Showing papers on "Granulite published in 1989"

The origins of granulites: a metamorphic perspective

Abstract: Although many recent reviews emphasize a uniformity in granulite pressure–temperature (P–T) conditions and paths, granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation. A thorough survey of over 90 granulite terranes or occurrences reveals that over 50% of them record P–T conditions outside the 7.5 ± 1 kbar and 800 ± 50 °C average granulite regime preferred by many authors. In particular, an increasing number of very high temperature (900−1000 °C) terranes are being recognized, both on the basis of distinctive mineral assemblages and geothermobarometry. Petrogenetic grid and geothermobarometric approaches to the determination and interpretation of P–T histories are both evaluated within the context of reaction textures to demonstrate that the large range in P–T conditions is indeed real, and that both near-isothermal decompression (ITD) and near-isobaric cooling (IBC) P–T paths are important. Amphibolite–granulite transitions promoted by the passage of CO2-rich fluids, as observed in southern India and Sri Lanka, are exceptional and not representative of fluid-related processes in the majority of terranes. It is considered, on the contrary, that fluid-absent conditions are typical of most granulites at or near the time of their recorded thermal maxima.ITD granulites are interpreted to have formed in crust thickened by collision, with magmatic additions being an important extra heat source. Erosion alone is not, however, considered to be the dominant post-collisional thinning process. Instead, the ITD paths are generated during more rapid thinning (1−2 mm/yr exposure) related to tectonic exhumation during moderate-rate or waning extension. IBC granulites may have formed in a variety of settings. Those which show anticlockwise P–T histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion, with or without additional crustal extension. IBC granulites at shallow levels (< 5 kbar) may also be formed during extension of normal thickness crust, but deeper-level IBC requires more complex models. Many granulites exhibiting IBC at deep crustal levels may have formed in thickened crust which underwent very rapid (5 mm/yr) extensional thinning subsequent to collision. It is suggested that the preservation of IBC paths rather than ITD paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust, and that hybrid ITD to IBC paths should also be observed.Most IBC granulites, and probably many ITD granulites, have not been exposed at the Earth's surface as a result of the tectonic episodes which produced them, but have resided in the middle and lower crust for long periods of time (100−2000 Ma) following these events. The eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation.

The Granulite Uncertainty Principle: Limitations on Thermobarometry in Granulites

Abstract: There are three geothermometers based on reversed experimental data and applicable to granulites: the two-pyroxene, two-oxide, and garnet-clinopyroxene thermometers. All have apparent closure temperatures below those of the granulite facies. This casts significant doubt on the concept that "peak" temperatures are routinely obtained from ion-exchange thermometers, particularly those that are empirically calibrated. The best way to recover high temperatures in granulite terrains is through the reintegration of exsolution lamellae or the use of relict mineral assemblages; we call these features fossil thermometers. Because fossil thermometers can be destroyed by deformation, in many terrains it may be impossible to tell whether the temperature they record is a true maximum or whether it is a temperature locked in during cooling. Low closure temperatures also affect the oxygen fugacites obtained from two-oxide equilibria. Compilation of the available data indicate that a number of granulite terrains either cr...

Sr, Nd, and Pb Isotopic Systematics in the Archean Low- to High-Grade Transition Zone of Southern India: Syn-Accretion vs. Post-Accretion Granulites

Abstract: Low-pressure charnockites, their tonalitic precursors, and syn-metamorphic granites in the granulite facies transition zone south of Krishnagiri, Tamil Nadu, India, define single Rb-Sr and Sm-Nd isochrons at, respectively, $$2463 \pm 65$$ and $$2455 \pm 121 Ma$$. Rb depletion occurred at this time, while Sm/Nd ratios were not significatively changed. Common lead ratios are identical for the granitic and tonalitic gneisses as well as the low-pressure charnockites; they are very unradiogenic ($$^{206}Pb/^{204}Pb$$: 14.5 to 15.5), indicating U depletion around 2500 Ma ago. These data suggest (1) that granulites in this part of the transition zone, as well as some high-grade charnockites south of the zone (Nilgiris and Sheravoy Hills) that yield similar results, derive from a 2.5 Ga-old crust and (2) the time elapsed between the crust-forming event and the metamorphism was short. The high-pressure charnockites of the Biligirirangan Hills, Karnataka, are different from those in the transition zone near Krishna...

