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 red 03 Heart-1 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.
Table 2. Rb-Sr analytical data
Sample Rb Sr 87Rb/86Sr 87Sr/86Sr (ppm) (ppm) Morton gneiss 02-1 W.R. 113 365 0.900 0.7402 02-2-1 W.R. 107 373 0.823 0.7371 Bi. 686 3.13 635.2 21.72 02-2-2 W.R. 133 401 0.960 0.7406 02-3 W.R. 113 350 0.932 0.7406 K-f. 232 573 1.172 0.7489 Pl. 7.35 423 0.0504 0.7094 Bi. 637 19.8 93.15 3.7881 03-1 W.R. 77.1 1359 0.164 0.7079 03-2 W.R. 63.8 1315 0.141 0.7051 03-3 W.R. 37.7 1328 0.0822 0.7035
Sacred Heart granite 05 W.R. 172 499 0.998 0.7380 K-f. 438 549 2.309 0.7804 Pl. 23.9 712 0.0965 0.7081 Bi. 580 93.7 17.93 1.2947
Montevideo gneiss 07-1 W.R. 46.5 391 0.345 0.7179 08-1 W.R. 107 469 0.661 0.7468 K-f. 382 435 2.545 0.7865 Pl. 11.8 332 0.107 0.7263 08-2 W.R. 124 417 0.860 0.7508 K-f. 385 444 2.504 0.7885 Pl. 21.7 291 0.218 0.7297 09 W.R. 94.0 499 0.545 0.7324 K-f. 308 674 1.325 0.7536 Pl. 31.4 541 0.162 0.7235 Bi. 655 24.0 79.10 2.6890
W.R. = whole rock K-f _ potassium feldspar Pl. = plagioclase Bi. = biotite 20 H. TSUNAKAWA and M. YANAGISAWA
In Table 1, the tonalitic phase of the Morton their five sample specimens, although their data gneiss (03-3) has the chemical composition points were considerably scattered. GRANT apparently distinct from that of the granitic (1972) indicated the significant inhomogeneity phase (02-1). The data points of samples 03-1, of the Sacred Heart granite. Hence, there is a 03-2 and 03-3 are scattered (Fig. 2). Therefore, possibility that each rock unit of the Sacred the Rb-Sr systems of these sample specimens Heart granite has different I value, causing the were not completely equilibrated in the meta scattered data points of their Rb-Sr analyses. morphism of 2.55b.y. ago. Plagioclase, potassium-feldspar and whole A good mineral isochron age of 2.37 ± 0.05 rock sample from sample 09 of the Montevideo b.y. with I = 0.7049 ± 0.0003 was obtained for gneiss constitute a well defined isochron with plagioclase, potassium feldspar and the whole an age of 1.82 ± 0.09b.y. and I = 0.7193 ± 0.003 rock sample from the pluton, Sacred Heart (Fig. 4). The 09 biotite also lies on this iso granite (05) (Fig. 3). The 05 biotite also lies chron. The very high I value of sample 09 must on this isochron. We conclude that the mineral show that the 1.82b.y. isochron age represents isochron age of 2.37 b.y. for sample 05 cor the time when this sample specimen of the responds to the time when the Sacred Heart Montevideo gneiss was completely re-equilibrat granite was intruded into the surrounding ed to reset the Rb-Sr mineral isochron age. This gneisses. The good agreement between this age result indicates that fairly intensive metamor (2.37 b.y.) and the biotite ages of the granitic phism occurred at the site of sample 09 1.82 b.y. phase of the Morton gneiss (2.38b.y.) may sug ago. On the other hand, the data for samples gest that. the Algoman orogeny persisted until 08-1 and 08-2 of the Montevideo gneiss, whose 2.37 2.38b.y. ago. We may speculate further sampling sites are only about 100m distant from that prior to 2.55 b.y. ago the original rocks of 09's site, do not lie on a straight line (Fig. 4). the Morton gneiss were burried and meta The apparent isochron ages constructed from a morphosed and that 2.3 7 2.3 8 