Occurrence and origin of andalusite in peraluminous felsic igneous rocks

Andalusite occurs as an accessory mineral in many types of peraluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and anatectic migmatites. Some published stability curves for And = Sil and the water-saturated granite solidus permit a small stability field for andalusite in equilibrium with felsic melts. We examine 108 samples of andalusite-bearing felsic rocks from more than 40 localities world-wide. Our purpose is to determine the origin of andalusite, including the T-P-X controls on andalusite formation, using eight textural and chemical criteria: size-compatibility with grain sizes of igneous minerals in the same rock; shape-ranging from euhedral to anhedral, with no simple correlation with origin; state of aggregation-single grains or clusters of grains; association with muscovite-with or without rims of monocrystalline or polycrystalline muscovite; inclusions--rare mineral inclusions and melt inclusions; chemical composition-andalusite with little significant chemical variation, except in iron content (0·08-1·71 wt % FeO); compositional zoning--concentric, sector, patchy, oscillatory zoning cryptically reflect growth conditions; compositions of coexisting phases-biotites with high siderophyllite-eastonite contents (Al iv ≈ 2·68 ± 0·07 atoms per formula unit), muscovites with 0·57-4·01 wt % FeO and 0·02-2·85 wt % TiO2, and apatites with 3· 53 ± 0·18 wt % F. Coexisting muscovite-biotite pairs have a wide range of F contents, and FBt = 1·612F Ms + 0·015. Most coexisting minerals have compositions consistent with equilibration at magmatic conditions. The three principal genetic types of andalusite in felsic igneous rocks are: Type 1 Metamorphic-(a) prograde metamorphic (in thermally metamorphosed peraluminous granites), (b) retrograde metamorphic (inversion from sillimanite of unspecified origin), (c) xenocrystic (derivation from local country rocks), and (d) restitic (derivation from source regions); Type 2 Magmatic-(a) peritectic (water-undersaturated, T↑) associated with leucosomes in migmatites, (b) peritectic (water-undersaturated, T↓), as reaction rims on garnet or cordierite, (c) cotectic (water-undersaturated, T↓) direct crystallization from a silicate melt, and (d) pegmatitic (water-saturated, T↓), associated with aplite-pegmatite contacts or pegmatitic portion alone; Type 3 Metasomatic-(water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement of feldspar and/or biotite, intergrowths with quartz. The great majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin. Of the many possible controls on the formation of andalusite (excess Al2O 3, water concentration and fluid evolution, high Be-B-Li-P, high F, high Fe-Mn-Ti, and kinetic considerations), the two most important factors appear to be excess Al2O3 and the effect of releasing water (either to strip alkalis from the melt or to reduce alumina solubility in the melt). Of particular importance is the evidence for magmatic andalusite in granites showing no significant depression of the solidus, suggesting that the And = Sil equilibrium must cross the granite solidus rather than lie below it. Magmatic andalusite, however formed, is susceptible to supra- or sub-solidus reaction to produce muscovite. In many cases, textural evidence of this reaction remains, but in other cases muscovite may completely replace andalusite leaving little or no evidence of its former existence.La lista completa de autores que integran el documento puede consultarse en el archivo.Centro de Investigaciones Geológica

