Charnockite Magmatism Research Articles

Hydrothermal dolomite marbles associated with charnockitic magmatism in the Proterozoic Bamble Shear Belt, south Norway

Deformed and metamorphosed dolomite marbles in the Kragero area of the Bamble Shear Belt occur as lenses within metasupracrustal sequences, as matrix in bodies resembling magmatic breccias, and as veins/dykes cutting amphibolites and metagabbros. A common origin is not evident from the field relationships, but is nevertherless probable due to great geochemical similarities between dolomite from the different occurrences. They are characterized by higher REE, Ni, Co, Cr and Sc, and lower Ba and Sr contents relative to metasedimentary marbles occurring nearby. Sm-Nd isotope data shows that the dolomites are of Sveconorwegian age (1175±37 Ma). The dolomite marbles are very weakly LREE-enriched and display in most cases positive Eu anomalies. Their stable isotope compositions are uniform (δ18O=+9.6 to +10.7‰; δ13C=-8.5 to-6.2‰), their initial 87Sr/86Sr ratios are high (0.706–0.709), whereas their age corrected ɛNd varies from +0.7 to-1.5. Geochemically the dolomitic marbles differ considerably from sedimentary or metasedimentary marbles as well as from carbonatites. The Kragero dolomite marbles represent deformed and metamorphosed hydrothermal veins or vein-complexes deposited in tensional fractures in the deep crust. Although the dolomitic marbles regionally are of minor volume, the dolomite deposition represents a specific and important event in the geological evolution of the Bamble Shear Belt. Their geochemical and isotopic homogeneity on a regional scale suggest that the hydrothermal solutions were supplied from a very large, homogeneous reservoir. Trace elements, stable and radiogenic isotopes, and field and isotopic age relationships are consistent with a deposition from hydrothermal solutions which were exsolved from crystallizing charnockitic intrusions and subsequently interacted with the crust. The parental magma to these intrusions underplated the crust and raised the geotherm of a carbonated ultramafic uppermost mantle, and imposed decarbonation and fluid release. These fluids were channelled into a large degassing zone now found as a deformed, regional zone with hydrothermal dolomite deposits, albitites, apatite-veins and widespread scapolitization. Whether the CO2-rich fluids, which precipitated the Kragero dolomites, pervasively infiltrated crustal rocks at a deeper level and caused granulitization is ambiguous, but possible.

Evidence for early fluid channelization, Pikwitonei granulite domain, Manitoba, Canada

The results of field mapping and carbon isotope and phase equilibria studies suggest that two different, locally controlled fluid regimes existed during at least the early phases of high-grade metamorphism in the north Cauchon Lake region, Pikwitonei granulite domain, Manitoba, Canada. During the prograde stages of high-grade "anticlockwise" regional metamorphism, rocks already metamorphosed to at least sillimanite grade were thermally metamorphosed at temperatures near 900 °C by the intrusion of a charnockitic magma. It is likely that this magma released an oxidizing, CO2-bearing, probably CO2-rich fluid phase while the region was still at relatively shallow depths. Fluid migration was channelized along the intrusive contact, and local fluid buffering characterized many of the country rocks. The light carbon isotope values of graphites (gr) and CO2 in cordierites (crd) in pelitic lithologies (δ13Cgr = −41.8 to −30.4; δ13Ccrd = −31.8 to −34.9), and the low oxygen fugacities in many samples rule out infiltration of these units by large amounts of an externally derived CO2-rich fluid phase. Texturally early CO2-rich fluid inclusions occur in the cores of garnets in a variety of rock types along the intrusive contact. These fluid inclusions were probably trapped during early garnet growth at high temperatures and relatively low pressures, and appear to have undergone limited or no subsequent reequilibration. They do not appear to provide direct information about the highest regional metamorphic temperature and pressure conditions to have affected the region (750 °C and 7 kbar (1 kbar = 100 MPa)) but may instead retain evidence of the prograde metamorphic path. These studies demonstrate the importance of local controls on the sources, compositions, timing, and transport of metamorphic fluids in the north Cauchon Lake region.

