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Numerous empirical and experimental studies have documented the consumption of orthopyroxene and production of clinopyroxene in mantle peridotite during carbonatite metasomatism, whereas little attention has been paid to whether and how olivine is compositionally modified through the same process of carbonatite metasomatism. Here, we present the discovery of well-preserved reaction features in olivine from typical calciocarbonatites from Sri Lanka. The olivine grains experienced serpentinization along the cracks and in their rims and are generally surrounded outwardly by serpentine, tremolite and diopside zones. The inner serpentine zone is dominated by serpentine with tiny magnetite, whereas the mantle tremolite zone is characterized by euhedral tremolite and intergrowth of dolomite with small amounts of serpentine relics and diopside. The outer diopside zone shows spongy texture and commonly encloses olivine, serpentine, tremolite and dolomite, which are also observed in interstitial diopside grains in the carbonatites. Compositional profile analyses reveal that these olivine grains display decreasing FeO and increasing MgO contents and thus elevating Fo values from their cores to the reaction rims, while the serpentine inherits compositions from the olivine. Elemental mapping images demonstrate outward diffusion of Si and Mg in the serpentine zones and contribution of Ca from carbonatite melts during tremolite formation. Further interaction with carbonatite melts produced Si- and Mg-poor and Ca-rich diopside relative to the reactant tremolite. These observations suggest that the interaction with carbonatite melts first transformed olivine to diopside through serpentinization, followed by tremolitization. The diopside formed through olivine-carbonatite melt interaction is anomalously depleted in Na and Al and enriched in Ca, which are distinct from those generated in interactions between orthopyroxene and both carbonatite and silicate melts. This implies that for the first time some wehrlites and clinopyroxenites in the mantle might originate from intense interaction between peridotites (including dunite) and carbonatite melts. This mechanism can be invoked to explain the formation of other global occurrences of wehrlite and clinopyroxenite suites. [ABSTRACT FROM AUTHOR]

In order to investigate element partitioning and Li‐O isotope fractionation between silicate minerals and carbonatite melts at variable levels from mantle to crust, we document elemental and Li‐O isotopic data for major minerals from crust‐derived carbonatites at Eppawala, Sri Lanka. Partition coefficients (D) of elements between olivine or clinopyroxene and carbonatite melts are consequently estimated. The estimated D values indicate that Li, Zn, Co, Cr, Mn, and Ni behave compatibly in olivine, while P and Sc are slightly compatible, and V and Al are mildly incompatible. Partition coefficients of elements between clinopyroxene and carbonatite melts are defined here, including highly compatible Li, Sc, Ti, V, Al, and Na, moderately compatible Zn, Co, Cr, and Ga, and incompatible Mn, Ni, P, and Cu. They are systematically higher than literature values obtained from mantle conditions, but their relative compatibilities at different systems are consistent. This indicates that element partitioning between silicates and carbonatite melts is highly temperature‐ and pressure‐dependent and can be used to evaluate geochemical proxies of carbonatite metasomatism, and evolution and mineralization of carbonatite melts. Profile analyses on olivine grains reveal that Fe‐loving elements in olivine could well preserve features of crystal growth and modal metasomatic interaction, while Li and O isotope fractionations are strongly controlled by element diffusion and Li isotopes are robust indicators of cryptic metasomatic interaction. Plain Language Summary: We derive partition coefficients of elements between olivine and carbonatite melts and between clinopyroxene and carbonatite melts based on analyses of crust‐derived carbonatites. The compatibilities of elements are consistent with experimental results from different systems, while the D values (ratio of element concentration in mineral against in melt) are temperature‐ and pressure‐dependent. Li and O isotope variations are strongly controlled by element diffusion. Thus, the derived D values and isotope fractionation patterns can be used to evaluate geochemical proxies of carbonatite metasomatism, and evolution and mineralization of carbonatite melts. Key Points: Good preservation of variable reaction features of olivine‐carbonatite melt interactionConstraints on elemental behaviors and Li‐O isotopic fractionations between olivine and carbonatite meltsElement partitioning between olivine, clinopyroxene, and carbonatite melts and implications for carbonatite metasomatism [ABSTRACT FROM AUTHOR]

