Abstract
Petrochemical, K-Ar dating, Sand Rb/Sr isotopes, metallogenic zoning, paleomagnetic and geotectonic studies of the Gyongsang basin were carried out to examine applicability of plate tectonics to the post-late Cretaceous igneous activity and metallogeny in the southeastern part of Korean Peninsula. The results obtained are as follows: 1. Bulgugsa granitic rocks range from granite to adamellite, whose Q-Ab-Or triangular diagram indicates that the depth and pressure at which the magma consolidated increase from coast to inland varying from 6 km, 0.5-3.3 kb in the coastal area to 17 km, 0.5-10 kb in the inland area. 2. The volcanic rocks in Gyongsang basin range from andesitic to basaltic rocks, and the basaltic rocks are generally tholeiitic in the coastal area and alkali basalt in the inland area. 3. The volcanic rocks of the area have the initial ratio of Sr^{87}/Sr^{86} varying from 0.706 to 0.707 which suggests a continental origin; the ratio of Rb/Sr changing from 0.079-0.157 in the coastal area to 0.021-0.034 in the inland area suggests that the volcanism is getting younger toward coastal side, which may indicate a retreat in stage of differentiation if they were derived from a same magma. The K_2O/SiO_2 (60%) increases from about 1.0 in the coastal area to about 3.0 in the inland area, which may suggest an increase indepth of the Benioff zone, if existed, toward inland side. 4. The K-Ar ages of volcanic rocks were measured to be 79.4 m.y. near Daegu, and 61.7 m.y. near Busan indicating a southeastward decrease in age. The ages of plutonic rocks also decrease toward the same direction with 73 m.y. near Daegu, and 58 m.y. near Busan, so that the volcanism predated the plutonism by 6 m.y. in the continental interior and 4 m.y. along the coast. Such igneous activities provide a positive evidence for an applicability of plate tectonics to this area. 5. Sulfur isotope analyses of sulfide minerals from 8 mines revealed that these deposits were genetically connected with the spacially associated ingeous rocks showing relatively narrow range of ${\delta}^{34}S$ values (-0.9‰ to +7.5‰ except for +13.3 from Mulgum Mine). A sequence of metallogenic zones from the coast to the inland is delineated to be in the order of Fe-Cu zone, Cu-Pb-Zn zone, and W-Mo zone. A few porphyry type copper deposits are found in the Fe-Cu zone. These two facts enable the sequence to be comparable with that of Andean type in South America. 6. The VGP's of Cretaceous and post Cretaceous rocks from Korea are located near the ones($71^{\circ}N$, $180^{\circ}E$ and $90^{\circ}N$, $110^{\circ}E$) obtained from continents of northern hemisphere. This suggests that the Korean peninsula has been stable tectonically since Cretaceous, belonging to the Eurasian continent. 7. Different polar wandering path between Korean peninsula and Japanese islands delineates that there has been some relative movement between them. 8. The variational feature of declination of NRM toward northwestern inland side from southeastern extremity of Korean peninsula suggests that the age of rocks becomes older toward inland side. 9. The geological structure(mainly faults) and trends of lineaments interpreted from the Landsat imagery reveal that NNE-, NWW- and NEE-trends are predominant in the decreasing order of intensity. 10. The NNE-trending structures were originated by tensional and/or compressional forces, the directions of which were parallel and perpendicular respectively to the subduction boundary of the Kula plate during about 90 m.y. B.P. The NWW-trending structures were originated as shear fractures by the same compressional forces. The NEE-trending structures are considered to be priginated as tension fractures parallel to the subduction boundary of the Kula plate during about 70 m.y. B.P. when Japanese islands had drifted toward southeast leaving the Sea of Japan behind. It was clearly demonstrated by many authors that the drifting of Japanese islands was accompanied with a rotational movement of a clock-wise direction, so that it is inferred that subduction boundary had changed from NNE- to NEE-direction. A number of facts and features mentioned above provide a suite of positive evidences enabling application of plate tectonics to the late Cretaceous-early Tertiary igneous activity and metallogeny in the area. Synthesizing these facts, an arc-trench system of continental margin-type is adopted by reconstructing paleogeographic models for the evolution of Korean peninsula and Japan islands. The models involve an extention mechanism behind the are(proto-Japan), by which proto-Japan as of northeastern continuation of Gyongsang zone has been drifted rotationally toward southeast. The zone of igneous activity has also been migrated from the inland in late-Cretaceous to the peninsula margin and southwestern Japan in Tertiary.