Introduction
The geology of Hong Kong is dominated by volcanic and intrusive rocks that belong to an extensive Mesozoic magmatic province along the continental margin of southeastern China. Volcanic rocks alone account for approximately 50% of Hong Kong's surface area (Figure 5.1), and form most of its mountainous ground. They comprise thick successions of tuff (pyroclastic rocks) with generally more minor lava flows. The volcanic rocks can be related in terms of their age, chemistry, petrography and structural control of distribution, to the subvolcanic plutonic (mainly granitic) rocks (Chapter 6). Interbedded sedimentary rocks of widely ranging grain size vary from dominantly volcaniclastic to epiclastic origin.
The intermediate to silicic character of this magmatism has resulted in a particularly complex volcanic–plutonic stratigraphy. In order to establish the sequence of magmatic events, it has been necessary to combine the results of field mapping with whole-rock geochemistry, precise radiometric dating and detailed petrographic studies. These studies have enabled a comprehensive revision of the volcanic–plutonic rock nomenclature for Hong Kong, which is summarized in Table 5.1.
The volcanic products were erupted in distinct phases during the Middle and Late Jurassic, and Early Cretaceous. The peak of volcanic activity occurred during the Late Jurassic and Early Cretaceous when several large volume eruptions took place within Hong Kong, mainly from east- and eastnortheast-trending, and to a lesser extent from northwest-trending fissures. This resulted in the formation of complex overlapping calderas.
The stratigraphic nomenclature for the volcanic rocks of Hong Kong has evolved considerably in recent years and these changes are summarized in Table 5.2. The latest groupings are based largely on high precision radiometric dating and whole-rock geochemistry which were not available at the time of earlier surveys, including that at 1:20 000 scale by the Hong Kong Geological Survey.
Classification
In terms of their physical characteristics, the tuffs of Hong Kong are classified according to the recommendations for pyroclastic rocks of the International Union of Geological Sciences (Schmid, 1981; Le Maitre, 1989) (Figure 5.2 & Figure 5.3). However, their compositional classification is based on whole-rock chemistry, since these rocks commonly contain extremely fine-grained matrix components (e.g. fine ash comprising devitrified glass and crystals) preventing accurate determination of their modal mineralogy.
Whole-rock chemical compositions are determined according to the widely used total alkali versus silica diagram (Na2O+K2O vs SiO2) of Le Maitre (1989) (Figure 5.4).
General volcanic features
The volcanic formations comprise lavas, tuffs (pyroclastic rocks), tuffites (reworked or secondary, mixed pyroclastic and epiclastic rocks) and sedimentary (epiclastic) rocks.
Lavas
Lavas of various compositions, including andesites, trachydacites, dacites and rhyolites, make up approximately 15% of the volcanic succession. Most individual lava flows are typically thick, and of limited lateral extent, suggesting that they were very viscous. These characteristics are consistent with the relatively high silica contents (>65 wt% SiO2) of the lavas.
Rhyolite lavas
Rhyolite lava is the most common lava-type in the volcanic sequences. Rhyolite lavas have been described in the Shing Mun, Lai Chi Chong, Ngo Mei Chau, Mang Kung Uk and Clear Water Bay formations, and within the Lantau Volcanic Group. They are typically flow banded (Plate 5.1), often flow folded, and locally autobrecciated. However, some of the units that have been previously interpreted as rhyolite lavas, are likely to be sills that were probably intruded relatively near to the surface at the time of their emplacement, and possibly within fine-grained sediments that were comparatively unconsolidated. The most convincing evidence for a sill of this type is the Sham Chung Rhyolite (Chapter 6). Other possible rhyolite sills occur within the Lantau Volcanic Group. As much work remains to be done on differentiating rhyolite lavas from near-surface sills in Hong Kong, the following description encompasses both.
The Shing Mun Formation on the south side of Tolo Channel and the Lai Chi Chong Formation include 35 m-thick flow-banded rhyolites that are dark grey with an aphanitic matrix and abundant euhedral quartz (c.2 mm with individual megacrysts up to 7 mm), alkali feldspar and plagioclase crystals (up to 3 mm). Although generally parallel to stratification, these thick rhyolites are locally transgressive and therefore are strongly suspected to be sills rather than lavas.
Rhyolites within the Lantau Volcanic Group are light to dark grey and typically contain white, euhedral, perthitic, alkali feldspar (1–5 mm, rarely to 25 mm) and quartz (1–7 mm) phenocrysts. Recrystallization of feldspar phenocrysts to quartz–sericite microcrystalline aggregates is typical and spherulites sometimes occur. On the flanks of Nei Lak Shan, the rhyolites are unusual in exhibiting very planar 'flow' banding (Plate 5.2), which is atypical of that in highly viscous lavas of this composition. Also, the presence of some shattered crystals is more typical of pyroclastic rocks than lava flows. There is some doubt therefore that these outcrops are true lavas. They could instead have been reconstituted from a hot, welded pyroclastic deposit. Alternatively, the outcrops may comprise highly silicified, bedded volcaniclastic rocks. Another notable feature of several rhyolites shown on the 1:20 000-scale geological maps is their occurrence, sandwiched between relatively thin siltstone or fine-grained sandstone units. Examples on Lantau Island include rhyolites on the southern and eastern flanks of Sunset Peak, Yi Tung Shan and Lin Fa Shan. This mode of occurrence may indicate that the rhyolites are sills rather than lavas, and that they were intruded preferentially within unconsolidated sedimentary units.
Autobrecciated, flow-banded spherulitic rhyolite lava occurs in the Mang Kung Uk Formation. The unit is thickest near Tai Mong Tsai, Clear Water Bay, and thins northwards for c.1.5 km. The lava flow is thought to have originated from the east-trending, Tin Ha Shan Fault and vent complex (see below, and Chapter 7).