Ky. ago a dis pair of plagioclase and whole rock data of 08-1 turbance which might have been associated with and 08-2 are 2.7 b.y. and 2.4 b.y., respectively. an upheaval occurred in this region with the Those between potassium-feldspar and whole intrusion of the Sacred Heart granite. The 05 rock data give a 1.6b.y. age for sample 08-1 isochron is younger and its I value is higher than those obtained by GOLDICHet al. (1970), who gave a Rb-Sr age of 2.70b.y. and I = 0.702 for 875r 08-2, 86Sr potassium-feldspar and whole rock sample from Montevideo gneiss 08-1
whole rock plagioclase n K-feldspar • biotite 08-2 87Sr a 0.750 09 86Sr Sacred Heart granite 05 08-1 ;.:
3 08-2,ti:=ef whole rock 08-1 1 5 y .~ 09 a plagioclase 2 e K-feldspar ti31/ / 09 1 0.750 • biotite 1.5 . 07-1 09 Biotite 0 0 50 100 05 ey 0.70C 0 0.5 1.0 1.5 2.0 87Rb/8651 1.0
05 Biotite
° 05 0.7 Fig. 4. Rb-Sr isochron diagram for the Montevideo 0 10 20
0.700 gneiss (samples 07-1, 08-1, 08-2, 09). 0 0.5 1.0 1.5 2.087R06Sr Solid line: Mineral isochron of sample 09. Dotted lines: Reference lines between the plagioclase Fig. 3. Rb-Sr mineral isochron of the Sacred Heart and whole rock and between the potas granite (sample 05). sium-feldspar and whole rock. Rb-Sr and 40Ar-39Ar geochronological studies 21
Table 3. Minimum age in 40Ar-39Ar age spectrum with the percentage of 39Ar and the temperature at the step which gives the minimum age Total fusion age corresponds to the K Ar age
Minimum age Percentage Temperature Total fusion Sample (b.y.) of 39Ar (%) (°C) age (b.y.) 01 W.R. 1.47 29 950-1000 1.82 02-1 W.R. 1.44 11 950 1.96 03-1 W.R. 2.04 14 1000 2.24 06 W.R. 1.95 64 1050-1100 2.16 07-1 W.R. 1.17 16 900-1000 1.47 07-2 W.R. 1.49 15 950-1000 1.94 08-1 W.R. 1.04 8 800 900 1.56 K-f. 1.19 32 750-1000 1.42 Pl. 1.02 8 950 2.20 08-2 W.R. 1.08 9 900 1.33 K-f. 1.27 16 800 1.63 Pl. 1.26 20 900 2.08 09 W.R. 1.16 19 950 1.81 K-f. 1.29 42 900-1000 1.67 P1. 1.39 16 1000 2.36 Correctionfactor: (36Ar/3'Ar)ca = 3 X 10-4, (39Ar/37Ar)ca= 1 X 10-3, (40Ar/39 Ar)K = 9 X 10-2. X(40K)= 5.543 X 10-10yr-i X (40K) = 0.581 X10-10yr-1 and a 1.7 b.y. age for sample 08-2. Those ap In Table 3, the minimum age in the age spec parent ages can be explained by the way that trum for each sample is shown with the cor although the plagioclase of samples 08-1 and responding temperature and percentage of the 39Ar released 08-2 retains the record of the Algoman orogeny, . In Fig. 5, it can be seen that all the loss of radiogenic 87Sr of the potassium minimum ages are older than 1.0b.y. and that a feldspar might have occurred in a metamor peak may be recognized at 1.0 1.2 b.y. This phism of 1.8b.y. ago, which is inferred from may suggest a geological disturbance in the the isochron age of sample 09. The results of Minnesota River Valley which occurred 1.0 b.y. samples 08-1, 08-2 and 09 of the Montevideo ago. The fact that the age of the last geological gneiss imply that the intensity of the meta disturbance inferred from the 40Ar-39Ar system morphism of 1.8 b.y. ago varied significantly atics does not appear in Rb-Sr systematics may from . place to place in the Montevideo area. be attributed to the general characteristic that From the whole rock data of samples 07-1, 08 the former systematics is more sensitive to the 1, 08-2 and 09, no isochron is obtained.
