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Rb-Sr and 40Ar 39Ar Geochronological Studies on the Precambrian Rocks in the Minnesota River Valley

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Rb-Sr and 40Ar 39Ar Geochronological Studies on the Precambrian Rocks in the Minnesota River Valley Geochemical Journal, Vol. 15, pp. 17 to 23, 1981 17 Rb-Sr and 40Ar 39Ar geochronological studies on the Precambrian rocks in the Minnesota River Valley HIDEO TSUNAKAWA 1 and MASAHISA Y ANAGISAWA2 Geophysical Institute, University of Tokyo, Bunkyo-ku, Tokyo 113,' and Institute of Space and Aeronautical Science, University of Tokyo, Meguro-ku, Tokyo 153,2 Japan Re( (Received May 26, 1980; Accepted March 3, 1981) Some granitic rocks and basaltic dikes of the Minnesota River Valley were dated by Rb-Sr and 40Ar-39Armethods . The obtained Rb-Sr mineral isochron ages are as follows: Morton gneiss2.55 ± 0.06 (la)b.y. and its biotite ages 2.38b.y., Sacred Heart granite 2.37 ± 0.05 (la)b.y., Montevideogneiss 1.82 ± 0.09 (1a)b.y. Each age may be correspondingto the respectivegeological event. Whole rock samples of the Morton and Montevideogneisses, however, do not define a Rb-Sr isochron age. From 40Ar-39Ar dating results, a mild geologicaldisturbance which might have occurred about 1.0b.y. ago was inferred. These results imply a complex history of the gneissesand a metamorphism in this region. INTRODUCTION (the Algoman orogeny) from the amphibolite to the granulite facies took place about 2.6 b.y. GOLDICHand HEDGE(1974) have reported a ago. About 1.8 b.y. ago, a mild metamorphism 3.80b.y. Rb-Sr whole rock isochron age for the reset only the K-Ar ages and the Rb-Sr biotite gneiss exposed in the Minnesota River Valley, ages of gneisses in the Granite Falls-Ortonville southwestern Minnesota, U.S.A. If this age is area. This event included the intrusion of established, the gneiss in the Minnesota River granitic stocks and mafic dikes into the gneiss Valley will be the oldest crustal rock ever found. in the vicinity of Granite Falls. A reconnaissance survey of the Precambrian GOLDICH and HEDGE (1974) reported a rocks in this region was made by LUND (1956). Rb-Sr whole rock isochron age of 3.80b.y. with CATANZARO(1963) reported the first radio an initial 87Sr/86Sr ratio of 0.700 for the Morton metric age over 3b.y. for North America. and Montevideo gneisses. However, FARHATand GOLDICH and his coworkers (GOLDICHet al., WETHERILL(1975) pointed out that, as these 1961, 1966, 1970; GOLDICHand HEDGE, 1962, gneisses were affected by metamrophism, it was 1974; GOLDICHand GAST 1966; GOLDICHand possible to obtain apparent ages either too old MUDREY, 1969) have reported data on gneisses or too young, if the rocks were only partially in the Minnesota River Valley by K-Ar, U-Pb equilibrated with respect to the Rb-Sr system. and Rb Sr methods. HANSONand HIMMELBERGThe latter authors found that their samples of (1967) performed a geochronological study of the Montevideo gneiss did not lie on the 3.80 dikes in this region. Mainly on the basis of b.y. isochron and reported an age of about 3.2 these results, SIMs and MOREY(1972) reviewed b.y. for the gneiss on the basis of their zircon the geochronological evolution of this region. data. MICHARD-VITRACet al. (1977) suggested According to these authors, the geological that their U Pb data on single zircons from the history may be summarized as follows: Sub Morton gneiss supported FARHATand WETHERILL sequent to the primary igneous processes (the (1975)'s interpretation. Mortonian event) which occurred prior to an We made Rb-Sr and 40Ar-39Ar geochrono event of 3.0b.y. ago, a high-grade metamorphism logical studies of the Minnesota River Valley 18 H. TSUNAKAWA and M. YANAGISAWA Precambrian rocks, hoping (1) to resolve more malized to the 86Sr/88Sr ratio of 0.1194. Re detailed metamorphic history of this region, (2) peated analyses of the 87Sr/86Sr ratio for the to compare the 40Ar-39Ar and Rb-Sr dating Eimer and Amend SrCO3 reagent during the results and (3), if possible, to establish the age course of this study gave an average of 0.7079 ± of the parent rocks of the gneisses in this region. 0.0002 (la). Used values of decay constant is X(87Rb) = 1.39 X 10-" yr-'. The isochron ages and the corresponding initial 87Sr/86Sr ratios SAMPLES AND ANALYTICAL METHODS (=I) were calculated by the York method (YORK, All the sample localities are shown in Fig. 1. 