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JOURNAL GEOLOGICAL SOCIETY OF INDIA

Vol.76, October 2010, pp.345-360

Petrography and Major Elements Geochemistry of Microgranular

Enclaves and Neoproterozoic Granitoids of South Khasi, Meghalaya:

Evidence of Magma Mixing and Alkali Diffusion

SANTOSH KUMAR 1 and THEPFUVILIE PIERU 2

1 Department of Geology, Kumaun University, Nainital - 263 002

2 Department of Geology, Patkai Christian College, Chumukedima, Patkai - 797 103

Email: skyadavan@yahoo.com

Abstract: Neoproterozoic (690±19 Ma) felsic magmatism in the south Khasi region of Precambrian northeast Indian

shield, referred to as south Khasi granitoids (SKG), contains country-rock xenoliths and microgranular enclaves (ME).

The mineral assemblages (pl-hbl-bt-kf-qtz-mag) of the ME and SKG are the same but differ in proportions and grain

size. Modal composition of ME corresponds to quartz monzodiorite whereas SKG are quartz monzodiorite, quartz

monzonite and monzogranite. The presence of acicular apatite, fine grains of mafic-felsic minerals, resorbed maficfelsic

xenocrysts and ocellar quartz in ME strongly suggest magma-mixed and undercooled origin for ME. Molar Al 2

O 3

/

CaO+Na 2

O+K 2

O (A/CNK) ratio of ME (0.68-0.94) and SKG (0.81-1.00) suggests their metaluminous (I-type) character.

Linear to sub-linear variations of major elements (MgO, Fe 2

O 3 t , P 2

O 5

, TiO 2

, MnO and CaO against SiO 2

) of ME and

SKG and two-component mixing model constrain the origin of ME by mixing of mafic and felsic magmas in various

proportions, which later mingled and undercooled as hybrid globules into cooler felsic (SKG) magma. However, rapid

diffusion of mobile elements from felsic to mafic melt during mixing and mingling events has elevated the alkali contents

of some ME.

Keywords: Neoproterozoic granitoids, Microgranular enclaves, Magma-mixing, South Khasi, Meghalaya.

INTRODUCTION

Granitoids may contain a variety of enclaves such as

(1) xenoliths of country rocks or deeper-derived lithology,

(2) surmicaceous enclaves as restite or remnant of protolith,

(3) autolith (cognate or cumulate) of felsic host magma, and

(4) microgranular enclaves (ME) which are fine- to mediumgrained,

light to dark coloured blobs of mafic to hybrid

magma (Vernon, 1983; Didier, 1984). The ME are common

and distinct features of many felsic plutons (e.g. Kumar et

al. 2004a, b, Kumar and Rino, 2006). The Assam-Meghalaya

plateau is a tectonically active Precambrian block in

northeast India. Late- to post-collisional, felsic igneous

plutons (479 – 881 Ma; Table 1) intrude the Basement

Gneissic Complex and Shillong Group (Medlicott, 1869;

Mazumder, 1976, 1986; Ghosh et al. 1991, 1994, 2005),

which were differentially uplifted (Harijan et al. 2003) as a

result of mantle upwelling during the Pan-African-Brasilino

orogeny. One of these in south Khasi region of West Khasi

Hill district of Meghalaya state is called here the south

Khasi granitoids (SKG; 690±19 Ma) pluton, which covers

an area of about 600 km 2 (Figs. 1b, c). In a recent short

communication, Kumar et al. (2005a) reported occurrences

and field relations of microgranular enclaves (ME) in SKG,

and have discussed the nature of coeval mafic and felsic

magma interactions in plutonic setting, but did not deal

with the likely processes in the evolution of ME, which is

the subject matter of this paper.

GEOLOGY AND FIELD RELATION

Neoproterozoic to Early Palaeozoic granitoid plutons

(South Khasi, Mylliem, Kyrdem and Nongpoh) are emplaced

proximal to regional Tyrsad-Barapani lineament (Fig. 1b),

broadly becoming younger in ages from southwest to

northeast (Kumar, 1998) reactivating the pre-existing fault

system (Nandy, 2001). The SKG pluton intrudes the

Basement Gneissic Complex and Proterozoic metasediments

of Shillong Group (Fig.1c). The SKG and ME belong to

magnetite series (oxidized) granites but reduced to ilmenite

series granites when marginally interacted with Shillong

Group of rocks (Kumar et al. 2005b). South Khasi granitoids

(SKG) are medium- to coarse-grained, equigranular and

0016-7622/2010-76-4-345/$ 1.00 © GEOL. SOC. INDIA


346 SANTOSH KUMAR AND THEPFUVILIE PIERU

Fig.1. (a) Political map of India and neighbouring countries. (b) Geological map of a part of the Khasi Hills in Meghalaya, showing

various granitoid plutons (after Mazumdar, 1976). (c) Geological map of south Khasi region of West Khasi Hills district

showing exposure of Neoproterozoic granitoids (SKG) and its relationship with adjacent lithotypes (simplified after Mazumder,

1976). Tyrsad-Barapani lineament is drawn after Sengupta and Agarwal (1998). The magnetic susceptibility values (x10 -3 SI

unit) of SKG/ME measured at and around various localities across the pluton are also shown (after Kumar et al. 2005b).

porphyritic in nature containing K-feldspar megacrysts (up

to 7 cm). Central part of the pluton is mainly dominated by

very coarse grained porphyritic variety of SKG.

Broadly two types of enclaves viz. xenoliths of country

rocks and microgranular enclaves (ME) can be recognized

hosted in SKG (Kumar et al. 2005a). The ME in equigranular

SKG are fine-grained, occasionally porphyritic, small and

circular in shape. The ME in porphyritic SKG are ubiquitous,

mesocratic to melanocratic, fine- to medium-grained and

porphyritic in nature. The size of ME in the interior parts of

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PETROGRAPHY AND MAJOR ELEMENTS GEOCHEMISTRY OF MICROGRANULAR ENCLAVES, MEGHALAYA 347

Table 1. Rb-Sr iotopic data of granitoid plutons in Meghalaya

Area Age (Ma) SrI Reference

Kyrdem 479±26 0.71482±0.00072 Ghosh et al. 1991

Songsak 500±40 0.72800±0.00020 Kumar, 1990

Nongpoh 550±15 0.70948±0.00047 Ghosh et al. 1991

Mylliem 607±13 0.71187±0.00047 Chimote et al. 1988

South Khasi 690±19 0.71074±0.00029 Ghosh et al. 1991

South Khasi 757±60 0.71069±0.00092 Selvam et al. 1995

South Khasi 748±26 0.71074±0.00043 Selvam et al. 1995

Rongjeng 788±21 0.70699±0.00020 Ghosh et al. 1994

Sindhuli 881±39 0.70517±0.00068 Ghosh et al. 1994

SrI= initial 87 Sr/ 86 Sr ratio

the pluton is larger than those occurring near the marginal

parts. The most frequent size of ME ranges between 3 and

30 cm, but occasionally noted upto 1.5 meter across. The

shapes (apparent geometry on 2D surface) of ME are subrounded,

ovate, discoidal, elliptical, tabular and angular to

sub-angular. The contacts between ME and SKG are usually

sharp and also display curved outline, but occasionally

diffuse contacts can be observed. Crenulated or serrated,

irregular (pillow-like), occasionally rimmed by felsic margin

towards felsic host SKG are also observed. K-feldspar

megacrysts of various shapes (tabular and square) are

frequently observed close to the ME-SKG contacts.

