南设得兰群岛利文斯顿岛鲍勒斯山组火山岩的~(40)Ar/~(39)Ar年龄及地质意义

采用４０Ａｒ／３９Ａｒ法和激光微区等时线方法，对南极南设得兰群岛利文斯顿岛东部产出的鲍勒斯山组火山岩重新进行了年龄测定。结果表明，４５Ｄ安山质熔岩和２６２玄武安山质熔岩的等时年龄分别为（１０５．６２±２．１１）Ｍａ和（１１１．４８±２．２３）Ｍａ，其４０Ａｒ／３９Ａｒ初始比与尼尔值相当，岩石中不含过剩氩，表明等时年龄代表岩石真正的形成年龄。脉状产出的Ａ９３５２细晶岩的４０Ａｒ／３９Ａｒ坪年龄为（９６．７±１．６）Ｍａ，计算得到等时年龄为９６．６Ｍａ，晚于熔岩喷发而形成。新的年龄数据证实了鲍勒斯山组火山岩形成于白垩纪。白垩纪火山活动曾影响了整个利文斯顿岛，在赫德半岛出露的鲍勒斯山组火山岩曾被较年轻火山岩所覆盖，构造抬升使其出露。

40 Ar/ 39 Ar AGES AND THEIR GEOLOGICAL SIGNIFICANCE OF THE VOLCANIC ROCKS OF MOUNT BOWLES FORMATION IN LIVINGSTON ISLAND, SOUTH SHETLAND ISLANDS

The isotopic age of the volcanic rocks of the Mount Bowles Formation (MBF) outcropped on the eastern part of Livingston Island, South Shetland Islands, Antarctica have been remeasured by 40 Ar/ 39 Ar and laser microarea isochron age dating techniques. The results indicate that the isochron ages of 45D andesitic lava and 262 basaltic andesite are (105.62±2.11) Ma and (111.48±2.23)Ma respectively. The two good fit isochron lines give the 40 Ar/ 36 Ar initial ratio of 295.3 and 294.6, which are almost the same as the ratio of atmospheric argon (Neir value=295.5). That images no excess argon in the rocks and the isochron age being the real forming age. The 40 Ar/ 39 Ar plateau age of A9352 aplite is (96.7±1.6) Ma and the calculated isochron age is 96.6Ma, which suggests that the aplite occurred after lava eruption. These new ages further verify that the MBF volcanic rocks were formed during Cretaceous. Therefore, Cretaceous volcanic activity should have created a great impression on the whole Livingston Island and the MBF volcanic rocks, which had been covered by younger overburden, were influenced by the thermal event of Tertiary intrusives.