Geochronology and Geodynamics of Scottish Granitoids From the Late Neoproterozoic Break-Up of Rodinia to Palaeozoic Collision
By Oliver, Grahame J H Wilde, Simon A; Wan, Yusheng
Abstract: Thirty-seven granitoids from Scotland have been dated using the sensitive high-resolution ion microprobe zircon method. Granitoids were intruded during: (1) crustal stretching at c. 600 Ma after Rodinia broke up (A-types); (2) the Grampian event of crustal thickening when the Midland Valley Arc terrane collided with Laurentia at c. 470 Ma (S-types); (3) erosion and decompression of the over-thickened Laurentian margin at c. 455 Ma (S-types); (4) subduction of Iapetus Ocean lithosphere under Laurentia starting at 430 Ma (I-types); (5) roll-back beginning at 420 Ma (I-types); (6) bilateral slab break-off and lithospheric delamination at 410 Ma (I- and S-type granites) when Baltica hard-docked against the Northern Highland terrane and Avalonia soft-docked against the Grampian Highland terrane. Far-field Acadian events at 390 Ma were recorded by I-type granites intruded along active sinistrally transpressive faults. I-types formed in lower crustal hot zones above subduction zones, whereas S-types formed in lower crustal hot zones above lithospheric windows through which hot asthenosphere had risen.
The aim of this paper is to report new U-Pb zircon ages of c. 600 to c. 400 Ma granites in Scotland and use these together with published data to refine a model for the geodynamic evolution of the Scottish Caledonides. This model integrates a general hypothesis for fast regional tectonometamorphism and the petrogenesis of S- and I- type granites in a geodynamic environment of slab break-off.
Figure 1 shows the division of Scotland into tectonostratigraphic terranes. The reader is referred to Trewin & Rollin (2002) for details. Briefly, the Hebridean terrane has a basement of Archaean and Palaeoproterozoic Laurentian continental (Lewisian) granulites and gneisses. East of the Moine Thrust, the Northern Highland (Mesoproterozoic Moine Supergroup) and Grampian Highland (Neoproterozoic and Early Palaeozoic Dalradian Supergroup) terranes are successive marine successions deposited on the Laurentian (early rifted and later passive) margin. South of the Highland Boundary Fault, the Midland Valley terrane contains relicts of Early Palaeozoic ocean crust and island arc from the margin of the Iapetus Ocean that formed between Laurentia and Gondwana when Rodinia broke up. The Southern Uplands terrane lies south of the Southern Upland Fault and is an Early Palaeozoic accretionary prism formed in the Iapetus Ocean. The Iapetus Ocean was consumed along the Iapetus Suture as the amalgamated Lautentia-related terranes collided with Gondwana-related terranes in England (Lakesman) and Wales. The closure of the Iapetus Ocean and the resulting island arc(s) and continental collisions is known as the Caledonian orogeny. In Scotland, the Ordovician island arc collision is known as the Grampian event and the Silurian continental collision is known as the Scandian event. Pre-, syn- and posttectonic Neoproterozoic to Devonian granitoids are associated with both these events.
Our strategy was to collect a wide variety of granitoids (Fig. 1, map references can be found in the Supplementary Publication; see below) based on the combined field, geochronological and geochemical classification scheme summarized by Oliver (2001, 2002). Granitoids were subdivided on the basis of their structural state (e.g. foliated and metamorphosed granites v. unfoliated and unmetamorphosed granites, i.e. Older Granites of Barrow (1893) and Newer Granites of Read (1961)), partly on their ^sup 87^/Sr whole- rock or mineral ages and more importantly on their ^sup 87^Sr/ ^sup 86^Sr initial ratios (acknowledging that these might not be accurate). In this way, four granite types were defined: (1) c. 600Ma foliated granites (i.e. Older Granites’ of Barrow 1893); (2) Grampian Event granites, 480-470 Ma, synmetamorphic foliated high ^sup 87^Sr/^sup 86^Sr initial ratio S-type granites (i.e. ‘Group 1 Newer Granites’ of Pankhurst & Sutherland 1982); (3) c. 460-435 Ma, post-tectonometamorphic, non-foliated high ^sup 87^Sr/^sup 86^ Sr initial ratio S-type granites; (4) c. 435-400 Ma non-foliated Andean- type subduction zone, calc-alkaline, low ^sup 87^Sr/^sup 86^ Sr initial ratio I-type granites (i.e. Group 2 and 3 Newer Granites of Pankhurst & Sutherland 1982).
