Trivandrum Block Research Articles

Decoding evolutionary history of provenance from beach placer monazites: A case study from Kanyakumari coast, southwest India

Mineral chemical and geochronological studies of beach placer monazites from Kanyakumari coast, SW India were carried out to understand growth history of monazite and its bearing on tectono-thermal evolution of the source rock. In chemically zoned monazites lowest backscattered electron (BSE) response domains (dark gray) comprising the thick core region are mantled by highest BSE response rim domains (light gray region). Dark gray BSE domains are enriched in LREEs (La and Ce), U and Y, and depleted in Th and Pb compared to the light gray BSE domains. Chemical variability between these two domains can be linked with dominantly huttonitic substitution. Two U–Th–total Pb chemical age clusters between 555–578Ma and 508–536Ma were obtained respectively for low Th/U monazite cores and high Th/U monazite rims.147Sm/144Nd ratios in the analyzed monazites vary from 0.0733 to 0.1039 with an average value of 0.0902. High negative εNd (t=0) values in the range of −26.5 to −30.8 indicate derivation of monazites from light LREE enriched igneous and metamorphic rocks. TCHUR and TDM model ages vary from 1687Ma to 2364Ma and 2035Ma to 2702Ma respectively. Average TDM age of 2375 (+96/−44) Ma for placer monazites fits with ~2.4Ga crustal accretionary episode in Southern Granulite Terrane (SGT). EPMA Sm–Nd ratios of beach placer monazites (~0.153) are similar with the monazites in granites and granodiorites (0.150) of Nagercoil (NG) and Trivandrum Block (TB) of SGT. A comparison with available Th–U–total Pb EPMA monazite ages from various tectonic units within SGT suggests that growth history and crystallization age of monazites also correlate well with the Pan-African granulite facies metamorphism (570Ma) and post-peak evolution (535Ma) of NG and TB.The results obtained in this study augment the growing evidences that beach placer monazites can be used as a proxy for provenance study. A corollary of the study further confirms similar geological history of NG and TB since 2.1Ga.

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.

Unravelling the complexities in high-grade rocks using multiple techniques: the Achankovil Zone of southern India

The Achankovil Zone of southern India forms a distinct isotopic and structural boundary separating the Madurai Block to the north from the Trivandrum Block to the south. We combine isotopic and trace element geochemistry of major and accessory phases with phase equilibria modelling to provide quantitative constraints on the timing and conditions of peak metamorphism and the nature of the protoliths within the Achankovil Zone. The results suggest a clockwise pressure–temperature path with peak metamorphic temperatures of up to 950 °C at pressures of around 0.7 GPa followed by high-temperature decompression. The metamorphic peak occurred at 545–512 Ma. U–Pb and Hf isotopic analysis of detrital zircon shows that the rocks have a strong affinity with the southern part of the Madurai Block. The Achankovil Zone is interpreted as the reworked southern margin of the Madurai Block, which was metamorphosed during the final stages of the assembly of Gondwana.

Post-peak, fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block, southern India

The quarry at Kottavattom in the Trivandrum Block of southern India contains spectacular examples of fluid-assisted alteration of high-grade metamorphic rocks. Garnet-biotite gneiss has undergone a change in mineral assemblage to form submetre scale orthopyroxene-bearing patches, later retrogressed to form an amphibole-bearing lithology. These patches, often referred to as arrested or incipient charnockite, crosscut the original metamorphic foliation and are typically attributed to passage of a low aH2O fluid through the rock. Whilst this conversion is recognised as a late stage process, little detailed chronological work exists to link it temporally to metamorphism in the region. Zircon and monazite analysed from Kottavattom not only record metamorphism in the Trivandrum Block but also show internal, lobate textures crosscutting the original zoning, consistent with fluid-aided coupled dissolution-reprecipitation during formation of the orthopyroxene-bearing patches. High-grade metamorphism at the quarry occurred between the formation of metamorphic monazite at ~585 Ma and the growth of metamorphic zircon at ~523 Ma. The fluid-assisted alteration of the garnet-biotite gneiss is poorly recorded by altered zircon with only minimal resetting of the U–Pb system, whereas monazite has in some cases undergone complete U–Pb resetting and records an age for fluid infiltration at ~495 Ma. The fluid event therefore places the formation of the altered patches at least 25 Myr after the zircon crystallisation in the garnet-biotite gneiss. The most likely fluid composition causing the modification and U–Pb resetting of zircon and monazite is locally derived hypersaline brine.

