CHAPTER THEE

PETROGRAPHY

Chapter III

Petrography

Basalts

Megascopic Characters: The basalt is melanocratic dark grey in colour. It is fine

grained with rare phenocrysts of plagioclase. Samples from the upper portion of the lava

flow show rounded to irregular vesicles. The vesicles are at places filled by secondary

silica, calcite and zeolites. Reddish brown to copper red grains of iddingsite are

commonly seen.

Microscopic Characters: In microsections the rock is hypocrystalline and

exhibits an inequigranular texture. It is composed of plagioclase, clinopyroxene,

pigeonite, olivine, opaques and glass. Olivine and iddingsite occur as accessories.

Secondary minerals are represented by a variety of silica minerals, calcite and zeolites.

Plagioclase and pyroxene occur both as microphenocrysts and constituents of

groundmass. Aggregates of plagioclase and clinopyroxene microphenocrysts form

glomeroporphyritic texture. The groundmass is usually hypocrystalline. Pyroxene and

plagioclase exhibit sub-ophitic relationship. At places, angular interstices between

felspars are occupied by fine granular pyroxene and iron ore resulting in an intergranular

texture. The groundmass, at places, also exhibits hyalo-ophitic texture in which glass

occupies minute interstices between felspar microlites in haphazard orientation.

Plagioclase belongs to two generations. Plagioclase-I occurs as microphenocryst.

It also occurs as a constituent of the groundmass which belongs to second generation.

Plagioclase-I is generally subhedral in form and show resorbed outlines. Twinning in

24

microphenocrysts is largely polysynthetic. Simple twinning is also observed.

Microphenocrysts exhibit zoning of normal and oscillatory type. Zoning is also exhibited

by multiple twins in some grains. The groundmass plagioclase occurs as subhedral laths.

Pyroxene belongs to two generations. Pyroxene-I is subhedral to anhedral and

shows resorption. It is colourless with a faint brownish tinge and nonpleochroic.

Extinction angle C A Z varies between 38- 45°, 2V is between 56-62° and it is optically

positive. The pyroxene may therefore, be identified as subcalcic augite to augite. The

groundmass pyroxene occurs as anhedral grains. Optical properties are similar to those

of the microphenocrysts.

Olivine rarely occurs as microphenocrysts as a constituent in glomeroporphyritic

aggregates. It is rarely fresh and is often replaced by serpentine forming pseudomorphs

after the former. Iddingsite is deep brown in colour pleochroic in shades of brown.

Opaques occur both as microphenocrysts as well as skeletal grains in the groundmass.

They are generally represented by magnetite and titanomagnetite.

Glass is generally represented by pallagonite yellowish to brownish in colour.

Glass, lining the walls of cavities and vesicles is generally chlorophaeite which is green

in colour. Secondary minerals filing the vesicles are represented by a variety of silica

minerals, calcite and zeolites.

Tholeiite-I

Megascopic Characters: The rock is melanocratic, fine grained and hard and

compact. The rock from the dyke selvages is almost glassy in hand specimen whereas the

one from the central parts of the dyke is relatively coarser in grain size.

25

Microscopic Characters: The rock exhibits glomeroporphyritic texture with

microphenocrystic aggregates of clinopyroxene and plagioclase in a finer grained

groundmass of clinopyroxene, microlites of plagioclase, opaques and glass. The

groundmass is microcrystalline and exhibits hyalo-ophitic, microlitic and at places, sub-

ophitic texture. The rock consists of microphenocrystic aggregates of plagioclase,

clinopyroxene and in some rocks olivine. The groundmass is dominated by plagioclase,

clinopyroxene, opaque oxides and palagonite. Olivine is invariably serpentinised and

chloritised. Glass is at places iddingsitised. Clinopyroxene is pinkish in colour and has

2V varying between 50-60° and is optically positive hence could be identified as

subcalcic augite to augite. Plagioclase is labradoritic. Majority of dykes show

hydrothermal alteration producing calcite, chlorite and zeolites. In some dykes the

groundmass contains tiny crystals of phlogopite which may be secondary. The

identification of phlogopite has been confirmed by electron probe microanalysis.

Tholeiite- II

Under this category of dykes, two petrographical types can be distinguished

namely : (i) Picrite Basalts and (ii) Tholeiitic Basalts. In the foregoing description, only

the petrographical characteristics of these two types are described separately. Elsewhere

in the thesis, both these types are treated under the broad category of Tholeiite-II (thol-II)

dykes; the distinguishing features being highlighted wherever necessary.

