ALBERT

Notes: Abstract At Kottavattam, southern Kerala (India), late Proterozoic homogeneous leptynitic garnet–biotite gneisses of granitic composition have been transformed on a decimetric scale into coarse-grained massive charnockite sensu stricto along a set of conjugate fractures transecting the gneissic foliation. Charnockitization post-dates the polyphase deformation, regional high-grade metamorphism and anatexis, and evidently occurred at a late stage of the Pan-African tectonothermal history. Geothermobarometric and fluid inclusion data document textural and chemical equilibration of the gneiss and charnockite assemblages at similar Plith–T conditions (650–700°C, 5–6 kbar) in the presence of carbonic fluids internally buffered by reaction with graphite and opaque mineral phases (XCO2= 0.7–0.6; XH2O= 0.2–0.3; XN2= 0.1; log fO2= -17.5).Mineralogical zonation indicates that charnockitization of the leptynitic gneiss involved first the breakdown of biotite and oxidation of graphite in narrow, outward-migrating transition zones adjacent to the gneiss, followed by the breakdown of garnet and the neoblastesis of hypersthene in the central charnockite zone. Compared to the host gneiss, the charnockite shows higher concentrations of K, Na, Sr, Ba and Zn and lower concentrations of Mg, Fe, Ti, V, Y, Zr and the HREE, with a complementary pattern in the narrow transition zones of biotite breakdown. The Plith–T–XH2O data and chemical zonation patterns indicate charnockitization through subsolidus-dehydration reaction in an open system. Subsequent residence of the carbonic fluids in the charnockite resulted in low-grade alteration causing modification of the syn-charnockitic elemental distribution patterns and the properties of entrapped fluids. We favour an internally controlled process of arrested charnockitization in which, during near-isothermal uplift, the release of carbonic fluids from decrepitating inclusions in the host gneiss into simultaneously developing fracture zones led to a change in the fluid regime from ‘fluid-absent’in the gneiss to ‘fluid-present’in the fracture zones and to the development of an initial fluid-pressure gradient, triggering the dehydration reaction.

Notes: High-MgAl rocks occur as xenoliths (up to 2 m in diameter) in mafic granulites at a newly discovered locality near Anakapalle. Following an early phase of deformation, ultrahigh-temperature (UHT) metamorphism and near-isothermal decompression, the rocks were intruded in a lit-par-lit manner by felsic melts (charnockite), which caused local-scale metasomatism. A subsequent deformation produced isoclinal folds and the distinct gneissic foliation of the charnockite still at granulite facies conditions.The sequence of multiphase reaction textures in the high-MgAl xenoliths reflects the changes of physico-chemical conditions during the polyphase evolution of the terrane; UHT metamorphism (stage 1, 〉 1000°C, c. 10 kbar) is documented by relics of extremely coarse grained domains with the assemblage orthopyroxene (opx)1 + garnet (grt)1 + sapphirine (spr)1 + spinel (spl)1 + rutile (rt). A subsequent phase of near-isothermal decompression in the order of 1–2 kbar (stage 2) resulted in extensive replacement of grt1 and opx1 megacrysts by lamellar (opx2 + spr2) symplectites. The intrusion of felsic melt (stage 3) led to the development of a narrow metasomatic black wall reaction zone (bt + sil + plg3 + opx2,3 + rt) at the immediate contact of the xenoliths and in melt infiltration zones to the partial replacement of (opx2 + spr2) symplectites by biotite and sillimanite and/or plg3, mainly at the expense of orthopyroxene, with concomitant coarsening of the intergrowth texture. The subsequent deformation (stage 4) further modified the symplectite textures through polygonization, recrystallization and grain-size coarsening. The deformation was followed by a period of cooling and decompression (stage 5, c. 800°C, 4–7 kbar) as indicated by local growth of late garnet (grt5) at the expense of (opx + spr + plg) domains at static conditions.Recently published isotope data suggest that the multistage evolution of the high-MgAl granulites at Anakapalle followed a discontinuous P–T trajectory that may be related to heating of the crust through magmatic accretion culminating in deep-crustal UHT metamorphism at 1.4 Ga (stage 1), fast uplift of the UHT granulites into mid-crustal levels as a consequence of extensional tectonics (stage 2), emplacement of felsic magmas in the Grenvillian (at c. 1 Ga, stage 3) resulting in reheating of the crust to high–T conditions followed by a phase of compressional tectonics (stage 4) and a period of cooling to the stable geotherm (stage 5) still in the Grenvillian.

