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SHRIMP analysis of zircon crystals provides a chronostratigraphic context on which the subsequent igneous petrology and geochemistry of the Assynt terrane is based. The TTG history of the Assynt terrane is unravelled using normalitive mineralogy and REE analysis. The Scourie dyke swarm is segregated based on silica content , REE patterns and mineralogy and subsequently placed in tectonic setting to provide an overall petrogenesis of the terrane prior to amalgamation with the Rhiconich terrane.
THE ASSYNT TERRANE
THE RHICONICH TERRANE
SCOURIE DYKE SWARM
SCOURIE DYKE SWARM
The Lewisian terrane which lies west of the Moine thrust (figure 1) represents the Archean and Proterozoic Eonothems (Eons), within mainland Britain. Structural relationships usually provide key evidence for chronological events. These relationships can be difficult to decipher as shearing can distort them. The complexities of these terranes, has resulted in fragmented terminology, making correlation of described terranes and metamorphic events difficult.
This problem is in-part resolved by structured terrane-based nomenclature proposed by Kinny et al. (2005). The proposition divides the mainland Lewisian into six terranes (figure 1). The terranes are defined on structural contacts (Sutton et al. 1951, Park et al 1993), geochemistry (Weaver et al. 1980, H.R Rollinson 1996) and geochronology (Friend et al. 1997&2001). The proposed nomenclature proves useful in correlating between authors, and thus will be adopted. The Northern Terrane is termed the Rhiconich Terrane and the Central Terrane is termed the Assynt Terrane.
Rare Earth Elements are incompatible in crystal structures and thus respond to variances in partial melting and fractional crystallisation. These parameters can be associated with tectonic settings.
This report will draw on geochemical and isotope chronology, to determine the tectonic setting and evolution of the Assynt terrane prior to amalgamation with the Rhiconich terrane. Unravelling the tectonic placement gives an indication of development of the Assynt terrane during the Late Archean and Paleoproterozoic.
A clear distinction between the Rhiconich and Assynt terranes must be drawn prior to the analysis of the Assynt Terrane.
U/Pb isotope analysis provides the most useful age determination as 235U and 238U decay at different rates to 207Pb and 206Pb, respectively. This provides a cross-check into whether a system has been affected by metamorphism, provided the system remains closed. Zircons have a high closure temperature about 1000Â°C, which prevents Pb diffusion to occur once the system has closed (Lee et al. 1997). This will record successive metamorphic events as concentric rings as the zircon crystals grow, but will remain a closed for temperatures below this threshold.
SHRIMP data gathered from the Rhiconich Terrane (Friend et al. 2001) and Assynt terrane (Friend et al. 1997) is used to calculate an age. This is done by using an isochron equation (see equation 1 and appendix table 1a and 1b).
Equation 1: Isochron equation utilising decay rates of Î»235= 0.00098485Ma-1 and Î»238= 0.00015513 Ma-1. t must be constant, and 207Pb/206Pb ratio determined by SHRIMP analysis.
A Wetherill Concordia diagram is constructed by plotting 206Pb/238U vs. 207Pb/235U. A Concordia, generated by plotting constant decay rate of a closed system (see fig 2a and b) reveals discordant curves. Discordant curves indicate radiogenic Pb loss, this loss is related to metamorphic events. It is important to note, that the standard deviation (Ïƒ) given by Kinny et al. has been corrected to 2Ïƒ, thus 95% confidence when quoting age.
THE ASSYNT TERRANE
The Assynt terrane is bounded by the Laxfordian shear zone to the North and the Strathan line (Evans et al. 1974) to the South (figure 1).
Meta-sedimentary rocks are present within the predominantly Tonalite-Trondhjemite-Granodiorite (TTG) gneisses. These meta-sediments, calc-silicates and banded-iron formations, may provide some insight into paleoclimate but will not be considered here.
The Assynt terrane has a maximum protolith age of 3097MaÂ±17Ma (appendix, table 1). The closure temperature of the zircon crystals is approximately 1000Â°C. Metamorphic events which exceed the closure temperature of zircons can clearly be seen on this Concordia diagram (figure 2a). The majority of initial zircon central growth occurs at 3027-2963Ma. This zircon growth has been correlated to the first granulite-facies metamorphism (A on figure 2a) by Friend et al. 1997, 1995. A second high-grade metamorphic event affected this terrane at 2760Â±12Ma (Zhu et al. 1997) (B on figure 2a). The final granulite-facies metamorphic event recorded (Badcallian) occurred at c. 2490Ma (C on figure 2a).
