Sapphirine Granulites Research Articles

Timing of UHT metamorphism and cooling in south Indian granulites: New P-T-t results from a sapphirine granulite

The Precambrian granulite terrane of south India is known for the preservation of regional-scale high to ultrahigh-temperature (HT-UHT) metamorphic rocks. In the Madurai block, the largest crustal block in the terrane, a significant proportion of these HT-UHT rocks occur along a narrow belt referred to as the Kambam ultrahigh-temperature belt. Understanding the P-T-t evolution of these HT-UHT granulites is vital in decoding the spatial and temporal evolution of the Madurai block in relation to other crustal blocks. Here we present the petrology, mineral chemistry, phase equilibrium modelling, and accessory mineral (zircon, monazite, rutile and apatite) geochronological and geochemical data for a hitherto unreported sapphirine granulite from the Kambam ultrahigh-temperature belt. Combined mineral reaction and phase equilibrium modelling indicate extreme P-T conditions up to 1130 °C at 11 kbar. The geometry of the P-T path is clockwise with initial isothermal decompression followed by near isobaric cooling. LA-ICPMS U-Pb geochronological studies on accessory phases report two distinct thermal events: (i) Paleoproterozoic high-T event and associated crustal anatexis at ∼2.5 Ga from zircons and (ii) Neoproterozoic UHT event at ∼550 Ma from monazites. The monazite ages suggest a 50–60 Ma prolonged thermal event, attributed to Ediacaran-Cambrian collisional orogenesis. Additional metamorphic pulses are also identified, demonstrating the polydeformed crustal evolution history of the terrane. U-Pb age dating of rutile and apatite in the sample provide ages of ∼458 Ma and ∼392 Ma and cooling rates of ∼6 °C/Ma and ∼0.3–2 °C/Ma respectively. This suggests a complex history of slow cooling followed by ultraslow cooling. The results bring forth the significance of the Kambam ultrahigh-temperature belt in understanding the tectonothermal evolution of Madurai block and provide additional evidence for long-lived Neoproterozoic UHT orogenesis in south India.

Constraining the pressure–temperature evolution and geodynamic setting of UHT granulites and migmatitic paragneisses of the Gruf Complex, Central Alps

Thermodynamic modeling of compositionally mapped microdomains and whole-rock compositions is used to constrain the pressure–temperature (P–T) evolution of sapphirine granulites and migmatitic paragneisses from the Gruf Complex of the Central Alps. The P–T paths and conditions estimated from granulite microdomains and whole-rock compositions are consistent with one another, indicating that the estimates from both types of compositions are accurate. The sapphirine granulites were heated to ultra-high temperature conditions of 900–1000 °C and 7.0–9.5 kbar as they were decompressed from ca. 800 °C and 9–12 kbar, resulting in garnet breakdown. In a subsequent step, nearly isothermal decompression led to the development of cordierite-bearing coronae and symplectites. By ca. 27 Ma, the sapphirine granulites had been exhumed to the midcrustal level of the migmatitic paragneisses, which were undergoing peak metamorphism at ca. 675–750 °C and 5–7 kbar. These results are consistent with a geodynamic model that invokes heat advection to the lower crust closely following the continental-subduction (ultra-high pressure) stage of the Alpine orogeny. The most plausible geodynamic model consistent with the results of this study is breakoff of a southward subducting lithospheric slab, resulting in asthenospheric mantle flow.

Late-Neoproterozoic ultrahigh-temperature metamorphism in the Highland Complex, Sri Lanka

The Sri Lankan crustal blocks preserve the records of Neoproterozoic tectonothermal events associated with the final assembly of the Gondwana supercontinent. Here we present petrological and U–Pb zircon geochronological data on a suite of ultrahigh-temperature (UHT) metapelites and mafic granulite from the Highland Complex in central Sri Lanka which provide convincing evidence for Late Neoproterozoic UHT metamorphism. Three sapphirine granulites, two corundum bearing metapelites and one mafic granulite were studied for petrology and zircon U–Pb geochronology. The petrographic observations coupled with conventional thermobarometric calculations indicate that the pelitic granulites attained P=11–12kbar and T=850°C during prograde evolution. Subsequently the rocks have undergone a prograde decompression stage with increasing temperature up to UHT conditions. The peak metamorphism was attained at P–T conditions around 900–975°C and 9–10kbar, followed by near isobaric cooling and rapid decompression.LA-ICPMS zircon U–Pb analyses of magmatic cores in detrital zircons from the sapphirine granulites show 206Pb/238U ages ranging from 834±12 to 722±14Ma, reflecting the timing of crystallization of these grains in their protoliths. Metamorphic zircons in these samples yield ages ranging 535±5 and 567±4Ma representing the age of the Neoproterozoic UHT metamorphism. The metamorphic zircons from corundum-bearing metapelites yield ages from 530±4 to 578±4Ma. An older age population between 1676Ma and 2365Ma from these metasediments suggests the timing of emplacement of the Paleoproterozoic sources. Our data suggest variable degree of input from multiple sources into the protolith sediments of the Highland Complex prior to the latest Neoproterozoic UHT metamorphism at ∼530–580Ma which is broadly coeval with the collisional assembly of the Gondwana supercontinent.

