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1.
Small pods of silica-undersaturated Al-rich and Mg-rich granulite facies rocks containing sapphirine, pleonastic spinel, kornerupine, cordierite, orthopyroxene, corundum, sillimanite and gedrite are scattered throughout the NE Strangways Range, Central Australia. These are divided into four distinct rock types, namely orthopyroxene-rich aluminous granofels and metapelitic gneisses containing sapphirine, spinel or kornerupine. Two granulite facies metamorphic events are recognized, of which only the first (M1) is considered in this paper. Peak metamorphic mineral parageneses indicate that the M1 thermal maximum occurred at approximately 900–950 °C and 8–9 kbar. All samples are characterized by profuse and diverse coronitic and symplectic reaction textures. These are interpreted as evidence for the sequential crossing of the following reactions in the system FMAS: cordierite + spinel + corundum = sapphirine + sillimanite, cordierite + spinel = orthopyroxene + sapphirine + sillimanite, sapphirine + spinel + sillimanite = orthopyroxene + corundum, sapphirine + sillimanite = cordierite + orthopyroxene + corundum. Phase stability relationships in FMAS and MASH indicate an anticlockwise P–T path terminated by isobaric cooling. Such a path is exemplified by early low-P mineral parageneses containing spinel, corundum and gedrite and the occurrence of both prograde and retrograde corundum. Reaction textures preserve evidence for an increase in aH2O and aB2O3 with progressive isobaric cooling. This hydrous retrogression resulted from crystallization of intimately associated M1 partial melt segregations. There is no evidence for voluminous magmatic accretion giving rise to the high M1 thermal gradient. The M1 P–T path may be the result of either lithospheric thinning after both crustal thickening and burial of the supracrustal terrane, or concomitant crustal thickening and mantle lithosphere thinning.  相似文献   

2.
ABSTRACT Sequential reaction textures in Archaean garnet-corundum-sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse-grained, high-pressure (P > 9.5 kbar) granulite-facies assemblages (M1) to late, low-pressure (P <6 kbar) granulite-facies sub-assemblages (M2). The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late-M1 boron-free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel). The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time-sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)-discontinuous reactions in the FMASH multisystem sapphirine-garnet-corundum-spinel-cordierite-gedrite-kornerupine, the facies-sequence indicates decompression at essentially constant T assuming constant a(H2O). Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the opposite dP from that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction of P-T vectors. Garnet-biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca. 850°C for the M1 stage. A minimum late-M1 pressure of ca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early-M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high-pressure granulite-facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. The P-T path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite-facies terranes.  相似文献   

3.
Silica-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 > gedrite ≥ anthophyllite ≥ biotite > sapphirine>orthopyroxene > 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.  相似文献   

4.
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.  相似文献   

5.
Sapphirine occurs in the orthopyroxene-cordierite and feldspar-sillimanite granulites in the Sipiwesk Lake area of the Pikwitonei granulite terrain, Manitoba (97°40W, 55°05N). The orthopyroxene-cordierite granulites have extremely high Al2O3 (24.5 wt%) and MgO (24.6 wt%) contents and contain sapphirine (up to 69.2 wt% Al2O3), aluminous orthopyroxene (up to 8.93 wt% Al2O3), cordierite, spinel, phlogopite, and corundum. Sapphirine forms coronas mantling spinel and corundum. Corona sapphirine is zoned and its composition varies through the substitution (Mg, Fe, Mn) Si=2 Al as a function of the phases with which it is in contact. Textural and chemical relationships of sapphirine with coexisting phases indicate that spinel + cordierite reacted to form orthopyroxene + sapphirine under conditions of increasing pressure. Moreover, decreasing core to rim variation of Al2O3 in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation. On the basis of experimentally determined P-T stability of the assemblage enstatite + sapphirine + cordierite, and the Al content of hypothetical Fe2+-free orthopyroxene associated with sapphirine and cordierite, metamorphic temperatures and pressures are estimated to be 860–890° C and 3.0–11.2 kbar.In the feldspar-sillimanite granulites, sapphirine occurs as a relict phase mantled by sillimanite and/or by successive coronas of sillimanite and garnet. These textural relations suggest the reaction sapphirine + garnet + quartz = orthopyroxene + sillimanite with decreasing temperature. Compositions of minerals in the assemblage garnet-orthopyroxene-sillimanite-plagioclase-quartz, indicate metamorphic P-T conditions of 780–880° C and 9±1 kb.The metamorphic conditions estimated in this study suggest that the sapphirine bearing granulites in the Sipiwesk Lake area represent Archean lower crustal rocks. Their formation might be related to the crustal thickening processes in this area as suggested by Hubregtse (1980) and Weber (1983).  相似文献   

