Wanni Complex Research Articles

Dolomitic Marble in a Context of UHT Metamorphism: Possible Signs of Melting, Lenadora Quarry, Sri Lanka

Abstract The Lenadora quarry, located 25 km north of Matale, in central Sri Lanka, exploits dolomitic marble on an artisanal scale. It is located in the Highland Complex but very close to its thrust contact with the Wanni Complex. The rocks have undergone metamorphism to the granulite facies; by analogy with metasedimentary rocks in a similar position south of Kandy, conditions of metamorphism may even have surpassed 900°C and 0.9 GPa. How did dolomitic marble fare in such an ultrahigh-temperature environment? High-resolution large-area image mosaics of imaged polished thin sections were acquired by using both light microscopy and scanning electron microscopy. We focus on fifty areas of interest. As expected, diopside and forsterite porphyroblasts appeared during prograde metamorphism. However, we did not anticipate to see subhedral porphyroblasts of forsterite mimetically replaced by diopside + dolomite. The conversion could have involved a fluid phase or a carbonate melt that had become locally enriched in silica. The presence of a melt accounts for composite calcite + dolomite globules trapped in forsterite. There is no evidence of breakdown products of dolomite, like periclase or brucite. The evidence that a carbonate melt was present is circumstantial; carbonate systems are notorious for erasing telltale evidence of melting. Other minerals encountered in the dolomitic matrix are phlogopite, pargasite, rutile, hydroxylapatite, zirconolite, lakargiite, a magnesiobeltrandoite-like phase, pyrrhotite, and calcite as a minor phase, some of which is of a second generation. Unreplaced relics of forsterite were partially serpentinized at a retrograde stage.

The influence of water\u2013rock interactions on household well water in an area of high prevalence chronic kidney disease of unknown aetiology (CKDu)

Poor drinking water quality in household wells is hypothesised as being a potential contributor to the high prevalence of chronic kidney disease of uncertain aetiology (CKDu) among the farming communities of the Medawachchiya area, Anuradhapura, Sri Lanka. One of the natural processes that can affect water quality is the dissolution of minerals contained within an aquifer by water–rock interactions (WRIs). Here we present a comprehensive assessment of WRIs and their influence on the water chemistry in household wells and spring waters in the Medawachchiya area by combining measurements of environmental isotopes, such as strontium, lithium and stable carbon isotopes and inorganic chemistry parameters, and modelling geochemical mass balance reactions between rainfall and groundwater samples. Our results reveal the presence of strontium, dissolved from both silicate and carbonate minerals, with high isotopic (87Sr/86Sr) ratios of up to 0.7316. Geochemical mass balance modelling and prior 87Sr/86Sr studies on the Wanni Complex bedrock suggest these strontium values may be the result of biotite dissolution. We also identify lithium and uranium contributed from the dissolution of silicates, albeit at concentrations too low to constitute a known health risk. In contrast, the levels of magnesium and calcium in our samples are high and demonstrate that, despite the felsic bedrock, well water chemistry in the Medawachchiya area is dominated by carbonate dissolution.

Prolonged Neoproterozoic high‐grade metamorphism of the Wanni Complex, Sri Lanka: New insights from petrology, phase equilibria modelling, and zircon U–Pb geochronology of partially melted cordierite gneiss from Walpita

We report new petrological and geochronological data of garnet‐cordierite (Grt‐Crd) gneiss as well as two types of leucocratic rocks (garnet‐bearing and cordierite‐bearing leucosomes) crystallized during prograde and near‐peak metamorphisms of the Wanni Complex (WC) in Sri Lanka, and investigate the pressure–temperature–time evolution of granulite‐facies metamorphism for unravelling the timing and tectonics of the Neoproterozoic collisional processes related to Gondwana amalgamation. Grt‐Crd gneiss, garnet‐biotite (Grt‐Bt) gneiss, garnet‐bearing leucocratic rock (Grt‐L), and cordierite‐bearing leucocratic rock (Crd‐L) discussed in this study were collected from Walpita in the margin of the WC along the boundary with the Highland Complex (HC). Grt‐L occurs as thin layers parallel to the foliation of matrix Grt‐Bt and Grt‐Crd gneisses. The occurrences of porphyroblastic garnet in quartzo‐feldspathic matrix and crystallised melt inclusions within the garnet suggest that the Grt‐L could have formed by partial melting of host Grt‐Bt gneiss during prograde to peak metamorphism. Crd‐L is coarse‐grained and massive, lacks obvious foliation, and crosscuts the foliation of host Grt‐Bt gneiss. These observations suggest that the Crd‐L could have intruded after the crystallization of the Grt‐L. Application of phase equilibria modelling in the system NCKFMASHTO for the rocks yielded three discrete thermal events: 600–800°C/4.0–6.0 kbar (Stage 1 from Grt‐L), 700–840°C/5.5–7.0 kbar (Stage 2 from Grt‐Crd gneiss), and 840–880°C/4.5–5.5 kbar (Stage 3 from Crd‐L), suggesting a clockwise P–T evolution. LA‐ICP‐MS zircon U–Pb dating of the leucosomes yielded several metamorphic events: >577 Ma (partial melting of Grt‐L), ~562 Ma (crystallization of Grt‐L), 537 ± 14 Ma (peak metamorphism and crystallization of Crd‐L), and 507–503 Ma (post‐peak cooling event). The results of this study therefore confirmed prolonged (>70 Myr) high‐grade metamorphism of the WC, although the duration might be shorter than that of the HC (>100 Ma). The continuous heat supply necessary for such long‐lived high‐grade metamorphism is explained by a recent tectonic model of Sri Lanka. The model suggests double‐sided subduction and continent‐continent collision, which could have given rise to subsequent slab delamination and differential heating of lower to middle crust. Such complex subduction–collision processes possibly controlled the difference in the style of P–T path and the duration of high‐grade metamorphism between the HC and the WC.