Three‐dimensional lithospheric electrical structure of Southern Granulite Terrain, India and its tectonic implications

AbstractThe crustal as well as the upper mantle lithospheric electrical structure of the Southern Granulite Terrain (SGT) is evaluated, using the magnetotelluric (MT) data from two parallel traverses: one is an ~ 500 km long N‐S trending traverse across SGT and another a 200 km long traverse. Data space Occam 3‐D inversion was used to invert the MT data. The electrical characterization of lithospheric structure in SGT shows basically a highly resistive (several thousands of Ohm meters) upper crustal layer overlying a moderately resistive (a few hundred Ohm meters) lower crustal layer which in turn is underlain by the upper mantle lithosphere whose resistivity shows significant changes along the traverse. The highly resistive upper crustal layer is interspersed with four major conductive features with three of them cutting across the crustal column, bringing out a well‐defined crustal block structure in SGT with individual highly resistive blocks showing correspondence to the geologically demarcated Salem, Madurai, and Trivandrum blocks. The 3‐D model also brought out a well‐defined major crustal conductor located in the northern half of the Madurai block. The electrical characteristics of this south dipping conductor and its close spatial correlation with two of the major structural elements, viz., Karur‐Oddanchatram‐Kodaikanal Shear Zone and Karur‐Kamban‐Painavu‐Trichur Shear Zone, suggest that this conductive feature is closely linked to the subduction‐collision tectonic processes in the SGT, and it is inferred that the Archean Dharwar craton/neoproterozoic SGT terrain boundary lies south of the Palghat‐Cauvery shear zone. The results also showed that the Achankovil shear zone is characterized by a well‐defined north dipping conductive feature. The resistive block adjoining this conductor on the southern side, representing the Trivandrum block, is shown to be downthrown along this north dipping crustal conductor relative to the Madurai block, suggesting a northward movement of Trivandrum block colliding against the Madurai block. The lithospheric upper mantle electrical structure of the SGT up to a depth of 100 km may be broadly divided into two distinctly different segments, viz., northern and southern segments. The northern lithospheric segment, over a major part, is characterized by a thick resistive upper mantle, while the southern one is characterized by a dominantly conductive medium suggesting a relatively thinned lithosphere in the southern segment.

Geochemistry and neodymium model ages of Precambrian charnockites, Southern Granulite Terrain, India: Constraints on terrain assembly

