Petrogenesis of ultramafics and mafics of batampudi ... · 1282 T Brahmaiah, O Vijaya Kumari, K Sai Krishna, Dr. Ch Ravi, & Dr. K S Sai Prasad and pyroxenites from the Layered and

International Journal of Applied Environmental Sciences

ISSN 0973-6077 Volume 12, Number 7 (2017), pp. 1281-1300

© Research India Publications

http://www.ripublication.com

Petrogenesis of ultramafics and mafics of batampudi

complex, Khammam district, Telangana state, South India.

1T Brahmaiah, 1O Vijaya Kumari 2K Sai Krishna

3Dr. Ch Ravi, 1Dr. K S Sai Prasad*

1Department .of Geology, Sri Venkateswara University, Tirupathi-517501,

2Dept. of Geology, Kakatiya University, Jangoan, Warangal. 3University PG College, Kakatiya University, Jangoan, Warangal.

Abstract

Present paper deals with the petrogenesis of gabbros and pyroxenites of

Batampudi Anorthositic complex, Khammam district, Telangana, South India.

The geochemical relations of gabbro and pyroxenite suggest tholeiitic to

calcalkaline signature. They are further characterised by low Th abundance

and a distinct Nb-Ta and Zr-Hf through, which are characterstic of tholeiitic

basalts produced at destrictive plate margins or within plate tholeiites

contaminated by continental crust.

Keywords: Bethampudi, Petrogenesis, Ultramafics, Mafics (gabbros) and

Pyroxenite

INTRODUCTION:

The study of ultramafics and mafic rocks is important for various aspect. It could

reveals the magmatic processes related to fractional crystallization and crystal

accumulation, both in continental lithospheric or oceanic environments. Additional,

these rocks could contain significant contents of Au, Ni, and PGE. Moreover, the

Ultramafic and mafic rocks stand out due to their ultramelonocratic nature, high

identity, less than 45% Silica, and high magnesium (MgO>12%) and low aluminium

(Al2O3 <10%) contents. Common examples of Ultramafic and mafic are peridotites

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1282 T Brahmaiah, O Vijaya Kumari, K Sai Krishna, Dr. Ch Ravi, & Dr. K S Sai Prasad

and pyroxenites from the Layered and alpine complex and Komatiites and ultramafic

basalts from greenstone sequences. In all these lithotectonic associations the

composition of the mantle from where they derived is recorded. They are associated

with extensional tectonics processes that have occurred in the earth’s crust since the

Paleoarchean. For the reason the distribution of this type of magmatism is worldwide

and not restricted to a specific geotectonic environment.

The Bethampudi Anorthositie Complex (BAC) is in northern portion to Chimalpahad

Layered Complex (CLC), situated within the NSB (Appavadhunulu et al 1976;

Narasimha Reddy and Leelanandam, 2004; Brahmaiah T et al 2016). is the largest,

deformed and metamorphosed Archaean anorthosite complex in southern Peninsular

India (Leelanandam, 1987; Ashwal, 1993; Narsimha Reddy and Leelanandam, 2004;

Bose, 2007; Brahmaiah T et al 2016). The Bethampudi anorthosite Complex (BAC)

includes anorthosites, Gabbroic anorthosites and Anorthositic gabbros and

subordinate gabbros and pyroxenites and mafic and ultramafic rocks and

Amphibolites.

GEOLOGICAL SETTING

The Bathumpudi Anorthosite complex extending >100 sq. km included in the survey

of India Toposheet Nos. 65C/7 and 65C/10; and boundary by latitudes 17°30' and

17°35' N, and longitudes 80°25' and 80°35' E (Fig. 1). The BAC is syntectonically

emplaced as a “sill-like” intrusive body trending NE – SW direction within the

Khammam Schist Belt (KSB).

