Fig. 2. Zircons from a metabasalt HB6596 and b metarhy- olite HF4990, Larji – Kullu – Rampur window.
Bhuntar and Manikaran. These metabasalts have also been studied by Bhat and LeFort 1992. The
original mineralogy and textures of these volcanic rocks have been destroyed by Tertiary deformation
and greenschist facies metamorphism. The meta- morphic assemblage is Amp ferri – potassian –
tschermakitic hornblende+Bio [MgMg+Fe =
0.47]+Plg Ab
80 – 82
+Qtz+sphene+ilmenite.
3
.
2
. Geochronology of the Rampur metabasalts Knowing the age of the Rampur metabasalts is
of major importance for the stratigraphic correla- tions in the LH. Analysis of subtype S24 and S25
Fig. 2a; Pupin and Turco, 1972 zircons from metabasalt HB6596 by the single grain evapora-
tion technique Kober, 1987; Klo¨tzli, 1997 yielded an age of 1800 9 13 Ma 1 s without
evidence of an older component Table 1. This age is interpreted as dating magmatic zircon
growth. However, if this zircon is inherited, this would represent a maximum crystallization age.
In contrast, Bhat and LeFort 1992 published a whole rock Sm – Nd isochron age of 2510 9 90
Ma 2 s, also from the Rampur metabasalts. In addition, Bhat et al. 1998 studied the Sm – Nd
systematics of the Garhwal and Bhowali mafic volcanic rocks of the Kumaun Lesser Himalaya
and interpreted the composite whole rock Sm – Nd reference line as indicating an age of 2510 9
80 Ma. Dating metabasaltic rocks by means of whole rock Sm – Nd isochrons, however, is highly
problematic. Several published Sm – Nd isochrons on old mafic rocks have subsequently been shown
by other methods to represent mixing arrays without any chronological meaning e.g. Chauvel
et al., 1985; Compston et al., 1986; Gruau et al., 1990.
Our zircon
207
Pb
206
Pb date implies that the 2.5 Ga age may be geologically meaningless and can-
not be correlated with the magma-forming event. Depleted mantle Nd model ages for the Rampur
metavolcanic rocks, including the data of Bhat and LeFort 1992, fall in the range 1.40 – 2.17 Ga
Table 2. These ages are younger than their Sm – Nd ‘isochron’ age, again indicating that the
Archean age is not meaningful. areas e.g. Bajaura Nappe: Frank et al., 1973;
Outer Granite Band: Bhatia and Kanwar, 1973; Baragaon Gneiss: Bhanot et al., 1978; Seawa
Paragneiss: Sharma et al., 1973; Gahr-Manjrot Formation: Bassi, 1989. The age of the Precam-
brian Haimanta Group metasediments is not well constrained, but several lines of evidence suggest a
Neoproterozoic age Frank et al., 1994; Draganits et al., 1998.
3. Lesser Himalaya: Rampur metabasalts
3
.
1
. Petrography In the LKRW metabasaltic rocks were emplaced
as lava flows, intrusive sheets, sills and dikes within the quartz-rich metasedimentary lithologies of the
Rampur formation. Good exposures can be stud- ied in the Sutlej Valley between Rampur and
Jhakri, in the Tirthan Valley near Banjar, in the Sainj Valley and along the Parvati River between
C .
Miller et
al .
