Peak Granulite Facies Metamorphism Research Articles

Contrasting metamorphic records and their implications for tectonic process in the central Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica

Abstract Metamorphic rocks in the central part of Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica, are classified into three types based on petrological characteristics. (i) The Austkampane area preserves c. 800 °C and 0.5–0.6 GPa peak metamorphic conditions followed by decompression and subsequent isobaric cooling and later hydration (A-type). (ii) The Brattnipene and eastern Menipa area preserve peak P–T conditions of c. 800 °C and 0.7–0.8 GPa with subsequent isobaric cooling and later hydration (B-type). (iii) The area including Lunckeryggen, southern Walnumfjella and western Menipa preserves an amphibolite–facies peak metamorphic condition with signatures of prograde metamorphism (L-type), which are typically unaffected by the retrograde hydration event. Peak granulite–facies metamorphism of A- and B-type rocks are contemporaneous at c. 640–600 Ma, but a difference in the P–T paths between these rocks can be explained by thrusting of the A-type rock unit onto the B-type rock unit. By contrast, the timing of the metamorphism of the L-type rocks is significantly younger at c. 550 Ma, possibly related to the intrusion of pegmatites and granitoids. These metamorphic records in the central part of the Sør Rondane Mountains can be a test ground for the regional tectonic processes proposed for the orogeny related to Gondwana formation. Supplementary material: Representative mineral compositions are listed at www.geolsoc.org.uk/SUP18623

Neoproterozoic granulites from the northeastern margin of the Tarim Craton: Petrology, zircon U–Pb ages and implications for the Rodinia assembly

We report the discovery of a suite of Neoproterozoic granulite facies rocks from the vicinity of the Boston Lake area at the northeastern margin of the Tarim Craton. Mineral parageneses show that these rocks witnessed peak granulite facies metamorphism followed by retrograde amphibolite facies overprinting. Pressure–temperature computations of the Grt–Opx granulites yield ca. 810°C and 1.0GPa, falling in the lower domain of high-pressure granulite-facies P–T field. Most of the zircons in our samples display core–rim textures with metamorphic overgrowths on detrital cores. LA-ICP-MS U–Pb dating on the zircon cores yields three major age populations at ca. 0.9–1.0Ga, 1.8–2.0Ga and 2.4–2.6Ga, suggesting the composite sources for the protolith sedimentary rocks. The metamorphic zircons from different samples record a narrow age range between 790 and 820Ma, marking the timing of high grade metamorphism. This is the first report of Neoproterozoic high-grade metamorphic event from the northeastern margin of the Tarim Craton. Our results could suggest either a major collisional event associated with the assembly of the Tarim Craton within the supercontinent Rodinia, or an Andean-type orogeny at the periphery of the Rodinia supercontinent.

Growth of the Greater Indian Landmass and its assembly in Rodinia: Geochronological evidence from the Central Indian Tectonic Zone

The Sausar Mobile Belt at the southern margin of the Central Indian Tectonic Zone (CITZ) is a key unit in constraining the timing of continent–continent collision between the North and South Indian cratonic blocks. It consists of three domains – northern, central and southern – that record the history of this collision. Rocks from the northern and central domains were metamorphosed under medium pressure–medium temperature conditions and define a clockwise P–T trajectory. Monazite spot ages in garnet–cordierite migmatite from the western extremity of the northern domain yield a weighted mean age of 1043±18Ma (MSWD=0.26) for monazite cores and 955±11Ma (MSWD=0.43) for monazite rims. These ages are correlated with peak granulite facies metamorphism and post-peak decompression, respectively. Garnet–orthoamphibole gneiss and garnet–cordierite migmatite from the central and eastern sectors of the northern domain, respectively, yield weighted mean ages (for monazite) of 974±9Ma (MSWD=0.53) and 989±9Ma (MSWD=1.3). These ages mark the timing of retrograde metamorphism in the northern domain. Charnockite in the northern domain, which was emplaced during post-peak decompression, records a SHRIMP U–Pb zircon concordia age of 938±3Ma (MSWD=1.9). Lu–Hf isotopic analyses of these zircons show a restricted range of εHf(T) values (from −2.9 to −5.8 with a mean of −4.3) and have TDMc model ages from 1995 to 2163Ma (with a mean of 2064Ma), implying derivation of charnockitic magma from a Paleoproterozoic crustal source. In contrast, garnet–staurolite–kyanite schist and garnet–biotite–muscovite–quartz schist from the central domain of the Sausar Mobile Belt yield weighted mean monazite ages of 1062±13Ma (MSWD=1.2) and 993±19Ma (MSWD=0.16), respectively, which are broadly correlated with the peak and retrograde stages of metamorphism from this domain. Geochronological data thus tightly bracket the collisional orogeny in the CITZ at between 1.06Ga and 0.94Ga. When combined with recent data from collisional belts further east of the CITZ, namely the Chhotanagpur Gneissic Complex and Shillong Plateau Gneissic Complex, a uniformity of Mesoproterozoic to Early Neoproterozoic events is evident, implying final amalgamation of the North and South Indian blocks at this time along a ~1500km long orogenic belt. This means that two or more separate blocks were in existence at the time of incorporation within Rodinia, not a single block as suggested in most current models.

