The Neoproterozoic-Early Paleozoic Crustal Evolution Of Sri Lanka

The Precambrian crustal basement of Sri Lanka is composed of three major tectonicunits namely,the Wanni Complex to the west,the Highland Complex at the middle,and the Vijayan Complex to the east.Although a small crustal fragment designated as the‘pendant of Gondwana',the rock records in Sri Lanka provide important windows to understand the Neoproterozoic tectonic events and crustal evolution history associated with the amalgamation of Gondwana supercontinent at Late Neoproterozoic–Cambrian boundary.In this thesis zircon U–Pb and Lu–Hf isotopes,bulk-rock geochemistry and calculated metamorphic phase diagrams are used to constrain the age and character of the Neoproterozoic magmatism,the Late Neoproterozoic–Cambrian metamorphism and the Early Cambrian mineralization process of Sri Lanka.The Wanni Complex,Kadugannawa Complex,and the Vijayan Complex experiencedconsecutive magmatism at¹100–700 Ma,which is an important period during the transition from Rodinia to Gondwana.A suite of magmatic rocks from all major terranes of Sri Lanka are collected in this thesis.Zircon U–Pb geochronology reveals that the charnockite from the Highland Complex,charnockite and granitic gneisses from the Wanni Complex,and the granitic gneisses from the Vijayan Complex show similar Neoproterozoic emplacement age at¹000–920 Ma,followed by new zircon growth through late Neoproterozoic,with peaks at⁸30,⁷80and⁷20 Ma,and all samples shows lower intercepted ages of 660–500 Ma.Intermediate to mafic enclaves are observed in those granitic intrusions from the Wanni Complex,they are dated at similar ages?⁹80–950 Ma?,suggesting the co-existence of mafic and felsic magma during early Neoproterozoic.Also,a suit of mafic magmatic rocks are first reported from the Vijayan–Highland tectonic boundary and dated at Middle to Late Neoproterozoic from 770–620 Ma.A subsequent Late Neoproterozoic–Cambrian metamorphism at⁶00–500 Ma is recorded in all rock types,which is associated with the final assembly of Gondwana.Our data also identify Ordovician ultramafic magmatism with CO₂ in fluxed mantle metasomatism at485±6 Ma,with the formation of magnesite deposits in the serpentinite.Zircon Lu–Hf isotope shows that the similar aged Neoproterozoic magmatic rocks havedifferent origin.Rocks from the Highland Complex possess negative?Hf?t?values?-13 to-7?with Archean–Paleoproterozoic model ages suggesting an evolved magma derivation through melting of Paleoproterozoic crust.In contrast,the large variation of?Hf?t?and Hf model ages from the Wanni Complex and Kadugannawa Complex suggest a mixed source of recycled and juvenile crust.The?Hf?t?values of samples from the VC are mostly positive with modal ages close to magmatic ages,which suggests minor input of crustal component.The variable?Hf?t?values are and the geochemical data show distinct characteristics of convergent margin setting and arc-related features for the Wanni and Kadugannawa Complex.Therefore,the data presented in our study also do not attest to an extensional setting of Rodinia breakup.Magmatic rocks from the Wanni and Vijayan show bulk-rock geochemical featuresconsistent 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 shows arc-related geochemical feature and the mafic suite shows island arc basalt affinity,suggesting that the protoliths of the rocks were derived from arc-related magmas in a convergent margin setting.Suggesting 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 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.The Highland Complex records Neoproterozoic–Cambrian metamorphism associatedwith the assembly of the supercontinent Gondwana.We combine phase equilibria modelling with isotopic and trace element geochemistry to provide a time-integrated P–T evolution using representative garnet-bearing mafic granulites from the Highland Complex.New zircon U–Pb geochronology and garnet trace element data from one locality in the EHC,together with a dataset compiled from published results,indicate garnet and zircon existed in equilibrium during long-lived high temperature metamorphism for possibly longer than 100 Ma?⁶60–520Ma?.Calculated pseudosections imply high-temperature to potentially ultrahigh temperature?>850°C?conditions for peak metamorphism.The Highland Complex is characterized by a small decrease in pressure and temperature following peak conditions.This was followed by an accelerated rate of exhumation,leading to the partial replacement of garnet-bearing assemblages by lower pressure paragenses.Although the post-peak cooling/modest pressure decrease is a comparatively minor part of the P–T evolution,compiled zircon U–Pb data suggest it was the temporally dominant part of the thermobarometric history,comprising more than 100Myr.The long-lived high post-peak temperatures require a stable in-situ heat source,which based on regional litho-tectonic correlations,was almost certainly elevated rates of crustal heat production.The minor cooling interval from P–T space is in fact is the temporally dominant part of the P–T evolution in the Highland Complex.This highlight that without time analysis,the P–T evolution does not convey the long-lived baric stability of the high-temperature history of the Highland Complex.A rare type of mineral iron oxide–apatite deposit associated with ultramafics at thecontact 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?.We also report LA–ICP–MS zircon U–Pb ages from the host ultramafic intrusion suggesting its emplacement at⁵30 Ma,which is younger than the regional high-grade metamorphism associated with the collisional assembly of the crustal blocks in Sri Lanka at⁵40 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.Reginal correlation with other Gondwana terranes with similar lithology andgeochronology suggested that?1?the Highland Complex is linked with the Androyen–Anosyen Domains,South Madagascar,the Lützow–Holm Complex,East Antarctica,and the Trivandrum Block,South India during Gondwana time.in terms of detritus,magmatic and metamorphic record.They are all characterized with a distinct Paleoproterozoic?¹.8 Ga?detrital and magmatic age peak,with minor/no Neoproterozoic record at¹000–800 Ma.Negative zircon?Hf?t?or bulk-rock?Nd?t?values and old crustal model ages suggest that they are mostly sourced from the reworked Archean–Paleoproterozoic crust.Furthermore,all these four terranes show evidence of a very long-lived Neoproterozoic–Cambrian?⁶60–480 Ma,more than 150 Myr?granulite facies metamorphism with peak temperature reaches UHT conditions?>900°C?.Also with broadly consistent clock-wise P–T trajectories,and metamorphic zircon age peaks at⁵50 Ma.which is associated with the final assembly of Gondwana.The Wanni Complex has strong similarity with the Southern Madurai Block,South India,they are both characterized with¹.0–0.7 Ga magmatic and detrital record with juvenile Hf and Nd isotopic at¹.1 Ga and become more evolved through time,they are interpreted to formed by the involvement of both ancient and juvenile crust component.We predict that the Wanni Complex and the Southern Madurai Block are located at the western Margin of Rodinia as a Neoproterozoic magmatic arc.The Vijayan Complex is equivalent with the Dronning Maud Land in East Antarctica,and the Lurio Foreland in East Africa in terms of magmatic and metamorphic records,according to the common widely occurred juvenile 1.1–0.7 Ga magmatism,with two peaks at¹.1 Ga and⁰.95 Ga,the radiogenic Hf and Nd isotopes suggest the derivation of juvenile component,the geochemical data suggested oceanic arc-like,subduction-related magma affinity.The offset of metamorphic ages from those terranes shows a west youngling trend which suggests a progressive zipping of Gondwana from east towards the west.