Sodic Amphibole Research Articles

Metamorphic Characteristics and Tectonic Implications of the Kadui Blueschist in the Central Yarlung Zangbo Suture Zone, Southern Tibet

The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone (YZSZ), southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides important clues for better understanding the evolution of the India-Asia convergence zone. In this paper, the systematical petrographic and mineral chemical studies were carried out on the Kadui blueschist, which reveal a mineral assemblage of sodic amphibole, chlorite, epidote, albite and quartz with accessory minerals of titanite, calcite and zircon. Electron microprobe analyses demonstrate that amphibole shows zoned from actinolite core to ferrowinchite/riebeckite rim composition indicating that the sodic amphibole has formed during a prograde metamorphic event. The protolith of the blueschist is an intermediate-basic pyroclastic rock. The calculated pseudosection indicates a clockwise P-T path and constrains peak metamorphic conditions of about 5.9 kbar at 345 °C. This condition is transitional between pumpellyite-actinolite, greenschist and blueschist facies with a burial depth of 20–22 km and a thermal gradient of 15–16 °C/km. This thermal gradient belongs to high pressure intermediate P/T facies series and is possibly related to a warm subduction setting of young oceanic slabs. Our new findings indicate that the Kadui blueschist in the central part of YZSZ experienced a rapid subduction and exhumation process as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage.

Magmatic evolution and controls on rare metal-enrichment of the Strange Lake A-type peralkaline granitic pluton, Québec-Labrador

Although it is well known that A-type granites are enriched in the rare earth elements (REE) and other high field strength elements (HFSE), the magmatic processes that concentrate these elements are still poorly understood. The 1.24 Ga Strange Lake pluton in northern Québec-Labrador provides an extraordinary example of hyper-enrichment in the REE, Zr, and Nb in a peralkaline A-type granite. The pluton consists of two hypersolvus granite units (southern and northern) and a transsolvus granite, all of which contain perthitic alkali feldspar as the earliest major mineral; the transsolvus granite also contains separate albite and microcline crystals. Arfvedsonite, a sodic amphibole, occurs exclusively as phenocrysts in the transsolvus granite, whereas in the hypersolvus granite it is present as a late, interstitial phase. The primary HFSE minerals are zircon, monazite-(Ce), gagarinite-(Ce) and the pyrochlore group minerals.Magma evolution was monitored by the alumina content in the bulk rock, which decreases from the southern to the northern hypersolvus granite and is lowest in the transsolvus granite. Alkalinity indices and bulk Si, Fe, Rb, REE, Zr, Nb concentrations show the opposite trend. Alkali feldspar compositions mirror the trend shown by the bulk rock, i.e., decreasing Al contents are accompanied by increasing Si, Fe3+, REE, Zr and Nb contents.The major driving forces for the evolution of the hypersolvus magma prior to emplacement were the early separation of a fluoride melt from the silicate melt and the crystallization of alkali feldspar and HFSE-rich phases (zircon, monazite-(Ce), pyrochlore group). An alkali feldspar-rich crystal-mush containing LREE-fluoride melt droplets was emplaced as the least evolved southern hypersolvus granite. Massive fractionation of alkali feldspar led to a sharp increase in ƒH2O and F− activity in the magma chamber that triggered the crystallization of arfvedsonite and was followed by emplacement of the northern hypersolvus granite, which contained a higher proportion of LREE-fluoride melt droplets. Further evolution in the magma chamber led to a transition from a miaskitic to an agpaitic composition. The transsolvus granite was intruded in the form of a low viscosity crystal mush of alkali feldspar, quartz, arfvedsonite (after appreciable crystallization of arfvedsonite) and LREE-fluoride melt droplets. Upon emplacement, arfvedsonite (and gagarinite-(Ce)) crystals segregated as cumulates in response to a combination of flow differentiation and gravity settling. The immiscible fluoride melt accumulated in a volatile-rich residual silicate magma, which migrated to the top of the pluton where it formed the F-REE-rich cores of highly mineralized pegmatites.

Fenitization in the giant Bayan Obo REE-Nb-Fe deposit: Implication for REE mineralization

