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Origin of garnet in aplite and pegmatite from Khajeh Morad in northeastern Iran: A major, trace element, and oxygen isotope approach
Abstract and Figures
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.
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... Secondly, the garnets in this study neither have any reaction rim nor contain any metamorphic mineral assemblages, unlike those garnets that enter S-type granitoids through selective entrainment (e.g., Taylor Manning, 1983;Müller et al., 2012;Samadi et al., 2014;Whitworth, 1992), the garnets in this study are predominantly almandine-spessartine solid solution with Sps content ranging from~30% to 50%. ...
... Recent studies show that garnets from NYF-type pegmatites are readily distinguishable from those from LCT-type pegmatites in terms of Y and HREE concentrations though garnets from both pegmatite types are almandine-spessartine garnets (e.g., Hönig et al., 2014;Müller et al., 2012;Samadi et al., 2014;Zhou et al., 2015). Although the fundamental reasons still need to be explored, the difference in Y and HREE signatures between garnets from NYF-type and LCT-type pegmatites may reflect the chemical difference between their sources. ...
... Data for constructing the field of NYF pegmatites are cited from Müller et al. (2012) and Hönig et al. (2014). Data for constructing the field of LCT pegmatites are from Zhou et al. (2015), Habler et al. (2007), and Samadi et al. (2014). It is noticeable that the majority of garnets from the Ziyugou pegmatites are similar to garnets from NYF-type pegmatites [Colour figure can be viewed at wileyonlinelibrary.com] ...
A group of Rb-rich granitic pegmatites near Ziyugou, Danfeng, in the North Qinling Orogen lack typical lithological zoning and have a simple mineralogy dominated by quartz + microcline + albite with garnet in minor quantities. Biotite is rare, and muscovite was mostly formed during greisenization. Graphic texture can be observed within the pegmatites near the contact with the country rock. The pegmatites have localized garnet-rich zones that contain 5–15 vol.% garnet and accessory zircon, columbite, xenotime, and thorite. The pegmatites are peraluminous (A/CNK = ~1.02–1.23) and contain low Ca but high alkali and elevated Rb (~600–1300 ppm). In REE patterns and spider diagrams, the pegmatites exhibit a pronounced negative Eu anomaly, strong Ti, P, Sr, and Ba depletion, and enrichment of Rb, HFSE, and HREE. In addition, the garnet-rich zones contain lower K and Rb but higher Na, Mn, Fe, Zr, Hf, Nb, Y, Th, and U concentrations than the part with graphic texture and accessory garnet. Two types of garnets occur in the pegmatites. Type 1 garnet is well-zoned and closely associated with the above-mentioned HFSE minerals in the garnet-rich zones, whereas Type 2 garnet included in either feldspar or quartz defining graphic texture has no associated HFSE minerals. Both types of garnets are almandine-spessartine solid solution and contain trace to minor amounts of pyrope, grossular, and uvarolite. Based on garnet chemistry, Type 1 garnet can be subdivided into two subtypes, namely, Type 1a with higher Alm content and Type 1b with higher Sps and Prp contents. Unlike garnets from LCT-type pegmatites but similar to those from NYF-type pegmatites, the garnets from the Ziyugou pegmatites contain up to 1.12 wt.% Y2O3 and 1.16 wt.% Yb2O3, which makes garnet the main carrier of Y and HREE. This explains why the garnet-rich part contains higher Y and HREE. Type 1 garnet has higher Mn/Fe ratio than Type 2 garnet, indicating that the garnet-rich part of the pegmatites likely represents a more evolved melt. Although the pegmatites somehow show S-type granite affinity (i.e., elevated Rb concentration and strong depletion in Sr and Ba), the HFSE and HREE enrichment of the pegmatites and garnet chemistry suggest that the pegmatites may have had a hybridized source.
... The composition of garnet has been used to support examples of both models (e.g. Whitworth 1992;Samadi et al. 2014;Shaw et al. 2016). Garnet-group minerals occur in a variety of metamorphic rocks (Baxter et al. 2013), but are rarer in igneous rocks and their origins are debated (e.g. ...
... Members of the garnet-group occur in granitic pegmatites, aplites, felsic peraluminous Stype granitoids (e.g. Miller and Stoddard 1981;Černý and Hawthorne 1982;du Bray 1988;Dahlquist et al. 2007;London 2008;Müller et al. 2012;Thöni and Miller 2004;Samadi et al. 2014;Heimann 2015), and in both A-type (e.g. Zhang et al. 2009Zhang et al. , 2012Hönig et al. 2012) and I-type granites (e.g. Green 1992;Wu et al. 2004;Yuan et al. 2008). ...
... Studies of compositional growth zoning in garnet minerals from granitic pegmatites have been used to provide insight into the crystallisation history of pegmatites and the evolution of granitic magma compositions (e.g. Baldwin and von Knorring 1983;Manning 1983;Whitworth 1992;Kano and Yashima 1997;Kleck and Foord 1999;Arredondo et al. 2001;Thöni and Miller 2004;Thöni et al. 2008;Sîrbu et al. 2010;Müller et al. 2012;Gharib 2012;Gadas et al. 2013;Samadi et al. 2014). Some garnet crystals from the Boroujerd pegmatites have inverse bellshaped Mn profiles, with core-to rim increases in Mn (Fig. 7a-e), whereas others have bell-shaped Mn profiles ( Fig. 7b and f). ...
Pegmatite-hosted garnets from four localities in the Boroujerd region, Lorestan (Western Iran), have been analysed for major and selected trace element compositions. The mineral assemblage of the granitic pegmatites is primarily quartz, plagioclase (albite), and alkali feldspar (orthoclase-microcline), as well as garnet, muscovite, fluorapatite, tourmaline (schorl-foitite), andalusite and zircon. The mineralogical and geochemical characteristics of the pegmatites indicate that they are peraluminous to slightly metaluminous I-type granites. Based on mineral assemblages and whole-rock geochemistry, the pegmatites are classified as muscovite-type pegmatites. Electron-probe micro-analysis reveals that garnets have concentric compositional zoning and are almandine-spessartine solid solutions with lesser pyrope, grossular and andradite components. Concentric zoning of major elements in the garnet is attributed to magmatic growth from a melt. On a MnO + CaO versus FeO + MgO (wt%) plot, the composition of garnet is consistent with crystallisation from weakly to moderately evolved melts. The garnets from the Boroujerd pegmatites are characterised by decreasing Y, HREE, Ti, Zr, Nb, Ta, Hf, and U abundances from core to rim. The garnets also have high chondrite normalized HREE abundances with nearly flat patterns (YbN/SmN = 0–508), lower LREE contents, and negative Eu anomalies (Eu/Eu* < 0.3). Variation in these elements from core to rim is attributed to increasing magma fractionation. The composition and major and trace element zoning patterns in the garnet of the Boroujerd pegmatites are compatible with a magmatic origin and crystallisation from variably fractionated I-type magmas demonstrating that garnet crystal-chemistry is an important tool for deciphering the origins of pegmatite magmas.
... Although garnet is a relatively common mineral phase in magmatic igneous rocks and it is mainly reported from very felsic peraluminous granitoids and from aplite and pegmatite granitoids (e.g., Gadas et al., 2013;Müller et al., 2012;Samadi et al., 2014), it is an uncommon accessory phase in volcanic rocks (e.g., Harangi et al., 2001 and references therein). In the last decade many studies have focused on the significance of garnet in felsic magmatic systems (e.g., Hönig et al., 2014;Lackey et al., 2012;Rubatto and Hermann, 2007;Samadi et al., 2014;Taylor et al., 2015;Taylor and Stevens, 2010;Zhou et al., 2017). ...
