菲尔德斯半岛南部新生界火山岩系古地磁及地质意义

PALEOMAGNETIC CHARACTERISTICS AND GEOLOGICAL SIGNIFICANCE OF CENOZOIC VOLCANIC ROCK SERIES FROM FILDES PENINSULA, KING GEORGE ISLAND, ANTARCTICA

菲尔德斯半岛南部火山岩系由下而上,玛瑙滩段和化石山段MI=-76.9°,MD=59.7°,属新生界下第三系始新统中下部极性(?)合带中的正向亚带;按磁偶极子模式,平均古地磁极(S)的位置是64.3°S,116.4°W;采点古纬度南纬约65°。长山段I=77.6°,D=211.6°,为上述极性混合带中的反向亚带;虚地磁极(N)位置是75.9°S.118.7°W;采点古纬度南纬约66°。位于化石山段之上,长山段之下的岩块山段,数据具有随机性,这与其形成的地质背景有关。结果表明:1、化石山段与岩块山段之间,是早、晚两个喷发旋回的界线所在。2、菲尔德斯半岛自早第三纪始新世以来。

During the Third Chinese National Antarctic Scientific Expedition from October 1986 to May 1987, a set of oriented paleomagnetic specimens were collected by Jin Qingmin and Zheng Xiangshen from Cenozoic volcanic sequences exposed on the south Fildes Peninsula of King George Island, Antarctica, which is located at latitute 62* 13' S , longitute 58° 58 ' W The samples were taken from different lithologic menbers from lower to upper which are Agate Beach basalt member, Fossil Hill tuffaceous sandstone member, Block Hill basaltic andesite member and Long Hill basalt member. Eighteen samples were treated in the paleomagnetic lab of Nanjing Institute of Geology and Mineral Resources. Three of them were demagnetized by thermal cleaning in steps of 50℃ or 30℃, the other were demagnetized by alternating-magnetic field cleaning in steps of 1 / Sπ × 105A/m. Demagnetism, the measure of natural remanent magnetization and anisotropy of magnetic susceptibility were made respectively in a model TSD-1 thermal demagnetometer, GSD-5A.C demagnetometer, DSM-2 Schonstedt digital spinner magnetometer, SSM-1A analoguespinner magnetometer made by U.S and a model KLY-2 accurate digital kappa bridge made by Czechoslovakia. The maximum value of natural remanent magnetization of these samples is 2.6 A / m ,the minimum value is 1.1 × 10-2A/m, usuall between 8.5× 10-2and 8.5× 10-2A/m. The average value of volume magnetic susceptibility is from 5.14× 10-3 to 3.978 × 10-3SI, A.M.S is not evident. Based on the study sof these samples in the laboratory ,the paleomagnetic direction of four lithologic members of middle and lower Eocene series was obtained: 1. Agate Beach member and Fossil Hill member from lower to upper is of the same paleomagnetic direction, average magnetic inclination I = -76.9° , average magnetic declination D = 59.7 ° , it lies at the sub-normal zone the twenty-third polarity mixed zone in the magnetic stratigraphy ; according to the model of aixed geocentric dipole, its mean paleo-magnetic poleposition (southern magnetic pole) lies at lat. 64.3 ° S, long. 116.4 ° W; Fisher's precision parameter K = 20.8° ; the cone of 95% confidence α95=10.9° , the paleolatitude of sampled sites lies at about lat. 65 ° S. 2. Long Hill member above Fossil Hill member, its magnetic inclination 1=77.6 ° , Imagnetic declination D = 211.6 ° . It lies at the sub-reversed zone of the twenty-third polarity mixed zone in the magnetic stratigraphy; its virtual paleo-magnetic pole position (northern magnetic pole ) lies at lat . 75.9° S, long. 118.7° W K = 1040, α95=3.8. The paleolatitude of sampled sites lies at about lat.66 ° S. 3. The results from Block Hill member which lies above Fossil Hill member and under Long Hill member are random ,it is related to the geologic background of the genesis of Block Hill member. Conclusions are as follows: (1) the bound which can divide into two different geologic cycles lies between Fossil Hill member and Block Hill member; (2) Fildes Peninsula has moved 3* to the equator since the Eocene epoch.