In the late twentieth century, the dating of Caledonian granite intrusions in Scotland relied on Rb-Sr, K-Ar and bulk zircon U-Pb radiometric dating. These methods have deficiencies; for example, lack of Sr isotope homogenization and Rb mobility produce errorchrons, Ar diffuses in and out of magmatic minerals giving apparent ages, and unabraded bulk zircon samples usually include inherited grains (see discussion by Oliver 2002). These methods have been bettered by U-Pb single grain zircon geochronology, using cathodoluminescence (CL), thermal ionization mass spectrometry (TIMS), secondary ion mass spectrometry (SIMS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) technology, which avoids problems such as mineral reheating, slow cooling, hydrothermal resetting or inheritance. The great advantage of LA- ICPMS and SIMS methods is the spatial resolution that allows zircon cores and rims to be analysed so that multiple events can be distinguished. Rogers et al. (1989), Rogers & Dunning (1991), Dempster et al. (2002) and Fraser et al. (2004) have published selected TIMS titanite and zircon granite ages from the Grampian terrane. Papers by Kinny et al. (1999, 2003a, b) have reported sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of granites from the Northern Highland terrane. However, more than half of Scottish granites have yet to be dated by the new zircon methods.
The SHRIMP II analyses were carried out at Curtin University m of Technology and the Chinese Academy of Geological Sciences, Beijing, following standard procedures (Nelson 1997; Williams 1998). Zircon crystals were obtained from crushed rock using a combination of Wilfley table, heavy liquid and magnetic separation techniques. Single crystals were hand picked and mounted, along with several pieces of the Curtin University standard CZ3 (which has a conventionally measured ^sup 206^Pb/^sup 238^U age of 564 Ma; Pidgeon et al. 1994) in epoxy resin discs. The discs were polished to effectively section the zircons in half, and then cleaned and gold coated. In Beijing, TEMORA (417 Ma, Black et al. 2003) and SL13 (572 Ma) standards were mounted separately from the unknowns. Spots for analysis were selected after optical and CL examination so that cracks and inclusions could be avoided.
Spot sizes averaged c. 25 [mu]m and each analysis site was rastered for 3 min to remove any surface contamination. An average mass resolution of c. 4700 (1% definition) was used to measure Pb/ Pb and Pb/U isotopic ratios, and Pb/U ratios were normalized to those measured on the standard zircons. The error in standard calibration during the analytical sessions for each sample is given in the Supplementary Publication (see below), and ranged from 3.99 to 0.23%. Measured 204Pb count rates in the unknowns were similar to those obtained from the standard zircon so common lead corrections were made assuming an isotopic composition of Broken Hill lead, as the common lead is considered to be associated mainly with surface contamination in the gold coating (Nelson 1997). Data reduction was performed using the SQUID and ISOPLOT programs of Ludwig (2003) and corrected using the measured 204Pb values. Uncertainties on single analyses are quoted in the Supplemetary Publication at the 1sigma level and plotted as 1sigma ellipsoids in Figures 2 and 3. Where the data have been grouped as concordia or mean ages, the uncertainties are quoted at 2sigma (95% confidence level).
Most granites contain significant quantities of zircons that have been inherited from both their melted source(s) and (assimilated) country rocks. New magmatic growth tends to be nucleated on these inherited grains as overgrowths. Our experience has been that core- less pristine magmatic prisms are difficult to find. Therefore, the new dates reported in Table 1 are based mainly on the analysis of zircon overgrowths, selected after optical and CL study. Magmatic overgrowths are recognized in CL by their fine, concentric, euhedral, oscillatory zones rimming contrasting inherited cores. They usually have Th/U ratios of >0.05. Metamorphic overgrowths are recognized by their subeuhedral outline, lack of zoning, high U contents (and therefore very dark CL signatures) combined with very low Th/U ratios
Table 1 summarizes the results. Figures 2 and 3 present Weatherill concordia diagrams (based on the 1sigma data in the Supplementary Publication and plotted with 1sigma error ellipses) for the new results, arranged in order, with oldest first. Concordia ages (2sigma errors), the number of analyses (n), MSWD of concordance and equivalence and average Th/U ratios are included. Where concordant ages could not be calculated, the weighted mean ^sup 206^Pb/^sup 238^U ages are quoted as they are generally considered more precise for near-concordant Phanerozoic zircons (Compston et al. 1992) because low count rates on ^sup 207^Pb result in larger statistical uncertainties, making the ^sup 207^Pb/^sup 206^Pb and ^sup 207^Pb/^sup 235^U ratios less sensitive measures of age. The U-Pb isotope data used in Figures 2 and 3, together with descriptions of the CL textures and notes on age interpretation, are available online at at http://www.geolsoc.org.uk/SUP18306. Because the strategy was to analyse as many rocks as possible using the limited machine time available, usually 10 spots were analysed from each sample carefully selected by detailed examination of the CL images: after data reduction, some discordant analyses were eliminated.