PETROGRAPHY AND MINERAL PARAGENESIS

The SKG exhibit medium- to coarse-grained,

inequigranular and hypidiomorphic textures with major

constituting minerals of amphibole as hornblende (~4 mm),

biotite (4 mm), plagioclase (~6 mm), K-feldspar (~7 mm,

groundmass) and quartz (~5 mm), in addition to K-feldspar

megacrysts. Epidote, titanite, apatite and opaques as Fe-Ti

oxides constitute the accessories. Early plagioclase, Fe-Ti

oxides and stuby apatite show poikilitic relation with

hornblende and biotite. Biotite is also found as inclusion in

hornblende. Biotite predominates over hornblende.

Plagioclase occurs as euhedral to subhedral, twinned and

zoned crystals.

The ME are fine- to medium-grained, hypidiomorphic

and variably porphyritic containing K-feldspar xenocrysts,

anhedral (ocellar) quartz and small xenophenocrysts of

biotite and hornblende. Groundmass minerals are

hornblende, biotite, plagioclase, K-feldspar (including

microcline), quartz and accessories like epidote, accicular

apatite, titanite and Fe-Ti oxides. Of these, apatite needles

are abundant and found as inclusions in mafic and interstitial

felsic minerals (Fig. 2a). Crystal habits of groundmass

hornblende and biotite are elongate, prismatic and granular

(Fig. 2b). Often, hornblende and biotite exhibit sub-poikilitic

to poikilitic textures (Fig. 2c). At microscopic level, the ME-

SKG contact is also sharp and wavy, without showing

reaction and recrystallization effects (Fig. 2d). Elongated

hornblende and biotite are aligned parallel to sub-parallel

along the contact outline. At ME-SKG contact, occasional

cumulate-like texture formed by cumulus plagioclase

(primocrysts) and inter-cumulus biotite can be observed.

Some relatively larger, anhedral, poikilitic grains (~2 mm)

of hornblende with apatite and plagioclase inclusions appear

identical to those observed in SKG. Similarly, K-feldspar

megacrysts and plagioclase phenocrysts (6 mm) in ME,

sometimes armoured by fine grains of mafic, appear identical

to those found in SKG except the crystal boundaries, which

suggest their xenocrystic origin in ME (Fig. 2e). Quartz

occurs as interstitial and xenocrystic (ocellar) (Fig. 2f) forms.

The interstitial quartz appears frozen or solidified between

early crystallized minerals of ME whereas the ocellar quartz

was partially corroded over which plagioclase, biotite and

hornblende were sometimes grown epitaxially.

Mineral paragenetic sequence of ME and SKG has been

deduced on the basis of degree of crystallinity, mineral

inclusions and textural relationships (Figs. 3a, b), excluding

the xenocrystic phases commonly present in the ME.

Apatite and Fe-Ti oxides were first to crystallize followed

by cotectic plagioclase, hornblende and/or biotite in ME.

Later, K-feldspar crystallized but prior to interstitial quartz.

Secondary biotite and sericite were formed near solidus

and subsolidus conditions. In SKG, the order of minerals’

crystallization was more- or- less similar, but rate of

crystallization was substantially at slower rate as compared

to the ME. Plagioclase was first to crystallize among

major rock-forming minerals, closely followed by

hornblende, biotite, K-feldspar and quartz. Plagioclase,

biotite, K-feldspar and quartz were also formed at later stages

of crystallization. Albitic rim over plagioclase formed below

the solidus of SKG.

MODAL MINERALOGY AND CLASSIFICATION

Modal mineral volume percent was determined counting

an average of 4000 points on rock-thin sections of 34

representative samples of ME and SKG, using model F-415C

point counter. The modal mineralogy (vol. %) and IUGS

nomenclature (Streckeisen, 1973; Le Maitre, 2002) of ME

and SKG are given in Tables 2 and 3 respectively. Modal

composition of ME largely corresponds to quartz

monzodiorite with an exception of tonalite, whereas the SKG

belong largely to quartz monzonite and a few to quartz

monzodiorite and monzogranite (Fig.4). The average

JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010


348 SANTOSH KUMAR AND THEPFUVILIE PIERU

a

a

a

ap ap

ap

b

b

b

Hbl

Mag

Hbl

Mag

Hbl Pl

Pl Mag

Pl

Pl

Pl

Pl

Hbl Hbl

Hbl

0.5 mm 0.5 mm

0.5 mm

1 mm

1 mm

Bt Bt

Bt

c

c

d

c

Bt

Bt

Bt

Mag

Mag

Mag

Hbl

Hbl

Hbl

Bt

Bt

1 mm

d

1 mm

Pl

Pl

SKG

Pl

SKG

SKG

Hbl

Hbl

Hbl

Bt

ME

ME

ME

Bt

1 mm

1 mm

Bt

e

1 mm

1 mm

f

Bt

e

f

e

pl

Bt

f

qtz

pl

pl

Bt

Bt

1 mm

1 mm

qtz

qtz

Fig.2. (a) Apatite needles in microgranular enclave. (b) Microgranular enclave showing granular (magmatic) textures. (c) Hornblende

showing poikilitic textures in microgranular enclave. 1 mm (d) Photomicrograph 1 mm from a single rock thin section cut across the contact

between microgranular enclave (ME) and south Khasi granitoid (SKG), where contact is sharp without recrystallization signatures.

