The intrusive igneous rocks of Hong Kong comprise major subvolcanic
intrusions of granodiorite and granite, and minor intrusions
of monzonite, quartz monzonite, rhyodacite, rhyolite,
microgranite, and mafic dykes. Collectively, these granitic
and related rocks occupy approximately 35% of the land
surface area (Figure
6.1) and probably underlie much of the offshore
area.
Systematic mapping of the granitoids began in the late
1960s with recognition by Allen
and Stephens (1971); (Table
6.1) of four main phases of Jurassic to Cretaceous
plutonism. They assigned widespread intrusive units
to an emplacement phase according to lithology and mutual
cross-cutting relationships. However, they made no attempt
to distinguish individual plutons (used here as contiguous
single intrusive bodies covering areas of generally
less than 100 km2 but greater than 10 km2),
or stocks (used here as contiguous single intrusive
bodies covering areas of less than 10 km2).
Mapping of intrusive rocks at scales of 1:20 000 and
1:5 000 was subsequently undertaken by the Hong Kong
Geological Survey (e.g. GCO,
1986a, b;
Chapter
1). A grain size-based classification was used to
map the intrusive units but these were not individually
defined. The first pluton-based nomenclature for Hong
Kong’s granitoid rocks was introduced by Strange
et al., 1990 following traverses
of intrusive contacts. Refinements to this nomenclature
were later made by Sewell
et al. (1992) who divided the granitoids
into two suites (groups of comagmatic rocks with characteristic
chemistry and mineralogy) on the basis of field, petrographic,
geochemical and age criteria. Nd and Sr isotope studies
(Darbyshire
& Sewell, 1997) and U–Pb dating
of the granitoids (Davis
et al., 1997)
have since led to further revisions of the pluton and
suite nomenclature (Table
5.3).
Plutonic and volcanic rocks are generally classified according
to their modal mineralogy, based on the relative proportions
of quartz (Q), alkali feldspar (A), plagioclase (P),
feldspathoids (F), and mafic (M) and related minerals
(Le
Maitre, 1989). In Hong Kong, however, many
of the minor intrusive rocks (rhyolite dykes) and volcanic
rocks are so fine grained that individual crystals are
impossible to distinguish, even under the microscope.
Therefore, to facilitate comparisons with the volcanic
rocks, the granitoid rocks are classified according
to their normative mineralogy based on whole-rock chemical
compositions. Since there are no known silica undersaturated
(or feldspathoid-bearing) plutonic rocks in Hong Kong,
the normative QAP diagram of Le
Maitre (1989) is used (Figure
6.2).
The grain size-based classification for intrusive rocks on the
published 1:20 000-scale and 1:5 000-scale geological maps was introduced
mainly for engineering purposes and the grain size definitions follow
those used in civil engineering (GCO,
1988a). In most instances,
this classification is sufficient for routine ground investigation
and rock core description.
Considerable textural variation exists among the plutonic rocks.
Grain size varies from coarse- (>6 mm), to medium- (2–6
mm), fine- to medium- (1–3 mm), fine- (0.06–2 mm), and
very fine-grained (<0.06 mm) lithologies. In pegmatites (see
below), the crystals may be unusually large. Strange et al., 1990 recommended
the use of very coarse-grained for those granitoids with grain sizes
of >20 mm, but this has not been generally applied. Although
Hong Kong granitoids have dominantly equigranular textures, porphyritic
texture is also relatively common and is particularly well developed
in feldsparphyric rhyolite dykes. Exceptionally large phenocrysts
are sometimes described as megacrysts. The most common microscopic
texture seen in the granitoids is granular where subhedral equant
interlocking crystals predominate. However, in strongly porphyritic
lithologies, such as the Lantau Dyke Swarm, a radiating intergrowth
of quartz and alkali feldspar, known as granophyric intergrowth,
may be present. This texture makes the distinction of modal proportions
very difficult. Perthitic intergrowth of plagioclase within an alkali
feldspar host grain, is also common in some granitoids. Bipyramidal
quartz phenocrysts are common in strongly differentiated rhyolite
dykes.
