Petrography of Bethampudi Anorthosites Layered Complex from the Khammam Schist Belt , Telangana , India

The Bethampudi layered anorthosite complex at the border zone of Archaean supracrustal rocks of Khammam district, Eastern Ghats shows normal stratification predominantly in the form of rhythmic layering and often exhibits of zebra layering. Graded bedding and cumulate structures are also noticed. The rocks of the study area are classified based on petrography into anorthositic rocks, gabbroic rocks and ultramafic rocks and amphibolites. The field relations and major element composition suggest that these anorthosite rocks are of calc-alkaline in nature and petrogenitically related to the gabbroic rocks by the fractional crystallization at <750 ̊C.


Introduction
Anorthosites of Bethampudi area are coarse grained, plagioclase-high rich (>70 vol.% of rock-modal composition) cumulates of plutonic igneous rocks that have variation of basaltic magmas of mantle origin [1] [2].The occurrences of anorthosites are volumetrically minor and spread the entire geological column, but more constrained to Precambrian terrains.Majority of anorthosites were mainly confined to high grade granulite-gneiss belts and low grade granite-greenstone belts of the Precambrian terrains [3] [4] [5] [6] [7].Minor anorthosite components in the younger mafic layered intrusions and in basaltic volcanic provinces of the Phanerozoic to recent periods are uncommon.
Archaean anorthosites comprise layered components of few or all the following: anorthosite, leuco-gabbro, gabbro, mela-gabbro and ultramafic rocks (including chromitites).They contain textures and structures similar to those of younger layered basic intrusions, but are generally distinguished by the abundance of anorthositic rocks over mafic (gabbros) and ultramafic rocks, the equidimensional high calcic plagioclase with a high anorthite content at An 80 -85 and affected by different degrees of metamorphism and deformation [10] [11].They also differ in overall structure and composition from Proterozoic massif-type anorthosites which commonly form large steep-sided plutons and are characterized by tabular crystals of intermediate plagioclase (An 40 -60 ) composition [7] [10] [12] [13].
The present paper discusses on petrography and geochemical interpretations of major elements of rocks from Bethampudi Anorthosite complex (BAC) of Khammam district, Telangana State.Based on geochemical studies, a genetic link between the rocks from BAC and associated mafic granulites and has been proposed.A possible parental magma was suggested along with geodynamic setting of the BAC and its associated mafic and felsic volcanic rocks in Khammam schist belt.

Geological Setting
The Bethampudi 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 (Figure 1).The BAC is syntectonically emplaced as a "sill-like" intrusive body trending NE-SW direction within the Khammam Schist Belt (KSB).Smaller outcrops of the BAC that are spatially separated from the main body by prominent shear zones, occur as tectonically dismembered and deformed sheets and lenses in the southern, northern and western parts of the complex.Further, the complex is intruded by younger intrusive dyke, quartz veins and pegmatites.The rocks of BAC have been affected by three phases of ductile-brittle deformation and metamorphism under upper amphibolite to lower granulite facies [11]; hence, clear-cut stratigraphic sequence of the magmatic disposition in the BAC could not be established; and the magmatic structures exhibits variably witnessed complex-folding and fracturing which was overprinted on the primary igneous layering during transpressive tectonism [11].
Anorthositic rocks constitute the bulk (>95%) of the BAC, and can be petrographically divided into two varieties: 1) garnet-bearing and 2) garnet-free anorthositic rocks.The garnet-bearing anorthositic rocks are comparatively less abundant than the garnet-free varieties; the former varieties generally confined to shear zones in the BAC.Garnet-Figure 1. Location & geology map of Bethampudi layered complex.free varieties are widespread over the hill ranges of the area.Layering with cumulus texture is the prominent primary feature of the anorthositic rocks and is extremely well preserved in these rocks at different localities of the marginal zone of the BAC.Gabbroic rocks (35% -65% of mafic minerals) are the distinct mafic rock units of the BAC.
They are sparsely distributed and have limited areal extent in the complex.They occur as younger concordant bands and lenses in the anorthositic rocks.Ultramafic rocks (>65% of mafic minerals) are seldom occur in the BAC.They occur as lenses and minor bands in the complex.

Amphibolites
Are the significant mafic rocks and associated with the rocks of the BAC and the tonalitic gneisses of the Khammam schist belt, Telangana State.The amphibolites are of texturally two types: 1) Banded and schistose type occurring as intercalated bands with tonalitic gneisses exposed in the plane country, and 2) massive type which occurs as sills and dikes of variable dimensions (few meters to hundreds of meters in length), lenses (few meters to tens of meters in diameter) and tectonic slices, rafts and flows (few meters to hundreds of meters in length) in and around BAC and in plane country of the KSB.They locally exhibit grain size variation from fine grained massive type to coarse grained schistose type.

