Age and origin of fluorapatite-rich dyke from Baranec Mt. (Tatra Mts., Western Carpathians): a key to understanding of the post-orogenic processes and element mobility
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Abstract On the southeastern slope of the Baranec Mount in the Western Tatra Mountains (Slovakia) an apatite-rich pegmatite-like segregation was found in the subvertical fault zone cutting metapelitic rocks. Two zones: felsic (F) and mafic (M) were found, differing in mineral assemblages and consequently in chemistry. Fluorapatite crystals yield a LA-ICP-MS U-Pb age of 328.6 ± 2.4 Ma. A temperature decrease from 634 °C to 454 °C at a pressure around 500 to 400 MPa with oxygen fugacity increasing during crystallization are the possible conditions for formation of the pegmatite-like segregation, while secondary alterations took place in the temperature range of 340 – 320 °C. The Sr-Nd isotope composition of both apatite and whole rock point toward a crustal origin of the dike in question, suggesting partial melting of (P, F, H 2 O)-rich metasedimentary rocks during prolonged decompression of the Tatra Massif. The original partial melt (felsic component) was mixed with an external (F, H 2 O)-rich fluid, carrying Fe and Mg fluxed from more mafic metapelites and crystallizing as biotite and epidote in the mafic component of the dyke.Keywords:
Felsic
Pegmatite
Baddeleyite
Massif
Fractional crystallization (geology)
Pegmatite
Muscovite
Fractional crystallization (geology)
Tourmaline
Incompatible element
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Felsic
Andesites
Fractional crystallization (geology)
Igneous differentiation
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Felsic magma petrogenesis was studied by analyzing 24 stratigraphically controlled Archean andesite-to-rhyolite lava flows of both tholeiitic and calc-alkalic affinity from the upper Noranda Subgroup, Quebec, using instrumental neutron activation and X-ray fluorescence techniques. The lavas have moderate values of [La/Yb] N (0.9–3.8) and low values of 100 × Th/Zr (~1). According to calculations following batch partial melting and Rayleigh fractional crystallization models, both the calc-alkalic and tholeiitic felsic volcanic rocks are probably products of shallow-level fractional crystallization of mafic parental magmas formed respectively by lower (~7 % for calc-alkalic) and higher (~14% for tholeiitic) degrees of partial melting of a primitive mantle source.Contribution to the magma genesis from plausible crustal materials was negligible. A back-arc-type diapirism is geochemically suggested for the tectonic model of origin of Noranda felsic magmas, in conformity with geological observations. Felsic volcanic rocks with compositions analogous to the studied samples exist in several other Archean terrains of the Canadian Shield, suggesting thereby that the late Archean sialic crust was at least in part produced by volcanic rocks ultimately derived from the primitive mantle.
Felsic
Fractional crystallization (geology)
Silicic
Petrogenesis
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Pegmatite
Fractional crystallization (geology)
Batholith
Igneous differentiation
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AssrRAcr Linkages between the fertile granite and associated pegmatites, linkages among pegmatites, and processes instrumental in defining their textural, chemical, and mineralogical uniqueness are obscured by their coarse grain-size and the transitional nature between magmatic and hydrothermal regimes. Our studies of the Harney Peak granite-pegmatite system in the Black Hills, South Dakota, indicate that these systems are a cuhnination of distinctive processes of partial melting and fractional crystallization. Both the Harney Peak Granite and the associated pegmatite field are mineralogically and chemically zoned. Superimposed on the general zoning in the pegmatite field are swanns of pegmatites that appear to define distinct compositional-textural arrays. These may be related to distinct fractional crystallization trajectories. The Harney Peak Granite and numerous pegmatites define a single trajectory of fractional crystallization, whereas Li-, Rb-, Cs-enriched zoned pegmatites and F-, Sn-, Be-enriched pegmatites represent trajectories of fractional crystallization involving chemically distinct magma-types. A fractionation trajectory (from biotite ganites to tourmaline granites) does not represent an evolutionary path of a single magma, but most likely represents paths of fractional crystallization of similar batches of magmas. Within each trajectory of fractional crystallization are coherent sequences of textural and mineralogical characteristics. The extreme rare-element enrichments observed in pegmatites can in part be modeled by moderate to high degrees of fractional crystallization (up to 70-90q0) of a suite of volatile-rich magmas. In addition to fractional crystallization, partial melting appears to be important in controlling the potential content and composition of the volatile component, and the incompatible element character of tle distinct magma-types. Varying dqrees of detrydrarion melting of compositionally diverse metasediments appear to be the most likely model for the production of these compositional diverse parental granitic magmas.
Pegmatite
Fractional crystallization (geology)
Tourmaline
Incompatible element
Igneous differentiation
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Felsic
Fractional crystallization (geology)
Silicic
Anatexis
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Felsic
Fractional crystallization (geology)
Igneous differentiation
Underplating
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