Mineralogically, these rocks could be categorised as alkali basalts following

Wilkinson (1967) as they contain microphenocrysts of olivine. However, as discussed

later under the chapter on geochemistry, the rocks could be classified as oversaturated-

tholeiites and mildly alkalic transitional tholeiites (e.g. Middlemost, 1985).

26

(i) Picrite Basalts

Megascopic Characters : The rock is melanocratic, dark in colour and fine

grained. The samples from the borders of dykes are finer grained to glassy than those

from the central part. At places, phenocrysts of olivine varying in size from 1-3 mm,

rarely over 5 mm, are seen in aphanitic groundmass. These can be readily made out due

to their yellowish to brownish alteration. Copper red coloured grains of iddingsite are

also seen.

Microscopic Characters: The rock is hypocrystalline and inequigranular under

the microscope. Thin sections from the border zones of dykes are fine grained to almost

glassy as compared to the central part. The former exhibits inequigranular vitrophyric

texture with phenocrysts of olivine in a cryptocrystalline to glassy groundmass. Olivine

is rarely fresh and idiomorphic (Plate V Photo 1) at places, corroded by reaction with the

groundmass. More often it is altered to yellowish serpentine. The rock from the central

part of the dyke exhibits glomeroporphyritic texture with aggregates of phenocrysts of

serpentinised olivine (Plate V Photo 2) in a fine grained partially crystalline groundmass

made up of microlites of plagioclase and granular clinopyroxene. The groundmass

exhibits intergranular texture, however, in more evolved types-the texture becomes sub-

ophitic. The phenocrysts in most rock are almost entirely made up of olivine which make

up more than 95 volume % of the total phenocryst assemblage. Clinopyroxene makes up

less than 5 % by volume, in a few relatively more evolved rocks. Such picrite basalts can

be classified as oceanites.

The rock is chiefly constituted of olivine, clinopyroxene, and plagioclase, with

opaques, iddingsite and glass in subordinate amounts. The plagioclase in evolved types

27

PLATE V

Photo 1: Idiomorphic partially resorbed microphenocryst of olivine in microcrystalline groundmass in picrite basalt (BxN, X 15).

Photo 2: Aggregates of serpentinised microphenocrysts of olivine in a partially crystalline groundmass.(BxN, X 20)

bears a sub-ophitic relationship with pyroxene. Plagioclase occurs as lath-shaped

microlites. Twinning is generally of polysynthetic type, the twin individuals being of

unequal width.

Pyroxene is anhedral and confined largely to the groundmass. It is faintly pinkish

in colour, non pleochroic and rarely shows development of prismatic cleavage. Pyroxene

is generally free of inclusions. Wavy extinction is shown by some grains. The 2V of

pyroxene varies from 55-65° and is positive. The extinction angle C A Z varies between

35-42°. Therefore the pyroxene may be identified as augite.

Olivine is found as phenocrysts. It is colourless when fresh, generally subhedral

rarely euhedral six sided in form, it is fractured and altered along the fractures. Majority

of grains are altered along borders, some are completely altered to yellowish serpentine.

The 2V of olivine is 60-80'. Rarely iddingsite occurs as pseudomorphs after olivine. It

is reddish brown in colour and pleochroic in shades of brown. Opaques are represented

by titanomagnetite and magnetite which occurs as skeletal grains.

Glass occurs interstial to plagioclase and clinopyroxene in the groundmass. It is

mainly represented by tachylite at places yellowish brown palagonite is also seen.

(ii) Tholeiitic Basalts

Megascopic characters: The rock is melanocratic, dark in colour and fine to

medium in grain size. The samples from the borders of dykes are finer grained than those

from the central part. At places phenocrysts of plagioclase varying in size from 2 mm to

over 5 mm are seen in aphanitic groundmass. Brownish grains of iddingsite are also

seen.