Fenitization, a unique type of alkali metasomatism related to carbonatite or alkaline igneous rocks, is widely distributed around the Mesoproterozoic carbonatite dykes in the Bayan Obo giant REE-Nb-Fe deposit. Meanwhile, represented by alkali silicates (sodic amphibole or aegirine) enriched REE-(Fe) ores, there is extensive alkali alteration in ores at Bayan Obo, whose similarity to fenitization has been proposed. Fenitization associated with carbonatite is an appropriate indicator of REE mineralizing process. Alkali silicates (sodic amphibole, aegirine and phlogopite) from the East and Main open pits, and fenite around the Wu dyke at Bayan Obo were collected for EMPA and laser ablation ICPMS analyses. Aegirine and phlogopite from the open pits and fenite around the Wu dyke contain similar major components. Sodic amphiboles from those two localities contain comparable trace element composition but with different Cao, MgO and F contents. Alkali silicates from fenites around the Wu dyke show systematic enrichment of Zr, Hf and Ti over alkali silicates from the open pits. Considering different wallrocks and other factors influencing the composition, combined with previous results of geochronology, mineralogy and fluid inclusion, the alkali alteration in the open pits is essentially fenitization in ore-hosting dolomite. A two-stage fenitization process is supported by the zonal texture of sodic amphiboles from Wu carbonatite dyke. The early stage fenitization was Mg, Na, K, F rich and Fe, La, Ce depleted, distributed in alkali silicates from the banded REE-Fe ores and in the core of sodic amphiboles from the Wu dyke. The late stage was Fe, LREE rich and Mg, Na, K, F depleted, distributed in the massive REE-(Fe) ores and the rim of sodic amphiboles from the Wu dyke. The initial alkali fluids derived from carbonatite commonly exhibits LREE-rich patterns. Therefore, the LREE-depletion and low La/Nd ratio of the early stage silicates indicates intense LREE-bearing mineral precipitation and the REE mineralization gradually weakened in the late stage when alkali fluids kept the initial REE patterns unchanged.

New archaeometrical results related to Neolithic blueschist stone tools from Borsod-Abaúj-Zemplén County, Hungary

26 blueschist Neolithic polished stone tools coming from archaeological sites in Borsod-Abaúj-Zemplén County (NE Hungary) were archaeometrically analysed. All items were described and documented macroscopically and magnetic susceptibility was measured on each tool. Non-destructive XRD analyses were made on all implements to separate blueschists from other metamorphic rocks, because this technique can make a difference between the diagnostic sodic amphiboles and other amphiboles, which was confirmed by EDS/SEM in each case. Electron microprobe analyses were carried out on polished sections and selected intact axes by using “original-surface” method. Non-destructive Prompt Gamma Activation Analysis (PGAA) was performed on selected pieces. Blueschist polished stone tools were classified into five groups based on their mineral components and metamorphic evolution. Mineral chemistry analyses suggest that blueschists suffered a polyphase metamorphism. Our data were compared to the nearest blueschist outcrop in the Meliata Unit (Slovakia); it is supposed that the source area of the blueschist implements is the Meliata Unit and/or the blueschist pebbles from Cretaceous conglomerate in the Pieniny Klippen Belt.

Origin of carbonatites of the Matcha alkaline pluton from Turkestan-Alai ridge, Kyrgyz Southern Tien Shan

Postorogenic alkaline intrusions in the Turkestan-Alai segment of the Southern Tien Shan coexist with dikes and veins of carbonatites dated at ∼220 Ma. They are primarily composed of calcite and dolomite (60–85 %), as well as sodic amphibole, phlogopite, clinopyroxene, microcline, albite, apatite, and magnetite, with accessory niobate, ilmenite, Nb-rutile, titanite, zircon, baddeleyite, monazite-(Ce), barite, and sulfides. The rocks share mineralogical and geochemical similarity with carbonatites that originated by liquid immiscibility at high temperatures above 500°C. Silicate and salt-carbonate melts are derived from sources with mainly negative bulk εND(t) ∼ from −11 to 0 and high initial 87Sr/86Sr ratios (∼ 0.7061-0.7095) which may be due to mixing of PREMA and EM-type mantle material. Pb isotopic ratios in accessory pyrrhotite (206Pb/204Pb = 18.38; 207Pb/204Pb = 15.64; 208Pb/204Pb = 38.41) exhibit an EM 2 trend. The intrusions bear signatures of significant crustal contamination as a result of magma genesis by syntexis and hybridism. Concordant isotope composition changes of δ13C (−6.5 to −1.9 ‰), δ18O (9.2-23 %„), δD (−58 to −41 %„), and δ34S (12.6-12.8 ‰) in minerals and rocks indicate inputs of crustal material at the stage of melting and effect of hot fluids released during dehydration of metamorphosed oceanic basalts or sediments. The observed HFSE patterns of the oldest alkaline gabbro may be due to interaction of the primary mafic magma with IAB-type material. The isotope similarity of alkaline rocks with spatially proximal basalts of the Tarim large igneous province does not contradict the evolution of the Turkestan-Alai Triassic magmatism as the «last echo» of the Tarim mantle plume.