Discussion of results
Table 1 summarizes U-Pb zircon ages of the Scottish late Neoproterozoic and Caledonian igneous rocks as well as including the less reliable Rb-Sr and K-Ar age dates from northern England. Figure 4 is a compilation of probability density distribution diagrams of granitoid and lava ages from the various UK terranes. In the Grampian Highland terrene there are six age spikes labelled as rifting, collision, decompression melting, start of subduction, roll- back and lithospheric slab break-off, as explained below. Figure 5 summarizes the ages of Neoproterozoic and Caledonian magmatism north of the Highland Boundary Fault, groups the ages according to Figure 4, and notes activity in neighbouring terranes and comments on the regional tectonic environment. The Grampian and Northern Highlands terrene are considered together, as the Great Glen Fault probably reflects only a few hundred kilometres of offset within the same terrane (Dewey & Strachan 2003).
c. 600 Ma: rift granites; during break-up of Rodinia
The c. 600 Ma granites (Older Granites of Barrow (1893) such as Ben Vuirich, Berridale, Breaval, Cam Chinneag, Keith, Muldery Hill, Portsoy) have epsilonNd -6, delta^sup 18^O 8-9[per thousand] (Harmon et al. 1984) and ^sup 87^Sr/^sup 86^ Sr initial ratios of 0.710- 0.718 (Long 1964; Pankhurst & Pidgeon 1976). The chemistry of the Ben Vuirich granite has been established as A-type (Tanner et al. 2006), typical of crustal sourced anorogenic granites formed during continental rifting (Loiselle & Wones 1979; Eby 1992). The Carn Chuinneag intrusion includes riebeckite-bearing granite (Pidgeon & Compston 1992) also typical of A-type granites (Loiselle & Wones 1979). The contemporaneous (U-Pb zircon age 601 +- 4 Ma, Dempster et al. 2002) Tayvallich basaltic lavas are typical of rifted environments (Graham 1986). This bimodal magmatism is contemporaneous with bimodal magmatism in the Appalachians and the Norwegian Caledonides and has been assigned to the break-up of the supercontinent Rodinia and the widening of the Iapetus Ocean (Soper 1994; Tanner et al. 2006). Figure 6a is an illustration of the expanding passive margin of Laurentia at c. 600 Ma. Although these granites and basalts are related to the break-up of Rodinia (Dalziel 1997; Dalziel & Soper 2001) and are pre-Caledonian, they were metamorphosed and tectonized during either the c. 470 Ma Grampian or the c. 430 Ma Scandian events (see below).
c. 470 Ma: Grampian event collisional granites
Figure 5 shows that there was c. 100 Ma gap in granite activity in Scotland between 588 and 477 Ma, while the Iapetus Ocean was spreading and the Laurentian margin changed from being actively rifted to becoming passive. By c. 490 Ma, the Midland Valley suprasubduction island arc complex and Highland Border back-arc had formed as part of the peri-Laurentian arcs (Oliver 2002; see Fig. 6b). The c. 25 km thick Dalradian sequence (Strachan et al. 2002) would have experienced burial metamorphism (Fig. 6b). By 470 Ma, Scotland was at latitude 20[degrees]S when the Midland Valley arc collided with southern Laurentia (Cocks & Torsvik 2002). Syn- regional metamorphic granites were intruded at c. 470 Ma. These Group 1 Newer Granites of Pankhurst & Sutherland (1982) are foliated granites, typically containing muscovite, biotite and rarely tourmaline, and have epsilonNd values of -10 to -13.9, delta^sup 18^O of 9-11[per thousand] and ^sup 87^Sr/^sup 86^Sr initial ratios >0.712 (Harmon et al. 1984), typical of S-type granites formed by the melting of sedimentary protoliths (Chappell & White 1974). Harrovian regional metamorphism in the central and SE Grampian terrane and Buchan regional metamorphism in the NE Grampian terrane have both been dated at 470 Ma. Oliver et al. (2000) and Baxter & Ague (2002) dated the peak of garnet growth during Harrovian high- pressure kyanite-sillimanite series metamorphism at 470 +- 3 Ma using garnet-whole-rock Sm/Nd isotopes. This is consistent with U- Pb zircon metamorphic overgrowth ages of 462 +-8.8 Ma obtained by Breeding et al. (2004) and U-Pb monazite ages as old as 470 Ma obtained by Barreiro (quoted by Phillips et al. 1999).
The synmetamorphic Strichen (473 +- 2 Ma) granite dates the Buchan low-pressure andalusite-sillimanite series metamorphism (Oliver et al. 2000). Carty (2001) and Condon & Martin (pers. comm.) (see Fig. 2e) have independently TIMS dated magmatic zircon from the syn-U2 metamorphosed Portsoy Gabbro in the Portsoy Shear Zone from the Buchan area at 471.5 +- 3 Ma and 474.3 +- 2.1 Ma, respectively. An undeformed quartzofeldspathic pegmatite that cross-cuts the deformed meta-gabbro gave an age of 473 +57-3 Ma (zircon, U-Pb, TIMS, Carty 2001). Consequently, magmatism, regional metamorphism and shearing had finished by 470 Ma. U-Pb dating of titanite (Carty 2001) gave ages of 466 +- 3 Ma and 471.5 +- 3 Ma, therefore cooling in the Portsoy Shear Zone was very rapid.
