Sapphirine-bearing Rocks Research Articles

The sapphirine-bearing rocks in contact with the Lherz peridotite body: New mineralogical data, age and interpretation

Sapphirine-bearing rocks are described in the Aulus Basin (Ariège, France) in a contact zone between the Lherz peridotitic body and Mesozoic metasediments which underwent the Pyrenean Cretaceous high-temperature, low-pressure metamorphic event (Monchoux, 1970, 1972a, 1972b). Sapphirine crystals occur in layered clastic deposits characterized by an uncommon suite of Al-Mg-rich minerals. A detailed petrographic study of sixteen samples representative of the diversity of the Lherz sapphirine-bearing rocks is presented. These rocks include breccias and microbreccias with various compositions. Some samples are composed of polymineralic clasts and isolated minerals that derive from regionally well-known protoliths such as ultramafic rocks, meta-ophites, “micaceous hornfels”, and very scarce Paleozoic basement rocks. Nevertheless, a large portion of the sapphirine-bearing clastic suite is composed of mono- and polymineralic debris that derive from unknown protolith(s). We define a "sapphirine-bearing mineral suite” (SBMS) composed of monomineralic debris including: sapphirine + enstatite + aluminous spinel + Mg-amphiboles + Ca-amphiboles + kornerupine + accessory minerals (apatite, diopside, rutile, serpentine, smectite, tourmaline, vermiculite and a white mica). We highlight the dominance of metamorphic Keuper clastic materials in the studied rocks and the presence of inclusions of anhydrite and F-, Cl-, Sr-rich apatite in minerals of the Al-Mg-rich suite. The brecciated texture and the presence of unequivocal sedimentary features suggest that the sapphirine-bearing rocks were mechanically disaggregated and then experienced winnowing in underwater conditions with poor mixing between the different sources. We measured U-Pb rutile age data in order to provide constraints on the age of (one of) the protolith(s) of those clastic deposits. The obtained age (98.6 + 1.2 Ma) is interpreted as the age of metamorphism of this protolith of the SBMS. Previous works interpreted the Lherz sapphirine-bearing rocks as crustal protoliths modified at depth along the contact with the ultramafic rocks of the Lherz body during their ascent towards shallower depths. These new data imply: (i) an Upper Triassic to Lower Jurassic origin for the main protolith of the sapphirine-bearing rocks; (ii) the metamorphism of this protolith along an active hot crust–mantle detachment during Cenomanian times with the involvement of metasomatic, brine-type fluids; and (iii) its brecciation during the exhumation of the material due to the evolution of the detachment, followed by subsequent sedimentary reworking of the metamorphic material.

Allanite-(Nd) from Sapfirinovaya Pit, Southern Urals: First Find in Russia

Allanite-(Nd) (Ca1.20Mn0.02Nd 0.33Ce 0.21Sm 0.07Pr 0.06La0.03Gd0.02Y0.01) Σ1.95(Al1.94 Fe2+Fe3+0.42Mg0.08Sc0.06Ti0.01)Σ2.97[Si2O7][SiO4]O(OH) was found frst in Russia as black crystals and grains up to 0.12 mm in size in a sapphirine-bearing rock of the Sapfrinovaya pit, near settlement of Tayginka in Kyshtym area of Chelyabinsk district, Southern Urals. According to strong bands of Raman spectrum of alla-nite-(Nd) (106, 205, 283, 321, 385, 444, 495, 568, 688, 874, 970, 1051 cm-1), it belongs to the allanite group of the epidote supergroup. Allanite-(Nd) is associated with davidite-(се) and monazite-(Nd) (frst fnds in Southern Urals).

