Showing papers on "Granulite published in 2004"

An extended episode of early Mesoproterozoic metamorphic fluid flow in the Reynolds Range, central Australia*

Abstract: The products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the Upper Calcsilicate Unit in the granulite portion of the Late Palaeoproterozoic Reynolds Range Group, northern Arunta Block, central Australia. The zones of retrogression, characterized by minerals such as wollastonite, grossular and clinohumite, local resetting of oxygen isotopic compositions and local major element metasomatism, were channelways for water-rich fluids derived from granulite facies metapelites. U–Th–Pb isotopic ages measured by the SHRIMP ion microprobe on zircon and monazite from a granulite facies semipelite, an early semiconcordant aluminous quartz-rich fluid-flow segregation and a late discordant quartz-rich segregation record some of the extended thermal history of the area. Zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 ± 11 Ma old, itself derived from a source containing zircon as old as 2.2 Ga. Low-Th/U overgrowths on the zircon grew during granulite facies metamorphism at 1594 ± 6 Ma. Monazite cooled to its blocking temperature at 1576 ± 8 Ma. Zircon cores from the semiconcordant segregation are dominantly >2.3 Ga old, indicating that the source of the fluids was not the particular metamorphosed semipelite studied. Two generations of low-Th/U overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 ± 8 and 1582 ± 8 Ma, respectively. The monazite age is the same, 1576 ± 12 Ma. Zircon from the late discordant segregation gave 1568 ± 4 Ma. Fluid flow occurred for at least 18 ± 3 (σ) Ma and ended 26 ± 3 (σ) Ma after the peak of metamorphism, suggesting a very slow cooling rate of ∼3°C Ma–1. The last regional high-grade metamorphism in the Reynolds Range occurred at ∼1.6 Ga, not ∼1.78 Ga as previously thought. The high-grade event at ∼1.78 Ga is a separate event that affected only the basement to the Reynolds Range Group.

Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from Akilia, Southwest Greenland

Abstract: A granodiorite from Akilia, southwest Greenland, previously suggested to date putative life-bearing rocks to greater than or equal to3.84 Ga, is re-investigated using whole-rock major and trace-element geochemistry, and detailed cathodoluminescence image-guided secondary ion mass spectrometer analyses of zircon U-Th-Pb and rare earth elements. Complex zircon internal structure reveals three episodes of zircon growth and/or recrystallization dated to c. 3.84 Ga, 3.62 Ga and 2.71 Ga. Rare earth element abundances imply a significant role for garnet in zircon generation at 3.62 Ga and 2.71 Ga. The 3.62 Ga event is interpreted as partial melting of a c. 3.84 Ga grey gneiss precursor at granulite facies with residual garnet. Migration of this 3.62 Ga magma (or melt-crystal mush) away from the melt source places a maximum age limit on any intrusive relationship. These early Archaean relationships have been complicated further by isotopic reworking in the 2.71 Ga event, which could have included a further episode of partial melting. This study highlights a general problem associated with dating thin gneissic veins in polyphase metamorphic terranes, where field relationships may be ambiguous and zircon inheritance can be expected.

Age and tectonic evolution of Neoproterozoic ductile shear zones in the Southern Granulite Terrain of India, with implications for Gondwana studies

