Formation of garnet+ corundum during isobaric cooling at UHT conditions: an example from pelitic granulites of the Highland Complex, Sri Lanka (original) (raw)
Related papers
Journal of Mineralogical and Petrological Sciences, 2011
Corundum in direct contact with quartz and associated with andalusite has been found as an inclusion in alkali feldspar obtained from a leucosome, which forms a part of migmatized pelitic granulite from the Kerala Khondalite Belt, southern India. Thermodynamically, corundum + quartz is metastable relative to andalusite. Textural features suggest that it is a rare example of an arrested reaction corundum + quartz → andalusite which may have been triggered off by the introduction of H 2 Orich fluids along the cracks of the host alkali feldspar during the uplift and cooling of the leucosome. The metastable corundum + quartz association most likely originated during the crystallization of the Al 2 O 3 -rich leucosome melt when early corundum came into contact with later quartz under conditions with a relatively low H 2 O activity.
Geoscience Frontiers, 2017
Here we report the occurrence of garnet porphyroblasts that have overgrown alternating silica-saturated and silica deficient microdomains via different mineral reactions. The samples were collected from ultrahigh-temperature (UHT) metapelites in the Highland Complex, Sri Lanka. In some of the metapelites, garnet crystals have cores formed via a dehydration reaction, which had taken place at silicasaturated microdomains and mantle to rim areas formed via a dehydration reaction at silica-deficient microdomains. In contrast, some other garnets in the same rock cores had formed via a dehydration reaction which occurred at silica-deficient microdomains while mantle to rim areas formed via a dehydration reaction at silica-saturated microdomains. Based on the textural observations, we conclude that the studied garnets have grown across different effective bulk compositional microdomains during the prograde evolution. These microdomains could represent heterogeneous compositional layers (paleobedding/laminations) in the precursor sediments or differentiated crenulation cleavages that existed during prograde metamorphism. UHT metamorphism associated with strong ductile deformation, metamorphic differentiation and crystallization of locally produced melt may have obliterated the evidence for such microdomains in the matrix. The lack of significant compositional zoning in garnet probably due to self-diffusion during UHT metamorphism had left mineral inclusions as the sole evidence for earlier microdomains with contrasting chemistry.
Contributions To Mineralogy and Petrology, 2004
The present study from the Sausar Mobile Belt (SMB) in the southern part of the Central Indian Tectonic zone (CITZ) demonstrates how microdomainal compositional variation of a single garnet porphyroblast in a metapelite granulite sample records the different segments of a near complete P-T path of metamorphic evolution. The microdomainal variation is ascribed to the preservation of growth zoning and heterogeneous distribution of diverse inclusion mineral assemblages. Subsequent mineral reactions under changing P/T conditions were controlled by this compositional heterogeneity. Four stages of metamorphic evolution have been deciphered. An early prograde stage (Mo) is implied by the rare presence of staurolite-biotite-quartz and in places of kyanite inclusion assemblages in other metapelite samples, together with the growth zoning preserved in garnet. The peak metamorphism (M1) at ~9.5 kbar, ~850 °C is consistent with the biotite dehydration melting that produced garnet-K-feldspar and granitic leucosomes. This was followed by near isothermal decompression (M2) at ~6 kbar, ~825 °C, during which different garnet segments behaved as separate microscale bulk compositions and decomposed both internally and externally to produce different retrograde mineral assemblages. In the quartz-bearing domain of almandine-rich and grossular-rich garnet core, grossular components in garnet reacted with included sillimanite and quartz to produce coronal plagioclase (XAn=0.90). By contrast, grossular-rich garnet in quartz-absent domain reacted with included sillimanite to produce layered spinelss {XMg (Mg/Mg+Fe2+) = 0.23–0.26}, XAl (Al/Al+Fe3+)=0.71–0.81}-plagioclase (XAn=0.91)-cordierite {XMg (Mg/Mg+Fe2+) = 0.80–0.83} coronas both in the core and inner rim region of garnet. During post-decompression cooling, reactions occurred at about 600 °C (M3), whereby quartz-bearing, sillimanite-absent microdomains of pyrope-rich, grossular-poor garnet outer rim decomposed to form relatively magnesian assemblages of cordierite-anthophyllite and cordierite-biotite-quartz. M2 spinelss decomposed to polyphase domains of spinel-magnetite±högbomite at this stage. Collating the textural and geothermobarometric results, a clockwise P-T path has been deduced. The deduced P-T loop is consistent with a model of crustal thickening due to continental collision, followed by rapid vertical thinning, which appears to be the general feature of the Sausar Mobile Belt. Using model calculations of the preserved growth and diffusion zoning in garnet, we demonstrate rather short-lived nature of this collision orogeny (in the order of 40–60 Ma).