Mid Carboniferous lamprophyres, Cobequid Fault Zone, eastern Canada, linked to sodic granites, voluminous gabbro, and albitization

Major intra-continental shear zones developed during the later stages of continental collision in a back-arc setting are sites of prolonged magmatism. Mantle metasomatism results from both melting of subducted sediments and oceanic crust. In the Cobequid Fault Zone of the northern Appalachians, back-arc A-type granites and gabbros dated ca. 360Ma are locally intruded by lamprophyric dykes dated ca. 335Ma. All the lamprophyres are kersantites with biotite and albite, lesser ilmenite, titanite and fluorapatite, and minor magmatic calcite, allanite, pyrite, magnetite, quartz and K-feldspar in some samples. The lamprophyres show enrichment in Rb, Ba, K, Th and REE and classify as calc-alkaline lamprophyre on the basis of biotite and whole rock chemistry. Pb isotopes lie on a mixing line between normal mantle-derived gabbro and OIB magma. Nd isotopes range from 1.3–3.5 εNdt, a little lower than in local gabbro. Most lamprophyres have δ18O=3.8–4.4‰. Country rock is cut by pyrite-(Mg)-chlorite veins with euhedral allanite crystals that resemble the lamprophyres mineralogically, with the Mg-chlorite representing chloritized glass.Early Carboniferous unenriched mafic dykes and minor volcanic rocks are widespread along the major active strike-slip fault zones. The lamprophyres are geographically restricted to within 10km of a small granitoid pluton with some sodic amphibole and widespread albitization. This was displaced by early Carboniferous strike-slip faulting from its original position close to the large Wentworth Pluton, the site of mantle-derived sodic amphibole granite, a major late gabbro pluton, and a volcanic carapace several kilometres thick, previously demonstrated to be the site of mantle upwelling and metasomatism. The age of the lamprophyres implies that enriched source material in upper lithospheric mantle or lower crust was displaced 50km by crustal scale strike-slip faulting after enrichment by the mantle upwelling before lamprophyre emplacement. This indicates a multi-stage process to emplace lamprophyric magma.

Mineralogy and geochemistry of triassic carbonatites in the Matcha alkaline intrusive complex (Turkestan-Alai Ridge, Kyrgyz Southern Tien Shan), SW Central Asian orogenic belt

Postorogenic intrusions of essexites and alkaline and nepheline syenites in the Turkestan-Alai segment of the Kyrgyz Southern Tien Shan coexist with dikes and veins of carbonatites dated at ∼220 Ma by the Ar–Ar and Rb–Sr age methods. They are mainly composed of calcite and dolomite (60–85%), as well as sodic amphibole, phlogopite, clinopyroxene, microcline, albite, apatite, and magnetite, with accessory niobate, ilmenite, Nb-rutile, titanite, zircon, baddeleyite, monazite-(Ce), barite, and sulfides. The rocks share mineralogical and geochemical similarity with carbonatites that originated by liquid immiscibility at high temperatures above 500 °C. Alkaline silicate and salt-carbonate melts are derived from sources with mainly negative bulk εNd(t) ∼ from −11 to 0 and high initial 87Sr/86Sr ratios (∼0.7061–0.7095) which may be due to mixing of PREMA and EM−type mantle material. Pb isotopic ratios in accessory pyrrhotite (206Pb/204Pb = 18.38; 207Pb/204Pb = 15.64; 208Pb/204Pb = 38.41) exhibit an EM2 trend. The intrusions bear signatures of significant crustal contamination as a result of magma genesis by syntexis and hybridism. Concordant isotope composition changes of δ13C (−6.5 to −1.9‰), δ18O (9.2–23‰), δD (−58 to −41‰), and δ34S (12.6–12.8‰) in minerals and rocks indicate inputs of crustal material at the stage of melting and effect of hot fluids released during dehydration of metamorphosed oceanic basalts or sediments. The observed HFSE patterns of the oldest alkaline gabbro may be due to interaction of the primary mafic magma with IAB-type material. The isotope similarity of alkaline rocks with spatially proximal basalts of the Tarim large igneous province does not contradict the evolution of the Turkestan-Alai Triassic magmatism as the “last echo” of the Tarim mantle plume.

A Nd- and O-isotope study of the REE-rich peralkaline Strange Lake granite: implications for Mesoproterozoic A-type magmatism in the Core Zone (NE-Canada)