1 mm 1 mm

(e) Large plagioclase xenocryst hosted in fine-grained microgranular enclave suggesting its magma-mixed origin. (f) Quartz

xenocryst in microgranular enclave also suggesting magma-mixed origin. Mineral symbols: ap-apatite; Hbl-horblende; Magmagnetite;

Bt-biotite; pl-plagioclase;

Fig. 2

qtz-quartz.

modal compositions of ME and SKG shown as Pi-diagrams (XRF) system on pressed powder pellets at Wadia Institute

(Figs.5a, b) reveal that the mineral assemblages (hbl+bt+pl+ Fig. 2 of Himalayan Geology, Dehradun. Several international

kf+qtz+Fe-Ti oxides) (mineral symbols after Kretz, 1983) rock standards were used for calibrations. The accuracy

Fig. 2

of ME and SKG are the same but differ in modal proportion. (% RSD) for major and minor oxides is


(a)

Magnetite/ Ilmenite

(Fe –Ti oxides)

Titanite

Apatite (accicular)

Hornblende

Biotite

Plagioclase

K-feldspar

Quartz

Epidote

(b)

Magnetite/ Ilmenite

(Fe –Ti oxides)

Titanite

Apatite

Hornblende

Biotite

Plagioclase

K-feldspar

Quartz

Epidote

PETROGRAPHY AND MAJOR ELEMENTS GEOCHEMISTRY OF MICROGRANULAR ENCLAVES, MEGHALAYA 349

Geochemical Classification

0…………PERCENT CRYSTALLIZED………100

0…………PERCENT CRYSTALLIZED………100

SUBSOLIDUS

SUBSOLIDUS

Fig.3. Paragenetic sequence in microgranular enclaves (a) and

south Khasi granitoids. (b) Some minerals even continued

to crystallize under sub-solidus condition.

Fig. 3

Based on variation of Fe-number (Fe*=FeO t /

FeO t +MgO) against SiO 2

content (not shown) the ME and

SKG belong to magnesian domain of granitoids showing

(a)

(b)

affinity with Cordilleran, I-type granitoids (Frost et al.

2001). The molar A/CNK ratio ranges from 0.68 to 0.94 in

ME and 0.81 to 1.00 in SKG (Fig. 6), which equivocally

correspond to metaluminous (I-type) granitoids (Chappell

and White, 1974; Maniar and Piccoli, 1989). In terms of A-

B parameters (Debon and Le Fort, 1983) the ME and SKG

belong to biotite+amphibole±pyroxene sector of

metaluminous domain, but the ME are more metaluminous

and higher in B (dark minerals) component compared to

that of SKG (not shown). Alkali-lime index (Peacock, 1931)

popularly known as Peacock’s index later modified by

Brown et al. (1984) suggest the variation of SKG similar to

alkali-calcic series (SiO 2

=51-56 wt%), whereas ME are

widespread from alkali to alkali-calcic series (Fig.7). In

AFM both ME and SKG show linear alkali enrichment trend

related to calc-alkaline series (Fig. 8). The most significant

characteristics of SKG are higher SiO 2

and lesser contents

of mafic oxides (Fe 2

O 3 t , MgO, MnO, TiO 2

etc) compared

to that of ME, which are in accordance with the modal

mineral abundances commonly hosting these oxides.

Normative Composition

Orthoclase has been recorded highest among all

normative minerals (Tables 4, 5). On an average 26.72 wt%

and 30.87 wt% of orthoclase norm have been observed in

ME and SKG respectively. Anorthite content is almost equal

Table 2. Modal mineral volume percentage of representative ME hosted in SKG

Sample No. ME ME ME ME ME ME ME ME ME ME ME ME ME ME ME ME X

10 14 15 16 17 18 20 23 24 27 34 35 36 38 41 42 (n=16)

Quartz 8.9 6.8 12.1 9.5 8.4 16.8 9.5 12.6 6.4 9.6 8.5 10.1 11.1 10.6 11.5 12.3 10.29

K-feldspar 14.1 12 8.4 15.4 12.8 4.2 10.9 6 15.8 12.1 14.9 10.7 9.7 9.3 9 8.7 10.88

Plagioclase 44.9 64.8 49 42.8 47.1 43.2 43 46.8 39.2 48.6 49.1 48.4 45.4 48.1 48.6 49.2 47.39

Biotite 20.7 12.3 16.8 25.3 16 22.4 21.5 18.5 24.6 18.9 14.6 21.1 24.1 23.3 20.8 18.4 19.96

Hornblende 9.7 3 12.1 6.2 13.2 12.8 12.6 15.2 10 8.7 9.1 8.5 8.2 8.2 8.9 9.5 9.74

Accessories 1.6 1 1.6 0.8 2.5 0.6 2.5 0.9 4 2.2 3.8 1.4 1.5 0.5 1.2 1.9 1.75

Total 99.9 99.9 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100.01

X =Average; Accessories include apatite, epidote, titanite and Fe-Ti oxides; ME18 is hornblende biotite tonalite and rest are hornblende biotite quartz

monzodiorite according to IUGS recommended nomenclature.

Table 3. Modal mineral volume percentage of representative SKG

Sample No. SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG X

1 5 10 14 15 16 17 18 20 23 24 27 34 35 36 38 41 42 (n=18)

Quartz 15.1 12.1 10.9 13.9 16.2 8 15.6 14.5 10.7 6.9 15 11.4 7.8 13.2 12 12.6 14.1 15.6 12.53

K-feldspar 38.1 34.1 36.7 40.7 35.5 32.6 32.4 35.4 33.3 31.2 30 31.7 33.4 30.9 35 38.6 27.2 15.7 32.92

Plagioclase 31.7 34.6 37.5 34.6 28.8 40.4 39.3 35.5 37.8 40.1 35 36.5 37.3 35.8 34.1 31.6 39.3 47.1 36.50

Biotite 7.4 11.1 8.8 5.1 9.7 12.5 8.2 10.2 11.1 12 15 12.9 10.8 14 11.6 10.3 11.8 13.3 10.88

Hornblende 5.9 4.6 4.3 5.5 6.3 3 3.5 2.3 5.3 8.3 5 6.8 8.6 5.9 6.8 6.9 6.8 6.8 5.70

Accessories 1.7 3.5 1.8 - 3.5 3.5 1 2.1 1.8 1.5 - 1.1 2.1 0.2 0.5 - 0.8 1.5 1.77

Total 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

X =Average; Accessories include apatite, epidote, titanite and Fe-Ti oxides; SKG14 is biotite hornblende quartz monzonite, SKG15 is hornblende

biotite monzogranite and rest are hornblende biotite quartz monzonite according to IUGS recommended nomenclature.

JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010


350 SANTOSH KUMAR AND THEPFUVILIE PIERU

SKG are olivine normative because of enrichment in

ferromagnesian minerals.

Fig.4. Quartz (Q)-Alkali feldspar (A)- Plagioclase (P) plot (vol.

% modes) of microgranular enclaves () and south Khasi

granitoids (O) showing the classification of individual rock,

qmD – quartzmonzodiorite, tg – tonalite, GD – granodiorite,

mG – monzogranite, qM – quartzmonzonite (after

Streckeisen, 1973; Le Maitre, 2002).

with average of 13.90 wt% and 13.33 wt% respectively for

ME and SKG. Average normative quartz of 7.31 wt% and

17.48 wt% respectively in ME and SKG has been observed,

which suggest relatively more mafic nature of ME than

the host SKG. Diopside, magnetite, ilmenite and apatite are

present in minor amount both in ME and SKG. Corundum

is present in a few samples of SKG whereas ME are

absolutely devoid of nepheline norm. A few ME and

Major Elements Variation

The major oxides such as MgO, Fe 2

O 3 t , MnO and to a

certain extent P 2

O 5,

TiO 2

, CaO of ME and SKG show

decreasing linear to sub-linear variation trends with

increasing SiO 2

content on Harker variation diagrams (Figs.