28

Microscopic Characters: The rock is hypocrystalline and equigranular under the

microscope. Thin sections from the border zones of dykes are fine grained as compared

to the central part. The former exhibit inequigranular texture. The rock from the central

part of the dyke is microporphyritic with microphenocrysts of clinopyroxene and

plagioclase. It is chiefly constituted of clinopyroxene, olivine and plagioclase, iddingsite,

opaques and glass occur in subordinate amounts. The plagioclase bears a subophitic

relationship with pyroxene. Plagioclase is subhedral and lath shaped. Twinning is

generally of polysynthetic type, the twin individuals being of unequal width. Zoning is

commonly observed and is of oscillatory type. Wavy extinction is exhibited in some

grains. Inclusions of glass and opaques are found within plagioclase.

Pyroxene is subhedral to anhedral in nature, it is faint brown in colour, non

pleochroic and shows prismatic cleavage. Pyroxene is generally free of inclusions.

Wavy extinction is shown by some grains. The extinction angle C A Z varies between 35-

42°. The 2V of pyroxene ranges from 55-65° and is optically positive. Therefore, the

pyroxene may be identified as augite.

Olivine is found as microphenocrysts in subordinate amounts. It is colourless,

subhedral, six sided in form, fractured and altered along the fractures. The 2V of olivine

is 60-80°. Iddingsite occurs as pseudomorphs after olivine. It is reddish brown in colour

and pleochroic in shades of brown. Opaques are represented by titanomagnetite and

magnetite which occurs as skeletal grains.

Glass occurs as patches and also as inclusions within plagioclase. Glass is mainly

represented by tachylite at places, yellowish brown palagonite is also seen

29

Lamprophyres

The lamprophyres include monchiquites and camptonites, however, the latter

predominate. The former at places, occur as selvages of hyalomonchiquite to

camptonites. They exhibit microporphyritic textures and carry euhedral resorbed

phenocrysts of olivine pseudomorphed by chlorite. Microphenocrysts and macrocrysts

(nongenetic term for larger crystals, sensu Mitchel, 1986) of clinopyroxene (Al-Fe-Ti

diopside), kaersutite and Ti-Cr phlogopite also occur. The groundmass consists of same

mineral phases except olivine. It is mafic in monchiquites, felsic in camptonites and

additionally contains Ti-magnetite, sphene, perovskite and zeolites. Globular structures

(ocelli) contribute a varying proportion of the mode. They are rounded or irregular with a

clear core of analcime and a border zone studded with ac oicular clinopyroxene and

kaersutite.

Megascopic Characters : The rock is melanocratic, dark coloured, hard and

compact, fine grained with megacrysts of phlogopite. The latter are rounded and vary in

size from 10-30 mm.

Microscopic Charaters In microsections two petrological types can be

recognised. Monchiquite and camptonite. Both exhibit porphyritic texture. Monchiquite

is characterised by macrocrysts and phenocrysts of phlogopite, clinopyroxene, kaersutite

and olivine in a fine grained groundmass of clinopyronxene kaersutite, phlogopite (Plate

VI Photo 1), opaques and zeolites. Camptonite carries macrocrysts and phenocrysts of

clinopyroxene, kaersutite and phlogopite set in fine grained groundmass of clinopyroxene

kaersutite, altered feldspar,opaques, chlorite and calcite.

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PLATE VI

Photo I: Microphenocryst and rounded ocilli in a fine grained groundmass dominated by clinopyroxene, phlogopite and kaersutite in monchiquite (ppl X 20).

Photo 2: Granophyric overgrowths of quartz-alkali felspar on plagiocla microphenocrysts in microdiorite (BxN, X 20).

Clinopyroxene occurs as microphenocrysts which are generally idiomorphic. It is

pale greenish in colour and nonpleochroic. The 2V of pyroxene varies from 50-60°, C A Z

is between 25-30° It is optically positive hence, identified as diopside. At places the

microphenocrysts are corroded with sharp reactions rims and embayed by groundmass

indicating resorption. Such grains may be referred to as xenocrysts. Rarely

clinopyroxene have irregular green pleochroic xenocrystic cores mantled by pale green

non pleochroic clinopyroxene. Such reversely zoned pyroxenes have also been reported

in lamprophyre from Murud-Janjira (Dessai et al 1990). The groundmass pyroxene

occurs as shdrt stout prisms and also as slender needles. It is generally colourless. In

monchiquites acicular clinopyroxene is included in amoeboid grains of phlogopite.

Amphibole is pleochroic in shades of brown, rarely occurs as phenocryst and is at

places corroded and resorbed. Some grains are rounded and embayed by groundmass,

constituents due to resorption. Generally amphibole is confined to groundmass. The 2V

varies from 75-80°, CAZ is between 5-12° and is optically negative, therefore, it has been

identified as kaersutite

Nephelinite

MegaScopic Characters: The rock is melanocratic dark greenish grey in colour.