Obduction of the Ballantrae ophiolite has been dated at 478 +- 8 Ma (K-Ar date on hornblende from the metamorphic sole, Bluck et al. 1980) and Maletz (2004) has refined the graptolite age of syn- obduction black shale-serpentinite conglomerates from Ballantrae as Ca4, which is 469.5 +- 1 Ma according to Gradstein (2004). Oliver (2001) related the collision and accretion of the Midland Valley arc terrane to the Laurentian margin (Grampian terrane) to this obduction. Therefore, according to Oliver (2001), there is a temporal link between collision, obduction, Newer Gabbro and S-type granite intrusion, regional metamorphism and crustal melting at 470Ma: this is the Grampian event of the Caledonian orogeny ( see Fig. 6c).
Geobarometry (Baker 1985) shows that Grampian Barrovian metamorphism occurred at depths to 35 km (1 GPa). As the present- day Grampian crust is a normal 35 km thickness (Hall et al. 1984) and 1 GPa metamorphic rocks are at the present-day surface, it can be assumed that the Grampian crust was double normal thickness at 470 Ma; that is, c. 70 km thick. The midLlanvim (465 Ma) Kirkland Conglomerate lies unconformably over the Ballantrae ophiolite and contains mostly ophiolite and some granite debris but also Barrovian high-grade garnet detritus, which Hutchison & Oliver (1998) and Oliver (2001) considered to be evidence for deep erosion of the Grampian terrane with a lag time of only 5 Ma after the peak of metamorphism. For metamorphosed crust at 35 km depth to reach the surface in 5 Ma requires a local erosion rate of 7.4 mm a^sup -1^, similar to the erosion rate being experienced in the 8.1 km high Naga Parbat region of the present-day Himalaya, where 15-20 km has been unroofed in the past 3 Ma (Zeitler et al. 2001). During collision, high mountains would have formed over the Grampian terrane and orographie monsoonal rain at these low latitudes would have focused fast erosion and fast exhumation of high-grade Barrovian metamorphic rocks in a narrow wedge (Fig. 6c). Carty (2001) has interpreted the Portsoy Shear Zone as a mid-crustal (0.5 GPa; equivalent to 20 km depth) extensional collapse structure, dated at 470 Ma (see above): this would have promoted fast exhumation. Therefore the situation in the Grampian terrane might have been analogous to that of the Tertiary and modern-day Himalayas where tectonics, metamorphism, magmatism and climate interact to focus fast exhumation of Barrovian regional metamorphic rocks (Beaumont et al. 2001; Zeitler et al. 2001). An obvious difference between Himalayan channel flow and the Grampian situation is the lack of extensive leucogranite sheets in the latter (Scaillet et al. 2006).
According to Thompson (1999) normal mantle heat flow and radioactive heating of a crust doubly thickened by collision would take at least 20 Ma to reach its metamorphic peak. However, the Barrovian and Buchan metamorphism coincided with the collisional thickening at 470 Ma; consequently, significant heat must have been added to the crust during collision.
As noted above, the 470 Ma granites are all two-mica S-types; that is, melted lower crustal (sedimentary) rocks formed during peak metamorphism. There were no I-types forming at this time. Annen et al. (2006) have postulated that I-type magma generation above subduction zones involves mantle-derived basalt intruding or underplating the lower crust, so providing a hot zone environment for partial melting, assimilation, storage and hybridization (Hildreth & Moorbath 1988). In this way intermediate and siliceous magmas can form and intrude the middle and upper crust. The lack of I-type magma at 470 Ma therefore suggests that mantle basalt has not been involved as a heat source. Instead, as in the model illustrated in Figure 6c, it is proposed that a lithospheric window formed as a result of lithospheric slab break-off or tear, and hot asthenosphere rose into contact with the lower crust (Oliver 2002). An alternative scenario might be that ridge subduction produced a lithospheric slab window under the Grampian terrane similar to that suggested for Central America in the Pliocene-Pleistocene (Johnson & Thorkelson 1997) and for Alaska in the Early Tertiary (Bradley et al. 2003). If ridge subduction was the case, then asthenospheric upwelling would also occur. It should be noted that there is no significant basalt melting of the asthenosphere in this model, so that only S-type lower crustal minimum melts (at c. 750[degrees]C, Thompson 1999) form from Al-rich metasediments at 470 Ma. Presumably this mechanism of asthenospheric rise did not transfer enough heat into the lower crust to cause the melting of igneous protoliths (e.g. amphibolites) to form I-type granites; that is, lower crustal temperatures remained below