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 to granulites in unbroken succes quartz and hence only quartz-bound fluids were sions on a regional scale, and is thereby taken considered for detailed examination. They are to represent the highest metamorphic "grade" generally distributed along healed fractures reached in a terrane, such an assumption is not (Fig. 3, insets). These arrays are, of two cate well founded especially where the reaction is gories, one pinching out in the middle of individ induced by fluid movement, as discussed later. ual grains (early arrays, depicted as E in Fig. 3) and the other cutting several grain boundaries, post-dating the former arrays (late arrays,

Fig. 2. Field photographs showing deep crustal path-ways exposed in the granulite terrane of Sri Lanka. A, The Kurunegala quarry showing discordant veins and patches and concordant layers of incipient charnockites developed within upper amphibolite facies gneisses. B, Dehydration and incipient charnockite formation along oriented patches at Kurunegala. C, A closer view of part of the above photo, showing the greasy green appearance of the incipient charnockites. The length of the photograph is about 2m. D, Enlarged view of an incipient charnockite patch showing the coarsening of grain size, disruption of gneissic foliation and formation of dry mineral assemblage (dark clots are orthopyroxene crystals). The length of the photo measures 30cm. E, Charnockite, formation along a mylonite band cutting small upright folds and associated foliation. Locality: Digana quarry, west of Kandy. F, An example of rehydra tion and "decharnockitization" along later pegmatite veins which served as path-ways for hydrous fluids. The dark veins on the right side of the photograph are the incipient charnock ites. This photograph shows that prograde and retrograde charnockitization occur in the same localities, within the scale of a few decimeters. Fluidcharacteristics across a gneiss-chamockitereaction front in Sri Lanka 31

B 32 M.Santosn, Masaru Yoshida and V. Nanda-Kumar

D Fluidcharacteristics across a gneiss-eharnockitereaction front in SriLanka 33

F 34 M. Santosh, Masaru Yoshida and V. Nanda-Kumar

Fig. 3. Sketches of fluid inclusions in gneisses (a) and charnockites (b) from the Kurunegala gneiss- incipient charnockite front. The insets show distribution pattern of inclusions and the in creased abundance of carbonic inclusions in the charnockites. Abbreviations: gb, grain boundary; Qz, quartz; E, early arrays; L, late arrays. The bar scales represent 15 microns.

depicted as L in Fig. 3). Both gneiss and char sure and temperature interval at some stage nockite contain fluid inclusions, but their abun during the uplift of the metamorphic rocks (cf. dance is contrastingly high in the latter (inset Roedder, 1984; Santosh, 1986). sketches in Fig. 3). Type III: The third category of inclusions are Type I: The common category in all cases is characterised by a vapor bubble and liquid at inclusions with a single fluid phase, which is room temperature and correspond to aqueous completely filled with a homogeneous phase at inclusions which trapped a water-rich fluid room temperature. These carbonic inclusions during uplift. These inclusions essentially (see below) are distributed along the early occur along late arrays, representing rehealed fractures in quartz. They are the most abun fractures which cut several grains. They are dantly represented category in the charnockite. more abundant in the gneisses than in char Type II: The second inclusion type is two nockites. phase at room temperature, in which a third Type IV: A fourth category of inclusions, fluid phase nucleates upon slight cooling. found in some gneiss quartz, is characterised by These phases consist of CO2(liquid)+CO2 highly irregular cavity shapes and containing (gas)+H2O(liquid). Such mixed carbonic cubic daughter crystals, with the fluid phase - aqueous inclusions occur mainly along early appearing to have leaked out. They represent arrays in the gneisses where they coexist with transposed brine inclusions which preserve the one phase carbonic inclusions. Such inclu premetamorphic fluid history of the gneisses. sions, although lesser in abundance, also occur Fluid inclusions in the samples show vary along late arrays in the charnokites where they ing size and shape patterns. In general , the coexist with water-rich inclusions. The car carbonic inclusions have various forms , bonic-aqueous inclusions within the same frac ovoidal, diamond-shaped, polygonal or irregu ture occasionally have various filling ratio lar. The water-rich inclusions are tabular , (vapor/liquid ratios), suggesting the presence elongate or polygonal . Carbonic and carbonic of heterogeneous fluids within a narrow pres aqueous inclusions have larger cavities ranging Fluid characteristics across a gneiss-charnockite reaction front in Sri Lanka 35

up to 30 microns in length as compared to ite is absent. Nitrogen has been detected in

aqueous inclusions, which are generally small fluid inclusions from high grade metamorphic