The Southern Granulite Terrain (SGT) of southern India comprises a collage of Archean and Neoproterozoic high-grade metamorphic terrains, where the regional tectonic and high-grade metamorphic events culminated at ca.2.52Ga in the northern granulite blocks and ca.0.55Ga in the Neoproterozoic Madurai and Trivandrum Blocks. These domains were presumed to be separated either by the crustal-scale Cauvery shear zone system (CSZ) or the less known Karur–Kambam–Painavu–Trichur shear zone (KKPT). The Neoproterozoic granulite domains comprise mainly the Madurai Block (MB) to the north and Trivandrum Block (TB) to the south separated by the Achenkovil shear zone (ASZ) system. The SGT has been central to several studies focusing on the amalgamation of Gondwana supercontinent. A favored view has been that the Neoproterozoic terrains of the SGT represent segments of orogen(s) that accreted to an Archean continental margin during the early Cambrian although whether the tectonic regime was collisional, accretionary or intracratonic remains enigmatic. This article presents a large data set of major, trace element, Rb–Sr and Sm–Nd isotopic compositions focusing on Sm–Nd model ages (TDM) signifying the crustal residence age of charnockitic orthogneisses from both Archean and Neoproterozoic domains of the SGT, especially from the data deficient MB and TB domains. We present a regional Sm–Nd model age map for the SGT that delineates four distinct isotopic provinces with ages in the range of ∼3.5–3.2Ga, 3.2–2.8Ga, 2.8–2.0Ga and 2.0–1.2Ga. Clearly, the KKPT shear zone separates two isotopic provinces, extending the Archean terranes further south across the CSZ. The Archean and Neoproterozoic charnockites show distinct compositional characteristics indicating contrasting conditions of magma genesis and tectonic setting. Most importantly, an overwhelming majority of the Neoproterozoic charnockite gneisses show Sm–Nd model ages significantly older than their crystallization age constrained by zircon dating. The charnockite gneisses bear a strong affinity to calc-alkaline magmas and compositions consistent with mixing of Neoproterozoic arc magmas and older crustal components prompting a close analogy with magmatism at convergent margin-collisional tectonic settings.

SHRIMP U–Pb age constraints on magmatism and high-grade metamorphism in the Salem Block, southern India

In this paper, we present Sensitive High Resolution Ion Microprobe (SHRIMP) U and Pb isotope analyses of zircon from a charnockite and a charnockite-hosted leucosome in order to determine the age of magmatism and anatexis related to high-grade metamorphism immediately to the north of the Palghat Cauvery Shear System (PCSS), a proposed Neoproterozoic terrane boundary within Southern India. Weakly luminescent, oscillatory-zoned cores in I06–128 and analyses with high Th/U ratios in I06–129 yield weighted mean 207Pb– 206Pb ages of 2538 ± 6 Ma and 2529 ± 7 Ma. These two ages are statistically indistinguishable and are interpreted to reflect the crystallisation age of the original magmatic protolith to the charnockite. Low Th, Th/U ratio and brightly luminescent overgrowths and complete zircon grains in I06–128 and I06–129 give weighted mean 207Pb– 206Pb ages of 2473 ± 8 Ma and 2482 ± 15 Ma respectively. The two ages are statistically distinct and are interpreted to constrain the timing of post-crystallisation high-grade metamorphism and partial melting of the magmatic rocks that crystallised at ~ 2530 Ma. The new ages of the charnockites are consistent with the suggestion that this activity is related to accretionary processes on the margin of the Dharwar craton and may be related to a second stage of convergent tectonics and collision on the margin of Kenorland. There is no evidence of a pervasive Neoproterozoic high-grade metamorphic event associated with the amalgamation of Gondwana recorded in these rocks. However, the possibility that deformation was localised along discrete shear zones at this time cannot be discounted. The data from this study add weight to the hypothesis that the PCSS represents a Neoproterozoic suture zone along which the Dharwar Craton and the microcontinent Azania, with its constituent Madurai and Trivandrum Blocks in the Southern Granulite terrane, collided at ca. 530 Ma Ga during the closure of the Mozambique ocean.

Age and sedimentary provenance of the Southern Granulites, South India: U-Th-Pb SHRIMP secondary ion mass spectrometry

Southern India lies at a junction in the Gondwana-forming orogenic belts, between the East African Orogen and the Kuunga Orogen. It contains voluminous high-grade metasedimentary gneisses that make up an important component of the record of collision and amalgamation of Gondwana. Here we present U-Pb Secondary Ion Mass Spectrometry (SIMS) isotopic data from detrital zircon cores from throughout southern India that demonstrate dominant Neoarchaean to Palaeoproterozoic age components that are incompatible with the known ages of potential southern and central Indian source regions. The original sediments to the Trivandrum Block gneisses were deposited between ∼1900 and ∼515 Ma, whereas a sample from the Achancovil Unit, and possible also a sample from the Madurai Block, were deposited in Neoproterozoic times. We speculate that these rocks broadly correlate with southern and western Malagasy metasedimentary rocks (including the Itremo and Molo Groups) and formed an extensive basin (or basins) that lay on the west side (present orientation) of the Neoproterozoic continent Azania. In addition, metamorphic zircon from four samples yielded an age of 513 ± 6 Ma that is interpreted as dating high-grade metamorphism throughout much of the Southern Granulite Terrane.

