南海北部及广东沿海新生代火山活动

CENOZOIC VOLCANIC ACTIVITY IN THE NORTHERN SOUTH CHINA SEA AND GUANGDONG COASTAL AREA

调查区新生代火山活动十分活跃。岩性以基性为主,少数属超基性Sr^(87/86)初始值最高0.706189,最低0.703178,陆区和海盆新生代火山岩均来自地幔。 K-Ar年龄最老62.86百万年,最新0.43百万年。从老至新可划分为8个活动期,分别为古新世早期、始新世早期、渐新世末期、中新世中期、中新世晚期、早更新世早期、早更新世晚期和中更新世中期。 海盆与陆区火山活动在岩性、同位素特征、活动时期、喷发方式以及岩浆来源等方面都十分相似,二者属统一的新生代火山岩区。

Cenozoic volcanic activity was very intensive in the South China Sea and in Guangdong coastal area. Volcanic rocks are widely distributed both in sea and land. The land distribution areas are, up to 4000km2, mainly represented by the north Hainan Island, Leizhou Peninsula, and Heyuan and Sanshui Basins, which make up an important part of the Cenozoic volcanic rock zone in Southeast Asia. These volcanic rocks have a rather unitary lithologic characters. The highest SiO2 is 52.82%, the general 47-50%, and the lowest 37.09%, indicating basic-ultrabasic rocks, among which, ultrabasic rocks (SiO2<45%) make up 15%±and the other 85% are basic rocks. Calc-alkali index (B) of these rocks is with - 13.32 as the minimum and13.46 as the maximum and generally between 2-8. There are different proportions of rock types according to Rittmann and Peacock principles. Initial Sr87/86 values of volcanic rocks in different parts of the South China Sea Basin are very similar to each other, only those in the Gaojianshi Island of the Xisha Islands are a little higher. All the volcanic rocks both in the land area and in the sea basin come from the mantle. According to K-Ar dating method, the youngest volcanic rock is Tianyang basalt, with an age of 0.43 Ma, beloging to middle middle Pleistocene; the second youngest is the upper mugearite in Suixi of Guangdong, with an age of 0.9043±0.5465 Ma, i. e. from early early Pleistocene to early middle Pleistocene. The oldest is the olivine basalt in the Sanshui Basin, with an age of 62.86 Ma, belonging to early Palaeocene. Volcanic rocks in the other land areas are between these ages. Limburgite in the Gaojianshi Island of the Xisha Islands is the youngest volcanic rock in the sea basin, with an age of 2.7 Ma and belonging to late Pliocene. The transitional tholeiite in the Daimao Sea-mount is the oldest one, 14.0 Ma (middle Miocene) in age. Through comprehensive analysis of petrology, stratigraphy and chronology, it is known that chronologic scale of Cenozoic volcanic activity in Guangdong coastal area had 8 stages, from No. 1 to No. 8 ascendantly. There were multiple eruptions in each stage.