about 850 [degrees]C (Annen et al. 2006), and more probably peaked at 750[degrees]C (Thompson 1999). From the work of Oxburgh & Turcotte (1974) it is implicit that the emplacement of hot asthenosphere under the Grampian terrane would reset the crustal geotherm to a higher gradient in a matter of 3-4 Ma. The hot asthenosphere is the heat source for the Harrovian regional metamorphism. One potential problem with this model might be the formation of Newer Gabbros at 470 Ma contemporaneously with the S- type granites (Fig. 6c). The gabbros have mantle-like ^sup 87^Sr/ ^sup 86^Sr initial ratios of 0.7032 (Pankhurst 1969) and flat heavy rare earth element patterns (Oliver et al., unpublished data), suggesting a spinel Iherzolite mantle source; that is, non- lithospheric slab modified mantle melting at
c. 455 Ma: Grampian decompression granites
According to Cocks & Torsvik (2002), between 460 and 455 Ma Scotland was still at 20[degrees]S and Avalonia had separated from Gondwana and drifted north, reducing the width of Iapetus from 4000 km to c. 1500 km (Fig. 6d). Figures 4 and 5 show that there might have been a c. 10 Ma pause after the Barrovian-Buchan metamorphic peak and before the next batch of granite intrusions. The non- foliated c. 455 Ma granites, such as the Kennethmont, Moy, Strathspey, Kyllachy and Inzie granites, have similar two-mica mineralogy and isotopic characteristics to the foliated 470Ma collisional granites: e.g. epsilonNd -10.0 to -12, delta^sup 18^O 8.5-11[per thousand], and ^sup 87^Sr/^sup 86^Sr initial ratios >0.712 (Harmon et al. 1984), typical of S-type granites.
Deposition started in the Southern Uplands terrane at 455 Ma: from then on the Southern Uplands record the deposition of Barrovian metamorphic (plus plutonic and volcanic arc) detritus into an accretionary prism (Oliver et al. 2002). Detrital garnet has the same 470 Ma (Sm-Nd) age as detrital mica (^sup 40^Ar/^sup 39^Ar) ages (Kelley & Bluck 1989), which is the same as in situ garnet (Oliver et al. 2002) and in situ mica ages (Dempster 1985) from the Grampian terrane, indicating Grampian terrane provenance. Grampian terrane mica Rb/Sr and K/Ar cooling ages between 455 and 420 Ma (Dempster 1985) imply that Grampian terrane cooling and exhumation coincided with erosion and deposition into the Southern Uplands accretionary prism (Hutchison & Oliver 1998). As there is an apparent lack of evidence for tectonic activity (e.g. crustal thinning by extension) in the Grampian terrane between 455 and 420 Ma, it is assumed that exhumation was driven by erosion. On this basis, Oliver (2002) suggested that the c. 450 Ma post-tectonic S- type granites would have been formed during decompression melting of the thickened Grampian continental crust as it was eroded and exhumed. Possibly, thermal input from the still hot asthenospheric root under the Grampians could have contributed to melting. As during the Grampian event, only the low melting point Al-rich sediments melted; amphibolites (igneous protoliths) did not, so that only S-type granites formed.
If the 70 km thick Grampian terrane crust was returned to a normal 35 km thickness between 465 and say 440 Ma then the erosion rate would have been 1.4 mm a^sup -1^ (Fig. 6d). Presumably the climate was wet enough at 20[degrees]S to facilitate this erosion. According to McKerrow et al. (1977), the Southern Uplands accretionary prism was built up on oceanic crust during the Caradoc to Wenlock (456-423 Ma). The crust under the Southern Uplands is now 35 km thick (Hall et al. 1984), thus c. 35 km of sediments were accreted during 456 and 423 Ma at a rate of 1.2 mm a^sup -1^, balancing the 1.4 mm a^sup -1^ erosion rate of the Grampian terrane. The accretionary process requires that there was northerly directed subduction, yet there is no in situ evidence of subduction zone I- type granites forming at this time in either the Grampian or Midland Valley terrane. The 451 +- 8 Ma Rb-Sr whole-rock mineral age of a hornblende granite boulder (Longman et al. 1979) in Caradoc conglomerate overlying the Ballantrae ophiolite is the only (circumstantial) evidence for subduction under the Midland Valley at this time (Fig. 6d).
Figures 4 and 5 show another gap in granite activity in Scotland between 450 and 430 Ma. Because the Southern Uplands accretionary prism was still active during this time, northward subduction would have continued. The lack of granite activity between 450 and 430 Ma suggests either that subduction was at too shallow an angle to cause mantle melting or that subduction rates were so slow that the lithospheric slab did not have time to extend under the Grampian terrane. Alternatively, the Laurentian margin was similar to present- day western California (i.e. largely transcurrent).
