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CHAPTER-V

Whole-Rock Geochemistry

5.1. Introduction

Geochemistry is the study of chemical changes on the Earth in terms of analyzing

absolute and relative abundances of chemical elements in minerals, soils, rocks, water and

atmosphere of the earth along with the distribution and movement of these elements in

various environments of the earth. Geochemical studies of ancient rocks, fluids and gases

provide insights into the evolution of Earth’s crust, oceans, primordial atmosphere, etc.

Magmatic processes such as partial melting, fractional crystallization and assimilation

affect the composition of the rocks during their evolution and formation. In metamorphism,

the bulk composition of a given rock under a variable P-T-t regime results in the distribution

of various elements in different phases that are in equilibrium with each other. These phases

can be identified and analyzed by the ratios and abundances of various elements by studying

them in great detail and applying various techniques to obtain reliable and accurate data,

which will in turn assist in interpreting the various processes and their environments.

Major, Trace and Rare Earth Elements (REE) have been used very widely and

successfully to model the origin and evolution of the rocks; the nature, composition and

differentiation of magmatic fluids at depth; and tectonic discrimination of geochemical

processes.

In the present context, granulite facies gneissic rocks are considered to be the

exhumed parts of the Earth’s lower crust while mafic-ultramafic complexes are considered as

the derivatives of upper mantle. Thus, the geochemical signatures of these units can throw

insights about the crustal evolution and changes in the upper mantle during Proterozoic time.

They also help in revealing the tectonic setting prevailed during the emplacement of these

mafic bodies. The present study area of NMC comprises heterogeneous litho units including

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periodtites, pyroxenites, gabbros, amphibolites and plagiogranites/trondhjemites. The

geochemical studies of these rocks help in understand the tectonic evolution of these oceanic

remnants of NMC.

This chapter presents geochemical characteristics of the representative samples, a

total of twenty five (peridotites-three samples; pyroxenites- three samples; hornblendites- two

samples; amphibolite-eleven samples and Plagiogranites/trondhjemite- six samples) were

collected for whole-rock chemical analysis. The major element compositions were estimated

on X-Ray Fluorescence Spectrometry (XRF), while the trace and Rare Earth Elements were

determined using Inductively Coupled Plasma Quad-Mass Spectrometer (ICP-MS) at the

Geological Studies Division, CSIR-NGRI, Hyderabad. The details of sample preparation are

outlined in the following sections. The precision and accuracy of the data are given by Govil

(1985) and by Balaram et al. (1992) for XRF and ICP-MS respectively. Absolute accuracy

has been assessed by comparison with international reference materials analyzed along with

the samples and is generally better than 2%. The results of these chemical analyses are given

in Table (1).

5.2. Geochemical Studies

The geochemical study is one of the important objectives of the present study. We

have carried out geochemical analyses of different samples representing ultramafic-mafic-

felsic rocks to understand the tectonic evolution of the rock types of NMC. The major

element compositions were estimated on X-Ray Fluorescence Spectrometry (XRF), while the

trace and Rare Earth Elements were determined using Inductively Coupled Plasma Quad-

Mass Spectrometer (ICP-MS) at the Geological Studies Division, NGRI, Hyderabad. The

details of sample preparation are outlined in the following sections. The precision and

accuracy of the data are given by Govil and Naquvi (1985) and by Balaram et al. (1992) for

XRF and ICP-MS respectively.

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Table 1: Whole rock chemistry of various rock samples in the complex Rock type Peridotite Peridotite Peridotite Pyroxenite Pyroxenite Pyroxenite Hnb.dite Sample ANP-20 ANP-85 ANP-22 ANPSS-1 ANP-21 ANP-23 ANP -65 SiO2 37.47 37.43 37.11 52.13 49.75 51.93 49.84 Al2O3 6.24 5.28 7.23 3.5 6.54 7.52 12.33 TiO2 0.69 0.55 0.68 0.31 0.51 0.19 0.34 Fe2O3 7.17 7.05 5.72 14.25 14.16 12.19 12.9 MnO 0.11 0.15 0.07 0.15 0.2 0.16 0.17 MgO 34.17 35.65 34.34 10.89 12.54 13.48 10.98 CaO 9.24 8.49 9.55 14.15 13.78 12.42 10.86 Na2O 0.58 0.69 1.29 0.59 0.88 0.76 0.98 K2O 0.16 0.08 0.16 0.05 0.03 0.03 0.03 P2O5 0.13 0.01 0.01 0.08 0.06 0.02 0.06 LOI 1.771 0.847 0.256 0.61 0.274 0.557 - Sum 97.731 96.227 96.416 96.71 98.724 99.257 98.49