U-Pb systematics of garnet: dating the growth of garnet in the late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes, Manitoba, Canada

Abstract: This study considers the potential of using the U-Pb dating of garnet for determining quantitative P-T-t paths for the late Archean metamorphism in the Pikwitonei granulite domain. Garnets for U-Pb dating were selected mainly from samples that also provide information on pressure and temperature. The garnets used for dating were clear and free of any visible inclusions. Pb concentrations range from 63 ppb to 966 ppb and U from 136 ppb to 1143 ppb. The measured 206Pb/204Pb ratios range from 52.8 to 529.4. The ages are generally discordant with U/Pb ages that may lie above or below concordia. The discordance is caused by a recent disturbance of the U/Pb ratio in the garnets as indicated by replicate analyses on the same garnet separates that reproduce 207Pb/206Pb ages well within analytical uncertainty and in most cases within ±1.5 Ma at 2600–2750 Ma. High grade metamorphism continued over a period of at least one hundred million years, but the garnet-K-feldspar Pb-Pb ages suggest that, during this time, garnet growth has been favored during three distinct periods in the Cauchon Lake area: 2700–2687 Ma 2660–2637 Ma 2605–2591 Ma The ca. 2695 Ma garnet ages from Cauchon Lake date the time of melting and staurolite breakdown during prograde metamorphism, the ca. 2640 Ma ages date the time of extensive migmatization and the last period of metamorphic garnet growth, the ca. 2600 Ma ages date the time of crystallization of igneous garnet in late granitic intrusions. Peak metamorphism occurred around 2640 Ma followed by the intrusions of pegmatites starting at 2629 Ma. The Pb-Pb ages for garnet are similar to the U-Pb ages for zircon that date a leucocratic mobilizate (2695 Ma), a plagioclaseamphibole mobilizate (2637 Ma) and pegmatite (2598 Ma) (Heaman et al. 1986 a; Krogh et al. 1986; this study). Xenocrysts of garnet from 2600 Ma old graphic granites give minimum ages of 2984 Ma and 2741 Ma which are minima for the times of garnet growth in the source of the granites. The agreement of the zircon and garnet ages suggests that the metamorphism may have been punctuated by events that led to the development of melts or encouraged mineral growth at specific times. If so, the prograde and retrograde paths of metamorphism in the area may have contained minor excursions in pressure, temperature or fluid fugacities. In the Natawahunan Lake area some 50 km northwest of Cauchon Lake, garnet growth associated with the prograde breakdown of staurolite occurred at ca. 2744–2734 Ma. This suggests that a similar style of metamorphism may have occurred earlier in the Natawahunan Lake area than at Cauchon Lake area, or higher grades of metamorphism were reached earlier and were of longer duration associated with the somewhat greater depths in the Natawahunan Lake area. These results indicate the these garnets, which are 0.1–1 cm in diameter, have maintained closed system behavior for U and Pb at peak metamorphic conditions, i.e. temperatures up to 800° C and pressures of 7.5 kb.

Origin of Granulite Terranes and the Formation of the Lowermost Continental Crust

Abstract: Differences in composition and pressures of equilibration between exposed, regional granulite terranes and suites of granulite xenoliths of crustal origin indicate that granulite terranes do not represent exhumed lowermost crust, as had been thought, but rather middle and lower-middle crustal levels. Application of well-calibrated barometers indicate that exposed granulites record equilibration pressures of 0.6 to 0.8 gigapascal (20 to 30 kilometers depth of burial), whereas granulite xenoliths, which also tend to be more mafic, record pressures of at least 1.0 to 1.5 gigapascals (35 to 50 kilometers depth of burial). Thickening of the crust by the crystallization of mafic magmas at the crust-mantle boundary may account for both the formation of regional granulite terranes at shallower depths and the formation of deep-seated mafic crust represented by many xenolith suites.