Abstract: [1] The high-grade rocks of the Southern Granulite Terrain (SGT) of Peninsular India are bounded to the north by the Archean Dharwar Craton. Another high-grade terrane, the Mesoproterozoic Eastern Ghats, occurs to the northeast of the SGT. The tectonic relationship between these crustal domains is complex. We present new geochronological and structural data that indicate a continuation of the Dharwar Craton into the Southern Granulite Terrain as far south as a newly identified Neoproterozoic shear zone, here named the Karur-Kamban-Painavu-Trichur Shear Zone (KKPTSZ). South of the KKPTSZ, Mesoproterozoic dates of the SGT are similar to those recorded in the Eastern Ghats, and the two domains may have been conterminous. Thirty-three new U/Pb/Th single zircon and monazite dates of samples from six structural transects across the regional shear zones indicate that the SGT has experienced at least seven thermo-tectonic events at 2.5 Ga, ∼2.0 Ga, ∼1.6 Ga, ∼1.0 Ga, ∼800 Ma, ∼600 Ma, and ∼550 Ma, and two distinct episodes of metasomatism/charnockitization between 2.50–2.53 and between 0.55–0.53 Ga. Deformation along a number of major shear zones in the SGT is Neoproterozoic to earliest Paleozoic in age, with an early phase (D2) concentrated between 700–800 Ma, and a later phase (D3) between 550 and 600 Ma. Major charnockitization (530–550 Ma) post dates D3, and is, in turn, overprinted by granitization, retrogression, and uplift between 525 and 480 Ma. The KKPTSZ, active between 560 and 570 Ma, is either a terrane boundary, or a tectonized decollement between cover and Archean basement rocks represented by predominantly paragneisses to the south and orthogneisses to the north, respectively. Other regional Neoproterozoic shear zones do not appear to separate allochthonous terranes as previously suggested on the basis of Nd model ages and Rb/Sr biotite/whole rock dates. The Neoproterozoic-Cambrian tectonothermal history of the SGT and Eastern Ghats is similar to that recorded in parts of Madagascar, East Africa, and Antarctica, and is used to reconstruct parts of central Gondwana, here named the Deccan Continent, with more robust confidence.

3.6 Ga lower crust in central China: New evidence on the assembly of the North China craton

Abstract: U-Pb and Hf isotope analyses of zircons from felsic granulite xenoliths in Mesozoic volcanics reveal Early Archean (≥3.6 Ga) lower crust beneath the younger (<2.85 Ga) southern margin of the North China craton, and suggest that the eastern part of the craton formed a coherent block by 3.6 Ga. Hf model ages indicate extraction of protoliths from the mantle ca. 4 Ga or earlier, followed by remelting at 3.6–3.7 Ga. Hf isotope data require both recrystallization of magmatic zircons, and growth of new zircon, up to ca. 1.9 Ga. One sample records 2.1–1.9 Ga remelting of a 2.5 Ga protolith. If large parts of the exposed upper continental crust elsewhere also are underlain by older lower crust, estimates of crustal growth rates through time will require revision.

Metastability, mechanical strength, and the support of mountain belts

Abstract: Exhumed high-pressure rocks from the Caledonian root zone in Norway provide analogues for processes occurring today under southern Tibet, allowing large-scale geophysical observations, from gravity and earthquakes, to be linked with the mechanical properties of metastable rocks under high mountains. Metastability is essential for the survival of thick mountain roots and, hence, of high mountains and is in turn controlled by water. If water is absent, dry granulite, formed from earlier melting episodes, is both stable and strong and likely to survive in the eclogite stability field of deep root zones for hundreds of million years. But if hydrous fluid is introduced, the transformation of granulite to eclogite is relatively rapid and accompanied by a dramatic loss of strength. In Norway, this transformation was initiated by water infiltration along fractures formed by earthquakes. The same process, as marked by deep earthquakes, may be occurring today beneath southern Tibet.

Dunk tectonics: A multiple subduction/eduction model for the evolution of the Scandinavian Caledonides