Exact timing of granulite metamorphism in the Namche-Barwa, eastern Himalayan syntaxis: new constrains from SIMS U–Pb zircon age

Zircon grains separated from 2 granulites from the eastern Himalaya were investigated by Raman spectroscopy, cathodoluminescence imaging, and secondary ion mass spectrometry. These grains have a thin homogeneous rim and an oscillatory inner zone domain with or without a relict inherited core. Garnet, kyanite, and rutile inclusions were identified within only the rim domain of zircon grains, indicating that the rim had formed during peak granulite-facies metamorphism. U–Pb zircon data record three distinct age populations: 1,805 Ma (for the inherited core), ca. 500 Ma (oscillatory inner zone), as well as 24–25 Ma and ca. 18 Ma (for the metamorphic rim). These new precision ages suggest that the peak metamorphic age for the HP granulite is at ca. 24–25 Ma, and subsequent amphibolite-facies retrograde metamorphism occurred at ca. 18 Ma.

Granulite facies anatexis in the Ancient Gneiss Complex, Swaziland, at 2.73 Ga: Mid-crustal metamorphic evidence for mantle heating of the Kaapvaal craton during Ventersdorp magmatism

The 3.7–3.2 Ga Ancient Gneiss Complex (AGC) of Swaziland, located S and SE of the Barberton Greenstone Belt (BGB), is a complex geological terrane where few studies have focussed on the timing and conditions of deformation and metamorphism, despite the fact that minor metasedimentary units within the AGC provide excellent opportunities for constraining metamorphic evolution. One of these, the Mkhondo Valley Metamorphic Suite consists of migmatitic metapelitic granulites, within which the peak metamorphic assemblage garnet + cordierite + biotite + plagioclase + quartz ± K-feldspar + melt is dominant. NCKFMASHTO pseudosection modelling of this anatectic assemblage constrains peak metamorphic conditions to 850–830 °C and 4.4 kbars, followed by near isobaric cooling of the terrane to 680 °C and 3.9 kbars. These conditions indicate a period of anomalously high heat flow within the crust due to the direct addition of mantle heat. U–Pb SHRIMP dating of zircons from an in situ leucosome indicates an age of ca. 2.73 Ga for peak granulite facies metamorphism, approximating the age of Ventersdorp Supergroup flood basalt volcanism on the craton and widespread intracratonic granitic magmatism. The current study suggests that substantial heat input to the Kaapvaal cratonic crust during underplating by Ventersdorp related magmas, possibly related to the presence of a ∼2.7 Ga mantle plume marginal to the craton, was responsible for high-grade metamorphism in the AGC and a period associated with regional crustal anatexis. The resultant mid-crustal ductility and weakening may have initiated mobilisation of the mid-crust as gneissic domes, producing basement uplift and sedimentary recycling of auriferous reefs of the Central Rand Group of the Witwatersrand Supergroup.

Low-density CO 2-rich fluid inclusions from charnockites of southwestern Madurai Granulite Block, southern India; implications on graphite mineralization

