Crystal chemistry and origin of epidote-(Sr) in alkaline rocks of the teschenite association (Silesian Unit, Outer Western Carpathians, Czech Republic)
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Allanite
Analcime
Titanite
Siliciclastic
Andradite
Recent studies on albitite rocks located in the granodiorite complex of Central Sardinia have revealed that epidote has a widespread occurrence as a light rare-earth element (LREE)-bearing accessory common phase. Titanite has been recorded as a heavy rare earth element (HREE)-bearing mineral. The Hercynian granodiorite complex of Central Sardinia is composed chiefly of quartz, Ca-plagioclase, K-feldspar and biotite and of a wide variety of secondary assemblages, mainly allanite, titanite and zircon. Albitic plagioclase and quartz are the main mineral components of the albitites. Additional minerals include, besides allanite and epidote, a more calcic-plagioclase (oligoclase), K-feldspar, chlorite, titanite and more rarely muscovite. The mineral assemblages and REE-bearing minerals of albitites were analysed by wavelength dispersive spectrometry (WDS). Chemical data suggest that there is a near complete solid-solution between epidote and allanite whereas little variations in HREE of titanites were detected. In epidote-group minerals a pronounced zoning in REE was observed while titanite was recorded unzoned. Textural relations were studied by SEM to distinguish primary from secondary epidotes. Chemical criteria to recognize magmatic from alteration epidotes were also applied. The alteration epidotes mainly occur and generally originate from plagioclase alteration and from leaching of magmatic allanite. Comparison of textures using both the SEM technique and EPMA data showed that the characteristic 'patchy zoning', observed in epidotes, corresponds with different amounts of REE in these minerals. The schematic model proposed for the epidote-forming reactions during the metasomatic processes that affected the granodiorites involves: (i) the instability of the anorthitic component of plagioclase; (ii) the simultaneous formation of albite; (iii) the leaching of the magmatic allanite with a redistribution of REE in the epidotes of the albitites. KEYWOROS: rare earth elements, albitite rocks, Sardinia, epidote, allanite, titanite.
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The current study presents new mineralogical, geochemical and geochronological data for a pegmatite body hosted in gneisses and marbles from the vicinity of the Strashimir Pb-Zn vein deposit, Madan ore district, South Bulgaria. The mineral composition of the studied pegmatite is represented by oligoclase–andesine (An10.1–33.2) and albite (An0–7.6), which prevail over K-feldspar (Or87.9–92.4), and quartz. The established accessory minerals are allanite-(Ce), titanite, apatite and zircon. The pegmatite/marble contact is affected by later hydrothermal silicate-carbonate alteration without detected ore mineralization, despite the spatial proximity with the Strashimir Pb-Zn vein deposit. Epidote-group minerals in pegmatite are defined as members of the clinozoisite–epidote series. As a major constituent of the hydrothermal alteration zone, they are manifested in two well-distinguished generations along with chlorite, quartz and carbonates. The calculated temperature of chlorite mineralization yields T° of crystallization in the range of 223–266 °C. As a result of the hydrothermal fluid circulation, the accessory allanite-(Ce) is transformed to REE-rich epidote-clinozoisite, marked by depletion of REE and Fe and enrichment of Si, Al, and Ca. Due to the limited mobility of REE in fluids, after leaching these elements are incorporated in nearby crystallized epidotes. According to the occurrence, mineral association and chemical properties, three titanite populations are distinguished in the pegmatite. Two of them were in-situ dated by the LA-ICP-MS U-Pb method and reveal overlapping ages of 39.9±2.1 Ma and 39.5±2.2 Ma (39.3±1.2 Ma combining all titanite analyses). The ages are interpreted as titanite growth in a pegmatite body related to granitic melts in the Late Alpine high-metamorphic units (Madan Unit) of the Central Rhodopes. Hydrothermal fluids either did not affect the U-Pb isotope system of the titanites or were derived from the same fluid-rich melt.