Sc 36.701 24.695 38.401 67.15 36.204 32.798 38.095 V 130.587 110.244 162.388 247.00 128.986 91.877 242.624 Cr 3429.557 2704.718 3268.767 1112.00 1073.190 1951.640 189.458 Co 82.212 109.498 77.234 132.90 62.527 55.493 63.891 Ni 950.265 1142.612 594.468 769.40 514.274 547.937 337.932 Cu 247.233 9.494 8.498 142.10 91.425 109.907 49.419 Zn 87.837 72.634 62.998 58.04 88.739 47.090 99.446 Ga 1.78 1.66 3.76 2.61 4.112 2.899 23.099 Rb 0.2 1.094 0.225 0.21 2.019 2.044 6.079 Sr 23.379 12.216 17.317 40.75 15.819 10.002 69.869 Y 3.402 2.981 5.848 15.27 7.424 4.167 30.981 Zr 0.053 0.06 0.095 67.77 4.599 4.567 188.541 Nb 0.004 0.054 0.175 0.21 0.069 0.069 3.971 Cs 0.017 0.058 0.037 0.00 1.303 1.230 1.911 Ba 4.143 6.401 5.437 11.30 18.612 17.126 24.887 Hf 0.002 0.002 0.003 3.58 0.123 0.121 5.202 Ta 0.001 0.006 0.009 0.17 0.021 0.021 0.552 Pb 3.424 2.493 2.256 3.00 5.290 5.184 7.483 Th 0.005 0.008 0.002 0.05 0.120 0.119 5.572 U 0.001 0.001 0.001 0.01 0.070 0.072 3.210

La 0.429 0.177 0.299 2.79 1.727 1.007 20.493 Ce 1.497 0.715 1.212 8.98 4.342 2.684 46.912 Pr 0.302 0.146 0.282 1.49 0.636 0.411 5.163 Nd 1.865 0.902 1.912 8.32 3.729 2.127 27.464 Sm 0.644 0.344 0.819 2.62 1.273 0.649 5.916 Eu 0.294 0.234 0.469 0.67 0.567 0.256 1.319 Gd 0.771 0.51 1.095 1.69 1.678 0.848 6.644 Tb 0.133 0.103 0.203 0.44 0.307 0.155 0.980 Dy 0.684 0.569 1.128 2.83 1.920 1.098 5.587 Ho 0.137 0.124 0.239 0.56 0.462 0.260 1.218 Er 0.332 0.306 0.589 1.47 1.366 0.693 3.488 Tm 0.047 0.044 0.08 0.22 0.202 0.107 0.527 Yb 0.219 0.211 0.381 1.36 1.338 0.719 3.580 Lu 0.029 0.028 0.047 0.22 0.213 0.107 0.596 Total REE 7.383 4.413 8.755 33.659 19.75913 11.12051 129.887

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Contd… Rock type Hnb.dite Amphibolite Amphibolite Amphibolite Amphibolite Amphibolite Sample ANP- 17 ANP-38 ANPSS-15 ANP-7 AS-20 ANPSS-7A SiO2 48.8 50.41 48.47 50.59 49.06 50.21 Al2O3 10.75 13.84 11.76 14.56 11.05 6.39 TiO2 0.29 0.65 0.5 0.5 1.11 0.57 Fe2O3 20.84 14.27 14.16 12.3 10.65 6.73 MnO 0.29 0.12 0.17 0.18 0.18 0.09 MgO 12.32 7.31 9.71 7.37 11.12 6.87 CaO 4.31 9.75 13.41 11.57 12.2 20.75 Na2O 0.65 1.3 1.16 1.3 2.38 3.03 K2O 0.04 0.31 0.41 0.11 0.41 0.52 P2O5 0.18 0.06 0.21 0.05 0.29 0.29 LOI 1.036 0.847 0.961 - 1.771 3.83 Sum 99.506 98.867 100.921 98.53 100.221 99.28

Sc 20.203 51.952 95.34 52.605 30.408 16.38 V 113.745 319.640 285.1 328.326 188.51 124.8 Cr 245.574 83.280 1443 95.946 55.016 32.48 Co 97.379 54.538 111.9 48.702 74.44 61.75 Ni 610.029 127.678 202 144.674 62.563 28.83 Cu 24.568 17.146 2.588 44.555 16.602 9.172 Zn 224.233 170.930 97.61 120.798 347.149 77.3 Ga 19.026 23.485 5.505 30.418 15.592 8.94 Rb 4.819 5.103 4.673 7.384 1.206 3.401 Sr 35.876 63.564 96.99 123.689 253.108 966.5 Y 24.891 24.153 41.2 35.796 16.954 19.11 Zr 142.825 93.435 77.9 144.982 22.607 36.34 Nb 3.058 3.959 0.281 6.209 2.673 3.3 Cs 1.756 1.583 0.145 2.237 0.1 0.146 Ba 18.467 20.991 62.03 31.981 45.717 242.5 Hf 3.940 2.578 3.218 4.000 0.871 2.183 Ta 0.441 0.354 0.228 0.775 0.661 0.304 Pb 5.818 6.284 3.08 10.319 8.594 0.916 Th 3.991 2.900 0.009 6.655 0.088 0.032 U 2.297 1.765 0.005 3.822 0.109 0.136

