Evidence from feldspar compositions of high temperatures in granite sheets in the Scourian complex, N.W. Scotland

Abstract

AI~STRACT 9 Mesoperthite feldspars from hypersolvus granites, with granulite-facies mineralogy from the Scourian complex, N.W. Scotland have an average bulk composition of (Abo.saOro.42)ssAn12. The mesoperthites have an Ab/Or ratio close to the critical composition of the alkali feldspar solvus for anorthite-free compositions. The critical temperature of the ternary feldspar solvus is estimated at high anorthite contents and used to indicate a temperature in excess of 1000 ~ for the crystallization of the granite sheets prior to granulite-facies metamorphism and permits a maximum water content in the melt of 1~.GRANITIC rocks (sensu stricto) represent a minor part of the granulite-facies gneisses of the Scourian complex. Samples were collected from the area between Badcall and Scourie, Sutherland, where granites form sheets two and three metres wide and up to a few hundred metres long which intrude tonalitic gneiss and metagabbro. Granites in this area represent the last stages of crystallization of a tonalite-trondhjemite-granite suite (Rollinson and Windley, 1980) 9 They have been subsequently metamorphosed to granulite facies and have the mineralogy bluish quartz + alkali feldspar + plagioclase feldspar+_iron-titanium oxides+orthopyro-xene or almandine garnet.Feldspars in the granitic rocks show a wide range of textures, compositions and structural states and include the assemblages orthoclase-mesoperthite, orthoclase-mesoperthite + plagioclase and orthoclase-mesoperthite + plagioclase + microcline. It should be noted, however, that not all the feldspar assemblages are in equilibrium 9Mesoperthite textures. Copyright the Mineralogical Societyoriented lamellae of orthoclase in plagioclase ( fig. la), which are either continuous over the whole 9 length of the grain or which bifurcate and taper to a point. The lamellae are uniformly developed throughout the grain and there is no significant variation in lamellar width. There are also broad (40 #m) irregular lameUae ( fig. lb) with no preferred crystallographic orientation 9 In fig. lc fine regularly spaced lamellae in the centre of the grain pass abruptly into coarse irregular lamellae at the edge of the grain indicating that the coarse lamellae developed from the fine. The texture in fig. lc suggests that the main control on perthite coarsening was the availability of a fluid phase which was unable to circulate freely 9The results of iron-titanium oxide thermometry and oxygen barometry in the same samples suggest that a hydrous fluid phase was introduced into the Scourian granites in the temperature interval 660 ~ to 530 ~ during cooling from the granulitefacies metamorphism (Rollinson, 1980). This agrees well with the results of two-feldspar thermometry (Stormer 1975; Whitney and Stormer 1977) on host-lamella pairs in mesoperthite, which indicate that alkali exchange ceased in the temperature interval 500 to 550 ~ Coarsening of mesoperthite lamellae therefore, probably took place in the presence of a hydrous fluid phase during the retrogression and ceased...

High-temperature Metamorphism and Crustal Anatexis

“…Additionally, relations between the leucogneisses and tectonic fabrics in the Stoer and Scourie areas indicate that the sheets were emplaced dose to the peak of the Badcallian event , O'Hara & Yarwood 1978, Rollinson & Windley 1980b). Rollinson (1982) furthered this idea by suggesting that the leucogneisses were formed at low (0.05 GPa, 0.5 kbar) pressure and represented melts with < 1 wtOJo H20. Veins from the basic gneisses (x), plagioclase amphibolite supracrustals (.…”

“…If the leucogneisses were derived from fractional crystallization (Rollinson & Windley 1980b), or even originated from a source outside the complex, they would still represent melts that existed at the peak of metamorphism (O'Hara & Yarwood 1978, Rollinson & Windley 1980b, Rollinson 1982, Sills & Rollinson 1987. Retrogression in regions which contain few leucogneisses is limited to relatively minor volumes of fluid channelled along the Inverian shears (possibly from underlying leucogneiss-rich zones).…”

“…As these lithologies dominate the Scourian Complex, this argument bears on the overall röle of partial melting in the shaping of the geochemical, isotopic, and mineralogical character of the terrain. However, Rollinson & Windley (1980b) and Rollinson (1982) proposed that these lithologies were derived from the fractional crystallization of the igneous precursors to the tonalites. Only if the tonalites were anhydrous during the later stages of pro grade metamorphism could partial melting have been prevented.…”

Diffusion-controlled and replacement microtextures in alkali feldspars from two pegmatites: Perth, Ontario and Keystone, South Dakota

“…2a) and will lie in or close to (100), not far from ( 601), the plane of coherent film perthites. Lenticular lamellae in non-turbid mesoperthites in granulite facies rocks (see Rollinson, 1982, fig. 1; Evangelekakis et al 1993, figs 2, 3;Cayzer, 2002;and Parsons and Lee, 2005, figs 16-18, for examples), which have periodicities up to ∼20 μm, are in ( 601).…”

Crystallization of melts, pegmatite intrusion and the Inverian retrogression of the Scourian complex, north‐west Scotland

“…The Inverian retrogression commenced when temperatures had fallen to 600650"C, at an ambient pressure of 3-6 kbar (Rollinson, 1980(Rollinson, ,1982Sills, 1982Sills, , 1983; Cartwright & Bamicoat, 1986). The precise age of the Inverian event is not known, but its commencement predates the formation of the earliest Scourie dykes at c. 2.39 Ga (Chapman, 1979).…”