Superficial Geology

Chapter 7
Metamorphic Rocks
The metamorphic rocks of the district are restricted to those formed by the metasomatic and
thermal alteration of the country rock in contact with or adjacent to granitic plutons. No
dynamically metamorphosed rocks have been recognized in the district apart from small
developments of mylonite in fault zones. Regional metamorphism, developed in the northern New
Territories, does not affect the rocks of this district.
Allen & Stephens (1971) described thermally metamorphosed volcanic rocks collected close to a
granite contact, but it was Ruxton (GCO, 1982) who first attempted a detailed examination of the
thermal metamorphic products. Ruxton produced a thermal metamorphic isograd map of the
Victoria Peak area of Hong Kong Island, and recognized four grades of thermal metamorphism.
Thermal (Contact) Metamorphism
The bulk of the pyroclastic volcanic rocks that constitute the country rock surrounding the
granitic intrusions are rhyolitic or rhyodacitic tuffs, possessing mineral assemblages stable at
temperatures as high as those of the intruding molten granite. Rocks within a few metres of the
contact usually show mesoscopic signs of thermal metamorphism, but away from this zone the
evidence of metamorphism is apparent only from the examination of thin sections. The fine and
coarse ash tuffs are metamorphosed to a lesser extent than the tuffaceous sandstone layers.
The area around Victoria Peak has been examined in detail, and the findings correspond
approximately to Ruxton's metamorphic zonation (GCO, 1982). The study of 72 thin sections in
this area by A. Lai (written communication, 1986) has enabled the division of the thermally
metamorphosed tuffs into four zones (Figure 21); Low-grade A (slightly metamorphosed), Lowgrade
B (albite-epidote hornfels fades), Medium-grade (hornblende hornfels facies) and Contact
rock (pyroxene hornfels facies).
Low-grade A (slightly metamorphosed facies). In the rocks of this facies the original
pyroclastic texture is preserved. Feldspar is commonly sericitized and the matrix recrystallised.
Incipient growths of muscovite and biotite are present interstitial to quartz in the matrix. In places
there are traces of garnet. These rocks are widespread and occur more than 500 m from the
nearest granite contact outcrop. Sample HK2225 (3149 1523) is typical; originally a coarse ash
crystal tuff, some recrystallization of the ground-mass has taken place, with quartz intergrown
with biotite and muscovite. Randomly oriented biotite aggregates are associated with garnet.
Low-grade B (albite-epidote-hornfels facies). The rocks of this facies are characterised by
an abundance of fine-grained sericite in the groundmass. Quartz is abundant and there are
variable amounts of garnet and biotite. The original pyroclastic texture is still visible, and in hand
specimen the thermally metamorphosed character is not readily apparent. Lack of available thin
sections covering the area south of the Peak and the Mount Gough area has prevented an accurate
southward delineation of this zone.
Medium-grade (hornblende-hornfels facies). The rocks whose outcrops are represented on
the 1:20 000 maps as thermally metamorphosed rock (hornfels) fall mainly within this category.
They are characterised by complete recrystallization of the matrix and the presence of higher
grade metamorphic minerals, such as andalusite. The original pyroclastic texture is not usually
preserved, apart from scattered larger quartz crystal fragments. Muscovite is abundant, and
clinozoisite and garnet are also common. Ruxton (GCO, 1982) considered this zone to be about
100 m thick in the area northeast of Victoria Peak. An example of this facies is seen in a sample
(HK 2447, 3192 1590) from near Hong Kong University that was probably originally a tuffaceous
sandstone (Plate 42). In thin section this rock consists mainly of inequigranular, interlocking
quartz crystals. Anhedral muscovite occurs interstitially to the quartz. Andalusite is present as
aggregates enclosed by quartz crystals up to 3 mm across.
Contact rock (pyroxene-hornfels facies). Rocks of this facies are found only at, or within a
few metres of, the granite contact. The incipient growth of sillimanite together with annealed
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