... Although garnet is a relatively common mineral phase in magmatic igneous rocks and it is mainly reported from very felsic peraluminous granitoids and from aplite and pegmatite granitoids (e.g., Gadas et al., 2013;Müller et al., 2012;Samadi et al., 2014), it is an uncommon accessory phase in volcanic rocks (e.g., Harangi et al., 2001 and references therein). In the last decade many studies have focused on the significance of garnet in felsic magmatic systems (e.g., Hönig et al., 2014;Lackey et al., 2012;Rubatto and Hermann, 2007;Samadi et al., 2014;Taylor et al., 2015;Taylor and Stevens, 2010;Zhou et al., 2017). Zhou et al. (2017) review the petrogenetic scenarios that result in the formation of garnet in magmatic systems: (i) restitic origin during a partial melting event; (ii) xenocrysts from metamorphic wall-rocks; (iii) peritectic genesis from wall-rock/xenolith materials reacting with magma; (iv) primary magmatic crystallization from ortho-magmatic to late-stage in fluid-rich aplite and pegmatite; and (v) secondary metasomatic origin by interaction between late-to post-magmatic fluids and the host SiO 2 -rich felsic rock. ...
... The integration of petrography with major and trace elements is considered to be the key to understanding the origin of garnet from these settings (Taylor et al., 2015;Villaros et al., 2009a;Villaros et al., 2009b;Zhou et al., 2017). However, whereas there is a wide literature dealing with the origin of garnets from granitoids (e.g., Villaros et al., 2009a;Villaros et al., 2009b;Ma et al., 2017), skarn (e.g., Rossetti et al., 2007), metapelitic (e.g., White et al., 2014) and ultramafic (e.g., Evans and Trommsdorff, 1978;Zhang et al., 1994) rocks, there has been much less work on garnet from aplite-pegmatite melts (e.g., London, 2008;Villaros et al., 2009b, Villaros et al., 2009aTaylor and Stevens, 2010;Müller et al., 2012;Gadas et al., 2013;Samadi et al., 2014;Taylor et al., 2015) and in felsic volcanism (Clemens and Wall, 1984;Kawabata and Takafuji, 2005;Oliver, 1956;Patranabis-Deb et al., 2009;Wood, 1974;Wyborn et al., 1981). Mn-rich garnets in rhyolitic explosive products have only been reported in a few instances (e.g., Caffe et al., 2012;Mitropoulos et al., 1999;Viramonte et al., 1984). ...
The Miocene “Corte Blanco Tuff” rhyolite deposit is the product of a large volume and high intensity Plinian eruption from the solitary and monogenetic Ramadas Volcanic Centre (Central Andes, Province of Salta, NW Argentina). The “Corte Blanco Tuff” consists of vitreous tube pumices with rare euhedral sub-millimetric Mn-garnet phenocrysts, typically hosting inclusions of U-phases as zircon and monazite. Here, we present new textural, major and trace elemental analyses of garnet, zircon and glass that, combined with in situ U-(Th)-Pb zircon and monazite dating, are used to reconstruct the thermobaric environment of formation, age and longevity of the magmatic plumbing system of the Ramadas magma. The results indicate to a crystallization path of a peraluminous rhyolitic melt at shallow crustal levels (≤6 km), as sequentially tracked by the initial nucleation of zircon (780 °C at 9.16 Ma) and garnet (above or at ca. 700 °C), to the final monazite growth (660–670 °C, at 8.70 Ma) in a water-saturated (H2O = 3–5 wt%) environment, shortly before the eruption started. These data (1) define for the first time the primary magmatic origin of Mn-garnet in a rhyolitic volcanic setting; (2) provide new partition coefficients of rare earth elements (REE) between natural garnet, zircon and rhyolitic melts; and (3) permit reconstruction of the magmatic processes that resulted in the Ramadas eruption. On a wider scale, our results document the spatio-temporal (P-T conditions, timing and longevity) time scales involved in the petrogenesis of a shallow peraluminous water-saturated rhyolitic magmatic plumbing system that is able to generate the conditions for extremely explosive Plinian eruptions.
... 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. ...
... Moretz et al. (2013) also showed that garnet from the least evolved melt has the lowest MnO, MgO, and CaO contents but the highest FeO content. Thus, magmatic garnets in less evolved granitoids are commonly Fe 2+ -rich, whereas garnets in highly evolved granitic aplites and pegmatites commonly have higher Mn contents ( Fig. 9) (e.g., Baldwin and Von Knorring 1983;du Bray 1988;Whitworth 1992;Arredondo et al. 2001;Samadi et al. 2014a;London 2008;Muller et al. 2012). Our specimens of garnet from both granite and pegmatite have high MnO contents of ~20-25 wt% (Online Material 1 Table OM2). ...
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.
... During post-tectonic phases of the Hercynian orogeny, the Paleo-Tethys meta-ophiolite and metaflysch strata were intruded by granitic rocks in the Triassic (Emami, 2000;Karimpour et al., 2010). The Mashhad batholith consists of granodiorite, monzogranite (biotite-muscovite granite), and dioritetonalite-granodiorite suites (Samadi et al., 2014). Mirnejad et al. (2013) classified the granitoids of the Mashhad area into I-and S-type granites that related to subduction and collision setting, respectively. ...
... In these tourmalines, the amounts of Na decrease from vein, nodular, radial, aplitic to pegmatitic tourmaline respectively, in consistent with increase of Al and X-site vacancy from radial, vein, nodular, aplitic to pegmatitic tourmalines. Samadi et al. (2014) stated that the origin of Mashhad aplitepegmatite melt is relative to granitic magma differentiation. Tourmaline formed from hydrothermal fluids shows fine-scale, oscillatory-type zoning (London and Manning, 1995;Samson and Sinclair, 1992) and magmatic tourmaline generally has little or no optical zoning (London and Manning, 1995). ...
Abundant tourmaline occurs in Late Triassic biotite-muscovite granites of Mashhad (NE Iran), with several distinct morphologies including tourmaline nodules in granite, pegmatitic tourmalines, aplitic tourmalines, and vein-related tourmalines (quartz-tourmaline vein, tourmaline-rich vein and radial tourmalines). Pegmatitic and aplitic tourmalines are compositionally Fe-rich schorl and the nodule, radial and vein tourmalines are Mg-rich schorl. The tourmalines have generally low abundances for most trace elements except for Sr, Ga, Li, Be, Sn, Pb and some transition elements. Tourmalines have low REE abundances and are enriched in LREE and depleted in HREE (mostly below detection limits) with positive Eu anomalies. The concentration of several trace elements (e.g., Pb, Be, Sn, Ga, Sr, Pb, and REE) correlates well with the major element composition of the tourmalines (e.g. Mg# (Mg/Mg + Fe), vacancy/)X-site vacancy + Na(and Na/)Na + Ca. Trace element data of coexisting minerals with tourmaline indicate that the oligoclase and albite show the most preference in Sr, Pb and Eu and LREEs, biotite readily incorporates Rb, Ba, Zn, Li, Cs and Nb; K-feldspar has high contents of Rb, Ba, Sr and Pb; and garnet incorporates HREEs and Y during magma differentiation. The B-O-H isotopic compositions for all tourmalines are consistent with an S-type granite source, that was derived from partial melting of the continental crust. The pegmatitic and aplitic tourmalines with Fe-rich schorl composition indicate magmatic conditions and the radial, vein, and nodule tourmalines with Mg-rich schorl composition formed during the transition from magmatic to hydrothermal conditions due to the internal evolution of the granitic magma.
... Most of the garnets in igneous and metamorphic rocks are chemically zoned; the zonings are typified as bell-shaped or inverse bell-shaped with core-to-rim increases in Mn. This compositional zoning in garnet has been used to provide us with insights into the protolith provenance, crystallization Tpy-calculated after Pyle et al. (2001) assuming that a total occupancy of 1 in a Y-HREE and considering that monazite crystallized in equilibrium with xenotime and garnet 1 3 history, and magma composition (e.g., Thöni et al. 2008;Samadi et al. 2014;Javanmard et al. 2018;Sami et al. 2020;Yu et al. 2021). Compositional growth zoning of the Wadi Muweilha garnets is characterized by a bell-shaped type (Fig. 7a). ...