Notes: At Deobhog, migmatitic gneisses and granulites of the Eastern Ghats Belt are juxtaposed against a cratonic ensemble of banded augen gneiss, amphibolite and calcsilicate gneiss, intruded by late hornblende granite and dolerite. In the migmatitic gneiss unit, early isoclinal folds (syn-D1M and D2M) are reoriented along N–S-trending and E-dipping shear planes (S3M), with (S1M–S3M) intersection lineations having steep to moderate plunges. The near-peak P–T  condition was syn-D3M (≥900 °C, 9.5 kbar), as inferred from syn-D3M Grt+Opx-bearing leucosomes in mafic granulites, and from thermobarometry on Grt (corona)–Opx/Cpx–Pl–Qtz assemblages. The P–T  values are consistent with the occurrence of Opx–Spr–Crd assemblages in spatially associated high-Mg–Al pelites. A subsequent period of cooling followed by isothermal decompression (800–850 °C, c. 7 kbar) is documented by the formation of coronal garnet and its decomposition to Opx+Pl symplectites in mafic granulites. Hydrous fluid infiltration accompanying the retrograde changes is manifested in biotite replacing Opx in some lithologies.The cratonic banded gneiss–granite unit also documents two phases of isoclinal folding (D1B & D2B), with the L2B lineation girdle different from the lineation spread in the migmatitic gneiss unit. Calcsilicate gneiss (Hbl–Pl–Cpx–Scap–Cal) and amphibolite (Hbl–Pl±Grt±Cpx) within banded gneisses record syn-D2B peak metamorphic conditions (c. 700 °C, 6.5 kbar), followed by cooling (to c. 500 °C) manifested in the stabilization of coronal clinozoisite–epidote. The D3B shear deformation post-dates granite and dolerite intrusions and is characterized by top-to-the-west movement along N–S-trending, E-dipping shear planes. Deformation mechanisms of quartz and feldspar in granites and banded gneisses and amphibole–plagioclase thermometry within shear bands in dolerites document an inverted syn-D3B thermal gradient with temperature increasing from 350 to 550 °C in the west to ≥700 °C near the contact with the migmatitic gneiss unit. The thermal gradient is reflected in the stabilization of chlorite after hornblende in S3B shears to the west, and post-D2B neosome segregation along D3B folds and shears to the east.The contrasting lithologies, early structures and peak metamorphic conditions in the two units indicate unconnected pre-D3P–T –deformation histories. The shared D3 deformation in the two units, the syn-D3 inverted thermal gradient preserved in the footwall cratonic rocks and the complementary cooling and hydration of the hanging wall granulites across the contact are attributed to westward thrusting of ‘hot’ Eastern Ghats granulites on ‘cool’ cratonic crust. It is suggested that the Eastern Ghats migmatitic gneiss unit is not a reworked part of the craton, but a para-autochthonous/allochthonous unit emplaced on and amalgamated to the craton.

Notes: In the Vizianagaram area (E 83°29.442′; N 18°5.418′) of the Eastern Ghats Belt, India, a suite of graphite-bearing calc-silicate granulites, veined by syenitic rocks, developed wollastonite-rich veins at 6–7 kbar and 〉 850 °C. During subsequent near-isobaric cooling wollastonite was replaced by calcite + quartz and a graphic intergrowth of fluorite + quartz ± clinopyroxene. Titanite with variable Al and F contents is present throughout the rock. Combining the compositional variation of titanite and recent experimental data, it is demonstrated that the mineral assemblage, the composition of coexisting fluids and the mobility of Al exert a far greater control on the composition of titanite than pressure, temperature or the whole rock composition. Thermodynamically computed isothermal–isobaric logfO2– logfCO2 and logfF2– logfO2 grids in the systems Ca–Fe–Si–O–F (CISOF; calcite-free) and Ca–Fe–Si–O–F–C–H (CISOFV; calcite-present) demonstrate the influence of bulk rock and fluid compositions on the stability of the fluorite-bearing assemblages in diverse geological environments and resolve the problem of the stability of titanite in fayalite + fluorite-bearing rocks in the Adirondacks. The mineralogy of the studied rocks and the topological constraints tightly fix the logfO2, logfF2 and logfCO2 at −15.8, −30.6 and 4.1, respectively, at 6.5 kbar and c. 730 °C. Because of the similarity in the P–T conditions, the compositions of pore fluids in the fluorite-bearing assemblages of the Adirondacks and the Eastern Ghats Belt have been compared.

