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Fluid Characteristics Across a Gneiss-Charnockite Reaction Front in Sri Lanka: Implications for Granulite Formation in Gondwanian Deep Crust

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Fluid Characteristics Across a Gneiss-Charnockite Reaction Front in Sri Lanka: Implications for Granulite Formation in Gondwanian Deep Crust J. Min. Petr. Econ. Geol. 86, 27-44, 1991 Fluid characteristics across a gneiss-charnockite reaction front in Sri Lanka: Implications for granulite formation in Gondwanian deep crust M. Santosh*,**, Masaru Yoshida* and V . Nanda-Kumar** * Department of Geosciences, Faculty of Science, Osaka City University, Osaka 558, Japan ** Centre for Earth Science Studies, P. B. 7250, Akkulam, Trivandrum 695 031, India Patches, veins and oriented zones of "incipient charnockites" occurring in the Precambrian granulite terrane of Sri Lanka provide compelling evidences for fluid-controlled granulite genesis. Transformation from gneiss to granulite involves breakdown of biotite or amphibole to orthopyr oxene, with the resultant coarse charnockitic assemblage testifying to increased reaction kinetics aided by the influx of fluids. Fluid inclusion studies across a typical gneiss-charnockite reaction front at Kurunegala reveal that carbon dioxide was the ambient fluid species during incipient charnockite formation, with a melting temperature close to that for pure CO2 and a density of 0.87 g/cm3. Fluid evolution is traced from early pure carbonic through intermediate mixed carbonic- aqueous to late aqueous regime. Visual decrepitation of carbonic inclusions in polished wafers indicate that the charnockite entrapped almost double the amount of CO2 as compared to the gneiss, indicating external addition of fluids which effected dehydration. Combined solid-fluid equilibria define a P-T path characterized by its convexity towards the temperature axis, suggest ing an isothemal uplift history. The close similarity between the decompression-related metamorphic uplift paths for Sri Lanka, South India and Antarctica strenghthen the current discussions on the juxtaposition of these continents in the Gondwana reconstruction. the anhydrous mineral, orthopyroxene, and INTRODUCTION represent an "arrested" stage of transformation Fluid processes and fluid-rock interaction of the gneisses to a dry granulite assemblage, in the deep crust are best documented from post-dating all penetrative deformational granulite facies terranes since granulites repre events. The granulite assemblage shows abun sent the uplifted portions of the roots of the dant CO2-rich inclusions, invoking carbon diox earth's crust. Classic examples of deep-crus ide as the major dilutant of water, derived from tal fluid pathways have recently been reported an external reservoir and pervaded by from southern India, where amphibolite facies advective flow from the centre of the charnock gneisses coexist on a decimeter scale with ite lenses (Jackson et al., 1988; Santosh et al., patches, veins and oriented zones of granulites, 1990). precluding a temperature-controlled origin and Recent investigations in the adjacent Sri invoking the role of fluids (Hansen et al., 1987; Lankan terrane have revealed the widespread Santosh et al., 1988; Newton, 1989). These occurrences of arrested charnockite formation granulite patches, commonly referred to as with striking evidence of fluid channels (Han "incipient charnockites" , are characterized by sen et al., 1987; Hiroi et al., 1990). This local (Manuscript received, September 14, 1990; accepted for publication, October 22, 1990) 28 M. Santosh, Masaru Yoshida and V. Nanda-Kumar granulite formation mechanism is distinctly of the country gneisses (Fig. 2A, B, C, D). In contrasted from the earlier regional granulite many cases, the foliation of the country rocks facies event, in that the former is characteristi gets warped and pulled into the shears which cally a late, low-pressure metamorphic over are filled with charnockitic pods or veins. The print along structurally-controlled locales of pods and veins occur either scatterred or inter fluid infiltration (Yoshida et al., 1990a). If linked, both representing an intricate network indeed the petrogenesis of these vein-type of fluid channels at depth. We could also find a granulites has been largely controlled by fluids, few cases where charnockitization proceeds the best evidences for this should come from along mylonitic shear bands (Fig. 2E). Pink fluid inclusions within minerals, because the granitic pegmatites, sometimes containing cor coarse-grained charnockite patches appear to dierite and rarely andalusite, are found cut by have formed in a fluid-rich environment where charnockite veins. Interestingly, there enhanced reaction kinetics have promoted the appears to have been a late rehydration along coarsening of grain size and such conditions are quartz+feldspar veins and influx of water-rich conducive for the entrapment of