Calc-silicate Granulites Research Articles

Metamorphic evolution and mineral assemblages of fassaite calc-silicate and sapphirine-quartz Al-Mg granulites from the Tekhamalt area (In Ouzzal terrane, Western Hoggar): Implications for ultra-high-temperature conditions and petrogeneses

Tekhamalt area (In Ouzzal terrane Western Hoggar) is remarkable for the occurrence of quartz-sapphirine granulites associated with fassaite-bearing calc-silicate granulitesand alkaline orthogneisses. The clinopyroxenes in these calc-silicate granulites contain various contents of aluminum, with concentrations reaching up to 11.69 wt%. The Al-Mg granulites are particularly notable for the presence of ultra-high temperature (UHT) assemblages, such as sapphirine-quartz and high-Al orthopyroxene. This sapphirine-quartz association has enabled a reevaluation of the peak metamorphic conditions in the region, which were previously studied using a simpler system. The new and more realistic systems (MnO-Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3) have been employed for this purpose and allowing us to constrain the peak metamorphic conditions at 7.5 ± 1 kbar and 980 ± 50 °C. Furthermore, the study of alkaline orthogneiss shows the assemblage of perthite-quartz-clinopyroxene with a notable addition of the rare mineral chevkinite. The alkaline orthogneiss provides valuable information for understanding the formation of the calc-silicates in Tekhamalt area, which result from metasomatic processes involving dolomitic limestones and fluids in equilibrium with protolith of alkaline orthogneiss.

Reactions in UHT calc-silicate granulites from the In Ouzzal terrane (Western Hoggar, South Algeria): Implication for pressure-temperature-fluid evolution

Calc-silicate granulites constitute a relatively small part of the whole granulitic material outcrops characterizing the In Ouzzal terrane (NW Hoggar, South Algeria). However, these rocks preserve a number of spectacular reaction textures that could be effectively used to infer their pressure-temperature-fluid history. These textures are interpreted using P-T and T-XCO2 grids in the simplified CaO-Al2O3-SiO2-Vapor system. In this process, sequences of reactions have been subdivided into two distinct stages: (i) the early prograde stage that was accompanied by significant rise of temperature from about 800 °C up to 1050 °C at around10 kbar followed by (ii) the decompression stage from about 9 to 6 kbar. During the prograde stage, coarse grained wollastonites were produced according to the reaction calcite + quartz → wollastonite + CO2. Furthermore, in the peak pressure temperature stage, the reaction producing wollastonite + scapolite from coarse primary garnet consumes CO2 with temperature increasing from 850 °C to 1000 °C according to the reaction 3grossular + 3CO2 → 3wollastonite + 2calcite + scapolite. The latest reactions have been occurred during the decompression stage from about 10 kbar to 5 kbar and cooling from 1000 °C to 800 °C. The growth of calcite + quartz around wollastonite besides to garnet coronas between wollastonite, calcite and scapolite are explained by the reaction: calcite + quartz → wollastonite + CO2 and 3wollastonite + scapolite +2calcite → 3grossular + 3CO2. The appearance of anorthite around scapolite occurs following a decrease of temperature independently to the fluids according to the reaction scapolite → 3anorthite + calcite. All reactions took place at CO2 low pressure which was estimated between 0.04 and 0.55.

On the factors controlling the incorporation of aluminium within titanites: a case study from medium pressure calc-silicate granulites in parts of the East Indian shield

On the factors controlling the incorporation of aluminium within titanites: a case study from medium pressure calc-silicate granulites in parts of the East Indian shield

Formation of aluminous clinopyroxene-ilmenite-spinel symplectic assemblage in a regionally metamorphosed calc-silicate granulite from the Chotanagpur Granite Gneiss Complex, East Indian shield

Formation of aluminous clinopyroxene-ilmenite-spinel symplectic assemblage in a regionally metamorphosed calc-silicate granulite from the Chotanagpur Granite Gneiss Complex, East Indian shield

Trace and Rare Earth Elements chemistry of detrital garnets in the SE Alps and Outer Dinarides flysch basins: An important tool to better define the source areas of sandstones