Characterization of fluid inclusions in graphite-bearing charnockites from the southwestern part of the Madurai Granulite Block in southern India reveals a probable relation with the formation and break down of graphite during the high-grade metamorphism. The first-generation monophase pure CO 2 inclusions, the composition of which is confirmed by laser Raman spectroscopy, recorded moderate density (0.77–0.87 g/cc) corresponding to low tapping pressure (around 2 kb) than that of the peak granulite-facies metamorphism. The precipitation of graphite, as inferred from graphite inclusions and δ 13C values of the graphite from the outcrops, is interpreted as the cause of this lowering of fluid density. An intermediate generation of pseudosecondary inclusions resulted from the re-equilibration or modification of the first-generation fluids and the CO 2 formed is interpreted to be the oxidation product from graphite. The youngest generation of fluids which caused widespread retrogression of the granulites is a low-temperature (∼350 °C) high-saline (32.4–52.0 wt% NaCl equivalent) brine. Carbon isotope data on the graphite from the charnockites show δ 13C values ranging from −11.3 to −19.9‰, suggesting a possibility of mixing of carbon sources, relating to earlier biogenic and later CO 2 fluid influx. Combining the information gathered from petrologic, fluid inclusion and carbon stable isotope data, we model the fluid evolution in the massive charnockites of the southwestern Madurai Granulite Block.

Dating Prograde Amphibolite and Granulite Facies Metamorphism Using In Situ Monazite U‐Pb SHRIMP Analysis

In situ SHRIMP U‐Pb analysis of monazite grains from pelites within an early‐formed high‐temperature shear zone in the southern Broken Hill Block, Australia, has been used to constrain the timing of prograde amphibolite facies and peak granulite facies metamorphism within the terrain. Geochronological analyses from grains inhabiting texturally distinct localities revealed two distinct age populations, ca. 1619 and ∼1600 Ma. The older age was obtained from grains that occur as inclusions completely enclosed within coarse‐grained K‐feldspar and garnet grains, which evidently armored the monazite inclusions against resetting during younger deformation and metamorphic events. The ca. 1619‐Ma monazite population occurs as part of an amphibolite facies inclusion assemblage hosted within the peak granulite facies mineral assemblage and constrains the timing of prograde amphibolite facies metamorphism within the Broken Hill Block. The younger ∼1600‐Ma monazite age population was from grains within the pervasive shear fabric of the pelites or adjacent to fractures or grain boundaries in the matrix. The ∼1600‐Ma age population represents either a later stage of monazite growth or isotopic resetting during deformation along the high‐temperature shear zone at peak granulite facies conditions.

A rare garnet-tourmaline-sillimanite-biotite-ilmenite-quartz assemblage from the granulite-facies region of south-central Massachusetts

A rare lithology consisting of garnet-tourmaline-sillimanite-biotite-ilmenite-quartz has been found within the granulite-facies region of south-central Massachusetts. The homogeneous, Ti-rich oxy-dravitic tourmaline, X Mg = Mg/(Mg + Fe) = 0.72.0.77, falls into the alkali group, and is similar in composition to lower grade tourmaline found in corresponding metapelitic rocks in Maine. Charge-balancing calculations and binary diagrams suggest that, like biotite in the region, tourmaline has undergone deprotonation by means of the exchange vectors AlOR -1 (OH) -1 and TiO 2 R -1 (OH) -2 , where R represents Fe + Mg. The restriction of a concordant tourmaline-rich horizon in otherwise tourmaline-free rocks of this granulite-facies region suggests that either: (1) B, released during prograde fluid-absent dehydration reactions of muscovite and biotite, was locally available in a fluid phase or melt for later crystallization near the peak of granulite-facies metamorphism along pathways that provided a conduit for fluid migration; or (2) that this is simply a B-rich compositional horizon (tourmalinite) that survived anatexis and granulite-facies metamorphism and that records the incipient conditions of tourmaline breakdown and subsequent recrystallization near or post-peak metamorphism.

Petrogenesis of silicic magmatism related to the ∼ 2.44 Ga rifting of Archean crust in Koillismaa, eastern Finland