9a-h, 10). Na 2

O, K 2

O and Al 2

O 3

contents of ME show data

scatter whereas for SKG these oxides show limited variations

with respect to SiO 2

, which could be partially a reflection

of varying modal abundance of K-feldspar. The ME show

greater degree of major oxides variations compared to SKG

host. The ME contain higher amount of CaO, MgO and

Fe 2

O 3

t

as compared to SKG, which suggest mafic nature of

ME. P 2

O 5

, TiO 2

and MnO contents are moderately higher

in ME than those of SKG, which could be due to high modal

apatite and titanite in ME.

DISCUSSION

Field Evidence of Enclave Origin

The enclaves (xenoliths and the ME) in SKG clearly

lack partial melting textures as reported elsewhere (Tindle

and Pearce, 1983) but xenoliths exhibit marginal reaction

as a result of thermal effect probably when country rocks

stopped and plunged into SKG melt. The SKG are devoid

of surmicaceous (biotite-rich) enclaves (Didier, 1984),

which may either represent residual after partial melting of

crustal protoliths or the source lithology that resisted anatexis

(e.g. Montel et al. 1991; Kumar, 1999).

Fig.5. Pi-diagrams showing average (N=17) modal mineral composition for microgranular enclaves (a) and average (N=18) mineral

composition for south Khasi granitoids (b).

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PETROGRAPHY AND MAJOR ELEMENTS GEOCHEMISTRY OF MICROGRANULAR ENCLAVES, MEGHALAYA 351

Table 4. Oxides of major elements (wt%) and CIPW normative minerals of representative microgranular enclaves (ME) hosted in south Khasi granitoids (SKG)

Sample ME ME ME ME ME ME ME ME ME ME ME ME ME ME ME ME X

No. 10 14 15 16 17 18 20 23 24 27 34 35 36 38 41 42 (N=16)

SiO 2

55.03 55.58 53.47 51.92 53.20 59.05 53.27 56.90 47.05 53.59 58.41 51.59 60.67 60.9 49.65 55.35 54.73

TiO 2

1.53 1.71 1.98 1.47 1.89 1.90 2.44 1.98 2.27 2.14 1.64 1.82 1.59 1.27 1.65 1.48 1.80

Al 2

O 3

16.43 13.53 14.16 13.64 14.21 14.04 13.37 13.77 13.28 14.29 14.07 15.16 13.86 15.83 11.3 15.13 14.13

Fe 2

O 3

t

7.85 8.14 10.6 11.6 10.38 8.96 11.94 9.08 12.36 10.07 8.02 10.89 8.27 5.78 13.98 6.78 9.60

MnO 0.09 0.11 0.15 0.18 0.13 0.12 0.18 0.13 0.13 0.16 0.11 0.15 0.11 0.06 0.2 0.09 0.13

MgO 3.88 4.21 4.72 5.75 4.89 3.91 4.71 4.40 7.26 3.94 3.55 5.10 3.68 2.19 7.53 4.02 4.61

CaO 3.60 4.56 5.75 5.2 6.46 4.55 5.52 5.80 6.16 6.70 4.64 5.56 4.22 3.79 4.72 4.41 5.10

Na 2

O 4.12 2.24 3.36 2.37 3.19 2.94 2.42 2.75 1.95 2.84 2.78 3.19 2.57 2.66 0.95 2.58 2.68

K 2

O 4.51 4.88 3.27 4.2 3.64 4.21 3.86 2.90 4.06 3.65 4.82 4.44 4.24 5.89 6.03 5.75 4.40

P 2

O 5

0.44 0.89 0.78 0.75 0.81 0.73 1.00 0.71 0.81 0.81 0.70 0.81 0.67 0.45 0.96 0.72 0.75

LOI 1.13 2.7 1.05 1.5 0.73 0.76 0.82 0.99 2.9 1.94 0.38 1.80 0.61 2 1.9 2.6 1.49

Total 98.61 98.55 99.29 98.58 99.53 101.17 99.53 99.41 98.63 100.13 99.12 100.51 100.49 100.8 98.9 98.9 99.51

Molar

A/CNK

0.90 0.76 0.73 0.76 0.68 0.79 0.73 0.76 0.72 0.68 0.77 0.75 0.84 0.94 0.69 0.83 0.77

CIPW NORM

qtz - 9.217 2.635 1.324 1.012 10.718 5.882 11.703 - 4.594 9.678 - 14.92 11.97 - 4.016 7.31

or 27.503 30.275 19.833 25.801 21.948 24.939 23.326 17.534 25.329 22.138 29.023 26.806 25.24 35.364 37.154 35.464 26.73

ab 35.979 19.902 29.176 20.85 27.543 24.937 20.934 23.811 17.423 24.666 23.964 27.577 21.907 22.872 8.377 22.787 23.29

an 13.431 13.071 14.265 14.733 13.981 12.713 14.54 17.051 16.354 15.874 11.897 14.234 13.86 14.08 9.135 13.277 13.91

ne - - - - - - - - - - - - - - - - -

crn - - - - - - - - - - - - - - - - -

di 1.652 3.709 8.005 5.719 10.877 4.181 5.563 6.226 8.042 10.505 5.618 6.978 2.299 1.658 7.138 3.755 5.77

hy 12.866 14.972 16.595 22.676 15.386 13.751 18.409 14.918 9.403 12.536 11.858 5.727 13.971 7.906 23.504 13.311 14.24

ol 1.409 - - - - - - - 12.408 - - 9.364 - - 4.042 - 6.81

mag 3.108 3.279 3.778 4.187 3.677 3.446 4.239 3.226 4.151 3.59 3.136 3.864 3.197 2.642 5.061 2.715 3.58

ilm 2.999 3.409 3.859 2.902 3.662 3.618 4.739 3.848 4.351 4.173 3.174 3.531 3.043 2.45 3.267 2.934 3.51

ap 1.052 2.164 1.854 1.807 1.914 1.696 2.368 1.682 1.981 1.925 1.652 1.918 1.564 1.059 2.319 1.74 1.79

Mg# 57.111 58.22 53.699 56.353 55.099 54.039 50.681 55.795 58.869 50.475 54.393 54.953 54.521 52.11 58.388 61.502 55.39

Salic 76.913 72.465 65.909 62.708 64.484 73.307 64.682 70.099 59.106 67.272 74.562 68.617 75.927 84.286 54.666 75.544 69.41

