Ultrahigh-Temperature Metamorphism in Madurai Granulites, Southern India: Evidence from Carbon Isotope Thermometry

Ultrahigh-temperature (UHT) metamorphism in the Madurai Block of the southern Indian granulite terrain has been verified using the calcite-graphite isotope exchange thermometer. Carbon isotope thermometry has been applied to marbles from a locality near the reported occurrence of sapphirine granulites that have yielded temperature estimates of around 1000 degrees C. The delta(13)C and delta(18)O values of calcite are homogenous, implying equilibration of the isotopes during metamorphism. However, the delta(13)C values of single graphite crystals show variations in the order of 1 per thousand within a hand specimen. Detailed isotopic zonation studies indicate that graphite preserves either the time-integrated crystal growth history or reequilibrium fractionation during its cooling history. The graphite cores preserve higher delta(13)C values than the rims. The fractionation between calcite and graphite cores gives the highest metamorphic temperature of about 1060 degrees C, which matches the petrologically inferred temperature estimates in the high-magnesian pelites. The fractionation between graphite rims and calcite suggests a temperature of around 750 degrees C, which is interpreted to reflect retrograde cooling. This event is also observed in the sapphirine granulites. Calcite-graphite thermometry thus provides a useful tool to define UHT metamorphism in granulite terrains.     

Sapphirine and Corundum-bearing Granulites from Karur, Madurai Block, Southern India

We report new occurrences of sapphirine- and corundum-bearing granulites intercalated within orthogneisses at Lachmanapatti and Malappatty in the northern part of Madurai Block. Sapphirine in these localities occurs either as needle-like intergrowth with cordierite and corundum in symplectites and medium- to fine-grained euhderal to subhedral crystals associated with cordierite and corundum (Lachmanapatti) or in association with plagioclase, corundum and gedrite (Malappatty). The sapphirine from Lachmanapatti is highly magnesian (X_Mg = 0.87-0.94) with higher Cr content (up to 0.9 pfu) as compared with those in other localities in the Madurai Block. The sapphirine-corundum association reported in this study has important bearing on the ultrahigh-temperature metamorphism and exhumation history of the northern Madurai Block as well as on the tectonic evolution of the continental deep crust in southern India.     

Lower Devonian Redeposited Serpentinite-Clastic Weathering Crust,Southern Urals

The structure and problems of origin of Lower Devonian breccia-conglomerates of the Sakmara–Voznesensk zone, southern Urals are considered. It was revealed that the breccia-conglomerates are composed of carbonatized serpentinite clasts and formed in a marine basin at the foothill of a relatively steep slope of a rather intensely dissected land. Clastic material of the breccia-conglomerates was mainly derived from carbonate weathering crusts of serpentinized ultramafic rocks.     

Petrology of Felsic Granulites, Metapelites, Metabasics, Ultramafics, and Metacarbonates from Southern Calabria (Italy): Prograde Metamorphism, Uplift and Cooling of a Former Lower Crust

A high-grade metamorphic terrane in the southern part of theCalabrian massif (South Italy) has been petrographically mappedand the dominant rock types petrologically investigated. Bothmethods of investigation have led to the recognition of a continuoussection through a former lower crust which is 7 km thick. Itslower part consists predominantly of metabasic rocks togetherwith minor felsic granulites, its upper part of metapeliteswith minor metabasic and metacarbonate rocks. The rocks experienced a common two-stage prograde metamorphicevolution in which the second stage occurred after the lastpenetrative deformation. The prograde metamorphism which, accordingto radiometric dates, ended in late Hercynian time, was of themedium-pressure type of Miyashiro (1961), and equilibrationoccurred in the ‘medium-pressure granulite field’(characterized by the instability of olivine-plagioclase aswell as garnet-clinopyroxene-quartz). Estimates of the highest_P—_T conditions of prograde metamorphism give 7–8kb and approximately 800°C at the base, but 5–6 kband 650–700°C at the top of the section, at whichthe paragenesis staurolite-quartz indicates the transition tothe amphibolite facies. The existence of a metamorphic gradientin the lower crust section is demonstrated by the systematicchange in the compositions of ferro-magnesian minerals in divariantmetapelitic assemblages. The metamorphic evolution during the excavation history of theformer lower crust has been reconstructed using the numerousdisequilibrium reaction textures preserved in most rock types.The highest metamorphic conditions ended with a pressure decreaseof approximately 1.5 to 2 kb, which was followed by a periodof quasi-isobaric cooling in the middle crust. During this cooling,the stability field of the ‘high-pressure granulites’(garnet-clinopyroxene-quartz) was reached. The pressure decrease, which induced the end of the high-temperaturehistory of the lower crust, is interpreted as reflecting theerosion of the uppermost crustal levels as a response to overlappingof large crustal segments during the Hercynian orogeny. Consequently,the deduced P—T path of the upper, i.e. overthrust crustalsegment is thought to have been tectonically controlled.     

