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Harald G. Dill - One of the best experts on this subject based on the ideXlab platform.
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kaolinization a tool to unravel the formation and unroofing of the pleystein pegmatite Aplite system se germany
Ore Geology Reviews, 2015Co-Authors: Harald G. Dill, Reiner Dohrmann, Stephan Kaufhold, Sorin-ionut BalabanAbstract:Abstract The Hagendorf–Pleystein Pegmatite Province, SE Germany, is known for the largest feldspar–quartz pegmatite in Central Europe and renowned for its rare elements, e.g., Li, Nb, and Ta, giving rise to a spate of exotic minerals, mainly phosphates. Argillaceous rocks are scarce and eclipsed by the numerous mineralogical investigations on rare phosphates. These phosphate pegmatites are for the first time subjected to a clay mineralogical study, rendered possible by the newly discovered strongly kaolinized Aplite near the Kreuzberg Quartz Pegmatite at Pleystein. The supergene kaolin akin to the residual kaolin deposit at Tirschenreuth, SE Germany, was analyzed for its major and minor elements by XRF and micro-chemically by EMPA. Mineralogical investigations involved XRD, IR spectroscopy, thermoanalytical studies, CEC analyses and SEM-EDX. Supergene kaolinization forms a repository for heavy minerals critical for the interpretation of the emplacement of the Late Paleozoic pegmatites as well as a matrix for pegmatite-related trace elements and thereby may be used as an ore guide during exploration of these rare metal pegmatites. The resultant kaolin is also the protagonist in the story of exhumation and destruction of a pegmatite by weathering and erosion. Irrespective of the strength of kaolinization, Nb–Ta–Ti heavy minerals can be identified in the regolith atop the host pegmatite or Aplite and used for genetic interpretation of the primary mineralization and the origin of the felsic intrusive. Nb–Ta solid solution series (s.s.s.) have to be treated cautiously because of the disposition of Ta-enriched Nb–Ta oxide s.s.s. to undergo corrosion in their tantalite lamellae more easily than in their niobium-enriched zones. Kaolinization may alter the primary Nb/Ta ratio but not to the extent that Ta is released completely. The most strongly kaolinized new Aplite is the youngest member of a series of felsic intrusive rocks in the Pleystein pegmatite–Aplite system. The supergene kaolinization extending from the Miocene through the Pliocene can easily be correlated by “minero-stratigraphy” with the larger Tirschenreuth kaolin deposit. The four stages established in the area furnish evidence of gradual alkalinization of the meteoric pore fluids throughout the Neogene and the Quaternary. Youngest stages are found at Tirschenreuth, the oldest regolith stage is present at Pleystein.
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The new Nb-P Aplite at Reinhardsrieth: A keystone in the lateral and depth zonations of the Hagendorf-Pleystein Pegmatite Field, SE Germany
Ore Geology Reviews, 2015Co-Authors: Harald G. Dill, Radek ŠkodaAbstract:Abstract The Hagendorf–Pleystein Pegmatite Field (HPPF) is the largest pegmatite concentration in Central-Europe, which has been intensively studied for its mineralogy, mainly at Hagendorf-South pegmatite stock. It was not until recently that the lateral and depth zonations of the HPPF could be clarified when the newly discovered tabular Aplite at Reinhardsrieth Aplite has been found at transition into the so-called barren zone devoid of rare metals. Thereby the theory of the derivation of the HPPF felsic mobilizates as a result of a successive differentiation of a pegmatitic melt from the Flossenburg Granite has to be discarded. From the structural and lithological point of view, stock-like, well zoned pegmatites, e.g. Hagendorf-South, developed in the hinge zone, whereas tabular Aplites, such as Reinhardsrieth, evolved along the limb zone of strongly folded basement rocks. In the latter case, the accommodation space was smaller and the cooling of the felsic mobilizates much faster, giving rise to fine-grained aplitic rather than coarse-grained felsic mobilizates (chilled margin effect). During the incipient stages of the evolution in the HPPF, at Reinhardsrieth, a stronger impact of granitization can be determined than at the neighboring Aplites and pegmatites based upon the REE fractionation. A tripartite subdivision may be established for the various pegmatites and Aplites in the HPPF: (1) Barren zone (minor amounts of LREE in a system otherwise dominated by feldspar and quartz), (2) marginal zone (mainly Fe–Al phosphates plus columbite-(Fe)), (3) central zone (Fe–Zn–Li phosphates plus