Precambrian
Research
103 2000
191 –
206
195 Table 1
Single zircon evaporation data of metabasalt HB6596 and of metarhyolite HF4990, Larji–Kullu–Rampur window, NW Himalaya No. of blocks
Evaporation
208
Pb
206
Pb
b 207
Pb
206
Pb 1 SE
b
1 SE
c
1 SE
c 207
Pb
206
Pb ThU
e
Sample 1 SE
1 SE
b
radiogenic
b
age Ma
d
temperature °C
a
Ma
b
HB
65 96
-A S
24
,
150
mm,
1:4
, sl. turbid, colourless, no inclusions 20
1254 1127Ac1
0.11156 0.00026
0.2 1825
4 4
1276 0.10960
0.00210 1127Ac2
1.9 1793
35 4
1296 0.11037
0.00078 0.7
1127Ac3 1806
13 0.11051
0.00081 0.7
1808 13
Mean HB
65 96
-B S
25
,
100
mm,
1:3
, sl. turbid, colourless 10
1360 0.10936
0.00030 1127Bc1
0.3 1789
5 20
1383 0.11238
0.00022 0.2
1838 4
0.076 0.001
0.211 1127Bc2
0.002 6
1449 0.10790
0.00232 2.2
1127Bc3 1764
39 0.093
0.001 0.259
0.002 Mean
0.10988 0.00187
1.7 1797
31 HF
49 90
-A J
5
,
150
m m,
1:4
, colourless 1
1400 0.11329
0.00126 1.1
1053AC1 1853
20 0.1277
0.0021 0.352
0.006 1053AC3
10 1420
0.11264 0.00098
0.9 1842
16 0.1215
0.0008 0.335
0.002 1053AC5
10 1440
0.11313 0.00070
0.6 1850
11 0.1279
0.0009 0.353
0.003 11
1461 0.11487
0.00095 0.8
1053AC7 1878
15 0.1305
0.0022 0.359
0.006 10
1480 0.11293
0.00116 1.0
1845 19
1053AC9 0.1211
0.0018 0.334
0.005 0.11335
0.00089 0.8
1854 14
Mean HF
49 90
-B J
5
,
120
m m,
1:5
, colourless 1053BC1
10 1301
0.11260 0.00841
7.5 1842
136 0.1118
0.0100 0.309
0.028 10
1340 0.11252
0.00205 1.8
1053BC2 1841
33 0.1084
0.0025 0.299
0.007 1053BC4
10 1461
0.11257 0.00111
1.0 1841
18 0.1114
0.0025 0.308
0.007 1053BC6
10 1380
0.11246 0.00143
1.3 1839
23 0.1138
0.0101 0.314
0.028 10
1400 0.11304
0.00614 5.4
1053BC8 1859
98 0.1170
0.0057 0.323
0.016 1053BC10
10 1420
0.11245 0.00446
4.0 1839
72 0.1354
0.0062 0.374
0.017 10
1440 0.11108
0.00455 4.1
1053BC12 1817
74 0.1443
0.0101 0.399
0.029 5
1460 0.11182
0.00156 1.4
1829 1053BC14
25 0.1541
0.0121 0.426
0.033 0.11232
0.00060 0.5
1837 Mean
10 HF
49 90
-C J
4
,
150
m m,
1:4
, colourless, sl. turbid 10
1340 0.11294
0.00563 5.0
1053CC1 1847
90 0.1319
0.0076 0.364
0.021 1053CC2
10 1380
0.11375 0.00262
2.3 1860
42 0.1288
0.0114 0.031
0.007 1053CC3
10 1400
0.11243 0.00229
2.0 1839
37 0.1206
0.0126 0.333
0.035 10
1400 0.11196
0.00236 2.1
1053CC4 1831
38 0.1208
0.0021 0.334
0.006 1053CC5
10 1420
0.11082 0.00304
2.7 1813
50 0.1151
0.0072 0.318
0.020 8
1420 0.11159
0.00165 1.5
1053CC6 1825
27 0.1186
0.0033 0.328
0.009 10
1440 0.10870
0.00825 7.6
1053CC7 1778
139 0.1188
0.0508 0.33
0.141 5
1440 0.11167
0.00156 1.4
1053CC8 1827
25 0.1057
0.0020 0.292
0.005
C .
Miller et
al .
Precambrian
Research
103 2000
191 –
206
Table 1 Continued Sample
No. of blocks 1 SE
b
Evaporation
207
Pb
206
Pb 1 SE
c
1 SE
c 207
Pb
206
Pb 1 SE
208
Pb
206
Pb
b
1 SE
b
ThU
e
temperature °C
a
age Ma
d
radiogenic
b
Ma
b
10 1460
0.11103 0.00162
1.5 1053CC10
1816 26
0.0953 0.0030
0.264 0.008
8 1480
0.10947 0.00316
2.9 1053CC12
1719 53
0.0775 0.0096
0.215 0.027
Mean excl. C7, C12 0.11202
0.00098 0.9
1832 16
HF
49 90
D J
4
,
130
m m,
1:3
, colourless, sl. turbid 1053DC1
10 1320
0.21539 0.00538
2.5 2947
40 0.4866
0.0353 1.214
0.085 10
1340 0.11414
0.00180 1.6
1053DC2 1866
29 0.1266
0.0082 0.349
0.022 1053DC3
5 1380
0.11147 0.00283
2.5 1824
46 0.1338
0.0076 0.370
0.021 0.11281
Mean excl. C1 0.00189
1.7 1845
30 0.11248
0.00100 0.9
1840 16
HF4990 mean of 4 grains
a
Error on evaporation temperature is estimated to be 9 10°C.