Occurrence and origin of andalusite in peraluminous felsic igneous rocks

Andalusite occurs as an accessory mineral in many types of peraluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and anatectic migmatites. Some published stability curves for And = Sil and the water-saturated granite solidus permit a small stability field for andalusite in equilibrium with felsic melts. We examine 108 samples of andalusite-bearing felsic rocks from more than 40 localities world-wide. Our purpose is to determine the origin of andalusite, including the T-P-X controls on andalusite formation, using eight textural and chemical criteria: size-compatibility with grain sizes of igneous minerals in the same rock; shape-ranging from euhedral to anhedral, with no simple correlation with origin; state of aggregation-single grains or clusters of grains; association with muscovite-with or without rims of monocrystalline or polycrystalline muscovite; inclusions--rare mineral inclusions and melt inclusions; chemical composition-andalusite with little significant chemical variation, except in iron content (0·08-1·71 wt % FeO); compositional zoning--concentric, sector, patchy, oscillatory zoning cryptically reflect growth conditions; compositions of coexisting phases-biotites with high siderophyllite-eastonite contents (Al iv ≈ 2·68 ± 0·07 atoms per formula unit), muscovites with 0·57-4·01 wt % FeO and 0·02-2·85 wt % TiO2, and apatites with 3· 53 ± 0·18 wt % F. Coexisting muscovite-biotite pairs have a wide range of F contents, and FBt = 1·612F Ms + 0·015. Most coexisting minerals have compositions consistent with equilibration at magmatic conditions. The three principal genetic types of andalusite in felsic igneous rocks are: Type 1 Metamorphic-(a) prograde metamorphic (in thermally metamorphosed peraluminous granites), (b) retrograde metamorphic (inversion from sillimanite of unspecified origin), (c) xenocrystic (derivation from local country rocks), and (d) restitic (derivation from source regions); Type 2 Magmatic-(a) peritectic (water-undersaturated, T↑) associated with leucosomes in migmatites, (b) peritectic (water-undersaturated, T↓), as reaction rims on garnet or cordierite, (c) cotectic (water-undersaturated, T↓) direct crystallization from a silicate melt, and (d) pegmatitic (water-saturated, T↓), associated with aplite-pegmatite contacts or pegmatitic portion alone; Type 3 Metasomatic-(water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement of feldspar and/or biotite, intergrowths with quartz. The great majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin. Of the many possible controls on the formation of andalusite (excess Al2O 3, water concentration and fluid evolution, high Be-B-Li-P, high F, high Fe-Mn-Ti, and kinetic considerations), the two most important factors appear to be excess Al2O3 and the effect of releasing water (either to strip alkalis from the melt or to reduce alumina solubility in the melt). Of particular importance is the evidence for magmatic andalusite in granites showing no significant depression of the solidus, suggesting that the And = Sil equilibrium must cross the granite solidus rather than lie below it. Magmatic andalusite, however formed, is susceptible to supra- or sub-solidus reaction to produce muscovite. In many cases, textural evidence of this reaction remains, but in other cases muscovite may completely replace andalusite leaving little or no evidence of its former existence.La lista completa de autores que integran el documento puede consultarse en el archivo.Centro de Investigaciones Geológica

Occurrence and origin of andalusite in peraluminous felsic igneous rocks

Andalusite occurs as an accessory mineral in many types of peraluminous felsic igneous rocks, including rhyolites, aplites, granites, pegmatites, and anatectic migmatites. Some published stability curves for And = Sil and the water-saturated granite solidus permit a small stability field for andalusite in equilibrium with felsic melts. We examine 108 samples of andalusite-bearing felsic rocks from more than 40 localities world-wide. Our purpose is to determine the origin of andalusite, including the T-P-X controls on andalusite formation, using eight textural and chemical criteria: size-compatibility with grain sizes of igneous minerals in the same rock; shape-ranging from euhedral to anhedral, with no simple correlation with origin; state of aggregation-single grains or clusters of grains; association with muscovite-with or without rims of monocrystalline or polycrystalline muscovite; inclusions--rare mineral inclusions and melt inclusions; chemical composition-andalusite with little significant chemical variation, except in iron content (0·08-1·71 wt % FeO); compositional zoning--concentric, sector, patchy, oscillatory zoning cryptically reflect growth conditions; compositions of coexisting phases-biotites with high siderophyllite-eastonite contents (Al iv ≈ 2·68 ± 0·07 atoms per formula unit), muscovites with 0·57-4·01 wt % FeO and 0·02-2·85 wt % TiO2, and apatites with 3· 53 ± 0·18 wt % F. Coexisting muscovite-biotite pairs have a wide range of F contents, and FBt = 1·612F Ms + 0·015. Most coexisting minerals have compositions consistent with equilibration at magmatic conditions. The three principal genetic types of andalusite in felsic igneous rocks are: Type 1 Metamorphic-(a) prograde metamorphic (in thermally metamorphosed peraluminous granites), (b) retrograde metamorphic (inversion from sillimanite of unspecified origin), (c) xenocrystic (derivation from local country rocks), and (d) restitic (derivation from source regions); Type 2 Magmatic-(a) peritectic (water-undersaturated, T↑) associated with leucosomes in migmatites, (b) peritectic (water-undersaturated, T↓), as reaction rims on garnet or cordierite, (c) cotectic (water-undersaturated, T↓) direct crystallization from a silicate melt, and (d) pegmatitic (water-saturated, T↓), associated with aplite-pegmatite contacts or pegmatitic portion alone; Type 3 Metasomatic-(water-saturated, magma-absent), spatially related to structural discontinuities in host, replacement of feldspar and/or biotite, intergrowths with quartz. The great majority of our andalusite samples show one or more textural or chemical criteria suggesting a magmatic origin. Of the many possible controls on the formation of andalusite (excess Al2O 3, water concentration and fluid evolution, high Be-B-Li-P, high F, high Fe-Mn-Ti, and kinetic considerations), the two most important factors appear to be excess Al2O3 and the effect of releasing water (either to strip alkalis from the melt or to reduce alumina solubility in the melt). Of particular importance is the evidence for magmatic andalusite in granites showing no significant depression of the solidus, suggesting that the And = Sil equilibrium must cross the granite solidus rather than lie below it. Magmatic andalusite, however formed, is susceptible to supra- or sub-solidus reaction to produce muscovite. In many cases, textural evidence of this reaction remains, but in other cases muscovite may completely replace andalusite leaving little or no evidence of its former existence.La lista completa de autores que integran el documento puede consultarse en el archivo.Centro de Investigaciones Geológica

