Semi-pelitic Gneisses Research Articles

Diverse subduction and exhumation of tectono-metamorphic slices in the Kalatashitage area, western Paleozoic Dunhuang Orogenic Belt, northwestern China

High-pressure mafic granulite and garnet amphibolite are identified as small-scale tectonic slices within pelitic or semi-pelitic gneiss in the Kalatashitage area, which is located in the western Paleozoic Dunhuang Orogenic Belt, northwestern China. These rocks retain three generations of metamorphic mineral assemblages: prograde assemblage (M1) preserved as inclusions within garnet porphyroblasts, metamorphic peak assemblage (M2) consisting of matrix minerals and garnet porphyroblasts, and retrograde assemblage (M3) mainly represented by the symplectic minerals surrounding the embayed garnet and the retrograded hornblende rimming matrix-type clinopyroxene. Metamorphic pressure and temperature (P-T) paths of high-pressure mafic granulite, amphibolite, and metapelite retrieved by thermobarometry are all clockwise, passing from 640 to 720 °C/6.2–12.6 kbar (M1) through 840–920 °C/14.6–16.2 kbar (M2) to 750–815 °C/5.5–7.9 kbar (M3) for high-pressure mafic granulite, from ~650 °C/5.7 kbar (M1) through ~750 °C/9.2 kbar (M2) to ~780 °C/8.1 kbar (M3) for amphibolite, and from ~615 °C/7.9 kbar (M1) through 730–820 °C/8.6–11.7 kbar (M2) to 675–740 °C/5.4–8.7 kbar (M3) for pelitic and semi-pelitic gneiss. Furthermore, pseudosection modeling of high-pressure mafic granulite indicates that the growth zonation of garnet porphyroblast exhibits prograde metamorphism in a P-T range of 510–800 °C/8.5–13 kbar and demonstrates peak metamorphic P-T conditions of ~850 °C/16 kbar, which are consistent with the thermobarometric estimates. The significant pressure differences in peak metamorphism observed in different rocks indicate that the rocks initially subducted to remarkably different depths and were subsequently juxtaposed at shallower crustal levels during exhumation. Sensitive high-resolution ion microprobe (SHRIMP) analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) UPb dating of metamorphic zircon indicates that the metamorphic events occurred at ca. 430–420 Ma (M2) and ca. 400–390 Ma (M3), respectively. Metamorphism was followed by the intrusion of granitic dykes at ca. 244 Ma. Moreover, the metamorphic evolution indicates that the Kalatashitage area was involved in the subduction, collision and subsequent tectonic exhumation in the Paleozoic. Combined with previous literature, it is inferred that the discrepant subduction and exhumation of high-grade metamorphic rocks is a universal phenomenon in the Paleozoic Dunhuang Orogenic Belt, supporting the ubiquitous existence of subduction-collision complexes in this orogenic belt.

Amphibolite facies metamorphism and geochronology of the Paleoproterozoic Aketashitage Orogenic Belt, northwestern China

Garnet amphibolite and semi-pelitic gneiss preserving three stages of metamorphic mineral assemblages, i.e., the prograde metamorphic inclusion assemblage (M1) preserved in the garnet porphyroblast, metamorphic peak assemblage (M2) and retrograde symplectic assemblage (M3), have been recognized in the Aketashitage area, northwestern China. Geothermobarometric computation shows that the M1, M2 and M3 assemblages record the metamorphic P-T conditions of 600–700 °C/4.2–5.3 kbar, 730–790 °C/6.3–7.1 kbar, and 770 °C/5.6 kbar, respectively. These three metamorphic stages define clockwise P-T paths including nearly isothermal decompression (ITD) process, indicating typical orogenic metamorphism. The peak metamorphic P-T conditions reach the transition zone between the high amphibolite facies and granulite facies, and belong to intermediate P/T facies series. The SHRIMP and LA-ICP-MS U-Pb dating of metamorphic zircon implies that the subduction might occur as early as ∼1.97 Ga and last as late as 1.84 Ga. Combined with literature data, it is concluded that the Aketashitage area was a Paleoproterozoic orogenic belt, which is possibly related to the assembly of the basements of the Columbia supercontinent.

Origin and tectonometamorphic history of the Repulse Bay block, Melville Peninsula, Nunavut: exotic terrane or deeper level of the Rae craton?

