Abstract:

The Middle Late Cretaceous volcanoclastic rocks outcropped on the central part of Livingston Island include heteromict lapilli tuffs,lapillistone and agglomeratic breccias predominantly andesitic in composition.Olivine basalts with holocrystalline porphyritic texture are mainly found in the Shirreff Point.Most basaltic andesite and andesite outcropped in Central Livingston and Hannah Point are finely crystalline with mino microporphyritic texture.The holocrystalline porphyritic olivine basalts and pyroclastic rocks are the components of Pleistocene to recent volcanic activities.On the whole,the mineral association,Plagioclase+pyroxene(+olivine)+iro opaque,is much similar in distinctive lavas,however,the content of minerals is different.The plagioclase phenocrysts are usually with normal zoning,the Or and Ab components increase and An decreases with increasing SiO 2 content of rocks and the composition of the phenocryst margin is almost the same as that of plagioclases in matrix.The predominant phenocrysts of basaltic adnesite and andesite are andesine labradorite,that of basalt is labradorite,whereas the core of plagioclase phenocryst of the olivine basalt from Shirreff Point or the dolerite from Siddons Point is bytownite.The plagioclase in the Pleistocene to recent olivine basalt,either as phenocryst or in matrix,is richer in CaO.The clinopyroxene phenocryst associates with plagioclase forming a porphyritic texture or glomerocryst texture.Augite is mainly found in basaltic andesite and diorite in olivine basalt and basalt,both pyroxene are depleted in Ti.The olivine composition of dolerite is similar to the matrix olivine in the olivine basalt from Sirreff Point,where chrysolite is the major phenocryst.There are two types of olivine phenocrysts in the olivine basalt with Pleistocene to recent age,one was crystallized under a general crystallizing condition and the other was the xenocryst with high Fo content.Above mentioned characteristics of the mineral associations suggested that the volcanic rocks were probably formed by an equilibrium crystallization.The olivine basalt from Shirreff Point has a genetic relationship with the dolerite in Siddons Point as showed by the olivine composition and the high An content of plagioclase,however,differs from the rocks of Central Livingston and Hannah Point.The olivine xenocryst found in the youngest olivine basalt indicates the magma was a primary basaltic magma which was directly derived from a deep magma chamber.