Mafic Granulites
Look similar gabbroic rocks in the field, but they vary from the latter by their typical characteristic of mineral assemblages; granoblastic texture, and nonappearance of cumulate characteristics.They arise as lenses, dykes and small mounds in the KSB and are generally located at the marginal portion of the BAC, Similar to those in high pressure retrograde granulites of Ungava orogen, Canada [34].Therefore, the mafic granulites around the BAC may be considered as high pressure retrogressive granulite facies metamorphic products of fine grained gabbros [11].
The BAC includes anorthosites, gabbroic anorthosites and anorthositic gabbros, and subordinate gabbros and pyroxenites.Spectacular phase layering of different styles and scales is observed, particularly in low-strain zones.The magmatic structures of AC include rhythmic grading, cyclic (cryptic) layers, zebra banding, cross bedding, trough structures, and cumulate textures [11]

Mineralogy
The anorthositic rocks from the BAC are mainly mixtures of two minerals: plagioclase and hornblende.However, both ortho-and clino-pyroxenes become more dominant in mafic and ultramafic rocks of the BAC (Table 1).Garnet, zoisite, clino-zoisite, epidote and scapolite form minor minerals, while quartz and calcite form accessory minerals.
Table 1 indicates the volume percent of plagioclase which gradually decreases with increasing volume percent of hornblende from pure anorthosite to leuco gabbroic mem- Hornblende is the main mafic mineral of the rocks in the BAC.It is fine grained in anorthosites, but its size increases with increasing modal content in the anorthositic rocks.Hornblende from gabbroic and ultramafic rocks is different from that of anorthositic rocks by its distinct colour and texture.Hornblende occurs as: 1) independent discrete grains; 2) coronas over pyroxene/garnet and 3) small anhedral inclusions and needles in coarse cumulus grains of plagioclases (Figure 3(c)) and is similar to that of the Fiskenaesset complex [14].Inclusions of the hornblende are not uncommon in ortho-and clino-pyroxenes and garnet porphyroblasts.Discrete hornblende is dark green in colour, displaying strong pleochroism, with pleochroic scheme of pale yellowish green, green and bluish green.Coronal and inclusion hornblendes are light green in colour with feeble pleochroism.All the hornblendes in the rocks of the BAC may be considered as metamorphic products of pyroxenes and precursor hornblende [27] [37].
Pyroxenes are dominant mafic minerals in gabbroic and ultramafic rocks.Orthopyroxene is totally absent and clinopyroxene is a minor mineral (it occurs as small anhe- Epidote (zoisite, clino-zoisite and epidote) is occasionally found in minor quantities.
They occur as tabular and euhedral crystals, and as small rounded grains.They are totally absent in gabbroic and ultramafic rocks.Scapolite is a common accessory mineral found all the rocks of the BAC.It occurs as altered product of plagioclase.Calcite is common in the anorthositic rocks and occurs as veins with radiating and tabular crystals.
Garnet is found in variable proportions in all the rocks of the BAC.It occurs in two forms: 1) coronal and 2) porphyroblastic types.The coronal garnet has width from 0.1 to 0.5 mm, while the porphyroblastic type with sieved micro-structures is very coarse (>2 mm).Garnet frequently exhibits euhedral crystals of chains/necklaces porphyroblasts, poiklioblasts and anhedral sieved grains (Figure 3(e)).
Amphibolites are mineralogically sub-divided into 1) clino-pyroxene bearing amphibolites; 2) sphene-bearing amphibolites; 3) garnet-bearing amphibolites mainly composed of hornblende and plagioclase as major minerals (Table 1).Garnet, epidote and scapolite occur as minor minerals.Clinopyroxene is considerably higher proportion in some amphibolites, but the orthopyroxene is completely absent in all amphibolites.Biotite, sphene, quartz and Fe-Ti oxides form the accessories in these rocks.
Mafic granulites have distinct mineralogy and textures, when compared to rocks from the BAC.Pyroxenes are dominant over amphiboles in these mafic granulites.
Clinopyroxenes are greater in proportions than orthopyroxenes and this feature is different from that of gabbroic rocks of the BAC.Absence of epidote group minerals and presence of biotite and sphene from mafic granulites are distinct mineralogic variations from those of the rocks from the BAC.Scapolite, and quartz form as accessories in the mafic granulites (Table 1).

