Ultrapotassic Rocks along Late Ductile Shear Zones from the Eastern Ghats Belt, India

Gondwana Research, Y 3, No. I, pp. 55-63. 02000 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Ultrapotassic Rocks along Late Ductile Shear Zones from the Eastern Ghats Belt, India Santanu Kumar Bhowmik Regional Petrology Laboratory, Geological Survey of India, Seminary Hills, Nagpur-440 006, India (Manuscript received March 3, 1999; accepted June 14,1999)

Abstract Ultrapotassic rock is reported for the first time from the polycyclic Eastern Ghats belt, India, near Borra, Visakhapatnam district, Andhra Pradesh. The rock, consisting of leucite, kalsilite, K-feldspar, graphite, apatite together with diopside, meionite and phlogopite, occurs as thin vein and veinlets in diopsidite, in close spatial association with a granulite facies carbonate ensemble of massive dolomitic carbonate rock and calc silicate granulite. It was emplaced in the midcrust along late ductile shear zones. Subsequent to its emplacement, the ultrapotassic melt with liquidus leucite interacted with the granulite wall rock, incorporating at least 40% of the crustal components mainly as Si, Al, Mg and Ca. After necessary correction of the crustal contaminant, the recalculated K,O/Na,O ratio of -12 (molar) and K,O/Al,O, ratio of -1 (molar) in the bulk rock composition indicates that the Borra ultrapotassic melt has a lamproitic affinity However, it is significantly modified as well, particularly being impoverished in mafic liquidus phases and depleted in incompatible (excepting Rb, Th and U) and compatible trace elements, compared to an average lamproite. Leucite later underwent subsolidus decomposition to K-feldspar kalsilite intergrowths. The emplacement of the ultrapotassic melt post-dates an early ultra high temperature metamorphism and also the 1000 Ma Grenvillian metamorphism in the Eastern Ghats Belt and is possibly of Pan-African age. The extensive K-feldspathisation in the Eastern Ghats belt could also be linked with this ultrapotassic melt.

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Key words : Ultrapotassic rock, ductile shear zone, Borra, Eastern Ghats, India.

Introduction Ultrapotassic (UP) rocks containing K-feldspathoidsare unique in terms of their very high WNa ratio (> 3 (molar)), silica undersaturation and enrichment in incompatible elements (Bergman, 1987). Though widespread in volcanic and subvolcanic environments, UP rocks are extremely rare in granulite terranes, having only been reported from Punalur area, Kerala Khondalite Belt, India (Sandiford and Santosh, 1991). Although a crustal metamorphic origin has been proposed for the Punalur occurrence (Sandiford and Santosh, 1991), research over the last three decades indicates that UP rocks are primarily derived either from a metasomatised lithospheric mantle or in the asthenosphere as extremely small melt fractions (< 1 volume%) (McKenzie, 1989) and require the involvement of a recycled crustal component through subduction, at some stages of its evolution (Nelson, 1992). The present communication reports an occurrence of UP rocks from the Eastern Ghats Granulite Belt, India, for the first time. Collating data on petrography, mineral

and bulk chemistry, an attempt has been made to understand i) the depth of emplacement of the UP melt, ii) its interaction with the wall rocks, and iii) its timing with respect to granulite facies metamorphism in the belt. The Eastern Ghats Belt, extending along the east coast of India (Fig.1) exposes a polydeformed and polymetamorphosed ensemble of metapelite, quartzofeldspathic, calcareous and mafic granulites. Recent petrological studies indicate at least three discrete metamorphic events (Dasgupta and Sengupta, 1998 and the references cited therein) being punctuated by alkaline, anorthosite, mafic-ultramafic and felsic magmatism. The earliest event is marked by an ultra high temperature (UHT) metamorphism (T> 950°C) at 8-9 kb along an ACW path, followed by near isobaric cooling to 700-750°C (MI) (Sengupta et al., 1990; Dasgupta et al., 1991,1995; Bhowmik et al., 1995; Mukhopadhyay and Bhattacharya, 1997). The second event (M,) which reworked the UHT granulites is characterised by near isothermal decompression down to 4 - 5 kb at 750-800"C, causing unroofing of nearly 15 km of the crust. The terminal,

S.K. BHOWMIK

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amphibolite facies event, M, is recorded in late ductile shear zones and was responsible for widespread hydration and carbonation of the granulites, along with emplacement of some late pegmatites at -600-650°C and 4 - 5 kb (Sengupta et al., 1990, 1991; Dasgupta et al., 1991, 1992; Bhowmik et al., 1995; Bhowmik, 1997). While, there is considerable agreement, regarding a Grenvillian age for M, and Pan African age for M, (Mezger and Cosca, 1999; Mezger et al., 1996; Shaw et al., 1997), the timing of UHT metamorphism is not known at this stage. In this belt, the alkaline rocks, ranging in age from 850 Ma to 1450 Ma (Sarkar and Paul, 1998 and the references cited therein) are restricted to the western margin of the belt, being confined to the junction between the cratons and the mobile belt (Leelanandam, 1993). The presently studied UP rocks, occurring at Borra (Lat. 18" 1 7 , Long. 83" 2') (Fig. 1) , in the interior of the Eastern Ghats Belt are therefore well away from the known occurrences of alkaline rocks.