Abstract Garnet is one of the most abundant heavy minerals present in the Cretaceous – Eocene flysch deposits of the Southeastern Alps and Outer Dinarides (Julian, Brkini and Istrian basins). About 300 detrital garnets from the Cretaceous-Eocene flysch deposits of the Southeastern Alps have been analysed by means of electron microprobe and LA-ICP-MS. In the Julian and Istrian basins, supplies are from amphibolite-facies rocks and mafic and ultramafic metamorphic rocks, while in the Brkini basin the latter are almost missing. Moreover, in the Julian and Istrian basins, supplies from skarns, very low-grade metabasites, or from ultra-high temperature metamorphosed calc-silicate granulites are present. Among these different groups, LA-ICP-MS analyses showed that trace element content can be very different in almandine-rich garnets from the different sources. In particular, the source that supplied the Julian Basin is significantly different from that of the Istrian Basin. From the Cretaceous to the Palaeocene the main supplies of Bi-type garnets derived from an area where feldspar-free garnet-bearing rocks were exposed. Successively, Bi-type garnets were supplied from an area where feldspar-garnet-bearing rocks were exposed. The presence of garnets from feldspar-free garnet-bearing rocks in the Brkini and Istrian basins can be ascribed to both recycling of material from the Julian Basin as well as direct input from the same areas that supplied the Julian Basin.

Petrology, mineralogy, and P–T path of calc-silicate granulites and fassaite-marbles from the Paleoproterozoic Gour Oumelalen area (Central Hoggar, Algeria)

The Gour Oumelalen area exposes Paleoproterozoic (1.9 Ga) marbles and calc-silicate granulites. Some marbles show a specific mineralogy characterized by the presence of a highly aluminous clinopyroxene with Al2O3 content exceeding16 wt%. This clinopyroxene shows a marked zoning with a hedenbergitic core rimmed by fassaite. Phase relations are expressed by spectacular reaction textures in calc-silicate granulites as Opx + Cpx + Pl + H2O == > Grt + Qtz ± Am and Cpx + Ilm + Pl ± Mt = > Grt + Qtz + Spn. In olivine-bearing marbles, clinopyroxene and dolomite occur around olivine and calcite. According to thermodynamic modeling in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCFMASHTO) system, the first stage of metamorphism is located at 800 °C and 6–7 kbar, which is followed by an increase in pressure at 9–10 kbar and 800 °C and an isobaric cooling at 680 °C. The deduced anticlockwise P–T path is consistent with a granulitic metamorphism occurring in an active continental margin context.

Mesoproterozoic suturing of Archean crustal blocks in western peninsular India: Implications for India–Madagascar correlations