Nature of crust beneath Sri Lanka using teleseismic receiver function

Sri Lanka occupied a unique position in the Eastern Gondwana assembly, surrounded by its Gondwana neighbours: Madagascar, India, Antarctica and Africa. It comprises four distinct lithologic units: the Highland Complex, the Wanni Complex, the Vijayan Complex and the Kadugannawa complex. Crustal properties (crustal thickness and velocities) in Sri Lanka have been estimated by inversion modeling of receiver functions computed using earthquake data from three stations that operated between 2011 and 2018. Results reveal ~38 km crustal thickness and high average shear wave velocity (Vs ~ 3.7–3.8 km/s) beneath Sri Lanka, low average Vp/Vs ratio (~1.72) in the Highland Complex, and high Vp/Vs ratio (1.79) in the Wanni Complex. This suggests that the crust beneath Sri Lanka is felsic to intermediate in nature and the probable rocks are of granite to granodioritic in composition. The probable rocks in Wanni Complex are felsic to mafic granulite in nature. The crustal thickness and average Vp/Vs ratio in the Highland Complex shows similarity with its Gondwana neighbours, denoting its unique position in Gondwana. The average crustal shear wave velocity in Sri Lanka is also found to be high (~3.7 km/s), which is also comparable with its Gondwana neighbours.

Detrital zircon and igneous protolith ages of high-grade metamorphic rocks in the Highland and Wanni Complexes, Sri Lanka: Their geochronological correlation with southern India and East Antarctica

The high-grade metamorphic rocks of Sri Lanka place valuable constraints on the assembly of central parts of the Gondwana supercontinent. They are subdivided into the Wanni Complex (WC), Highland Complex (HC) and Vijayan Complex (VC), but their correlation with neighbouring Gondwana terranes is hindered by a poor understanding of the contact between the HC and WC. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb dating of remnant zircon cores from 45 high-grade metamorphic rocks in Sri Lanka reveals two domains with different age characteristics that correlate with the HC and WC and which help constrain the location of the boundary between them. The HC is dominated by detrital zircon ages of ca. 3500–1500 Ma from garnet–biotite gneiss, garnet–cordierite–biotite gneiss, some samples of garnet–orthopyroxene–biotite gneiss and siliceous gneiss (interpreted as paragneisses) and igneous protolith ages of ca. 2000–1800 Ma from garnet–hornblende–biotite gneiss, other samples of garnet–orthopyroxene–biotite gneiss, garnet–two-pyroxene granulite, two-pyroxene granulite and charnockite (interpreted as orthogneisses). In contrast, the WC is dominated by detrital zircon ages of ca. 1100–700 Ma from paragneisses and igneous protolith ages of ca. 1100–800 Ma from orthogneisses. This clearly suggests the HC and WC have different origins, but some of our results and previous data indicate their spatial distribution does not correspond exactly to the unit boundary proposed in earlier studies using Nd model ages. Detrital zircon and igneous protolith ages in the HC suggest that sedimentary protoliths were eroded from local 2000–1800 Ma igneous rocks and an older Paleoproterozoic to Archean craton. In contrast, the WC sedimentary protoliths were mainly eroded from local late Mesoproterozoic to Neoproterozoic igneous rocks with very minor components from an older 2500–1500 Ma craton, and in the case of the WC precursor sediments there was possibly additional detritus derived from early to middle Neoproterozoic metamorphic rocks. The relic zircon core ages in the HC are comparable with those of the Trivandrum Block and Nagercoil Block of southern India. In contrast, those ages in the WC match the Achankovil Shear Zone and Southern Madurai Block of southern India. These comparisons are also supported by Th/U ratios of detrital zircon cores from paragneisses (Th/U ratios of >0.10 for the former and not only >0.10 but also ≤0.10 for the latter). Comparisons with the Lützow-Holm Complex of East Antarctica indicate that the geochronological characteristics of the HC and WC broadly match those of the Skallen Group, and the Ongul and Okuiwa Groups, respectively.

Detrital zircon geochronology of the Lützow-Holm Complex, East Antarctica: Implications for Antarctica–Sri Lanka correlation

The Lützow-Holm Complex (LHC) of East Antarctica has been regarded as a collage of Neoarchean (ca. 2.5 Ga), Paleoproterozoic (ca. 1.8 Ga), and Neoproterozoic (ca. 1.0 Ga) magmatic arcs which were amalgamated through the latest Neoproterozoic collisional events during the assembly of Gondwana supercontinent. Here, we report new geochronological data on detrital zircons in metasediments associated with the magmatic rocks from the LHC, and compare the age spectra with those in the adjacent terranes for evaluating the tectonic correlation of East Antarctica and Sri Lanka. Cores of detrital zircon grains with high Th/U ratio in eight metasediment samples can be subdivided into two dominant groups: (1) late Meso- to Neoproterozoic (1.1–0.63 Ga) zircons from the northeastern part of the LHC in Prince Olav Coast and northern Sôya Coast areas, and (2) dominantly Neoarchean to Paleoproterozoic (2.8–2.4 Ga) zircons from the southwestern part of the LHC in southern Lützow-Holm Bay area. The ca. 1.0 Ga and ca. 2.5 Ga magmatic suites in the LHC could be proximal provenances of the detrital zircons in the northeastern and southwestern LHC, respectively. Subordinate middle to late Mesoproterozoic (1.3–1.2 Ga) detrital zircons obtained from Akarui Point and Langhovde could have been derived from adjacent Gondwana fragments (e.g., Rayner Complex, Eastern Ghats Belt). Meso- to Neoproterozoic domains such as Vijayan and Wanni Complexes of Sri Lanka, the southern Madurai Block of southern India, and the central-western Madagascar could be alternative distal sources of the late Meso- to Neoproterozoic zircons. Paleo- to Mesoarchean domains in India, Africa, and Antarctica might also be distal sources for the minor ∼2.8 Ga detrital zircons from Skallevikshalsen. The detrital zircons from the Highland Complex of Sri Lanka show similar Neoarchean to Paleoproterozoic (ca. 2.5 Ga) and Neoproterozoic (ca. 1.0 Ga) ages, which are comparable with those of the LHC, suggesting that the two complexes might have formed under similar tectonic regimes. We consider that the Highland Complex and metasedimentary unit of the LHC formed a unified latest Neoproterozoic suture zone with a large block of northern LH–Vijayan Complex caught up as remnant of the ca. 1.0 Ga magmatic arc.

