Genesis of porphyry and plutonic mineralisation systems in metaluminous granitoids of the Grampian Terrane, Scotland

1994 ◽  
Vol 85 (3) ◽  
pp. 221-237 ◽  
Author(s):  
David Lowry ◽  
Adrian J. Boyce ◽  
Anthony E. Fallick ◽  
W. Edryd Stephens
Keyword(s):  
Source Region ◽  
Crustal Contamination ◽  
Magma Ascent ◽  
Magmatic Fluid ◽  
Magmatic System ◽  
Low Salinity ◽  
Deep Faults ◽  
Supergene Enrichment ◽  
Propylitic Alteration ◽  
O 10

AbstractMineralisation associated with Late Caledonian metaluminous granitoids in the Grampian Terrane has been investigated using stable isotope, fluid inclusion and mineralogical techniques.A porphyry-stock-related style of mineralisation in the Grampian Terrane is characterised by a stockwork of veinlets and disseminations in dacite prophyries, consisting of quartz, dolomite, sulphides and late calcite, and well-developed wallrock alteration dominated by zones of phyllic, sericitic and propylitic alteration. On the basis of δ34S (+0·4±l·0‰), δ13C (−5·7‰ to + l·4‰) and δ18O (+10·8‰ to +19·9‰) it is likely that initial mineralising components were orthomagmatic with an input of external fluids during the later parageneses. Fluids were saline, boiling (up to 560°C), deficient in CO2, and ore deposition took place at depths of less than 3 km.Plutonic-hosted mineralisation in appinites, diorites, tonalites and monzogranites is commonly represented by sporadic disseminations and occasional veins consisting of quartz, calcite and sulphides. Wallrock alteration is generally propylitic with phyllic vein selvages. Deposition from a cooling magma sourced fluid is indicated by δ34S (+2·6±l·5‰), δ13C (−7·2‰ to −4·5‰) and δ18O (+9·5‰ to + ll·8‰) data. Fluids were CO2-rich and of low salinity; inclusions were trapped below ≈460°C, and formed at estimated depths of 3–5 km.Differences between these styles of mineralisation may due to multiple factors, the most important being the nature of the fluid: porphyry systems are dominated by greater volumes and much higher temperatures of hydrothermal fluids. Other controlling factors are likely to be the compositional characteristics of the melt source region, the mechanism of magma ascent, the level of emplacement, and the nature of the host metasediments. Variations in δ34S between the two groups are related, for the most part, to redox processes during magma and fluid genesis and not by crustal contamination.Nolarge porphyry-related mineral deposits have been found in the Grampian Terrane, unlike those in Mesozoic and Tertiary continental margin environments. This is largely due to a combination of detrimental factors which massively reducesthe probability of economic mineralisation. These include the already metamorphosed nature of the host Dalradian, the absence of seawater (which entered many subduction-related magmatic systems), a poorly-developed system of deep faults (most deposits too deep to be influenced by surface-derived fluids), and the absence of supergene enrichment. The main processes which aid the concentration of mineralisation involve encroachment of external fluids (formation, meteoric and seawaters) into the magmatic system, but these fluids were largely absent from the Grampian host block at the time of granitoid intrusion.The results of this study can be used in characterising the sources of fluids in sedimentary-hosted ore veins known (or considered) to be underlain by metaluminous granitoid batholiths, particularly in estimating the degree of magmatic fluid inputs into the vein systems: an example where this interaction has occurred (the Tyndrum Fault Zone) is discussed.

On the volcanic architecture, petrology and geodynamic setting of the 3.48 Ga Barberton komatiite suite, South Africa

2021 ◽  
Author(s):  
E.G. Grosch ◽  
J. Slama
Keyword(s):  
South Africa ◽  
Mantle Plume ◽  
Source Region ◽  
Crustal Contamination ◽  
Primitive Mantle ◽  
Geochemical Data ◽  
Geodynamic Setting ◽  
Crustal Material ◽  
Type Section ◽  
Formation Type