It is well established that A-type granites enriched in high field strength elements, such as Zr, Nb and the REE, form in anorogenic tectonic settings. The sources of these elements and the processes controlling their unusual enrichment, however, are still debated. They are addressed here using neodymium and oxygen isotope analyses of samples from the 1.24 Ga Strange Lake pluton in the Paleoproterozoic Core Zone of Quebec-Labrador, an A-type granitic body characterized by hyper-enrichment in the REE, Zr, and Nb. Age-corrected eNd values for bulk rock samples and sodic amphiboles (mainly arfvedsonite) from the pluton range from −0.6 to −5.7, and −0.3 to −5.3, respectively. The eNd values for the Napeu Kainiut quartz monzonite, which hosts the pluton, range from −4.8 to −8.1. The 147Sm/144Nd ratios of the suite and the host quartz monzonite range from 0.0967 to 0.1659, large variations that can be explained by in situ fractionation of early LREE-minerals (Strange Lake), and late hydrothermal HREE remobilization. Oxygen isotope analyses of quartz of both Strange Lake and the host yielded δ18O values between +8.2 and +9.1, which are considerably higher than the mantle value of 5.7 ± 0.2‰. Bulk rock oxygen isotope analyses of biotite-gneisses in the vicinity of the Strange Lake pluton yielded δ18O values of 6.3, 8.6 and 9.6‰. The negative eNd values and positive δ18O values of the Strange Lake and Napeu Kainiut samples indicate that both magmas experienced considerable crustal contamination. The extent of this contamination was estimated, assuming that the contaminants were sedimentary-derived rocks from the underlying Archean Mistinibi (para-) gneiss complex, which is characterized by low eNd and high δ18O values. Mixing of 5–15% of a gneiss, having an eNd value of −15 and a δ18O value of +11, with a moderately enriched mantle source (eNd = +0.9, δ18O = +6.3) would produce values similar to those obtained for the Strange Lake granites. Based on analogies between the Nain Plutonic Suite and the Gardar alkaline igneous province (SW-Greenland), we conclude that the Strange Lake pluton and associated REE-mineralized anorogenic bodies formed from a combination of subduction-induced fertilization of the sublithospheric mantle, crustal extension and in situ magma evolution.

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge, SW Japan

ABSTRACTTo understand tectono‐metamorphic processes within or close to the brittle–ductile transition of quartz‐rich crustal rocks in an accretionary wedge, an integrated field, petrological, geochronological and Raman spectroscopic study was conducted on the Mikabu‐Northern Chichibu belt in SW Japan. Field mapping in central Shikoku reveals that the Northern Chichibu belt is comprised of a pile of four tectono‐stratigraphic units, referred to as A, B, C and D units. The A unit (dominated by pelagic sedimentary rocks) represents the structurally lowest and youngest accretionary complex that forms a composite unit with the Mikabu ophiolitic suite. The B unit (consisting of chert‐clastic rock sequences) overlies the A unit and is overlain by the C and D units (mudstone‐matrix mélange units). Raman spectroscopy of carbonaceous material constrains the peak temperature of each unit to be ~290°C for the A unit, 270–290°C for the B unit, 230–250°C for the C unit and ~220°C for the D unit. Ductile deformation and pervasive metamorphism are limited to rocks in the Mikabu, A and B units. Alkali pyroxene and sodic amphibole occur in metabasite from the Mikabu, A and B units, and the widespread occurrence of prograde veins containing lawsonite+quartz pseudomorphs after laumontite was newly recognized from the C unit. Phase petrological data constrain the peak pressure of each unit to be ~0.65 GPa for the Mikabu‐A unit (aragonite stable), ~0.45–0.6 GPa for the B unit (jadeite+albite stable in the structurally lower part), and ~0.35 GPa for the C unit (prehnite+lawsonite stable). The peak metamorphic pressure increases towards structurally lower and younger accretionary complexes, but the thickness of the preserved strata is insufficient to account for the inferred pressure range. The structural–metamorphic relations imply thickening of the accretionary wedge by underplating was followed by a significant phase of thinning by both ductile and brittle processes.

Fingerprints of forearc element mobility in blueschist-facies metaconglomerates, Catalina Schist, California

ABSTRACTMetaconglomerates in the lawsonite–blueschist facies unit of the Catalina Schist (California) contain gabbroic and dioritic clasts exhibiting evidence for extensive metasomatism during high-P/T metamorphism. We performed whole-rock and in situ analyses of these metaconglomerate clasts to better constrain the composition of infiltrating fluids and to elucidate the history of chemical alteration. Petrographic evidence for this alteration includes replacements of plagioclase by phengite and sodic amphibole rims developed on igneous hornblende. These observations regarding mineral replacement are reinforced by corresponding shifts in chemical compositions. Relative to compositions of presumed protoliths, whole-rock compositions of the metaconglomerate clasts show enrichments in elements that are relatively mobile in aqueous fluids (LILE: K, Rb, Cs, and Ba; Li, B, N), and elevated δ15N, and show depletions in Ca and Sr. Electron and ion microprobe data, and analyses of mineral separates, show that phengite and sodic amphibole are enriched in LILE and Li and B, respectively, relative to the igneous phases they have replaced. Oxygen and C isotope compositions of finely disseminated calcite in the clasts, and of calcite in veins cross-cutting or mantling the clasts, are consistent with crystallization from fluids previously equilibrated with metasedimentary rocks within the same unit. The same fluids are implicated as the source for the Li, B, N, and LILE enrichments. These metaconglomerate clasts provide unique records of forearc metasomatism due to the presumed extremely low and well-constrained concentrations of fluid-mobile elements in their protoliths and the previously published, larger-scale fluid–rock context into which the observed metasomatic changes can be placed.