Abstract: [1] The evolution of the Caledonides of Scandinavia included at least four events that resulted in high-pressure (HP) and ultrahigh-pressure (UHP) eclogite facies metamorphism and the introduction of peridotite from the mantle into the crust. Two are the classic Finnmarkian orogeny at ∼500 Ma and the Scandian orogeny between 400 and 425 Ma. We have dated a third HP metamorphism in the Seve Nappe Complex in Jamtland, central Sweden, at ∼454 Ma. Identical Upper Ordovician ages from eclogite of the Troms region, northern Norway, date a simultaneous fourth HP event that occurred in a different tectonic setting, probably western Iapetus. Recently reported Late Silurian ages from eclogite of the Bergen Arcs District, southern Norway, suggest a possible fifth event. We propose that all HP/UHP events in the Scandinavian Caledonides occurred through the subduction of crustal slabs into the mantle and their subsequent buoyancy-enhanced exhumation toward the surface (eduction). The slabs are recognized as intensely deformed, high-grade (eclogite and HP granulite facies) metamorphic rocks although HP metamorphism is not necessarily penetrative and retrogression to medium grade assemblages is common. The HP/UHP metamorphic terranes are bounded by thrust faults at the base and low-angle normal detachments at the top and are separated by these shear zones from less deformed terranes of lower pressure metamorphic grade. The peridotite bodies were introduced from the overlying mantle wedge during subduction or eduction. The evolution of metamorphic terranes in other orogens might be better understood through successive subduction/eduction events, a process we provisionally call “dunk” tectonics.

Trace element signature and U–Pb geochronology of eclogite-facies zircon, Bergen Arcs, Caledonides of W Norway

Abstract: Secondary-ion mass spectrometry (SIMS) U–Pb and trace element data are reported for zircon to address the controversial geochronology of eclogite-facies metamorphism in the Lindas nappe, Bergen Arcs, Caledonides of W Norway. Caledonian eclogite-facies overprint in the nappe was controlled by fracturing and introduction of fluid in the Proterozoic—Sveconorwegian—granulite-facies meta-anorthosite-norite protolith. Zircon grains in one massive eclogite display a core–rim structure. Sveconorwegian cores have trace element signatures identical with those of zircon in the granulite protolith, i.e. 0.31≤Th/U≤0.89, heavy rare earth element (HREE) enrichment, and negative Eu anomaly. Weakly-zoned to euhedral oscillatory-zoned Caledonian rims are characterized by Th/U≤0.13, low LREE content (minimum normalized abundance for Pr or Nd), variable enrichment in HREE, and no Eu anomaly. A decrease of REE towards the outermost rim, especially HREE, is documented. This signature reflects co-precipitation of zircon with garnet and clinozoisite in a feldspar-absent assemblage, and consequently links zircon to the eclogite-facies overprint. The rims provide a mean 206Pb/238U crystallization age of 423±4 Ma. This age reflects eclogite-forming reactions and fluid–rock interaction. This age indicates that eclogite-facies overprint in the Lindas nappe took place at the onset of the Scandian (Silurian) collision between Laurentia and Baltica.

Pre-Variscan geological events in the Austrian part of the Bohemian Massif deduced from U-Pb zircon ages

Abstract: In an attempt to elucidate the pre-Variscan evolution history of the various geological units in the Austrian part of the Bohemian Massif, we have analysed zircons from 12 rocks (mainly orthogneisses) by means of SHRIMP, conventional multi-grain and single-grain U–Pb isotope-dilution/mass-spectrometry. Two of the orthogneisses studied represent Cadomian metagranitoids that formed at ca. 610 Ma (Spitz gneiss) and ca. 580 Ma (Bittesch gneiss). A metagranite from the Thaya batholith also gave a Cadomian zircon age (567±5 Ma). Traces of Neoproterozoic zircon growth were also identified in several other samples, underlining the great importance of the Cadomian orogeny for the evolution of crust in the southern Bohemian Massif. However, important magmatic events also occurred in the Early Palaeozoic. A sample of the Gfohl gneiss was recognised as a 488±6 Ma-old granite. A tonalite gneiss from the realm of the South Bohemian batholith was dated at 456±3 Ma, and zircon cores in a Moldanubian metagranitic granulite gave similar ages of 440–450 Ma. This Ordovician phase of magmatism in the Moldanubian unit is tentatively interpreted as related to the rifting and drift of South Armorica from the African Gondwana margin. The oldest inherited zircons, in a migmatite from the South Bohemian batholith, yielded an age of ca. 2.6 Ga, and many zircon cores in both Moravian and Moldanubian meta-granitoid rocks gave ages around 2.0 Ga. However, rocks from the Moldanubian unit show a striking lack of zircon ages between 1.8 and 1.0 Ga, reflecting an ancestry from Armorica and the North African part of Gondwana, respectively, whereas the Moravian Bittesch gneiss contains many inherited zircons with Mesoproterozoic and Early Palaeoproterozoic ages of ca. 1.2, 1.5 and 1.65–1.8 Ga, indicating a derivation from the South American part of Gondwana.