b
Weighted mean from indivial scan ratios.
c
All errors reported are 1 standard errors of the mean.
d
Mean ages derived from individual scan ratios and not from individal scan ages.
e
ThU at apparent 207Pb206Pb age.
C .
Miller et
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Research
103 2000
191 –
206
197
Table 2 Sr and Nd isotopic data for Proterozoic igneous rocks from the NW Himalaya
Unit
a
Sample no.: Locality
T Nd
DM
Lithology Age
Nd Rb
Sm Sr
87
Sr
86
Sr 9 2s
87
Rb
86
Sr
147
Sm
144
Nd
143
Nd
144
Nd 9 2s o
tNd ppm
ppm ppm
Ma ppm
Ga Proterozoic mafic rocks
LH LKRW, Sainj
Metabasalt 1800
19 HF4590
180 0.305
0.727636 9 9 3.6
11.8 0.1844
0.51277 9 3 5.4
2.00 LH
LKRW, Rampur Metabasalt 1800
21 180
0.338 0.716951 9 7
5.2 24.7
0.1273 0.51187 9 3
1.0 HB6596
1.40 LH
LKRW, Rampur Metabasalt 1800
Rj25
b
0.132 0.511859 9 23
− 0.3
2.12 Rj21
b
LH LKRW, Rampur Metabasalt
1800 0.377
0.71637 9 49 0.134
0.511864 9 17 −
0.6 2.16
Rj15
b
LH LKRW, Rampur Metabasalt
1800 0.136
0.511885 9 28 −
0.7 2.17
LH LKRW, Rampur Metabasalt
1800 0.391
Rj2
b
0.72803 9 45 0.153
0.512191 9 25 1.4
2.03 LH
LKRW, Rampur Metabasalt 1800
0.189 0.72803 9 27
4.7 1.51
Rn6
b
Proterozoic felsic rocks LH
LKRW Leucogranite
1840 445
HF3790 70
19.460 1.19248 9 22
LH LKRW
Granite 1840
243 501
1.412 0.74311 9 9
10.3 66.4
0.094 0.511104 9 8
− 6.2
HF3990 2.38
LH LKRW
Leucogranite 1840
483 69
21.620 HF4090
1.31165 9 17 HF4190
LH LKRW
Granite 1840
309 330
2.735 0.78370 9 2
LH LKRW
Bio-granite 1840
212 HF4290
152 4.105
0.83411 9 8 LH
LKRW Qtz-syenite
1840 309
201 4.519
HF4390 0.84727 9 36
HF4690 LH
LKRW Leucogranite
1840 348
62 16.970
1.15080 9 8 HF4890
LH LKRW
Qtz-monzonite 1840
151 176
2.495 0.74302 9 10
LH LKRW, Sainj
Metarhyolite 1840
260 143
5.381 HF4990
0.86610 9 12 12.0
65.9 0.110
0.511310 9 9 −
5.9 2.44
HF3592 LH
JWGC, Wangtu Granitic gneiss 1860
298 72
3.083 0.78958 9 6
LH JWGC, Wangtu Granitic gneiss
1860 296
139 12.460
HF3692 1.04450 9 8
HF10290 LH
KW, E Kishtwar Granitic gneiss 1840
352 130
8.018 0.93982 9 10
HF10390 LH
KW, E Kishtwar Qtz-monzonite 1840
291 630
1.343 0.74238 9 14
LH KW, E Kishtwar Granite
1840 282
357 2.313
HF10690 0.76737 9 11
7.1 39.1
0.109 0.511151 9 7
− 8.8
2.63 HF10890
LH KW, E Kishtwar Granite
1840 274
257 3.120
0.80476 9 7 LH
KW, E Kishtwar Granitic gneiss 1840
372 100
11.148 HF11090
1.01302 9 6 LH
KW, E Kishtwar Granite 1840
315 130
9.109 HF11190
0.95020 9 8 MCT
Bajaura Granitic mylonite 1840
345 64
16.320 HF5090
1.14520 9 8 21.9
130.4 0.102
0.511219 9 9 −
5.8 2.40
MCT Bajaura
Granitic mylonite 1840 227
98 6.860
0.90582 9 5 HF5290
MCT Bajaura
Granitic mylonite 1840 180
56 9.440
HF5490 0.84630 9 6
HF3792 MCT
Baragaon Granitic mylonite 1840
