Charnockitic Rocks Research Articles

The early evolution of the Southwest Swedish Gneiss Province: Geochronological and isotopic evidence from southernmost Sweden

Abstract The continental crust in southwestern Sweden has a complicated history of formation, apparently commencing ∼ 1.75 Ga ago and continuing until the end of the Sveconorwegian orogeny ∼ 0.9 Ga ago. Most of the rocks are polymetamorphic gneisses, but massifs of intrusive rocks can commonly be recognized. This study reports new UPb zircon ages from the province of Skane in the southernmost part of the region. Two gneiss samples and four samples of granitoid rocks were dated by the UPb zircon method. In the two gneiss samples, selected single zircon crystals were analyzed in addition to the standard multi-grain size fractions. The multigrain zircon analyses yielded an upper intercept age of 1613 ± 6 Ma for a grey gneiss from the central part of the province of Skane, and 1557−27+32 Ma for a red aplitic gneiss from southern Skane. Individual zircon crystals from these samples gave 207Pb 206Pb -ages between 1335 and 1669 Ma. For the grey gneiss, the individual zircons define a separate discordia with an upper-intercept age of 1640±16 Ma; in the red gneiss sample they scatter around a line with an upper intercept at 1675±25 Ma. Core-bearing zircons from a red gneissic granite in central Skane yielded an age of 1575−61+77 Ma, while similar zircons from a strongly foliated red granite from the Kullaberg horst in northwestern Skane gave an age of 1497−34+47 Ma. Core-free zircons from the Beden granodiorite in southern Skane have an age of 1449−11+23 Ma, and core-bearing zircons from a gneissic charnockite near Orkelljunga in northern Skane yielded an age of 1452−50+350 Ma. Although the latter age is poorly defined, it allows for similar intrusion ages of the Orkelljunga charnockite and the Beden granodiorite. SmNd isotopic analyses yield TDM ages of 1.84 o 1.99 Ga, and TCHUR ages of 1.48 to 1.60 Ga for these rocks, while RbSr analyses give TUR ages between 1.46 and 1.71 Ga. The combined evidence suggests that the main crust-forming episode in this part of southwestern Sweden occurred 1.6 to 1.7 Ga ago, with presumably subduction-related magmatism forming the protoliths of the red and grey gneisses. Around 1.5 to 1.6 Ga ago red anatectic granites were generated. The Beden granodiorite and the Orkelljunga charnockite, as well as the Varberg charnockite farther north, were all formed ∼ 1.45 Ga ago, possibly by mantle-generated anorogenic magmatism. Some 0.9 Ga ago, after the Sveconorwegian crustal thickening and high-pressure metamorphism, uplift within the Southwest Swedish Gneiss Province exposed lower- to mid-crustal granulitic and charnockitic rocks in its southern part.

Response of zircon U?Pb isotopes and whole-rock geochemistry to CO2 fluid-induced granulite-facies metamorphism, Kabbaldurga, Karnataka, South India

The arrested, prograde amphibolite- to granulite-facies transition at Kabbaldurga, south India, overprints Archacan amphibolite-facies nebulitic gneisses and the late Archaean Closepet granite. Previous studies have shown that this facies transition was controlled by a channelled influx of a dehydrating fluid, assumed to be CO2, at ∼750°C and 5.5 kbar confining pressure. The effect of this type of prograde transition on zircon U−Pb isotopic systematics and whole-rock geochemistry has been studied using ∼1 kg amphibolite-facies, transitional and granulite-facies domains from a single block of gneiss. The zircon populations from all three domains have essentially similar morphology and U−Pb systematics. This similarity shows that at the conditions under which the prograde granulite-facies transition took place via fluid influx, the zircon U−Pb systematics were not disturbed by the process. Using the pooled data from all three domains, it is concluded that the protolith of the gneiss formed at 2965±4 Ma (2σ), and that zircons also grew during an anatectic event common to all domains at 2528±5 Ma. The granulite-facies metamorphism has not been dated directly due to the lack of response to the zircon U−Pb isotopic systematies to it. However, field and petrographic criteria dictate that its maximum age is 2528±5 Ma, the age of the anatectic event common to each domain in the gneiss block, which was overprinted during the granulite-facies event. For most major and trace elements, consistent enrichment or depletion trends associated with the transition to granulite facies cannot be identified with confidence. However, the granulite-facies portion is LREE (light-rare-earth-element)-enriched and H (heavy) REE-depleted compared with the amphibolite-facies domain, and the transitional domain is at intermediate values. The isotopic and geochemical evidence presented supports the conclusion that the granulite-facies charnockitic rocks at Kabbaldurga were not formed by removal of an anatectic melt, but that they formed later by simple metamorphic overprint of amphibolite-facies rocks.