The Kumta and Mercara suture zones welding together Archean crustal blocks in western peninsular India offer critical insights into Precambrian continental juxtapositions and the crustal evolution of eastern Gondwana. Here we present the results from an integrated study of the structure, geology, petrology, mineral chemistry, metamorphic P–T conditions, zircon U–Pb ages and Lu–Hf isotopes of metasedimentary rocks from the two sutures. The dominant rocks in the Kumta suture are greenschist- to amphibolite-facies quartz–phengite schist, garnet–biotite schist, chlorite schist, fuchsite schist and marble. The textural relations, mineral chemistry and thermodynamic modelling of garnet–biotite schist from the Kumta suture indicate peak metamorphic P–T conditions of ca. 11kbar at 790°C, with detrital SHRIMP U–Pb zircon ages ranging from 3420 to 2547Ma, εHf (t) values from −9.2 to 5.6, and TDMc model ages from 3747 to 2792Ma. The K–Ar age of phengite from quartz–phengite schist is ca. 1326Ma and that of biotite from garnet–biotite schist is ca. 1385Ma, which are interpreted to broadly constrain the timing of metamorphism related to the suturing event. The Mercara suture contains amphibolite- to granulite-facies mylonitic quartzo-feldspathic gneiss, garnet–kyanite–sillimanite gneiss, garnet–biotite–kyanite–gedrite–cordierite gneiss, garnet–biotite–hornblende gneiss, calc-silicate granulite and metagabbro. The textural relations, mineral chemistry and thermodynamic modelling of garnet–biotite–kyanite–gedrite–cordierite gneiss from the Mercara suture indicate peak metamorphic P–T conditions of ca. 13kbar at 825°C, followed by isothermal decompression and cooling. For pelitic gneisses from the Mercara suture, LA-ICP-MS U–Pb zircon ages vary from 3249 to 3045Ma, εHf (t) values range from −18.9 to 4.2, and TDMc model ages vary from 4094 to 3314Ma. The lower intercept age of detrital zircons in the pelitic gneisses from the Mercara suture ranges from 1464 to 1106Ma, indicating the approximate timing of a major lead-loss event, possibly corresponding to metamorphism, and is broadly coeval with events in the Kumta suture. Synthesis of the above results indicates that the Kumta and Mercara suture zones incorporated sediments from Palaeoarchean to Mesoproterozoic sources and underwent high-pressure metamorphism in the late Mesoproterozoic. The protolith sediments were derived from regions containing juvenile Palaeoarchean crust, together with detritus from the recycling of older continental crust. Integration of the above results with published data suggests that the Mesoproterozoic (1460–1100Ma) Kumta and Mercara suture zones separate the Archean (3400–2500Ma) Karwar–Coorg block and Dharwar Craton in western peninsular India. Based on regional structural and other geological data we interpret the Kumta and Mercara suture zones as extensions of the Betsimisaraka suture of eastern Madagascar into western India.

UHT granulites of the Highland Complex, Sri Lanka II: Geochronological constraints and implications for Gondwana correlation

The regional ultrahigh-temperature (UHT) metamorphism of the Highland Complex, Sri Lanka is well established and has an important role in our understanding of the tectonic history of the Gondwana supercontinent. U-Pb zircon dating of sapphirine-bearing Mg-Al granulites yielded two major metamorphic age populations at approximately 620-590 and 563-525 Ma with no evidence of older zircon cores. Pelitic granulite samples with a Grt-Sil-Spl-Crd assemblage have similar metamorphic ages with concordant data clusters at similar to 602, 563, and 526 Ma and inherited zircon cores aged from 2040 to 1600 Ma. The pelitic granulites also underwent two stages of metamorphism (565-520 and 622-580 Ma). Some of these pelitic granulite samples have inherited zircon cores ranging from 3060 to 760 Ma. Zircons in mafic granulite samples have age ranges of 566-533 and 620578 Ma. A calc-silicate granulite sample also has similar age populations at 591, 541, and 524 Ma. Combining these new results with previously published ages from Sri Lanka and formerly adjacent continental fragments of Gondwana, we propose that the terranes in southern Madagascar (south of Ranotsara Shear Zone), Northern and Southern Madurai and the Trivandrum Blocks of southern India, the Highland Complex of Sri Lanka, and the Skallen Group in the Lutzow-Holm Complex of East Antarctica represent a unique metamorphic belt that regionally experienced the Ediacaran-Cambrian UHT event during the amalgamation of the Gondwana super-continent.

Rb-Sr Isotopic Dating and Metamorphism of a Suite of Migmatite and Granites from Sandmata Granulite Complex, Rajasthan: An Evidence of Mesoproterozoic Thermal Event