276 108
7.510 0.88259 9 14
HB5296 MCT
Luhri Granitic mylonite 1840
286 163
5.140 0.84983 9 6
MCT Kotlu
Granitic mylonite 1840 231
95 7.130
HB6196 0.84483 9 9
MCT Machad
Granitic mylonite HB6696
1840 267
65 12.130
0.88907 9 7 Paleozoic intrusi6es
HHC Mandi
Bi–Ms granite 496
HF6791
c
3.7 14.9
0.149 0.511954 9 9
2.41 HHC
Jaspa Ms-granite
496 T41
c
2.6 9.5
0.166 0.512020 9 11
2.96 HHC
Kaplas Bi–Ms granite
554 12.5
HF14490
c
75.4 0.099
0.511836 9 6 1.58
a
LH = Lesser Himalaya, MCT = Main Central thrust, HHC = High Himalaya Crystalline, LKRW = Larji–Kullu–Rampur Window, KW = Kishtwar Window, JWGC = Jeori–Wangtu gneiss complex.
b
Data from Bhat and LeFort 1992, 1993.
c
Data from Miller et al. in preparation.
3
.
3
. Geochemistry of the Rampur metabasalts Major, trace element and rare earth element
REE data of four metabasic rocks from the Rampur – Jhakri section and Sainj are given in
Table 3. The analyzed samples are quartz-tholei- itic in composition. In addition to high SiO
2
and relatively low MgO, the major element composi-
tion is characterized by high FeO. The Mg-num- bers range between 29 and 55 [Mg-number =
MgMg + Fe
2 +
, setting Fe
3 +
Fe
2 +
= 0.15]. The
chondrite-normalized REE patterns are shown in Fig. 3a. All samples plot within the field previ-
ously reported for the Rampur metavolcanic rocks Bhat and LeFort, 1992. The three evolved
samples from Jhakri are LREE enriched La
N
Yb
N
= 4.9 – 5.4 with relatively flat HREE pat-
terns at approximately 17 times chondrite and small negative Eu anomalies EuEu = 0.86 –
0.94, quite similar to samples Rj15, Rj21 and Rj25 from the same section analyzed by Bhat and
LeFort 1992. In contrast, the more primitive metabasalt HF4590 from Sainj has lower REE
contents, a lower La
N
Yb
N
ratio of 2.3 and does not possess a depletion in Eu. Another illustration
of the trace element signature is given by the primitive mantle normalized diagram Fig. 3b.
Apart from the greater spread for the more in- compatible elements, this diagram shows negative
Ta, Nb, Sr and P anomalies for the samples from Jakri. The distinct negative Nb anomaly is also
seen in the sample from Sainj and in the meta- basalts analyzed by Bhat and LeFort 1992. Sr
and Nd isotopic data are given in Table 2 and in Bhat and LeFort 1992, 1993. Accepting a 1.8 Ga
eruption age, our samples yielded initial oNd val- ues of + 5.4 and + 1.0, the samples analyzed by
Bhat and LeFort 1992 range from + 4.7 to −
0.7. Only four Sr isotopic data exist so far. Calculated with an age of 1.8 Ga, our initial
87
Sr
86
Sr ratios range from 0.7082 to 0.7197, those reported by Bhat and LeFort 1993 range from
0.7066 to 0.7179.
3
.
