Advective heat transfer and fabric development in a shallow crustal intrusive granite — the case of Proterozoic Vellaturu granite,south India

Syntectonic plutons emplaced in shallow crust often contain intermediate-to low-temperature deformation microstructures but lack a high-temperature, subsolidus deformation fabric, although the relict magmatic fabric is preserved. The Proterozoic Vellaturu granite emplaced at the eastern margin of the northern Nallamalai fold belt, south India during the late phase of regional deformation has a common occurrence of intermediate-to low-temperature deformation fabric, superimposed over magmatic fabric with an internally complex pattern. But high-T subsolidus deformation microstructure and fabric are absent in this pluton. The main crystal plastic deformation and fluid enhanced reaction softening was concentrated along the margin of the granite body. Resulting granite mylonites show Y-maximum c-axis fabric in completely recrystallized quartz ribbonds, dynamic recrystallization of perthites, and myrmekite indicative of fabric development under intermediate temperature (∼ 500–400°C). The weakly-deformed interior shows myrmekite, feldspar microfracturing and limited bulging recrystallization of quartz. The abundance of prism subgrain boundaries is indicative of continuing deformation through low-temperature (∼ 300°C). The relative rates of cooling influenced by advective heat transfer and deformation of the pluton seem to control the overall subsolidus fabric development. The rapid advective heat transfer from the interior in the early stages of subsolidus cooling was followed by slow cooling through intermediate temperature window as a well-developed phyllosilicate rich mylonitic skin around the granite body slowed down conductive heat loss. Low-T crystal plastic deformation of quartz was effected at a late stage of cooling and deformation of the shallow crustal granite body emplaced within the greenschist facies Nallamlai rocks.     

Crystallization conditions of the Wiborg rapakivi batholith, SE Finland: an evaluation of amphibole and biotite mineral chemistry

Summary The wiborgite and dark wiborgite rapakivi granite phases of the Wiborg batholith in southeastern Finland compose about 80% of the total batholith area. A new study of the dominant mafic silicate minerals, in comparison with mafic silicates from more evolved granite phases, hybridized granite and mafic magmatic enclaves provide insights into the overall petrogenesis of the Wiborg batholith. All of the mafic silicate minerals are iron-rich, reflective of the whole rock compositions. Biotite is annitic, calcic amphibole is ferro-edenite to hastingsite, and subsolidus Fe-Mg amphibole is found as accessory grunerite. Temperatures derived from amphibole-plagioclase thermometry suggest crystallization at about ∼ 740 °C. Pressure estimates derived from Al in amphibole barometry range between 2.5 and 5.4 kilobars. This is noticeably higher than the previous estimates of 1 kbar for the Wiborg batholith. Oxygen fugacity estimates from biotite suggest low fO₂ initial values and increase from FMQ to above NNO for late stage granite phases. Received February 29, 2000; revised version accepted December 27, 2000     

Magmatic history and geophysical signature of a post-collisional intrusive center emplaced near a crustal-scale shear zone: the Plechý granite pluton (Moldanubian batholith,Bohemian Massif)

The Plechy pluton, southwestern Bohemian Massif, represents a late-Variscan, complexly zoned intrusive center emplaced near the crustal-scale Pfahl shear zone; the pluton thus provides an opportunity to examine the interplay among successive emplacement of large magma batches, magmatic fabric acquisition, and the late-Variscan stress field associated with strike-slip shearing. The magmatic history of the pluton started with the emplacement of the porphyritic Plechy and Haidmühler granites. Based on gravity and structural data, we interpret that the Plechy and Haidmühler granites were emplaced as a deeply rooted, ∼NE–SW elongated body; its gross shape and internal fabric (steep ∼NE–SW magmatic foliation) may have been controlled by the late-Variscan stress field. The steep magmatic foliation changes into flat-lying foliation (particularly recorded by AMS) presumably as a result of divergent flow. Magnetic lineations correspond to a sub-horizontal ∼NE–SW finite stretch associated with the divergent flow. Subsequently, the Třístoličník granite, characterized by steep margin-parallel magmatic foliation, was emplaced as a crescent-shaped body in the central part of the pluton. The otherwise inward-younging intrusive sequence was completed by the emplacement of the outermost and the most evolved garnet-bearing granite (the Marginal granite) along the southeastern margin of the pluton. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ore geochemistry,zircon mineralogy,and genesis of the Sakharjok Y-Zr deposit,Kola Peninsula,Russia

