Geochemistry

The major and trace element geochemical data of the Bomdila Orthogneisses is presented in Table 1. The silica content of the samples varies from 70-76 wt.%. Total alkali concentrations are generally variable and range from 5.5 to 8.5 wt.%, which are mainly controlled by K2O abundance. However, the tourmaline granites show slightly higher total alkalies when compared to the two-mica granites; this may be because the former are enriched in both K and Na contents. All the Bomdila samples are peraluminous with aluminium saturation index [ASI, molecular Al2O3/CaO+Na2O+K2O] of more than 1.1 and also contain abundant normative corundum. These characteristics and high Al2O3 (>13 wt.%) contents indicate that they correspond to S-type granites of Lachlan Fold Belt (Chappell and White, 1974). The pertinent major and trace element compositions of the granites are represented in Harker variation diagram for ease of interpretation and comparison with data from the literature. Large variation in both the suites and intra- and inter-suite variations in some of the elements can be observed (Fig. 2) emphasizing the heterogeneity of the granites. Moreover there are no discernible trends of the major elements (with silica) which could potentially indicate fractional crystallization. However, Al2O3, Fe2O3 and MgO exhibit poor decreasing linear trends with increasing silica, which may indicate fractionation of early formed aluminous and ferromagnesian phases from the magma. It is noteworthy that two-mica granites have low silica content and high FeO+MgO content (up to 6 wt.%) when compared to tourmaline granites, where the latter is characterized by relatively high silica and low FeO + MgO (up to 2 wt.%), TiO2 and CaO abundances. Tourmaline granites are enriched in K2O and hence in normative Or/Ab in contrast to the other tourmaline bearing granites. For example, in the Himalayan Badrinath-Gangotri and Manaslu plutons, tourmaline is generally confined to the most sodic granites (Scaillet et al., 1990; France-Lanord and LeFort, 1988).

Table 1. Major and trace element concentrations of Bomdila orthogneisses, Arunachal Pradesh, NE Lesser Himalaya