'OA r-39Ar dating results Although the 40Ar U) 39Ar stepwise-heating dating on the samples from a E 8 0 the Minnesota River Valley does not yield a r 6 definite age, we can still obtain some useful 0 information about the record of the geological 4 disturbances which affected the 40Ar-39Ar system 2 of the samples. It is generally regarded that the Z 0 minimum age in the 40Ar-39Ar age spectrum 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 b.y. represents an upper limit of the age of the last Minimum Age geological disturbance, as discussed by FITCH et al. (1969) and LANPHERE and DALRYMPLE(1971) Fig. 5. Histogram for the minimum ages in the 40Ar and also inferred from the diffusion model of 39Ar age spectra. This may suggest that a geological the 40Ar-39Ar age spectrum by TURNER (1968). disturbance occurred about 1.0b.y. ago. 22 H. TSUNAKAWA and M. YANAGISAWA geological disturbance. This 1.Ob.y. event coin= Ages of the Duluth gabbro complex and of the cides with the age of the Duluth gabbro and the Endion sill, Dulth, Minnesota. J. Geophys. Res. 74, North Shore volcanic group in the west of Lake 720-725. GOLDICH,S. S. and GAST, P. W. (1966) Effects on Superior (FAURE et al., 1969). weathering on the Rb-Sr and K-Ar ages of biotite from the Morton gneisses, Minnesota. Earth Planet. Sci. Lett. 1, 372-375. CONCLUSION GOLDICH, S. S. and HEDGE, C. E. (1962) Dating of The Rb-Sr mineral isochron ages and 40Ar the Precambrian of the Minnesota River Valley, 39Ar dating results in this study reveal more Minnesota. J. Geophys. Res. 67, 3561-3562. GOLDICH,S. S. and HEDGE, C. E. (1974) 3800-Myr detailed metamorphic event in the Minnesota granitic gneiss in southwestern Minnesota. Nature River Valley, as follows. 252, 467-468. (1) The activity of the Algoman orogeny in GOLDICH,S. S., HEDGE, C. E. and STERN, T. E. (1970) this region reached a climax prior to 2.55 b.y. Age of the Morton and Montevideo gneisses and related rocks, southwestern Minnesota. Geol. Soc. ago resulting a high-grade metamorphism and Am. Bull. 81, 3671-3696. persisted until 2.3 2.4b.y. ago. The upheaval GOLDICH, S. S., LIDIAK, E. G., HEDGE, C. E. and in this region might have occurred with the WALTAHLL,F. G. (1966) Geochronology of the intrusion of the Sacred Heart granite 2.3 2.4 midcontinent region, United States, Part 2, northern b.y. ago. area. J. Geophys. Res. 71, 5389-5408. (2) About 1.8 b.y. ago, a relatively intensive GOLDICH, S. S. and MUDREY, M. G., JR. (1969) Dilatancy model for discordant U-Pb zircon ages. metamorphism occurred in the Montevideo area. Geol. Soc. America Abs. with Programs 80. The intensity of this metamorphism varied GOLDICH,S. S., NIER, A. 0., BAADSGAARD,H., significantly from place to place. HOFFMAN,J. H. and KRUGER,H. W. (1961) The (3) A last mild geological disturbance may be Precambrian geology and geochronology of Minne inferred from the 40Ar-39Arsystematics to have sota. Minn. Geol. SurveyBull. 