Early Palaeozoic ultrahigh-temperature (UHT) metamorphism in eastern Gondwana was an event that started with Gondwana amalgamation at c. 580 Ma and lasted at least 50 Myr. Sapphirine + quartz, Mg–Al granulites preserve a record of the timing and duration of the event along the metamorphic belt. U–Th–Pb dating of zircon and monazite shows that metamorphism peaked almost simultaneously in Antarctica (554.0 ± 4.7 Ma), Sri Lanka (555.5 ± 4.6 Ma), southern India (548.1 ± 8.1 Ma) and Madagascar (550.6 ± 6.0 Ma), and ended in all locations at the same time, 530–520 Ma. Rare earth element (REE) compositions of the metamorphic zircon zones can be matched to the REE zoning in the associated garnet. Phase-diagram modelling indicates that the peak UHT P – T conditions in Antarctica, Sri Lanka, and India were very similar, 1020–1040 °C at 0.8 GPa. Peak conditions in Madagascar were at higher T and similar P : 1090 °C and 0.8 GPa. The East African Orogeny before 600 Ma preconditioned the crust of the eastern Gondwanan terranes by thickening it and harbouring heat-producing elements, heating the crust over c. 60 Myr; such that UHT conditions were reached when East and West Gondwana collided. [ABSTRACT FROM AUTHOR]

The Vijayan Complex (VC) of Sri Lanka is a Neoproterozoic arc terrain having no convincingly proven counterpart elsewhere in the Gondwana supercontinent. Therefore, little is known about the nature of its pre-Gondwanan ancestry. We present here new elemental and Sr-Nd isotope systematics of orthogneisses to envisage the origin and evolution of the VC. Samples with high and variable Sr-initials and negative εNd > +4 are compatible with their derivation by melting of older rocks with a long crustal residence history. Most of the studied rocks show depleted-mantle-model ages (1.4–3.3 Ga) considerably older than the ∼1 Ga magmatic crystallization age of the VC. The rest of the samples show model ages of ∼0.9–1.1 Ga, coeval with the crystallization age of the VC, indicating separation from the mantle as juvenile inputs. Therefore, protolith sources of the majority of these rocks are mixtures of distinctive components formed much earlier than the time of predominant magmatism (∼1 Ga) in the VC. Major and trace elements and rare earth element ratios divide these samples into two categories. The "Depleted Group" represents protoliths similar to Proterozoic oceanic island basalt–like mantle materials that intensively interacted/mixed with different proportions of melts derived from juvenile and ancient lower crust. The "Enriched Group" represents mid-ocean ridge basalt–like mantle materials subsequently interacted/mixed with variable amounts of melts derived from Archean felsic and mafic crust. Geochemical-mixing modeling with 87Sr/86Sr and 143Nd/144Nd confirms the involvement of above multiple source components. Thus, our findings illustrate evidence for hybridization of mantle-derived melts from various juvenile and ancient crustal components. Therefore, this first finding of Paleoproterozoic to early Archean–aged components within the VC provides new insights to elucidate its pre-Gondwanan geodynamic setting, revising the previous understanding of it as an "exotic terrain" to Gondwana with only largely juvenile Neoproterozoic materials. [ABSTRACT FROM AUTHOR]

Here we attempt to constrain the P–T evolution of ultrahigh‐temperature granulites using textures coupled with multiple thermobarometric approaches. Sapphirine‐bearing granulites were collected from a quarry in the central part of the Highland Complex of Sri Lanka. Three sapphirine‐bearing domains were selected and petrographically studied. Homogeneous sample domains were thermodynamically modelled using their bulk compositions (forward phase equilibria modelling). One heterogeneous sample from a single domain, composed of irregularly distributed residuum and melt, was also used. The bulk composition of its residual part was calculated using mineral compositions and their respective modes. Equilibrium T–X(Fe2O3) phase diagrams were constructed in the chemical system NCKFMASHTO to estimate the bulk ferric/ferrous iron ratio, and conventional geothermometers (garnet–orthopyroxene and Al in orthopyroxene) were applied. The Ti in zircon trace element thermometer was also applied to calculate peak metamorphic conditions. Modal abundance isopleths of each mineral in equilibrium phase diagrams and textural observations were combined to constrain the retrograde P–T path. Our hybrid approach of forward and inverse phase equilibria modelling and conventional thermobarometric calculations indicate that the sapphirine‐bearing granulites have reached their peak T of 920–940°C at P ~10 kbar under relatively highly oxidizing conditions. Subsequently, the rocks followed a near‐isobaric cooling path down to 890–860°C, prior to near‐isothermal decompression up to 6 kbar. The results highlight the importance of dealing with Fe3+. Multiple thermobarometric approaches on carefully observed mineral textures are required to retrieve the most reliable P–T conditions of HT/UHT mineral assemblages. [ABSTRACT FROM AUTHOR]