The Wadi Muweilha muscovite pegmatite in the Central Eastern Desert (CED) of Egypt is strongly influenced by a strike-slip shearing and fault system related to the final stage of the East African Orogen (EAO) in the Arabian Nubian Shield (ANS). It intrudes the volcano-sedimentary succession, occurs in large pegmatitic plugs, and is comprised of K-feldspar,
quartz, plagioclase, and muscovite as rock-forming minerals, while abundant garnet, monazite-(Ce), Nb-depleted rutile, ilmenite, zircon, xenotime-(Y), and Y–thorite represent the rare metal minerals. Most minerals show several signatures of ductile/brittle deformations. Garnets form almandine–spessartine solid solutions and exhibit Mn bell-shaped type zoning, due to temperature decrease and fractionation of Fe–Mn during crystallization. The monazite occurs in three phases: pris�tine (Mnz1), recrystallized (Mnz2), and hydrothermal (Mnz3). The Y–Th-enriched Mn1 was altered into Y–Th-depleted Mnz3 associated with xenotime–thorite solid solutions (Y–thorite) via dissolution–reprecipitation mechanism. The mineral reaction happened along the preexisting microfractures (resulting from the former brittle deformation) that were used as pathways for interfacial fluids. The chemical and textural characteristics of monazite, garnet, and other minerals enhance the igneous origin of pegmatite and the relation to syn-tectonic I-type granitic magma source. The 641±5 Ma (2σ) age of Mnz1 indicates that the post-collisional stage began after~640 Ma in the north of the ANS. The date range (584–590 Ma) of Mnz2 is simultaneous with the rare metal-bearing post-collisional A-type granite in the CED, while the 538 to 663 Ma date range of Mnz3 may suggest a prolonged period (100 Ma) of the tectonic rejuvenation in the Mubarak–Barramiya Shear Belt at ca. 640–540 Ma.
... Garnets from less fractionated pegmatites are typically Fe rich (Müller et al. 2012). Garnets from aplites and pegmatites are often Fe-Mn rich, and exhibit obvious core to rim decrease in Mn (Baldwin and von Knorring 1983;Whitworth 1992;Gadas et al. 2013), whereas garnets from granitoids are mostly Fe rich and show weak core-to-rim increase in Mn (Day et al. 1992;Harangi et al. 2001;Koepke et al. 2003;Samadi et al. 2014). The wide variations noted above in almandine and spessartine components indicate low to moderate degrees of pegmatite evolution, suggesting that high Mn content in garnet reflects more fractionated magmas. ...
Deposits of semi-gemstones tourmaline, beryl, and garnet associated with Jurassic granites are found in the northern Sanandaj-Sirjan Zone (SaSZ) of western Iran, defining a belt that can be traced for about 400 km. Granitic magmas strongly interacted with or were derived from melts of continental crust and/or sediments. Based on morphologies, size, mineral assemblage, and contact relationships with host granite and associated metamorphic aureoles, these deposits are categorized into six types: (1) garnet in skarns, (2) tourmaline, beryl, and garnet in pegmatite and aplitic dikes, (3) disseminations and patches of tourmaline in leucogranites, (4) quartz-tourmaline veins in granite, (5) tourmaline and garnet in metamorphic aureoles, and (6) tourmaline orbicules in aplite. Tourmalines are mostly schorl and dravite, and garnets are mostly almandine, spessar-tine, and grossular. Tourmaline, beryl, and garnet from pegmatites in the contact aureole of Jurassic granites reflect segregations of Be, B, Mn, and Al bearing melts from the Jurassic peraluminous granites. Quartz-tourmaline veins and hydrothermal garnets in skarns reflect fluids exsolved from the surrounding metasediment and pegmatite melt. In contrast, tourmaline patches and orbicules developed from boron-rich aqueous fluids exsolved from cooling granitic magma. Distribution of semi-gemstones in the SaSZ shows that these are mostly related to pegmatites associated with Jurassic granitic intrusions. Mineral equilibrium considerations indicate that SaSZ semi-gemstones crystallized at P = 3.5-7.5 kbar (11.5-25 km deep) and temperatures of 550-650°C. SaSZ pegmatites fall in the muscovite (MS) and MS-rare element classes. They are Lithium Cesium Tantalum (LCT)-type pegmatites. Fluids responsible for gem mineralization were exsolved from cooling granite bodies and released by metamorphosed sediments. Further studies are needed to better understand the northern Sanandaj-Sirjan tourmaline-garnet-beryl semi-gemstone Province. ARTICLE HISTORY
... Garnet can preserve chemical zoning below closure temperature due to slow diffusion rates for most cations and its resistance to alteration (Samadi et al., 2014). The studied garnet is distinguished by its poorly expressed zoning (Fig. 5) which is considered as a typical of liquidus garnet (Green, 1977). ...
The highly fractionated garnet‐bearing muscovite granite represents the marginal granitic facies of Abu‐Diab multiphase pluton in the Central Eastern Desert of Egypt. New electron microprobe analyses (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) data of garnets are reported in order to constrain their origin and genesis. Garnet in the Abu‐Diab host granite is euhedral to subhedral, generally homogenous and in rare cases, it shows weak zonation. The Garnet contains appreciated amounts of MnO and FeO, with lesser amounts of MgO and CaO, yielding an end‐member formula of Sps61–72Alm25–35Prp1–4Adr0–1. Moreover, it is depleted in large ion lithophile elements (LILE) with lower values of Ba, Nb and Sr relative to the primitive mantle. Additionally, it contains high concentrations of HREE and Y and their REE pattern shows strong negative Eu anomalies. Garnet was crystallized under relatively low temperature (646°C–591°C) and pressure (< 3 kbar) conditions. The textural and chemical features indicated that garnet is of magmatic origin and is chemically similar to that from highly fractionated A‐type granite. It was probably formed at the expense of biotite in a highly evolved MnO‐rich magma and/or by hydroxyl complexing of Mn during the ascending fluid phases.
... The garnet crystals in the garnet-bearing muscovite granite are unlikely to be of xenocrystic origin from the wall rocks or a residual phase of the source, as no garnet- and strong negative Eu anomaly resembling that of garnet crystallizing from a high-SiO 2 melt (e.g., Dahlquist et al., 2007;Samadi et al., 2014;Taylor et al., 2015;Villaros et al., 2009;Fig. 9). ...
Peraluminous granitoids can provide crucial insights into crustal reworking. Here we report a geochemical study of two Late Paleozoic peraluminous granitic plutons in the South Tianshan Belt, NW China, to constrain the crustal reworking history. Zircon U-Pb analyses yield ages of 311 ± 4 Ma for the garnet-bearing biotite granite and 283 ± 4 Ma for the garnet-bearing muscovite granite. The garnet-bearing biotite granites exhibit peraluminous affinity (ASI = 1.04–1.11), high CaO/Na2O, low Rb/Sr, and negative εNd(t) (−8.7 to –7.5) and zircon εHf(t) (–8.4 to –0.8) values with ancient crustal model ages together suggesting the dehydration melting of metagraywacke. The garnet-bearing muscovite granites, containing manganese garnet, show high SiO2 (72.5–76.8 wt.%) and strongly peraluminous (ASI = 1.14–1.22), high Rb/Sr (37.8–128.9), low Zr/Hf (16.61–31.80) and Nb/Ta (3.05–5.71), and exhibit tetrad REE patterns (TE1,3 = 1.05–1.27), which can be ascribed to high-degree fractional crystallization accompanied by the interaction with fluids. They show negative εNd(t) values (–8.6 to –6.6) and variable zircon εHf(t) values (–4.9 to +2.7), implying ancient crustal with minor juvenile crustal sources. In combination with the occurrence of other peraluminous magmatic rocks of the South Tianshan Belt, we propose that the two studied peraluminous magmatic rocks were formed by the partial melting of ancient crustal rocks from the passive continental margin of the Tarim Craton, which represents an important crustal reworking event in the Central Asian Orogenic Belt.
