Lesser Himalaya: Rampur metabasalts

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 al . Precambrian 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