Paleoproterozoic (ca. 1.8 Ga) arc magmatism in the Lützow-Holm Complex, East Antarctica: Implications for crustal growth and terrane assembly in erstwhile Gondwana fragments

The Lützow-Holm Complex (LHC) of East Antarctica forms a part of the latest Neoproterozoic–Cambrian high-grade metamorphic segment of the East African–Antarctic Orogen. Here we present new petrological, geochemical, and zircon U–Pb geochronological data on meta-igneous rocks from four localities (Austhovde, Telen, Skallevikshalsen, and Skallen) in the LHC, and evaluate the regional Paleoproterozoic (ca. 1.8Ga) arc magmatism in this terrane for the first time. The geochemical features reveal a volcanic-arc affinity for most of the meta-igneous rocks from Austhovde and Telen, suggesting that the protoliths of these rocks were derived from felsic to mafic arc magmatic rocks. The protoliths of two mafic granulites from Austhovde are inferred as non-volcanic-arc basalt such as E-MORB, suggesting the accretion of remnant oceanic lithosphere together with the volcanic-arc components during the subduction–collision events. The weighted mean 206Pb/238U ages of the dominant population of magmatic zircons in felsic orthogneisses from Austhovde and Telen show 1819±19Ma and 1830±10Ma, respectively, corresponding to Paleoproterozoic magmatic event. The magmatic zircons in orthogneisses from other two localities yield upper intercept ages of 1837±54Ma (Skallevikshalsen), and 1856±37Ma and 1854±45Ma (Skallen), which also support Paleoproterozoic magmatism. The earlier thermal events during Neoarchean to Early Paleoproterozoic are also traced by 206Pb/238U ages of xenocrystic zircons in the felsic orthogneisses from Austhovde (2517±17Ma and 2495±15Ma) and Telen (2126±16Ma), suggesting partial reworking of the basement of a 2.5Ga microcontinent during ca. 1.8Ga continental-arc magmatism. The timing of peak metamorphism is inferred to be in the range of 645.6±10.4 to 521.4±12.0Ma based on 206Pb/238U weighted mean ages of metamorphic zircon grains. The results of this study, together with the available magmatic ages as well as geophysical and lithological data from the region, suggest that the LHC can be divided into three units: Neoarchean (ca. 2.5Ga) unit in the southern LHC (Shirase Orthogneiss or “Shirase microcontinent”), Neoproterozoic (ca. 1.0Ga) unit in the northern LHC, and supracrustal unit in the central LHC with fragments of Paleoproterozoic (ca. 1.8Ga) and minor Neoarchean (ca. 2.5Ga) and Neoproterozoic (ca. 1.0Ga) magmatic arcs. The 1.8Ga arc magmatism inferred in this study has also been reported from adjacent Gondwana fragments such as the Highland Complex in Sri Lanka, and the Trivandrum and Nagercoil Blocks in southern India. Although the ca. 1.8Ga arc-magmatic event is coeval in these regions, the Paleoproterozoic supracrustal unit in the central LHC may not be contiguous with those in the Highland Complex of Sri Lanka because recent studies have shown that the Vijayan Complex in Sri Lanka and the ca. 1.0Ga northern LHC possibly were part of a single crustal unit (northern Lützow-Holm–Vijayan Complex) within the Kalahari Block. The supracrustal unit possibly marks part of a discrete suture formed by the collision of the ca. 2.5Ga southern LHC (Shirase microcontinent) and the ca. 1.0Ga northern Lützow-Holm–Vijayan Complex during the latest Neoproterozoic–Cambrian Gondwana amalgamation, which might be coeval with the collision of the Vijayan and Wanni Complexes and the formation of the Highland Complex in Sri Lanka. Our study provides new insights on crustal growth and terrane assembly in the ancient continental blocks of Gondwana.

Silica-undersaturated reaction zones at a crust–mantle interface in the Highland Complex, Sri Lanka: Mass transfer and melt infiltration during high-temperature metasomatism

Sri Lanka is a crucial Gondwana fragment mostly composed of granulitic rocks in the Highland Complex surrounded by rocks with granulite to amphibolite grade in the Vijayan and Wanni Complex that were structurally juxtaposed during Pan-African orogeny. Fluids associated with granulite-facies metamorphism are thought to have controlled various lower crustal processes such as dehydration/hydration reactions, partial melting, and high-temperature metasomatism. Chemical disequilibrium in the hybrid contact zone between a near peak post-tectonic ultramafic enclave and siliceous granulitic gneiss at Rupaha within the Highland Complex produced metasomatic reaction zones under the presence of melt. Different reaction zones observed in the contact zone show the mineral assemblages phlogopite+spinel+sapphirine (zone A), spinel+sapphirine+corundum (zone B), corundum (~30%)+biotite+plagioclase zone (zone C) and plagioclase+biotite+corundum (~5%) zone (zone D). Chemical potential diagrams and mass balance reveal that the addition of Mg from ultramafic rocks and removal of Si from siliceous granulitic gneiss gave rise to residual enrichment of Al in the metasomatized mineral assemblages. We propose that contact metasomatism between the two units, promoted by melt influx, caused steady state diffusional transport across the profile. Corundum growth was promoted by the strong residual Al enrichment and Si depletion in reaction zone whereas sapphirine may have been formed under high Mg activity near the ultramafic rocks. Modelling also indicated that metasomatic alteration occurred at ca. 850°C at 9kbar, which is consistent with post-peak metamorphic conditions reached during the initial stage of exhumation in the lower crust and with temperature calculations based on conventional geothermometry.

