Study On Stratigraphic Chronology And Sedimentary Environment Of Port Island,Hong Kong

Port Island is an important part of Northeast New Territories Sedimentary Rock geo-region of Hong Kong UNESCO Global Geopark(HKUGG).There are various typical geoheritages of significant scientific value and science popularization value in HKUGG.It has great theoretical and practical significance to research the scientific value of the geoheritages,which can promote the scientific implications and realize the popular science education function of HKUGG.Port Island is an isolated island in northeastern New Territories,Hong Kong.Previous studies on rocks,strata and volcanic activities of Hong Kong are mainly focused on the main land,and there have been relatively few studies on Port Island.This thesis takes the volcanic rocks of Long Harbor Formation and the sedimentary rocks of Port Island Formation as the research objects.We researched the petrological characters by thin section examination,discussed the petrogenesis and tectonic settings of the volcanic rocks by geochemical analysis,identified the formation age of Long Harbor Formation and Port Island Formation and explored the volcanic activities of Hong Kong by using zircon U-Pb dating.Based on the field investigation and measured sections,we analyzed the stratigraphic characteristics of Port Island Formation and divided the sedimentary facies.Main achievements are as follow:1.The volcanic rocks of Long Harbor Formation on Port Island are pyroclastic rock.These rocks are enriched in large ion lithophile elements(LILEs)and light rare earth elements(LREEs),and depleted in high field strength elements(HFSEs),belonging to the high K calc-alkaline to shoshonite series.All volcanic rocks have high ?REE contents,and show obvious negative Eu anomaly.2.The volcanic rocks of Long Harbor Formation were formed in a back-arc extension setting in response to the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate.And the acid rhyolitic tuffs of Long Harbor Formation were derived from deep melting in the continental crust.3.The first precise zircon U-Pb age of rhyolitic tuff from Long Harbor Formation is about 139.5±1.1 Ma and 141.1±1.3 Ma,indicating that these volcanic rocks were formed in the Early Cretaceous.The Early Cretaceous volcanism in Hong Kong may be more continuous than previously thought.The Long Harbor Formation may be the youngest stratigraphic unit in the Repulse Bay Volcanic Group.4.This study found a tuffite interlayer in Port Island Formation for the first time,with the zircon U-Pb age of 128.2±2.7 Ma.We thought there were no volcanic activities in Hong Kong area at ca.128 Ma.The tuffite was formed of the volcanic ash deposition and the volcanic ash came from the volcanic eruption of Hong Kong surroundings.5.The age of the tuffite interlayer made Port Island Formation obtained a precise chronology constraint for the first time,and the sedimentary age of Port Island Formation is Early Cretaceous.The dinosaur skeletal fossil found in the lower Port Island Formation stops the gap that there is no dinosaur in Hong Kong.6.Port Island Formation is a clastic sedimentary formation with reddish brown conglomerate,sandstone and brick-red siltstone and argillaceous siltstone,and it was divided into three members.The sedimentary environment of Port Island Formation was fan-delta deposit in arid to semi-arid climate.The subfacies are fan-delta plain and fan delta front without pro-fan delta.