Geological Background The area around Borra exposes an interlayered sequence of different types of carbonate granulites, khondalite and leptynite. This is separated from a linear orthopyroxene granulite unit in the east by a NE-SW trending ductile shear zone that contains small pods and lenses of khondalite, two pyroxene granulite and orthopyroxene granulite (Fig. 1). Since the UP rocks (described later) occur in close spatial association with the carbonate granulites, the geology of the carbonate ensemble is described in more detail below. Four distinct types of carbonate rocks are present in the area, each with characteristic bulk composition, mineral assemblage and retrograde reaction textures (Bhowmik et al., 1995). These are: 1) calc silicate granulite (Cpx + Scp Qtz Grt + P1 + Cal Kfs + Ttn f Wo f Mag +_ Amph +_ Ep/Zo), 2) massive dolomitic carbonate rock (MDC) ( Cal + Do1 + Di It 01 k Spl f Scp f P1 f Kfs f Phl +_ Ttn), 3) diopsidite (Di + Scp k An f 01 +_ Spl f Cal f Phl f Kfs), and 4) pegmatitic calcite rock (abbreviations after Kretz, 1983). The MDC, which occurs at the center of the complex, has a triangular shaped outcrop pattern, and is bounded on three sides by the calc silicate granulites (Fig.1). The pegmatitic carbonate rock occurs as coarse veins and apophyses within the MDC, and locally in diopsidite. Small oval to elliptical bodies of the pegmatitic variety also crop out well within the calc silicate granulite, in the northern part. Diopsidite generally occurs as rafts and enclaves of various sizes and shapes within the MDC. Small, detached masses of it also border the MDC at its northern and south-western

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periphery. In majority of these occurrences, there is millimeter to centimeter-scale interbanding of the two units. This is particularly prominent in the largest, NESW trending diopsidite outcrop. The rock also displays a pervasive gneissic banding, demarcated by alternate pyroxene-rich and scapolite plagioclase-rich layers. Economically important phlogopite mineralization in diopsidite is noted in the northern periphery of the MDC, along a narrow E-W shear zone (Bhowmik, 1994). Further away from the center, the calc silicate granulite occurs as a continuous mappable unit bordering MDC and/or diopsidite. The association of pyroxenite (= diopsidites), syenite (= K-feldspathised rock) and massive dolomitic carbonate rock and the presence of rafts and enclaves of diopsidite within the MDC, led Ramam and Viswanathan (1977) to conclude that the trio represents an igneous association and that the MDC is a carbonatite. This was, however, discounted by later petrographic (Rao, 1989), mineral chemical and stable isotopic studies of the carbonate rocks (Bhowmik et al., 1995). The results suggest a sedimentary origin for the MDC and the diopsidite. Recent petrological and structural studies from the Borra area have documented a polyphase deformation and metamorphic history (Bhowmik, 1997). The relevant features, pertinent to this study are briefly described below. The Borra granulites have recorded five episodes of deformation (Dl, D,, D,, D, and DJ and four phases of metamorphism (MI, M,, M, and MJ. The sequence of PT evolution of an early UHT metamorphism (T 975"C, P 9 kb), followed by isobaric cooling (T - 730°C, P 7 kb), isothermal decompression (down to 5 kb), and terminal shear zone reconstitution is similar to that recorded elsewhere in the Eastern Ghats belt (Bhowmik, 1997). The terminal episode of reworking (D,) at Borra was localized along E-W and NE-SW trending shear zones which acted as channelways for infiltration of carbonic fluids and potash-metasomatism (Bhowmik et al., 1995). The ultrapotassic rocks in question are scattered along or in close proximity to the said shear zones (Fig. 1).These are confined within the diopsidite, the latter occurring either as small detached masses within a soil covered tract, as thin discontinuous bands at the northern periphery of the MDC or as large enclaves within the MDC. Locally, the UP rock also occurs at the centre of a thick diopsidite unit. In majority of these occurrences, UP rocks occur as 10-15 centimeter thick and nearly 100 meter long veins, broadly conforming to the gneissic foliation of the host diopsidite (Fig. 2 ) . The veins are discontinuous. There are numerous millimeter-thick veinlets which branch out from the main vein wall. Petrography of the wall rock diopsidite and the UP rock, occurring in veins and veinlets are described in detail below.