La 18.048 10.105 3.18 18.530 7.208 17.68 Ce 38.791 23.087 11.02 38.802 18.777 40.8 Pr 4.117 2.452 2.116 4.097 2.247 5.405 Nd 21.378 13.120 10.56 21.007 13.451 24.69 Sm 4.293 3.093 3.115 4.586 3.018 5.161 Eu 0.687 0.905 1.052 1.062 1.504 1.321 Gd 4.837 3.840 2.41 5.566 3.437 2.871 Tb 0.748 0.658 0.735 0.938 0.548 0.6 Dy 4.478 4.366 5.64 6.238 2.736 3.41 Ho 1.054 1.062 1.327 1.535 0.587 0.674 Er 3.021 2.917 3.654 4.258 1.886 1.793 Tm 0.464 0.441 0.727 0.651 0.307 0.283 Yb 3.208 3.006 4.639 4.472 1.685 1.74 Lu 0.561 0.490 0.775 0.768 0.263 0.285 Total REE 105.6848 69.543 50.95 112.510 57.654 106.713

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Contd… Rock type Amphibolite Amphibolite Amphibolite Amphibolite Amphibolite Amphibolite Sample ANPSS-16 ANPSS-6 ANP-35 ANP-41 ANP-87 ANP-44 SiO2 47.23 50.43 47.05 48.47 48.79 48.7 Al2O3 12.13 11.15 10.98 7.84 10.75 9.93 TiO2 1.52 1.26 0.51 0.93 0.69 0.76 Fe2O3 13.48 12.71 11.31 14.54 11.04 11.56 MnO 0.17 0.24 0.15 0.18 0.19 0.17 MgO 7.34 9.76 16.53 14.51 14.82 12.83 CaO 11.34 10.25 11.89 12.04 12.41 13.11 Na2O 4.01 2.25 1.04 0.96 0.99 1.21 K2O 0.5 0.43 0.16 0.4 0.07 0.03 P2O5 0.56 0.19 0.07 0.18 0.03 0.02 LOI 0.388 0.642 - - 0.739 - Sum 98.668 99.312 99.69 100.05 100.519 98.32

Sc 46.44 48.55 59.341 53.59 57.631 44.034 V 368.8 349.1 222.721 221.902 360 250.918 Cr 112.7 366.7 236.629 205.432 80.895 125.495 Co 165.9 125.1 72.395 49.021 66.93 59.653 Ni 74.01 138.6 152.933 48.781 65.077 61.401 Cu 158.9 25.75 48.207 34.275 120.984 34.861 Zn 146.2 135.3 106.211 352.102 213.299 74.141 Ga 12.57 6.894 13.317 17.94 17.106 14.685 Rb 0.455 1.61 1.071 1.074 1.166 0.474 Sr 1293 149.1 73.947 36.704 64.438 59.75 Y 40.38 29.96 16.097 17.55 22.223 16.437 Zr 58.41 61.97 32.082 24.515 33.132 21.216 Nb 6.42 3.036 1.807 7.385 0.878 0.186 Cs 0.018 0.062 0.066 0.028 0.072 0.039 Ba 246.9 52.99 44.594 46.333 46.924 83.453 Hf 3.512 3.825 1.068 1.196 1.273 0.698 Ta 0.735 0.503 0.748 1.441 1.151 0.117 Pb 1.216 2.314 4.371 4.304 5.165 2.865 Th 0.136 0.222 0.785 0.857 0.106 0.023 U 0.055 0.106 0.221 0.187 0.135 0.024

La 22.15 5.081 9.13 12.985 2.332 0.962 Ce 47.54 14.83 19.592 42.478 9.422 3.818 Pr 7.901 2.212 2.102 5.462 1.465 0.652 Nd 37.75 11.64 11.367 31.489 9.805 4.765 Sm 9.116 3.538 2.491 6.664 2.87 1.684 Eu 2.559 1.051 1.293 1.305 0.879 0.644 Gd 5.407 2.502 2.888 6.441 3.476 2.254 Tb 1.179 0.706 0.497 0.81 0.613 0.431 Dy 6.863 4.904 2.603 3.41 3.378 2.582 Ho 1.318 1.062 0.548 0.602 0.755 0.553 Er 3.138 2.995 1.719 1.78 2.525 1.782 Tm 0.536 0.505 0.28 0.265 0.432 0.282 Yb 3.132 3.193 1.436 1.337 2.444 1.517 Lu 0.48 0.548 0.232 0.202 0.391 0.233 Total REE 149.069 54.767 56.178 115.23 40.787 22.159