Structure and tectonics of the Achankovil Shear Zone, southern India

An integrated approach to resolve the kinematics of the controversial Achankovil Shear Zone (AKSZ) has been attempted involving remote sensing data, shaded relief topo-maps, ground details of lithology and mesoscopic structures. An excellent correlation of structural trends exists on all scales of observation. The AKSZ is distinctly defined by NW–SE trending foliation fabrics with steep dips to southwest. The adjacent Madurai block and Trivandrum block show contrasting lithological and structural characteristics as shown in structural cross-sections. The mesoscopic structural studies reveal the presence of sub-horizontal stretching lineations, asymmetric structures like S–C′ fabrics, porphyroclasts, ‘S’ shaped folds and shear bands confirming the strike-slip component of shear along AKSZ. The deformation undergone by the AKSZ could be described in terms of an initial dextral deformation — D 1, reactivated and superimposed by sinistral kinematics — D 2, which is also supported by megascopic structural interpretation of remote sensing data. The megascopic structural interpretation of AKSZ displays en-echelon pattern of lineaments with right overstepping arrangement, which can be interpreted as an evidence of the latest sinistral transpressional deformation.

Kaolin deposits at Melthonnakkal and Pallipuram within Trivandrum block, southern India

The kaolin deposits at Melthonnakkal and Pallipuram mines form part of the Warkalli Formation belonging to the Tertiary sequence in southern Kerala and occur at the boundary between the Tertiary sequence and Precambrian granulite facies metapelites (khondalites). The sedimentary clays are composed mainly of kaolinite, quartz and gibbsite. XRD and SEM studies have revealed that kaolinite is well-crystallized variety and the platy crystals are scarcely broken in the sedimentary clays. These sedimentary kaolins are considered to have been formed by intense tropical weathering of the khondalites, and subsequently transported and deposited with high organic input into lakes near the weathering crust over the basement rock. Besides, the surficial parts of the sedimentary deposits are extensively lateritized with the formation of goethite and hematite by Quaternary tropical weathering processes.

Geochronology of the khondalite belt of Trivandrum Block, Southern India: Electron probe ages and implications for Gondwana tectonics

