Geochemistry and Geochronology of the Cretaceous Igneous Rocks in the Mogpo Area

Igneous rocks of Mogpo area composed of mainly Cretaceous volcanic and granitic rocks, especially volcanic rocks included with some glassy rock. The glassy rock corresponded to pitchstone with waxy and resinous luster because of absorbed water from neighboring hydrous rock formations. The pitchstone occur as dike in Gajido, Momilhang and Gohado of Mogpo area, and as small stock in Geumdangdo area. The Cretaceous igneous rocks plot on trend of calc-alkaline and corresponded to sub-alkalic rock series, but pitchstone show different trend from these igneous rocks, and are a series of differentiated products from cogenetic magma. In minor elements, igneous activity of Mogpo area start from volcanic activity, and continued on intrusive rocks at end of the Cretaceous. Pitchstone with relatively high differentiation intrued into hydrous rock formation at early Tertiary (Eocene). In chondrite normalized REE pattern, REE pattern are similar to micrographic granite-pitchstone and rhyolite-tuff, respectively. The former enriched than the latter in LREE, and have high Eu(-) anomaly. Parental magma type of these igneous rocks correspond to mantle fractionation+Syn-collision(volcanics). VAG field in Pearce diagram, and NCA field in volcanic arc maturity. Isotopic age of igneous rocks in the Mogpo area are 82.8∼86.4 Ma, 61.6∼81.4 Ma, 71.8∼75 Ma and 81.2∼86.9 Ma and 41.4∼42.3 Ma in tuff, rhyolite, porphyries and pink feldspar granite, respectively. These isotopic and geochemical data suggest the Cretaceous igneous rocks of Mogpo area were intruded and emplaced at two orogeny associated with igneous activity (Bulkuksa and Youngil disturbance). Pitchstone intrued into hydrous rock formations at late Eocene (in MogPo area) and late Cretaceous (in Geumdangdo area). and concluded pitchstones of both area have similar geological environment that rhyolitic magma intruded into neighbouring hydrous rock formation.