in order to constrain their ages and tectonic significance. Based on their zircon U–Pb ages, these dykes can be divided into four types: 2.52 Ga metadiabase, 2.45 Ga K‐feldspar granite, 2.44 Ga granitic pegmatite, and 1.99 Ga metagabbro. Geochemically, the 2.52 Ga metadiabase rocks have low SiO2, high MgO, and Na2O/K2O ratios, with enrichment in LILE and depletion in HFSE, which suggests these rocks originated from melting of the lithosphere mantle components metasomatised by subducted slab‐derived fluids. The 2.45 Ga K‐feldspar granite and the 2.44 Ga granitic pegmatite are characterised by high SiO2 and Na2O + K2O contents, low MgO, Ni and Cr contents, and high A/CNK ratios, similar to typical features of strongly peraluminous granites. Additionally, these rocks show a variation of Eu anomalies and strong depletion of Th, U, Nb, and Ta elements, which characteristics are considered as a feature of anatectic origin. The 1.99 Ga metagabbro rocks belong to calc‐alkaline rock series, and contain low SiO2 contents, high MgO, Cr and Ni contents, negative Nb, Ta, and Ti anomalies, and high LILE abundances without Eu anomalies. Geological features of the metagabbro dykes indicate that they are likely derived from partial melting of the subduction‐related metasomatised lithospheric mantle. Combined with the evidence of mafic‐felsic dykes in the Hadamengou and magmatism in the adjacent area, a complex tectonic evolution history during Late Neoarchean to Middle Palaeoproterozoic is presented in the Daqingshan region, which is associated with the amalgamation of micro‐blocks within the North China Craton (NCC), regional extension after cratonisation and subduction of the old ocean between the Yinshan Block and the Ordos Block.

From Decompression Melting to Mantle-Wedge Refertilization and Metamorphism: Insights from Peridotites of the Alag Khadny Accretionary Complex (SW Mongolia)

This study reports on mineral and bulk rock compositions of metaperidotites from the Alag Khadny accretionary complex in SW Mongolia, to reveal their nature and relationships with associated eclogites. The peridotites preserved original porphyroclastic textures and are composed of olivine, orthopyroxene relics, Cr-spinel, interstitial (not residual) clinopyroxene, and secondary chlorite, tremolite, olivine, Cr-magnetite, clinopyroxene, and antigorite. Cr-spinel has Cr# of 0.3–0.5, and primary olivine shows Mg# of 0.90–0.92. The pyroxenes are high-magnesian with low Al2O3 and Cr2O3. The bulk rocks have U-shaped normalized trace-element patterns with enrichment in LILE, L-MREE relative to HREE, and weak Pb–Sr peaks and Nb–Zr–Hf minima. Interstitial clinopyroxene exhibits V- and U-shaped normalized REE patterns with (La/Yb)N > 1 (Yb = 1.2–3 of chondritic values) and enrichment in fluid-mobile elements and Zr. HREE abundances of clinopyroxene can be simulated by 23–26% partial melting of depleted mantle starting at garnet-facies (6–8%) depths, followed by hydrous or anhydrous melting at spinel-facies depths L-MREE characteristics of clinopyroxenes can be simulated by further interaction of harzburgites with an island-arc basaltic melt in a supra-subduction environment. The association of hydrous secondary minerals in the Alag Khadny peridotites suggests their retrograde metamorphism at 1.6–2.0 GPa and 640–720 °C, similar to P–T conditions reported earlier for the spatially associated eclogites. This supports metamorphism of the Alag Khadny peridotites in a mantle wedge, followed by joint exhumation of peridotites and eclogites. Given the findings above and implying the regional geological background, we advocate for a sequential Neoproterozoic evolution the Alag Khadny harzburgites from (1) their formation by decompression partial melting in an Early Neoproterozoic or older spreading center of a mid-ocean or back-arc setting, and (2) refertilization by supra-subduction melts, followed by (3) Late Neoproterozoic–Early Cambrian hydrous-fluid metamorphism and juxtaposition with eclogites.