In a comprehensive U–Pb electron microprobe study of zircon and monazite from the khondalite belt of Trivandrum Block in southern Kerala, we present age data on five key metapelite locations (Nedumpara, Oottukuzhi, Kulappara, Poolanthara and Paranthal). The rocks here, characterized by the assemblage of garnet–sillimanite–spinel–cordierite–biotite–K–feldsapr–plagiocalse–quartz–graphite, have been subjected to granulite facies metamorphism under extreme thermal conditions as indicated by the stability of spinel + quartz and the presence of mesoperthites that equilibrated at ultrahigh-temperature (ca. 1000 °C) conditions. The oldest spot age of 3534 Ma comes from the core of a detrital zircon at Nedumpara and is by far the oldest age reported from this supracrustal belt. Regression of age data from several spot analyses in single zircons shows “isochrons” ranging from 3193 ± 72 to 2148 ± 94 Ma, indicating heterogeneous population of zircons derived from multiple provenance. However, majority of zircons from the various localities shows Neoproterozoic apparent ages with sharply defined peaks in individual localities, ranging between 644–746 Ma. The youngest zircon age of 483 Ma was obtained from the outermost rim of a grain that incorporates a relict core displaying ages in the range of 2061–2543 Ma. The cores of monazites also show apparent older ages of Palaeo-Mesoproterozoic range, which are mantled by late Neoproterozoic/Cambrian rims. The oldest monazite core has an apparent age of 2057 Ma. Extensive growth of new monazite during latest Neoproterozoic to Cambrian–Ordovician times is also displayed by grain cores with apparent ages up to 622 Ma. The homogeneous core of a sub-rounded monazite grain yielded a maximum age of 569 Ma, markedly younger than the 610 Ma age reported in a previous study from homogenous and rounded zircon core from a metapelite in Trivandrum Block. These younger ages from abraded grains that have undergone fluvial transport are interpreted to indicate that deposition within the khondalite belt was as young as, or later than, this range. Probability density plots indicate that majority of the monazite grain population belong to Late Proterozoic/Cambrian age (ca. 560–520 Ma) with major peaks defining sharp spikes in individual localities. The age data presented in this study indicate that the metasediments of the Trivandrum Block sourced from Archaean and Paleo-Mesoproterozoic crustal fragments that were probably assembled in older supercontinents like Ur and Columbia. The largest age population of zircons belong to the Neoproterozoic, and are obviously related to orogenies during the pre-assembly phase of Gondwana, possibly from terrains belonging to the East African Orogen. Several prominent age spikes within the broad late Neoproterozoic–Cambrian age range displayed by monazites denote the dynamic conditions and extreme thermal perturbations attending the birth of Gondwana. Our study further establishes the coherent link between India and Madagascar within the East Gondwana ensemble prior to the final assembly of the Gondwana supercontinent.

Depositional constraints and age of metamorphism in southern India: U–Pb chemical (EMPA) and isotopic (SIMS) ages from the Trivandrum Block

We report U–Pb electron microprobe (zircon and monazite) and Secondary Ion Mass Spectrometry (SIMS) U–Pb (zircon) ages from a granulite-facies metapelite and a garnet–biotite gniess from Chittikara, a classic locality within the Trivandrum Block of southern India. The majority of the electron-microprobe data on zircons from the metapelite define apparent ages between 1500 and 2500 Ma with a prominent peak at 2109±22 Ma, although some of the cores are as old as 3070 Ma. Zircon grains with multiple age zoning are also detected with 2500–3700 Ma cores, 1380–1520 mantles and 530–600 Ma outer rims. Some homogeneous and rounded zircon cores yielded late Neoproterozoic ages that suggest that deposition within the Trivandrum Block belt was younger than 610 Ma. The outermost rims of these grains are characterized by early Cambrian ages suggesting metamorphic overgrowth at this time. The apparent ages of monazite grains from this locality reveal multiple provenance and polyphase metamorphic history, similar to those of the zircons. In a typical case, Palaeoproterozoic cores (1759–1967 Ma) are enveloped by late Neoproterozoic rims (562–563 Ma), which in turn are mantled by an outermost thin Cambrian rim (∼515 Ma). PbO v. ThO*2 plots for monazites define broad isochrons, with cores indicating a rather imprecise age of 1913±260 Ma (MSWD=0.80) and late Neoproterozoic/Cambrian cores as well as thin rims yielding a well-defined isochron with an age of 557±19 Ma (MSWD=0.82). SIMS U–Pb isotopic data on zircons from the garnet–biotite gneiss yield a combined core/rim imprecise discordia line between 2106±37 Ma and 524±150 Ma. The data indicate Palaeoproterozoic zircon formation with later partial or non-uniform Pb loss during the late Neoproterozoic/Cambrian tectonothermal event. The combined electron probe and SIMS data from the metapelite and garnet–biotite gneiss at Chittikara indicate that the older zircons preserved in the finer-grained metapelite protolith have heterogeneous detrital sources, whereas the more arenaceous protolith of the garnet–biotite gniess was sourced from a single-aged terrane. Our data suggest that the metasedimentary belts in southern India may have formed part of an extensive late Neoproterozoic sedimentary basin during the final amalgamation of the Gondwana supercontinent.