C-type magmas: igneous charnockites and their extrusive equivalents

ABSTRACTIgneous charnockites are characterised by distinctively high abundances of K2O, TiO2, P2O5 and LIL elements and low CaO at a given SiO2 level compared to metamorphic charnockites, and I-, S- and A-type granites. They form a distinctive type of intrusive igneous rocks, the Charnockite Magma Type (CMT or C-type), which generally lack hornblende and consist of pyroxene, alkali feldspar, plagioclase, quartz, biotite, apatite, ilmenite and titanomagnetite. Although this mineral assemblage superficially resembles that of metamorphic charnockites, magmatic charnockites are characterised by inverted pigeonite, exceptionally calcic alkali feldspar, potassic plagioclase, and coexisting opaque oxides, all with crystallisation temperatures of 950-1050°C. Apatite is a ubiquitous phase which, together with the very high concentrations of Zr and TiO2 over a wide silica range, is consistent with the derivation of the Charnockite Magma Type by very high temperature partial melting and fractionation.The credibility of intrusive charnockites as a magmatic type has historically foundered because of their apparent restriction to granulite belts and the absence of any reported extrusive equivalents. We report examples of volcanic rocks, of various ages, with the same distinctive major and trace element compositions, mineral assemblages and high temperatures of crystallisation as intrusive chamockites.The Charnockite Magma Type is considered to be derived by melting of a hornblende-free or poor, LILE-enriched fertile granulite source which had not been geochemically depleted by a previous partial melting event but which was dehydrated in an earlier metamorphism. Whereas H2O-saturated melting produces migmatites or "failed" granites, and vapour-absent melting of an amphibolite can produce I-type granites, according to this model the vapour-absent melting of a hornblende-free or hornblende-poor granulite at even higher temperatures produces charnockites.

Fluid inclusions in charnockites from the Bjerkreim-Sokndal massif (Rogaland, southwestern Norway): fluid origin and in situ evolution

Fluid inclusions and mineral associations were studied in late-stage charnockitic granites from the Bjerkreim-Sokndal lopolith (Rogaland anorthosite province). Because the magmatic and tectonic evolutions of this complex appear to be relatively simple, these rocks are a suitable case for investigation of the origin and evolution of “granulitic fluids”. Fluid inclusions, primarily contained in quartz, can be divided into four types: carbonic (type I), N2-bearing (type II), CO2+H2O (type III) and aqueous inclusions (type IV). For each type, the role of leakage and fluid mixing are discussed from microthermometric and Raman spectrometric data. The most striking features of CO2-rich inclusions (the predominant fluid) is the presence of graphite in numerous, trail-bound inclusions (Ib) and its absence in a few isolated, very dense (d=1.16), pure CO2 inclusions (Ia) and in the late carbonic inclusions (Ic). Fluid chronology and mineral assemblages suggest that carbonic Ia inclusions represent the first fluid (pure CO2) trapped at or close to magmatic conditions (T=780–830° C, fO2=10-15 atm and P=7.4±1 kb), outside the graphite stability field. In contrast, type Ib inclusions enclosed graphite particles from a channelized fluid during retrograde rock evolution (P=3–4 kb and T=600° C). Decreases in T-fO2 could explain a progressive evolution from a CO2-rich fluid to an H2O-rich fluid in a closed C−O−H system. However, graphite destabilization observed in type Ic inclusions implies some late introduction of external water during the last stage of retrogression. The main results of this study are the following: (1) a carbonic fluid was present in an early stage of rock evolution (probably in the charnockitic magma) and (2) this granulite occurrence offers good evidence of crossing the graphite stability field during post-magmatic evolution.

Sveconorwegian Granulite Facies Metamorphism of Polyphase Migmatites and Basic Dikes, South Norway

Migmatites in the Nelaug-Krossvatnet-Ubergsmoen area in the amphibolite facies part of the Bamble Sector were intruded by the Ubergsmoen Augen Gneiss (UAG) and by younger basic dikes. Both are critical time markers and show that the area underwent two phases of deformation and migmatization during the Gothian Orogeny (1600-1500 Ma) before emplacement of the augen gneiss, and four subsequent phases of deformation during the Sveconorwegian Event (1200-950 Ma), accompanied by amphibolite facies metamorphism and a minor third migmatization. Mineral assemblages in the basic dikes and in the migmatites indicate local granulite facies conditions resulting from lowered at nearly constant temperature of at 8.7 kb. It is suggested that this Sveconorwegian granulite facies overprinting was due to the introduction of fluids released during the crystallization of another charnockitic magma at a deeper level.