The IUGS systematics of igneous rocks

In order to create a sustainable classification of igneous rocks which all geologists might use, an international body was set up by the IUGS: the IUGS Subcommission on the Systematics of Igneous Rocks. In the course of creating the classification, the Subcommission has established ten principles for its construction and for defining an appropriate nomenclature. The principles are: (1) use descriptive attributes; (2) use actual properties; (3) ensure suitability for all geologists; (4) use current terminology; (5) define boundaries of rock species; (6) keep it simple to apply; (7) follow natural relations; (8) use modal mineralogy; (9) if mode not feasible, use chemistry; (10) follow terminology of other IUGS advisory bodies. These principles and their rationale have not previously been enunciated. The classification separates and individually classifies the pyroclastic, carbonatitic, melititic, lamprophyric and charnockitic rocks before entering the main QAPF classification for plutonic and volcanic rocks which is based on the modal mineral proportions of quartz (Q), alkali feldspar (A) and plagioclase (P) or of alkali feldspar (A), plagioclase (P) and feldspathoids (F). Rocks with mafic content >90% have their own classification. If the mineral mode cannot be determined as is often the case for volcanic rocks, then a chemical classification of total alkalis versus silica (TAS) is used. The nomenclature for these classifications necessitates only 297 rock names out of the c . 1500 that exist.

Study of geology and structure of perungulathur area in thiruvannamalai sambuvarayar district, tamil nadu, using remote sensing techniques

Remote sensing technique can play a vital role in geological, geomorphological and structural studies. Geology and structure of Perungulathur area in Thiruvannamalai Sambuvarayar district was studied by using Landsat imagery and aerial photographs with limited field checks. Charnockite, which forms the country rock of the area, and the interbedded pink granite and amphibolite exhibit typical tonal and textural characters on aerial photographs. The study has brought out a partly exposed structural basin which is represented by the folded structure of amphibolite and pink granite within the charnockitic country rock.

Magnetic Anomalies and Basement Structure Around Vizianagaram, Andhra Pradesh

Abstract Magnetic anomalies in an area of about 1500 sq km around Vizianagaram town in Andhra Pradesh bring out a shallow magnetic basement of charnockitic rocks, which is highly faulted. The magnetic basement exhibits horst and graben-like structures and broadly maintains conformable relations with the antiformal and synformal structures of the overlying khondalite suite of rocks. The development of these faults is attributed to the earlier folding of the Eastern Ghats and their refolding about a N-S axis to change their initial NE-SW trend to E-W and NW-SE in the nonhern part of Andhra Pradesh. The development of the horst-graben type structure is explained by movements of the faulted blocks along the fault planes at different stages or tectonic activity.

Magnetic Anomalies and Tectonics of Gosthani River Basin in the Eastern Ghats of India

Abstract Magnetic anomalies in the Gosrhani river basin in Andhra Pradesh indicate a suite of charnockitic rocks at depth. The emplacement of these rocks is explained by folding leading to an apical graben development followed by subsidence, uplift and erosion.

Origin and metamorphic history of an Early Cretaceous polybaric granulite terrain, Fiordland, southwest New Zealand

