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The Geology of Hong Kong (Interactive On-line)
Tei Tong Tsui Quartz Monzonite - Klt

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.