The igneous charnockite?high-K alkali-calcic I-type granite?incipient charnockite association in Trivandrum Block, southern India

The Pan-African (640 Ma) Chengannoor granite intrudes the NW margin of the Neoproterozoic high-grade metamorphic terrain of the Trivandrum Block (TB), southern India, and is spatially associated with the Cardamom hills igneous charnockite massif (CM). Geochemical features characterize the Chengannoor granite as high-K alkali-calcic I-type granite. Within the constraints imposed by the high temperature, anhydrous, K-rich nature of the magmas, comparison with recent experimental studies on various granitoid source compositions, and trace- and rare-earth-element modelling, the distinctive features of the Chengannoor granite reflect a source rock of igneous charnockitic nature. A petrogenetic model is proposed whereby there was a period of basaltic underplating; the partial melting of this basaltic lower crust formed the CM charnockites. The Chengannoor granite was produced by the partial melting of the charnoenderbites from the CM, with subsequent fractionation dominated by feldspars. In a regional context, the Chengannoor I-type granite is considered as a possible heat source for the near-UHT nature of metamorphism in the northern part of the TB. This is different from previous studies, which favoured CM charnockite as the major heat source. The occurrence of incipient charnockites (both large scale as well as small scale) adjacent to the granite as well as pegmatites (which contain CO2, CO2-H2O, F and other volatiles), suggests that the fluids expelled from the alkaline magma upon solidification generated incipient charnockites through fluid-induced lowering of water activity. Thus the granite and associated alkaline pegmatites acted as conduits for the transfer of heat and volatiles in the Achankovil Shear Zone area, causing pervasive as well as patchy charnockite formation. The transport of CO2 by felsic melts through the southern Indian middle crust is suggested to be part of a crustal-scale fluid system that linked mantle heat and CO2 input with upward migration of crustally derived felsic melts and incipient charnockite formation, resulting in an igneous charnockite – I-type granite – incipient charnockite association.

Outcrop-scale silicate liquid immiscibility from an alkali syenite (A-type granitoid)-pyroxenite association near Puttetti, Trivandrum Block, South India

The Pan-African (572 Ma) Puttetti syenite (A-type granitoid)-pyroxenite association intrudes the high-grade metamorphic terrain of the Trivandrum Block, South India. Field evidence indicates the contemporaneous nature of syenitic and pyroxenitic liquids. The occurrence of a mixed-rock (~70% syenite and 30% pyroxenite) with an emulsion-like texture, and the occurrence of pyroxenite globules in syenite, is interpreted as relics of immiscible magmas. Both syenite and pyroxenite show similar mineral assemblage (with major minerals having overlapping compositions), but the relative proportions differ. Major element and trace element partitioning trends, parallel REE patterns, and similar Sr initial isotope compositions are in accord with behavior either predicted or measured for immiscible melts in experimental and/or natural systems. The more pronounced Eu anomalies and LREE/HREE ratios of syenite and pyroxenite (relative to the mixed-rock) is related to fractionation caused by immiscible separation. The proposed origin of the Puttetti pluton involves the intrusion of a magma whose bulk composition is that of the mixed-rock. This melt behaved immiscibly and split into two fractions, which produced the syenite and pyroxenite magmas.

Constraints on the application of carbon isotope thermometry in high‐ to ultrahigh‐temperature metamorphic terranes