Early Paleoproterozoic extension in the Archean craton of the Fennoscandian shield led to the emplacement of several 2.44 Ga layered gabbroic intrusive complexes in northern Finland and adjacent Russia. Closely associated with them are felsic rocks of similar age: (1) the Sirniö Group volcanic rocks on top of the Koillismaa layered complex; (2) a quartz alkali feldspar syenite at Kynsijärvi near the Koillismaa complex; and (3) an aluminous A-type granite at Nuorunen near the Oulanka layered complex. In the Koillismaa area, the ruptured Archean crust consists of ortho- and paragneisses that were intruded and migmatized by somewhat younger granites. U–Pb zircon data indicate that the gneisses are at least ∼2.8 Ga old and that the granites were crystallized at ∼2.7 Ga. Both rock types show a common monazite age of 2695 Ma that registers the peak of granulite facies metamorphism and, possibly, the intrusion of the granites. The local Neoarchean crust has ɛ Nd(at 2440 Ma) values between − 5 and − 8.5. The mafic rocks of the Koillismaa complex show initial ɛ Nd(at 2440 Ma) values around − 1.5 and those of the Oulanka complex range from − 2.1 to 0. The ɛ Nd value (− 4.8) and T DM model age (2.9–3.0 Ga) of the Kynsijärvi quartz alkali feldspar syenite are within the limits of the evolution path of the local Archean crust. The corresponding values for the Nuorunen granite are − 2.0 and 2.76 Ga and are thus closer to those of the mafic rocks. The volcanic rocks of the Sirniö Group show more scatter with initial ɛ Nd(at 2440 Ma) values of − 1.1 to − 5.3; the lowest ɛ Nd values probably reflect later disturbance–magmatic values cluster around − 2. Major and trace element modeling shows that fractional crystallization of the Koillismaa complex parental magma or partial melting of the Archean crust cannot account for the ∼ 2.44 Ga silicic rocks of Koillismaa. The geochemical and Nd isotope characteristics of the volcanic rocks and the Kynsijärvi quartz alkali feldspar syenite are best explained by a combined assimilation–fractional crystallization model involving a contaminated komatiitic parental magma and intermediate-mafic lower crustal assimilant. Differences in the initial Nd isotope composition of the 2.44 Ga rocks reflect varying source components and evolutionary history for the Archean lithosphere in eastern Fennoscandia.

Metamorphic petrology and zircon geochronology of high‐grade rocks from the central Mozambique Belt of Tanzania: crustal recycling of Archean and Palaeoproterozoic material during the Pan‐African orogeny

AbstractNew data on the metamorphic petrology and zircon geochronology of high‐grade rocks in the central Mozambique Belt (MB) of Tanzania show that this part of the orogen consists of Archean and Palaeoproterozoic material that was structurally reworked during the Pan‐African event. The metamorphic rocks are characterized by a clockwiseP–Tpath, followed by strong decompression, and the time of peak granulite facies metamorphism is similar to other granulite terranes in Tanzania. The predominant rock types are mafic to intermediate granulites, migmatites, granitoid orthogneisses and kyanite/sillimanite‐bearing metapelites. The meta‐granitoid rocks are of calc‐alkaline composition, range in age from late Archean to Neoproterozoic, and their protoliths were probably derived from magmatic arcs during collisional processes. Mafic to intermediate granulites consist of the mineral assemblage garnet–clinopyroxene–plagioclase–quartz–biotite–amphibole ± K‐feldspar ± orthopyroxene ± oxides. Metapelites are composed of garnet‐biotite‐plagioclase ± K‐feldspar ± kyanite/sillimanite ± oxides. Estimated values for peak granulite facies metamorphism are 12–13 kbar and 750–800 °C. Pressures of 5–8 kbar and temperatures of 550–700 °C characterize subsequent retrogression to amphibolite facies conditions. Evidence for a clockwiseP–Tpath is provided by late growth of sillimanite after kyanite in metapelites. Zircon ages indicate that most of the central part of the MB in Tanzania consists of reworked ancient crust as shown by Archean (c. 2970–2500 Ma) and Palaeoproterozoic (c. 2124–1837 Ma) protolith ages. Metamorphic zircon from metapelites and granitoid orthogneisses yielded ages ofc. 640 Ma which are considered to date peak regional granulite facies metamorphism during the Pan‐African orogenic event. However, the available zircon ages for the entire MB in East Africa and Madagascar also document that peak metamorphic conditions were reached at different times in different places. Large parts of the MB in central Tanzania consist of Archean and Palaeoproterozoic material that was reworked during the Pan‐African event and that may have been part of the Tanzania Craton and Usagaran domain farther to the west.

A Grenvillian arc on the margin of Amazonia: evidence from the southern Oaxacan Complex, southern Mexico