columbite-(Fe)). The afore-mentioned zonation shows an E–W trend and a bilateral symmetric arrangement of the various areas. There are significant indications of an oxidizing facies to substitute for a more reducing facies at the transition from the central into the marginal zone of the HPPF. The key mineral for the primary pegmatite mineralization in the marginal and central zones is columbite-(Fe). Its Mn/Fe ratios plotted vs. the Ta/Nb ratios allow for an assessment to what extent metamorphic and magmatic processes have contributed to the built-up of the pegmatites and Aplites in the HPPF. Even the most primitive “nigrine”-hosted columbites fit well into this pattern, forming a lithochemical halo in the roof rocks of the pegmatites. The heavy mineral aggregated of rutile–ilmenite (“nigrine”) giving host to the columbite as armored relicts got released from the basement rock on top of the pegmatites into the stream sediments where they are operative as an ore guide. The morphology of zircon in the stream sediments has been utilized as an economic ore indicator. The Zr silicate is also indicative of the temperature of formation and closely supports the tripartite zonation established above. Manganiferous apatite is a good depth-of-emplacement indicator for each Aplite and pegmatite, respectively. The higher the FeO + MnO content of apatite, the shallower the depth of emplacement of its host pegmatite or Aplite. Vivianite, in turn, marks the onset of the hydrothermal alteration of the pegmatites and Aplites and its Mn contents relate to the depth where alteration has taken place or emplacement of the secondary phosphates. The higher the Mn content the shallower the depth of alteration. Peculiar element ratios were selected to design some key diagrams so as to corroborate the field-based mineralogical and geological results in terms of zonation of genetic economic geology as well as to cater for applied economic geology as proximity indicators and ore guides when exploring for rare element pegmatite.
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Kaolinization — a tool to unravel the formation and unroofing of the Pleystein pegmatite–Aplite system (SE Germany)
Ore Geology Reviews, 2015Co-Authors: Harald G. Dill, Reiner Dohrmann, Stephan Kaufhold, Sorin-ionut BalabanAbstract:Abstract The Hagendorf–Pleystein Pegmatite Province, SE Germany, is known for the largest feldspar–quartz pegmatite in Central Europe and renowned for its rare elements, e.g., Li, Nb, and Ta, giving rise to a spate of exotic minerals, mainly phosphates. Argillaceous rocks are scarce and eclipsed by the numerous mineralogical investigations on rare phosphates. These phosphate pegmatites are for the first time subjected to a clay mineralogical study, rendered possible by the newly discovered strongly kaolinized Aplite near the Kreuzberg Quartz Pegmatite at Pleystein. The supergene kaolin akin to the residual kaolin deposit at Tirschenreuth, SE Germany, was analyzed for its major and minor elements by XRF and micro-chemically by EMPA. Mineralogical investigations involved XRD, IR spectroscopy, thermoanalytical studies, CEC analyses and SEM-EDX. Supergene kaolinization forms a repository for heavy minerals critical for the interpretation of the emplacement of the Late Paleozoic pegmatites as well as a matrix for pegmatite-related trace elements and thereby may be used as an ore guide during exploration of these rare metal pegmatites. The resultant kaolin is also the protagonist in the story of exhumation and destruction of a pegmatite by weathering and erosion. Irrespective of the strength of kaolinization, Nb–Ta–Ti heavy minerals can be identified in the regolith atop the host pegmatite or Aplite and used for genetic interpretation of the primary mineralization and the origin of the felsic intrusive. Nb–Ta solid solution series (s.s.s.) have to be treated cautiously because of the disposition of Ta-enriched Nb–Ta oxide s.s.s. to undergo corrosion in their tantalite lamellae more easily than in their niobium-enriched zones. Kaolinization may alter the primary Nb/Ta ratio but not to the extent that Ta is released completely. The most strongly kaolinized new Aplite is the youngest member of a series of felsic intrusive rocks in the Pleystein pegmatite–Aplite system. The supergene kaolinization extending from the Miocene through the Pliocene can easily be correlated by “minero-stratigraphy” with the larger Tirschenreuth kaolin deposit. The four stages established in the area furnish evidence of gradual alkalinization of the meteoric pore fluids throughout the Neogene and the Quaternary. Youngest stages are found at Tirschenreuth, the oldest regolith stage is present at Pleystein.