Fig. 1: Location& Geology map of Bethampudi Layered Complex

(Modified after M N Reddy et al 2006)

Petrogenesis of ultramafics and mafics of batampudi complex, Khammam.. 1283

The Bethampudi complex is intruded by younger intrusive dyke, and sills of mafic

nature and preserved as enclaves amphibolites. The rocks of BAC have been affected

by three phases of ductile-brittle deformation and metamorphism under upper

amphibolite to lower granulite facies (Narsimha Reddy and Leelanandam, 2004;

Brahmaiah T et al, 2016);

Gabbros are the distinctive mafic rocks of the study area, but they are parsley

distributed, typically in isolated patches that have limited aerial extent. Gabbros

occur as thin, sheet-like bodies and lenses, parallel to the anorthosites and as cross

cutting dykes in the surrounding by amphibolite and other country rocks as well as

across patches. The sheet like bodies trend NE – SW. parallel to the anorthosites and

enclosing country rocks while the dykes’ trend in many directions. The rocks are

generally garentiferous, medium-to coarse-grained and dark coloured and have

hypidiomorphic textures. Gabbros are characterized by color indices ranging from

approximately 65 to 85, with the mafic minerals being dominated by pyroxene,

amphibole, and garnet. Plagioclase feldspar is the dominant light colored mineral in

these rocks (Fig 2 A&B). Magnetite and ilmenite can also be readily identified. All

of these rocks are strongly deformed and metamorphosed and show consistent

geneissic banding. Colors range from dark grey to green-grey and grain sizes range

from very coarse to medium. The plagioclase feldspar crystals seem to preferentially

resist weathering leaving more plagioclase, whereas the hand samples tend to be

much darker, epically on freshly cut surfaces.

Ultramafic rocks (Pyroxenites) occur as scattered outcrops, as short lenses and

disconnected bands, and are surrounded by the supracrustals. The color index of the

pyroxenite generally >85% and displays green to dark green in color. These rocks are

generally coarse-grained, highly weathered and stained with yellowish tint, thus

making poor outcrops (Fig 2 C&D).

A B


Fig. 2: A) Highly weathered isolated pyroxenite mound having contact with Gabbro in the study area, B) Foliation in the pyroxinites, C) Coarse grained gabbros in the study area and D) Sharp contact between gabboro and anorthosite in the study area.

PETROGRAPHY:

Petrographic analysis was performed on the pyroxenite and gabbro lithiologies of the

study area. Sixteen thin sections were prepared from the rock samples. The

petrographic descriptions of these rocks reveal the mineral content micro-textural

relationships between the grains and deformational as well as alteration structures.

Pyroxenites contains clinopyroxene and orthopyroxene in more or less equal

proportion exhibiting cumulous texture (Fig.3A). Both clino- and ortho-pyroxenes

have exsolution lamellae (Fig.3B). Plagioclase is generally absent. Alteration of

clinopyroxene into amphibole is very common. Fractured clinopyroxene is filled with

iron oxide. The accessory phase being the magnetite and ilmenite. In thin section

gabbro is relatively plagioclase poor, with average plagioclase abundances of 10 to

20%. Orthopyroxene is typically coarse grained and is characterized by subhedral

crystals provided indication of magmatic origin (Fig.3C). The plagioclase is mainly

equigranular; medium grained and display polygonal texture with angular crystal

margins (Fig.3D). The modal composition of ultramafic and gabbros are given in

Table- 1.

Table-1: Model composition of Mafic and Ultramafic rocks

S.No Mafics (Gabbros) Ultramafics (Pyroxenites)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Plagioclase 56 62 55 56 61 68 62 63 1.9 --- 2.2 --- 1.6 --- --- 1.2