Deciphering the petrogenesis of deeply buried granites: whole-rock geochemical constraints on the origin of largely undepleted felsic granulites from the Moldanubian Zone of the Bohemian Massif

Abstract: The prominent felsic granulites in the southern part of the Bohemian Massif (Gfohl Unit, Moldanubian Zone), with the Variscan (∼340 Ma) high-pressure and high-temperature assemblage garnet+quartz+hypersolvus feldspar ± kyanite, correspond geochemically to slightly peraluminous, fractionated granitic rocks. Compared to the average upper crust and most granites, the U, Th and Cs concentrations are strongly depleted, probably because of the fluid and/or slight melt loss during the high-grade metamorphism (900–1050°C, 1·5–2·0 GPa). However, the rest of the trace-element contents and variation trends, such as decreasing Sr, Ba, Eu, LREE and Zr with increasing SiO2 and Rb, can be explained by fractional crystallisation of a granitic magma. Low Zr and LREE contents yield ∼750°C zircon and monazite saturation temperatures and suggest relatively low-temperature crystallisation. The granulites contain radiogenic Sr (87Sr/86Sr340 = 0·7106–0·7706) and unradiogenic Nd ( = − 4·2 to − 7·5), indicating derivation from an old crustal source. The whole-rock Rb–Sr isotopic system preserves the memory of an earlier, probably Ordovician, isotopic equilibrium.Contrary to previous studies, the bulk of felsic Moldanubian granulites do not appear to represent separated, syn-metamorphic Variscan HP–HT melts. Instead, they are interpreted as metamorphosed (partly anatectic) equivalents of older, probably high-level granites subducted to continental roots during the Variscan collision. Protolith formation may have occurred within an Early Palaeozoic rift setting, which is documented throughout the Variscan Zone in Europe.

Calculated phase equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O for sapphirine-quartz-bearing mineral assemblages

Abstract: Sapphirine, coexisting with quartz, is an indicator mineral for ultrahigh-temperature metamorphism in aluminous rock compositions. Here a new activity-composition model for sapphirine is combined with the internally consistent thermodynamic dataset used by THERMOCALC, for calculations primarily in K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH). A discrepancy between published experimentally derived FMAS grids and our calculations is understood with reference to H2O. Published FMAS grids effectively represent constant aH2O sections, thereby limiting their detailed use for the interpretation of mineral reaction textures in compositions with differing H2O. For the calculated KFMASH univariant reaction grid, sapphirine + quartz assemblages occur at P–T in excess of 6–7 kbar and 1005 °C. Sapphirine compositions and composition ranges are consistent with natural examples. However, as many univariant equilibria are typically not ‘seen’ by a specific bulk composition, the univariant reaction grid may reveal little about the detailed topology of multi-variant equilibria, and therefore is of limited use for interpreting the P–T evolution of mineral assemblages and reaction sequences. Calculated pseudosections, which quantify bulk composition and multi-variant equilibria, predict experimentally determined KFMASH mineral assemblages with consistent topology, and also indicate that sapphirine stabilizes at increasingly higher pressure and temperature as XMg increases. Although coexisting sapphirine and quartz can occur in relatively iron-rich rocks if the bulk chemistry is sufficiently aluminous, the P–T window of stability shrinks with decreasing XMg. An array of mineral assemblages and mineral reaction sequences from natural sapphirine + quartz and other rocks from Enderby Land, Antarctica, are reproducible with calculated pseudosections. That consistent phase diagram calculations involving sapphirine can be performed allows for a more thorough assessment of the metamorphic evolution of high-temperature granulite facies terranes than was previously possible. The establishment of a a-x model for sapphirine provides the basis for expansion to larger, more geologically realistic chemical systems (e.g. involving Fe3+).