Abstract This study presents new field and petrological observations combined with geochemical data on a range of komatiitic to tholeiitic volcanic rocks from the ca. 3.48 Ga mid-lower Komati Formation type-section of the Barberton Greenstone Belt, South Africa. A range of mafic-ultramafic rocks is identified across a 1.44 km profile, leading to the proposition of a new preliminary volcanic architecture for the mid-lower Komati Formation type-section. Major, trace and rare earth element (REE) data in conjunction with Lu-Hf isotopic constraints indicate that the tholeiites, newly recognized high-magnesium basalts, basaltic komatiites and komatiites in the volcanic sequence have a primitive mantle signature with no geochemical affinity to Archaean or modern-day supra-subduction zone boninites. The whole rock initial εHf values of spinifex and massive komatiite flows in the lowermost part of the Komati type-section are negative, ranging between -1.9 and -3.1, whereas the second overlying spinifex and massive flow unit records positive initial εHf values between +0.5 and +4.7. A new geodynamic model involving crustal contamination of the mafic-ultramafic lavas is proposed for the Barberton mid-lower Komati Formation type-section, involving mantle plume-crust interaction. The new observations and data indicate that the komatiites erupted as a result of a mantle plume from a hot (>1 600oC) mid-Archaean mantle, in which the earliest volcanic flows were variably affected by crustal contamination during their ascent and eruption. The possibility of incorporation of lower crustal material and/or recycled crust residing in the mantle source region cannot be excluded. This indicates that modern-style plate tectonic processes, such as subduction, may not have been a requirement for the formation of the 3.48 Ga Barberton komatiite suite, with implications for the hydration state, geodynamic processes and secular thermal evolution of the Archaean mantle.

Mafic and ultrapotassic rocks from the Canyon domain (central Grenville Province): geochemistry and tectonic implications

2012 ◽  
Vol 49 (2) ◽  
pp. 412-433 ◽  
Author(s):  
Carolina Valverde Cardenas ◽  
Aphrodite Indares ◽  
George Jenner
Keyword(s):  
Tectonic Setting ◽  
Source Region ◽  
Crustal Contamination ◽  
Granulite Facies ◽  
Grenville Province ◽  
Mafic Rocks ◽  
Isotopic Signatures ◽  
Late Evolution ◽  
Grenvillian Orogeny ◽  
Laurentian Margin

The Canyon domain and the Banded complex in the Manicouagan area of the Grenville Province preserve a record of magmatic activity from ∼1.4 to 1 Ga. This study focuses on 1.4–1.2 Ga mafic rocks and 1 Ga ultrapotassic dykes. Geochemistry and Sm–Nd isotopic signatures were used to constrain the origin of these rocks and evaluate the changing role of the mantle with time and tectonic setting from the late evolution of the Laurentian margin to the Grenvillian orogeny, in the Manicouagan area. The mafic rocks include layers inferred to represent flows, homogeneous bodies in mafic migmatite, and deformed dykes, all of which were recrystallized under granulite-facies conditions during the Grenvillian orogeny. In spite of the complexities inherent in these deformed and metamorphosed mafic rocks, we were able to recognize suites with distinctive geochemical and isotopic signatures. Integration of this data along with available ages is consistent with a 1.4 Ga continental arc cut by 1.2 Ga non-arc basalts derived from depleted asthenospheric mantle, with varied degrees of crustal contamination and inferred to represent magmatism in an extensional environment. The 1 Ga ultrapotassic dykes postdate the Grenvillian metamorphism. They are extremely enriched in incompatible elements, have negative Nb anomalies, relatively unradiogenic Sr-isotopic compositions (initial 87Sr/86Sr ~ 0.7040) and εNd –3 to –15. Some dykes have compositional characteristics consistent with derivation from the mantle, ruling out crustal contamination as a major process in their petrogenesis. The most likely source region for the ultrapotassic dykes is a metasomatized subcontinental lithospheric mantle, with thermal input from the asthenosphere in association with post-orogenic delamination.

scholarly journals The tholeiites of the Valaisan domain (Versoyen, western Alps): a Carboniferous magma emplaced in a small oceanic basin

2008 ◽  
Vol 179 (4) ◽  
pp. 357-368 ◽  
Author(s):  
Jean-Louis Mugnier ◽  
Sebastien Cannic ◽  
Henriette Lapierre
Keyword(s):  
Crustal Contamination ◽  
Western Alps ◽  
Black Shales ◽  
Unconsolidated Sediments ◽  
Magma Ascent ◽  
Tholeiitic Magma ◽  
Oceanic Basin ◽  
Crustal Thinning ◽  
The Alps ◽  
Ree Patterns

