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.
General plutonic features
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.
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 (Figure 6.3, Figure 6.4, Figure 6.5 & Figure 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.
Middle Jurassic to Early Cretaceous
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.
- - 'A-type' Subsuite
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 ).
Kwai Chung Suite
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.
- - Lantau Dyke Swarm
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.
Cheung Chau Suite
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.
Lion Rock Suite
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.
- - 'Granitic' Subsuite
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.