Autobrecciated rhyolite lava is common in the Clear Water Bay Formation (Plate 5.3), particularly towards the base of the formation northeast of Sai Kung. Here, the rhyolite lava sequence is up to 400 m thick. The unit as a whole can be traced laterally for up to c.10 km from south to north. Its northern limit coincides with an easterly-, varying to northeasterly-trending subvertical, fault-bounded zone. This zone is nearly 1 km wide and is infilled with intrusive, flow-banded rhyolites (Plate 5.4) which are similar to the rhyolites within the Clear Water Bay Formation. The zone extends for more than 4 km from northeast of Pyramid Hill to Shek Nga Shan. Beyond there, to the southwest, individual rhyolite dykes can be traced along the same trend for a further 1.5 km. The fault-bounded zone is interpreted as a major fissure from which the Clear Water Bay Formation rhyolites were erupted. The rhyolites within the zone are considered, therefore, to be the infilling of a wide, fissure-like vent. They are very fine-grained with few phenocrysts that are mostly of flow-aligned euhedral feldspars (up to 3 mm). The matrix is recrystallized to a microlitic fabric and spherulites are locally developed.
Dacite and trachydacite lavas
Dacite lava has only been mapped in the Sai Lau Kong Formation. Dacites are also shown on the 1:20 000-scale geological map west of Tai Mo Shan, within the Yim Tin Tsai and Shing Mun formations. However, it is almost certain that these are intrusive rocks and are fine-grained variants of the Tai Po Granodiorite (see Chapter 6).
Trachydacite, transitional to rhyolite lava, occurs in the basal part of the Pan Long Wan Formation (formerly the Tai Miu Wan Member of the Clear Water Bay Formation) around Tai Miu Wan on the Clear Water Bay Peninsula. The unit is up to 150 m thick and can be traced northwards, gradually diminishing in thickness towards the coast at Bayside Beach (Pik Sha Wan). It is bluish grey when fresh, and comprises abundant, characteristically twinned, plagioclase and alkali feldspar phenocrysts (c.2 mm), and scattered small quartz phenocrysts, set in a very fine-grained matrix. Occasional, large, microperthite megacrysts (up to 7 mm) occur, as do magnetite and pyrite accessory minerals, and secondary chlorite, sericite and calcite. Finely laminated flow banding is typical and there is some complex flow folding. The upper parts of the formation comprise more trachydacite and rhyolite lavas of broadly similar characteristics. These are interbedded with fine ash tuff of similar composition. The thickness variations, especially of the basal flow unit, suggest that the lavas were derived from an easterly-trending fissure just to the south of Tai Miu Wan. This fissure was one of the most important and repeatedly reactivated fault controls of volcanism in the southern part of Hong Kong.
Andesite lava
Andesites are relatively common in the upper part of the Tuen Mun Formation. The andesites are interpreted as lava flows that are interbedded with crystal tuff and volcaniclastic sedimentary rocks. They are dark grey or greenish grey, massive, and are variably altered and commonly epidotized. The andesite lavas contain abundant plagioclase phenocrysts (1–3 mm), set in an aphanitic matrix. Scattered, altered, mafic phenocrysts, probably originally of clinopyroxene and possibly of hornblende, also occur. The geometry and lateral extent of individual andesite flows are not readily indentifiable. This is mainly due to the limited exposure in the vicinity of Tuen Mun. However, some interbedded lithic-bearing ash crystal tuffs are observed, particularly in boreholes in the area. A possible vent source for the formation has been inferred to the north of Tuen Mun (Langford et al., 1989).
Pyroclastic rocks
The pyroclastic nature of Hong Kong's volcanic rocks is evident from a number of field and petrographic features. The most significant of these is the common occurrence of welding fabrics (Plate 5.5). These result from the fusing and attenuation of originally hot glassy fragments (shards and pumice), manifested by eutaxitic foliation. The foliation is often emphasised by fiamme (collapsed and commonly altered pumice; Plate 5.6), and parataxitic foliation (extreme attenuation of fused pyroclastic debris; Plate 5.7). Other pyroclastic characteristics include: very high crystal contents, up to and exceeding the limits (c.55% by volume) at which a magma could flow (Plate 5.8); fragmentation (during eruption and emplacement) on a wide range of scales (fine ash to blocks (Plate 5.9), and especially the presence of common angular broken crystals (Plate 5.10); and poor sorting of the deposits.
Most primary pyroclastic rocks in Hong Kong were probably deposited from pyroclastic flows, some of which were generated by eruptions of vast proportions. For example, the High Island Formation alone comprises >60 km3 of preserved pyroclastic rocks, implying a considerably greater original eruption volume. Fall deposits are often suspected but are harder to confirm. Surge deposits have not yet been confirmed, although some are suspected (see below).
Pyroclastic flow deposits
In Hong Kong, primary pyroclastic flow emplacement of ash deposits was probably commonplace, but the limitations of exposure make it difficult to identify complete flow units (especially upper and lower contact regions). Similarly, there appears to be limited preservation of the fine ash tuffs that would normally characterise the uppermost parts of flow units. Pyroclastic flow deposits laid down further from their source are poorly represented.
Most of the large volume pyroclastic flow deposits are rhyolitic in composition, but rhyodacites and dacites also occur. These compositional differences appear to make little difference to the form of the individual pyroclastic flow deposits.
