The Tei Tong Tsui Quartz Monzonite (Table 6.5) is the name given to several east- to northeast-trending stocks and dykes of quartz monzonite and monzonitic granite outcropping in the central and southeastern New Territories, and on Hong Kong Island (Figure 6.6). At the type locality in Tai Miu Wan, the unit intrudes the Chek Kwu Shan, Pan Long Wan and Clear Water Bay formations and is closely associated with tuff breccias and related volcanic deposits. In the Sai Kung area, small stocks of quartz monzonite form a discontinuous ring around the Sai Kung Caldera (see Figure 7.3), while along the main divide between Sha Tin and Kowloon, dykes of quartz monzonite form a northeast-trending swarm from Lai King to Three Fathoms Cove (Figure 6.6). On Hong Kong Island, the Tei Tong Tsui Quartz Monzonite intrudes the Mount Davis and Ap Lei Chau formations, and Kowloon and Mount Butler granites forming a discontinuous dyke up to 50 m wide from Lei Yue Mun in the northeast to Wong Chuk Hang in the southwest. A 200 m-wide fine-grained monzonitic granite dyke near Shek Miu Wan in Tseung Kwan O may be an extension of this dyke.

The quartz monzonites are typically porphyritic fine to medium grained but may vary to non-porphyritic lithologies. Large (5–10 mm) euhedral alkali feldspar (orthoclase) phenocrysts predominate with some smaller (3–5 mm) strongly concentrically-zoned plagioclase (andesine to oligoclase) phenocrysts. Euhedral green amphibole and reddish brown biotite are the chief mafic minerals. The granular matrix consists of euhedral alkali feldspar laths, zoned plagioclase and minor interstitial quartz. Accessory minerals include allanite, zircon, apatite, and Fe-oxide. In more fractionated dykes, quartz is abundant and is often accompanied by microcline, but there are few mafic minerals.

The Tei Tong Tsui Quartz Monzonite has not been dated isotopically, but geochemical correlation with the Clear Water Bay and High Island formations, and field relationships, suggest it was emplaced at c.140 Ma.

Details

Kowloon Reservoir - Sha Tin Wai. Quartz monzonite is well exposed in the catchwater between the reservoir and Amah Rock, and in roadside outcrops by the entrance to Lion Rock Tunnel. Contacts are rarely seen and when exposed are usually sheared. However, at Lion Rock Tunnel the quartz monzonite can be seen intruding coarse-grained granite with an irregular contact. The quartz monzonite here is itself intruded by a dyke of feldsparphyric rhyolite. At Sha Tin Wai the quartz monzonite intrudes coarse-grained granite with a vertical contact. The monzonite is chilled against the granite with a darker grey, biotite rich marginal facies. In roadside exposures farther south the quartz monzonite contains xenoliths of coarse-grained granite. In borrow areas southeast of Tsang Tai Uk (838150 825800 Klt-1) and south of Sha Tin Wai (838500 825940 Klt-2) a 10 m thick sheet of dark grey, porphyritic, fine-grained quartz monzonite intrudes fine-grained granite, dipping N at about 40^o.

In the area south of Tsang Tai Uk (838200 825900 Klt-3) and at To Shek (838900 826150 Klt-4) the medium-grained granite includes an interstitial fine-grained matrix, particularly in the vicinity of intercalations of fine-grained granite. South of Tsang Tai Uk xenoliths of medium-grained granite occur within the fine-grained variety. At To Shek, sharp, chilled margins of fine-grained granite occur at the contacts. A fine-grained matrix is also seen west of Tai Che (840600 826670 Klt-5). There are sheets of fine-grained granite on the southeastern contact of the medium-grained granite. However, whereas on the northwestern contact the fine-grained granite is chilled against, and clearly younger than, the medium-grained granite, on the southern contact the interbanding of the two lithologies could be a normal marginal feature of the medium-grained granite. Sheets of quartz monzonite run parallel to the contact and are younger than the granites.

South of Sha Tin, fine-grained granite forms a sheet like body striking east northeast between medium- and coarse-grained granites. However, around To Shek (839000 826200 Klt-6) and Tsang Tai Uk (838200 825800 Klt-7) the granite has more complex field relations. It contains xenoliths of medium-grained granite and intrudes the medium-grained granite with sharp contacts. Adjacent to the contacts, the medium-grained variety has developed a fine-grained matrix, and the fine-grained granite has absorbed megacrysts from the medium-grained granite.

Southeast of the Sha Tin Valley. In the areas around Man Tau Tun (839000 824600 Klt-8), Mau Tso Ngam and Wong Ngau Shan (841300 826150 Klt-9), contacts have rarely been observed and outcrops have been plotted from the occurrence of corestones. This is also the case around Ngong Ping. In Ma On Shan Mine (843580 829210 Klt-10) the quartz monzonite forms an irregular vertical dyke 4 m thick intruding chloritised, altered sedimentary rocks which form abundant xenoliths within the dyke. Nearby outcrops show very irregular thin dykes of quartz monzonite cross cutting quartz veins, brecciating the country rock and carrying rounded blocks of garnetiferous skarn.

