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AUDEN, J. B. (1949) Dykes in Western India-A discussion of their relationship with the Deccan Traps. Trans. Nat. Inst. Sc. India, v.III(3), pp.123-157. BACHINSKI ...

INDIAN DYKES: Geochemistry, Geophysics and Geochronology Editors: Rajesh K. Srivastava, Ch. Sivaji and N. V. Chalapathi Rao © 2008, Narosa Publishing House Pvt. Ltd., New Delhi, India

Compositional Variation of Micas From the Lamprophyre Dykes of Bakhatgarh - Phulmal Area, Jhabua District, M. P., India K. R. RANDIVE* Post-graduate Department of Geology, RTM Nagpur University, Nagpur 440 001, India

Abstract Micas occur as phenocrysts as well as in the groundmass of the lamprophyre dykes of Bakhatgarh – Phulmal and surrounding areas. These micas were analysed using electron probe micro-analyser, and it is observed that they are compositionally zoned with the cores being rich in MgO, NiO and Cr2O3, compared to the rims, which are rich in FeO and TiO2. However, there is no significant variation in SiO2 and Mg# within the phenocrysts. Analyses of micas plotted in TiO2 vs. Al2O3 and FeOT vs. TiO2 diagrams fall within the lamprophyre field, and comply well with the lamprophyres world over. The fractionation trend, which is from monchiquite to minette, suggests evolution of magma from alkaline to calc-alkaline varieties. Keywords: Lamprophyre, Phlogopite, Bakhatgarh - Phulmal, Fractionation, Zoning.

Introduction The Bakhatgarh – Phulmal sector (Lat. 21°55’-22°10’ and Long. 74°05’-74°10’) covering an area of about 200 km2 (Fig. 1), exposes a dyke swarm in the lower reaches of the Narmada valley (Blanford, 1869; Bose, 1884; Auden, 1949; Narain, 1978, 1985; Gwalani et al., 1993). These rocks particularly lamprophyres, has been a subject of research in the past decade (Chawade, 1996; Hari, 1998; Randive, 2005a). The dykes occurring in this area are grouped on the basis of their composition into four types viz. tholeiitic, picrobasalt, lamprophyre and calcareosiliceous. The general trend of these dykes is E-W, but locally swerves around NESW and ENE-WSW. They occur in several outcrops of limited lateral extent; however, majority of the linear outcrops tend to represent the same individual dykes that are manifested in stretches along the prevailing structural grain. They vary in width from 1 meter to 50 meters and from 50 meters to 3 kilometers in length; but traced up to 12 km if collectively measured (Randive, 2005a). Nine lamprophyre dykes were traced in the area, ____________________ *e-mail: [email protected]

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which are confined to north, NW and NE parts near Biswani, Kundwat and Undri. Some of these dykes show effects of Fenitisation (Randive, 2005b).

Figure 1: Geological map of the Bakhatgarh – Phulmal area showing intrusive dykes (dark), host lava flows and basement rocks (after Randive, 2005a).

Petrography The lamprophyres of this area are grouped into calc-alkaline and alkaline; each of these are represented by two varieties each namely, minette and kersantite (calc-alkaline) and

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camptonite and monchiquite (alkaline). Although, detail information on petrography of lamprophyres is available in Randive (2005a) and Randive et al. (2005), a concise description of micas is given here. Dark colored, melanocratic, basaltic rocks having distinct phenocrysts of mica can be easily identified as lamprophyres in the field. However, those varieties that contains mica only in the groundmass needs to be checked under the microscope. Mica phenocrysts encloses crystals of other minerals such as spinels, pyroxenes and other inclusions; and show considerable variation in size and abundance. Overall, the phenocrysts are idiomorphic, cleaved and strongly pleochroic; they often exhibit weak zoning. However, a distinctly birefringent, pale-yellow to brown colored zone is developed towards the rim, which is accentuated by presence of opaque minerals. High power microscopic examination of the rim shows innumerable pleaochroic needles of phlogopite or kaersutite along with spinel and glass. In some varieties these phenocrysts acquired appreciable roundedness. Groundmass micas can be confused with the pale-yellow colored glassy material, particularly in minette. Micas also occur as plates or flakes or needles in the groundmass. Sometimes these needles surround the early-formed phenocrysts (olivine and pyroxene) or occur as inclusions within them.