Petrogenesis of Archean TTG-series rocks from the Jiaodong complex, eastern China: Implications for crustal evolution in the North China Craton

Rocks of the tonalite-trondhjemite-granodiorite (TTG) series with associated supracrustal rocks are dominant components of many Archean terranes, but their petrogenesis remain controversial. Here, we present whole-rock major and trace elements, zircon U-Pb ages, Sm-Nd and zircon Hf isotopes of the three phases of TTG gneisses (2.9, 2.7 and 2.5 Ga) and amphibolites in the Jiaobei terrane, China, to investigate their petrogenesis and implications for crustal evolution. The 2.5 Ga TTGs are characterized by high Sr, Sr/Y and (La/Yb)N values, enrichment in LILE and depletion in HFSE, positive or negligible Eu anomalies and similarity to medium- to high-pressure (MP-HP) TTGs. In combination with their geochemical modeling results and high contents of MgO, Mg#, Cr and Ni, it is considered that the 2.5 Ga TTGs were subduction-related and mainly from melting of rutile-bearing eclogite and garnet amphibolite. In contrast, the 2.7 and 2.9 Ga TTGs show similarity to low- to medium-pressure (LP-MP) TTGs and have lower Sr contents and negligible Eu anomalies. Combining their Y/Yb and (Ho/Yb)N ratios, the geochemical modeling results and their MgO, Mg#, Cr and Ni contents, it can be inferred that they were mostly produced by melting of garnet amphibolite and amphibolite at the base of thickened lower crust. The available geochronological and zircon Hf isotopic data suggest that the major crustal growth period was around 2.7 Ga with the subordinate growth peroid at ~2.9–3.0 Ga. Synthesizing all lines of evidence, we propose that the Jiaodong complex possibly witnessed a transition of tectonic regime during the Meso-Neoarchean.

Geochemistry, Sr-Nd-Pb Isotopic Compositions and Zircon U-Pb Geochronology of Neoproterozoic Mafic Dyke in the Douling Complex, South Qinling Belt, China

Mafic dyke swarm is widely developed in Proterozoic continental lithosphere. The Gangou diabase dyke from the Xixia area, intruded into the Douling complex of the South Qinling belt, yields zircon UPb age of 731 Ma. It has low SiO2 content (49.02 wt.%-49.37 wt.%) and Mg# (34.0–37.7) and shows characteristics of subalkaline tholeiite series. They show high ΣREE (155.5×10 6–184.7×106), weak negative Eu anomaly (SEu=0.88-0.93), slight depletion of Nb and Ta, and enrichment of LILE. Their major and trace element, Sr-Nd-Pb isotope and clinopyroxene compositions indicate that it originated from the partial melting of the asthenospheric mantle within-plate extension setting and was slightly contaminated by crust compositions. The similarities of formation age, petrogenesis, source characteristic and tectonic setting for Gangou diabase with mafic dykes in Wudang Block and mafic volcanic rocks in Yaolinghe Group indicate that the South Qinling belt underwent strong continental extension in Neoproterozoic during 796–685 Ma most likely correspondence to the breaking-up of the Rodinia supercontinent.

Polymineralic inclusions as tracers of multistage metasomatism in a paleo mantle wedge