Aberdeen, Wong Chuk Hang and Shouson Hill. Both fine- and medium-grained phases of quartz monzonite are present in this area, and these crop out as impersistent elongate bodies intruding both granite and volcanic rocks. Behind the Technical School at Wong Chuk Hang (834700 812230 Klt-11) the quartz monzonite grades into a chilled margin with an aphanitic groundmass and a few scattered feldspar megacrysts. Quartz monzonite excavated from temporary trenches nearby (834700 812190 Klt-12) is medium-grained and has a distinctive bluish grey colour. Some feldspar crystals display a schiller effect, and in hand specimen the rock resembles larvikite (a syenite that grades to monzonite).

Although quartz monzonite intrudes medium-grained granite, for example along Deep Water Bay Road (837470 813080 Klt-13), it is in turn intruded by fine- to medium-grained granite; this is well seen along the stream bed east of Shouson Hill (837450 812880 Klt-14). Here, the granite has a chilled margin against the medium-grained quartz monzonite, and thin discontinuous dykes of fine-grained granite extend several metres into the quartz monzonite. Small patches of granite pegmatite were noted at the contact.

Mount Butler and Chai Wan. Fine-grained quartz monzonite forms a prominent ridge extending from Mount Parker westwards to Mount Butler (839800 814310 Klt-15). Its relationship with the surrounding granite is unclear, with no exposures of the contact, but a contact between chilled quartz monzonite and tuff is exposed (840000 814280 Klt-16). It is possible that the quartz monzonite is older than the surrounding granite at Mount Butler, as is the case at Lei Yue Mun Barracks (842200 815520 Klt-17). In this outcrop the quartz monzonite forms a sill cutting tuffs of the Mount Davis Formation; these in turn are intruded by granite, the contact having been well exposed in slope remedial works at A Kung Ngam Road (841940 815580 Klt-18).

Southern Clear Water Bay Peninsula. On the eastern side of Tai Miu Wan (848000 814400 Klt-19) a small quartz monzonite pluton surrounded by a chilled margin of quartz trachyte is exposed in coastal sections. Here, rounded monzonite corestones stand out in the weathered cliff exposures (Plate 6.A24). The rock is essentially fine grained and pinkish grey, with abundant alkali feldspar megacrysts which are often aligned to form the characteristic trachytoid texture. Striking east west across the southern slopes of Tin Ha Shan is a quartz monzonite dyke that reaches 60 m in width and contains several long enclaves or screens of coarse-grained granite (847500 814750 Klt-20, 847720 814770 Klt-21). In places, the dyke appears to bulge, as for example at the helicopter landing site above the Tin Hau Temple (847730 814750 Klt-22).

Sharp Island and Tai Mong Tsai. Quartz monzonite outcrops on the western side of Sharp island. The rock is pink, fine-grained or fine- to medium-grained, with abundant euhedral alkali feldspar megacrysts. Similar rock occurs on the small island of Tai Tsan Chau (847900 826300 Klt-23) where it is seen intruding tuffaceous sedimentary rocks of the Mang Kung Uk Formation. In the northern part of Sharp Island the outcrop of quartz monzonite is marked by abundant rounded boulders along the shoreline (848060 826080 Klt-24). In the vicinity of Tai Mong Tsai village (849000 828300 Klt-25) a small pluton of quartz monzonite and quartz trachyte intrudes tuff and tuffaceous sedimentary rocks. Boulders of quartz monzonite and quartz trachyte are scattered across the surface and exposures are poor.

Three Fathoms Cove and Long Harbour. Small isolated outcrops of quartz monzonite have been noted intruding High Island Formation tuff along the shoreline southwest of Yung Shue Au (847200 831500 Klt-26). It seems probable that this coastline parallels a southwestward extension of the Cheung Sheung - Chek Keng Fault. A quartz monzonite body some 200 m by 100 m in outcrop is exposed in the catchwater excavations (847270 831410 Klt-27, 847350 831460 Klt-28).

Small irregular shaped quartz monzonite bodies form discontinuous outcrops between Uk Tau, Wong Shek Pier (852500 832200 Klt-29), To Kwa Peng (853100 832200 Klt-30) and Chek Keng Hau (854500 832400 Klt-31), along or parallel to the line of the Cheung Sheung - Chek Keng Fault. There is no evidence to show that the quartz monzonite is faulted. The rock has widely spaced joints, and it seems probable that the intrusions postdate the main fault movements. The intrusions at To Kwa Peng vary between fine-grained and medium-grained, with abundant alkali feldspar megacrysts up to 20 mm in length. At Chek Keng Hau the rock has a fine-grained pink groundmass with feldspar megacrysts averaging 10 mm in length and quartz up to 6 mm across. Single fresh biotite flakes are scattered throughout.