6.
Sapphirine granulite occurring as lenses in charnockite at Anantagiri,Eastern Ghat, India, displays an array of minerals which developedunder different P-T-X conditions. Reaction textures in conjunctionwith mineral chemical data attest to several Fe-Mg continuousreactions, such as
  1. spinel+rutile+quartz+MgFe–1=sapphirine+ilmenite
  2. cordierite=sapphirine+quartz+MgFe–1
  3. sapphirine+quartz=orthopyroxene+sillimanite+MgFe–1
  4. orthopyroxene+sapphirine+quartz=garnet+MgFe–1
  5. orthopyroxene+sillimanite=garnet+quartz+MgFe–1
  6. orthopyroxene+sillimanite+quartz+MgFe–1=cordierite.
Calculated positions of the reaction curves in P-T space, togetherwith discrete P-T points obtained through geothermobarometryin sapphirine granulite and the closely associated charnockiteand mafic granulite, define an anticlockwise P-T trajectory.This comprises a high-T/P prograde metamorphic path which culminatedin a pressure regime of 8?3 kb above 950?C, a nearly isobariccooling (IBC) path (from 950?C, 8?3 kb, to 675?C, 7?5kb) anda terminal decompressive path (from 7?5 to 4?5 kb). Spinel,quartz, high-Mg cordierite, and sapphirine were stabilized duringthe prograde high-T/P metamorphism, followed by the developmentof orthopyroxene, sillimanite, and garnet during the IBC. Retrogradelow-Mg cordierite appeared as a consequence of decompressionin the sapphirine granulite. Deformational structures, reportedfrom the Eastern Ghat granulites, and the available geochronologicaldata indicate that prograde metamorphism could have occurredat 30001?00 and 2500?100 Ma during a compressive orogeny thatwas associated with high heat influx through mafic magmatism. IBC ensued from Pmax and was thus a direct consequence of progrademetamorphism. However, in the absence of sufficient study onthe spatial variation in P-T paths and the strain historiesin relation to time, the linkage between IBC and isothermaldecompression (ITD) has remained obscure. A prolonged IBC followedby ITD could be the consequence of one extensional mechanismwhich had an insufficient acceleration at the early stage, orITD separately could be caused by an unrelated extensional tectonism.The complex cooled nearly isobarically from 2500 Ma. It sufferedrapid decompression accompanied by anorthosite and alkalinemagmatism at 1400–1000 Ma.  相似文献   

7.
ULIANOV  A.; KALT  A. 《Journal of Petrology》2006,47(5):901-927
Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg–Aland Ca–Al granulite xenoliths. Their protoliths are interpretedas troctolitic cumulates; however, the original mineral assemblageswere almost completely transformed by subsolidus reactions.Mg–Al granulites contain the minerals spinel, sapphirine,sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxeneand garnet, whereas Ca–Al granulites are characterizedby hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase,quartz, clinopyroxene, corundum, and garnet. In the Mg–Algranulites, the first generation of orthopyroxene and some spinelmay be of igneous origin. In the Ca–Al granulites, hibonite(and possibly some spinel) are the earliest, possibly igneous,minerals in the crystallization sequence. Most pyroxene, spineland corundum in Mg–Al and Ca–Al granulites formedby subsolidus reactions. The qualitative P–T path derivedfrom metamorphic reactions corresponds to subsolidus cooling,probably accompanied, or followed by, compression. Final equilibrationwas achieved at T 600–740°C and P <8 kbar, inthe stability field of sillimanite. The early coexistence ofcorundum and pyroxenes (± spinel), as well as the associationof sillimanite and sapphirine with clinopyroxene and the presenceof hibonite, makes both types of granulite rare. The Ca–Alhibonite-bearing granulites are unique. Both types enlarge thespectrum of known Ca–Al–Mg-rich granulites worldwide. KEY WORDS: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift  相似文献   