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Contd… Rock type Trondhjemite Trondhjemite Trondhjemite Plagiogranite Trondhjemite Plagiogranite Sample AS-17 AS-16 AS-31A AS-14 AS-14A ANPSS-14 SiO2 66.37 69.7 68.17 67.36 68.3 66.33 Al2O3 18.94 17.8 15.09 16.47 16.77 19.08 TiO2 0.04 0.05 0.05 0.15 0.01 0.01 Fe2O3 0.011 0.13 1.48 1.73 1.12 0.23 MnO 0.01 0.01 0.01 0.03 0.002 0.01 MgO 0.1 0.05 1.99 1.02 0.64 0.14 CaO 4.9 4.07 4.54 3.12 2.77 4.81 Na2O 7.12 6.24 6.28 5.35 4.9 6.11 K2O 0.47 0.29 0.83 2.91 4.28 0.63 P2O5 0.01 0.01 0.01 0.08 0.05 0.02 LOI 1.036 0.388 - 0.735 - - Sum 99.007 98.738 98.45 98.955 98.842 97.37 Sc 1.292 1.958 1.175 2.2 1.57 0.49 V 5.123 8.257 3.432 13.578 10.543 19.39 Cr 7.202 12.344 8.286 8.812 8.607 6.66 Co 76.242 142.906 80.755 92.303 96.672 133 Ni 1.555 3.697 1.721 3.312 1.885 - Cu 0.26 0.419 0.272 0.267 0.429 - Zn 84.877 19.046 78.207 22.557 13.47 - Ga 14.813 14.379 13.177 12.317 11.256 9.284 Rb 1.897 2.297 3.124 23.264 28.788 3.427 Sr 355.005 564.651 219.424 400.465 388.453 1561 Y 0.522 0.294 0.261 2.766 3.571 0.459 Zr 14.017 26.419 11.541 21.105 6.346 23.57 Nb 0.749 0.264 0.223 1.506 1.771 0.234 Cs 0.07 0.039 0.045 0.114 0.114 0.092 Ba 219.598 130.122 49.064 1043.074 1715.548 283.6 Hf 0.366 0.621 0.383 0.517 0.159 0.748 Ta 0.623 0.56 0.315 0.348 0.398 0.513 Pb 19.184 22.479 35.061 13.547 12.458 18.87 Th 0.115 0.243 0.048 0.26 0.121 0.069 U 0.195 0.153 0.198 0.119 0.091 0.117 La 1.391 2.797 0.201 12.119 4.94 0.697 Ce 1.77 4.482 0.42 21.763 8.73 1.049 Pr 0.218 0.396 0.043 2.217 0.886 0.131 Nd 0.786 1.43 0.16 8.415 3.489 0.51 Sm 0.129 0.222 0.033 1.351 0.622 0.124 Eu 0.19 0.574 0.157 0.58 0.712 0.533 Gd 0.11 0.148 0.033 1.027 0.569 0.072 Tb 0.019 0.019 0.008 0.133 0.085 0.012 Dy 0.089 0.065 0.036 0.611 0.572 0.064 Ho 0.012 0.012 0.006 0.061 0.071 0.013 Er 0.035 0.026 0.019 0.187 0.228 0.035 Tm 0.009 0.009 0.005 0.025 0.031 0.007 Yb 0.068 0.046 0.035 0.164 0.291 0.041 Lu 0.016 0.017 0.012 0.034 0.052 0.008 Total REE 4.842 10.243 1.168 48.687 21.278 3.296

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5.2.1. Sample preparation

Based on petrographic characteristics 25 fresh rock samples were selected for

geochemical studies from NMC. The selected samples were crushed in a jaw crusher to

granules of ~10mm size and finally by conning and quartering, a homogenous fraction of

about 100 grams of the coarse powder was pulverized in a SPEX mixture mill using a pre-

cleaned fused alumina vial and balls to ‘fine powder’ (<0.075mm) and stored in cleaned

polypropylene screw capped vials with neat labeling slips for whole rock geochemical

analysis. The necessary care was taken to maintain non contamination of the samples.

5.3. Analytical Techniques employed

5.3.2 X-ray Fluorescence Spectrometry

Sample Preparation, instrumentation and operating parameters are described below.

Collapsible aluminium cups were filled with 9 gm of boric acid, which acts as binding

material. 1 gm of ~200 mesh homogenized sample powder was sprayed upon it by covering

the boric acid uniformly and about 15 tons of pressure was applied using Herzog hydraulic

press (H/100) to obtain a pressed pellet of 40 mm diameter. The samples were analyzed for

major elemental compositions by Philips PW 1400 microprocessor controlled wavelength

dispersive, sequential X-Ray Fluorescence spectrometer (Phillips, Holland). The system was

interfaced to a Phillips P851 online minicomputer for preparing calibration curves relating the

concentration and intensity levels in a standard as well as the unknown samples after due

matrix corrections. Software available in the computer was able to take care of dead time;

background and line overlap corrections after regression and converting the counts into

corrections with the help of the calibration curves finally giving the output directly as

concentration in oxide percentages (or) in ppm as required. A spinner was used to spin the

samples inside the spectrometer while measuring to have uniform counts. Certain elements

were analyzed using a Rhodium target X-ray tube while a Chromium X-ray target tube

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estimated Na, Al and Mg since the concentration levels of these elements were very low. All

the elements were estimated under a high vacuum condition (10-6 Torr). The major and

minor elemental data estimated by XRF are reproducible with a precision range of ± 5%.