The Sakharjok Y-Zr deposit in Kola Peninsula is related to the fissure alkaline intrusion of the same name. The intrusion ∼7 km in extent and 4–5 km² in area of its exposed part is composed of Neoarchean (2.68–2.61 Ma) alkali and nepheline syenites, which cut through the Archean alkali granite and gneissic granodiorite. Mineralization is localized in the nepheline syenite body as linear zones 200–1350 m in extent and 3–30 m in thickness, which strike conformably to primary magmatic banding and trachytoid texture of nepheline syenite. The ore is similar to the host rocks in petrography and chemistry and only differs from them in enrichment in zircon, britholite-(Y), and pyrochlore. Judging from geochemical attributes (high HSFE and some incompatible element contents (1000–5000 ppm Zr, 200–600 ppm Nb, 100–500 ppm Y, 0.1–0.3 wt % REE, 400–900 ppm Rb), REE pattern, Th/U, Y/Nb, and Yb/Ta ratios), nepheline syenite was derived from an enriched mantle source similar to that of contemporary OIB and was formed as an evolved product of long-term fractional crystallization of primary alkali basaltic melt. The ore concentrations are caused by unique composition of nepheline syenite magma (high Zr, Y, REE, Nb contents), which underwent subsequent intrachamber fractionation. Mineralogical features of zircon-the main ore mineral—demonstrate its long multistage crystallization. The inner zones of prismatic crystals with high ZrO₂/HfO₂ ratio (90, on average) grew during early magmatic stage at a temperature of 900–850°C. The inner zones of dipyramidal crystals with average ZrO₂/HfO₂ = 63 formed during late magmatic stage at a temperature of ∼500°C. The zircon pertaining to the postmagmatic hydrothermal stage is distinguished by the lowest ZrO₂/HfO₂ ratio (29, on average), porous fabric, abundant inclusions, and crystallization temperature below 500°C. The progressive decrease in ZrO₂/HfO₂ ratio was caused by evolution of melt and postmagmatic solution. The metamorphic zircon rims relics of earlier crystals and occurs as individual rhythmically zoned grains with an averaged ZrO₂/HfO₂ ratio (45, on average) similar to that of the bulk ore composition. The metamorphic zircon is depleted in uranium in comparison with magmatic zircon, owing to selective removal of U by aqueous metamorphic solutions. Zircon from the Sakharjok deposit is characterized by low concentrations of detrimental impurities, in particular, contains only 10–90 ppm U and 10–80 ppm Th, and thus can be used in various fields of application.     

Mafic and Felsic Magma Interaction in Granites: the Hercynian Karkonosze Pluton (Sudetes, Bohemian Massif)

The Hercynian, post-collisional Karkonosze pluton contains severallithologies: equigranular and porphyritic granites, hybrid quartzdiorites and granodiorites, microgranular magmatic enclaves,and composite and lamprophyre dykes. Field relationships, mineralogyand major- and trace-element geochemistry show that: (1) theequigranular granite is differentiated and evolved by smalldegrees of fractional crystallization and that it is free ofcontamination by mafic magma; (2) all other components are affectedby mixing. The end-members of the mixing process were a porphyriticgranite and a mafic lamprophyre. The degree of mixing variedwidely depending on both place and time. All of the processesinvolved are assessed quantitatively with the following conclusions.Most of the pluton was affected by mixing, implying that hugevolumes (>75 km³) of mafic magma were available. This maficmagma probably supplied the additional heat necessary to initiatecrustal melting; part of this heat could have also been releasedas latent heat of crystallization. Only a very small part ofthe Karkonosze granite escaped interaction with mafic magma,specifically the equigranular granite and a subordinate partof the porphyritic granite. Minerals from these facies are compositionallyhomogeneous and/or normally zoned, which, together with geochemicalmodelling, indicates that they evolved by small degrees of fractionalcrystallization (<20%). Accessory minerals played an importantrole during magmatic differentiation and, thus, the fractionalcrystallization history is better recorded by trace rather thanby major elements. The interactions between mafic and felsicmagmas reflect their viscosity contrast. With increasing viscositycontrast, the magmatic relationships change from homogeneous,hybrid quartz diorites–granodiorites, to rounded magmaticenclaves, to composite dykes and finally to dykes with chilledmargins. These relationships indicate that injection of maficmagma into the granite took place over the whole crystallizationhistory. Consequently, a long-lived mafic source coexisted togetherwith the granite magma. Mafic magmas were derived either directlyfrom the mantle or via one or more crustal storage reservoirs.Compatible element abundances (e.g. Ni) show that the maficmagmas that interacted with the granite were progressively poorerin Ni in the order hybrid quartz diorites—granodiorites—enclaves—compositedykes. This indicates that the felsic and mafic magmas evolvedindependently, which, in the case of the Karkonosze granite,favours a deep-seated magma chamber rather than a continuousflux from mantle. Two magma sources (mantle and crust) coexisted,and melted almost contemporaneously; the two reservoirs evolvedindependently by fractional crystallization. However, maficmagma was continuously being intruded into the crystallizinggranite, with more or less complete mixing. Several lines ofevidence (e.g. magmatic flux structures, incorporation of granitefeldspars into mafic magma, feldspar zoning with fluctuatingtrace element patterns reflecting rapid changes in magma composition)indicate that, during its emplacement and crystallization, thegranite body was affected by strong internal movements. Thesewould favour more complete and efficient mixing. The systematicspatial–temporal association of lamprophyres with crustalmagmas is interpreted as indicating that their mantle sourceis a fertile peridotite, possibly enriched (metasomatized) byearlier subduction processes. KEY WORDS: Bohemian Massif; fractional crystallization; geochemical modelling; hybridization; Karkonosze     