Abstract The mafic-ultramafic assemblages of the Versoyen complex exposed in the Valaisan domain is close to the boundary between the Internal and the External domains of the western Alps. Zircons extracted from the Versoyen complex suggest an emplacement during Paleozoic times, and probably during the Visean (~337 Ma). The base of the Versoyen complex is formed of laccoliths and sills associated with black shales, while pillow basalts and tuffs predominate at the uppermost levels. Locally, basaltic dikelets intruded leucocratic gneiss. Ultramafic-mafic cumulates form the bottom of the thickest intrusions while diabases are present along the chilled margins. All these rocks have been affected by a polyphased metamorphism under eclogitic to blueschist and greenschist facies conditions. Magmatic textures have been destroyed and the igneous mineralogy is seldom preserved. The mafic rocks of the Versoyen complex show tholeiitic to alkali-transitional affinities. The pillow basalts and the sill cores have flat REE patterns characteristic of N-MORB and T-MORB. Their εNd (assuming an age of 337 Ma) ratios range from + 5.7 to + 9 which suggest a mixing of N-MORB and OIB sources. The sill margins show Th, U and LREE-enrichments and negative εNd ratios. These features are likely related to contamination when hot mafic magmas intruded unconsolidated sediments rich in water. The high Th, U, LREE abundances and low εNd ratio of the basaltic dikelet are probably related to crustal contamination occurring during magma ascent. The geochemical characteristics of the Versoyen rocks are compatible with a tholeiitic magma emplaced into a small oceanic basin in the vicinity of a continent. The importance of pre-Mesozoic crustal thinning evidenced in one segment of the boundary between the Internal and External zones of the Alps suggests that the Pennine Front is an Alpine mega-thrust inherited from a Variscan suture.

Petrology and geochemistry of volcanic rocks of the Cerro Galan caldera, northwest Argentina

1989 ◽  
Vol 126 (5) ◽  
pp. 515-547 ◽  
Author(s):  
P. W. Francis ◽  
R. S. J. Sparks ◽  
C. J. Hawkesworth ◽  
R. S. Thorpe ◽  
D. M. Pyle ◽  
...  
Keyword(s):  
Fractional Crystallization ◽  
Source Region ◽  
Crustal Contamination ◽  
Crustal Material ◽  
Magma Chambers ◽  
Northwest Argentina ◽  
Incompatible Elements ◽  
High K ◽  
Uniform Region ◽  
Compositional Zonation

AbstractAt least 2000 km3 of relatively uniform dacitic magma have been erupted from the Cerro Galan caldera complex, northwest Argentina. Between 7 and 4 Ma ago several composite volcanoes predominantly of dacitic lava were constructed, and several large high-K dacitic ignimbrites were erupted. 2.2 Ma ago the > 1000km3 Cerro Galan ignimbrite was erupted. The predominant mineral assemblage in the ignimbrites is plagioclase-biotite-quartz-magnetite-ilmenite; the Cerro Galan ignimbrite also contains sanidine. Fe-Ti oxide minerals in the Cerro Galan ignimbrite imply temperatures of 801–816 °C. Plagioclase phenocrysts in the ignimbrites typically have rather homogeneous cores surrounded by complex, often oscillatory zoned, rims. Core compositions show a marked bimodality, with one population consisting of calcic cores surrounded by normally zoned rims, and a second of sodic cores surrounded by reversely zoned rims. The older ignimbrites do not show systematic compositional zonation, but the Cerro Galan ignimbrite exhibits small variations in major elements (66–69% SiO2) and significant variations in Rb, Sr, Ba, Th and other trace elements, consistent with derivation from a weakly zoned magma chamber, in which limited fractional crystallization occurred. The ignimbrites have 87Sr/86Sr = 0.7108–0.7181; 143Nd/144Nd = 0.51215–0.51225, and δ18O = + 10 to + 12.5, consistent with a significant component of relatively non-radiogenic crust with high Rb/Sr and enriched in incompatible elements. Nd model ages for the source region are about 1.24 Ga. 87Sr/86Sr measurements of separated plagioclases indicate that Anrich cores have slightly lower 87Sr/86Sr than less calcic plagioclases, suggesting a small degree of isotopic heterogeniety in different components within the magmas. Pb isotope data for plagioclase show restricted ranges (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb = 18.87–18.92, 15.65–15.69 and 39.06–39.16 respectively), and suggest derivation from Proterozoic crustal material(> 1.5 Ga).Contemporaneous satellite scoria cones and lavas are high-K basalts, basaltic andesites and andesites with SiO2 = 51–57%; K2O = 2–3% and normative plagioclase compositions of An37–48, and may be derived from a mantle source containing both ‘subduction zone’ and ‘within plate’ components. 87Sr/86Sr ranges from 0.7055 to 0.7094 and 143Nd/144Nd from 0.51250 to 0.51290. Variation diagrams such as MgO: SiO2 show two trends, one indicating closed system fractional crystallization and the other crustal contamination. AFC modelling of the open system rocks indicates a parental mantle-derived mafic magma which is itself enriched in K, Rb, Ba, U, Ta/Sm, Ta/Th and Sr, and has 87Sr/86Sr = 0.705–0.706, while the contaminant need not be more radiogenic than the dacitic ignimbrites.The Cerro Galan dacitic magmas are interpreted in terms of a deep and uniform region of the central Andean continental crust repeatedly melted by emplacement of incompatible-element-enriched, mantle-derived mafic magmas, a proportion of which may also have mixed with the dacite magmas. A component of the crustal material had a Proterozoic age. The magmas derived by crustal melting were also enriched in incompatible elements either by crystal/liquid fractionation processes, or by metasomatism of their source regions just prior to magma generation. Much of the crystallization took place in the source region during the melting process or in mid-crustal magma chambers. The magmas may have re-equilibrated at shallow levels prior to eruption, but only limited compositional zonation developed in high-level magma chambers.

