The Chek Mun Rhyolite (Table 6.2) is the name given to a swarm of quartzphyric rhyolite dykes that intrude volcanic rocks of the Tsuen Wan Volcanic Group, the Tai Po Granodiorite, the Tsing Shan Granite and pre-volcanic sedimentary lithologies in the New Territories (Plate 6.8). The dykes are generally northeast-trending and mostly vary from 2 to 5 m wide, although rarely they can be up to 60 m wide. The type locality is at Pak Kok Tsui in Tolo Channel where a 20 m-wide quartzphyric rhyolite dyke intrudes the Tolo Channel Formation. A U–Pb age of 160.8 ± 0.2 Ma has been obtained for the dyke at Pak Kok Tsui (Sewell et al., 1998).

In thin section, euhedral to subhedral phenocrysts of bipyramidal quartz and altered alkali feldspar are seen in a fine-grained aphanitic matrix of quartz, feldspar, and altered biotite. Accessory minerals include zircon and apatite.

Details

Yim Tin Tsai. The best exposed, densest swarm of quartzphyric rhyolite dykes occurs on and around Yim Tin Tsai. Individual dykes vary from 3 to 20 m wide, and strike generally in a northeasterly direction. Several dykes crop out on the northern coast of Yim Tin Tsai (840500 834720 Jmm-1) where they intrude tuffs of the Yim Tin Tsai Formation. They occur either as single or as multiple intrusions. On Ma Shi Chau (841200 834430 Jmm-2) quartzphyric rhyolite intrudes along the fault separating Permian rocks of the Tolo Harbour Formation and Jurassic rocks of the Tolo Channel Formation. North of Yim Tin Tsai (839770 835450 Jmm-3) quartzphyric rhyolite dykes intrude granodiorite; the direction of strike in this area varies from northerly to easterly.

Tai Po - Chinese University. An easterly or east northeasterly dyke trend is predominant in the area south of Tai Po. Here a group of quartzphyric dykes, although displaced by faults, can be traced for over 3 km. The outcrops above Sheung Wun Yiu (834800 833400 Jmm-4) are deeply weathered to a white clay soil which, in the past, has been extensively excavated for use in the manufacture of pottery. Northeast of the Chinese University, near Tsiu Hang and Pai Mun (838800 831500 Jmm-5), quartzphyric rhyolites intrude siltstones and sandstones of the Tai O Formation while, at the Chinese University itself, a rare instance occurs of quartzphyric rhyolite intruding medium-grained granite. As on Yim Tin Tsai the trend of these dykes is northeasterly. The dykes of the Tsiu Hang and Pai Mun outcrops appear to be offset sinistrally across the northwest trending faults of Tai Po Mei (838450 831230 Jmm-6). Dykes crop out in the Cove Hill borrow area (838900 830300 Jmm-7), intruding granodiorite and tuffs of the Repulse Bay Volcanic Group, and similar dykes crop out again on Cove Hill (837800 829900 Jmm-8).

Kwai Chung. Few instances of quartzphyric rhyolite occur southwest of Cove Hill until the outcrops of Kwai Chung are reached. Here, quartzphyric rhyolite intrudes granodiorite and fine-grained granite in faulted complexes at Sheung Kwai Chung (832640 826880 Jmm-9) and Sam Tsuen (831000 826020 Jmm-10).

Tsing Shan (Castle Peak). A swarm of eastnortheast trending quartzphyric rhyolite dykes crosses the southern part of the Tsing Shan range, intruding granites and rocks of the Tsing Shan and Tuen Mun formations. In its fresh state the rhyolite is very pale grey, though in most exposures it appears pale yellowish green, saccharoidal and locally chalky. It contains quartz phenocrysts, generally subhedral to euhedral, in the range 1 to 2 mm (811550 825910 Jmm-11); exceptionally, quartz grains up to 10 mm have been recorded (812060 826000 Jmm-12). The rock is generally foliated, in places to the extent that quartz phenocrysts may be obliterated or recrystallized; the dykes are, with a few exceptions, associated with parallel or subparallel quartz veins (812480 825230 Jmm-13), and the adjoining host rock is commonly strongly sheared. The thickest dyke of the swarm, about 10 m across, was noted in excavations around a landfill site north of Mong Hau Shek (Pillar Point) (812590 826340 Jmm-14).