Reply to “Comment on “Isotopic insight on paleodiet of …” by Bocherens et al. (Gondwana Research, 48(1), 7–14)”

We report here for the first time the occurrence of sapphirine-bearing rocks within the Paleoproterozoic granulite complex of the Angara-Kan block (SW Siberian craton) and provide strong evidence for crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT rocks contain sapphirine, high alumina orthopyroxene, orthopyroxene + sillimanite assemblage, mesoperthitic feldspar and pseudomorphoses after osumilite crystals. The peak temperature of metamorphism was estimated using Al2O3 contents in Opx (up to 10 wt%) and the ternary feldspar composition, and falls in a range of 1050–1100 °C. Observed mineral microstructures indicate post-peak cooling of rocks without a considerable decrease in pressure down to a temperature of about 900 °C. Another type of rock related with UHT metamorphism is represented by orthopyroxene-sillimanite gneisses, which recorded a peak temperature of about 950 °C, decreasing down to 900 °C during post-peak stage. The UHT gneisses record microstructural evidence of melt injection and matrix-melt interaction near the peak metamorphic conditions. Sapphirine grains occur in the gneiss with relatively low Mg bulk composition. It is suggested that sapphirine occurs in the Mg-Al-rich domains which have been formed as a result of partial melting and precipitation of Fe-oxides from the melt. High concentrations of Al2O3 in orthopyroxene are the only sign of UHT metamorphism in Fe-rich rock matrix which is not affected by melt. New SHRIMP U-Pb zircon data from two UHT gneisses indicate the presence of two well-defined age peaks at ca. 1.8 and 1.9 Ga from rims and metamorphic grains, and older dates from ca. 1.91 to 3.15 Ga from detrital cores. The prominent feature of the metamorphic zircons is depletion of U with high Th/U ratio up to 6.0. Steep HREE patterns of the metamorphic zircons suggest their growth in the presence of a melt. The metamorphic zircons from the UHT gneiss record a broad range of temperatures between 1000 °C and 810 °C indicating protracted zircon growth from near peak conditions through retrograde stage of PT-path. The growth of the youngest zircons occurred at higher temperature indicating that the UHT metamorphic conditions were reached at 1.78–1.8 Ga. The timespan of the granulite-UHT metamorphic event is near 50 Ma and is limited by intrusions of late leucogranites and charnockites at 1.75–1.73 Ga. Protracted interval of granulite-UHT metamorphism conforms with isobaric cooling P-T path constructed by mineral assemblages.

Sapphirine-bearing Fe-rich granulites in the SW Siberian craton (Angara-Kan block): Implications for Paleoproterozoic ultrahigh-temperature metamorphism

We report here for the first time the occurrence of sapphirine-bearing rocks within the Paleoproterozoic granulite complex of the Angara-Kan block (SW Siberian craton) and provide strong evidence for crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT rocks contain sapphirine, high alumina orthopyroxene, orthopyroxene+sillimanite assemblage, mesoperthitic feldspar and pseudomorphoses after osumilite crystals. The peak temperature of metamorphism was estimated using Al2O3 contents in Opx (up to 10wt%) and the ternary feldspar composition, and falls in a range of 1050–1100°C. Observed mineral microstructures indicate post-peak cooling of rocks without a considerable decrease in pressure down to a temperature of about 900°C. Another type of rock related with UHT metamorphism is represented by orthopyroxene-sillimanite gneisses, which recorded a peak temperature of about 950°C, decreasing down to 900°C during post-peak stage. The UHT gneisses record microstructural evidence of melt injection and matrix-melt interaction near the peak metamorphic conditions. Sapphirine grains occur in the gneiss with relatively low Mg bulk composition. It is suggested that sapphirine occurs in the Mg-Al-rich domains which have been formed as a result of partial melting and precipitation of Fe-oxides from the melt. High concentrations of Al2O3 in orthopyroxene are the only sign of UHT metamorphism in Fe-rich rock matrix which is not affected by melt. New SHRIMP U-Pb zircon data from two UHT gneisses indicate the presence of two well-defined age peaks at ca. 1.8 and 1.9Ga from rims and metamorphic grains, and older dates from ca. 1.91 to 3.15Ga from detrital cores. The prominent feature of the metamorphic zircons is depletion of U with high Th/U ratio up to 6.0. Steep HREE patterns of the metamorphic zircons suggest their growth in the presence of a melt. The metamorphic zircons from the UHT gneiss record a broad range of temperatures between 1000°C and 810°C indicating protracted zircon growth from near peak conditions through retrograde stage of PT-path. The growth of the youngest zircons occurred at higher temperature indicating that the UHT metamorphic conditions were reached at 1.78–1.8Ga. The timespan of the granulite-UHT metamorphic event is near 50Ma and is limited by intrusions of late leucogranites and charnockites at 1.75–1.73Ga. Protracted interval of granulite-UHT metamorphism conforms with isobaric cooling P-T path constructed by mineral assemblages.