Crystallization of the Little Three layered pegmatite-aplite dike, Ramona District, California

Subhorizontally layered pegmatite-aplite bodies are characterized by fine-grained, sodic to granitic aplite that is usually juxtaposed abruptly above by much coarser-grained, commonly graphic potassic pegmatite. Although well studied, there currently is little concensus as to how such dikes form. The Little Three dike near Ramona, California, is representative of such zoned bodies in this and other regions, and contains discontinuous miarolitic pockets near the base of the graphic pegmatite zone. Tourmaline, garnet, biotite, and muscovite show no changes in major- or minor-element compositions indicative of progressive magmatic fractionation until the immediate vicinity of the main miarolitic zone, where they record abrupt and extreme enrichments in Li, F, and Mn. There is no correlation of chemical changes in the dike with the appearance of small miarolitic vugs well below the main miarolitic zone, nor is there any indication that the aplite, graphic pegmatite, or miarolitic pockets represent separate magma injections. The chemistries of tourmaline, garnet, and micas, however, preclude conventional models of Rayleigh fractionation or traditional zone refining. Textural features and modeled cooling histories indicate that the dike cooled quickly and might have solidified partially or totally to glass before crystallization commenced. Geothermometry based on the compositions of coexisting plagioclase and homogeneous, nonperthitic K-feldspar indicates inward crystallization of the dike, from ∼400–435 °C at the margins to ∼350–390 °C within 20–30 cm of the pocket horizon, then a sharp decrease to 240–275 °C in the pockets where K-feldspar is perthitic. We interpret the feldspar geothermometry (except perhaps in the miarolitic cavities) to reflect the temperatures at crystallization fronts that advanced into the pegmatite, first from the foot wall and eventually joined by a similar front downward from the hanging wall. Crystallization down from the hanging wall may have commenced after ∼70–80% of the foot wall aplite had crystallized. The very abrupt increases of Li, Mn, and F in tourmaline and garnet near the miarolitic zone appear to be explained best by the process of constitutional zone refining, in which a fluxed crystallization front sweeps an incompatible element-enriched boundary layer through a solid or semi-solid. After these two highly fluxed boundary layers merged near the main miarolitic zone, compositional evolution could have proceeded by crystal-melt fractionation. Received: 24 March 1998 / Accepted: 10 March 1999     

Metamorphic charnockite in contact aureoles around intrusive enderbite from Natal, South Africa

In the Port Edward area of southern Kwa-Zulu Natal, South Africa, charnockitic aureoles up to 10 m in width in the normally garnetiferous Nicholson's Point Granite, are developed adjacent to intrusive contacts with the Port Edward Enderbite and anhydrous pegmatitic veins. Mineralogical differences between the country rock and charnockitic aureole suggest that the dehydration reaction Bt + Qtz → Opx + Kfs + H₂O and the reaction of Grt + Qtz → Opx + Pl were responsible for the charnockitization. The compositions of fluid inclusions show systematic variation with: (1) the Port Edward Enderbite being dominated by CO₂ and N₂ fluid inclusions; (2) the non-charnockitized granite by saline aqueous inclusions with 18–23 EqWt% NaCl; (3) the charnockitic aureoles by low-salinity and pure water inclusions (<7 EqWt% NaCl); (4) the pegmatites by aqueous inclusions of various salinity with minor CO₂. As a result of the thermal event the homogenization temperatures of the inclusions in charnockite show a much larger range (up to 390 °C) compared to the fluid inclusions in granite (mostly <250 °C). Contrary to fluid-controlled charnockitization (brines, CO₂) which may have taken place along shear zones away from the intrusive body, the present “proximal” charnockitized granite formed directly at the contact with enderbite. The inclusions indicate contact metamorphism induced by the intrusion of “dry” enderbitic magma into “wet” granite resulting in local dehydration. This was confirmed by cathodoluminescence microscopy showing textures indicative for the local reduction of structural water in the charnockite quartz. Two-pyroxene thermometry on the Port Edward Enderbite suggests intrusion at temperatures of ∼1000–1050 °C into country rock with temperature of <700 °C. The temperature of aureole formation must have been between ∼700 °C (breakdown of pyrite to form pyrrhotite) and ∼1000 °C. Charnockitization was probably controlled largely by heat related to anhydrous intrusions causing dehydration reactions and resulting in the release and subsequent trapping of dehydration fluids. The salinity of the metamorphic fluid in the contact zones is supposed to have been higher at an early stage of contact metamorphism, but it has lost its salt content by K-metasomatic reactions and/or the preferential migration of the saline fluids out of the contact zones towards the enderbite. The low water activity inhibited the localized melting of the granite. Mineral thermobarometry suggests that after charnockite aureole genesis, an isobaric cooling path was followed during which reequilibration of most of the aqueous inclusions occurred. Received: 8 November 1998 / Accepted: 21 June 1999     

