Reaction history of garnet-sapphirine granulites and conditions of Archaean high-pressure granulite-facies metamorphism in the Central Limpopo Mobile Belt, Zimbabwe

ABSTRACT Sequential reaction textures in Archaean garnet-corundum-sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse-grained, high-pressure (P > 9.5 kbar) granulite-facies assemblages (M1) to late, low-pressure (P <6 kbar) granulite-facies sub-assemblages (M2). The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late-M1 boron-free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel). The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time-sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)-discontinuous reactions in the FMASH multisystem sapphirine-garnet-corundum-spinel-cordierite-gedrite-kornerupine, the facies-sequence indicates decompression at essentially constant T assuming constant a(H₂O). Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the opposite dP from that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction of P-T vectors. Garnet-biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca. 850°C for the M1 stage. A minimum late-M1 pressure of ca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early-M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high-pressure granulite-facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. The P-T path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite-facies terranes.     

Empirical tests of carbon isotope thermometry in granulites from southern California

Abstract ‘Peak’metamorphic carbon isotope fractionations between calcite and graphite (Δ_Cal–Gr) in marbles and calc-silicates from the Cucamonga granulite terrane (San Gabriel Mountains, California) range from 3.48 to 2.90%. The data are used to test three previously published calibrations of the calcite–graphite carbon isotope thermometer. An empirical calibration of the calcite–graphite carbon isotope thermometer gives temperatures of 700–750°C; a theoretical–experimental calibration of the system gives temperatures of 760°–870°C; an experimental calibration gives temperatures of 870–1300°C. Temperatures calculated using the empirical calibration are in agreement with those calculated from garnet-based cation exchange thermometry when uncertainty is considered. Temperatures calculated using the theoretical–experimental calibration overlap the upper range of cation exchange thermometry temperatures and range to 50°C higher. The experimental calibration yields temperatures from 50 to 480°C higher than those from cation exchange thermometry. Moreover, temperatures from the experimental calibration are also inconsistent with mineral and melt equilibria in the granulite phase assemblage. Despite the better agreement between cation exchange thermometry and the empirical calibration of the calcite–graphite system, temperatures calculated using the theoretical–experimental calibration may be real peak metamorphic temperatures. If retrograde diffusion partially reset garnet-based cation exchange thermometers by c. 50°C, then the cation exchange temperatures are consistent with those from the theoretical–empirical calibration. Thermometric evidence from biotite dehydration melting equilibria is consistent with either the empirical calibration if melting was fluid-present, or the theoretical–experimental calibration if melting was fluid-absent.     

Charnockitic alteration: evidence for CO2 infiltration in granulite facies metamorphism

Charnockitic alteration (arrested orthopyroxene formation in biotite- and amphibole-bearing rocks) occurs in high-grade terranes of all ages. Three criteria are used to show that this alteration was produced in many locations by a migrating fluid phase: (i) diffuseness of the alteration—the alteration zones are often quite unlike discrete migmatitic veins; (ii) relation to deformation—most occurrences show alteration closely associated with warping of foliation or dilation cracks; (iii) open-system alteration—whilst some occurrences represent nearly isochemical alteration, slight changes in bulk composition, often loss of mafic constituents and gain of Na and Si, are evident in detailed mass-balance analysis. Y and sometimes Rb are characteristically depleted. Partial melting sometimes accompanied volatile infiltration, as evidenced by more discrete veins and euhedral orthopyroxene. It is quite unlikely, however, that open-system alteration was produced by escape of viscous quartzo-feldspathic melts. Pervasive migration of low-T lamprophyric (mafic–alkaline, CO₂-charged) interstitial liquids is a possibility by virtue of their extreme fluidity, but CO₂ infiltration was needed to generate these liquids. Vapour-deficient dehydration melting is another feasible mechanism of orthopyroxene formation which may have operated in conjunction with CO₂ infiltration. Characteristic development of charnockitic alteration in some prograde amphibolite to granulite facies transitions, as in the Dharwar Craton of South India, suggests that the alteration is a fundamental feature of the granulite facies metamorphism, implying active and causal participation of migrating fluids. In other high-grade terranes like the Adirondack Mountains of New York, this kind of alteration is rare, and fluid action does not seem to have been important in the metamorphism. A vapour phase participating in charnockitic metamorphism was necessarily one of relatively low H₂O, therefore presumably rich in CO₂. Consideration of possible large CO₂ sources leads to the conclusion that emanations from volatile-rich basalts emplaced in the lower crust are the most probable source of charnockitizing fluids. The ultimate source would therefore be enriched subcontinental lithosphere or asthenosphere. The Rb-depleted pyroxene gneiss (charnockitic) terranes may be characteristic of zones of large-scale transcurrent or oblique-motion faults which tap such great depths.     