AbstractNine marble horizons from the granulite facies terrane of southern India were examined in detail for stable carbon and oxygen isotopes in calcite and carbon isotopes in graphite. The marbles in Trivandrum Block show coupled lowering of δ13C and δ18O values in calcite and heterogeneous single crystal δ13C values (− 1 to − 10‰) for graphite indicating varying carbon isotope fractionation between calcite and graphite, despite the granulite facies regional metamorphic conditions. The stable isotope patterns suggest alteration of δ13C and δ18O values in marbles by infiltration of low δ13C–δ18O‐bearing fluids, the extent of alteration being a direct function of the fluid‐rock ratio. The carbon isotope zonation preserved in graphite suggests that the graphite crystals precipitated/recrystallized in the presence of an externally derived CO2‐rich fluid, and that the infiltration had occurred under high temperature and low fO2 conditions during metamorphism. The onset of graphite precipitation resulted in a depletion of the carbon isotope values of the remaining fluid+calcite carbon reservoir, following a Rayleigh‐type distillation process within fluid‐rich pockets/pathways in marbles resulting in the observed zonation. The results suggest that calcite–graphite thermometry cannot be applied in marbles that are affected by external carbonic fluid infiltration. However, marble horizons in the Madurai Block, where the effect of fluid infiltration is not detected, record clear imprints of ultrahigh temperature metamorphism (800–1000 °C), with fractionations reaching <2‰. Zonation studies on graphite show a nominal rimward lowering δ13C on the order of 1 to 2‰. The zonation carries the imprint of fluid deficient/absent UHT metamorphism. Commonly, calculated core temperatures are > 1000 °C and would be consistent with UHT metamorphism.

The application of single zircon evaporation and model Nd ages to the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India

The technique of single zircon dating from the thermal evaporation of 207Pb/206Pb (Kober 1986, 1987) provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages. In contrast Nd model ages may provide a measure of the period of crustal residency for the sample or its protolith. These two techniques have been combined to elucidate the tectonic history of the Proterozoic mobile belt of southern India, exposed south of the Palghat-Cauvery Shear Zone that marks the southern boundary of the Archaean craton of Karnataka. The two main tectonic units of this mobile belt comprise the Madurai and Trivandrum Blocks, both of which are characterised by massive charnockite uplands and low-lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the Proterozoic. Evidence for early Palaeoproterozoic magmatism is restricted to the Madurai Block where single zircon evaporation ages from a metagranite (2436 ± 4 Ma) are similar to model Nd ages from a range of lithologies suggesting crustal growth at that time. The Trivandrum Block, to the south of the Achankovil shear zone, is comprised of the Kerala Khondalite Belt, the Nagercoil charnockites and the Achankovil metasediments. Single zircon evaporation ages, together with conventional zircon and garnet chronometry, suggest that all three units underwent upper-amphibolite facies metamorphism at ∼1800 Ma, an event unrecorded in the metagranite from the Madurai Block. This implies that the Madurai and Trivandrum blocks represent distinct terrains throughout the Palaeoproterozoic. Model Nd ages from the Achankovil metasediments are much younger (1500–1200 Ma) than those from the adjacent Kerala Khondalite Belt and Madurai Blocks (3000–2100 Ma), but there is no evidence for zircon growth in these metasediments during the Mesoproterozoic. Hence the comparatively young model Nd ages of the metasediments are indicative of a mixed provenance rather than a discrete period of crustal growth. Zircon overgrowths from the Madurai Block (547 ± 17 Ma) and Achankovil metasediments (530 ± 21 Ma) suggest that all tectonic units of the Proterozoic mobile belt of South India shared the same metamorphic history from the early Palaeozoic. This event has been recognised in the basement lithologies of Sri Lanka and East Antarctica, confirming that the constituent terrains of East Gondwana had assembled by this time.

Carbon Isotope Fractionation Between Calcite and Graphite During High Temperature Metamorphism