Paleosubduction zones are key areas where slab-derived fluids interact with the overlying mantle wedge. Slab fluids play therefore an important role in the deep element cycle and can metasomatize ultramafics showing particular mineral assemblages. The nature of slab fluids is usually investigated from fluid inclusions and/or multiphase solid inclusions, as well as from incompatible elements in mantle minerals. We have made a study on a garnet-peridotite from the Variscan Ulten Zone (Eastern Alps, Italy) in which peculiar polymineralic inclusions (PI) in garnet provide a wealth of information on the interaction between mantle wedge and slab fluids. The selected sample shows a fine-grained amphibole-olivine-orthopyroxene-spinel-clinopyroxene matrix and a fractured large garnet porphyroclast, surrounded by a kelyphitic corona. Garnet core and rim host randomly distributed PI, which may be as large as 284,000 μm2. The PI show irregular, fracture-controlled shapes and often contain unusual mineral associations. Many PI are directly associated with Mg-hornblende- or chlorite-sealed micro-fractures crosscutting garnet. The most abundant PI contain spinel and amphibole as the main minerals. Spinel-bearing PI consists of high-Cr spinel (Cr# = 0.31–0.41) surrounded by amphiboles (magnesio-hornblende), locally associated with dolomite, sapphirine, calcite, sulphides and kinoshitalite (i.e. a Ba-rich phlogopite). Amphibole-bearing PI contain zoned amphiboles (tschermakitic cores surrounded by Mg-hornblende, which is rimmed by tremolite and Mg-amphibole), dissakisite (an REE-bearing epidote), apatite, dolomite, calcite, spinel, sapphirine, chlorite, kinoshitalite, and sulfides. The retrieved mineral reaction history and the major and some incompatible elements (Cl, Ba, Sr) of PI minerals provide clues about the composition of metasomatic fluids. The studied PI formed in garnet fractures during exhumation by reactions between garnet and (1) slab-derived COH fluids and (2) late-stage saline brines. Metasomatic fluids/brines were rich in volatile elements and had a crustal signature (enrichment in LILE and LREE) with variable Cl contents and CO2/H2O ratios. Products of early garnet-fluid reactions became reactants of later reactions, thus generating complex textures, including replacement textures of sapphirine over spinel, calcite over dolomite and chlorite over Mg-hornblende. Textures and compositions of the PI testify to a multistage metasomatic overprint at peak P-T conditions above 2 GPa (M1), during the first exhumation stage at 775–1000 °C, below 2 GPa (M2) and finally at 775 °C and 1 GPa (M3). In this study we demonstrate that Gibbs free energy minimization may aid in distinguishing between multiphase solid inclusions and polymineralic inclusions; the latter do provide valuable information on the evolution of retrograde P-T paths in the paleosubduction channel, the relative chronology of metasomatic events and clues on the chemical signature of the fluids.

Petrogenesis of Neogene polymagmatic suites at a monogenetic low-volume volcanic province, Bahariya depression, Western Desert, Egypt

Monogenetic volcanoes can yield eruptive suites displaying substantial complexity in compositional characteristics. The Bahariya monogenetic volcanoes (BMV) in the Western Desert, Egypt are a good example. They comprise complex stratigraphic deposits involving scoria cone, lava flows, and subvolcanic sills and dikes related to diverse eruptive styles. Whole-rock and mineral chemistry and 40Ar–39Ar geochronology are used here to document the petrogenesis, source characteristics, and evolution of the Bahariya volcanoes. The architecture of the BMV is the product of two alkali magma batches: pyroclastics and lava flows forming explosive scoria cone (batch 1) and subvolcanic sills (batch 2). The two batches show contrast in the concentrations of incompatible trace elements and REE as well as element ratios such as Nb/Yb, Gd/Yb, Nb/U, and Ce/Pb (36, 5.0, 44, 30 vs. 17, 4.0, 39, 24 for batch 1 and 2, respectively). New whole-rock 40Ar/39Ar dating displays consistent age of 23.71 ± 0.06 and 23.73 ± 0.01 Ma for magma emplacement of batch 2. Batches 1 and 2 share common LILE and LREE enrichments and HFSE depletions, analogous to a HIMU-like, mantle-derived OIB source described from other Paleogene–Oligocene intraplate magmatic provinces in North Africa and worldwide. Trace element modeling proposes a derivation of the Bahariya volcanoes from parental melts generated by 8–12% partial melting of garnet lherzolite and amphibole-bearing garnet lherzolite at 2.18 ± 0.33 and 1.77 ± 0.33 GPa for batch 1 and batch 2, respectively, across the lithosphere–asthenosphere boundary at c. 70–90 km depth (2.14–2.76 GPa). These sources had been earlier metasomatized by volatile-, LILE- and HFSE-rich fluid(s) originating from Neoproterozoic subduction or a Phanerozoic plume. Fractional crystallization involved olivine + clinopyroxene in both batches followed by Fe–Ti oxides + apatite in batch 2. Furthermore, crust contamination/assimilation was an irrelevant process at crustal level during magma ascend to the surface. Data results of the geo-barometric computations disclose two magma storage levels involving intermediate to lower crustal levels at c. 35 km (1.05 GPa) for batch 1 and mid-crustal level at c. 25 km depth (0.75 GPa) for batch 2. This study delivers proof that magmas emitted at Bahariya depression can undergo complex polymagmatic processes during their storage and passage in the crust, mainly due to the existence of a multilevel plumbing system. The origin of the BMV, as with other within-plate volcanoes in North Egypt, appears to be allied to extension-induced asthenosphere upwelling activated by limited exclusion of thickened lithospheric root under a passive rift tectonic regime coupled with the development of lithospheric thinning and continental breakup in North Africa.