... Garnets from less fractionated pegmatites are typically Fe rich (Müller et al. 2012). Garnets from aplites and pegmatites are often Fe-Mn rich, and exhibit obvious core to rim decrease in Mn (Baldwin and von Knorring 1983;Whitworth 1992;Gadas et al. 2013), whereas garnets from granitoids are mostly Fe rich and show weak core-to-rim increase in Mn (Day et al. 1992;Harangi et al. 2001;Koepke et al. 2003;Samadi et al. 2014). The wide variations noted above in almandine and spessartine components indicate low to moderate degrees of pegmatite evolution, suggesting that high Mn content in garnet reflects more fractionated magmas. ...
Deposits of semi-gemstones tourmaline, beryl, and garnet associated with Jurassic granites are found in the northern Sanandaj-Sirjan Zone (SaSZ) of western Iran, defining a belt that can be traced for about 400 km. Granitic magmas strongly interacted with or were derived from melts of continental crust and/or sediments. Based on morphologies, size, mineral assemblage, and contact relationships with host granite and associated metamorphic aureoles, these deposits are categorized into six types: (1) garnet in skarns, (2) tourmaline, beryl, and garnet in pegmatite and aplitic dikes, (3) disseminations and patches of tourmaline in leucogranites, (4) quartz-tourmaline veins in granite, (5) tourmaline and garnet in metamorphic aureoles, and (6) tourmaline orbicules in aplite. Tourmalines are mostly schorl and dravite, and garnets are mostly almandine, spessar-tine, and grossular. Tourmaline, beryl, and garnet from pegmatites in the contact aureole of Jurassic granites reflect segregations of Be, B, Mn, and Al bearing melts from the Jurassic peraluminous granites. Quartz-tourmaline veins and hydrothermal garnets in skarns reflect fluids exsolved from the surrounding metasediment and pegmatite melt. In contrast, tourmaline patches and orbicules developed from boron-rich aqueous fluids exsolved from cooling granitic magma. Distribution of semi-gemstones in the SaSZ shows that these are mostly related to pegmatites associated with Jurassic granitic intrusions. Mineral equilibrium considerations indicate that SaSZ semi-gemstones crystallized at P = 3.5-7.5 kbar (11.5-25 km deep) and temperatures of 550-650°C. SaSZ pegmatites fall in the muscovite (MS) and MS-rare element classes. They are Lithium Cesium Tantalum (LCT)-type pegmatites. Fluids responsible for gem mineralization were exsolved from cooling granite bodies and released by metamorphosed sediments. Further studies are needed to better understand the northern Sanandaj-Sirjan tourmaline-garnet-beryl semi-gemstone Province. ARTICLE HISTORY
... For their leucogranites, Clemens et al. (2017) reported a LA-ICP-MS U-Pb monazite age of 534 ± 5 Ma. The Lu-Hf garnet age of 530.6 ± 0.8 Ma from this study has been obtained on an igneous garnet: The major element zoning of garnet is similar to the zoning seen in moderate-temperature igneous garnet (Samadi et al., 2014) and the grains have not been compositionally homogenized by post-crystallization diffusion. Consequently, this date is then interpreted as an intrusion age that is more precise than the U-Pb monazite date presented by Clemens et al. (2017). ...
Syn-tectonic 530.6 ± 0.8 Ma pegmatites and aplites from the Donkerhoek batholith in the Damara orogen (Namibia) are moderately to strongly peraluminous, ferroan, alkalic to calc-alkalic leucogranites. Major and trace element variations and strongly fractionated REE patterns with positive Eu anomalies indicate that the leucogranites represent highly fractionated melts that accumulated or retained feldspar that may account, in part, for their alkalic composition. Elemental variations imply that biotite, garnet, and feldspar were the main fractionating minerals. The pegmatites have lower ⁸⁷Sr/⁸⁶Sr (0.7053–0.7097) than—but similar unradiogenic initial εNd (−4.1 to −10) to—the least evolved Donkerhoek granites. Two aplites have similar εNd values but unreasonably unradiogenic ⁸⁷Sr/⁸⁶Sr ratios as a result of late-stage disturbance and associated overcorrection due to their extremely high ⁸⁷Rb/⁸⁶Sr ratios. Subtle variation in Nd isotope compositions coupled with LREE fractionation indicate limited AFC or contamination processes. Lead isotope compositions are more radiogenic than those from published Donkerhoek samples, indicating derivation from or involvement of a component with a considerable crustal residence time. Based on the alkalic to calc-alkalic and ferroan composition and the similarity in Nd–Sr isotopes, meta-igneous basement rocks from the nearby Kalahari craton are likely sources. This study confirms previous studies on the Donkerhoek batholith that have shown that giant batholiths consist of distinct magma batches that are derived from various sources. The new age constraints in conjunction with published ages show that the large-scale Donkerhoek batholith, with a spatial extent of >5000 km², grew incrementally over a period of at least 30 Myr.
... Therefore, it tends to produce distinct minerals such as beryl [3]. Beryl ( ,Na,Cs,H 2 O)(Al,Sc,Fe,Mg) 2 (Be, Li) 3 (Si 6 O 18 ) is the main beryllium mineral produced in a few geological settings: (1) highly evolved S-type granites and granitic [33] with positions of beryl-bearing pegmatites including (1) Khajeh Morad [26,27], (2) Kamari and Zaman Abad [28,29], and (3) Ebrahim-Attar granitic pegmatite (this study). ...
Ebrahim-Attar granitic pegmatite, which is distributed in southwest Ghorveh, western Iran, is strongly peraluminous and contains minor beryl crystals. Pale-green to white beryl grains are crystallized in the rim and central parts of the granite body. The beryl grains are characterized by low contents of alkali oxides (Na2O = 0.24–0.41 wt.%, K2O = 0.05–0.17 wt.%, Li2O = 0.03–0.04 wt.%, and Cs2O = 0.01–0.03 wt.%) and high contents of Be2O oxide (10.0 to 11.9 wt.%). The low contents of alkali elements (oxides), low Na/Li (apfu) ratios (2.94 to 5.75), and variations in iron oxide (FeO= 0.28–1.18 wt.%) reveal a poorly evolved magmatic source of the beryl grains. Low abundances of rare earth elements (ΣREE = 0.8–4.9 ppm) with high 87Sr/86Sr(i) ratios of 0.739 ±0.036 for the beryl grains and 0.7081 for the host granites infer that the primary magma was directly produced by partial melting of the upper continental crust (UCC). The crystallization temperature of the Ebrahim-Attar granitic pegmatite changes from 586 to 755 °C (average = 629 °C), as calculated based on the zircon saturation index. Furthermore, the quartz geobarometer calculation shows that crystallization occurred at pressures of approximately 233–246 MPa. This pressure range is a promising condition for saturation of Be in magma. During granitic magma crystallization, the melt was gradually saturated with Be, and then beryl crystallized in the assemblage of the main minerals such as quartz and feldspar. Likewise, the host granite is characterized by high ratios of Nb/Ta (4.79–16.3) and Zr/Hf (12.2–19.1), and peraluminous signatures are compatible with Be-bearing LCT (Li-Ce and Ta) pegmatites.
... Garnet composition has previously been used as an indicator in the exploration of diamonds and Pb-Zn deposits [11]. Over the years, some studies have also been developed on the compositional variations of garnets from different pegmatites around the world [1,[11][12][13][14][15][16][17], giving interesting results. These variations may reflect the chemistry of pegmatitic magmas, the reactions between garnets and their associated minerals, as well as the pressure and temperature conditions during crystallization. ...