太古宙下部地壳流体研究现状

太古宙下部地壳流体的研究对于阐明麻粒岩相变质作用成因及深成地质作用过程具有重要意义。下部地壳流体的性状，成因，运移方式及其与麻粒岩相变质作用成因联系等方面取得了一定的研究成果。采取流体包裹体研究与脱水变质反应平衡热力学计算，矿物对地质温压计估算等相结合的方法才能获得更可信的结果，实验研究是今后重要的发展方向。     

Petrology of the Basic Granulites from Kodaikanal, South India

Basic granulites occurring as small enclaves and pods within charnockites contain predominantly orthopyroxene, clinopyroxene, hornblende, plagioclase feldspar and quartz. Chemical composition of coexisting orthopyroxene, clinopyroxene, plagioclase and hornblende has been represented in ACF and AFM diagrams. The mineral assemblages and the textural relationships of the basic granulites have been described. Garnet is notably absent in the basic granulites and this is explained as due to lower (< 8 kbar) pressure and relatively magnesian bulk composition.     

Evidence from Xenoliths for a Dynamic Lower Crust, Eastern Mojave Desert, California

Garnet-rich xenoliths in a Tertiary dike in the eastern MojaveDesert, California, preserve information about the nature andhistory of the lower crust. These xenoliths record pressuresof 10–12 kbar and temperatures of 750–800C. Approximately25% have mafic compositions and bear hornblende + plagioclase+ clinopyroxene + quartz in addition to garnet. The remainder,all of which contain quartz, include quartzose, quartzofeldspathic,and aluminous (kyanitesillimanite-bearing) varieties. Mostxenoliths have identifiable protoliths—mafic from intermediateor mafic igneous rocks, quartzose from quartz-rich sedimentaryrocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathicfrom feldspathic sediments and/or intermediate to felsic igneousrocks. However, many have unusual chemical compositions characterizedby high FeO(t), FeO(t)/MgO, Al₂O₃, and Al₂O₃/CaO, which correspondto high garnet abundance. The mineralogy and major-and trace-elementcompositions are consistent with the interpretation that thexenoliths are the garnet-rich residues of high-pressure crustalmelting, from which granitic melt was extracted. High ⁸⁷Sr/⁸⁶Srand low ¹⁴³Nd/¹⁴⁴Nd, together with highly discordant zirconsfrom a single sample with Pb/Pb ages of 1.7 Ga, demonstratethat the crustal material represented by the xenoliths is atleast as old as Early Proterozoic. This supracrustal-bearinglithologic assemblage may have been emplaced in the lower crustduring either Proterozoic or Mesozoic orogenesis, but Sr andNd model ages> 4 Ga require late Phanerozoic modificationof parent/daughter ratios, presumably during the anatectic event.Pressures of equilibration indicate that peak metamorphism andmelting occurred before the Mojave crust had thinned to itscurrent thickness of <30 km. The compositions of the xenolithssuggest that the lower crust here is grossly similar to estimatedworld-wide lower-crustal compositions in terms of silica andmafic content; however, it is considerably more peraluminous,has a lower mg-number, and is distinctive in some trace elementconcentrations, reflecting its strong metasedimentary and restiticheritage. ^* Author to whom correspondence should be addressed. Present address: Rensselaer Polytechnic Institute, Department of Earth and Environmental Sciences, Troy, New York 12180, USA. Fax: 518–276–8627; email: hanchj{at}rpi.edu.     

