Kerala Khondalite Belt Research Articles

A petrological and fluid inclusion study of calc-silicate–charnockite associations from southern Kerala, India: implications for CO2 influx

Recent discovery of wollastonite-bearing calc-silicate assemblages adjacent to gneiss–charnockite horizons in the supracrustal terrain of the Kerala Khondalite Belt, southern India, provides an opportunity to evaluate the carbonic fluid infiltration model proposed for charnockite formation. Petrological and fluid inclusion studies across these horizons in three representative localities are presented in this study. The calc-silicate assemblages define peak metamorphic conditions of ∼800°C at 5 kbar and define a low aCO2. Adjacent charnockite assemblages developed through dehydration involving the breakdown of garnet, biotite and quartz to produce orthopyroxene under low aH2O conditions. Retrograde reactions preserved in the calc-silicate rocks, such as scapolite–quartz symplectites, and the partial breakdown of wollastonite previously has been attributed to a near isothermal decompression during which infiltration of CO2-rich fluids occurred. Fluid inclusion studies indicate that the earliest generation of fluids preserved in the calc-silicate assemblages are aqueous (with salinity ∼8 wt% NaCl equivalent), consistent with mineral phase equilibria defining low aCO2. The estimation of NaCl content in brines coexisting with scapolite, based on the Cl content of the scapolite, indicates the presence of up to 20 wt % NaCl during the formation of scapolite consistent with the saline primary fluid inclusions. Primary carbonic inclusions occur within the retrogressed calcite+quartz assemblage after wollastonite, and are considered to represent the post-peak metamorphic carbonic fluid infiltration event, synchronous with the development of charnockites in the adjacent gneisses. These inclusions have identical characteristics to those in the charnockites. We envisage that the Kerala Khondalite Belt fluid regime was largely internally buffered during the prograde path, and that CO2 infiltration post-dated peak metamorphism. Influx of CO2 was mostly structurally controlled, and occurred along a near-isothermal uplift path. Graphite-bearing pegmatitic dykes with abundant CO2-rich inclusions in these localities attest to the transfer of carbonic fluids through magmatic conduits.

Granulite Facies Spinel-Cordierite Assemblages from the Kerala Khondalite Belt, Southern India

Granulite facies metapelites at Koliacode in the Kerala Khondalite Belt show compositional bands of cordierite, spinel, sillimanite and biotite alternating with quartz-feldspar rich bands. Reaction textures involving cordierite and garnet show evidence for decompression. Mineral phase equilibria computations provide temperatures of ca. 700-750C and pressures of 4.5-5.0 kbar. Spinel in these rocks is remarkably enriched in Zn (ZnO up to 9.4 wt. %). The metamorphic P-T conditions and mineral reactions indicate that the terrain underwent rapid and near isothermal decompression.

PbPb dating of apatite by a stepwise dissolution technique

Apatites from different geologic settings have been dated by a new sequential dissolution method. The procedure is based on stepwise exposure of apatite to weak-intermediate acids. A pegmatite sample from the Kerala Khondalite Belt in southern India yielded an age of 509 ± 25 Ma. Apatites from an enderbite sample from the Northern Marginal Zone of the Limpopo Belt in southern Africa yielded an age of 1970 ± 26 Ma which is in agreement with the high-grade metamorphism at around 2.0 Ga, reported from the Triangle Shear Zone and amphibolite-facies metamorphism in the Transition Zone. Apatites from an amphibolite-facies gneiss from the Transition Zone south of the NMZ yielded an age of 1878 ± 18 Ma. This age is somewhat younger than a sphene age on the same rock of 1971 ± 17 Ma and is believed to reflect the time of retrograde metamorphism in the presence of a fluid phase. The sequential dissolution method is a new tool that allows dating of apatites which have relatively unradiogenic Pb ratios because the spread can be significantly increased and no correction for common lead is needed. This study further demonstrates that apatites from magmatic and high-grade metamorphic rocks yield ages very close to those of monazite, zircon or garnet, unless the rock subsequently interacted with a fluid phase.

Ultra‐high‐temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the Palni Hill Ranges, southern India

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.