Anisian granodiorites and mafic microgranular enclaves in the eastern Kunlun Orogen, NW China: Insights into closure of the eastern Paleo–Tethys

Granitoids and their mafic microgranular enclaves (MMEs) provide important insights into magmatic activity and tectonic evolution associated with accretionary orogens. Here we investigate the origin of Middle Triassic granodiorites and MMEs in the central segment of the eastern Kunlun Orogen (EKLO) in NW China, which is an important region to evaluate the history of closure of the Paleo–Tethys, through zircon U–Pb geochronology, major and trace element geochemistry, and Sr–Nd–Hf isotopes. Zircon U–Pb ages indicate the coeval formation of granodiorites and their MMEs during the Anisian (ca. 245 Ma). The granodiorites are calc‐alkaline and metaluminous with high SiO2 contents and Mg# values. They are characterized by enrichment of LREEs and LILEs, and depletion of Nb, Ta, and Ti. Their Sr–Nd –Hf isotopic compositions indicate that the magmas were derived from enriched mantle and Paleoproterozoic basement. The monzodioritic‐dioritic MMEs exhibit low SiO2 contents and high Al2O3 and MgO with high Mg# values, enrichment in LREEs and LILEs, and depletion in HFSEs with negative Nb, Ta, Ti, and P. In contrast, the gabbroic MMEs show higher Mg# values and Cr contents. Both types of MMEs have Sr–Nd–Hf isotopic compositions similar to the host granodiorites, and originated from mixing of lower crust‐ and enriched mantle‐derived magmas. The mantle‐derived sources were metasomatized by slab‐derived melts. The Middle Permian to Middle Triassic magmatism, metamorphism, sedimentation, and tectonics in the EKLO suggest northward subduction of the Paleo–Tethys Ocean as represented by the A'nyemaqen Ocean. The magma mixing that generated the Anisian granodiorites and MMEs was likely associated with slab break‐off of the Paleo–Tethys Ocean. The formation of the Anisian magmatic rocks in the EKLO suggest that the cessation of subduction of the Paleo–Tethys Ocean and onset of collision took place during the late stage of Middle Triassic.

Geochemical characteristics of igneous host rocks of Lubin-Zardeh Au-Cu deposit, NW Iran

The Lubin-Zardeh Au-Cu ± Pb ± Zn deposit is located in the Azerbaijan-Western Alborz zone, 45 km NE of Zanjan, Iran. It is situated within the Eocene-Oligocene volcanic-plutonic belt of the Tarom subzone of the Alborz mountains range. Quartz monzonite and tuffs are the main host rock of the Lubin-Zardeh Au-Cu ± Pb ± Zn deposit. Granitoid rocks are predominantly high- K, calc-alkaline with shoshonitic affinities. They are I-type, metaluminous, magnesian and are characterized by LILE, LREE and Th enrichment relative to HFSE, depletion in Nb, Ta, Ti and weak depletion in HREE and Y. These granitoids show some affinities with adakites, e.g., high Sr/Y and La/Yb ratios coupled with low Yb and Y, and enrichment in LREE and LILE with weak Eu anomalies. Their relatively low to moderate initial 87Sr/86Sr values (0.7047–0.7051), positive Nd (t = 36 Ma) values (0.39–2.1) and TDM ages of 0.69 to 1.06 Ga, with Pb isotopic ratios of initial 206Pb/204Pb = 18.49–18.68, initial 207Pb/204Pb = 15.58–15.61 and initial 208Pb/204Pb = 38.33–38.77, are consistent with a phlogopite-bearing, amphibole-rich, and enriched lithospheric mantle source metasomatized by earlier subducted sediments. The geochemical and isotopic compositions of the granitoids are similar to the host rocks of porphyry and epithermal deposits in the Urumieh–Dokhtar zone.