Ca. 820–640 Ma SIMS U-Pb age signal in the peripheral Vijayan Complex, Sri Lanka: Identifying magmatic pulses in the assembly of Gondwana

Sri Lanka comprises three roughly north-south trending amphibolite- to granulite-facies lithotectonic complexes, from west to east the Highland Complex, the Wanni Complex, and the Vijayan Complex. These terranes were correlated with other East Gondwana continental terranes with similar lithologies forming at similar ages. The Wanni Complex and the Vijayan Complex have been interpreted as volcanic arc terranes brought together by a double-sided subduction. The Highland Complex represents the metamorphosed accretionary prism within the suture when the Wanni and Vijayan Complexes were juxtaposing against each other. In contrast to the Wanni and Highland Complexes, the Vijayan Complex has yielded only a few geochronological data with satisfactory precision. Previous studies suggested that the Vijayan Complex comprises ∼1100–924Ma granitic gneisses, which were metamorphosed during ∼590–456Ma. More recently, ∼772–617Ma mafic intrusions have been identified. This study divides the Vijayan granitic gneisses and the associated melt products geochemically into a low-Nb series and a more primitive high-Nb series. Our SIMS U-Pb zircon data suggested that both series have protolith magmatic ages of ∼1062–935Ma, and metamorphic ages of ∼580–521Ma, which is consistent with previous work. However, some of the Vijayan granitic gneisses and granitic anatectic melt products at the Highland-Vijayan tectonic mixed zone preserve an additional Tonian-Cryogenian (∼820–630Ma) age signal. This age signal suggested that felsic magmatism also occurred when mafic granulites were emplaced along the Highland-Vijayan boundary, which is broadly coeval with to the bimodal magmatism occurring along the Highland-Wanni boundary. This study also suggests that charnockitisation in the Vijayan Complex occurred at 562±6Ma during the Neoproterozoic regional metamorphism. The Tonian-Cryogenian signal preserved in the Highland-Vijayan tectonic mixed zone can also be found in the alkaline intrusion hosted by the Namuno Terrane and the Lurio Belt in Mozambique. This indicates a relationship between the Vijayan granitic gneisses and the Lurio foreland metagranitic basement, while the Namuno Terrane and the Lurio Belt are correlated with the Highland-Vijayan tectonic mixed zone. The ages and the isotope signatures of these granitic bodies further suggest a genetic relationship of these granitic bodies with various magmatic intrusions in East Antarctica.

Neoproterozoic arc accretion along the ‘eastern suture’ in Sri Lanka during Gondwana assembly

Sri Lanka is composed of four major tectonic units: the Wanni, Highland, Kadugannawa and Vijayan complexes, with important records of Neoproterozoic tectonic events including subduction-related vertical and lateral continental growth associated with the final assembly of the Gondwana supercontinent. Here we investigate the boundary between the Highland (HC) and Vijayan (VC) complexes, which we define as the ‘eastern suture’ in Sri Lanka. We report granitoid gneisses, diorite, clinopyroxenite, garnet-bearing mafic granulites and serpentinized ultramafics with bands of chert and quartzite. Our petrologic and geochemical data suggest subduction-related magmatism, marking the boundary between the HC and VC as an active continental margin. We report zircon U–Pb ages and Lu–Hf isotopic data which show that the protoliths of granitoid gneisses including hornblende biotite gneiss, granodioritic gneiss and meta-granite were emplaced at ca. 924–966Ma, followed by new zircon growth during repeated thermal events through late Neoproterozoic–Cambrian. The zircons from garnet-bearing mafic granulite and clinopyroxenite show magmatic core ages of 617±27Ma and 772±6Ma respectively, with minor to significant lead-loss at about 550–580Ma. Our data also identify Ordovician ultramafic magmatism with CO2 influx and mantle metasomatism at 485±6Ma, with the formation of magnesite deposits in the serpentinite. The zircon Hf isotopic data from the Vijayan gneisses and the mafic–ultramafic intrusions show depleted mantle signature, with only minor input of older continental material. The data obtained in this study, and those from recent studies suggest that the Vijayan and Wanni Complexes are similar aged Neoproterozoic magmatic arcs, and that the Vijayan–Highland boundary marks an active convergent margin similar to the western boundary between the Wanni and Highland Complexes. The Cambrian high-grade metamorphism is extensively recorded in all rock types. The Neoproterozoic records from Sri Lanka are consistent with double-sided subduction and accretion of arcs on both sides through the closure of the HC segment of the Mozambique Ocean, with the HC representing a Cambrian Himalaya in Sri Lanka constructed during the collisional event associated with the final assembly of Gondwana. Comparable rock suites with similar ages and tectonic setting in East Antarctica and southern India support arc assembly and closure of intervening oceans along multiple subduction systems.

Early to late Neoproterozoic magmatism and magma mixing–mingling in Sri Lanka: Implications for convergent margin processes during Gondwana assembly

