LILE Contents Research Articles

Geochemistry and petrogenesis of radioactive Palaeoproterozoic granitoids of Kinwat crystalline inlier, Nanded and Yeotmal districts, Maharashtra

Kinwat crystalline inlier exposes Palaeoproterozoic granitoids belonging to the northern extensions of younger phase of Peninsular gneissic complex (PGC) within Deccan Trap country in Eastern Dharwar Craton (EDC) and bounded in south by a major NW-SE trending lineament (Kaddam fault). Geochemically, the Kinwat granitoids are similar to high-K, calc-alkaline to shoshonite magnesian granitoids and subdivided into two major groups, i.e. felsic group (pink and grey granites) and intermediate to felsic group (hybrid granitoids). The felsic group (∼67–74% SiO2) shares many features with Neoarchaean to Palaeoproterozoic high potassic granites of PGC such as higher LILE and LREE content and marked depletion in Eu, P and HFSE, especially Nb, Ti, relative to LILE and LREE. The hybrid granitoids (∼58–67% SiO2) have comparatively higher Ca, Mg and Na contents and slightly lower REE content than the granitoids of felsic group. Both, felsic and hybrid granitoids are metaluminous to weakly peraluminous and belong to highly fractionated I-type suite as evidenced by negative correlation of SiO2 with MgO, FeOt, CaO, Na2O, Al2O3, whereas K2O, Rb and Ba show sympathetic relationship with SiO2. Moderate to strong fractionated REE patterns (Ce/YbN: ∼54–387) and strong negative Eu anomalies (Eu/Eu*: 0.13–0.41) are quite apparent in these granitoids. The geochemical characteristics together with mineralogical features such as presence of biotite±hornblende as the dominant ferromagnesian mineral phases point towards intracrustal magma source, i.e. derivation of magma by partial melting of probably tonalitic igneous protolith at moderate crustal levels for felsic granites, whereas hybrid granitoids appear to be products of juvenile mantle-crust interaction, in an active continental margin setting.

Bimodal tholeiitic and mildly alkalic basalts from Bhir area, central Deccan Volcanic Province, India: Geochemistry and petrogenesis

Bimodal tholeiitic and mildly alkalic basalts occur near Bhir, in the central part of Deccan Volcanic Province (DVP). Major and trace element concentrations show that, of the ten flows, nine are tholeiitic and one is an alkalic basalt. The Bhir basalts have a wide range of chemical composition. Geochemical variations in the stratigraphic section define three distinct phases of evolution (zones 1 to 3). Crystal fractionation of plagioclase, clinopyroxene, olivine and Fe–Ti oxide expanded the compositional range. Low Mg#s (39–55), low concentrations of Ni and Cr and high Zr suggest the evolved nature of the Bhir basalts. Fractionation modeling suggests about 42% fractional crystallization. In spite of the dominant role of fractional crystallization in the evolution of Bhir basalts, some other processes must be sought to explain the chemical variations. Crustal contamination, magma mixing and degree of partial melting are suggested to explain the observed chemical variations. Resorption, reverse zoning and compositional bimodality in plagioclase phenocrysts indicate magma mixing. Samples of flows one and four suspected of being contaminated all have enriched SiO 2 and LILE (K, Rb, and Ba) contents and depletion in Ti and P, believed to be due to ‘granitic’ crustal contamination. As compared to tholeiitic basalts, the alkalic basalts are characterized by low SiO 2 and high TiO 2, Na 2O, K 2O and P 2O 5. Alkalic basalts are richer in LILE (Rb and Ba), HFSE (Nb, Zr, and Y) and REE than the tholeiitic basalts. The alkalic basalt occurrence is important from a petrogenetic point of view and also suggests that the sources of alkalic basalt magmas may be of variable ages under different parts of the DVP. Based on major, trace and rare earth element distributions it is suggested that asthenospheric mantle having affinities with the source of OIB was the source material of the magmas and the range in the composition of tholeiitic and alkalic basalts was probably controlled by different degrees of melting and/or inhomogeneities in the mantle source.

Discrimination and origin of the sanukitoid series: Geochemical constraints from the Neoarchean western Karelian Province (Finland)

Neoarchean late to post-tectonic sanukitoid intrusions are found in the western part of the Karelian Province in Finland. Variable-sized, even-grained to K-feldspar porphyritic intrusions form a series of diorites, tonalites, and granodiorites, that are calc-alkaline to alkali-calcic, magnesian, and mostly peraluminous. The major and trace element geochemistry of the intrusions show typical sanukitoid affinities: a mantle signature (high content of Mg, Ni, Cr and high Mg#) and enrichment in LILE (especially K, Ba, and Sr). The intrusions form a distinctive series that can be distinguished from the TTG series of the Karelian Province by their high Ba–Sr signature, low Na 2O/K 2O ratio, and uniform HREE pattern. The sanukitoid series can be compositionally constrained as follows: SiO 2 = 55–70 wt.%, Na 2O/K 2O = 0.5–3, MgO = 1.5–9 wt.%, Mg# = 45–65, K 2O = 1.5–5.0 wt.%, Ba + Sr > 1400 ppm, and (Gd/Er) N = 2–6. The probable source was enriched subcontinental lithosphere mantle with high LILE concentrations in phlogopite and/or sanidine. The accumulation of K, Ba, and Sr in the mantle source may have occurred as a consequence of two separate metasomatic events: 1) by subduction-related fluids/melts and 2) by metasomatism related to asthenospheric mantle upwelling caused by slab breakoff. The slab breakoff at the end-stage of subduction is also a viable trigger for the sanukitoid magmatism.