8.
A suite of spinel–cordierite granulites from Viziangram, Eastern Ghats Belt, India preserve mineral assemblages and reaction textures indicative of peak metamorphic conditions of >1000 °C, >8<10 kbar, followed successively by near isobaric cooling (down to 750–800 °C), near isothermal decompression (to 4–5 kbar), and late hydration. P–T conditions of each stage are evaluated through a combination of petrogenetic grid approach and thermobarometry. Sapphirine is developed in sillimanite‐bearing acid pegmatite veins that intruded the spinel–cordierite granulite close to peak metamorphic conditions, and also in the host rock in immediate contact with the pegmatite. Both sillimanite and sapphirine in the pegmatite are considered to be magmatic phases. Field observations and textural characteristics suggest that Al‐metasomatism of the spinel–cordierite granulite due to the intrusion of pegmatite was responsible for sapphirine formation in the spinel granulite.  相似文献   

9.
Sapphirine occurs with humite-group minerals and forsteritein Precambrian amphibole-facies rocks at Kuhi-lal, SW PamirMountains, Tajikistan, a locality also for talc+kyanite magnesiohornblendewhiteschist. Most of these sapphirine-bearing rocks are graphiticand sulfidic (pyrite and pyrrhotite) and contain enstatite,clinohumite or chondrodite, spinel, rutile, gedrite, and phlogopite.A phlogopite schist has the assemblage with XFe = Fe/(Fe+Mg)increasing as follows: chlorite (0-003)<phlogopite (0.004–0.005)sapphirine (0.004–0.006) enstatite (0-006)forsterite (0-006–0-007)<spinel (0-014). This assemblage includes the incompatiblepair sapphirine+forsterite, but there is no textural evidencefor reaction. In one rock with clinohumite, XFe increases asfollows: clinohumite (0-002) <sapphirine (0-003) <enstatite(0-004–0-006) <spinel (0-010). Ion microprobe and wet-chemicalanalyses give 0-57–0-73 wt.% F in phlogopite and 0-27wt.% F in chlorite in the phlogopite schist; 0-04, 1.5–1.9,and 4.4 wt.% F in forsterite, clinohumite, and chondrodite,respectively; and 0-0-09 wt.% BeO and 0-05–0-21 wt.% B2O3in sapphirine. Stabilization of sapphirine+clinohumite or sapphirine+chondroditeinstead of sapphirine+phlogopite is possible at high F contentsin K-poor rocks, but minor element contents appear to be toolow to stabilize sapphirine as an additional phase with forsterite+enstatite+spinel.Although sapphirine+forsterite is metastable relative to spinel+enstatitein experiments conducted at aH2O=1 in the MgO-Al2O3-SiO2-H2Osystem, it might be stabilized at aH2O0.5, P4 kbar, T650–700C.Textures in the Kuhi-lal whiteschists suggest a polymetamorphicevolution in which the rocks were originally metamorphosed atT650C, P 7 kbar, conditions under which sapphirine+clinohumiteand sapphirine+chondrodite are inferred to have formed, andsubsequently affected by a later event at lower P, similar T,and lower aH2O. The latter conditions were favorable for sapphirine+forsteriteto form in a rock originally containing chlorite+forsterite+spinel+enstatite.  相似文献   

10.
An Al‐rich, SiO2‐deficient sapphirine–garnet‐bearing rock occurs as a metapelitic boudin within granulite facies Proterozoic charnockitic gneisses and migmatites on the island of Hisøy, Bamble Sector, SE Norway. The boudin is made up of peraluminous sapphirine, garnet, corundum, spinel, orthopyroxene, sillimanite, cordierite, staurolite and biotite in a variety of assemblages. Thermobarometric calculations based on coexisting sapphirine–spinel, garnet–corundum–spinel–sillimanite, sapphirine–orthopyroxene, and garnet–orthopyroxene indicate peak‐metamorphic conditions near to 930 °C at 10 kbar. Corundum occurs as single 200 to 3000 micron sized skeletal crystal intergrowths in cores of optically continuous pristine garnet porphyroblasts. Quartz occurs as 5–60 micron‐sized euhedral to lobate inclusions in the corundum where it is in direct contact with the corundum with no evidence of a reaction texture. Some crystal inclusions exhibit growth zoning, which indicates that textural equilibrium was achieved. Electron Back‐Scatter Diffraction (EBSD) studies reveal that the quartz inclusions share a common c‐axis with the host corundum crystal. The origin of the quartz inclusions in corundum is enigmatic as recent experimental studies have confirmed the instability of quartz–corundum over geologically realistic P–T ranges. The combined EBSD and textural observations suggest the presence of a former silica‐bearing proto‐corundum, which underwent exsolution during post‐peak‐metamorphic uplift and cooling. Exsolution of quartz in corundum is probably confined to fluid‐absent conditions where phase transitions by coupled dissolution–precipitation mechanisms are prevented.  相似文献   