Abstract: Sandmata Granulite complex (SGC) of Rajasthan of Western India is a multiply deformed, metamorphosed and migmatised granulite units representing a Mesoproterozoic collisional orogen. It consists of migmatised metapelites, calcsilicate granulites, pyroxene granulites and charnockitic intrusives at 1.7 Ga. The granulites mostly occur as dismembered, isolated enclaves in garnet bearing migmatites and quartzofeldspathic gneiss. Intrusion of small scale post-tectonic granite in granulite documents the last thermal activity in the area. At the eastern part of SGC, greenschist to lower amphibolite facies rocks of Manglawar Complex (MC) are exposed. The P-T conditions of garnet formation during metamorphism in migmatite, leptynite and quartzo-feldspathic gneiss are 780-850°C and 7–8 kbar. Retrograded biotite forming reactions occur at much lower temperature of 500 to 600°C and pressure of 3.5-4.5 kbar. The intrusive granite is weakly deformed, calc-alkaline to sub-alkaline, peraluminous in nature, having trace element signature of volcanic-arc to syn-collisional granite. The chondrite-normalised REE patterns indicate a garnet-free source. The whole-rock Rb-Sr age of 1715 ± 24 Ma deduced for the granite mark its emplacement age. Intrusive nature of granite suggests that the age of granulite metamorphism and migmatization should be >1715 Ma, which is much closer to the whole-rock Rb-Sr age of migmatite and quartzofeldspathic gneiss. The whole-rock Rb-Sr analysis of migmatite gives an age of 1735±150 Ma with an initial 87Sr/86Sr ratio of 0.704 which is interpreted as an age when the temperature of migmatization passed below the blocking temperature of Rb and Sr for biotite and plagioclase; an upper age limit of migmatization. SGC represents a major 1.9-1.7 Ga Palaeoproterozoic, tectonothermal event. The granulite facies metamorphism is succeeded by 1.7 Ga migmatisation, intrusion of calc-alkaline granite and emplacement of charnockite protolith. The whole granulite complex is subsequently cooled and later exhumed, and became juxtaposed with MC during a late major orogeny.

Mineralogy and Mineral Chemistry of Placer Deposit around Jhatipodar, Odisha

Abstract: The heavy mineral placer sands of Jhatipodar deposit, Odisha, India extends in NE-SW parallel to the coast over a width of 750-980m. A total of 195 samples were collected vertically in a grid pattern from 46 bore holes in 5 transects from beach and dunes covering an area of 75 hectares. The total heavy minerals (THM) content of the samples vary from 3.01% to 44.38%. The heavy minerals as observed from field and laboratory investigations are ilmenite, leucoxene, garnet, sillimanite, zircon, rutile, monazite, magnetite and corundum together with hornblende, diopside, tourmaline, sphene, apatite and epidote. The heavy minerals show similar pattern of grain size distribution from beach to dunes and are dominantly in the size of 90 - 350 μm with a peak distribution in -180, +125 μm. A bulk sample of dunal sands analysed 20.48% THM, contain 7.85% ilmenite, 7.65% garnet, 4.09% sillimanite, 0.20% zircon, 0.18% rutile and 0.13% monazite. The mineral grains are well liberated, rounded to sub-rounded and mostly free from surface coatings. The ilmenites contain 47.87-56.59% TiO2 with an average of 51.76% TiO2. The high TiO2 content (9.86-16.30%) in exsolved hematite is due to Fe2O3-FeTiO3 solid solution. Compared to ilmenite, leucoxene has higher amounts of Ti, Al, Si, Cr and lower amounts of Fe and Mn. The Ti/(Ti+Fe) in ilmenite varies between 0.46% and 0.55% while that in leucoxene has a range from 0.64% to 0.96%. The provenance rocks for this deposit are the khondalites, quartzites, calcsilicate granulites, granitic gneisses, charnockite, basic granulite, anorthosite and pegmatites of the Eastern Ghats Mobile Belt in the catchment areas of Rushikulya river. This paper deals with the distribution of heavy minerals in beaches and dunes, mineralogical association and chemistry of different minerals to evaluate the economic potentiality of Jhatipodar placer deposit and to decipher the amenability of different heavy minerals for mineral processing.

Early Permian mantle–crust interaction in the south-central Altaids: High-temperature metamorphism, crustal partial melting, and mantle-derived magmatism