  Two-mica granites Tourmaline granites
  BG-2 BG-3 BG-4 BG-6 BG-8 BG-28 BG-10 BG-20 BG-33 BG-33A BG-34 BG-35 BG-36
SiO2 73.13 72.84 76.17 74.55 74.71 73.03 70.16 70.49 74.97 75.21 75.16 75.11 76.18
TiO2 0.44 0.46 0.13 0.41 0.23 0.23 0.55 0.8 0.09 0.12 0.1 0.12 0.11
Al2O3 13.4 13.5 13.45 13.16 13.79 14.97 16.02 15.46 14.61 14.24 14.58 14.11 13.61
Fe2O3 3.85 4.08 1.27 3.71 2.5 2.89 5.16 4.62 1.34 1.48 1.45 1.64 1.37
MgO 0.67 0.72 0.18 0.78 0.45 0.61 1.46 1.55 0.19 0.37 0.39 0.42 0.31
CaO 1.32 1.34 0.63 1.47 0.89 0.48 1.04 1.11 0.47 0.36 0.38 0.36 0.36
Na2O 2.51 2.41 2.66 2.53 2.41 4.01 1.42 2.34 3.44 3.01 3.11 2.93 2.87
K2O 5.05 4.66 5.83 3.86 5.38 3.23 4.1 3.7 5.02 5.25 5.43 5.29 5.19
P2O5 0.14 0.15 0.09 0.12 0.11 0.11 0.09 0.12 0.2 0.16 0.17 0.15 0.14
MnO 0.04 0.05 0.01 0.05 0.03 0.03 0.02 0.05 0.02 0.02 0.02 0.02 0.02
Trace elements (ppm)
Sc 5.76 5.92 2.33 5.48 3.91 3.44 7.41 6.92 4.48 4.16 3.73 4.66 4.17
Rb 278.68 308.26 290.27 273.36 367.28 177.59 178.55 156.43 555.61 465.57 426.03 459.68 436.14
Sr 79.76 57.74 38.77 52.67 42.48 31.39 90.61 109.02 20.21 21.14 23.05 19.76 20.90
Y 42.22 46.55 17.69 36.94 48.65 36.30 26.37 28.67 18.81 19.24 18.21 28.45 21.47
Zr 3.60 2.32 3.10 3.01 2.47 19.15 2.90 6.40 3.27 3.10 3.62 2.73 3.27
Nb 11.75 12.18 8.64 10.37 9.39 13.64 10.51 13.59 14.37 12.57 8.96 11.70 10.27
Ba 696.31 470.61 211.90 332.50 374.33 290.90 663.46 599.20 115.42 128.89 171.05 118.90 125.41
Hf 0.19 0.15 0.12 0.17 0.15 1.06 0.12 0.23 0.13 0.14 0.14 0.12 0.15
Ta 1.63 2.35 1.65 1.82 1.56 2.86 1.28 1.68 3.69 2.54 3.24 3.85 2.24
Pb 26.01 25.88 31.63 21.94 26.01 14.97 21.61 23.74 16.30 15.25 15.87 14.22 16.59
Th 27.81 28.18 13.57 26.19 23.37 21.19 15.44 23.35 6.68 9.27 8.31 9.89 9.79
U 5.91 6.60 3.00 7.73 6.39 4.49 2.65 4.55 8.06 7.35 17.72 30.15 9.14
La 54.13 55.00 24.77 46.80 35.10 33.11 49.93 63.56 8.30 11.07 12.61 13.75 12.35
Ce 113.02 113.98 52.94 97.40 74.05 69.04 97.73 130.31 17.74 23.48 25.57 26.09 26.24
Pr 11.96 12.15 5.73 10.25 7.85 7.30 9.98 13.46 1.81 2.41 2.76 2.95 2.67
Nd 45.45 46.04 21.18 37.93 29.01 27.22 36.62 51.02 6.91 8.77 10.25 10.87 9.63
Sm 8.70 9.01 4.84 7.32 6.14 5.58 6.07 8.28 1.68 1.79 2.20 2.42 1.98
Eu 1.11 1.05 0.54 0.83 0.62 0.62 1.02 1.41 0.19 0.21 0.27 0.25 0.20
Gd 7.39 7.65 3.75 5.96 5.43 4.77 4.64 6.33 1.55 1.64 1.81 2.22 1.70
Tb 1.18 1.26 0.60 1.00 1.05 0.87 0.68 0.86 0.35 0.35 0.38 0.50 0.39
Dy 7.38 8.14 3.38 6.45 7.49 6.03 4.24 4.96 2.75 2.88 2.82 4.03 3.04
Ho 0.78 0.88 0.31 0.67 0.88 0.66 0.47 0.53 0.32 0.34 0.32 0.47 0.37
Er 2.47 2.75 0.88 2.05 2.85 2.11 1.62 1.73 1.12 1.23 1.08 1.59 1.32
Tm 0.30 0.34 0.10 0.24 0.36 0.25 0.21 0.20 0.17 0.19 0.15 0.22 0.19
Yb 2.86 3.29 0.81 2.27 3.39 2.28 2.18 2.00 1.88 1.97 1.56 2.20 2.08
Lu 0.45 0.53 0.12 0.35 0.51 0.32 0.36 0.30 0.29 0.30 0.22 0.32 0.31
Eu/Eu* 0.42 0.39 0.39 0.38 0.33 0.37 0.59 0.60 0.35 0.37 0.41 0.33 0.33
CIPW norms
Quartz 0.50 0.53 0.47 0.56 0.51 0.49 0.65 0.55 0.45 0.47 0.46 0.48 0.49
Orthoclase 0.43 0.39 0.49 0.33 0.45 0.28 0.35 0.52 0.42 0.44 0.46 0.45 0.44
Albite 0.32 0.31 0.34 0.32 0.31 0.52 0.18 0.13 0.44 0.39 0.40 0.38 0.37
Anorthite 0.09 0.10 0.04 0.10 0.06 0.03 0.07 0.05 0.03 0.03 0.03 0.03 0.03

