lain by Phanerozoic sedimentary rocks, and are locally intruded by Cretaceous granitic rocks in
the southern part of the area. The Wonnam group, the oldest unit in the study area, is mainly
composed of mica schists, garnet-mica schists, biotite gneisses, quartzite, and aplitic gneisses to-
gether with subordinate calcsilicates and amphi- bolites. The Pyeonghae gneiss comprises mainly
well-foliated biotite gneisses and aplitic gneisses, showing augen and banded structures. In the fel-
sic interlayer, K-feldspar porphyroblasts about 2 cm in length are commonly observed. Kim et al.
1991 suggested an upper amphibolite facies metamorphic condition for the Pyeonghae gneiss,
but quantitative estimates of temperature and pressure are not available yet. No geochronologi-
cal data have been reported for the Precambrian rocks in the Pyeonghae area.
3. Samples and experimental procedures
Pb and Nd whole rock isotopic compositions were measured for selected samples of the Won-
nam group and the Pyeonghae gneiss. The loca- tions of analyzed samples are shown in Fig. 1C.
The rock chosen for the PbSL garnet dating PH13 is a fresh specimen of garnet-biotite schist
collected from the central part of the Wonnam group Fig. 1C. The garnet ranges from 1 to
4 mm in diameter. The garnets are predomin- antly
almandine-pyrope solid
solutions with
minor spessartine and grossular components Alm
66 – 71
Pyr
17 – 25
Spe
1.7 – 2.7
Gro
4.4 – 9.1
. Pure garnet separates were hand-picked under a binocular mi-
croscope from rock fragments ranging from 20 to 60 mesh in size. Garnet separates were repeatedly
rinsed with acetone and Millipore
®
water in an ultrasonic cleaner for 30 min.
All the analyses including chemical separation and mass spectrometry were performed at the
Korea Basic Science Institute. About 100 mg of rock powder was mixed with a
150
Nd –
149
Sm mixed spike and then dissolved with a mixed acid
HF: HClO
4
: HNO
3
= 4:1:1 in Teflon vessels.
REE rare earth element fractions were collected by the conventional cation column chemistry. Sm
and Nd fractions were separated from each other by the second step cation column chemistry using
0.2 M HIBA alpha-hydroxy-iso butyric acid Makishima et al., 1993.
Three 120°C leaching steps were performed on the garnet separate. The first step was treatment
with a mixed acid of 12:1 1N HBr + 2N HCl for 30 min. The second and third steps were per-
formed with 4.5N HBr for 3 h and 9N HBr for 18 h, respectively. 30 ml Savillex
®
screw-top beakers were used in the leaching experiment. The residue
was rinsed three times with purified water and dried between steps. Sm, Nd, Th, and U concen-
trations of the leachates were measured using a VG PQ III
®
inductively coupled plasma mass spectrometer ICP-MS. For Pb isotope analysis,
whole rock powders and PH 13 garnet were di- gested using the same method as above but with-
out spikes. The Pb of the PbSL leachates, unleached garnet, and whole rock samples was
separated by the anion exchange column chem- istry using an HBr medium.
Isotopic ratios were measured on a VG 54-30
®
thermal ionization mass spectrometer TIMS equipped with nine Faraday buckets. The Nd and
Pb isotopic compositions were measured with dynamic and static modes, respectively. The
143
Nd
144
Nd ratios
were normalized
to
146
Nd
144
Nd = 0.7219, and further corrected for Nd contribution from added spikes. Replicate
analyses of La Jolla Nd gave
143
Nd
144
Nd = 0.511833 9 0.000005 2s
m
, N = 13. The Pb iso- tope ratios were corrected for instrumental
fractionation using average measured values of the NBS 981 standard. The measured isotopic
ratios of the NBS 981 showed mass fractionation of around 0.1 per atomic mass unit relative to
the recommended value. Total blank levels were about 10 pg for Sm and 50 pg for Nd. Pb blanks
were about 0.3 ng for the PbSL and below 1 ng for the whole rock experiment. Isochron parame-
ters were calculated using the computer program of Ludwig 1994. In the isochron calculation, we
assumed 2s error of 0.1 = external reproduci- bility of NBS981 data, N = 13 for most of
207
Pb
204
Pb and
206
Pb
204
Pb data because internal errors for individual data were smaller than 0.1. Only
for the third leaching step data, internal errors of
Table 1 Pb isotope data for Precambrian basement rocks from the Pyeonghae area, northeastern Yeongnam massif, South Korea
206
Pb
204
Pb 9
2s
a 207
Pb
204
Pb 9
2s
a
Rock types
208
Pb
204
Pb Sample
9 2s
a
Wonnam group 30.303
17.215 Amphibolite
YH01 40.009
16.770 15.638
PH03 Amphibolite
35.673 16.833
15.599 Biotite gneiss
b
YH03 36.265
22.995 YH04
Biotite gneiss
b
16.347 42.951
17.782 15.703
Garnet-mica schist PH13
40.955 17.177
15.563 PH14
Garnet-mica schist 39.488
20.565 16.056
Garnet-mica schist PH25-1
41.777 Garnet-mica schist
PH26 20.066
15.842 39.821
Pyeonghae gneiss PH04
Augen gneiss 20.674
16.129 41.915
22.383 16.319
42.566 PH09
Augen gneiss 19.345
15.950 Porphyroblastic gneiss
PH11 42.969
19.585 15.970
PH15 Porphyroblastic gneiss
42.316 17.038
15.640 Augen gneiss
PH19 37.303
23.945 16.545
PH20 49.406
Augen gneiss PbSL for garnet in PH13
17.758 0.06
15.687 Leaching step[1]
0.06 40.946
0.06 20.636
0.07 15.979
0.07 Leaching step[2]
49.740 0.06
Leaching step[3] 79.447
0.25 22.617
0.24 236.273
0.24 26.094
0.09 16.582
0.09 50.851
0.09 Unleached garnet bulk
a
Internal errors SD, N = 60. For whole rock data, within run errors are sufficiently smaller than 0.1.
b
Gneissic part of schist-gneiss-quartzite interlayer.
ca. 0.25 were given. The residue after the PbSL gave a very poor signal during the mass spectro-
metric run, probably indicating little Pb remained after the leaching. So no Pb isotopic data are
reported for the residue. Errors of calculated ages were reported at the 95 confidence level.
4. Results and discussion