Basites of the polychronous magmatic center with the Erdenetiyn-Ovoo porphyry Cu-Mo deposit ( northern Mongolia): petrogeochemistry, 40Ar/ 39Ar geochronology, geodynamic position, and related ore formation

The Erdenetiyn-Ovoo magmatic center (EMC) with a porphyry Cu-Mo deposit includes the following intrusive complexes: Selenga, Shivota, ore-bearing porphyry, and post-ore dike. The EMC formed at 260–200 Ma. The geologic evolution of northern Mongolia in that period was much determined by the effect of a mantle plume, which showed two periods of activity: Late Paleozoic and Early Mesozoic. The long multistage evolution of the EMC was due to its localization on the periphery of the Late Paleozoic and Early Mesozoic areas of the plume’s influence. The Shivota and post-ore basites are considered to be comagmatic to the Late Permian–Early Triassic trachyandesite-basalt and Late Triassic–Early Jurassic trachyandesite series, respectively, which are similar to the products of Late Paleozoic and Early Mesozoic within-plate magmatism in northern Mongolia. The Selenga complex, which formed before the Shivota one, and the porphyry complex, which formed before the post-ore dike one, are differentiated gabbro-granite series. Gabbro-granitoid magmatism was initiated by the melting of rocks of continental lithosphere under the action of a plume. Later on, as the plume ascended to the surface and the lithosphere became thinner, the conditions were created favoring the lithosphere breakthrough and within-plate basaltoid magmatism. In geochemical features (high contents of LILE and LREE, low contents of HFSE and HREE) the studied basites are similar to the products of subduction magmatism. But this contradicts the geologic position of basites formed after the completion of subduction during the transition of the region to the rifting stage and during the rifting. The mantle metasomatized during the preceding subduction is regarded as the main source of basites. The high contents of alkalies and LREE in the volcanics of the post-ore dike complex and the REE patterns similar to the OIB ones evidence the influence of the plume on the magma formation. The high contents of incompatible trace elements and the Nd isotope composition corresponding to the weakly depleted mantle do not exclude a possible plume effect during the formation of the Selenga complex gabbroids. The geochemical features of the Shivota gabbros, comagmatic to volcanics produced during the Late Paleozoic within-plate activity, are partly transformed during the melt evolution in crustal chambers. The REE patterns of the EMC basites evidence that the evolution of ascending magma was accompanied by the fractionation of amphibole. During this process, ore elements were redistributed into mineral and concentrated in amphibole-containing rocks, from which metals were later mobilized by late melts and fluids. The evolution of basaltoid magmatism of the Selenga, Shivota, and porphyry complexes is regarded as a preliminary stage of ore formation, which was considerably responsible for the EMC productivity.

Geochemistry and petrogenesis of basaltic rocks from the Develidağ volcanic complex, Central Anatolia, Turkey

The Develidağ volcanic zone (DVZ) is a part of the Middle Miocene-Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia. The DVZ is located at the eastern side of the Plio-Quaternary Sultansazlığı pull-apart basin, that opened along the Ecemiş left-lateral strike-slip fault. Towards the south, the volcanic rocks of the DVZ overlie the Paleozoic metamorphic rocks of the Taurus range. Regional stratigraphic studies indicate that volcanic rocks of the DVZ crop out in a ∼N–S trending rectangular-shaped exposure in the middle of which Middle Miocene andesitic rocks are found surrounded by Upper Miocene basaltic rocks. The purpose of this paper is to present basic geochemical data for the DVZ rocks and discuss possible processes of magma generation. The Develidağ basalts are characterized by low LILE (Rb, K, Ba, Th) and high HFSE (Nb,Zr,Hf,Y) contents, whereas the andesites generally have high LILE and HFSE values except for Nb and Zr. Variable abundances of Pb (3.11–12.09 ppm) and U (0.36–2.64 ppm) are associated with high Ba content within the rock suites. Although low Nb/La (0.6–0.7) and relatively high Ba/Nb ratios indicate crustal involvement for the basalts, high Zr/Ba (0.5), Zr/Hf (42–47) and Th/U (3.13–4.69) values imply contributions from an asthenospheric source component. Furthermore, the high Zr/Hf values (>36) are the diagnostic feature of metasomatized mantle (Dupuy et al., 1992; Rudnick et al., 1993). Moreover, multi-element patterns show that Develidağ basalts have similar trace element signatures to those of the US Cascades tholeiites. The ratios of Zr/Hf, Zr/Ba, Nb/Th and Sr/Ce indicate that basaltic rocks are derived from a MORB-like mantle, and calculated melting model reflects generation from a spinel peridotite source (3–4% melting), but the combined effects of melting and assimilation and fractional crystallization (AFC) processes seem to be partially responsible for the relatively evolved rocks. Typical tholeiitic-calk-alkaline associations are also observed among the volcanic rocks as a consequence of melting processes and crustal involvement, or due to the changes in the melting rate and degree. The lack of alkaline rocks that is conventionally observed during the final stage of many central Anatolian volcanic centers may be attributed either to the lack of extensional development or to a deficient amount of extension before the late Miocene.

