A Reappraisal of the Pressure-Temperature Path of Granulites from the Kerala Khondalite Belt, Southern India (original) (raw)
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Journal of Southeast Asian Earth Sciences
This paper reports several new localities of wollastonite-and scapolite-bearing talc-silicate assemblages from the granulite-facies supracrustal Kerala Khondalite Belt (KKB), southern India. Based on mineralogy, these talc-silicate rocks are classified into four types: Type I, lacking wollastonite and grossular; Type II, wollastonite-bearing but grossular-absent; Type III, wollastoniteand grossular-bearing; and Type IV, dolomitic marbles. Detailed petrographic studies reveal a variety of reaction textures overprinting the polygonal granoblastic peak metamorphic assemblages in these rocks. The Type II talc-silicate rocks preserve reaction textures, including meionite breaking down to anorthite-calcite-quartz, wollastonite breaking down to calcite-quartz and meionite-quartz symplectites after K-feldspar and wollastonite. Type III talc-silicate rocks have porphyroblastic and coronal grossular. Grossular-quartz coronas separating wollastonite and anorthite and the development of grossular within the anorthite-calcite-quartz pseudomorphs of meionite form important retrograde reaction textures in this type. In Type. IV dolomitic marble assemblages, meionite forming in grain boundaries of calcite and feldspars, forsterite rimmed by diopside-dolomite and the formation of grossular in feldspar-rich zones are the important textures. Calculated partial petrogenetic grids in the CaO-AlzO,-SiO&02 system are used to deduce the pressure-temperature fluid evolution of the talc-silicate rocks. The Type II assemblages provide CO, activity estimates of >0.5, with a peak metamorphic temperature of about 790°C. Initial cooling followed by later COZ influx can be deduced from reaction modelling in these talc-silicate rocks. Type III assemblages are characterized by internal fluid buffering throughout their tectonic history. The formation of coronal grossular indicates an initial cooling from peak metamorphic temperatures of about 830°C deduced from vapour-absent meionite and grossular equilibria. Type IV marble assemblages also indicate internal fluid buffering followed by localized CO* influx. Overall, the talc-silicate rocks of the KKB define peak metamorphic temperatures in the range of 790_85o"C, with an internally buffered fluid composition during the peak conditions. Initial cooling was followed by localized carbonic fluid influx that also postdated decompression deduced from other rock types in the KKB.
Journal of Mineralogical and Petrological Sciences, 2004
Granulite facies metapelites (khondalites) from Chittikara are characterized by the assemblage Grt + Sil + Bt + Crd + Spl + Kfs + Pl + Qtz ± graphite. The rocks show a series of mineral reactions such as: (1) Bt + Sil + Qtz = Grt + Crd ± Kfs, (2) Grt + Sil + Qtz = Crd, (3) Grt + Crd + Sil = Spl + Qtz , (4) Spl + Qtz = Crd, and (5) Grt + Sil = Spl + Crd. Spinel and quartz occur in direct grain contact and their stability considerations in appropriate reaction grids indicate > 950°C at 8 kbar. This, together with the ca. 1000°C temperatures derived from mesoperthites suggests ultrahigh temperature metamorphism in the khondalite belt. Electron microprobe data on the various minerals have been used to compute pressure temperature estimates from mineral phase equilibria thermobaromtery. Temperature estimates from spinel cordierite thermometry are around 900°C, while garnet cordierite thermometer yields an upper estimate of 818°C. Estimates based on garnet sillimanite plagioclase quartz barometer indicate pressures up to 8 kbar. Evaluation of the microstructures and reactions in the khondalites using available reaction grids for pelitic rocks provide evidence for peak metamorphism at high or ultrahigh temperatures followed by steep decompression from around 8 kbar down to 4 kbar. The P T path attests to a rapid exhumation history probably associated with the extensional collapse following collision of continental blocks during the assembly of Gondwana in the late Pan African. * Fe 2 O 3 was calculated based on stoichiometry. SC, Spl + Crd; SQ, Spl + Qtz grain contact; SQC, Spl rimmed by Crd against Qtz.