The most readily defined pyroclastic flow units are generally eutaxitic, i.e. are welded pyroclastic deposits. Numerous individual eutaxites, some with unwelded bases and tops, have been mapped, or are recognisable on a local basis, within the Lantau Volcanic Group, and the Shing Mun, Ap Lei Chau, Mount Davis, Mang Kung Uk, Ngo Mei Chau, Pan Long Wan and High Island formations. Virtually continuous, widespread eutaxitic sequences, comprising several units in succession, occur within parts of the Ap Lei Chau, Che Kwu Shan and High Island formations. Other indications of the hot emplacement of these rocks include columnar cooling joints (Plate 5.11), and devitrification features such as perlitic fracturing and spherulitic recrystallisation. Welding is known to occur elsewhere in the world within both pyroclastic flow and fall deposits. However, known examples of welded airfall deposits are generally no more than a few metres thick and they extend less than c.5 km from their eruptive source (e.g. Cas & Wright, 1987 and references therein). In addition, they exhibit mantling bedforms (see below) draping pre-existing topography. Many of the eutaxites in Hong Kong appear too thick (tens of metres) and, or, too laterally extensive (up to 10 km or more) to have formed during airfall deposition. Furthermore, mantling bedforms have not been identified. The eutaxites are interpreted, therefore, as the products of pyroclastic flows. In the case of very thick successions of welded pyroclastic flow deposits, such as comprise the High Island Formation, the sequence probably represents several flow units deposited within a sufficiently short time in order for the whole sequence to have behaved as a single cooling unit.
Possible unwelded pyroclastic flow units occur within the Lai Chi Chong Formation. Other examples, typically of coarse ash crystal tuffs varying to fine ash vitric tuffs, occur within the Yim Tin Tsai, Shing Mun, Tai Mo Shan, Long Harbour and Mount Davis formations. The units within the Lai Chi Chong Formation have been mapped as coarse ash tuffs, but include block-bearing and lapilli-rich units, often with abundant locally derived sedimentary rock clasts (Plate 5.12). These demonstrate many features of rapid emplacement, including loading at the bases of flow units, flame structures, and complex soft sediment deformation of the substrate. Normal grading and capping with fine ash tuff are also typical. However, all of these features could equally occur within a sediment gravity flow deposit mainly comprising volcaniclastic debris that has been redistributed. In the absence of evidence for emplacement while the volcaniclastic debris was still hot, the primary pyroclastic nature of such units remains uncertain.
Block-rich megabreccias, with blocks up to several metres or more across, and with pyroclastic matrices, are seen locally, within the Shing Mun, Lai Chi Chong, Long Harbour, Ap Lei Chau, Mount Davis, Che Kwu Shan, Mang Kung Uk and Pan Long Wan formations. These represent proximal pyroclastic facies, in some instances ponded within structurally discordant vent structures.
Other features typical of both welded and unwelded pyroclastic flow deposits in Hong Kong, but seen more sporadically, include: preferred concentrations of lithics, crystals, and blocks towards the bases of units due to sorting during flow; indistinct and discontinuous bedding within flow units related to subflow units or internal shearing within flows; fine ash accumulation towards the tops of units reflecting fallout of ash; and coarse tuff breccias with variably developed normal grading of blocks contained within a volcaniclastic matrix. Some breccias of this type may be the infillings of vents, but stratified tuff breccias occur within the Lantau Volcanic Group, and the Shing Mun, Tai Mo Shan, Ap Lei Chau, Mount Davis, Long Harbour, Che Kwu Shan, Mang Kung Uk and Pan Long Wan formations.
Pyroclastic fall deposits
Identifying pyroclastic fall deposits in volcanic rock sequences depends to a large degree on establishing a laterally continuous, mantling style of geometry. This requires very good exposure over a wide area. As a result, it is generally difficult to identify pyroclastic fall deposits in ancient volcanic sequences. In subaerial volcanic environments, preservation of pyroclastic fall deposits is generally poor because the deposits are easily eroded. However, in subaqueous environments, particularly within well stratified lacustrine and alluvial sequences, these deposits have a much greater likelihood of preservation.
In Hong Kong, possible pyroclastic fall deposits include finely laminated, white to light grey and orangish brown, very fine to fine ash vitric tuffs with variable crystal components. Their thickness lies on a scale of millimetres to hundreds of millimetres. Pyroclastic fall deposits have been interpreted in many formations in Hong Kong, and most commonly within the Shing Mun, Lai Chi Chong, and Mang Kung Uk formations (Plate 5.13). Within the Pak Kok Member of the Lantau Volcanic Group, and in the Lai Chi Chong Formation, the tuffs are interbedded with fossiliferous grey to dark grey epiclastic sandstones, siltstones and mudstones, indicating repeated deposition of tuffs in a lacustrine environment. However, some of these could have been deposited from turbidites or have formed in other ways. A further possible indication of airfall deposition is the preservation of accretionary lapilli, for example in the Shing Mun Formation (formerly the lower part of the Lai Chi Chong Formation) on the south side of Tolo Channel (Strange et al., 1990). However, accretionary lapilli (Plate 5.14) can also form in pyroclastic surge deposits and their association with cross-bedded tuffaceous sandstones could be consistent with this. Accretionary lapilli have also been observed locally in the Mang Kung Uk Formation on the east side of Junk Bay.
Secondary volcaniclastic rocks
The distinction between volcaniclastic rocks that are primary, i.e were laid down directly following a volcanic eruption, and secondary, i.e. comprise volcaniclastic material that has been reworked by sedimentary processes, is often difficult, even in recent volcanic environments. In many rocks there are no absolutely diagnostic criteria of the emplacement process. This is particularly the case where there are no indications of emplacement of hot pyroclasts. Therefore, many of the tuff sequences are likely to include secondary volcaniclastic deposits, emplaced by mass flow, mass wasting, and fluvial and alluvial processes. This is especially the case in mixed volcaniclastic and epiclastic sequences such as in the Shing Mun, Lai Chi Chong, and Mang Kung Uk formations. Where there are substantial mixtures of volcaniclastic and epiclastic debris, (>25% of both) the rocks are termed tuffites. These have been identified and mapped locally. Where there is less than 25% of volcaniclastic material, the rocks are regarded as being essentially sedimentary, i.e. epiclastic, in origin. Rocks of this type occur in a wide range of grain sizes, including boulder- and cobble-bearing conglomerates and breccias, conglomerates, sandstones, siltstones and mudstones.