Neoproterozoic granites of Sharm El-Sheikh area,Egypt: mineralogical and thermobarometric variations

Calc-alkaline and alkaline intrusions of the late Neoproterozic form essential part of the Arabian–Nubian Shield. They were formed during the collision between East- and West-Gondwana. Sharm El-Sheikh area, Sinai, includes wide compositional array of these intrusions that can be considered as a case study. Variations in both tectonic and thermobarometric condition for granitic intrusions are studied. Four mappable granitic types are recognized namely monzogranite, syenogranite, alkali feldspar granites, and riebeckite-bearing granites. The monzogranite and the syenogranite of the study area are mostly I-type, whereas the alkali feldspar granite and the riebeckite-bearing granite belong to A-type granitoid. The calc-alkaline intrusions were formed in compressional setting due to decompressional melting of mafic lower crust. Partial melting and anatexing of crustal rocks are suggested to explain the protolith of the alkaline intrusions. The transition from the calc-alkaline magma to the alkaline one occurred as a result of the tectonic transition from compression regime to tectonic relaxation (extension setting) during the last stage of the Pan-African Orogeny. The amphiboles of the studied granites are classified as calcic- and alkali-amphiboles. The calcic-amphiboles are ferro-edenite while the alkali-amphiboles are typically riebeckite. Both amphibole types are of magmatic nature. Coexisting amphiboles and plagioclases are used to estimate the physicochemical parameters of magma crystallization. The syenogranite underwent temperature and pressure of formation range of 520–730 °C, <3 kbar. The alkali feldspar granite records 450–830 °C, <4 kbar, while the riebeckite-bearing granite records the lowest temperature condition among all varieties and estimate formation at 350–650 °C, <4 kbar.     

Evaluation of phase chemistry and petrochemical aspects of Samchampi-Samteran differentiated alkaline complex of Mikir Hills,northeastern India

The Samchampi-Samteran alkaline complex occurs as a plug-like pluton within the Precambrian granite gneisses of Mikir Hills, Assam, northeastern India and it is genetically related to Sylhet Traps. The intrusive complex is marked by dominant development of syenite within which ijolitemelteigite suite of rocks is emplaced with an arcuate outcrop pattern. Inliers of alkali pyroxenite and alkali gabbro occur within this ijolite-melteigite suite of rocks. The pluton is also traversed by younger intrusives of nepheline syenite and carbonatite. Development of sporadic, lumpy magnetite ore bodies is also recorded within the pluton. Petrographic details of the constituent lithomembers of the pluton have been presented following standard nomenclatorial rules. Overall pyroxene compositions range from diopside to aegirine augite while alkali feldspars are typically orthoclase and plagioclase in syenite corresponds to oligoclase species. Phase chemistry of nepheline is suggestive of Na-rich alkaline character of the complex. Biotite compositions are typically restricted to a uniform compositional range and they belong to ‘biotite’ field in the relevant classification scheme. Garnets (developed in syenite and melteigite) typically tend to be Ti-rich andradite, which on a closer scan can be further designated as melanites. Opaque minerals mostly correspond to magnetite. Use of Lindsley’s pyroxene thermometric method suggests an equilibration temperature from ∼450°–600°C for melteigite/alkali gabbro and ∼400°C for syenite. Critical assessment of other thermometric methods reveals a temperature of equilibration of ∼700°–1350°C for ijolite-melteigite suite of rocks in contrast to a relatively lower equilibration temperature of ∼600°C for syenite. Geobarometric data based on pyroxene chemistry yield an equilibration pressure of 5.32–7.72 kb for ijolite, melteigite, alkali pyroxenite, alkali gabbro and nepheline syenite. The dominant syenite member of the intrusive plug records a much higher (∼11 kb) equilibration pressure indicating a deeper level of intrusion. Major oxide variations of constituent lithomembers with respect to differentiation index (D.I.) corroborate a normal magmatic differentiation. A prominent role of liquid immiscibility is envisaged from field geological, petrographic and petrochemical evidences. Tectonic discrimination diagrams involving clinopyroxene chemistry strongly suggest within plate alkaline affinity for the parental magma which is in conformity with the regional plume tectonics.     