Chemical Composition Micas were analysed using Electron Probe Micro Analyser JEOL JA-8600 MX Superprobe (WDS) at the Mineral Physics Division, GSI, Hyderabad. At the accelerating voltage of 15 KV and probe current of 1 x 10-8 Amperes. The counting time was 10 seconds, whereas background-counting time was 5 seconds. Probe diameter was 1 micron and the correction procedure adopted is Bence and Albee (BAA) and the standards used were natural and synthetic compounds supplied by M/S/JEOL Ltd., Japan and ASTIMAX Australia. Micas from alkaline (monchiquite, KR/55) and from calc-alkaline (minette, KR/73) lamprophyres were selected for analysis. The analyses are given in Tables 1 and 2. Total 18 numbers of analyses includes those of phenocrysts (core Æ rim) and the groundmass. The cations were calculated on the basis of 22-oxygen. It is observed that micas from monchiquite are high in SiO2, Cr2O3, MgO, Na2O, K2O and NiO compared to minette, which are relatively high in TiO2, FeOT, MnO and CaO. Mg# of monchiquite mica is higher than minette mica, suggesting primitive nature of the former. When these analyses are plotted in Mg-Al-FeT triangular diagram of Mitchell (1995), majority of the analyses plot in phlogopite field, whereas rims of some mica phenocrysts from both the varieties plot in siderophillite field (Fig. 2a). It is also observed that TiO2 invariably increases from core to rim. The groundmass micas possess higher content of TiO2, which is similar to the rims of large phenocrysts. It is interesting to note that the TiO2 content of a rim of mica (MiC) in monchiquite is as high as 9.24 wt% (Table 1). Five crystals were analysed to know their compositional variation from core to rim including 3 from monchiquite (MiA, MiB, MiC) and 2 from minette (MiD and MiE). It is observed that the micas exhibit compositional zoning in various chemical components, Crystals MiC and MiB records maximum variations and thereby, signify the fluctuations in magma reservoir. There is a general decrease in MgO and Ni, whereas increase in FeO and

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TiO2. Na2O decreases from core to rim in minette, but do not follow a particular pattern in monchiquite. Similarly, Fe# remains almost constant for minette, but increases for monchiquite from core to rim.

Figure 2: (a) Diagram showing scatter of data points of the micas from Bakhatgarh – Phulmal lamprophyres in phlogopite and siderophillite fields, arrow indicates possible fractionation trend. (b) Enlarged view (Mg60Al60Fe2+30) shows variation from Core and Rim in mica phenocrysts of monchiquite (MiA) and minette (MiD).

Evolutionary Trends Mitchell and Bergman (1991) have summarised the compositional variation of minette micas and shown that the characteristic evolutionary trend is one of increasing Fe with slightly increasing Al, whereas the ultramafic lamprophyres exhibit an extremely wide range in composition with respect to their Al, Fe, Ti and Ba contents (Rock, 1986, 1990), but as such no characteristic trends were identified (Mitchell, 1995). There is paucity of data available on the evolutionary trends of micas from alkaline lamprophyres.

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Table 1: Chemical analyses of mica phenocrysts from monchiquite (KR/55). Crystal