Textural characteristics of pyroclasts
Glass and pumice fragments
The vitric component of the tuffs comprises two main size ranges: fine ash, varying to very fine ash and to a lesser extent coarse ash, and lapilli- varying to block-sized fragments. Vitroclastic forms are often evident in the finer fraction. Originally, these would have been shards of volcanic glass resulting from rapid chilling and extreme fragmentation of an originally highly vesiculose magma. Commonly, the shards became fused together and attenuated (drawn out during welding) as a result of compaction after deposition. Pumice, ranging up to block size, formed from fragments of highly vesiculose magma, chilled to glass. The pumice contains a variable component of crystals, formed prior to the eruption. The pumice fragments commonly collapsed during compactional welding to form fiamme. Subsequently, however, all of the glass has undergone devitrification and recrystallization leaving microcrystalline mosaics of quartz and feldspar and their alteration products. Textural features associated with early stages of devitrification include perlitic fractures and spherulites.
Crystals
Crystals vary in size from fine ash to lapilli. They may comprise up to c.80% of some tuffs, but typically are c.30% by volume. Most crystals are either isolated pyroclasts separated from the parent magma during eruption and emplacement, or are compounded within larger, variably vesiculose clasts of the parental magma (i.e. cognate lithics). Crystals can also occur within accidental or accessory lithic clasts (see below). In many tuffs, the isolated crystal pyroclasts have been physically broken before incorporation into the rock. Disintegration probably occurred during explosive eruption, and only to a limited extent during pyroclastic flow. Resorption of crystals is shown by their embayed margins and hollow forms (Plate 5.15). This was caused by disequilibrium between the crystals in their host magma as a result of the change in physical and chemical conditions as the magma ascended rapidly through the crust before and during eruption.
Lithic fragments
Lithic fragments are very variable in size, ranging from ash to very large blocks. They are a relatively minor component in many of the tuffs but lithic tuffs are not uncommon. Cognate, accessory and accidental lithic fragments are represented.
Cognate lithics are primary fragments of the erupted magmas and include variably vesiculose (pumiceous) to non-vesiculose (magmatic) fragments of parental rhyolite, rhyodacite and dacite magma in most rocks. However, in classifying tuffs (Figure 5.2), pumice fragments are regarded as a glass component rather than a lithic component.
Accessory lithics, torn from the country rock in the sides of the vents during eruption, vary from common to sporadic in most pyroclastic units. The best examples of accessory lithics are blocks, up to 200 mm in size, comprising coarse ash tuffs, eutaxites and occasionally granites within tuff breccias in the Shing Mun and Che Kwu Shan formations and the Lantau Volcanic Group. Marble clasts are an important and locally abundant accessory lithic component in the Tuen Mun Formation. This suggests that marble is more widespread at depth beneath the New Territories than is apparent from the local areas of marble subcrop proven to date (see Chapter 3). Marble clasts also occur in the Shing Mun Formation.
Accidental lithics, incorporated during emplacement of pyroclastic deposits, include siltstone and mudstone especially towards the bases of coarse ash tuffs in the Lai Chi Chong Formation.
Matrix
The volcanic matrices typically comprise microcrystalline aggregates of quartz, feldspar, sericite, chlorite, Fe-oxides and calcite. They represent the devitrified and variably recrystallized very fine ash component of eruptions. Where the matrix was originally a compact glass, perlitic fracturing and spherulites can be seen.
Structures
Welding fabrics
Welding is the most commonly observed fabric within the volcanic rocks. It is manifested by fused, attenuated, and consequently aligned shards. They were originally glass in composition, but subsequently they became devitrified and recrystallized. Typically, the welding fabric is referred to as eutaxitic foliation (Plate 5.6), but in extreme examples of attenuation, as in the Tai Tun Member of the Clear Water Bay Formation, the fabric is described as parataxitic foliation (Plate 5.7). Eutaxitic fabrics are visible in hand specimen, especially in weathered rock, as well as in thin section. Fiamme are usually the most obvious indication of welding in natural exposures. They are variably pumiceous (vesiculose) fragments of coarse ash to block size that have collapsed or have been flattened and attenuated parallel to the welding fabric. The fiamme are often preferentially chloritized, in which case their dark green colour contrasts strongly with the grey tones of the host rock. In other instances they may be extensively kaolinized, carbonatized or preferentially silicified. In the former two cases they weather preferentially to form depressions on exposed surfaces, whereas in the lattermost case they resist erosion and stand proud on weathered surfaces. Well-exposed examples of fiamme occur in the Shing Mun, Ngo Mei Chau, Ap Lei Chau, Long Harbour, Che Kwu Shan, Clear Water Bay and High Island formations. Discrete units of welded tuff with fiamme are classified as eutaxite. Absence of fiamme, however, does not preclude welding of a pyroclastic unit. Although fine-scale welding fabrics may be determined in hand specimen, they are best confirmed in thin section (Plate 5.5 & Plate 5.15).
Remobilisation, or continued movement during emplacement of the welded tuffs has sometimes resulted in locally complex secondary flow structures, including disharmonic folds and thrusts (Plate 5.16). These are categorized under the general heading of rheomorphism. Although rheomorphic structures can be used to infer the topography at the time of emplacement of the tuff, no such studies have been carried out in Hong Kong.
Deformational fabrics
Deformation fabrics occur sporadically in most volcanic formations. These are generally restricted to discrete and narrow zones of shearing that caused broadly parallel alignment of clay minerals, mainly chlorite and sericite, and of tabular crystals, including feldspars and biotite. In northwestern parts of Hong Kong, however, these deformation zones can be tens and even hundreds of metres wide, and are sufficiently distinct to be shown on the 1:20 000-scale geological maps. Care must be taken in distinguishing between primary compactional fabrics in tuffs, and secondary tectonic deformational features.
