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contain porph>Titic lapilii t>pical of the formation, as well as small, dark, aphanitic lapilli with reaction (resorptionor alteration) rims.Around the shoreline of long Fuk Mui Wan, there are scattered exposures of mafic-rich, lapilli-ash crystal tuff withelongate porphyiitic lapilli. On the southwest side (0992 0923) of the bay these elongate lapilli define a fabricwhich dips northnortheast at 71°. Feldsparph>ric rhyolite dykes cut the tuff on the northeast coast. On the southside (103S 0889) of the bay, the tuffs include a porphyritic lapilli-bearing tuff, with a fabric dipping approximately80° to the southsoutheast. They are intruded by a trachytic quartz syenite. Elsewhere, this mafic-bearing, lapilli-ashcrystal tuff contains dark lithic clasts, up to 20 mm across, and pale porphyritic lapilli and blocks up to 100 mmlong.Yi O. On the Tsing Lam Kok headland (0210 1080), west of Yi O, exposures of crystal lapilli-rich tuff includeporphyiitic lava lapilli with diffuse margins, and small aphanitic lapilli with reaction rims. Similar tuffs, withprominent mafic pyroclasts, up to 4 mm, and varying contents of porphyritic lapilli, occur around the coast to thenorth and west of Yi O. West of Kai Kung Shan (015 099), tuffs of the Yim Tin Tsai Formation pass transitionallyup into finer grained, ash crystal tuffs of the Shing Mun Formation.North of Yi O, around Nga Ying Kok (027 115), exposures of tuff are interpreted as lying close to the contact withthe overling Shing Mun Formation. These tuffs contain quartz crystals up to 4 mm, relict feldspars up to 5 mm,and common euhedral pseudomorphs after amphibole. The relatively coarse, mafic-rich nature of the tuff, combinedwith the very rare occurrence of porphyritic lava lapilli, are characteristics of the Yim Tin Tsai Formation.PetrographyLight grey, crystal vitric tuffs from Tsing Yi (HK 5828, 2656 2370) contain disseminated maficminerals up to 5 mm across. They comprise hornblende, biotite and chlorite, whereas thedominant crystal pyroclasts are quartz, microcline and plagioclase. The matrix comprises smallquartz crystals and interlocking grains of quartz, feldspar and chlorite.Typical vitric crystal tuff from Shui Hau Wan (HK 9986, 9845 9245) is light brownish-grey inhand specimen, with white feldspar crystal pyroclasts up to 5 mm and small porphyritic lapilli.Dark flakes of mafic minerals, up to 3 mm across, are composed of pleochroic, light brown toreddish-brown biotite, and light olive-brown hornblende. The devitrified matrix ismicrocrystalline and forms about half of the rock.Lapilli-ash crystal tuff from Yi 0 (HK 9338, 02670 10285) contains subhedral to broken quartzcrystal pyroclasts up to 4 mm across, set in a recrystallized quartz-sericite matrix. Feldspar isgenerally altered to fine-grained sericite, while pseudomorphs after biotite and hornblendecomprise sericite and opaque minerals. Relict lithic lapilli, up to 3mm, are composed ofmicrocrystalline quartz. Unaltered samples from the Yi 0 area contain strongly zoned plagioclaseand perthitic alkali feldspar, as well as fresh hornblende and biotite. The devitrified matrix iscryptocrystalline or microcrystalline.Volcanic EnvironmentThe Yim Tin Tsai Formation is notable for its thickness and lateral extent; it stretches beyond thedistrict to the north and east, into the Sha Tin district (Addison, 1986), as well as to the south ofHong Kong Island (Strange & Shaw, 1986). Its base is not seen in the district, but its top isgradational into the overlying Shing Mun Formation. Epiclastic units are absent, except withinthe uppermost transitional zone, and most of the tuffs are welded. Flow may be indicated byelongation of the lapilli, although this could equally be due to post-depositional, down-sloperheomorphism. No regional flow directions can therefore be assumed; especially considering thelimited outcrop evidence. The high temperature of the flows probably caused the partialresorption of lapilli, giving them their characteristic diffuse margins. The vent from which theseflows were erupted has not been located, and need not have been within the district. Thesecharacteristics suggest that the tuffs were deposited from very large pyroclastic ash flows.40