La Geoquímica en la FCNyM: un ejemplo pionero de interdisciplina

Desde mediados del siglo 20, la geoquímica es una disciplina científica consolidada con objetivos propios que la distinguen. En 1958 se creó la Licenciatura en Geoquímica en la FCNyM de la UNLP y en 1969 se la incluye como materia obligatoria de la Licenciatura en Geología. Es significativo que estas iniciativas hayan surgido de esta facultad, que se caracteriza por su formación naturalista antes que exacta. Ambas iniciativas estuvieron relacionadas a inquietudes de personalidades relevantes, que no se sujetaron a limitaciones en apariencia difíciles de sortear ni a esquemas rígidos para practicar la ciencia.Fundación Museo de La Plat

50 anos da Geologia Isotópica na América do Sul: Destaques do 9º Simpósio-Sul Americano de Geologia Isotópica

On April 2014, over 250 members of the South America Earth Sciences community gathered at the Universidade de São Paulo, Brazil, for the 9th South American Symposium on Isotope Geology – 9th SSAGI. The symposium is a traditional scientific meeting in South America and it has been organized since 1997 in different countries of our continent, including Brazil, Argentina, Chile, Uruguay and Colombia. The motivation to hold the 9th SSAGI in São Paulo was to celebrate the 50th anniversary of the Geochronological Research Center (CPGeo).Centro de Investigaciones Geológica

Dating the Triassic continental rift in the southern Andes : The Potrerillos Formation, Cuyo Basin, Argentina

The Triassic successions of western Argentina commonly show thin pyroclastic levels intercalated within thick fluvial and lacustrine terrigenous deposits. The Potrerillos Formation is the thickest Triassic unit in the Cuyo Basin. It is composed of alternating cycles of gravelly- sandy- and muddy-dominated intervals, in which several laterally-continuous tuff horizons occur. U-Pb SHRIMP ages were determined on zircon grains from three tuff levels located between the lowermost and the middle sections of the Potrerillos Formation. The ages for the time of deposition of the tuffs are 239.2 ± 4.5 Ma, 239.7 ± 2.2 Ma and 230.3 ± 2.3 Ma (Middle Triassic). Chemical data indicate that these acid to intermediate volcaniclastic rocks are derived from coeval basic magmas displaying tholeiitic to slightly alkaline signatures. They are associated with the rift stage that followed the extensive post-orogenic volcanism of the Choiyoi Group, that in turn has been ascribed to slab break-off in neighbouring areas. Two of the studied samples also record a subpopulation of inherited zircon grains with crystallisation ages of 260-270 Ma. The latter are considered to be an indirect measurement for the age of the Choiyoi Group in the Cuyo basin. The rift-related Triassic event represents the culmination of the Gondwanian magmatic cycle, and is interpreted as the result of subduction cessation and anomalous heating of the upper mantle previous to the western Gondwana break-up.Centro de Investigaciones Geológica