The dominant mafic mineral in the granitoids is biotite with subordinate
hornblende present in the less silicic compositions. There are no
known alkali mafic minerals (e.g. arfvedsonite or aegirine) in Hong
Kong granitoids. Accessory minerals include zircon, apatite, titanite,
allanite, xenotime and monazite. The Fe-oxides include ilmenite
and magnetite.
Recognition of intrusive bodies (plutons, stocks and dykes) in Hong Kong has
relied upon a combination of field and laboratory observations,
supported by geochemical and age data. Owing to the variety of intrusive
lithologies present, and the considerable textural variation within
some bodies, the broad plutonic distinctions were first established
by tracing out intrusive contacts. In some cases, isolated granitoid
outcrops were difficult to assign because their textural variation
meant that different areas of the same intrusive body could resemble
more than one other intrusion. In such cases, petrography and geochemistry
have proved valuable because each intrusive body has its own distinctive
mineralogical and chemical signature.
The main compositional categories represented amongst Hong Kong
granitoids are monzogranite, granodiorite, monzonite, and quartz
monzonite. The most useful textural and mineralogical features for
identifying these granitoids include grain size, biotite habit,
colour, presence of amphibole, zoning in plagioclase, twinning in
alkali feldspar and presence of accessory minerals such as allanite,
titanite, monazite, and fluorite. Useful field characteristics include
presence or absence of miarolitic cavities (see below), rhyolite
dykes and foliation (see below). Some plutons (e.g. Kowloon Granite)
are texturally more uniform than others (e.g. Sha Tin Granite).
Therefore, in most cases, a combination of field, textural, and
mineralogical observations have been needed to identify a particular
intrusive body with confidence. Where this was not possible, geochemistry
was used as a final resort.
General field relationships
The close spatial and temporal associations between the volcanic
and plutonic rocks of Hong Kong suggest that the granitoids were
emplaced at a relatively shallow crustal level, probably within
a few kilometres of the ground surface. Many plutons were intruded
directly into volcanic rocks of similar age and composition. Thermal
metamorphic aureoles in the country rocks are not well developed
and are generally restricted to relatively narrow (10–20 m)
zones.
The shape and size of the intrusive bodies in Hong Kong suggests
that a combination of factors controlled their emplacement. The
Tai Po Granodiorite and Lamma Granite plutons are largely elliptical
and their shape is controlled by the major northnortheast- to northeast-trending
faults. The margins of these plutons are commonly foliated due to
contemporaneous deformation. Large Z-shaped outcrop patterns for
the granodiorite also suggest emplacement during a time when oblique
shearing was occurring (see below). Contacts with volcanic country
rocks commonly dip at a low angle suggesting that the granodiorite
was partly emplaced as a series of high level sills. These are likely
to be connected at depth to a larger batholithic body.
A slightly different tectonic regime, possibly indicating relaxation,
accompanied emplacement of the younger Tai Lam and Tsing Shan granites.
Although these plutons show a tendency to be elongate in a northeast
orientation, they also commonly contain large foundered blocks of
country rock suggesting that stoping may have played an important
role in their emplacement. A return to a strong control of emplacement
by northeast-trending faults is shown by the plutons of the Kwai
Chung Suite. The southwestward transition from these plutons, through
the northeast- to eastnortheast-oriented Lantau Dyke Swarm and the
overlying Lantau Caldera, both of approximately the same age as
the plutons, may depict an oblique section through an upper crustal
volcanic plumbing system (see below). The youngest plutons of the
Lion Rock Suite are largely subcircular in outcrop but their margins
preserve a rectilinear outline. This suggests that both northeast-
and northwest-oriented faults controlled the distribution of the
magma. In places, dyke-like intrusions of granitic magma invaded
the volcanic country rock along faults and spread out as minor sills
between volcanic layers. In other places, contacts with the surrounding
country rock are sinuous while there is annular folding in the country
rock. These features suggest that intrusions of the granitoids by
ballooning and stoping mechanisms occurred at the same time as faulting.