Boron and lithium isotopic compositions as provenance indicators of Cu-bearing tourmalines

2010 ◽  
Vol 74 (2) ◽  
pp. 241-255 ◽  
Author(s):  
B. M. Shabaga ◽  
M. Fayek ◽  
F. C. Hawthorne
Keyword(s):  
Late Stage ◽  
Isotope Fractionation ◽  
Large Range ◽  
Source Region ◽  
Magma Ascent ◽  
Magma Source ◽  
B Values ◽  
Granite Pegmatite ◽  
Isotopic Compositions ◽  
Li Isotope

AbstractThe Li and B isotopic compositions of gem-quality Cu-bearing tourmalines were used (1) to distinguish among Paraiba tourmalines from Brazil and Cu-bearing tourmalines from Nigeria and Mozambique; and (2) to identify the likely source of Li and B for these gem-quality tourmalines. The δ11B values of tourmaline from Paraiba, Brazil, range from –42.4‰ to –32.9‰, whereas the δ11B values of Cu-bearing tourmaline from Nigeria and Mozambique range from –30.5‰ to –22.7‰ and –20.8‰ to –19.1‰ respectively. Tourmalines from each locality have relatively homogeneous δ11B values and display no overlap. There is slight overlap between δ7Li values of Paraiba tourmaline (+24.5‰ to +32.9‰) and Cu-bearing tourmaline from Nigeria (+32.4‰ to +35.4‰), and δ7Li values of Cu-bearing tourmaline from Nigeria and Mozambique (+31.5‰ to +46.8‰). Nevertheless, Cu-bearing tourmalines from each locality can be fingerprinted using a combination of their δ11B and δ7Li values. The very small δ11B values are consistent with a non-marine evaporite source, and are among the smallest reported for magmatic systems, expanding the global range of B isotopicc omposition for tourmaline by 12‰. The corresponding large δ7Li values are among the largest reported, although they are less diagnostic of the source of the Li. The large δ7Li values in conjunction with the small δ11B values suggest a non-marine evaporite or brine as a source for Li and B, either as constituent(s) of the magma source region or, by assimilation during magma ascent. The large range in δ11B and δ7Li values suggests that B and Li isotope fractionation occurred during magmatic degassing and late-stage magmatic-hydrothermal evolution of the granite-pegmatite system.

scholarly journals Preliminary Fluid Geochemical Survey in Tete Province and Prospective Development of Geothermics in Mozambique

2021 ◽  
Author(s):  
Monia Procesi ◽  
L. Marini ◽  
D. Cinti ◽  
A. Sciarra ◽  
P. Basile ◽  
...  
Keyword(s):  
Geothermal Energy ◽  
Circulation Model ◽  
Thermal Springs ◽  
Geothermal System ◽  
Reservoir Temperature ◽  
Magmatic System ◽  
Isotopic Data ◽  
Prospective Development ◽  
Geochemical Characterization ◽  
Low Salinity

Abstract An evaluation of the feasible development of geothermal energy in Mozambique is proposed based on some thermal springs geochemical characterization in the Tete region. Chemical and isotopic data suggest that the springs have a meteoric origin and do not show connection with any active magmatic system. The proposed circulation model suggests high depths infiltration of meteoric waters along faults and fractures in a system characterised by discrete permeability and reservoir temperature between 90 and 120°C. These results, jointly with low salinity fluids and corrosive components absence suggest that the geothermal system may be conveniently exploited for direct and indirect uses.