New Finding of Sillmanite in Sapphirine-bearing Granulites from Pedapalli, NE Part of the Eastern Dharwar Craton, India

Abstract We report the occurrence of sillimanite in sapphirine-bearing granulites near Pedapalli town in the north eastern part of the Eastern Dharwar Craton (EDC). The sapphirine-bearing granulites of the Pedapalli area display evidence of metamorphic reactions involved in the formation of diverse mineral assemblages documented in different types of reaction textures. The common stable assemblage contains sapphirine-spinel-orthopyroxene-cordierite-plagioclage-corundum-sillimanite. The P-T evolution of these sapphirine granulites has been constrained by using the TWEEQU program. Temperature of formation of sapphirine-spinel assemblages is high, around 800 °C, and pressure ca. 8 kbar. The assemblage reported in this study has important bearing on the high-temperature metamorphism and exhumation history of the EDC.

A New Occurrence of Sapphirine-Spinel-Corundum-bearing Granulite from NE of Jagtiyal, Eastern Dharwar Craton, Andhra Pradesh

Abstract: We report a new occurrence of sapphirine-spinel-corundum bearing granulites enclosed in granitic gneisses near Jagtiyal in the Eastern Dharwar Craton (EDC). These granulites are very important in deciphering the prehistory of the thermal peak of metamorphism due to the presence of refractory phases. The appearance of sapphirine is related to the following reactions: (a) Spl1 + Crd1 = Spr1 (2:2:1) (b) Phl (in Bt1) + Crn1 = East (in Bt2) + Spr2 (7:9:3) + Crd2 (c) Bt2 + Crd1 +Crn1 = Spr2 (7:9:3) + Kfs + H2O (d) Crd1 + Spl1 + Crn1 = Spr2 (7:9:3). The P–T evolution of these sapphirine granulites has been constrained through the use of THERMOCALC program. Temperature of formation of sapphirine-spinel assemblages is high, around 800 °C, and pressure ranges from 5-7 kbars, suggesting that sapphirine formation took place during decompression stage.

Phase equilibrium modelling of Palaeoproterozoic ultrahigh‐temperature sapphirine granulite from the Inner Mongolia Suture Zone, North China Craton: implications for counterclockwise P–T path

We report new petrological data on the sapphirine granulite from Tuguiwula in the Inner Mongolia Suture Zone in the North China Craton which further confirm extreme crustal metamorphism associated with collisional orogeny. The peak metamorphic conditions of the rock have been estimated by sapphirine + quartz assemblage at T > 950 °C and P > 7.5 kbar (M1) based on mineral equilibrium modelling in the NCKFMASHTO system. These conditions are slightly lower than those reported in previous studies (~1050 °C and 10.5 kbar) due to the consideration of Fe2O3 content in the sapphirine granulite in this study, although we confirm a peak ultrahigh‐temperature metamorphism in this region. The peak event was followed by a cordierite‐forming retrograde event (M2), and biotite‐forming retrograde stage (M3) which are constrained in the P–T pseudosection by the stability of cordierite (<7.5 kbar) and biotite + sillimanite + K‐feldspar (<890 °C). Combining the results of this study and the petrological observations of spinel + quartz assemblage formed prior to sapphirine + quartz, and orthopyroxene + sillimanite + quartz assemblage after sapphirine + quartz reported in previous studies, a counterclockwise P–T history for these rocks is further confirmed, reflecting heat input into the lower crust from asthenospheric upwelling and underplating associated with the Palaeoproterozoic collisional orogeny in the North China Craton. Copyright © 2013 John Wiley & Sons, Ltd.