Regionally extensive two-pyroxene granulites in Fiordland, southwest New Zealand, are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes. The granulite protolith (igneous charnockitic rock) synkinematically intruded metasediment and other orthogneiss in an Early Cretaceous subduction-related magmatic arc, and during cooling experienced deformation-induced recrystallization to form granoblastic gneiss. The granulites occur side by side with coeval rocks of amphibolite facies. Mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization: 1) an early high temperature, comparatively low pressure event accompanying magmatic intrusion (andalusite-sillimanite facies series recorded locally in the country rock), followed by 2) high pressure metamorphism under conditions of ∼650°–700° C at ∼12–13 kbar. Garnet granulite locally overprinted earlier formed two-pyroxene granulite during the latter event. The pressure increase (∼6 kbar) between the two events is attributed to crustal thickening by overthrusting, and is equivalent to unloading of a ∼20 km thick slab over rocks already buried at mid-crustal depths. Both events occurred over a < 20 m.y. interval, between the time of magmatic emplacement of the granulite protolith and uplift-controlled final cooling of the terrain. The Phanerozoic granulites in Fiordland share some petrologic similarities with Precambrian granulite terrains, suggesting that at least some aspects of the former may serve as a useful model for development of the latter.

History of Successive Deformations in Relation to Metamorphism-Migmatitic Events in the Schirmacher Hills, Queen Maud Land, East Antarctica

Abstract The basement complex in the Schirmacher Hills of East Antarctica records the impress of multiple episodes of metamorphism, migmatization and deformation. In the earliest event there was a regional metamorphism under granulite facies conditions and synkinematic migmatization leading to the development of charnockitic rocks. An amphibolite facies metamorphism was superimposed on them. This late event was closely associated with widespread granitization leading to the development of the majority of the quartzofeldspathic gneisses. In the earliest deformation, a migmatitic banding and a crude foliation formed in the charnockitic rocks. This was followed by a period of strong deformation during which two broadly coaxial sets of isoclinal folds (F2A and F2B) formed. The folding movement was also associated with widespread ductile shearing. The periods of amphibolite facies metamorphism, extensive granitization, F2 folding and the major phase of ductile shearing were overlapping. Localized pegmatite bodies were also emplaced during the later and much weaker folding movements of F3 and F4 and along some discordant ductile shear zones cutting across the axial surfaces of F2 folds.

Chemical constraints on the origin of the charnockites in the Eastern Ghat mobile belt, India

Charnockitic rocks of intrusive origin form an important lithological unit in the Eastern Ghat granulite belt, the others being khondalites, migmatites and anorthosites. The charnockitic rocks of the belt can be broadly divided into two groups based upon field relations and overall major- and trace-elemental chemistry: (1) basic granulites; and (2) charnockites. The charnockites comprise two distinct sub-groups. There is an overall increase of both K and Rb from the basic granulites to the charnockites, with few charnockites showing strong enrichment of both these elements. The two sub-groups in the charnockites can be distinguished by other elemental parameters like CaO, Y, V, Zr and FeO T. It is suggested that the protoliths for these charnockitic rocks are the fractionated products of a melt which was derived from metasomatised mantle and that a depletion event, probably coeval with granulite-facies metamorphism, has affected the entire terrain. This was followed by a metasomatic event which restored to some extent the depleted large-ion lithophile elements (LILE) to different levels in the different litho-units. The available rare-earth element (REE) data on these rocks support this mode of origin. Strong enrichment of K and Rb in one group of the charnockites indicates that they were subjected to post-metamorphic migmatisation/anatexis processes coupled with metasomatic processes, resulting in the abnormal K and Rb enrichment. The occurrence of numerous pegmatitic veins and migmatic gneisses in close proximity to these charnockites supports this conclusion.

Zircon in charnockitic rocks from rogaland (southwest Norway): Petrogenetic implications

Zircons separated from monzonoritic rocks and quartz mangerites (igneous and metamorphic) have been studied for various mineralogical, chemical and physical properties (colour, zoning, presence of core, U distribution, morphology, lattice parameters, thermoluminescence) to determine the relationships with chemical composition of the host rocks and geological setting. The zircons grade from nearly perfectly crystalline to slightly metamict types. The crystal morphology shows prismatic habits and a striking lack of well-developed pyramidal faces, possibly indicating growth in a dry formation environment. The zircons can be divided into two groups. The first one comprises zircons from monzonoritic rocks. They are unzoned, devoid of brownish core, with a uniform U distribution, and low U content (except in Hidra). The second group comprises zircons from igneous and metamorphic quartz mangerites; they contain brownish, zoned, U-rich cores. In rocks where temperature of crystallization can be determined, Watson and Harrison's (1983) experimental data indicate strong zircon saturation. The cores are considered as inherited, a fact that can be accounted for either by dry anatexis or bulk contamination. In the Bjerkreim-Sokndal layered lopolith, previous isotopic data are confirmed, which indicate that the acidic upper part of the massif is not comagmatic with the anorthosito-noritic lower part and the monzonoritic rocks.