内蒙古赤峰维拉斯托大型锡多金属矿的地质地球化学特征

2014年发现的维拉斯托锡锌矿是继20世纪末该矿区铜锌矿之后的重要找矿进展,已控制Sn金属资源量10万t。成矿作用与隐伏花岗岩体有关,该岩体侵入于前寒武纪变质岩中。矿化类型包括岩体顶部的花岗岩型锡锌矿、岩体外侧的石英脉型锡锌矿以及外围的铜锌矿。针对花岗岩、各类矿体开展了岩石学、矿床学、主微量元素地球化学、年代学等研究,初步查明岩浆演化机制、矿床成因及三类矿化的关系。细粒斑状碱长花岗岩La-ICPMS锆石UPb年龄(139.5±1.2)Ma(MSWD=3.3)。岩石中发育多级斑晶,结晶(沉淀)顺序为钠长石→石英→钾长石→钠长石→石英、黄玉、锡石、闪锌矿。花岗岩富Si O2贫Al2O3、Ti O2、TFe2O3、Ca O等,高Rb、Cs、Nb、Ta及W、Mo、Bi、Cu、Zn、In等元素,低Sr、Ba等,钠长石An0.3,与锡钨多金属矿成矿花岗岩性质相似。岩浆晚期经历了岩浆-热液过渡阶段(浆液过渡态流体),自硅酸盐相中分离出富Si、富F和富S的流体相,分别形成花岗岩型矿石中的石英、黄玉、锡石-闪锌矿囊状体(珠滴),伴随熔融包裹体和熔流包裹体,晚期逐渐、连续地向热液阶段过渡。岩浆-热液过渡阶段在岩体顶部形成花岗岩型锡锌矿石,热液阶段在岩体外侧和外围形成石英脉型锡锌矿及铜锌矿、铅锌银矿。这些矿体连同成矿花岗岩共同构成岩浆-热液型锡多金属矿床成矿系统。锡林郭勒—赤峰地区,很多脉状铅锌银矿的成矿作用与酸性侵入岩有关,深部可能存在大规模岩浆-热液型锡(钨)多金属矿。     

Palaeoproterozoic A-type felsic magmatism in the Khetri Copper Belt, Rajasthan, northwestern India: petrologic and tectonic implications

Summary A number of small Palaeoproterozoic granitoid plutons were emplaced in the Khetri Copper Belt, which is an important Proterozoic metallogenic terrane in the northeastern part of Aravalli mountain range. Contiguous Biharipur and Dabla plutons are located about 15 km southeast of Khetri, close to a 170 km long intracontinental rift zone. The plutons are composed of amphibole-bearing alkali-feldspar granites, comprising microcline-albite granite, albite granite and late-stage microgranite. The albite granite in Biharipur is confined to the margins of the pluton, and shows extensive commingling with the synchronous mafic plutonics. Geochemically, the albite granites are characterised by low K₂O (∼0.5 wt.%) and elevated Na₂O (∼7.0 wt.%) abundances. By contrast, the microcline-albite granite does not show any significant mafic-granite interactions and shows normal concentrations of alkali elements. The granitoids display high concentrations of the rare earth (except Eu) and high field strength elements, high values of Ga/Al (>2.5), agpaitic index and Fe^*-number. These features together with their alkaline metaluminous and ferroan nature classify the rocks as typical A-type within-plate granites. All the granitoid facies display similar REE and incompatible element profiles indicating their cogenetic nature. These granitoids were emplaced in a shallow crustal chamber under relatively low pressures, high temperature (≥850 °C) and relatively oxidising conditions. The oxidised nature, HFSE concentrations and Nd isotope data (ɛNd = −1.3 to −2.9) favour derivation of these granitoid rocks from crustal protoliths. The generation of albite granite is attributed to the replacement of alkali feldspar and plagioclase of the original granite by pure albite as a consequence of pervasive infiltration of a high Na/(Na + K) fluid at the late-magmatic stage. This model may have wider significance for the generation of albite granites/low-K granites or albitites in other areas. The A-type plutonism under consideration seems to be an outcome of ensialic rifting of the Bhilwara aulacogen.     

The origin and evolution of the parental magmas of frontal volcanoes in Kamchatka: Evidence from magmatic inclusions in olivine from Zhupanovsky volcano