The rocks in the southern Oaxacan Complex of Mexico form part of the basement of Oaxaquia, a terrane that underlies most of eastern Mexico. They have undergone granulite facies metamorphism at ∼1 Ga, that was initially overprinted by lower amphibolite facies metamorphism accompanied by extensive hydration, and then by greenschist facies metamorphism. Geochemical data indicate that protoliths include volcanic arc lavas and sediments intruded by a rift-related granite. The metavolcanic rocks have SiO 2 ranging between 55 and 72%, are typically enriched in highly incompatible elements such as Ba, Rb and light rare earth elements, and are accompanied by relative depletion of Nb and Ti characteristic of subduction-related magmas. The granite has SiO 2 ranging between 67 and 70%, is enriched in strongly incompatible elements, and depleted in Nb and Ti indicative of either a crustal source or a subduction-related tectonic setting. Concordant, U-Pb analyses of single zircons from this granite yield an age of 1117±4 and 988±5 Ma interpreted as the age of intrusion and the time of peak granulite facies metamorphism, respectively. Combined with published information from other parts of Mexico, these data indicate that Oaxaquia was a juvenile arc metamorphosed during a relatively young Grenvillian metamorphism at 990 Ma. Similar characteristics may also be found in the Andean massifs of Colombia and in the Appalachian terranes (Carolina–Piedmont and Avalon) of Amazonian provenance. This suggests that Oaxaquia may also have lain adjacent to northwestern Amazonia at ∼1 Ga, a location that may allow Neoproterozoic subduction as the origin of the hydrous, lower amphibolite facies metamorphism. It would also provide a proximal source for low grade to unmetamorphosed ∼1 Ga and Neoproterozoic igneous boulders in northern Oaxaquia and 1000–900 Ma detrital zircons in Neoproterozoic and Paleozoic sedimentary rocks in Avalonia and Iberia.

Oxygen isotope composition of granulites from dabieshan in eastern China and its implications for geodynamics of yangtze plate subduction

The oxygen isotope composition of whole-rock and mineral separates was measured for granulite rocks from Dabieshan. According to their whole-rock δ18O values relative to the normal mantle δ18O values of 5.7±0.5‰, two groups are classified: (1) mafic granulite which exhibits lower δ18O values of 3.5 to 4.7‰, and (2) felsic granulite which shows higher δ18O values of 7.6 to 7.8‰. Consistent isotope temperatures of 800 to 900 °C are obtained for mineral pairs containing such refractory minerals as pyroxene, garnet, hornblende and iron oxides, suggesting the achievement and preservation of oxygen isotope equilibrium at the conditions of the peak granulite-facies metamorphism. This not only points to a rapid cooling and ascent for the granulite rocks in the early stage of exhumation, but also precludes the infiltration of external fluids during exhumation as the cause for the 18O-depletion in the mafic granulite. It is evident that the granulite rocks acquired the low δ18O values before the granulite-facies metamorphism by interaction with a certain 18O-depleted surface fluid. The surface fluid is assumed to exchange oxygen isotopes with the granulite protoliths prior to plate subduction. Fluid-absent metamorphism is suggested for the formation of the granulites on local scales. It is likely that the granulites together with the ultrahigh pressure eclogites and gneisses in Dabieshan were part of a single tectonic entity in the processes of subduction and Triassic metamorphism but experienced differential two-stage uplifts prior to amphibolite-facies retrogression.

Zircon isotopic ages from magnetite quartzites of the Jianping metamorphic complex, western Liaoning Province

Using the zircon Pb evaporation method and high precise ion microprobe analyses, zircon isotopic ages in supracrustal magnetite quartzites from Jianping metamorphic complex have been obtained. The results show that the deposition of the supracrustal rocks occurred around 2 552–2 520 Ma and the peak of granulite facies metamorphism reached at about 2 487 Ma; about (1 806±16) Ma, i.e. Mid-Proterozoic, the other possible metamorphic event with relatively low intensity happened in the study area. It is speculated that a likely tectonic setting where the Jianping metamorphic complex formed is an active continental margin that is involved in continental collision and crustal thickening shortly after its formation.

Relationships between magmatism, metamorphism and deformation in the Fraser Complex, Western Australia: Constraints from new SHRIMP U–Pb zircon geochronology