Journal of Petrology, 2020
High-grade metamorphic rocks underlying the intrusive layered dunite-pyroxenite-gabbronorite East-Khabarny Complex (EKC) are integrated in the complex Khabarny mafic-ultramafic Massif in the Sakmara Allochthon zone in the Southern Urals. These rocks are associated with hightemperature shear zones. Garnetites from the upper part of the metamorphic unit close to the contact with EKC gabbronorite are chemically and texturally analysed to estimate their formation conditions and fluid regime. Fluids provide crucial information of formation conditions and evolution of these garnetites during high-grade metamorphism, and are preserved in channel positions within Si 6 O 12À 18 rings of cordierite, and in fluid inclusions in quartz and garnet. Minerals and fluid inclusions of the garnetites are studied with X-ray fluorescence spectrometry, electron microprobe analyses, Raman spectroscopy, and microthermometry. The garnetites mainly consist of garnet (up to 80 vol. %), cordierite and quartz. Accessory minerals are rutile, ilmenite, graphite, magnetite and cristobalite. Granulite-facies metamorphic conditions of the garnetites are estimated with the garnet-cordierite-sillimanite-quartz geothermobarometer: temperatures of 740 to 830˚C and pressures of 770-845 MPa. The average garnet composition in end-member concentrations is 48Á5 mole % almandine (63Á9), 34Á7 mole % pyrope (63Á3), 10Á3 mole % spessartine (61Á1), 1Á8 mole % grossular (61Á5), and 1Á5 mole % andradite (61Á5). The cordierite electron microprobe analyses reveal an average Mg 2þ fraction of 0Á79 6 0Á01 in the octahedral site. Relicts of a strong positive temperature anomaly (up to 1000˚C) are evidenced by the preservation of cristobalite crystals in garnet and the high titanium content of quartz (0Á031 6 0Á008 mass % TiO 2) and garnet (0Á31 6 0Á16 mole % endmember Schorlomite-Al). The fluid components H 2 O, CO 2 , N 2 and H 2 S are detected in cordierite, which correspond to a relatively oxidized fluid environment that is common in granulites. In contrast, a highly reduced fluid environment is preserved in fluid inclusions in quartz nodules, which are mono-fluid phase at room temperature and composed of CH 4 (>96 mole %) with locally minor amounts of C 2 H 6 , N 2 , H 2 S and graphite. The fluid inclusions occur in homogeneous assemblages with a density of 0Á349 to 0Á367 gÁcm-3. The CH 4-rich fluid may represent peak-temperature metamorphic conditions, and is consistent with temperature estimation ($1000˚C) from Ti-in-garnet and Ti-in-quartz geothermometry. Tiny CH 4-rich fluid inclusions (diameter 0Á5 to 2 mm) are also detected by careful optical analyses in garnet and at the surface of quartz crystals that are included in garnet grains. Graphite in fluid inclusions precipitated at retrograde metamorphic conditions around 300-310 6 27˚C. Aragonite was trapped simultaneously with CH 4-rich fluids and is assumed to have crystallized at metastable conditions. The initial granulite facies conditions that led to the formation of a cordierite and garnet mineral assemblage must have occurred in a relative
Journal of Petrology
A suite of high-grade rocks including felsic gneiss, aluminous granulite, charnockite and calcsilicate granulite is exposed at Phulbani, which belongs to a petrologically little understood crustal domain (Phulbani domain) of the Eastern Ghats Belt. The aluminous granulite is constituted of corundum þ spinel þ ilmenite þ garnet þ sillimanite þ quartz 6 K-feldspar 6 plagioclase 6 biotite. Textural analysis indicates that corundum, spinel, garnet and/or K-feldspar were formed as a result of biotite dehydration melting of a Si-poor protolith during prograde metamorphism. Although corundum and quartz coexist in micro-scale domains, phase diagram modelling suggests that garnet þ corundum þ spinel þ ilmenite þ sillimanite (up to 800 C at 8 kbar) and garnet þ spinel þ sillimanite þ ilmenite þ quartz (above 950 C at 8 kbar) assemblages were stabilized in two different temperature intervals while attaining the ultrahigh-temperature metamorphic peak. The transformation from corundum-to quartz-bearing assemblages was principally governed by chemical reactions. Quartz, formed at the peak stage, produced complex reaction textures involving spinel, corundum, garnet and sillimanite during near-isobaric cooling. Intersection of the same mineral reactions during the prograde and the retrograde paths implies the near-closed-system behaviour of the lower crust, at least at microdomain-scale, possibly achieved after large-scale melt loss along the prograde-to-peak stage of evolution. The pressure-temperature path remained near-isobaric during the prograde and the retrograde evolution of the assemblages. High-density (up to 1Á03 g cm-3) CO 2-rich fluid inclusions in aluminous granulite, coarse-grained charnockite and felsic gneiss indicate that peak metamorphism and subsequent evolution occurred under a CO 2-dominated fluid regime. The pressure-temperature-fluid evolutionary history of the Phulbani domain shows similarity to that of the adjacent Visakhapatnam domain of the Eastern Ghats Belt and poses questions on the status of the boundary separating these two domains.
