Carboniferous to Triassic strata of the Pyeongan Group crop out in the Gohan area, northeastern South Korea. These strata consist, in ascending order, of the Hongjeom, Sadong, Gobangsan, and Nogam formations. Sandstones in these formations comprise three distinct framework-mineral suites. Suite 1 sandstones (Hongjeom Formation through the Middle Member of Gobangsan Formation) are quartz and quartzose arenites/wackes that contain abundant quartz, little or no feldspar, and few acid volcanic clasts. Heavy minerals are mainly tourmaline, rutile, and zircon. Suite 2 sandstones (Upper Member of Gobangsan Formation) are lithic arenites/wackes characterized by moderate quartz content, low amounts of feldspar, and moderate to abundant volcanic clasts. Sphene is the predominant heavy mineral. Suite 3 sandstones (Nogam Formation) are feldspathic arenites/wackes distinguished by low quartz content, moderate to abundant feldspar, and low amounts of acid volcanic clasts. The heavy-mineral assemblage of the Nogam Formation is characterized particularly by epidote and sphene. Feldspar-poor Suite 1 sandstones were derived from a source terrain rich in quartzite and quartzose sandstone. Abundant quartz and stable heavy minerals were supplied initially during Late Carboniferous time to a shallow-marine shelf, which changed gradually near the end of Carboniferous time to a nonmarine, paralic platform setting. Electron probe microanalysis shows that Suite 1 tourmaline was derived originally from Li-poor granitoids, pegmatites, and Ca-poor aplite and metapsammite. Petrographic evidence demonstrates that the scarcity of feldspars in Suite 1 sandstones is not the result of diagenetic intrastratal solution. Major changes in mineralogy from Suite 1 into Suite 2 and Suite 3 sandstones indicate changes in provenance. Suite 2 sandstones were derived from diorite, granodiorite, acid volcanic rocks, and possibly some metamorphic rocks, which furnished feldspars and sphene. The appearance of feldspars and volcanic rock fragments in these sandstones reflects orogenic changes and uplift resulting from incipient collision of the Asian mainland and the Honshu Block of southwest Japan. Compositionally immature Suite 3 sandstones were derived mainly from granite, granodiorite and granitic gneiss, indicating continuing uplift. These source rocks provided quartz, K-feldspar, plagioclase, epidote, and sphene. Acid volcanic rocks supplied minor amounts of rock fragments.
Bibliographical noteFunding Information:
This researchw as supportedb y the Basic Science ResearchI nstituteProgram, Ministry of Education, Korea, 1994, Project No. BSRI-94-5415 and during 1995-1996 by a sabbatical-leavger ant to the first authorf rom the Ministry of EducationR esearchP ro-gram Abroad, Korea. We are very grateful for this support. The first author wishes to expressh is sincere appreciationto the Departmenot f Geological Sciences,U niversityo f Oregon, USA, for their gracious cooperationa nd assistanced uring his 1995-1996 sabbatical at the University of Oregon. He wishes also to express special thanks to Mr. Chang Woo Han of the Maruhan Corporation for partial financial support of this project. We also wish to thank Keith Crook, Henrik Friis, V.P. Nechaev,a nd an anonymousr eviewerfor critically reviewing an earlierv ersiono f this paper.
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