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The Triassic Dehnow pluton of NE Iran is a garnet-bearing I-type calc-alkaline metaluminous diorite-tonalite-granodiorite intrusion. The parental magma formed as the result of partial melting of intermediate to felsic rocks in the lower crust. Petrological and geochemical evidence which favors of a magmatic origin for the garnet includes: large size (~10 to 20 mm) of crystals, absence of reaction rims, a distinct composition from garnet in adjacent metapelitic rocks, and similarity in the composition of mineral inclusions (biotite, hornblende) in the garnet and the same minerals in the matrix. Absence of garnet-bearing enclaves in the pluton and lack of sillimanite (fibrolite) and cordierite inclusions in magmatic garnet suggests that the garnet was not produced by assimilation of meta-sedimentary country rocks. Also, the δ18O values of garnet in the pluton (8.3 to 8.7‰) are significantly lower than δ18O values of garnet in the metapelitic rocks (12.5 to 13.1‰). Amphibole-plagioclase and garnet-biotite thermometers indicate crystallization temperatures of 708 and 790 °C, respectively. A temperature of 692 °C obtained by quartz-garnet oxygen isotope thermometry points to a closure temperature for oxygen diffusion in garnet. The composition of epidote (Xep) and garnet (Xadr) indicate ~800 °C for the crystallization temperature of these minerals. Elevated andradite-content in the rims of garnet suggest that oxygen fugacity increased during crystallization.
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... There are also many reported examples of garnet in aplites and pegmatites (Arredondo et al. 2001;Gadas et al. 2013;Samadi et al. 2014aSamadi et al. , 2014b, which is generally considered magmatic in origin (Manning 1983;Deer et al. 1992;Muller et al. 2012). In most cases, however, zoning of major and trace elements in the garnets of aplites and pegmatites differs from those in the other magmatic garnets (e.g., Samadi et al. 2014aSamadi et al. , 2014b, implying possibly different origins. ...
... There are also many reported examples of garnet in aplites and pegmatites (Arredondo et al. 2001;Gadas et al. 2013;Samadi et al. 2014aSamadi et al. , 2014b, which is generally considered magmatic in origin (Manning 1983;Deer et al. 1992;Muller et al. 2012). In most cases, however, zoning of major and trace elements in the garnets of aplites and pegmatites differs from those in the other magmatic garnets (e.g., Samadi et al. 2014aSamadi et al. , 2014b, implying possibly different origins. Pegmatites have long been viewed as essentially igneous rocks because of their bulk compositions. ...
... Data for the garnets from low-evolved melts are from Wang et al. (2008), Xia et al. (2019), and some unpublished data obtained from melting experiments. Data for the HP garnets from M/I-type magmas are from Samadi et al. (2014b). Data for other magmatic garnets are from Thöni and Miller (2004). ...
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Two generations of garnet are recognized in a granite and a pegmatite from the Gangdese orogen in southeastern Tibet on the basis of a combined study of petrography, major and trace element profiles, and garnet O isotopes. Zircon U-Pb dating and Hf-O isotope compositions also help constrain the origin of both granite and pegmatite. The first generation of garnet (Grt-I) occurs as residues in the center of garnet grains, and it represents an early stage of nucleation related to magmatic-hydrothermal fluids. Grt-I is dark in backscattered electron (BSE) images, rich in spessartine, and poor in almandine and grossular. Its chondrite-normalized rare earth element (REE) patterns show obvious negative Eu anomalies and depletion in heavy REE (HREE) relative to middle REE (MREE). The second generation of pegmatite garnet (Grt-II) occurs as rims of euhedral garnets or as patches in Grt-I domains of the pegmatite, and it crystallized after dissolution of the preexisting pegmatite garnet (Grt-I domains) in the presence of the granitic magma. Compared with Grt-I, Grt-II is bright in BSE images, poor in spessartine, and rich in almandine and grossular contents. Its chondrite-normalized REE patterns exhibit obvious negative Eu anomalies but enrichment in HREE relative to MREE. The elevation of grossular and HREE contents for Grt-II relative to Grt-I domains indicate that the granitic magma had higher contents of Ca than the magmatic-hydrothermal fluids. The garnets in the granite, from core to rim, display homogenous profiles in their spessartine, almandine, and pyrope contents but increasing grossular and decreasing REE contents. They are typical of magmatic garnets that crystallized from the granitic magma. Ti-in-zircon temperatures demonstrate that the granite and pegmatite may share the similar temperatures for their crystallization. Grt-II domains in the pegmatite garnet have the same major and trace element compositions as the granite garnet, suggesting that the pegmatite Grt-II domains crystallized from the same granitic magma. Therefore, the pegmatite crystallized at first from early magmatic-hydrothermal fluids, producing small amounts of Grt-I, and the fluids then mixed with the surrounding granitic magma. The U-Pb dating and Hf-O isotope analyses of zircons from the granite and pegmatite yield almost the same U-Pb ages of 77–79 Ma, positive eHf(t) values of 5.6 to 11.9, and d18O values of 5.2 to 7.1‰. These data indicate that the granite and pegmatite were both derived from reworking of the juvenile crust in the newly accreted continental margin prior to the continental collision in the Cenozoic.
... For example, garnet xenocrysts in granitoids and metapelites typically exhibit normal zoning, with Mn-rich and Fe-poor cores and core-to-rim decrease of Mn whereas phenocrysts and magmatic garnet in plutonic rocks tend to show reversed zoning with core-to-rim increase of Mn, with increasing differentiation in a melt (e.g. Abbott, 1981aAbbott, , 1981bAllan and Clarke, 1981;Day et al., 1992;Green and Ringwood, 1968;Harangi et al., 2001;Harris and Vogeli, 2010;Kawabata and Takafuji, 2005;Koepke et al., 2003;Lackey et al., 2008Lackey et al., , 2011Lackey et al., , 2012Leake, 1967;Stoddard, 1981a, 1981b;Mirnejad et al., 2008;Patranabis-Deb et al., 2008;Samadi et al., 2014b;Schwandt et al., 1996;Spear and Kohn, 1996;Vielzeuf et al., 2005). In contrast, garnets within zoned pegmatite bodies are often characterized by Mn-rich cores and Fe 2+ -rich rims (Arredondo et al., 2001;Baldwin and Von Knorring, 1983;Manning, 1983;Whitworth, 1992). ...
... Aydar and Gourgaud, 2002;Mirnejad et al., 2008;Patranabis-Deb et al., 2008) or have oscillatory compositional zoning (e.g. Day et al., 1992;Kano, 1983;Kawabata and Takafuji, 2005;Samadi, 2009Samadi, , 2014Samadi et al., 2014b). Garnets from aplite-pegmatite are often Fe-Mn rich, and exhibit pronounced core-to-rim decrease in Mn (e.g. ...
... Baldwin and Von Knorring, 1983;Gadas et al., 2013;Manning, 1983;Thöni et al., 2003;Whitworth, 1992), whereas garnets from granitoids are mostly Fe-rich and exhibit weak core-to-rim increase in Mn (e.g. Allan and Clarke, 1981;Day et al., 1992;Green and Ringwood, 1968;Harangi et al., 2001;Kawabata and Takafuji, 2005;Koepke et al., 2003;Mirnejad et al., 2008;Samadi, 2014;Samadi et al., 2014b;Spear and Kohn, 1996;Vielzeuf et al., 2005;Wang et al., 2003). Other chemical characteristics of magmatic, Li 97 177 188 213 187 136 96 107 104 93 107 128 Na 75 94 143 168 155 107 62 73 86 75 87 81 Ti 222 426 737 594 745 326 229 294 272 264 333 Xenocrystic garnets in igneous rocks are characterized by Mnnormal zoning with core-to-rim depletion in Mn, whereas magmatic garnets exhibit reversed zoning with core-to-rim enrichment in Mn (Allan and Clarke, 1981). ...
Article
Triassic monzogranites and granodiorites of the Khajeh Morad region in northeastern Iran are cut by two types of garnet-bearing intrusive veins: (1) aplite and (2) granitic pegmatite. The former is composed of quartz, feldspar, muscovite, with minor garnet, biotite, and ilmenite. The latter contains quartz, plagioclase (± quartz and muscovite inclusions), alkali feldspar, and muscovite, with minor amounts of garnet, tourmaline, beryl, columbite, and ilmenite. Garnet in both rock types has MnO > 12 wt% and CaO < ~2 wt% with spessartine-rich cores, and a core-to-rim increase in Fe, Mg, and Ca. Garnet cores are enriched in Y, REE, Zr, Nb, Ta, Hf, and U. The Y, HREE, and Mn concentrations show strong positive correlations in both types of garnet associations and decrease from core-to-rim. These core-to-rim elemental variations can be explained by increasing fluid content and H2O activity in magma, together with decreasing Mn contents of an evolved host melt. Aplite and pegmatite garnet δ18O values are nearly identical (~10.3‰, n=7, SD=0.09) and are similar to magmatic garnets in granitoids elsewhere. On the basis of calculated δ18O values for magma (~12.5 and 12.6‰) and quartz (~13.6‰, n=7, SD=0.08) as well as the major and trace element characteristics, we suggest that the Khajeh Morad garnets crystallized from a variably fractionated S-type monzogranitic magma.
... In metaluminous cordilleran-type granites as well as in metaluminous volcanic rocks, garnet is extremely rare and few natural occurrences have been reported in the literature (e.g. Bach et al., 2012;Barnes and Allen, 2006;Dawes and Evans, 1991;Day et al., 1992;Evans and Vance, 1987;Harangi et al., 2001;Samadi et al., 2014). Crystallization experiments (e.g. ...
... In addition, epidote appears to crystallize together with garnet only above approximately 1.2 GPa (Schmidt and Thompson, 1996). We combined information on occurrences of magmatic garnet (Samadi et al., 2014) and epidote (Schmidt and Poli, 2004) in metaluminous intrusives and volcanic rocks worldwide and there appear to be only few localities where magmatic garnet and epidote have crystallized together. These include: the garnet-epidote bearing dikes cropping out in the Front Range, Colorado (Dawes and Evans, 1991;Evans and Vance, 1987); the Dehnow pluton, north east Iran (Samadi et al., 2014); the Bushy Point Granites, south eastern Alaska (Arth et al., 1988;Hammarstrom, 1984a, 1984b); the Jinshan Intrusion associated with the Dabie Orogenic belt, China (e.g. ...
... We combined information on occurrences of magmatic garnet (Samadi et al., 2014) and epidote (Schmidt and Poli, 2004) in metaluminous intrusives and volcanic rocks worldwide and there appear to be only few localities where magmatic garnet and epidote have crystallized together. These include: the garnet-epidote bearing dikes cropping out in the Front Range, Colorado (Dawes and Evans, 1991;Evans and Vance, 1987); the Dehnow pluton, north east Iran (Samadi et al., 2014); the Bushy Point Granites, south eastern Alaska (Arth et al., 1988;Hammarstrom, 1984a, 1984b); the Jinshan Intrusion associated with the Dabie Orogenic belt, China (e.g. Xu et al., 2013) and the subject of this study, the 30,000 km 2 Neoproterozoic Cordilleran-type Galiléia Batholith, located within the Araçuaí Orogen, Brazil (Mondou et al., 2012;Nalini, 1997;Nalini et al., 2000Nalini et al., , 2005Nalini et al., , 2008Vauchez et al., 2007). ...