(less than 10 microns). terranes (eg: Swanenberg, 1980). In the Sri

Lankan granulites, the dominant ambient fluid Microthermometry species has a composition approximating to

Carbonic inclusions were frozen into solid pure carbon dioxide as indicated by the melting aggregates and on slow warming, they showed temperatures of majority of the inclusion popu

first melting temperatures in the range of lation. The gneiss-bound fluids are also near -55 .5•Ž to -58•Ž, with a peak near -56.6•Ž pure carbonic. (Fig. 4A), which is the triple point for pure Upon continued heating, the fluid homoge

carbon dioxide, indicating that the trapped fluid nizes into the liquid phase in all cases. The

phase has an almost pure carbon dioxide com one phase carbonic inclusions (Type I) in the

position. The carbonic phase within carbonic charnockites yield the lowest homogenization - aqueous inclusions also melted close to this temperatures (6•Ž; Fig. 4B), the peak

temperature. Slight depression in the melting homogenization temperatures translating into a

temperatures in some inclusions might indicate density of about 0.87g/cm3. Homogenization

the presence of traces of additional fluid temperatures of the carbonic inclusions in the

species. Methane and nitrogen are the com gneisses and mixed carbonic-aqueous inclu mon candidates which lower the melting tem sions in the charnockites overlap, with a peak

peratures of carbonic inclusions in high grade around 22-26•Ž. Some of the mixed carbonic- metamorphic rocks. Phase equilibria consider aqueous inclusions in the gneisses homogenize

ations require graphite to be present in order to at higher temperatures (up to 28•Ž), indicating

have CO2-CH4 mixtures at the P-T conditions lower densities.

of charnockite formation (cf. Lamb and Valley, The coexistence of carbonic-aqueous (type

1984). Therefore, methane can be ruled out as II) and aqueous (type III) inclusions along late

an unlikely species in the present case as graph fractures in the charnockites is important in

Fig. 4. Microthermometric data on carbonic and mixed carbonic-aqueous inclusions in gneisses (GN) and charnockites (CH) from Kurunegala. A, CO2 melting temperatures; B, CO2 homogenization temperatures. N represents number of observations. 36 M. Santosh, Masaru Yoshida and V. Nanda-Kumar constructing the evolutionary history of the varying CO2-H2O filling ratios were observed fluids vis a vis the uplift tectonics of the to coexist, implying that at some stage during metamorphic rocks (cf. Santosh, 1985, 1987). metamorphic uplift, the fluids were heter

Especially, in the present case, there is compel ogenous. Homogenization temperatures of ling optical evidence for a genetic correlation H2O-rich inclusions and the total homogeniza between the two categories, as inclusions with tion temperatures of CO2-H2O inclusions are

shown in Fig. 5. In the latter case, most of the

inclusions decrepitated before attaining com

plete homogenization of the two phases, which reflects high internal pressures. Aqueous inclu

sions are homogenized into the liquid phase,

denoting entrapment from a hydrothermal

medium. Some of these which have high

vapor/liquid ratios were homogenized into

vapor phase, having been trapped from a

pneumatolytic medium. In Fig. 5, it is interest ing to note that the homogenization tempera

tures of the two categories of inclusions overlap

at the range of 350-400•Ž. Entrapment tem Fig. 5. Homogenization temperatures of aqueous inclusions and total homogenization (CO2+ perature and pressure conditions of coexisting H2O) temperatures of carbonic-aqueous carbonic-aqueous and aqueous inclusions can inclusions in the incipient charnockites be derived simultaneously from the intersec from Kurunegala. N, number of inclu sions. tion point of their corresponding isochores in a

