The Yangkou Beach – A Proposed Reserve Region for the
Study of Ultrahigh-P Terrane in China
J. G. Liou & Ruyuan Zhang (Dept of Geological & Environmental Sci., Stanford
University)
One of the most provocative revelations of the post-plate-tectonic-discovery era is the
realization that continental crust was subducted to depths of > 100 km and exhumed,
at numerous locations worldwide, throughout much of Earth’s history. The recent
discovery of former majoritic garnet within crustal rocks in Yangkou beach (Ye et al.
2001) of the Sulu terrane requires subduction and exhumation of continental crust
from depths > 150-200 km, further highlights the role ultrahigh-pressure (UHP)
processes must have played in Earth evolution. Such processes encompass, but are not
limited to, exchange of material between the crust and mantle, generation and
collapse of mountain belts, the formation of continental crust, and the influence of
UHP rock formation and exhumation on tectonic plate motions.
The Dabie-Sulu terrane is famous as the coesite county and has attracted a great deal
of national and international attentions during last decade. A small UHP slab (200 x
300 m) at Yangkou Beach, northeast of Qingdau, occurs numerous unique UHP rocks
and minerals in addition to the majoritic garnet mentioned above. For example,
intergranular coesite crystals along garnet-omphacite grain boundary in eclogites have
only been documented in this area (Liou & Zhang 1996). Similar intergranular coesite
grains have been documented in some mantle-derived xenoliths (Smyth 1977) but not
other UHP terranes. Moreover, metagabbroic and metagranitic rocks with preserved
relict igneous minerals and textures also occur at this locality (Hirajima et al. 1993;
Zhang & Liou 1997). The occurrence of intergranular coesite grains and the
preservations of igneous textures and minerals together with the preservation of
rhythmic layered garnet peridotite in Yangkou suggest lack of a discrete fluid phase
during and after the Triassic UHP metamorphism.
These features are so unique that we cannot lose the outcrops. However, recent
development of fishery industry in this locality has destroyed some precious structural
and petrotectonic features. We propose to preserve this area to be prohibited for
further development of housing and fishing sites as several well-known eclogite
localities in the world. This area should be considered as equally important as the 44
National Geoparks recently proposed in China (Zhao and Zhao 2003). Each site
preserves many important physical features from which we can trace the evolutionary
history of the Earth, and records “geological heritage” bestowed by Mother Earth.