Figure 2a) A Wetherill Concordia diagram that indicates metamorphic events within the Assynt terrane. The three >1000Â°C (granulite) facies metamorphic events, labelled A, B and C respectfully. Data gathered from Friend et al. 1997 Figure 2b): Wetherill Concordia diagram indicating metamorphic events affecting Rhiconich Terrane. A high temperature metamorphic event (A) and subsequent granite-pegmatite intrusion (B) is seen.
Seven events occurred within the Assynt terrane, in which four did not occur in the Rhiconich terrane (Appendix, Table 2). This indicates Archean development was independent of the Rhiconich terrane, and gives an indication of the time of amalgamation (possibly pre-Laxfordian, definitely by end of Laxfordian shear event, c. 1700Ma).
THE RHICONICH TERRANE
The Rhiconich terrane lies north of the Laxford shear zone (figure 1).
The grey and mafic gneisses of this terrane indicate that these hydrous assemblages (contain biotiteÂ± hornblende) are of a relatively lower metamorphic grade.
The zircon indicates a protolith of 3002Â±26Ma. This terrane remains stable until c. 2760Â±12 Ma, whilst granulite metamorphism affects the Assynt terrane (see Figure 2a & 2b).
There is a radiogenic Pb "spike" at 2760Â±12Ma which may indicate the dioritic component of this granodiotite gneiss, but this is unclear. The Concordia (figure 2b) reveals a high-grade metamorphism at 2535Â±10Ma, this may be the date of the yet undated amphibolite metamorphic event. Hydrothermal fluids cause large radiogenic Pb loss (figure 2b).
Granite-pegmatites intruded the Rhiconich terrane c. 1855 (B, figure 2b). This was ensued by the Laxfordian event c. 1750Ma.
Understanding the petrogenesis of the Assynt terrane can help re-assemble the various tectonic settings that this terrane encountered during its independent formation.
Geochemistry of the Basic, Intermediate, Tonalitic and Trondhjemitic gneisses (TTG) (Weaver et al. 1980) within the Assynt terrane will provide a foundation the parental chemistry of these 2900Ma gneisses. Utilising Rare Earth Elements, the overall tectonic facies in which the terrane developed may support a model on the presumed development of this terrane.
Twenty four samples of gneisses where analysed by Weaver et al. 1980, eighteen of which will be reviewed here (Appendix, Table 3). Two methods of analysis where used in determining composition.
Neutron Activation Analysis (see figure 3) which envolves bombardment of a sample with neutrons. This induces a radioactive state of the elements' nucleus (neutron capture) within the sample. The radioactive element decays into its radiogenic daughter, and thus releases gamma rays, which is unique to the element analysed.
X-ray fluorescence is used to analyse the trace elements within the samples. This involves ionization of the sample by x-ray which dislodges inner electrons. An outer electron replaces this inner vacancy. This causes an energy release in the form of a fluorescent x-ray, which is unique to each element.
Figure 3: The basis of the Neutron Activation Analysis method. Adapted from Dr. Michael D. Glascock, University of Missouri Research Reactor
The results of this analysis are classed on SiOâ‚‚ content. Basic gneisses have 45-55% SiOâ‚‚; Intermediate gneisses, 55-62% SiOâ‚‚; Tonalite gneises, ~62-69% SiOâ‚‚; and Trondhjemitic gneisses, >70% SiOâ‚‚ (Appendix, Table 3). Normalitive mineral assemblages were calculated (Appendix, Table 4) on the basis of Cross et al. 1903 classification method. The Albite (NaAlSiâ‚ƒOâ‚ˆ), Anorthite (CaAlâ‚‚Siâ‚‚Oâ‚ˆ) and Orthoclase (KAlSiâ‚ƒOâ‚ˆ) relative abundances are plotted on a ternary diagram (Figure 4), with the aid GCD Plot (Wang et al. 2008).
Figure 4: Ternary diagram representing the relative abundances of Anorthite, Albite and Orthoclase in the Assynt terrane gneisses. Red triangles represent Basic gneisses; green squares represent Intermediate gneisses; blue squares represent Tonalitic gneisses; and turquoise circles represent Trondhjemitic gneisses. The arrow indicates direction of cooling. Generated using GCD Plot V0.40 (Wang et al. 2008)
Anorthite has the highest crystallisation temperature, followed by that of Albite, with Orthoclase crystallising at the lower temperature (Bowen, 1913) figure 4. This difference in crystallisation temperatures signifies that:
The Basic gneisses formed at a the highest temperature, as the Anorthite percentage is highest (Anâ‚†â‚€) (red triangles) ;
Intermediate gneisses (green squares) and Tonalitic gneisses (blue squares) share similar Anâ‚ƒâ‚…:Abâ‚…â‚… ratio.