5.3.2. Inductively Coupled Plasma Quad-Mass Spectrometer

Rare Earth Elemental data has become a vital component in the modern geochemical

investigations. Inductively coupled plasma-mass spectrometry (ICP-MS) is being extensively

used for accurate and precise determination of REE in silicate rocks (Date and Hutchison,

1987). For the present study trace and rare earth elemental analysis of the rock samples were

carried out by ELAN DRC II (Perkin- Elmer, Sciex, USA) ICP-MS. The instrument is

equipped with Dynamic Reaction Cell and other advancements leading to extremely low

background, better sensitivity and striking improvement in measurement precision which

takes the detection limits for the most of the elements in the periodic table to pg/ml (ppt) and

fg/ml (ppq) levels.

5.3.3. Sample Preparation for trace and REE estimations

Accurately weighed 50 mg of the rock sample powder is taken in a 100 ml Teflon

beaker and 10 ml of acid mixture consisting of 7:3:1 proportion of HF, HNO3 and HClO4

was added and kept overnight with lids on the beakers. Next day the beakers were transferred

on to a hot plate at 2500C, heated for 30 minutes and then the lids were removed and the

beakers re-heated until dryness. Another 10 ml of the acid mixture is added in the same

proportions to the sample in the beakers and dried completely. Then 5 ml of 1-ppm rhodium

(Rh) is added with 20 ml of 1:1 HNO3 and the beakers were kept on the hot plate under a

warm condition for 15 minutes. Then the solution was cooled and transferred to a 250 ml

volumetric flask and the required volume is made. 60 ml of the solution is taken in a

polypropylene bottle for analysis using ICP-MS.

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5.3.4. Loss-On-Ignition (LOI)

Loss-On-Ignition (LOI) used in the whole rock analysis is the measurement of total

volatiles (i.e. H2O, CO2, F, Cl, F etc.) usually added to the other oxides to get total up to 100

± 1%.

Before heating, the samples were dried at 110 – 120 °C. Then dried samples were

heated for two hours in a platinum crucible at 950°C. The difference in weight between

ignited and unignited sample was considered as LOI. The Loss-on-ignition is calculated by

using the following formula:

LOI % = (W2-W3/W2-W1) X 100

Where,

W1= weight of empty crucible

W2= weight of crucible + sample

W3= weight of crucible + sample after ignition.

5.4. Whole rock geochemistry

Twenty five representative samples of varied lithologies within the NMC were

analyzed for major, trace and REE. The results are listed in the Table.1

5.4.1. Ultramafic rocks

The ultramafic rocks include periodtites and pyroxenites. The geochemical results of

Periodotites show relatively high Mg# (100xMgO/MgO+FeOt) ranges from 82.65-85.72 and

low FeO (14.25-12.19) and CaO (8.49-9.55 wt%) concentrations along with low SiO2

(37.11-37.43 wt.%), and moderate TiO2 (0.55-0.69 wt%) and Al2O3 (5.28-7.23 wt%). The

total alkalis are low (K2O+Na2O >2 wt%) and trace element concentrations include high Ni

(595-1142 ppm) and moderate concentration of Cr (2705-3430 pm). The binary plots with

reference to MgO show positive correlation with SiO2, Al2O3, CaO, TiO2, P2O5 and slight

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negative correlation with FeOt and TiO2 and no difference with Ni, Cr (Fig. 5.1). The rocks

show relatively low REE, ranging from 4.413-8.75 ppm and moderate La/YbN ratio (0.58-

3.97). Ce/YbN (0.95-3.54) and La/SmN (0.22-0.84) are depleted with an average value of 0.91

and 0.58. On normalized chondrite, the REE patterns of these rocks show slight HREE

enrichment with reference to LREE showing positive Eu anomalies (Fig. 5.2a) with flat

trends indicating less fractionation. On primitive mantle-normalized diagram the rocks show

slight LILE enrichment (Rb, Ba) and depletion of HFSE (Ti, Nb, Yb), together with strong

negative Sr, Zr, and Y anomalies (Fig. 5.2b). The pyroxenites show similar geochemical

characteristics with peridotites but the Mg# is very low that ranges from 42.3 -53.5. The

rocks show relatively high SiO2 (49.75-52.13 wt%), FeO (12.19-14.25 wt.%), CaO (12.42-