Early Permian mafic magmatism, the partial melting of the crust, and high-temperature metamorphism in the Chinese Altai, south-central Altaids, provide an excellent case study of exchange of energy and mass between the mantle and crust. Field and petrographic observations, together with microprobe mineral analyses, have, for the first time, allowed us to identify scapolite-bearing calc-silicate granulites along the southern margin of the Chinese Altai. The conditions of metamorphism were 680–800°C and 6–7 Kbar, based on mineral phase relationship and compositions, as well as the results of previous studies. New LA-ICP-MS zircon U-Pb geochronology demonstrates that the high-T metamorphism in the Chinese Altai was accompanied by the emplacement of leucogranites at approximately 295Ma. SHRIMP II dating results, combined with previously published data, reveal a mafic magmatic event at around 275Ma in a large region of northern Xinjiang, NW China. This mafic magma was derived from N-MORB-like depleted mantle, as deduced from our new bulk geochemical and zircon Hf isotopic data. Synthesizing available geochemical (including isotopic) and geochronological data, we propose a two-stage model of mantle–crust interaction to explain the early Permian geology of northern Xinjiang. The early stage of interaction involved high-temperature metamorphism and the coeval partial melting of crustal rocks, indicating solely a heat exchange between the mantle and crust; in contrast, the later stage involved the exchange of both mass and energy between the mantle and crust.

Origin of graphite, and temperature of metamorphism in Precambrian Eastern Ghats Mobile Belt, Orissa, India: A carbon isotope approach

The carbon isotope composition of graphite and carbon and oxygen isotope composition of associated calcite from different locations of the Eastern Ghats Mobile Belt (EGMB) of Orissa have been measured in order to understand the origin of graphite. The δ 13C values of graphite range from −2.4‰ to −26.6‰. Forty-four of sixty-one samples have δ 13C values less than −20‰. Most of these low δ 13C values graphite corresponds to schists and disseminations in khondalite and calc-silicate granulites, thus indicating graphitization of organic matter. The remaining light-carbon–graphite occurs as veins which is the result of graphitization of transported organic matter. The graphite with intermediate δ 13C value (−13‰ to −19‰) indicates carbon contributions from both organic and carbonates sources and/or mantle sources. The higher δ 13C values graphite (−2.4‰ to −8.8‰) represent mantle carbon and/or carbonate sources without significant contribution from organic carbon. The temperatures of metamorphism have been estimated using carbon isotope ratios of graphite and associated calcite of calc-silicate granulites, where typical cation exchange thermometer assemblages are lacking and significant mineral reaction textures used to calculate pressure–temperature of metamorphic events are absent. Metamorphic temperatures obtained 945 °C are close to the ultrahigh-temperature reported from the EGMB. The minimum temperature estimated using the graphite–carbonate carbon isotope ratio is 90 °C. The lower estimates of temperatures probably indicate changes in the carbon isotope ratio of calcite by decarbonation reaction or armoring of carbonaceous matter in silicates during metamorphism preventing continuous exchange with calcite.

Solubility of andradite, Ca3Fe2Si3O12, in a 10 mol% NaCl solution at 800 C and 10 kbar: Implications for the metasomatic origin of grandite garnet in calc-silicate granulites

The solubility of andradite garnet was determined at 800 °C and 10 kbar in a solution of 10 mol% NaCl and 90 mol% H 2 O. Experiments were syntheses carried out with high-purity natural wollastonite, reagent hematite or natural specular hematite containing ~1 wt% TiO 2 , and fluid. All experiments were performed with Mn 2 O 3 -Mn 3 O 4 or Fe 2 O 3 -Fe 3 O 4 oxygen buffers in a piston-cylinder apparatus with NaCl pressure media for 1–3 days. Andradite saturation was determined by the presence or absence of garnet in quenched charges. Andradite dissolves incongruently to hematite and fluid with CaSiO 3 molality ( m CS ) of 0.0838 ± 0.0015 for the reagent hematite and both buffers. Slightly higher m CS of 0.0895 ± 0.0005 for the natural hematite and Mn-oxide f O 2 buffer is interpreted as due to incomplete equilibration and/or ~9 mol% Ti in the run-product andradite. Dissolved Fe molality could be determined only approximately, but must be at least ten times lower than m CS . Quenched fluids were very basic (pH 11–12). The solubility of Fe 2 O 3 in andradite-saturated H 2 O-NaCl fluids is lower than that of Al 2 O 3 at grossular saturation at the same pressure ( P ), temperature ( T ), and fluid composition. The results permit a test of a model of CaSiO 3 dissolution in NaCl solutions to three dominant aqueous species: CaCl + , OH , and H 3 NaSiO 4 . Combination of the CaSiO 3 molality at andradite saturation with wollastonite solubility at the same conditions (0.1253 ± 0.0047 molal; Newton and Manning 2006) leads to a Gibbs free energy change of the reaction 3 wollastonite + hematite = andradite at 800 °C and 10 kbar of −32.21 ± 2.45 kJ. The good agreement between this value and that derived from previous studies supports the dissolution model of CaSiO 3 in NaCl solutions. The low solubility of the Fe 2 O 3 component of andradite contrasts with the high solubility of magnetite and of Fe in pelitic and granitic mineral assemblages measured in acidic chloride solutions by previous workers at lower P and T . The results imply that Fe 2 O 3 is conserved during metasomatic processes affecting calc-silicates at high metamorphic grades.