Vertical extrusion and middle crustal spreading of omphacite granulite: a model of syn-convergent exhumation (Bohemian Massif, Czech Republic)

Abstract: The exhumation of eclogite facies granulites (Omp–Plg–Grt–Qtz–Rt) in the Rychleby Mts, eastern Czech Republic, was a localised process initiated by buckling of crustal layers in a thickened orogenic root. Folding and post-buckle flattening was followed by the main stage of exhumation that is characterized by vertical ductile extrusion. This process is documented by structural data, and the vertical ascent of rocks from a depth of c. 70 to c. 35 km is documented by metamorphic petrology. SHRIMP 206Pb/238U and 207Pb/206Pb evaporation zircon ages of 342 ± 5 and 341.4 ± 0.7 Ma date peak metamorphic conditions. The next stage of exhumation was associated with sideways flat thrusting associated with lateral viscous spreading of granulites and surrounding rocks over indenting adjacent continental crust at a depth of c. 35–30 km. This stage was associated with syntectonic intrusion of a granodiorite sill at 345–339 Ma, emplaced at a crustal depth of c. 25 km. The time required for cooling of the sill as well as for heating of the country rocks brackets this event to a maximum of 250 000 years. Therefore, similar ages of crystallization for the granodiorite magma and the peak of eclogite facies metamorphism of the granulite suggest a very short period of exhumation, limited by the analytical errors of the dating methods. Our calculations suggest that the initial exhumation rate during vertical extrusion was 3–15 mm yr−1, followed by an exhumation rate of 24–40 mm yr−1 during further uplift along a magma-lubricated shear zone. The extrusion stage of exhumation was associated with a high cooling rate, which decreased during the stage of lateral spreading.

Ultrahigh-temperature metamorphism followed by two-stage decompression of garnet–orthopyroxene–sillimanite granulites from Ganguvarpatti, Madurai block, southern India

Abstract: The Mg–Al granulites from Ganguvarpatti consist of orthopyroxene–sillimanite–garnet ± quartz as peak assemblage, with a few porphyroblasts of cordierite and sapphirine. These assemblages were strongly overprinted by late symplectites and coronas. Orthopyroxene inclusions in garnet and porphyroblast cores have the highest X Mg (0.80) and Al2O3 content (10.7 wt%). The estimated near-peak metamorphic conditions (1,000±50°C and 11 kbar) using garnet–orthopyroxene geothermobarometry are consistent with those determined using a petrogenetic grid. The proposed multi-stage evolution process implies an initial decompression, deduced from multi-phase symplectites, followed by cooling during biotite formation. Further late decompression is explained from the orthopyroxene rims on biotite. This proposed P–T path thus suggests a unique and complex evolution history for the UHT granulites of southern India. Present results are comparable with similar adjacent terranes in the Gondwana supercontinent, but the lack of structural and geochronological data makes a link with any major metamorphic event uncertain.