Femic 21.489 24.207 29.203 33.295 29.607 23.534 30.729 26.078 36.029 26.63 21.814 26.836 21.691 14.104 40.396 21.662 26.71

DI 63.482 59.394 51.644 47.975 50.503 60.594 50.142 53.048 42.252 51.398 62.665 54.383 62.067 70.206 45.531 62.267 55.50

Fe 2

O 3 t = as total iron; LOI = loss on ignition; Molar A/CNK = Molar Al 2

O 3

/CaO+Na 2

O+K 2

O; Symbols for CIPW normative minerals are taken after Kretz (1983) and are

mentioned in small letters for weight norms (Mc Birney, 1984); Salic = sum of salic normative minerals; Femic = sum of femic normative minerals; DI = Differentiation Index;

Mg# = 100 Mg/(Mg + Fe), atomic; CIPW norms are calculated on an anhydrous 100% adjusted basis and Fe 2

O 3

/FeO ratio after Middlemost (1989), using the SINCLAS

computer program (Verma et al., 2002). X=Average.

Fig.6. Molar Al 2

O 3

/(CaO+Na 2

O+K 2

O) versus molar Al 2

O 3

/

(Na 2

O+K 2

O) diagram (Maniar and Piccolli, 1989),

showing metaluminous nature of microgranular enclaves

() and South Khasi granitoids (O).

The ME hosted in SKG are mesocratic to melanocratic,

fine to medium grained and porphyritic in nature, typically

showing igneous textures. The sharp, crenulate ME-SKG

contacts and also irregular shapes (rounded to ellipsoidal)

of ME have been thought to be originated by mingling of

hotter ME magma within cooler SKG melt, and therefore

all ME globules were emplaced before the SKG solidified

(Kumar et al. 2005a). The ME containing variable amounts

of mafic-felsic xenocrysts, most likely represent different

degrees of hybridized ME sampled from hybrid magma

zone, yet heterogeneous, formed by mixing of coeval mafic

(enclave) and felsic (SKG) magmas. Flinders and Clemens

(1996) argued that within any choosen volume of magma,

moving as a result of either mass flow or convection, a

certain portion will experience chaotic flow resulting in

elongation of less viscous ME globules whereas some

parts will experience non-chaotic flow, where the ME

JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010


352 SANTOSH KUMAR AND THEPFUVILIE PIERU

Table 5. Oxides of major elements (wt%) and CIPW normative minerals of representative south Khasi granitoids (SKG)

Sample SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG SKG X

No. 1 5 10 14 15 16 17 18 20 23 24 27 34 35 36 38 41 45 (n=18)

SiO 2

65.56 62.81 65.32 61.36 63.16 62.60 63.64 60.79 60.74 61.73 60.70 59.32 60.99 64.72 63.92 64.16 62.85 60.81 62.51

TiO 2

0.70 0.84 0.83 1.09 0.87 0.85 0.95 1.19 1.35 1.18 1.62 1.69 1.31 0.98 1.08 0.97 0.89 1.45 1.10

Al 2

O 3

13.84 14.57 13.69 14.09 14.95 15.26 14.11 14.42 14.05 14.7 13.54 13.86 14.26 14.79 14.08 14.73 14.26 14.40 14.31

t

Fe 2

O 3

4.94 4.87 4.86 6.66 4.89 4.80 5.43 6.86 7.31 5.99 7.54 8.36 7.08 5.39 6.51 4.62 5.62 6.64 6.02

MnO 0.07 0.07 0.06 0.08 0.06 0.07 0.07 0.08 0.09 0.07 0.08 0.09 0.09 0.08 0.10 0.06 0.07 0.08 0.08

MgO 1.61 3.19 2.10 3.14 1.98 2.05 2.35 3.11 3.31 2.65 3.45 3.63 3.23 3.26 3.20 2.09 2.65 3.64 2.81

CaO 2.19 3.23 2.54 3.35 2.94 3.65 3.19 3.86 4.38 3.68 4.40 4.57 4.48 2.95 2.83 3.37 3.39 4.50 3.53

Na 2

O 2.22 2.20 2.34 2.10 2.35 2.79 2.34 2.27 2.17 2.36 2.07 2.14 2.45 2.60 2.57 2.47 2.28 2.47 2.34

K 2

O 5.20 5.54 5.16 5.63 6.36 5.29 5.11 5.14 4.78 5.27 4.44 4.65 4.54 5.60 4.92 5.47 5.01 4.18 5.13

P 2

O 5

0.25 0.43 0.28 0.45 0.38 0.36 0.40 0.51 0.61 0.49 0.67 0.72 0.64 0.36 0.41 0.37 0.41 0.64 0.47

LOI 2.20 1.40 1.30 2.30 1.90 2.20 1.70 1.30 1.70 1.40 1.70 0.70 0.09 0.85 1.00 1.90 1.20 1.20 1.45

Table 98.78 99.15 98.48 100.25 99.84 99.92 99.29 99.53 100.49 99.52 100.2 99.7 99.2 101.6 100.6 100.2 98.6 100.0 99.74

Molar

A/CNK

1.00 0.88 0.87 0.93 0.88 0.83 0.93 0.88 0.82 0.82 0.81 0.88 0.82 0.94 0.96 0.88 0.93 0.88 0.89

CIPW NORM

qtz 25.34 17.25 23.36 15.50 15.98 15.66 20.17 14.62 15.53 16.21 17.17 13.94 15.15 16.471 18.724 18.92 19.12 15.50 17.479

or 31.85 33.52 31.40 33.98 38.41 32.02 30.97 30.95 28.62 31.76 26.67 27.77 27.10 32.976 29.317 32.91 30.41 25.03 30.870

ab 19.46 19.06 20.39 18.15 20.32 24.18 20.30 19.57 18.60 20.37 17.80 18.30 20.94 21.925 21.932 21.28 19.81 21.18 20.198

an 9.57 13.53 11.10 12.65 11.71 13.82 13.23 14.24 14.67 14.23 14.78 14.63 14.65 12.1 11.461 13.17 14.25 16.07 13.326

ne - - - - - - - - - - - - - - - - - - -

crn 1.22 0.11 0.31 - - - - - - - - - - - 0.364 - - - 0.501

di - - - 1.07 0.54 1.89 0.26 1.53 2.70 0.92 2.42 2.86 2.93 0.108 - 1.11 0.22 1.89 1.461

hy 8.51 11.51 8.76 12.20 8.09 7.59 9.62 12.72 12.82 10.16 13.62 14.07 12.28 11.318 12.224 7.61 10.72 13.56 10.966

ol - - - - - - - - - - - - - 2.415 2.953 - - - 2.684

mag 2.09 2.37 2.38 3.27 2.38 2.34 2.66 2.86 3.03 2.92 2.83 3.47 2.93 1.854 2.068 2.23 2.76 2.48 2.607

ilm 1.38 1.63 1.62 2.11 1.69 1.65 1.85 2.30 2.60 2.29 3.13 3.24 2.51 0.832 0.957 1.88 1.74 2.79 2.011

ap 0.60 1.02 0.67 1.07 0.90 0.86 0.95 1.21 1.43 1.16 1.58 1.69 1.50 63.465 58.54 0.87 0.98 1.50 7.778