Sveconorwegian Mid-crustal Ultrahigh-temperature Metamorphism in Rogaland, Norway: U-Pb LA-ICP-MS Geochronology and Pseudosections of Sapphirine Granulites and Associated Paragneisses

MgAl-rich sapphirine granulites (bulk X Mg 0·71–0·75) occur as boudinaged layers in migmatitic garnet–orthopyroxene–cordierite–spinel gneisses and migmatitic garnet–sillimanite metapelites in the vicinity of the c. 930–920 Ma Rogaland anorthosite–mangerite–charnockite complex, SW Norway. Investigation of the mineral reaction history of the sapphirine granulites and the surrounding paragneisses, combined with geothermobarometric calculations and constraints from pseudosections calculated in the Na 2 O–CaO–K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 (NCKFMASHT) system, indicates a clockwise P–T path that reached peak-metamorphic ultrahigh-temperature (UHT) conditions of c. 1000°C at c. 7·5 kbar by prograde heating. UHT peak metamorphism is followed by near-isothermal (ultra)high-temperature decompression to P 850–940°C as estimated from Ti-in-zircon thermometry, suggesting that UHT metamorphism and the deduced clockwise P–T evolution is linked to regional Sveconorwegian metamorphism at c. 1010 Ma. Most of the metamorphic zircon surrounds largely resorbed inherited oscillatory zoned zircon cores ( 207 Pb/ 206 Pb apparent ages 1220–1841 Ma), testifying to the sedimentary origin of the sapphirine granulites. Epitactic growth of xenotime on metamorphic zircon at 933 ± 5 Ma is suggested to be related to crystallization of anatectic melt during post-decompressional cooling. The clockwise P–T path culminating at mid-crustal UHT conditions at c. 1010 Ma followed by (U)HT decompression is interpreted to result from collisional tectonics during the early stages of the Sveconorwegian Orogeny, followed by gravitational collapse of the mountain plateau.

Sapphirine granulites from Panasapattu, Eastern Ghats belt, India: Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin

We report equilibrium sapphirine + quartz assemblage in biotite–orthopyroxene–garnet granulites from a new locality in Panasapattu of Paderu region in the Eastern Ghats granulite belt, which provide new evidence for ultrahigh-temperature (UHT) metamorphism at 1030–1050 °C and 10 kbar in this region. The development of migmatitic texture, stabilization of the garnet–orthopyroxene–plagioclase–K-feldspar association, prograde biotite inclusions within garnet and sapphirine as well as sapphirine and cordierite inclusions within garnet in these granulites indicate that the observed peak assemblages probably formed during prograde dehydration melting of a Bt–Sill–Qtz assemblage, and constrain the prograde stage of the P– T path. The core domains of orthopyroxene porphyroblasts have up to wt(Al 2O 3) 9.6%, which suggest that the temperatures reached up to 1150 °C suggesting extreme crustal metamorphism. These conditions were also confirmed by the garnet–orthopyroxene thermobarometery, which yields a P– T range of 1012–960 °C and 9.4 kbar. The P– T phase topologies computed using isochemical sections calculated in the model system Na 2O–CaO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O (NCKFMASH) for metapelites, garnet-free sapphirine granulites and garnet-bearing sapphirine granulites match the melt-bearing assemblages observed in these rocks. Isochemical sections constructed in the NCKFMASH system for an average sub-aluminous metapelite bulk composition, and contoured for modal proportions of melt and garnet, as well as for the compositional isopleths of garnet, predict phase and reaction relations that are consistent with those observed in the rocks. Garnet and orthopyroxene contain Ti-rich phlogopite inclusions, suggesting formation by prograde melting reactions at the expense of phlogopite during ultrahigh-temperature conditions. These P– T results underestimate ‘peak’ conditions, in part as a result of the modification of garnet compositions in the domains where some melt was retained. The post-peak evolution is constrained by a succession of melt-present reactions that occur at P < 10 kbar, inferred from micro-structural relations among various minerals. After high-temperature decompression from the metamorphic peak, the P– T path followed a near isobaric cooling stage to T < 900 °C. The UHT rocks investigated in this study occur within a continental collision suture which witnessed prolonged subduction–accretion history prior to the final collision. We correlate the extreme metamorphism and the stabilization of UHT mineral assemblages to heat and volatile input from an upwelled asthenosphere during subduction–collision tectonics in a Proterozoic convergent plate margin.

UHT sapphirine granulite metamorphism at 1.93–1.92 Ga caused by gabbronorite intrusions: Implications for tectonic evolution of the northern margin of the North China Craton