Article
Magmatic garnet, together with epidote, is a rare mineral association in cordilleran-I-type granitoids and of special petrogenetic significance. The metaluminous to slightly peraluminous (ASI = 0.97–1.07) Galiléia batholith (Brazil) is a large (ca. 30,000 km²), Neoproterozoic (ca. 632–570 Ma) weakly foliated calc-alkaline granitoid body, characterized by the widespread occurrence of garnet (grossular 25–43 mol%) and epidote (pistacite 9.3–22.7 mol%). Field, petrographic and mineral chemical evidence indicate that garnet, epidote, biotite as well as white mica crystals (low-Si phengite), are magmatic. There is no difference in bulk rock major and trace element composition between the Gailiéia granitoids and other garnet-free cordilleran-type granitoids worldwide. This evidence strongly suggests that the origin of the uncommon garnet + epidote parageneses is related to the conditions of magma crystallization, such as pressure, temperature and water content. Comparison between the mineral assemblages and mineral compositions from this study and those recorded in crystallization experiments on metaluminous calc-alkaline magmas, as well as within garnet-bearing metaluminous volcanic rocks and granitoids, indicate that the supersolidus coexistence of grossular-rich garnet, epidote and white mica are consistent with magma crystallization at pressures greater than 0.8 GPa (above 25 km depth) and at temperatures below 700 °C, i.e. near the water saturated solidus. Furthermore, resorption textures around garnet (plagioclase ± quartz coronas) and epidote suggest that these minerals have been partially consumed prior to complete crystallization. These findings demonstrate that at 630 Ma the crust underneath the Araçuaí Orogen was already at least 25 - 30 km thick and relatively cool. However, this contrasts with the marked high heat flow registered from the neighbour Carlos Chagas Batholith located 50 km to the east. In fact such granitoids records granulite-facies metamorphism at the same pressure and time (ca. 570 Ma) of Galiléia granitoids crystallisation. Thus, a more suitable geodynamic scenario is required in order to explain these two contrasting thermal regimes within the same orogen. Eventually, field, petrographic and mineral chemical analogies with similar garnet-bearing granitoids located in the fore-arc settings of the British Columbia subduction zone, possibly imply that the Galiléia granitoids represent “rare” garnet- and epidote-bearing metaluminous Cordilleran-I-type granites which can only form in a fore-arc setting.
... A peculiar feature is the occurrence of garnet in the Reifnitz tonalite, which is typical for S-type granites and rare but present in some I-type granitoids(e.g., Pe-Piper 2000; Harangi et al. 2001;Samadi et al. 2014 and references therein). René & Stelling (2007) summarized potential models for the occurrence of garnet in granitoids, which is more common in S-type granitoids and rare in I-type granitoids (see also Harangi et al. 2001): (1) garnet could represent a refractory restite phase transported within the magma from the area of partial melting, or (2) a refractory xenocryst phase from high-grade metasedimentary country rocks, or (3) could have crystallized in the marginal facies of a granitic intrusion as a result of reaction between granitic melt and pelitic xenoliths rich in Al and Mn compared to the melt. ...
... These garnet aggregates likely interacted with magma forming the outer rim with a CaO content of ca. 5 percent implying crystallization at middle crustal level. The almandine-rich garnet outer rim composition with ca. 5 percent CaO is very similar to magmatic garnet, which crystallized in an I-type granite in Iran (Samadi et al. 2014). The CaO content requires an elevated pressure of garnet crystallization corresponding to middle to lower crustal depth (Harangi et all. ...
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Abstract: In the south-eastern Eastern Alps, the Reifnitz tonalite intruded into the Austroalpine metamorphic basement of the Wörthersee half-window exposed north of the Sarmatian–Pliocene flexural Klagenfurt basin. The Reifnitz tonalite is dated for the first time, and yields a laser ICP-MS U–Pb zircon age of 30.72 ± 0.30 Ma. The (U–Th–Sm)/He apatite age of the tonalite is 27.6 ± 1.8 Ma implying rapid Late Oligocene cooling of the tonalite to ca. 60 °C. The Reifnitz tonalite intruded into a retrogressed amphibolite-grade metamorphic basement with a metamorphic overprint of Cretaceous age (40Ar/39Ar white mica plateau age of 90.7 ± 1.6 Ma). This fact indicates that pervasive Alpine metamorphism of Cretaceous age extends southwards almost up to the Periadriatic fault. Based on the exhumation and erosion history of the Reifnitz tonalite and the hosting Wörthersee half window formed by the Wörthersee anticline, the age of gentle folding of Austroalpine units in the south-eastern part of the Eastern Alps is likely of Oligocene age. North of the Wörthersee antiform, Upper Cretaceous–Eocene, Oligocene and Miocene sedimentary rocks of the Krappfeld basin are preserved in a gentle synform, suggesting that the top of the Krappfeld basin has always been near the Earth’s surface since the Late Cretaceous. The new data imply, therefore, that the Reifnitz tonalite is part of a post-30 Ma antiform, which was likely exhumed, uplifted and eroded in two steps. In the first step, which is dated to ca. 31–27 Ma, rapid cooling to ca. 60 °C and exhumation occurred in an E–W trending antiform, which formed as a result of a regional N–S compression. In the second step of the Sarmatian–Pliocene age a final exhumation occurred in the peripheral bulge in response to the lithospheric flexure in front of the overriding North Karawanken thrust sheet. The Klagenfurt basin developed as a flexural basin at the northern front of the North Karawanken, which represent a transpressive thrust sheet of a positive flower structure related to the final activity along the Periadriatic fault. In the Eastern Alps, on a large scale, the distribution of Periadriatic plutons and volcanics seems to monitor a northward or eastward shift of magmatic activity, with the main phase of intrusions ca. 30 Ma at the fault itself.
... Many authors have investigated the genesis of garnet, commonly present in peraluminous granites (Dahlquist et al., 2007;Green, 1977), less common in metaluminous (Day et al., 1992;Harangi et al., 2001;Samadi et al., 2014) and ferroan granites (du Bray, 1988;Zhang et al., 2012). They also have investigated the genesis of the host granites. ...
... In the grossular vs. spessartine diagram of Samadi et al. (2014), most of the garnets of Tilaboni granite fall outside the field of either M-(metamorphic xenocrysts) or I-type (magmatic phenocrysts) garnets (Fig. 14b) except one sample which plots within the field of I-type garnet. Zhang et al. (2012) discriminated the fields of magmatic garnet in the 10*MgO-FeO-MnO diagram. ...
Article
We report for the first time magmatic Ca-rich (CaO = 7-11.3 wt%) almandine garnet-bearing ferroan granites. The almandine (Alm66.6Grs24.2Pyr4.9Spes3.9Uvr0.1) garnet-bearing Tilaboni granite pluton was emplaced in a major regional shear zone of Chhotanagpur Gneissic Complex, Eastern India. Garnets show partial dissolution, corona, and symplectitic textures. Petrography and composition of minerals suggest the garnets are magmatic. Mineral chemistry of these garnets differs from those of calc-alkaline I-type, S-type, A-type, mantle-type, and metamorphic garnets. Geochemically, the host granites show high 10,000Ga/Al ratios (average 3.2), high K2O (average 4.91 wt%), high total alkalis (average 7.5wt.%), high Ta+Yb (average 10.2ppm), high Ce+Nb+Zr+Y (average 668ppm), high Ce/Yb (average 27.3) and strong negative Eu-anomalies (average 0.3). These granites are classified as ferroan, calc-alkalic to alkalic, metaluminous to weakly peraluminous, and highly fractionated I-type. Early crystallization of magnesian amphiboles/biotites enriched the FeO in the derivative melts. The granite magma had high liquidus temperature (800−950 °C), low oxygen fugacity (ΔQFM = +1 to -1.6), and solidified at around 5 to 6 kb pressure (Al-in hornblende barometers). Pseudosection modelling shows that the garnets crystallized from a hydrous melt (H2O= 6−9 wt%) at around 760°C temperature, 6 kb pressure, and fO2 -15 log unit (bar). The pluton emplaced in a post-collisional tectonic setting. Low Mg# (average 0.24), low Nb/U (average 9.8), and Ce/Pb (average 7.1) ratios but high Th/U (average 9.8) ratios of the Tilaboni granites strongly suggest their crustal source. The granite magma was derived by the 20−40% partial melting of an old high-K high alumina shoshonitic hornblende granulite protolith at around 7 kb. The shear zone facilitated the fast upward movement of the magma and incomplete dissolution of the garnets before solidification at lower pressure.
... Ma, U-Pb zircon ages in Mirnejad et al., 2013) granitoid occur west, south, and southwest of Mashhad city (Hashemi and Samadi, 2017) (Fig. 1b). These intruded the Mashhad Metamorphic Complex (MMC) during the final stages of Paleo-Tethys subduction and early stages of Turan-Iran Plate collision to south of the Kopeh Dagh zone (Mirnejad et al., 2013;Samadi et al., 2014). Dehnow, Khalaj, and Khajeh Morad study area exposures trend NW-SE, over a distance of 20 km, southwest of Mashhad city (Fig. 1b). ...
... This garnet phenocryst has recorded crystallization of a metaluminous intermediate tonalitic melt. Similar to the host garnet (Samadi et al., 2014), Al and Fe decrease and Ti (and Mg) increase in the biotite inclusions from the core to middle and rim of the garnet crystal (Fig. 8a) as a result of fractional crystallization and decreasing temperature. This progression indicates that biotite formed in association or following garnet crystallization will have lower Al and Fe and higher Ti than earlier formed biotite or biotite crystallizing in the absence of garnet. ...
Article
Chemical compositions of Fe-Mg biotite have been used to understand the petrogenesis of metamorphic and igneous rocks. However, biotite is affected by sub-solidus hydrothermal alteration, metamorphism, and chemical exchange with other common coexisting phases such as garnet and muscovite. Therefore, the interpretation of igneous and metamorphic processes using biotite compositions is not always straightforward. Here we compare biotite compositions in igneous rocks, meta-igneous rocks, and meta-sedimentary rocks from localities in northeast (Dehnow, Khalaj, Khajeh Morad) and central (Jandaq and Airekan) Iran, with similar rock types in the global GEOROC database and from other localities, in order to constrain associated petrogenetic classification schemes. We find important compositional contrasts in biotite associated with muscovite and/or garnet (in both igneous and metamorphic rocks), suggesting careful use of common discrimination schemes. For example, magmatic biotite associated with garnet and/or muscovite (i.e., Bt + Ms, Bt + Ms + Grt, Bt + Grt) is often enriched in Al and depleted in Fe, Mg, and Ti, likely due to crystallization prior to muscovite but synchronous with or following garnet crystallization. Metamorphic biotites in garnet- and/or muscovite-bearing rocks tend to be enriched in Ti, Fe, and Mg and depleted in Al. The contrasting compositional behavior of magmatic and metamorphic biotites also poses problems for garnet-biotite, biotite-muscovite, and Ti-in-biotite thermometers. Our analysis indicates that biotite rare earth and trace element concentrations are strongly influenced by co-existing garnet and muscovite. When magmatic biotite crystallization occurs with muscovite and garnet, HREE concentrations respectively decrease and increase.
... In the Binaloud area, this deformation generated several thrust fault systems, resulting in the imbrications of Paleozoic and Mesozoic successions and Neogene sediments (Sheikholeslami and Kouhpeyma, 2012). The Mashhad granitoids which have been the focus of many studies (e.g., Alavi and Majidi, 1972;Alberti and Moazez, 1974;Majidi, 1978;Valizadeh and Mirnejad, 1992;Valizadeh and Karimpour, 1995;Karimpour et al., 2010;Mirnejad et al., 2013;Samadi et al., 2014) intruded into metamorphic rocks. Valizadeh and Karimpour (1995) suggested that these intrusions originate from the lower continental crust during an early stage of collision. ...