Mg# 45.26 64.01 53.98 55.91 52.38 53.73 53.96 53.35 53.32 54.49 52.80 52.26 53.53 - 0.364 55.27 56.00 57.35 51.057

Salic 86.21 83.37 86.25 80.27 86.41 85.67 84.67 79.37 77.42 82.56 76.42 74.65 77.85 83.472 81.434 86.30 83.59 77.78 81.872

Femic 11.97 15.51 12.77 18.28 12.49 12.80 14.29 18.84 20.14 15.97 21.09 22.58 19.56 15.667 17.245 12.47 15.36 20.08 16.506

DI 76.65 69.82 75.15 67.63 74.70 71.86 71.43 65.14 62.75 68.34 61.64 60.02 63.20 71.372 69.973 73.11 69.34 61.72 68.547

Fe 2

O 3

t

= as total iron; LOI = loss on ignition; Molar A/CNK = Molar Al 2

O 3

/CaO+Na 2

O+K 2

O; Symbols for CIPW normative minerals are taken after Kretz (1983) and are

mentioned in small letters for weight norms (Mc Birney, 1984); Salic = sum of salic normative minerals; Femic = sum of femic normative minerals; DI = Differentiation Index;

Mg# = 100 Mg/(Mg + Fe), atomic; CIPW norms are calculated on an anhydrous 100% adjusted basis and Fe 2

O 3

/FeO

ratio after Middlemost (1989), using the SINCLAS computer program (Verma et al., 2002). X=Average.

0.3

F

Log CaO/(Na a

2 O+K 2

O)

0

0.3

A A - C C - A C

51

56

61

50 60 SiO

70

2

(wt%)

Tholeiitic

0.6

Calc-alkaline

Fig.7. Calc-alkaline (log CaO/Na 2

O+K 2

O) ratio versus Silica (SiO 2

wt. %) diagram showing alkali-calcic trend (broken line)

for south Khasi granitoids (O). The microgranular enclaves

() are widespread from alkali to alkali-calcic series. The

igneous series (A: alkali series 61) are shown after Brown et

al. (1984).

A

Fig.8. A (Na 2

O+K 2

O) - F(FeO t ) - M(MgO) triangular diagram

showing linear alkali enrichment trend for both

microgranular enclaves (Ï%) and south Khasi granitoids

(O). The boundary line separating tholeiitic and calcalkaline

fields is taken after Irvine and Baragar (1971).

M

JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010


PETROGRAPHY AND MAJOR ELEMENTS GEOCHEMISTRY OF MICROGRANULAR ENCLAVES, MEGHALAYA 353

8

15

4

9

0

1.5

(a)

3

3

(b)

.75

1.5

0

(c)

0

(d)

.3

8

.15

4

0

(e)

(f)

5

18

2.5

14

(g)

0

40 55 70

SiO (wt%)

Fig.9. (a-h) Harkers’ variation diagrams as SiO 2

versus major oxides plotted for microgranular enclaves () and south Khasi granitoids

(O).

JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010

(h)

40 55 70

SiO (wt%)


354 SANTOSH KUMAR AND THEPFUVILIE PIERU

globules are much less affected. The processes of mingling

and mixing thus correspond to isolated mixing region (IMR)

and active mixing region (AMR) respectively (e.g. Perugini

et al. 2003). It is therefore probable that the ME in

crystallizing granite system most likely have experienced

only limited mingling, as observed in the case of SKG

pluton.

Petrographic Features of Mixing, ME Mingling and

Undercooling

Microscopic features such as presence of xenophenocrysts,

ocellar quartz, fine grained texture, occasional

chilled margin, coarsening of grains inward without grain

deformation and recrystallization, and presence of accicular

(needle-shaped) apatite in ME are strong evidences of

mixing, quenching and mingling processes. Ocellar quartz

and patchy zoned plagioclase in ME strongly favour the

state of disequilibration after mechanical transfer of crystals

from felsic to hybrid (ME) magma zone (e.g. Barbarin,

1990b; Hibbard, 1981, 1991). The growth of mafics

(hornblende and biotite) over quartz ocelli most probably

resulted from marginal dissolution of the unstable quartz in

hybrid magma, and armouring by stable minerals during

undercooling of ME magma (e.g. Reid et al. 1983; Vernon,

1983, 1984, 1990; Pesquera and Pons, 1989). Fine grained

(chilled) margin and relatively coarser interior of ME have

been formed as a result of variations in nucleation and growth

rate of crystals along thermal gradient with increased

undercooling of ME liquid (Vernon, 1983; Kumar, 1995).

Rapid temperature equilibration between ME globules of

relatively smaller size and host SKG melt may, however,

lead to uniform undercooling of ME and therefore, finegrained

textures are uniformly developed. Accicular

apatite should have been rapidly crystallized in ME

magma (Wyllie et al. 1962) saturated in P 2

O 5

content

(Kumar, 1995). Since the accicular apatites are found

mostly as poikilitic inclusions in early crystallized

hornblende and biotite, and therefore must represent the

primary liquidus phase, not the restite phase as argued

elsewhere (Chappell et al. 1987; Chen et al. 1989).

Modal Mineral Disequilibrium

The modal mineral disequilibrium between ME and SKG

is reflected in their colour indices, but the observed modal

quartz equilibration is considered a feature resulted by

binary (mafic-felsic) magma mixture (e.g. Van Der Laan

and Wyllie, 1993). The ME are more common between

Jashiar and Mawthaphdah regions, where the colour index

(mesocratic) of ME and SKG becomes more-or-less similar,

but their grain size still differs. This region of SKG pluton

appears most hybridized parts compared to other regions.

Some crystals are either incorporated into the ME or aligned

along the ME-SKG interface depending upon their prismatic

and tabular habits during ME mingling event (e.g. Perugini

et al. 2003).

Igneous Series and Nature of Protoliths

In terms of K 2

O and SiO 2

(Fig. 10), the ME and SKG

are recognized as shoshonite (Peccerillo and Taylor, 1976),

the term originally coined by Iddings (1892) for orthoclasebearing

basalt from Yellowstone Park, Wyoming. Further

Joplin (1965) in Jiang et al. (2002) described shoshonite

magma series for a suite of basaltic to trachytic rocks that

are potassic-rich equivalent of alkali basalt magma series.

