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the majority of rocks in the district are of rhyolitic and rhyodacitic composition, and are ofsimilar mineralogy to the main intrusive rocks. Hence, their mineralogy was relatively stable atthe temperatures of the intruding magmas. The one notable exception is on Green Island (297162), in the extreme east of the district, where hornfelsing related to a granitoid intrusion affectedcoarse ash crystal tuffs of the Mount Davis Forrmation (but not younger medium-grained quartzsyenite dykes).Ruxton (1957) identified tourmaline, muscovite, iron-stained clinozoisite and incipient andalusitein rocks on The Brothers islands and attributed their presence to contact metamorphism, althoughno major intrusion is exposed nearby. Allen and Stephens (1971) recorded a similar mineralassemblage in Lok Ma Chau Formation sediments in northwestern Lantau Island, in strata theyconsidered to be 'hard and baked 1 . They interpreted the assemblage as being due to contactmetamorphism related to underlying intrusive rocks at no great depth. However, the spatialrelationship with possible overthrusting onto the Jurassic volcanic sequence suggests a dynamicmetamorphic association, or hydrothermal alteration associated with the adjacent LantauCaldera.Graphite beds which occur on West Brother island were mined from an area in the core of asyncline. There, the graphite has lustrous foliation surfaces and is interpreted as thermallymetamorphosed by an uncxposed intrusion.Various examples of skara mineralization have been noted in boreholes sunk through buriedmarble cast of { Tung Chung Wan, e.g. boreholes TC65 (12196 16639), H12/17357 (1211216599) and east of East Brother Island (Borehole NS1/14460, 17111 21750). The skams,comprising greenish-grey assemblages of magnetite, variously with epidote, chlorite, augite andplagioclase, occur where marble is intruded by feldsparphyric dykes and granites, and is mainly aconsequence of contact metamorphism, Skarn, including magnetite, garnet, vesuvianite, diopsideand epidote, has also been reported by Peng (1978) at San Shek Wan (0714 1612) in a zone 5 mwide at the contact between foliated sandstones and megacrystic, fine-grained granite.Regional MetamorphismAllen and Stephens (1971) considered that regional metamorphism was of greater significance inthe Territory as a whole than had earlier workers, They proposed that the Carboniferous Lok MaChau Formation had been regionally metamorphosed during the Mesozoic, but prior to granitoidemplacement. Bennett (1984b) interpreted the regional metamorphism of the Lok Ma ChauFormation as of greenschist facies and also suggested the possibility that the formation couldhave been metamorphosed at an earlier date, before it was technically emplaced during theJurassic.Regional metamorphism is not widely developed at surface within the district. However, inboreholes to the north of Lantau Island, marble has been recognised, and much of the offshorearea to the north of Lantau Island may be underlain by the marble, which also extensivelyunderlies the western New Territories and extends northwards into Guangdong Province. Therecrystallization of limestones of Carboniferous age to marble, is therefore a regional-scalemetamorphic process.Regional metamorphism in the western New Territories is primarily associated with thrusting ofthe Palaeozoic basement onto the Jurassic Tsuen Wan Volcanic Group (Langford et al, 1989). Asimilar thrust relationship has been interpreted in northwest Lantau Island, although the thrustfault was subsequently modified during development of the Lantau Caldera. However, regionalmetamorphism related to this structure is less evident in the district than in the New Territories,although the sediments of the Lok Ma Chau Formation have been described on the publishedj105