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Nodular tourmalines composed of dark cores and white haloes occur in the Mashhad leucogranite. The cores are made up of tourmaline and quartz, and the haloes of quartz, muscovite, microcline and orthoclase. The host granite consists of quartz, pla-gioclase, microcline, muscovite and biotite. Geochemical analyses show that the studied tourmalines are schorl. They are characterized by oscillatory zoning, increasing concentrations of Ca and Mg from core to rim, moderate Fe/Fe + Mg ratios (56–63), and REE distribution patterns similar to those of the host granite. The tourmalines exhibit high Ti content due to breakdown of biotite. Petrographic and geochemical results show that the formation of tourmaline can be attributed to two stages. In the first stage, bubbles containing vapor-rich liquid, Fe, and B were generated by magma differentiation. During rise and cooling of the magma, core tourmaline was produced with Fe rich schorl composition. In the second stage, the tourmaline composition changed to that of Mg rich schorl because of the decomposition of biotite (biotite is considered as a source of Mg), and the mixing with vapor-rich liquid in bubbles.
... In the Binaloud area, this deformation generated several thrust fault systems, resulting in the imbrications of Paleozoic and Mesozoic successions and Neogene sediments (Sheikholeslami and Kouhpeyma, 2012). The Mashhad granitoids which have been the focus of many studies (e.g., Alavi and Majidi, 1972;Alberti and Moazez, 1974;Majidi, 1978;Valizadeh and Mirnejad, 1992;Valizadeh and Karimpour, 1995;Karimpour et al., 2010;Mirnejad et al., 2013;Samadi et al., 2014) intruded into metamorphic rocks. Valizadeh and Karimpour (1995) suggested that these intrusions originate from the lower continental crust during an early stage of collision. ...
Article
Nodular tourmalines composed of dark cores and white haloes occur in the Mashhad leucogranite. The cores are made up of tourmaline and quartz, and the haloes of quartz, muscovite, microcline and orthoclase. The host granite consists of quartz, plagioclase, microcline, muscovite and biotite. Geochemical analyses show that the studied tourmalines are schorl. They are characterized by oscillatory zoning, increasing concentrations of Ca and Mg from core to rim, moderate Fe/Fe + Mg ratios (56–63), and REE distribution patterns similar to those of the host granite. The tourmalines exhibit high Ti content due to breakdown of biotite. Petrographic and geochemical results show that the formation of tourmaline can be attributed to two stages. In the first stage, bubbles containing vapor-rich liquid, Fe, and B were generated by magma differentiation. During rise and cooling of the magma, core tourmaline was produced with Fe rich schorl composition. In the second stage, the tourmaline composition changed to that of Mg rich schorl because of the decomposition of biotite (biotite is considered as a source of Mg), and the mixing with vapor-rich liquid in bubbles
... Garnets from plutonic rocks are generally Fe-rich and exhibit a weak increase in Mn content from core to rim (e.g. Mirnejad et al., 2008;Samadi et al., 2014;Ismail et al., 2014;Xu et al., 2016), whereas garnets originating from skarns are often characterized by Al-rich cores and Fe 3+ -rich rims (Gaspar et al., 2008). Trace elements in skarn minerals are useful geochemical tracers (Cheng et al., 2012;Schmidt et al., 2011). ...
Article
The Sangan iron skarn deposit is located in the Sabzevar-Dorouneh Magmatic Belt of northeastern Iran. The skarn contains zoned garnet, clinopyroxene and magnetite. Cores and rims of zoned garnets are generally homogeneous, having a relatively high ΣREE, low ΣLREE/ΣHREE ratios, and positive Eu anomalies. The cores of the zoned clinopyroxenes are exceptionally HREE-rich, with relatively high ΣREE and HREE/LREE ratios, as well as positive Eu anomalies. Clinopyroxene rims are LREE-rich, with relatively low ΣREE contents and HREE/LREE ratios, and do not have Eu anomalies. Magnetite grains are enriched in LREEs in comparison with the HREEs and lack Eu anomalies. Variations of fluid composition and physicochemical conditions rather than YAG-type substitution mechanism are considered to have major control on incorporating trace elements, including REE, into the skarn mineral assemblage. Based on baro-acoustic decrepitation analysis, the calc-silicate and magnetite dominant stages were formed at similar temperatures, around 350–400 °C. In the Sangan skarns, hydrothermal fluids shifted from near-neutral pH, reduced conditions with relatively high ΣREE, low LREE/HREE ratios, and U-rich characteristics towards acidic, oxidized conditions with relatively low ΣREE, high LREE/HREE ratios, and U-poor characteristics.
... Figura 14. (A) Comparação entre granadas magmáticas cristalizadas em equilíbrio com líquidos metaluminosos a fracamente peraluminosos com as do Maciço Capela (diagramas adaptados de Narduzzi et al., 2017); (B) diagrama MnO versus CaO (após Harangi et al., 2001;Samadi et al., 2014), com áreas normalmente ocupadas por granadas metamórficas de metapelitos e ígneas cristalizadas a partir de magmas peraluminosos tipo S e metaluminosos tipo I ou M de alta e baixa pressão. durante a cristalização e em estágio subsolidus. ...
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O Maciço Capela é intrusivo nas rochas metassedimentares do Domínio Macururé, Sistema Orogênico Sergipano, no sul da Província Borborema. Ele é constituído por dioritos, hornblenditos, gabros e granitos, que hospedam enclaves tonalíticos e hornblendíticos. As formas e os contatos dos enclaves tonalíticos, aliados à presença de texturas de zoneamento inverso e oscilatório em cristais de plagioclásio e ao hábito acicular da apatita nos enclaves e dioritos sugerem coexistência de magmas máfico e félsico. Os piroxênios identificados nos gabros apresentam composições de enstatita, augita e diopsídio. Os anfibólios das rochas máficas são cálcicos e correspondem a pargasita, tschermakita e magnésio-hornblenda. A mica marrom dos hornblenditos, gabros e dioritos é rica na molécula de flogopita, enquanto a dos granitos é mais enriquecida em ferro. A granada, de ocorrência restrita aos hornblenditos e dioritos, é rica na molécula de almandina. O plagioclásio varia de albita a bytownita e o feldspato potássico é a microclina. A presença de titanita e epídoto magmáticos, coexistentes com silicatos máficos magnesianos, indica a cristalização sob condições de alta fO2, próximas ao tampão NNO. As estimativas de pressão forneceram um valor médio de 8,5 kbar, que corresponde a uma profundidade aproximada de 30 km. As temperaturas liquidus, obtidas com as composições de piroxênios e anfibólios, variam de 1.261 a 831°C. Temperaturas solidus, estimadas com o par anfibólio-plagioclásio, situam-se entre 775 e 614°C. Dados de química mineral e estimativas termobarométricas sugerem que as rochas do Maciço Capela cristalizaram-se a partir de magmas basálticos hidratados, em ambiente de arco continental.
... This sample also shows entirely correct assignment to the amphibolite-facies group (B) with high individual percentages of probability (66-98%). The third example (B-c) is garnet from mica schist surrounding the Dehnow Pluton in NE Iran (Samadi et al., 2014). These schists experienced amphibolite-facies metamorphism at around 570°C and 5.3 kbar (Samadi et al., 2012) and their garnets are assigned correctly to amphibolite facies for 95% of the analyses (Table 7). ...
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Garnet chemistry provides a well-established tool in the discrimination and interpretation of sediment provenance. Current discrimination approaches, however, (i) suffer from using less variables than available, (ii) subjective determination of discrimination fields with strict boundaries suggesting clear separations where in fact probabilities are converging, and (iii) significant overlap of compositional fields of garnet from different host-rock groups. The new multivariate discrimination scheme is based on a large database, a hierarchical discrimination approach involving three steps, linear discriminant analysis at each step, and the five major host-rock groups to be discriminated: eclogite- (A), amphibolite- (B) and granulite- (C) facies metamorphic rocks as well as ultramafic (D) and igneous rocks (E). The successful application of statistical discrimination approaches requires consideration of the a priori knowledge of the respective geologic setting. This is accounted for by the use of prior probabilities. Three sets of prior probabilities (priors) are introduced and their advantages and disadvantages are discussed. The user is free to choose among these priors, which can be further modified according to the specific geologic problem and the level of a priori knowledge. The discrimination results are provided as integrated probabilities of belonging to the five major host-rock groups. For performing calculations and results a supplementary Excel® spreadsheet is provided. The discrimination scheme has been tested for a large variety of examples of crystalline rocks covering all of the five major groups and several subgroups from various geologic settings. In most cases, garnets are assigned correctly to the respective group. Exceptions typically reflect the peculiarities of the regional geologic situation. Evaluation of detrital garnets from modern and ancient sedimentary settings of the Western Gneiss Region (Norway), Eastern Alps (Austria) and Albertine Rift (Uganda) demonstrates the power to reflect the respective geologic situations and corroborates previous results. As most garnet is derived from metamorphic rocks and many provenance studies aim at reconstructing the tectonic and geodynamic evolution in the source area, the approach and the examples emphasize discrimination of metamorphic facies (i.e., temperature-pressure conditions) rather than protolith composition.
... PM to aplitic dikes. Garnet is generally reported from both peraluminous and metaluminous granitoids (e.g., Patranabis-Deb et al., 2008;Samadi et al., 2014a), however, the absence of garnet from some peraluminous granites could be related to low MnO content of magma and/or pressures lower than lower crustal depths (~<5-7 kbar). MnO in the peraluminous S-type granite samples of Airekan (<0.05wt%; ...
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In the center of Iran, Central-East Iranian Microcontinent (CEIM) was a part of Gondwana supercontinent in pre-Palaeozoic during Pan-African Orogeny. It is a zone of several tectonomagmatic and metamorphic episodes from Neoproterozoic to earliest Palaeozoic. In the north of CEIM, Airekan granite is a relic of Paleozoic magmatism in northern Gondwanaland. It is potentially a significance pluton that preserved the magmatic/metamorphic evolution of the active continental margin of the vanished Ocean of Proto-Tethys. This pluton is characterized by SiO2> 70 wt%, A/CNK>1, Rb >~160 ppm, Y <50 ppm, Th <30 ppm, Th/Ta >5. The δ18O value of quartz (average ~11.86 ‰; n=8), calculated δ18O value of whole rock (average ~10.75 ‰), absence of hornblende, presence of biotite, muscovite and inherited zircon, higher content of orthoclase, and microgranular granitic enclaves are all consistent with it being a continental collision-related peraluminous S-type granite. Th+U versus 206Pb/238U ratios of zircons correlate with decreasing crystallization temperatures related to the Cambrian-Ordovician magmatic events preserved in the inherited and magmatic zircons, toward their Devonian metamorphic overgrowth occurred via Caledonian Orogeny. It is probably formed by mica-dehydration melting at ~ 690-820 °C/ 10-15 kbar, and it is geochronologically and geochemically comparable with other Gondwanan collision-related granitic plutons (along north of Africa, Turkey, Iran to Himalaya).
... Metamorphic Garnet (Stevens et al., 2007;Kohn et al., 1997) Metamorphic Garnet (Dehnow Mica Schist) (Samadi et al., 2014) Metamorphic Garnet (Khalaj Mica Schist) (Samadi et al., 2014) Magmatic Garnet (Vielzeuf et al., 2005) Magmatic Garnet (S-type granitoids) (Harris and Vogelli, 2010) Magmatic Garnet (Harangi et al., 2001) Magmatic Garnet(I-Type Dehnow Pluton) (Samadi et al., 2014b ...