Caracterização geoquímica e isotópica do batólito Serra do Catu e sua evolução da interface dos domínios Sergipano e Pernambuco-Alagoas, Província Borborema

The Serra do Catu Batolith is located in the interface between the Pernambuco-Alagoas Domain and the Sergipano Belt/Domain. The Pernambuco-Alagoas Domain show rocks with T DM model ages ranging from 2.4Ga to 0.90Ga, while the Sergipano Belt/Domain show record of a ~1.0 Ga orogeny (Maranco-Poco Redondo Domain), and record of Brasiliano Orogeny (Caninde Domain). The Neoproterozoic Serra do Catu Batolith is tardi- to post-tectonic and it was emplaced into a sinistral transpressive shear zone in pull-apart structure. It comprises three coeval felsic facies: quartz monzonite to monzogranite, discontinuously distributed in the periphery of the intrusion (the oldest); (quartz) alkalifeldspar gray syenite, occupying the central portion of the batolith, and the quartz syenite to quartz alkalifeldspar pink syenite (the younger), disposed among the two previous units. The lithotypes belong to the shoshonitic-ultrapotassic series, they present high LILE (Rb, Ba and Sr) and low HFSE (Nb, Zr, Y) content, and exhibit outstanding negative anomalies of Nb, Ta, P and Ti and geochemical signature related to subducction in continental arc setting. The isotopic results of this Batolith indicate eNd (0,6Ma.) of -3.8 to -5.45 and model ages T DM ranging from 1.4 to 1.6 Ga. Considering the geologic and isotopic data, it is proposed that the protholith of the studied sienitoids involved an enriched lithospheric mantle and orthorocks from the Sergipano Domain . The available data suggest that the activity of the shear zone was possibly related for enriched lithosferic mantle partial melt, by decompression melting, generating the magmas with the observed isotopic signatures

Caracterização geoquímica e isotópica do batólito Serra do Catu e sua evolução da interface dos domínios Sergipano e Pernambuco-Alagoas, Província Borborema

The Serra do Catu Batolith is located in the interface between the Pernambuco-Alagoas Domain and the Sergipano Belt/Domain. The Pernambuco-Alagoas Domain show rocks with T DM model ages ranging from 2.4Ga to 0.90Ga, while the Sergipano Belt/Domain show record of a ~1.0 Ga orogeny (Marancó-Poço Redondo Domain), and record of Brasiliano Orogeny (Canindé Domain). The Neoproterozoic Serra do Catu Batolith is tardi- to post-tectonic and it was emplaced into a sinistral transpressive shear zone in pull-apart structure. It comprises three coeval felsic facies: quartz monzonite to monzogranite, discontinuously distributed in the periphery of the intrusion (the oldest); (quartz) alkalifeldspar gray syenite, occupying the central portion of the batolith, and the quartz syenite to quartz alkalifeldspar pink syenite (the younger), disposed among the two previous units. The lithotypes belong to the shoshonitic-ultrapotassic series, they present high LILE (Rb, Ba and Sr) and low HFSE (Nb, Zr, Y) content, and exhibit outstanding negative anomalies of Nb, Ta, P and Ti and geochemical signature related to subducction in continental arc setting. The isotopic results of this Batolith indicate εNd (0,6Ma.) of -3.8 to -5.45 and model ages T DM ranging from 1.4 to 1.6 Ga. Considering the geologic and isotopic data, it is proposed that the protholith of the studied sienitoids involved an enriched lithospheric mantle and orthorocks from the Sergipano Domain . The available data suggest that the activity of the shear zone was possibly related for enriched lithosferic mantle partial melt, by decompression melting, generating the magmas with the observed isotopic signatures

Geochemistry and geochronology of migmatites of the Kurul’ta-Nyukzha segment and the problem of correlation between metamorphic events in the Dzhugdzhur-Stanovoi folded area, Eastern Siberia

