The Precambrian basement beneath much of Illinois, Michigan, southwestern Ontario, Indiana, western Ohio, western Kentucky, and southeastern Missouri (including exposures in the S t Francois Mountains) is known as the eastern granite-rhyolite prov ince. This crust is characterized by undeformed and mostly unmetamorphosed highsilica rhyolite to dacite with epizonal granite of equivalent composition having zircon UPb ages of 1,470 ± 30 Ma. The Precambrian subcrop beneath parts of southwestern Mis souri, southern Kansas, northern Arkansas, most of Oklahoma (including small areas of outcrop), and the panhandle of Texas is known as the southern granite-rhyolite province. This crust is also characterized by undeformed and mostly unmetamorphosed high-silica rhyolite to dacite with epizonal granite, but zircon U-Pb ages from this province are about 100 m.y. younger, 1,370 ± 30 Ma. We have obtained additional zircon U-Pb ages and whole-rock Sm-Nd data for many drill-hole samples and a few outcrop samples from these provinces to (1) define their geographic limits more accurately, and (2) use them as a means of defining the age of the crust which presumably underlies these two provinces and from which the granite and rhyolite magmas were derived.These granite-rhyolite provinces were previously thought to have formed from anatectic melting of preexisting Paleoproterozoic crust, which is a southward and eastward extension of the 1,600 to 1,800 Ma Central Plains orogen. However, recent Sm-Nd stud ies, including additional data reported here, indicate that some of the rhyolite and gran ite from both the eastern granite-rhyolite province and the southern granite-rhyolite province have Nd crustal residence ages indicating magma derivation from juvenile, mantle-derived sources that are only slightly older than the granites and rhyolites them selves. As a result, a relatively sharp boundary can be defined that separates Mesoproterozoic granites and rhyolites (regardless of age) having Nd crustal residence ages indicative of Paleoproterozoic crustal sources (TDM > 1,550 Ma) from ones having younger crustal sources (TDM < 1,550 Ma). This boundary extends northeasterly from southeastern Oklahoma through southeastern Missouri, southern Illinois, central Indi ana, southeastern Michigan, and into southwestern Ontario; its southwestern extension is not defined, but it probably parallels, and may be close to, the northwestern boundary of the Llano province in Texas. The underlying crust northwest of boundary line is inferred to be Paleoproterozoic, whereas the underlying crust southeast of this bound- Van Schmus, W. R., Bickford, M. E., and Turek, A., 1996, Proterozoic geology of the east-central Midcontinent basement, in van der Pluijm, B. A., and Catacosinos, P. A., eds., Basement and Basins of Eastern North America: Boulder, Colorado, Geological Society of America Special Paper 308. 7 on May 15, 2015 specialpapers.gsapubs.org Downloaded from 8 W. R. Van Schmus and Othersary is inferred to be Mesoproterozoic. We interpret this boundary ...

The variance in isotope‐dilution measurements of Rb87/Sr86in geological samples is shown to be proportional in general to (Rb87/Sr86)2, so that simple regression methods for estimating Rb‐Sr isochrons are not formally valid. A new regression method which allows for known experimental error in both Sr87/Sr86 and Rb87/Sr86 and which weights the data to allow for the nonuniform Rb87/Sr86 variance is given. The method is illustrated with the data for the Heemskirk granite, western Tasmania. Analysis of the data shows that, although the simple method gives quite accurate estimates of the isochron parameters, it tends to underestimate the uncertainty in the age and to overestimate that in the initial Sr87/Sr86. Models with geological variation over and above the experimental variation were also examined. Alternative models considered are geological variation independent of Rb87/Sr86 or proportional to (Rb87/Sr86)2, or a compromise between these.

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The above ages together with existing published zircon ages for other plutons in the region indicate three periods of igneous activity. Foliated mafic and felsic plutons were emplaced between 205-230 Ma and 180-200 Ma and nonfoliated, also mafic and felsic, plutons were intruded at 170-180 Ma. The Honam shear zone, which separates the Ryeongnam massif from the Ogcheon fold belt to the north, developed at ca. 180 Ma. K-Ar and Ar-Ar ages in the area are younger than zircon ages and probably apply to meta morphic activity related to the deformation along the Honam shear zone. Ages for Mesozoic plutons in the Ryeongnam massif are similar to ages reported in the Qinling-Dabie belt of China and indicate coeval igneous activity and possibly similar tectonic evolution. INTRODUCTIONThe Korean peninsula is situated on the eastern margin of the Eurasian plate close to the boundary with the Pacific plate. The Eurasian plate in East Asia is a mosaic of tectonic terrains. Korea is composed of three massifs separated by major fold belts (Fig. 1). The Nangrim massif in the north is separated from the Gyeonggi massif to the south by the Imjingang belt. The Ogcheon belt separates the Gyeonggi massifs from the Ryeongnam massif. The Ryeongnam massif is composed of Precam brian crystalline rocks of basic to intermediate composition, and minor metasedimentary rocks. These basement rocks have been intruded by mafic to felsic Mesozoic plutons. The northern boundary of the Ryeongnam massif with the Ogcheon belt is a major ductile deformation zone, the Honam shear zone (Yanai et al., 1985). This mylonitic zone is about 400 km long and varies in width from several kilometres to several hundreds of meters (Kim and Kee, 1994). To the south the Ryeongnam massif is overlain by Mesonic volca nic and volcanoclastic rocks of the Gyeongsang basin.It is generally accepted that the tectonic terrains of Korea extend into mainland China, but there is uncertainty in matching the Korean and the Chi nese terrains. -The Qinling-Dabie belt of China represents a continental collision in Triassic time between the Sino-Korean platform or North China block and the Yangtse platform or South China block (Ames et al., 1993). The Qinling-Dabie belt could correspond to the Imjingang belt or the Ogcheon belt in Korea. If the Ogcheon belt is a continuation of the Qinling-Dabie belt then the Ryeongnam massif of Korea would correspond to the Fuchian fold belt of the Southeast China terrain (Fig. 1) GENERAL GEOLOGYThe Precambrian rocks of the Ryeongnam massif fall into three distinct units (Table 1). The oldest, unit 1, is a gneissic complex composed of migmatitic gneisses, banded gneisses, augen Intrusive contact Honam shear zone. Ultramylonite and protomylonite. Ductile dextral strike slip fault zone, 400 x 1-20 km.Foliated granite. Composition: tonalite to alkali granite. Syntectonic pluton, intrudes the Gneiss Complex-unit 1, and diorite-unit 8, and is intruded by Namwon granite-unit 12. Syenite stock. Intrudes the anorthosite-unit 4, and diorite-unit 8. The Meso...

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