Fig. 6. Isochores (with density values) of CO2-H2O and H2O inclusions within late fractures in char nockite quartz shown in P-T space. The intersection point of the two isochores yield the P- T conditions at which they were simultaneously entrapped. The inset shows the solvus crest of CO2-H2O system at various salinities. Data sources and discussion are given in the text . Fluid characteristics across a gneiss-charnockite reaction front in Sri Lanka 37

P-T space. However, this requires a precise (7000; 7 kbar). They found that although knowledge on the compositions and densities of inclusion shape had a minor effect on Td, inclu the two categories of inclusions in order to sion size was a dominant factor. Applying the constrain their isochores. We have selected a empirical relations derived from experimental pair of representative CO2-H2O and H2O inclu studies to natural carbonic inclusions, as the sion in the charnockite for this procedure. carbonic inclusions from the Bamble granulites

Microthermometric measurements indicated studied by Swanenberg (1980), pressures of 3- that the aqueous fluid in the water-rich inclu 3.5 kbar at 400-500•Ž are indicated for the sion has a salinity of about 6 wt per cent NaCl decrepitation of 10 micron size inclusions. equivalent as inferred from the ice melting Here, we employ a visual decrepitation temperature. The CO2 isochore for the obser technique developed by Santosh et al. (1988) to ved degree of fill and H2O isochore for the obtain a semi-quantitative estimate of the estimated salinity are shown in Fig. 6. Their abundance of carbon dioxide in the gneiss-char intersection at ca. 485•Ž and 1.8 kbar pressure nockite reaction fronts in Sri Lanka. The yields the P-T values under which they were results are assembled in the histograms in Figs. simultaneously entrapped. Experimental 7a and b. Decrepitation of carbonic inclusions determinations in the CO2-H2O system indicate in most cases involved the rupture of groups or that although the solvus crest is only at 275•Ž at arrays of inclusions by crack formation. How

1 kbar for pure carbon dioxide and water ever, some individual explosions were observed mixtures (Todheide and Franck, 1963), the at lower temperatures. Each crack propaga presence of electrolytes greatly expands this tion in the field of view of the microscope is solvus (Takenouchi and Kennedy, 1965) due to documented as one decrepitation event (DE), strong partitioning of salts in the aqueous phase although it may involve the rupture of up to 20 relative to the carbonic phase. Experimental inclusions in the peak rupture region. Decre studies of Gehrig et al. (1979) showed that for pitation at the lower temperature region (up to a fluid containing 6 wt per cent NaCl in the 500•Ž) is mainly contributed by the rupture of aqueous phase, the solvus crest is at about mixed carbonic-aqueous inclusions. In gen

420•Ž at 1.5 kbar, which is close to the estimate eral, the highest density one phase carbonic obtained from microthermometric measure inclusions present in the samples decrepitate ments of natural fluid inclusions in this study. above 500•Ž. The 500-800•Ž peak range (cf. Fig. 7) results from crack propagation and

Visual Decrepitation Studies rupturing of such inclusions. Decrepitation

Because of high internal pressures, inclu modelling suggests that the inclusion size is one sions generally explode when they are heated of the major factors that control the tempera above their temperatures of homogenization. ture of rupturing. The sharp increase in decre

The temperature at which a fluid inclusion pitation rate with temperature may correspond decrepitates (Td) depends on several variables to the beginning of rupture of volumetrically such as inclusion shape and size of the cavity, important larger carbonic inclusions. fluid composition and density of the fluid, struc Although the temperatures of decrepitation in ture and grain size of the host mineral etc. both gneiss and charnockite are overall similar,

Bodnar et al. (1989) carried out a decripitation the gneiss-bound inclusions yield much broader study of hydrous fluid inclusions in quartz peak than those in charnockite. synthesized under metamorphic conditions Of particular relevance is the large con 38 M. Santosh, Masaru Yoshida and V. Nanda-Kumar