General Stratigraphy
Detailed accounts of the lithological characteristics of individual formations at specific locations are contained in the six memoirs, which accompany the 1:20 000-scale geological maps, and these need not be repeated here. However, general descriptions are given below for all lithostratigraphic units. Emphasis is placed on synthesising the available information into a coherent geological model. Generalized vertical sections of the volcanic stratigraphy exposed in various parts of Hong Kong are given in Figure 5.5.
Evidence of what is thought to be the earliest Jurassic volcanism is preserved only in northwestern parts of Hong Kong and comprises andesitic lavas and tuffs of the Tuen Mun Formation. With the exception of this formation, all other volcanic formations are now assigned to one of four revised and newly defined volcanic groups (Campbell & Sewell, 1998). These are based on the ages and chemical compositions of the different volcanic phases (Table 5.3) in addition to their broad lithological characteristics and stratigraphy. Collectively, these groups record predominantly pyroclastic volcanism during the Middle Jurassic to Early Cretaceous. Each of the volcanic groups has an equivalent intrusive granitoid suite of similar age and composition (see below).
Stratigraphic correlation based on geochemistry
Many workers have cautioned against regarding the bulk composition of pyroclastic rocks, especially those in ancient sequences, as an indication of their parental magma composition (Ui, 1971; Walker, 1972; Cas & Wright, 1987) because several factors may affect the bulk chemistry. These include the potential for a large accidental lithic component, the common effects of alteration after deposition (e.g. vapour phase crystallization, fumarolic activity, ground water leaching). Sorting during eruption, transport and deposition of the volcaniclastic fragments also typically results in differential concentrations and depletions of crystals and glass in volcanic deposits. Under certain circumstances, however, bulk rock chemistry may be of value in correlating pyroclastic rocks if used in conjunction with detailed mapping and petrographic observations (e.g. Howells et al., 1991). In such studies, pyroclastic units forming thick sequences of homogenous tuff within calderas have been shown to have relatively consistent geochemical signatures (e.g. certain major and trace element ratios). Laterally extensive fine ash vitric tuffs, and tuffs lacking a significant lithic component, may also exhibit similar geochemical coherence. To limit the influence of alteration processes affecting bulk rock chemistry, greater reliance can be placed on those elements that are relatively immobile under many conditions of alteration.
During the ten-year period of the 1:20 000-scale geological mapping of Hong Kong, the need for modifications to the lithostratigraphy became apparent. Some of these changes were incorporated in later published 1:20 000-scale maps in the series while others have been included in the 1:100 000-scale geological map accompanying the Pre-Quaternary Geology of Hong Kong (Sewell et al., 2000). In part, these stratigraphic refinements have been based on analysis of a comprehensive whole-rock geochemical database. This became available only after most of the 1:20 000-scale maps had been published. The most significant adjustments have resulted from the ability to discriminate geochemically between some of the coarse ash crystal tuffs, which otherwise lack diagnostic lithological characteristics. The most useful geochemical signatures in this regard have proved to be those based on ratios of relatively immobile elements, most notably Ti (expressed conveniently as wt% TiO2) and Zr (Figure 5.6). Similar geochemical characterisation of coeval plutons is also possible (see below).
Geochronology
Chandy and Snelling (in Allen & Stephens, 1971) reported the first radiometric dating of a volcanic rock in Hong Kong. They obtained a K–Ar biotite mineral age of 154 ± 4 Ma from a coarse ash crystal tuff, now assigned to the Shing Mun Formation. Darbyshire (1990) obtained Rb–Sr whole-rock isochron ages of 140 ± 2 Ma for a vitric tuff from the Ap Lei Chau Formation and 135 ± 8 Ma for vitric tuff of the High Island Formation. Subsequently, Darbyshire (1993) also reported a Rb–Sr whole-rock age of 144 ± 2 Ma for a rhyolite from the Lantau Volcanic Group.
New U–Pb age data derived from analyses of single zircon crystals in several volcanic formations (Table 5.3) have enabled further refinement of the ages of the volcanic rocks. Together with whole-rock geochemistry (Sewell & Campbell, 1997), they have confirmed that volcanism occurred in at least four distinct episodes (Davis et al., 1997). In addition, one compositionally distinct andesitic unit is thought to pre-date the main period of Middle Jurassic to Early Cretaceous volcanic activity (see below). A significant aspect of the U–Pb age results is the comparatively short duration of the volcanic episodes. These lasted 1 to 4 million years, while the intervening periods of relative quiescence lasted 2 to 12 million years. Significant unconformities separate the episodes. This has been used as further grounds for subdividing the volcanic sequence into four groups (Campbell & Sewell, 1998) (Table 5.3). These encompass all of the volcanic formations except for the Tuen Mun Formation.
It should be emphasized that the volcanic stratigraphy of Hong Kong continues to evolve as new data and new techniques become available. Further refinements are inevitable. In addition, the heirarchy of formations and groups may require modification as correlation with the vast Mesozoic volcanic hinterland of Guangdong and neighbouring provinces is improved. For example, units regarded as formations and groups in Hong Kong may equate more to those of member and formation status respectively in Guangdong Province. However, until geological maps of Guangdong Province become generally available at the same scale and level of stratigraphical subdivision as has been achieved in Hong Kong on the 1:20 000-scale geological maps, the issue of detailed regional correlation must remain unresolved.
Details of the volcanic formations in Hong Kong and their more general stratigraphic relationships are given below.
Lithostratigraphy
Tuen Mun Formation - Ju
Tsuen Wan Volcanic Group (TWVG)
The Tsuen Wan Volcanic Group (Table 5.4) represents the earliest phase of the mainly intermediate–silicic Middle Jurassic to Early Cretaceous volcanism. The group comprises four formations, the Yim Tin Tsai, Shing Mun, Tai Mo Shan and Sai Lau Kong formations. Coarse ash crystal tuffs are dominant within the group and these rocks typically weather to an orange to brown saprolitic soil with large corestones. The main area of outcrop of the group is in the northeast and northwest New Territories, but the group extends to western Lantau Island, Lamma Island and southern Hong Kong Island (Figure 5.7).