... Considering this, plus the absence of typical mafic alkaline minerals (e.g., arfvedsonite, riebeckite, aegirine) and the emplacement age of the CLG before the typical alkaline rocks in the Sul-Riograndense Shield (Nardi & Bitencourt, 2009), the geochemical classification that best describes the CLG chemistry is of a highly differentiated I-type granite. Even though garnet-bearing I-type granites are rare and sometimes related with specific origins (Narduzzi, Farina, Stevens, Lana, & Nalini, 2017), authors report the occurrence of this mineral around the world both in volcanic and intrusive calc-alkaline associations (Arth, Barker, & Stern, 1988;Bach, Smith, & Malpas, 2012;Barnes & Allen, 2006;Day, Green, & Smith, 1992;Harangi et al., 2001;Samadi et al., 2014). Oyhantçabal et al. (2007) also report rocks with transitional characteristics between calc-alkaline and alkaline chemical fingerprints in the southernmost DFB, in Uruguay. ...
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The Capão do Leão Granite (CLG; 583 ± 3 Ma) occurs as two NE‐oriented bodies, near the namesake city and Pedro Osório, Rio Grande do Sul, Brazil. The CLG is linked to the voluminous Ediacaran magmatism in the SE Dom Feliciano Belt (Pelotas Batholith), intruded in post‐collisional settings in the set of the non‐deformed younger plutons in the area. This work presents field characterization, petrography, mineral chemistry and whole‐rock geochemical affinities for the CLG. This granite exhibits miarolitic cavities, suggesting an epizonal emplacement, as well as mafic enclaves, which may represent source material, while mylonitic and S–C foliations are localized and post‐magmatic. The analysed samples are massive, leucocratic, coarse‐ to medium‐grained hypidiomorphic alkali‐feldspar granites composed of quartz, orthoclase (Or89–98), albite (An1–10), oligoclase (An11–16), garnet (Sps31–64 and Alm24–56), siderophyllite/annite, muscovite and Fe‐chlorite (secondary), apatite, epidote/zoisite, zircon, and opaques. The dispersion in mineral compositions in each occurrence and between them suggest different magmatic evolutionary trends. The diverse textures, zoning patterns and chemistry in garnet indicate both equilibrium with highly evolved Al2O3‐rich liquids, as well as a xenocrystic/restitic origin. The geochemistry shows subalkaline, high‐K, and metaluminous to weakly peraluminous fingerprints. Samples are commonly enriched in Rb, Th, U, Pb, Nd, and Sm, and depleted in Ba, Sr, Eu, Nb, P, Zr and Ti, due to advanced fractional crystallization. The classification of the CLG into some traditional chemical patterns is difficult, due to the high differentiation and the overlap of petrologic processes, although its derivation from I‐type liquids is proposed. The dispersion both in mineral and whole‐rock chemistry indicates different evolutionary trends for the CLG, where fractionation coupled with crustal recycling by assimilation/contamination processes were responsible for the granite differentiation.
... similar to many crystals from the Galiléia batholith, central sector of the Rio Doce magmatic arc, studied by Narduzzi et al. (2017). Experiments and studies of metaluminous igneous rocks have systematically shown that garnet with CaO content over 4 wt% crystallizes at pressures above 8 kbar (Alonso-Perez et al., 2009;Bach et al., 2012;Barnes and Allen, 2006;Dawes and Evans, 1991;Day et al., 1992;Green, 1972Green, , 19771992;Green and Ringwood, 1968;Harangi et al., 2001;Samadi et al., 2014). As in our samples, the CaO content is between 9.34 and 10.36 wt %; we can estimate crystallization depths of at least 29 km, assuming an average density of the continental crust of 2.75 g/cm 3 . ...
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The Rio Doce magmatic arc, developed from ca. 630 to ca. 580 Ma on the active continental margin, links the Araçuaí and Ribeira orogens in southeastern Brazil. The plutonic portion comprises the G1 supersuite, a calc-alkaline, magnesian, I-type pre-collisional rock-assemblage, mostly composed of tonalite to granodiorite, frequently containing dioritic to mafic enclaves, and their metamorphosed equivalents. We carried out field, petrographic, lithochemical and isotopic (Sm–Nd,-Sr) studies on a segment of the Rio Doce arc located in the transition area between the Araçuaí and Ribeira orogens. The studied samples include metamorphic granitic rocks (referred to by their igneous names in the QAP diagram), consisting of syenogranite, monzogranite, granodiorite, tonalite, quartz diorite, orthopyroxene-bearing tonalite and orthopyroxene-bearing quartz diorite. This rock assemblage defines an I-type, magnesian, metaluminous to slightly peraluminous, medium-to high-K, expanded calc-alkaline series. The numerous mafic to dioritic enclaves and related features indicate magma mixing processes. Isotopic data show moderately to strongly negative εNd(t) values (−2.9 to −13.6) and intermediate to high 86Sr/87Sr ratios (0.7067–0.7165) suggests assimilation of older crustal material (i.e., the Juiz de Fora and Pocrane complexes, enclosing paragneisses), which is also indicated by Nd TDM model ages from 1.19 Ga to 2.13 Ga. Magmatic orthopyroxene and high content of CaO in garnet could indicate magma crystallization in the deep crust. Together, our data point out to a combination of partial melting of mantle wedge in the subduction zone, deep crustal anatexis and host rock assimilation, and crystal fractionation for magma genesis in the southeastern Rio Doce arc.
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Plutonic suite of Khajeh Morad in southeast of Mashhad is including of granodiorites cut across by younger aplite, granitic pegmatite dykes and monzogranites. Aplites are mineralogically including quartz, feldspar (albite to orthoclase and microcline), muscovite, and accessory minerals of garnet (almandine-spessartine), tourmaline, biotite, and ilmenite. Pegmatites are composed of quartz, feldspar (albite to oligoclase, orthoclase, and microcline), muscovite, and minor amounts of garnet (almandie-spessartine), tourmaline, ilmenite, beryl, and columbite. Based on mineralogical and geochemical evidences, Khajeh Morad pegmatites are related to Li-rare elements (RE-Li) and Lithium-Cesuim-Tantalum (LCT) pegmatite family. According to the field evidences and whole rock geochemistry, origin of garnet-bearing aplite-pegmatite melts could be related to the S-type monzogranites, as their differentiation products at late stages, occurred in continental collision belts.
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Plutonic suite of Khajeh Morad in southeast of Mashhad is including of granodiorites cut across by younger aplite, granitic pegmatite dykes and monzogranites. Aplites are mineralogically including quartz, feldspar (albite to orthoclase and microcline), muscovite, and accessory minerals of garnet (almandine-spessartine), tourmaline, biotite, and ilmenite. Pegmatites are composed of quartz, feldspar (albite to oligoclase, orthoclase, and microcline), muscovite, and minor amounts of garnet (almandie-spessartine), tourmaline, ilmenite, beryl, and columbite. Based on mineralogical and geochemical evidences, Khajeh Morad pegmatites are related to Li-rare elements (RE-Li) and Lithium-Cesuim-Tantalum (LCT) pegmatite family. According to the field evidences and whole rock geochemistry, origin of garnet-bearing aplite-pegmatite melts could be related to the S-type monzogranites, as their differentiation products at late stages, occurred in continental collision belts.
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Fibrolite garnet staurolite mica schist and staurolite garnet mica schist cropped out around the northwest of Khalaj (in the south of Mashhad) in a southeast to northwest direction along the metamorphic complex of Kuh-e-Majuni. They have similar mineralogy and consist of quartz, annite, staurolite, almandine, muscovite, zircon, and ilmenite; however, fibrolite in fibrolite garnet staurolite mica schist, and chlorite and tourmaline in the staurolite garnet mica schist are additionally found. Application of garnet - biotite thermometry and GBMAQ barometry indicates temperatures and pressures of 560 and 605 °C / 3.5 and 5 kilobar for fibrolite garnet staurolite mica schist and temperatures of 489 and 547 °C (in 3.5 to 5 kilobar) for the staurolite garnet mica schist. Pressure and temperature increase during the garnet growth indicates the effect of regional and contact thermal metamorphism on these rocks. Based on mineral paragenesis in KFMASH system, the metamorphic degree of regional metamorphism was about lower amphibolite (in staurolite garnet mica schist) to middle amphibolite facies (in fibrolite garnet staurolite mica schist). Meanwhile, intrusion of Khalaj granitoid and its thermal diffusion raised the metamorphic temperature up to lower amphibolite facies (in staurolite garnet mica schist) and middle amphibolite facies (in fibrolite garnet staurolite mica schist), and consequently this caused the fibrolite formation in the sample close to the pluton (i.e. fibrolite garnet staurolite mica schist).
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Plutonic suite of Khajeh Morad at southeast of Mashhad includes granodiorites, which are cut across by younger aplite, granitic pegmatite dykes and monzogranites. Aplites are mineralogically including quartz, feldspar (albite to orthoclase and microcline), muscovite, and accessory minerals of garnet (almandine-spessartine), tourmaline, biotite, and ilmenite. Pegmatites are composed of quartz, feldspar (albite to oligoclase, orthoclase, and microcline), muscovite, and minor amounts of garnet (almandine-spessartine), tourmaline, ilmenite, beryl, and columbite. Based on mineralogical and geochemical evidence, Khajeh Morad pegmatites are related to Li-rare elements (RE-Li) and lithium-cesuim-tantalum (LCT) pegmatite family. According to the field evidence and whole rock geochemistry, origin of garnet-bearing aplite-pegmatite melts could be related to the S-type monzogranites, as their differentiation products at late stages, occurred in a continental collision belts.
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The granitoids of Dehnow in NE Iran are part of a calc-alkaline plutonic series (diorite-tonalite-granodiorite) that intruded the remnants of the Paleo-Tethys oceanic crust during the Triassic. New major and trace element data together with isotopic compositions elucidate their I-type nature and a deep magma origin. P-T calculations based on amphibole and plagioclase suggest crystallization stages in the upper lithosphere at an approximate pressure of 6.4 kbar and temperature of 708°C. The Dehnow granitoids are characterized by high concentrations of LILE, LREE, HFSE and low concentrations of HREE, similar to some worldwide I-type granites, including examples from Harsit (along the Alpine-Himalayan suture zone), Iberia and the Martins Pereira plutons. The new geochemical data in combination with mineral parageneses and field observations suggest that the origin of the low temperature, Caledonian-type, arc-related granitoids of Dehnow resulted from the subduction of the Paleo-Tethys oceanic slab beneath the Turan block (along the Alpine-Himalayan suture zone) and involved the contribution of lower crust and mantle melts in this collisional setting.