The paper presents data on naturally quenched melt inclusions in olivine (Fo 69–84) from Late Pleistocene pyroclastic rocks of Zhupanovsky volcano in the frontal zone of the Eastern Volcanic Belt of Kamchatka. The composition of the melt inclusions provides insight into the latest crystallization stages (∼70% crystallization) of the parental melt (∼46.4 wt % SiO₂, ∼2.5 wt % H₂O, ∼0.3 wt % S), which proceeded at decompression and started at a depth of approximately 10 km from the surface. The crystallization temperature was estimated at 1100 ± 20°C at an oxygen fugacity of ΔFMQ = 0.9–1.7. The melts evolved due to the simultaneous crystallization of olivine, plagioclase, pyroxene, chromite, and magnetite (Ol: Pl: Cpx: (Crt-Mt) ∼ 13: 54: 24: 4) along the tholeiite evolutionary trend and became progressively enriched in FeO, SiO₂, Na₂O, and K₂O and depleted in MgO, CaO, and Al₂O₃. Melt crystallization was associated with the segregation of fluid rich in S-bearing compounds and, to a lesser extent, in H₂O and Cl. The primary melt of Zhupanovsky volcano (whose composition was estimated from data on the most primitive melt inclusions) had a composition of low-Si (∼45 wt % SiO₂) picrobasalt (∼14 wt % MgO), as is typical of parental melts in Kamchatka and other island arcs, and was different from MORB. This primary melt could be derived by ∼8% melting of mantle peridotite of composition close to the MORB source, under pressures of 1.5 ± 0.2 GPa and temperatures 20–30°C lower than the solidus temperature of “dry” peridotite (1230–1240°C). Melting was induced by the interaction of the hot peridotite with a hydrous component that was brought to the mantle from the subducted slab and was also responsible for the enrichment of the Zhupanovsky magmas in LREE, LILE, B, Cl, Th, U, and Pb. The hydrous component in the magma source of Zhupanovsky volcano was produced by the partial slab melting under water-saturated conditions at temperatures of 760–810°C and pressures of ∼3.5 GPa. As the depth of the subducted slab beneath Kamchatkan volcanoes varies from 100 to 125 km, the composition of the hydrous component drastically changes from relatively low-temperature H₂O-rich fluid to higher temperature H₂O-bearing melt. The geothermal gradient at the surface of the slab within the depth range of 100–125 km beneath Kamchatka was estimated at 4°C/km.     

Coexistence of two distinct mantle sources during formation of ophiolites: a case study of primitive pillow-lavas from the lowest part of the volcanic section of the Troodos Ophiolite, Cyprus

We present a detailed mineralogical, petrological and melt inclusion study of unusually fresh, primitive olivine + clinopyroxene phyric Lower Pillow Lavas (LPL) found near Analiondas village in the northeastern part of the Troodos ophiolite (Cyprus). Olivine phenocrysts in these primitive LPL show a wide compositional range (Fo_82–92) and have higher CaO contents than those from the Upper Pillow Lavas (UPL). Cr-spinel inclusions in olivine are significantly less Cr-rich (Cr/Cr + Al = 28–67 mol%) compared to those from the UPL (Cr# = 70–80). These features reflect differences in melt compositions between primitive LPL and the UPL, namely higher CaO and Al₂O₃ and lower FeO* compared to the UPL at a given MgO. LPL parental melts (in equilibrium with Fo₉₂) had ∼10.5 wt% MgO and crystallization temperatures ∼1210 °C, which are significantly lower than those previously published for the UPL (14–15 wt% MgO and ∼1300 °C for Fo₉₂). The fractionation path of LPL parental melts is also different from that of the UPL. It is characterized initially by olivine + clinopyroxene cotectic crystallization joined by plagioclase at ∼9 wt% MgO, whereas UPL parental melts experienced a substantial interval of olivine-only crystallization. Primitive LPL melts were formed from a mantle source which was more fertile than that of tholeiites from well-developed intra-oceanic arcs, but broadly similar in its fertility to that of Mid-Ocean Ridge Basalt (MORB) and Back Arc Basin Basalts (BABB). The higher degrees of melting during formation of the LPL primary melts compared to average MORB were caused by the presence of subduction-related components (H₂O). Our new data on the LPL coupled with existing data for the UPL support the existing idea that the LPL and UPL primary melts originated from distinct mantle sources, which cannot be related by progressive source depletion. Temperature differences between these sources (∼150 °C), their position in the mantle (∼10 kbar for the colder LPL source vs 15–18 kbar for the UPL source), and temporal succession of Troodos volcanism, all cannot be reconciled in the framework of existing models of mantle wedge processes, thermal structure and evolution, if a single mantle source is invoked. Possible tectonic settings for the origin of the Troodos ophiolite (forearc regions of intra-oceanic island arc, propagation of backarc spreading into arc lithosphere) are discussed. Received: 20 May 1996 / Accepted: 25 March 1997     

Geochemical characteristics of some rare element-bearing granites in southeast China and physico-chemical conditions of their formation

The formation of ore-bearing granites in the Yenshanian movement in southeast China and the geochemical characteristics of some RE-bearing granites have been studied through multivariate statistical analysis and physicochemical approach. The main conclusions have Been drawn as follows: (1) The granites are believed to be products of anatexis of the crustal materials. The formation temperature of granitic magma is estimated at about 600°C and the crystallization temperatures range from 600° to 500°C. The temperature of Li-, Fand H₂O-rich residual magma in the latest stage of magmatism is probably below 500°C. (2) A rock series from early lepidomelane-granite through protolithionite-and zinnwalditegranite to lithioniteor lepidolite-granite is considered as a result of actual crystallization. (3) The mineral paragenesis and fades zonation of granite plutons are mainly controlled by Ph₂o, μNa₂O and μK₂O in the magma. (4) During the magmatic evolution the ore-forming elements (REE, Nb, Ta, etc.) are variable in geochemistry. REEs, similar to mafic components, were highly concentrated at the early stage of the magmatic evolution and deposited under favourable conditions in the zinnwaldite-and protolithionite-granites; Nb and Ta have a preference for felsic and volatile components, thus are mainly concentrated at later stages of the magmatic evolution. Nb ore deposits are formed in the lithionite and lepidolite granites, for Ta is intimately associated with Na₂O, Li₂O, F and H₂O.     