Evolution and assembly of Archean Gneiss Terranes in the Godthåbsfjord Region, southern west Greenland: Structural, metamorphic, and isotopic evidence

Abstract: The Godthabsfjord region of southern West Greenland comprises several terranes that were assembled between 2750 and 2550 Ma and folded during amphibolite facies metamorphism. The terranes, which are dominated by gneisses of different ages and show different preassembly metamorphic and structural histories, are (1) the Faeringehavn terrane containing the 3820–3600 Ma Amitsoq gneisses, with granulite facies metamorphism at circa 3600 Ma, (2) the Akia terrane containing the 3070–2940 Ma Nuk gneisses, with granulite-amphibolite facies metamorphism at circa 2980 Ma, (3) the Tasiusarsuaq terrane containing circa 2900 Ma gneisses, with granulite facies metamorphism at circa 2800 Ma, and (4) the Tre Brodre terrane containing the 2800–2750 Ma Ikkattoq gneisses, with amphibolite facies metamorphism at 2800–2750 Ma. Metamorphic assemblages and structures formed prior to terrane assembly, were variably overprinted during amphibolite facies metamorphism and heterogeneous strain associated with assembly. Recognition of the region as consisting of several terranes indicates that the anatomy of some Archean high-grade gneiss complexes may resemble that of orogens of Proterozoic and Phanerozoic age that formed as a consequence of plate tectonic processes.

Fluid and enthalpy production during regional metamorphism

Abstract: Models for regional metamorphism have been constructed to determine the thermal effects of reaction enthalpy and the amount of fluid generated by dehydration metamorphism. The model continental crust contains an average of 2.9 wt % water and dehydrates by a series of reactions between temperatures of 300 and 750° C. Large scale metamorphism is induced by instantaneous collision belt thickening events which double the crustal thickness to 70 km. After a 20 Ma time lag, erosion due to isostatic rebound restores the crust to its original thickness in 100 Ma. At crustal depths greater than 10 km, where most metamorphism takes place, fluid pressure is unlikely to deviate significantly from lithostatic pressure. This implies that lower crustal porosity can only be maintained if rock pores are filled by fluid. Therefore, porosities are primarily dependent on the rate of metamorphic fluid production or consumption and the crustal permeability. In the models, permeability is taken as a function of porosity; this permits estimation of both fluid fluxes and porosities during metamorphism. Metamorphic activity, as measured by net reaction enthalpy, can be categorized as endothermic or exothermic depending on whether prograde dehydration or retrograde hydration reactions predominate. The endothermic stage begins almost immediately after thickening, peaks at about 20 Ma, and ends after 40 to 55 Ma. During this period the maximum and average heat consumption by reactions are on the order 11.2·10−14 W/cm3 and 5.9·10−14 W/ cm3, respectively. The maximum rates of prograde isograd advance decrease from 2.4·10−8 cm/s, for low grade reactions at 7 Ma, to 7·10−10 cm/s, for the highest grade reaction between 45 and 58 Ma. Endothermic cooling reduces the temperature variation in the metamorphic models by less than 7% (40 K); in comparison, the retrograde exothermic heating effect is negligible. Dehydration reactions are generally poor thermal buffers, but under certain conditions reactions may control temperature over depth and time intervals on the order of 1 km and 3 Ma. The model metamorphic events reduce the hydrate water content of the crust to values between 1.0 and 0.4 wt % and produce anhydrous lower crustal granulites up to 15 km in thickness. In the first 60 Ma of metamorphism, steady state fluid fluxes in the rocks overlying prograde reaction fronts are on the order of 5·10−11 g/cm2-s. These fluid fluxes can be accommodated by low porosities (<0.6%) and are thus essentially determined by the rate of devolitalization. The quantity of fluid which passes through the metamorphic column varies from 25000 g/cm2, within 10 km of the base of the crust, to amounts as large as 240000 g/cm2, in rocks initially at a depth of 30 km. Measured petrologic volumetric fluid-rock ratios generated by this fluid could be as high as 500 in a 1 m thick horizontal layer, but would decrease in inverse proportion of the thickness of the rock layer. Fluid advection causes local heating at rates of about 5.9·10−14 W/cm3 during prograde metamorphism and does not result in significant heating. The amount of silica which can be transported by the fluids is very sensitive to both the absolute temperature and the change in the geothermal gradient with depth. However, even under optimal conditions, the amount of silica precipitated by metamorphic fluids is small (<0.1 vol %) and inadequate to explain the quartz veining observed in nature. These results are based on equilibrium models for fluid and heat transport that exclude the possibility of convective fluid recirculation. Such a model is likely to apply at depths greater than 10 km; therefore, it is concluded that large scale heat and silica transport by fluids is not extensive in the lower crust, despite large time-integrated fluid fluxes.