Structures
Various structures are present within the Hong Kong granitoids.
These are particularly useful in characterising the different plutons
and in determining the tectonic stresses at the time of their emplacement.
Miarolitic cavities are typically found in the fine-grained granites
such as the Mount Butler Granite and the Needle Hill Granite. These
small (<30 mm) irregular cavities, often filled with drusy quartz,
represent exsolved gas pockets within the late crystallizing magma.
They also affirm the shallow level of emplacement of these plutons.
Pegmatites and aplites are relatively common within the granitoids.
The pegmatites vary from a few tens of millimetres up to 1 m wide,
whereas the aplites vary from veins typically tens of millimetres
thick, up to dykes, as much as 25 m wide. Pegmatites may occur sporadically
in any of the plutons, although they tend to be concentrated along
the roof margins.
Magmatic foliations are present in many of the granites. These
include mineral banding and layering (known as schlieren)
and alignment of megacrysts in rhyolite dykes. Schlieren
is relatively common, and comprises bands of aggregated
biotite crystals up to 30 mm wide. In rare instances,
partial assimilation of country rock by granitoid magma
has produced flow foliation in the form of a crystal–melt
mush (Plate
6.1).
In the northwest of Hong Kong, tectonic foliations are developed
in granitoids of the Lamma Suite that have undergone ductile deformation.
The resulting structure in these rocks is extensive microfracturing
which is accompanied by weakly aligned biotite, and stretched quartz
and feldspar crystals. Shear zones within the granites, representing
zones of brittle deformation, are also frequently encountered.
Joints are ubiquitous in the granitoid rocks and are often related
to cooling and contraction. Sometimes, the joints have
exploited planar foliations in the rock, such as schlieren.
Two to four major subvertical joint sets are generally
present in granitoids in Hong Kong. These may be accompanied
by a further low-angle joint set. Low-angle sheeting
joints are common, and are particularly well seen on
the outlying islands (Plate
6.2). These are considered to form as a result of
stress release during removal of overburden pressure
(e.g. during weathering and erosion).
Dark-coloured inclusions (or enclaves) are common amongst
the granitoids of the Cheung Chau and Lion Rock suites,
and are found in small concentrations in older plutons
of the Lamma Suite. Most of the enclaves in the Cheung
Chau and Lion Rock suites are ellipsoidal and less than
0.5 m in diameter, and are considered to represent immiscible
globules of mafic magma of the same age. A concentration
of angular rock fragments in granodiorite exposed on
Stanley Peninsula (Plate
6.3) is thought to represent incorporation of country
rock from an earlier magmatic phase (Tuen Mun Formation).
On Lamma Island, there is abundant evidence of mingling
of mafic and granitoid magma in the youngest plutons
of the Lion Rock Suite. Composite mafic–felsic
dykes in northern Lantau Island, and on Sung Kong Island
(Plate
6.4), also indicate bimodal magmatism, which is
typical of extensional environments.
Quartz veins are ubiquitous within the plutonic rocks width from
millimetres up to several metres and occasionally tens of metres.
Individual veins can sometimes be traced along strike for several
hundred metres. Throughout Hong Kong, the broadest quartz veins
are commonly found associated with major fault zones. On Lantau
Island, swarms of minor quartz veins have a consistent northnorthwest
strike suggesting that they are structurally controlled. The quartz
veins are most strongly mineralized where they occur within highly
silicic (fractionated) granitoid rocks. These granitoids typically
have high concentrations of trace elements. Although the quartz
veins have not been dated, many are probably associated with the
main episodes of volcano–plutonism.
Geochronology and geochemistry
The earliest attempts to date the granitoids of Hong Kong isotopically
were made by Chandy and Snelling (in Allen
& Stephens, 1971) who reported seven K–Ar cooling
ages which range from 143 ± 4 Ma to 117 ± 3 Ma, with
a tendency to cluster about 135 Ma. However, the sequence of emplacement
of the granitoids was poorly understood and many plutons were not
analysed.