Mineralogy and Petrology, 2010
The Madurai Block, southern India, lies between the Palghat-Cauvery and the Achankovil shear zones. The Karur area represents a portion of the granulite-facies terrain of the Madurai block. Highpressure (HP) and ultrahigh-temperature (UHT) mafic granulites have been found as enclaves within the gneisses. The peak assemblage (M1) consists of garnet, orthopyroxene, clinopyroxene, quartz, and plagioclase. Garnet breaking down during isothermal decompression is indicated by the development of pyroxene+plagioclase symplectites, which characterize the M2 stage of metamorphism. Late stage hornblende-plagioclase symplectites rimming garnet is related to the decompressioncooling M3 stage of metamorphism. Peak metamorphism M1 occurs at~12 kbar pressure and temperatures in excess of 1,000°C. This was followed by a retrograde M2 stage when the mafic granulites suffered isothermal decompression to 6 kbar to 7 kbar at 800-900°C. At the terminal retrograde stage M3 solid-melt back reaction took place at 4.5-5.5 kbar and 650-700°C. The proposed clockwise P-T path implies that rocks from the study area could have resulted from thickened continental crust undergoing decompression. The SHRIMP data presented here from the Karur area provide evidence for a Neoproterozoic (521±8 Ma) metamorphic event in the Madurai block. The formation of symplectic assemblages during near isothermal decompression can be attributed to tectonic activity coinciding with the Pan-African phase of a global orogeny.
Mineralogy and Petrology, 1996
The Kodaikanal massif is part of a Precambrian terrane characterized by granulite facies rocks. It is dominated by the widespread occurrence of charnockites. The observed textural relationships in these rocks are consistent with the following main reactions: i. Biotite + Quartz = Orthopyroxene + Alkali feldspar + Garnet + Vapour ii. Garnet + Quartz = Orthopyroxene + Plagioclase iii. Pyrope = Mg Tschermaks + Enstatite (in Opx) Garnet consuming reactions and the preservation of biotite-quartz-orthopyroxeneplagioclase symplectites are indicative of a decompression event. Progress of such reactions with decreasing pressure together with fluid inclusion data has implications for the construction of P-T vectors. Quartz from the charnockites contains the following fluid inclusions: (1) monophase high density CO2-rich (0.968~l.014g/cm 3) as the dominant fluid phase; (2) aqueous biphase CO2-H20 (0.888-0.915 g/cm 3) and (3) late minor aqueous H20 inclusions with no visible CO 2. CO2-isochores for the high density fluid inclusions yield a pressure limit of ca. 6.5 kbars, at granulite facies temperatures of ca. 800°C, which is in accordance with the estimation from mineralogical thermobarometry. The P-T path delineated from combined mineralogical and density data on carbonic inclusions is characteristically T-convex suggesting an isothermal decompression path and rapid uplift followed by cooling of a tectonically thickened crust. Zusammenfassung Fliissigkeitseinschliisse in Charnockiten des Kodaikanal Massivs (Sfidindien) und ihre Bedeutung ffJr die P-T Entwicklung und Krustenhebung Granatabbaureaktionen und das Auftreten yon Symplektiten mit Biotit, Quarz, Orthopyroxen und Plagioklas, belegen ein Dekompressionsereignis. Das Ablaufen dieser Reaktionen mit fallendem Druck und die Daten der Untersuchungen an Fliissigkeitseinschlfissen sind ftir die Konstruktion des P-T Pfades bedeutsam. Quarz in Charnockiten f/ihrt folgende Fltissigkeitseinschliisse: (1) Einphasige, sehr dichte, CO2-reiche Fluide (0.968-1.014g/cm 3) dominieren. (2) Zweiphasige, wfissrige, COz-HaO (0.888-0.915 g/cm 3) und (3) sprite, CO 2-freie, H20-Einschliisse treten untergeordnet auf. Aus den Isochoren der Einschliisse hoher Dichte 1/il3t sich ein Druck yon 6.5 kbar bei granulitfaziellen Temperaturen von ca. 800 °C ableiten, was mit den geothermobarometrischen Ergebnissen {ibereinstimrnt. Der aus den mineralogischen und F1-Daten abzuleitendene P-T Pfad ist charakteristischerweise T-konvex ausgebildet und ist mit isothermischer Dekompression und rascher Hebung einer tektonisch vedickten Kruste erklfirbar.