Fig. 7. Histograms showing visual decrepitation results of fluid inclusions in (a) incipient charnockites (CH) and (b) gneisses (GN) from Kurunegala. Note the contrastingly higher decrepitations in the charnockites as compared to the gneisses (c). DE, number of decrepitation events; TD, temperature of decrepitation; CY, yield of carbon dioxide. See text for discussion. Fluid characteristics across a gneiss-charnockite reaction front in Sri Lanka 39 trast in the DE values between gneiss and char symplectites after garnet, both of which are nockite, yielding a significantly higher peak for indicative of formation under a decompres the charnockite than the gneiss. Although the sional regime. Consequently, the dehydration temperature range for both peaks overlaps, the reaction appears to have been attained follow peak height is almost double in charnockite ing peak pressure conditions, an inference (Fig. 7c). The DE value corresponds to the supported by the observation of Hiroi et al. number of decrepitation events, which in turn is (1990) and the P-T estimates in cordierite- a measure of the yield of carbon dioxide (de bearing equilibria computed from the data picted as Cy in Fig. 7c). Thus it follows that presented in Asami et al. (1990). All of them the charnockite-bound inclusions yield carbon indicate low pressure conditions of about 3-5 dioxide which is quantitatively almost double kbar at 650-800•Ž. However, it should be than that yielded by the gneiss-bound ones. noted that these P-T conditions correspond to Stepped thermal experiments in the south In the incipient charnockites and cordierite-bear dian incipient charnockite horizons have also ing younger granulites only, while the older yielded a similar picture, with 2-3 times more regional granulites were formed at relatively abundnce in CO2 in the charnockites than in the higher pressures as compared to these. Incipi gneisses (Santosh et al., 1988). ent chrnockites frequently cross-cut deforma tion fabrics such as gneissic foliation or mig

DISCUSSION matitic structures in the rock. The charnock The intimate association of gneiss and ites are generally coarser grained than the granulite as observed in the incipient charnock gneisses and have a granular texture, developed ite horizons in Sri Lanka precludes a purely as linear formations or dendritic arrays around temperature controlled transformation from shear zones and brittle fractures. These fea gneiss to granulite and also argues against an tures suggest that the incipient charnockites origin by metamorphism of anhydrous lith can develop under brittle conditions as opposed ologies. Since the charnockite-forming reac to the earlier ductile dominated gneiss-forming tion involves biotite and/or amphibole conver regime. Evidences from both microtextures sion to orthopyroxene, which is the character and fabric analysis imply that the incipient isitic reaction for the transformation from charnockite formation was a fluid controlled gneiss to granulite facies assemblages, these process, which occurred along deformationally dehydration reactions have often been taken to enhanced zones of fluid flow during uplift. We represent the highest metamorphic "grade" seek to associate this period with fluid entrap reached. However, where the reaction is in ment. duced by fluid movement as in the case of the Fluid inclusion evidences indicate that car incipient charnockites, such an assumption may bon dioxide was the ambient fluid species which not be well founded. In the areas where garnet effected dehydration. The significant increase is present, it is generally seen to be consumed in the abundance of CO2 in the charnockites by the charnockite-forming reaction in a num relative to the precursor gneisses, as observed ber of localities (eg: near Kandy). In cases from inclusion petrography and visual decre where cordierite is present in the incipient pitation, must be related to charnockite forma charnockites, we have observed reaction tex tion and hence carbonic fluids were not captur tures including cordierite-quartz symplectites ed during a low-temperature post-granulite around garnet and cordierite/hypersthene event. If the fluids were retrogressive, pervad 40 M. Santosh, Masaru Yoshida and V. Nanda-Kumar

Fig. 8. (A), The H2O contents of C-O-H fluids in equilibrium with the incipient charnockites (as inferred from the biotite-orthopyroxene-K-feldspar-quartz equilibria by Hansen et al., 1987). The hatched box represents the P-T conditions of the charnockite formation discussed in the text. (B), T-XH2O equilibria for the gneiss-incipient charnockite front (constructed from activities as computed from the biotite-quartz-orthopyroxene-alkali feldspar assemblage by Santosh et al., 1990). See text for details.