Thermal and baric evolution of garnet granulites from Sri Lanka
Journal of Metamorphic Geology, 1988
Garnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700-750°C and pressures in the vicinity of 6-8 kbar. Associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening, with the prograde P-T conditions probably reflecting a combination of the conductive and advective transport of heat at the mid-levels of tectonically thickened crust. The occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions, or fluid absent conditions, during peak metamorphism and precludes pervasive advection of a C02-rich fluid. The advective heat component is therefore likely to have been provided by the transport of silicate melt. Intricate symplectitic textures record partial re-equilibration of the garnet granulites to lower pressures (-4-6 kbar) at high temperatures (6O0-75O0C), and testify either to the erosional denudation of the overthick crust prior to significant cooling (i.e. quasi-isothermal decompression) or to a subsequent static heating possibly of early Palaeozoic age (Pan-African). The metamorphic history of the Sri Lankan granulites is compared with high grade terrains in the neighbouring fragments of Gondwana, with the emphasis on similarities with Proterozoic, granulites of the East Antarctic craton.
American Journal of Science, 2002
The P-T-fluid histories of aluminous garnet؉clinopyroxene-bearing calc-silicate granulites are commonly constrained through interpretation of sequential reaction textures based on activity-corrected petrogenetic grids for model systems, for example the widely used CAS(CO 2 -H 2 O) topologies. Application of these topologies to calc-silicates of diverse bulk composition is limited by the fact that the role of clinopyroxene and hence of garnet composition in the garnet-forming reactions is ignored or not adequately considered. In this contribution the complexities that arise from the involvement of clinopyroxene in the garnet-forming reactions are critically evaluated. As an example, the paragenetic relations and compositional features of clinopyroxene and coronitic garnet across paired scapolite-and wollastonitedominated reaction zones (scp؉cpx؎qtz,cal,qtz -wo؉cal؉cpx؎scp) developed between marble layers (cal؎cpx,wo,scp) and their host calc-silicate granulite (cpx؉pl؎qtz,scp) at an exposure in the high-grade Eastern Ghats Belt (India) are examined. The microtextural features and systematic changes of modal and phase compositions across the profile indicate that clinopyroxene, scapolite, plagioclase, quartz and calcite as well as wollastonite participated in the garnet-forming reactions. Using the sub-assemblage qtz؉cal؉cpx؉scp؉grt؉v in the system CaO-FeO-Al 2 O 3 -SiO 2 -CO 2 it is demonstrated that at the ambient P-T-X fluid conditions, the reaction coefficients of the participating phases are uniquely defined by the garnet composition. As a result, the P-T slope of a garnet-forming reaction changes with the garnet composition, with sharp changes at "singular points" across which one or more phases change their sides with respect to garnet in a given reaction.
Journal of Metamorphic Geology, 2010
An Al-rich, SiO 2 -deficient sapphirine-garnet-bearing rock occurs as a metapelitic boudin within granulite facies Proterozoic charnockitic gneisses and migmatites on the island of Hisøy, Bamble Sector, SE Norway. The boudin is made up of peraluminous sapphirine, garnet, corundum, spinel, orthopyroxene, sillimanite, cordierite, staurolite and biotite in a variety of assemblages. Thermobarometric calculations based on coexisting sapphirine-spinel, garnet-corundum-spinel-sillimanite, sapphirine-orthopyroxene, and garnet-orthopyroxene indicate peak-metamorphic conditions near to 930°C at 10 kbar. Corundum occurs as single 200 to 3000 micron sized skeletal crystal intergrowths in cores of optically continuous pristine garnet porphyroblasts. Quartz occurs as 5-60 micron-sized euhedral to lobate inclusions in the corundum where it is in direct contact with the corundum with no evidence of a reaction texture. Some crystal inclusions exhibit growth zoning, which indicates that textural equilibrium was achieved. Electron Back-Scatter Diffraction (EBSD) studies reveal that the quartz inclusions share a common c-axis with the host corundum crystal. The origin of the quartz inclusions in corundum is enigmatic as recent experimental studies have confirmed the instability of quartz-corundum over geologically realistic P-T ranges. The combined EBSD and textural observations suggest the presence of a former silica-bearing proto-corundum, which underwent exsolution during post-peak-metamorphic uplift and cooling. Exsolution of quartz in corundum is probably confined to fluid-absent conditions where phase transitions by coupled dissolution-precipitation mechanisms are prevented.
Journal of Metamorphic Geology, 1996
Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual highgrade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxenesillimanite-garnet-quartz, sapphirine-quartz and spinel-quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+Qtz Opx+Sil, (2) Opx+Sil Grt+Spr+Qtz, (3) Grt+Sil+Qtz Crd, (4) Grt+Sil Crd+Spr, (5) Grt+Sil+Spr Crd+Spl, (6) Grt+Sil Crd+Spl, (7) Grt+Crd+Sil Spl+Qtz and (8) Grt+Sil Spl+Qtz. Minerals in quartz Al-Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe2+) range of the main minerals is as follows: cordierite (0.81-0.97), sapphirine (0.77-0.88), orthopyroxene (0.65-0.81), garnet (0.33-0.64) and spinel (0.23-0.56). The reaction textures and the evolution of the mineral assemblages in the quartz Al-Mg granulites indicate a clockwise P-T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.