江苏东海新生代橄榄霞石岩及纯霞岩包体

ON THE MESOZOIC OLIVINE NEPHELINITE AND ULTRAURTITE XENOLITH IN DONGHAI,JIANGSU

本文研究了江苏省东海县安峰山和平明山两地的新生代火山岩,认为原确定的“霞石云玄武岩”应为“橄榄霞石岩”。并且还发现了一种特殊岩石—呈包体产出的纯霞岩,几乎全由霞石组成,属碱性超基性岩。

There is a suit of pleistocene volcanic rocks in Donghai,Northern Jiangsu Province.Previously, the rock was named after nepheline basalt by Jiangsu Geological Survey.But the rock is composed of 5—10 percent olivine,40—55 percent pyroxene,30—50 percent nepheline and other minerals,such as magnetite,ilmentite,apatite etc.without plagioclase. The chemical composition of the rock is characterized by low silicon,high alkaline and titanium contents indicating that they belong to the ultrabasic rocks.The strontium content has reached 1130 ppm in the rocks which reflects mantle component. There used to be many nepheline basalt in China,but now gradually many of them are confirmed to be olivine nephelinite because their common characteristics are lake of plagioclase. Another important fact is that the rock contains many ultraurtite xenoliths.The xenolith is,gen- erally 2—7 cm in diameter,irregular shaped and massive in structure,almost wholly composed of nephelin(>94 percent)and the rest are ilmenite and apatite. Therefore,the author considers that the olivine nephelinite must come from mantle.It firstly forms ultraurtite in the deeprer place and then erupts onto ground and forms ultraurtits xenolith-bear- ing olivine nephelinite.