The Glen Dessary syenite intrusion is enigmatic in having a zircon U/Pb upper intercept age of 456 +- 5 Ma, low I-type ^sup 87^Sr/ ^sup 86^Sr initial ratio of 0.7041 +-0.0001, and combined Sr and Pb isotopic evidence that is consistent with an entirely mantle derivation (Van Breemen et al. 1979) and is therefore unrelated to the S-type decompression granites described above. The original U- Pb dating (Van Breemen et al. 1979) was carried out using bulk zircon separates and thus the 456 Ma intercept age is perhaps suspect. Bulk titanite fractions gave ^sup 206^Pb/^sup 238^U ages of 447 +- 5 Ma and 443 +- 5 Ma and these may be nearer the intrusion age. Perhaps the Glen Dessary syenite indicates slighly earlier subduction zone related magmatism in the Northern Highlands than in the Grampian Highlands (see below).
430-420 Ma: Convergence of Laurentia with Baltica-Avalonia: start of subduction granites
During a gap of c. 20 Ma between c. 450 and c. 430 Ma, there was apparently no granite activity in Scotland. Scotland was still at 20[degrees]S (Fig. 6e) when suddenly at 430 Ma, the Laurentian continental margin became active again as new granites were intruded. These non-foliated Newer Granites of Read (1961) (Group 2 and 3 Newer Granites of Pankhurst & Sutherland 1982) are calc- alkaline hornblende and biotite granites that strongly contrast with the earlier two-mica granites described above; that is, epsilonNd is zero to -13.3, delta^sup 18^O is 7-11[per thousand] and ^sup 87^Sr/ ^sup 86^Sr initial ratios are
420-400 Ma: terminal Caledonian collision, slab roll-back, slab break-off and lithospheric delamination
Sedimentation in the Southern Uplands accretionary prism ceased at the end of the Wenlock at c. 420 Ma, indicating that a subduction- related trench ceased to exist, probably because the leading edge of Avalonia (i.e. the Lakesman terrane) had finally docked against Laurentia along the lapetus Suture (see Fig. 6e). A lack of penetrative foliation in the youngest Southern Uplands sediments signifies that this was a soft docking (Soper et al. 1992; Soper & Woodcock 2003). At the same time as subduction ceased, Old Red Sandstone fluvial sedimentation and coeval calc-alkaline volcanism commenced in extensional (transtensional) basins in the NE Midland Valley (Marshall et al. 1994). Extension and volcanism would have been initiated in the Midland Valley if the subduction zone began to roll back after 420 Ma (Fig. 6e).
About 80% of the volume of exposed granite in the Grampian, Southern Uplands and the Lakesman terranes appears to been intruded between 410 and 405 Ma (compare Fig. 1 and Table 1). Stephens & Halliday (1984) have distinguished the Argyll, Cairngorm and South of Scotland suites on the basis of chemical and isotopic criteria (see Fig. Ib). These different suites presumably reflect the different nature of the lower crust under these regions. Palaeoproterozoic Rhinns-type crust underlies the Argyll Suite (Dickin & Bowes 1991), Grenvillian and Knoydartian crust probably underlies the Cairngorm Suite (Oliver et al. 2000), whereas Palaeozoic Lakesman crust underlies the Southern Uplands (Soper et al. 1992; Anderson & Oliver 1996).
Late Caledonian calc-alkaline lamprophyre dykes occur throughout Scotland. They are most commonly spatially and temporally associated with the Newer Granites but also occur west of the Moine Thrust (associated with alkaline intrusions, e.g. Borolan), in the Southern Uplands and south of the Iapetus Suture in the Lake District (e.g. associated with the Shap granite) (Rock 1991). The chemistry of these lamprophyres has been described by Canning et al. (1998) and is similar to that of Cretaceous calc-alkaline lamprophyres from the Sulu collisional orogen in eastern China; for example, significant large ion lithophile element and light rare earth element enrichment, NbTa depletion and enriched Sr-Nd isotopic signatures (compare data of Rock (1991), Canning et al. (1998) and Guo et al. (2004)). According to Guo et al. (2004), these characteristics, including moderate Zr/Hf and Nb/Ta fractionations, favour a lithospheric slab break-off model, which induces low-degree melting (
Davies & von Blackenburgh (1995) first proposed the slab break- off model to explain the magmatism and deformation of collisional orogens in the Alps, Aegean and Dabie Shan. Slab break-off has subsequently been proposed for various ancient and modern orogens; for example, the Himalaya (Maheo et al. 2001; Kohn & Parkinson 2002), the Andes (Haschke & Scheuber 2002), New Guinea (Cloos & Sapii 2005) and Turkey (Altunkaynak 2007). Atherton & Ghani (2002) were the first to propose a unilateral slab break-off for the northern British Caledonides. The bilateral slab break-off model proposed here accounts for contemporaneous granite petrogenesis in the northern and southern British Caledonides.