ing the rocks after charnockite formation, no ite formation. such systematic difference between gneisses Fig. 8A shows the water contents of C-O-H and charnockites would be expected. In fact, fluids in equilibrium with incipient charnockites some of the field evidences for late rehydration based on biotite-orthopyroxene-K-feldspar- as shown in Fig. 2F indicate that the post fluid equilibrium as given by Hansen et al. metamorphic fluids were dominantly water (1987). For the P-T conditions recorded by -rich. The close correlation between granulite the Sri Lankan rocks, the mol fraction of water assemblages and total CO2 abundance must be (XH2O)in a vapor phase in equilibrium with the inherited from the time of charnockite forma charnockites was less than 0.35. Santosh et al. tion. It is therefore probable that the char (1990) calculated the XH2Oin a gneiss-charnock nockite formation and entrapment of carbonic ite reaction boundary in southern India based fluids were broadly synchronous events. There on mineral and fluid phase equilibria modelling. are two possibilities to explain why the char As shown in Fig. 8B, a shift of about 0.1 XH2O nockites contain more carbon dioxide than the could drive the dehydration reaction at the adjacent gneisses. The most obvious is that observed temperature. the pore fluids were richer in CO2, but it is Fluid evolution in these rocks can be traced possible that during the charnockite recrystal from an early near-pure carbonic regime lization, inclusion fluids were readily trapped in through an intermediate mixed carbonic-aque the growing minerals. Even if the second pos ous stage to a late aqueous stage . The present sibility is true, the capture of CO2-rich fluids study does not resolve whether the P-T condi must have occurred at the time of the charnock tions derived for the simultaneous entrapment Fluid characteristics across a gneiss-charnockite reaction front in Sri Lanka 41

Fig. 9. The metamorphic uplift path at Kurunegala based on combined P-T data from solid and fluid phase equilibria. Thin lines with numerical values indicate CO2 isochores (from Touret and Bottinga, 1979) for corresponding densities. The hatched area showing regional P-T range represents the regime of early carbonic fluids. The late fluid unmixing (as derived in Fig. 6) is shown as stippled area where mixed inclusions coexist. The inset shows uplift paths for charnockites from the Kerala region in South India (Santosh, 1985, 1987) and Lutzow-Holm Bay region in East Antarctica (Motoyoshi, 1990). See text for discussion. 42 M. Santosh, Masaru Yoshida and V. Nanda-Kumar of CO2-H2O and H2O inclusions represent a shi, 1990; Yoshida et al., 1990b). The close stage of unmixing of an originally homogenous similarity between the P-T trajectories for the carbonic-aqueous fluid or mixing of late aque Sri Lankan and Lutzow-Holm Bay granulites ous fluids with the early carbonic fluids. How could be an added evidence for their, juxtaposi ever, the pressure and temperature constraints tion during the early geologic history. The thus derived offer an important estimate of the occurrence of the incipient charnockites and low temperature path which the rocks might cordierite granulites in close similarity, with have followed during the metamorphic uplift. comparable isothermal decompression histories The uplift path derived from the combined solid and uplift paths in Sri Lanka and the Kerala and fluid data is shown in a P-T space in Fig. 9. Khondalite Belt at the southern tip of the In For a regional granulite facies temperature dian Peninsula, also calls for a radically ren range of 650-800•Ž, the isochores for the highest ewed look at the present configuration patterns density carbonic inclusions indicate an entrap of the Gondwanian jigsaw puzzle of continental ment pressure of nearly 4.8 kbar, an estimate juxtapositions. Similar tectonothermal his which is closely corresponding to that of the tories among the deep crusts in Sri Lanka low-pressure type granulite metamorphism - Lutzow-Holm Bay (Antarctica)-South India, responsible for the generation of the incipient comprising earlier isoclinal folding and regional charnockites of Sri Lanka (Hiroi et al., 1990; granulite metamorphism, subsequent passive Santosh and Yoshida, in prep.). The uplift folds with amphibolite facies overprinting and path, drawn-in through the peak fluid entrap later quasi-ductile to brittle deformation as ment at high P-T conditions and mixed fluid sociated with the incipient charnockite forma entrapment at intermediate to low P-T condi tion make a strong case for a correlation of the tions (Fig. 9) define a piezothermic array with high grade metamorphic terranes in these conti its convexity towards the temperature axis. nents (cf. Yoshida and Santosh, 1987; Yoshida Similar T-convex paths were obtained for the et al., 1990b; Yoshida and Hiroi, in press). south Indian charnockites (Santosh, 1985, 1987) One of the important corollaries of our present and for Antarctic granulites from the Lutzow- work is to predict the possibility of occurrence Holm Bay area (Motoyoshi, 1990) (shown as of gneiss-granulite reaction fronts along struc inset in Fig. 9). Such T-convex paths result turally enhanced zones of fluid flow in Antar when uplift takes place from higher to lower ctica. This calls for intensified field investiga pressures at a near-constant temperature in tions in the Antarctican granulite terranes to response to decompression in a distensional resolve the interesting question of whether tectonic regime. A virtually isothermal uplift incipient charnockite formation is a common history is also in keeping with the field relations phenomenon in all the Gondwanian continental of the incipient charnockites and the mineral fragments. If so, it would be of great rele reaction textures. vance in understanding the deep crustal and