Derivation of A-type Granites from a Dehydrated Charnockitic Lower Crust: Evidence from the Chaelundi Complex, Eastern Australia

Triassic I- and A-type granites of the Chaelundi Complex, NewEngland Fold Belt, eastern Australia, were generated in a subduction-relatedtectonic setting. Although isotopic ages of the suites are indistinguishable,field relations indicate that the A-type is younger. The mostmafic granitoids from each suite have similar silica contents(66–68% SiO₂), slightly LREE enriched patterns withoutEu anomalies, low Rb/Sr and K/Ba ratios, and high K/Rb ratios,suggesting that both represent parental magmas. The A-type isdistinguished mineralogically by abundant orthoclase and sodicplagioclase (total >60%), ferro-hornblende, annite and allanite.In contrast, the I-type has more hornblende and biotite, whichare more magnesian in composition, and less feldspar. The parentalmagmas of both suites have many similar geochemical characteristics,although the A-type has slightly higher alkalis, Zr, Hf, Znand LREE, and lower CaO, MgO, Sr, V, Cr, Ni and Fe³⁺/Fe. Thegeochemical properties characteristic of leucocratic A-typegranites, such as high Ga/Al, Nb, , HREE and F contents, areonly manifest in the more felsic members of the A-type suite.These features were produced by 70% fractional crystallizationof feldspar, hornblende, quartz and biotite. Both granite suites were generated by water-undersaturated partialmelting of a similar source, but the A-type parent magma resultedfrom lower aH₂O conditions during partial melting. Generationand rapid ascent of the earlier 1-type magma during disequilibriumpartial melting produced a relatively anhydrous, but not refractory,charnockitic lower crust. Continued thermal input from mantle-derivedmagmas, during continuing subduction, partially melted the ‘charnockitized’lower crust at temperatures in excess of 900C, to produce A-typemagmas. Charnockitic magmas (C-type) form in a similar way toA-type magmas, although their different composition reflectsvariations in the anhydrous lower-crustal mineral assemblagesthat remain after the previous (1-type) granite-forming event. The New England Fold Belt was a subduction—accretion complexuntil the late Carboniferous, when the deeper parts underwentpartial melting to produce S-type granites. As the I-and A-typegranites intruded penecontemporaneously, a tonalitic sourcemodel for genesis of the Chaelundi A-type is untenable. KEY WORDS: A-type; charnockitization; eastern Australia ^*Corresponding author.     

Remnants of Early Continental Crust in the Amgaon Gneisses, Central India: Geochemical Evidence

The Central Indian continental crust is postulated to have formed around the Archean nuclei of the Bastar Craton (Radhakrishna, 1993). Around 3.5 Ga. Old, high-Al 2 O 3 trondhjemite gneisses have been reported from the southern part of the Bastar Craton (Sarkar et al., 1993). However, neither isotopic nor geochemical evidence exists in the literature for the presence of rocks older than 2.5 Ga from the northern part of the Bastar Craton (Sarkar et al., 1990). The absence of tonalite-trondhjemite-granodiorite (TTG) suites from the Amgaon Gneisses (Rao et al., 2000), were considered to indicate substantial geochemical differences between the Amgaon gneisses and the TTG basement gneisses of the Dharwar Craton (i.e., the peninsular gneisses). Accordingly the mode of the tectonomagmatic evolutionary patterns of the Bastar Craton was considered to be different, both in time in space from the bordering Dharwar and Bundelkhand Cratons, respectively. In this communication we report the presence of high-Al 2 O 3 trondhjemite from the Amgaon gneisses, along with calc-alkaline and peraluminous granites that are geochemically similar to the late granitoids (2.5 to 2.6 Ga old) of the Dharwar Craton, suggesting that the two cratons were nearest neighbours at least during the late Archean.     

Lamprophyres from Southern Karnataka,Dharwar Craton,India: Insight on the Rodinia Break-up and Addition of Juvenile Crust

正The late Archean Dharwar Craton is an important part of the Archean and Proterozoic terrains in Peninsular India.Dharwar Craton consists of Western and Eastern Blocks,separated by the Chitradurga Shear Zone.Eastern     

Remnants of Early Continental Crust in the Amgaon Gneisses,Central India: Geochemical Evidence

The Central Indian continental crust is postulated to have formed around the Archean nuclei of the Bastar Craton (Radhakrishna, 1993). Around 3.5 Ga. Old, high-Al 2 O 3 trondhjemite gneisses have been reported from the southern part of the Bastar Craton (Sarkar et al., 1993). However, neither isotopic nor geochemical evidence exists in the literature for the presence of rocks older than ∼2.5 Ga from the northern part of the Bastar Craton (Sarkar et al., 1990). The absence of tonalite-trondhjemite-granodiorite (TTG) suites from the Amgaon Gneisses (Rao et al., 2000), were considered to indicate substantial geochemical differences between the Amgaon gneisses and the TTG basement gneisses of the Dharwar Craton (i.e., the peninsular gneisses). Accordingly the mode of the tectonomagmatic evolutionary patterns of the Bastar Craton was considered to be different, both in time in space from the bordering Dharwar and Bundelkhand Cratons, respectively. In this communication we report the presence of high-Al 2 O 3 trondhjemite from the Amgaon gneisses, along with calc-alkaline and peraluminous granites that are geochemically similar to the late granitoids (∼2.5 to 2.6 Ga old) of the Dharwar Craton, suggesting that the two cratons were nearest neighbours at least during the late Archean.     