14.15 wt.%) and varied Al2O3 (3.5- 7.52 wt.%) with low TiO2 (<0.3). The total alkalis are

very low (K2O+Na2O >1 wt%) and trace element concentrations include high Ni (595-1142

ppm) and low concentration of Cr (1073.20-1951.64 ppm). The binary plots with reference to

MgO indicate positive correlation with SiO2, FeOt, TiO2, CaO, P2O5, and slight negative

correlation with Al2O3 and no difference with Ni, Cr and Y (see Fig. 5.1). The REE ranges up

to 33.66 ppm with depleted La/YbN (0.89-1.43), La/SmN (0.32-0.97) and Ce/YbN ratios (0.87-

1.79). On normalized chondrite, the REE patterns show flat pattern with slight +v Eu

anomalies and without any significant fractionation of magmas. On primitive mantle-

normalized plot shows slight LILE enrichment (Rb, Ba, Th) and depletion of HFSE (Ti, Y,

Yb) with negative anomalies of Sr and Zr, (Fig.5.8b). Most of the peridotites and pyroxenites

examined in this study are usually altered and serpentinized with slightly high LOI. The LOI

of these samples correlates well with the degree of alteration determined petrographically.

Normalization of the major oxides on a dry-weight basis of these rocks indicates that the

alteration mainly involved hydration with little gain or loss of major oxides. The distinct

positive Eu anomalies of these rocks could be the effect of alteration on the REE.

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Fig. 5.1: Binary plots for pyroxenites and gabbros: MgO (wt%) vs SiO2 (wt%); MgO (wt%) vs TiO2 (wt%); MgO (wt%) vs Al2O3 (wt%); MgO (wt%) vs FeO(t) (wt%); MgO (wt%) vs CaO (wt%); MgO (wt%) vs Ni (ppm); MgO (wt%) vs Cr (ppm); MgO (wt%) vs Zr (ppm), Diamond represents peridotite, square represents pyroxenite and triangle represents hornblendite.

5.4.2. Mafic rocks

5.4.2.1. Hornblendites

The hornblendites relatively show lower SiO2 concentration ranging from 48.80-49.84

wt% with high Al2O3 (10.75-12.33 wt%) and FeO (12.9-20.84) and lower CaO (4.31-10.86

wt%) along with low Mg# (100xMgO/MgO+FeOt) ranging from 37.15-45.97. The rocks

show lower TiO2 concentration varying from 0.29-0.34 wt% and lower total alkalis (0.69-

1.01 wt%). The trace element characteristics have low compatible elements Cr (189.46-

245.57 ppm), Co (63.89-97.34 ppm), V(113.74-242.62 ppm) and Sc (22.20-38.12 ppm), but

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Ni concentration is varying between 337-610 ppm and relatively some higher incompatible

elements Zr (142.83- 188.54), Nb (3.06-3.97 ppm) Hf (3.94-5.20 ppm) Y(24.89-30.981ppm).

Fig. 5.2: (a) Chondrite normalized diagram for the peridotites, pyroxenites and hornblendites (after Sun and Mac Donough, 1989). (b) Rock/Primitive mantle normalized plots of peridotites, pyroxenites and hornblendites (after Sun and Mac Donough, 1989); symbols are similar to that of Fig. 5.1. The binary plots with reference to MgO show positive correlation with SiO2, Al2O3 FeOt,

CaO, TiO2, P2O5, and Yr and -ve correlation with Ni and Cr (see Fig. 5.1). The chondrite-

normalized REE patterns (see Fig. 5.2a) show slight LREE enrichment with a general flat

trend and slight +ve Eu anomaly is observed in these samples, suggest plagioclase

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fractionation as a dominant mode of differentiation. A la/YbN and Ce/YbN ratio varies from

0.544-3.90, 0.86-3.36 and La/SmN ratio ranges from 2.16-2.63. Primitive mantle-normalized

trace element patterns (see Fig. 5.2b) show negative anomalies of K, Ti, Y and Sr, and with

enrichment of LILE relative to HFSE.

5.4.2.2. Amphibolites

The amphibolites show relatively narrow range in SiO2 (47.05-50.43 wt%), Al2O3

(6.73-14.27 wt%), FeO (6.73-14.54 wt%), and CaO (9.75-13 45 wt%) except one sample

(20.75 wt%t%). The Mg# varies from 33.87-59.38 and total alkalis (Na2O+K2O) range from

1.06-4.51 wt%. These rocks are also relatively low in TiO2 similar to gabbros and ultramafic

cumulates. The Al2O3/TiO2 ratio ranges from 7.98-29.12. The trace element concentrations

vary with low Ni (28.83-202 ppm) and Co (48.7-165.9). The Cr and V concentrations are up

to 1443-360 ppm. The large Ion lithophile elements like Ba and Sr show considerable

chemical variation between 20.99 and 246.9 and 36.70-253 (except 2 samples 966.5-12903).