SHRIMP U–Pb zircon isotopic data have been obtained for four samples collected from granitoids and paragneisses in the Fraser Complex, a large composite metagabbroic body cropping out in the Mesoproterozoic Albany‐Fraser Orogen of Western Australia. The data are combined with the results of field mapping and petrographic analysis to revise a model for the geological evolution of the Fraser Complex. Three main phases of deformation are recognised in the Fraser Complex (D1–3) associated with two metamorphic events (M1–2), which involve four distinguishable episodes of recrystallisation. The first metamorphic event recognised (M1a/D1) reached granulite facies and is characterised by peak T ≥800°C and P = 600–700 MPa. A syn‐M1a/D1 charnockite has a U–Pb SHRIMP zircon age of 1301 ± 6 Ma, which also provides an estimate for the age of intrusion of Fraser Complex gabbroic rocks. Disequilibrium textures comprising randomly oriented minerals (M1b), consistent with approximately isobaric cooling, formed in various lithologies in the interval between D1 and D2. Post‐D1, pre‐D2 granites intruded at 1293 ± 8 Ma and were foliated during the D2 event, which culminated in the burial of the Fraser Complex to depths equivalent to 800–1000 MPa. Following burial, pyroxene granulites on the western boundary of the complex were pervasively retrogressed to garnet amphibolite (M2a). An igneous crystallisation age of 1288 ± 12 Ma from a syn‐M2a aplite dyke suggests that retrogression may have occurred only a few millions of years after the peak of granulite facies metamorphism. Exhumation to depths of less than ∼400 MPa occurred within ∼20–30 million years of the M2a pressure peak. Associated deformation (D3) is characterised by the development of mylonite and transitional greenschist/amphibolite facies disequilibrium textures (M2b).

Melt-related metasomatism in mafic granulites of the Quetico subprovince, Ontario: constraints from O-Sr-Nd isotopic and fluid inclusion data

Mafic granulites in the Archean Quetico subprovince, north of Manitouwadge, Ontario, occur as isolated lenses or discontinuous layers in spatial association with tonalitic leucosomes in metasedimentary rocks and exhibit concentric zoning from a biotite-rich margin to an orthopyroxene-rich outer zone and a clinopyroxene-rich central zone, with internal orthopyroxene-bearing leucosomes and, rarely, patches of relict amphibolites within the clinopyroxene-rich zone. Microstructural and microchemical evidence suggests that the mafic granulites formed from amphibolites by combined infiltration-diffusion processes in the presence of a P-F-bearing silicate melt ((P2O5)melt = 0.24-0.28 wt.%) and a CO2-rich (hypersaline?) fluid. The whole-rock and mineral δ18O values of the mafic granulites (8-9‰ V-SMOW) indicate oxygen-isotope equilibration between amphibolites (6.6-6.9‰) and associated tonalitic leucosomes (9.5-10‰) at 700-800°C. Strontium- and Nd-isotope data and U-Pb zircon ages confirm isotopic homogenization at the leucosome-amphibolite boundaries during the peak granulite-facies metamorphism at about 2650 Ma. Texturally, early CO2-rich fluid inclusions in quartz and garnet yield P-T conditions similar to those of the peak granulite-facies metamorphism. Hypersaline fluid inclusions occur in textural coexistence with the early CO2-rich inclusions, but are invariably low in homogenization temperatures (178-234°C). This study shows that silicate melts not only provide a conduit for CO2-rich fluids but also interact directly with country rocks for the formation of granulites. Also, the O-Sr-Nd isotope data show that the documented mobility of rare-earth elements in the Quetico granulite zone is localized in scale and related to anatexis of local metasedimentary rocks during the granulite-facies metamorphism.

Sm–Nd, Rb–Sr and U–Pb geochronology of a Grenville Terrane in Southern Mexico: origin and geologic history of the Guichicovi Complex

The Guichicovi complex forms the metamorphic basement near the western edge of the Maya terrane in southern Mexico. Petrographic field studies show that the Guichicovi complex, like other high grade metamorphic terranes in southern Mexico, consists of paragneisses and a igneous suite of rocks metamorphosed under granulite facies conditions. The Sm–Nd isotopic results yield depleted mantle model ages of 1.4–1.6 Ga for the metaigneous rocks and of 1.5–2.0 Ga for the metasediments. On a Sm–Nd isochron diagram the isotopic results of the metaigneous suite of rocks define an isochron whose slope corresponds to an age of ∼1.44 Ga. This age can be interpreted as the time of a major crust forming event. Discordant U–Pb zircon data from a felsic orthogneiss plot on a chord intersecting the concordia at 1231±43 and at 975±35 Ma. The upper intercept age is interpreted as the time of zircon crystallization, hence the age of intrusion of the granitic precursor. The lower intercept age reflects the time of peak granulite facies metamorphism. Concordant U–Pb isotope ratios of a zircon fraction from a charnockite (991±4 Ma) gives the time of crystallisation of the charnockite. Garnet/whole rock ages (Sm–Nd) between ∼911 and ∼933 Ma and biotite/whole rock ages (Rb–Sr) between ∼866 and ∼882 Ma document the cooling history of the Guichicovi complex. Age and petrography of the Guichicovi complex confirm its connection with other granulite exposures in Mexico, which comprise the Oaxaquia microcontinent. The results support the hypothesis that the Precambrian crust in Mexico together with the Colombian Andes and the Adirondacks originated during the mid-Proterozoic. The Grenville orogenic event depressed these terranes into the roots of the orogen. Our data indicate that the Guichicovi complex stayed for >70 Ma at a deep crustal level before uplift. Subsequent breakup of Rodinia left Oaxaquia including the Guichicovi complex together with Gondwana. Further collisions of Gondwana and Laurentia and repeated transpressional interactions in the Paleozoic and early Mesozoic led to the present day position of Grenville fragments in Mexico.