11.
Experiments in the quartz-saturated part of the system KFMASHunder fO2 conditions of the haematite–magnetite bufferand using bulk compositions with XMg of 0·81, 0·72,0·53 define the stability limits of several mineral assemblageswithin the P–T field 9–12 kbar, 850–1100°C.The stability limits of the mineral assemblages orthopyroxene+ spinel + cordierite ± sapphirine, orthopyroxene + garnet+ sapphirine, sapphirine + cordierite + orthopyroxene and garnet+ orthopyroxene + spinel have been delineated on the basis ofP–T and T–X pseudosections. Sapphirine did not appearin the bulk composition of XMg = 0·53. A partial petrogeneticgrid applicable to high Mg–Al granulites metamorphosedat high fO2, developed in our earlier work, was extended tohigher pressures. The experimental results were successfullyapplied to several high-grade terranes to estimate P–Tconditions and retrograde P–T trajectories. KEY WORDS: KFMASH equilibria; experimental petrogenetic grid at high fO2  相似文献   

12.
Sapphirine/kornerupine-bearing rocks occur within the anorthosites of the Messina layered intrusion in the Limpopo mobile belt of Zimbabwe. The XMg range of the major minerals is as follows: cordierite (0.98-0.93); enstatite (0.97-0.86); chlorite (0.98-0.92); phlogopite (0.98-0.90); sapphirine (0.98-0.86); kornerupine (0.94-0.88); gedrite (0.96-0.85); spinel (0.92-0.78). There are four rock types, the constituent minerals of which have different values, which decrease in the above mineral order; other minerals are corundum, sillimanite and relict kyanite. We recognise twenty reactions without phlogopite and nine reactions involving phlogopite. The textural relations and the plots of the microprobe data of coexisting minerals in the MgO-Al2O3-SiO2-(H2O) system are consistent with the following sequence of main reactions: (1) enstatite+corundum cordierite+sapphirine; (4) sapphirine+sillimanite cordierite+corundum; (8) kornerupine+corundum cordierite+sapphirine; (13) kornerupine cordierite+sapphirine+enstatite; (15) enstatite+spinel chlorite+sapphirine; (18) cordierite+sapphirine chlorite+corundum; (20) sapphirine chlorite+corundum+spinel. The early reactions are shown by coarse-grained reaction intergrowths, kornerupine and gedrite breakdown is shown by finer-grained symplectites, and the latest reactions by very fine-grained products in micro-fractures. These selected reactions illustrate a remarkably steep trajectory from thePT peak close to 10 kbar and 800° C to the minimum observable at 3.5–4.5 kbar and 700° C as indicated by the pure MASH system. Very rapid uplift took place under nearly isothermal conditions. The protolith of this material was possibly sedimentary, derived from altered volcanic rocks. The bulk composition is close to the composition of kornerupine or to a mixture of alunite, chlorite and pyrophyllite. These texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history.  相似文献   