Clinopyroxene exsolution in wollastonite from Namaqualand granulite, South Africa

Chemical and crystallographic properties of clinopyroxene exsolution in wollastonite are described from metamorphosed calc-silicate granulite, Namaqualand, South Africa. The wollastonite is Ca1.96Fe0.01Al0.01Si2.01-O6 belonging to space group P21/a (2M polytype) and the clinopyroxene is Ca0.99Mg0.75-0.80Fe0.17-0.21Na0.02Al0.03Si1.99-2.00O6 belonging to C2/c. An electron backscattered diffraction investigation suggests that the clinopyroxene lamellae elongated along [11̅1] lie on (120) and (100) of the wollastonite-2M, and [11̅0] of both lamellae are parallel to [001] of the wollastonite-2M. The formation of the exsolution probably results from the relatively high peak metamorphic temperature (800-860ºC) of the Namaqualand granulite and its slow cooling rate.

Origin of Grandite Garnet in Calc-Silicate Granulites: Mineral–Fluid Equilibria and Petrogenetic Grids

The role of clinopyroxene in producing grandite garnet is evaluated using data from an ultrahigh-temperature metamorphosed calc-silicate granulite occurrence in the Eastern Ghats Belt, India. ‘Peak’ pressure–temperature conditions of metamorphism were previously constrained from associated high Mg–Al granulites as c. 0·9 GPa, >950°C, and the rocks were near-isobarically cooled to c. 750°C. Grandite garnet of variable composition was produced by a number of reactions involving phases such as clinopyroxene, scapolite, plagioclase, wollastonite and calcite, in closely spaced domains. Compositional heterogeneity is preserved even on a microscale. This precludes pervasive fluid fluxing during either the peak or the retrograde stage of metamorphism, and is further corroborated by computation of fluid–rock ratios. With the help of detailed textural and mineral compositional studies leading to formulation of balanced reactions, and using an internally consistent thermodynamic dataset and relevant activity–composition relationships, new petrogenetic grids are developed involving clinopyroxene in the system CaO–Al2O3–FeO–SiO2–CO2–O2 in T–aCO2–fO2 space to demonstrate the importance of these factors in the formation of grandite garnet. Two singular compositions in garnet-producing reactions in this system are deduced, which explain apparently anomalous textural relations. The possible role of an esseneite component in clinopyroxene in the production of grandite garnet is evaluated. It is concluded that temperature and fO2 are the most crucial variables controlling garnet composition in calc-silicate granulites. fO2, however, behaves as a dependent variable of CO2 in the fluid phase. External fluid fluxing of any composition is not necessary to produce chemical heterogeneity of garnet solid solution.

Fluid compositions and local fluid buffering during the retrograde metamorphism of Al-bearing dolomitic calc-silicate granulites in the Limpopo Central Zone, southern Africa

