Study of petrographic characteristics and rock engineering properties of contact metamorphosed rocks in western Hong Kong Island

Abstract

Contact metamorphism affects a rock by changing its mineralogy and texture, attributes that influence its engineering properties. In Hong Kong, volcanic rocks close to granitic intrusions are commonly affected by contact metamorphism, although their engineering properties remain scantily known. This study combined petrographic analysis and standard rock tests (point load, uniaxial compressive strength and Cerchar) to investigate the rock strength and abrasivity of contact metamorphosed crystal tuffs retrieved from two existing boreholes in western Hong Kong Island. The petrographic analysis found that the crystal tuff had been altered by recrystallisation, chloritisation/sericitisation and/or formation of new minerals such as andalusite. Four classes of metamorphic grade were subsequently defined based on the observed mineral assemblages (chlorite ± seriticte ± andalusite) and degree of recrystallization. The rock test results suggest that higher metamorphic grade is generally associated with lower rock strength, reflecting the increasing influence of chloritisation and/or sericitisation. However, higher degree of recrystallization tends to strengthen the rocks. In contrast, the effect of contact metamorphism on rock abrasivity is imperceptible, as this property also depends on other factors, such as size of groundmass, relative abundance of hard/soft minerals, degree of recrystallization and weathering. The study provides better understanding of the implications of textural and mineralogical changes on the engineering properties of contact metamorphosed tuff, and offers a useful framework for future studies of other metamorphic rocks.