Textures
The rocks in BAC are, in general, coarse grained and granular in form.The anorthositic Cumulus textures (Figure 3(f)) are common in anorthositic, gabbroic and ultramafic rocks.In anorthositic and gabbroic rocks plagioclase is a primary cumulus phase and mafic silicates (hornblende and pyroxenes) are inter cumulus phases.However, these mafic silicates form the as cumulus phases in the ultramafic rocks.
The rocks of the complex exhibit, by and large, metamorphic equilibrium textures.

(b)).
When the anorthosite rock samples are plotted in Harker diagram (Figure 7), they show gentle variation trends, where oxides of major elements (Al 2 O 3 , CaO, MgO, TiO 2 and P 2 O 5 ) are negatively correlated with SiO 2 , only Na 2 O and FeO t despite some scatter exhibits positively correlated trend.Major element compositions are further illustrated in a series of variation diagrams (Figure 7).Clusters as well as spreads of points of major element data of anorthositic rocks in binary variation diagrams suggesting that these rocks are formed by the accumulation of different proportions of cumulus high calcic plagioclase and intercumulus minerals as hornblende (± clinopyroxene and ± Fe-Ti oxides).The use of MgO as the index of variation is based on the assumption that the rocks were assembled as mixtures of plagioclase and mafic constituents.On the MgO-FeO t plot (Figure 8) the anorthosite samples are plotted at origin and gabbroic anorthosites show a positive intercept, which suggests that the primary plagioclase in these rocks was Fe-bearing, and/or, that a small amount of an essentially Mg-free, and Fe-bearing phase (magnetite) was present in the protolith [42].On the CaO and Na

Discussion
The formations of the study area show magmatic cross-bedding, graded bedding and cumulate structures.These field evidences indicate an undoubted igneous origin of the complex.This view is further corroborated by the twinning of plagioclase on Carlsbad and albite laws [43].Layering is predominantly rhythmic type with alternating thin plagioclase-rich and hornblende rich layers.Layering involving different members of the complex i.e. anorthosite, gabbroic anorthosites, anorthositic gabbro, gabbros and ultramafic rock, is observed only at a few places of limited extent.The other layered complexes having cross bedding and graded bedding are skaergard intrusions and the Messina layered intrusion of Limpoo belt [23].Opined that convection current was responsible for magmatic cross-bedding in skaergard.[35] considered the cross-stratification in chamber.[44] has discussed the role of the magmatic density currents in the cumulus processes in layered rocks.
Mose [45] stated that the magma will convict because it cools more rapidly at the top.
It is therefore possible that crystals would form first at the top and those heavier than the liquid start settling downwards.The simplest convection pattern would be downward motion along the walls and inward convergence along the floor and outward dispersion along the roof.The convection, continuous or periodic, would influence the crystal deposition.Variation in the velocity of convection current will modify the rate between minerals of different settling velocities producing variations in the proportion of heavy and light minerals settling in the magma chamber.This would be a better working model to explain the rhythmic layering in the study area.
Batampudi complex has many characteristic features of the metamorphosed complexes.The plagioclase is strikingly rich in calcium with the anorthite molecular percentage ranging from An 70 to An 91 as in the case of most of the Archean anorthosites.
Both orthopyroxene and clinopyroxene in different degrees of alteration are noticed.
There are two types of hornblende viz; the pale green and olive green hornblende free from relict pyroxenes and blue green type with relict pyroxene and in some cases forming rims or coronas around pyroxenes could be of primary igneous origin.The whole rock chemistry reveals that the BAC is rich in CaO, Al 2 O 3 and poor in alkalies, in conformity with the Archaean layered anorthosites.The AFM diagram plots show a pure calc-alkalin and amphiblites show tholiietic differentiation trend.This is also in conformity with the views of [14] that the differentiation trends in metamorphosed layered complexes are believed to be similar and display a calc-alkaline trend.

