SiO2 precipitation in olivine: ATEM investigation of two dunites annealed at 300 MPa in hydrous conditions

Two natural dunites were annealed at pressure P=300 MPa, temperature T=1373, 1473 and 1573 K, and fO₂ within the stability field of olivine. The starting materials contained small amounts of hydroxyls in the form of minor phases of hydrated minerals, which released an aqueous phase during the experiments. A detailed analytical transmission electron microscopy (ATEM) investigation of these materials revealed that small quantities of two types of silica-rich glass formed during heat treatment. The first type of glass, found at triple junctions as rare partially crystallized glass pockets, results from melting dehydration reactions involving the hydrous phases. The second type of glass is found as pure silica precipitates (0.1–0.5 μm in size, for a total of a few 0.1 vol%) within the olivine grains of specimens heated to ≥1473 K. From considerations of the kinetics of the precipitation at 1473 K, we interpret this silica precipitation as resulting from the condensation of olivine metallic vacancies promoted by increasing fluid fugacities during the runs. Our observations, thus, demonstrate that metastable silica can precipitate in olivine from dunites experiencing rapid changes in their thermodynamical environment.     

Geology of the High Grade Proterozoic Terrains of Sri Lanka, and the Assembly of Gondwana: an Update on Recent Developments

Nd model ages determined for the high-grade rocks of Sri Lanka delineate three crustal units, viz., the Highland Complex (HC), the Wanni Complex WC), and the Vijayan Complex (VC). The distribution of these three units differs considerably from the three geological divisions demarcated previously on the basis of geological mapping. The centrally located HC comprises mainly granulite grade charnockitic rocks, and metasediments characterized by older Nd model ages (2.0–3.4 Ga). The Highland sedimentary pile was thickened by intermittent granitoid intrusions, most of which are now charnockitic gneiss, and granulites, and basaltic sills, and dikes. All these metaigneous rocks now occur as conformable bands or layers due to intense polyphase deformation. The HC is bounded on the east by the amphibolite grade VC, composed mainly of granitic gneisses, basic gneisses, and migmatites, and they have ‘younger’ Nd model ages (1.1–1.8 Ga). The isotopic, and geochemical characteristics identify the precursors to the Vijayan rocks as I-type calc-alkaline granitoids originated at an ‘arc’-related tectonic environment. Thus, the earlier interpretation that the Vijayan rocks represent reworked HC was rejected. The granulite inliers within the VC, earlier considered as “resisters” to re-working, are now shown as overthrust klippen or rotated rafts of the HC. The WC, demarcated on the basis of Nd model ages (1.1–1.8 Ga) similar to those of the VC, lies west of the HC. It consists mainly of granitic gneisses, charnockitic gneisses, and migmatites, and the metamorphic grade ranges from amphibolite to granulite.Comprehensive geothermobarometric surveys constrain the P-T evolution of the three crustal units, and indicate that both the HC, and WC underwent near isobaric cooling, followed by a decompression with decreasing temperature. Extensive isotopic studies (U-Pb, Pb-Pb, Sm-Nd, Rb-Sr) have established a new geochronological framework for these high-grade rocks of Sri Lanka. The new framework has bracketed the age of high grade metamorphism in the three crustal units at 550–600 Ma.The recent advances in knowledge of the geology of Sri Lanka favour a strong geological correlation of the HC, and the VC of Sri Lanka, respectively, with the Lutzöw-Holm Complex, and the Yatmato-Belgica Complex in the East Antarctica. The geology of the WC suggests a possible correlation with Madagascar, and East Africa. The amalgamation of the three crustal units of Sri Lanka, is apparently related to the two distinct orogenic events that resulted in the assembly of the Gondwana supercontinent.     

Arrested charnockite formation in southern India and Sri Lanka

Arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high-grade terrains of southern India and Sri Lanka. Two major kinds of orthopyroxene-producing reactions are recognized. Breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic/granitic gray gneiss produced the regional orthopyroxene isograd, manifest in charnockitic mottling and veining of mixed-facies exposures, as at Kabbal, Karnataka, and in the Kurunegala District of the Sri Lanka Central Highlands. Chemical and modal analyses of carefully chosen immediately-adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of CaO, MgO and FeO and gains of SiO₂ and Na₂O. Rb and Y are depleted in the charnockite. Another kind of charnockitization is found in paragneisses throughout the southern high-grade area, and involves the reaction of biotite and quartz±garnet to produce orthopyroxene and K-feldspar. Although charnockite formation along shears and other deformation zones at such localities as Ponmudi, Kerala is highly reminiscent of Kabbal, close pair analyses are not as suggestive of open-system behavior. This type of charnockite formation is found in granulite facies areas where no prograde amphibole-bearing gneisses exist and connotes a higher-grade reaction than that of the orthopyroxene isograd. Metamorphic conditions of both Kabbaltype and Ponmudi-type localities were 700°–800° C and 5–6 kbar. Lower P(H₂O) in the Ponmudi-type metamorphism was probably the definitive factor.CO₂-rich fluid inclusions in quartz from the Kabbaltype localities support the concept that this type of charnockite formation was driven by influx of CO₂ from some deep-seated source. The open-system behavior and high oxidation states of the metamorphism are in accord with the CO₂-streaming hypothesis. CO₂-rich inclusions in graphitebearing charnockites of the Ponmudi type, however, commonly have low densities and compositions not predictable by vapor-mineral equilibrium calculations. These inclusions may have suffered post-metamorphic H₂ leakage or some systematic contamination.Neither the close-pair analyses nor the fluid inclusions strongly suggest an influx of CO₂ drove charnockite formation of the Ponmudi type. The possibility remains that orthopyroxene and CO₂-rich fluids were produced by reaction of biotite with graphite without intervention of fluids of external origin. Further evidence, such as oxygen isotopes, is necessary to test the CO₂-streaming hypothesis for the Ponmudi-type localities.     