U–Pb and Hf-isotope analysis of zircons in mafic xenoliths from Fuxian kimberlites: evolution of the lower crust beneath the North China craton

Abstract: Mafic xenoliths from the Paleozoic Fuxian kimberlites in the North China craton include garnet granulite, and minor pyroxene amphibolite, metagabbro, anorthosite and pyroxenite. The formation conditions of the amphibolites are estimated at 745–820 °C and 7.6–8.8 Kb (25–30 km); the granulites probably are derived from greater depths in the lower crust. LAM-ICPMS U–Pb dating of zircons from four granulites reveals multiple age populations, recording episodes of magmatic intrusion and metamorphic recrystallisation. Concordant ages and upper intercept ages, interpreted as minimum estimates for the time of magmatic crystallisation, range from 2,620 to 2,430 Ma in three granulites, two amphibolites and two metagabbros. Lower intercept ages, represented by near-concordant zircons, are interpreted as reflecting metamorphic recrystallisation, and range from 1,927 to 1,852 Ma. One granulite contains two metamorphic zircon populations, dated at 1,927±55 Ma and 600–700 Ma. Separated minerals from one granulite and one amphibolite yield Sm–Nd isochron ages of 1,619±48 Ma (143Nd/144Nd)i=0.51078), and 1,716±120 Ma (143Nd/144Nd)i=0.51006), respectively. These ages are interpreted as recording cooling following metamorphic resetting; model ages for both samples are in the range 2.40–2.66 Ga. LAM-MC-ICPMS analyses of zircon show a range in 176Hf/177Hf from 0.28116 to 0.28214, corresponding to a range of eHf from −34 to +12. The relationships between 207Pb/206Pb age and eHf show that: (1) the granulites, amphibolites and metagabbro were derived from a depleted mantle source at 2.6–2.75 Ga; (2) zircons in most samples underwent recrystallisation and Pb loss for 100–200 Ma after magmatic crystallisation, consistent with a residence in the lower crust; (3) metamorphic zircons in several samples represent new zircon growth, incorporating Hf liberated from breakdown of silicates with high Lu/Hf; (4) in other samples metamorphic and magmatic zircons have identical 176Hf/177Hf, and the younger ages reflect complete resetting of U–Pb systems in older zircons. The Fuxian mafic xenoliths are interpreted as the products of basaltic underplating, derived from a depleted mantle source in Neoarchean time, an important period of continental growth in the North China craton. Paleoproterozoic metamorphic ages indicate an important tectonic thermal event in the lower crust at 1.8–1.9 Ga, corresponding to the timing of collision between the Eastern and Western Blocks that led to the final assembly of the North China craton. The growth of metamorphic zircon at 600–700 Ma may record an asthenospheric upwelling in Neoproterozoic time, related to uplift and a regional disconformity in the North China craton.

The Problem of Dating High-pressure Metamorphism: a U–Pb Isotope and Geochemical Study on Eclogites and Related Rocks of the Mariánské Lázně Complex, Czech Republic

Abstract: This study presents new geochemical (major and trace element, Nd–Sr isotope) and U–Pb zircon, monazite, titanite and rutile data for various rock types (eclogite, high-pressure granulite, amphibolite, orthogneiss, leucosome) of the high-grade metamorphic Marianske Lazně Complex in the western Bohemian Massif. Concordant U–Pb zircon analyses from the mafic to intermediate samples disclose the Marianske Lazně Complex as a mixture of c. 540 Ma oceanic rocks juxtaposed at depth with lower-crustal rocks of the structurally overlying Tepla–Barrandian Unit. This interpretation refutes earlier models in which the Marianske Lazně Complex was interpreted as a dismembered Cambro-Ordovician ophiolite complex affected by Variscan subduction. Metamorphic zircon in mafic rocks of the Marianske Lazně Complex, as well as monazite from orthogneiss, and titanite in leucosome yield ages around 380 Ma. On the basis of detailed petrography and cathodoluminescence imaging we conclude that the c. 380 Ma age reflects the timing of Variscan exhumation and associated decompression melting under upper amphibolite-facies to granulite-facies conditions. Thereby, the data derived from metamorphic zircon of eclogites and high-pressure granulite, unexpectedly, do not date the timing of eclogitization, which could have happened just before Variscan exhumation, or even shortly after Late Cadomian protolith formation.

