江西广丰早白垩世中晚期盆地火山-侵入杂岩活动序列确认及地质意义

ACTIVE SUCCESSION ESTABLISHMENT FOR VOLCANIC-INTRUSIVE COMPLEX IN MIDDLE-LATE LOWER CRETA-CEOUS IN GUANGFENG,JIANGXI AND ITS GEOLOGICAL IMPLICATION

江西广丰早白垩世盆地座落在124.51±11.9Ma(Rb-Sr等时线)石溪组火山岩系之上,是一个由湖进到湖退、具完整2级层序的湖相盆地;盆地湖进期沉积岩中穿插有斑岩岩脉群(正长斑岩、花岗斑岩),Rb-Sr等时线年龄为110.83±3.16Ma,U-Pb不一致曲线下交点105.4±1.3Ma,盆地最大湖面时期伴有碱性玄武岩的溢出,K-Ar年龄为100～104Ma,盆地湖退时期伴有亚碱性拉斑玄武岩溢出,K-Ar年龄为98～103Ma。同位素年代学无法区分其早晚,但野外迭置关系及沉积岩序可判别出早晚,这就构成了伴随盆地形成过程的火山-侵入杂岩活动序列,即中酸性岩脉→碱性玄武岩→拉斑岩武岩,该序列对探索该区早白垩中晚期拉张盆地的成因机制具有启迪意义。

The Lower Cretaceous Guangfeng in Jiangxi lies above the Shixi Formation of volcanic rock system with its Rb -Sr isochron a ge 124.51±11.9Ma,and it is a lake basin with a complete second grade stratigraphic sequence from prograding to degrading.Basinal sedimentary rocks are intercalated porphyry vein rock gro ups involving syenite porphyry and g ranite porphyry in the pro-grading time,the porphyries have 110.83±3.16Ma age of Rb -Sr isochron an d 105.4±1.3Ma age of U -Pb discordant chart lower ac ross.Eruption of alkalic basalt and subalkalic basalt re-spectively took place when the lake h ad the largest surface and had cut dow n the surface.the K -Ar age of the basalt is 100～104Ma and 98～103Ma.They hardly can be distinguis hed sequently by isotopic age,but by their overlying and the succession of sedimentary ro cks.So,a active succession of volcanic-intrusive complex was c onstituted by them,namely,from int ermediate-acid vein rock to alkalic basalt,then to subalkalic b asalt.This succession has an instru ctive significance to explore the genesis of extensional basic in middle-late Lower Cretaceous period.