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A Newly Discovered Swarm of Shear-Zone-Hosted Bi–As–Fe–Mg–P-Rich Aplites and Pegmatites in the Hagendorf–Pleystein Pegmatite Province, Southeastern Germany: A Step Closer to the Metamorphic Root of Pegmatites
The Canadian Mineralogist, 2012Co-Authors: Harald G. Dill, Radek Škoda, Berthold Weber, Zsolt Berner, Axel H. E. Müller, Ronald J. BakkerAbstract:The Miesbrunn pegmatite-Aplite swarm (MPAS), part of the Hagendorf-Pleystein pegmatite province, southeastern Germany, was emplaced syn- to postkinematically relative to the Variscan tectonometamorphic processes (stage I). The MPAS developed along the contact between the autochthonous Moldanubicum and the nappes of the Bohemicum. Supercritical mobilization of REE, Zr, Mg, Zn, Li, Be and B during stage II led to a silicate-phosphate mineral association between 500 and 600 degrees C. Below 475 degrees C and a pressure of 3.8 kbar, contact-metasomatic reactions caused Mg phosphates to appear. Subsequently to the emplacement of the felsic intrusives bodies, the shear structure was reactivated and used as a conduit for fluids bearing S, As, Bi, Sb, Hg and U, from which sulfides and arsenides precipitated at 350 degrees C. Hypogene alteration during stages III and IV affected the primary phosphates at temperatures below 100 C, with fluctuating redox conditions when the MPAS was brought closer to the paleosurface as a result of strong uplift of the crystalline basement. Phosphates of Fe2+ formed during this epithermal mineralization in the pH range 5 to 10, whereas the stability field of Fe3+ phosphates extended toward lower values, below a pH of 5. The mineral assemblages of the MPAS are on one hand representative of mineralization at the deepest parts of the pegmatite, close to its root zone, and on the other, within the uppermost parts of pegmatites as they were pervasively altered by near-surface fluids. Sulfur isotopes of sulfides found in the MPAS and its wallrocks show a significant trend toward more strongly negative values away from the pegmatites and Aplites. A survey of sulfides using their S isotopes may well be a useful exploration tool for "blind" pegmatites (i.e., not exposed).