Horizontal structures in granulite terrains: A record of mountain building or mountain collapse?

Abstract: In many high-temperature (>800°C) granulite terrains, the development of characteristic horizontal structures occurred during the metamorphic culmination and was followed by isobaric cooling. The absolute magnitude of isobaric cooling (commonly >300°C) implies cooling intervals of the order of the thermal time constant of the continental lithosphere (~100 m.y.). Such prolonged isobaric cooling implies that no significant erosional denudation followed the development of the horizontal structures and thus precludes the prograde deformation being responsible for significant crustal thickening. Rather, the prograde deformation more probably records bulk crustal thinning during extensional collapse of a previously thickened crust possibly triggered by detachment of a thickened thermal boundary layer at the base of the lithosphere. -Author

Empirical calibration of geobarometers for the assemblage garnet + hornblende + plagioclase + quartz

Abstract: Four geobarometers for garnet amphibolites and granulites have been calibrated and are based on the equilibrium 6 anorthite + 3 albite + 3 tremolite = 2 grossular + 1 pyrope + 3 parga site + 18 quartz and its Fe end-member equivalent. Literature data representing 19 natural samples that equilibrated from 3.5 to 14 kbar and from 520 to 800°C are fitted to the general equation -RT In Keq= A - BT + (P - 1)C, where T is in kelvins and P is in bars, using a weighted least-squares method. Isopleths of In Keqin P- T space are quite flat; slopes at 600 OCand 5 to 10 kbar range from - 7 to + 13 bars/°C. Propagated analytical errors produce a corresponding error in pressure of :t200 to 600 bars (10-).When errors in T and the regressed parameters are also propagated, the accuracy is estimated to be not better than :t5000 bars. Applications show that these calibrations are consistent with other calibrated mineral equilibria, but caution is advised in applying the barometers outside the range of mineral compositions employed here, or to rocks that equilibrated at high temperatures (> -700 0c), because of the potential for re-equilibration of garnet and hornblende on cooling.

Origin and metamorphic history of an Early Cretaceous polybaric granulite terrain, Fiordland, southwest New Zealand

Abstract: Regionally extensive two-pyroxene granulites in Fiordland, southwest New Zealand, are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes. The granulite protolith (igneous charnockitic rock) synkinematically intruded metasediment and other orthogneiss in an Early Cretaceous subduction-related magmatic arc, and during cooling experienced deformation-induced recrystallization to form granoblastic gneiss. The granulites occur side by side with coeval rocks of amphibolite facies. Mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization: 1) an early high temperature, comparatively low pressure event accompanying magmatic intrusion (andalusite-sillimanite facies series recorded locally in the country rock), followed by 2) high pressure metamorphism under conditions of ∼650°–700° C at ∼12–13 kbar. Garnet granulite locally overprinted earlier formed two-pyroxene granulite during the latter event. The pressure increase (∼6 kbar) between the two events is attributed to crustal thickening by overthrusting, and is equivalent to unloading of a ∼20 km thick slab over rocks already buried at mid-crustal depths. Both events occurred over a < 20 m.y. interval, between the time of magmatic emplacement of the granulite protolith and uplift-controlled final cooling of the terrain. The Phanerozoic granulites in Fiordland share some petrologic similarities with Precambrian granulite terrains, suggesting that at least some aspects of the former may serve as a useful model for development of the latter.