Shear-zone hosted graphite in southern Kerala, India: implications for CO 2 infiltration

Highly crystalline, coarse graphite flakes fill mesosopic shear zones and pegmatitic veins which cut across the regional penetrative fabric of granulite-facies supracrustals in the Kerala Khondalite Belt (KKB) of southern India. Carbon stable isotope analysis yield δ 13C values of −8.2 to −12.4% for the shear-hosted graphites and −10.1 to −15.1% for the pegmatite-hosted graphites. These values are distinctly from the lighted carbon enriched values obtained from the disseminated graphite in the host gneisses (up to −32.1%). From the mode of occurrence and carbon isotopic composition, the graphite disseminations in the KKB supracrustals can be linked to the process of graphitisation of organic material trapped within sediments during regional high-grade metamorphism. A mechanism for the formation of shear- and vein-hosted graphites is discussed. It involves the influx of CO 2-rich fluids from a possible juvenile igneous source along structural pathways and precipitation of graphite under low oxygen fugacity conditions. Where the fluid pathways intersect impervious lithologies, fluid ponding and copious precipitation of graphite can be noticed at the lithologic contact. The graphite precipitation is considered to be synchronous with, and complementary to, the formation of arrested “incipient charnockites” elsewhere in the KKB, where upper amphibolite-facies gneisses transform to anhydrous granulite-facies assemblages through the influx of CO 2-rich fluids.

Calc-silicate assemblages from the Kerala Khondalite Belt, southern India: implications for pressure-temperature-fluid histories

This paper reports several new localities of wollastonite- and scapolite-bearing calc-silicate assemblages from the granulite-facies supracrustal Kerala Khondalite Belt (KKB), southern India. Based on mineralogy, these calc-silicate rocks are classified into four types: Type I, lacking wollastonite and grossular; Type II, wollastonite-bearing but grossular-absent; Type III, wollastonite- and 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 calc-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 calc-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Al 2O 3SiO 2CO 2 system are used to deduce the pressure-temperature-fluid evolution of the calc-silicate rocks. The Type II assemblages provide CO 2 activity estimates of > 0.5, with a peak metamorphic temperature of about 790°C. Initial cooling followed by later CO 2 influx can be deduced from reaction modelling in these calc-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 2 influx. Overall, the calc-silicate rocks of the KKB define peak metamorphic temperatures in the range of 790–850°C, with an internally buffered fluid composition during the peak conditions. Initial cooling was followed by localized carbonic fluid influx that also post-dated decompression deduced from other rock types in the KKB.

Neodymium isotope constraints on the tectonic evolution of East Gondwana

East Gondwana incorporates a collage of polymetamorphic terrains with long-lived tectonic histories from the Early Archaean to the Neoproterozoic. The oldest cratonic areas have been identified in South India (north of the Palghat-Cauvery shear zone) and East Antarctica (the Napier Complex). These terrains are remnants of an East Gondwana craton that underwent initial crustal growth during the Early Archaean and granulite-facies metamorphism at ∼2.5 Ga. Both were virtually unaffected by the Pan-African orogeny (1.1-0.5 Ga). In contrast, Proterozoic terrains were subject to high-grade metamorphism during the Pan-African event. On the basis of published Nd model ages, a direct correlation can be made between southern Madagascar (south of the Ranotsara shear zone), southern India (the Madurai Block and Kerala Khondalite Belt) and the Highland/Southwestern Complex of Sri Lanka, which comprise a Later Archaean-Palaeoproterozoic (3.2-2.0 Ga) mobile belt that may extend eastwards into East Antarctica. The youngest period of crustal growth in East Gondwana has been recognised at 1.5-0.8 Ga from isotopic studies of the Mozambique Belt of East Africa, the Vijayan Complex of Sri Lanka and the Yamato-Belgica Complex/Sør Rondane Mountains of East Antarctica. Small slivers of terrain of intermediate age (1.9-1.2 Ga) have been recognised in South India (Achankovil metasediments) and Sri Lanka (Wanni Complex) that may represent mixed-age contributions to clastic sedimentary basins.

A Pan-African thermal event in southern India

UPb zircon data from five igneous suites confirm previous studies that demonstrated widespread Pan-African magmatism in the Granulite Terrain of southern India. Ages determined here are ∼560 Ma for the Peralimala Granite and ∼555 Ma for the Kalpatta Granite, both north of the Palghat-Cauvery lineament, and ∼585 Ma for a charnockite in the Cardamom massif south of the lineament. Zircon from a pegmatite in the Kerala khondalite belt at Melankode yields an age of 512 Ma. Resetting of zircons in the 2500-Ma Arsikere Granite of the western Dharwar craton probably occurred at ∼450 Ma. These ages and the concentration of Pan-African granitic magmatism around the Indian portion of a broad region of granulite-facies metamorphism in East Gondwana demostrates generation of a restricted area of high temperature either above a rising plume or a zone of rifting. Mantle-derived fluids continued to move upward through the crust of southern India for at least 100 m.y. after the peak of magmatism, and the entire region was still cooling at 400 Ma.