Almandine-spessartine garnets, from the Oxford County pegmatites and the Palermo No. 1 pegmatite, record significant compositional variations according to the degree of evolution of their hosting rock. Garnets from the most fractionated pegmatites (Mt. Mica, Berry-Havey, and Emmons) show the highest Mn, Nb, Ta, Zr, and Hf values, followed by those from the intermediate grade pegmatites (Palermo No. 1) and, finally, garnets from the barren pegmatites show the lowest values (Perham and Stop-35). Iron, Ca, and Mg contents follow an inverse order, with the highest contents in the latter pegmatites. Major element zoning shows increasing Mn values from core to rim in most garnet samples, while trace element zoning is not systematic except for some crystals which show a core to rim depletion for most of these elements. Chondrite normalized HREE (Heavy Rare Earth Elements) spectra show positive slopes for garnets from barren pegmatites, both positive and negative slopes for those associated with the intermediate pegmatite, and negative or flat slopes in garnets from the highly fractionated pegmatites. Ion exchange mechanisms, including Fe2+−1Mn2+1, (Fe2+, Mn2+)−1Si−1Li1P1; and, (Y, Ho3+)2(vac)1(Fe2+, Mn2+)−3, could explain most of the compositional variations observed in these garnets. These compositional variations are the reflection of the composition of the pegmatitic magma (barren pegmatites originate from a more ferromagnesian magma than fractionated pegmatites); and of the coexisting mineral phases competing with garnets to host certain chemical elements, such as biotite, schorl, plagioclase, apatite, Fe-Mn phosphates, Nb-Ta oxides, zircon, xenotime, and monazite.
Leucogranitic rocks, including two-mica granite, muscovite granite, and granitic pegmatite (aplite), crop out in the eastern Himalayan Cuonadong dome, southern Tibet. These rocks have high SiO2 (∼73 wt.%), Al2O3 (∼16 wt.%), and total alkali contents (∼9 wt.%), and are peraluminous with A/CNK ratios of 1.0–1.1. Whole-rock major and trace element data and mineral compositions reveal strong differentiation and fractionation trends for the two-mica granite, muscovite granite, and pegmatite–aplite dikes. Albite, muscovite, and garnet contents increased as the leucogranitic magma evolved, whereas K-feldspar and biotite contents decreased. Muscovite in aplite has higher Rb2O (∼0.4 wt.%), Nb (∼219 ppm), and W + Sn (∼737 ppm) contents, and lower Nb/Ta ratios (∼6) than muscovite in the two-mica and muscovite granites. Garnet varies in composition from almandine in the leucogranites to spessartine in the aplites. The muscovite granite and pegmatite are associated with rare-metal mineralization. Rare-metal-bearing minerals include beryl, columbite-group minerals, tapiolite, Nb–Ta-bearing rutile, cassiterite, and wolframite. Skarn and altered (i.e., chloritized) rocks along the margins of the leucogranites are the products of hydrothermal reaction between leucogranite and marble. Beryl, scheelite, and cassiterite occur in the skarn and altered rocks. Rare-metal mineralization in the Cuonadong dome can be divided into two types: magmatic Be–Nb–Ta mineralization related to pegmatite–aplite dikes and hydrothermal Be–W–Sn mineralization hosted in the skarn and associated with the two-mica and muscovite granites. Monazite U–Th–Pb dating reveals two episodes of leucogranitic magmatism in the Cuonadong dome at ca. 20 and 17 Ma. Columbite U–Pb dating indicates two stages of rare-metal mineralization at ca. 17 and 14 Ma (including tapiolite). It is inferred that magmatism was related to the exhumation of the Higher Himalayan Crystalline Basement, with the degree of differentiation of the associated leucogranites being the key factor controlling the type of mineralization.
Abstract:
Study of pegmatites has special importance in geological, mineralogical, and economic geology researches..Mineralogy of pegmatite from the south Mashhad is simple and it is mostly including of major minerals of quartz, feldspar, muscovite, and biotite in some cases, and different minor minerals of beryl, tourmaline, some apatite, garnet (spessartine - almandine), lepidolite, and columbite.These Mesozoic pegmatites have a similar composition with the S-type and peraluminous granitoids of collisional environment. Regarding the pegmatites classification based on the formation depth, pegmatites of the south Mashhad are of high depth pegmatites. In addition, these pegmatites are structurally of simple pegmatite type, except for Khajeh Morad samples which are composite pegmatite type. Regarding the economic geology, these pegmatites contain rare elements of lithium, cesium, and tantalum. Pegmatite dikes have occurred in several stages. They have structurally different dimension, with low thickness and high slope, and it is not possible to extract them by vein mining.
Study of pegmatites has special importance in geological, mineralogical, and economic geology researches. .Mineralogy of pegmatite from the south Mashhad is simple and it is mostly including of major minerals of quartz, feldspar, muscovite, and biotite in some cases, and different minor minerals of beryl, tourmaline, some apatite, garnet (spessartine - almandine), lepidolite, and columbite. Khajeh Morad pegmatit whith Mesozoic age are composite pegmatite type. Based on the mineralogy, these pegmatites can be classified in LCT pegmatite family and rare element Li-bearing (RE-Li) pegmatite class.
The Baiganhu W–Sn orefield in the southeastern Xinjiang Uygur Autonomous Region is associated with Caledonian S-type syenogranites and metasediments of the Paleoproterozoic Jinshuikou Group. Four types of garnets have been identified in the orefield using petrographic and major and trace element data. Grt-I garnets are generally present as inclusions within magmatic quartz in the syenogranites, with end-member formulas of Sps45–53Alm46–53Adr0–1Grs0–1Prp0–1 and rare earth element (REE) patterns that are enriched in heavy REE (HREE) and contain strong negative Eu anomalies. Grt-II garnets are associated with tourmaline and quartz and occur in interstices between feldspars within the syenogranites. In general, the Grt-II garnets have end-member formulae (Sps64–70Alm29–34Adr0–1Grs0–2Prp0) and REE patterns that are similar to the Grt-I garnets although they are more spessartine-rich and contain higher concentrations of HREE. Grt-III garnets coexist with clinopyroxenes and Mo-rich scheelites within skarns developed along the syenogranite and marble contact. Their compositions are Adr62–88Grs1–18Sps3–12Alm0–8Pyr0 and they have relatively flat REE patterns with no negative Eu anomalies. Grt-IV garnets are present as massive aggregates that are often cross-cut by Mo-poor scheelite-bearing calcite veins. Their end-member formulas are Adr4–22Grs62–73Sps5–16Alm2–10Pyr0 and they have slightly domed REE patterns without negative Eu anomalies. Both Grt-III and Grt-IV garnets contain lower concentrations of the HREE (2–3 and 4–32 ppm, respectively) than Grt-I and Grt-II garnets (682–1352 ppm with Y = 1558–2159 ppm, and 6051–12831 ppm with Y =9663–13333 ppm, respectively).
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.
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.
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.
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.
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.
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.
Pegmatites accentuate the trace element signatures of their granitic sources. Through that signature, the origin of pegmatites can commonly be ascribed to granites whose own source characteristics are known and distinctive. Interactions with host rocks that might modify the composition of pegmatites are limited by the rapid cooling and low heat content of pegmatite-forming magmas. The trace element signatures of most pegmatites clearly align with those of S-type (sedimentary source, mostly postcollisional tectonic environment) and A-type (anorogenic environment, lower continental crust ± mantle source) granites. Pegmatites are not commonly associated with I-type (igneous source) granites. The distinction between granites that spawn pegmatites and those that do not appears to depend on the presence or absence, respectively, of fl uxing components, such as B, P, and F, in addition to H 2O, at the source.
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.
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.