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The intercrystalline distribution of Al and Fe3 + between grossular - andradite and clinozoisite - epidote solid solution series were determined experimentally within the temperature range of 500 - 650 "C at 0.3 GPa, HM-buffer. The AI - Fe exchange reaction exhibits a remarkable dependence on composition and temperature due to the existence of two intermediate solvi along the join clinozoisite - epidote. The immiscibility is produced by ordering phenomena, resulting in different local environments for the octahedral (M 3) sites of the different epidote phases. The exsolved phases can be detected only by methods sensitive to short-range order phenomena. In the presence of grandite and abundant fluid phase these domains act as seeds and macroscopic phases crystallize. The solvi project in the range of xFe epi= 0.54 - 0.76 and xFe epi = 0.24 - 0.50 at 500 "C/O.3 GPa, HM buffer, respectively. The miscibility gaps are separated by a narrow range of single-phase epidote at about xFe epi= 0.52. The solvi exhibit steep flanks, resulting in critical temperatures of 740 "C and 790 "C. The closing temperatures are metastable with respect to the upper thermal stability of intermediate epidotes. As a consequence 5 sets of intercrystalline distribution coefficients for AI - Fe3 + between grandite and epidote occur. At Fe-rich and AI-rich epidote compositions the intercrystalline KD' s vary significantly with temperature, at Fe-rich compositions KD' s increase with temperature and at AI-rich compositions KD' s decrease with temperature. The intercrystalline partition data exhibit an azeotropic behaviour due to the non ideal thermodynamic mixing properties of grandites and epidotes and a reversal in preference of Al and Fe3 + occur at xFe(grt) = O.41/xFe(epi)= 0.90. According to ENGI & WERSIN (1987) and FEHR (1992) the mixing properties of grandites are asymmetric and have to be described by the means of a 2-parameter or a 3-parameter Redlich-Kister equation, respectively . The thermodynamic mixing model for stable ordered epidotes by B IRD & HELGESON (1980) predicts degrees of disorder which are too high and has to be revised. The application of the distribution curves on grandit elepidote pairs of a skarn deposit (KITAMURA 1975) demonstrates that the intercrystalline AI - Fe exchange reaction
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The introduction of a fifth amphibole group, the Na-Ca-Mg-Fe-Mn-Li group, defined by 0.50 < (B)(Mg,Fe2+,Mn2+,Li) < 1.50 and 0.50 less than or equal to (B)(Ca,Na) less than or equal to 1.50 apfu (atoms per formula unit), with members whittakerite and ottoliniite, has been required by recent discoveries of (B)(LiNa) amphiboles. This, and other new discoveries, such as sodicpedrizite (which is herein slightly, but significantly changed from the original idealized formula), necessitate amendments to the IMA 1997 definitions of the Mg-Fe-Mn-Li, calcic, sodic-calcic and sodic groups. The discovery of obertiite and the finding of an incompatibility in the IMA 1997 subdivision of the sodic group, requires further amendments within the sodic group. All these changes, which have IMA approval, are summarized.
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The liquidus relationships projected in the AFM system (A = Al2O3-K2O-Na2O-CaO, F = FeO, M = MgO) have been described by Abbott & Clarke (1979) for silicate liquids (Liq) saturated with respect to quartz, alkali feldspar, plagioclase and one or more of the AFM minerals. Als (andalusite or sillimanite), garnet (Gar), biotite (Bio) and cordierite (Cdt). Three improvements are presented in this paper: (1) The liquidus relationships in part of the metaluminous region (A < 0) are deduced by considering the equilibrium Liq-Bio-Hnb (Hnb = hornblende). The liquidus boundary for this equilibrium is believed to extend from the metaluminous region, where the reaction is even (Hnb + Bio = Liq), into the peraluminous region, where it is odd (Bio = Hnb + Liq). Liquids on the Hnb-Bio-Liq equilibrium may change from metaluminous to peraluminous during normal fractional crystallization. (2) A plausible sequence of changes in the AFM liquidus topology is presented for the disappearance of a liquidus field for FeBio from the AF join. The breakdown of FeBio results in the appearance of a liquidus field for fayalite. (3) The effects of adding MnO to the AFM system are examined. Whereas at low Mn/(Fe+Mg+Mn) the equilibrium Bio-Gar-Liq is interpreted to be even (Bio+Gar = Liq), at high Mn/(Fe+Mg+Mn) this equilibrium is believed to be odd (Gar = Bio+Liq). This behavior may account for the disappearance of Bio during the final stages of fractional crystallization. leading to garnetiferous aplites and pegmatites.
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The introduction of a fifth group of amphiboles, the Na-Ca-Mg-Fe-Mn-Li group, defined by 0.50 < (B)(Mg,Fe2+,Mn2+,Li) < 1.50 and 0.50 less than or equal to (B)(Ca,Na) less than or equal to 1.50 atoms per formula unit, with members whittakerite and ottoliniite, has been required by recent discoveries of (B)(Li,Na) amphiboles. These, and other new discoveries, such as sodicpedrizite (which is herein slightly, but significantly, changed from the original idealized formula), necessitate amendments to the IMA 1997 definitions of the Mg-Fe-Mn-Li, calcic, sodic-calcic and sodic groups. The discovery of obertiite and the finding of an incompatibility in the IMA 1997 subdivision of the sodic group require further amendments within the sodic group. All these changes, which have IMA approval, are summarized.
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The Upper Cretaceous Beypazari granitoid of the western Ankara, Turkey, is composed of two different units, on the basis of petrography and geochemical composition; these are granodiorite and diorite. The granitoid is subalkaline, belonging to the high-K calc-alkaline I-type granite series, which have relatively low initial Sr-87/Sr-86 ratios (0.7053-0.7070). All these characteristics, combined with major, trace element geochemical data as well as mineralogical and textural evidence, reveal that the Beypazari granitoid formed in a volcanic arc setting and was derived from a subduction-modified and metasomatized mantle-sourced magma, with its crustal and mantle components contaminated by interaction with the upper crust. The rocks have epsilon Nd-(75Ma) values ranging from -5.5 to -2.0. These characteristics also indicate that a crustal component played a very important role in their petrogenesis. The moderately evolved granitoid stock cropping out near Beypazari, Ankara, was studied using the oxygen and hydrogen isotope geochemistry of whole rock, quartz and silicate minerals. PO values of the Beypazari granitoid are consistently higher than those of normal I-type granites. This is consistent with field observations, petrographic and whole-rock geochemical data, which indicate that the Beypazari granitoid has significant crustal components. However, the delta O-18 relationships among minerals indicate a very minor influence of hydrothermal processes in sub-solidus conditions. The oxygen isotope systematics of the Beypazari granitoid samples results from the activity of high-delta O-18 fluids (magmatic water), with no major involvement of low-delta O-18 fluids (meteoric water) evident. The analysed four quartz-feldspar pairs have values of Delta(qtz-fsp) between 0.5-2.0, which are consistent with equilibrium under close-system conditions. No stable isotope evidence was found to suggest that extensive interaction of granitoids with hydrothermal fluids occurred and this is consistent with the lack of large-scale base-metal mineralization.
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Garnet is an accessory mineral in the Cape Granite Suite, and garnet δ18O values in the Peninsula Granite range in from 10.0 to 11.4‰ (mean 10.6 ± 0.6‰, n = 15). These values are consistent with the garnet being produced during incongruent melting of a metapelitic source that has a similar O-isotope composition to the Malmesbury Group. Peninsula Granite quartz δ18O values range from 13.2 to 14.0‰ (mean 13.6 ± 0.3‰, n = 17), at the high end of the range previously observed for the Cape Granite Suite. These high δ18O values are consistent with the source of the Peninsula Granite magma having a greater component of clay minerals, which have inherently high δ18O values. Garnet has a high closure temperature (>800 oC) to oxygen diffusion and its δ18O value should, therefore, correlate closely with that of the source. Quartz has a significantly lower closure temperature (~550 oC) than garnet, and sub-solidus oxygen isotope re-equilibration between quartz and feldspar during slow cooling ought to result in a greater variation in quartz δ18O values compared to that of garnet. That the reverse is the case suggests that granite magmas were derived from a moderately heterogeneous source, as expected for metasedimentary rocks. This source underwent melting to produce different batches of granitic magma containing entrained garnets of slightly different δ18O value. Magma batches were subsequently mixed and homogenized before and/or during the emplacement process, resulting in a narrower spread of quartz δ18O values.
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The 2·05 Ma Bushveld magmatic event culminated in the production of >90 000 km3 of granite and granophyres. In these granitic rocks, high-temperature equilibrium O-isotope fractionations are generally preserved between quartz and zircon, but not between quartz and feldspar, or between biotite and amphibole. Quartz separated from four granite samples shows no significant difference in core and rim δ18O values, which indicates that quartz is not significantly zoned, and provides further evidence that it is unaffected by alteration. Quartz can, therefore, be used as a proxy for the magma δ18O value, leading to estimates of 6·9‰ for both the granites (assuming Δquartz–magma = 1·11‰) and granophyres (assuming Δquartz–magma = 0·62‰). Similar magma δ18O values (6·6‰) were obtained using zircon δ18O values, assuming Δzircon–magma = −1·3‰. The initial Nd-isotope ratio of the granitic rocks ranges from 0·509676 to 0·509822, with an average value of 0·509655 (n = 12). This corresponds to average εNd values of −5·9 and −4·8 for the granites and granophyres, respectively. The similarity in isotope composition between the granites and granophyres, and between the granitic rocks from each of the three major lobes of the Bushveld complex, is consistent with a common origin. The δ18O values of the granitic rocks suggest derivation from mantle-derived magmas by either fractional crystallization or partial melting, but this hypothesis is incompatible with their crustal εNd values (average −5·5). The associated Rustenburg Layered Suite (RLS) rocks have average δ18O values of 7·1‰, which is within error of the average estimate for the Bushveld granitic rocks, and similar εNd values. However, granitic magma derived from the same paretal magmas that produced the RLS would have had an average magma δ18O of about 7·9‰, 1‰ higher than observed. We therefore suggest that the granitic magmas were produced by fractional crystallization of RLS magma (or by partial melting of solidified RLS magma at depth) followed by assimilation, at a shallower level, of a significant quantity of hydrothermally altered low δ18O material from the since eroded volcanic edifice.
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An unusual andesitic suite from the Miocene volcanic arc in Northland, New Zealand, comprises pyroxene andesite and garnet-bearing hornblende-pyroxene, hornblende and biotite-hornblende andesites. Garnet crystals occur as 1-10 mm single crystals or more commonly as two or more annealed crystals and as garnetite lenses. The andesitic rocks also contain enclaves of high-MgO pyroxenite, hornblendite, and pyroxene-hornblende gabbro as well as high-Al2O3 hornblende gabbro, garnet-hornblende gabbro, and anorthosite. Garnet crystals in the andesitic volcanic rocks and in the enclaves show comparable compositional ranges, zoning patterns and inclusions, which indicate that they share a common petrogenetic history. They can be grouped into four distinct types on the basis of mode of occurrence, chemical composition and zoning patterns, which leads to their interpretation as antecrysts rather than orthocrysts. The compositions of the garnets, as well as their included mineral assemblages, reflect a petrogenetic trend from high-temperature pyroxene-bearing high-Mg garnet to low-temperature Fe-rich garnet at relatively constant pressure. Well-preserved zoning patterns, in particular those of the Ca- and Mg-rich garnets, reflect processes within a deep crustal arc environment. Later assimilation is suggested by some zoning patterns that show decreasing Ca and increasing Fe and Mn contents. The garnets are interpreted as being derived by disintegration of discrete but closely related cumulate material that formed at pressures of 8-10 kbar. The host volcanic rocks and their garnet crystals together with the enclaves thus represent a consanguineous mixture of liquid and solid components that developed where subduction-related magmas ponded and interacted at or near the base of the crust. Together they represent a rare snapshot of the processes and components that produce arc-type rocks.
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The granitoids of Dehnow in NE Iran are part of a calc-alkaline plutonic series (diorite-tonalite-granodiorite) that intruded the remnants of the Paleo-Tethys oceanic crust during the Triassic. New major and trace element data together with isotopic compositions elucidate their I-type nature and a deep magma origin. P-T calculations based on amphibole and plagioclase suggest crystallization stages in the upper lithosphere at an approximate pressure of 6.4 kbar and temperature of 708°C. The Dehnow granitoids are characterized by high concentrations of LILE, LREE, HFSE and low concentrations of HREE, similar to some worldwide I-type granites, including examples from Harsit (along the Alpine-Himalayan suture zone), Iberia and the Martins Pereira plutons. The new geochemical data in combination with mineral parageneses and field observations suggest that the origin of the low temperature, Caledonian-type, arc-related granitoids of Dehnow resulted from the subduction of the Paleo-Tethys oceanic slab beneath the Turan block (along the Alpine-Himalayan suture zone) and involved the contribution of lower crust and mantle melts in this collisional setting.
