Thermal Evolution of Archean Basement Rocks from the Eastern Part of the North China Craton and Its Bearing on Tectonic Setting
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Abstract The basement rocks in the eastern zone of the North China craton are composed predominantly of pretectonic tonalitic-trondhjemitic-granodioritic gneisses and syntectonic granitoids, with rafts of supracrustal rocks consisting of ultramafic to felsic volcanic and sedimentary rocks, metamorphosed over a range of conditions from greenschist to granulite facies. Most mafic granulites, amphibolites, and some pelitic gneisses and schists preserve the prograde, peak, and post-peak textures. The prograde metamorphic stage is indicated by mineral inclusions within minerals of the peak stage, represented by the assemblages of hornblende + plagioclase + quartz ± biotite in mafic granulites, chlorite + actinolite + epidote + plagioclase + quartz in amphibolites, and biotite + plagioclase + quartz in pelitic gneisses. The peak stage is shown by assemblages of orthopyroxene + clinopyroxene + garnet + plagioclase + quartz in the mafic granulites, hornblende + plagioclase + quartz + garnet in garnetiferous amphibolites, and garnet + sillimanite + plagioclase + quartz + biotite in pelitic gneisses. The post-peak stage is represented by garnet + quartz symplectic coronas in mafic granulites, actinolite + garnet retrogressive rims around garnet or hornblende grains in amphibolites, and kyanite replacing sillimanite or staurolite replacing sillimanite + garnet in pelitic gneisses. These textural relations and their P-T estimates define counterclockwise P-T paths. The P-T-t(ime) paths of metamorphic rocks from the eastern zone reflect an origin related to the intrusion and underplating of large amounts of mantle-derived magmas that not only provided heat for the metamorphism but also added a large volume of mostly mafic material to the base of the crust. Crustal growth therefore was dominated by vertical addition of mantle-derived magmas to the crust. Combined with lithologic, structural, metamorphic, and geochronologic considerations, these large volumes of underplated magma are considered to be related to upwelling mantle plumes (hotspots).Keywords:
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An example of cordierite-bearing gneiss that is part of a high-grade gneiss-migmatite sequence is described from the Hatch Plain in the Read Mountains of the Shackleton Range, Antarctica, for the first time. The cordieritebearing rocks eonstitute the more melanosomic portions of the metatectic and migmatitic rocks that are associated with relict granulite facies rocks such as enderbitic granulite and enderbitic garnet granulite. The predominant mineral assemblage in the eordierite-bearing rocks is chemically homogeneous eordierite (X 0.61) and biotite (Xo 0.47), strongly zoned garnot (X 0.18-0.11), silli;llanite, K-feldspar (Or:'.94Ab5.,sAn06)' plagioclase (An28)~ and quartz. Inclusions of sillimanite and biotite relics in both gamet and cordierite indicate that garnet and cordierite were produced by the coupled, discontinuous reaction
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ABSTRACT 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. Fe 2+ /(Fe 2+ + 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 H 2 O‐poor conditions.