Garnet in Nova Scotia's peraluminous South Mountain Batholith (SMB) displays diversity in its texture and composition that has challenged a comprehensive explanation of its origins. In this study, we have employed oxygen isotope analysis to "fingerprint" magmatic, peritectic, and xenocrystic SMB garnets to take advantage of contrasting delta(18)O of SMB and metasedimentary country-rocks and the slow rates of diffusion of oxygen in garnet. Among texturally well-characterized garnet, values of delta(18)O distinguish magmatic (8.21 +/- 0.19%; n = 10), metamorphic (9.38 +/- 0.13%; n = 6), and peritectic garnet (8.67 +/- 0.20%; n = 6). Values of delta(18)O of magmatic garnet are in equilibrium with coexisting zircon (delta(18)O = 8.14 +/- 0.23%; n = 21) in the SMB, confirming direct magmatic crystallization of garnet. Entrained metamorphic garnet porphyroblasts preserve high-delta(18)O values, confirming a slow rate of intracrystalline diffusion of oxygen in garnet. Averaging of metamorphic and magmatic contributions is evident from the intermediate delta(18)O of peritectic garnet, and corresponds to textural evidence that garnet crystallized dynamically, and that metamorphic wallrocks were partially melted and disaggregated by magmas. In the case of texturally ambiguous garnet found on the margin of the Halifax pluton, delta(18)O varies by 2.5 parts per thousand among closely spaced (separated by mm to cm) crystals, signaling heterogeneous populations of magmatic, peritectic, and xenocrystic garnet, and thorough mixing of the host magma. In total, delta(18)O analysis provides a powerful complement to existing methods of determining garnet provenance and a new means to deconvolute garnet assemblages in peraluminous magmas.
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.
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
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
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).
Five distinct paragenetic, morphological and compositional types of grossular garnet (G1, G2, G3, G4, G5) were distinguished within the individual (sub)units of the zoned leucotonalitic pegmatite cutting serpentinized lherzolite with rodingite dikes at Žďár near Ruda nad Moravou, Staré Město Unit, Northern Moravia. Detailed study using Electron Microprobe Analysis, Laser Ablation Inductively Coupled Plasma Mass Spectrometry, Cathodoluminiscence and Infrared Spectroscopy revealed distinct compositional trends in major, minor and trace elements. The contents of Fe3+, Mn, Mg and Ti increase from early garnet (G1) in the outermost grossular subunit through the interstitial garnet (G2) in the leucocratic subunit to graphic intergrowths of quartz+garnet (G3) in the coarse-grained unit. Then these constituents decrease in inclusions of garnet (G4) from the blocky unit and large crystals of garnet (G5) from the quartz core. Some trace elements (V, Ni, Y) exhibit the same trends, only Be evidently increases in garnet from border zone to the centre. Fluorine has negative correlation with Fe3+ as well as some trace elements (Ta, Pb). Concentrations of H2O in garnets, up to 0.22 wt.% H2O, are comparable with spessartine-almandine garnets from the Rutherford No. 2 pegmatite, Virginia, and grossular garnets from high-temperature calc-silicate rocks (skarns). Water contents correlate positively with Fe3+, but inversely with F. The use of water contents in garnet to elucidate the fluctuations of activity of H2O during the pegmatite formation is only limited; the incorporation of hydrous defects seems to be controlled instead by crystal-structural constraints. However, the sum of all volatile components (H2O + F) increases about twice from the outermost subunit to the centre of the pegmatite body.
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.
Trace element (REE, Cr, Ti, Y, Y, and Zr) analysis of garnet from the garnet, staurolite, and lower sillimanite zones of an aluminous schist of the Black Hills, South Dakota, indicates that REE zoning varies as a function of grade. Garnet-zone garnet has high concentrations of REEs, Cr, Ti, Y, Y, and Zr in the cores and low concentrations in the rims. Profiles of heavy REEs contain inflections between the cores and rims, which are approximately symmetric about the cores. Staurolite-zone garnet contains cores enriched with Y and heavy REEs, which decrease toward the rim and increase again at the rim edges but to lower concentrations than in the cores. Cr, Y, Ti, Zr, and light REE zoning is less pronounced than heavy REE zoning and is less symmetric about the garnet cores. Almandine-rich garnet of the lower sillimanite zone displays no major element zonation. Trace element (Ti, Cr, Y, and Zr) concentrations are minimal, and the zoning is irregular and not symmetric about the garnet cores. Garnet from all three zones has core-to-rim Fe/(Fe + Mg) profiles that suggestgarnet growth was uninterrupted with respect to major element components and that Mn zoning formed by a fractionation process. Analysis of trace element zoning in this garnet reveals that the major element zoning was relatively unaffected by volume-diffusion reequilibration. Trace element zonation of all samples of garnet is best explained by a fractionation mechanism in conjunction with limited inter- granular diffusion and changing partition coefficients during garnet growth. Heavy REE partitioning is especially dependent on the major element composition of garnet. This research complements previous research by others on the use of trace elements as meta- morphic petrogenetic indicators, which demonstrated the importance of bulk-rock com- position and phase assemblage on trace element partitioning.
In this contribution we provide evidence for the extraction of volatile and incompatible element enriched melts from common granites. This provides a mechanism showing that at least a large proportion of granitic pegmatites could be genetically directly connected to a main granite body. In granites there are often two principal types of melt inclusions: (i) those that represent the bulk chemistry of the granite and (ii) those with very different compositions. In the Variscan Erzgebirge granites, the second type is characterized by the abundance of fluorine. However, in other geodynamic settings inclusions in granites can contain high concentrations of other elements which may take over the function of fluorine. From textural relationships the second inclusion type represents intergranular melts enriched in all elements incompatible with the ideal haplogranite system. Due to the high volatile content of such melts, the viscosity can be several orders of magnitude lower than the quasi-solid bulk system and can therefore move rapidly through the partially or totally crystallized host, and flow together into a separate system forming pegmatite bodies inside or outside the granite body. Another important effect of the high volatile content is the phase separation resulting from the speciation changes OH- -> H2O or CO32- -> CO2 due to temperature and/or pressure changes at different locations within the granite-intergranular melt system. Since melt inclusions provide a means of conserving original un-degassed compositions, they yield important evidence for closing the gap between granites and granitic pegmatites. The paper is dedicated to two Czech colleagues - Petr Cerny and Milan Novak - who have devoted their lives to the study of granitic pegmatites.
No one universally accepted model of pegmatite genesis has yet emerged that satisfactorily explains all the diverse features of granitic pegmatites. Genesis from residual melts derived from the crystallization of granitic plutons is favoured by most researchers. Incompatible components, fluxes, volatiles and rare elements, are enriched in the residual melts. The presence of fluxes and volatiles, which lower the crystallization temperature, decrease nucleation rates, melt polymerization and viscosity, and increase diffusion rates and solubility, are considered to be critical to the development of large crystals. A number of new concepts have shed light on problems related to pegmatite genesis.
Cooling rates calculated from thermal cooling models demonstrate that shallow-level pegmatites cool radically more rapidly than previously believed. Rapid cooling rates for pegmatites represent a quantum shift from the widely held view that the large crystals found in pegmatites are the result of very slow rates of cooling and crystal growth.
Experimental and field evidence both suggest that undercooling and disequilibrium crystallization dominate pegmatite crystallization. London's constitutional zone refining model of pegmatite evolution involves disequilibrium crystallization from an undercooled, flux-bearing granitic melt. The melt is not necessarily flux–rich and the model does not require the presence of an aqueous vapor phase.
Experimental studies of volatile- and flux-rich melts and fluid inclusion studies suggest that volatile-rich silicate melts may persist to temperatures well below 500 °C and even down to 350 °C.
Studies of melt inclusions and fluid inclusions have led some researchers to suggest that the role of immiscible fluids must be considered in any model regarding pegmatite genesis. Fluid saturation is thought to occur early in the crystallization history of pegmatites. Two types of melt inclusions along with primary fluid inclusions have been found coexisting in pegmatite minerals. Advances by Petr Černý in pegmatite classification are in wide use and the fractionation trends of Nb, Ta and other HFSE and K, Rb, Cs, Li, Ga and Tl are now well understood.