Metapelites of the Kanskiy Granulite Complex (Eastern Siberia): Kinked P-T Paths and Geodynamic Model

The Southern Yenisey Range (Eastern Siberia) consists of thegranulite-facies Kanskiy complex bordered by the lower-gradeYeniseyskiy and Yukseevskiy complexes. Samples of metapeliteof the Kanskiy complex typically show characteristic garnet-formingreaction textures and near-isobaric cooling P–T paths.An important new result of this study concerns the differencein shape of the P–T paths from different parts of theKanskiy granulite complex: metapelites collected 8 km from theboundary with the Yeniseyskiy complex followed a linear pathwith dP/dT 0·006 kbar/°C; metapelites collected3 km from this boundary reveal a kinked P–T path withan interval of burial cooling (dP/dT –0·006 kbar/°C).The difference in the shape of the P–T paths is supportedby the chemical zoning of garnet studied in the second groupof samples. A mechanism of buoyant exhumation of granulite issuggested by comparison with the results of numerical modelling,which indicate that such a diversity of P–T paths mayresult from a transient disturbance of the thermal structureby rapid differential movement of material from different crustallevels. To arrive at a correct tectonic interpretation, thewhole assemblage of interrelated P–T paths of metamorphicrocks collected from different localities within the same complexmust be studied. KEY WORDS: crustal diapirism; exhumation; granulites; numerical modelling; P–T path     

Inherent gravitational instability of thickened continental crust with regionally developed low- to medium-pressure granulite facies metamorphism

Petrological arguments show that regionally developed low- to medium-pressure, high-temperature granulite facies metamorphism may critically enhance the lowering of crustal density with depth. This leads to gravitational instability of homogeneously thickened continental crust, mainly due to changes in mineral assemblages and the thermal expansion of minerals in conjunction with the exponential lowering of the effective viscosity of rocks with increasing temperature. It is argued that crustal processes of gravitational redistribution (crustal diapirism) contributing to the exhumation of granulite facies rocks may be activated in this way.     

Oxide and sulphide minerals in highly oxidized, Rb-depleted, Archaean granulites of the Shevaroy Hills Massif, South India: Oxidation states and the role of metamorphic fluids

The Shevaroy Hills of northern Tamil Nadu, southern India, expose the highest-grade granulites of a prograde amphibolite facies to granulite facies deep-crustal section of Late Archaean age. These highly oxidized quartzofeldspathic garnet charnockites generally show minor high-TiO₂ biotite and amphibole as the only hydrous minerals and are greatly depleted in the incompatible elements Rb and Th. Peak metamorphic temperatures (garnet–orthopyroxene) and pressures (garnet–orthopyroxene–plagioclase–quartz) are near 750 °C and 8 kbar, respectively. Pervasive veinlets of K-feldspar exist throughout dominant plagioclase in each sample and show clean contact with orthopyroxene. They are suggested to have been produced by a low H₂O activity, migrating fluid phase under granulite facies conditions, most likely a concentrated chloride/carbonate brine with high alkali mobility accompanied by an immiscible CO₂-rich fluid. Silicate, oxide and sulphide mineral assemblages record high oxygen fugacity. Pyroxenes in the felsic rocks have high Mg/(Mg+Fe) (0.5–0.7). The major oxide mineral is ilmenite with up to 60 mole per cent exsolved hematite. Utilizing three independent oxygen barometers (ferrosilite–magnetite–quartz, ferrosilite–hematite–quartz and magnetite–hematite) in conjunction with garnet–orthopyroxene exchange temperatures, samples with XIlmHm>0.1 yield a consistent oxygen fugacity about two log units above fayalite stability. Less oxidized samples (XIlmHm<0.1) show some scatter with indications of having equilibrated under more reducing conditions. Temperature-f (O₂ ) arrays result in self consistent conditions ranging from 660 °C and 10^?16 bar to 820 °C and 10^?11.5 bar. These trends are confirmed by calculations based on the assemblage clinopyroxene–orthopyroxene–magnetite–ilmenite using the QUIlF program. In the most oxidized granulite samples (XIlmHm>0.4) pyrite is the dominant sulphide and pyrrhotite is absent. Pyrite grains in these samples have marginal alteration to magnetite along the rims, signifying a high-temperature oxidation event. Moderately oxidized samples (0.1no coexisting magnetite. Chalcopyrite is a common accessory mineral of pyrite and pyrrhotite in all the samples. Textures in some samples suggest that it formed as an exsolution product from pyrrhotite. Extensive vein networks of magnetite and pyrite, associated principally with the pyroxene and amphibole, give evidence for a pervasive, highly oxidizing fluid phase. Thermodynamic analysis of the assemblage pyrrhotite, pyrite and magnetite yields consistent high oxidation states at 700–800 °C and 8 kbar. The oxygen fugacity in our most oxidized pyrrhotite-bearing sample is 10^?12.65 bar at 770 °C. There are strong indications that the Shevaroy Hills granulites recrystallized in the presence of an alkali-rich, low H₂O-activity fluid, probably a concentrated brine. It cannot be demonstrated at present whether the high oxidation states were set by initially oxidized protoliths or effected by the postulated fluids. The high correspondence of maximally Rb-depleted samples with the highest recorded oxidation states suggests that the Rb depletion event coincided with the oxidation event, probably during breakdown of biotite to orthopyroxene+K-feldspar. We speculate that these alterations were effected by exhalations from deep-seated alkali basalts, which provided both heat and high oxygen fugacity, low a_H2O fluids. It will be of interest to determine whether greatly Rb-depleted granulites in other Precambrian terranes show similar highly-oxidizing signatures.     