13.
Abstract Sapphirine-bearing rocks occur in three conformable, metre-size lenses in intrusive quartzo-feldspathic orthogneisses in the Curaçà valley of the Archaean Caraiba complex of Brazil. In the lenses there are six different sapphirine-bearing rock types, which have the following phases (each containing phlogopite in addition): A: Sapphirine, orthopyroxene; B: Sapphirine, cordierite, orthopyroxene, spinel; C: Sapphirine, cordierite; D: Sapphirine, cordierite, orthopyroxene, quartz; E: Sapphirine, cordierite, orthopyroxene, sillimanite, quartz; F: Sapphirine, cordierite, K-feldspar, quartz. Neither sapphirine and quartz nor orthopyroxene and sillimanite have been found in contact, however. During mylonitization, introduction of silica into the three quartz-free rocks (which represent relict protolith material) gave rise to the three cordierite and quartz-bearing rocks. Stable parageneses in the more magnesian rocks were sapphirine–orthopyroxene and sapphirine–cordierite. In more iron-rich rocks, sapphirine–cordierite, sapphirine-cordierite–sillimanite, cordierite–sillimanite, sapphirine–cordierite–spinel–magnetite and quartz–cordierite–orthopyroxene were stable. The iron oxide content in sapphirine of the six rocks increases from an average of 2.0 to 10.5 wt % (total Fe as FeO) in the order: C,F–A,D–B,E. With increase in Fe there is an increase in recalculated Fe2O3 in sapphirine. The four rock types associated with the sapphirine-bearing lenses are: I: Orthopyroxene, cordierite, biotite, quartz, feldspar tonalitic to grandioritic gneiss; II: Biotite, quartz, feldspar gneiss; III: Orthopyroxene, clinopyroxene, hornblende, plagioclase meta-norite; IV: Biotite, orthopyroxene, quartz, feldspar, garnet, cordierite, sillimanite granulite gneiss. The stable parageneses in type IV are orthopyroxene–cordierite–quartz, garnet–sillimanite–quartz and garnet–cordierite–sillimanite. Geothermobarometry suggests that the associated host rocks equilibrated at 720–750°C and 5.5–6.5 kbar. Petrogenetic grids for the FMASH and FMAFSH (FeO–MgO–Al2O3–Fe2O3–SiO2–H2O) model systems indicate that sapphirine-bearing assemblages without garnet were stabilized by a high Fe3+ content and a high XMg= (Mg/ (Mg+Fe2+)) under these P–T conditions.  相似文献   

14.
假蓝宝石是Mg-Al质麻粒岩中一种特殊的高温矿物,对超高温变质作用的研究有重要的意义。本文通过对全球66个超高温麻粒岩中47个含假蓝宝石麻粒岩地区的文献调研,总结了几种最常见的含假蓝宝石矿物组合产出的结构位置和变质反应关系,以及假蓝宝石的矿物化学特征。假蓝宝石的化学成分一般位于7∶9∶3端元左右,X_(Mg)大于0. 7,XFe_(3+)变化范围很宽,为0～0. 7。含假蓝宝石矿物组合的形成和演化指示了岩石经历的P-T轨迹。岩石中保留的假蓝宝石取代尖晶石、Grt/Opx+Sil取代Spr+Qz组合,以及随后的Spr+Crd±Opx后成合晶取代Grt/Opx+Sil组合的结构,一般可能指示了逆时针P-T轨迹中冷却和随后减压的部分;岩石中Grt/Opx+Sil/Ky或富Mg十字石反应形成Spr+Qz组合的结构可能指示了顺时针P-T轨迹中减压升温的部分。超高温变质岩不同的P-T轨迹暗示着它们的成因机制并不单一,前者可能是幔源基性岩浆底侵或增生作用的结果,后者可能与长期的热造山作用相关。  相似文献   

15.
Spinel granulites, with or without sapphirine, occur as lensesin garnetiferous quartzofeldspathic gneisses (leptynites) nearGokavaram in the Eastern Ghats Belt, India. Spinel granulitesare mineralogically heterogeneous and six mineral associationsoccur in closely spaced domains. These are (I) spinel–quartz–cordierite,(II) spinel–quartz–cordierite–garnet–orthopyroxene–sillimanite,(III) spinel–cordierite–orthopyroxene–sillimanite,(IV) spinel–quartz–sapphirine–sillimanite–garnet,(V) spinel–quartz-sapphirine–garnet and (IV) rhombohedral(Fe–Ti) oxide–cordierite–orthopyroxene–sillimanite.Common to all the associations are a porphyroblastic garnet(containing an internal schistosify defined by biotite, sillimaniteand quartz), perthite and plagioclase. Spinel contains variableamounts of exsolved magnetite and is distinctly Zn rich in thesapphirine-absent associations. XMg in the coexisting phasesdecreases in the order cordierite–biotite–sapphirine–orthopyroxene–spinel–garnet–(Fe–Ti)oxides. Textural criteria and compositional characteristicsof the phases document several retrograde mineral reactionswhich occurred subsequent to prograde dehydration melting reactionsinvolving biotite, sillimanite, quartz, plagioclase and spinel.The following retrograde mineral reactions are deduced: (1)spinel + quartz cordierite, (2) spinel + quartz garnet + sillimanite,(3) garnet + quartz cordierite + orthopyroxene, (4) garnet+ quartz + sillimanite cordierite, (5) spinel + cordierite orthopyroxene + sillimanite, (6) spinel + sillimanite + quartz sapphirine, (7) spinel + sapphirine + quartz garnet + sillimanite,and (8) spinel + quartz sapphirine + garnet. A partial petrogeneticgrid for the system FeO–MgO–Al2O3–SiO2–K2O–H2Oat high fo2, has been constructed and the effects of ZnO andFe2O3 on this grid have been explored Combining available experimentaland natural occurrence data, the high fo2 invariant points inthe partial grid have been located in P–T space. Geothermobarometricdata and consideration of the deduced mineral reactions in thepetrogenetic grid show that the spinel granulites evolved throughan anticlockwise P–T trajectory reaching peak metamorphicconditions >9 kbar and 950C, followed by near-isobaric cooling(dT/dP = 150C/kbar). This was superimposed by an event of near-isothermaldecompression (dT/dP = 15C/kbar). The studied spinel granulites,therefore, preserve relic prograde mineral associations andreaction textures despite being metamorphosed at very high temperatures,and bear evidence of polymetamorphism. KEY WORDS: spinel granulite; Eastern Ghats; India; polymetamorphism; geothermometry; geobarometry Corresponding author  相似文献   