Crustal thickening of the lower crust of the Kohistan arc (N. Pakistan) deduced from Al zoning in clinopyroxene and plagioclase

The lower‐crustal rocks of the Kohistan complex (northern Pakistan) are mostly composed of metabasic rocks such as pyroxene granulites, garnet granulites and amphibolites. We have investigated P–T trajectories of the relic two‐pyroxene granulites, which are the protolith of the amphibolites within the Kamila amphibolite belt. Aluminous pyroxene retains igneous textures such as exsolution lamellae developed in the core. The significant amount of Al in clinopyroxene is buffered by breakdown reactions of plagioclase accompanied by film‐like quartz as a product at grain boundaries between plagioclase and clinopyroxene. Distinct Al zoning profiles are preserved in pyroxene with exsolution lamellae in the core and in plagioclase adjacent to clinopyroxene in pyroxene granulites. In the northern part of the Kamila amphibolite belt, Al in clinopyroxene increases towards the rim and abruptly decreases at the outer rim, and anorthite in plagioclase decreases towards the rim and abruptly increases near the grain boundary between plagioclase and clinopyroxene. In the southern part of the Kamila amphibolite belt, Al in clinopyroxene and anorthite in plagioclase simply increase towards the margins of the grains. The anorthite zoning in plagioclase is in agreement with the zoning profiles of Ca‐Tschermaks and jadeite components inferred from variations of Al, Na, Ti and Fe3+ in clinopyroxene. Assuming that the growth surface between them was in equilibrium, geothermobarometry based on Al zoning in clinopyroxene coexisting with plagioclase indicates that metamorphic pressures significantly increased with increasing temperature under granulite facies metamorphism. The peak of granulite facies metamorphism occurred at conditions of about 800 °C and 800–1100 MPa. These prograde P–T paths represent a crustal thickening process of the Kohistan arc during the Early to Middle Cretaceous. The crustal thickening of the Kohistan arc was caused by accretion of basaltic magma at mid‐crustal depths.

Carbon Isotope Fractionation Between Calcite and Graphite During High Temperature Metamorphism

The carbon isotope exchange thermometer between calcite and graphite have been recently widely applied to amphibolite and granulite facies marbles (e.g. Kitchen and Valley, 1995). It has been proved that the temperature dependant fractionation is reliable up to temperatures of the order of 800~ The fractionation between calcite and graphite nears unity at higher tempertures and hence is difficult to calibrate the thermometer at temperatures higher than 800~ Here we try to emperically test the high tempertaure fractionation between calcite and graphite using marble samples from the high temperature granulite facies terrains of southern India and East Antarctica. Marbles from the Trivandrum and the Madurai Block, from the southern granulite terrain were examined in this study. The Trivandrum Block comprises of granulite facies supracrustal rocks, while in the Madurai block massive charnockitic rocks predominate. Marbles bands occur in these two terrains as conformable units intercalated with quartzites. Texturally these are very coarse (up to several centimeters) and highly crystalline graphite crystals can be seen embedded within calcite. Calcite constitutes more than 80 modal % in most of the samples. The silicate mineral phases include phlogopite, forsterite, clinopyroxene and spinel; with occasional clinohumite, pargasite, and other calc-silicate mineral phases. Marbles from LutzowHolm complex, which is a granulite to amphibolite facies terrain with a regional peak granulite facies metamorphism at temperatures in the range 760830~ and pressures of 7 • kbar were also examined. Peak metamorphic temperatures were determined from fractionation of C-isotopes between calcitegraphite pairs using the recent calibration of Kitchen and Valley (1995). For example the marbles from the Trivanrum Block in Southern India gave fractionation range from 2.46%0 to 3.76%0, corresponding to a temperature range of 700 to 800~ Out of the ten calcite-graphite pairs, 6 gave similar results. These results are significant that the carbon isotope thermometer can be used to determine accurate peak metamorphic temperatures in granulite facies rocks. The marbles form the Madurai block and the Lutzow-Holm Bay gave temperatures between 800 and 900~