The Rae craton on Melville Peninsula, Nunavut, comprises several lithotectonic domains, including a structurally and lithologically distinct yet poorly known crustal terrane, the Repulse Bay block (RBb). This study presents new lithological and petrographic observations, combined with U–Pb zircon data, to better understand the Archean and Paleoproterozoic crustal evolution of the RBb. The new data demonstrate that the central-eastern RBb consists of the following: (i) upper amphibolite- to granulite-facies, ca. 2.73–2.71 Ga intermediate granitoid gneisses and gabbroic sheets; (ii) ca. 2.69 Ga two-pyroxene charnockite to enderbite intrusions; and (iii) thin slivers of both Archean and Paleoproterozoic supracrustal rocks. Inherited zircon also attests to the presence of a Mesoarchean to Paleoarchean substrate. A semi-pelitic gneiss from one of the Paleoproterozoic supracrustal panels was deposited sometime after ca. 1.89 Ga and shows a similar detrital zircon age profile to a <1.92 Ga semi-pelitic gneiss from the Lyon Inlet boundary zone at the northern extent of the RBb. Zircon in most rocks analyzed record metamorphism related to the Trans-Hudson orogeny between ca. 1.84 and 1.82 Ga. Results from this study are most consistent with the RBb, representing a piece of lower to middle crustal level of the Rae craton, rather than a distinct and separate crustal entity (i.e., an exotic block).

Structural and geochronological constraints on the evolution of the eastern margin of the Tanzania Craton in the Mpwapwa area, central Tanzania

A study of the position, nature and geochronology of the eastern margin of the Tanzania Craton near Mpwapwa yields new constraints on Archaean to Neoproterozoic orogenesis of central Tanzania. The eastern part of the craton comprises typical Neoarchaean grey granodioritic orthogneisses dated with the LA-ICP-MS method by U–Pb zircon at ca. 2.7Ga. A gradual eastward increase in strain in these rocks culminates in a 1–2km wide, locally imbricated, north–south-trending, ductile thrust/shear zone with an oblique top-to-the-NW sense of movement. East of the craton-edge shear zone, high-grade supracrustal rocks are termed the “Mpwapwa Group” in view of uncertain regional correlations. There is an apparent lithological zonation of the Mpwapwa Group parallel to the craton margin shear zone. In the west, the group consists of typical “shelf facies” metasedimentary rocks (marbles, calc-silicates, quartzites, etc.). U–Pb dating of detrital zircons from two quartzites reveal only Archaean detritus, constraining their maximum depositional age to <ca. 2.6Ga. The shelf rocks pass eastwards into semi-pelitic gneisses interlayered with bimodal mafic-felsic gneisses, which may have volcanic protoliths and are intruded by multiple mafic sills. Dating of a felsic gneiss showed it to probably be a Neoarchaean rock which underwent Palaeoproterozoic metamorphism during the Usagaran event at ca. 1930Ma. This date likely represents initial juxtaposition of the (Archaean) Mpwapwa Group against the Tanzania Craton. Consequently the Group are not considered to be stratigraphic correlates of the Usagaran Konse Group or Isimani Suite seen further south, but rather form part of the Neoarchaean crust of the “Western Granulite” terrane of the East African Orogen. The Mpwapwa Group was intruded by weakly foliated biotite granite at 1873±31Ma. Zircons in the granite have metamorphic rims dated between 550 and 650Ma that grew during the East African orogenic event.Distinctive para- and orthogneisses, including charnockite dated at 2707±21Ma, outcrop in the southern part of the area. A sliver of southern tonalitic orthogneiss, entrained in the craton margin shear zone, is an igneous rock of probable early Palaeoproterozoic or Archaean age (>ca. 2300Ma) with a strong metamorphic overprint at ca. 1960Ma (zircon), confirmed by a metamorphic titanite age of ca. 1990Ma, again believed to date the initial phase of craton-margin shearing and juxtaposition of the Archaean crustal blocks. The role of the Neoproterozoic East African orogeny in the evolution of the craton margin is unclear, but the geometry of the shear zone, the presence of Neoproterozoic zircon rims in the Palaeoproterozoic granite and published studies from nearby, all suggest that the latest movements on the shear zone may be Neoproterozoic in age and that the structure may represent the local western front of the East African Orogeny.