4
. Petrogenesis of the Rampur metabasalts The low Mg-numbers of the Rampur meta-
basalts clearly indicate that they could not have been in equilibrium with mantle olivine. Even the
least evolved sample in terms of incompatible elements has a much lower Mg-number 55 and
Ni content 125 ppm than expected for un- modified primary basalts. MgOFeO, a monitor
of ferromagnesian silicate fractionation, ranges from 0.61 to 0.21. The development of small
negative Eu anomalies in the more evolved sam- ples is consistent with fractionation of plagioclase.
The primitive mantle normalized patterns of the Rampur metabasalts Fig. 3b resemble those ob-
served in other Proterozoic mafic dikes Tarney, 1992 and in many continental flood basalts
Fig. 3. Whole rock data from metabasic rocks from Jakri and Sainj, Larji – Kullu – Rampur window, NW Himalaya. a
Chondrite-normalized Boynton, 1984 REE patterns. b Primitive mantle normalized trace element variation diagram.
Values for ocean island basalts OIB and normalization fac- tors are from Sun and McDonough 1989. Shaded fields
represent samples analyzed by Bhat and LeFort 1992.
C .
Miller et
al .
Precambrian
Research
103 2000
191 –
206
199
Table 3 Major wt, trace element and REE ppm concentrations of Rampur metabasalts and granitic rocks from the Larji–Kullu–Rampur and Kishtwar windows, the Jeori–Wangtu gneiss complex and the MCT zone, NW
Himalaya HF4590
HB6596 HB3397
HB3497 HF4990
HF3990 HF4690
Sample no.: HF10290
HF10690 HF3592
HF3792 HF5090
Metabasalt Metabasalt
Metabasalt Metabasalt
Metarhyolite Lithology
Granite Granite
Granite Granite
Granite Mylonite
Mylonite Unit
LKRW LKRW
LKRW LKRW
LKRW LKRW
LKRW KW
KW JWGC
MCT MCT
Sainj W Jakri
W Jakri W Jakri
Sainj Sainj
Location Sainj
E Kishtwar E Kishtwar
Wangtu Baragaon
Bajaura 49.7
51.6 50.1
53.5 68.2
68.5 SiO
2
71.3 69.4
68.2 74.6
66.8 74.5
1.4 1.5
2.6 1.8
0.6 0.5
TiO
2
0.3 0.4
0.4 0.2
0.2 0.2
13.6 13.7
11.3 12.1
13.7 14.8
Al
2 O
3
14.4 15.4
14.1 13.4
14.4 11.7
12.7 16.5
20.4 17.7
4.9 3.6
Fe
2 O
3
2.6 2.3
3.7 1.7
5.8 2.5
0.2 0.2
0.2 0.2
0.1 0.0
MnO 0.0
0.0 0.1
0.0 0.1
0.0 MgO
7.0 4.3
3.8 4.0
0.5 1.2
0.9 0.7
1.7 0.3
1.5 0.2
11.1 7.5
7.6 7.2
2.1 3.0
CaO 0.4
0.8 2.6
1.0 3.0
1.1 Na
2 O
2.0 2.9
1.9 3.4
2.4 2.5
1.9 3.0
2.4 2.7
2.1 1.8
K
2 O
0.4 0.8
1.9 0.5
5.5 4.7
5.3 6.0
4.7 5.1
4.7 5.8
0.1 0.2
0.2 0.2
0.2 0.2
P
2 O
5
0.2 0.2
0.2 0.1
0.2 0.0
LOI 1.4
0.3 0.3
0.3 0.5
0.5 1.4
0.7 0.5
0.3 1.0
1.1 99.5
99.5 100.3
100.8 98.6
99.5 Total
98.7 98.8
98.6 99.5
99.7 99.0
n.d. n.d.