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پیامبر خدا (صلی‌الله ‏علیه و ‏آله و سلّم) فرمودند: هر گاه مؤمن یک برگه که روى آن علمى نوشته شده باشد از خود برجاى گذارد، روز قیامت آن برگه پرده میان او و آتش می ‏شود و خداوند تبارک‌وتعالی به ازاى هر حرفى که روى آن نوشته شده، شهرى هفت برابر پهناورتر از دنیا به او میدهد. سلام علیکم؛ ایزد دانا را سپاس می‌گویم که بنده را یاری بخشید تا بتوانم در زمینه تحقق اهداف خویش، گام بردارم. برای پاسداشت و ترویج علم مقدس زمین‌شناسی، رساله دکتری خود را به همه فرهیختگان جامعه علمی زمین‌شناسی ایران تقدیم می‌نمایم. شایسته است انشالله همه بزرگواران امانت داری کامل علمی را رعایت بفرمایند. پیروزی و موفقیت شما را در تمامی امور زندگی آرزومندم. دکتر رامین صمدی
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پیامبر خدا (صلی‌الله ‏علیه و ‏آله و سلّم) فرمودند: هر گاه مؤمن یک برگه که روى آن علمى نوشته شده باشد از خود برجاى گذارد، روز قیامت آن برگه پرده میان او و آتش می ‏شود و خداوند تبارک‌وتعالی به ازاى هر حرفى که روى آن نوشته شده، شهرى هفت برابر پهناورتر از دنیا به او میدهد. سلام علیکم؛ ایزد دانا را سپاس می‌گویم که بنده را یاری بخشید تا بتوانم در زمینه تحقق اهداف خویش، گام بردارم. برای پاسداشت و ترویج علم مقدس زمین‌شناسی، پایان نامه کارشناسی ارشد خود در دانشگاه تهران را به همه فرهیختگان جامعه علمی زمین‌شناسی ایران تقدیم می‌نمایم. شایسته است انشالله همه بزرگواران امانت داری کامل علمی را رعایت بفرمایند. پیروزی و موفقیت شما را در تمامی امور زندگی آرزومندم. دکتر رامین صمدی
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Metamorphic rocks of Dehnow area mainly consist of gray to black fine-grained schists. Garnet schists are closer to the tonalitic body than the garnet chloritoid schists. There is a thin layer of staurolite and andalusite bearing hornfels between these schists and the Dehnow tonalitic body. Garnet schists and garnet chloritoid schists of Dehnow area are mineralogically comprised of quartz, biotite, muscovite, garnet, chlorite, chloritoid, tourmaline and ilmenite. Geothermobarometry results indicate that hornfels (550oC, 4.3 kbar) and garnet chloritoid schist (486-497oC) have formed in lower equilibrium condition in comparison with garnet schist (569oC, 5.3 kbar).
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The Erlangmiao granite intrusion is located in the eastern part of the East Qinling Orogen. The granite contains almost 99 vol.% felsic minerals with accessory garnet, muscovite, biotite, zircon, and Fe-Ti oxide. Garnet is the dominant accessory mineral, shows zoned texture, and is rich in w(FeO) (14.13%–16.09%) and w(MnO) (24.21%–27.44%). The rocks have high SiO2, alkalis, FeOt/MgO, TiO2/MgO and low Al2O3, CaO with w(Na2O)/w(K2O)> 1. Their Rb, Ga, Ta, Nb, Y, and Yb contents are high and Sr, Ba, Eu, Zr, P, and Ti contents are low. These features indicate that the Erlangmiao granite is a highly evolved metaluminous A-type. Garnet crystallized at the expense of biotite from the MnO-rich evolved melt after fractionation of biotite, plagioclase, K-feldspar, zircon, apatite, and ilmenite. The relatively high initial 87Sr/86Sr ratios (0.706–0.708), low and negative ɛNd (120 Ma) values (−6.6 to −9.0), and old Nd model ages (1.5–1.7 Ga) suggest that the rocks were probably formed by partial melting of the Paleoproterozoic granitic gneisses from the basement, with participation of depleted mantle in an extensional setting.
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Values of δ18O of zircon from the central Sierra Nevada batholith (SNB), California, yield fresh insight into the magmatic evolution and alteration history of this classic convergent margin batholith. Direct comparison of whole-rock and zircon (Zrc) δ18O provides evidence for modest (0.5‰), but widespread, alteration, which has complicated interpretation in previous whole-rock δ18O studies. Four discrete belts of δ18O values are recognized in the central Sierra. A small belt of plutons with relatively low δ18O(Zrc) values (5.2-6.0‰) intrudes the foothills, with a sharp increase of δ18O revealing the concealed Foothills Suture; high δ18O(Zrc) values (7.0-8.5‰) dominate the rest of the western SNB. East of the axis of the Sierra, δ18O is distinctly lower (6.75-5.75‰), and decreases monotonically to the Sierra Crest. A sharp 1‰ increase of δ18O in the eastern Sierra reveals a second crustal boundary, with the fourth belt hosted in high-δ18O North American crust in the White Mountains and Owens and Long Valleys. Correlated O, Sr, and Pb isotope ratios reveal differences in magma generation between the western and eastern Sierra. The western Sierra experienced massive crustal recycling, with substantial melting and mobilization of accreted oceanic and volcanic arc rocks; crustal contamination affects many western SNB plutons. In contrast, the eastern Sierra was dominated by voluminous recycling of the lithospheric mantle and lower crust, with minimal crustal contamination. Batholith-wide shifts in δ18O occur between pulses of Cretaceous magmatism that may be linked to tectonic reorganizations of magma sources. Within intrusive suites, δ18O may be unchanged (Tuolumne); increase (Sonora and Whitney); or decrease (Sequoia and John Muir) with time. These trends show stable long-lived sources, or those where recycling and contamination may increase or decrease with time. Overall, δ18O reveals diverse magma system behavior at a range of scales in the Sierran arc. © The Author 2008. Published by Oxford University Press. All rights reserved.
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Miocene to Pleistocene calc-alkaline volcanism in the East Carpathian arc of Romania was related to the subduction of a small ocean basin beneath the continental Tisza–Dacia microlate. Volcanic products are predominantly andesitic to dadtic in composition, with rare basalts and rhyodacites (51–l71% SiO2; mg-number 0.65–0.26) and have medium- to high-K calcalkaline and shoshonitic affinities. Mg, Cr and Ni are low in all rock-types, indicating the absence of primary erupted compositions. Detailed trace element and Sr, Nd, Pb and 0 isotope data suggest that magmas were strongly crustally contaminated. Assimilation and fractional crystallization (AFC) calculations predict the consumption of 5–35% local upper-crustal metasediments or sediments from the palaeo-accretionary wedge. Variations in the isotopic composition of the contaminants and parental magmas caused variations in the mixing trajectories in different parts of the arc The most primitive isotopic compositions are found in low-K dacites of the northern Cdlimani volcanic centre and are interpreted as largely mantle derived. A second possible mantle reservoir of lower 149 Nd/144 Nd and lower 206 Pb/204 Pb is identified from back-arc basic calc-alkaline rocks in the south of the arc Both magmatic reservoirs have elevated isotopic characteristics, owing either to source bulk mixing (between depleted or enriched asthenosphere and <1% average subducted local sediment) or lower-crustal contamination.
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Thermodynamic oxygen isotope factors for anhydrous silicate minerals have been calculated by means of the modified increment method. The obtained order of O-18 enrichment in common rock-forming minerals is quartz > albite greater-than-or-equal-to K-feldspar > sillimanite greater-than-or-equal-to leucite > andalusite > jadeite > kyanite > anorthite greater-than-or-equal-to cordierite > diopside > wollastonite > zircon almost-equal-to garnet > olivine. Two sets of self-consistent fractionation factors between quartz and the silicate minerals and between the silicate minerals and water have been respectively obtained for a temperature range of 0 to 1200-degrees-C. The present results on the quartz-mineral systems are in good agreement with high-temperature data derived from semi-empirical calibrations and calcite-exchange experiments, demonstrating that the present calculated fractionation factors for mineral pairs are applicable to isotopic geothermometry and as a test of disequilibrium in natural mineral assemblages over all temperature ranges of geological interest. The silicate-water fractionations obtained from the present calculations also match those from hydrothermal experiments (except 3-isotope exchange results). The present calculations provide a potential insight into the quantitative dependence of oxygen isotope fractionation upon chemical composition and structural state of minerals. Minerals may be depleted in O-18 with increasing pressure as a result of the change in crystal structures.
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Xenoliths, considered to be of igneous origin and consisting of hornblende±garnet±plagioclase ±clinopyroxene, occur in association with high-pressure phenocrysts in early Miocene high-silica andesites and dacites, Northland, New Zealand. Microstructures of these xenoliths range from coarse, even-grained sub-ophitic types to others with coarse glomerocrysts set in a finer-grained mesostasis. The xenoliths are commonly flow-banded and are argued to represent direct crystallization products and crystal aggregations from the calc-alkaline host or related magmas at depth. Many garnets within these high-pressure aggregates and also discrete garnet phenocrysts are rimmed by medium-coarse-grained, interlocking hornblende±plagioclase, representing partial adjustment to an assemblage stable at shallower levels. The garnets are typically pyrope-almandine with 17-28 mol.% grossular and show normal, reverse and oscillatory zoning; the associated amphibole is pargasite trending to hornblende in phenocryst rims and reaction rims. Metamorphic xenoliths with plagioclase-hornblende-quartz assemblages are also found in the rocks and are characterized by fine-grained granoblastic mosaic microstructures with well-developed foliation defined by preferred orientation of elongate grains and a mineral layering. These metamorphic xenoliths are interpreted as fragments of lower-crustal country rocks accidentally incorporated into rising andesitic magma.Application of established experimental high-pressure phase diagrams for andesites indicates crystallization of these assemblages at depths corresponding to 10-20-kb pressure, and appropriate geothermometers indicates the following temperatures for equilibration of assemblages at a nominal pressure of 12 kb: garnet-augite ∼980°C; garnet-augite-hornblende ∼920-1020 °C. Geobarometry on a single garnet-orthopyroxene-bearing xenolith indicates a pressure of 10-12 kb for a likely temperature range of 950-1000°C. Thus the xenoliths point to the generation of host andesite-dacite magmas at suberustal depths of 35-45 km, from fractional crystallization of more mafic mantlederived magmas, and demonstrate that relatively silicic calc-alkaline magmas may evolve in the mantle. The rarity of evidence for such a process may be linked with the obduction-related tectonic events operative in Northland just before the magmatic episode, and to the unusually high water content in the magma.