As during the Grampian event at 470 Ma (Fig. 6c) the ascent of hot asthenosphere would initiate high-grade metamorphism in the adjacent lower crust and promote partial melting and the formation of minimum melt S-type granites from Al-rich metasediments at c. 750[degrees]C (Thompson 1999). There is evidence for S-type granites in the Southern Uplands; for example, the Fleet and Griffel plutons in the Southern Uplands have muscovite-bearing granite varieties with ^sup 87^Sr/^sup 86^ Sr initial ratios up to 0.7109 and delta^sup 18^O up to 12[per thousand] (i.e. transitional S-type) attained either by strong fractional crystallization of mantle- derived magmas or, as we prefer, first time melting of the local Silurian greywackes (Stephens 1992). However, the Southern Uplands also exhibit contemporaneous I-type granites (e.g. Loch Doon (Stephens 1992) and Portencorkie (Stone 1995)), which suggests that lower crustal temperatures were high enough in places (e.g. 850- 1050 [degrees]C, Annen et al. 2006) to melt igneous protoliths: a possible candidate could be the Lakesman terrane Borrowdale Volcanic Series, which underlies the Southern Uplands (see Fig. 6f; Anderson & Oliver 1996). Because the climate was relatively dry at this time of Lower Old Red Sandstone deposition, erosion failed to exhume the lower crust and expose the high-grade regional metamorphic rocks formed in the lower crust.
The I-type subduction-related magmas in this model (Fig. 6f) would have evolved in a similar way to the Scandian I-types described above (Fig. 6e); that is, intrusion of mantle-derived basalt magma at the base of or within the lower crust forming an 850- 1050[degrees]C hot zone with melting and subsequent fractionation, hybridization and contamination (Davidson & Arculus 2006; Annen et al. 2006; Kemp et al. 2007). High eNd (0 to -5) and low ^sup 87^Sr/ ^sup 86^Sr initial ratios (
420-400 Ma: Scandian event and batholithic buoyancy of the Grampian terrane
From Figures 1, 4 and 5 it can be seen that c. 80% by volume of Newer Caledonian granite in the northeastern Grampian terrane was intruded between 410 and 405 Ma. The effects of batholithic buoyancy on terrane uplift as a result of the reduction of crustal density with additional emplacement of granite has been explored by Robinson (pers. comm.) using both an Airy and a 2D flexure model. The buoyancy effect of this additional granite would have raised the surface of the central section of the Grampian terrane by as much as 2.6 km. Because an unknown amount of lower crust would have been melted to granite in the hot zone and moved to the upper crust, the 2.6 km of uplift is a maximum estimate. If 5 km of mantle-derived basalt magma (gabbro) was added to the lower crust then the Airy- type isostatic uplift would be c. 0.5 km on top of that caused by the granite buoyancy (Robinson, pers. comm.). In addition, removal of the downgoing slab would lead to rapid isostatic uplift (Davies & von Blackenburgh 1995). The combined result of this uplift would initiate erosion into the Midland Valley: evidence for this is the mega-conglomerates of the 410-400 Ma Dunnottar, Crawton and Arbuthnott Groups, which are full of Grampian-like detritus (Haughton et al. 1990).
By c. 400 Ma Scotland had become part of a large continental landmass, having drifted to 30[degrees]S (Cocks & Torsvik 2002). Compared with modern-day climatic belts, Scotland would have lain under a dry subtropical high-pressure cell. Therefore batholithic buoyancy would have lifted a high, dry, Lower Old Red Sandstone semi- arid or desert plateau over Scotland and northern England as more and more granites were added to the crust (Fig. 6f). Contemporaneous red-bed alluvial fans that were deposited in the stretched Midland Valley and northern England crust (Soper & Woodcock 2003) confirm a semi-arid climate. In the north Midland Valley these conglomerates are dominated by clasts of contemporary Midland Valley andesite, Grampian terrane Barrovian metamorphic rocks and granite. In the south, alluvial fans are dominated by clasts of contemporary Midland Valley andesitic lava plus greywacke debris derived from the Southern Uplands terrane. In the north, as much as 8 km of Lower Old Red Sandstone was deposited in extensional and pull-apart basins (Marshall et al. 1994). In the south, 1.2 km of Lower Old Red Sandstone was deposited (Trewin & Thirlwall 2002). The Highland Boundary and Southern Upland faults would have been active at this time. Thus there is a link between granite intrusion, buoyancy driven uplift and erosion in the Grampian terrane, and deposition and contemporary andesite volcanism in the extending neighbouring Midland Valley terrane (Fig. 6f). This is Andean-type orogenesis (i.e. mountain building) driven by subduction and not continent- continent collision. Scandian orogeny
As noted above, collision of the extended Laurentian margin (i.e. the amalgamated Grampian, Midland Valley and Southern Upland terranes) with Avalonia was a soft collision involving a dominant sinistral strike-slip vector (Soper & Woodcock 1990). In contrast, Strachan et al. (2002) and Dewey & Strachan (2003) have summarized how the Scandian orogenic event involved hard collision of the Northern Highland terrane with Baltica; that is, NW-WNW-directed thrusting along the Naver, Sgurr Beag and Moine thrusts. Evidence for this includes the 435-425 Ma Rb/ Sr, K/Ar and ^sup 40^Ar/^sup 39^Ar mica ages from mylonites and schists (Freeman et al. 1998), isoclinal folding and amphibolite-facies metamorphism of the Western Moine units (Dallmeyer et al. 2001), and formation of the North Highland Steep Belt (Strachan et al. 2002). As the Grampian terrane did not experience these events, the two terranes must have been separated by some hundreds of kilometres (Dewey & Strachan 2003). Sinistral strike-slip faulting along the Great Glen Fault brought the two terranes together during the time of the intrusion of the 425-400 Ma granites into both terranes.