ClinoPyroxene 23 15 34 20 18 10 18 19 83 82 82 85 81 84 83 85

Orthopyroxene 16 13 9 14 9.2 9.1 10 11 5.2 7.6 8.3 6.6 5.4 7.3 8.4 4.8

Hornblende 4.6 7.9 1 4 1.6 2.2 3.2 3.2 1.2 3.2 1.4 2.4 4.7 3.8 2.2 3.2

Biotite 0.4 0.8 --- 5 0.6 2.4 2.1 1.8 --- 1.4 --- 1.4 --- 1.8 1.6 ---

C D


Olivine 0.5 2.1 --- 1.2 4.3 1.2 --- --- 2.1 --- 0.3 --- --- 1.2 --- ---

Garnet 1 0.8 1.2 --- --- --- --- --- --- --- ---

Magnetite/

ilmanite

0.3 0.5 1 2.5 4.2 5 2 --- 4.3 5.8 5.2 5.2 4.6 3.1 4.6 3.8

Apatite --- --- --- --- 0.3 0.6 --- 0.8 1.6 --- --- --- 1.1 --- --- 1.3

Fig. 3: A) Pyroxenite with cumulate olivine and chromite and intercumulus plagioclase, B) Holocrystalline pyroxenite, coarse-grained dominated by orthopyroxene, clinopyroxene and olivine, with an inequigranular texture. In some places intergranular areas are filled with a fine-grained assemblage of minerals replacing intercumulus phases, C) An olivine gabbro that contains cumulate olivine and plagioclase in a matrix of intercumulate pyroxene and magnetite (black) that crystallized from trapped intercumulate liquid and D) Plagioclase occurs as euhedral to subhedral stubby tabular laths and exhibits well developed polysynthetic twinning. Pyroxene occurs as both subhedral elongate prismatic crystals. and as smaller rounded or intergranular crystals. Both augite and orthopyroxene (hypersthene) contain well developed exsolution lamellae. Biotite is spatially associated with pyroxene occuring both as inclusions and along the margins of grains.

A B

C D


PETROCHEMISTRY:

Major, trace and rare earth element (REE) data of gabbro and ultramafic roks of

Bathampudi complex are given in Table-2&3. The geochemical signatures of both

gabbro and ultramafics show consistent and complementary variation in major trace

and REE. The SiO2 abundance covers a narrow compositional range, gabbros (46.3 to

47.1 wt%) and ultramafics (43.5 to 53.7 wt%), Al2O3 from 15.1 – 201.5; MgO from

5.05 – 16.3; CaO from 9.2 – 11.0; Total iron from 9.0 – 11.0 and TiO2 from 0.09 –

1.7, while the composition of Al2O3, MgO, CaO, Total Iron and TiO2 in ultramafics

varies from 2.11 – 2.75; 10.12 – 15.06; 11.2 – 20.68; 8.96 – 11.04; 0.13 – 0.28. The

relatively high Al2O3 and Na2O) can be attributed the presence of plagioclase in

gabbros.

The Cao and Al2O3 relationship of both gabbro and ultramafics show their trend

towards primordial mantle (Al2O3/CaO= >1). The CaO/ Al2O3 ratio ranging from

0.50 to 0.60 in gabbros and from 0.11 to 0.16 in ultramafics which conforms to the

primordial mantle, where CaO/ Al2O3 ratio is 0.79 (Halfmann, 1988).

On the alkali (Na2O + K2O) vs SiO2 diagram (Fig.4) all samples are broadly classified

as gabbro and gabbroic diorite. A plot for R1 – R2 (Fig.5) after De la Roche et al

(1980) indicated that these samples are of Gaggro and ultramafic rocks. According to

peccerillo and Taylor (1976) SiO2 – K2O plot these rock samples belong to tholeiitic

series (Fig.6). Same has been conformed when all samples plot in Y vs, Zr diagram

(Fig.7). The chemical analysis indicate that the magma type is tholeiitic but trending

towards a calc-alkaline nature, this view is also supported by AFM diagram (after

Irven and Bargar, 1975) (Fig.8). Jenson (1976) Al2O3 - Feo+TiO2 – Mgo diagram

conforms that gabbros cluster on high-Fe (HFT) tholeiitic basalts and ultramafic rocks

are of basaltic komatitic (BK) in nature(Fig.9). The plot again indicates that there is

an overall tholeiitic affinity of magma trending towards calc-alkaline nature.

Fig.3.Total – alkali Silica Diagram (Le Bas et al. 1986)


Fig. 5.R1-R2 plot (De la Roche et al. 1980)

Fig.6. SiO2 Vs K2O plot (Peccerillo and Taylor 1976)


Fig.7: Zr vs. Y plot of Ross and Bedard (2009).