Rocks from the peripheral part of the body are finer grained as compared to those from

the cental parif, which are coarser in grain size. The texture is porphyritic with dark

green, lath shaped, prismatic megaphenocrysts of pyroxene. They range in size from few

mm to over 8 mm and are embedded in a finer grained groundmass which is dominated

by pyroxene and nepheline. The samples from the peripheral part of the intrusion are

31

veined by calcite and contain pinkish altered nephelines which can be distinguished in a

largely aphanitic groundmass.

Microscopic Characters : In microsections the rock is microporphyritic with

microphenocrysts of clinopyroxene and Fe-Ti oxides with accessory proportions of

olivine and perovskite in a fine microcrystalline groundmass dominated by

clinopyroxene, nepheline, Fe-Ti oxides, perovskite and sphene. Zeolites occur as

amygdules with chlorite smectite intergrowths.

Clinopyroxene is the most abundant constituent of the rock. It makes up as much

as 46-52 volume % of the mode. In peripheral samples the groundmass is extremely fine

grained. Invariably the microphenocrysts have resorbed, corroded, rounded to irregular,

greeni strongly pleochroic xenocrystic cores mantled by pale green borders. The boundary

between the core and the mantle is generally sharp. Invariably the boundary zone is

studded with fine inclusions of sphene and iron oxide whereas the core and the

surrounding mantles are free from inclusions. The phenocrysts are complexly zoned and

are corroded and embayed by the groundmass constituents. Zoning is invariably of

oscillatory type, some grains, however, show convolute zoning. Cores of

microphenocrysts provide 2V varying from 55 to 60° and C A Z from 16 to 35° and hence

could be identified as aegerine augite. The mantle-clinopyroxene has 2V between 55-62°

and CAZ varying between 35-45° and hence could be identified as diopsidic augite.

Clinopyroxene included within olivine is pale green in colour as compared to the

microphenocryst cores, shows anomalous interference colours but is otherwise similar to

the mantle-clinopyroxene. It could be identified as chrome diopside. This clinopyroxene

is generally unzoned.

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Olivine occurs as megacrysts, is fresh, resorbed, corroded and embayed by the

groundmass. It shows a 2V of 70-80°. Along fractures and grain boundaries it is

replaced by phlogopite. Felspathoids are mainly represented by nepheline. It rarely

occurs as microphenocrysts, invariably it is confined to the groundmass. It is usually

found as idiomorphic, colourless, clear six sided crystals showing low relief and dull

interference colours. At places, it is altered to cancrinite. Opaques are present as

phenocrysts and as skeletal grains in the groundmass. Phlogopite has developed around

grains of olivine. It is strongly pleochroic in shades of brown. 2V varies between 0-10°.

Patches and blebs of calcite are seen in the groundmass. Calcite in veins is associated

with clinopyroxene. It is possible therefore that calcite is of magmatic origin.

Tephriphonolite

Megascopic Characters : The rock from the central part of the intrusion is

mesocratic, light grey in colour with a brownish tinge. It has megaphenocryst of felspars

and needles of clinopyroxene in a fine grained groundmass. Tabular crystals of felspar

range in size from 1-5 mm. Needles of pyroxene vary between 3-6 mm. They occur

either singly or as clusters which at places are included within clusters of felspars. The

groundmass is fine grained and aphanitic. The rock from the border zone is fine grained,

mesocratic, light green in colour and is brecciated. It is traversed by a network of veins

of silica and calcite. Rarely needles of pyroxene can be seen in an aphanitic groundmass.

At places nodular aggregates of stumpy clinopyroxene are seen in the groundmass.

Microscopic Characters : In microsections the rock from the border zone is

highly brecciated and contains nodular clusters of needles of clinopyroxene which ranges

in size from 2-8 mm. The clinopyroxene is deep green in colour strongly pleochroic in

33

shades of green, 2V ranges from 52-60° and C A Z from 10-32°, therefore, the pyroxene

can be identified as aegerine. Opaques occur intergranular to clinopyroxene. Alkali

felspar is perthitic orthoclase in which the intergrown sodic phase shows braided pattern.

Apatite occurs as stumpy, tabular crystals.