Petro-tectonic Imprints in the Sapphirine Granulites from Anantagiri, Eastern Ghats Mobile Belt, India

Sapphirine granulite occurring as lenses in charnockite at Anantagiri,Eastern Ghat, India, displays an array of minerals which developedunder different P-T-X conditions. Reaction textures in conjunctionwith mineral chemical data attest to several Fe-Mg continuousreactions, such as

1. spinel+rutile+quartz+MgFe_–1=sapphirine+ilmenite
2. cordierite=sapphirine+quartz+MgFe_–1
3. sapphirine+quartz=orthopyroxene+sillimanite+MgFe_–1
4. orthopyroxene+sapphirine+quartz=garnet+MgFe_–1
5. orthopyroxene+sillimanite=garnet+quartz+MgFe_–1
6. orthopyroxene+sillimanite+quartz+MgFe_–1=cordierite.

Calculated positions of the reaction curves in P-T space, togetherwith discrete P-T points obtained through geothermobarometryin sapphirine granulite and the closely associated charnockiteand mafic granulite, define an anticlockwise P-T trajectory.This comprises a high-T/P prograde metamorphic path which culminatedin a pressure regime of 8?3 kb above 950?C, a nearly isobariccooling (IBC) path (from 950?C, 8?3 kb, to 675?C, 7?5kb) anda terminal decompressive path (from 7?5 to 4?5 kb). Spinel,quartz, high-Mg cordierite, and sapphirine were stabilized duringthe prograde high-T/P metamorphism, followed by the developmentof orthopyroxene, sillimanite, and garnet during the IBC. Retrogradelow-Mg cordierite appeared as a consequence of decompressionin the sapphirine granulite. Deformational structures, reportedfrom the Eastern Ghat granulites, and the available geochronologicaldata indicate that prograde metamorphism could have occurredat 30001?00 and 2500?100 Ma during a compressive orogeny thatwas associated with high heat influx through mafic magmatism. IBC ensued from P_max and was thus a direct consequence of progrademetamorphism. However, in the absence of sufficient study onthe spatial variation in P-T paths and the strain historiesin relation to time, the linkage between IBC and isothermaldecompression (ITD) has remained obscure. A prolonged IBC followedby ITD could be the consequence of one extensional mechanismwhich had an insufficient acceleration at the early stage, orITD separately could be caused by an unrelated extensional tectonism.The complex cooled nearly isobarically from 2500 Ma. It sufferedrapid decompression accompanied by anorthosite and alkalinemagmatism at 1400–1000 Ma.     

Multi-stage Melting in the Lower Crust of the Serre (Southern Italy)

The lower-crustal section exposed in the Serre, southern Italy,consists mainly of Al-rich metasediments, which underwent granulite-faciesmetamorphism, partial melting and melt extraction. The paperconsiders the formation of melts in metapelites and metagreywackes.Leucosomes and host rocks have been studied to investigate themelting process. Biotite-rich and biotite-free melanosomes withscarce felsic components are present; the biotite-rich typesare widespread in the upper part of the section and the twotypes may occur side by side in the lower part. Na-rich andK-rich leucosomes including residual phases are interspersedwithin the metasediments; on the whole they do not show geochemicalsignatures suggestive of magmatic fractionation. Leucotonalitictypes prevail among the sampled leucosomes, which generallyare rare earth element (REE) depleted with positive Eu anomalieswhereas the host rocks are REE enriched with overall negativeEu anomalies. Melanosomes and migmatites show restitic chemistries.The precursor metagreywackes underwent depletion in Na₂O andenrichment in K₂O. The precursor metapelites document generaldepletion in Na₂O and they may be enriched or depleted in K₂O.All the characteristics of the migmatites and of their componentsreflect a two-stage melting: (1) H₂O-present melting, involvingmainly plagioclase, and (2) dehydration melting of micas. Allthe metasediments underwent H₂O-present melting, forming mostlysodic melts which, owing to their removal from the source asfast as they formed, did not accumulate in such proportionsas to allow migration and mostly remained within the lower-crustalmetasediments; metapelites also underwent variable dehydrationmelting, depending on chemical features and physical conditions,forming larger volumes of mobile granitic melts, most of whichmigrated far from the source. Extractions of 57–66 vol.% of total melts (sodic + potassic) from the most residual metapeliticmelanosomes and of about 27–44 vol. % of potassic meltsfrom metapelitic migmatites have been calculated. Higher volumesof the extracted melts have been calculated for the metapelitesof the lower part of the section; the most depleted metagreywackesunderwent melt extraction of about 9–13 vol. %. The two-stagemelting occurred during the prograde metamorphism and continuedduring the isothermal decompression. KEY WORDS: Calabria; lower crust; multi-stage melting     

