Petrographic study and geological model of the contact between meta-tuff and meta-sedimentary rock under Liantang boundary control point (BCP) area

Abstract

Throughout the history of geological mapping in the northern New Territories, the contact relationship between meta-sedimentary rocks and meta-tuff has been interpreted differently. The contact was encountered during a ground investigation project involving closely-spaced drillholes located to the north of Ta Kwu Ling. Owing to the good quality of rock core recovered from the drillholes, an ideal opportunity arose to examine the contact in detail, and to unravel the metamorphic and structural history. Data from 272 drillhole logs, including 219 logs from project drillholes and 53 drillhole records from previous ground investigation projects were reviewed. Twenty-seven rock samples and thin sections were examined to study the mineral composition and micro-structure of the rocks. The meta-sedimentary rocks and meta-tuff have experienced low-grade regional metamorphism to greenschist facies, with the typical mineral assemblage of Qtz + Ser/Ms + Chl + Bt + Ep +Ca. The occurrence of garnet in isolated areas of the project site suggests that the rocks may locally have experienced a higher metamorphic grade. The project site is underlain by one north-northeast trending fault and three northwest-trending faults, which are consistent with previously mapped structural orientations. The rockhead within the project site is controlled by the heavily fault subcrop. Two types of contact between meta-sedimentary rocks and meta-tuff have been identified within the project site: (1) a conformable contact and (2) a structural (faulted) contact. 2D and 3D modelling of the contact zone suggests that the conformable contact is dipping northwest-west at 30° to 40°, whereas the strikeslip faulted contact is oriented steeply (70° to 80°) and is accompanied by a narrow crushed zone. At least three deformations have been identified based on the micro-structure analysis in meta-sedimentary rocks and meta-tuffs. The first two deformations (F1 and F2) were associated with folding and regional metamorphism while the third deformation was associated with dynamic metamorphism. The dynamic metamorphism, both brittle and ductile, probably occurred during localized shearing, strike-slip faulting and/or thrusting. Metasomatism and/or hydrothermal alteration may have accompanied the dynamic metamorphism, or may have been associated with emplacement of nearby granitoid bodies. The field observations, petrographic and structural analysis of the contact zone have shown agreement with the observations made by early workers and support the possibility that the stratigraphic sequence has been overturned during regional folding and metamorphism.