δD and δ18O variations of the magmatic system beneath Deception Island volcano (Antarctica): Implications for magma ascent and eruption forecasting

2020 ◽  
Vol 542 ◽  
pp. 119595 ◽  
Author(s):  
A.M. Álvarez-Valero ◽  
G. Gisbert ◽  
M. Aulinas ◽  
A. Geyer ◽  
G. Kereszturi ◽  
...  
Keyword(s):  
Magma Ascent ◽  
Deception Island ◽  
Magmatic System ◽  
Eruption Forecasting ◽  
Δd And Δ18o

scholarly journals Magmatic Fluids Implicated in the Formation of Propylitic Alteration: Oxygen, Hydrogen, and Strontium Isotope Constraints from the Northparkes Porphyry Cu-Au District, New South Wales, Australia

2020 ◽  
Vol 115 (4) ◽  
pp. 729-748 ◽  
Author(s):  
Adam Pacey ◽  
Jamie J. Wilkinson ◽  
Adrian J. Boyce ◽  
Ian L. Millar
Keyword(s):  
Hydrogen Isotope ◽  
Isotope Composition ◽  
New South ◽  
New South Wales ◽  
Strontium Isotope ◽  
Magmatic Fluid ◽  
Porphyry Cu ◽  
South Wales ◽  
Magmatic Fluids ◽  
Propylitic Alteration

Abstract In porphyry ore deposit models, the propylitic alteration facies is widely interpreted to be caused by convective circulation of meteoric waters. However, recent field-based and geochemical data suggest that magmatic-derived fluids are likely to contribute to development of the propylitic assemblage. In order to test this hypothesis, we determined the oxygen and hydrogen isotope compositions of propylitic mineral separates (epidote, chlorite, and quartz), selected potassic mineral separates (quartz and magnetite), and quartz-hosted fluid inclusions from around the E48 and E26 deposits in the Northparkes porphyry Cu-Au district, New South Wales, Australia. In addition, the strontium isotope composition of epidote was determined to test for the potential contribution of seawater in the Northparkes system given the postulated island-arc setting and submarine character of some country rocks. Oxygen isotope geothermometry calculations indicate potassic alteration occurred between ~600° and 700°C in magmatic/mineralized centers, persisting to ~450°C upon lateral transition into propylitic alteration. Across the propylitic facies, temperature progressively decreased outward to <250°C. These temperature estimates and additional data from chlorite geothermometry were utilized to calculate the oxygen and hydrogen isotope composition of the fluid in equilibrium with the sampled minerals. Results show that propylitic fluids spanned a range of compositions with δ18O between 0.5 and 3.7‰ and δD between –49 and –17‰. Comparison of these results with the modeled compositions of meteoric and/or magmatic fluids during their evolution and isotopic exchange with local country rocks shows that a magmatic fluid component must exist across the propylitic halo during its formation. Strontium isotope data from propylitic epidote provide initial (based on formation at ~450 Ma) 87Sr/86Sr values in the range of 0.704099 to 0.704354, ruling out the presence of seawater as a second fluid in the system. Although we cannot exclude magmatic-meteoric mixing, especially toward the fringes of the system, our results support a model in which magmatic-derived fluid is the primary driver of propylitic alteration as it undergoes cooling and chemical equilibration during outward infiltration into country rocks. This is consistent with chemical mass transfer calculations for Northparkes and published chemical-thermodynamic models that only require a magmatic fluid for the production of propylitic assemblages. In view of this and supporting data from other deposits, we suggest that magmatic fluids are essential drivers of propylitic alteration in porphyry systems.

scholarly journals The Passage of the Bomb Radiocarbon Pulse into the Pacific Ocean