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The Governor Lake area, Meguma terrane, Nova Scotia, is underlain mainly by ca. 373 Ma granitoid rocks of the Trafalgar plutonic suite that intruded metasedimentary rocks of the Goldenville and Halifax groups. Garnet is abundant in the Beaverbank Formation of the Goldenville Group and its enclaves in the granitoid rocks. Single crystals of garnet, locally up to 3 cm in diameter, are also present in the Twin Lakes granodiorite and Bog Island Lake tonalite. On the basis of petrographic examination and electron-microprobe data, we have identified four different types of garnet. Type-1 garnet, generally associated with coticule xenoliths, forms small spessartine-rich (Sps(19-74)) grains concentrated in irregular to planar aggregates. Crystals range from homogeneous (type 1A) to zoned toward a Mn-enriched rim (type 1B). Type-2 garnet is restricted to metasedimentary host rocks, where it forms small, spessartine-rich (Sps(22-71)) grains zoned from a Mn-rich core to a Mn-poor rim. Type-3 garnet contains abundant metamorphic inclusions (e. g., sillimanite) and commonly has an inclusion-rich core with an inclusion-poor rim. Type-3A crystals have a Mn-rich core (Sps(21-52)), whereas type-3B crystals have a Mn-rich rim (Sps(15-21)). Type-4 garnet forms large, subhedral to euhedral crystals with abundant inclusions (e. g., apatite, plagioclase) of probable igneous origin. Type-4A garnet is spessartine-poor (Sps(4-11)) and weakly zoned, whereas zoning in type-4B crystals suggests that a Mn-poor core (Sps(8-16)) was partially resorbed and overgrown by a Mn-rich rim (Sps(11-26)). Of the various types of garnet identified in the granitic samples, types 1A and 3B are interpreted as orthoxenocrysts, type 1B, as paraxenocrysts, type 3A, as having an orthoxenocrystic core overgrown by an orthomagmatic or paraxenocrystic rim, and types 4A and 4B, as orthomagmatic. The presence of Mn-rich xenocrystic garnet suggests that the Twin Lakes and Bog Island Lake plutons were contaminated by manganiferous rocks, probably derived from the Beaverbank Formation. Incorporation and assimilation of Mn-rich material may also have led to crystallization of orthomagmatic garnet in the granitic magmas.
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In order to improve understanding of how accessory garnet crystallizes in igneous rocks, and evaluate it as a mineral recorder of magma history, we analyzed δ18O of garnets from the Hallowell and Togus plutons in south-central Maine (United States) by laser fluorination, and in situ by ion microprobe. Two types of garnet are recognized, magmatic and locally derived peritectic. Traverses of some single crystals show both gradual and abrupt changes of δ18O(garnet), commonly >1‰, while other garnet grains are isotopically homogeneous. Rimward increase of δ18O in many crystals indicates that garnet grew while high δ18O metamorphic wall rocks were assimilated. Peritectic grains have a complementary record of the transfer of high δ18O melts to the plutons. In some rocks, δ18O varies among neighboring grains, evidence that crystals grew episodically or were juxtaposed from different sources during magma mixing. Garnet faithfully records changing magmatic δ18O, and is a valuable tool to decipher magma petrogenesis.
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As demonstrated by the chapters in this short course, stable isotope techniques are an important tool in almost every branch of the earth sciences. Central to many of these applications is a quantitative understanding of equilibrium isotope partitioning between substances. Indeed, it was Harold Urey’s (1947) thermodynamically based estimate of the temperature-dependence of 18O/16O fractionation between calcium carbonate and water, and a recognition of how this information might be used to determine the temperatures of ancient oceans, that launched the science of stable isotope geochemistry. The approach pioneered by Urey has since been used to estimate temperatures for a wide range of geological processes (e.g. Emiliani 1955; Anderson et al. 1971; Clayton 1986; Valley, this volume). In addition to their geothermometric applications, equilibrium fractionation data are also important in the study of fluid-rock interactions, including those associated with diagenetic, hydrothermal, and metamorphic processes (Baumgartner and Valley, this volume; Shanks, this volume). Finally, a knowledge of equilibrium fractionation is a necessary first step in evaluating isotopic disequilibrium, a widespread phenomenon that is increasingly being used to study temporal relationships in geological systems (Cole and Chakraborty, this volume). In the fifty-four years since the publication of Urey’s paper, equilibrium fractionation data have been reported for many minerals and fluids of geological interest. These data were derived from: (1) theoretical calculations following the methods developed by Urey (1947) and Bigeleisen and Mayer (1947); (2) direct laboratory experiments; (3) semi-empirical bond-strength models; and (4) measurement of fractionations in natural samples. Each of these methods has its advantages and disadvantages. However, the availability of a variety of methods for calibrating fractionation factors has led to a plethora of calibrations, not all of which are in agreement. In this chapter, we evaluate the major methods for determining fractionation factors. …
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New empirical calibrations for the fractionation of oxygen isotopes among zircon, almandine-rich garnet, titanite, and quartz are combined with experimental values for quartz-grossular. The resulting A-coefficients (‰K 2) are: for the relation 1000 ln Y-X A Y-X (10 6 /T 2). The fractionation of oxygen isotopes between zircon and coexisting minerals can provide otherwise unavailable evidence of magmatic processes, including crystalli-zation, remelting, and assimilation-fractional crystallization.
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Most granitic batholiths contain plutons which are composed of low-variance mineral assemblages amenable to quantification of the P – conditions that characterise emplacement. Some mineral thermometers, such as those based on two feldspars or two Fe–Ti oxides, commonly undergo subsolidus re-equilibration. Others are more robust, including hornblende–plagioclase, hornblende–clinopyroxene, pyroxene–ilmenite, pyroxene–biotite, garnet–hornblende, muscovite-biotite and garnet–biotite. The quality of their calibration is variable and a major challenge resides in the large range of liquidus to solidus crystallisation temperatures that are incompletely preserved in mineral profiles. Further, the addition of components that affect K d relations between non-ideal solutions remains inadequately understood. Estimation of solidus and near-solidus conditions derived from exchange thermometry often yield results >700°C and above that expected for crystallisation in the presence of an H 2 O-rich volatile phase. These results suggest that the assumption of crystallisation on an H 2 O-saturated solidus may not be an accurate characterisation of some granitic rocks. Vapour undersaturation and volatile phase composition dramatically affect solidus temperatures. Equilibria including hypersthene–biotite–sanidine–quartz, fayalite–sanidine–biotite, and annite–sanidine–magnetite (ASM) allow estimation of Estimates by the latter assemblage, however, are highly dependent on . Oxygen fugacity varies widely (from two or more log units below the QFM buffer to a few log units below the HM buffer) and can have a strong affect on mafic phase composition. Ilmenite–magnetite, quartz–ulvospinel–ilmenite–fayalite (QUILF), annite–sanidine–magnetite, biotite–almandine–muscovite–magnetite (BAMM), and titanite–magnetite–quartz (TMQ) are equilibria providing a basis for the calculation of . Granite barometry plays a critical part in constraining tectonic history. Metaluminous granites offer a range of barometers including ferrosilite–fayalite–quartz, garnet–plagioclase–hornblende–quartz and Al-in-hornblende. The latter barometer remains at the developmental stage, but has potential when the effects of temperature are considered. Likewise, peraluminous granites often contain mineral assemblages that enable pressure determinations, including garnet–biotite–muscovite–plagioclase and muscovite–biotite–alkali–feldspar–quartz. Limiting pressures can be obtained from the presence of magmatic epidote and, for low-Ca pegmatites or aplites, the presence of subsolvus versus hypersolvus alkali feldspars. As with all barometers, the influence of temperature, , and choice of activity model are critical factors. Foremost is the fact that batholiths are not static features. Mineral compositions imperfectly record conditions acquired during ascent and over a range of temperature and pressure and great care must be taken in properly quantifying intensive parameters.
Article
Rare garnet phenocrysts and garnet-bearing xenoliths occur in high-silica, metaluminous to peraluminous andesites and dacites (and their high-level intrusive quartz diorite equivalents) from a Miocene calc-alkaline province in Northland, New Zealand. These garnets are among the most Ca-rich (17–28 mol% grossular) garnets of igneous origin so far recorded in calc-alkaline suite rocks. Associated minerals are dominant hornblende and plagioclase and minor augite, occurring as phenocrysts in xenoliths and as inclusions in the garnet. This mineralogy points to the I-type character of the garnet-bearing host magma compositions, and contrasts this garnet occurrence with the more frequently recorded grossular-poor (3–10 mol%) garnets with hypersthene, plagioclase, biotite and cordierite, found in S-type volcanic and intrusive host rocks. Detailed experimental work on a glass prepared from one of the garnet-bearing dacites closely constrains the conditions under which the natural phenocryst and xenolith mineral assemblages formed. This work was conducted over a pressure-temperature range of 8–20 kbar, 800–1050°C with 3–10 wt% of added H 2 O, defining overall phase relationships for these conditions. Importantly, amphibole only appears at temperatures of 900°C or less and clinopyroxene at >900°C (with 3wt% H 2 O). Orthopyroxene occurs with garnet at lower pressure (∼15 kbar with 3wt% H 2 O; ∼>10kbar with 5wt% H 2 O). Absence of orthopyroxene from the natural garnet-bearing assemblages indicates pressures above these limits during crystallisation. Plagioclase is markedly suppressed (with respect to temperature) with increasing H 2 O content, and for pressures of 10–15 kbar, the maximum H 2 O content possible in the magma with retention of clinopyroxene and plagioclase together (as evident in xenoliths) is 5–6 wt%. Finally, the lack of quartz in any of the xenoliths suggests magma H 2 O content higher than 3% (where quartz appears with amphibole at 900°C), since the quartz liquidus temperature decreases with increasing H 2 O content, and with decreasing pressure. In experiments with 5wt% H 2 O, a quartz-free field of crystallisation of garnet-clinopyroxene-amphibole-plagioclase occurs between 10 and 15 kbar and temperatures between 850 and 900°C. In addition, detailed experimentally-determined garnet compositional trends, together with ferromagnesian mineral compositional data for specific experiments with 5 wt% H 2 O added and run at 10-13 kbar and ∼900°C, suggest that the natural assemblages formed at these conditions. This implies that the parental dacitic magma must have been derived at mantle depths (the Northland crust is ∼25 km thick), and any basaltic or basaltic andesite precursor must have contained ∼2–3 wt% H 2 O. The unique nature of the Northland volcanics and high-level intrusives, preserving evidence of relatively grossular-rich garnet fractionation in the high-pressure crystallisation history of an originally mantle-derived magma, is attributed to a combination of unusually hydrous conditions in the source region, complex tectonic history involving obduction and subduction, possible incorporation of crustal slivers in a mantle-crust interaction zone, and relatively thin (∼25 km) crust.
Article
Petrological investigations of granite commonly reveal multiple periods of growth punctuated by resorption for many of the constituent minerals. Complementary to such textures are mineral compositional heterogeneity manifested by zoning or grain to grain variability. These features ultimately reflect changes in the intensive parameters or activities of components during melt solidification. Such complexities of granite crystallisation can be simultaneously modelled in a reaction space constructed from the set of linearly independent reactions describing the equilibria among all phases and components in the system of interest. The topology of the linearly independent reactions that define the reaction space for garnetmuscovite-biotite granites yields the following insights: (1) there is no one unique reaction that produces or consumes aluminous minerals (e.g. garnet); (2) minerals can alternate as reactants or products in different reactions accounting for textures indicating multiple periods of crystallisation separated by resorption; (3) mineral compositions are regulated by the reaction(s) producing them and vary as the stoichiometry of the reaction(s) producing them varies; (4) resorption of early crystallising garnet is likely to reflect decreasing pressure, presumably during magma ascent; (5) late crystallisation of garnet, at the expense of biotite, reflects an increase in melt aluminosity and does not necessarily require high Mn activities for the melt and (6) increasing melt H 2 O, at H 2 Oundersaturated conditions, favours the formation of biotite–muscovite granite. Application of the reaction space method to other granite types holds considerable promise for elucidating reactions that regulate mineral assemblages and compositions during crystallisation.