Liegeois et al. (1998) observed that shoshonitic magma is

assumed to represent transitional stage between calc-alkaline

(I-type) and alkaline (A-type) magmas formed during lateto

post-collisional environment. Chemical variability of

shoshonite is poorly understood in geodynamic sense and

true source characterization (Lôpez-Moro and Lôpez-Plaza,

2004). However, shoshonite magma can be formed by

combined processes of fractional crystallization and magma

mixing of contrasted magma types.

The ME in SKG are chemically modified, and hence

cannot be utilized for protolith evaluation of mafic (enclave)

magma. The K 2

O content of SKG, to a certain extent, can

be used to infer the nature of protolith involved in the

generation of felsic magma. Comparing with the

experimental melt compositions derived from various crustal

Fig.10. K 2

O versus SiO 2

plot for microgranular enclaves () and

south Khasi granitoids (O) pairs. The microgranular

enclaves and respective host south Khasi granitoids are

joined by tie lines. See text for discussion.

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PETROGRAPHY AND MAJOR ELEMENTS GEOCHEMISTRY OF MICROGRANULAR ENCLAVES, MEGHALAYA 355

sources (compilation of Roberts and Clemens, 1993), the

most likely source materials for SKG melt appear hydrated,

high-K calc-alkaline andesite and basaltic andesite (lower

crustal rocks), which may evolve into shoshonite field

(Fig. 10). The inferred source materials could be derived

from enriched subcontinental lithospheric mantle, and are

broadly consistent with the optimism of Selvam et al. (1995)

that the felsic magmas (ca 480-760 Ma) of Meghalaya can

be derived from partial melting of lower crustal rocks. The

ME in SKG are not the restite of source region because

they lack partial melting texture as described elsewhere

(Tindle and Pearce, 1983), and moreover surface expression

of source terrain is not an essential feature of felsic plutons

(e.g. Roberts and Clemens, 1993). It is likely that voluminous

andesite and basaltic andesite source rocks at deeper level

existed, below which hot mafic magma underplated and

initiated the melting events.

Evidence of K 2

O Diffusion

Most ME-SKG tie-lines on K 2

O vs SiO 2

(Fig. 10) show

a positive slope because K 2

O content of ME is relatively

lower at low SiO 2

values as compared to respective SKG

suggesting low degree of K 2

O diffusion between them. Few

ME-SKG tie-lines show negative slope because of higher

K 2

O content in ME than their respective SKG, which are

strong indicative of K 2

O migration from felsic to mafic

magma during ME mingling. Van Der Laan et al. (1994)

have estimated the diffusion co-efficient for K 2

O as 2x10 -7

cm 2 sec -1 which is capable to explain rapid K 2

O migration

from SKG to ME within a shorter time, i.e. before the ME

undercooled, and the degree of K-diffusion may be related

with size of ME (e.g. Kumar et al. 2004b; Kumar and Rino,

2006). However, K-feldspar megacrysts mechanically

transferred from host SKG to ME during mixing and

mingling may have partly induced the high K 2

O content of

ME.

Geochemical Variations as Evidence of Magma-mixing

The observed linear to sub-linear variations of major

oxides against SiO 2

for ME and SKG most likely suggest

that the various ME in SKG are formed by mixing of mafic

and felsic magmas in various proportions (Kumar and

Rino, 2006). However, the same variation trends in many

other granitoid suites have been thought to be originated

either by varying degrees of restite-melt separation (e.g.

Chappell et al. 1987; Chappell and White, 1991) or crystal

fractionation of host felsic magma (e.g. Dodge and Kistler,

1990). It has been further argued that the linear elemental

variation has resulted due to heterogeneous distribution of

mafic minerals and greater abundance of enclaves in more

mafic member of suites (White et al. 2001), which favour

the restitic origin for most enclave types hosted in granitoids

(White et al. 1999). If so, then enclaves must not show

magmatic textures anyway (Clemens, 2003), who further

emphasized that there is still no whisper from the enclaves

regarding the dominant source material. Since field,

textural and other geochemical evidences support the view

of magma-mixing and mingling as dominant processes in

the evolution of ME in SKG, therefore the observed linear

variation for ME can be explained in favour of magmamixing.

However, a low degree of fractionation within the

SKG magma cannot be ruled out. Some of the noted data

scatter in ME and SKG for K 2

O, Na 2

O, Al 2

O 3

and to a certain

extent CaO may be due to combined effect of diffusion

and mineral sorting during magma-mixing and mingling

events. Experiments predict that volatiles and some other

mobile, large-ion lithophile elements (LILE) may also

migrate towards ME during magma mingling events

(Watson and Jurewicz, 1984; Johnson and Wyllie, 1988;

Baker, 1990 in Tate et al. 1997). The AFM variation trend

of ME and SKG resembles as high-K, calc-alkaline suite

(e.g. Janoušek et al. 2000), which most likely generated by

the process of felsic-mafic magma mixing. The observed

compositional overlap of ME and SKG suggests that

not only the ME, but a few SKG may also be of hybrid in

nature.

Composition of Pristine Mafic (enclave) Magma

Fe 2

O 3 t vs MgO variations of ME and SKG can recognize

the most mafic composition of ME (Fig. 11). The ME24

(Fe 2

O 3 t =12.76 wt%, MgO=7.26 wt%, SiO 2

=47.05 wt%) can

be considered parental to several ME, equivalent to highiron

tholeiite (Jensen, 1976) in terms of (FeO+TiO 2

)-Al 2

O 3

-

MgO (Fig. 12). However, pristine mafic (enclave) magma

may be hidden below the hybrid (ME) magma zone.

Geotectonic Setting

World-wide there seems to be a common temporal and

spatial association between major granite plutonism and

crustal extension (Clemens, 2003), which should be based

on broad regional geology rather than focussing on

geochemical and isotopic features of granitoid in isolation

(e.g. Roberts et al. 2000). Nevertheless, tectonic segments

of Batchelor and Bowden (1985) in multiple cationic R 1

-R 2

parameters (De La Roche et al., 1980) suggest the nature of

most SKG similar to post-collisional uplift tectonic setting

(Fig. 13) consistent with geological observations (e.g.

Sengupta and Agarwal, 1998; Harijan et al. 2003). Since

the ME bear modified compositions and hence cannot be

utilized for tectonic inferences.

JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010


t

R 2 = 6Ca+2Mg+Al

356 SANTOSH KUMAR AND THEPFUVILIE PIERU

14

2500

1 - Mantle Fractionate

2 - Pre-plate Collision

12

2000

1500

3 - Post-collision Uplift

4 - Late-orogenic

5 - Anorogenic

6 - Syn-collision

7 -Post-orogenic

1

2

1000

3

10

4

t

Fe 2 O 3

8

500

5

0

0 500 1000 1500 2000 2500 3000

R

1

= 4Si - 11(Na + K) - 2(Fe+Ti)

6

7

6

4

2

FeO

2 4 6 8

MgO (wt%)

MgO

Fig.11. Fe 2

O 3

t

versus MgO diagram showing evolutionary trend

of microgranular enclave () and South Khasi granitoid

(O). Inset figure (FeO t vs MgO) is taken from Zorpi et al.