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Abstract A study of the distribution of REE in epidote-bearing metaluminous granitoids from Sierra de Chepes, Sierras Pampeanas, Argentina, reveals that a large proportion of the REE reside in the accessory minerals (allanite, epidote, titanite, apatite and zircon), and therefore these minerals control the behaviour of REE in granitic magmas. Well-developed chemical zonation in titanite indicates that the REE content decreases in the melt during crystallization of this mineral. The textural and chemical characteristics of euhedral epidote suggest a magmatic origin, and in that case it may have played an important role in the fractionation of the REE . The amount of silica and any other geochemical parameter indicative of fractionation progress in the dominant granodioritic-tonalitic facies (gtf) do not correlate with observed variations in the REE patterns. When many accessory minerals are involved, as in the gtf, the differentiated melts (e.g. aplites) are REE poor. Thus, the presence/absence of accessory minerals in granitoids can be indicative of the generation of differentiated melt enriched or poor in REE and other trace elements. This may have an economic significance, as it may allow us to predict the probable geochemistry of the differentiated melts (i.e. those that tend to develop mineralization) from the textural analysis of the ‘regional’ granitic rock. Finally, the type and abundance of accessory minerals in the granitic suite can also help us to define the geotectonic environment where magmas were generated.
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Abstract Silicate minerals enriched in V, Cr and Mn including garnets and epidote-supergroup members, in association with amphiboles, albite, hyalophane, titanite, chamosite, sulfides and other minerals occur in Devonian black shales near Čierna Lehota in the Strážovské vrchy Mountains, Slovakia. The garnets have high concentrations of V, Cr and Mn (up to 17 wt.% V 2 O 3 , ≤11 wt.% Cr 2 O 3 and ≤ 21 wt.% MnO) and several compositional types. Vanadian-chromian grossular (Grs 1) usually preserves primary metamorphic oscillatory zoning, whereas solid solutions between goldmanite (Gld 2A,B), V- and Cr-rich grossular and spessartine (Grs 2A,B, Sps 2) form irregular domains or crystals with variable zoning. Dominant substitutions in the garnets include CaMn –1 and (V,Cr)Al –1 , resulting in coupled Ca(V,Cr)Mn –1 Al –1 . Epidote-supergroup minerals occur as abundant anhedral crystals with variable compositional zoning. Nearly all crystals have a complete zoning sequence beginning with REE -rich allanite-(La), followed by mukhinite and by V- and Cr-rich clinozoisite to mukhinite and V- and Cr-poor clinozoisite. In common with garnets, the epidote-supergroup minerals are enriched in V, Cr and Mn (<7 wt.% V 2 O 3 , <5 wt.% Cr 2 O 3 and <3 wt.% MnO). Lanthanum is the dominant REE (up to 11.5 wt.% La 2 O 3 ) in allanite-(La). The composition of epidote-supergroup minerals is controlled by REE Fe 2+ (CaAl) –1 , REE Mg(CaAl) –1 , REE Mn 2+ (CaAl) –1 and REE Fe 2+ (CaFe 3+ ) –1 substitutions introducing REE , together with VAl –1 and CrAl –1 substitutions. The negative Ce and slightly positive Eu anomalies displayed in chondrite-normalized patterns and enrichment in V, Cr and Mn are ascribed to the geochemical properties of the protolith. The minerals investigated exhibit multi-stage evolution: (1) presumed low-grade greenschist-facies metamorphism; and (2) development of V- and Cr-rich zones in both garnet- and epidote-supergroup minerals which result from late-Variscan contact metamorphism due to granitic intrusion of the Suchý Massif. Decreased temperature following the metamorphic peak probably resulted in the formation of REE -, V- and Cr-poor clinozoisite and secondary garnet.