Determination of fluorine by proton-induced gamma-ray emission (PIGE) spectrometry in igneous and metamorphic charnockitic rocks from Rogaland (S. W. Norway)

More than 200 specimens from different occurrences of the Rogaland igneous complex and surrounding granulite facies metamorphic rocks (S. W. Norway) have been analysed by a direct non-destructive proton induced gamma-ray emission (PIGE) technique. The fluorine contents vary from <25 ppm to 3500 ppm. There is a good correlation between the concentration of fluorine and that of phosphorus for igneous rocks, suggesting a control of apatite on the F content. In metamorphic rocks, amphibole and biotite besides apatite are the principal cencentrators of fluorine indicating that fluorine in the system is controlled by granulite facies metamorphism conditions.

Charnockite-granite association in SW Nigeria: rapakivi granite type and charnockitic plutonism in Nigeria?

A plutonic complex containing both charnockitic and non-charnockitic granite rocks (Older Granites) occurs within the amplhibolite facies rocks of gneisses and migmatites in the Ado Ekiti-Akure region of southwestern Nigeria. This complex comprises three petrographic types of charnockitic rocks and three of granitic rocks. These are the coarse-grained charnockitic variety, massive fine-grained and the gneissic fine-grained types, while the granitic rocks consist of the fine-grained biotite granite, medium- to coarse-grained and the porphyritic biotite-hornblende granites. Field observation shows remarkable close association of these charnockitic and non-charnockitic components of the complex, and also geochemical evidence provides indications for a petrogenetic link between the rocks. The coarse charnockitic rock type and the granitic rocks in the area have high K 2O levels relative to SiO 2, high K 2O/Na 2O and high FeO/MgO ratios. There are striking chemical similarities which also characterize rocks of the rapakivi suite. Comparable petrogenetic processes are therefore thought to have been in operation, and the granites of the area are linked to rapakivi granite types which could be a product of charnockitic plutonism. All the above features are reminiscent of the rapakivi granite-massive anorthosite-charnockitic rock series, the close association of whic is well documented in some parts of the world. On the basis of trace element geochemistry, the charnockitic rocks are divided into two groups which are the ‘normal’ -LIL (large-ion-lithophile) and LIL-deficient types. The ‘normal’ -LIL type is represented by the coarse-grained charnockitic type while the LIL-deficient type is the massive fine-grained variety. The LIL-deficient variety has low REE while the ‘normal’ -LIL charnockitic rock type and the granites are enriched in REE and exhibit fractionated patterns. Both the LIL and REE patterns are consistent with fractionation processes involving separation of LIL-deficient phases from a basic magma emplaced under high grade conditions, and the ‘normal’ -LIL rock type with the granites represent equivalents of rapakivi granites that crystallized from the residual melt at higher structural levels.

Genetic implications of Pb isotopic geochemistry in the Rogaland anorthositic complex (southwest Norway)

110 samples from different massifs of the Rogaland igneous complex have been analyzed for whole-rock Pb isotopic compositions. These data show no evidence of rocks older than 1500 Ma. This is probably the time of extraction of the magmatic precursors from the mantle. The Pb data indicate a mantle origin for the parental magmas of the massiftype anorthosites and of the jotunitic series. Nevertheless, an origin by partial melting of mantle-derived basic material in the lower crust cannot be excluded, especially for the latter. The charnockitic rocks associated with the anorthosites could either be anatectic melts of the enclosing gneisses or residual liquids of anorthosite differentiation contaminated by slightly U-depleted lower-crustal material (although, in some cases, upper-crustal material has also contributed to the contamination).

Charnockites and the genesis of gem minerals

Charnockitic rocks that occur in many Proterozoic high-grade metamorphic terrains correspond to regions of crustal thickening and are often associated with continental collision processes. The charnockites of the Highland Group of Sri Lanka represent this type and show genetic relationships to the central High-land Group. Geochemical investigations indicate that charnockites play a key role in the formation of the gem minerals. It seems probable that the contact metamorphic effects of charnockitic plutonic activity in the highly aluminous metasediments such as those found in Sri Lanka result in the formation of an assemblage of gem minerals such as sapphirine, cordierite, spinel, ruby and corundum. The pegmatites with which gem minerals like beryl, chrysoberyl and tourmaline are associated can also be considered as being derived from a charnockitic parent.