Sapphirine granulites occur in the Daqingshan and Jining areas in the Palaeoproterozoic Khondalite belt, which divides the Western Block of the North China Craton into the Yinshan block to the north and the Ordos block to the south. The sapphirine granulites in the Daqingshan area are always in contact with meta-gabbronorite dykes, implying a causal relationship. The sapphirine-bearing rocks are divided into spinel–garnet–sillimanite–biotite–plagioclase–sapphirine gneiss, UHT sapphirine granulite, and spinel–garnet granulite. The sapphirine granulite contains up to 30% sapphirine, garnet (30–50%), spinel (5–15%), sillimanite (5–15%), biotite (10–20%) and plagioclase (10–20%) with minor cordierite, rutile and ilmenite, but without quartz and orthopyroxene. Bulk chemical compositions show that the sapphirine granulites have very low SiO2 contents (39wt.%), high Al contents, and low XMg. Biotite contains very high TiO2 contents up to 7.6wt.%. Detailed petrographic examination of the sapphirine granulites reveals five mineral assemblages (M0–M4): (1) an assemblage (M0) of mineral inclusions within garnet cores, (2) a matrix (peak) assemblage (M1) represented by coarse-grained garnet, sapphirine, spinel, sillimanite, biotite and plagioclase, (3) sapphirine+plagioclase symplectite (M2), (4) spinel+plagioclase symplectite (M3), and (5) retrogressive biotite (M4). The P–T stability field in the pseudosection of the NCKFMASH system indicates that the temperature of the peak UHT metamorphism of the Daqingshan sapphirine granulites is in the range 910–980°C (this compares with the peak regional metamorphic temperature of the khondalites of 700–820°C). The P–T path inferred from the P–T stability fields of the mineral assemblages (M1–M4) suggests that the peak UHT metamorphism (M1) was followed by nearly isothermal decompression (M2 and M3) and later cooling (M4). Field relations and geochronological data suggest that the high-heat flow necessary for the UHT metamorphism of the sapphirine granulites from the Daqingshan area was provided by coeval ∼1.93–1.92Ga gabbronorite intrusions that were most probably generated by ridge subduction, which was also responsible for abundant garnet-rich granites by crust melting the area.

New constraints on the metamorphic evolution of the Eastern Ghats Belt, India, based on relict composite inclusions in garnet from ultrahigh‐temperature sapphirine granulites

AbstractThe sapphirine granulites from Gangaraju Madugula, Eastern Ghats Belt (EGB), India preserve a rich variety of mineral assemblages and unique isolated and composite mineral inclusions within garnet that provide robust evidence for extreme crustal metamorphism at ultrahigh temperature (UHT) conditions (&gt;900°C). Diagnostic UHT assemblages in these rocks include sapphirine + quartz, spinel + quartz and high alumina orthopyroxene + sillimanite + quartz. The stability of spinel + quartz, sapphirine + quartz and orthopyroxene + sillimanite + quartz assemblages provides evidence for temperatures exceeding 960°C at moderate pressures. The mineral association of garnet–orthopyroxene is indicative of a subsequent high P–UHT metamorphic event as indicated by the high alumina contents of orthopyroxene (&gt;10 wt% Al2O3) coexisting with garnet. Peak P–T conditions of ∼970°C and 9.5 kbars are calculated from conventional garnet–orthopyroxene geothermobarometry. Calculated isochemical sections constructed in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) for the sapphirine granulites and garnet–orthopyroxene granulites adequately predict phase relationships that are consistent with those observed in the rocks. An evaluation of the assemblages and textures and P–T estimates indicate a three‐stage evolution of the sapphirine granulites and associated garnet–orthopyroxene granulites: (1) inclusion assemblages with sapphirine, spinel and quartz on the low‐pressure prograde path (M1 stage); (2) a peak UHT assemblage of porphyroblastic garnet–orthopyroxene (M2 stage) and (3) a retrogression that resulted in orthopyroxene–cordierite intergrowths and biotite rims on Grt (M3 stage). The sapphirine granulites and associated garnet–orthopyroxene granulites indicate that they grew during the prograde and retrograde stage. The thermo‐barometric estimates from mineral compositions and the complimentary isochemical sections approached from bulk rock compositions allow tighter constraints to be placed on the P–T evolution of this sector of the EGB. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Metamorphic evolution of the Karimnagar granulite terrane, Eastern Dharwar Craton, south India