Ultrahigh-temperature Metamorphism (1150°C, 12 kbar) and Multistage Evolution of Mg-, Al-rich Granulites from the Central Highland Complex, Sri Lanka

Abstract: Mgand Al-rich granulites of the central Highland Complex, Sri Lanka preserve a range of reaction textures indicative of a multistage P–T history following an ultrahigh-temperature metamorphic peak. The granulites contain a near-peak assemblage of sapphirine–garnet–orthopyroxene–sillimanite–quartz–K-feldspar, which was later overprinted by intergrowth, symplectite and corona textures involving orthopyroxene, sapphirine, cordierite and spinel. Biotite-rims, kornerupine and orthopyroxene-rims on biotite are considered to be late assemblages. Thermobarometric calculations yield an estimated P–T of at least 1100 C and 12 kbar for the near-peak metamorphism. Isopleths of Al2O3 in orthopyroxene are consistent with a peak temperature above 1150 C. The P–T path consists of four segments. Initial isobaric cooling after peak metamorphism (Segment A), which produced the garnet–sapphirine–quartz assemblage, was followed by near-isothermal decompression at ultrahigh temperature (Segment B), which produced the multiphase symplectites. Further isobaric cooling (Segment C) resulted in the formation of biotite and kornerupine, and late isothermal decompression (Segment D) formed orthopyroxene rims on biotite. This evolution can be correlated with similar P–T paths elsewhere, but there are not yet sufficient geochronological and structural data available from the Highland Complex to allow the tectonic implications to be fully assessed.

Eclogites of the Snowbird tectonic zone: petrological and U-Pb geochronological evidence for Paleoproterozoic high-pressure metamorphism in the western Canadian Shield

Abstract: Eclogite occurs within the southern domain of the East Athabasca mylonite triangle in northern Saskatchewan. Situated at the boundary between the Archean Rae and Hearne Provinces of the western Canadian Shield, the East Athabasca mylonite triangle is a fundamental exposure of the ~3,000-km-long Snowbird tectonic zone. The eclogite occurs in association with a variety of lower crustal high-pressure granulites that record a complex metamorphic history from 2.6 to 1.9 Ga. Temperatures of the eclogite facies metamorphism are constrained by garnet-clinopyroxene exchange thermometry at 920–1,000 °C. Minimum pressure conditions are recorded by the jadeite+quartz=albite geobarometer at 1.8–2.0 GPa. A near-isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene-plagioclase and pargasite-plagioclase. U-Pb IDTIMS zircon geochronology of the eclogite yields a weighted mean 207Pb/206Pb date of 1,904.0±0.3 Ma, which we interpret as the time of peak eclogite facies metamorphism. SHRIMP in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207Pb/206Pb date of 1,905±19 Ma. Inclusion suites of high-pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism. Zircon from one eclogite sample has older cores that are 2.54 Ga, which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith. U-Pb rutile data indicate slow cooling at ~1°C/Ma below ~500 °C from 1.88 to 1.85 Ga. The formation and exhumation of the eclogites at ca.1.9 Ga has important implications for the tectonic significance of the Snowbird tectonic zone during the Paleoproterozoic. The eclogites described here are consistent with transport of continental crust to mantle depths during the Paleoproterozoic, followed by rapid buoyancy-driven exhumation to normal lower crustal depths.