The Trondhjemitic gneisses (turquoise dots) formed at coolest temperature, as the Albite (Abâ‚‡â‚€) and Orthoclase (Orâ‚â‚€) percentage is highest.
Rare Earth Elements (REE) from Weaver et al. 1980 (Appendix, table 3) are normalised to C1 chondrite (Sun et al. 1989) and plotted (Appendix, figure 5).
A fractionation history of the gneisses can be deduced, assuming that they fractionally crystallised from a chondritic magma. REE have similar chemical behaviour and incompatibility. Light-REEs behave more incompatibly than heavy-REEs, due to the larger radius as light-REEs have fewer protons in the nucleus than heavy-REEs (Crystal chemistry, Geochemistry lecture, May 2009). The REE patterns (Appendix, figure 5) are described below:
The Basic gneisses' have a flat REE pattern with a slight enrichment of light-REE.
Tonalite gneisses' are enriched in light-REE and depleted in heavy-REE, with a slight positive Europium (Eu) anomaly.
The Intermediate gneisses' REE pattern is enriched in light-REE, and has a flat mid-REE and heavy-REE slope.
The Trondhjemite gneisses' REE pattern shows enrichment in light-REE, depletion in heavy REE and noticeable positive Eu anomaly.
SCOURIE DYKE SWARM
Four sets of dykes (bronzite-picrites, norites, olivine-gabbros and quartz dolerites) are identified by Tarney et al. (1987), based on petrology and geochemistry. A Total Alkali Silica (TAS) plot (Appendix, figure 6) of the dykes give a general overview of source of melt, with Olivine-gabbros dykes having a more primitive source than Bronzite-picrite dykes. Normalitive mineral assemblages are calculated using Cross et al 1903 sequence of crystallisation. Weight oxide percent of the analysed samples (Tarney et al. 1987) are normalised using Fe(III)/Total iron ratio. The sequence of crystallisation of an anhydrous mineral assemblage (as stated by Cross et al. 1903) is calculated. A brief summary of the dykes are given below:
Bronzite-picrite dykes, represented here by the Northern Leothaid dyke, c. 2418Ma. Concentrated north of Lochinver. These dykes do not show chilled margin, have a high Magnesium concentration in modal Olivine (Foâ‚ˆâ‚…â‚‹â‚ˆâ‚‚) and Orthopyroxene (Enâ‚‰â‚€â‚‹â‚ˆâ‚‡).
Norites dykes- constitute a minor proportion of the overall swarm. The dykes show chilled margins. Badcall norites' have a high Magnesium composition of modal orthopyroxene (Enâ‚ˆâ‚). The late stage orthopyroxenes show a more Iron rich composition.
Olivine-gabbro dykes, represented here by the Loch Sionascaig dyke, c. 1992Ma. Concentrate near Loch Sionascaig, south of Lochinver. These dykes show thin chilled margins, there is a higher concentration of Iron in modal Olivine (Foâ‚…â‚‰â‚‹â‚‡â‚€) and Orthopyroxene (Enâ‚‡â‚€â‚‹â‚‡â‚ƒ). The overall normalitive mineral assemblage have Feldspathoids, which suggest a more alkali basalt composition.
Dolerite dykes- constitutes the majority of the swarm. These dykes have chilled margins and have a tholeiitic bulk composition (Tarney, 1973). The overall mineral compositions show the greatest variance of the swarm.
The Rare Earth Element patterns for the four dyke swarms (figure 7) reveal two distinct melt derivatives. The Bronzite-picrites and Norites have similar patterns (figure 7a), with enriched light-REEs and flat mid- and heavy-REE. The Norites are more heavily fractionated, as the patterns are more enriched with the incompatible REEs, than that of the Bronzite-picrites. The Olivine gabbros and Quartz dolerites (figure 7b) are less fractionated as the patterns are more flat. The Olivine gabbros are depleted in heavy-REE, thus garnet is likely to be residual in the melt source (Tarney 1992).
It is assumed that the dykes within the Rhiconich terrane are of the same swarms. Data from the Rhiconich terrane is limited, thus REE patterns could not be attained and compared with dykes of the Assynt terrane.