Low-Pressure Granulites from the Ketilidian Mobile Belt of Southern Greenland

Early Proterozoic cordierite-garnet–orthopyroxene–K–feldsparmetasedimentary gneisses are developed in the flat—lyingcomplex (FLC) of the southern part of the Ketilidian mobilebelt (1?8–1?7GA), Greenland. These granulites containlow—pressure assemblages and are developed in both regionalmetamorphic rocks and in some thermal aureoles of contemporaneousrapakivi granite plutons. Thermobarometry shows that both theregional and contact metamorphism took place at 2–4 kband 650–800?C. The granulites were developed in an extensionaltectonic regime, and appartently record the culmination of asingle thermotectonic event. There is a close temporal associationbetween the peak of high—grade metamorphism and the emplacementof synorogenic rapakivi granites, and melts from the metasedimentarypile probably contributed largely to the granites. The low pressuresand high temperatures for regional metamorphism require geothermalgradients in excess of 60?C/km, and are consistent with thepresence of regional extensional tectonics, synorogenic magmatism,and underplating of mafic magma in this area during the Ketilidian. ^*Offprint requests to T. J. Dempster. Present address: Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK     

Mantle Input to the Crust in Southern Gangdese, Tibet, during the Cenozoic: Zircon Hf Isotopic Evidence

The Quxu (曲水) complex is a typical Intrusive among the Gangdese batholiths. Two sets of samples collected from the Mianjiang (棉将) and Niedang (聂当) villages in Quxu County, Including gabbro, mafic micro-enclaves (MME), and granodiorites in each set, were well dated in a previous SHRIMP zircon U-Pb analysis (47-51 Ma). In this article, the same zircons of the 6 samples were ap-plied for LA ICP-MS Hf isotopic analysis. The total of 6 samples yields 176Hf/177Hf ratio ranging from 0.282 921 to 0.283 159, corresponding to εHf(t) values of 6.3-14.7. Their Hf depleted-mantle modal ages (TDM) are in the range of 137-555 Ma, and the zircon Hf isotope crustal model ages (TDMC) range from 178 to 718 Ma. The mantle-like high and positive εHf(t) values in these samples suggest a mantle-dominated input of the juvenile source regions from which the bathollth originated. The large varia-tions in εHf(t) values, up to 5-e unit among zircons within a single rock and up to 15-e unit among zir-cons from the 6 samples, further suggest the presence of a magma mixing event during the time of magma generation. We suggest that the crustal end-member involved in the magma mixing is likely from the ancient basement within the Lhasa terrane itself. The zircon Hf isotopic compositions further suggest that magma mixing and magma underplating at about 50 Ma may have played an important role in creating the crust of the southern Tibetan plateau.     

Metamorphism and Isotopic Evolution of Granulites of Southern India: Reference to Neoproterozoic Crustal Evolution