High-Al garnet-sillimanite-graphite gneisses (khondalites) from the Matale area in central Sri Lanka show evidence for open system formation of symplectites. Three types of khondalite (type A, B, and C) were collected from two localities (Lo1 and Lo2) in this area. In Lo1, khondalite type A shows a stable mineral assemblage with garnet (Grt), ribbon quartz (Qtz), prismatic sillimanite (Sil), alkali feldspar (Akfs) and minor graphite. The abundant leucosome is interpreted as (modified) crystallised melt (Liq). Qtz is invariably rimmed by Akfs double corona. In type B, Grt and Sil are partially rimmed by spinel (Spl)-Akfs symplectites. Type C samples show the extensive development of Spl-Akfs symplectites, locally with Bt, as well as corundum (Crn)-Akfs symplectites after Sil in domains relatively remote from Grt. Mineral inclusions in Grt suggest advanced biotite dehydration melting producing Grt ± Spl + Liq, similar to other Sri Lankan metapelitic granulites. Peak P-T conditions are ~8–9 kbar and ~ 850 °C. The symplectites developed on the retrograde path, with Ti in Bt thermometry indicating 700–720 °C. The chemical potential calculations indicated that the chemical potential gradient from garnet to sillimanite could well have produced the AKfs+Spl symplectite. Hence the most critical point is the initiation of Grt breakdown reactions which can facilitate the formation of Spl + Akfs and Crn + Akfs symplectites after sillimanite via the chemical potential gradient. Textural observations coupled with mass balance of symplectitic reaction textures indicate that addition of external Na and K to reaction sites is needed to trigger the symplectite formation. Excess Fe and Mg from Grt breakdown probably diffused away from garnet along a chemical potential gradient. As a result, Sil in the vicinity of Grt has broken down to Spl-Akfs symplectites, whereas Sil further away from Grt produced Crn-Akfs symplectites. Spl-Akfs and Crn-Akfs symplectites after Grt and Sil in khondalites are restricted to the boundary zone between the Wanni and Highland Complexes. The formation of symplectites is inferred to have taken place during or just after a decompression stage with minor cooling, under the influence of an alkaline fluid derived from nearby pegmatites, when the assemblage Grt + Sil was already metastable. The combination of fluids and symplectites indicates a very fast process rate. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Kiruna-type iron oxide-apatite associations occur in a variety of rock types and their origin has remained controversial. Most of the Kiruna-type deposits are associated with intermediate to felsic rocks, and in rare cases with ultramafic rocks. Here we investigate the Seruwila iron oxide-apatite deposit at the contact between the Highland and Vijayan complexes that has been defined as the "eastern suture" in Sri Lanka, which formed during the late Neoproterozoic assembly of the Gondwana supercontinent. The ore deposit is hosted in an ultramafic intrusion and comprises massive and disseminated mineralization. The ore-bearing rocks are mainly composed of low-Ti magnetite and chlor-fluorapatite. Our petrological and geochemical studies suggest a magmatic-hydrothermal model for the mineralization wherein: (1) the Cl-rich magmatic-hydrothermal fluid scavenged iron and P from the ultramafic magma, transported iron to shallower levels in the crust and deposited along the suture zone to form the massive type magnetite and apatite; and (2) the cooling of the hydrothermal fluids resulted in the growth of disseminated magnetite and the precipitation of sulfide minerals, followed by a calcic metasomatism (scapolitization and actinolitization). This model is in conformity with the genetic relation between Kiruna-type deposits and iron oxide-copper-gold (IOCG) deposits. We also report LA-ICP-MS zircon U-Pb ages from the host ultramafic intrusion suggesting its emplacement at ca. 530 Ma, which is younger than the regional high-grade metamorphism associated with the collisional assembly of the crustal blocks in Sri Lanka at ca. 540 Ma. By analogy with the common occurrence of Kiruna-type deposits in extensional tectonic settings, and the geochemical features of the studied rocks including low silica, high Mg, Fe, Ca with high field strength elements (HFSEs such as Nb, Ta, Zr, Hf, Ti) depletion and strong LREE and F enrichment, we theorize that the ultramafic magmatism occurred in a post-collisional extensional setting derived from a volatile- and LREE-rich metasomatized lithospheric mantle. [ABSTRACT FROM AUTHOR]