Yim Tin Tsai Formation - Jty
Shing Mun Formation - Jts
Tai Mo Shan Formation - Jtm
Sai Lau Kong Formation - Jtl
Lantau Volcanic Group (LVG)
The Lantau Volcanic Group (Table 5.5) represents the second phase of Middle Jurassic to Early Cretaceous volcanism (Figure 5.13). The group includes the revised Lai Chi Chong Formation in the northeast of the New Territories. The group has not yet been divided into constituent formations in its main area of outcrop on Lantau Island although one member, the Pak Kok Member (High Island Formation - Kkh
Interpretation of volcanic environments
In order to identify eruptive sources, and reconstruct volcanic environments in general, facies-based interpretation has been used. This approach emphasizes distinctive associations of rock types rather than their individual characteristics. Rock sequences are, therefore, considered in terms of their lithology, geometry, sedimentary structures, palaeo-movement indicators, distribution patterns and fossil contents.
Facies analysis
The accuracy with which volcanic environments in Hong Kong can be reconstructed using facies analysis is constrained by the varying preservation of units and poor exposure in general. Many volcanic deposits, especially pyroclastic deposits, are poorly consolidated after their deposition and have little likelihood of being preserved in the form in which they were laid down. Instead, they are commonly reworked before final deposition and lithification, and may pass through many such cycles of redistribution. Pyroclastic flow and fall deposits on steep-sided volcanoes, and thin distal subaerial volcaniclastic facies, in general, are particularly susceptible to reworking and are poorly represented in the rock record compared to their frequency of eruption. However, thick, volcanic deposits, such as occur within calderas and other depressions, and subaqueous (lacustrine and marine) volcanic deposits in general, are preferentially preserved. It is the former type of caldera-related volcanic deposits that appear to be best represented in Hong Kong.
Further complications arise in facies interpretation in Hong Kong because of post-depositional changes to geometries. These are caused, for example, by deformation associated with regional extension, especially associated with the intrusion of dykes in large volumes, regional strike-slip faulting, and pluton emplacement. Alteration, including hydrothermal alteration and dynamic metamorphism, also makes interpretation difficult as it has often obscured primary features of the rocks.
Eruptive centres
Volcanic deposits near source are likely to be hotter, coarser and thicker at the time of deposition, than their distal equivalents. On the basis of several recurring features, it is clear that many of the volcanic rocks in Hong Kong were deposited at, or near, their eruptive sources (i.e. proximally). For example, some volcanic formations, such as the Long Harbour and High Island formations, are hundreds of metres or more thick, but lack significant variations in lithology and are poorly stratified, suggesting rapid accumulation near source.
Many formations were clearly very hot when deposited. This is confirmed by the presence of: welding (e.g. in the Lantau Volcanic Group (undifferentiated), and the Ap Lei Chau, Long Harbour, Che Kwu Shan, Mang Kung Uk, Clear Water Bay and High Island formations); columnar cooling joints (e.g. in the Ap Lei Chau, Che Kwu Shan, Pan Long Wan, Clear Water Bay and High Island formations); and diffuse margins of volcaniclasts (e.g. in the Yim Tin Tsai Formation) suggesting thermal and chemical disequilibrium.
There are several examples of spatial associations between outcrops of volcanic formations and related subvolcanic intrusions. These indicate eruptive centres that lie above or close to their magmatic source region. For example, granodioritic intrusions commonly intrude the Yim Tin Tsai Formation and other formations of the Tsuen Wan Volcanic Group. Some intrusions are distributed along faults that bound major volcanic outcrops, as is the case with the Long Harbour Formation, whereas other dykes pervade the general terrain around the outcrop as with the Lantau Volcanic Group. Rarely, fault-bounded intrusions can be traced in outcrop continuity into extrusive equivalents, as with rhyolites within the Clear Water Bay Formation.
Fault-bounding of outcrops suggests the possibility of a tectonic constraint on the primary distribution of the volcanic deposits, most notably with respect to the Lantau Volcanic Group, and the Ap Lei Chau, and Long Harbour formations. However, care must be taken in interpreting fault relationships in this way, as in some cases the faulting substantially post-dates the volcanic rocks.
Locally thick, very coarse pyroclastic breccias and tuff-breccias are present within the Shing Mun, Long Harbour, Che Kwu Shan, Mang Kung Uk and Clear Water Bay formations. These deposits are likely to have been erupted very close to their eruptive sources.
The common and widespread occurrences of coarse ash crystal tuff may be an indication of proximity to source. As fine ash vitric components are progressively sorted and transported further from their source during eruption and emplacement (by pyroclastic fall, flow or surge processes), greater concentrations of coarse ash crystals are likely to characterise the environments closer to the volcanic sources. However, crystal concentrations can also result from secondary sedimentary processes. Coarse ash crystal tuffs dominate the Yim Tin Tsai, Shing Mun, Tai Mo Shan, Mount Davis and Long Harbour formations.
Calderas
Large volume rhyolitic and rhyodacitic pyroclastic deposits in the geological record are almost always deposited from pyroclastic flows and are commonly associated with the formation of calderas. Calderas are structurally-bounded depressions that form during, and as a direct consequence of, an eruption. Rapid evacuation of a near-surface magma chamber during eruption and loading of the overlying crust destabilize the crust, so much so that it can collapse into the newly created cavity below. The process of collapse may continue during the later phases of an eruption or may post-date it, depending on the scale and duration of the eruption. If the collapse occurs during eruption, as is often thought to be the case, the structurally-defined surface depressions can trap thick sequences of pyroclastic flow deposits (intracaldera deposits). They contrast with the much thinner sequences of pyroclastic flow and fall deposits outside the caldera These result from eruptions that occurred prior to caldera development, as well as from later airfall events and pyroclastic flows that escape the confines of the caldera (outflow deposits).