The 390 +- 5 Ma Middle Devonian Glen Tilt granite is in the margin of the dioritic part of the Glen Tilt Igneous Complex (Fig. 1). Locally, the granite shows weak east-west-striking, subvertical foliation and parallel ellipsoidal enclaves of (unfoliated) diorite and (foliated) psammite: this evidence for deformation, plus the elongate outcrop pattern of the diorite and the offset pattern of the local Argyll Group metasediments, suggests that intrusion was controlled by a sinistral Loch Tay-Glen Tilt Fault active at 390 +- 5 Ma. The sinistrally transpressive mid-Devonian Acadian orogeny in Midland and Northern Britain has been attributed to the effects of flat-slab subduction of Rheic Ocean lithosphere (Woodcock et al. 2007). Movement on the Loch Tay-Glen Tilt Fault may be a far-field effect of that orogeny.
Different granites types were intruded into the Scottish crust during the period c. 600 to c. 400 Ma as follows.
(1) A-types were intruded during crustal stretching and rifting of the Laurentian margin at c. 600 Ma after Rodinia broke up and as the Iapetus Ocean expanded.
(2) S-types formed when the Midland Valley Arc terrane collided with the margin of Laurentia at c. 470 Ma and hot asthenosphere rose and occupied a lithospheric slab window, thereby regionally metamorphosing and locally melting the lower crust of the Grampian Highland terrane. High rainfall focused fast exhumation of Barrovian high-grade regionally metamorphosed rocks. This is the collisional Grampian event of the Caledonian orogeny.
(3) More S-types formed during exhumation and decompression of the over-thickened Grampian Highland terrane at c. 455 Ma. The products of this erosion were deposited in the Southern Uplands accretionary prism.
(4) I-type granites appeared at 430 Ma when subduction of Iapetus Ocean lithosphere under Laurentia caused lithospheric slab and mantle wedge melting. These basalt-andesite melts intruded the lower crust of the Grampian Highland terrane, forming a hot zone where the processes of fractionation, partial melting, mixing, assimilation and hybridization produced a range of intermediate and siliceous I- type diorites and granites.
(5) I-type granites continued to form as the slab rolled back, beginning at 420 Ma as the Midland Valley was stretched.
(6) When Baltica ‘hard-docked’ against the Northern Highland terrane and Avalonia ‘soft-docked’ against the Grampian Highland terrane, the subducting lithospheric slabs broke off and effectively delaminated the lithosphere. The bulk of Scottish granites were intruded during this time (c. 410 Ma) as I-types continued to form in the lower crustal hot zones above the sinking slabs (both in the Northern-Grampian Highland and Lakesman terranes) whereas transitional S-types formed where hot asthenosphere rose into contact with the lower crust of the Southern Uplands terrane. Batholithic buoyancy initiated uplift of a high, dry, Old Red Sandstone intercontinental plateau.
(7) Scandian events of the Caledonian orogeny are represented by (4), (5) and (6).
(8) The far-field effects of the Acadian orogeny (c. 390 Ma, flat- slab subduction of Rheic Ocean lithosphere) were recorded by granites that intruded along active faults during sinistral transpressive movements.
The Caledonian orogeny should be regarded as a series of erogenic events rather than as a single orogeny: granites form episodically through the Wilson Cycle of continental drift (stretching and rifting) and oceanic closure (subduction, collision, slab roll- back, slab break-off, and lithospheric delamination). Similar processes apply to the Andean and Himalayan orogenies.
We are grateful for funding from the Carnegie Trust for the Universities of Scotland, the Royal Society of Edinburgh and the Royal Society of London. We thank G. Steinhofe] for help in selecting and collecting samples, T. Kinnard for separating and mounting zircons, D. Condon and M. Martin for dating the Portsoy Gabbro at MIT, Fu-Yuan Wu for many discussions on the origin of granites, R. Parrish for advice on the interpretation of concordia diagrams, E. Hegner for helpful insights on lower crustal heating, and especially Liu Dunyi for providing SHRIMP time and technical support at the Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences. Lastly, we thank M. Whitehouse, R. A. Strachan and C. Kirkland for constructive reviews, which greatly improved the paper.
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