MiA

Position

Core

SiO2 TiO2 Al2O3 Cr2O3 FeOt MnO MgO CaO Na2O K2O NiO

35.96 5.39 16.04 0.25 8.42 0.02 18.45 0.03 0.65 9.72 0.13

Total

MiB Rim

Core

36.64 5.38 16.13 0.14 7.37 0.00 19.03 0.09 0.57 9.8o 0.09

36.75 5.29 15.89 0.16 7.66 0.08 19.18 0.04 0.50 9.59 0.06

35.74 5.72 16.43 0.07 9.77 0.04 17.84 0.06 0.75 9.87 0.00

35.95 5.73 16.27 0.04 9.01 0.00 18.20 0.06 0.60 10.02 0.05

95.06

95.24

95.2

96.29

95.93

Si Ti Al Cr Fe2+ Mn Mg Ca Na K Ni

5.271 0.594 2.771 0.015 1.033 0.003 4.033 0.005 0.046 0.455 0.015

5.321 0.588 2.760 0.008 0.896 0.000 4.121 0.014 0.040 0.454 0.011

5.34 0.578 2.721 0.009 0.932 0.010 4.156 0.006 0.035 0.445 0.007

5.205 0.627 2.820 0.004 1.191 0.005 3.874 0.009 0.053 0.459 0.000

Total

14.241

14.213

14.239

14.247

MiC Rim

Core

35.36 6.16 16.41 0.06 10.63 0.15 17.4 0.05 0.72 9.63 0.05

35.35 6.29 16.02 0.00 10.65 0.00 16.97 0.08 0.74 9.63 0.00

35.85 5.57 16.55 0.46 8.30 0.13 19.12 0.02 0.46 8.43 0.05

36.22 5.97 16.33 0.32 7.96 0.11 18.98 0.05 0.47 9.02 0.10

34.51 9.24 16.25 0.00 13.27 0.19 13.67 0.14 0.76 7.64 0.00

96.62

95.73

94.94

95.53

95.67

Based on 22 oxygens 5.236 5.154 0.628 0.675 2.792 2.819 0.002 0.004 1.098 1.297 0.000 0.019 3.953 3.782 0.009 0.008 0.042 0.051 0.466 0.448 0.006 0.006

5.197 0.696 2.775 0.000 1.311 0.000 3.721 0.013 0.053 0.452 0.000

5.221 0.610 2.840 0.027 1.012 0.016 4.153 0.003 0.033 0.392 0.006

5.246 0.651 2.787 0.018 0.965 0.014 4.100 0.008 0.033 0.417 0.012

5.093 1.026 2.826 0.000 1.639 0.024 3.009 0.022 0.054 0.360 0.000

14.218

14.313

14.251

14.053

14.232

14.263

Rim

Figure 3: (a) Monchiquite micas showing variation of titanium against Fe#, arrow joins core to rim of the phenocryst. (b) Minette micas from groundmasss (open stars) and phenocrysts (solid stars), arrow joins core to rim.

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Table 2: Chemical analyses of mica from phenocrysts and groundmass of minette (KR/73). Crystal

MiD

Position

Core

SiO2 TiO2 Al2O3

MiE Groundmass

Rim

Core

Rim

33.34

35.3

34.23

32.77

29.93

32.87

5.94

6.00

6.94

6.93

6.40

7.28

7.53

16.27

16.06

18.09

14.6

14.47

15.88

16.57

34.67

Cr2O3

0.01

0.01

0.00

0.00

0.02

0.00

0.00

FeOt

10.27

10.39

10.11

13.64

8.68

11.19

9.72

MnO

0.05

0.08

0.17

0.20

0.16

0.14

0.02

MgO

16.78

17.27

14.87

11.61

14.46

14.37

16.47

CaO

0.06

0.07

0.17

4.76

4.51

0.72

0.85

Na2O

0.71

0.67

0.53

0.50

0.44

0.48

0.48

K2O

9.36

9.49

8.21

8.19

7.56

8.21

9.01

NiO

0.00

0.07

0.04

0.02

0.00

0.02

0.00

Total

92.79

95.41

93.36

93.22

86.63

91.16

95.32

Si

5.067

5.199

5.101

5.095

4.909

5.087

5.092

Ti

0.679

0.665

0.778

0.811

0.790

0.847

0.832

Al

2.914

2.787

3.177

2.675

2.797

2.896

2.868

Cr

0.001

0.001

0.000

0.000

0.001

0.000

0.000

Fe2+

1.306

1.281

1.261

1.775

1.192

1.449

1.195

Mn

0.006

0.010

0.022

0.026

0.022

0.018

0.003

Based on 22 oxygens

Mg

3.803

3.793

3.305

2.692

3.537

3.316

3.607

Ca

0.010

0.011

0.027

0.793

0.793

0.119

0.134

Na

0.052

0.048

0.038

0.038

0.035

0.036

0.034

K

0.454

0.446

0.390

0.406

0.396

0.405

0.422

Ni Total

0.000

0.008

0.005

0.003

0.000

0.003

0.000

14.292

14.249

14.104

14.314

14.472

14.176

14.187

Figure 4: Evolutionary trends in lamprophyres (open triangles – monchiquite, solid triangles – minette) as seen from variation of fractionation index (Fe#) against (a) titanium and (b) tetrahedral aluminium. Arrows in both the diagrams shows variation from core to rim.