In Hong Kong, the approximate locations of large calderas can be inferred in relation to several of the volcanic formations (Figure 5.13 & Figure 5.16). Nearly complete, ellipsoidal to polygonal calderas are interpreted as confining the Lantau Volcanic Group (undifferentiated) and Long Harbour Formation, and possibly the Yim Tin Tsai and Shing Mun formations together. Incomplete calderas can be proposed for the Ap Lei Chau, Clear Water Bay and High Island formations. These calderas are described in greater detail in relation to individual eruptive phases (see below).
Synthesis of volcanic environments
Tsuen Wan Volcanic Group Phase
The early and later parts of the Tsuen Wan Volcanic Group were dominated by relatively continuous tuff accumulation, resulting in the deposition of widespread, comparatively homogeneous, coarse ash crystal tuff (Yim Tin Tsai and Tai Mo Shan formations). The intervening deposits (Shing Mun Formation) reflect more sporadic volcanism with periods of quiescence marked by the accumulation of fine-grained volcaniclastic and epiclastic sediments.
The homogeneity of the Yim Tin Tsai and Tai Mo Shan formations suggests rapid, largely uninterrupted eruption and deposition from substantial pyroclastic flows fairly close to source. A principal source of eruption is inferred to be in the vicinity of Tai Mo Shan. However, as this interpretation is based in part on the association and spatial distribution of coeval intrusions, further discussion is delayed until Chapter 7, which addresses the synvolcanic structural environment.
No major uplift or submergence appears to have occurred during deposition of the Shing Mun Formation, and this formation probably represents a period of largely waning volcanism. Thick accumulations of debris flow and laharic material (Shek Lung Kung Member), and pyroclastic breccia, suggest degradation of an existing volcanic centre or centres, interspersed with periods of renewed eruption.
The main outcrop of the Ngau Liu Member west of Tai Mo Shan may also represent a near-source volcanic facies. The source, however, of the Cheung Shan Member, whose thickest outcrop is on the northwest side of Lantau Island, is unknown.
The dominance of comparatively viscous dacitic lavas within the Sai Lau Kong Formation, suggests an eruptive centre, or centres, in the northeastern New Territories. The outcrop pattern of the Sai Lau Kong Formation is generally elongate in a northwesterly direction, and bedding attitudes suggest a basin-like structure. Sporadic accumulations of tuff-breccia and tuffaceous sediments within the sequence, coupled with thick piles of dacitic lava, suggest an extensional fissure in which lavas and other eruptive products ponded. Similar volcanic deposits in the neighbouring Shenzhen Special Economic Zone (Yang et al., 1986) probably mark the continuation of this fissure vent system yet further to the north.
Lantau Volcanic Group Phase
The largely fault-bounded outcrop pattern of the Lantau Volcanic Group on Lantau Island is interpreted as a caldera. The entire structure developed within a narrow, elongate, northeast-trending rift zone that extended to the northeast and incorporated the outcrop of the Lai Chi Chong Formation (Figure 5.27).
The internal structure of the caldera is markedly asymmetrical, and appears to reflect the pattern of bounding faults. In contrast to the northward-dipping, and disconformable succession in the south, the internal structure in the north is more complex, although exposure is poorer. This complexity is expressed by abrupt changes in bedding orientation, lithology, the degree of hydrothermal alteration, folding, local unconformities and faulting. These features are interpreted as being largely primary and related to the evolution of the caldera, rather than to any later event. The degree of complexity also suggests the piecemeal creation and infilling (cf. Branney & Kokelaar, 1992) of an elongate, asymmetric, rift-like volcanotectonic depression rather than the single collapse of a circular structure (cf. Smith & Bailey, 1968).
The relative abundance of lavas as opposed to tuffs in the Lantau Volcanic Group (excluding the Lai Chi Chong Formation) is unusual for a rhyolitic volcanic centre. This could suggest the likely prevalence of high temperature, low viscosity lava fountains extruding molten rock in pulses, as opposed to slightly lower temperature ash columns that collapsed and coalesced near the vent forming welded flows. Quiescent phases were sometimes long enough for sediments to accumulate. The abundant flora indicate a warm, wet palaeoclimate with swampy basins on the flanks of the volcano. Thick, laterally extensive tuffaceous units, representing mass flows, and including primary lahars, redeposited tuffaceous material, causing rapid infilling of small basins within the caldera. The sedimentary and tuffaceous rocks, characterised by their restricted outcrop and rapid gradational changes, accumulated as sediments in small basins that were filled during flash floods and by debris flows. During quiescent periods, graded silts were deposited. The grading may reflect seasonal fluctuations, or local rainstorms redistributing poorly consolidated volcanic deposits.
The Lai Chi Chong Formation represents sedimentation within a shallow, non-marine, probably lacustrine environment. Periodic volcanism within or adjacent to the lake caused frequent inundation of the lake by primary volcanic and secondary volcaniclastic detritus transported and deposited from mass flows. The secondary mass flows mainly included low and high concentration turbidites with some debris flows also. The primary volcanic deposits comprise possible surge and airfall deposits and lavas. The presence of surge deposits implies the proximity of maar or tuff ring volcanoes occurring as small islands within, or close to the margin of, the lake. Instability associated with the emplacement of the thicker deposits caused extensive downslope disturbance of the partially dewatered substrate. This suggests the relatively rapid accumulation of the sequence as a whole. At Lai Chi Chong, the palaeoslope was inclined generally southwards. However, further south the formation also includes fluviatile conglomerates that thicken southwards towards the Chek Keng Fault. This suggests that there was uplift on the south side of the fault. These conflicting slope orientations suggest that the basin was only a few kilometres across.
