THE INFLUENCE AND TECTONIC SIGNIFICANCE OF THE EARLY CRETACEOUS VOLCANIC ACTIVITY ON SEDIMENTATION IN THE RIQINGWEI BASIN

The Ri-Qing-Wei basin is a late Mesozoic rift basin developed in the middle of the Sulu orogenic belt. In the early Cretaceous, the basin filling sequence mainly developed from Laiyang Group sedimentary strata to Qingshan Group volcanic strata.The rock types of the Laiyang Group, from bottom to top, are glutenite, sandstone, siltstone, mudstone, and sandstone, with interbedded siliceous rocks distributed among strata, and the basalt layer is exposed at the top of the formation. The Qingshan Group is mainly composed of continental volcanic rocks and shallow water continental sedimentary strata. The basin filling sequence performs transgression-regression characteristics.

( 1 ) The petrographic and geochemical analysis for the siliceous rocks of the Laiyang Group reveals that it is a hot water genesis, forming under a continental rift sedimentary environment, indicating that the Laiyang Period was mainly deep magmatic activity which indirectly represents deep magmatic activity. ( 2 ) The rapid deposition of thick strata in the middle of the Laiyang Group indicates the rapid cracking of the basin, and the average content of SiO₂ in the top basalt is 50.86 %; K₂O and Na₂O are 4.50 %-4.94 %; Mg# is 74-88. The trace elements in the sample showed the enrichment of large ion lithophile elements (Ba, K, Pb) and the light rare earth elements, and the loss of high field strength elements (Ta and Nb) and heavy rare earth elements. The Th/Yb value (mean 2.87) and the tectonic environment discrimination diagram (Y-Zr, Th/Ta-La/Yb) demonstrate that this set of basalts originated from the enriched lithospheric mantle. It can be concluded that the rapid rifting may be caused by lithospheric mantle delamination. ( 3 ) A large number of volcanism in the Qingshan Period implies thermal uplift and detachment. In summary, the evolution process of the Riqingwei Basin was jointly affected by the root subsidence of the lithosphere and subduction of the paleo Pacific plate. The basin evolution in the Laiyang period experienced the process of the ‘cracking-subsidence-closure’ rift basin; The Qingshan Period was transformed into a volcanic lake basin, and the influence of magmatic activities on surface sedimentary rocks was also changed from the early indirect participation to late intrusive transformation.