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     

冀北滦平—承德一带晚太古代高压麻粒岩的发现及其构造地质意义

在冀北地区原太古代＂单塔子群＂内多处发现了高压基性麻粒岩构造岩片或透镜体,围岩为花岗片麻岩类,在区域上呈构造带出现。高压基性麻粒岩早期矿物组合为单斜辉石＋石榴石＋石英,退变组合为斜方辉石＋斜长石＋单斜辉石及更晚期的角闪石＋斜长石,明显经历了快速的抬升过程。上述高压麻粒岩在区域上可能对应于晋北－冀西北的高压麻粒岩带,代表克拉通基底的重要构造边界,对于认识区域早前寒武纪构造演化及深部地质作用过程将产生重要影响。     

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.     

Mineralogy and chemistry of banded iron formations (BIF) of Tiruvannamalai area,Tamil Nadu

The mineralogy and chemistry of banded iron formations (BIF) of Archaean high grade granulite gneiss belt of Tiruvannamalai area are presented here. The BIF of this area is chemically different from those around the world. The iron formations and associated granulites are of different origin namely metasedimentary and metavolcanic respectively.     

Crustal architecture above the high-pressure belt of the Grenville Province in the Manicouagan area: new structural, petrologic and U–Pb age constraints

Recent U–Pb age determinations and P–T estimates allow us to characterize the different levels of a formerly thickened crust, and provide further constraints on the make up and tectono-thermal evolution of the Grenville Province in the Manicouagan area. An important tectonic element, the Manicouagan Imbricate zone (MIZ), consists of mainly 1.65, 1.48 and 1.17 Ga igneous rocks metamorphosed under 1400–1800 MPa and 800–900 °C at 1.05–1.03 Ga, during the Ottawan episode of the Grenvillian orogenic cycle, coevally with intrusion of gabbro dykes in shear zones. The MIZ has been interpreted as representing thermally weakened deep levels of thickened crust extruded towards the NW over a parautochthonous crustal-scale ramp. Mantle-derived melts are considered as in part responsible for the high metamorphic temperatures that were registered.New data show that mid-crustal levels structurally above the MIZ are represented by the Gabriel Complex of the Berthé terrane, that consists of migmatite with boudins of 1136±15 Ma gabbro and rafts of anatectic metapelite with an inherited monazite age at 1478±30 Ma. These rocks were metamorphosed at about the same time as the MIZ (metamorphic zircon in gabbro: 1046±2 Ma; single grains of monazite in anatectic metapelite: 1053±2 Ma) and under the same T range (800–900 °C) but at lower P conditions (1000–1100 MPa). They are mainly exposed in an antiformal culmination above a high-strain zone, which has tectonic lenses of high P–T rocks from the MIZ and is intruded by synmetamorphic gabbroic rocks. This zone is interpreted as part of the hangingwall of the MIZ during extrusion. A gap of 400 MPa in metamorphic pressures between the tectonic lenses and the country rocks, together with the broad similarity in metamorphic ages, are consistent with rapid tectonic transport of the high P–T rocks over a ramp prior to the incorporation of the mafic lenses in the hangingwall.Between the antiformal culmination of the Gabriel Complex and the MIZ 1.48 Ga old granulites of the Hart Jaune terrane are exposed. They are intruded by unmetamorphosed 1228±3 Ma gabbro sills and 1166±1 Ma anorthosite. Hart Jaune Terrane represents relatively high crustal levels that truncate the MIZ-Gabriel Complex contact and are preserved in a synformal structure.Farther south, the Gabriel Complex is overlain by the Banded Complex, a composite unit including 1403+32/−25 Ma granodiorite and 1238+16/−13−1202+40/−25 Ma granite. This unit has been metamorphosed under relatively low-P (800 MPa) granulite-facies conditions. Metamorphic U–Pb data, limited to zircon lower intercept ages (971±38 Ma and 996±27 Ma) and a titanite (990±5 Ma) age, are interpreted to postdate the metamorphic peak.The general configuration of units along the section is consistent with extrusion of the MIZ during shortening and, finally, normal displacement along discrete shear zones.