Chapter 1. Time interval of sedimentation and volcanic activity in the Cretaceous Jinan Basin
The Jinan Basin is one of the Cretaceous pull-apart Basins in South Korea located along the Yongdong-Gwangju fault system. Field works were conducted for 4 sites along the line crossing the Jinan Basin. Each site consists of sedimentary rocks with interlayered volcanic rocks and is named as J1, J2, J3 and J4 respectively from the northwest to southeast along the line. The J1, J2, J3 and J4 sedimentary outcrops locate in the Manducsan, Dalgil, Sansudong and Maisan formations. (LA-MC-ICPMS) age dating was carried out on the zircons from sedimentary and igneous rocks in those sites. The youngest detrital ages obtained from the bottom and top of sedimentary sequences near to J1 in the northwestern margin are 93.8Ma and 89.47 Ma respectively and that from the bottom of J3 in the center is 97.7 Ma. Basaltic andesite in the northwestern margin of the Jinan Basin gives age of 90.30Ma. Basalt in J3 gives an eruption age of 91.81Ma. The ages obtained from the rhyolitic sill and rhyolite in J3 in the center part of Jinan Basin, are 89.6 Ma, 88.87 Ma respectively. 84.56Ma was obtained from andesite intrusion in center part of the Jinan Basin. Andesite in the southwestern part of Jinan Basin gives 84.2Ma.
The Jinan Basin began to form asymmetrically with the deepest part in the J3 locating in the southwestern part of the Basin from 97-93Ma. At the beginning stage of the Basin, conglomerate was mainly deposited in both margins while sandstone was mainly deposited in the center; the coarser cobble size in the J4 compared to J1 indicates that slope of the southeastern margin of the Basin was steeper than that of the northeastern margin. In the second stage, sandstone to shale or mud were deposited in the J2 and J3 and from conglomerate and less coarse conglomerate and sandstone in the J1 and J4 implying fining upward. In the third stage shale and sandstone deposited alternatively deposited in the J2 and J3 and conglomerate mainly deposited in J1 and J4. At the beginning of this stage basalt or basaltic andesite extruded at 90-92 Ma and later rhyolitc and andesitic magma intruded the Jinan Basin at 90-89 Ma and 85-84 Ma respectively. These data indicate that the deposition had started from 97-92 Ma and continued at least until 85-84 Ma implying that the sedimentary sequence of the Jinan Basin can be correlated to the Yucheon Group in the Gyeongsang Basin.
Rhyolitic volcanic rocks in and around the Jinan Basin are plotted on the VAG (Volcanic Arc Granite) field and basaltic to intermediated volcanic rocks are plotted on the CAB (continental arc basalt) fields in the tectonic discrimination diagrams. From the 84.5 Ma and 84.2 andesite in the center and southeastern parts of the Jinan Basin, 7.9~8.5kbar and 2.1~3.7kbar are for the depth of magma chamber These data suggest that the igneous rocks in the Jinan Basin had formed from mantle or crustal rocks due to the Cretaceous subduction and moved up to the depth of middle to upper crust forming magma chamber. The fault around the Jinan Basin may be the path of magma from deep part to middle to upper crust.