(1989). I (dashed arrows) – hybridization trend of enclaves;

II (solid arrow) – tholeiitic trend of more mafic (less

hybridized) enclaves; Fields shown by solid lines represent

host granitoid composition.

Fig.12. Al 2

O 3

- (FeO t +TiO 2

) - MgO ternary plot showing the

evolutionary trend of microgranular enclaves () and south

Khasi granitoids (O). The various fields are taken from

Jensen (1976). CD – calc-alkaline dacite, CA – calc-alkaline

andesite, CB – calc-alkaline basalt, HFT – high-Fe tholeiitic

basalt, HMT – high-Mg tholeiitic basalt.

12

8

4

II

I

2 4 6 8

Fig.13. R 1

-R 2

diagram (after De La Roche et al. 1980) showing

various tectonic fields (after Batchelor and Bowden, 1985),

used to discriminate the tectonic setting of south Khasi

granitoids (O) only. Microgranular enclaves () are also

shown and joined by tie-lines with their respective host

granitoids. See text for discussion.

Magma-Mixing Test

In order to understand what fraction of mafic component

in the mixture can form the hybrid ME samples, three

representative ME samples (ME15, ME10, ME38) with low,

moderate and high SiO 2

contents respectively were chosen

to test the mixing calculations using the mixing equations

(Fourcade and Allçgre, 1981); according to which the

content of an element i in a mixture CiM follows the

equation:

CiM-CiA = X(CiB-CiA) (1)

where, CiA is the content of i in the felsic component A (i.e.

taken as average of SKG); CiB is the content of i in the

mafic component B (i.e. ME24); X is the fraction of mafic

component in the mixture (i.e. in hybrid ME samples).

Graphical representation of mixing parameters provide

slopes (m) of 0.59, 0.41 and 0.06 for hybrid ME15, ME10

and ME38 respectively (Figs. 14a, b, c), which represent

the estimated proportions of mafic component in the mixture

(hybrid). To a certain extent, the estimated proportions are

consistent with the observed mafic and felsic xenocrysts

present in these ME samples. The elements such as Al 2

O 3

,

Na 2

O and K 2

O plotting off the trend lines might have been

either less affected during hybridization or behaved

erratically during mixing and mingling events occurred in

plutonic setting.

Genetic Model of ME Evolution in SKG

Field-petrography and major elements geochemical

evidences suggest that felsic magma that crystallized as

coarse SKG was having some initial crystallinity at the time

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PETROGRAPHY AND MAJOR ELEMENTS GEOCHEMISTRY OF MICROGRANULAR ENCLAVES, MEGHALAYA 357

(a) Ci mME15 -Ci A av.SKG 6

Fe O

4

MnO

2

CaO

CiBME24 - Ci

MgO

A av.SKG Na 2 O

SiO

2

-4

-8

-12

m=1

Fe O 2 3

TiO 2

-16 -12 -8 -4 Al 2

O 3 P 2 O 5

4 8

K 2

O

(b)

Ci BME24 - Ci A av.SKG

-16

SiO 2

m=0.59

-12

Ci mME10 -Ci A av.SKG 6

4

m=0.41

-8

-4

Al O 2 3 P

2 2 O 5

Na 2 O

CaO MgO

4 8

MnO TiO2

K O 2

-4

-8

m= =1

Fe O 2 3

produced convective overturn (Huppert et al. 1984) or other

dynamic forces such as shear forces. For obtaining thermal

equilibration of a mixture of magmas depends upon initial

temperature, heat capacity, heat of fusion, change in

crystallinity and mass fraction of each magma participating

in mixing (Sparks and Marshall, 1986). The ME as globules

may get separated from heterogeneous hybrid magma

through convection current and were incorporated in the

relatively cooler crystallizing granite magma (e.g. Dorais et

al. 1990; Neves and Vauchez, 1995). The large ME may

eventually disaggregate into smaller ME (few centimeters

to 30 centimeters across) and further elongation of ME due

to shear forces associated with viscosity contrast and relative

velocity (flowage) of interacting melts. The crystal contents,

viscosity and yield strength of felsic host (Furman and Spera,

1985 in Blundy and Sparks, 1992) prevent the ME from

settling out over the time-scale required for complete

solidifications. Transfer of heat and volatile from mafic

magma to felsic not only prompted remobilizations leading

to formation of ME swarms (e.g. Blundy and Sparks, 1992)

but also acted as carrier to mobile elements, particularly the

alkalies.

(c) Ci mME38 -Ci A av.SKG

6

4

Al 2 O 3

2

Ci BME24 - Ci A av.SKG

-16

SiO 2

-12

m=0.06

-8

MnO

K 2

O

TiO 2 CaO -4

Na O P O 4 8

2 2 5 MgO Fe 2 O 3

Fig.14. Magma mixing plots (Cim-CiA versus CiB-CiA) using

major elements for ME15 (a), ME10 (b) and ME38 (c).

Slopes (m) of line represents fraction of mafic component

in hybrid ME samples. Mixing was not possible in the

shaded regions (after Fourcade and Allégre, 1981). See

text for detailed discussion.

it interacted with mafic (enclave) magma. Some of the maficfelsic

minerals were therefore mechanically mixed and

formed hybrid (enclave) magma zone, after attainment of

thermal equilibration and minimal rheological contrast

between mafic and felsic magmas, which subsequently

-4

-8

m= =1

CONCLUSIONS

Petrographic features of ME in SKG indicate coeval

nature of mafic and felsic magmas, which upon mixing

produced hybrid (ME) zone in a deep-seated environment.

Linear to sub-linear geochemical variations and mixing test

suggest that the ME are mixed product of mafic and felsic

magmas in various proportions. There was mingling of

hybrid (ME) globules within a relatively cooler host SKG.

Diffusion of alkalies (Na 2

O, K 2

O) from felsic to mafic

components has occurred during mafic-felsic magma mixing

and hybrid (ME) magma mingling prior to complete

solidification of ME-SKG system.

Acknowledgements: Financial assistance from DST,

New Delhi (ESS/23/VES/046/98) supported this work. N.

K. Saini, Wadia Institute of Himalayan Geology, Dehradun

is thanked for extending helps during XRF analysis.

Generous comments and suggestions provided by an

anonymous reviewer greatly improved the earlier version

of paper.

JOUR.GEOL.SOC.INDIA, VOL.76,OCT.2010


358 SANTOSH KUMAR AND THEPFUVILIE PIERU

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