AbstractThe Karimnagar granulite terrane is an integral part of the Eastern Dharwar Craton (EDC), India, having been the subject of much interest because of the only reported granulite facies rocks in the EDC. It shows a large variety of rock types with a wide range of mineral parageneses and chemical compositions, namely charnockites (Opx+Pl+perthite+Qtz±Bt±Grt), gneisses (Opx+Crd+Bt+Pl+Qtz+perthite±Sil±Grt±Spl; Bt+Qtz+Pl±Crd±Hbl±Spl), mafic granulites (Cpx+Pl+Qtz±Opx±Hbl), quartz-free granulites (Spr+Spl+Bt+Crd+Kfs+Crn; Bt+Crd+Kfs±Crn±Spl±Krn; And+Bt+Kfs+Chl), granites (Qtz+Pl+Kfs±Bt±Hbl), altered ultramafic rocks (Chl+Trem+Tlc), metadolerites (Cpx+Pl±Bt±Qtz±Chl), banded magnetite quartzites and quartzites. Andalusite- and chlorite-bearing assemblages presumably suggest a retrograde origin. Investigation of quartz-free granulites of the area brings out some interesting and important observations, reflecting the presence of refractory phases. These granulites are devoid of sillimanite and contain corundum instead. Reaction textures in the gneisses include breakdown of garnet to form coronas and symplectites of orthopyroxene+cordierite, formation of cordierite from garnet+sillimanite+quartz and late retrograde biotite and biotite+quartz symplectites. In the mafic granulites, inclusions of quartz and hornblende within orthopyroxene are interpreted as being a part of the prograde assemblage. At a later stage orthopyroxene is also rimmed by hornblende. The quartz-free granulites display a variety of spectacular coronas, for example, successive rims on corundum consisting of spinel+sapphirine+cordierite±orthopyroxene, rare skeletal symplectitic intergrowth of sapphirine+cordierite+potash feldspar, and late retrograde formation of chlorite, corundum, spinel and andalusite from sapphirine±cordierite. Based on chemographic relationships and petrogenetic grids, a sequence of prograde, isothermal decompressive and retrograde reactions have been inferred. Quartz-free sapphirine granulites and mafic granulites record the highest P–T conditions (~7 kbar, 850°C), whereas the gneisses were formed at lower P–T conditions (~5 kbar, 800°C). In addition, the presence of andalusite-bearing rocks suggests a pressure of around 2.5 kbar. This change in pressure from 7 kbar to around 2.5 kbar suggests a decompressive path for the evolution of granulites in the study area, which indicates an uplift for the granulite-facies rocks from lower crustal conditions. The implications for supercontinent history are also addressed in light of available geochronological data.

Ultrahigh-temperature metamorphism and decompression history of sapphirine granulites from Rajapalaiyam, southern India: implications for the formation of hot orogens during Gondwana assembly

AbstractSapphirine-bearing Mg–Al granulites from Rajapalaiyam in the southern part of the Madurai Block provide critical evidence for Late Neoproterozoic–Cambrian ultrahigh-temperature (UHT) metamorphism in southern India. Poikiloblastic garnet in quartzo-feldspathic and pelitic granulites contain inclusions of fine-grained subidioblastic to xenoblastic sapphirine associated with quartz, suggesting that the rocks underwent T &gt; 1000°C peak metamorphism. Quartz inclusions in spinel within garnet are also regarded as clear evidence for a UHT condition. Inclusions of orthopyroxene within porphyroblastic garnet in the sapphirine-bearing rocks show the highest Al2O3 content of up to 10.3 wt%, suggesting T = 1050–1070°C and P = 8.5–9.5 kbar. Temperatures estimated from ternary feldspar and other geothermometers (T = 950–1000°C) further support extreme thermal metamorphism in this region. Xenoblastic spinel inclusions in sapphirine coexisting with quartz suggest that the spinel + quartz assemblage pre-dates the sapphirine + quartz assemblage, probably implying a cooling from T ~ 1050°C or an anticlockwise P–T path. The FMAS reaction sapphirine + quartz + garnet → orthopyroxene + sillimanite indicates a cooling from the sapphirine + quartz stability field after the peak metamorphism. Corona textures of orthopyroxene + cordierite (± sapphirine), orthopyroxene + sapphirine + cordierite, and cordierite + spinel around garnet suggest subsequent near-isothermal decompression followed by decompressional cooling toward T = 650–750°C and P = 4.5–5.5 kbar. The sapphirine–quartz association and related textures described in this study have an important bearing on the UHT metamorphism and exhumation history of the Madurai Block, as well as on the tectonic evolution of the continental deep crust in southern India. Our study provides a typical example for extreme metamorphism associated with collisional tectonics during the Late Neoproterozoic–Cambrian assembly of the Gondwana supercontinent.