An interesting implication of this observa crust-mantle interaction processes and fluid tion is its bearing on the geologic correlation flow mechanisms during the Pan-African times . invoked among the Gondwanian crustal seg ments. The juxtaposition of Sri Lanka with Acknowledgements: The senior author's the Lutzow-Holm Bay region of Antarctica research in Japan was supported by a Fellow during the Gondwanian period is a topic of ship from the Japan Society for Promotion of current intense perusal (eg: Hiroi and Motoyo Science. The idea contained in this paper has Fluid characteristics across a gneiss-charnockite reaction front in Sri Lanka 43 benefitted from our field experience in the south Kagami, H., Owada, M., Osanai, Y. and Hiroi, Y. Indian granulite terrain and collaborative (1990), Preliminary geochronological study of Sri Lankan rocks. In Study of geological research work with various teams, especially correlation between Sri Lanka and Antarctica the Open University (U. K.) team. We thank (Hiroi, Y. and Motoyoshi, Y. Eds.). Interim the two Referees of this Journal, especially Dr. Report of Japan-Sri Lanka Joint Research, Makoto Watanabe of the Hiroshima University 55-70. Kroner, A., Williams, I. S., Compston, W., Baur, N., for the helpful comments which greatly aided in Vitanage, P. W. and Perera, L. R. K. (1987), improving an earlier version of this manuscript. Zircon ion microprobe dating of high-grade rocks in Sri Lanka. J. Geol., 95, 775-791. Motoyoshi, Y, (1990), A review of P-T evolution References of high-grade metamorphic terranes in East Asami, M. (1990), Secondary cordierite in a sil Antarctica. In Study of geologic correlation limanite-garnet gneiss from the Kegalla dis between Sri Lanka and Antarctica (Hiroi, Y. trict, Sri Lanka. In Study of geologic corre and Motoyoshi, Y. Eds.). Interim Report of lation between Sri Lanka and Antarctica Japan-Sri Lanka Joint Research, 132-139. (Hiroi, Y. and Motoyoshi, Y. Eds.). Interim Newton, R. C. (1989), Metamorphic fluids in the Report of Japan-Sri Lanka Joint Research, 3- deep crust. Ann. Rev. Earth Planet. Sci., 17, 43. 385-412. Bodnar, R.J., Binns, P.R. and Hall, D.L. (1989), Roedder, E. (1984), Fluid inclusions. Rev. in Synthetic fluid inclusions-VI. Quantitative Mineralogy 12, Mineral. Soc. Amer., 644p. evaluation of the decrepitation behaviour of Santosh, M. (1985), Fluid evolution characteristics fluid inclusions in quartz at one atmosphere and piezothermic array of south Indian char confining pressure. J. Matamorphic Geol., 7, nockites. Geology, 13, 361-363. 229-242. Santosh, M. (1986), Carbonic metamorphism of Gehrig, M., Lentz, H. and Franck, K. U. (1979), charnockites in the southwestern Indian Thermodynamic properties of water-carbon shield: a fluid inclusion study. Lithos, 19, 1-10. dioxide-sodium chloride mixtures at high tem Santosh, M. (1987), Cordierite gneisses of southern peratures and pressures. In High pressure Kerala, India: petrology, fluid inclusions and science and technology I, properties and mate implications for crustal uplift history. rial synthesis (Timmerhaus, K.D. and Barber, Contrib. Mineral. Petrol., 96, 343-356. M.S. Eds). Plenum, New York, 539-542. Santosh, M., Jackson, D. H., Mattey, D. P. and Hansen, E.C., Janardhan, A.S., Newton, R.C., Harris, N. B.W. (1988), Carbon stable isotopes Prame, W.K. B. N. and Kumar, G.R. R. (1987), of fluid inclusions in the granulites of southern Arrested charnockite formation in southern Kerala: implications for the source of CO2. India and Sri Lanka. Contrib. Mineral. Pet J. Geol. Soc. India, 32, 477-493. rol., 96, 225-244. Santosh, M., Harris, N. B.W., Jackson, D. H. and Hiroi, Y. and seventeen co-authors (1990), Arrest Mattey, D.P. 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etat pour le CO2: application aux inclusions dwana. In Study of geologic correlation carboniques. Bull. Mineral., 102, 635-649. between Sri Lanka and Antarctica (Hiroi, Y. Yoshida, M., Kehelpannala, K. V. W., Hiroi, Y. and and Motoyoshi, Y. Eds.). Interim Report of Vitanage, P.W. (1990a), Sequence of deforma Japan-Sri Lanka Joint Research, 118-131. tion and metamorphism of granulites of Sri Yoshida, M. and Santosh, M. (1987), Charnockite Lanka. J. Geosci., Osaka City Univ., 33, 69- "in the breaking" and "making" in Kerala , 102. south India: Tectonic and microstructural Yoshida, M., Funaki, M. and Vitanage, P.W. evidences. J. Geosci. Osaka City Univ., 30, 23- (1990b), Juxtaposition of India-Sri Lanka- 49. Antarctica in Proterozoic to Mesozoic Gon