16.
Integrated metamorphic and geochronological data place new constraintson the metamorphic evolution of a Neoproterozoic orogen in eastAntarctica. Granulite-facies rocks from a 150 km stretch ofthe Kemp Land coast reflect peak conditions involving T 870–990°Cat P 7·4–10 kbar, with pressure increasing westwardtowards an Archaean craton. Electron microprobe-derived (Th+ U)–Pb monazite ages from metapelitic assemblages indicatethat the major mineral textures in these rocks developed duringthe c. 940 Ma Rayner Orogeny. Complex compositional zoning inmonazite suggests high-T recrystallization over c. 25 Myr. Diversityin metapelitic reaction textures reflects silica and ferromagnesiancontent: Si-saturated Fe-rich metapelites contain garnet thatis partially pseudomorphed by biotite and sillimanite, whereasSi-saturated Mg-rich metapelites and Si-undersaturated metapeliticpods have reaction microstructures involving cordierite enclosingorthopyroxene, garnet and/or sapphirine, cordierite + sapphirinesymplectites around sillimanite and coarse-grained orthopyroxene+ corundum separated by sapphirine coronae. Interpretationsbased on P–T pseudosections provide integrated bulk-rockconstraints and indicate a clockwise P–T–t pathcharacterized by a post-peak P–T trajectory with dP/dT 15–20 bar/ °C. This moderately sloped decompressive-coolingP–T path is in contrast to near-isothermal decompressionP–T paths commonly cited for this region of the RaynerComplex, with implications for the post-collisional tectonicresponse of the mid- to lower crust within this orogenic belt. KEY WORDS: electron microprobe monazite dating; granulite facies; Rayner Complex; sapphirine; THERMOCALCMinerals abbreviations: q, quartz; g, garnet; sill, sillimanite; ky, kyanite; opx, orthopyroxene; cd, cordierite; ksp, alkali feldspar; pl, plagioclase; bi, biotite; sp, spinel; ilm, ilmenite; mt, magnetite; ru, rutile; sa, sapphirine; cor, corundum; osm, osumilite; liq, silicate melt; mnz, monazite  相似文献   