Lam Tei. A swarm of quartzphyric rhyolite dykes with an eastnortheast trend intrudes the granite east of Lam Tei. The largest dyke is 1.2 km long and is exposed in the northern part of the quarry, where it varies from 3 to 6 m wide. Further east (817870 830760 Jmm-15) the dyke is 20 m wide and has quartz megacrysts up to 2 mm, with minor amounts of biotite and feldspar. The margins of this dyke are aphanitic, non megacrystic and flow banded (Plate 6.A10). When traced further east to the ridge (818500 831000 Jmm-16) this dyke divides into two thin dykes of 3 m and 2 m.

To the north (818300 831800 Jmm-17), in a temporary excavation, a quartzphyric rhyolite dyke varies in width from 6 to 17 m and is accompanied by further thin dykes of around 0.1 m width. To the northeast (818800 832000 Jmm-18) a quartzphyric rhyolite up to 10 m wide is exposed along a new road; the dyke gradually thins to 2 m or less to the west.

Man Kam To. The most prominent quartzphyric rhyolite dyke in the district, which is up to 60 m wide, can be traced for nearly 1 km, extends from near Kong Nga Po (832000 843200 Jmm-19) towards the north-northeast (832240 843810 Jmm-20). The dyke intrudes Carboniferous metasedimentary strata of the Mai Po Member of the Lok Ma Chau Formation and can be traced close to (831900 842900 Jmm-21), but not apparently as far as, the thrust-faulted contact between the metasedimentary rocks and the underlying Jurassic volcanic rocks of the Tai Mo Shan Formation. A minor dyke occurs within metasedimentary rocks further to the southwest (831000 842600 Jmm-22). North of the main dyke outcrop, a photolineament of similar trend continues for about 1 km, albeit offset slightly to the west, and there is also a more northeast-trending dyke (832360 843900 Jmm-23).

Northeastern New Territories. The Tai Mo Shan Formation is intruded by quartzphyric rhyolite dykes at two localities. At one locality, on the northern flank (835780 837810 Jmm-24) of Kau Lung Hang Shan, the dyke strikes in the same general direction as metamorphosed zones in the area, although its strike is slightly oblique to the nearest such zone shown on the published map (Sheet 3). The other dyke also obliquely intrudes a metamorphosed zone west (834940 839880 Jmm-25) of Lau Shui Heung. These dykes also intrude the Shing Mun Formation south of Pak Kung Au (839550 845450 Jmm-26).

Tolo Channel. Dykes striking variably northeast to east-northeast, and extending for distances of up to several hundred metres, occur at several localities between Wong Wan Tsai and Wong Chuk Kok Tsui on the northern side of Chek Mun Hoi Hap (Tolo Channel). The most laterally extensive of these dykes intrudes black mudstone and siltstone of the Lower Jurassic Tolo Channel Formation on the coast between Wong Wan Tsai (848900 837770 Jmm-27) and Fung Wong Wat (849740 838270 Jmm-28, Plate 6.A11). Subparallel dykes intrude the Devonian Bluff Head Formation further to the north (848440 838470 Jmm-29, 848900 838860 Jmm-30) and northeast (850200 839300 Jmm-31), and on the coast southeast (852700 840930 Jmm-32) of Wong Chuk Kok Tsui (Bluff Head).

Dykes of feldsparphyric rhyolite have only been observed at two localities, near Pak Kok Shan (850800 839120 Jmm-33) and at Wong Wan Tsai (848500 837700 Jmm-34), both of which are on the northern coast of Chek Mun Hoi Hap (Tolo Channel). At both localities, the northeast-trending dyke, 5 m wide, intrudes black mudstone and siltstone of the Tolo Channel Formation. The feldsparphyric rhyolite is rather weathered and consists of phenocrysts of euhedral alkali feldspar and subhedral quartz in a fine-grained granular matrix.