Conclusion
The Bethampudi complex is essentially a leucogabbro (gabbroic anorthosite and anorthositic gabbro) with minor volumes of anorthoistes, gabbro, amphibolites, pegmatites and quartz veins occur as concordant or discordant bodies in the country rock.The layered leucogabbros show magmatic cross-bedding, graded bedding and cumulate structures.These field evidences indicate an undoubted igneous origin of the complex.
The geochemical data suggest that the anothosites are of calc-alkaline and amphibolites have tholeiitic affinity.The presence of well-developed rhythmic layering indicates that fractional crystallization is generated by gravity settling of successively crystallizing cumulus minerals from a primitive basaltic magma, clinopyroxenite, gabbro, anorthositic gabbro and anorthosite, in this order to build up the study area complex, BAC.The parent magma, in general is basaltic with high alumina and low silica and alkalies.The plots on SiO 2 and MgO versus alumina and alkalis indicate the tholeiitic balsalt composition.
[35].The cyclic layering is made up of different proportions of felsic (plagioclase) and mafic (hornblende + clinopyroxene) phases (Figure2(a) and Figure 2(b)), the felsic and mafic layers have either gradational or sharp contacts.In general, the layering is very similar to that in the Fiskenaesset Complex, West Greenland [10] [14] [29] [36].The BAC is a polydeformed igneous body with three events of deformation.Ductile-brittle deformation (D1, D2 and D3) is recorded in the rocks of the Bethampudi layered complex during and after emplacement.The first deformation D1 is represented by strong vertical/sub-vertical planer foliation (S1) (Figure 2(c)), and vertical (or horizontal) stretching minerals lineation (L1) in the rocks of the Bethampudi
bers (gabbroic anorthosite to anorthositic gabro) of anorthositic rocks[1] [5] [6][7].It indicates an inverse relationship between volume percent of plagioclase verses hornblende from these anorthositic rocks.Plagioclase inclusions in hornblende and viceversa from anorthositic rocks indicate that they are in mutual coexisting primary phases.Plagioclase, in general is coarse grained (>1 mm) and subhedral to anhedral and grain size decreases (greater than 1 mm to less than 1 mm) with decreasing modal content (98 to 40 volume percent) from anorthositic-to gabbroic-rocks.The plagioclase occurs in different textural forms in the rocks of BAC due to subsolidus reaction process and subsequent deformation and metamorphism, as: 1) big strained cumulus grains entrapped in post cumulus matrix of hornblende/pyroxene/garnet; 2) small grains of strain-free recrystallized polygons and 3) very small anhedral inclusions in hornblende and in porphyroblastic (sieved) garnet.Plagioclase commonly exhibits twinning on the albite and pericline laws but untwined plagioclase is not rare in the rocks of the BAC.Bent twin lamellae and saw-tooth lamellae with feeble to strong zoning are frequently noticed in plagioclases from the deformed Anorthositic and gabbroic rocks (Figure3(a) & Figure 3(b)).It is occasionally replaced by scapolite, and zoisite/cli-no-zoisite; and is the sole reactant felsic phase for the growth of garnet porphyroblasts and garent coronas.