REE geochemistry of early Precambrian charnockites and tonalitic-granodioritic gneisses of the Qianan Region, eastern Hebei, North China

The charnockites and gneisses from the Qianan area, eastern Hebei Province, China, were investigated for their isotope and trace element abundances.The majority of the charnockites are acid in composition, only a few samples have a composition corresponding to intermediate to basic rocks. A Rb-Sr whole-rock isochron of the charnockites gives an age of 2.65 ± 0.05 Ga. The highly fractionated REE patterns and the variable (Ce/Yb)N ratios in these rocks are interpreted as being a result of igneous processes. It is believed that the charnockitic rocks are related to each other by fractional crystallization under high temperatures and low PH2O conditions; the major precipitating phases are thought to be orthopyroxene and plagioclase.By contrast, the Qianan gneisses are mainly of tonalitic and granodioritic composition. However, a few samples have composition corresponding to granite (s.s.), probably produced by metasomatic processes. A Rb-Sr whole-rock isochron of the Qianan gneisses yields an age of 2.13 ± 0.06 Ga and this age probably represents a remobilization event. The Qianan gneisses do not contain hypersthene; they have strongly fractionated REE patterns similar to those found in many Archaean tonalitic rocks of the world. In addition, the total REE abundances and the magnitudes of the Eu anomaly show regular variation with the SiO2 content. This may suggest that the Qianan gneisses might have evolved under hydrous conditions with hornblende exerting a major control on their trace element chemistry.

Evidence for fluid pathways through Archaean crust and the generation of the Closepet granite, Karnataka, South India

In the Archaean amphibolite—granulite facies transition zone of southern Karnataka anatexis of amphibolite facies Peninsular gneiss occurred. Based on cross-cutting relationships this melting event, which produced the Closepet granite, was contemporaneous with the formation of charnockitic rocks. This formation of charnockite took place initially due to channelized CO2-rich volatiles moving along an anastamosing network of shears and micro-fissures. These fluid pathways are now represented by brown veins of charnockite. Later, the fluid became more pervasive and resulted in areas of total inversion to charnockite. A lateral change along the fluid pathways from charnockite through incipient charnockite to either a thin granitic vein or a vein of reddening, can be traced through the country rocks. These different effects along the same, laterally continuous, vein are interpreted to represent the lateral change of volatile composition which, in front of the CO2-rich fluids, were increasingly H2O-dominated.

Thermometry and barometry of some amphibolite–granulite facies rocks from the Otter Lake area, southern Quebec

Grenville rocks from a 2500 km2 area centered on Otter Lake, Quebec (some 75 km northwest of Ottawa) are in the uppermost amphibolite to lower granulite facies; orthopyroxene occurs occasionally in both metabasic and charnockitic rocks. The temperature of metapmorphism was approximately 675 °C, based upon oxide, feldspar, and garnet–clinopyroxene thermometry. Little thermal gradient could be detected across the area. Carbonate thermometry, using reintegrated calcite compositions, yielded lower temperatures of 600 °C (maximum), while garnet–biotite and other Kd thermometers yielded scattered and for the most part unreasonable results. Metamorphic pressure, calculated from the reaction anorthite = grossular + sillimanite + quartz, was 5.0 ± 0.5 kbar(500 ± 50 MPa). Similar calculations based upon the reactions garnet + quartz = anorthite + orthopyroxene and garnet + quartz = anorthite + clinopyroxene yielded pressures of 5.5–7.0 kbar (550–700 MPa). Pressure calculations based upon assemblages of cordierite–garnet–sillimanite–quartz were less precise, but agreed with the outer estimates. Similar metamorphic temperatures and slightly lower pressures have been estimated for the Adirondack Lowlands of New York. In the Morin Highlands, 100 km east of Otter Lake, and in the Adirondack Highlands, 100 km east of the Adirondack Lowlands, temperatures of metamorphism (700–800 °C) and pressures of metamorphism (6–9 kbar (600–900 MPa)) are both higher. Thus it appears that over an approximate 300 km north–south direction nearly constant metamorphic conditions prevailed at Grenville time. In the east–west direction significant variations in metamorphic grade are recorded; both temperature and pressure markedly increase to the east.