Figure 7: REE patterns of Scourie dykes. The bronzite-picrites and Norites (a) have a more fractionated pattern, with Heavy-REE enrichment and flat Mid- and Light-REE. The Quartz dolerites and Olivine gabbros (b) are less fractionated, with a flat REE pattern. Figures adapted from Tarney 1992.
The feldspars of the Basic Gneisses are Anorthite rich (Anâ‚†â‚€), which is indicative of a high temperature crystallisation environment (Bowen, 1913). The flat REE pattern indicates that it fractionally crystallised from a chondritic source. The slight enrichment in light-REEs suggests that it crystallised in low pressure environment. The overall REE pattern resembles that of the Lower Crust, yet less fractional crystallisation has taken place, (see figure 8a) and E-type MORB, (see figure 8b). The petrogenetic environment during the genesis of the Basic gneisses is most likely deep crustal extensional setting where there was some degree of decompression has occurred, perhaps by extension.
Figure 8: Rare Earth Element pattern of a) The Lower crust normalised to chonditic composition (Rudnick et al. 2003) and (b) Mid-Ocean Ridge Basalts (MORB) normalised to C1 chondrite (Klein 2003). Basic gneiss pattern is similar and lies between these two plots.
The feldspar composition (Anâ‚ƒâ‚…) of the Tonalite gneisses is indicative of a cool crystallisation temperature (Bowen 1913). The REE pattern is more fractionated than that of the Basic gneiss. The overall pattern of the REE resembles that of the Middle crust (see figure 9). The heavy-REEs, which are more compatible than light-REEs, are depleted. This depletion is speculated to be caused by two stages of crystallisation, whereby garnet is left within the residual melt (Weaver et al. 1980). The positive Europium anomaly, which is not present in the mid-crustal REE pattern, is explained by the substitution of Europium for Calcium in the Anorthite plagioclase crystal structure (Best 2003), as Europium has a lower melting point (826Â°C) than Calcium (Gschneidner et al. 1987). Europium remains in the melt and crystallises at a late stage within Anorthite crystal structure.
Figure 9: REE pattern for Middle crust normalised to chondritic composition (Rudnick et al. 2003). This resembles the Tonalite gneiss, but does not reveal Europium anomaly seen in the Tonalite gneisses.
The feldspar composition and REE pattern for the Intermediate gneisses are similar to that of the Tonalite gneisses apart from the Intermediate gneisses having a flat mid- and heavy-REE pattern. The amount of fractionation is higher than that of the Tonalite gneisses. The chemical components of garnet may not have been present, thus preserving the chondritic REE pattern. This may be due to extension within the crust at the time of formation, allowing magma with a chondritic composition to ascend.
Trondhjemitic gneisses have the most Albite and Orthoclase rich feldspar compositions (Abâ‚‡â‚€) as well as the highest SiOâ‚‚ (>70%), thus indicating the coolest crystallisation temperature of these gneisses. The REE pattern indicates a complex fractionation history, with extremely enriched light-REE and heavily depleted heavy-REE. The positive Eu anomaly is caused by the substitution of Eu for Ca within the Anorthite crystal structure, as the ionic radii and charge are similar (Kimata, 1988). This is due to the large amount of fractionation that has occurred. The overall heavily fractionated pattern indicates re-melting of the upper crust. The direct source remains heavily debated as Cartwright et al. (1992) suggests that this may be caused by in situ melting of Tonalite gneiss and Rollinson (1996) suggests the melt source is of basic origin.
SCOURIE DYKE SWARM
The Total Alkali Silica (TAS) diagram (Appendix, figure 6) indicates that this swarm of dykes are from a primitive source. The REE patterns (figure 7) suggest these primitive dykes come from 2 different parental melts. The Bronzite-picrites (c. 2418Ma) and Norites share similar steep sloped REE pattern. The Quartz dolerites and Olivine gabbros (c. 1992Ma.) share a flattened REE pattern, with slight light-REE enrichment.
The absence of a chilled margin in the Bronzite-picrite dykes signifies that they intruded country rock of similar heat. The Magnesium rich Olivine (Foâ‚ˆâ‚…â€â‚ˆâ‚‚) and Orthopyroxene (Enâ‚‰â‚€â‚‹â‚ˆâ‚‡) are conclusive evidence that the crystals formed at high temperatures. The assemblages are hypersthenes normalative and thus are Tholeiitic (Appendix, Table 4).
