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

Abstract

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.