Dating the Triassic continental rift in the southern Andes : The Potrerillos Formation, Cuyo Basin, Argentina

The Triassic successions of western Argentina commonly show thin pyroclastic levels intercalated within thick fluvial and lacustrine terrigenous deposits. The Potrerillos Formation is the thickest Triassic unit in the Cuyo Basin. It is composed of alternating cycles of gravelly- sandy- and muddy-dominated intervals, in which several laterally-continuous tuff horizons occur. U-Pb SHRIMP ages were determined on zircon grains from three tuff levels located between the lowermost and the middle sections of the Potrerillos Formation. The ages for the time of deposition of the tuffs are 239.2 ± 4.5 Ma, 239.7 ± 2.2 Ma and 230.3 ± 2.3 Ma (Middle Triassic). Chemical data indicate that these acid to intermediate volcaniclastic rocks are derived from coeval basic magmas displaying tholeiitic to slightly alkaline signatures. They are associated with the rift stage that followed the extensive post-orogenic volcanism of the Choiyoi Group, that in turn has been ascribed to slab break-off in neighbouring areas. Two of the studied samples also record a subpopulation of inherited zircon grains with crystallisation ages of 260-270 Ma. The latter are considered to be an indirect measurement for the age of the Choiyoi Group in the Cuyo basin. The rift-related Triassic event represents the culmination of the Gondwanian magmatic cycle, and is interpreted as the result of subduction cessation and anomalous heating of the upper mantle previous to the western Gondwana break-up.Centro de Investigaciones Geológica

El cratón del Río de La Plata en la provincia de Córdoba

Fil: Rapela, Carlos Washington. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Baldo, Edgardo G.. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Centro de Investigaciones en Ciencia de La Tierra; Córdob

Geochemistry in Argentina: from pioneers to the present

Argentine geochemistry evolved during the nineteenth century hand in hand with other sciences. The Scotsman John J. Kyle was the first chemist to arrive in Argentina in 1862, contributing during his lifetime reports that expanded the geochemical knowledge of local natural resources. After visiting the USA and Europe (1868), Argentina’s President Sarmiento requested Hermann Burmeister (a prestigious biologist) to engage European scientists to foster the teaching and research of Natural Sciences (sensulato) in Argentina. The first to arrive, in August 1871 at the National Academy of Sciences and the university in Cordoba, was Max Siewert, a chemist from the German Martin Luther University. Siewert set up a state-of-the-art laboratory and analyzed, as Kyle had a few years before, a range of materials from waters and minerals to natural salts and biological materials. Some years later, Adolf Doring replaced Siewert. In the twentieth century, Gustavo Fester is the personality to highlight as a chemist/geochemist because he accomplished a vast task as teacher and researcher at the Universidad Nacional del Litoral and other institutions. During the 1950s and 1960s Argentine Geochemistry experienced slow but sustained growth, promoted by competent university professors such as Felix Gonzalez Bonorino, Jose Catoggio, Mario Teruggi and Carlos Gordillo. The first Geochemistry curriculum was initiated in 1958 at the Universidad Nacional de La Plata as a result of the bold initiative of Catoggio and Teruggi. Nowadays, Geochemistry is solidly established in Earth Sciences curricula and Argentine geochemical papers are found in all the international journals of the specialty.Facultad de Ciencias Naturales y MuseoCentro de Investigaciones Geológica

Las cordierititas orbiculares y masivas cámbricas del Complejo El Pilón, Sierra de Córdoba : ejemplo único a nivel mundial de acumulación de cordierita vinculada a procesos magmáticos

Fil: Rapela, Carlos Washington. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Centro de Investigaciones Geológicas; ArgentinaFil: Baldo, Edgardo G.. Universidad Nacional de Córdoba. Facultad de Ciencias Exactas, Físicas y Naturales. Centro de Investigaciones en Ciencia de La Tierra; Córdob