200 180
962 1273
891 1446
1247 680
470 2620
F B
20 B
20 B
20 B
20 3.8
5.6 Be
4.2 11.1
6.1 3.2
4.8 8.8
39 33
40 37
9 4
5 4
7 4
10 5
Sc 339
292 520
306 36
50 V
23 28
58 8
112 12
278 63
22 38
104 80
Cr 85
89 146
8 52
96 55
46 56
47 15
11 Co
5 5
12 2
38 4
Ni 125
50 41
52 12
9 7
4 19
3 20
6 156
118 278
121 6
Cu 3
4 3
20 4
Zn 88
106 158
140 73
50 25
38 47
33 66
56 Ga
21 22
22 22
n.d. 21
n.d. 23
20 20
n.d. 24
19 21
57 17
259 241
Rb 350
349 282
298 276
342 180
180 144
191 140
498 Sr
64 128
355 72
108 63
13 31
36 36
35 11
Y 13
3 18
25 33
70 Zr
73 168
207 190
372 167
121 165
164 150
483 314
3 8
9 9
19 11
Nb 24
14 10
16 10
12 Ba
48 289
474 148
911 1222
286 408
886 294
485 487
Hf n.d.
4.7 5.5
5.3 n.d.
n.d. n.d.
n.d. n.d.
4.7 5.3
n.d. n.d.
0.6 0.7
0.7 9.3
8.8 Ta
17.0 10.2
14.0 2.0
1.1 10.4
Pb n.d.
5 8
13 16
18 28
34 21
58 42
48 0.64
4.49 5.2
5.1 32
24 Th
26 78
26.7 34.4
40 63
n.d. 1
1 1
6.5 15
U 10
10 18
13 3.6
7.3 6
24 26
26 58
48 La
24 23
43 46
57 130
16 50
59 58
106 103
Ce 54
62 89
96 121
200 2
6 7
7 14
11 Pr
7 8
10 10
14 30
11.8 24.7
28 29
51 40
Nd 25
34 37
36 48
101 3.6
5.2 6
6 10
7 Sm
6 6
7 7
9 18
Eu 1.3
1.6 1.8
1.6 1.6
1.2 0.4
0.5 1.5
0.6 0.8
1.0 4
5 6
6 11
7 Gd
5 5
7 5
7 19
Tb 0.7
0.9 1.0
0.9 1.3
0.59 0.6
0.2 0.7
0.9 1.1
2.3 Dy
4.1 5.0
6.0 5.6
7.5 2.6
2.9 0.9
3.5 4.6
6.1 13.1
0.8 1.1
1.3 1.3
1.4 0.42
Ho 0.47
0.12 0.62
0.97 1.3
2.5 Er
2.2 3.0
3.7 3.3
3.7 1.1
1.3 0.3
1.7 2.4
3.5 7.1
0.31 0.46
0.57 0.53
0.50 0.16
0.18 Tm
0.04 0.25
0.36 0.54
1.03 1.7
2.9 3.6
3.5 2.9
1.0 Yb
1.1 0.04
1.3 2.5
3.7 6.5
Lu 0.26
0.48 0.56
0.60 0.34
0.13 0.14
0.02 0.22
0.35 0.57
0.88
Fig. 4. oNd versus 1Nd diagram at the time of eruption 1.8 Ga with fractional crystallization FC and AFC DePaolo,
1981 trends for the metabasic rocks from the LKRW includ- ing samples analyzed by Bhat and LeFort, 1992. The AFC
vectors represent fractional crystallization of metabasalt Rn6 with assimiliation of granitoid HF5090 r = 0.3. Note that
this is not a unique solution, just a possible explanation for the crustal contamination model.
high initial
87
Sr
86
Sr ratios that are far outside the variation range of the mantle trend.
Essentially two options are available for the introduction of a crustal component: either i
magmas were derived from the mantle and be- came contaminated during ascent through the
crust crustal-level contamination or ii recycling of older crust mantle source contamination. The
evidence at the moment is far from clear-cut although the correlation between Nd content and
o tNd Fig. 4 would be consistent with increas-
ing degrees of contamination of the least evolved mafic lavas by LREE-enriched crust such as the
Proterozoic granitoids. The correlation R = 0.93 between 1Nd and
143
Nd
144
Nd in the Rampur metabasalts also indicates that the 2.5 Ga Sm – Nd
array Bhat and LeFort, 1992 is an artifact of mixing between depleted mantle melts generated
at 1.8 Ga and an older enriched lithospheric component.
4. Lesser Himalaya and MCT zone: felsic magmatic rocks