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THE ORIGIN AND ZONING OF HYPOGENE AND SUPERGENE Fe–Mn–Mg–Sc–U–REE PHOSPHATE MINERALIZATION FROM THE NEWLY DISCOVERED TRUTZHOFMÜHLE Aplite, HAGENDORF PEGMATITE PROVINCE, GERMANY
The Canadian Mineralogist, 2008Co-Authors: Harald G. Dill, Frank Melcher, Axel Gerdes, Berthold WeberAbstract:An Aplite containing Fe–Mn–Mg–Sc–U–REE phosphates, some Cu–Pb–Zn sulfides, barite, and U–Nb–Ta–Ti–Fe–Mn oxides was recently discovered near Trutzhofmuhle (THM) at the western border of the Hagendorf Pegmatite Province, Germany. We describe the sequence of phosphate crystallization in six stages of mineralization (I to VI) covering the time span from the Late Carboniferous through the Recent supergene alteration, and six sequences (1a/1b to 5) reflecting the reaction of phosphate-bearing solutions with the gneissic country-rocks (exo-aplitic) and with intra-aplitic rock-forming minerals that formed during crystallization. Age dating was carried out on columbite-(Fe) and torbernite using laser-ablation techniques. Precipitation of columbite-(Fe) and early magmatic phosphates (Mn-rich apatite, monazite) in the THM Aplite is correlated with a thermal event around 302 Ma postdating the intrusion of the post-kinematic Flossenburg granite. The sequences 1a and 1b, containing the lazulite solid-solution series, gordonite and childrenite–eosphorite series, reflect late magmatic and early hydrothermal exo-aplitic processes. The late magmatic and early hydrothermal stages of the intra-aplitic sequences 2 to 5 are characterized by triplite, wolfeite, triploidite, an unnamed K–Ba–Sc–Zr phosphate, an unnamed Zr–Sc phosphate–silicate, phosphoferrite, Mn-rich vivianite, and lermontovite – vyacheslavite (?). Complexing agents such as fluorine and phosphate control the formation of Sc phosphates and silicates. In contrast with the neighboring Hagendorf pegmatite, the magmatic and hydrothermal phosphate mineralization of the THM Aplite does not contain any Li-bearing phosphates and is very low in F. Rockbridgeite, whitmoreite, “ferrolaueite”, Al-bearing rockbridgeite, mitridatite, “metamitridatite”, kolbeckite and strunzite appear during late hydrothermal processes and weathering. Kolbeckite formed at the transition from hypogene to supergene processes. Its morphology varies from a rather simple combination of faces (platy kolbeckite I) under hydrothermal conditions to complex mineral aggregates (stubby kolbeckite II) produced under weathering conditions. The latest supergene alteration consists of wavellite, beraunite, cacoxenite, strengite, P- and Mn-bearing “limonite”, autunite, Sc-bearing vochtenite, Sc-bearing churchite-(Y) and diadochite. The latter phosphates with predominantly Fe, Al and U in close association with kaolinite are the representatives of supergene alteration, which is related in time and space to the Miocene peneplanation between 4.8 and 6.9 Ma. The boron- and phosphate-bearing THM Aplite is not directly linked to any of the granitic plutons nearby, and is not easily classified within the scheme of rare-element pegmatites.
Jeremy T. Ross - One of the best experts on this subject based on the ideXlab platform.
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Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within Aplite dikes
Contributions to Mineralogy and Petrology, 2018Co-Authors: Charles R. Stern, Julien Allaz, Markus B. Raschke, G. Lang Farmer, M. Alexandra Skewes, Jeremy T. RossAbstract:Aplite dikes intruding the Proterozoic 1.42(± 3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the Aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride + phosphate (+ S + CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich Aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride + phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(± 25) and 1.442(± 8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, eNd1.42Ga values for the granites and related granitic pegmatites range from − 3.3 to − 4.7 (average − 3.9), and differ from the values for both the Aplites and REE-rich segregations, which range from − 1.0 to − 2.2 (average − 1.6). Furthermore, the granites and pegmatites have (La/Yb)N 100 and absence of an Eu anomaly in both the Aplites and segregations. These data are consistent with the Aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher eNd1.42Ga values for the Aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.
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formation by silicate fluoride phosphate melt immiscibility of ree rich globular segregations within Aplite dikes
Contributions to Mineralogy and Petrology, 2018Co-Authors: Charles R. Stern, Julien Allaz, Markus B. Raschke, Lang G Farmer, Alexandra M Skewes, Jeremy T. RossAbstract:Aplite dikes intruding the Proterozoic 1.42(± 3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the Aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride + phosphate (+ S + CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich Aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride + phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(± 25) and 1.442(± 8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, eNd1.42Ga values for the granites and related granitic pegmatites range from − 3.3 to − 4.7 (average − 3.9), and differ from the values for both the Aplites and REE-rich segregations, which range from − 1.0 to − 2.2 (average − 1.6). Furthermore, the granites and pegmatites have (La/Yb)N 100 and absence of an Eu anomaly in both the Aplites and segregations. These data are consistent with the Aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher eNd1.42Ga values for the Aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.