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Abstract A new occurrence of epidote-(Sr) [CaSrAl 2 Fe 3+ (Si 2 O 7 )(SiO 4 )(OH)], Sr-REE-rich epidote and Sr-rich allanite-(Ce) is located in Lower Cretaceous, Sr-rich hydrothermally altered leucocratic dykes penetrating alkaline igneous rocks (teschenites) near the Nový Jičín town (the Silesian Unit, Outer Western Carpathians). The dykes contain phenocrysts of pyroxene, amphibole, biotite, fluorapatite and dominant felsic groundmass consisting mostly of alkali feldspars and zeolites (analcime, natrolite and thomsonite-Ca). Accessory minerals include Ti-rich magnetite, prehnite, chamosite, pyrite, calcite, (OH, F)-rich grossular, epidote-group minerals, HFSE-, REE-rich minerals, Sr-rich baryte and slawsonite. The Sr-rich epidote forms columnar crystals or irregular aggregates which are mostly spatially related to chamosite pseudomorphs; it contains 0.15–0.81 apfu Sr, <0.53 apfu REE; Fe 3+ /(Fe 3+ + Al) = 0.16–0.48. The Sr 2+ substitutes Ca 2+ in the A2 site by a coupled substitution involving other A2 (REE 3+ , Y 3+ , Th 4+ ) or M (Al 3+ , Fe 3+ , Fe 2+ ) cations. The Sr-rich epidote crystallized from hydrothermal solutions, probably at temperatures between ~250–300 °C, during cooling of the host rock. The dykes show higher Sr contents (5680–7830 ppm) and 87 Sr/ 86 Sr i(120 Ma) ratios (~0.7046–0.7047) compared to host mesocratic teschenites (1310–1470 ppm Sr and ~0.7038–0.7045, respectively). The Sr isotopes indicate origin of most Sr from primary magmatic plagioclase in parent teschenite. Nevertheless, there participated fluids of external origin, derived from the Lower Cretaceous seawater or diagenetic waters related to associated siliciclastic sediments with 87 Sr/ 86 Sr i(120 Ma) = ~0.7073–0.7083. These more radiogenic sources contributed at least 6–21 % of the bulk Sr budget of the studied Sr-rich epidote-bearing leucocratic dykes.
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Abstract Recent studies on albitite rocks located in the granodiorite complex of Central Sardinia have revealed that epidote has a widespread occurrence as a light rare-earth element ( LREE )-bearing accessory common phase. Titanite has been recorded as a heavy rare earth element ( HREE )-bearing mineral. The Hercynian granodiorite complex of Central Sardinia is composed chiefly of quartz, Ca-plagioclase, K-feldspar and biotite and of a wide variety of secondary assemblages, mainly allanite, titanite and zircon. Albitic plagioclase and quartz are the main mineral components of the albitites. Additional minerals include, besides allanite and epidote, a more calcic-plagioclase (oligoclase), K-feldspar, chlorite, titanite and more rarely muscovite. The mineral assemblages and REE -bearing minerals of albitites were analysed by wavelength dispersive spectrometry (WDS). Chemical data suggest that there is a near complete solid-solution between epidote and allanite whereas little variations in HREE of titanites were detected. In epidote-group minerals a pronounced zoning in REE was observed while titanite was recorded unzoned. Textural relations were studied by SEM to distinguish primary from secondary epidotes. Chemical criteria to recognize magmatic from alteration epidotes were also applied. The alteration epidotes mainly occur and generally originate from plagioclase alteration and from leaching of magmatic allanite. Comparison of textures using both the SEM technique and EPMA data showed that the characteristic ‘patchy zoning’, observed in epidotes, corresponds with different amounts of REE in these minerals. The schematic model proposed for the epidote-forming reactions during the metasomatic processes that affected the granodiorites involves: (i) the instability of the anorthitic component of plagioclase; (ii) the simultaneous formation of albite; (iii) the leaching of the magmatic allanite with a redistribution of REE in the epidotes of the albitites.
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