The formation of the giant Huayangchuan U-Nb deposit associated with carbonatite in the Qingling Orogenic Belt

Carbonatitic magmatism plays a significant role in outgassing carbon from mantle and the formation of rare earth element (REE), rare metal (e.g., Nb and Th) and other types of deposits. The mechanism of REE mineralization associated with carbonatite have been widely studied. However, it is hard to understand U-Nb mineralization without Th enrichment associated with carbonatite. Here we report a carbonatite-hosted U-Nb deposit in Huayangchuan, located in the north Qinling Orogenic Belt. Field observation, mineralogy and geochemical analysis on a suite of drillhole samples were conducted to decipher the mineralization mechanism and its relationship with carbonatite. Huayangchuan carbonatite samples mainly consist of calcite and augite with small volume of accessory minerals (e.g., allanite, fluorapatite, barite and celestite). Betafite [(Ca,U)2(Ti,Nb,Ta)2O6(OH)] is the major ore-bearing mineral in Huayangchuan deposit.The carbonatite shows high CaO, low MgO and alkali contents, which should be products to be differentiated from primary carbonatite (high MgO and alkali contents). The immiscibility and crystallization processes could explain the high CaO/(CaO + MgO + FeO) ratios and the enrichment of LILE. Numerical modeling also indicates positive δ18OSMOW (7.29 to 15.53‰) and negative δ13CPDB (−5.26 to −10.08‰) shifts are induced by reduced sediments assimilation from source consistent with there being enriched Sr-Nd and low Mg isotopic compositions. LA-ICP-MS zircon U-Pb dating of Huayangchuan carbonatite yielded Triassic ages of 229 ± 3 Ma, which corresponds to the post-collision stage of Qinling Orogen during the middle-late Triassic. We then proposed that the recycling of subducted sediments and later re-melting of those materials in shallow mantle generated the Huayangchuan carbonatite and subsequently formed the Huayangchuan deposit. Fluorine concentration decrease, caused by fluorapatite crystallization, ultimately resulted in betafite mineralization.

Late Paleozoic back-arc basin in the Indochina block: Constraints from the mafic rocks in the Nan and Luang Prabang tectonic zones, Southeast Asia

Southeast Asia was mainly created by the late Paleozoic assemblage of the Eastern Cimmeria with other continental fragments. However, the tectonic nature and geodynamic setting of the key tectonic zones in the Indochina block are still in dispute, hindering the understanding of the Eastern Paleotethyan evolution of Southeast Asia. This study presents a set of new geochronological, elemental and Sr-Nd isotopic data for the late Paleozoic mafic rocks in the Nan and Luang Prabang tectonic zones. The mafic rocks from the Nan and Luang Prabang zones give zircon U-Pb ages of 279–294 Ma and 265–294 Ma, respectively, with xenocryst ages of 302–2524 Ma. They have SiO2 = 45.99–57.47 wt%, Al2O3 = 12.74–21.06 wt%, FeOt = 8.34–15.03 wt% and MgO = 2.79–14.74 wt% with mg-number of 34–74, and fall into the fields of basalt/basaltic andesite and its equivalents. These rocks are characterized by enrichment in LILEs and LREEs, depletion in HFSEs, low initial 87Sr/86Sr ratios (0.7032–0.7057) and high εNd(t) values (+4.8 to +7.4). They might be derived from a mantle wedge source modified by slab-derived components, and formed in a continental back-arc basin setting. Our analysis indicates the development of the Carboniferous-Permian mafic igneous event in the Nan and Luang Prabang tectonic zones, and that the Nan tectonic zone linked northerly with the Luang Prabang zone during the late Paleozoic, tectonically comparable to the Banpo-Nanlinshan back-arc basin in SW Yunnan, SW China. The back-arc basin finally closed in the early-middle Triassic and the subsequent assemblage continued till the latest Triassic in response to the eastward subduction of the Paleotethyan slab.