Water-deficient Calc-alkaline Plutonic Rocks of Northeastern Superior Province, Canada: Significance of Charnockitic Magmatism

Calc-alkaline batholiths of the Archaean Minto block, northeasternSuperior Province, Canada, have pyroxene- and hornblende-bearingmineral assemblages inferred to have crystallized from hot,water-undersaturated magmas at 2·729–2·724Ga. A regional amphibolite- to granulite-facies tectonothermalevent at 2·70 Ga resulted in mild to negligible metamorphiceffects on the dominantly granodioritic units. Geochemical,textural and thermobarometric studies define the crystallizationhistory in compositions ranging from cumulate pyroxenite throughquartz diorite, granodiorite, granite, and syn-magmatic gabbroicdykes. Early magmatic assemblages include orthopyroxene, clinopyroxene,plagioclase, biotite, Fe–Ti oxides and ternary feldspar,indicating crystallization from magmas containing <2 wt %H₂O at 1100–900°C. Water enrichment in the residualmelt induced hornblende crystallization at 5 ± 1 kbar,800–600°C. Characterized by a continuum of large ionlithophile element (LILE)-enriched, high field strength element(HFSE)-depleted compositions, the I-type suite resembles moderncontinental arc batholiths in composition and size but not primarymineralogy. Magmatic arcs produced between 2·75 and 1·85Ga commonly have charnockitic components, possibly because slab-derivedfluids interacted with mantle wedges at ambient temperatureshigher by 100°C than at present, producing large volumesof water-deficient magma. KEY WORDS: granitoid rocks; igneous pyroxenes; water-undersaturated magma; charnockite     

Geochemistry and fluid inclusions study of highly fractionated garnet-bearing granite of Gabal Abu Diab, central Eastern Desert of Egypt

The Neoproterozoic granite of Gabal Abu Diab, central Eastern Desert of Egypt, comprises mainly garnet-bearing granite and alkali feldspar granite intruded into calc-alkaline granodiorite–tonalite and metagabbro–diorite complexes. The garnet-bearing granite is composed mainly of plagioclase, K-feldspar, quartz, garnet and primary muscovite ± biotite. The presence of garnet and primary muscovite of Abu-Diab granite suggests its highly fractionated character. Geochemically, the garnet-bearing granite is highly fractionated as indicated from the high contents of SiO₂ (74.85–77.5%), alkalis (8.27 to 9.2%, Na₂O+K₂O) and the trace elements association: Ga, Zn, Zr, Nb and Y. This granite is depleted in CaO, MgO, P₂O₅, Sr and Ba. The alumina saturation (Shand Index, molar ratio A/CNK) of 1.0 to 1.1 indicates the weak peraluminous nature of this garnet-bearing granite. The geochemical characteristics of the Abu Diab garnet-bearing granite are consistent with either the average I-type or A-type granite and also suggest post-orogenic or anorogenic setting. A fluid inclusions study reveals the presence of three fluid generations trapped into the studied granite. The earlier is a complex CO₂–H₂O fluid trapped in primary fluid inclusions with CO₂ contents >?60 vol.%. These inclusions were probably trapped at minimum temperature >?400°C and minimum pressure >?2 kb. The second is immiscible water–CO₂ fluid trapped in secondary and/or pseudo-secondary inclusions. The trapping conditions were estimated at temperature between 400°C and 170°C and pressure between 900 and 2000 bar. The latest fluid is low-salinity aqueous fluid trapped in secondary two-phase and mono-phase inclusions. The trapping conditions were estimated at temperature between 90°C and 160°C and pressure <?900 bar. The origin of the early fluid generation is magmatic fluid while the second and third fluids are of hydrothermal and meteoric origin, respectively.     

Significance of mineral chemistry of syenites and associated rocks of Elagiri complex,Southern Granulite Terrane of the Indian shield

The Elagiri complex (12°31′ N: 78°35′ E) represents one of the important silica — oversaturated syenite plutons of the Southern Granulite Terrane of the Indian shield. This article for the first time reports the mineral chemistry of the Elagiri complex and brings out important petrogenetic significance. The litho-members of Elagiri complex are intrusive into high grade country rocks viz. granite gneiss, amphibolite and pyroxene granulite. The country rocks are foliated bearing evidences of multiple folding and deformation. On the other hand, the constituent litho members of the Elagiri complex (syenites, gabbro and later intrusives marked by lamprophyre and carbonatite) show preservation of igneous layering in terms of discernible parallelism of the constituent minerals. The Elagiri complex shows presence of sharp contacts among litho members and marked absence of chilled facies peripheral to the margin. Electron microprobe data have been critically used to systematize the constituent mineral-phases of the different lithomembers of the complex. Geothermobarometric data indicate a temperature of equilibration in the range of ∼700° to 500°C at ∼2.0 to 5.2 kb which corresponds to shallow level (cf. 18.2 km) equilibration-depth of the complex. Field observations and mineral chemistry data suggest that liquid immiscibility plays an important role during the evolution of the Elagiri complex.     