The Sri Lankan fragment of Gondwana preserves the records of Neoproterozoic tectonothermal events associated with the final assembly of the supercontinent. Here we investigate a suite of magmatic rocks from the Wanni, Kadugannawa and Highland Complexes through geological, petrological, geochemical and zircon U–Pb and Lu–Hf isotopic techniques. The hornblende biotite gneiss, charnockites, metagabbro and metadiorites investigated in this study show geochemical features consistent with calc-alkaline affinity and subduction-related signature including LILE enrichment relative to HFSE coupled with distinct Nb–Ta depletion and weak negative Zr–Hf anomalies. The felsic suite falls in the volcanic arc granites (VAGs) field and the mafic suite shows island arc basalt affinity in tectonic discrimination plots, suggesting that the protoliths of the rocks were derived from arc-related magmas in a convergent margin setting. LA-ICPMS zircon U–Pb analyses show crystallization of charnockite and dioritic mafic magmatic enclave from the Highland Complex during ca. 565 and 576Ma corresponding to bimodal magmatism. The diorite also contains metamorphic zircons of ca. 525Ma. Hornblende–biotite gneiss from the Kadugannawa Complex shows protolith emplacement age at 973–980Ma, followed by new zircon growth during repeated thermal events through late Neoproterozoic. The dioritic enclaves in these rocks are much younger, and form part of a deformed and metamorphosed dyke suite with emplacement ages of 559Ma, broadly coeval with the bimodal magmatism in the Highland Complex at that time. The youngest group of zircons in this rock shows ages of 508Ma, corresponding to the latest thermal event. A charnockite from this locality shows oldest group of zircons at 962Ma, corresponding to emplacement age similar to that of the magmatic protolith of the hornblende biotite gneiss. This rock also shows zircon growth during repeated thermal events at 832Ma, 780Ma, 721Ma and 661–605Ma. The lower intercept age of 543Ma marks the timing of collisional metamorphism. Charnockite from the Wanni Complex shows emplacement age at 1000Ma, followed by thermal event at 570Ma, the latter correlating with the bimodal magmatic event in the Highland Complex. The dioritic enclave within this charnockite shows an age of ca. 980Ma, suggesting intrusion of mafic magma into the felsic magma chamber. Zircons in the diorite also record multiple zircon events during 950 to 750Ma. Zircons in the Highland Complex charnockite possess negative εHf(t) values in the range −6.7 to −12.6 with TDMC of 2039–2306Ma suggesting magma derivation through melting of Paleoproterozoic source. In contrast, the εHf(t) range of −11.1 to 1.6 suggests a mixed source of both of older crustal and juvenile material. The εHf(t) values of −4.5 to 4.5 and TDMC of 1546–1962Ma for the hornblende biotite gneiss also shows magma derivation from mixed sources that included Paleoproterozoic components. The younger dioritic intrusive, however, has a more juvenile magma source as indicated by the mean εHf(t) value of 1.3. The associated charnockite shows a tight positive cluster of εHf(t) from 0.6 to 5.1, suggesting juvenile input. Charnockite from the Wanni Complex shows clearly positive εHf(t) values of up to 13.1, and TDMC in the range 937–1458Ma suggesting much younger and depleted mantle source. The diorite enclave also has positive εHf(t) values with an average value of 8.5 and TDMC in the range of 709–1443Ma clearly suggesting younger juvenile sources. The early and late Neoproterozoic bimodal suites are correlated to convergent margin magmatism associated with the assembly of Sri Lanka within the Gondwana supercontinent.

Neoproterozoic crustal evolution in Sri Lanka: Insights from petrologic, geochemical and zircon U–Pb and Lu–Hf isotopic data and implications for Gondwana assembly

Sri Lanka, the ‘pendant’ of Gondwana, is a collage of distinct crustal blocks that preserve important records of major Neoproterozoic tectonothermal events. Here, we present the petrology, geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes on a suite of meta-igneous rocks including granodiorite, diorite and garnet amphibolite from the Kadugannawa Complex (KC), granodiorite from the Wanni Complex (WC) and mafic granulites, gabbros and garnet-bearing charnockite from the Highland Complex (HC) along a NW–SE transect. The regional metamorphic peak P–T conditions were estimated from garnet-clinopyroxene-plagioclase-quartz assemblage in the metagabbro as 830–860°C and 9.4–9.8kbar. Slightly lower temperature ranges of 700–780°C were obtained from garnet amphibolite, metagranodiorite and metadiorite, corresponding retrograde conditions. Trace element and rare earth element patterns as well as Rb-Y-Nb and Rb-Yb-Ta discrimination plots show volcanic arc affinity for the granodiorite, diorite and garnet charnockite suggesting that the protoliths of the rocks were formed from felsic to intermediate arc magmas. The mafic granulites and magnesian metagabbro also suggest volcanic arc affinity and indicate subduction-related mafic magmatism and magma underplating. The garnet-bearing metagabbro shows N-MORB signature, whereas the garnet amphibolite displays oceanic island alkali basalt affinity. These rocks therefore represent accretion of the remnants of oceanic lithosphere during the subduction-collisional event.Zircons in a metadiorite and the surrounding metagranodiorite from the KC yield ages of 980±16Ma to 916±57Ma marking early Neoproterozoic magmatism followed by metamorphism at 532±18Ma. Zircons in the garnet amphibolite from this complex show extensive metamorphic recrystallization yielding a weighted mean 206Pb/238U age of 520.7±6.6Ma. From the WC, zircons in a metagranodiorite define three groups of weighted mean 206Pb/238U ages at 805±12Ma (emplacement of the magmatic protolith), 734.0±4.6Ma (Cryogenian thermal event) and 546.0±5.7Ma (latest Neoproterozoic-Cambrian metamorphism). Zircons from the HC record multiple late Neoproterozoic-Cambrian thermal events with weighted mean 206Pb/238U ages of 576.8±9.3Ma and 523.0±7.1Ma (metagabbro) and 579±10Ma, 540.4±6.0Ma and 511.1±5.9Ma (garnet charnockite), 553.0±3.2Ma (mafic granulite), and 539.1±4.4Ma (mafic granulite sill). Lu–Hf data reveal dominantly positive ɛHf(t) values for zircons in the metadiorite and metagranodiorite from the KC (−1.1 to 7.2) and Hf crustal model ages (TDMC) in the range of 1206–1733Ma suggesting a mixed source from both juvenile and Paleo-Mesoproterozoic components. However, zircons in the garnet amphibolite from this complex show dominantly negative ɛHf(t) values (mean −16.5) with TDMC in the range of 2356–2828Ma suggesting reworked Neoarchean-Paleoproterozoic crustal source. Zircons in metagranodiorite of the WC also possess negative ɛHf(t) values (mean −6.2) with TDMC in the range of 1799–2498Ma suggesting reworked Paleoproterozoic crust as the magma source. The HC rocks also preserve distinct imprints of reworking of older crust. The zircon ɛHf(t) values in mafic granulite show a tight cluster from −2.2 to 0.1 with TDMC in the range of 1501–1651Ma suggesting a mixed source from both juvenile Neoproterozoic and reworked Mesoproterozoic components. Zircons in the metagabbro from this complex show negative ɛHf(t) values (mean −6.3) and TDMC of 1847–1978Ma. Zircons in the garnet charnockite also display highly negative ɛHf(t) values (mean −17.7) and older TDMC) (mean 2614Ma) suggesting reworked Paleoproterozoic crust. Zircons in the mafic granulite sample show negative ɛHf(t) values (mean −14.1) and TDMC between 2263 and 2790Ma indicating that the source material for the magma evolved from the Neoarchean-Paleoproterozoic crust. In summary, the 916–980Ma ages from the KC represent arc magmatism during early Neoproterozoic, followed by the 805Ma granodioritic magma emplacement in the WC. Repeated thermal events during mid and late Neoproterozoic are also recorded from the Wanni and Highland Complexes, culminating in Cambrian high-grade metamorphism that reached ultrahigh-temperature conditions. We propose a model of double-sided subduction during the Neoproterozoic, where the Wanni Complex to the west and the Vijayan Complex to the east represent continental arcs, culminating in collision along the HC during late Neoproterozoic-Cambrian.