Nd Isotopes and the Origin of Grenville-Age Rocks in Texas: Implications for Proterozoic Evolution of the United States Mid-Continent Region

Abstract: Nd isotopic data were obtained for Precambrian Grenville-belt rocks in Texas. The samples represent most components of the crust of the Llano, Van Horn, and Franklin Mountains exposed terranes. Almost all Precambrian igneous, metamorphic, and sedimentary rocks from the three regions document addition to North America of mantle-derived crustal materials in the 1.6-1.0 Ga interval. The exception is a quartzite from the westernmost (Franklin Mountains) exposure, which was derived from {approximately}1.8 Ga crust of the southwestern United States. The initial {epsilon}{sub Nd} values of all rocks except the quartzite lie in the +1 to +6 range for igneous/metamorphic ages of 1.37 to 1.06 Ga. These results can be interpreted in two ways: (1) as documenting 0-20% additions of older crustal material to mantle-derived products 1.4-1.0 Ga ago; or (2) as documenting derivation of the Grenville exposures by recycling of older crustal protoliths separated from the mantle 1.6-1.3 Ga ago. The Nd data in isolation do not resolve these two interpretations. The model mantle separation ages (T{sub DM}) of the rocks are very similar to published values of granulites in Mexico and Virginia: all these regions of the United States and Mexico show a strong peak of T{sub DM} around 1.4more » Ga ago. If the ages represent older crustal protoliths, then they would have formed coevally with the 1.5-1.3 Ga Granite-Rhyolite Terranes of the continental interior USA. This would imply that the Granite-Rhyolite Terranes were formed during orogenic/accretionary processes in the adjoining Grenville Belt, and are not anorogenic in association, as conventionally assumed.« less

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.

Ion probe U-Th-Pb zircon dating of polymetamorphic orthogneisses from northern Labrador, Canada

Abstract: Rounded zircons included within igneous zircons from the Uivak I gneisses, northern Labrador, have ages of up to 3863 ± 12 Ma (2σ), showing the (former) existence of rocks that predate the Uivak gneisses by more than 100 Ma. The igneous zircons themselves have experienced varying amounts of early Pb loss, but age estimates based on maximum 207Pb/206Pb from three separate gneiss samples agree to within error at 3732 ± 6 Ma (2σ), which is taken as the age of emplacement of the igneous precursor to the dominant component of the Uivak gneisses. A new generation of zircons was formed during migmatization at ca. 3620 Ma. The volumetrically less important Lister gneiss was emplaced at 3235 ± 8 Ma (2σ). In the late Archaean, different parts of the gneiss complex were subjected to different grades of metamorphic overprinting. The zircon chronology of gneisses affected by granulite facies metamorphism suggests a rapid sequence of events, with simultaneous recrystallization of old zircons and growth of new U-poor zi...

A regional perspective of the Quetico metasedimentary belt, Superior Province, Canada

Abstract: Alternating greenstone–granite and metasedimentary gneiss belts are a first-order tectonic feature of the southern Superior Province. The tectonic development of the Quetico metasedimentary belt is reviewed with regard to depositional, structural, and metamorphic–plutonic history. Over its 1200 km length, the belt consists of marginal metasedimentary schists of turbiditic origin and interior metasedimentary migmatite and peraluminous leucogranite. Polyphase deformation has resulted in a steep easterly-striking foliation and regional, gently east-plunging stretching lineation. Metamorphic grade varies in a low-P facies series from greenschist at the belt margins to upper amphibolite and local granulite in the central migmatite – intrusive granite zone. Mineral assemblages in the central zone yield estimates of metamorphic pressure that increase systematically eastward over 800 km from about 250 MPa (2.5 kbar) near the Canada – United States border to 600 MPa (6 kbar) in granulites adjacent to the Kapuskasi...