Age (U-Pb, SHRIMP-II), Geochemistry of zircon, and conditions of the formation of sapphirine-bearing rocks of the Central Kola granulite-gneiss area

Sapphirinebearing rocks are observed in the cen� tral part of the Central Kola granulite-gneiss area (CKGGA), ~10 km to the west of the Kitsa railway station (1), where two types of rocks are abundant, namely orthopyroxene-biotite orthogneiss separated by aluminous garnet-biotite and garnet-cordierite- sillimanite-biotite plagiogneisses related to the Kola Series. The contacts between the two rock types are tectonic. Several outcrops of sapphirinebearing rocks occur in the shear zone of the contact between the Vezhetundra orthogneiss body and aluminous gneiss. Sapphirinebearing rocks often compose elongated veinlike bodies with a length of >100 m and a thickness of < n m. In each area sapphirinebearing rocks are quite variable by the structure and mineral composi� tion: from acid rocks enriched in quartz to quartzfree melanocratic rocks with a remarkable content of sap� phirine, spinel, and corundum. A typical feature of sapphirinebearing rocks is their zoning, which may be described by infiltration metasomatic columns with a decrease in the number of phases towards their axial zones, with various reaction relationships between minerals; some of them provide evidence for the allochemical character of alterations.

The genesis of mantle-derived sapphirine

Sapphirine, a typical ultrahigh-temperature metamorphic mineral, is rarely found in mantle xenoliths. Here we report the occurrence and characteristics of sapphirine in a mantle-derived xenolith from the Cenozoic basalts of Hannuoba in the North China Craton. The xenolith consists of a clinopyroxene, spinel, and sapphirine assemblage, with the sapphirine occurring as the reaction rim surrounding spinel. The mineral compositions of this sample are all characterized by high Mg contents, similar to those of minerals from other sapphirine-bearing rocks reported from high-Mg-Al granulites elsewhere in the world. Clinopyroxene is relatively rich in Al and Ca in comparison to pyroxene in peridotite and pyroxenite xenoliths in the Hannuoba basalts, as well as in global mafic and felsic granulites in other terranes, a feature that is consistent with the bulk composition. The P-T compilations from both experimental and natural rock data show a restricted stability field for the coexisting clinopyroxene + spinel + sapphirine assemblage of around 8–15 kbar and 800–900 °C. The rare occurrence of sapphirine in a mantle-derived xenolith therefore suggests specific bulk composition, restricted P-T range and possible melt-peridotite interaction. Such conditions are best satisfied in a tectonic setting with basaltic magma underplating and interaction between the infiltrating melts and the wall-rock peridotite.

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.

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.

First Report of Sapphirine+Quartz Assemblage from Southern India: Implications for Ultrahigh-temperature Metamorphism

We report here for the first time, the occurrence of sapphirine+quartz assemblage in textural equilibrium from quartzo-feldspathic and pelitic granulites from southern India. The sapphirine-bearing rocks occur as layered gneisses associated with pink granite within massive charnockite in Rajapalaiyam area in the southern part of Madurai Block. Sapphirine occurs in three associations: (i) fine-grained subhedral mineral associated with quartz enclosed in garnet, (ii) intergrowth with Al-rich orthopyroxene (up to 9.7 wt.% Al 2O 3), and (iii) in symplectitic intergrowth with orthopyroxene (Al 2O 3= 5.9–6.7 wt.%) and cordierite surrounding garnet. The sapphirine in association with quartz is slightly magnesian (X Mg = 0.79–0.80) and low in Si content (1.55–1.56 pfu) as compared with those associated with orthopyroxene and cordierite (X Mg= 0.77–0.79, Si = 1.59–1.63 pfu). The sapphirine+quartz assemblage suggests that the granulites underwent T>1050 °C peak metamorphism. Cores of porphyroblastic orthopyroxene in the sapphirine-bearing rocks shows high-Al 2O 3 content of up to 9.7 wt.%, suggesting T = 1040–1060°C and P = 8 kbar. FMAS reaction of sapphirine+quartz→garnet+sillimanite+cordierite indicates a cooling from sapphirine+quartz stability field after the peak ultrahigh-temperature metamorphism. Slightly lower temperature estimates from ternary feldspar and sapphirine-spinel geothermometers (T = 950–1000°C) also support a post-peak isobaric cooling. Corona textures of orthopyroxene+cordierite (±sapphirine), orthopyroxene+sapphirine, and cordierite+spinel around garnet suggest subsequent decompression. The sapphirine-quartz association and related textures reported in this study have important bearing on the ultrahigh-temperature metamorphism and exhumation history of the Madurai Block as well as on the tectonic evolution of the continental deep crust in southern India.