Article
Mashhad granitoids in northeast Iran are part of the so-called Silk Road arc that extended for 8300 km along the entire southern margin of Eurasia from North China to Europe and formed as the result of a north-dipping subduction of the Paleo-Tethys. The exact timing of the final coalescence of the Iran and Turan plates in the Silk Road arc is poorly constrained and thus the study of the Mashhad granitoids provides valuable information on the geodynamic history of the Paleo-Tethys. Three distinct granitoid suites are developed in space and time (ca. 217-200 Ma) during evolution of the Paleo-Tethys in the Mashhad area. They are: 1) the quartz diorite-tonalite-granodiorite, 2) the granodiorite, and 3) the monzogranite. Quartz diorite-tonalite-granodiorite stock from Dehnow-Vakilabad (217 ± 4-215 ± 4 Ma) intruded the pre-Late Triassic metamorphosed rocks. Large granodiorite and monzogranite intrusions, comprising the Mashhad batholith, were emplaced at 212 ± 5.2 Ma and 199.8 ± 3.7 Ma, respectively. The high initial 87Sr/86Sr ratios (0.708042-0.708368), low initial 143Nd/144Nd ratios (0.512044-0.51078) and low ɛNd(t) values (- 5.5 to - 6.1) of quartz diorite-tonalite-granodiorite stock along with its metaluminous to mildly peraluminous character (Al2O3/(CaO + Na2O + K2O) Mol. = 0.94-1.15) is consistent with geochemical features of I-type granitoid magma. This magma was derived from a mafic mantle source that was enriched by subducted slab materials. The granodiorite suite has low contents of Y (≤ 18 ppm) and heavy REE (HREE) (Yb < 1.53 ppm) and high contents of Sr (> 594 ppm) and high ratio of Sr/Y (> 35) that resemble geochemical characteristics of adakite intrusions. The metaluminous to mildly peraluminous nature of granodiorite from Mashhad batholiths as well as its initial 87Sr/86Sr ratios (0.705469-0.706356), initial 143Nd/144Nd ratios (0.512204-0.512225) and ɛNd(t) values (- 2.7 to - 3.2) are typical of adakitic magmas generated by partial melting of a subducted slab. These magmas were then hybridized in the mantle wedge with peridotite melt. The quartz diorite-tonalite-granodiorite stock and granodiorite batholith could be considered as arc-related granitoid intrusions, which were emplaced during the northward subduction of Paleo-Tethys Ocean crust beneath the Turan micro-continent. The monzogranite is strongly peraluminous (Al2O3/(CaO + Na2O + K2O) Mol. = 1.07-1.17), alkali-rich with normative corundum ranging between 1.19% and 2.37%, has high initial 87Sr/86Sr ratios (0.707457-0.709710) and low initial 143Nd/144Nd ratios (0.512042-0.512111) and ɛNd(t) values (- 5.3 to - 6.6) that substantiate with geochemical attributes of S-type granites formed by dehydration-melting of heterogeneous metasedimentary assemblages in thickened lower continental crust. The monzogranite was emplaced as a consequence of high-temperature metamorphism during the final integration of Turan and Iran plates. The ages found in the Mashhad granites show that the subduction of Paleo-Tethys under the Turan plate that led to the generation of arc-related Mashhad granites in late-Triassic, finally ceased due to the collision of Iran and Turan micro-plates in early Jurassic.
Article
Mesozoic tonalites were emplaced in the axial zones of F2 folds in low to medium grade metamorphic rocks. Tonalite has assimilated Fe-rich pelite and, at the contact, tonalite contains refractory garnet xenocrysts. Digestion of the xenocrysts to biotite-quartz-plagioclase assemblages has produced a decrease in amphibole, K-feldspar and garnet and an increase in biotite und quartz from the margins towards the centre of the tonalite mass. The magma probably intruded hot regionally metamorphosed rocks at high pressures. In the schistose thermal aureole, almandine porphyroblasts decrease in size and abundance away from the contact. Garnet grew from pre-existing nuclei in the regional metamorphic rocks and was armoured from reaction by quartz-sillimanite sheaths. In the aureole, chloritoid and staurolite are confined to rocks with a restricted bulk rock composition, sillimanite is present in rocks with a high iron content and andalusite is common in more aluminous pelites.
Article
The ability to determine the thermal and barometric history during crystallization and emplacement of granitic plutons has been enhanced by several new calibrations applicable to granitic mineral assemblages. Other existing calibrations for granitic plutons have continued to be popular and fairly robust. Recent advances include the trace element thermometers Ti-in-quartz, Ti-in-zircon, and Zr-in-sphene (titanite), which need to be further evaluated on the roles of reduced activities due to lack of a saturating phase, the effect of pressure dependence (particularly for the Ti-in-zircon thermometer), and how resistive these thermometers are to subsolidus reequilibration. As zircon and sphene are also hosts to radiogenic isotopes, these minerals potentially also provide new insights into the temperature - time history of magmas. When used in conjunction with pressure-sensitive mineral equilibria in the same rocks, a complete assessment of the P-T-t (pressure-temperature-time) path is possible given that the mineralogy of plutons can reflect crystallization over a range of pressure and temperature during ascent and emplacement and that many intrusions are now seen as forming over several millions of years during the protracted history of batholith construction. Accessory mineral saturation thermometers, such as those for zircon, apatite, and allanite, provide a different and powerful perspective, specifically that of the temperature of the onset of crystallization of these minerals, which can allow an estimate of the range of temperature between the liquidus and solidus of a given pluton. In assessment of the depth of crystallization and emplacement of granitic plutons, the Al-in-hornblende remains popular for metaluminous granites when appropriately corrected for temperature. For peraluminous granites, potential new calibrations exist for the assemblages bearing garnet, biotite, plagioclase, muscovite, and quartz. Other thermometers, based on oxygen abundance, and including Fe-Ti oxides, pyroxene, fayalitic olivine, quartz, sphene, and/or biotite, some of which have been recently revised, can provide additional information on temperature and oxygen fugacity. Oxygen fugacity can range over several orders of magnitude in different magmatic systems and can have profound influence on the mineralogy and mineral compositions in granitic magmas. It also forms the foundation of the popular magnetite- versus ilmenite-series granite classification.
Article
Calc-alkaline dacites from the Setouchi volcanic belt contain garnet crystals that are classified petrographically and chemically into two types: type I and type M. Type-M garnets are characterized by acicular sillimanite inclusions or dissolution textures, and may be accompanied by xenolith fragments. They exhibit extensive compositional zoning with an increase in MgO/FeO and decrease in MnO content towards the margin. These petrographical and compositional features are identical to those of garnets from metamorphic xenoliths entrained in the Setouchi volcanic rocks, suggesting a xenocrystic origin for the type-M garnets. In contrast, type-I garnets lack sillimanite inclusions and have different rim compositions from the type-M garnets. Transmission electron microscope analysis has revealed the presence of minute glass inclusions in the type-I garnets, which indicate conclusively that these garnets grew in the presence of a melt. Type-I garnets have oscillatory zoning characterized by an antipathetic variation between FeO and MgO. This zoning was probably caused by magma heterogeneity within magma batch. Differences in rim compositions between the two types of garnets, and the presence of reaction rims indicate that the xenocrystic type-M garnets were incorporated into the magma after phenocrystic type-I garnet became unstable due to decompression during magma ascent.
Article
Small, euhedral Mn-rich garnets (32-52 mol. % spessartine) from the Cairngorm granite, Eastern Grampian Highlands, Scotland, are considered to be of magmatic origin and have not been derived from the assimilation of metasedimentary material, despite their occurrence largely at the margins of the pluton. Similar garnets also occur in a late cross-cutting aplite sheet. The garnets in the granite crystallized early in the sequence and are thought to have formed in response to the ponding of Mnrich fluids against the wall of the pluton. This Mn enrichment of the fluid phase continued throughout the evolution of the pluton, resulting in Mn-rich biotites and opaque oxides and the localized crystallization of Mn-rich garnets in aplite. Garnet contains up to 1.67 wt. % Y, but has not played a major role in the geochemical evolution of the Cairngorm granite, which has high SiO2 (72-77%) and is enriched in Y and HREE. Chemical analyses of garnets, biotites and rocks are given.
Article
Microprobe analyses of eight almandine-spessartines from pegmatites and aplites of the Hub Kapong batholith and Phuket Island show three types of zoning. Garnets from pegmatites have Mn-rich cores (approx 80% spessartine) and Mn-poor rims whereas those from aplites are either unzoned or show Mn-enriched rims. The pegmatitic garnets grew under conditions favourable for the development and preservation of concentration gradients (low nucleation density, rapid growth rate and slow cation-diffusion rates for the crystal, and rapid diffusion rates for the pegmatite liquid) whereas the aplitic garnets had slower growth rates and faster diffusion rates for the crystal coupled with slower diffusion rates for the aplite magma.-R.A.H.
Article
Garnet occurs as megacrysts or in xenoliths within volcanic rocks of andesitic-rhyolitic composition, as detrital grains in the associated terrestrial sediments, and as an accessory mineral in many plutonic rocks from the E coast of the Antarctic Peninsula. The primary igneous garnets are almandine-rich (6 probe analyses show Fe71-76Mn3-5 with one Fe53Mn39) whereas those occurring as xenocrysts or in xenoliths are almandine- pyrope. Comparison with published experimental work suggests that the Fe-rich, Mn-poor garnets of the volcanic rocks are remnant from high-P crystallization from magma at >7 kbar. The garnets richer in pyrope have been derived from garnet-bearing country rocks at depth, either as accidental inclusions or through direct partial melting of the lower crust (implying that >25 km of crustal material was in existence before the generation of the Mesozoic magmatic arc). The origin of these calc-alkaline magmas may thus be due in part to partial melting of pre-existing sialic crustal material. -R.A.H.
Article
Remnants of the Paleo-Tethys oceanic realm in the Binalood region include not only ophiolite complexes but also a pile of upward-coarsening, pre-Late Triassic metasedimentary rocks that are here interpreted to be abyssal plain and deep-sea flysch deposits. Obduction of the accretionary assemblage over the Iranian microcontinent took place prior to Late Triassic time. -from Author
Article
A new geobarometer based on the Al content of igneous hornblendes in equilibrium with melt, fluid, biotite, quartz, sanidine, plagioclase, sphene, and magnetite or ilmenite has been calibrated experimentally. The calibration was performed by equilibrating the required phase assemblage over the pressure range 2-8 kbar at 740-780C, and then analyzing euhedral hornblendes in equilibrium with glass (melt). Experiments were performed on natural samples of both volcanic and plutonic rocks. Earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks. The experimental calibration differs from the empirical calibrations, especially above 5 kbar, and shows that the Al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought. The geobarometers uncertainty is dramatically reduced. The derived equation is P ({plus minus}0.5 kbar) = 3 -3.46 ({plus minus}0.24) + 4.23 ({plus minus}0.13) (Al{sup T}). The geobarometer is applied to post-Bishop Tuff volcanic rocks from Long Valley caldera, California, and reveals that most rhyodacites in this complex erupted from depths of about 6 km. These eruptions occurred over 500,000 yr, suggesting that the rhyodacitic magma reservoir beneath Long Valley had reached a steady P (depth)-T state.
Article
Entrainment of restite is commonly invoked to explain both the origin of relatively mafic granites and granodiorites, as well as the chemical connection between granite magmas and their sources. This concept has become linked to models for magma migration out of the source, as restite entrainment is considered to take place when diatexitic sources mobilise en masse. This is at odds with the common occurrence of relatively mafic granites as high level intrusions in the crust or their eruptive equivalents that must have formed from markedly water-undersaturated magmas that ascended through narrow conduits. We investigate pelitic migmatites from the Mkhondo Valley Metamorphic Suite (MVMS) in Swaziland, where a mid-crustal heating event produced metatexitic migmatites with minimal post-anatectic recrystallisation. In these rocks all the garnet is peritectic, having arisen through biotite fluid-absent melting, which produced garnet poikiloblasts characterised by inclusions of melt, quartz and biotite. Leucosomes that represent sites of melt transfer carry similar, smaller (typically