Rare-earth element fractionation and petrogenetic modelling in charnockitic rocks, southwest Norway

Quantitative analysis of REE distribution in differentiated, clearly intrusive charnockites from southwest Norway, reveal extreme variation in both absolute abundances and fractionation patterns. The pyroxene-bearing, charnockitic facies show uniform REE patterns with slight enrichment of the light-REE (about 150x chondrites) and positive, neutral or negative Eu-anomalies. Subsequent amphibole-bearing, adamellitic facies show progressive, preferential enrichment of light-REE, reaching La-values higher than 500x chondrites, and increasingly negative Euanomalies. Finally, highly differentiated biotite-granites show a marked depletion of ight-REE, ending with chondrite-normalized La/Lu ratios about 1 and Eu/ Eu* ratios less than 0.2.

Stark Complex (Dexter Lake area): Petrology, chemistry, structure, and relation to other green rock complexes and layered gneisses, northern Adirondacks, New York: Summary

INTRODUCTION The origin of green rock complexes in the Adirondacks, their interrelations, and their relations to other Precambrian rock types are obscure. These green rocks have been called syenites (Smyth, 1897), older granites (Buddington, 1939), and charnockites (Berry and Walton, 1962). Buddington (1939), after working in the northern Adirondacks, thought that anorthosite and two generations of granite intruded the Grenville sequence of sediments. De Waard and Walton (1967), after working in the southern Adirondacks, thought that the sequence of sediments was deposited on meta-anorthosite and charnockitic gneiss. Buddington thought that all granitic and charnockitic rocks in the Adirondacks were intruded. De Waard and Walton thought that some belonged to the basement complex upon which the Grenville sediments were deposited, some were initially deposited as Grenville sediments, and some were produced by local melting. The diverse characters and occurrences of granitic and charnockitic gneisses in the Adirondacks show that these rocks formed by several processes. Recent field studies have investigated two green rock complexes: the Tupper Complex (Crosby, 1968; Davis, 1969; Seifert, 1978) and the Diana Complex (Hargraves, 1969; Wiener, 1979). These studies have generally supported Buddington9s conclusions. The Stark Complex is the only green rock complex in the northern Adirondacks that has not been examined since de Waard and Walton published their interpretation of Adirondack geology. Rocks around Jennings Mountain, Lyon Mountain, Taylor Pond, and Loon Lake and between Lake Titus and Blue Mountain are part of the Stark Complex. This paper presents the results of a study on the Stark Complex around Dexter Lake and relates that complex to other green rock complexes, the Grenville sequence, and granitic gneisses that Buddington (1939, p. 135–149) labeled the younger granites.

Some charnockitic rocks in the Nagssugtoqidian of West Greenland

Several charnockite bodies occur in the Precambrian gneiss terrain between Nordre Strømfjord and Agto, West Greenland. The country rocks are mainly enderbitic gneisses, granodioritic gneisses and almandine-sillimanite bearing schists. The largest charnockite body forms the mountain of Kingigtoq between Nordre Strømfjord and Gieseckes Sø. The body has been deformed by at least two fold phases prior to the sinistral simple shear of the Nordre Strømfjord shear zone. Most parts have a gneissose structure, but undisturbed porphyritic textures also occur. The main rock types are charnockite, quartz mangerite and quartz jotunite. The charnockites in the Agto area may be of anorthosite kindred because of chemical similarities: Fe tot./Fe tot. + MgO is about 0.9, average K/Rb is 358, and K2O/SiO2 is high. Average U is 0.52 ppm and average Th is 11.0 ppm. Metamorphism subsequent to intrusion has led to assemblages with ubiquitous gamet and hornblende, commonly accompanied by ortho- and clinopyroxene. Gamet and clinopyroxene stably coexist in felsic charnockites in the Agto archipelago, but on Kingigtoq only in mafic members of the suite. Element distributions indicate an equilibration temperature of 750°C - 850°C, at 8-10 kb solid pressure.