Petrography and mineral chemistry of the pelitic and semi-pelitic gneisses of the merelani tanzanite mining area, northeastern Tanzania

Sillimanite/kyanite + K-feldspar + quartz ± garnet + biotite ± Fe-Ti oxide-bearing rocks occur associated with calc-silicate rocks in the Merelani tanzanite mining area north-eastern Tanzania. The whole rock chemistry shows varying rock oxidation ratios, which are reflected in the mineralogical assemblages. Almandine garnet is oxidized to mixtures of hematite, quartz, and hydrous aluminosilicates in oxidized rocks and is absent in highly oxidized rocks. In the latter, ilmenite is oxidized into mixtures of rutile and hematite. Such relatively high temperature alteration/oxidation effects could have occurred with the influx of water. Absence of staurolite in these aluminium-rich rocks is indicative of high fugacity of oxygen during metamorphism. These rocks were regionally metamorphosed to the high-grade granulite facies with estimated formation temperatures and pressures of 596 – 726o C and 7.7 – 9.1 kbars and later, underwent retrograde metamorphism to the amphibolite facies. Tanzania Journal of Science Vol. 31 (2) 2005: pp. 81-92

Petrology and lithogeochemistry of the mineralized tanzanite-grossular bearings rocks in the Merelani-Lelatema area, northeastern Tanzania

The Merelani-Lelatema area, which is occupied by late Proterozoic metasedimentary rocks, lies along the Lelatema fault zone. The rocks are mainly pelitic and semi-pelitic gneisses, psammitic gneisses, hydrothermally altered rocks, crystalline dolostones, pegmatites and quartz veins. The occurrence of gem-quality tanzanite and grossular has been found to occur mainly in boudinaged pegmatitic veins and hydrothermal fracture fillings. These fracture fillings occur in brecciated and hydrothermally altered graphite-bearing gneiss in a mineral association containing glass-clear quartz, diopside, zoisite, graphite and calcite. The psammitic gneisses are more siliceous and enriched in feldspars. They are grouped into graphite-kyanite gneiss with thin interlayers of quartz-feldspar fels. The pelitic and semi-pelitic gneisses are aluminium-rich and thus have been grouped into garnet-biotite-sillimanite and/or kyanite graphite gneisses. The hydrothermally altered rocks are strongly deformed and contain varying amounts of clay minerals, which characteristically show distinct banded layers due to alterations of minerals. The rocks have been folded and the fold structures have undergone three deformation episodes (D1, D2 and D3) D1 and D2 were accompanied by high-grade metamorphism whereas D3 was associated with retrograde metamorphism and the formation of retrograde shear zones. Evidence from metamorphosed graded bedding confirms the existence of the antiform and also shows that the macroscopic F2 folds are superimposed on a previously inverted stratigraphic succession. This inversion is the consequence of D1 deformation. The polyphase nature of these rocks with which pegmatites and quartz veins occur is shown by rotation of boudins, presence of overturned tight isoclinal folds resembling F1, presence of F2 folds and refolded folds shown by quartz veins. Features like boudins shown by quartz veins and pegmatites at the limbs and along the axial planes of F1 folds with S1 development in the host rocks and disruption of limbs of tight folds resembling F1, shown by these veins also point to significant amount of flattening during F1 folding phase and longitudinal stretching of the late stage of their development. Slippage in the later stages of deformation phase (D1) is also distinct in displaced veins. Lithogeochemical studies reveal that the gneissses originated from miogeosynclinal sediments composed of sand; organic matter rich in V, U and fine Al-rich materials. These underwent advanced weathering, deposited in a lake and during metamorphism and after the Pan-African tectonothermal event, the silicates were altered, trace elements released and migrated with hydrothermal solutions rich in Ca, Mg, CO2 and SO3. Finally these elements together with V, U, Sr, Zn and heavy REE elements were deposited in the hydrothermal zone. Emplacement of gemstones (tanzanite and green grossular) in the area is structurally controlled along the Lelatema fault system. Tanz. J. Sci. Vol.29(2) 2003: 55-70

Trace elements characterization of the hydrothermally deposited tanzanite and green grossular in the Merelani – Lelatema shear zone, northeastern Tanzania

