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The Geology of Hong Kong (Interactive On-line) |
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Kau Sai Chau Volcanic Group (undifferentiated) - Kku
At the summit of Lantau Peak in the centre of Lantau Island,
there is a sequence of up to 120 m of fine ash tuff and rhyolite.
This sequence differs geochemically from the underlying Lantau Volcanic
Group in several respects and most notably in its low TiO2-content
(So, 2000). In this respect it resembles the Kau Sai Chau Volcanic
Group in general. High precision U–Pb zircon age-dating of
a fine ash tuff near the base of this sequence has returned an age
of 141.1 ± 0.2 Ma (GEO, unpublished data), which is within
error of other dates from the Kau Sai Chau Volcanic Group (see below).
This, in addition to the geochemical similarities, indicates that
the outlier should be correlated with the Kau Sai Chau Volcanic
Group, but as yet it cannot be assigned to a specific formation.
Therefore, the outlier unconformably overlies the Lantau Volcanic
Group.
A narrow, east-striking outcrop of volcanic rocks that have been
assigned to the Kau Sai Chau Volcanic Group (undifferentiated) lies
within the Tin Ha Shan Fault towards the southern end of the Clear
Water Bay Peninsula. The outcrop probably contains representatives
of both the Clear Water Bay and High Island formations. The fault
may have been a conduit used during the emplacement of both formations
(Figure
5.28).
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