Granulites are important component of high-grade metamorphic rocks reflecting intense conditions observed for crustal rocks in terms of temperature, and pressure. This review paper demonstrates how these high-grade granulites are critical to understanding the evolution of the lower continental crust with special reference to southern India. Geothermobarometric traverse across different granulite blocks in southern India shows wide ranging P-T conditions of metamorphism (700–1000 ° C, and 5–10 kbar). The sapphirine-, orthopyroxene-sillimanite, and spinel -bearing quartz-deficient granulites recognised from parts of southern granulite terrain (Ganguvarpatti, Kiranur, and Palani hill ranges etc.) show oriented sillimanite aggregates pseudomorph after course twinned kyanite, staurolite + kyanite assemblages, and corroded blebs of gedrite within orthopyroxene, suggesting a prograde stage of a clockwise P-T evolution. Evidence of ITD history comes from the textures in which an early Mg-rich garnet (X_Mg 52–60) with orthopyroxene (up to 10% Al₂O₃) involving sillimanite breakdown forming variety of symplectites having combinations of orthopyroxene, sapphirine, cordierite, and spinel. These spectacular reaction textures, and mineralogic sensors from the high-grade rocks establish a prograde clockwise P-T-t path with notable decompressive history (ITD) in the southern granulite terrain. The inferred P-T-t paths have been further integrated with the recent geochronological, and isotopic data to constrain the timing, and duration of metamorphism, emplacement of the magmatic protolith for characterising the evolution of the granulites, and their bearing on the geodynamic implications. Based on the emerging evidence for Neoproterozoic tectonothermal imprints in the southern granulite terrain, history of the assembly of dispersed fragments is also addressed within the East Gondwana framework.     

Petrology of Granulites from Anakapalle--Evidence for Proterozoic Decompression in the Eastern Ghats, India

The granulite complex at Anakapalle, which was metamorphosedat 1000 Ma, comprises orthopyroxene granulites, leptynite, khondalite,mafic granulites, calc-silicate rock, spinel granulites, andtwo types of sapphirine granulites—one quartz-bearingand migmatitic and the other devoid of quartz and massive. Reactiontextures in conjunction with mineral-chemical data suggest severalcontinuous and discontinuous equilibria in these rocks. In orthopyroxenegranulites, dehydration-melting of biotite in the presence ofquartz occurred according to the reaction biotite+quartz= garnet (Py₃₇)+K-feldspar+orthopyroxene + liquid. Later, this garnet broke down by the reaction garnet (Py₃₇)+quartz= orthopyroxene + plagioclase. Subsequently, coronal garnet (Py₃₀) and quartz were producedby the same reaction but proceeding in the opposite direction.In spinel granulites, garnet (Py₄₂) and sillimanite were producedby the breakdown of spinel in the presence of quartz. In thetwo types of sapphirine granulites, garnet with variable pyropecontent broke down according to the reaction garnet = sapphirine + sillimanite + orthopyroxene. The highest pyrope content (59 mol %) was noted in garnets fromquartz-free sapphirine granulites compared with the quartz-bearingone (53 mol % pyrope). The calculated positions of the mineralreactions and diserete P-T points obtained by thermobarometrydefine a retrograde P-T trajectory during which a steep decompressionof 1.5 kbar from P-T_max of 8 kbar and 900C was followed bynear-isobaric cooling of 300C. During this decompression, garnetwith variable pyrope contents in different rocks broke downon intersection with various divariant equilibria. Near-isobariccooling resulted in the formation of coronal garnet around second-generationorthopyroxene and plagioclase replacing earlier porphyroblasticgarnet in orthopyroxene granulites. It has been argued thatthe deduced P-T trajectory originated in an extensional regimeinvolving either a crust of near-normal thickness of a slightlyoverthickened crust owing to magmatic underaccretion.     

Petrogenesis of Olivine-phyric Basalts from the Aphanasey Nikitin Rise: Evidence for Contamination by Cratonic Lower Continental Crust

In this work we investigate the olivine-phyric basalt suiteof the Aphanasey Nikitin Rise, an intraplate volcanic structureformed during the Late Cretaceous in the Indian Ocean. The parentalmelt of the basalt suite has a hypersthene-normative tholeiiticcomposition with low H₂O content (0·3–0·5wt %) and high SiO₂/Al₂O₃ (3·5). The basalt suite ischaracterized by Nb, Ta, Th and U depletion, and uniquely low²⁰⁶Pb/²⁰⁴Pb and ¹⁴³Nd/¹⁴⁴Nd among the Cretaceous tholeiiticbasalts of the Indian Ocean. Our modelling demonstrates thatfractional crystallization of depleted mantle-derived melt andlower continental crust assimilation is a suitable model forthe genesis of the parental magma of this suite. The continentalcrustal material involved is characterized by long-term Rb,U and Th depletion and probably remained isolated for >10⁹years in cratonic Gondwanan lithosphere. On a broader scale,two geochemical groups can be distinguished among tholeiitesformed in the Indian Ocean basin during the period 115–75Ma, from the Aphanasey Nikitin Rise, the southern Kerguelenand Naturaliste plateaux and the Broken Ridge. Both groups havea compositional range from hypersthene-normative basalt to basalticandesite and are characterized by Nb–Ta depletion, extremelylow