Geochemical investigations of trace element contents of different rock samples were determined in the late Proterozoic metamorphic sequences of the Mozambique Belt along the Merelani–Lelatema shear zone, northeastern Tanzania. The area is composed of the following rock types: pelitic and semi-pelitic gneisses, psammitic, hydrothermally altered including carbonate-rich, carbonate-gypsum-rich, and iron-rich rocks, pegmatite dykes and crystalline limestones. Trace element characteristics of the principal rock units in and outside the shear zone have been compiled and studied. With the exception of higher barium contents in pelites, trace element contents in the psammitic and pelitic gneisses show some similarities while in the hydrothermally altered rocks where gemstones (tanzanite and green grossular) are localized reveal that the following trace elements: Ba, Cu, Mo, Ni, Rb, Sr, U, V and Zn have been enriched, while As, Ag, Co, Cr, Cs, Pb, Th and Zr were relatively depleted compared to the other rock types. Calculated CI/ Br ratios are at maximum of 900, which are the same as found in waters of deep drill holes and in salt waters associated with oil deposits. Calculated Co/Ni ratios are low, less than unity in most rocks except the psammitic gneisses and carbonate-rich rocks whose values are approximately 3.4 and 6.5 respectively. The low Co and Ni values in these rocks may be a result of low a co 2+ and a ni 2+ in the hydrothermal fluids and / or lower temperature of crystallization. The high V contents result in an unusual degree of V substitution in a number of silicate and oxide phases. The high levels of V in the metasediments, together with that of Ni and Sr, is reflected in their enhanced concentration in the hydrothermally altered zone indicating that they may have been partly re-distributed by the hydrothermal fluids. Vanadium contents in the hydrothermally altered rocks are strongly enriched especially in the calcium-magnesium-aluminium-rich rocks but low in carbonate-sulphate-rich rocks. It is most likely that at the stage when tanzanite and green grossular were crystallized Al, Ca, Mg, Si, and V were migrating together and were incorporated into the minerals precipitated. Vanadium (V 3+ ) substitutes for iron (Fe 3+ ) and aluminium (Al 3+ ) diadochically in the crystal lattices of silicates mainly zoisite and green grossular. The data suggest that the gemstone deposits are the result of a sequence of long-continued and overlapping geological processes culminating in intense shearing, considerable mobilization of trace elements and syn-tectonic hydrothermal intrusion.The Proterozoic metasedimentary rocks of the shear zone, as well as those enclosing it, served as the source of trace elements, while diffusion of the trace element constituents and their precipitation in physico-chemically favorable traps, formed in response to folding and shearing, resulted in the gemstone deposition. Tanz. J. Sci. Vol.29(1) 2003: 45-60

Spinel-bearing assemblages and P-T-t evolution of the Central Zone of the Damara Orogen, Namibia

The Central Zone of the Damara Orogen in Namibia is dominated by intrusive granitoids and upper amphibolite to granulite-facies metasediments. High-temperature low-pressure mineral assemblages near the Atlantic Coast have not been previously described with spinel symplectites. In semi-pelitic gneisses, the symplectites are composed of hercynitic spinel and Fe-Ti oxides occurring within the cores of zoned porphyroblasts of antiperthite+sillimanite. In pelitic schists, hercynite-sillimanite symplectites occur together with minor quartz in the core of Mg-cordierite porphyroblasts (X Mg = 0.66–0.72). Thermobarometry determines the P-T conditions of spinel symplectites to be 527 ± 112°C and 4 ± 1 kbar. Textural relationships indicate that the symplectite post-dates the metamorphic peak conditions dated by syn-metamorphic granite at 534 ± 7 Ma (published U-Pb zircon age) but are still relatively early within the metamorphic history. Subsequent isobaric regional heating and partial melting occurred in association with biotite breakdown and garnet growth. Thermobarometry gives 700 ± 50°C and 4 ± 1 kbar for this episode. This regional isobaric heating is equated with the emplacement of voluminous granitoid magmas, dated at post-534 ± 7Ma and pre-508 ± 2 Ma (published U-Pb zircon and monazite ages) and generated during decompression caused by the exhumation of the Central Zone. The system then cooled through 550 ± 25°C and 2 ± 1 kbar at 465 ± 5 Ma (published Rb-Sr and Ar 40/Ar 39 biotite-muscovite ages) on its way to the surface. The occurrence of a later separate thermal overprint as evidenced by spinel symplectites and garnet growth argues in favour of a two-stage metamorphic model. This study confirms the clockwise P-T path for the Central Zone of the Damara Orogen.