Early Paleozoic subduction in the Indochina interior: Revealed by Ordo-Silurian mafic-intermediate igneous rocks in South Laos

The early Paleozoic tectonic nature and subsequent orogenic process remain poorly known in the Indochina interior. This study introduces the general geology of South Laos and presents a set of new zircon UPb and HfO isotopic analyses, and whole-rock elemental and Sr-Nd-Pb isotopic data for the mafic-intermediate igneous rocks in South Laos. The representative samples from the previously-mapped Ordo-Silurian strata in the southern Truong Son zone give the formation ages of 434–438 Ma. Five samples from the Tam Ky-Phuoc Son tectonic zone yield zircon UPb ages of 447–485 Ma. The corresponding zircon εHf(t), δ18O and TDM values are in range of +1.6~ −6.5, 6.1–8.6‰, and 1.02–1.79 Ga, respectively. Forty-five mafic-intermediate igneous rocks in South Laos with mg-number = 42–69 have SiO2 = 53.04–61.66 wt%, Al2O3 = 12.13–20.92 wt% and MgO = 2.73–9.55 wt%, and are characterized by enrichment in LILEs and depletion in HFSEs with NbTa negative anomalies. Their (87Sr/86Sr)i ratios range from 0.7044 to 0.7109 and εNd(t) values from −6.2 to −2.6, with (206Pb/204Pb)i = 18.08–18.68, (207Pb/204Pb)i = 15.50–15.77, and (208Pb/204Pb)i = 38.04–38.97. Ten gabbroic samples, with mg-number = 77–79, show similar incompatible elemental ratios and Sr-Nd-Pb-Hf-O isotopic compositions with the other mafic-intermediate igneous rocks, but have lower SiO2 and higher MgO, Cr, Ni and V contents, reflective of a primary mafic magma. Such characteristics suggest a newly-modified wedge source by recycled sediment-derived components. In combination with other geological observations in Central Vietnam, we propose a northerly-younging early Paleozoic igneous event in response to the on-growing accretionary orogenesis in the Indochina interior. The southward consumption of the early Paleozoic Tam Ky-Phuoc Son Ocean in the Indochina interior continued until at least ~430 Ma and the assemblage of the Northern Indochina with the Kontum terrane likely terminated at the early Devonian.

Geochemical and Sr–Nd–Pb isotopic constraints on the origin and petrogenesis of Paleozoic lamproites in the southern Yangtze Block, South China