Notes: Sapphirine granulites from a new locality in the Palni Hill Ranges, southern India, occur in a small enclave of migmatitic, highly magnesian metapelites (mg=85–72) within massive enderbitic orthogneiss. They show a variety of multiphase reaction textures that partially overprint a coarse-grained high-pressure assemblage of Bt+Opx+Ky+Grt+Pl+Qtz. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid considerations, records a clockwise P–T evolution with four distinct stages. (1) Equilibration of the initial high-P assemblage in deep overthickened crust (12 kbar/800–900 °C) was followed by a stage of near-isobaric heating, presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives. During heating, kyanite was converted to sillimanite, and biotite was involved in a series of vapour-phase-absent melting reactions, which resulted in the ultra-high-temperature assemblage Opx+Crd+Kfs+Spr±Sil, Grt, Qtz, Bt, coexisting with melt (equilibration at c. 950–1000° C/11–10 kbar). (2) Subsequently, as a result of decompression of the order of 4 kbar at ultra-high temperature, a sequence of symplectite assemblages (Opx+Sil+Spr/Spr+Crd→Opx+Spr+Crd→Opx+Crd→Opx+Crd+Spl/Crd+Spl) developed at the expense of garnet, orthopyroxene and sillimanite. This stage of near-isothermal decompression implies rapid ascent of the granulites into mid-crustal levels, possibly due to extensional collapse and erosion of the overthickened crust. (3) Development of late biotite through back-reaction of melt with residual garnet indicates a stage of near-isobaric cooling to c. 875 °C at 7–8 kbar, i.e. relaxation of the rapidly ascended crust to the stable geotherm. (4) A second period of near-isothermal exhumation up to c. 6–5 kbar/850 °C is indicated by the partial breakdown of late biotite through volatile phase-absent melting reactions. Available isotope data suggest that the early part of the evolutionary history (stages 1–3) is presumably coeval with the early Proterozoic metamorphism in the extended granulite terrane of the Nilgiri, Biligirirangan and Shevaroy Hills to the north, while the exhumation of the granulites from mid-crustal levels (stage 4) occurred only during the Pan-African thermotectonic event, which led to the accretion of the Kerala Khondalite Belt to the south.

Notes: Zusammenfassung Schachbrettoligoklas (Ab83An17) wurde erstmals in alpidisch metamorphen Gneisen der Zillertaler Alpen (Tirol, Österreich) gefunden. Er entstand durch metasomatische Verdrängungen von prämetamorphem Kalifeldspat I durch Oligoklas ausschließlich im Bereich der niedrig temperierten Amphibolitfazies (B 2.1 nachWinkler, 1967) gemäß der Reaktionsgleichung: $$\begin{gathered} 1,17KAlSi_3 O_8 + 0,17Ca^{2 + } + 0,83Na^ + \to \hfill \\ \to (0,17CaAl_2 Si_2 O_8 /0,83NaAlSi_3 O_8 ) + 0,68SiO_2 + 1,17K^ + \hfill \\ \end{gathered} $$ Die Gültigkeit dieser Reaktionsgleichung ist durch die Integration des Schachbrettoligoklases und seiner amöboiden Quarz-Einschlüsse in Dünnschliffen bestätigt worden. Schachbrettoligoklas entspricht in seinen optischen und röntgenographischen Daten maximal geordnetem Oligoklas (Ab83An17).

