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SynopsisThree narrow bands of chloritoid-bearing pelitic schist cross Shetland and are distinguishable both in field appearance and chemical composition (high Al and/or high Fe 2+ /Mg) from adjacent non-chloritoid-type pelitic schists. Of these three, the easternmost band, the Dunrossness Phyllites (upper Dalradian), is a chloritoid-muscovite phyllite also containing chlorite, quartz, kyanite and paragonite, overlying muscovite-chlorite-quartz pelitic phyllites, the Clift Hills Phyllites. Both types of pelite are cut by the Spiggie Granite. Thermal metamorphism extends 2 km from the granite in the Dunrossness Phyllites, but only 0.5 km in the adjacent Clift Hills Phyllites. In the Dunrossness Phyllites thermal zones based on the incoming of chloritoid, chloritoid-staurolite, staurolite-biotite, and retrograde chloritoid (near the granite contact) are recognizable, as are thermal zones based on the incoming of kyanite, andalusite, and fibrolite (near the contact). In the Clift Hills Phyllites, thermal zones based on the incoming of garnet, staurolite-biotite (in continuity with that in the Dunrossness Phyllites), andalusite and fibrolite occur. In Unst, in the northward continuation of the Dunrossness Phyllites, the Skaw Granite produces similar zones, except that thermal andalusite is absent, indicating metamorphism at greater pressure. The difference in response to metamorphism of the chloritoid and the non-chloritoid-type pelites in terms of reactivity and mineralogy, is' further emphasized by the widespread presence of andalusite, chloritoid, white mica and chlorite as retrograde minerals in the Dunrossness Phyllites.

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The study area is underlain predominantly by syenite, monzonite and gabbro. The area lies on the western flank of Benom batholith. Field mapping has proven the intrusive nature of the suite of rocks into the surrounding country rocks. The country rocks were contact metamorphosed into various hornfelses of hornblende hornfels facies. The occurrence of a crystal settling layer within the suite of rocks is particularly convincing of an igneous origin. Moreover, the occurrence of accidental xenoliths in these igneous rocks, which correspond to the aureole rocks, indicate that the xenoliths were incorporated during magma intrusion. The presence of different types of cognate xenoliths ranging from pyroxinite to gabbroic and syenitic in composition, is also widespread in the study area. The occurrence of planar flow structure in the syenite and monzonite defined by megacrystic K-feldspar, is evident of magmatic flow. Petrographic studies reveal that igneous foliation in the suite of rocks is made up of euhedral to subhedral plagioclase and K-feldspar crystals. Abstrak: Kawasan kajian terdiri daripada batuan syenit, monzonit dan gabro. Kawasan ini terletak di sebelah barat Komplek Benom. Pemetaan lapangan menunjukkan batuan ini menerobos batuan sekelilingnya yang terdiri daripada metamorfisma hornfel daripada fasis hornblende hornfel. Kewujudan lapisan pemendapan hablur di dalam suit batuan ini mencadangkan asalan igneus. Tambahan lagi kewujudan xenolit yang mempunyai ciri-ciri persamaan dengan batuan sekeliling mencadangkan xenolit tersebut telah terjatuh ke dalam magma semasa penerobosan magma tersebut. Kewujudan pelbagai jenis xenolit kognat berjulat daripada piroksinit ke gabroik dan syenitik juga banyak dijumpai di kawasan kajian. Kajian petrografi menunjukkan bahawa foliasi igneus di dalam batuan ini dibentuk oleh plagioklas yang berbentuk euhedral to subhedral plagioclase dan alkali feldspar.

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