Lamproites and kimberlites are natural probes of the subcontinental lithospheric mantle providing insights into the Earth’s continental lithosphere. Whole-rock major-, trace-element and Sr–Nd–Pb isotopic compositions of the Paleozoic (~ 253 Ma) lamproite dikes from the Baifen zone of the Zhenyuan area in southeastern Guizhou Province (in the southern Yangtze Block, South China) are presented. The Baifen lamproites are characterized by high MgO (7.84–14.1 wt%), K2O (3.94–5.07 wt%) and TiO2 (2.69–3.23 wt%) contents, low SiO2 (41.3–45.7 wt%), Na2O (0.21–0.28 wt%) and Al2O3 (6.10–7.20 wt%) contents. All lamproites have elevated Cr (452–599 ppm) and Ni (485–549 ppm) abundances, as well as high Ba (1884–3589 ppm), La (160–186 ppm), Sr (898–1152 ppm) and Zr (532–632 ppm) concentrations. They show uniform REE distribution patterns that are strongly enriched in light REEs relative to heavy REEs [(La/Yb)N = 71.1–87.6], and exhibit OIB-like geochemical features with obvious enrichment of both LILEs and HFSEs in the primitive mantle-normalized multi-element distribution diagram. Moderately radiogenic Sr (87Sr/86Sri = 0.706336–0.707439), unradiogenic Nd (143Nd/144Ndi = 0.511687–0.511704 and eNd(t) = − 12.2 to − 11.9), and low initial Pb (206Pb/204Pbi = 16.80–16.90, 207Pb/204Pbi = 15.34–15.35 and 208Pb/204Pbi = 37.43–37.70) isotopic compositions are obtained from the rocks. They yield old model ages of TDM(Nd) = 1.48–1.54 Ga. These signatures suggest that the Baifen lamproite magmas are alkaline, ultrapotassic and ultramafic in character and mainly represent mantle-derived primary melts, which have undergone insignificant crustal contamination and negligible fractional crystallization. The Baifen lamproites originated from a veined metasomatized lithospheric mantle source. We envisage that they were derived by partial melting of old, mineralogically complex metasomatic vein assemblages in the subcontinental lithospheric mantle beneath the southern Yangtze Block. The source region experienced ancient mantle metasomatism with complex modification by enriched fluids and melts. The metasomatic agents are most likely to originate from pre-existing slab subduction beneath the southeastern margin of the Yangtze Block. Tectonically, the Baifen lamproites were emplaced at the southern margin of the Yangtze Block, and they formed in an intraplate extensional setting, showing an anorogenic affinity. In terms of time and space, the genesis of Baifen lamproites is presumably related to the Emeishan large igneous province. The Emeishan mantle plume is suggested as an effective mechanism for rapid extension and thinning of the lithosphere, followed by decompression melting of the subcontinental lithospheric mantle. Combined with the thermal perturbation from asthenospheric upwelling induced by the Emeishan mantle plume, the lamproite magmas, representing small volume and limited partial melts of ancient enriched mantle lithosphere, arose. We propose that the generation of the Baifen lamproite dikes probably was a consequence of the far-field effects of the Emeishan mantle plume.

Geochemical characteristics of Cretaceous granitoids and their implication for tectonics in Vietnam

Vietnam is located between the Pacific subduction zone and Tethys orogenic belt. There are numerous Cretaceous granitoids in Vietnam, belonging quartz-diorite-granosyenite- granite-leucogranite magmatic series. According to geochemical data, granitoids are sub- alkaline. Samples in SV are calc-alkalic, with lower A/CNK and (Zr+Nb+Ce+Y) values, which belong to I-type granite. And those in NV are allali-calcic, with higher A/CNK values but lower (Zr+Nb+Ce+Y) values, which belong to S-type granite. Granitoids exhibit fractionated REE patterns with negative Eu anomalies, and show enrichment in LILEs and depletion in HFSEs. Because of the absence of alkaline dark-colored minerals, abundant intermediate igneous rocks and assemblage of I- and S-type granite with orogen-related geochemical characteristics, it is believed that there was compressive structure in Vietnam in Cretaceous due to Meso-Tethyan subduction.

Petrological and geochemical study of the Sylhet trap basalts, Shillong plateau, N.E. India: Implications for petrogenesis

Sylhet Traps exposed along the southern margin of Shillong plateau, Northeast India are subalkaline tholeiitic basalts. The basalts are generally massive but occasionally contain large amygdules of zeolites and chalcedony. Microscopically, some basalts show porphyritic texture with olivine phenocrysts. Phenocryst assemblage of plagioclase ± clinopyroxene ± olivine implies crystallization at shallow level. SEM-EDX analysis shows occurrences of spinel with Ni and Cr within the basalts therefore indicating partial melting of the subcontinental lithospheric mantle as the possible source materials for the basalts. The multi-element plot for the basalts shows two distinct trends: one with significant enrichment of LILE and depletion of HFSE and plot similar to OIB (Type 1) while the other trends are chara cterized by slight enrichment of LILE and negative anomalies at Nb, P and Ti (Type 2). Chondrite-normalized REE patterns for Type 1 basalt shows very high enrichment of LREE and a strong right dip HREE pattern and also plots similar to typical OIB while Type 2 show a slight enrichment of LREE over HREE with small Eu anomaly. The geochemical signatures suggest crustal contamination by plume-derived magma produced by low degree of partial melting for Type 1 basalt. Type 2 basalt was produced by partial melting of subcontinental lithospheric mantle, which may be triggered by plume upwelling.