Geochemical evolution of high-pressure mafic granulites from the Bacariza formation (Cabo Ortegal complex, NW Spain): an example of a heterogeneous lower crust

 The Cabo Ortegal complex (northwestern Iberian massif) is a klippen formed of several structural units stacked during the Hercynian collision. All these units include ultramafic rocks, metabasites and quartz-feldspathic gneisses affected by different metamorphic conditions. The Bacariza formation is heterogeneous showing a conspicuous layering mainly defined by alternate high-pressure ultrabasic-to-basic granulites, retrogressed garnet amphibolites of intermediate composition and rare acid rocks forming garnet trondhjemitic gneisses. This layering is inherited from a gabbroic protolith showing a composition rich in Fe and Ti. Major and trace elements of these rocks can be correlated to continental tholeiitic series of extensional settings. These high-pressure granulites are situated in normal contact between±serpentinised ultramafic rocks and other high-grade metabasites with lessevolved and more-depleted composition comparable to T-type and N-type MORB. It is suggested that the layered gabbro-type protolith was part of a continuous mafic crust. This crust was initially formed during Early Ordovician in a continental extensional setting and progressively evolved to oceanic spreading. Received: 9 February 1996/Accepted: 10 February 1997     

Coexisting pyroxenes from the basic granulites of Visakhapatnam area in the Eastern Ghats terrain

Fifteen pairs of coexisting pyroxenes from basic granulites associated with leptynites in the khondalite suite of rocks are analysed and the distribution of Mg and Fe²⁺ ratios is presented. Temperature estimates for the coexisting pyroxenes from the basic granulites of Visakhapatnam may be expressed as 750±100†C corresponding to intermediate pressure granulites.     

From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean

The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre-Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished.
(a) A medium- to low- P granulite stage giving rise to the development of orthopyroxene + garnet + plagioclase + brown amphibole + ilmenite ± biotite in basic granulites and garnet + K-feldspar + plagioclase + cordierite + sillimanite + biotite + ilmenite in acid granulites.
(b) A post-granulite re-equilibration, associated with the development of shear zones, producing discrete amphibolitization of the basic granulites and widespread development of biotite + sillimanite + cordierite + spinel in the acid rocks.
(c) An eo-Alpine eclogite stage giving rise to the crystallization of high- P and low- T assemblages.
In an effort to quantify this evolution, independent well-calibrated thermobarometers were applied to basic and acid rocks. For the granulite event, P-T estimates are 7–9 kbar and 700–800° C, and for subsequent retrograde evolution, P-T was 4–5 kbar and 600° C. For the eo-Alpine eclogite metamorphism, pressure and temperature conditions were 14–16 kbar and 550° C.
The inferred P-T path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre-Alpine granulite and post-granulite retrograde evolution to the Permo-Jurassic extensional regime. The complex granulite-eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean.     

Two-stage decompression in garnet-bearing mafic granulites from Sostrene Island, Prydz Bay, East Antarctica

Mafic garnet-bearing granulites from Sostrene Island, 150 km southwest of Davis Station on the coast of Prydz Bay, East Antarctica, exhibit two-stage symplectic coronas on garnet, formed after peak metamorphic conditions (M1). An outer corona of Opx (Mg₆₆) + Pl (An_94–97) + minor Hbl mantles a finer-grained inner corona of Opx (Mg₆₇) + Pl (An_95–96) + Spl (Mg₃₆). Both symplectites contain minor ilmenite–magnetite intergrowths. The finer-grained symplectite also occurs along a fracture cleavage in the garnet. The outer corona originated during a second metamorphic event (M2) via the reaction Grt + Cpx (Hbl) + SiO₂= Opx + Pl (1), whereas the inner corona formed later in response to decompression and minor deformation, resulting in the fracture cleavage in the garnet, according to the reaction Grt = Opx + Pl + Spl (2). The grossular content of the garent (X_Grs= 0.168) is almost exactly that which is required for the stoichiometric breakdown by reaction (2) (calculated X_Grs= 0.167). The mafic rocks are silica undersaturated, and the SiO₂ for reaction (1) was most probably derived externally from the surrounding felsic gneisses. Preferred P–T estimates for M1 based on garnet core (Prp₄₀Alm₄₂Grs₁₇Sps₁)–matrix Opx–Cpx–Hbl pairs are c. 10 kbar at 980°C. The fine-grained symplectite formed post-peak M2 at c. 7 kbar and 850°C. The enclosing felsic gneisses yield pressure estimates of between 5 and 7 kbar, which compare with conditions of c. 6 kbar and 775°C in the nearby Bolingen Islands. These lower P–T estimates are considered to be representative of the widespread 1100-Ma metamorphic event recognized in outcrops along the Prydz Bay coast. The high-P, high-T estimates derived from the garnet relics provide evidence for an earlier, possibly Archaean, high-grade metamorphic event.