Low-pressure granulite facies metapelitic assemblages and corona textures from MacRobertson Land, east Antarctica: the importance of Fe2O3and TiO2in accounting for spinel-bearing assemblages

Abstract: Low-pressure granulite facies metasedimentary gneisses exposed in MacRobertson Land, east Antarctica, include hercynitic spinel-bearing metapelitic gneisses. Peak metamorphic mineral assemblages include spinel + rutile + ilmenite + sillimanite + garnet, spinel + ilmenite + sillimanite + garnet + cordierite, ortho-pyroxene + magnetite + ilmenite + garnet, spinel + cordierite + biotite + ilmenite and orthopyroxene + cordierite + biotite, each with quartz, K-feldspar and melt. The presence of garnet + biotite- and cordierite + orthopyroxene-bearing assemblages implies crossing tie-lines in AFM projection for the K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) system. This apparent contradiction, and the presence of spinel, rutile and ilmenite in the assemblages, is acounted for by using the KFMASH-TiO2-O2 system, i.e. AFM + TiO2+ Fe2O3. We derive a petrogenetic grid for this system, applicable to low-pressure granulite facies metamorphic conditions. Retrograde assemblages are interpreted from corona textures on hercynitic spinel and Fe-Ti oxides. The relative positions of the peak and retrograde metamorphic assemblages on the petrogenetic grid suggest that corona development occurred during essentially isobaric cooling.

Nitrogen-bearing, aqueous fluid inclusions in some eclogites from the Western Gneiss Region of the Norwegian Caledonides

Abstract: Minerals in eclogites from different localities in the Western Gneiss Region of the Norwegian Caledonides (age ≈425 Ma) contain a variety of fluid inclusions. The earliest inclusions recognized are contained in undeformed quartz grains, protected by garnet, and consist of H2O+N2 (with \(X_{{\text{H}}_{\text{2}} {\text{O}}} \geqslant 0.8\)). The reconstructed P-V-T-X properties of these fluid inclusions are compatible with peak or early-retrograde metamorphic conditions. Matrix minerals (quartz, garnet, apatite, plagioclase) contain a complex pattern of mostly truly secondary inclusions, dominated by CO2 and N2. The textural patterns and P-V-T-X properties of these inclusions are incompatible with the high pressures of the eclogite-forming metamorphic event, but suggest that they were formed during uplift, by a combination of remobilization of preexisting inclusions and influx of external fluids. The fluid introduced at a late stage was dominated by CO2, and did not contain N2. The present data agree with theoretical predictions of eclogite fluids from mineral equilibria, and highlight the differences between granulite (CO2) and eclogite (H2O+N2) fluid regimes. The provenance of the nitrogen in the eclogite fluid inclusions represents an important, but unsolved question in the petrology of high-pressure metamorphic rocks.

Geochemistry of exposed granulite facies terrains and lower crustal xenoliths in Mexico

Abstract: The trace element compositions of 13 samples from exposed granulite facies terrains and 17 granulitic lower crustal xenoliths were determined using an inductively coupled plasma-mass spectrometer to constrain the composition and evolution of the Mexican lower crust. Most xenoliths found are pyroxeneplagioclase orthogneisses, although quartzo-feldspathic paragneisses form up to 50% of the xenolith population in some areas. Lithologies in the exposed terrains consist of pelitic and carbonate-rich paragneisses, charnockites, and less abundant basic, intermediate, and felsic orthogneisses. Both xenoliths and exposed granulites generally have low abundances of incompatible elements such as K, Rb, Th, and U. The xenoliths, however, often have lower abundances of these elements than the exposed granulites. High K/Rb ratios, coupled with an inverse correlation between K2O and K/Rb, indicate that most samples from the Mexican lower crust are depleted in both K and Rb. Furthermore, Th/U ratios similar to upper crustal materials and high La/Th ratios suggest depletion in both Th and U relative to average crustal material. Removal of a melt phase, either after the crystallization of cumulates or following a partial melting event, can explain many of the chemical features observed and may be responsible for the elemental depletions. Some samples, however, appear to have been unaffected during their residence in the lower crust and chemically resemble their presumed protoliths. Available geobarometric data suggest that xenoliths equilibrated near the crust/mantle boundary (∼10 kbar) and represent the lowermost crust, while the exposed granulites equilibrated at shallower crustal levels (∼7 kbar). Thus the differences observed between the xenoliths and exposed granulites indicate that there may be chemical and lithologic zonation in the Mexican lower crust. The lowermost crust in Mexico consists of crystallized basaltic liquids, cumulates, and/or residue after the removal of a partial melt as well as garnet-rich metasediments.

