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
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International Journal of Applied Environmental Sciences
ISSN 0973-6077 Volume 12, Number 7 (2017), pp. 1281-1300
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
1284 T Brahmaiah, O Vijaya Kumari, K Sai Krishna, Dr. Ch Ravi, & Dr. K S Sai Prasad
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
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
1286 T Brahmaiah, O Vijaya Kumari, K Sai Krishna, Dr. Ch Ravi, & Dr. K S Sai Prasad
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