How pegmatitic melts are produced, the types of source rocks involved and how melt generation relates to plate tectonic models are challenging areas for future investigations. Also, the roles of regional zoning, anatexis, and chemical quenching in pegmatite genesis are areas for future pegmatite research.
Meta-pegmatites and meta-pegmatite-hosted garnet from three localities in the western/northern Ötztal basement, Tyrol (Eastern Alps), have been analysed for major and selected trace element chemistry and for Sm–Nd isotope systematics. The garnets have frequently retained a euhedral shape, even though they are now set in a strongly foliated matrix consisting of dynamically recrystallized quartz and feldspar. They are characterized by high Fe and Mn contents (up to 29 mol% spessartine) and a simple major element zoning with decreasing MnO and increasing FeO from core towards the rim, suggesting a single-phase crystallization and primary, magmatic chemistry. Chondrite-normalized REE patterns display a strong enrichment of HREE relative to LREE, and pronounced negative Eu anomalies. Based on ID-TIMS analysis, neodymium concentration in all but one of the optically clean garnet separates varies between 0.12 and 5.5 ppm, at concomitant high Sm/Nd ratios (1.2–4.9). LA-ICP-MS analysis revealed very strong within-grain variation both for Nd concentration (
Experimental melt (glass) compositions indicate that most of the S-type granites of the Cape Granite Suite in South Africa have ferromagnesian contents too high to represent melts. Consequently, the composition of the more mafic granites demands the addition of an Fe- and Mg-rich component to the magma. The compositions of the granites evolve along well-defined trends away from the likely melt composition for many components plotted against Mg + Fe. An increase in A/CNK, Mg#, Ca, and HREEs, as well as a decrease in K and Si, as a function of increasing Mg + Fe appears to limit the contaminant to garnet (up to 20 wt%). The rate of Ti increase, as a function of Mg + Fe increase in the granites, matches that defined by the stoichiometry of high-temperature biotite, but cannot be the product of accumulation of biotite (± other phases) in the magma because the chemical trends are inconsistent with this, particularly those portrayed by K and Ti as a function of Mg + Fe. This, in conjunction with the fact that no large, counterbalancing population of very leucocratic material exists in the Cape Granite Suite, suggests that the relatively mafic granites are not the products of garnet fractional crystallization. Rather, these appear to be the result of selective entrainment of peritectic garnet and ilmenite. Thus, this work indicates that much of the compositional variation in the granites is primary, reflecting the magma composition that ascended from the source, and is controlled by the proportion of peritectic products entrained into the melt. There is no indication of entrainment of a mineralogically diverse residuum (restite).
Reply to discussion by Abbott (preceding abstract).-I.M.S.
The hydroxide contents of spessartine-almandine garnets from the Rutherford no. 2 (Virginia), Himalaya (California), and George
Ashley Block (California) pegmatites were determined by infrared spectroscopy. The hydroxide content of garnet increases from
the wall zone to the core zone of the Rutherford no. 2 and Himalaya pegmatites, consistent with increasing H2O activity during pegmatite crystallization. However, the absolute OH contents differ by about two orders of magnitude for
these two suites of garnets, possibly due to the elevated Ca content of the Rutherford no. 2 and the differences in the depth
of emplacement. The garnets from the George Ashley Block show significant excursions from this correlation at the positions
within the pegmatite where Kleck and Foord (1999) identified disruptions in major- and minor-element trends that they associated with re-injections of magma and subsequent
flushing of the dike system. Ease of measurement as well as a relative amplified sensitivity compared to the Mn and Fe trends,
make hydrogen an excellent tracer for the evolution of granitic pegmatites.
Heat capacities (5-380 K) have been measured by adiabatic calorimetry for five highly disordered alkali feldspars (Ab99Or1, Ab85Or15, Ab55Or45, Ab25Or75 and Ab1Or99). The thermodynamic and mineralogical implications of the results are discussed. The new data are also combined with recent data for plagioclases in order to derive a revised expression for the two-feldspar thermometer. T calculated from the revised expression tend to be higher than previous calculations.-J.A.Z.
Garnet occurs as a characteristic mineral in granodiorite, monzogranite and leucocratic monzogranite host rocks near the contact of the South Mountain batholith in Nova Scotia, Canada. Three textural groups of garnets are defined on the basis of their crystal size, shape, inclusion content and relationship to biotite. These textural groups correspond with differing mineral chemistries (8 chemical analyses), group I garnets are 10-15 mm, anhedral, inclusion-rich and rimmed by biotite; they have MnO contents of 1.55-4.62 wt.% and normal zoning, and are considered xenocrysts. Group III garnets are <2 mm, euhedral and inclusion-free, have no relationship with biotite, have MnO contents averaging 6.91 wt.% and reverse zoning, and are considered magmatic. Group II garnets are similar to those of group III in that they are reversely zoned and have spessartine contents >10%, but have complex textural relationships. They are considered to be a mixture of magmatic and highly modified xenocrystic types. Projections of coexisting garnet-biotite pairs of magmatic origin into the AFM diagram permits delineation of the liquidus path followed by the South Mountain batholith magma during the late stages of differentiation.-S.A.K.
Three improvements are offered for the projection of liquidus relationships in the AFM system (M.A. 80-4287) for silicate liquids saturated with respect to quartz, alkali feldspar, plagioclase and one or more of the AFM minerals, andalusite or sillimanite, garnet, biotite and cordierite. These modifications, which extend the projection from peraluminous to metaluminous compositions, are as follows: 1) the liquidus relationships in part of the metaluminous region involve the equilibrium biotite-hornblende-liquid. As the liquids move from the metaluminous region (A < 0) of the AFM diagram to the peraluminous region, the reaction on the liquidus changes from liquid = hornblende + biotite, to hornblende + liquid = biotite; 2) a sequence of reactions leading to the disappearance of biotite from the A-F join is presented; and 3) the effects of increasing MnO content are to shift the liquidus reaction from liquid = garnet + biotite to liquid + biotite = garnet, accounting for the occurrence of aplites and pegmatites, derived from hornblende-biotite granites, which contain spessartine as the sole ferromagnesian mineral.-S.A.K.
This paper reports the results of 20 experiments in which mixes of two or three feldspars were reacted to produce coexisting plagioclase feldspar (PF) and alkali feldspar (AF). Starting materials with similar bulk compositions were prepared using different combinations of two and three minerals, and experiments were designed to produce similar AF and PF minerals in the experimental products from different starting binary and ternary compositions. The coexisting AF and PF compositions produced as products define compositional fields that are elongate parallel to the ternary solvus. In 11 experiments reaction was sufficient to product fields of coexisting AF and PF, or AF, PF, and melt with a bulk composition close to that of the starting mixture. In six experiments significant reaction occurred in the form of reaction rim overgrowths on seeds of the starting materials. Three experiments produced AF, PF, and melt from a natural granite starting material. A two-feldspar thermometer is presented in which temperature is constrained by equilibria among all three components - Albite, Orthoclase, and Anorthite - in coexisting ternary feldspars. -from Authors
The Mn content of garnets in zoned pegmatites increases from the outer contact zones to the inner core and replacement zones. The highest Mn contents occur in core and replacement zones in Li-rich pegmatites. Variable zoning patterns were found in individual garnets. Thirty-one chemical analyses of garnets are included. The association garnet + muscovite + quartz sets narrow limits on the P-T conditions of formation.-M.I.C.
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.
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.
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.
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.