Geology and structure of the Larsemann Hills area, Prydz Bay, East Antarctica

Abstract: The Larsemann Hills area in Prydz Bay, East Antarctica, underwent low pressure granulite facies metamorphism about 1100 Ma ago This peak metamorphic age is similar to that of large areas west of the Larsemann Hills, but contrasts with the Archaean age of the Vestfold Hills only 100 km to the northeast The dominant rock types in the Larsemann Hills are metapelitic cordierite‐ and Fe‐Ti oxide‐rich gneisses and various leucogneisses Felsic, garnet‐bearing, variably foliated ‘yellow gneiss’ (60% of outcrop) and the extremely cordierite‐rich ‘blue gneiss’ (10% of outcrop) constitute the two major metasedimentary units Mafic dykes, charnockites and evidence of brittle deformation are absent, and there are minor orthogneisses as well as mafic two pyroxene gneisses that lack garnet and ultramafic rocks These features distinguish the Larsemann Hills not only from the Archaean Vestfold Hills to the northeast, but also from the 1000 Ma old ‘transition zone’ in the Rauer Group and outcrops of similar age to the

Low-pressure granulite facies metamorphism in the Larsemann Hills area, East Antarctica; petrology and tectonic implications for the evolution of the Prydz Bay area

Abstract: Thermobarometric studies on various granulite facies areas along the Prydz Bay coast, East Antarctica (73°-79°E, 68°-70°S), show that, at around 1100 Ma, during a late Proterozoic orogeny, the rocks of the Larsemann Hills suffered a lower pressure metamorphic peak than the surrounding areas. Along the Prydz Bay coast, the rocks affected by this event include parts of the Vestfold Hills block plus all of the Rauer Group, the Larsemann Hills and the Munro Kerr Mountains. The dykes in the south-west corner of the Vestfold Hills were recrystallized during this event with little deformation at temperatures not quite as high as in the areas further south-west (650°C, 6.5 kbar) (Collerson et al., 1983), the Rauer Group was metamorphosed at 800°C and 7.5 kbar (Harley, 1987a), the Larsemann Hills at 750°C and 4.5 kbar, and the Munro Kerr Mountains probably at around 850°C and 5 kbar. Retrograde equilibration in the different areas occurred during decompression to about 10 km depth in all areas, followed by isobaric cooling at this depth. This paper shows that the peak metamorphism in the Larsemann Hills occurred at a pressure which is too low to have been the consequence of thermal relaxation of overthickened crust with normal mantle heat flow. Although other areas in Prydz Bay were metamorphosed at sufficiently high pressures so that their decompression paths are not inconsistent with a continental collision model, the inferred pre-metamorphic peak histories and the requirement of consistency with the Larsemann Hills, make it unlikely that collision followed by erosion-driven decompression is an appropriate model. We suggest that the thermal regime of the crust in the Larsemann Hills region was controlled by a perturbation in the asthenosphere, with magma invasion of the crust. We suggest that the 500 Ma event, represented in Prydz Bay by granitic outcrops at Landing Bluff and by several K/Ar ages from the Larsemann Hills area, was responsible for the final excavation of the terrane.