The Norite dykes' Orthopyroxenes (Enâ‚ˆâ‚) have a similar high Magnesium content to that of the Bronzite-picrite dykes. This is overgrown by more Iron rich Orthopyroxene, suggesting either a cooler magma source or depletion of Magnesium by fractional crystallisation. The chilled margins of the Norite dykes suggest that at time of intrusion the country rock had cooled. The dykes are sparse within the terrane.
The Olivine-Gabbros are nepheline-normalitive (Appendix, Table 4) and are thus classified as Alkali- Basalts. Chilled margins are proof of injection into cool country rock, thus cannot be produced by deep melting at high temperature. REE plots suggest chondritic source, as the geometry is flat. The depletion of heavy-REE suggests that garnet may be present in the residue that generated the source melt (Weaver et al., 1980), as heavy-REEs are more compatible in its lattice than light-REE. This indicates the Olivine-Gabbros formed due to small degree of partial melting of chondritic source or possibly caused by melting in the presence of a carbonate.
Quartz dolerites form the majority of the swarm, are from a similar source as the Olivine-Gabbros, but are not related, as the REE patterns could not result from fractional crystallisation of the same magma (Weaver et al., 1980). The dykes show the greatest geochemical variance of the swarm (Weaver et al. 1980). The REE pattern reveals enrichment in both light-REE and heavy-REE. These dykes are Tholeiitic.
The overall swarm indicates that during the Assynt terrane was:
Initially at great depth, where Bronzite-picrite dykes intruded into this hot terrane. The source was REE depleted, indicating a depleted source
Rose, as the crust cooled, thus chilled margin where present upon intrusion of the Norite dykes. This movement causes light-REE enrichment as the source melt is more heavily fractionated.
The Olivine gabbros indicate the crust had thickened significantly, such that the small amount of magma (as it has Alkali-basalt assemblage) fractionally crystallises garnet, within the garnet stability field c. 85km (Robinson et al., 1998). A light-REE enriched and heavy-REE depleted pattern remains.
The Quartz dolerite suggests a lower mantle source, as the patterns show enrichment of both light-REE and heavy-REE. The overall pattern is similar, but more REE enrichment, to that of the Kilauea Tholeiites. This explains the abundance of dykes within the terrane.
Basic gniesses show that the initial development of the crust was similar to low crustal region, decompression on the upper mantle may have generated these gneisses. This decompression or exhumation to the middle crustal region brought about the development of the Tonalite gneisses. The source of the melt had a garnet residue which indicates iether a thick crust (Rollinson 1996) had developed of the source was that of a subducted plate(Weaver et al. 1980). Trondhjemite gneisses are most likely caused by the melting of a subducted slab and various phases of fractional crystalisation, leading to this extremely fractionated pattern.
Subsequent metamorphic events affected the terrane as orgogenies occured between various terranes. The movement pf the high grade metamorphic events may possibly be traced via zircon analysis. An example is the GST 10 zircon sample (9.1 central) from Scouriemore, which crystallized at 2860Â±25Ma, and was reset before the GST 8 zircon at Scourie, (9.1 central) with an age of 2769Â±10Ma. The movement of this metamorphic front is in a North Easterly direction. Further analysis of zircons within this area would be required to track the movement of this metamorphic front more precisely.
These metamorphic events may signify the subduction of this terrane, which would explain the succession of intrusions from a deep crustal level that gradually becomes exhumed over a period of c. 426Ma (2418Ma (Bronzite-picrite) to 1992Ma (Olivine gabbro)). This exhumation may be due to the buoyancy of the gneiss compared to that of the mantle. The final stages of intrusions suggest a possible plume-like source intruding through the margin of the North Atlantic Craton, to which Assynt was a part of (Bridgewater et al., 1973).
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Table 1a): 207Pb/206Pb isotopic age determination of the Northern Terrane (Fanagmore and Laxford Brae). This is a modification of Friend & Kinny 2001. Standard deviation doubled to quote age with 95% confidence.
Fanagmore, Northern region Scotland. (Granodiorite gneiss)
Table 1a) continued
Laxford Brae (Granite sheet)
Table 1a) continued
Laxford Brae (Granite sheet)
Table 1b): 207Pb/206Pb isotopic age determination of the Assynt Terrane (GST 10-Scouriemore, GST 8-Scourie). This is a modification of Friend & Kinny 1997. Standard deviation doubled to quote age with 95% confidence
Table 1b) continued
Table 2: A summary of events affecting Assynt and Rhiconich terranes. Adapted and edited from Kinny, Friend & Love (2005).