The carbon isotope exchange thermometer between calcite and graphite have been recently widely applied to amphibolite and granulite facies marbles (e.g. Kitchen and Valley, 1995). It has been proved that the temperature dependant fractionation is reliable up to temperatures of the order of 800~ The fractionation between calcite and graphite nears unity at higher tempertures and hence is difficult to calibrate the thermometer at temperatures higher than 800~ Here we try to emperically test the high tempertaure fractionation between calcite and graphite using marble samples from the high temperature granulite facies terrains of southern India and East Antarctica. Marbles from the Trivandrum and the Madurai Block, from the southern granulite terrain were examined in this study. The Trivandrum Block comprises of granulite facies supracrustal rocks, while in the Madurai block massive charnockitic rocks predominate. Marbles bands occur in these two terrains as conformable units intercalated with quartzites. Texturally these are very coarse (up to several centimeters) and highly crystalline graphite crystals can be seen embedded within calcite. Calcite constitutes more than 80 modal % in most of the samples. The silicate mineral phases include phlogopite, forsterite, clinopyroxene and spinel; with occasional clinohumite, pargasite, and other calc-silicate mineral phases. Marbles from LutzowHolm complex, which is a granulite to amphibolite facies terrain with a regional peak granulite facies metamorphism at temperatures in the range 760830~ and pressures of 7 • kbar were also examined. Peak metamorphic temperatures were determined from fractionation of C-isotopes between calcitegraphite pairs using the recent calibration of Kitchen and Valley (1995). For example the marbles from the Trivanrum Block in Southern India gave fractionation range from 2.46%0 to 3.76%0, corresponding to a temperature range of 700 to 800~ Out of the ten calcite-graphite pairs, 6 gave similar results. These results are significant that the carbon isotope thermometer can be used to determine accurate peak metamorphic temperatures in granulite facies rocks. The marbles form the Madurai block and the Lutzow-Holm Bay gave temperatures between 800 and 900~

Dextral Pan‐African Shear Along the Southwestern Edge of the Achankovil Shear Belt, South India: Constraints on Gondwana Reconstructions

The Achankovil shear belt of the southern Indian Peninsular shield is prominent on Landsat images. It coincides with the boundary between charnockites of the Madurai block to the north and khondalites of the Trivandrum block to the south, and with a major change in the aeromagnetic pattern that can be traced across southern India. Field investigations reveal a major shear zone along the southwestern edge of the Achankovil shear belt, the Tenmala shear zone. Rocks in the Tenmala shear zone include charnockite, garnet‐biotite gneiss, garnet‐sillimanite gneiss, cordierite gneiss, and granite. Kinematic indicators include stretched and asymmetric garnet, feldspar and quartz porphyroclasts, shear bands, asymmetric folds, extensional and contractional composite structures, hook folds on rotated and deformed gash veins, and lineations. Kinematic analysis of these features along 60 km of the shear zone indicates primarily dextral shear, with a minor component of reverse shear. Textures, as well as mineral assemblages, are consistent with deformation under granulite facies conditions. Some overprint by the most recently formed charnockite postdates the shearing. Limited geochronologic data suggest a late Proterozoic (Pan‐African) age of shearing. Dextral shear along the Achankovil shear belt is opposite to the sinistral shear reported for the Bongolava‐Ranotsara shear zone in southerhem Magascar; hence these two shear zones cannot be correlated in reconstructions of these parts of Gondwana.

Decompressional P-T history in sapphirine-bearing granulites from Kodaikanal, southern India

The sapphirine-bearing granulites exposed around the Kodaikanal region of the Madurai block, southern India, present a variety of mineral parageneses involving garnet, spinel, sillimanite, cordierite, orthopyroxene, phlogopite, potash feldspar and plagioclase. interpretation of multiphase reaction textures in conjunction with mineral chemical data and topology in the (FM)AS system is consistent with the main sapphirine-forming reactions: orthopyroxene + sillimanite = sapphirine + cordierite phlogopite + sillimanite = sapphirine + cordierite + K-feldspar + vapour Mg-tschermak = sapphirine + cordierite. The P-T evolution of these sapphirine granulites has been constrained through the use of conventional geothermobarometry, internally consistent TWEEQU programme and thermodynamically calibrated MAS equilibria. The P-T estimates define a retrograde trajectory with substantial decompression of c.4 kbar from P-T max of c.8 kbar at c.800°C. On the basis of the available evidence for a Pan-African granulite-facies event in the Madurai and Trivandrum blocks, the emerging concept of East Gondwana assembly is endorsed.