Halogen (F, Cl, Br, I) behaviour in subducting slabs: A study of lawsonite blueschists in western Turkey

Abstract We examined the F, Cl, Br and I abundance of minimally retrogressed lawsonite blueschists from the Tavsanli Zone in northwest Turkey to evaluate the behaviour of halogens in subduction zones, and to determine the role coexisting high pressure minerals may play in transporting the halogens to the Earth's mantle. The blueschists contain sodic amphibole and lawsonite, with variable amounts of phengite and chlorite, and minor apatite. A positive correlation between Cl, Br and I contents in bulk rocks suggests their overall coherent behaviour in subduction zones, although high ratios of I/Cl and Br/Cl compared to altered oceanic crust indicate that Cl is preferentially lost relative to Br and I before or during blueschist metamorphism. Iodine and F are enriched relative to altered oceanic crust, suggesting incorporation from marine sediments. In situ analyses of minerals in thin sections reveal F preferentially concentrates in apatite (avg. 3.13 wt%), over phengite (482 ppm), lawsonite (avg. 413 ppm) and Na-amphibole (257 ppm). Chlorine also preferentially resides in apatite (138 ppm), followed by equal partitioning between phengite (59 ppm) and Na-amphibole (56 ppm), and lower concentrations in lawsonite (27 ppm). Upon apatite decomposition at a depth of ∼200 km, F may redistribute into lawsonite and phengite in slabs, whilst Cl is likely expelled to the overlying mantle wedge. Given the stability of lawsonite and phengite to a depth of 280–300 km in cold subduction zones, they may transport F beyond subarc depths, contributing to the high F in magmas derived from the deep mantle.

Geochemistry and geochronology of the blueschist in the Heilongjiang Complex and its implications in the late Paleozoic tectonics of eastern NE China

The Paleozoic to early Mesozoic tectonic framework and evolution of Northeast China, especially the Jiamusi block and its related structural belts, are highly debated. In this paper, geochemical, geochronological and isotopic analyses were carried out on the blueschist in the Heilongjiang Complex to address these issues. The Heilongjiang Complex defines the suture belt between the Jiamusi block and the Songliao block in NE China, and the blueschist is a major composition for this complex, coexisting with mafic-ultramafic rocks, greenschist, quartzite and mica schist. The blueschist has a mineral association of sodic amphibole, epidote, chlorite, phengite, albite and quartz with accessory phases of apatite, titanite, zircon and ilmenite. Together with the lithological association, the major and trace element compositions present that the protoliths of the blueschist can be divided into the alkaline and tholeiitic basalts and have OIB affinities, formed in an ocean island setting, indicated by the (La/Yb) N values of 3.57 – 11.54, and the (La/Sm) N values of 0.69 – 3.64. The high and positive εNd (t) values of+3.7 to +9.0, and relative enrichment in Nb (vs. Th) and Ta (vs. U) show that both the alkaline and tholeiitic basalts may be derived from the asthenospheric mantle with insignificant crustal contamination. Magmatic zircons from the blueschist in Yilan area yield a 206Pb/238U age of 281±3Ma, interpreted as its protolithic age. The youngest ages of~200Ma of the detrital zircons in the associated mica schist from Mudanjiang area place constraints on the timing of metamorphism for the blueschist. These indicate that a big ocean existed between the Jiamusi and Songliao blocks at least since the early Permian, and the blueschist formed since the late Triassic to late Jurassic by the subduction of this ocean. Such an ocean during the Permian – Jurassic is difficult to be interpreted by the tectonic evolution of the Paleo-Asian Ocean.

FLUORNATROPYROCHLORE, A NEW PYROCHLORE SUPERGROUP MINERAL FROM THE BOZIGUOER RARE EARTH ELEMENT DEPOSIT, BAICHENG COUNTY, AKESU, XINJIANG, CHINA