Age and origin of the annular charnockitic complex at Toro, Northern Nigeria: UPb and RbSr evidence

The Toro Charnockitic Complex (T.C.C.) is composed essentially of hypersthene diorite and biotite hornblende granites intrusive in a basement composed mainly of migmatites and orthogneisses. The latter have been deformed and metamorphosed during the Pan-African orogeny. UPb data from T.C.C. zircons indicate that the hypersthene diorite and the associated porphyritic granite were emplaced at 585 ± 7 Ma. and 607 ± 11 Ma. respectively. These ages disagree with previous assumptions of Kibaran of Archaean time of emplacement for this type of charnockitic complex in Nigeria. Our results place the Toro Complex precisely in the series of Pan-African granitoids. UPb geochronology and Sr isotope results neither imply different sources for the granites and the charnockites, nor suggest an entirely mantle source at 600 Ma.; the initial isotope ratios ( 87Sr/ 86Sr)i range from 0.70619 to 0.71015 for the two rock types. It is in effect a case of significant crustal contamination. We envisage the melting of lower crustal material during the Pan-African. Charnockitic magma, produced by fusion of tonalitic restite under dry granulite facies conditions, was contaminated by granitic magma under more hydrous amphibolite facies conditions at higher crustal levels; a phenomenon corroborated by the occurence of hybrid rocks at the contact between the two rock types. This model is reinforced by UPb data which implies a contemporaneous emplacement for both granites and charnockites of the Toro Complex.

Archaean and Proterozoic crustal evolution in Lofoten–Vesterålen, N Norway

The region exposes an unusually deep section through the continental crust. Gravity and seismic surveys show a ridge-like NE-SW up-warping of the Moho to within 25 km of the surface. The oldest rocks are migmatitic gneisses of generally intermediate composition, probably largely of supracrustal origin; these were intruded by a granodiorite/granite pluton at about 2600 Ma. Pb isotope data indicate that the migmatites formed c. 2700 Ma ago, and the isotopic systems in some rocks have been disturbed by Proterozoic granulite-facies metamorphism. A Proterozoic supracrustal series is composed dominantly of felsic metavolcanic gneisses but includes marble, quartzite, graphite schist and iron formations. An Rb-Sr whole-rock isochron age of 1830 ±35 Ma is interpreted as the age of a metamorphism which reached intermediate-pressure granulite facies in the western part of the area. An early low-P, high-T event was followed by PT conditions near 900°C and 10 kbar at the metamorphic maximum. This thermal peak was essentially post-tectonic and coincided with the intrusion of gabbro, anorthosite and huge volumes of mangeritic to charnockitic magma between c. 1800 and c. 1700 Ma ago. A swarm of alkali-olivine dolerites was intruded shortly after the mangerites. The Lefdingen granitic batholith gives an Rb-Sr isochron age of 1380 ±80 Ma and may represent remobilized basement rocks. Pelitic schists of the Leknes Group were tectonically emplaced against the mangerites and gneisses, and metamorphosed in amphibolite facies 1140 ±135 Ma ago. Rb-Sr and K-Ar mineral ages suggest that the extensive retrograde metamorphism observed in the older rocks occurred at this time. Small granitic pegmatites were also emplaced during this episode. The Caledonian orogeny is recorded by some K-Ar mineral ages, the development of local W-dipping thrust zones, and the local intrusion of large pegmatites. Lofoten-Vesteralen was probably part of the Baltic plate during the Caledonian orogeny, but escaped deformation because it lay at a high tectonic level and consisted largely of massive granulite-facies rocks.

Charnockitic and Related Cordierite-Bearing Rocks from Dangin, Western Australia

VI. SummaryA series of charnockitic rocks and associated cordierite-bearing rocks from Western Australia has been described and compared with charnockitic rocks from other localities. This charnockitic suite is considered to be a series of basic intrusions into an older series of paraschists. It has, in places, been considerably contaminated by assimilation of the aluminous metasediments and in such cases is now represented by cordierite-bearing rocks. The basic charnockitic rocks and their contaminated equivalents have been subsequently invaded, under deep-seated conditions by granitic magma or emanations, and now appear as recrystallized remnants of the original masses. They have suffered extensive granitization which has yielded the acid charnockites (hypersthene-alkali felspar gneisses) in which the hypersthene is considered to have been derived from the older hypersthene-bearing basic charnockites. In view of the similarity to the Indian and Antarctic charnockites it appears possible that the charnockitic rocks of these areas are of similar origin, and that charnockite (hypersthene granite) rather than being the result of crystallization from a magma, is due to the granitization of older hypersthene-bearing rocks. If charnockite is of magmatic origin then it is most probable that this magma was of palingenetic origin and that the hypersthene is a relict from the earlier rocks and not the product of crystallization of the charnockite magma. Dunn (1942, p. 23) considers that the evidence afforded by Indian rocks is not opposed to the view that the hypersthene gneisses (charnockites) are the product of partial or complete palingenesis of very deep-seated rocks which had lost their water content.