Figure 2. Harker variation diagram (major elements vs. silica)

Harker variation diagram (major elements vs. silica)

Harker variation diagram (major elements vs. silica) for Bomdila orthogneisses. Solid triangles = two-mica granites and Cross hair symbol = tourmaline-bearing leucogranite.


The Bomdila orthogneisses are enriched in incompatible elements such as Rb, Ba, K and Th, and depleted in high field strength elements (HFSE) like Zr, Hf, Ta, Y and Nb (Table 1). The enrichment of the Bomdila samples in the incompatible elements and the depletion in the HFSE strongly supports their postulated crustal source. Both the granite suites are characterized by high incompatible elements/HFSE ratios, which is in agreement with many intra-crustally derived granites. Tourmaline granites are clearly distinguished by their low Sr and Ba contents compared with the two-mica granites and are generally depleted in Sc, Y and Nb (Table 1). The discriminant Rb vs. (Nb + Y) diagram (Fig. 3) shows that the tourmaline granite generally lies within the syn-collision field (as do other Himalayan leucogranites), whereas the two-mica granites straddles the field boundary between collision granite and volcanic-arc granite. The extensive studies on Himalayan leucogranites by Harris et al. (1986), suggest that the granites that form in the syn-collision zones are generally peraluminous leucogranites and may be derived from the hydrated bases of continental thrust sheets. Primitive mantle normalized spidergrams for the Bomdila rocks are presented in Fig. 4. Like most crustal granitoids, they show negative Ti, Sr and Nb anomalies, reflecting the influence of some accessory phases such as rutile as a residual phase in high pressure melting during the event that formed juvenile sialic crust (Gill, 1981).

Figure 3. Tectonic discrimination diagram

Tectonic discrimination diagram

Tectonic discrimination diagram of Pearce et al. (1984). Syn-COLG – syn-collision granites, WPG-within-plate granites, VAG-volcanic arc granites. Symbols as in figure 2.


Figure 4. Primitive mantle normalized trace element spider diagram

Primitive mantle normalized trace element spider diagram

Primitive mantle normalized trace element spider diagram of Bomdila orthogneisses. Normalising values after Sun and McDonough, 1989.


In contrast with other Himalayan leucogranites which are generally characterized by unusually low REE contents, the Bomdila rocks show higher concentrations of all REEs than published averages from other such leucogranites (Vidal et al., 1982; Scaillet et al., 1990). The two-mica granites show higher total REE concentrations (up to 294 ppm) than the tourmaline granites (sum = 67 ppm). The REE contents of two suites indicate light REE (LREE) enrichment over heavy REE (HREE) and have variable LREE/HREE ratios [(La/Yb)N = 3–23]. The patterns show steeply inclined LREEs with flatter and little-fractionated HREEs resulting in overall concave patterns (Fig. 5). The REE abundances of the Bomdila granites coincide with typically crustally derived granites (i.e. La = 20-100X chondrite, Yb = 0.5–8X chondrite, Holtz, 1989) and show consistent fractionation patterns within the LREE group [(La/Sm)N = 3–5]. Negative Eu anomalies are pronounced in both the suites and reveal a very narrow range of difference (Eu/Eu* = 0.35–4) indicating that plagioclase fractionation has been essential in their petrogenesis.

Figure 5. Chondrite-normalised REE patterns of Bomdila orthogneisses.

Chondrite-normalised REE patterns of Bomdila orthogneisses.

Chondrite-normalised REE patterns of Bomdila orthogneisses. Normalising values after Sun and McDonough, 1989. Symbols as in figure 2.