17.
The aluminous enclaves occur in gedrite-cordierite-gneissesof the Middle Ordovician Ammonoosuc Volcanics, and are composedof combinations of the aluminous minerals sillimanite (Sill),kyanite, corundum (Cor), staurolite (St), sapphirine (Sa), andspinel (Sp), which are set in a matrix of cordierite (Crd) orplagioclase (Plag). Generally, where plagioclase is present,both it and the aluminous minerals are separated from gedrite(Ged) and rare hornblende (Hbl) by cordierite. The enclavesarc interpreted to have formed near the peak of Acadian (Devonian)metamorphism at sillimanite-staurolite-muscovite grade by reactionsthat were encountered during the pressure decrease which accompaniedthe rise of gneiss domes in the region. The enclaves are divided into two main types: (1) enclaves ofcordierite surrounding aluminous minerals; and (2) enclavesof cordierite and plagioclase surrounding aluminuous minerals.Sapphirine grains contain between 9?2 and 9?3 Al atoms per formulacalculated to 14 cations. Staurolites from the enclaves areMg-rich and have (Fe2++ Mn)/(Fe2++Mn+Mg) ratios of 0-59–0?64. The textures and mineralogy of the enclaves suggest that theserocks originally consisted of Ged+Sill?Qz?Hbl?Sp?Plag. Theseminerals reacted to form Crd+Aluminous Minerals?Plag. The mineralogyof both main types of enclaves can be explained by two analogoussets of continuous Fe-Mg reactions:The structure of the enclavessuggests that the mineral growth by the above reactions wasdiffusion controlled, which would have resulted from oversteppingthe above reactions (i.e. the P change exceeded the reactionrate). Therefore, chemical potential gradients (relative mobilityof diffusing components) between gedrite and sillimanite controlledthe location of mineral growth. The Fe-Mg ratio of the bulkcomposition and the proportions of non-Fe-Mg minerals (quartzand sillimanite) appear to determine which continuous Fe-Mgreactions were encountered. Examples of mineral sequences in the cordierite enclaves are:Sill (core)/St+Crd/Ged (matrix); Cor+Crd (core)/Ged (matrix),and Sill (core)/St+Crd/Sa+Crd/Ged (matrix). Examples of themineral sequences in the cordierite-plagioclase enclaves are:Sill (core)/St+Plag/Plag+Crd/Hbl+Ged (matrix); Cor+Plag (core)/St+Plag/Sa+Plag/Ged+ Hbl (matrix); and St+Plag (core)/Plag+Crd/Ged+Hbl (matrix). P–µFeMg–1 diagrams proved to be an importanttool for understanding and illustrating the development of theenclaves. These diagrams allow one to view simultaneously allthe discontinuous and continuous Fe-Mg reactions along a P–µH2O(or T) rock path. With this information it is possible to determinequalitatively which reactions and what sequence of reactionsmight be encountered by bulk compositions with variable Fe-Mgratios and modal proportions of phases.  相似文献   

18.
The granulite complex at Anakapalle, which was metamorphosedat 1000 Ma, comprises orthopyroxene granulites, leptynite, khondalite,mafic granulites, calc-silicate rock, spinel granulites, andtwo types of sapphirine granulites—one quartz-bearingand migmatitic and the other devoid of quartz and massive. Reactiontextures in conjunction with mineral-chemical data suggest severalcontinuous and discontinuous equilibria in these rocks. In orthopyroxenegranulites, dehydration-melting of biotite in the presence ofquartz occurred according to the reaction biotite+quartz= garnet (Py37)+K-feldspar+orthopyroxene + liquid. Later, this garnet broke down by the reaction garnet (Py37)+quartz= orthopyroxene + plagioclase. Subsequently, coronal garnet (Py30) and quartz were producedby the same reaction but proceeding in the opposite direction.In spinel granulites, garnet (Py42) and sillimanite were producedby the breakdown of spinel in the presence of quartz. In thetwo types of sapphirine granulites, garnet with variable pyropecontent broke down according to the reaction garnet = sapphirine + sillimanite + orthopyroxene. The highest pyrope content (59 mol %) was noted in garnets fromquartz-free sapphirine granulites compared with the quartz-bearingone (53 mol % pyrope). The calculated positions of the mineralreactions and diserete P-T points obtained by thermobarometrydefine a retrograde P-T trajectory during which a steep decompressionof 1.5 kbar from P-Tmax of 8 kbar and 900C was followed bynear-isobaric cooling of 300C. During this decompression, garnetwith variable pyrope contents in different rocks broke downon intersection with various divariant equilibria. Near-isobariccooling resulted in the formation of coronal garnet around second-generationorthopyroxene and plagioclase replacing earlier porphyroblasticgarnet in orthopyroxene granulites. It has been argued thatthe deduced P-T trajectory originated in an extensional regimeinvolving either a crust of near-normal thickness of a slightlyoverthickened crust owing to magmatic underaccretion.  相似文献   