Crystalline schists of the El’gakan unit (Nyukzha River) were affected by penetrative (volume) replacement by plagiogneisses and granite-gneisses (Lc1) and were then transformed into a polymigmatite complex with successively developing leucosomes Lc2, Lc3, and Lc4. After a thrust-nappe structure was formed in response to collision processes, a new generation of granite veins was produced (Lc5), and then tonalite gneisses Lc6avt and branching migmatites with leucosomes Lc6all were formed along strike-slip fault zones. Zircons from granite-gneisses Lc1 were classified into four types (populations) based on SHRIMP II data. Type I (rhythmically zonal cores) were dated at 2960 and 3010 Ma, which is correlated with the age of the magmatic (predominantly volcanic) protolith. Types II and III were dated at 2703 Ma, which corresponds to granitization under amphibolite-facies conditions and the origin of the Stanoi granite-gneiss. This event is correlated with granulite metamorphism and ultrametamorphism over the whole territory of the Dzhugdzhur-Stanovoi folded area. The most widely spread type IV of the zircons has an age of 1915 Ma, which corresponds to the metamorphism coeval with overthrusting and, hence, with the collision of the Stanovoi plate and a margin of the Siberian Platform. Concentrations of REE, U, and Th and the Th/U ratio were determined to systematically decrease from type I to IV of the zircons (except their type III, whose Th/U ratio increases to >1). Zircons from Lc5 have a concordant age of 139 Ma, which is comparable with the age of the Late Stanovoi granites. The compositional changes from the older cores to younger rims of zircons from Lc5 are analogous to those mentioned above for zircon from Lc1. The concordant age of zircons from Lc6avt is 127–130 Ma. Their Th/U ratio increases from cores ( 1), which suggests that melt may have appeared when Lc6avt was formed. ICP-MS analyses of 53 rock samples reveal differences in the character of the trend (increase/decrease) and magnitude of the changes in the concentrations of trace elements in the distinguished granitization and migmatization series; correlations were revealed between the concentrations of elements and composition of the rock groups. For example, the development of Lc1 was associated with enrichment in Rb, Sr, Ba, LREE, Th, Zr, and Hf at depletion in Nb, Ta, U, and HREE relative to the original rocks. The leucosomes of the Lc2, Lc3, and Lc4 migmatites are depleted in all of these elements except LILE, which is thought to be explained by infiltration-controlled granitization with volume replacement and partial melting at the development of vein leucosome and the subsequent mobilization of the melts together with residues. The different signs of the changes in the LREE and LILE concentrations is unusual for anatectic processes and can be modeled by equilibrium or disequilibrium melting.

Fluid/mineral interaction in UHP garnet peridotite

We present two case studies of metasomatised garnet peridotite from the Sulu (Zhimafang) and of garnet orthopyroxenite from the Dabie Shan (Maowu) ultrahigh-pressure terranes (Eastern China). The mantle-derived peridotite from Zhimafang shows two ultrahigh-pressure (UHP) mineral assemblages. The older one is made of porphyroclastic garnet rich in inclusions (Grt 1), coarse exsolved clinopyroxene (Cpx 1) and coarse phlogopite flakes (Phl 1). The younger paragenesis consists of fine-grained olivine + clinopyroxene (Cpx 2) + orthopyroxene ± magnesite ± Phl 2 equilibrated with neoblastic garnet (Grt 2). The inclusions inside porphyroclastic Grt 1 are polyphase secondary inclusions related to microfractures cutting the garnet core. They display irregular shapes and contain microcrystals of calcic-amphibole, chlorite, phlogopite and rare talc, associated with pyrite and/or spinel. The low Al 2O 3 content (< 0.2 wt.%) in orthopyroxene coexisting with garnets and clinopyroxenes indicates equilibration at P = 4.0–6.0 GPa and T = 700–1000 °C. The trace element composition of Cpx 1 and Phl 1 combined with the petrologic and isotopic data of Yang and Jahn [Yang, J.J., Jahn, B.M., 2000. Deep subduction of mantle-derived garnet peridotites from the Su-Lu UHP metamorphic terrane in China. Journal of Metamorphic Geology 18, 167–180.] suggests that the Zhimafang garnet peridotite experienced metasomatism by a melt with alkaline character at high-temperature conditions ( T ∼ 1000 °C and P > 5.0 GPa). The microtextural identification of pseudosecondary inclusions in the porphyroclastic garnet core and their geochemical characterisation indicate that an incompatible element- and silicate-rich fluid subsequently metasomatised the garnet peridotite and equilibrated with the newly formed Cpx 2 probably during Triassic UHP metamorphism. Ultramafic metasomatic layers at Maowu Ultramafic Complex (Dabie Shan) consist of layered websterite and orthopyroxenite which preserve an old olivine + orthopyroxene (Opx 1) + garnet (Grt 1) ± Ti-clinohumite paragenesis, overgrown by poikilitic Opx 2. Grt 2 is associated with Opx 2 + phlogopite along the foliation, and fine-grained idiomorphic clinopyroxene also occurs. Grt 2 cores contain disseminated primary polyphase inclusions. The textural and geochemical analyses of the primary polyphase inclusions indicate that they derive from a homogeneous fluid characterised by high LILE concentrations with spikes in Cs, Ba, Pb and high U/Th. These inclusions are interpreted as remnants of the LILE- and LREE-enriched residual fluid produced when a crust-derived Si-rich metasomatic agent reacted with a previous harzburgite to form garnet orthopyroxenite. The in-situ trace element analyses of the major phases garnet, clinopyroxene and phlogopite that formed at the same time as the polyphase inclusions at Maowu, permit the determination of empirical mineral/fluid partitioning at pressures relevant for element recycling in subduction zones. Our estimated D Cpx/fluid suggests that all LILE are highly incompatible, Th and U are moderately incompatible, Pb is close to unity and Sr is moderately compatible. Phlogopite preferentially incorporates Rb and K with respect to Ba and Cs, and Th with respect to U. The similarity between the residual Maowu fluid with the secondary inclusions in the UHP wedge-type garnet peridotite from Sulu, indicates that the fluids produced from reactions at the slab–mantle interface may be effective metasomatic agents in the mantle wedge. Such reactions may produce phlogopite, which plays an important role in controlling the LILE characteristics of the slab-derived fluid in subduction zones.