Aluminous sapphirine granulites from the Eastern Ghats Belt (India): Phase relations and relevance to counterclockwise P-T history

In the Eastern Ghats Belt, 7-9-3 sapphirine in biotite-free Crd + Crn ± Sil ± Spl restitic domains occurs interlayered with Opx + Crd + K-feldspar assemblages extensively replaced by biotite. Natural settings and experimental phase relations indicate the restitic assemblages were derived by low pressure ( 800 °C) anatexis of pelitic protoliths. In the restitic domain, sapphirine formed by the reaction Crn + Crd + Spl = Spr. Volumetrically minor amounts of sillimanite associated with sapphirine were produced by the ancillary reaction Crn + Crd = Spr + Sil. The decomposition of sapphirine + cordierite to En + Sil symplectites is attributed to the reaction Spr + Crd = En + Sil; occasional spinel in the symplectite may have formed by the reaction Spr = Spl + Sil + En. A quantitative activity-corrected M(F)AS grid constructed for Grt and Qtz-absent Mg, Al-rich metapelites suggests that sapphirine stabilization was induced by loading at ∼4.5 kbar, 900 °C. The decomposition of sapphirine occurred during cooling down to 700 °C at 8 Kbar in a thickened crust. The record of low-P prograde heating that led to high T (> 800 °C, P max attained broadly synchronous with P max at ∼9 kbar. The phase petrological relations in the sapphirine granulites strongly indicate that near-peak T was attained prior to the attainment of P max value. The initial high thermal anomaly is reconciled with asthenospheric heating below a thinned crust, and does not correspond with the thermal condition in the upper zone of a crust of normal thickness experiencing asthenospheric perturbation. Since loading induces temperature increase due to shallowing of geotherms, the cooling of the thickened crust can only be explained by conductive heat loss to “cooler” thrust slices above the hot crust, and/or asthenospheric depression. The post-T max cooling in the CCW orogen suggests the continuance of thermal relaxation in the thickened crust in its attempt to attain isostatic stability with its environs.

Petrology of sapphirine granulite and associated sodic gneisses from the Indian Head Range, Newfoundland

Migmatitic granulites from the Indian Head Range (IHR) are dominated by granoblastic, Opx-bearing (quartz) dioritic gneiss with subordinate garnet+orthopyroxene+biotite+albite (±quartz±microcline±cordierite±sillimanite) gneiss and comparatively biotite-rich, sapphirine+cordierite+orthopyroxene+albite (±microcline±sillimanite±corundum) gneiss. The latter contains at least two generations of sapphirine and cordierite, one apparently predating migmatization, the other associated with or following this event. Mineral thermobarometers yield temperatures up to ∼800 °C at 8 kbar. The IHR granulites have very high δ 18O values (+10.8 to +14.5) which most likely indicate a sedimentary precursor. Their REE and HFSE contents resemble the trace element signature of post-Archean Australian shale (PAAS). Lithological analogues from other Grenvillian inliers in western Newfoundland have similar oxygen isotopic and immobile-element signatures. They too are interpreted as metasedimentary rocks. The IHR rocks, however, have unusually sodic compositions (e.g., Na 2O/CaO and Na 2O/K 2O=2.1–22.5 and 0.93–13.8, respectively) and contain albite or sodic oligoclase despite their high metamorphic grade. The geochemical data indicate that these rocks were albitized prior to high-grade metamorphism. No counterpart of this event is recorded in granulites from the other inliers, indicating that the IHR may be highly allochthonous with respect to these other Grenvillian terranes.

High-temperature crustal anatexis in a clockwise P – T – t path: isotopic evidence from a granulite–granitoid suite in the Eastern Ghats belt, India

The high-grade metamorphic belt of the Eastern Ghats, India, displays polyphase deformation structures, a complex metamorphic record and dehydration melting in different crustal protoliths. High-temperature melting in pelitic and greywacke-like precursors in a clockwise P – T path, decompression and subsequent isobaric cooling have recently been described from the south–central sector of this belt. In this contribution, high-temperature melting has been dated at c . 1162 Ma by Sm–Nd whole-rock isotopic compositions of granitoids. Subsequent isobaric cooling could have occurred between 988 and 900 Ma as indicated by U–Pb ages of zircon and monazite in pelitic granulites. Thus, a prolonged orogenic cycle of at least 250 Ma is envisaged. Additionally, isotopic disequilibrium between whole rock and minerals in sapphirine granulites could have resulted from rapid melting and been further enhanced by a prolonged reaction history of the sapphirine granulite.

Very high-density carbonic fluid inclusions in sapphirine-bearing granulites from Tonagh Island in the Archean Napier Complex, East Antarctica: implications for CO2 infiltration during ultrahigh-temperature (T&gt;1,100 °C) metamorphism