The tectonostratigraphical significance of pre-750 Ma metagabbros within the northern Central Highlands, Inverness-shire

Introduction Amphibolite suites within the late-Proterozoic metasediments of the Northern Highlands are mostly regarded as being of igneous origin, reflecting the emplacement of tholeiitic magmas at an early stage of their tectonothermal history (Winchester 1984; Rock et al. 1985). To the south of the Great Glen Fault the presence of amphibolites, similarly of igneous origin, is also recognized within migmatized Precambrian metasediments assigned by Piasecki (1980) to the Central Highland Division (Highton 1986). This communication describes a suite of metagabbro and amphibolite bodies which crop out within sub-Appin Group metasediments of the northern Central Highlands. Their position within this tectonostratigraphy provides an important reference point for the timing of orogenic activity in the Grampian terrain. General geology Within the northern Central Highlands the presence of Moine-like metasediments, formerly referred to as the Central Highland Granulites, has long been recognised (Home and Wilson 1902). Here, Piasecki (1980) defined two tectonostratigraphical units; the Central Highland Division, a polymetamorphic ‘basement’ complex comprising migmatized psammitic and semi-pelitic gneisses with subordinate quartzites; and the Grampian Division (or ‘Group’, sensu Harris et al. 1978), described as a structurally simpler ‘cover’ sequence of mostly non-migmatitic micaceous psammites, psammitic flags and massive quartzites. These two units were perceived as being separated by a major structural discontinuity, the Grampian Slide. Current resurvey work by the British Geological Survey (BGS) under its Monadhliath Regional Survey Project, of which the present study forms part, has identified a regional suite of metabasic rocks associated with linear belts of high strain, similar . . .

Charnockites, pyroxene granulites, and garnet-cordierite gneisses at a boundary between Early Svecofennian rocks and Småland-Värmland granitoids, Karlskoga, southern Sweden

Abstract A lobe of Early Svecofennian high-grade, metamorphic rocks surrounded and intruded by rocks of the Smaland-Varmland batholith east of Karlskoga, central southern Sweden, has been studied. Application of geothermobarometry reveals that these rocks have suffered granulite facies metamorphism at conditions constrained to 670–770°C, 4.0–4.5 kbar and aH2O0.1–0.3. The metamorphism has transformed biotite granite into charnockite, intermediate volcanite into pyroxene granulite, and lower grade presumably semipelitic gneiss into garnet-cordierite gneiss. Extensive partial melting accompanied the metamorphism in the garnet-cordierite gneisses and granulites, but not in the charnockites. The metamorphism is attributed to a local contact metamorphic peak, associated with the emplacement of the Smaland-Varmland granitoids and related mafic plutonics, in the penecontemporaneous, amphibolite facies, regional “serorogenic Svecofennian” episode.

Early Proterozoic history of the Karari Fault Zone, northwest Gawler Craton, South Australia

A 300 km-long linear aeromagnetic anomaly associated with the Karari Fault Zone defines a subvertical, Early Proterozoic, intracratonic shear zone within the Gawler Craton. The shear zone contains various mylonites and separates quartz + sapphirine-bearing granulites to the northwest from upper amphibolite facies gneisses to the southeast. The aeromagnetic anomaly is produced by magnetite-rich metasediments (iron formation), semi-pelitic gneisses and thin pegmaties which have become interlayered on all scales during deformation. All have undergone intense non-coaxial strain with development of both Type I and Type II S-C proto- to ultra-mylonite fabrics. Kinematic indicators, including asymmetrical megacryst tails, elongation lineations, vergence relationships of S-C foliations, and mica ‘fish’ indicate downthrow of the northwestern block with a sinistral strike-slip component. The shear zone represents a major zone of crustal weakness which has remained intermittently active until the Tertiary. Earlier, ...