Crustal evolution of the Southern Granulite Terrane, south India: New geochronological and geochemical data for felsic orthogneisses and granites

This article presents LA-ICPMS U-Pb zircon and U-Th-Pb monazite ages and geochemical data for felsic orthogneisses and granites from the Southern Granulite Terrane of India, a key area for reconstructing the evolution of the late-Neoproterozoic Gondwana supercontinent. The data identify two distinct crustal domains in the Madurai Province, the major part of the Southern Granulite Terrane, and we present a new geotectonic model for their crustal evolution. The Western Madurai Domain is widely composed of late-Neoarchaean (2.53–2.46Ga) subduction-related, magnesian charno-enderbites, which were reworked in the early-Palaeoproterozoic (2.47–2.43Ga) during granulite facies metamorphism and partial melting. The Eastern Madurai Domain is dominated by a vast supracrustal sequence, which was deposited on a late-Palaeoproterozoic (1.74–1.62Ga) basement of magnesian charnockites and Hbl-Bt gneisses that formed through reworking of underlying Archaean rocks. Both domains of the Madurai Province were intruded by voluminous mid-Neoproterozoic A-type charnockites and felsic orthogneisses (0.83–0.79Ga), which formed through reworking of Archaean to Palaeoproterozoic rocks, together with minor leuco-gabbros and rhyolites, during a major phase of extensional rifting. Regional mid-Neoproterozoic (0.82–0.78Ga) metamorphism is temporally and spatially related to A-type granite magmatism. Convergence of the two domains in the late-Neoproterozoic (ca. 0.55Ga) culminated in their collision along a SSW-NNE-trending belt of ultrahigh-temperature metamorphic rocks, the Kambam UHT Belt, and was associated by the localized emplacement of leucogranites (0.56Ga).Accretion of the Madurai Province at the Archaean Dharwar Craton occurred in the earliest Palaeoproterozoic along the Moyar-Bhavani-Cauvery Suture, a distinct crustal domain characterized by late-Neoarchaean (2.50Ga) subduction-related, magnesian enderbites. They resemble the age and composition of the Western Madurai Domain charno-enderbites and intrude Mesoarchaean charnockites (2.85Ga) and metabasites, representing remnants of oceanic crust. The suture zone was, coeval with the Western Madurai Domain, affected by early-Palaeoproterozoic (2.49–2.45Ga) HP granulite facies metamorphism. Just as the Madurai Province the suture zone was intruded by ferroan mid-Neoproterozoic (0.84Ga) metagranitoids but was only locally affected by an early-Cambrian overprint (0.54Ga). The inferred early-Palaeoproterozoic accretion of the Madurai Province at the Dharwar Craton and the extension-related origin of the voluminous mid-Neoproterozoic A-type charnockites are difficult to match with the recently established concept of a Neoproterozoic subduction zone and suture between the craton and the Madurai Province.In a Palaeogeographic model the widely Neoarchaean Western Madurai Domain is correlated with the Antananarivo Domain of central Madagascar, whereas the Proterozoic Eastern Madurai Domain is linked with the Ongole Domain, the southernmost segment of the Eastern Ghats Belt, and the Wanni Complex of Sri Lanka.

Zircon ages and Hf isotopic systematics reveal vestiges of Mesoproterozoic to Archaean crust within the late Neoproterozoic–Cambrian high-grade terrain of southernmost India