Fluornatropyrochlore is a new pyrochlore supergroup mineral species found in the intrusive alkali granite of the Boziguoer rare earth element deposit in Baicheng County, Akesu, Xinjiang Autonomous Region, China. This new mineral is found to be closely associated with microcline, albite, aegirine, a sodic amphibole, biotite, zircon, rutile, thorite, fluorite, fluocerite-(Ce), columbite-(Fe), xenotime-(Y), astrophyllite, chevkinite-(Ce), and fergusonite-(Y). Fluornatropyrochlore occurs mostly as anhedral crystals, rarely as subhedral grains, with a grain size of 0.02 to 0.25 mm. The color is brownish yellow to reddish orange. Other physical properties are: brittle, translucent to transparent, non-fluorescent, adamantine luster, light yellow streak, with Mohs hardness 4–4½, and calculated density 5.275 g cm−3. Fluornatropyrochlore is isotropic, n = 2.10(5) (589.9 nm). It has a cubic crystal structure, with unit-cell parameters determined using single-crystal X-ray diffraction as: a 10.5053(10) A, V 1159.4(2) A3, Z = 8, space group Fd 3 m . The strongest ten X-ray diffraction (XRD) peaks in the powder XRD pattern are [ d in A( I )( hkl )]: 6.0741(3)(111), 3.0416(100)(222), 2.6280(38)(004), 1.8568(34)(044), 1.5820(15)(226), 1.5148(4)(444), 1.3137(2)(008), 1.2045(3)(266), 1.1726(2)(048), and 1.0712(1)(448). Chemical analyses by a combination of electron microprobe, Fourier transform infrared spectroscopy (FTIR), and single crystal-structure refinement give its chemical composition as: Na2O 6.80, K2O 0.01, CaO 2.01, MgO 0.01, FeO 0.05, SrO 0.03, PbO 16.17, Ce2O3 4.29, La2O3 1.65, Nd2O3 0.41, Y2O3 0.42, SiO2 0.03, TiO2 1.36, UO2 5.81, Ta2O5 3.00, Nb2O5 53.42, F 3.19, Cl 0.02, ThO2, Sb2O5 0.01, ZrO2 0.01, MnO 0.04, SnO2 0.34, H2O not detected, with a sum of 98.21 wt.%. The empirical formula derived from its averaged composition is (Na1.03Pb0.34Ca0.17U0.10Th0.01Ce0.12La0.05Y0.02Nd0.01)Σ1.85(Nb1.88Ti0.08Ta0.06Sn0.01)Σ2.03O6.21F0.79 based on 7 anions per formula unit. The crystal structure was refined to an R index of 0.053 using Mo Ka X-ray intensity data. Site-scattering refinement analysis shows that the new mineral is characterized by F dominance at the Y site, Na dominance at the A site, and Nb dominance at the B site, based on the overall electroneutrality requirement of the structural formula.

Systematic mineralogical diversity in A-type granitic intrusions: Control of magmatic source and geological processes

The origin and causes of mineralogical diversity of A-type granites are debated. The series of A-type granite plutons, with distinct mineralogical differences, emplaced along an Upper Paleozoic crustal-scale shear zone in the Cobequid Highlands, Nova Scotia, provide an opportunity to examine the origin of different A-type plutons in a similar tectonic setting. Based on the ferromagnesian minerals present, the plutons are classified into sodic granites with sodic amphibole, calcic granites with calcic amphibole, and biotite granites. Sodic and calcic granites occur exclusively in complex intrusions with subequal amounts of gabbro in the eastern shear zone, whereas plutons in the western shear zone, with lesser gabbro, are solely biotite granites. Trace elements and radiogenic isotopes show that the three granite types have different sources. Intensive parameters including temperature, pressure, and water-in-melt contents were estimated from mineralogical and geochemical data. Modeling of these geochemical data suggests that the biotite and calcic granites were derived by 20%–40% partial melting of intracrustal feldspathic rocks, whereas the sodic granites are extreme fractionates (90%) of coeval mafic magma. We propose that supply of Upper Paleozoic mafic magma, probably related to regional extension and decompression melting beneath the Magdalen Basin, created a deep crustal hot zone in the eastern Cobequid Highlands, and extreme fractionation of underplated mafic sills produced the sodic granites. Heat transfer from crystallizing mafic magma induced partial melting of the surrounding crust, creating batches of biotite and calcic granitic melts in different depths. Fractionated and crustally derived melts segregated along crustal-scale faults, constructing the complex plutons in the east. Melting of the crust was further facilitated by the release of water from the crustal rocks upon heating. In the eastern shear zone, water was released predominantly by magmatic rocks and in lesser amounts compared to the west, where Neoproterozoic sedimentary and volcaniclastic rocks are more abundant. The volatile-rich granitic melts in the western part of the shear zone were crystallized rapidly, stabilizing only biotite. This study demonstrates that the mineralogical variations in A-type granites arise from rather similar magma compositions, but they are important petrogenetic indicators of varying sources, specific magmatic processes, and emplacement conditions.

Emplacement and geochemical evolution of highly evolved syenites investigated by a combined structural and geochemical field study: The lujavrites of the Ilímaussaq complex, SW Greenland

Structural mapping and the combined study of magmatic to solid-state deformation textures and mineral compositions in highly evolved nepheline syenites (lujavrites) of the alkaline to peralkaline Ilímaussaq complex (South Greenland) reveal detailed insight into the emplacement and geochemical evolution of the melts they crystallized from.Based on magmatic to solid-state flow textures such as foliations and lineations, we propose that the investigated rock sequence forms a sill-like structure with a steep feeder zone that flattens out over a short distance and intrudes into less evolved overlying units as sub-horizontal sheets by roof uplift. Systematic compositional variation of early-magmatic eudialyte-group minerals (EGM) in the investigated rock sequence monitors the geochemical evolution of the lujavrite-forming melt(s). The chlorine contents of EGM decrease successively upwards within the rock sequence, which probably indicates a successive increase of water activity during differentiation, consistent with a change from sodic pyroxene (aegirine) to sodic amphibole (arfvedsonite) in the mineral assemblage. Both REE contents and Fe/Mn ratios of EGM are promising differentiation indicators, which increase and decrease, respectively, upwards within the sequence due to fractional crystallization. This closed-system evolution is interrupted by a shift towards less evolved melt compositions in one lujavrite unit, for which we assume magma recharge.Our study demonstrates the strength of a combined structural and petrological approach to understand the petrogenesis of an igneous body in more detail and highlights their close connection.