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Figure 2b shows the variation of data points from core to rim in the mica phenocrysts of lamprophyres in the Mg-Al-FeT+2 diagram. Monchiquite (MiC, MiD) and minette (MiE) display a trend of increasing Fe content. This trend is similar to the evolutionary trend in minettes, which is from phlogopite towards titanian aluminous biotite and represents solid solution between phlogopite-biotite and ‘estonite’-siderophyllite molecules (Mitchell, 1995). Figures 3 (a) and (b) shows variation of titanium against fractionation index Fe# (Fe/Fe+Mg), in the monchiquite (a) and minette (b) these variations in both rock types are between groundmass and phenocryst micas. It is interesting to note that the phenocryst micas are more fractionated compared to the groundmass micas (more clearly in Fig. 3a). Another important evolutionary trend emerges after plotting micas from monchiquite and minette; Figures 4 (a and b), which shows that monchiquite is more primitive as compared to minette. However, the rims of phenocrysts in both the rocks fall away from main cluster indicating simultaneous differentiation of magma from monchiquite to minette with fractionation from core to rim.

Figure 5: TiO2 vs Al2O3 plot for micas of lamprophyres from Bakhatgarh – Phulmal area, diagram of Mitchell (1995).

Discussion The Mg/Fe ratio for phlogopites shows a wide variation from core to rim and indicate that the magma have undergone fractional crystallisation. The cores having high Mg/Fe and Ni, and low TiO2 compared to the rims, is a characteristic feature of these micas. This trend is common to micas in the lamprophyres worldwide and has been reported from the diverse localities such as Arizona, USA (Jones and Smith, 1983), Cornwall, UK (Hall, 1982), Devon, UK (Jones and Smith, 1985), Shaw’s Cove, Canada (Bachinski and Simpson, 1984), Bohemia, Czech republic (Schulze et al., 1985), Cross Fell and Taythes (Meyer et al., 1994),

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etc. The compositional trend of micas depicted for lamprophyres world over, is of almost constant Al2O3 with increasing FeOT and TiO2, and is also observed for the BakhatgarhPhulmal lamprophyres. Mitchell (1995) suggested evolutionary trends for the Kimberlite and related rocks based on TiO2 vs. Al2O3 and FeOT vs. TiO2 diagrams. When the data from Bakhatgarh-Phulmal micas were plotted in these diagrams, they comply well with the trend suggested for minettes and alnoites (Figs. 5 and 6). It is observed that the fractionation trend, which is from monchiquite to minette, suggests evolution of magma from alkaline to calcalkaline lamprophyres.

Figure 6: FeOT vs Al2O3 variation observerd micas of Bakhatgarh – Phulmal lamprophyres showing fractionation trends of related rock types. It can be observed that minette (solid triangles) are more fractionated compared to monchiquite (open triangles), diagram after Mitchell (1995).

Acknowledgements I thank the Dy. Director General, Geological Survey of India, Southern region, Hyderabad for permitting me to carryout chemical analyses of micas and other mineral phases using EPMA. I am also thankful to Mr. A. Anil Kumar, Mr. G. J. S. Prasad and R. Rama Rao for the help rendered during analyses, to Mr. Parijat Roy for supply of reprints, to Shri. P. B. Sarolkar and anonymous referees for critical comments, to Prof. Rajesh Srivastava for inviting me to contribute a paper for the Silver Jubilee volume of IAG, and to wife Chetana for all the help and support during preparation of this paper.