Uranium mobility in non-oxidizing brines: field and experimental evidence

Solution, transport and precipitation mechanisms for U which are dependent on the formation of soluble uranyl U 6+ ions and complexes under oxidizing conditions and of insoluble uranous U 4+ species under reducing conditions do not adequately explain U mobility under consistently reducing conditions. Uranium deposits formed under deep-seated metamorphic conditions and some hydrothermal deposits fall into this category. This paper presents field and experimental evidence supporting the mobility of U in strongly reduced polycationic brines. The Wollaston Group of Early Proterozoic sediments in northern Saskatchewan, Canada, locally contains series of associated semi-pelitic gneiss, calc-silicate gneiss and marble which have major and trace element characteristics and identify them as having formed in an evaporative environment. Associated with the evaporative metasediments are Na-rich and Cl-rich metasomes, albitite and scapolite calc-silicate rocks, respectively, which are the products of brine-rock reactions centred on the saliferous zones. Uranium and Mo appear to have been deposited syngenetically with the primary sediments but have been redistributed during metamorphism resulting in the formation of numerous mineralized zones which have attracted considerable prospecting attention. The present distribution of U can be related to the movement of the brines as revealed in the Na Ca Mg Cl-metasomes. Experiments were conducted at 60 and 200°C under stringently non-oxidizing conditions using solvents ranging from distilled water to a Ca Na K brine formulated to simulate the major element composition of the Salton Sea geothermal brines. The experiments were conducted on natural pitchblende (UO 2.67) and synthetic uraninite (UO 2.01). Natural pitchblende was more strongly dissolved than the synthetic uraninite, and the synthetic Salton Sea brine was a more potent solvent than distilled water, 1:4 diluted Salton Sea brine, or pure NaCl brine. Within analytical limits of detection the dissolved U is present in the uranous (U 4+) state. The evidence demonstrates empirically the mechanism of dissolution of naturally occurring U minerals in reduced brines and describes a geological case where this appears to have happened. Redeposition of the U presumably occurs in zones where the uraniferous brines are diluted or experience any changes in their cationic or anionic compositions which lessen U solubility factors.

Structural evolution of the Harts Range Area and its implication for the development of the Arunta Block, central Australia

A division of the high grade metamorphics of the Arunta Block in the Harts Range Area into two main groups on the basis of lithology, structure, metamorphism and igneous activity is suggested as a result of detailed mapping. The lower group is regarded as a basement terrain and comprises major layered felsic or granitic gneiss, minor amphibolite and mafic granulite, while the structurally overlying group is regarded as a cover sequence (Irindina supracrustal assemblage) and comprises predominantly metasediments (pelitic and semi-pelitic gneiss, calc-silicate and quartzite) and amphibolite, anorthosite and ultramafic rock of the Harts Range meta-igneous complex. The cover sequence might have been deposited in an aulacogen or adjacent to a continental margin which developed by rifting of a pre-existing basement.The two groups both show early major ductile tectonothermal stages with four major fold generations and a later discrete shear event in the basement and two major fold generations and a later series of stacked nappes developing in the cover. The basement's early orogenic evolution with associated granulite facies metamorphism is typical of intensely reworked early Precambrian terrains and developed spatially and or temporally separated from the cover. Cover deformation closely resembles continental margin Cordillera type overthrusting and thus thin-skinned tectonics associated with large scale crustal shortening.During the later Harts Range remobilization, the basement and cover were juxtaposed by displacement on a fundamental crustal dislocation (recumbent contraction fault). Mylonites repeatedly developed along the dislocation which was also isoclinally folded, intruded by a major megacrystic granitoid (Bruna gneiss) sheet and repeatedly rejuvenated. The history of development of the dislocation is documented and described as a process of underthrusting of basement beneath cover which led to the final termination of the orogenic activity.

Metamorphic petrology of the northeast Front Range, Colorado: The Pingree Park area

The Pingree Park area in the northeast Front Range, Colorado, is underlain by Precambrian pelitic and semipelitic gneiss and schist that were metamorphosed about 1.75 b.y. ago, forming a distinct zonal pattern of minerals indicating an increase in grade of metamorphism to the west and north. Three isograds have been identified. The andalusite-sillimanite isograd is defined by the first occurrence of sillimanite in the prograde direction and is interpreted to mark the reaction of andalusite to sillimanite. A band as much as ∼1 km across contains both andalusite and sillimanite and represents a divariant band or reaction overstepping of 20 °C or less. The K-feldspar–sillimanite isograd separates microcline + sillimanite from prograde muscovite + quartz assemblages and is interpreted to represent the reaction (K, Na) muscovite + quartz = (K, Na) microcline + sillimanite + H 2 O. Despite the presence of significant amounts of sodium in both muscovite and microcline, the reaction shows no perceptible divariant band. There has been extensive reaction of microcline + sillimanite to retrograde muscovite + quartz. The migmatite-in isograd is defined by the first occurrence of migmatitic rocks in the prograde direction and is interpreted to represent the inception of partial melting. The isograds form an eastward-plunging set of synformal surface the south limb of which has a gentle dip and the north limb of which apparently has a much steeper dip. Based on the chemical composition of the minerals and comparison with experimentally calibrated equilibria, the conditions of metamorphism at the K-feldspar–sillimanite isograd are estimated to have been P H 2 O ≅ P total = 3 to 4 kbar, T = 650 ± 30 °C, f O 2 ≅ 10 −12 to 10 −15 bars, and f HF ≅ 1/4 to 1 bar. The geothermal gradient at the peak of metamorphism was ∼60 °C/km.