Calc-silicate granulites were examined to evaluate the fluid composition and retrograde metamorphic conditions in the Central Zone of the Limpopo Belt, southern Africa. Quartz deficient assemblages are characterized by minerals such as diopside, forsterite, spinel and/or magnesiohornblende and tremolite in the presence of calcite and dolomite. Although the granulites are Al-poor (Al 2O 3 is less than or equal to 1.0 wt.%) and dolomitic in composition, they include Al-bearing phases. Phase analyses for the assemblages in the two model systems CaO–MgO–SiO 2–H 2O–CO 2 and CaO–MgO–SiO 2–Al 2O 3–H 2O–CO 2 provide constraints on fluid compositions in the granulite facies and retrograde metamorphisms in the Limpopo Central Zone. In the presence of amphiboles, isobaric T–X(CO 2) phase relations suggest that high X(CO 2) conditions were established in the calc-silicate rocks of present study. The phase relations with tschermakitic amphiboles at 0.35 GPa restrict diopside-spinel occurrences in the presence of calcite, dolomite and forsterite within very-high X(CO 2) with low a(H 2O). The fluid compositions, X(CO 2), were effectively buffered by the mineral assemblages during granulite facies metamorphism to subsequent decompression and cooling stages. The presence or absence of retrograde magnesiohornblende and tremolite appeared to be controlled not only by infiltration of H 2O-rich fluid during retrograde metamorphism but also Al content in the local bulk rock compositions. The presence of the two-amphibole phases shows that the fluid compositions were locally buffered in the Al-bearing dolomitic granulites. Comparing the calculated X(CO 2) values in the present study area and in the Alldays area, a difference of retrograde hydration effects is observed.

Garnet composition as a petrogenetic indicator: An example from a marble -- calc-silicate granulite interface at Kondapalle, Eastern Ghats Belt, India

TheP-T-fluid histories of aluminous garnet+clinopyroxene-bearing calc-silicate granulites are commonly constrained through interpretation of sequential reaction textures based on activity-corrected petrogenetic grids for model systems, for example the widely used CAS(CO2-H2O) topologies. Application of these topologies to calc-silicates of diverse bulk composition is limited by the fact that the role of clinopyroxene and hence of garnet composition in the garnet-forming reactions is ignored or not adequately considered. In this contribution the complexities that arise from the involvement of clinopyroxene in the garnet-forming reactions are critically evaluated. As an example, the paragenetic relations and compositional features of clinopyroxene and coronitic garnet across paired scapolite- and wollastonite-dominated reaction zones (scp+cpx±qtz,cal,qtz — wo+cal+cpx±scp) developed between marble layers (cal±cpx,wo,scp) and their host calc-silicate granulite (cpx+pl±qtz,scp) at an exposure in the high-grade Eastern Ghats Belt (India) are examined. The microtextural features and systematic changes of modal and phase compositions across the profile indicate that clinopyroxene, scapolite, plagioclase, quartz and calcite as well as wollastonite participated in the garnet-forming reactions. Using the sub-assemblage qtz+cal+cpx+scp+grt+v in the system CaO-FeO-Al2O3-SiO2-CO2 it is demonstrated that at the ambient P-T-Xfluid conditions, the reaction coefficients of the participating phases are uniquely defined by the garnet composition. As a result, the P-T slope of a garnet-forming reaction changes with the garnet composition, with sharp changes at “singular points” across which one or more phases change their sides with respect to garnet in a given reaction. To consider the consequences of these complexities on the reaction topology of calc-silicate granulites, a new petrogenetic grid for the system CaO-FeO-Al2O3-SiO2-CO2 involving clinopyroxene has been constructed. The effects of fluid composition (CO2-H2O-O2), fluid pressure (Pfluid

Ultrapotassic Rocks along Late Ductile Shear Zones from the Eastern Ghats Belt, India