Fig.8: AFM triangular diagram, showing the demarcation in the calc-alkaline divisions. A = Wt% Na2O + K2O, F = Wt% FeO+Fe2O3 and M = Wt% MgO.


Fig.9: Al2O3–Fe2O3 + TiO2–MgO (Jensen 1976) triangular plot

Condrite normalized REE plot (after Boynoton, 1984) of ultramafic rocks reflects

negative Eu anomaly with slight enrichment of LREE. The negagive Eu anomaly in

these rocks may be interpreted as due to the fractionation of plagioclase + hornblende

can be imposed when the melt phase enters the stability field of plagioclase. Gabbros

show a low LREE and slight enrichment of HREE pattern with low Al2O3, CaO, Sr

content and absence or positive Eu anomaly suggest the removal of plagioclase from

basic parent magma or may be due to the magma that might have segregated at such

depth where plagioclase is not stable and hance could not be fractionated (Barker et

al., 1976), (Fig.10)

Fig.10: REE Chondrite-normalized multi-element diagram (Boynton 1984)


Table-2 Major element concentration of Mafic and Ultramafic rocks

S.No Mafics (Gabbros) Ultramafics (Pyroxenites)

1/34 2/39 3/33 4/31 5/38 6/44 7/46 8/58 9/61 10/62

SiO2 47.01 46.52 46.28 46.60 47.10 52.06 53.72 43.48 48.60 49.47

TiO2 0.13 0.18 0.28 0.23 0.21 0.26 0.09 0.72 0.36 1.78

Al2O3 20.49 15.08 17.24 17.60 17.46 12.22 19.08 20.68 19.88 17.97

Fe2O3t 11.92 9.94 11.06 10.97 11.06 8.96 10.08 11.04 10.56 10.16

MnO 0.18 0.18 0.16 0.17 0.15 0.17 1.11 0.46 0.79 0.63

MgO 5.05 16.63 10.87 10.85 7.66 21.36 10.12 15.06 12.59 14.78

CaO 11.00 9.22 9.60 9.94 10.25 2.11 2.75 2.31 2.53 2.43

Na2O 2.72 0.98 1.53 1.74 3.04 0.59 0.70 0.70 0.70 0.67

K2O 0.18 0.05 0.14 0.12 0.43 0.09 0.11 0.03 0.07 0.08

P2O5 0.01 0.01 0.06 0.03 0.04 0.02 0.10 0.01 0.06 0.05

Total 98.82 98.70 97.66 98.39 98.97 97.71 97.95 94.05 96.00 96.43

CIPW

Q 1.88 0.00 1.86 0.60 0.00 2.72 12.17 0.00 3.01 2.94

Or 1.06 0.30 0.83 0.73 2.54 0.53 0.65 0.00 0.41 0.44

Ab 23.02 8.29 12.95 14.75 25.72 4.99 5.92 0.00 5.92 5.69

An 43.17 36.60 39.76 39.84 32.73 2.84 4.04 3.07 3.55 3.38

Lc 0.00 0.00 0.00 0.00 0.00 0.00 0.00 67.95 0.00 0.00

Ne 0.00 0.00 0.00 0.00 0.00 0.00 0.00 3.21 0.00 0.00

Kp 0.00 0.00 0.00 0.00 0.00 0.00 0.00 49.14 0.00 0.00

Di 8.67 7.07 4.36 6.80 9.54 44.89 57.70 0.00 69.69 66.68

Wo 0.00 0.00 0.00 0.00 0.00 0.00 6.84 0.00 2.31 0.00

Hy 8.56 31.59 25.06 23.88 9.19 32.50 0.00 0.00 0.00 6.66

Ol 0.00 4.73 0.00 0.00 3.84 0.00 0.00 26.59 0.00 0.00

Dcs 0.00 0.00 0.00 0.00 0.00 0.00 0.00 30.79 0.00 0.00