There is much variation in ratio ranges like Zr/Y (1.29-4.05), Ti/Zr (20.67-294.36), Nb/Th

(0.088-0.59), and Zr/Nb (3.32-37.73), two samples show very high ratio of Zr/Nb (114.06;

277.22). In binary plots of the amphibolites with reference to MgO (Fig. 5.3) these rocks

show positive correlations between SiO2, Al2O3, TiO2, P2O5 and Y but with FeO and CaO, Ni

and Cr no distinct variation is observed.

The major and trace element geochemistry of amphibolites of this complex were

plotted in various geochemical variation diagrams. The total alkali vs silica diagram, (Fig.

5.4a) many of the samples plot in the basaltic field and similarly, Zr/Ti vs Nb/Y plot all the

samples plot in the same field and two samples plot in andesitic filed (Fig. 5.4b). The K2O vs

SiO2 plot the entire rock units show tholeiitic trend with overlapping cal-alkaline field (Fig.

5.4c). Similarly, in the AFM diagram (Fig. 5.4d) many of these rocks follow a tholeiitic

fractionation trend. REE patterns of the amphibolites show flat pattern and relatively slight

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LREE enrichment resulting slight fractionation. The La/YbN ratio slightly elevated from of

0.439-7.04 and Ce/YbN ratio ranges between 0.68-8.56 with an average of 3.19, and without

any pronounced Eu anomalies (Fig. 5.5a). The La/SmN ranges from 0.36-2.53 and Gd/YbN

ranges from 0.43-3.98. On Spider plots normalized MORB (Fig. 5.5b) these amphibolites

show LILE (K, Rb, Ba, Th) enrichment and HFSE (Ti, Nb, Hf, Zr) with distinct Nb depletion

in some of the garnet rich amphibolites. Four samples show slightly lower Th content.

5.4.3. Felsic rocks

The geochemistry of plagiogranites/trondhjemites from the complex show uniformly

higher SiO2 (66.33-69.70 wt %), Al2O3 (15.09-19.08 wt %) and Na2O (4.9-7.12 wt %) and

very low K2O (0.47-4.28 wt%) with an average A/CNK ratios of 0.75 and 0.95 indicate

metaluminous to peraluminous nature. The FeOt/(FeOt+MgO) ratio ranges from 0.43-0.72

which a typical of tholeiitic rocks. The CIPW normative anorthite content of these rocks

ranges from An25 to An33. On The ACF diagram (Na2O-K2O-CaO, Fig. 5.6a) and normative

Ab-An-Or plot (Fig. 5.6b) these rocks plot on trondhjemite field except one sample. The

chondrite-normalized REE patterns of these rocks (Fig. 5.7a) show a sub-parallel distribution

with flat trends showing +ve Eu anomalies and show LREE enrichment. The rocks are low in

HREE, the La/YbN ratio varies from 3.98 to 14.18 and but two samples show higher ratio

(42-51.2). The Ce/YbN ratio ranges from 3.24-14.18 with higher ratio of two samples (26-35)

and Eu/E* is a relatively high range from 2.73-8.66. The Ocean ridge normalized plots show

enrichment of LIL (K2O, Rb, Ba) and depletion of Ce, Hf and Zr with -ve Nb anomalies (Fig.

5.13b).

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Fig.5.3: Binary diagrams of amphibolites: MgO (wt%) vs SiO2 (wt%); MgO (wt%) vs TiO2 (wt%); MgO (wt%) vs Al2O3 (wt%); MgO (wt%) vs FeO(t) (wt%); MgO (wt%) vs CaO (wt%); MgO (wt%) vs Ni (ppm); MgO (wt%) vs Cr (ppm); MgO (wt%) vs Zr (ppm). 5.5. Tectonic significance

The geochemical data from the amphibolites and trondhjemites/plagiogranites of the

Anniyapuram complex were plotted on various major, trace and REE tectonic discrimination

diagrams, which are typically used for distinguishing Mid Oceanic Ridge Basalt (MORB)

and Island Arc Tholeiite (IAT). In the TiO2-MnO-P2O5 ternary tectonic diagram (Fig. 5.8a)

except two samples all plot in Island arc tholeiite field, similarly in MgO-FeOt-Al2O3 ternary

diagram all samples show ocean island signatures. In TiO2 vs. Al2O3 tectonic discrimination

plot (Fig. 5.8b.) except three samples all sample show arc signature and in Ti/Cr vs Ni

diagram all samples plot in Isalnd arc field (Fig. 5.8c). In Zr/Y vs. Zr discrimination plot

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(Fig. 5.8d) except three samples all are plot in Island arc basalts and in V vs. Ti/1000 plot

(Fig. 5.9a) few samples plot in arc field and few samples plot in ocean floor basaltic field

(OFB). In Ti vs. Zr (Pearce, 1980) plot (Fig. 5.9b) and Cr vs. Y (Fig. 5.9c) many samples

show major IAT source (Mullen, 1983) and few overlap with MORB.