スリランカのチャルノカイト生成反応における流体相の役割ゴンドワナ地殻深部におけるグラニュライト形成の考察

M.サントシ・吉田勝・V.ナンダクマ-ル

スリランカ先カンブリア代グラニュライト地域の,各所の片麻岩類中に産出する脈状,プール状,斑点状などの"Incipient charnockite"は,グラニュライト生成が明らかに流体の関与によって行われたよい例である。片麻岩中の黒雲母や角閃石は斜方輝石へ壊変し,岩石は最終的に粗粒なチャルノカイトになる。模式的露頭であるクルネガラで,片麻岩がチャルノカイトに変化する微小部分の流体包有物の変化を検討した。チャルノカイト生成時における流体相は,溶解温度からは殆ど純粋なCO2で,比重0.87g/cm3である。流体組成は早期から後期のものへと,純粋なCO2-CO2とH2O混合相-H2Oの変化を示す。研磨切片の鏡下での流体包有物破裂観察結果の解析では,チャルノカイト中のCO2の量は片麻岩類中の2倍近くであった。このことは,系外起源の流体の侵入が脱水効果をもたらし,チャルノカイト生成の原因となったことを示唆する。流体相平衡の考察を固相平衡から得られている結果と重ね併せると,スリランカのグラニュライトは温度軸にたいして上に凹なP-Tカーブをもつ,等温上昇的な経過をたどったことが示される。このような等温減圧を特徴とする上昇過程は,ゴンドワナにおいてスリランカと対置される南インドや南極昭和基地付近の変成岩類にも認められており,ゴンドワナ全域にわたる巨大造構過程を反映している。