Domainal fabric development,associated microstructures and <Emphasis Type="Italic">P-T</Emphasis> records attesting to polymetamorphism in a granulite complex of the Eastern Ghats granulite belt,India

The granulite complex around Jenapore, Orissa, Eastern Ghats granulite belt, bears the imprint of two episodes of strong deformation (D₁ and D₂) attended with foliation (fabric) development (S₁ and S₂). Two distinct metamorphic events at P-T conditions of ∼900°C at ∼9 kbar and ∼600°C at ∼6 kbar are correlated with D₁ and D₂ respectively. The reaction textures in S₁-microdomains are interpreted to be the product of near isobaric cooling at ∼9 kbar from 950°C to 600°C, whereas those in the S₂-microdomains are considered to be the result of an up-pressure trajectory from ∼6 kbar at 600°C. The D₁-M₁ high P-T granulite event is interpreted to be Archean in age (ca. 3 Ga) on the basis of the isotopic data obtained from the charnockite suite of the area. The later relatively low P-T granulite facies event, attendant to D₂-S₂ is considered to be related to the Grenvillian orogeny as represented by the dominant isotopic record in the belt.     

Beginning of melting in the granite system Qz-Or-Ab-An-H2O

The beginning of melting in the system Qz-Or-Ab-An-H₂ O was experimentally reversed in the pressure range kbar using starting materials made up of mixtures of quartz and synthetic feldspars. With increasing pressure the melting temperature decreases from 690° C at 2 kbar to 630° C at 17 kbar in the An-free alkalifeldspar granite system Qz-Or-Ab-H₂O. In the granite system Qz-Or-Ab-An-H₂O the increase of the solidus temperature with increasing An-content is only very small. In comparison to the alkalifeldspar granite system the solidus temperature increases by 3° C (7° C) if albite is replaced by plagioclase An 20 (An 40). The difference between the solidus temperatures of the alkalifeldspar granite system and of quartz — anorthite — sanidine assemblages (system Qz-Or-An-H₂O) is approximately 50° C. With increasing water pressures plagioclase and plagioclase-alkalifeldspar assemblages become unstable and are replaced by zoisite+kyanite+quartz and zoisite+muscovite-paragonite_ss +quartz, respectively. The pressure stability limits of these assemblages are found to lie between 6 and 16 kbar at 600° C. At high water pressures (10–18 kbar) zoisite — muscovite — quartz assemblages are stable up to 700 and 720° C. The solidus curve of this assemblage is 10–20° C above the beginning of melting of sanidine — zoisite — muscovite — quartz mixtures. The amount of water necessary to produce sufficient amounts of melt to change a metamorphic rock into a magmatic looking one is only small. In case of layered migmatites it is shown that 1 % of water (or even less) is sufficient to transform portions of a gneiss into (magmatic looking) leucosomes. High grade metamorphic rocks were probably relatively dry, and anatectic magmas of granitic or granodioritic composition are usually not saturated with water.     

内蒙古红山子复式岩体地质时代的厘定及其地质意义

通过岩相学研究和LA-ICP-MS锆石U-Pb测年,对红山子岩体的主要侵入期次和侵入岩形成时代进行了厘定,明确了红山子岩体为复式岩体并探讨了其地质意义。岩相学研究表明,红山子复式岩体由粗粒碱长花岗岩、斑状黑云母花岗岩、细粒黑云母碱长花岗岩和花岗斑岩等组成; LA-ICP-MS锆石U-Pb测年结果显示:粗粒碱长花岗岩的年龄为153. 6±1. 2 Ma,斑状黑云母花岗岩年龄为151. 4±1. 1 Ma,与周边火山盆地赋矿火山岩的年龄一致,属晚侏罗世早期;细粒黑云母碱长花岗岩的年龄为131. 5±1. 8 Ma,花岗斑岩年龄为133. 3±1. 4 Ma,与侵入红山子盆地的花岗斑岩的年龄一致,属早白垩世早期。因此,红山子岩体是由燕山期2个相隔20 Ma的不同期次侵入体组成的复式岩体。研究表明,红山子铀矿床赋存在早白垩世早期花岗斑岩与晚侏罗世火山岩的内、外接触带中,暗示复式岩体早白垩世早期细粒黑云母花岗岩、花岗斑岩与晚侏罗世粗粒碱长花岗岩、斑状黑云母花岗岩的内、外接触带是有利的找矿部位,并得到了铀矿勘查实践的初步证实。