Repulse Bay Volcanic Group 'Rhyolitic' Phase
Various features suggest that the Long Harbour Formation was deposited relatively close to its eruptive source. Most notable among these are the abundance of coarse accidental lithic clasts, the virtual absence of sedimentary intercalations within a thick sequence with limited lithological variation, and the evidence of hot emplacement of the tuffs (e.g. columnar jointing). This evidence, taken in conjunction with the largely fault-bounded nature of the outcrop, has been used to argue (Strange et al., 1990; Campbell & Sewell, 1997) that the Long Harbour Formation to the northeast of Sai Kung represents the infilling of a caldera or similar volcanotectonic depression.
To the west of Sai Kung, a discrete ellipsoidal outcrop of the formation is interpreted as another caldera. This contains two separate intracaldera facies, one, in the southwest, that is rhyolite-rich, and the other which is rhyolite-poor, cropping out mainly in the northeast. The rhyolite-rich facies is probably the stratigraphically lower of the two, but given the lack of stratigraphic control within the Sai Kung Caldera, the relationship is uncertain.
The Long Harbour Formation west of Sai Kung and the Mount Davis Formation are lithologically and geochemically very similar. They also appear to occupy similar stratigraphic positions although they outcrop in separate areas, or are separated by faulting. These relationships suggest that the Long Harbour Formation west of Sai Kung is largely an intra-caldera facies and that the Mount Davis Formation may be its equivalent outflow facies. If this can be demonstrated with greater confidence, it may prove necessary to rationalize the stratigraphy so that the Mount Davis and Long Harbour formations are combined into one formation only.
The presence of eutaxite units within the Mount Davis Formation, which are more typical of the older Ap Lei Chau Formation, suggests the possibility that the source of the Ap Lei Chau Formation continued to erupt periodically during the early stages of accumulation of the Mount Davis Formation.
Repulse Bay Volcanic Group 'Trachytic' Phase
The thick accumulation of welded pyroclastic deposits within the Ap Lei Chau Formation suggests that they were deposited near to their eruptive source. The formation is restricted to the area northeast of the inferred northwest-trending 'East Lamma Channel fault' and southeast of the eastnortheast-trending Chek Keng Fault (see Chapter 7). Both faults may have been active during deposition of the formation (Campbell & Sewell, 1997). Owing to the subsequent intrusion of the Kowloon Granite (see Chapter 6) and related deformation, including annular folding, little progress has yet been made in identifying eruptive sources for the Ap Lei Chau Formation.
The restricted, partially fault-bounded, northwest-trending outcrop of the Ngo Mei Chau Formation may be, at least in part, a primary volcanic feature (Sewell et al., 1998). In this case, the formation could be interpreted as a proximal accumulation of pyroclastic deposits within an elongate depression or vent controlled by a fissure. The formation is approximately coeval with the lithologically and geochemically similar Ap Lei Chau Formation.
Locally within the Che Kwu Shan Formation, there are coarse ash tuffs within or close to vent. These indicate periodic reactivation of sources that gave rise to the older Long Harbour and/or Mount Davis formations. Hence, during deposition of the Che Kwu Shan Formation, the area was characterised by broadly coeval eruptions from a trachytic source (Che Kwu Shan Formation) and a rhyodacite to rhyolite source (Long Harbour and Mount Davis formations).
The Mang Kung Uk Formation was deposited in an unstable environment, with sediments and derived volcaniclastic detritus washed into a shallow, possibly volcanotectonic, lacustrine basin. There are indications of volcanic sources contributing to the Mang Kung Uk Formation both from the north and from the south.
Near the Tin Ha Shan Fault, the pronounced thickness increase of the Tai Miu Wan Member of the Pan Long Wan Formation has been interpreted (Campbell & Sewell, 1997) as indicating that the fault was the feeder and eruptive source of the lava. An increase in thickness of the fine ash tuff units within the formation towards the south suggests a similar interpretation.
Kau Sai Chau Volcanic Group Phase
The lavas in the Clear Water Bay Formation are likely to have been erupted locally since the travel distances of rhyolite flows are unlikely to have exceeded c.10 km. The eastnortheast-trending, subvertical, fault-bounded intrusive zone is interpreted as the main fissure-like vent from which most of the rhyolite lavas were erupted (Figure 5.28). The structure fed a subsiding zone bounded to the north by an east-striking section of the Chek Keng Fault with upthrow to the north. The easterly-dipping extrusive flows therefore ponded along and to the south of the fault, with maximum accumulation of the lavas close to the fault. A further eruptive centre may have developed at the eastern end of the Chek Keng Fault in the vicinity of Sharp Peak. The subsiding ground south of the fault was the first phase in the development of a caldera that further evolved during eruption of the High Island Formation. A further structure has been inferred subparallel to, and to the south of, the Chek Keng Fault (Figure 5.28). This structure may have limited the southerly spread of the rhyolite lavas. The narrow outcrop of the formation within the Tin Ha Shan Fault in the south of the area, suggests that the fault acted as a second fissure-like source of the lavas.
The High Island Formation was deposited within a caldera-like, fault-bounded depression, bounded by the east-striking Chek Keng Fault to the north and the Tin Ha Shan Fault to the south. The same structure inferred to have limited the southerly spread of the Clear Water Bay Formation rhyolite lavas, may have acted as a collapse structure during deposition of the High Island Formation. In this instance, however, the greater collapse appears to have occurred to the south of the structure (Figure 5.28).
Small-scale vents were identified by Strange et al., 1990, within rocks now identified as Clear Water Bay Formation. These lie within the Tin Ha Shan Fault and may represent feeders to the High Island Formation.
Further interpretation of the magmatic evolution of Hong Kong, especially with regard to the common structural controls of volcanism and plutonism, is presented in Chapter 7.