Il 0.39 0.17 0.34 0.37 0.32 0.25 0.34 0.53 0.44 0.39

Tn 0.14 0.00 1.33 0.40 3.96 0.00 0.00 0.00 0.00 0.00

Ap 0.02 0.02 0.14 0.06 0.10 0.05 0.24 0.02 0.13 0.11

Sum 86.91 88.77 86.61 87.43 87.92 88.76 87.90 83.03 85.46 86.29

Niggle

si 137.09 104.10 120.66 119.15 130.70 110.88 138.32 92.40 113.16 112.55

al 35.21 19.88 26.48 26.52 28.55 2.65 4.17 2.89 3.47 3.25

fm 22.40 55.82 42.60 41.73 32.04 68.13 41.27 48.54 45.25 51.36

c 34.37 22.10 26.81 27.23 30.47 27.88 52.63 47.08 49.59 43.80

alk 8.02 2.20 4.10 4.52 8.94 1.34 1.93 1.48 1.68 1.59

k 0.04 0.03 0.06 0.04 0.09 0.09 0.09 0.03 0.06 0.07

mg 0.98 0.99 0.99 0.99 0.99 1.00 0.94 0.98 0.97 0.98

c_fm 0.57 0.15 1.41 0.69 3.72 0.21 0.35 0.45 0.40 0.35

ti 0.01 0.01 0.07 0.03 0.05 0.02 0.11 0.01 0.05 0.05

p 1.53 0.40 0.63 0.65 0.95 0.41 1.28 0.97 1.10 0.85

qz 5.00 -4.69 4.26 1.07 -5.05 5.52 30.61 13.53 6.43 6.19


Table-3: Trace and Rare Earth Element concentration of Mafic and Ultramafic rocks.

S.No Gabbros Ultramafics

1/34 2/39 3/33 4/31 5/38 6/44 7/46 8/58 9/61 10/62

Trace (ppm)

Ni 300.40 606.20 113.00 339.87 23.00 1479.60 263.80 562.40 413.10 679.73

Cr 471.00 1534.60 41.00 682.20 41.00 2921.10 472.50 643.70 558.10 1148.85

Co 93.20 84.90 56.00 78.03 35.00 91.10 17.70 48.20 32.95 47.49

V 357.20 188.90 70.00 205.37 70.00 128.40 70.90 48.90 59.90 77.03

Sc 33.20 27.60 16.00 25.60 31.00 25.90 12.20 5.30 8.75 13.04

Rb 0.30 0.80 1.00 0.70 1.00 0.30 3.80 1.60 2.70 2.10

Sr 235.90 57.80 649.00 314.23 692.00 24.90 22.20 32.70 27.45 26.81

Ba 134.30 32.20 154.00 106.83 285.00 14.40 64.20 4.90 34.55 29.51

Y 4.92 1.61 5.50 4.01 13.50 12.16 41.31 13.76 27.54 23.69

Zr 3.60 1.45 25.00 10.02 41.00 7.76 34.11 32.40 33.26 26.88

Nb 0.06 0.00 2.00 0.69 7.00 1.92 4.13 0.46 2.30 2.20

Hf 0.07 0.05 0.40 0.17 0.70 0.26 0.98 2.20 1.59 1.26

Ta 0.47 0.87 0.16 0.50 0.34 0.07 1.07 0.35 0.71 0.55

Th 0.00 0.00 0.09 0.03 0.06 0.00 3.64 1.21 2.43 1.82

U 0.00 0.00 0.03 0.01 0.01 0.00 1.30 0.60 0.95 0.71

REE(ppm)

La 1.65 0.63 2.53 1.60 5.37 3.35 16.53 15.26 15.90 12.76

Ce 2.90 0.78 1.60 1.76 1.96 13.64 39.43 53.51 46.47 38.26

Nd 1.61 0.36 4.58 2.18 10.62 8.18 16.03 16.72 16.38 14.33

Sm 0.42 0.07 1.05 0.51 2.75 2.10 4.10 4.48 4.29 3.74

Eu 0.36 0.04 0.55 0.32 1..97 0.72 0.41 0.48 0.45 0.51

Gd 0.56 0.11 0.69 0.45 2.70 2.01 4.27 2.53 3.40 3.05

Tb 0.12 0.03 0.19 0.11 0.23 0.28 0.77 0.28 0.53 0.46

Yb 0.46 0.21 0.67 0.45 1.19 0.82 2.61 0.94 1.78 1.54

Lu 0.08 0.04 0.10 0.07 0.18 0.13 0.38 0.13 0.26 0.22

TECTONIC SETTING:

The gochemistry of mafic and ultramafic rocks is most commonly used to

discriminate tectonic setting. The idea of trying to fingerprint magmas from different

tectonic setting chemically is best attributed to Pearce and Cann (1971 & 1973). This

study show that it is possible to use geochemistry to distinguish between basalts

produced in different tectonic settings. The basalts are formed in almost every

tectonic environment and they are believed to be geochemically sensitive to the

changes in plate tectonic frame work. The Mg# ranges from 30 to 60 for gabbros and

from 50 to 70 for ultramafics indicating crystallisation from both primitive and

evolved magmas. Inorder to undetstand the tectonic environment of the studied

samples plotted in Hf – Rb/30 – 3Ta (Harris et al., 1986) diagram (Fig.11) and in


different plots of Schandl and Gorton (2002) depicts that the these rocks fall in the

within plate volcanic zones (Fig.12).

Fig. 11: Triangle diagram of Hf – Rb/30 – 3Ta (Harris et al., 1986)

Fig.12: Tectonic classification Diagram of Volcanic rocks after (Schandl and Gorton

2002)


The NMORB and EMORB normalised trace element plots for ultramafic and gabbro

rocks are presented (Fig.13&14). Relative to primitive mantle the both types of rocks

are enriched in LILE and LREE. They further characterised by a low Th, abundance

and a distinct Nb – Ta; Zr – Hf through. These features are characterstic of tholeiitic

basalt produced at distructive plate margins or within plate tholeiitics contaminated by

continental crust (Hawkesworth et al., 1994; Ambarasu et al., 2011; Satyanarayana et

al., 2011; and Mohammed Dar et al., 2014). The depletion of Nb is not affected by

fractional crystallisation and it is known that Nb anomaly in modern arc volcanic is

independ of the degree of crystallisation (Mohammed Dar et al., 2014).

Fig.13: N-MORB-normalized multi-element diagram (Sun and McDonough 1989)

Fig.14: E-MORB-normalized multi-element diagram (Sun and McDonough 1989)

PETROGENSIS:

Chemical data can provide useful information on the course of fractional

crystallisation/magmatic evolution. Harker variation diagrams prepared for study area

rocks (Fig.15). In the diagram Al2O3, MgO, CaO, and Na2O/K2O ratio shows a

positive correlation with SiO2, where as FeOt, P2O5, TiO2 show a negative correlation,

which is characteristics of an igneous rock. Plots of MgO versus major oxides

(Fig.16) show strong negative correlation with FeOt, Al2O3, Na2O, K2O and strong

positive correlation with SiO2, CaO and K2O/Na2O ratio. A positive correlation

between CaO and MgO supports the fractionation of clinopyroxenes. During the


plagioclase removal, CaO/Al2O3 ratio increases whereas it remains constant during

olivine fractionation (Dungan and Rnodes, 1978).

Fig.15: Harkers’ Variation diagram of SiO2 vs. Major elements for Gabbros and Ultramafics of the study area.


Fig.16: Binary Plots of MgO versus major oxides for Gabbros and Ultramafics of the

study area.

High values of CaO/ Al2O3 (> 0.5) in the studied gabbros may be due to plagioclase

fractionation. Fractional crystallisation associated with crustal contamination is an

important process during magmatic evolution (De-Paolo, 1981). Low concentration

and narrow range of K2O and Na2O in the study area samples suggests in favour of

their minimal crustal contamination.

The Nb/La values for gabbros and ultramafic rocks (0.51 and 0.23 respectively) are

not only very low compared to those of primitive mantle (1.02) of Taylor and

McLennan (1985) and 1.04 of Sun and McDonouth (1989), but also lower than the

average bulk crust (0.69). Such lower values are not expected to be produced by

processes of contamination by an average crustal component. Thus it can be inferred

that the enrichment of LILEs and depletion of HFSEs may also occur in within plate

tectonic setting rocks due to crustal contamination.