Magmatic origin of low-T mafic blueschist and greenstone blocks from the Franciscan mélange, San Simeon, California

The Franciscan mélange exposure near San Simeon contains abundant greenstone and minor blueschist blocks that were tectonically boudinaged while encased in the shale-matrix. Tectonic deformation of the blueschists is evident from variable amounts of cataclastic flow along their margins and in pinched tails. Major, trace, and rare earth elemental analyses indicate that blueschist and greenstone blocks in this area of the mélange were derived from sources with MORB-like composition along with some having trace element and REE patterns similar to OIB compositions. Most blocks are low LREE basalts that probably formed in an open ocean ridge setting, but some blocks have high LREE contents similar to off-axis seamounts. Linear trends of incompatible elements for both blueschist and greenstone blocks indicate that both lithologies were probably derived from a similar, variably fractionated, tholeiitic magma.Blueschist blocks with sodic amphibole+lawsonite±epidote were pervasively recrystallized at 300–350°C and foliated during ductile deformation that included folding. Their protolith can only be identified as mafic. A few blocks contain very small amounts of metasedimentary materials indicating some were probably seafloor basalts, but some may have been diabase or even gabbro. Where interlayered sediment was present, the mafic protolith was enriched in K, Rb, and Na. Greenstones, on the other hand, contain abundant pseudomorphic evidence of magmatic textures. Alteration to albite, chlorite and pumpellyite at temperatures of 100–200°C is intense, especially in cataclastic margins and pinched tails. Some of the basaltic greenstones have attached radiolarian chert, and a few have relict diabasic textures.The mafic blueschists and greenstones in the mélange near San Simeon are probably fragments from the uppermost part of the Farallon plate. The blueschists may be mostly mafic slabs uprooted from the subducting crust, underplated to the base of the North American plate and later detached and carried back towards the surface in upwelling mélange. The greenstones are most likely fragments detached from seamounts subducted during times of mélange upwelling.

Early Tertiary deformation of the Zhongba–Gyangze Thrust in central southern Tibet

As the boundary thrust between India and Asia in southern Tibet, the Zhongba–Gyangze Thrust (ZGT) emplaced the Yarlung Zangbo Suture Zone (YZSZ) units in the hanging wall southward onto Tethyan Himalaya sequences (THS) of the northern Indian continental margin in the footwall. Detailed field investigation, electron backscatter diffraction (EBSD) analysis, detrital zircon U–Pb geochronology and 40Ar–39Ar thermochronology were conducted to understand the evolution of the ZGT in Sangsang area, central southern Tibet. The shear zone of ZGT is located within the sedimentary-matrix mélange of YZSZ that is mainly composed of matrix of blueschist with meta pelagic–hemipelagic siliceous and siliciclastic rocks and blocks of basalt, limestone and sandstone. Penetrative F1 foliation and kink band structure were recorded within the matrix both on outcrop and under microscope. Strong lattice preferred orientation (LPO) fabric initiated by the low-temperature (350−450°C) (010)[001] slip system was detected by EBSD in the sodic amphiboles of the blueschist. The 40Ar–39Ar ages of the phengites from blueschist and sericites from the phyllite in the shear zone indicate that the activity of ZGT occurred between 71 and 60Ma. In the THS, a newly documented younger unit preserving detrital zircons from the southern Asian margin lies above the Triassic–Cretaceous sequences that carries only detrital zircons from the Indian continent. This unit is dated to be ~61Ma by the detrital zircon ages, similar to the Sangdanlin Formation and representing the Yarlung Zangbo Foreland Basin (YZFB) system. The ZGT had probably been active due to the initial India–Asia collision and acted as the frontal thrust controlling the development of YZFB.