small portions fluids, resulting in the bleaching and retrogres of the ambient fluid species within growing sion of charnockites along the margins of these crystal faces or microfractures. Apart from veins (Fig. 2F). Field and microstructural evi the nature of fluids which attended the dehydra dences offer a clear case for incipient charnock tion process, study of the various inclusion ite formation postdating the major tectonother categories could also yield valuable informa mal events in the region, including the regional tion on the fluid evolution characteristics and granulite-amphibolite facies metamorphism uplift mechanism of these granulite veins. The (Hiroi et al., 1990; Yoshida et al., 1990a). aim of the present study is hence to systemat The age of the incipient charnockite formation ically characterize the nature, role and evolu has recently been constrained at 430 Ma based tionary characteristics of trapped fluid phases on whole-rock and mineral isochron dating involved in gneiss-charnockite reaction fronts from the Kurunegala quarry (Kagami et al., in Sri Lanka and to trace their metamorphic 1990). The age of regional metamorphism, on uplift path. the other hand, has been dated at ca. 1,000 Ma based on U-Pb zircon geochronology (Kroner CHARNOCKITE FORMATION IN SRI et al., 1987), suggesting at least two distinct LANKA charnockite formation events with the incipient Precambrian crystalline rocks constitute charnockites belonging to the younger event. the dominant portion of the Sri Lankan crustal For our present study, we have selected a segment, which have been divided into the gneiss-charnockite reaction front at the classic Highland Group, the Southwestern Group and Kurunegala quarry, which was described by the Vijayan Complex. Vein-type charnockite Hansen et al. (1987), as it offers spectacular formation is observed mainly in the central to field evidences for channelised fluid flow (Fig. western segment of Sri Lanka, which is under 2). Here, coarse charnockite veins , pools and lain by rocks belonging to the Highland Group, layers running nearly N-S and NNE with steep the Southwestern Group and the Western Vi dips have developed within biotite and horn jayan Complex (Fig. 1). Incipient charnocki blende-bearing grey gneisses which have NW tization takes place as pods, veins or branching trending foliation dipping moderately towards out "trees", regardless of the general foliation NE. Some of the N-S trending veins show Fluid characteristics across a gneiss-charnockite reaction front in Sri Lanka 29 Fig. 1. Geological outline of Sri Lanka showing incipient charnockite localities (filled circles). C, Colombo; K, Kandy; Ku, Kurunegala. shear-derived characteristics as judged from which have been subjected to upper amphibolite the distortion of foliation of the gneisses. facies metamorphism show a regional alterna Charnockite formation along the foliation tion with metapelitic garnet biotite gneisses, planes is also not rare. The country gneisses partly inheriting small isoclinal folding struc 30 M. Santosh, Masaru Yoshida and V. Nanda-Kumar ture and northerly plunging undulation linea tion. The alternation of gneiss-charnockite- FLUID INCLUSIONS metapelite shows macroscopic linear folding Doubly polished rock wafers ranging in with vertical axial plane running NNW, render thickness from 0.3 to 0.5mm were prepared ing evidence for the superposed folding episodes from gneiss-incipient charnockite close pair as the earlier isoclinal folding (D1-D2) and samples for fluid inclusion studies. Detailed later upright folding (D3) as demonstrated by microscopic studies were carried out to charac Yoshida et al. (1990a). The mode of occur terize the distribution pattern of inclusions and rence and the trend of shear veins and pools the nature of trapped fluids. Heating-freezing filled with the incipient charnockite offer com experiments to estimate the composition and pelling evidences for charnockite formation to densities of inclusions were carried out using a be unrelated to, and postdating D1-D2 and D3 temperature-calibrated CHAIXMECA micro events. thermometry apparatus. High temperature The charnockite veins and patches at Kur visual decrepitation experiments were perfor unegala show a coarse crystalline assemblage, med using a LEITZ-1350 Heating Stage for which is discordant with the regional foliation semi-quantitative estimation of the yield of of the gneisses (cf. Figs. 2C, D). Bands of carbon dioxide (cf. Santosh et al., 1988; Jack biotite+hornblende in the gneisses break down son et al., 1988). to coarse orthopyroxene when traced into these veins. The charnockite-forming reaction Inclusion petrography accompanies the breakdown of biotite and Fluid inclusions were found in quartz, feld amphibole to orthopyroxene. Though this spars (both perthite and plagioclase) and ortho reaction characterizes the transformation from pyroxene. Inclusions are more common in amphibolites
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