Ultrapotassic rock is reported for the first time from the polycyclic Eastern Ghats belt, India, near Borra, Visakhapatnam district, Andhra Pradesh. The rock, consisting of leucite, kalsilite, Khyphen;feldspar, graphite, apatite together with diopside, meionite and phlogopite, occurs as thin vein and veinlets in diopsidite, in close spatial association with a granulite facies carbonate ensemble of massive dolomitic carbonate rock and calc silicate granulite. It was emplaced in the midhyphen;crust along late ductile shear zones. Subsequent to its emplacement, the ultrapotassic melt with liquidus leucite interacted with the granulite wall rock, incorporating at least 40% of the crustal components mainly as Si, Al, Mg and Ca. After necessary correction of the crustal contaminant, the recalculated K 2O/Na 2O ratio of ∼12 (molar) and K 2O/Al 2O 3 ratio of ∼1 (molar) in the bulk rock composition indicates that the Borra ultrapotassic melt has a lamproitic affinity. However, it is significantly modified as well, particularly being impoverished in mafic liquidus phases and depleted in incompatible (excepting Rb, Th and U) and compatible trace elements, compared to an average lamproite. Leucite later underwent subsolidus decomposition to Khyphen;feldspar + kalsilite intergrowths. The emplacement of the ultrapotassic melt posthyphen;dates an early ultra high temperature metamorphism and also the 1000 Ma Grenvillian metamorphism in the Eastern Ghats Belt and is possibly of Panhyphen;African age. The extensive Khyphen;feldspathisation in the Eastern Ghats belt could also be linked with this ultrapotassic melt.

Evidence of Superposed Metamorphism from the Gokavaram Area, Eastern Ghats Belt, and its Relation with the Kemp Land Coast, East Antarctica

In this paper, we compare the petrological histories of the Kemp Land Coast (east Antarctica), and Gokavaram area (Eastern Ghats), that were supposed to have been juxtaposed. The area around Gokavaram is dominated by different varieties of paragneisses (pelitic, quartzofeldspathic, and calcareous composition) with relatively minor amounts of orthogneisses (mafic, enderbitic, and granitic composition). The rocks were involved in three major phases of deformation, and were finally affected by localised shear movement. On the basis of reaction textures, well preserved in high Mg-Al granulites, and calc-silicate granulites, and geothermobarometric data we deduce a polymetamorphic evolution of the rocks. Following an early M 1 metamorphism culminating at 9.2–9.4 kbar, > 950°C, the rocks cooled nearly isobarically down to 850°C. During a subsequent M 2 metamorphism, near isothermal decompression to 5–6 kbar occurred. This was followed by near isobaric cooling down to 600–650°C. M 3 is a weak amphibolite facies overprint, largely restricted to late shears, which involved hydration as well. Available radiometric data from this area can be interpreted in terms of partial resetting of U-Pb systematics in older sphenes due to M 3 metamorphism at ca. 550 Ma. Despite the absence of sufficient isotopic data on the Eastern Ghats granulites, we document a remarkable similarity in the petrological history of the two supposedly erstwhile neighbours.

Controls of mineral reactions in high‐grade garnet‐wollastonite‐scapolite‐bearing calcsilicate rocks: an example from Anakapalle, Eastern Ghats, India

ABSTRACT A suite of garnet‐wollastonite‐scapolite‐bearing calcsilicate granulites from the Eastern Ghats has been investigated to document the controls of mineral reactions during the metamorphic evolution of the deep continental crust. The rocks studied show heterogeneity in modal mineralogy and phase compositions in millimetre‐sized domains. Textural relations, and the compositional plots of the phases, established that the clinopyroxene exerts a strong influence on the formation and composition of garnet in the complex natural system. P‐T estimates using the vapour‐independent equilibria involving garnet define a near isobaric cooling path from c. 850C at c. 5.5–5.2 kbar. The deduced trajectory tallies well with the terminal segment of the overall retrograde P‐T path construed from the associated rocks using well‐calibrated thermobarometers. The ubiquitous occurrence of wollastonite and scapolite in the main calcsilicate body suggests low aCO2 during peak metamorphic condition. Fluid compositions constrained from mineral‐fluid equilibria of the garnet‐bearing assemblages show domainal variations as a function of the compositions of the solid phases, e.g. garnet and clinopyroxene. A quantitative log/CO2‐log/O2 diagram has been constructed to depict the stability of the different calcsilicate assemblages as functions of the compositions and the behaviour of these fugitive species. The results of the mineral‐fluid equilibria and the quantitative fluid/rock ratio calculations, in conjunction with the topological constraints, imply vapour‐deficient meta‐morphism in the rocks studied. It is argued that fO2 during peak metamorphism was monitored by the ambient fO2. Subsequently, during retrogression, different domains evolved independently, whereas the fluid composition was controlled by the mineral‐fluid equilibria.