Geochemistry and genesis of granitoid rocks from Southern Sri Lanka

Summary Pyroxene granulites of intermediate to felsic composition comprise a major part of the Precambrian terrain exposed in southern Sri Lanka. Chemical characteristics and field relations reveal that there are two closely associated groups of granitoids both occurring as discrete layers or deformed bodies. Granitic-adamellitic rocks are more common and can be distinguished from tonalitic-trondhjemitic rocks. Granitoids of these two groups and minor metabasalt and metadiorite/meta-andesite have been subjected to high-grade metamorphism and were mapped as charnockite or charnockitic rocks. Rocks of granitic composition are mostly meta-aluminous and, in the AFM diagram, fall in the tholeiitic field. These rocks are mainly A-type granites characterized by high K₂O/Na₂O, FeO/MgO and high Zr, Nb and LREE contents, most belonging to the relatively Ce- and Y-enriched A₂-type. Tonalitic rocks are meta-aluminous to per-aluminous and display a calc-alkaline trend. These rocks are classified as I-type, although some of them have very low K₂O and REE contents.Tonalites and granites are not products of a consanguineous fractionation process. In spite of their close field association, they appear to be polygenetic. The parent magmas of the A-type granites seem to have been differentiated melts derived from felsic continental crust. They were emplaced into normal or attenuated continental crust in a non-Compressive (extensional or trans-tensional) tectonic regime. Tonalites most probably represent volcanic arc granitoids emplaced in a compressive tectonic setting. Present intercalation of these polygenetic granitoids, generated in different tectonic environments, is a result of the complex tectonic activity which prevailed before or during Pali-African (550–610 Ma) granulite facies metamorphism.

---

Geochemie und Genese von Granitoiden aus dem süchen Sri Lanka

Zusammenfassung Pyroxengranulite intermediärer bis saurer Zusammensetzung bilden einen Hauptbe standteil des präkambrischen Terrains im südlichen Sri Lanka. Chemische Untersuchungen und Feldbeziehungen ermöglichen die Unterscheidung von zwei räumlich eng assoziierten Gruppen von Granitoiden, die beide lagenförmig oder in Form deformierter Körper auftreten. Granitisch-adamellitische Gesteine sind häufiger und sind von tonalitisch-tronhjemitischen Gesteinen zu unterscheiden. Diese Granitoide und untergeordnet vorkommende Metabasalte und Metadiorite/Metaandesite wurden von hochgradiger Metamorphose erfaßt und als Charnockite bzw. charnokitische Gesteine kartiert. Gesteine granitischer Zusammensetzung sind meist metaluminös und fallen im AFM Diagramm ins Feld der Tholeiite. Es sind A-Typ Granite des an Ce und Y angereicherten A2-Subtyps mit hohem K₂O/Na₂O, FeO/MgO und hohen Gehalten an Zr, Nb und LREE. Tonalitische Gesteine sind met-bis peraluminös und folgen einem kalk-alkalischen Trend. Sie werden als I-Typ Granite klassifiziert, obwohl einige Proben sehr niedrige K₂O und REE Gehalte aufweisen. Die Tonalite und Granite sind nicht die Produkte eines cogenetischen Fraktionie rungsprozesses und sind, trotz ihrer räumlichen Assoziation, polygenetischen Ursprungs. Die Ausgangsmagmen der A-Typ Granite scheinen differentierte Schmelzen saurer krustaler Herkunft zu sein, die in normal dicker oder verdünnter Kruste in einem nicht kompressionellen tektonischen Regime (ex- oder transtensional) intrudierten. Die Tonalite sind wahrscheinlich in einem Kompressionsregime enstandene Granitoide eines Vulkanbogens. Das heutige Nebeneinander dieser polygnetischen, in unterschiedlichen geotektonischen Bereichen gebildeten, Granitoide ist das Ergebnis komplexer tektonischer Aktivität vor oder während der panafrikanischen (550–610 Ma) granulitfaziellen Metamorphose.

---

---

With 11 Figures