Blueschist-facies rehydration of eclogites (Tian Shan, NW-China): Implications for fluid–rock interaction in the subduction channel

Chemical changes associated with the rehydration of dry eclogites to form blueschists were studied to obtain information about the chemistry of the fluids infiltrating during this retrograde metamorphic overprint. The studied eclogites of the Tian Shan were formed during Carboniferous subduction of pillow basalts that show typical ocean island basalt geochemical signatures. The retrograde P– T path is characterised by decompression associated with cooling, typical for fragments of a subducting slab that are ascending in the cool and hydrated subduction channel. The fluids infiltrating the eclogites are interpreted to represent dehydration fluids of the down going slab that ascended to the overlying subduction channel. The rehydration of the eclogites proceeded from the pillow margins producing two concentric shells consisting of glaucophane-dominated blueschists (inner shell) and phengite–ankerite blueschists (outer shell) replacing omphacite and garnet of the eclogite. Mass-balance calculations based on major and trace element compositions of eclogites and blueschists point to high element mobility during fluid infiltration and associated metamorphic reactions. Blueschists show lower contents of REE, Y, Sr, Pb, U, P, Ca, Na, Al and Si compared to eclogites, indicating a loss of these elements, but they show higher contents of volatiles, Mg, transition metals and LILE, indicating a gain of these elements. The chemical changes point to a composition of the infiltrating fluid that has an affinity to fluids derived from serpentinites, i.e. with low concentrations in Si, Al, Ca and Na and high concentrations in Mg, Co and Ni. The gain of Cu, Mn, Zn and LILE in the blueschists points to an addition of a sediment-derived component in the serpentinite-derived fluid. The pronounced enrichment of K 2O and of Ba at constant Ba/Th ratios in the blueschists resembles those of island arc basalts derived from a fluid-enriched source. The low element load of the fluid equilibrated with Mg-rich mélange zones induces the mobility of most of the elements during the eclogite–blueschist transformation to compensate for disequilibrium between the mafic rock and the infiltrating fluid. The associated volume loss enhances the fluid infiltration into the eclogite interior. Increasing precipitation rates and/or diminishing diffusion rates for element removal brought the eclogite–blueschist transformation to an end, which resulted in the preservation of eclogite in the pillow cores.

Geochemistry of migmatite-granite connection: a case study from the Central Rhodope, Bulgaria

Extensive crustal melting and intrusive granite magmatism accompanied the extensional stage of the Rhodope massif Alpine evolution. Granites of different structural position and time of crystallization could reveal compositional and temporal relations between anatectic migmatization and granite magma generation. We have studied the post-kinematic Smilyan pluton and smaller syn-kinematic granite bodies hosted by the Madan unit metatexitic gneisses in the southwestern periphery of a metamorphic core complex known as Central Rhodopian Dome (CRD). The dominant geochemical features of the Madan unit granites display remarkable similarities with in situ formed anatectic melts from the CRD diatexitic core (Arda unit): felsic peraluminous compositions, low HFSE and REE, high LILE contents and LREE/HREE ratios, and negligible to positive Eu anomaly. Some deviations of the syn-kinematic granites geochemistry (metaluminous compositions, REE and HFSE enrichment, LREE/HREE ratios and Eu/Eu* variation) support an idea of separate partial melt batches extracted from different precursor compositions. Age data available (Smilyan pluton 43 Ma, and CRD core anatectic melts 37–38 Ma) preclude direct feedback relations between Madan unit intrusive granites and Arda unit migmatites. We infer that the migmatite-granite connection should be considered a common process of protracted Tertiary crustal melting that operated during CRD evolution and affected different crustal sources to produce discrete portions of granite melts. Major and trace elements geochemistry reveals potential lines of descent amongst groups of spatially related granitic rocks due to the same mechanism of melt generation.