The ultrahigh-temperature (UHT) metamorphism of the Napier Complex is characterized by the presence of dry mineral assemblages, the stability of which requires anhydrous conditions. Typically, the presence of the index mineral orthopyroxene in more than one lithology indicates that H2O activities were substantially low. In this study, we investigate a suite of UHT rocks comprising quartzo-feldspathic garnet gneiss, sapphirine granulite, garnet–orthopyroxene gneiss, and magnetite–quartz gneiss from Tonagh Island. High Al contents in orthopyroxene from sapphirine granulite, the presence of an equilibrium sapphirine–quartz assemblage, mesoperthite in quartzo-feldspathic garnet gneiss, and an inverted pigeonite–augite assemblage in magnetite–quartz gneiss indicate that the peak temperature conditions were higher than 1,000 °C. Petrology, mineral phase equilibria, and pressure–temperature computations presented in this study indicate that the Tonagh Island granulites experienced maximum P–T conditions of up to 9 kbar and 1,100 °C, which are comparable with previous P–T estimates for Tonagh and East Tonagh Islands. The textures and mineral reactions preserved by these UHT rocks are consistent with an isobaric cooling (IBC) history probably following an counterclockwise P–T path. We document the occurrence of very high-density CO2-rich fluid inclusions in the UHT rocks from Tonagh Island and characterize their nature, composition, and density from systematic petrographic and microthermometric studies. Our study shows the common presence of carbonic fluid inclusions entrapped within sapphirine, quartz, garnet and orthopyroxene. Analysed fluid inclusions in sapphirine, and some in garnet and quartz, were trapped during mineral growth at UHT conditions as 'primary' inclusions. The melting temperatures of fluids in most cases lie in the range of –56.3 to –57.2 °C, close to the triple point for pure CO2 (–56.6 °C). The only exceptions are fluid inclusions in magnetite–quartz gneiss, which show slight depression in their melting temperatures (–56.7 to –57.8 °C) suggesting traces of additional fluid species such as N2 in the dominantly CO2-rich fluid. Homogenization of pure CO2 inclusions in the quartzo-feldspathic garnet gneiss, sapphirine granulite, and garnet–orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of –34.9 to +4.2 °C. This translates into very high CO2 densities in the range of 0.95–1.07 g/cm3. In the garnet–orthopyroxene gneiss, the composition and density of inclusions in the different minerals show systematic variation, with highest homogenization temperatures (lowest density) yielded by inclusions in garnet, as against inclusions with lowest homogenization (high density) in quartz. This could be a reflection of continued recrystallization of quartz with entrapment of late fluids along the IBC path. Very high-density CO2 inclusions in sapphirine associated with quartz in the Tonagh Island rocks provide potential evidence for the involvement of CO2-rich fluids during extreme crustal temperatures associated with UHT metamorphism. The estimated CO2 isochores for sapphirine granulite intersect the counterclockwise P–T trajectory of Tonagh Island rocks at around 6–9 kbar at 1,100 °C, which corresponds to the peak metamorphic conditions of this terrane derived from mineral phase equilibria, and the stability field of sapphirine + quartz. Therefore, we infer that CO2 was the dominant fluid species present during the peak metamorphism in Tonagh Island, and interpret that the fluid inclusions preserve traces of the synmetamorphic fluid from the UHT event. The stability of anhydrous minerals, such as orthopyroxene, in the study area might have been achieved by the lowering of H2O activity through the influx of CO2 at peak metamorphic conditions (>1,100 °C). Our microthermometric data support a counterclockwise P–T path for the Napier Complex.

High-temperature dehydration melting and decompressive P-T path in a granulite complex from the Eastern Ghats, India

The granulite complex of Paderu, in the south central sector of the Eastern Ghats belt, India, consists of closely related pelitic granulites and peraluminous granitoids which could be linked via dehydration melting in pelitic and greywacke-like precursors. The pelitic granulites, including high-Mg-Al sapphirine granulites with early deformation microstructures, also record a high-temperature decompression from ~10 to ~8 kbar at ~1,000 °C, preceding isobaric cooling from above 900 to ~600 °C at 8 kbar. Highly magnesian biotite in the pelitic granulites, the presence of spinel in some of the granitoids, and granitoids of two distinct compositions, namely granite and quartz-monzonite, all suggest dehydration melting in highly magnesian pelitic and greywacke-like precursors. Moreover, high-temperature melting in highly magnesian pelitic precursors is indicated by the migmatitic spinel-bearing layers which, besides having significant abundance of quartz and feldspar, also contain aluminous orthopyroxene and cordierite. These melting reactions, occurring above 9 kbar, may constrain the prograde arm of the P–T trajectory. This and the high-temperature decompression constitute a clockwise P–T path. This clockwise P–T path is consistent with the tectonic model in which crustal thickening and granulite metamorphism in the Eastern Ghats belt is interpreted as the result of homogeneous shortening in a compressional setting.

Ultrahigh-Temperature Metamorphism in Madurai Granulites, Southern India: Evidence from Carbon Isotope Thermometry.