In microsections the rock from the central part is hypidiomorphic granular. It

exhibits microporphyritic texture with euhedral microphenocrysts of alkali felspar,

clinopyroxene, nepheline, biotite and rarely apatite. The last is generally confined to the

groundmass which is made up of kaolinised felspar, needles of aegerine, cancrinite and

opaques. The K-felspar is perthitic orthoclase with which the sodic phase is intergrown

forming a braided pattern. K-felspar phase is invariably kaolinised as compared to the

Na-felspar phase. The microphenocrysts of K-felspar are often surrounded by a cluster of

needles of clinopyroxene. The clinopyroxene is strongly pleochroic in shades of dark

green (X), green (Y) and yellowish green (Z). The 2V ranges from 60-75° and C AZ from

5-12°, hence the pyroxene can be identified as aegerine. It is invariably corroded and

embayed by the groundmass along grain boundaries and along cleavages. Biotite is

present in the groundmass, rarely occurs as microphenocrysts. It is pleochroic in shades

of straw yellow (X) and dark reddish brown (Y & Z). At places it replaces

clinopyroxene. Needles of aegerine from the groundmass are clustered around biotite

and are arranged parallel and tangential to it. Nepheline is present in subordinate

amounts and occurs as microphenocrysts that are invariably altered to cancririite. Rarely

microphenocrysts of sodalite are seen. Euhedral microphenocrysts of apatite are either

found discretely within groundmass or they are enclosed within felspar, aegerine and

biotite. Opaques are mainly represented by magnetite and titanomagnetite. Deuteric

34

calcite is seen to replace both the microphenocrysts as well as the groundmass

constituents.

Andesite/Microdiorite

Megascopic Characters: In hand specimen the rock is mesocratic, medium

grained and shows a variety of grey to pale grey shades depending upon the extent of

mixing between tholeiitic and trachytic magma. Thin veins of trachyte traverse the rock

at places.

Microscopic Characters: In microsections the rock is holocrystalline and

inequigranular. It exhibits microporphyritic texture wherein the microphenocrysts of

plagioclase and pyroxene occur in a granophyric groundmass. The rock is constituted of

plagioclase, pyroxene, brown amphibole, alkali feldspar and quartz. The plagioclase

feldspar occur as microphenocrysts. It is invariably zoned from core to margin. The

microphenocrysts have grown at the expense of granophyric intergrowth of quartz and

alkali feldspar (Plate VI Photo 2). Plagioclases are encased by the latter intergrowth.

Granophyric intergrowth also occur filling interstices between plagioclase crystals.

Different types of granophyric intergrowth e.g. cuneiform, vermicular, radiating are

observed. They often occur in combination. At places, these intergrowths become coarse

and form bulbous quartz units. Plagioclase is surrounded by alkali felspar. It shows

normal as well as reverse zoning. In some grains, the cores have An content of An 57

whereas the borders have An48. In others, the cores have An28 whereas the rims Ann.

Plagioclase crystals invariably display resorbed xenocrystic cores around which a rim of

plagioclase has grown. Plagioclase included in orthoclase has An content varying

between Ani4-2E.

35

Pyroxene is pale yellowish green in colour and is nonpleochroic. Extintion angle

C AZ varies from 30-42° , 2V is between 50-65° and it is optically positive. The pyroxene

may therefore, vary in composition between augite and diopside. It is invariably

corroded and resorbed. It exhibits reaction coronas of brown amphibole. The amphibole

is pleochroic in shades of light brown to dark brown. The C A Z varies between 5-12°, 2V

is between 70-80° and it is optically negative. Hence the amphibole may be identified as

kaersutite. It is invariably propylitised to a granular mineral aggregate.

Fine needles of apatite, zircon, and opaque oxides occur throughout the rock. The

modal proportion when plotted in the QAP ternary diagram (Streckeisen, 1979) the rock

varies in composition from quartz monzodiorite to diorite.

Xenoliths

The lamprophyres have entrained a variety of lithospheric xenoliths and

megacrysts (non-genetic term for crystals > 5 mm in diameter including those from

fragmented and disagregated xenoliths). The xenoliths could be broadly categorised

under three broad petrological groups:

Ultramafic Group: The group is mainly represented by (i) peridotites (extremely

rare), and (ii) pyroxenite which include spinel clinopyroxenite, garnet clinopyroxenite,

and rare websterites.