Late Neoproterozoic to Cambrian ages have been widely recorded from the Southern Granulite Terrain (SGT) of India and were correlated with global tectono-thermal (Pan-African) events associated with amalgamation of the Gondwana supercontinent. We report results of single zircon dating from a charnockite and a granitic augen–gneiss from the SGT using SHRIMP and evaporation techniques, complemented by zircon Hf isotopic systematics. The SHRIMP data for zircons in the charnockite are spread along a well-defined discordia line suggesting variably strong lead–loss during the late Neoproterozoic Pan-African event. A concordant grain yielded a 207Pb/206Pb age of 1893±13Ma, and the upper intercept of the discordia line for all analyses with Concordia is at 1881±14Ma. The concordant grain age is considered to most closely reflect the time of emplacement of the charnockite protolith. The imprecise lower Concordia intercept at 572±25Ma is likely to reflect Pb-loss during granulite-facies metamorphism. Evaporation of four zircons yielded consistent 207Pb/206Pb isotopic ratios at high evaporation temperatures of ca. 1598°C, suggesting an age of ca. 1450Ma. This date is clearly much younger than the SHRIMP result and suggests that the evaporated grains no longer contained their original, magmatic 207Pb/206Pb ratios but record strong Pb-loss as also shown by the SHRIMP data. Lu–Hf isotopic analysis of ten spots on zircon grains previously analyzed by SHRIMP II yielded strongly negative εHf(t)-values of −6.1 to −9.2 and mean Hf crustal residence ages of 2.95 to 3.14Ga.Eight grains from the augen–gneiss analyzed on SHRIMP II also show variable but considerable Pb-loss. A concordant grain yielded an age of 1557±16Ma, and the other grains define a discordia line with an upper Concordia intercept at 1553±21Ma. Two unzoned grains produced a combined mean 206Pb/238U age of 583±6Ma and reflect the high-grade late Neoproterozoic metamorphic event. A concordant 207Pb/206Pb age of 2043±13Ma was obtained for a xenocryst. Evaporation of five large zircons from this sample yielded a precise mean 207Pb/206Pb age of 1556.0±0.5Ma, identical to the result for the SHRIMP-dated concordant grain. Lu–Hf isotopic analysis of ten spots on zircon grains from where SHRIMP data were gathered yielded strongly negative εHf(t)-values of −6.0 to −17.0 and mean Hf crustal residence ages of 2.68 to 3.37Ga.Our results suggest that not all rocks in the so-called Pan-African domain of southern India are Neoproterozoic in age, and that there could be more Mesoproterozoic to Archaean crust hidden in the “sea” of Neoproterozoic granulites, granitoids and migmatites than so far recognized. The Hf isotopic data reveal both granitoid samples to be derived from crustal melts generated from Archaean continental crust, and the chemistry of these rocks is inherited from their ancient source. We compare the zircon ages with those of similar rocks in the Highland Complex of Sri Lanka and discuss their relevance as vestiges of an older continental terrain incorporated within the late Neoproterozoic–Cambrian collision belt of East Gondwana. In view of rock and age similarities we tentatively suggest links between the Indian Trivandrum and Madurai Blocks and the Sri Lankan Highland and Wanni Complexes.

Cryogenian (∼830 Ma) mafic magmatism and metamorphism in the northern Madurai Block, southern India: A magmatic link between Sri Lanka and Madagascar?

The northern Madurai Block, southern India, lies directly south of, and partly deformed by, the Palghat–Cauvery Shear Zone System (PCSS) – a potential suture of the Neoproterozoic Mozambique Ocean. The Kadavur gabbro-anorthosite complex lies south of the PCSS, in the northern Madurai Block, and crystallized at 829 ± 14 Ma (LA-ICPMS zircon data) in a supra-subduction zone setting. The complex contains zircon ε Hf(t) values of −12.5 to −8.6 that represent Palaeoproterozoic T (DM) model ages (2.3–2.5 Ga). These broadly agree with a whole rock neodymium T (DM) model age of 2287 Ma. Oxygen isotope δ 18O ratios range from 5.82‰ and 6.74‰. The parental magma for the gabbro-anorthosites are interpreted to be derived from a juvenile Neoproterozoic mantle contaminated by Mesoarchaean igneous infra-crustal sources. The gabbro-anorthosites intrude quartzites with dominantly Palaeoproterozoic detrital zircons that contain Neoarchaean and Mesoarchaean hafnium model ages. These quartzite zircons contain metamorphic rims that yield an age of 843 ± 23 Ma demonstrating the autochthonous nature of the gabbro-anorthosite complex. Later felsic magmatism is recorded by the 766 ± 8 Ma crystallisation age of the protolith of a felsic gneiss. Cryogenian magmatism in the Madurai Block is interpreted to form part of an extensive arc magmatic province within the southern East African Orogen that can be traced from central Madagascar, through southern India to the Wanni Complex of Sri Lanka. This province is interpreted to have formed above a south/west dipping subduction system as the Mozambique Ocean was subducted under the Neoproterozoic continent Azania.

Statistical evaluation of stream sediment geochemistry in interpreting the river catchment of high-grade metamorphic terrains

Stream sediment geochemistry is extensively used in mineral exploration and environmental studies. However, quantitative assessments of the effectiveness of stream sediment geochemistry for describing upstream lithologies are rare, especially in high-grade metamorphic terrains. This study statistically evaluates whether stream sediment geochemistry can aid in recognizing variations in upstream geology in several high-grade metamorphic lithotectonic units having different metamorphic and tectonic histories, including the Highland Complex, Vijayan Complex, Wanni Complex, and Kadugannawa Complex of Sri Lanka. For this study, concentrations of 21 elements were measured in 2080 stream sediment samples collected from the Walawe River, Maha Oya, Gala Oya basins located on above lithotectonic units and Uma Oya, Belihul Oya, Badulu Oya basins situated adjacent to each other on the Highland Complex. These rivers flow across dry, intermediate and wet zones of Sri Lanka, with river courses having both slope (> 20°) and flat (< 20°) areas. Elemental concentrations, averaged over each river basin, show patterns of enrichment and depletion which may relate to localized mineralization conditions, local lithological changes, anthropogenic activities and environmental factors such as local variations in climate and morphology among river basins. Comparison of element concentrations in sediments from the four different lithotectonic units shows that enrichment – depletion patterns can be partly related to rock geochemistry of the associated lithotectonic unit. However, climate and basin morphology also seem to play an important role. Results of Kruskal–Wallis H tests show that both major and trace element levels in sediments from the four different lithotectonic units, as well as from adjacent Uma Oya, Badulu Oya and Belihul Oya basins, are significantly different. Discriminant function analysis appropriately classifies sediments into the four different lithotectonic units with an accuracy of 91.9%. This method also classifies sediments into river basins which share common lithology being situated adjacent to each other in the same lithotectonic unit with an accuracy of 89.5%. This strongly suggests that stream sediment geochemistry is capable of describing the upstream regional scale as well as local scale lithological changes at a great accuracy in complex high-grade metamorphic terrains. In both cases use of channel slope and basin climatic zone as additional variables does not significantly increase overall or individual accuracy in classification.