Petrogenesis of Permian alkaline lamprophyres and diabases from the Spanish Central System and their geodynamic context within western Europe

Basic to ultrabasic alkaline lamprophyres and diabases intruded within the Spanish Central System (SCS) during Upper Permian. Their high LREE, LILE and HFSE contents, together with positive Nb–Ta anomalies, link their origin with the infiltration of sublithospheric K-rich fluids. These alkaline dykes may be classified in two distinct groups according to the Sr–Nd isotope ratios: (1) a depleted PREMA-like asthenospheric component, and (2) a BSE-like lithospheric component. A slight enrichment in radiogenic 207Pb and 208Pb allows the contribution of a recycled crustal or lithospheric component in the mantle sources. The intrusion of this alkaline magmatism is likely to have occurred due to adiabatic decompression and mantle upwelling in the context of the widespread rifting developed from Carboniferous to Permian in western Europe. The clear differences in the geochemical affinity of Lower Permian basic magmas from north-western and south-western Europe might be interpreted in terms of a more extensive separation of both regions during that period, until they were assembled during Upper Permian.

Geochemistry and Sr-Nd isotope systematics of metabasites in the Tunchang area, Hainan Island, South China: implications for petrogenesis and tectonic setting

The metabasites in Tunchang area, East-central Hainan Island, South China generally display strongly LREE-depleted patterns, extremely low HFSE concentrations and relatively high LILE contents; a few boninite-like samples have a concave-up, slightly LREE-enriched pattern. The low but variable Ti/V ratios between 6 and 20, and the high mg-numbers and Cr, Co, Ni contents, are indicative of plume-influenced intraoceanic arc magmas. It accord with this model are moderately high ɛNd(t) values ranging from +2.16 to +6.75, and 87Sr/86Sri values ranging from 0.70239 to 0.70824. These geochemical features imply that the protoliths for the Tunchang area metabasites are relics of an intra-oceanic island arc and the parental melts of these rocks were generated from a depleted, primitive mantle arc source rather than from a N-MORB source in a supra-subduction zone-like setting. The minor geochemical differences between the two identified groups of metabasites are due to a combined result of the variable degree of partial melting and the crystal fractionation of clinoproxene ± olivine ± plagioclase. The Tunchang area metabasites are distinctive in geochemical and isotopic compositions, and likely also in age from the Bangxi area metabasites in the northwestern part of Hainan Island. We assume that the NE–SW trending Baisha Fault Zone likely marks an early suture zone due to the collision of an Early Paleozoic intra-oceanic arc with the South China continent. In contrast, the Bangxi area metabasites more likely are formed in an extensional back-arc basin due to subduction of the Paleo-Tethys beneath the South China continental margin during the Late Paleozoic.

Geochemistry and age of metamorphic rocks of the Khavyven Highland, eastern Kamchatka

The metamorphic rocks of the Khavyven Highland in eastern Kamchatka were determined to comprise two complexes of metavolcanic rocks that have different ages and are associated with subordinate amounts of metasediments. The complex composing the lower part of the visible vertical section of the highland is dominated by leucocratic amphibole-mica (±garnet) and epidote-mica (±garnet) crystalline schists, whose protoliths were andesites and dacites and their high-K varieties of the island-arc calc-alkaline series. The other complex, composing the upper part of the vertical section, consists of spilitized basaltoids transformed into epidote-amphibole and phengite-epidote-amphibole green schists, which form (together with quartzites, serpentinized peridotites, serpentinites, and gabbroids) a sea-margin ophiolitic association. The high LILE concentrations, high K/La, Ba/Th, Th/Ta, and La/Nb ratios, deep Ta-Nb minima, and low (La/Yb)N and high 87Sr/86Sr ratios of the crystalline schists of the lower unit are demonstrated to testify to their subduction nature and suggest that their protolithic volcanics were produced in the suprasubduction environment of the Ozernoi-Valaginskii (Achaivayam-Valaginskii) island volcanic arc of Campanian-Paleogene age. The green schists of the upper unit show features of depleted MOR tholeiitic melts and subduction melts, which cause the deep Ta-Nb minima, and low K/La and 87Sr/86Sr ratios suggesting that the green schists were formed in a marginal basin in front of the Ozernoi-Valaginskaya island arc. Recently obtained K-Ar ages in the Khavyven Highland vary from 32.4 to 39.3 Ma and indicate that the metamorphism of the protolithic rocks occurred in the Eocene under the effect of collision and accretion processes of the arc complexes of the Ozernoi-Valaginskii and Kronotskii island arcs with the Asian continent and the closure of forearc oceanic basins in front of them. The modern position of the collision suture that marks the fossil subduction zone of the Ozernoi-Valaginskii arc and is spatially restricted to the buried Khavyven uplift in the Central Kamchatka Depression, which is characterized by well-pronounced linear gravity anomalies.