Dehydration—Melting Phenomena in Leptynitic Gneisses and the Generation of Leucogranites: a Case Study from the Kerala Khondalite Belt, Southern India

Dehydration—Melting Phenomena in Leptynitic Gneisses and the Generation of Leucogranites: a Case Study from the Kerala Khondalite Belt, Southern India

Nd isotopic evidence for the position of southernmost Indian terranes within East Gondwana

Sm-Nd model ages of orthopyroxene-bearing massif charnockites from the Cardamom Hills Massif and adjoining supracrustal rocks from the Kerala Khondalite Belt in southernmost India are used to infer some of the relationships within these rocks and between them and neighboring areas. Most of these rocks have model ages of 2.1–2.8 Ga with most charnockites in the range 2.2–2.6 Ga. Thus, 3.0–3.4 Ga Archean rocks to their north did not contribute material to either suite and the two suites may have been juxtaposed after formation of the supracrustal rocks. The similarity of Sm-Nd isotope systems in the two units studied here supports an argument that the massif charnockites were the primary sole source of the detritus incorporated into the supracrustal rocks. A cordierite gneiss, representative of a relatively minor lithology in the supracrustal belt, has a model age of ∼ 1.3 Ga. The protolith of this gneiss not only formed from much younger material than the rest of the belt but also formed significantly after the other metasedimentary rocks. The source material of the gneiss protolith may have been located in the Wanni and Vijayan Complexes of Sri Lanka. The overlap of the model ages of rocks in this area and those in the Highland Complex of Sri Lanka supports the notion that these two sets of rocks were joined to each other in Gondwana. They belong to a belt that ran from Antarctica through Sri Lanka and India into Madagascar. This belt was involved in Pan-African tectono-metamorphism, as reflected in the ∼ 550 Ma age of the last, granulite-forming, event throughout the belt.

The Leptynitic Gneisses of the Kerala Khondalite Belt, Southern India: Phenomena and Mechanism of Dehydration/Melting at Granulite Facies Conditions

Within the late-Proterozoic Kerala Khondalite Belt in southern India, the widespread biotitebearing granitic gneisses have been subjected to insitu dehydration and melting at upper amphibolite to granulite facies conditions (Srikantappa et al. 1985, Chacko et al. 1987). Segregation of melts occurred in the centimetre to metre scale as well as over outcrop distances and gave rise to a complete sequence of garnet-bearing leueosome types. This ensemble (leptynitic gneiss) was formed under essentially static conditions and fortunately has not been overprinted by subsequent ductile shear deformation. Several active quarries have been selected to study three major aspects of crustal anatexis: (1) the generation of granitic melts at conditions of a fluid-absent granulite facies regime, (2) the mechanisms of melt segregation and melt migration and (3) the geochemical affinity of these melts with granites from distinct geotectonic settings. Here we report the field relations, describe the phenomena of dehydration/melting and give first results of the petrological and geochemical investigation. coarse-grained graphic intergrowths of plagioclase and alkalifeldspar. Plagioclase as single matrix phase is rare. The graphic intergrowths often surround the mesoperthite grains from which they may have been formed either through fluid-enhanced recrystallization or crystallization of an intergrannlar melt at subsolvus conditions. The in-situ leucosomes (L1) and the gneiss domains have similar granodioritie compositions, although some complementary features are obvious: significantly higher concentrations of FeO, MgO, Y and the HREE characterize the leucosomes, whereas the gneiss domains are moderately enriched in CaO, NazO, K20, Rb, Sr and Ba. These chemical characteristics document small-scale element transfer between the two domains, which was mainly controlled by the continuous biotite breakdown in the gneiss domains and the simultaneous growth of garnet in the centre of the leucosomes. It is suggested that the leucosomes (L1) were formed through flnidabsent biotite dehydration-melting according to the model reaction

A Carbon Isotope Study of Graphites from the Kerala Khondalite Belt, Southern India: Evidence for $CO_{2}$ Infiltration in Granulites

Field relations of arrested charnockite patches, felsic pegmatite dikes, and coarse cordierite + graphite veins in the granulite facies supracmstal belt of southern Kerala suggest paleo-fluid channels. We have investigated the stable isotopic composition of graphites associated with these structures, as well as those in association with charnockites adjacent to calcsilicate horizons. The remarkably high values displayed by graphites from the structurally controlled zones ( - 8.6 to - ), together with the narrow spread in individual localities (), correlate with precipitation from isotopically homogeneous -rich fluids that either infiltrated along structural pathways (faults/ shears) or were transferred through magmatic conduits. Graphites from charnockite-calcsilicate associations show extreme enrichment ( up to -4.8%). Changing fluid regimes are detected from up to variation in among graphites sampled adjacent to calcsilicate horizons and within single crystals of coarse graphite associated with cordierite-rich veins. Steep isotopic gradients across fluid channels, and the preservation of biogenic carbon signatures in disseminated graphite flakes within adjacent metapelites (), suggest that influx was strongly channelized. Our study provides evidence for graphite precipitation in reduced crustal lithologies infiltrated by -rich fluids and illustrates that graphite can be used as a potential monitor to detect infiltration in granulites.