Cryptic metasomatic agent measured in situ in Variscan mantle rocks: Melt inclusions in garnet of eclogite, Granulitgebirge, Germany

AbstractGarnet of eclogite (formerly termed garnet clinopyroxenite) hosted in lenses of orogenic garnet peridotite from the Granulitgebirge, NW Bohemian Massif, contains unique inclusions of granitic melt, now either glassy or crystallized. Analysed glasses and re‐homogenized inclusions are hydrous, peraluminous, and enriched in highly incompatible elements characteristic of the continental crust such as Cs, Li, B, Pb, Rb, Th, and U. The original melt thus represents a pristine, chemically evolved metasomatic agent, which infiltrated the mantle via deep continental subduction during the Variscan orogeny. The bulk chemical composition of the studied eclogites is similar to that of Fe‐rich basalt and the enrichment in LILE and U suggest a subduction‐related component. All these geochemical features confirm metasomatism. In comparison with many other garnet+clinopyroxene‐bearing lenses in peridotites of the Bohemian Massif, the studied samples from Rubinberg and Klatschmühle are more akin to eclogite than pyroxenites, as reflected in high jadeite content in clinopyroxene, relatively low Mg, Cr, and Ni but relatively high Ti. However, trace elements of both bulk rock and individual mineral phases show also important differences making these samples rather unique. Metasomatism involving a melt requiring a trace element pattern very similar to the composition reported here has been suggested for the source region of rocks of the so‐called durbachite suite, that is, ultrapotassic melanosyenites, which are found throughout the high‐grade Variscan basement. Moreover, the Th, U, Pb, Nb, Ta, and Ti patterns of these newly studied melt inclusions (MI) strongly resemble those observed for peridotite and its enclosed pyroxenite from the T‐7 borehole (Staré, České Středhoři Mountains) in N Bohemia. This suggests that a similar kind of crustal‐derived melt also occurred here. This study of granitic MI in eclogites from peridotites has provided the first direct characterization of a preserved metasomatic melt, possibly responsible for the metasomatism of several parts of the mantle in the Variscides.

Geochronological and Geochemical Constraints on the Petrogenesis of Early Paleoproterozoic (2.40-2.32 Ga) Nb-Enriched Mafic Rocks in Southwestern Yangtze Block and Its Tectonic Implications

Recent geological survey has identified the Early Paleoproterozoic meta-mafic intrusions in the southwestern Yangtze Block. We present geochronological, whole-rock geochemical and Nd isotopic data for these meta-mafic rocks to better address the tectonic evolution of the Yangtze Block during the Early Paleoproterozoic Period. Geochronological data show that the meta-mafic rocks have zircon ages of 2 395-2 316 Ma. They have high TiO2 contents of 1.40 wt.%3.66 wt.% and Nb concentrations of 13.7 ppm45.5 ppm, thus aregrouped as Nb-enriched mafic rocks. These mafic rocks are characterized by tholeiitic compositions with enrichment of LREEs and LILEs, and can be divided into two groups. Group 1 samples display E-MORB-like geochemical characteristics. Group 2 samples have positive eNd(t) values of 4.0–5.0. Geochemical data indicate that all meta-mafic rocks were likely derived from a depleted asthenospheric mantle. REE modeling indicates lower degree of partial melting for Group 2 samples (3%–10%) relative to Group 1 samples (15%–20%). Taking into account contemporaneous post-collisional granitoids in southwestern Yangtze Block, we propose that these meta-mafic rocks were formed in a post-collisional extension setting. These meta-mafic rocks can be compared with those in Africa, South America and Europe, and might be linked with the Arrowsmith orogenic belt. KEY WORDS: zircon dating, geochemistry, Early Paleoproterozoic, Nb-enriched rocks, Yangtze Block.