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The Bakhuis belt, one of the major granulite-facies domains in the Guiana Shield, consists of a core of banded rocks of the charnockite suite, metamorphosed and migmatized under granulite-facies conditions. Pelitic gneiss intercalations locally show sapphirine-quartz and orthopyroxene-sillimanite-quartz assemblages, with up to 10% Al2O3 in the orthopyroxene. These assemblages point to ultrahigh-temperature (UHT) metamorphism. P-T conditions are difficult to deduce, because the assemblages contain Fe3+ in sapphirine and sillimanite, and do not contain coeval garnet, but spinel, Feand Fe-Ti oxides instead. P-T conditions for the peak UHT metamorphism are estimated to have been 950 °C and 8.5-9 kb. An assemblage of impure corundum associated with quartz may also have formed during the UHT metamorphism. Unravelling the assemblages indicates a counterclockwise P-T path from an early cordierite-sillimanite assemblage via a subsequent sapphirine-quartz assemblage to the peak metamorphic assemblage orthopyroxene-sillimanitequartz. Retrogressive assemblages show an isobaric to near isobaric cooling path for the UHT occurrence. Single zircon Pb-evaporation and whole rock Sm-Nd dating were carried out on the Bakhuis granulites. Three zircons from a garnet-sillimanite-bearing gneiss yielded an age of 2072 ± 4 Ma, but a fourth grain provided increasing ages up to 2.10 Ga. An enderbitic granulite gave zircon ages in the range of 2.15-2.09 Ga. Zircons from a garnet-bearing gneiss defined an age of 2055 ± 3 Ma, while those from a garnet-bearing pegmatite layer gave ages ranging between 2085 Ma and 2058 Ma. A mylonitic orthopyroxene-bearing granite (a true charnockite) furnished a Pb-Pb zircon age of 2065 ± 2 Ma this is the first indication that the high-grade metamorphism was (at least locally) associated with melting and production of magmatic charnockite. Zircons from two samples of a discordant recrystallized basic dyke yielded ages of 2060 ± 4 Ma and 2056 ± 4 Ma, respectively. A discordant sheared pegmatite vein provided an age of 2059 ± 3 Ma. In conclusion, the 2072–2055 Ma ages are interpreted as the age of granulite metamorphism in the Bakhuis Mountains, and ages older than 2.07 Ga, and up to 2.15 Ga, are considered to reflect an inherited component from the early Transamazonian protoliths of the granulite. TDM model ages for the Bakhuis samples range from 2.40 to 2.19 Ga. These data, together with positive to slightly 175 GEOLOGIE DE LA FRANCE, N° 2-3-4, 2003 GEOLOGY OF FRANCE AND SURROUNDING AREAS, N° 2-3-4, 2003 The Bakhuis ultrahigh-temperature granulite belt (Suriname): I. petrological and geochronological evidence for a counterclockwise P-T path at 2.07-2.05 Ga La ceinture de granulites d’ultra-haute temperature des Monts Bakhuis (Suriname) : I. mise en evidence d’un chemin P-T anti-horaire a 2,07-2,05 Ga Geologie de la France, 2003, n° 2-3-4, 175-205, 12 fig., 6 tabl. Mots cles : Roche metamorphique, Facies granulite, Condition pression temperature, Datation, Pb-Pb, Sm-Nd, Zircon, Roche totale, Paleoproterozoique, Suriname, Bouclier guyanais.
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Abstract The Beaverdam Amphibolite occurs as several exposures of metamorphosed mafic rocks in the Ashland-Wedowee belt in the Eastern Blue Ridge of Eastern Alabama. The exact stratigraphic position of these exposures is unclear and this research cannot resolve that question, but we can determine the character of the parent rock and begin to clarify the differences in metamorphism. Major and trace element geochemistry shows the rocks from the three outcrops have the similar protoliths and are consistent with interpretations that the parent basalts originated in a rifting environment, most likely in a back-arc setting. The rocks have complex structural and metamorphic histories. Samples from Beaverdam Creek are well foliated, schistose to gneissic rocks composed primarily of quartz, plagioclase, epidote, and hornblende, typical of the epidote amphibolite facies. The hornblende is, green to yellow green to light brown. Occasional grains of relict pyroxene from the original basalt are present. This likely represents the earliest phase of meta-morphism in the rocks. Samples from the Cornhouse antiform are amphibole schists to phyllites Dominant minerals are blue green hornblende, quartz, and epidote, typically a greenschist facies assemblage. The rocks are finely laminated, with hornblendes lying in the foliation. Tight microfolds bend the foliation and individual hornblende grains are deformed by bending or extension fractures. These rocks likely formed during later shearing of the coarser amphibolites. Greenschist facies temperature and pressure conditions persisted after the deformation as indicated by the presence of blue green hornblende and chlorite fans that cut across the foliation.
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Actinolite
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