1969). All errors of the calculated values are Based on the classification of rock types in the quoted at 1a level. For the 40Ar-39Aranalyses, Minnesota River Valley by GRANT(1972), the samples were irradiated with a fast neutron dose samples can be divided into the five groups: of about 1018nvt in a JMTR reactor with a (1) granitic phase of the Morton gneiss (sample standard sample (Bern 4M muscovite; the age Nos. 02-1, 02-2-1, 02-2-2, 02-3), (2) tonalitic = 18.7 Ma), K2SO4 and CaF2. The irradiated phase of the Morton gneiss (sample Nos. 03-1, samples were heated in seven steps from 600°C 03-2, 03-3), (3) Sacred Heart granite (sample to the melting point in a vacuum. The detailed No. 05), (4) Montevideo gneiss (sample Nos. experimental procedures for the 40Ar-39Ardating 07-1, 08-1, 08-2, 09) and (5) dikes (sample Nos. are described elsewhere (SAITO and OZIMA, 01, 06, 07-2). Detailed description of these 1977). The decay constants of X(40K)= 5.543 X samples is given in the appendix. 10-10yr' and Xe(40K) = 0.581 X 10-'Oyr-' are In all cases, sample sizes of 1-2kg were used in the calculation of ages. crushed and the fraction of 80-100 mesh or 100-150 mesh was taken for analysis. Plagio RESULTS AND DISCUSSION clase, potassium-feldspar and biotite were separated from several samples. Plagioclase Rb-Sr dating results Chemical composition fractions contain some quartz grains. Chemical of the representative samples and all the data compositions of the whole rock samples were of Rb-Sr analyses are shown in Tables 1 and 2, measured by the X-ray fluorescence method. respectively. Rb and Sr concentrations were determined by Plagioclase, potassium-feldspar and the whole the isotope dilution method. The precision for rock sample from the granitic phase of the 87Rb/86Sr ratios is estimated to be less than Morton gneiss (02-3) yielded a good mineral ± 3 %(1 a). All the 87Sr/86Sr ratios were nor isochron with an age of 2.55 ± 0.06b.y. and I = 0.7076 ± 0.0002 (Fig. 2). We conclude that the age of 2.55 b.y. corresponds to the metamor phism during the Algoman orogeny in the L.Superior Morton area. The whole rock samples 02-1, MINNESOTA X Ortonville \1 02-2-1 and 02-2-2 also lie on this 2.55 b.y. iso \ 1100Minneapolis chron within the analytical error. Hence, these four sample specimens (02-1, 02-2-1, 02-2-2, 02 g 2 Montevideo 07-1 SacFalls niteo 3) are likely to have been equilibrated with Gra °05 red03 -1Heart 03-2 respect to the Rb-Sr system in the metamor 06 ~,~ Morton • samplelocality 02 1" phism of 2.55b.y. ago. However, the data points 2-,i 1Q, Minnesota for the 02-2-1 and 02-3 biotites lie systematically 0 40 km 02-2-2 River ~~ 02-3 below the 2.55b.y. isochron. Both dotted lines in Fig. 2 connecting the points of the whole Fig. 1. Map of the Minnesota River Valley showing the rock and biotite of 02-2-1 and 02-3 samples localities of samples. give the same age of 2.38b.y. It is very likely Rb-Sr and 40Ar-39Ar geochronological studies 19 that the Rb-Sr system of the 02-2-1 and 02-3 2.3 8 b.y. ago. Geological significance of this biotite did not become completely closed until age is discussed later. Table 1. Chemical composition (water free, recalculated to 100%) of the representative samples 87Sr Sample 01 02-1 03-3 05 07-1 86Sr Morton gneiss Si02 49.4 72.4 56.9 71.8 73.8 wholerock 6 O~ TiO2 1.07 0.15 0.77 0.31 0.10 o plagioclase e K-feldspar 'Lhh A1203 13.2 14.7 21.0 15.8 16.3 biotite FeO 13.6 1.62 4.46 2.48 1.05 0.750 a 02-3 / 022120 02-3 MnO 0.18 0.00 0.00 0.00 0.00 02-1..». 02-2-2 ti5~e;~y. MgO 7.46 0.60 1.59 0.54 0.14 02-2-1 CaO 10.5 1.53 6.28 1.49 1.54 10 Na20 2.38 2.52 5.17 2.90 3.31 02-2-1.02-3 Biotite K20 1.42 4.95 0.94 4.48 2.63 02~ A'02-3 P205 0.14 0.09 0.07 0.12 0.08 ;p . 03-1 0 •03-2 0 200 400 600 ' 03-3 Total 99.4 98.5 97.2 99.9 98.9 0.700 0 0.5 1.0 1.5 2.0 87Rb/86Sr 01: Dike, 02-1: granitic phase of the Morton gneiss, 03-3: tonalitic phase of the Morton gneiss, 05: Sacred Fig. 2. Rb-Sr isochron diagram for the Morton gneiss. Heart granite, 07-1: Montevideo gneiss. These chemical Solid line: Mineral isochron of sample 02-3. compositions are measured by the X-ray fluorescence Dotted lines: Mineral age measured on the biotite and method. the whole rock of samples 02-2-1 and 02-3.
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