Ultrahigh-temperature (UHT) metamorphism in the Madurai Block of the southern Indian granulite terrain has been verified using the calcite-graphite isotope exchange thermometer. Carbon isotope thermometry has been applied to marbles from a locality near the reported occurrence of sapphirine granulites that have yielded temperature estimates of around 1000 degrees C. The delta(13)C and delta(18)O values of calcite are homogenous, implying equilibration of the isotopes during metamorphism. However, the delta(13)C values of single graphite crystals show variations in the order of 1 per thousand within a hand specimen. Detailed isotopic zonation studies indicate that graphite preserves either the time-integrated crystal growth history or reequilibrium fractionation during its cooling history. The graphite cores preserve higher delta(13)C values than the rims. The fractionation between calcite and graphite cores gives the highest metamorphic temperature of about 1060 degrees C, which matches the petrologically inferred temperature estimates in the high-magnesian pelites. The fractionation between graphite rims and calcite suggests a temperature of around 750 degrees C, which is interpreted to reflect retrograde cooling. This event is also observed in the sapphirine granulites. Calcite-graphite thermometry thus provides a useful tool to define UHT metamorphism in granulite terrains.

Anomalously Low Temperature Orthopyroxene, Spinel, and Sapphirine Occurrences in Metasediments From the Bamble Amphibolite‐To‐Granulite Facies Transition Zone (South Norway): Possible Evidence For Localized Action of Saline Fluids

The question has been raised whether isolated granulite facies “islands,” which occur throughout the amphibolite facies of the regional amphibolite‐to‐granulite facies transition zone in the central part of the Bamble sector, southern Norway are the result of prograde or retrograde metamorphism. We have studied one of these islands, the sapphirine granulites at Hauglandsvatn. Despite the preponderance of amphibolite facies assemblages in the surrounding rocks and the small scale of distribution of the granulite facies assemblages, the current assemblage is considered to be prograde, and has developed at the expense of biotite in the regional main foliation, leaving relict biotite separated from quartz by a continuous corona of orthopyroxene and K‐feldspar. Other high temperature, low aH2O reactions resulted in the formation of spinel ± corundum + albite/oligoclase symplectites at the expense of biotite and sillimanite. The symplectites have partially been replaced by symplectites of tiny blue sapphirine ± albite/oligoclase. Quartz contains fluid inclusions of highly saline brines. Relatively small inclusions, usually without a salt cube being present at room temperature, have salinities of c. 25 wt% NaCl, but larger inclusions containing a salt cube with little fluid indicate that actual salinities may have been considerably higher. CO2‐rich fluid inclusions are absent. As no feasible heat transport mechanism is available to cause an intense, local increase in temperature, we suggest that breakdown of biotite and development of granulite facies assemblages is due to the localized presence of highly concentrated brines at temperatures close to or lower than peak metamorphic temperatures. Recent experimental and theoretical results demonstrate that salts will strongly reduce aH2O while maintaining alkali transport properties. The latter capacity of the fluids enabled K+ derived from biotite breakdown to be transferred to quartz‐feldspar leucosomes, which, however, are non‐anatectic. The occurrence lends support to recent petrogenetic grids that demonstrate that sapphirine is not indicative of extremely high temperatures, even in quartz‐rich rocks, and provides a possible illustration of the action of saline fluids in high‐grade metasediments.

Reaction history of sapphirine granulites and a decompressional P-T path in a granulite complex from the Eastern Ghats

The sapphirine granulites from G. Madugula, Eastern Ghats preserve a variety of mineral textures and reactions. Corona and reaction textures are used in conjunction with mineral compositions to construct a sequence of metamorphic reactions describing the mineralogical evolution of sapphirine granulites. An early stage is characterized by the development of sapphirine + quartz, spinel + quartz in textural equilibrium, and possible relicts after osumilite during peak metamorphic conditions. Sapphirine/spinel crystals were later detached from quartz in the form of mineral coronas. During a subsequent sapphirine-cordierite stage, several cordierite forming reactions reflect decreasingP-T conditions. Finally during the late stage, a few samples show evidence of retrogressive hydration. Sapphirine is rather iron-rich (12.8 wt%) and the Mg number in the analysed minerals varies in the order: cordierite > phlogopite > sapphirine > orthopyroxene > spinel > garnet.P-T conditions of metamorphism have been constrained through the application of geothermobarometry and thermodynamically calibrated MAS equilibria.P-T vectors from granulite facies rocks in the G. Madugula area indicate that the rocks experienced substantial decompression (up to 3 kbar) and moderate cooling (150–200°C) subsequent to peak conditions of metamorphism (8.4 kbar, > 900°C). The decompressionalP-T history of sapphirine granulites interpreted from textural features and thermobarometric estimates suggest that they may have eventually resulted from exhumation of thickened crust.