Geochemistry of high-grade supracrustal rocks from the Kerala Khondalite Belt and adjacent massif charnockites, South India

The Kerala Khondalite Belt (KKB) comprises a suite of supracrustal rocks of probable Proterozoic age that was metamorphosed to the granulite facies. The KKB consists of an interlayered sequence of: (1) garnet-biotite ± orthopyroxene ± graphite gneisses; (2) khondalites (graphite-garnet-biotite-sillimanite ± cordierite gneisses); and (3) cordierite gneisses (garnet-biotite-cordierite ± orthopyroxene), as well as less abundant mafic granulites, calc-silicates and quartzites. Petrologic and geochemical data suggest that these rocks were originally arkosic and argillaceous sediments (possibly accompanied by felsic volcanic rocks), along with minor intercalations of basalts, carbonates and quartz-rich sandstones. The REE patterns of the metasediments are typically LREE enriched with large negative Eu anomalies, indicating a continental provenance. The major- and trace-element compositions of some of the arkosic rocks of the KKB resemble those of the massif charnockites that bound the KKB to the north which suggests that the massif charnockites may have been the source of some of the KKB sediments. Modern-day analogues of the pelite-arkose lithologic association observed in the KKB occur in intracratonic rifts and rifted continental margins. This association is clearly distinct from the graywacke-mafic volcanic association found in the southern Indian greenstone belts and many other lower-grade Archean sedimentary sequences. The KKB supracrustals show both lithologic and chemical similarities to many Proterozoic supracrustal successions.

Metamorphic P-T Conditions of the Kerala (South India) Khondalite Belt, A Granulite Facies Supracrustal Terrain: A Discussion

Metamorphic P-T Conditions of the Kerala (South India) Khondalite Belt, A Granulite Facies Supracrustal Terrain: A Discussion

Metamorphic P-T Conditions of the Kerala (South India) Khondalite Belt, a Granulite Facies Supracrustal Terrain

The Khondalite Belt of southernmost India is an immense section of granulite facies metasediments of probable early Proterozoic age. Dominant rock types are migmatized khondalites (garnet-sillimanite $$\pm$$ cordierite metapelites), acid biotite-garnet graphitic gneisses and charnockites (quartzofeldspathic gneisses with orthopyroxene). Throughout much of southern Kerala, the biotite gneisses are in a state of arrested conversion to charnockite. Partially decrepitated $$CO_{2}$$ inclusions in quartz at the Ponmudi mixed biotite gneiss-charnockite quarry have significant $$CH_{4}$$ and/or $$N_{2}$$ and are texturally late. The probable sequence of events in metasediments was (1) migmatization, (2) charnockite formation, (3) symplectitic cordierite formation, and (4) late $$CO_{2}$$ entrapment. Phase equilibrium diagrams of anatexis in aluminous systems and Fe-Mg exchange between garnet and biotite indicate metamorphic temperatures of 650-850°C for the khondalites. Temperatures based on garnet-orthopyroxene...

Adirondack gold and platinum sands

Water (H2O) has been identified during Raman analyses at ~ 150–200 °C of fluid inclusions from granulites that were previously thought to contain pure CO2. At room temperature, H2O in a CO2-rich fluid inclusion forms an optically unresolvable liquid wetting the inclusion walls. During heating, the H2O phase evaporates into the CO2 phase to produce a homogeneous H2O–CO2 fluid throughout the inclusion volume, and the H2O Raman peak at ~ 3640 cm− 1 is clearly resolvable at elevated temperatures. To test and confirm the method, synthetic H2O–CO2 FI of known composition (87.5 mol% and 95 mol% CO2) were also analyzed.Previously studied granulite samples from the Adirondack Mountains, USA, and from the Kerala Khondalite Belt, Sittampundi Complex and Sevitturangampatti, India were tested for the presence of H2O in CO2-rich fluid inclusions. Traces of H2O were found in CO2-rich fluid inclusions in quartz crystals from the Adirondacks and from the Kerala Khondalite Belt. H2O was not detected in CO2-rich secondary fluid inclusions in garnet from the Sittampundi Complex and from Sevitturangampatti. Rather, hydrogarnet was detected along the walls of inclusions in garnet, as evidenced by a peak at ~ 3661 cm− 1. The hydrogarnet is interpreted to be a “step-daughter” phase that formed by reaction of the H2O in the fluid inclusion with the garnet host during retrogression. These observations confirm the presence of an H2O-bearing, CO2-rich fluid at some time during granulite petrogenesis, but do not resolve the question of whether these CO2-rich fluids were trapped at peak metamorphic conditions or during retrogression, or whether the fluid inclusions have reequilibrated following entrapment.