Fig.5.4: Geochemical Variation diagrams of amphibolites: (a) total alkali content (Na2O + K2O) vs. SiO2 classification plot (after Le Maitre et al., 1989). (b) Zr/Ti versus Nb/Y diagram after (Pearce, 1996). (c) Classification diagram of K2O vs SiO2 (after Peccerillo and Taylor 1976). (d) Fe2O3-Na2O+K2O-MgO geochemical variation diagram of amphibolites (after Irvine and Baragar, 1971).

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Fig. 5.5: (a) Chondrite normalized diagram for the amphibolites (after Sun and Mac Donough, 1989). (b) MORB normalized incompatible element diagram for amphibolites (after Sun and Mac Donough, 1989; Alabaster et al., 1982).

Similarly Ti/Cr vs. Ni Plot (Beccauluva et al., 1973) clearly indicates that the rocks were

derived in an Island arc environment (Fig. 5.9d). On spider plot with normalized MORB (Fig.

5.7b), these amphibolites show LILE (K, Rb, Ba, Th) enrichment and HFSE (Ti, Nb, Hf,)

depletion with negative Nb anomalies consistent with suprasubduction zone (SSZ) origin.

The Lu/Hf vs. La/Sm ratio (Regelous et al., 2003) of the mafic lavas (amphibolites) suggests

they might be derived from higher degrees of mantle melting and moderate pressures within

the spinel stability field (Fig. 5.10).

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Fig. 5.6: Geochemical variation and tectonic discrimination of plagiogranites: (a) K2O+Na2O+CaO ternary plot for classification of plagiogranites (after Barker and Arth, 1976). (b) Ternary diagram of showing CIPW normative content of Ab-An-Or plot for classification plagiogranites (after O’connor, 1965). (c) Y+Nb (ppm) vs Rb (ppm) of trondhjemites plotted in tectonic environment discrimination diagram for granitic rocks, ORG: ocean ridge granites; VAG: volcanic arc granites; WPG: within plate granites (after Pearce et al., 1984b). (d) Rb (ppm) vs Sr (ppm) of plagiogranites plotted in tectonic environment discrimination diagram for granitic rocks (after Coleman and Peterman, 1975).

Plagiogranites of this complex (NMC) were evaluated using various tectonic discrimination

diagrams such as Rb vs. Nb+Y (Fig. 5.6c) and Rb vs. Sr these tectonic plots suggest volcanic

arc granite (VAG) affinity similar to oceanic palgiogranites (Fig. 5.6d). Ocean ridge granite

normalized patterns (normalized values are from Pearce et al., 1984b) (Fig. 5.6b) of these

rocks display relatively low HFS element contents and highly variable LIL elements. The

HFS elements are particularly depleted relative to those of ORG and show negative Nb-Ta

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anomalies. These geochemical characteristics are typical of SSZ ophiolite sequence of arc

granitoids and these are similar to the plagiogranites of the MOC and DOC described in this

terrain.

Fig. 5.7: (a) Chondrite normalized diagram for the trondjhemites (after Sun and Mac Donough, 1989). (b) Ocean ridge granite normalized diagram for the trondhjemites (after Pearce et al., 1984b).

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Fig.5. 8: Tectonic discrimination diagrams of amphibolites: (a) TiO2-MnO-P2O5 ternary diagram (Mullen, 1983). (b) MgO–FeOt–Al2O3 ternary diagram (Pearce et al., 1977). (c) Al2O3 vs TiO2 (wt%) discrimination diagram (after Mullen, 1983). (d) Zr/Y vs. Zr tectonic diagram (after Pearce and Norry, 1979).

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Fig. 5.9: Tectonic discrimination diagrams of amphibolites: (a) Ti (ppm) versus V (ppm) discrimination diagram after (after Shervais, 1982). (b) Y (ppm) versus Cr (ppm) discrimination diagram (after Pearce and Norry, 1979). (c) Zr (ppm) versus Ti (ppm) discrimination diagram (after Pearce, 1980) IAT: island arc Tholeiite, MORB: mid ocean ridge basalt, WPB: within-plate-basalt; (d) Ni vs Ti/Cr diagram (after Beccaluva et al., 1979).

1

10

100

1000

10000

1 10 100 1000

Cr (ppm)

Y (ppm)

IAT BSA MORB

100

1000

10000

100000

10 100 1000

Ti (ppm)

Zr (ppm)

MORB

WPM

IAT

1

10

100

1000

1 10 100 1000

Ti/Cr

Ni

Island arc

Ocean floor tholeite

a b

c d

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Fig. 5.10: La/Sm Vs Lu/Hf ratio of amphibolites for partial melting curves of spinel and garnet peridotites (Regelous, 2003).

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 2 4 6 8 10

Lu/H

f

La/Sm

Spl-peridotite

Grt-peridotite

0.30.5

0.2

1

35

10

20

20

10

5 3

2 10.5

0.2