19.
The upper Triassic Karmutsen metabasites from northeast VancouverIsland, B.C., are thermally metamorphosed by the intrusion ofthe Coast Range Batholith. The amygdaloidal metabasites developedin the outer portion of the contact aureole show a progressivemetamorphism from zeolite to prehnite-pumpellyite facies. Thesize of an equilibrium domain is extremely small for these metabasites,and the individual amygdule assemblages are assumed to be inequilibrium. Two major calcite-free assemblages (+chlorite+quartz)are characteristic: (i) laumontite+pumpellyite+epidote in thezeolite facies and (ii) prehnite+pumpellyite+epidote in theprehnite-pumpellyite facies. The assemblages and compositionsof Ca-Al silicates are chemographically and theoretically interpretedon the basis of the predicted P-T grid for the model basalticsystem, CaO-MgO-A12O3-Fe2O3-SiO2-H2O. The results indicate:(1) local equilibrium has been approached in mineral assemblagesand compositions; (2) the XFe3+ values in the coexisting Ca-Alsilicates decrease from epidote, through pumpellyite to prehnite;(3) with increasing metamorphic grade, the Fe3+ contents ofepidotes in reaction assemblages decrease in the zeolite facies,then increase in the prehnite-pumpellyite facies rocks. Suchvariations in the assemblages and mineral compositions are controlledby a sequence of continuous and discontinuous reactions, andallow delineation of T-XFe3+ relations at constant pressure.The transition from the zeolite to prehnite-pumpellyite faciesof the Karmutsen metabasites is defined by a discontinuous reaction:0·18 laumontite+pumpellyite+0·15 quartz = 1·31prehnite+ 0·78 epidote+0·2 chlorite+ 1·72H2O, where the XFe3+ values of prehnite, pumpellyite and epidoteare 0·03, 0·10 and 0·18, respectively.These values together with available thermodynamic data andour preliminary experimental data are used to calculate theP-T condition for the discontinuous reaction as P = 1·1±0·5 kb and T = 190±30°C. The effectsof pressure on the upper stability of the zeolite facies assemblagesare discussed utilizing T-XFe3+ diagrams. The stability of thelaumontite-bearing assemblages for the zeolite facies metamorphismof basaltic rocks may be defined by either continuous or discontinuousreactions depending on the imposed metamorphic field gradient.Hence, the zeolite and prehnite-pumpellyite facies transitionboundary is multivariant.  相似文献   

20.
Mineral chemistries and textures are described from a suiteof sapphirine-bearing granulites from the Gruf Complex of theItalian Central Alps. The granulites contain combinations ofgarnet, orthopyroxene, sapphirine, sillimanite, cordierite,biotite, quartz, spinel, corundum, staurolite, plagioclase,K-feldspar, ilmenite and rutile, in assemblages with low (usuallynegative) variance. They are outstanding in that they preservea textural and chemical record of a protracted metamorphic evolution. Reaction textures are common and include: (i) pseudomorphs (e.g.of sillimanite after kyanite); (ii) relatively coarse-grainedmonomineralic reaction rims (e.g. of cordierite between sapphirineand quartz); (iii) fine-grained symplectitic coronas (e.g. oforthopyroxene + sapphirine round garnet); (iv) inclusions, ingarnet cores, of minerals (e.g. staurolite) not found elsewherein the rocks. Detailed microprobe study has revealed large chemical variationswithin each phase. Different textural types of each phase havedifferent compositions, and strong zoning is preserved in garnet(Mg/(Mg + Fe) from 0.30 to 0.61) and coarse sapphirine. Inclusionpopulations in garnet correlate with host composition. The textural and chemical features are interpreted in termsof successive equilibrium assemblages and reactions. Metamorphicconditions operative at each stage in the evolution are calculatedusing published geothermometers and geobarometers as well asthermodynamically calibrated MAS and FASH equilibria. The resultsare used to construct a P—T-time path for the sapphirine-granulites,which can be summarized as follows: (i) Increasing T at high P (>7 kb). Partial melting. (ii) A maximum T of 830 ?C attained at 10 kb. (iii) Almost isothermal decompression, reaching 750 ?C at 5kb, under conditions of low µH2O. (iv) Further cooling, and decompression. Localized hydration.Rocks exposed. The P—T-time path is interpreted as the product of a singlemetamorphic cycle (the tertiary ‘Lepontine’ event)and is extrapolated to the Gruf Complex as a whole. When combinedwith published geochronological data, the results indicate anaverage uplift rate in excess of 2 mm/yr for the Gruf Complexbetween 38 and 30 Ma ago. An in situ partial melting origin for the sapphirine-granulitesis favoured. Extraction of an iron-rich granitic liquid froma normal pelitic palaeosome could generate a refractory residuewith the required Mg, Al-rich composition. The change in bulksolid composition during partial melting is thought to accountfor the extraordinarity strong zoning in the garnets.  相似文献   

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