Julien Allaz - One of the best experts on this subject based on the ideXlab platform.
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Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within Aplite dikes
Contributions to Mineralogy and Petrology, 2018Co-Authors: Charles R. Stern, Julien Allaz, Markus B. Raschke, G. Lang Farmer, M. Alexandra Skewes, Jeremy T. RossAbstract:Aplite dikes intruding the Proterozoic 1.42(± 3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the Aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride + phosphate (+ S + CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich Aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride + phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(± 25) and 1.442(± 8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, eNd1.42Ga values for the granites and related granitic pegmatites range from − 3.3 to − 4.7 (average − 3.9), and differ from the values for both the Aplites and REE-rich segregations, which range from − 1.0 to − 2.2 (average − 1.6). Furthermore, the granites and pegmatites have (La/Yb)N 100 and absence of an Eu anomaly in both the Aplites and segregations. These data are consistent with the Aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher eNd1.42Ga values for the Aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.
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formation by silicate fluoride phosphate melt immiscibility of ree rich globular segregations within Aplite dikes
Contributions to Mineralogy and Petrology, 2018Co-Authors: Charles R. Stern, Julien Allaz, Markus B. Raschke, Lang G Farmer, Alexandra M Skewes, Jeremy T. RossAbstract:Aplite dikes intruding the Proterozoic 1.42(± 3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the Aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride + phosphate (+ S + CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich Aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride + phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(± 25) and 1.442(± 8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, eNd1.42Ga values for the granites and related granitic pegmatites range from − 3.3 to − 4.7 (average − 3.9), and differ from the values for both the Aplites and REE-rich segregations, which range from − 1.0 to − 2.2 (average − 1.6). Furthermore, the granites and pegmatites have (La/Yb)N 100 and absence of an Eu anomaly in both the Aplites and segregations. These data are consistent with the Aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher eNd1.42Ga values for the Aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.
Charles R. Stern - One of the best experts on this subject based on the ideXlab platform.
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Formation by silicate–fluoride + phosphate melt immiscibility of REE-rich globular segregations within Aplite dikes
Contributions to Mineralogy and Petrology, 2018Co-Authors: Charles R. Stern, Julien Allaz, Markus B. Raschke, G. Lang Farmer, M. Alexandra Skewes, Jeremy T. RossAbstract:Aplite dikes intruding the Proterozoic 1.42(± 3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the Aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride + phosphate (+ S + CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich Aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride + phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(± 25) and 1.442(± 8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, eNd1.42Ga values for the granites and related granitic pegmatites range from − 3.3 to − 4.7 (average − 3.9), and differ from the values for both the Aplites and REE-rich segregations, which range from − 1.0 to − 2.2 (average − 1.6). Furthermore, the granites and pegmatites have (La/Yb)N 100 and absence of an Eu anomaly in both the Aplites and segregations. These data are consistent with the Aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher eNd1.42Ga values for the Aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.
