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Chapter 8Metamorphism and AlterationIntroductionMetamorphism comprises a broad spectrum of post-emplacement fluid, thermal, and mechanicalprocesses which change the primary chemistry, mineralogy and texture of rocks. Alterationprocesses are similar, but involve less extreme physical and chemical gradients, and are usuallymore localised. However, there is no clear demarcation between metamorphism and alteration,and they can be regarded as end-members of a continuous spectrum. Metasomatism is commonlyused to describe processes of alteration involving the passage of hot fluids, derived internally orexternally through rocks, but the term alteration is preferred in the following descriptions.It is beyond the scope of this memoir to describe comprehensively all forms of metamorphismand alteration in the district. It has, however, been possible on the published sheets to indicate,using overprint, the spatial limits of some areas in which the rocks have, in broad terms, beenmetamorphosed. In most instances, these rocks have undergone limited mineralogical andchemical changes, and can equally be regarded, therefore, as altered.Hydrothermal alteration is probably more widespread than any other form of alteration in thedistrict, but examples of contact metamorphism, caused by heat from adjacent intrusions, arealso common, albeit generally localised. Regional metamorphism affected limestones, known onlyin boreholes, and possibly Carboniferous rocks in general. Many rocks have been texturallychanged by mechanical processes under the influence of far-field, or local stress, and aretherefore dynamically metamorphosed.Superficial weathering also alters mineralogy and chemistry, but its effects are moreappropriately described in Chapter 9, within the context of other surface processes.Hydrothermal AlterationHydrothermal alteration is probably the dominant secondary alteration process in the district. Itcan be related principally to the evolution of the Lantau Caldera, and to the emplacement ofgranites during the Jurassic and Cretaceous. In addition, it was associated with the emplacementof other intrusions, including the fcldsparphyric rhyolite dyke complex in northeast LantauIsland, and is spatially related to faults in the district. Further types of hydrothermal alteration,resulted in the kaolinization of granites on Chek Lap Kok, and greisenization of granites.Hydrothermal alteration is extensive within the confines of the Lantau Caldera, and affects allrock types, and has been mapped in several areas. It is depicted on the published maps under thebroad category metamorphism and is characterised by silicification, the development of quartzveins, iron staining and recrystallization.One of the largest mapped areas of metamorphism is to the south and east of Tai 0. It occurswithin a corridor, up to 2 km wide, which stretches from Fan Lau in the southwestern tip ofLantau Island, northeastwards to Sze Shan (0512 1270), and beyond, tapering rapidly to thesoutheast of Sham Wat. It affects a wide range of rock types within the Shing Mun and Lantauformations, The northwestern limit of this area of metamorphism coincides in part with thefaulted boundary of the Lantau Caldera, but has not been recognised in all Lantau Formation103