The ascent of the ultramafic magma through the continental lithosphere to the surface

makes crustal contamination inevitable, either through assimilation-fractional

crystallisation or thermal erosion of floor rocks, due to their higher liquidus

temperature (Halama et al. 2004; Tang et al. 2012; and Chandran kumar and Ugarkar,


2017). Sylvester et al (1977) was the first person to say the use of Nb/Th ratios in

Archaean basalts as a monitor of the extent and timing of extraction of continental

crust from the mantle. The primitive mantle has Nb/Th ratio of 8, and Phanerozoic

upper continental crust a value of 1.1 (Sun and McDonogh, 1989; Rudnink and Gao,

2003; and Condie, 2005). Nb/Th value of the studied ulatramafic rocks varies

between 0.38 to 1.21 with an average of 0.73, which is similar to that of Archaean

continental crust has a value of 0.76 (Condie, 2005). The ultramafic rocks of the study

area exhibit Nb/Th value less than 1, and are characterised by negative Zr anomalies

coupled with Nb-Ta negative anamalies and enriched LREE suggests probable crustal

contamination enroute to the surface.

Contrarily, the Nb/Th values more than 8 for gabbros suggest crustal contamination

incurred by these litho units. Contamination of these litho units by the older salic

crustal basement en route to the surface is a possibility. Alternatively, contamination

of the magma source of the studied basement and gabbros due to the interaction of

mantle plumes with metasomatized continental mantle lithosphere also cannot be

ruled out. Said et al (2012) have stated that the contamination by subduction

metasomatized mantle lithosphere dominates due to interaction of mantle plumes with

the base of the lithosphere than crustal contamination. All the samples of ultramafic

and Gabbros exhibit negative Nb-Ta anomalies implying assimilation of subduction-

processed lithospheric mantle material by plume derived magma (Song et al. 2008).

Fan and Kerrich (1977) used Zr and Hf anomalies have been used to constrain the

nature of sources and melt residues. Study area gabbors and ultramafics exhibit slight

to moderave Zr and Hf anomalies in NMORB normalized spider diagram (Fig…..),

implying their derivation from a deep mantle source (~ 350 -250 km), where garnet

fractionated or was retained in the residue (Fan and Kerrilch, 1977). Strong negative

Nb anomalies on the spider diagram have been accounted for magma generation at

shallower mantle in arc environments or even crustal contamination processes (Polat

and kerrich, 2000).

CONCLUSIONS:

Present paper deals with the petrogenesis of gabbros and pyroxenites of Batampudi

Anorthositic complex, Khammam district, Telangana, South India. Pyroxinites are

composed of coarse grained clinopyroxenes, orthopyroxenes and hornblende, olivine

and magnetite as accessary minerals. Gabbros are mainly composed of plagioclase

and pyroxenes with minor amounts of amphibole and apatite and magnetite as

accessory minerals. Geochemically these gabbros and pyroxinites are broadly

classified as gabbros and ultramafics respectively and their magma type is tholeiitic

but trending towards a calc-alkaline nature. Condrite normalized REE plot of

ultramafic rocks reflects negative Eu anomaly with slight enrichment of LREE. The

negagive Eu anomaly in these rocks may be interpreted as due to the fractionation of

plagioclase. Gabbros show a low LREE and slight enrichment of HREE pattern with


low Al2O3, CaO, Sr content and absence or positive Eu anomaly suggest the removal

of plagioclase from basic parent magma. Tectonic setting discrimination diagrams, in

addition to their geochemical characters such as low Th abundance and very low

Nb/La ratios, enrichment of LILE and depletion of HFSE may also indicates their

occur in within plate tectonic setting rocks due to crustal contamination. Both

pyroxenite and gabbros exhibit negative Nb-Ta, Zr-Hf anomalies, implying

assimilation of subduction-processed lithospheric mantle material by plume derived

magma.

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