Palaeoproterozoic UHT metamorphism in the Lewisian complex and North Atlantic region

Palaeoproterozoic ultra-high temperature (UHT) metamorphism of the Lewisian complex in South Harris and the North Atlantic regions were examined. The characteristic features of the UHT metamorphism in the South Harris can be summarized as a) mineral textures and constituent mineral associations are concordant with anti-clockwise metamorphic history, b) presence of subduction-related various precursory rocks lead to formation of various types of sapphirine-bearing rocks, and c) heat source that established UHT metamorpism in South Harris is the emplacement of igneous suite in the magmatic arc at c. 2.18-1.87 Ga. In the North Atlantic region, possible UHT metamorphism were reported from the Boothia-Somerset granulite terrane, Taltson-Thelon orogen and Kolvista-Umba zone during 1.9-2.0 Ga. Precise correlation between UHT in South Harris and these terranes are not clear, although magmatic arc or island arc are considered to be reasonable tectonic setting for the UHT metamorphism in the Palaeoproterozoic North Atlantic region.

Högbomite in sapphirine-bearing rocks from the Bamble Sector, south Norway

AbstractHögbomite is reported from two upper-amphibolite and granulite-facies, sapphirine-bearing, Al–Mg–Fe-rich and silica-poor lens-shaped layers within the Bamble Sector, south Norway. Primary assemblages, indicating peak metamorphic conditions of 773-844°C at 7 kbar (Mg–Fe exchange thermometry), are spinel–sapphirine–biotite–gedrite, spinel–corundum–sapphirine–cordierite and orthopyroxene–biotite–cordierite–plagioclase. Högbomite formed by hydrous alteration and oxidation of primary spinel and rutile and/or ilmenite according to the generalised reaction:Suggested conditions of högbomite formation are 550–620°C and 6–7 kbar. The högbomites contain 10.2–14.7 wt.% MgO, 04).3 wt.% ZnO, 58.9–62.1 wt.% Al2O3 and 15.6-17.6 wt.% Fe as FeO. The two högbomite may belong to different polytypes, as suggested by their differing TiO2 (9.9-10.1 versus 5.7–5.8 wt. %) and calculated Fe3+ – and H2O–contents. The partitioning of Zn between spinel and högbomite is not uniform and is considered to depend upon prevailing fo2 and aH2O.

Orthopyroxene–sillimanite–sapphirine granulites from the Bamble granulite terrane, southern Norway

ABSTRACTSilica‐deficient sapphirine‐bearing rocks occur as an enclave within granulite facies Proterozoic gneisses and migmatites near Grimstad in the Bamble sector of south‐east Norway (Hasleholmen locality). The rocks contain peraluminous sapphirine, orthopyroxene, gedrite, anthophyllite, sillimanite, sapphirine, corundum, cordierite, spinel, quartz and biotite in a variety of assemblages. Feldspar is absent.Fe2+/(Fe2++ Mg) in the analysed minerals varies in the order: spinel &gt; gedrite ≥ anthophyllite ≥ biotite &gt; sapphirine&gt;orthopyroxene &gt; cordierite.Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene‐sillimanite‐sapphirine‐cordierite‐corundum developed during a high‐pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium‐pressure granulite facies generated various sapphirine‐bearing assemblages. The diagnostic assemblage of this stage is sapphirine‐cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum.The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H2O‐poor conditions.