Journal of Metamorphic Geology, 1997
Migmatitic semipelitic granulites of the Proterozoic Epupa Complex, NW Namibia, underwent ultrahigh-temperature metamorphism as is indicated by the high alumina contents of orthopyroxene (8±11 wt % Al 2 O 3 ) coexisting with garnet. Peak P±T conditions of 970Cand970 C and 970Cand9Á5 kbar are calculated from conventional garnet±orthopyroxene geothermobarometry. Conspicuous reaction textures document a multistage retrograde uplift±cooling path: post-peak decompression initially under still ultrahigh temperatures ($940 C and 8kbar)isrecordedbycoronasofaluminousorthopyroxeneplagioclasearoundgarnet.Continueddecompression(8 kbar) is recorded by coronas of aluminous orthopyroxene plagioclase around garnet. Continued decompression (8kbar)isrecordedbycoronasofaluminousorthopyroxeneplagioclasearoundgarnet.Continueddecompression(6 kbar and 800C)isevidentfromsubsequentlyformedsymplectitesofcordieritelower−aluminaorthopyroxeneandcordieritelower−aluminaorthopyroxenespinel,bothreplacinggarnet.Subsequentregrowthofgarnetandbiotite,mainlyformedattheexpenseofthesymplectiticphases,presumablyreflectsback−reactionswithcrystallizingmeltsduringnear−isobariccoolingtoupperamphibolite−faciesconditions(800 C) is evident from subsequently formed symplectites of cordierite lower-alumina orthopyroxene and cordierite lower-alumina orthopyroxene spinel, both replacing garnet. Subsequent regrowth of garnet and biotite, mainly formed at the expense of the symplectitic phases, presumably reflects back-reactions with crystallizing melts during near-isobaric cooling to upper amphibolite-facies conditions (800C)isevidentfromsubsequentlyformedsymplectitesofcordieritelower−aluminaorthopyroxeneandcordieritelower−aluminaorthopyroxenespinel,bothreplacinggarnet.Subsequentregrowthofgarnetandbiotite,mainlyformedattheexpenseofthesymplectiticphases,presumablyreflectsback−reactionswithcrystallizingmeltsduringnear−isobariccoolingtoupperamphibolite−faciesconditions(660 C and $5 kbar). Rims of low-alumina orthopyroxene around retrograde biotite point to renewed decompression subsequent to cooling. The deduced clockwise retrograde P±T path reflects the thinning and later cooling of former thickened lower crust. Because of the limited structural and geochronological data it remains uncertain whether initial ultrahigh-temperature metamorphism was induced by a collision event or by crustal extension.
The Island Arc, 2000
To investigate the regional thermobaric structure of the diamondiferous Kokchetav ultrahigh-pressure and high-pressure (UHP-HP) massif and adjacent units, eclogite and other metabasites in the Kulet and Saldat-Kol regions, northern Kazakhstan, were examined. The UHP-HP massif is subdivided into four units, bounded by subhorizontal faults. Unit I is situated at the lowest level of the massif and consists of garnet-amphibolite and acidic gneiss with minor pelitic schist and orthogneiss. Unit II, which structurally overlies Unit I, is composed mainly of pelitic schist and gneiss, and whiteschist locally with abundant eclogite blocks. The primary minerals observed in Kulet and Saldat-Kol eclogites are omphacite, sodic augite, garnet, quartz, rutile and minor barroisite, hornblende, zoisite, clinozoisite and phengite. Rare kyanite occurs as inclusions in garnet. Coesite inclusions occur in garnet porphyroblasts in whiteschist from Kulet, which are closely associated with eclogite masses. Unit III consists of alternating orthogneiss and amphibolite with local eclogite masses. The structurally highest unit, Unit IV, is composed of quartzitic schist with minor pelitic, calcareous, and basic schist intercalations. Mineral assemblages and compositions, and occurrences of polymorphs of SiO 2 (quartz or coesite) in metabasites and associated rocks in the Kulet and Saldat-Kol regions indicate that the metamorphic grades correspond to epidote-amphibolite, through high-pressure amphibolite and quartz-eclogite, to coesite-eclogite facies conditions. Based on estimations by several geothermobarometers, eclogite from Unit II yielded the highest peak pressure and temperature conditions in the UHP-HP massif, with metamorphic pressure and temperature decreasing towards the upper and lower structural units. The observed thermobaric structure is subhorizontal. The UHP-HP massif is overlain by a weakly metamorphosed unit to the north and is underlain by the low-pressure Daulet Suite to the south; boundaries are subhorizontal faults. There is a distinct pressure gap across these boundaries. These suggest that the highest grade unit, Unit II, has been selectively extruded from the greatest depths within the UHP-HP unit during the exhumation process, and that all of the UHP-HP unit has been tectonically intruded and juxtaposed into the adjacent lower grade units at shallower depths of about 10 km.
Journal of Metamorphic Geology, 1995
Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P-T record. A high-T decompression from above 10 kbar to 8 kbar around 1100" C has been determined from Mg-rich metapelites (XMg > 0.60) with quartz-cordieriteorthopyroxene-sillimanite and cordierite-orthopyroxene-sapphirine-spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670" C at 8 kbar is evident. These changes are registered by the rim Compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene-quartz-garnet-plagioclase-cordierite assemblage, and are further supported by the garnet + quartz .$ orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650" C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P-T values are obtained from metapelites with a quartz-plagioclase-garnet-sillimanite-cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700" C and the rims 5 kbar, 650" C, suggesting a third decompression. The earliest deformation (Fl) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P-T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking. Two segments of the present P-T path replicate parts of the P-T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats.