Palaeoproterozoic granulite-facies metamorphism and granitoid intrusions in the Ubendian-Usagaran Orogen of northern Malawi, east-central Africa

The Paleoproterozoic basement of northern Malawi shows evidence for granulite-facies metamorphism older than the 1930±30 Ma-old Nyika Granite. Low-pressure granulite-facies metamorphism in regionally coherent and extensively exposed cordierite-garnet granulite reached 750–850°C and 5-5.5 kbar. The Chelinda Granite intruded during this event and has been dated by single zircons at 1995±0.4 Ma (207Pb206Pb age; 2σ-mean errors). Anatectic melt which formed concurrently with cordierite growth in the cordierite-garnet granulite yielded a 207Pb206Pb zircon age of 1988±0.6 Ma. These ages are interpreted to date the peak of regional low-pressure granulite-facies metamorphism. The Nyika Granite, and other smaller collisional-type two-mica granites, intruded after the peak of granulitefacies metamorphism. Their zircon ages range from 1930±30 Ma to 1969±0.4 Ma and constrain the end of regional high-temperature conditions. Enderbitic gneiss, which is tectonically intercalated with the cordierite-garnet granulite, records peak-metamorphic conditions of ca 850–880°C and 9–11 kbar. Metamorphic zircons from the enderbitic gneiss yielded a 207Pb206Pb age of 2002±0.3 Ma. This age for high-pressure granulite-facies metamorphism in northern Malawi is similar to the recently reported age for eclogite-facies metamorphism in the Usagaran Belt of central Tanzania and indicates that orogenic activity during the Ubendian-Usagaran Orogeny culminated at ca 2000 Ma.Pre-2000 Ma magmatic events encompass the intrusion of the enderbitic gneiss precursor at 2093±0.6 Ma. Furthermore, a late granite intrusion of the Rumphi Igneous Complex (RIC) yielded a single-zircon age of ca 2048±0.7 Ma. The chemistry of those late-stage granites of the RIC displays an affinity to volcanic-arc granites. Therefore, the age of 2048 Ma is interpreted to date a mature stage of volcanic-arc magmatism. The nature of the pre-2000 Ma magmatic events remains open, however; the granitoids might have been emplaced in a long-lived Andean-type subduction zone. Tectonic juxtaposition of both granulite-facies rocks and of the latter two with the RIC occurred after Paleoproterozoic granulite-facies metamorphism in northern Malawi.

Retrograde fluids in the Archean Shawmere anorthosite, Kapuskasing Structural Zone, Ontario, Canada

The Archean Shawmere anorthosite lies within the granulite facies portion of the Kapuskasing Structural Zone (KSZ), Ontario, and is crosscut by numerous linear alteration veins containing calcite + quartz ± dolomite ± zoisite ± clinozoisite ± margarite ±paragonite ± chlorite. These veins roughly parallel the trend of the Ivanhoe Lake Cataclastic Zone. Equilibria involving clinozoisite + margarite + quartz ± calcite ± plagioclase show that the vein minerals were stable at T 0.9. Thus, vein formation, while clearly retrograde, spanned a range of temperatures, and fluid compositions evolved from H2O-rich to CO2-rich. The calcite in the retrograde veins has δ18O values that range from 8.4 to 11.2‰ (average = +9.7 ± 0.9‰) and δ13C values that range from −3.9 to −1.6‰ (average = −3.1 ± 0.6‰). These values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies. The fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent Abitibi subprovince. Vein quartz contains CO2-rich fluid inclusions (final melting T = −57.0 to −58.7 °C) that range in size from 5 to 17 μm. Measured homogenization temperatures (T h) range from −44.0 to 14.5 °C, however for most inclusions (46 of S1), T h = −44.0 to −21.1 °C (ρCO2 ≈ 1.13 to 1.05 g/cm3). At 400 to 600 °C, these densities correspond to pressures of 3.5 to 7 kbar, which is the best estimate of pressures of vein formation. It has been argued that some high density CO2-rich fluid inclusions found in the KSZ were formed during peak metamorphism and thus document the presence of a CO2-rich fluid during peak granulite facies metamorphism (Rudnick et al. 1984). The association of high density CO2-rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism. Thus, the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment. All fluid inclusion results are consistent with an initially semi-isobaric retrograde P–T path.