Archaean granulite facies metamorphism of the Lewisian of Tiree, Inner Hebrides, north‐west Scotland

Abstract

The Lewisian of Tiree, north-west Scotland, underwent granulite facies metamorphism prior to 2.4 Ga. The temperatures and pressures estimated from garnet-clinopyroxene, garnet-orthopyroxene, hornblende-plagioclase and garnet-biotite geothermometers and clinopyroxene-plagioclase-garnetquartz and orthopyroxene-plagioclase-garnet-quartz geobarometers are 810 f 50" C and 10.5 f 1.5 kbar.The imprecision of pressure estimates stems largely from uncertainties in garnet activity models. Calculations of blocking temperatures for Fe-Mg interdiffusion in clinopyroxene and garnet suggest that these temperatures and pressures represent only slightly reset peak-metamorphic conditions. Down-temperature re-equilibration resulted in chemical zoning over the outer 50-100 pm of the mafic minerals. P -T paths calculated from this mineralogical zoning suggest nearly isobaric cooling. However, the growth of late sillimanite in metapelites requires that the retrograde P-T path had a significant decompression component, suggesting that the mineralogical zonation does not define the retrograde P-T path. The discrepancy between the P-T path calculated from mineralogical zonation and that implied by mineral reactions probably results from the net-transfer geobarometry reactions closing at higher temperatures than the exchange geothermometers.The Tiree rocks have a similar history to the mainland Scourian complex. Granulite facies metamorphism accompanied by partial melting occurred prior to the intrusion of the Scourie dykes at c. 2.4 Ga, and the rocks underwent retrogression both prior to and after dyke emplacement. However, peak metamorphic temperatures and pressures on Tiree were lower than those recorded in the Scourian complex, and the Tiree rocks may have been at a different crustal level at that time.

Garnet compositions in Scottish and Norwegian basement terrains: a framework for interpretation of North Sea sandstone provenance

“…Similar type A garnets occur in river and beach sediments from southern India, derived from charnockitic and high-grade metasedimentary rocks (Sabeen, Ramanujam, & Morton, 2002), as well as in high-grade Lewisian metasedimentary rocks of South Harris (Fig. 2) and Tiree (Cartwright, 1992). The type A garnets characterising the river sediments from area N5 therefore confirm that high Mg, low Ca garnet is derived from charnockites or granulites-facies metasedimentary rocks.…”

“…Since chloritoid and chrome spinel are scarce in sediments sourced from other Scottish basement areas, the combination of abundant chrome spinel, chloritoid and Field B garnets is a clear indication of a Shetland provenance. Cartwright (1992) and Fettes et al (1992). Filled circles are garnets from metabasic gneisses, filled squares are garnets from metapelitic gneisses.…”

Origin of felsic sheets in the Scourian granulites: new evidence from rare earth elements Published in Scottish Journal of Geology, Vol. 30(2), 1994, pp. 121–129.

“…I agree with the spirit of his comment, namely that the Scourian granulites should have melted given the calculated P-T conditions for the granulite-facies metamorphism. Further, it is probable that in restricted localities, notably in the rare metasediments, the Scourian granulites did melt, as is seen for example on Tiree (Cartwright 1992). Nevertheless the felsic sheets in the Scourian granulites, the subject of this discussion, are not the product of such partial melting.…”

Mixed metamorphic and fluid graphite deposition in Palaeoproterozoic supracrustal rocks of the Lewisian Complex, NW Scotland

“…The combination of graphitic sediments, marbles and rust zones occur in each of six outliers: Gairloch‐Loch Maree, South Harris, Tiree, Iona, Glenelg‐Loch Duich and Scardroy (Figure 1). The similarities have been inferred as evidence that all are of similar age (Cartwright, 1992; Coats et al., 1997; Rock, 1987; Tilley, 1936). The outliers were formerly assumed to be in the main Lewisian succession, i.e.…”

“…The outliers were formerly assumed to be in the main Lewisian succession, i.e. Archean (Cartwright, 1992; Whitehouse & Russell, 1997). Three of the six outliers now have age constraints in the mid‐Palaeoproterozoic.…”