Radiocarbon ◽  
2010 ◽  
Vol 52 (3) ◽  
pp. 1182-1190 ◽  
Author(s):  
William J Jenkins ◽  
Kathryn L Elder ◽  
Ann P McNichol ◽  
Karl von Reden
Keyword(s):  
Water Mass ◽  
Source Region ◽  
South Pacific ◽  
Intermediate Water ◽  
Positive Anomaly ◽  
Low Salinity ◽  
Difference Field ◽  
The Pacific ◽  
The Pacific Ocean ◽  
The Antarctic

We report and compare radiocarbon observations made on 2 meridional oceanographic sections along 150°W in the South Pacific in 1991 and 2005. The distributions reflect the progressive penetration of nuclear weapons-produced 14C into the oceanic thermocline. The changes over the 14 yr between occupations are demonstrably large relative to any possible drift in our analytical standardization. The computed difference field based on the gridded data in the upper 1600 m of the section exhibits a significant decrease over time (approaching 40 to 50‰ in Δ14C) in the upper 200–300 m, consistent with the decadal post-bomb decline in atmospheric 14C levels. A strong positive anomaly (increase with time), centered on the low salinity core of the Antarctic Intermediate Water (AAIW), approaches 50–60‰ in Δ14C, a clear signature of the downstream evolution of the 14C transient in this water mass. We use this observation to estimate the transit time of AAIW from its “source region” in the southeast South Pacific and to compute the effective reservoir age of this water mass. The 2 sections show small but significant changes in the abyssal 14C distributions. Between 1991 and 2005, Δ14C has increased by 9‰ below 2000 m north of 55°S. This change is accompanied overall by a modest increase in salinity and dissolved oxygen, as well as a slight decrease in dissolved silica. Such changes are indicative of greater ventilation. Calculation of “phosphate star” also indicates that this may be due to a shift from the Southern Ocean toward North Atlantic Deep Water as the ventilation source of the abyssal South Pacific.

Petrological and geochemical characteristics of Mesoproterozoic dyke swarms in the Gardar Province, South Greenland: Evidence for a major sub-continental lithospheric mantle component in the generation of the magmas

2015 ◽  
Vol 79 (4) ◽  
pp. 909-939 ◽  
Author(s):  
Alexander Bartels ◽  
Troels F. D. Nielsen ◽  
Seung Ryeol Lee ◽  
Brian G. J. Upton
Keyword(s):  
Rare Earth ◽  
Lithospheric Mantle ◽  
Source Region ◽  
Crustal Contamination ◽  
Coarse Grained ◽  
Stability Field ◽  
Time Interval ◽  
Continental Lithospheric Mantle ◽  
Melt Generation ◽  
Dyke Swarms

AbstractThe Mesoproterozoic Gardar Province in South Greenland developed in a continental rift-related environment. Several alkaline intrusions and associated dyke swarms were emplaced in Archaean and Ketilidian basement rocks during two main magmatic periods at 1300–1250 Ma and 1180–1140 Ma. The present investigation focuses on mafic dykes from the early magmatic period ('Older Gardar') and the identification of their possible mantle sources.The rocks are typically fine- to coarse-grained dolerites, transitional between tholeiitic and alkaline compositions with a general predominance of Na over K. They crystallized from relatively evolved, mantle-derived melts and commonly show minor degrees of crustal contamination. Selective enrichment of the large ion lithophile elements Cs, Ba and K and the light rare-earth elements when compared to high field-strength elements indicate significant involvement of a sub-continental lithospheric mantle (SCLM) component in the generation of the magmas. This component was affected by fluid-dominated supra-subduction zone metasomatism, possibly related to the Ketilidian orogeny ∼500 Ma years prior to the onset of Gardar magmatism. Melt generation in the SCLM is further documented by the inferential presence of amphibole in the source region, negative calculated εNd(i) values (–0.47 to –4.40) and slightly elevated87Sr/86Sr(i) (0.702987 to 0.706472) ratios when compared to bulk silicate earth as well as relatively flat heavy rare-earth element (HREE) patterns ((Gd/Yb)N= 1.4 –1.9) indicating melt generation above the garnet stability field.The dyke rocks investigated show strong geochemical and geochronological similarities to penecontemporaneous mafic dyke swarms in North America and Central Scandinavia and a petrogenetic link is hypothesized. Considering recent plate reconstructions, it is further suggested that magmatism was formed behind a long-lived orogenic belt in response to back-arc basin formation in the time interval between 1290–1235 Ma.

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