Metamorphic fluids in the deep crust

Abstract: Most of the inferences about fluid action in the deep crust are drawn from field, petrographic, and geochemical studies in the granulite facies terrains. Deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks, calculations of fluid-mineral equilibria based on observed assemblages, fluid inclusions, and field evidence of fluid pathways. Discussion of these features has focused attention on several major problems concerning the timing, flow mechanisms, and origin of the fluids. A key issue concerns episodicity versus secular activity of fluids. There are strong arguments that most fluid action in the deep crust is linked to thermal/deformational events, including magmatism and regional meta­ morphism. Discrete radiometric ages of high-grade terrains often define relatively short periods of crystallization or recrystallization. The late Archean granulite facies metamorphism of southern India is an example, where U-Pb systems of zircons (Buhl et al 1983), whole-rock U-Pb data (Peucat et al 1 987), and Sm-Nd systems of whole-rocks and minerals (Bernard-Griffiths et al 1 987) all show a major high-temperature recrys­ tallization event, inferred by several workers to have involved pervasive fluid action, centered closely around 2.5 Ga. Another geochemical indi­ cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences, as in the Damara orogen, Namibia, where whole­ rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds (Hoernes & Hoffer 1 985).

Metamorphic evidence for the heating and cooling path of Namaqualand granulites ; In Evolution of Metamorphic Belts

Abstract: The Namaqua Province is one of the Proterozoic belts surrounding the Archaean Kaapvaal craton in southern Africa. It contains a wide and well-exposed zone of low-pressure granulite-facies rocks, produced during the Namaqua thermal event (c. 1150 Ma). This paper summarizes evidence for the P-T conditions and timing of metamorphism, and suggests that parts of the province may have followed an ‘anticlockwise’ P-T loop. It complements an earlier paper (Waters 1986a) which described the metamorphic zonation in part of the province, and discussed the thermal and tectonic setting of the high-grade metamorphism. The geology, structure and age relationships of the Namaqua Province are summarized by Tankard et al. (1982) and Botha (1983). The Bushmanland Subprovince (Fig. 1), the subject of this study, contains one or more sequences of supracrustal rocks, and a great deal of granitic gneiss. Locally, certain strongly deformed gneiss units may represent the 2000–1700 Ma basement to the supracrustals. The majority of granite gneisses, however, are sheet-like intrusions of batholithic proportions with ages in the range 1200–1050 Ma. The supracrustal rocks are repeated by thrusting, and infolded with granitic gneisses in east-west-trending belts. The Bushmanland Subprovince, in contrast to neighbouring areas, shows a relatively simple metamorphic zonation of amphibolite to granulite facies, with little evidence for more than one metamorphic event (Waters 1986a). Timing of deformation and mineral growth The structural sequence over most of the Bushmanland Subprovince was established by Joubert (1971), and his scheme has been followed, with local modifications, by most other workers. In the

Timing of Continental Arc-Type Magmatism in Northwest India: Evidence from U-Pb Zircon Geochronology

Abstract: Charnockites and granodiorites, which occur within granulite facies metasediments of the Bhilwara Supergroup of Rajasthan, northwest India, are cogenetic and exhibit petrologic characteristics indicative of a magmatic derivation. Zircon U-Pb data yield a common crystallization age of 1723 +14/-7 Ma. These rocks, earlier believed to represent oldest (Archean) and deepest crust, actually mark a major Proterozoic event. They are believed to be the products of continental arc magmatism, where a basic layer underplating the lower crust led to intrusion of differentiated melts in crustal domains undergoing dry metamorphism.

Archean protoliths within Early Proterozoic granulitic crust of the west European Hercynian belt: Possible relics of the west African craton

Abstract: Granulites from the Bay of Biscay area have been studied by using U-Pb and Sm-Nd methods. Results on four granulites suggest two major events of crustal growth and high-grade metamorphism. The first is Archean (2.7 Ga), and the second is Early Proterozoic (1.9 Ga). In the Hercynian belt, the oldest basement remnants known so far are about 2.0 Ga and are located in the northern Armorican massif. The 2.7 Ga orthogneiss protolith age from the Bay of Biscay is the first direct evidence of Archean crust involved in the Hercynian belt. Major events at 2.7, 1.8, and 0.6 Ga in the Hercynian belt suggest that European Precambrian microplate(s) were part of the west African craton, where the same sequence of events is observed.