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formation by silicate fluoride phosphate melt immiscibility of ree rich globular segregations within Aplite dikes
Contributions to Mineralogy and Petrology, 2018Co-Authors: Charles R. Stern, Julien Allaz, Markus B. Raschke, Lang G Farmer, Alexandra M Skewes, Jeremy T. RossAbstract:Aplite dikes intruding the Proterozoic 1.42(± 3) Ga Longs Peak-St. Vrain Silver Plume-type peraluminous granite near Jamestown, Colorado, contain F, P, and rare earth element (REE)-rich globular segregations, with 40–46% REE, 3.7–4.8 wt% P2O5, and 5–8 wt% F. A combination of textural features and geochemical data suggest that the Aplite and REE-rich globular segregations co-existed as two co-genetic liquids prior to their crystallization, and we propose that they are formed by silicate–fluoride + phosphate (+ S + CO2) melt immiscibility following ascent, cooling, and decompression of what was initially a single homogeneous magma that intruded the granite. The REE distribution coefficients between the silica-rich Aplites and REE-rich segregations are in good agreement with experimentally determined distribution coefficients for immiscible silicate–fluoride + phosphate melts. Although monazite-(Ce) and uraninite U–Th–Pb microprobe ages for the segregations yield 1.420(± 25) and 1.442(± 8) Ga, respectively, thus suggesting a co-genetic relationship with their host granite, eNd1.42Ga values for the granites and related granitic pegmatites range from − 3.3 to − 4.7 (average − 3.9), and differ from the values for both the Aplites and REE-rich segregations, which range from − 1.0 to − 2.2 (average − 1.6). Furthermore, the granites and pegmatites have (La/Yb)N 100 and absence of an Eu anomaly in both the Aplites and segregations. These data are consistent with the Aplite dikes and the REE-rich segregations they contain being co-genetic, but derived from a source different from that of the granite. The higher eNd1.42Ga values for the Aplites and REE-rich segregations suggest that the magma from which they separated had a more mafic and deeper, dryer and hotter source in the lower crust or upper mantle compared to the quartzo-feldspathic upper crustal source proposed for the Longs Peak-St. Vrain granite.
J.a. Davies - One of the best experts on this subject based on the ideXlab platform.
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The transition from granite to banded Aplite-pegmatite sheet complexes: An example from Megiliggar Rocks, Tregonning topaz granite, Cornwall
Lithos, 2018Co-Authors: Karel Breiter, Axel H. E. Müller, Jana Ďurišová, Tomas Hrstka, Zuzana Korbelová, M. Vašinová Galiová, Beth Simons, Robin K. Shail, Ben J. Williamson, J.a. DaviesAbstract:Abstract The genetic relationship between a granite pluton and adjacent complex of rare-metal pegmatite-Aplite-banded sheets (Megiliggar Sheet Complex - MSC) has been studied at the border of the Tregonning topaz granite at Megiliggar Rocks, Cornwall, SW England. Similarities in whole-rock chemical and mineralogical compositions, together with a gradual change in textures away from the granite margin, provide strong evidence for a genetic link between the Tregonning Granite and MSC. The sheets are likely to represent apophyses of residual melt which escaped from the largely crystallized roof of the granite pluton. The escaping melt was peraluminous, had a composition near the F, B, Li slightly enriched granite minimum, and, in comparison with other Cornish granites, was enriched in F, Li, Rb, Cs, Sn, W, Nb, Ta, and U, and depleted in Fe, Mg, Ca, Sr, Th, Zr, and REE. With increasing distance from the Tregonning Granite, the silicate melt crystallized as homogeneous leucogranite sheets and banded complex sheets (i.e. combinations of bands with granitic, aplitic and pegmatitic textures), then layered Aplite-pegmatites; this sequence becoming progressively more depleted in the fluxing and volatile elements F, Li, Rb, and Cs, but showing no change in Zr/Hf ratios. The fixed Zr/Hf ratio is interpreted as indicating a direct genetic link (parental melt) between all rock types, however the melt progressively lost fluxing and volatile elements with distance from the granite pluton, probably due to wall-rock reaction or fluid exsolution and migration via fractures. Differentiation of the primary melt into Na-Li-F-rich and separate K-B-rich domains was the dominant chemical process responsible for the textural and mineral diversity of the MSC. On a large (cliff-section) scale, the proximal Na-Li-F-rich leucogranite passes through complex sheets into K-B-rich Aplite-pegmatites, whilst at a smaller (