Figure 3 .
Figure 3. (a): Bent twin lamellae and saw-tooth lamellae in plagioclase from the deformed anorthositic gabbro; (b): Stained cumulus grains of plagioclase entrapped in post cumulus matrix of hornblende and pyroxene; (c): Small anhedral grains of hornblende in cumulus grains of plagioclase; (d): Sub-hderal clinopyroxene grains mantiled by hornblende and garnet in anorthosites; (e): Porphyroblastic garnet; (f): Pleagioclase is primary cumulus and hornblende and pyroxene are intercumulus in anorthosites; (g): Garnet corona at the interface between plagioclase, orthopyroxene and hornblende and (h): Development of epidote minerals on plagioclase.
rocks occasionally exhibit plaigoclase-rich felsic layers and hornblende (garnet/clinopyroxene)-rich mafic layers, and they also display parallel alignment of lenticular hornblende and randomly distributed garnet porphyroblasts.The rocks of the BAC exhibit a variety of textural features ranging from relict magmatic to metamorphic (and deformation) fabric.The textures of the BAC are broadly classified into: 1) relict igneous cumulus textures; 2) metamorphic recrystallized equili-brium (and rarely disequilibrium) textures; 3) replacement textures and 4) deformation fabrics.
Metamorphic disequilibrium textures are locally developed due to diffusion-controlled and interface-controlled metamorphic reactions.Thin zone of white plagioclase moat around garnet porphyroblasts and rapid growth of sieved garnets with "inclusions" of plagioclase, hornblende, pyroxene and Fe-Ti oxide minerals represent diffusion-controlled metamorphic reaction textures.While garnet coronas at the interface between plagioclase and clinopyroxene/orthopyroxene/Fe-Ti oxide minerals, and composite coronas of garnet, hornblende and quartz between plagioclase and clinopyroxene (Figure3(g)) represent interface-controlled metamorphic reaction textures[39].Retrogressive replacement textures are were observed in the rocks and appears to be an account of as above: 1) simple structural transformation due to decline of temperature; and 2) chemical alteration due to fluid action.A rare feature of epitaxial overgrowth of hornblende over clinopyroxene is an example of structural transformation.Frequent occurrence of hornblende rims over clinopyroxene/orthopyroxene/garnet/Fe-Ti oxide minerals and the development of scapolite and epidote minerals on plagioclase are the examples of retrogressive alteration by fluid action (Figure 3(h)).Many of the textural features observed in the BAC mimic those exhibited by the Fiskenaesset complex [14] [23] [33] [40], Sittampundi complex [30] [31].However, the typical garnet symplectitic textures are totally absent in BAC, unlike those present in the Sittampundi anorthosite complex [30] [41].Mafic granulites typically show granoblastic texture with sieved as well as coronal garnets, flakes of biotite, rounded grains of sphene.Some of the mafic granulites exhibit exsolution lamellae of ilmenite in clinopyroxene, and these lamellae occur only in the core portion of clinopyroxene.Relict exsolution lamellae of thin clinopyroxene in orthopyroxene, orthopyroxene in clinopyroxene, and relict sub-ophitic textures are occasionally preserved in some of the mafic granulites.The mineralogic assemblage and textural features of the mafic granulites are distinctly different from those of the gabbroic rocks of the BAC.Amphibolites commonly exhibit both non-foliated and foliated metamorphic textures.They rarely show microfolding mylonitic and garnet coronal microtextures.Relict magmatic textures like ophitic are rarely observed in some of the amphibolites.
and fresh samples, showing the widest possible variation in modal mineralogy, were analyzed for major elements.Cleaned chips were powdered to ~200 mesh using a steel jaw crusher and a ring mill.The rock powders were analyzed by X-ray fluorescence spectrometry (XRF) using Philips MAGIX PRO-Model 244O for the major elements.Representative whole rock analyses are presented in Table2(a) & Table 2(b).

6
Anorthosite rocks, 6 Gabbroic anorthosites, 6 Anorthosites, 2 Gabbroic rocks, 3 ultramafic rocks and 5 Amphibolites Reprehensive samples are analyzed for geochemistry purpose of Bethampudi Anorthosite Complex are presented in Table2(a).It appears that the bulk chemistry of the anorthosite rocks from the BAC is controlled by the nature, composition and model proportion of cumulus high calcic plagioclase and of inter-cumulus mafic phases as hornblende, clinopyroxene and Fe-Ti oxides.The high plagioclase modes (>65%), very high total normative feldspar (72% -92%) and very high normative plagioclase compositions (An 70-An 95, see Table2 (a)) clearly suggest a very high Cao (11.54 -16.02 wt%) and Al2O3 (24.12 -32.82 wt%) in their bulk compositions.Other major elements in anorthosite rocks, are small in amount and show large variations in MgO (0.11 -5.21 wt%) and in Fe 2 O 3 (0.25 -6.74 wt%).Alkalies (Na 2 O + K 2 O) and SiO 2 have restricted range from 1.91% -4.7% and from 44.23 -50.23 wt% respectively.These geochemical variations in anorthositic rocks (Table 2(a)) are in a way controlled by the nature, composition and model proportion of intercumulus mafic constituents, particularly hornblende.TiO 2 and P 2 O 5 have far less in amount <0.6

Figure 6 .
Figure 6.AS plot of silica saturation (represented as % normative olivine or quartz) versus total amount of normative feldspar (Dymek and Owens, 2001).
2 O plots these rocks are formed as two groups as anorthosites and both gabbroic anorthosites/anorthositic gabbros.Low MgO with variable CaO and Na 2 O, in anorthosites suggests variable plagioclase composition.In gabbroic anorthosite and anorthositic gabbro rocks an increase in MgO is accompanied by a general decrease in CaO and Na 2 O, where as the data for K 2 O define a crude horizontal array.Such patterns indicate that

Figure 8 .
Figure 8. Weight % oxide vs. MgO variation trends for anorthosite rocks of the study area.

Figure 9 .
Figure 9. AFM triangular diagram, showing the demarcation between the tholeiitic and calc-alkaline divisions.

Table 2 .
(a) Major element chemistry (wt.%) and norms of the anorthositic rocks from the bethampudi layered complex.(b) Major element chemistry (wt.%) and norms of the gabbroic, ultramafic and amphibolites associated with Bethampudi layered complex.