A horizontal tectonic regime in the Archaean of Greenland and its implications for early crustal thickening — a reply

Abstract The Archaean complex of Greenland consists of a layered sequence of gneisses whose history can be traced back from 2,600 m.y. to before 3,750 m.y. ago. Quartzo-feldspathic gneisses that are considered to be derived from intrusive granitic rocks make up more than 80% of the complex. They were emplaced during at least two separate periods of plutonism. The remainder of the complex consists of supracrustal rocks, mainly amphibolites and semipelitic gneisses with smaller amounts of quartzite and calcarous rocks, and metamorphosed basic igneous rocks dominated by leucogabbros and anorthosites. Lithologic units of different ages and provenances were intercalated by nappe-like folding and thrusting and were intruded by syntectonic sheets of calc-alkaline granitic rocks. The resulting sequence was repeatedly folded and deformed under high grade metamorphic conditions and the layering was further emphasised. This combination nappe-like folding, thrusting and granite injection is considered to have resulted in a considerable amount of crustal thickening. Granulite facies metamorphism ensued at depth as the base of the thickened sialic mass dried out. In this way a stable continental mass was formed with a refractory base of high grade rocks depleted in radioactive elements. It is suggested that the driving force responsible for the folding, thrusting and generation and intrusion of calc-alkaline magmas was some form of sub-horizontal movement within the mantle and between the mantle and thin crust. This dominantly horizontal tectonic regime is contrasted with the dominantly vertical tectonic regime described from the greenstone belt—granite terrain of southern Africa.

A horizontal tectonic regime in the Archaean of Greenland and its implications for early crustal thickening

The Archaean complex of Greenland consists of a layered sequence of gneisses whose history can be traced back from 2,600 m.y. to before 3,750 m.y. ago. Quartzo-feldspathic gneisses that are considered to be derived from intrusive granitic rocks make up more than 80% of the complex. They were emplaced during at least two separate periods of plutonism. The remainder of the complex consists of supracrustal rocks, mainly amphibolites and semipelitic gneisses with smaller amounts of quartzite and calcarous rocks, and metamorphosed basic igneous rocks dominated by leucogabbros and anorthosites. Lithologic units of different ages and provenances were intercalated by nappe-like folding and thrusting and were intruded by syntectonic sheets of calc-alkaline granitic rocks. The resulting sequence was repeatedly folded and deformed under high grade metamorphic conditions and the layering was further emphasised. This combination nappe-like folding, thrusting and granite injection is considered to have resulted in a considerable amount of crustal thickening. Granulite facies metamorphism ensued at depth as the base of the thickened sialic mass dried out. In this way a stable continental mass was formed with a refractory base of high grade rocks depleted in radioactive elements. It is suggested that the driving force responsible for the folding, thrusting and generation and intrusion of calc-alkaline magmas was some form of sub-horizontal movement within the mantle and between the mantle and thin crust. This dominantly horizontal tectonic regime is contrasted with the dominantly vertical tectonic regime described from the greenstone belt—granite terrain of southern Africa.

Field investigations in the Julianehåb granite on the nunataks north-west, north and east of Narssarssuaq, South Greenland

Before the compilation of the Narssarssuaq sheet (1:100 000) could be finished, about 900 km2 of high mountainous terrain in the north-western, northern and eastern parts of the sheet area had to be mapped. Important occurrences of supracrustal rocks have been found in three different places. One, north-west of Narssarssuaq, consists of interbanded ampbibolite, semipelitic gneiss and aplitic gneiss. The two other occurrences, both in the north, consist of acid and also some intermediate porphyhtic metavolcanics alternating with layers of pyrodastic rocks and aplitic gneiss.

Corundum in Biotite-Sillimanite Gneiss From near Polgahawela, Ceylon

AbstractIdiomorphic, pale blue corundum crystals (S.G. =4·02, n0=1·770 ± 0·002) up to one inch long occur as porphyroblasts in a finegrained biotite-sillimanite gneiss cut by pegmatite veins at Gangoda, 5½ miles south of Polgahawela. The field relations and petrography of the corundum-bearing rocks are described briefly. The corundum is thought to have crystallized by metamorphic differentiation in alumina-rich silica-poor bands in a semipelitic gneiss.