Mafic Group: This group consist of granulites and garnet granulites. Some of

them contain layers of clinopyroxenite, others are veined by clinopyroxene forming

composite xenoliths.

Felsie Group: The group comprise granites and rare syenites.

36

I) Peridotites: These are extremely rare and are represented by wehrlites (Dessai, 1985);

They are usually very small (5 mm-< 1 cm) fragmented and commonly occur as

aggregate of few mineral grains distributed throughout the rock. Rarely they may be

about 5 cm in size. Petrographical characteristics of these are described by Dessai

(1987). Spinel lherzolite xenoliths from Deccan are, however, reported from Kutch

(Krishnamurthy et al., 1988).

ii) Pyroxenites: They are subrounded and usually range in size from 5 mm to 2 cm.

Some are resolvable only under the microscope (Plate VII Photo 1). They are dominated

by clinopyroxenites along with rare websterites. The former consist of clinopyroxene (>

80 vol %) + opaque oxides (4-11 vol %) + phlogopite (11 vol %) ± sulphides (Table 3.1).

The latter are dominated by clinopyroxene + opaque oxides. Orthopyroxene occurs both

as discrete grains as well exsolution blebs in clinopyroxene. Rare polygonal grains of

orthopyroxene represent recrystallised exsolved material. Websterites may be

gradational to the pyroxenites and appear to have evolved through aluminous

pyroxenites. The clinopyroxenites consist of clinopyroxene + garnet + opaque oxides +

phlogopite. These are traversed by veins of sulphides, carbonates and apatite They

exhibit texture transitional between porphyroclastic and equigranular. The pyroxenites

belong to the Al-augite group (Wilshire and Shervais, 1975) which is equivalent to Type

II xenoliths of Frey and Prinz (1978). These are considered to represent cumulates of

basaltic melts, re-equilibrated to varying degrees.

The clinopyroxenites exhibit textures transitional between porphyroclastic

(Mercier and Nicholas, 1975) and equigranular still retaining some relict porphyroclasts.

The latter have serrated and curvilinear outlines, strain-shadows, deformation lamellae

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PLATE VII

Photo 1: Clinopyroxenite microxenolith in lamprophyre at Murud-Janjira (ppl X 25).

Photo 2: Foliated fabric of clinopyroxenite xenolith defined by preferred orientation of clinopyroxene (BxN, X 60).

Table 3.1: Modal compositions (vol %) of xenoliths from Murud-Janjira

Smp 1 Cpx Opx Phig Plg Gnt Opaq Apa Rut. Spn

Clinopyroxenites

1 281X 91

2 281V11 81 11

3 281S 89

Granulites

4 281C1 52 46

5 267RH Tr 30 22 48

6 281W 51 Tr 2 28 18 Tr

7 281VI 53 Tr 31 15 Tr

8 281P 81 Tr 3 16

Br1/1 16 52 29 2

9 Krl/X1 13 Tr Tr 54 32 Tr

10 Kr1/X2 13 Tr Tr 68 17 Tr

Eclogitic granulite

11 281V 58 9 33

Disaggregated xenoliths

12 281A +

13 281B +

14 281C

15 281D +

16 281E +

17 281G +

Tr : trace + : present Cpx : Clinopyroxene Opx : Orthopyroxene Phlg : Phlogopite Plg :Plagioclase Gut : Garnet Opaq : Opaque Apa : Apatite Rut : Rutile Spn : Sphene (after Dessai and Vaselli, 1999; Dessai et al., 1999).

and exsolutions of orthopyroxene. These are surrounded by smaller recrystallised equant

grains. Such clinopyroxenes are in places included within the porphyroclasts. Some

samples exhibit a foliated fabric (281 X) defined by preferred orientation of

clinopyroxene. In these textural types too, relict porphyroclasts are seen (Plate VII Photo

2). Secondary minerals have formed by reaction with silicate melt. Glass veins contain

tiny clinopyroxene crystals which also occur in melt pockets. This clinopyroxene is here

referred to as "secondary". Phlogopite is anhedral and intergranular (Plate VIII Photo 1).