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BLANFORD, W. T. (1869) On the geology of the Taptee and Lower Narmada valleys and some adjoining districts. Geol. Surv. India Mem. 6, pp.163-384. BOSE, P. N. (1884) Geology of the lower Narmada valley between Nimawar and Kawant. Geol. Surv. India Mem. 21(1), pp.1-72. CHAWADE, M. P. (1996) The petrology and geochemistry of dykes in Deccan basalts in part of Lower Narmada Valley, around Chhaktala, Jhabua district, M.P. In: Deshmukh, S.S. and Nair, K.K.K. (Eds.) Deccan Basalts, Gond. Geol. Mag. Spl. Vol. 2, pp.185-200. GWALANI, L. G., ROCK, N. M. S., CHANG, W-J, FERNANDEZ, S., ALLEGRE, C-J. and PRINZHOFER, A. (1993) Alkaline rocks and carbonatites of Amba Dongar and adjacent areas, Deccan Igneous Province, Gujarat, India. 1. Geology, petrography and petrochemistry. Mineral. Petrol., v.47, pp.219-253. HALL, A. (1982) The Pendennis peralkaline minette. Miner. Mag., v.45, pp.257-266. HARI, K. R. (1998) Mineralogical and petrochemical studies of the lamprophyres around Chhaktalao area, Madhya Pradesh. Jour. Geol. Soc. India., v.51, pp.21-30. JONES, A. P. and SMITH, J. V. (1983) Petrological significance of mineral chemistry in the Agathia Peak and the Thumb Minettes, Navajo volcanic field. Jour. Geol., v.91, pp.643-656. JONES, A. P. and SMITH, J. V. (1985) Phlogopites and associated minerals from pwermian minettes in Devon, South England. Bull. Geol. Soc. Finland, v. 57, pp. 89 – 102. MEYER, H. O. A., MITCHELL, R. H. and JAYAGANAPATHY, S. (1994) Phlogopite in the calc-alkaline lamprophyres of Northern England. Mineral. Petrol., v.51, pp.227-237. MITCHELL, R. H. (1995) Kimberlites, Orengeites and related rocks. Plenum Press, New York, 410p. MITCHELL, R. H. and BERGMAN, S. C. (1991) Petrology of lamproites. Plenum Press, New York, 440p. NARAIN, AHILESH (1978) Acid differentiates in Deccan Traps of Bakhatgarh region, Jhabua district, M.P., Indian Mineral., v.32(1), pp.71-72. NARAIN, AKHILESH (1985) Petrology and geochemistry of Deccan Trap and associated intrusives of Bakhatgarh region in the Narmada rift zone, Alirajpur, Jhabua district, Madhya Pradesh. Rec. Geol. Surv. India, v.114(6), pp.31-44. RANDIVE, K. R. (2005a) Intrusive dykes of Chhaktala, Jhabua district, Madhya Pradesh: Field relations, petrography and petrochemical characteristics with reference to their PGE concentration. Jour. Econ. Geol Georesource. Manag., v.2(1-2), pp.117-137. RANDIVE, K. R. (2005b) Fenitization of the rocks of rocks of Bakhatgarh-Phulmal area, Deccan Igneous Province, Jhabua district, Madhya Pradesh: A preliminary investigation. Gond. Geol. Mag., v.20(2), pp.147-154. RANDIVE K. R., PRASAD, G. J. S., ANIL KUMAR, A., RAMA RAO, R., BHASKARA RAO, Y. J. and DAYAL, A. M. (2005) Picrobasalt and lamprophyre dyke swarm of Bakhatgarh-Phulmal area, Chhota Udaipur alkaline complex, Deccan Igneous Province. Indian Jour. Geochem., v.20, pp.191-212. ROCK, N. M. S. (1986) The nature and origin of lamprophyres: Alnoites and allied rocks (ultramafic lamprophyres). Jour. Petrol., v.27, pp.193-227. ROCK, N. M. S. (1990) Lamprophyres. Blackie and Son, Edinburgh, 256p. SCHULZE, D. J., SMITH, J. V. and NEMEC, D. (1985) Mica chemistry of lamprophyres from the Bohemian Massif, Czechoslovakia. Neus. Jaharbuch. Miner. Abh., v.152, pp.321-334.

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