Petrology and geochemistry of Zijinshan alkaline intrusive complex in Shanxi Province, western North China Craton: Implication for magma mixing of different sources in an extensional regime

In situ zircon U–Pb ages and Hf isotopic compositions and whole rock geochemical and Sr–Nd–Pb isotopic data are presented for the Zijinshan alkaline intrusive complex from the Shanxi Province, western North China Craton. Salic rocks dominate the complex with the monzonite occurring in the outermost and pseudoleucite phonolitic breccia in the center. The intrusion took place 127 Ma ago with the earliest emplacement of monzonite and the termination of cryptoexplosive pseudoleucite phonolitic breccia. All rocks from this complex show LREE enrichment and HFSE depletion and exhibit enriched to depleted Sr–Nd isotopic features. The presence of inherited zircons and enriched Hf isotopic compositions in zircon rims, along with the enriched whole rock Sr–Nd isotopic compositions, indicate that the monzonite was formed through the mixing of lithospheric mantle-derived magma with lower crust-derived melts. The diopside syenite and nepheline-bearing diopside syenite are more depleted than the monzonite in terms of the Sr and Nd isotopes, together with their very high concentrations of LILE, we proposed that they originated from a mixed mantle source of enriched lithospheric mantle and depleted asthenosphere. The nepheline syenite has very low concentrations of MgO, Ni, Cr, suggesting that the magma underwent significant crystal fractionation. The most depleted Sr and Nd isotopic compositions (( 87Sr/ 86Sr) i = 0.7036–0.7042, ε Nd( t) = − 0.2–0.3) among all rock types indicate a great contribution of asthenosphere to the nepheline syenite. The Zijinshan complex and its related crust-mantle interaction occurred in an extensional environment which resulted in continuously asthenospheric upwelling. Such an extensional environment might have been developed during the post-orogenic stage of the Late Paleozoic amalgamation of North China Craton with Mongolian continents and subsequent Mongol–Okhotsk ocean closure.

Experimental constraints on element mobility from subducted sediments using high-P synthetic fluid/melt inclusions

A series of hydrothermal piston-cylinder experiments have been performed to determine the composition of representative fluids and fluid/melt/rock interaction in subduction zones. Experiments were conducted under H 2O saturated conditions at 2.2 GPa over a temperature range from 600–750 °C. The experiments contained synthetic, trace-element-doped pelitic starting material and fractured quartz chips to trap and preserve synthetic fluid/melt inclusions. Pelite residues from the subsolidus experiments (600–650 °C) consist of an eclogite-facies mineral assemblage including quartz, phengite, epidote, rutile, garnet, amphibole, apatite, and zircon. Coexisting hydrous fluids are expected to be completely buffered for trace elements by this mineral assemblage. At 2.2 GPa the wet solidus for the pelitic starting material is located at approximately 675 °C and hydrous fluid and melt coexist as immiscible phases at least up to 750 °C. Residue phases in the supersolidus experiments (700–750 °C) are garnet, rutile, and zircon, which suggest that HREE and HFSE are largely retained in slab residues during very high degrees of H 2O saturated melting. Laser ablation ICPMS analysis and quantification of trapped fluid inclusions from the experiments indicate that subsolidus hydrous fluids released from subducted sediments have relatively high LILE contents compared to REE and HFSE, but overall are remarkably dilute. Total solute contents are approximately 5 wt.%, of which > 75% is SiO 2 and around 15% is Na 2O + Al 2O 3. The experimental results are used to show that subducting sedimentary rocks do not undergo significant element loss during metamorphic dehydration up to eclogite facies. If these fluids are representative of aqueous fluids released at sub-arc depths then simple slab dehydration models may be unable to account for element transfer from the slab to arc magmas. Instead, element recycling through subduction zones may be a product of complex fluid-melt-rock interaction processes involving multiple slab components.

Geochemistry and geodynamic setting of Late Cretaceous-Miocene basalts in the southern Korean Peninsula

Newly obtained data highlight strong geological and geochemical differences between Late Cretaceous-Paleogene and Eocene-Middle Miocene volcanic rocks in the southern Korean Peninsula. The rocks are spatially separated and differ in the proportions of acid and basic varieties. The Late Cretaceous-Paleogene basalts are similar to suprasubduction rocks in having high Al2O3, LILE, and Th contents, and low TiO2 and HFSE contents. The Miocene basalts have a composition intermediate between those of subduction and within-plate rocks. Compared to subduction rocks, they are lower in radiogenic Sr, K, LILE (Cs, Rb, Ba), and Th and higher in MgO, Ni, Ti, and HREE. A drastic change in U, Ba, Rb, Ce, Th, and 87Sr/86Sr in the basic volcanic rocks of the southern Korean Peninsula at the Late Cretaceous-Paleogene boundary suggests a decreasing sedimentary contribution to the magma. The latter testifies to a change in the direction of the motion of the oceanic and continental plates, increasing compressional forces and, finally, the cessation of subduction. The synthesis of the original authors and published data on Cenozoic volcanism of the southern Korean Peninsula and the eastern Sikhote Alin showed that the tectonic evolution of the eastern Eurasian margin occurred in four stages: Late Cretaceous-Paleogene, Eocene-Oligocene, Early, and Middle-Miocene.

