浙江早白垩世晚期火山活动与火山—沉积岩系地层划分

VOLCANIC ACTIVITIES AND STRATIGRAPHIC DIVISION OF VOLCANO-SEDIMENTARY ROCKS DURING LATE PERIOD OF EARLY CRETACEOUS EPOCH IN ZHEJIANG

系统研究浙江下白垩统“永康群”、“横山组”火山—沉积岩系地层层序，岩石组合，古生物，火山岩同位素年龄和岩石化学、地球化学资料表明：浙江永康群中火山岩成岩时间在距今115～100Ma之间，属早白垩世晚期。火山岩具双峰式特征。大致以江山—上虞和景宁—宁海一线为界，分成衢州—诸暨、永康—宁波、文成—临海3个火山活动带，自西向东火山活动渐次增强，火山活动起始时间稍有后移。根据火山—沉积岩系岩石组合，古生物群特征，火山岩同位素年龄及古地磁资料，提出横山组、中截组、永康群和文成群是早白垩世晚期同一构造发展阶段的产物。

The upper part of the lower cretaceous in Zhejiang, generally called 'red layer', is a seriesof red sedimentary rocks interbedded with volcanic rocks, which is mainy distributed in the south-east of Zhejiang. Commonly, its lower part which is interbedded with greyisn green or greyishblack siltstone and mudstone is called Guantou Formation, the layer of the red sedimentary rockis called Chaochuan Formation and Fangyan Formation. As a whole, it's called Yongkang Group.Studies have revealed that the features of the rock combination are different in different areas,and have certain variation regularity in time and space, i. e., from west to east, the volcanic activi-ties are strengthened, the volcanic rocks are thickened and the sedimentary rocks are thinned rela-tively.The Jujiang Group in Jinhua-Juzhou is doninated by sedimentary rocks with a thicknessmore than 4 000m of which the Zhongdai Formation sedimentary rocks at the lower part are 840- 1160m thick, interealated with 20-100m basalt and thin - layered rhyolitic tuff at the bot-tom. The Younkang Group in Yongkang-Ningbo is 800- 1200m thick; of which the GuantouFormation at its lower part is well developed in volcanic rocks, taking 30- 60% of the total thick-ness of this strum, and the basalt and rhyolitic volcanic rock form a typical combination of double-peak; the Chaochuan Formation and Fangyan Formation at its upper part are dominated by redsedimentary rocks, intercalated wich a few rhyplitic pyroclastic rocks. The volcano-sedimentaryrock series in Wenchen-Linhai at the eastern coast is called Wenchen Group, the Meiqiang For-mation at its lower part is mainly composed of sedimentary rocks, interbedded with basalt and afew andesite; the Longtanshan Formation at its upper part is rhyolitic pyroclastic rock of 200-500m thick, occasionally interbedded with purplish red sedimentary rocks.Data of isotopic age for the volcanic rocks show that the basalts from Zhongdai Formation oflower Jujiang Group, Guantou Formation and Chaochuan Formation of lower Yongkang Group,and Meiqlang Formation of lower Wenchen Group are very approximate in age (103 - 110Ma).The volcanic rocks tend to be younger in age from west to east, which means that they are erup-tive materials of the same volcanic stage. According to the data of the isotopic age of the volcanicrocks, palaeontogic fossils and palaeomagnetism, the age is determined as late period of early Cre-taceous epoch to early period of late Cretaceous epoch.