Rb–Sr whole-rock age dating undertaken by Sewell et al. (1992)
on a number of newly recognized intrusive units yielded ages in
the range 155–136 Ma. Older plutonic bodies were identified
on the basis of cross-cutting relationships, but were not analysed
due to problems in obtaining fresh material.
Using U–Pb single zircon analysis, Davis
et al. (1997) dated the oldest Jurassic
intrusive rocks at 164–161 Ma. They also identified
a Triassic granitoid (236 Ma) using core obtained offshore
in the northwestern part of Hong Kong. Several other
plutonic bodies were also dated using the same technique,
confirming at least four main episodes of Middle Jurassic
to Early Cretaceous magmatic activity (Table
6.1).
Whole-rock geochemistry has further indicated that each episode
represents a compositionally distinct intrusive pulse (Sewell
& Campbell, 1997). Each intrusive pulse has now been
accorded the status of a suite to encompass widely separated bodies
with similar age, petrography and geochemistry (Campbell
& Sewell, 1998). Two subgroups of granitoids with
similar age and isotope characteristics, but slightly different
whole-rock chemical signatures, are recognized in the oldest suite.
These have been termed the 'I-type' Subsuite and the 'A-type' Subsuite
respectively (Campbell
& Sewell, 1998), the names reflecting slightly different
compositional characteristics. In the youngest suite, two further
compositional subgroups are also recognized. These have been termed
the ‘Monzonitic’ Subsuite and the ‘Granitic’
Subsuite respectively (Campbell
& Sewell, 1998).
The granite suites generally correlate well with volcanic groups,
which are of similar composition and age (Table
5.1). More recent U–Pb radiometric dating
(GEO, unpublished data) has indicated the presence of
a magmatic event between the first and second major
episodes of Middle Jurassic to Early Cretaceous intrusive
activity.
Crustal sources of the granites
The variations in field characteristics, geochemistry, and age
among the Middle Jurassic to Early Cretaceous granitoid plutons
indicate changes in composition and style of emplacement through
time. These changes are consistent with a transition from a subduction-related
to an extension-related tectonic regime (Sewell
et al., 1992).
The earliest plutonic pulse comprises granitoids belonging to the
Lamma Suite which have isotope, age and zircon characteristics indicating
the involvement of late Archaean crust in the source region (Darbyshire
& Sewell, 1997).
The Cheung Chau and Lion Rock suites comprise granite plutons with
isotope, age and zircon characteristics indicating the involvement
of dominantly Mesoproterozoic crust in the source area.
The Kwai Chung Suite includes granitoids with isotope and chemical
characteristics that are transitional between the Lamma Suite on
the one hand, and the Cheung Chau and Lion Rock suites on the other.
The isotope, age and zircon characteristics of these rocks suggest
derivation from late Archaean, Mesoproterozoic crustal and mantle
sources.
Twenty major and six minor intrusive units have been
identified in Hong Kong (Figures
6.3, 6.4,
6.5
& 6.6).
The major units consist of granitoid plutons and stocks,
whereas the minor units essentially comprise dyke swarms.
With the exception of the Triassic Deep Bay Granite,
the granitoids are described in terms of plutonic suites
according to established field, petrographic and age
relationships.
Granitoids belonging to the earliest pulse of Middle
Jurassic to Early Cretaceous plutonism are assigned
to the Lamma Suite (Sewell
et al., 1991; Campbell
& Sewell, 1997). They include the Tai Po
Granodiorite, Lantau Granite, Chek Lap Kok Granite,
Tai Lam Granite and Tsing Shan Granite, and a swarm
of quartzphyric rhyolite dykes (Chek Mun Rhyolite).
Details of the evolution of intrusive rock nomenclature
are given in Table
6.1. The granitoids of the Lamma Suite are fragmentary
and widely distributed in outcrop in Hong Kong. This
is due in part to later magmatism and extension. However,
most intrusions are concentrated in the central and
western New Territories (Figure
6.3). Two subsuites, the 'I-type' Subsuite and the
'A-type' Subsuite, have been identified (Campbell
& Sewell, 1998).