Notes: Zusammenfassung Der archaische Kraton Südindiens ist durch eine komplexe, noch nicht vollständig aufgeklärte geologische Entwicklungsgeschichte gekennzeichnet. Sie umfaßt mehrere Zyklen von Sedimentation und Vulkanismus, Deformation und Metamorphose in der Zeitspanne von etwa 3400 bis 2500 Ma. Die großregionale metamorphe Zonierung wird im wesentlichen als das Ergebnis eines Metamorphose-Ereignisses vor ca. 2600 Ma angesehen, das den gesamten archaischen Komplex aus granitoiden Gneisen, Migmatiten und „Greenstone“-Serien erfaßte. Das südliche Gebiet ist durch Bedingungen der Granulitfazies (700–750° C/8–10 kb) charakterisiert. Hier entstand eine ausgedehnte Charnockit-Khondalit-Zone durch post-tektonische Umwandlung der Migmatit-Gneisserien infolge Zufuhr einer CO2-reichen fluiden Phase. Nach Norden zu nimmt der Metamorphosegrad bis zur niedrigtemperierten Amphibolitfazies (600° C/6–8 kb) und in den Dharwar-„Greenstone Belts“ sogar bis zur Grünschieferfazies (400° C) ab. Eine jüngere Metamorphose von Amphibolit-bis Grünschieferfazies ist auf proterozoische Scherzonen beschränkt. Im südlichen Gebiet führte sie zu einer örtlich unterschiedlich intensiven retrograden Überprägung der Charnockit-Khondalit-Serie. Die P-, T-Bedingungen der Regionalmetamorphose wurden anhand von Mineralstabilitätsdaten und mit Methoden der Geothermometrie und Geobarometrie für das Gebiet zwischen Shimoga-Chitradurga (Norden) und Coimbatore-Karur (Süden) abgeleitet.

Notes: Abstract Let be $$G \subseteq \mathbb{R}^n $$ . A ball with centerx and radiusr is denoted by (x, r). The set of all balls contained inG is writtenK(G). AnalogousK (ℝ n ) is defined. For a given functionf:G→ℝ m isF:K(G) →K (ℝ m ) a centered ball extension if the restriction on balls with radius 0 isf, and the center of the ballF(x, r) is equal tof(x). It is called inclusion isotonic if $$(x,r) \subseteq (y,s) \Rightarrow F(x,r) \subseteq F(y,s)$$ is true. The problem of existence and uniqueness of inclusion isotonic centered ball extensions is treated. It is shown that if only one such an extension is existing, there exist arbitrary many ones, but one and only one “smallest” is given. Further, there are given sufficient conditions of existence and nonexistence. In the case that the domainG off is open sufficient and necessary conditions are found. Finally, some examples finish the paper.

Notes: Abstract The development of Fe-Ti oxide assemblages in basic rocks from the Penninic series of the southern Venediger rea, Austria, during polyphase Alpine metamorphism has been studied. Textural and compositional relations indicate thorough reequilibration of the opaque mineral assemblages during late Barrovian metamorphism at essentially static conditions of lower amphibolite to greenschist facies. In contrast, silicate mineralogy of the preceeding blueschist to eclogite facies metamorphism might still be preserved to a large extent. Chemical adjustment of the Fe-Ti oxide minerals to decreasing P-T conditions is characterized by (1) formation of complex intergrowths of ilmenite and hematite solid solutions (〈550° C), (2) the decomposition of hemo-ilmenite 1 to ferrianilmenite2+magnetite+rutile and of ilmeno-hematite1 to titanhematite2+rutile±magnetite (〈450° C), and (3) low-grade oxidation of ferrianilmenite2 to magnetite+hematite-rutile intergrowths or hematite +rutile and of titanhematite2 to hematite-rutile intergrowths (≦400° C). Chemical equilibrium is suggested by the regular partitioning of Cr, V, Mg and Mn between coexisting hemo-ilmenite, ilmeno-hematite, and magnetite. The hematite-ilmenite miscibility gap has been delimited on the basis of the bulk compositions of the exsolved phases and the temperature estimates obtained from Fe-Ti oxide thermometry.