Origin of a mica megacryst in an alkaline dyke from the Veneto Volcanic Province, Italy

We report the first finding of a mica megacryst included in an alkaline basic dyke belonging to the Tertiary Veneto Volcanic Province (northern Italy). The megacryst is surrounded by a corona with a mineral assemblage consisting of fine-grained mica flakes + Ti-magnetite. The megacryst is a biotite with a mg# of 0.75, containing 1.35 wt. % TiO 2 , 1041 ppm Ba and 3.8 wt. % H 2 O. The mica flakes have a mg# of 0.79 and contain 4.33 wt. % TiO 2 ,21 968 ppm Ba and 3.0 wt. % H 2 O. Both mica types show high concentrations of LILE and Nb (461–135 ppm). REE contents are typically below the analytical detection limit. Fractional crystallization modelling shows that the mica megacryst crystallized at depths in the mantle consistent with the spinel-facies stability field. The unusually high Nb N /Sr N and Sr N /Zr N The calculated structural formulae based on 24(O+F+Cl) indicate that full occupancy of the tetrahedral site can only be attained by allowing the entry of a small fraction of Fe 3+ or Ti 4+ . In addition, Ti-oxy (likely coupled with Ba-oxy) substitution occurs to provide charge balance with the excess negative charge at the O4 position. The occurrence of such exchange mechanisms can enhance the thermal stability of the mica flakes that most likely crystallized at shallower depths under high temperature and high f (O 2 ) conditions.

CHARACTERISATION OF THE LITHOSPHERE BENEATH THE NORTH-WESTERN ETHIOPIAN PLATEAU: SPINEL LHERZOLITES FROM INJIBARA (LAKE TANA PROVINCE)

We have investigated xenoliths of spinel lherzolites in a Quaternary basanitic flow near Injibara (Tana Lake Province) to characterise the mantle beneath the north-western Ethiopian Plateau. Previous studies of mantle rocks from the Main Rift (e.g. Mega, South Ethiopia; Bedini et al., 1997), have indicated that peridotites suffered modal (apatite) or cryptic (enrichment/depletion in LILE) metasomatism. The metasomatic agents were identified as silicate melts migrating through the lithospheric mantle. At Injibara, lherzolites [Ol (46-69), Opx (19-31), Cpx (9-22), and Spl (2-7) in vol.%] show porphyroclastic to equigranular textures. These rocks suffered a modal metasomatic event, testified by growth of Ti-pargasite, and enrichment in major (Mg,Na,Ti) and trace elements (LREE,Rb,Ba,Th,U,Pb). Equilibration temperatures are 907-1015 °C (median values 960-970°C), in agreement with previous data obtained from Injibara mantle xenoliths (954-985°C, Conticelli et al., 1999). Fluid inclusions (<1-40 _m) occur only in the most preserved rocks, along intragranular trails within porphyroclasts, and as rare primary inclusions in Opx neoblasts. At room temperature, monophase (L) and biphase (L+V) inclusions are observed with rare birefringent solids, recognised as magnesite by Raman spectroscopy. Microthermometric data reveal the presence of CO2 that melts (TmCO2) instantaneously at @ -56.6°C. Most of inclusions homogenise to the liquid (ThLCO2) between –39.2 and 31°C, and only rare homogenise to the vapour (ThVCO2; 28.3 - 30.9°C). For a temperature of 970°C, a minimum pressure of 0.95 GPa is obtained by the highest density (1.11 g/cm3) fluid isochore. In conclusion, our data indicate that the mantle under Injibara (North-Western Ethiopian Plateau) underwent a different evolution with respect to the mantle beneath the Rift. In mantle rocks beneath Injibara, growth of amphibole and the LILE enrichment are strictly associated to the migration of a CO2-rich fluid, whereas in rift peridotites, apatite growth and variable LILE contents are related to interaction with silicate melts.

Multiple-mineral inclusions in diamonds from the Snap Lake/King Lake kimberlite dike, Slave craton, Canada: a trace-element perspective

Multiple inclusions of minerals in diamonds from the Snap Lake/King Lake kimberlites of the southeastern Slave craton in Canada have been analyzed for trace elements to elucidate the petrogenetic history of these inclusions, and of their host diamonds. As observed worldwide, the harzburgitic-garnet diamond inclusions (DIs) possess sinusoidal REE patterns that indicate an early depletion event, followed by metasomatism by LREE-enriched, HREE-depleted fluids. Furthermore, these fluids appear to contain appreciable concentrations of LILE and HFSE, based on the increasing abundances of these elements in the olivine inclusion that occurs at the outer portion of a diamond compared to that near the core. The compositions of these fluids are probably a mixture of hydrous-silicic melt, carbonatitic melt, and brine, similar to the compositions of micro-inclusions in diamonds reported by Navon et al. (2003). Comparison between the compositions of majoritic and normal harzburgitic garnets shows that the former are more depleted in terms of major/minor elements (higher Cr#) but significantly more enriched in the REE (up to ∼10×). This characteristic may indicate the higher susceptibility for metasomatic enrichment of previously more depleted garnets. Garnets of eclogitic paragenesis show strong LREE-depleted patterns, whereas the coexisting omphacite inclusion has relatively flat light- and middle-REE but depleted HREE. Whole-rock reconstruction from coexisting garnet and omphacite inclusions indicates that the protolith of these inclusions was probably the extrusive section of an oceanic crust, subducted beneath the Slave craton.