Sapphirine-Bearing Quartzite from the Eastern Ghats Granulite Terrain, Vizianagaram, India

Sapphirine occurs in a quartzite in the Eastern Ghats granulite province near Vizianagaram, India. Sapphirine is developed at the contact of quartzite and spinel-magnetite-cordierite bands and occurs as small grains throughout the quartzite. Bulk chemical analyses of sapphirine-bearing rock suggest that the protolith was a quartzite with aluminous and ferruginous cements. The presence of monazite and rare earth element (REE) analyses also indicate a sedimentary origin. Textural relations indicate that spinel, sapphirine, hypersthene, and sillimanite formed in an early metamorphic event at very high temperature and elevated pressure. Low activity of water and high oxygen fugacities accompanied reactions forming these minerals. Later metamorphic reactions accompanied by higher activity of water produced cordierite and biotite. Mineralogic geothermometry also supports a multi-stage metamorphic history. Experimental evidence for temperatures of 850°C or more required to stabilize sapphirine and quartz indicates that some parts of the Eastern Ghats province are among the highest temperature metamorphic terrains which have been described, comparable to Napier Complex of Endeby Land, Antarctica and the Wilson Lake area of Labrador.

P-T-X Relationships Deduced from Corona Textures in Sapphirine--Spinel--Quartz Assemblages from Paderu, Southern India

The sapphirine (Sa)-spinel (Sp)-quartz (Qz)-bearing rocks from Paderu occur as lenticular enclaves within the Precambrian khondalitecharnockite terrane of southern India. In addition these rocks contain orthopyroxene (Opx), sillimanite (Sill), garnet (Gt), cordierite (Cd), biotite, potash feldspar (Kf), plagioclase, and symplectites of Cd-Kf-Qz-Opx. The symplectites may have formed from the breakdown of osumilite. Grain contacts of sapphirine and spinel with quartz are rarely observed and the incompatibility with quartz during later stages is displayed by the development of several types of polymineralic reaction coronas. The coronas in the different rock types A, B, etc. are (minerals listed from core to rim of corona): (A-1) sapphirine-bearing rock type without spinel: Sa-Sill-Opx, Sa-Sill-Cd, Sa-Cd-Opx (A-2) sapphirine and spinel-bearing: Sp-Sa-Sill-Opx-Qz, Sp-Sa-Sill, Sp-Sa-Opx, Sp-Sill-Opx, Sp-Sa-Sill-Gt-Qz, Sa-Sill-Opx, Sp-Sa-Sill-Opx, Sa-Sill-Opx-Gt, Sp-Sa-Opx-Gt, Sp-Sa-Sill-Gt; and (B) spinel-bearing but sapphirine free: Sp-Sill-Opx, Sp-Sill-Gt, Sp-Cd. Commonly the coronas in the rock type A 2 and B also contain ilmeno-hematite±corundum in the core in association with spinel. These rock types also provide textural evidence for later crystallization of Cd, Cd + Sa, and Gt + Qz from Opx+Sill±Qz and Gt+Sill+Qz. Sapphirine is aluminous (near 7(Mg, Fe 2+ )O.9(Al, Fe 3+ ) 2 O 3 .3SiO 2 ) and contains up to 12.2 wt. per cent iron as FeO. Orthopyroxene is also aluminous, containing up to 10.4 wt. per cent Al 2 O 3 . Sapphirine and spinel have relatively high contents of Fe 2 O 3 . X Mg in the Fe-Mg minerals increases from rock type B to A2 to A1. A sequence of reactions has been deduced from coronas and other reaction textures, and from the phase compatibility relations in the FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O system. The P-T-X relationships from geothermobarometry and petrogenetic grids, viz. μ Fe 2 O 3 vs. μ FeO and μ H 2 O vs. μ Fe 2 O 3 , suggest: (1) a retrograde, mildly decompressive trajectory from 900±60°C/65±0.7 kb (core) to 760±50°C/5 ± 0.6 kb (rim); and (2) the observed mineralogy of the coronas and reactions deduced from them are dependent on the relative FeO, Fe 2 O 3 , and H 2 O contents of the rocks (μ FeO 3 , μ Fe 2 O 3 ), and μ H 2 O ).