Age, protoliths and tectonic implications of the Toudaoqiao blueschist, Inner Mongolia, China

The Toudaoqiao blueschist is located in the northern Great Xing’an Range of China, part of the eastern Central Asian Orogenic Belt, and was discovered in the 1980s, but the age of its formation and metamorphism and the nature of its protolith, have long remained uncertain. Thin-section examination and microprobe analyses show that the Toudaoqiao blueschist has a mineral assemblage of sodic amphibole+chlorite+epidote+albite+phengite±apatite±quartz. Electron microprobe analyses attest that sodic amphibole is magnesioriebeckite in composition. The blueschist-facies metamorphism occurred under metamorphic P–T conditions of ∼7kbar and 450–480°C, estimated from pseudosection as well as the compositions of the Na-amphibole and phengite. The protolith of the blueschist is mainly composed of alkali basalt, with a minor amount of tholeiitic basalt. The alkali basalt shows similarities in chemical composition to oceanic island basalt (OIB), suggesting derivation from an OIB-like asthenospheric mantle source in a within-plate setting. The tholeiitic basalt has geochemical features transitional between mid-ocean ridge basalt and island arc tholeiite, which implies an origin from a depleted mantle source that would have been selectively metasomatized in a supra-subduction setting. Sensitive high resolution ion microprobe (SHRIMP) zircon dating confirms that the protolith of the Toudaoqiao blueschist formed at 516±11Ma, in the middle Cambrian, and experienced blueschist facies metamorphism during the Silurian at ca. 436Ma. The juxtaposition of the two distinct rock types and the association of the blueschist with mélange suggest that the origin of the Toudaoqiao blueschist belt involve collision–accretionary processes. Based on these new results and regional data, we propose that the Toudaoqiao blueschist belt represents the suture zone between the Erguna and Xing’an terranes and that suturing took place during the Early Silurian period. The southwestern extension of the suture in Mongolia is represented by the Middle Gebi suture.

Paleotethyan subduction process revealed from Triassic blueschists in the Lancang tectonic belt of Southwest China

The subduction of the Paleotethyan Ocean and subsequent continental collision along the Lancang tectonic belt of the southeastern Paleotethyan belt is a major tectonic event in Southwest China, but the event of the subduction preceding the final collision is still not well-constrained. The mafic blueschists exposed in the Lancang accretionary complex provide crucial records of the Paleotethyan subduction process. In this paper, we present a set of new petrologic, geochronological and geochemical data for the Suyi mafic blueschists in the Lancang metamorphic zone. The mineral assemblage of these blueschists consists of zoned sodic amphibole (25–30%), albite (15–20%), epidote (25–30%), phengite (5–10%), chlorite (~5–10%), and minor amounts of actinolite, apatite, sphene, zircon, ilmenite, quartz and secondary limonite. This suggests a prograde metamorphism from ~0.5 to ~0.9GPa and retrograde metamorphic overprinting (back to ~0.6GPa) within the temperature range of 300–450°C. The Suyi blueschists give a zircon U–Pb age of 260±4Ma and glaucophane minerals formed during prograde metamorphism yield a 40Ar/39Ar plateau age of 242±5Ma (MSWD=0.77; P=0.54). The blueschists have geochemical compositions of subalkaline basalt and show typical OIB-type REE and multi-elemental patterns and εNd(t) values ranging from +3.35 to +4.85. Based on available data, it is inferred that the protolith formed at 260Ma and originated from a basaltic seamount. The basaltic rocks subducted down to 30–35km depths beneath the Lincang arc to form the epidote blueschists at ~242Ma. The blueschists were subsequently transported to shallower crustal levels in response to the continuous underthrust of the subducted slab and the continent–continent collision in the middle–late Triassic. These results provide a systematic constraint on the tectonic evolution and temporal framework of the southeastern Paleotethyan belt in Southwest China.

Experimentally determined stability of alkali amphibole in metasomatised dunite at sub-arc pressures

The phase and melting relationships of olivine mixed with 25 % of hydrous felsic slab melt have been determined in piston-cylinder experiments between 2.5 and 4.5 GPa and 800 to 1,050 °C to constrain metasomatic processes in the mantle wedge above subduction zones. At sub-solidus conditions, olivine, orthopyroxene, phlogopite, a Na-rich amphibole and an aqueous fluid are present. Na-rich amphibole is still observed at 950 °C at 4.5 GPa, providing evidence that this hydrous phase might be stable at sub-arc depths in an alkali-rich, Ca-poor mantle wedge. The maximum temperature stability is reached at 1,000 °C at 3.5 GPa, where amphibole coexists with hydrous melt. A sodium-rich phlogopite is stable over the whole range of P–T conditions investigated. At 2.5 GPa, 850 °C, aspidolite (Na analogue of phlogopite) has been observed as a sodium-bearing phase in the peridotite. The wet solidus in the metasomatised dunite lies between 850 and 900 °C at 2.5 GPa and between 950 and 975 °C at 3.5 GPa. At 4.5 GPa, melting relations are ambiguous and no clear solidus was found. The consumption of amphibole and minor phlogopite at the wet solidus produced Na- and H2O-rich phonolitic melts. The presence of phlogopite and sodic amphibole in the metasomatised dunite has implications on alkali and water storage in the part of the mantle wedge that is coupled to the down-going slab and might play a role on alkali and trace element recycling through arc magmatism.