It occurs within clinopyroxene and is also seen to enclose it. Sulphides occur as

megacrysts (> 5 mm) and are represented by pyrite, chalcopyrite, pyrrhotite and

pentlandite. In fragmented xenoliths spinel has reacted with the host magma to form

opaque oxides.

iii) Granulites: They range in size from 2-20 cm and consist of two-pyroxene granulites

and garnet granulites. The former are made up of clinopyroxene (> 50 vol %) +

plagioclase (2-46 vol %) + garnet (2-28 vol %) ± orthopyroxene ± spinel + rutile. They

invariably contain trace amounts of phlogopite. The garnet granulites consist of

clinopyroxene + garnet + plagioclase. Kaersutite, phlogopite, apatite and sulphides vein

the granulites. Some of the granulites (e.g. 281 W) are composite with layers of

clinopyroxenites and are also veined by the latter. The mafic granulites are made up of

clinopyroxene + plagioclase + garnet ± orthopyroxene ± spinel ± rutile + sphene. They

are traversed by veins of phlogopite, kaersutite, K-felspar, sulphides and apatite. They

exhibit porphyroclastic to granoblastic texture with a metamorphic overprint.

The mafic granulites are medium grained. Rocks dominated by plagioclase (281

VI) exhibit equant granoblastic texture. Those dominated by clinopyroxene and garnet

38

PLATE VIII

Photo 1: Anhedral, intergranular phlogopite in clinopyroxenite xenolith (ppl X 60).

Photo 2: Garnet porphyroblasts with keliphytic rims in an equant plagioclase dominated groundmass in granulite xenolith (BxN, X 30).

show porphyroclastic (Plate VIII Photo 2) and meta-igneous textures that often appear to

be cumulates with a mild metamorphic overprint. Preferred orientation of clinopyroxene

and plagioclase in some samples (281 C1) may suggest original igneous layering (Plate

IX Photo 1). Clinopyroxene is pale green to almost colourless except in 281C1 in which

it is deep green. Pale pink garnet is altered along its borders to double rims of

cryptocrystalline kelyphite in which relicts of fresh garnet are visible. The inner rim is

pale brown, fibrous or acicular whereas the outer one is vermicular and is studded with

opaque oxides. The garnet contains inclusions of clinopyroxene and may be also veined

by the latter. Plagioclase shows stretching, bending, it is granulated along borders (Plate

IX Photo 2) and exhibits deformation lamellae giving rise to protomylonitic texture. In

some xenoliths plagioclase is replaced by scapolite. In reacted xenoliths, minerals show

resorbed margins (Plate X Photo 1) and at palces irregular grains of -sphene have

developed in the vicinity of garnet. Rutile is generally present in trace amounts but rarely

may form up to 5% by volume. In some samples (eg. 267 RHA) rutile + magnetite +

plagioclase form the bulk of the rock with trace amounts of clinopyroxene. The rutiles

are rounded and resorbed.

Orthopyroxene occurs as both exsolutions in clinopyroxene (Plate X Photo 2) and

as recrystallised grains. Phlogopite is anhedral and intergranular. It is found enclosing

clinopyroxene and is also included in it. Spinel is anhedral in porphyroclastic types and

intergranular in recrystallised xenoliths. In some xenoliths, veins of plagioclase,

phlogopite, kaersutite, sulphides and rarely apatite traverse clinopyroxene.

Composite xenoliths (e.g. 281W) are rare. In such xenoliths granulite contain

layers of clinopyroxenite. The former shows equant granoblastic texture whereas the

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PLATE IX

Photo 1: Modal layering in granulite defined by alternate layers of clinopyroxene and plagioclase. The latter is stretched and bent (BxN, X 60).

Photo 2: Protomylonitic texture in granulite xenolith. Plagioclase porphyroclasts show bending, deformation lamellae and granulation along borders (BxN, X 60).

PLATE X

Photo 1: Clinopyroxene resorbed due to reaction with the melt in granulite xenolith (BxN, X 30).

Photo 2: Exsolution of orthopyroxene in clinopyroxene in granulite xenolith (BxN, X 60).

latter are xenomorphic granular. The contact between the two lithologies is generally

irregular and is marked by the presence of phlogopite which decreases away from the

contact.

(iv) Granites: They vary in size from 2- > 20 cm in diameter. Under this class are

included hypidiomorphic, granular sodic granites and felsic syenites. The former consist

of microcline, perthitic sodic plagioclase, quartz, biotite, epidote, sphene and opaques.

The latter are equigranular and are made up of plagioclase and aegerine augite, with

biotite epidote and opaque as accessories.

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