Constraints on the Timing of Pan‐African Granulite‐Facies Metamorphism in the Kerala Khondalite Belt of Southern India: SHRIMP Mineral Ages and Nd Isotopic Systematics

The supracrustal Kerala Khondalite Belt (KKB; southern India) constitutes an important but little understood terrain in the evolution of East Gondwana. Here we report SHRIMP U‐Pb zircon and titanite ages for a suite of metasedimentary rocks from the KKB. Metamorphic ages at $$530.2\pm 4.4$$ and $$525\pm 4.5$$ Ma were obtained from metapelitic and metapsammitic migmatites, respectively. These ages are interpreted to date peak metamorphism because both rock types have extensively melted under granulite‐facies conditions. A similar age ($$520\pm 4.7$$ Ma) was obtained from titanite from a granulite‐facies calc‐silicate rock. These ages are within error of SHRIMP ages for granulite‐facies assemblages in Lützow‐Holm Bay, East Antarctica, the Highland and Wanni Complexes of Sri Lanka, and Southern Madagascar. We propose that this timing reflects a short span (∼10 m.yr.) of high‐temperature granulite‐facies metamorphism and concomitant widespread migmatization that marks the final stage of collision between East and West Gondwana.

Geochemical evidence for a Neoproterozoic magmatic continental margin in Sri Lanka—relevance for the Rodinia–Gondwana supercontinent cycle

Abstract Zircon ages of Neoproterozoic crustal blocks in Sri Lanka indicate crystallization and metamorphism contemporaneously with the development of the supercontinent Rodinia as well as the early amalgamation of Gondwana, respectively [Asian J. Earth Sci., in press and references therein]. In this context, the geodynamic significance of the ca. 890 Ma mafic Kandy Complex is enigmatic, as it may represent a magmatic arc during the final assembly of Rodinia as well as a rifting-related complex belonging to its dispersal or even early assembly of Gondwana. Here we interpret geochemical and Nd isotopic data of the Kandy Complex as evidence for its former tectonic setting and its role in the development of the Rodinia–Gondwana supercontinent cycle. A representative sample suite of the Kandy Complex comprises amphibolites, metagabbros, and metadiorites. Some of the samples show relicts of primary magmatic layering. Initial eNd values of −2.8 to +1.3 are interpreted as evidence for melting of a heterogeneous, and ultimately depleted mantle, overprinted by subducted old crustal material. Combined chemical and Nd isotopic data show that the crustal signatures were inherited from the mantle rather than by melt contamination at crustal levels. The samples show enrichment of light rare-earth elements, negative Nb-, and positive Pb-anomalies. These characteristics are similar to those found in modern arcs containing a subducted sedimentary component. The evidence suggests that the Kandy Complex represents a deep crustal level of a magmatic arc. Isotopic as well as geochronological data suggest magmatism over a period of ca. 50 Ma [Asian J. Earth Sci. in press] by repeated rejuvenation of an Andean-type continental margin. The Sm–Nd isotopic systematics indicate that the Kandy Complex represents an integral part of the ca. 1.1–0.75 Ga Wanni Complex in NW Sri Lanka, and that it evolved separately from the presently juxtaposed metasedimentary Highland Complex.

Multistage graphite precipitation through protracted fluid flow in sheared metagranitoid, Digana, Sri Lanka: evidence from stable isotopes

The Digana metagranitoid, situated within the Digana shear zone, a critical junction between the Highland and Wanni complexes of Sri Lanka, hosts a variety of graphite types with structural evidence showing formation during different stages prior to and after deformational events affecting this terrain. Here we discuss their occurrence, association and stable isotopes. The carbon isotopes of graphite reported in this study from the Sri Lankan high-grade metamorphic terrain differ significantly from the composition of graphite reported elsewhere in other Precambrian terrains. The stable isotope composition from random crystals depicts an enriched carbon isotopic composition in the range −1.6‰ to −8‰, suggesting precipitation by reduction from CO 2-rich fluids. Intracrystalline microscale traverses made on graphite crystals indicate the δ 13C composition of the rims becoming progressively lighter in 13C, in metagranitoid hosted samples, with the variation reaching up to 1.5‰, consistent with the changing fluid regimes induced by decreasing temperature during the graphite growth, and shows the preservation of core at higher temperatures. A highly enriched carbon isotopic composition (−0.6‰) with a rimward enrichment of 13C observed in migmatites, contrary to the values in metagranitoid, indicates preferential enrichment of heavy isotopes as a result of the partial melting and dissolution/reprecipitation of graphite. The oxygen isotopic composition of a quartz–calcite vein show values in the range 18.5‰ to 20‰, typical for recycled crustal source. The tight constraint in the isotopic data from different graphite types studied indicates a homogeneous fluid source for the deposits. Prolonged fluid flow in this region is envisaged in this study, which is related to the multistage deformation of the crustal scale shear zone.

Proterozoic crustal evolution of southernmost India and Sri Lanka

Abstract The Southern Granulite Terrain of India comprises four Proterozoic domains (from north to south): the Madurai Block, and the Achankovil, Ponmudi and Nagercoil Units. All but the Achankovil Unit share similarities with the Highland Complex of Sri Lanka: (1) evidence for Palaeoproterozoic crust formation (from U-Pb ages and 2.0–3.0 Ga Nd model ages); and (2) Pan-African ultrahigh-temperature metamorphism between 610 and 550 Ma. More specifically, the Ponmudi Unit and the Madurai Block share lithological and petrological characteristics with the low- P and high- P domains of the Highland Complex, respectively. The Achankovil Unit is correlated with the Wanni Complex of Sri Lanka on the basis of lithologies and Nd model ages (1.0–2.0Ga). These domains and the Vijayan and Kadugannawa Complexes of Sri Lanka represent Late Mesoproterozoic-Early Neoproterozoic crustal domains. Similarities in tectonic style, degree of metamorphism and Neoproterozoic U-Pb ages, suggest a common Pan-African tectonothermal evolution for the lower crustal domains of southern India, Sri Lanka and the Lützow-Holm Bay of Eastern Antarctica. Sri Lanka may have been located at the junction between the Mozambique Belt and a second belt partly involving areas that underwent a Grenvillian high-grade event during the formation of Rodinia. Madagascar shows a similar Pan-African metamorphic overprint, but is located in a different position and lacks the influence of the second belt. Alternative views are discussed.