41, 193. occurred over the Minnesota River Valley region GRANT,J. A. (1972) MinnesotaRiver Valley, south western Minnesota. in Geologyof Minnesota, P. K. about 1.Ob.y. ago. SIMSand G. B. MOREY,eds. (MinnesotaGeological Survey) 177-196. Acknowledgments-We greatfully acknowledge the con HANSON,G. N. and HIMMELBERG,G. E. (1967) Ages tinuing encouragement and guidance of Prof. M. OZIMA, of mafic dikes near Granite Falls, Minnesota. Geol. University of Tokyo. We thank Prof. P. W. WEIBLEN Soc. Am. Bull. 78, 1429-1432. and Dr. R. BAYER, University of Minnesota for their LANPHERE,M. A. and DALRYMPLE,G. B. (1971) A collaboration in sampling. We would like to thank Prof. test of the 40Ar-39Arage spectrum technique on some S. ARAMAKI, University of Tokyo, Dr. K. SAITO, Uni terrestrial materials. Earth Planet. Sci. Lett. 12, versity of Yamagata and colleagues in Geophysical 359-372. Institute, University of Tokyo for valuable help and LUND,E. N. (1956) Igneousand metamorphicrocks counsel. of the MinnesotaRiver Valley. Geol. Soc. Am. Bull. 67,1475-1490. REFERENCES MICHARD-VITRAC,A., LANCELOT,J., ALLEGRE,G. J. and MOORBATH,S. (1977) U-Pb ages on single CATANZARO,E. J. (1963) Zircon ages in south zircons from the early Precambrian rocks of west western Minnesota. J. Geophys. Res. 68, 2045 Greenland and the Minnesota River Valley. Earth 2048. Planet. Sci. Lett. 35, 449-453. FARHAT,3. S. and WETHERILL,G. W. (1975) Inter SAITO, K. and OZIMA, M. (1977) 40Ar-39Ar geo pretation of apparent ages in Minnesota. Nature257, chronological studies of submarine rocks from the 721-722. western Pacific area. Earth Planet. Sci. Lett. 33, FITCH, F. J., MILLER,J. A. and MITCHELL,J. G. (1969) 353-369. A new approach to radio-isotopic dating in orogenic SIMS, P. K. and MOREY, G. B. (1972) Resume of belts. in Time and place in orogeny (Geological geology of Minnesota. in Geology of Minnesota, P. Society of London) 157-195. K. SIMSand G. B. MOREY,eds. (Minnesota Geolog FAURE, G., CHAUDHURI,S. and FENTON,M. D. (1969) ical Survey) 3-17. Rb-Sr and 40Ar-39Ar geochronological studies 23
Sacred Heart granite (Sample No. 05) TURNER, G. (1968) The distribution of pottasium 05 is a salmon-pink granite which was intruded and argon in chondrites. in Origin and distribution of into the surrounding gneisses. Main minerals are potas elements, L. H. AHRENS,ed. (Pergamon) 387-398. sium-feldspar, quartz, plagioclase, biotite and chlorite. YORK, D. (1969) Least squares fitting of a straight line with correlated errors, Earth Planet. Sci. Lett. 5, Montevideo gneiss (Sample Nos. 07-1, 08-1, 08-2, 09) 320-324. These rocks are medium-grained,equigranular, pink to red quartzofeldspathic gneisses. The sampling site APPENDIX for 07-1 is almost in contact with the dike (07-2). 08-1 and 08-2 were collected within 1 m distance from each Morion gneiss other. Samplingsites for 09 is about 100m apart from (i) Granitic phase (Sample Nos. 02-1, 02-2-1. 02-2-2, those of 08-1 and 08-2. Mainminerals of these Monte 02-3) videogneisses are potassium-feldspar,plagioclase, quartz, 02-1, 02-2-1. 02-2-2 and 02-3 are medium myrmekite, biotite and apatite. grained, equigranular and gray to pink quartzofeld spathic gneisses. These were collected from a block Dikes (Sample Nos. 01, 06, 07-2) less than 2m in size. Main minerals are plagioclase, Sample 01 was collected from a basaltic dike which quartz, biotite, potassium-feldspar, myrmekite and was intruded into the Morton gneiss. Main minerals apatite. of 01 are hornblende and plagioclase. Samples 06 and (ii) Tonalitic phase (Sample Nos. 03-1, 03-2, 03-3) 07-2 are tholeiitic diabase including plagioclase pheno 03-1, 03-2 and 03-3 are medium-grained, equi cryst, which were intruded into the Montevideo gneiss. granular and gray quartzofeldspathic gneisses, which were collected from a block less than 2m in size. Main minerals are plagioclase, quartz, hornblende and biotite.