Petrology and in situ U–Th–Pb Monazite Geochronology of Ultrahigh-Temperature Metamorphism from the Andriamena Mafic Unit, North–Central Madagascar. Significance of a Petrographical P–T Path in a Polymetamorphic Context

Abstract: Petrological studies and electron microprobe dating of monazite from the mafic Andriamena unit, north–central Madagascar, indicate that an apparently continuous P–T path inferred for Mg-granulites is actually discontinuous, resulting from the superposition of two distinct metamorphic events at 2 5 Ga and 750Ma. The late Archaean event corresponds to an ultrahigh-temperature metamorphism (1000 C, 10 5 kbar) characterized by a sapphirine–garnet–orthopyroxene–quartz assemblage. Neoproterozoic ages are associated with the development of a sapphirine–cordierite-bearing assemblage, symplectites of orthopyroxene–sillimanite and partial melting at 850 C and 7 kbar. This sequence of reactions and mineral assemblages could be interpreted as the result of near-isothermal decompression to about 4 kbar followed by isobaric cooling to 650 C. However, geodynamic constraints suggest that the granulites underwent a phase of cooling to the stable geotherm following the ultrahightemperature metamorphism at 2 5 Ga. Consequently, we suggest that the ‘petrographical path’ inferred from the Mg-granulites is not representative of the actual P–T–t path. The decompression, in particular, is an artefact of the P–T path with no geological meaning; it results from the equilibration of the refractory late Archaean ultrahigh-temperature assemblages at a lower pressure during the middle Neoproterozoic event.

Decoupling of the Lu-Hf and Sm-Nd isotope systems during the evolution of granulitic lower crust beneath southern Africa

Abstract: The Lu-Hf and Sm-Nd isotope systems in kimberlite-borne, Archean to Proterozoic lower-crustal granulite xenoliths from southern Africa display significant parent-daughter fractionations and corresponding isotopic anomalies resulting in the decoupling of these generally well correlated systems. Systematic compositional and mineralogical controls on decoupling have also been identified. Opposite senses of divergent evolution from the terrestrial Hf-Nd isotope array are demonstrated in mafic granulite samples versus felsic and metasedimentary granulite samples. The former have superchondritic Lu/Hf and 176Hf/177Hf ratios at subchondritic Sm/Nd and 143Nd/144Nd ratios, whereas the latter have subchondritic Lu/Hf and 176Hf/177Hf ratios at a wide range of subchondritic to superchondritic Sm/Nd and 143Nd/144Nd ratios. This decoupling is shown to be related to the mineralogical sensitivity of crustal protolith Lu/Hf ratios to fractionation upon anatexis in the presence not only of residual garnet and accessory zircon, but also the residual oxide minerals (rutile versus iron-titanium oxides). The isotope diversity observed in these deep-crustal melt residues supports suggestions that the Hf-Nd systematics of complementary crustal melts may be useful fingerprints for deciphering the mineral assemblage and geochemical character of granitoid magma sources, and that together these systematics can yield insights into the origin and differentiation of continental crust.

U–Pb Zircon Calendar for Namaquan (Grenville) Crustal Events in the Granulite-facies Terrane of the O'okiep Copper District of South Africa

Abstract: s, XVI Geokongres, Geological Society of South Africa, Stellenbosch, pp. 160---163. Van Zyl, D. (1978). A petrological approach towards the ore-bearing potentialities of the Okiep basic intrusives in Namaqualand. In: Verwoed, W. J. (ed.) Mineralization in Metamorphic Terranes. Geological Society of South Africa, Special Publication 4, 323---329. Vellet, V. (1958). The geology of the ‘Copper District’ Namaqualand, Cape Province, South Africa. Unpublished report of the O’okiep Copper Company, 110 pp. Waters, D. J. (1989). Metamorphic evidence for the heating and cooling path of Namaqualand granulites. In: Daly, J. S., Cliff, R. A. & Yardley, B. W. D. (eds) Evolution of Metamorphic Belts. Geological Society, London, Special Publication 43, 357---363. Williams, I. S. & Claesson, S. (1987). Isotopic evidence for the Precambrian provenance and Caledonian metamorphism of high grade paragneisses from the Seve Nappes, Scandinavian Caledonides. II Ion microprobe zircon U---Th---Pb. Contributions to Mineralogy and Petrology 97, 205---217. CLIFFORD et al. CRUSTAL EVENTS IN O’OKIEP DISTRICT