No abstract

No abstract

ABSTRACT. Scourian ultramafic granulites from the Assynt region, of Sutherland were retrogressed to amphibolite facies during the Inverian and in Laxfordian shear zones. Inverian assemblages are doniinantly chlorite and tremolite with minor anthophyUite and dolomite. Laxford~an assemblages are cummingtonite, chlorite, and dolomite with only minor amounts of tremolite. Retrogression took place on a falling temperature path with temperatures between 750 and 550~ and with H20 forming more than 90~ of the fluid phase. Initially retrogression was caused by the influx of large volumes of hydrous fluid associated with the development of Inverian folds. During the Laxfordian shearing Pco2 was increased slightly. Serpentinization of olivine occurred at a much later stage. T n I s paper considers the retrogression of Scourian ultramafic granulites from the Assynt region of Sutherland during both the Inverian and Laxfordian events. The ultramafic granulites concerned form part of layered ultramafic-gabbro bodies (Bowes et al., 1964) metamorphosed at granulite facies during the Scourian event at c. 2.7 b.y. (Chapman and Moorbath, 1977). The Lewisian gneisses of Assynt have suffered two further tectono-metamorphic events (Sheraton et al., 1973;Tarney, 1973); the first of these, the Inverian, caused widespread retrogression to amphibolite facies associated with the development of north-westtrending monoclinal folds. Most of the retrogression occurred before the intrusion of the Scourie dykes. The gneisses were further affected by localized, discrete Laxfordian shear zones which caused a substantial reduction in grain size and the development of a new penetrative foliation and lineation. There are three types of retrogressive metamorphism affecting the ultramafic rocks:(i) Retrogression without complete recrystallization in or near Inverian fold belts.(ii) Recrystallization accompanied by shearing in Laxfordian shear zones.(iii) Serpentinization of olivine and orthopyroxene.In this paper the mineralogical changes, the P-T 9 Copyright the Mineralogical Society conditions, and the composition of the fluid phase that caused the retrogression and consequent chemical changes will be discussed. PetrographyThe ultramafic granulites have an equigranular, granoblastic texture typical of granulite-facies rocks ( fig. la). They have the assemblage: orthopyroxene-clinopyroxene-olivine-pargasite-spinelmagnetite with harzburgite occurring locally. This assemblage is recognized as being entirely metamorphic with the normative mineralogy classifying the rocks as plagioclase-peridotites.The first sign of retrogression is the exsolution of opaque oxides from hornblende and pyroxene along cleavage planes and grain boundaries, this effect being most pronounced in hornblende ( fig. lb). This is due to the exsolution of excess TiOz from granulite-facies pargasite and is associated with a change in colour from brown to green. When the rock has completely retrogressed, the Inverian assemblage is typically: tremolite-chlorite-magnetite + anthophyllit...

No abstract

Summary Layered gabbros and ultramafics, well exposed in Val Sesia, have been interpreted as a layered complex. The lower part (LLG; up to 1 km thick) comprises well layered pyroxenites and gabbros with minor peridotite and harzburgite. The upper part consists of fairly homogeneous gabbros (MG), which grade into diorite (up to 5.5 km thick). This gabbro body intrudes already highly deformed metapelites, but is itself relatively undeformed. These two groups (LLG and MG-diorite) are distinct on the basis of isotope, trace element and major element chemistry and cannot have been derived from the same liquid. The LLG has ɛ Nd ranging from +1.5 to +2.5 and ɛ Sr from +7 to −8 and was derived from a depleted source. The MG-diorite body has ɛ Sr ranging from +40 to +60 and a wide range of ɛ Nd from −2 to −6. The LLG pyroxenites and gabbros are dominantly cumulates of clinopyroxene with lesser amounts of orthopyroxene. Cumulus plagioclase appears towards the top of the LLG. The LLG is strongly depleted in LREE and LIL elements and was derived from a basaltic liquid with relatively depleted trace element characteristics. It probably fractionated at moderate pressures and was emplaced into the Ivrea Zone along with the adjacent Balmuccia peridotite. The MG comprises plagioclase ± olivine cumulates with minor cumulus apatite, ilmenite and pyroxene. The diorites additionally contain cumulus biotite. The whole body has strongly LREE-enriched patterns (Ce N 4.5–20) and large positive Eu anomalies (Eu/Eu* from 2–12). The MG has calc-alkaline features with high Ba and low Nb. The parental magma was also LREE-enriched, with low Ni, Cr and MgO and high Ba. It was possibly an andesite. The spread in ɛ Nd cannot be accounted for by contamination by any of the likely crustal contaminants. The spread in intial isotope ratios either results from a heterogeneous enriched mantle source or from a two stage process involving remelting of an intermediate mafic reservoir with moderate enrichment in Rb and LREE. The MG-diorite body was emplaced into sediments already undergoing amphibolite-facies metamorphism at pressures of 5–7 kbar. It appears as if the development of magma chambers in the crust was an important process of crustal growth in the Ivrea Zone.