琼北峨蔓地区火山地质与喷发历史

STUDY ON VOLCANIC GEOLOGY AND HISTORY OF ERUPTION IN EMAN AREA,NORTHERN HAINAN ISLAND

琼北作为中国最大的第四纪火山区之一,其火山活动表现出多期、多旋回的特征,但位于其西北角的峨蔓火山岩的喷发时代依然存在争议。文中对峨蔓火山区展开了详细的火山地质及地貌调查,并结合全岩主量元素分析及火山岩K-Ar年代学和贝壳^(14)C年代学研究,揭示了其火山活动时代和喷发特征等。峨蔓火山区包括笔架岭、春历岭、兵马角、龙门激浪、龙门灯塔及张屋等火山,其火山喷发方式主要为溢流式喷发、射汽-岩浆爆破式喷发和弱岩浆爆破式喷发。熔岩流几乎遍及整个火山区,分布面积约为26.3km^(2);基浪堆积物、溅落堆积物以及火山渣分布范围较小,仅限于龙门激浪-五彩湾和张屋村附近。峨蔓火山岩成分变化较大,整体偏中性,主要由玄武安山岩、玄武粗安岩和粗安岩组成,岩浆演化过程经历了橄榄石和单斜辉石的结晶分异。综合火山岩风化程度(球形风化、红土层)、火山地质与地貌特征(锥体形貌、坡度)、岩石学和地球化学特征(橄榄石斑晶蚀变、全岩成分差异)以及K-Ar年龄(0.12~0.44Ma)和贝壳^(14)C年代学结果((43.27±0.67)ka BP),分析认为峨蔓火山区的火山活动时代应属于中、晚更新世。

As one of the largest Quaternary volcanic clusters in China,the volcanic activities of Qiongbei are characterized by multi-stage and multi-cycle.However,the eruption era of Eman volcanic rocks located in the northwest of Qiongbei volcanic cluster is still controversial.In this paper,we present a comprehensive study of volcanic geology and geomorphology,whole-rock major elements,K-Ar geochronology of volcanic rocks and ^(14)C geochronology of conch,in order to reveal the epoch of volcanic activity and eruption characteristics of Eman volcanic field.According to the field geological survey,it is found that Eman volcanic field has many craters,such as Bijialing,Chunliling,Bingmajiao,Longmenjilang,Longmen Pharos,and Zhangwu.The main types of eruptions are effusive eruption,phreatomagmatic explosive eruption and weakly magmatic explosive eruption.Lava flows almost cover the entire volcanic field,with an area of about 26.3km^(2),which are mainly formed by the eruptions of Bijialing and Chunliling volcanoes in the central-south of the volcanic field.Among them,Bijialing volcano consists of five volcanoes,with steep-slope cones and grayish-black block lavas.Chunliling volcano is located in the east of Bijialing,with gentle slope cone,few lava outcrops and spherical weathering.However,the distribution of base-surge deposits,spattering deposits and scoria is relatively small,and limited to the vicinity of Longmenjilang to Wucaiwan and Zhangwu Village.They were formed by phreatomagmatic explosive eruptions of Bingmajiao and Zhangwu volcano,as well as weakly magmatic explosive eruptions of Longmenjilang and Longmen Pharos volcanoes.Moreover,compared with the Holocene Shishan and Late Pleistocene Daotang basalts in Haikou,Eman volcanic rocks have a wider range of silicon(SiO2=51.39%~55.00%)and alkali(K_(2)O+Na_(2)O=3.51%~8.48%)content.Nevertheless,they are general intermediates,mainly composed of basaltic andesite,basaltic trachyandesite and trachyandesite,and experienced fractional crystallization of olivine and clinopyroxene in the process of magmatic evolution.Combining the weathering degree of volcanic rocks(spherical weathering,red clay layer),volcanic geology and geomorphology(cone morphology,slope),petrology and petrogeochemistry(difference of major elements,olivine phenocryst alteration),K-Ar age of volcanic rocks(0.12~0.44Ma)and ^(14)C age of conch((43.27±0.67)kaBP),we conclude that the eruption era in Eman volcanic field belongs to the Middle and Late Pleistocene.