Formation of sapphirine during retrogression of a basic high-pressure granulite, Roan, Western Gneiss Region, Norway

Sapphirine-bearing rocks occur in the northern part of the Western Gneiss Region, Vestranden, central Norway. The sapphirine-bearing rocks are characterized by a high MgO/(MgO + FeO) ratio, high Al2O3, MgO and CaO, and low SiO2 contents. These rocks form layers within larger complexes which originated as layered magmatic rocks. High PT-metamorphism produced a cpx+ky+gt assemblage. The P and T estimates are P = 14.5±2 kbar and T= 870±50° C. During retrogression, the high-P granulite assemblage broke down to form an intermediate-P granulite mineralogy comprising orthopyroxene, spinel, anorthite, andesine, sapphirine and corundum. Textural relationships suggest that sapphirine formed by the reaction: spinel+kyanite → sapphirine+corundum, and probably also by a reaction between corundum, spinel and orthopyroxene. All reactions took place within the stability field of kyanite. Textural and micro-chemical relationships indicate equilibrium, conditions during the peak metamorphism, whereas pronounced disequilibrium characterizes the mineral associations formed during the early retrogression at low PH2O. The investigation shows that parts of the northern segment of the Western Gneiss Region underwent a metamorphic evolution similar to the Caledonian one recorded from eclogite/granulite terrains further south.

Chemographic relationships in sapphirine-bearing rocks of the Limpopo Belt, Southern africa

Silica-deficient metasedimentary rocks in which sapphirine coexists with several minerais in various paragenesis occur in lenses or as thin layers within the Messina Layered lntrusion. Following the intrusion of some tholeiitic dykes, the Messina Layered Intrusion was emplaced into a sequence of supracrustal rocks (~ 3.2 Ga). The intrusion consists of poorly exposed chromite layered anorthosites, leucogabbros and gabbros. After the intrusion of some granitic rocks there was a widespread period of deformation and a high-grade metamorphism (~1.9 Ga). The average XMg of all minerals in the silica-deficient metasedimentary rocks is rather high and varies in the folowing order: cordierite (~ 0.96) > enstatite (~0.96) > chlorite (~ 0.94) >> phlogopite (~ 0.93) > sapphirine (0.96-0.86) > kornerupine (~ 0.90) > gedrite (~ 0.85) > spinel (0.92-0.80). Other minerals are corundum, sillimanite, relict kyanite and plagioclase. The textural relations and the plots of the microprobe data of coexisting minerals in the MgO-Al 2 0 3 -SiO 2 (H 2 0) system are consistent with the following sequence of main reactions: (1) enstatite + corundum → cordierite + sapphirine; (4) sapphirine + sillimanite → cordierite + corundurn; (13) kornerupine → cordierite + sapphirine + enstatite; (17) sapphirine + enstatite → cordicrtte + spinel; (20) sapphirine → chlorite + corundum + spinel. These selected reactions illustrate the movement path from the PT peak elose to 10 kb/800°C to the minimum observable at 3.5 kb/700°C as indicated by experimental data for the pure system MgO-Al 2 0 3 -Si0 2 -(H 2 0). The protolith of this material was possibly sedimentary, with a bulk composition close to the composition of kornerupine and probably was composed of chlorite, pyrophyllite and alunite.

Sapphirine-bearing rocks with sedimentary and volcanogenic protoliths from the Arunta Block

AT least 15 separate localities for sapphirine-bearing rocks have been found in the Arunta Block, a Precambrian metamorphic complex in central Australia (Fig. 1), although only two have been documented in any detail1–3. Such rocks are the most spectacular examples of a suite of rocks with a distinctive, characteristic chemistry. This suite includes, inter alia, the better known cordierite–anthophyllite rocks, and is widely distributed both within the Arunta Block and world-wide. From field observations it is proposed here that two different geological processes, one sedimentary or diagenetic and the other volcanic, generated rocks that are now indistinguishable in their whole-rock chemistry and mineral assemblages. It had formerly been proposed that sapphirine-bearing rocks of volcanogenic and sedimentary origins were mineralogically and chemically distinct4.