High mantle heat flow in a Precambrian granulite province: Evidence from southern India
Journal of Geophysical Research, 2003
1] Twelve new heat flow values determined at nine sites and heat production estimated from radioelemental measurements at 330 sites in the southern granulite province (SGP) bring out contrasting crustal and subcrustal thermal characteristics between the SGP and the adjacent Archaean Dharwar greenstone-granite-gneiss province (DP) in south India. A twolayer granulitic crust of Late Archaean charnockites and gneisses characterizes the northern block (NB), north of the Palghat-Cauvery lineament (PCL). The heat production of the upper, 7-10 km thick, metasomatized granulitic layer ranges between 0.2 and 0.75 mW m À3 (mean 0.5 ± 0.3 (SD) mW m À3 ). This layer overlies radioelement-depleted granulites characterized by very low heat production ranging from 0.14 to 0.2 mW m À3 (mean 0.16 ± 0.07(SD) mW m À3 ). In a large sector of the NB, erosion of the upper metasomatized granulite layer has laid bare the depleted granulitic rocks, which represent one of the lowest heat-producing crustal sections. The mean heat flow in the NB is 36 ± 4 mW m À2 (N = 10). The southern block (SB), south of PCL, in contrast to the NB, comprises complexly interlayered charnockites, gneisses, granites, khondalites, and leptynites, which have variable and much higher levels of heat production ranging between 1.11 and 2.63 mW m À3 . The heat flow in the SB is 47 ± 8 mW m À2 (N = 3). Overall, the range of heat flow values in the SGP is within the range for the DP. Mantle heat flow in the NB, both from the lowest heat-producing sector and other areas, is deduced in the light of heat production and heat flow data, at 23-32 mW m À2 , whose values are distinctly higher than 11-16 mW m À2 for the adjacent DP. The higher mantle heat flow in the NB appears to be a consequence of higher heat production in the subjacent mantle.
Lithos
Garnet-bearing migmatites and associated leucogranites and leucosomes of the Trivandrum Block in the Kerala Khondalite Belt were formed through granulite facies dehydration melting of metasedimentary protoliths. Significant trace element depletions in Cs, Zr, Nb and the compatible transition metal elements V, Cr, Ni, Cu and Zn relative to average post-Archaean shales and model middle and upper crust, recorded in all samples, require their sedimentary protoliths to have been impure sandstones and greywackes, rather than shales. Leucogranites (70-75wt %) and leucosomes (SiO 2 : 68-70wt %), which are uniformly peraluminous and classed as S-type on the basis of their A/NK and ASI relations, form a compositional array that shows strong correlated variations between TiO 2 and Fe 2 O 3 , and TiO 2 or Fe 2 O 3 with Co and Y. These reflect coupled variations in modal garnet and ilmenite and require the presence of up to 15-20 wt% of entrained peritectic garnet in the higher Y and HREE leucosomes. The leucosomes have REE patterns with normalised La/Sm of 10, negative Eu anomalies (Eu* < 0.8) and flat HREE at 6-20 times chrondrite, whereas leucogranites range to much lower HREE (1-5 times chondrite) with higher La/Sm and large positive Eu anomalies (Eu* = 1.4-3.4). Despite broadly similar major element compositions that lie within the granite minimum melt field in terms of felsic components, leucogranite Zr contents are highly variable, ranging down to 4-20 ppm in the lowest HREE and high Eu* cases, resulting in Zr saturation temperatures (544-647ºC) that are lower than any feasible melt. These geochemical features, coupled with covariations between Nb,-Ta and Y-Yb, collectively support petrological and field observations that the leucosomes are mixtures between former melt and entrained peritectic garnet and ilmenite. The leucogranites are the products of melt extraction and migration on metres to several metres lengthscales. Leucogranite Nb-Y, Ta-Yb, Eu* and Sr-Ba relationships demonstrate that their chemical
2008
The Bakhuis belt, one of the major granulite-facies domains in the Guiana Shield, consists of a core of banded rocks of the charnockite suite, metamorphosed and migmatized under granulite-facies conditions. Pelitic gneiss intercalations locally show sapphirine-quartz and orthopyroxene-sillimanite-quartz assemblages, with up to 10% Al2O3 in the orthopyroxene. These assemblages point to ultrahigh-temperature (UHT) metamorphism. P-T conditions are difficult to deduce, because the assemblages contain Fe3+ in sapphirine and sillimanite, and do not contain coeval garnet, but spinel, Feand Fe-Ti oxides instead. P-T conditions for the peak UHT metamorphism are estimated to have been 950 °C and 8.5-9 kb. An assemblage of impure corundum associated with quartz may also have formed during the UHT metamorphism. Unravelling the assemblages indicates a counterclockwise P-T path from an early cordierite-sillimanite assemblage via a subsequent sapphirine-quartz assemblage to the peak metamorphic as...