Cameron et al. (1949) devised the nomenclature and delineated the patterns of internal zonation within granitic pegmatites that are in use today. Zonation in pegmatites is manifested both in mineralogy and in fabric (mineral habits and rock texture). Although internal zonation is a conspicuous and distinctive attribute of pegmatites, there has been no thorough effort to explain that mineralogical and textural evolution in relation to the zoning sequence presented by Cameron et al. (1949), or in terms of the comprehensive petrogenesis of pegmatite bodies (pressure, temperature, and whole-rock composition). This overview of internal zonation within granitic pegmatites consists of four principal parts: (1) a historic review of the subject, (2) a summary of the current understanding of the pegmatite-forming environment, (3) the processes that determine mineralogical and textural zonation in pegmatites, and (4) the applications of those processes to each of the major zones of pegmatites. Based on the concepts presented in London (2008), the fundamental determinates of the internal evolution of pegmatite zones are: (1) the bulk composition of melt, (2) the magnitude of liquidus undercooling prior to the onset of crystallization, (3) subsolidus isothermal fractional crystallization, by which eutectic or minimum melts fractionate by sequential, non-eutectic crystallization, (4) constitutional zone refining via the creation of a boundary layer liquid, chemically distinct from but continuous with the bulk melt at the crystallization front, and (5) far-field chemical diffusion, the long-range and coordinated diffusion of ions, particularly of alkalis and alkaline earths, through melt.
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.
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.
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.
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.
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
X-ray composition maps of Sc, Y, P, and Cr in garnet have been measured
using the electron microprobe. Zoning is discontinuous and is correlated
with fluid-absent crustal melting reactions. The magnitude of the
discontinuity is a function of the amount of muscovite that melts, and
the partition coefficients of solids versus melt. Trace element zoning
is apparently not modified by diffusive processes, so the zoning can be
used to monitor the extent of melting and to examine kinetics of melting
and the participation of accessory phases in crustal anatexis.
Synchrotron radiation X-ray fluorescence microanalysis (mu-SRXRF) was applied to products of experimental geochemistry to determine (1) trace element diffusivities in andesite melts and (2) trace element partitioning behaviour between garnet and melt. To achieve sufficient spatial resolution, non-focusing and focusing glass capillaries reduced the incoming synchrotron beam down to sizes of 20 and 2.7 mum respectively. (1) Diffusion couples of trace element-doped and undoped andesite melts were prepared in internally heated pressure vessels. A special sample setup allowed the pencil-shaped synchrotron beam to irradiate volume elements showing identical diffusion behaviour. Eighteen trace elements were measured simultaneously and quickly, resulting in diffusion profiles well suited for evaluating diffusion coefficients. (2) Garnet and andesitic melt were synthesized and equilibrated in a piston cylinder apparatus. The garnets were exceptionally large due to specially designed dehydration-melting experiments with monazite as a trace element source. Coexisting garnets and melt were analyzed with mu-SRXRF, and new distribution coefficients for Sr (0.126), Y (5.27), Zr (0.533), La (0.014), Cc (0.020), Nd (0.245), Sm (1.21), Eu (1.18), Gd (5.29), Yb (52.5), and Lit (76) were determined. However, the general use of mu-SRXRF for experimental partitioning studies is limited due to the relatively poor spatial resolution caused by the penetrating character of the synchrotron beam and due to limited count rates at high energies.
The Lower Carboniferous Culm Basin is part of the European Variscan foreland basin. Garnet is an abundant heavy mineral at all stratigraphic levels of the Culm sediments (the Protivanov, Rozstání and Myslejovice Formation from base to top). This paper compares the major element (electron microprobe) and trace element (LA-ICP-MS) compositions of garnet for the purpose of studying the provenance of the Culm sediments. The polymict garnet assemblages ranging from the Protivanov to lower part of the Myslejovice Formation (Goniatite zones Peγ-Goα) pass into oligomict ones dominated by pyrope–almandine garnets (uppermost part of the Myslejovice Formation; Goniatite zones Goβ-Goγ). As the predominant low-grossular pyrope–almandines from oligomict garnet assemblages are very uniform and homogeneous in major element composition, we studied their trace element composition and zoning. Pyrope–almandine garnets are poor in LREE and show enrichment in HREE; the chondrite-normalized patterns are almost flat from Dy to Lu and show a significant negative Eu anomaly, typical for granulite-grade garnets. Major element compositions of detrital low-grossular pyrope–almandines can only be matched with some granulites and garnetiferous felsic gneisses of the Bohemian Massif. Garnets from granulites cropping out at the present-day erosion level of the Bohemian Massif have usually higher Ca and/or lower Mg contents. The major and trace element compositions and zoning patterns of pyrope–almandine garnets from the upper part of the Myslejovice Formation compare well with garnets from the Miroslav granulites and small Moldanubian granulite bodies west of the Třebíč Massif. The Moldanubian granulites are commonly associated with mantle-derived peridotites and eclogites. The presence of detrital Cr-spinels with high Cr# and low Mg# (single grains of Cr-spinels and fine-grained kelyphitic intergrowths of Cr-spinel with chlorite) comparable with spinel types from Moldanubian peridotites could indicate the joint occurrence of peridotites, serpentinized peridotites, and granulites at the erosion level of the Bohemian Massif during the deposition of the uppermost Culm sediments (325 Ma). While the older Culm sediments (polymict garnet assemblages) represent the accumulation of clastic material from the northern and north-western areas (Moravo-Silesian and Lugian Zone), the uppermost Culm sediments (oligomict garnet assemblages) contain material from western and south-western areas (Moldanubian Zone and Moravian Nappe); the general influx of clastic material changed as a result of clockwise rotation of the eastern part of the Variscan belt during this time span (340–325 Ma).
The concept of I‐ and S‐type granites was introduced in 1974 to account for the observation that, apart from the most felsic rocks, the granites in the Lachlan Fold Belt have properties that generally fall into two distinct groups. This has been interpreted to result from derivation by partial melting of two kinds of source rocks, namely sedimentary and older igneous rocks. The original publication on these two granite types is reprinted and reviewed in the light of 25 years of continuing study into these granites.
Unusually large δ18O heterogeneities (≥4‰) within single crystals are reported in garnets from dioritic migmatites in the Pyrenees. These heterogeneities,
together with contrasting Ca and P zoning, allow the identification of different growth zones. Garnet cores with high δ18O values (12–14‰) are relatively poor in Ca (7–9 mol% Grs) and rich in P (400–900 ppm P2O5). In contrast, garnet rims with lower δ18O values (7–12 ‰) are richer in Ca (12–14% Grs) and poorer in P (100–200 ppm). These growth zones can be ascribed to a metamorphic
event followed by crustal partial melting and contamination by magmas from the mantle. High δ18O intra-crystalline contrasts result from mineral growth in an open magmatic system involving the interaction of partial melts
with distinct δ18O signatures. At the garnet core-rim interface, compositional profiles in major divalent cations are consistent with the relaxation
of an initial sharp step in Ca, Fe, and Mg by Ca ↔ (Fe, Mg) interdiffusion. At the same interface, an O-isotope profile is
documented. The analogy of Ca and O isotope profiles suggests that the δ18O distribution may also result from a diffusion process. In this particular case (temperature, garnet composition, oxygen
fugacity), O diffusion appears to be of the same order of magnitude as Ca ↔ (Fe, Mg) interdiffusion. Considering a duration
of 10 Ma for the plutono-metamorphic event in the Pyrenees, Ca and O diffusivities in the range 10−22 m2/s (at 850 °C) are retrieved from the measured profiles. Like Ca, O diffusion in garnet at magmatic temperatures (850–900
°C) is both slow enough to preserve large δ18O heterogeneities and fast enough to generate relaxation profiles.
The Xihuashan granitic complex is character- ized by enrichment in rare-earth elements (REE). In par- ticular, the second-stage granite (G-b) is markedly enriched in yttrium, and therefore contains complex associations of Y-bearing minerals. In this granite, garnet displays specific yttrium zoning with an Y-rich core and a "clean" rim. Be- sides minute inclusions of Y-bearing minerals, garnet in- volves a striking amount of Y and HREE in its central area. It is suggested that enrichment in Y in the garnet core ac- cords with that in the melt as a result of REE magmatic frac- tionation. However, the "clean" rim may be the direct result of accumulation of fluid phases in the magma, which is vir- tually unfavorable for the entrance of REE in the garnet structure.