A SAPPHIRINE-ORTHOPYROXENE-SPINEL OCCURRENCE IN THE PIEN AREA, PARANÁ, SOUTHERN BRAZIL

A minor outcrop of a sapphirine-bearing rock within migmatites is geologically associated with the large mafic-ultramafic complex at Piên, State of Paraná, S, Brazil. The rock, petrographycally classified as a sapphirine meta-norite, is characterized by a fine-grained equilibrium mosaic texture, with plagioclase (An 60%), bronzite, sapphirine and green spinel as main minerals. The sapphirine is observed as simplectitic intergrowths with orthopyroxene and as isolated grains associated with_ plagioclase and orthopyroxene. Accessory minerals, not associated texturally with sapphirine crystallization, are garnet and some retrograde minerals. Chemically, the rock presents high Al 2 O 3 , and MgO contents and low SiO 2 , ratios, with a strongly undersaturated normative mineralogy. It is suggested that the original material was a spinel-rich ultramafic rock. The sapphirine probably crystallized by a combination of reactions involving mainly Al-pyroxenes, olivine and spinel. A 2-pyroxene geothermometer, applied to associated mafic and ultramafic rocks, indicates for the granulitic metamorphism a temperature of crystallization between 750 and 880°C and minimum pressures of 7 Kb, while temperatures estimated from the Al-content of orthopyroxenes coexisting with sapphirine are much too high. Similar conclusions are drawn from the coexistence of sapphirine, bronzite and spinel, which suggests a crystallization temperature of about 800°C or more, at a minimum pressure of 4 Kb.

A SAPPHIRINE-ORTHOPYROXENE-SPINEL OCCURRENCE IN THE PIEN AREA, PARANÁ, SOUTHERN BRAZIL

A minor outcrop of a sapphirine-bearing rock within migmatites is geologically associated with the large mafic-ultramafic complex at Pien, State of Parana, S, Brazil. The rock, petrographycally classified as a sapphirine meta-norite, is characterized by a fine-grained equilibrium mosaic texture, with plagioclase (An 60%), bronzite, sapphirine and green spinel as main minerals. The sapphirine is observed as simplectitic intergrowths with orthopyroxene and as isolated grains associated with_ plagioclase and orthopyroxene. Accessory minerals, not associated texturally with sapphirine crystallization, are garnet and some retrograde minerals. Chemically, the rock presents high Al 2 O 3 , and MgO contents and low SiO 2 , ratios, with a strongly undersaturated normative mineralogy. It is suggested that the original material was a spinel-rich ultramafic rock. The sapphirine probably crystallized by a combination of reactions involving mainly Al-pyroxenes, olivine and spinel. A 2-pyroxene geothermometer, applied to associated mafic and ultramafic rocks, indicates for the granulitic metamorphism a temperature of crystallization between 750 and 880°C and minimum pressures of 7 Kb, while temperatures estimated from the Al-content of orthopyroxenes coexisting with sapphirine are much too high. Similar conclusions are drawn from the coexistence of sapphirine, bronzite and spinel, which suggests a crystallization temperature of about 800°C or more, at a minimum pressure of 4 Kb.

Chemographic relationships in sapphirine-bearing rocks from Sonapahar, Assam, India

Silica-deficient rocks in which sapphirine coexists with cordierite, gedrite, chondrodite, spinel, corundum and phlogopite in various parageneses, occur in association with cordierite-garnet-sillimanitebearing granulitic gneisses in a Precambrian metamorphic terrain displaying amphibolite-granulite transition facies. Brucite coexists with spinel and corundum and its unusual stability in this environment has been attributed to its significant fluorine content. Kornerupine occurs as armoured relics within cordierite in one sample of the sapphirine-spinel-cordierite-phlogopite-bearing rock. Sapphirines (X Mg 0.77 to 0.92) plot near the 7∶9∶3 composition.X Mg varies in the order: chondrodite and brucite>gedrite>cordierite >phlogopite>sapphirine>spinel.