This implies that two slightly compositionally different
source rocks contributed to the genesis of the Lamma
Suite. Petrographical and morphological characteristics
of the suite are summarized in Table
6.2.
- - 'I-type' Subsuite
The granitoids of the 'I-type' Subsuite include the Tai Po Granodiorite and the Lantau Granite. These granitoids form large irregular plutons that occur across central and southern parts of Hong Kong. They may extend also into mainland waters farther south. U - Pb single zircon age-dating indicates that the granitoids were emplaced between 164.6 and 161.5 Ma.
Three granitoids and a rhyolite dyke swarm are assigned to a second
intrusive pulse of the Lamma Suite (Table
6.2). These intrusions have slightly different petrography
and geochemistry from those of the 'I-type' Subsuite,
and include strongly fractionated fluorite-bearing compositions
with high concentrations of Rare Earth Elements, Y and
Nb. U–Pb zircon ages indicate that emplacement
of the 'A-type' Subsuite occurred between 160.8 and
159.3 Ma (Davis
et al., 1997).
Granitoids of the second pulse of Middle Jurassic to Early Cretaceous
plutonism form extensive outcrops in central and southern
parts of Hong Kong (Figure
6.4). They comprise subcircular to elliptical plutons
of biotite monzogranite, and a swarm of porphyritic
microgranite, rhyodacite and rhyolite dykes (Lantau
Dyke Swarm) (see below). These intrusive units were
emplaced between 148 and 146 million years ago. A swarm
of quartzphyric rhyolite dykes in the southeastern extremity
of Hong Kong Island (Hok Tsui Rhyolite), recently dated
at 151.9 ± 0.2 Ma (GEO, unpublished data), is
described with this suite.
Felsic dykes belonging to a dense swarm of eastnortheast-trending
felsic dykes concentrated in northern Lantau Island
(Plate
6.14) were described as feldspar porphyry by Allen
and Stephens (1971). They are by far the
most voluminous of all dyke rocks in Hong Kong and have
since been referred to as feldsparphyric rhyolite on
published 1:20 000-scale geological maps (GEO
1991, 1994,
1995,
1996).
On the basis of field, petrographic and geochemical
criteria, two main generations of dykes are now recognized
(Sewell
& James, 1995). The older swarm comprises
relatively broad dykes (>5 m wide) of dominantly
rhyodacitic composition (East Lantau Rhyodacite), whereas
dykes of the younger swarm are narrower (<5 m wide)
and dominantly rhyolitic in composition (East Lantau
Rhyolite). Porphyritic microgranite is a textural variant
of the feldsparphyric rhyolite and is compositionally
equivalent to rhyodacite dykes.
Granitoids of the Cheung Chau Suite represent the third pulse of
Middle Jurassic to Early Cretaceous magmatism. They form a compositionally
coherent group of biotite monzogranite plutons, rhyodacite dykes,
and quartz monzonite stocks that were emplaced approximately at
143 Ma.
Intrusive units of the fourth pulse of Middle Jurassic to Early
Cretaceous plutonism can be split into a ‘Monzonitic’
Subsuite and a ‘Granitic’ Subsuite (Campbell
& Sewell, 1998). These intrusions were emplaced at
around 140 Ma and represent the final pulse of plutonic activity
recorded in Hong Kong.
- - 'Monzonitic' Subsuite
The 'Monzonitic' Subsuite mostly comprises stocks and dykes of quartz
monzonite exposed in southern and western areas of Hong Kong. The
subsuite includes three main intrusive units: the D'Aguilar Quartz
Monzonite, the Tong Fuk Quartz Monzonite, and the Tei Tong Tsui
Quartz Monzonite.
The 'Granitic' Subsuite comprises granite intrusions that form subcircular
plutons and irregular stocks in the south and west of Hong Kong.
The subsuite includes five main intrusive units: the Kowloon Granite,
the Po Toi Granite, the Mount Butler Granite, the Sok Kwu Wan Granite
and the Fan Lau Granite.