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Fig. 1. Geological map of Borra and adjoining areas in the Eastern Ghats belt, India. Inset map shows the location of Borra in the Eastern Ghats belt (EGB)

Petrography and Mineral Chemistry

Fig. 2. Sketch showing the occurrence of the UP rock as veins (Association I) and veinlets (Association 11) within the host diopsidite.

Gondwana Research, K 3, No. 1, 2000

Electron Microprobe analyses of the Borra UP rock and associated diopsidite are presented in Table 1. Analyses were performed with a Cameca SX 51, wave dispersive, EPMA at the Faridabad laboratory of tE. . Geological Survey of India, operated at 15 KV accelerating voltage, 2 mm beam diameter and 10 nA specimen current. Natural mineral standards were used and the raw data were corrected by a PAP procedure. The wall rock diopsidite, away from UP veins, is generally composed of diopside (50-60%) and meionite (50-40%) with accessory Fe-sulphides and does not contain graphite. It shows a granoblastic mosaic texture.

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Table 1. Representative chemical analyses of important phases in the UP rock and Diopsidite.

Anal. No. Mineral SiO, TiO, A203

Cr2 0 3 FeO MnO MgO CaO Na,O

1 Di (C)

Oxygen basis Si Ti

Al C13' Fe3+ Mn Mg Ca Na K Ba Total

(R)

3 Di (C) 48.45 1.94 8.71 0.09 0.27

46.85 1.94 9.93 0.06 0.37 0.14 13.74 25.87 0.34

46.78 2.04 11.23

99.24

100.15

6 (0) 1.714 0.053 0.428 0.002 0.011 0.004 0.749 1.014 0.024

6 (0) 1.694 0.056 0.480

0.738 1.001 0.020 0.002

0.784 1.000 0.024

3.999

3.999

4.001

ro

BaO Total

2 Di

0.29 0.01 13.66 25.80 0.29 0.05

0.008

14.50 25.74 0.34 ma. 100.04

6(0) 1.757 0.053 0.372 0.003 0.008

Association I 4 5 Ks Di (R) 37.92 46.67 2.12 32.45 11.72 0.04 0.06 0.07 0.38 0.08 0.02 13.37 25.92 0.09 0.29 0.06 27.59 n.a. 0.24 100.67 98.42

6 0 1.684 0.058 0.498 0.002 0.012 0.002 0.719 1.002 0.020 0.003 4.000

6 Kfs,

7 Lc

8 Lc

9 Kfs,

10 Kfs,

65.16

54.81 0.07 23.28 0.01 0.11 0.07

54.63

59.10

60.56

23.48

19.83

19.21

0.10 0.04

0.02

0.26 0.02 0.02 0.08 13.86 4.83 98.84

18.62 0.01 0.05

0.05 0.06 15.50 0.35 99.80

4 (0) 1.003

8 (0) 3.006

1.021 0.001 0.002

1.012 0.002

0.04 20.29 98.58

0.06 13.01 7.27 99.29

6 (0) 2.008 0.002 1.005

6 (0) 2.000

8 (0) 2.874

8 (0) 2.914

1.013

1.137

1.089

0.003 0.002

0.003 0.001

0.001

0.010

0.006 0.807 0.139 4.964

0.002 0.001 0.007 0.851 0.091 4.965

0.145 0.848

0.096 0.895

0.001 0.005 0.931 0.002 2.966

L

0.003 0.005 0.912 0.006 4.946 0.007 0.985

&r#

Diopside and meionite (EqAn = 96)constitute peak M, assemblage. Meionite is locally replaced by delicate anorthite + calcite intergrowths, during an episode of near isobaric cooling (Bhowmik et al., 1995).Diopsidite occurring along the shear zone shows extensive fracturing and fracture-controlled replacement of diopside by phlogopite (Bhowmik, 1994).All along this zone, there is terminal development of K-feldspar which poikiloblastically encloses diopside, meionite, anorthite + calcite intergrowths, phlogopite and even calcite. Locally, relic blebs of meionite in lobate to amoeboid form also occur within this K-feldspar (Fig. 3) indicating resorption. Depending on its mode of occurrence, the Borra UP rock is subdivided into two associations. Association I, constituting thicker veins is dominated by ultrapotassic phases, mainly leucite (50-55%), together with diopside (40-45%),minor K-feldspar, kalsilite (together 5%) and accessory graphite, Fe-sulphide and apatite. Association 11, representing finer veinlets, is composed of kalsilite, Kfeldspar, phlogopite, diopside and meionite. Importantly, the ultrapotassic phases in this association never exceed 20% by volume.

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0.06 19.92 0.01 98.34

0.004 0.931

0.003 0.947

3.955

3.967

Diopsidite 11 12 Di Di

47.71 0.81 8.57

48.74 0.71 7.20

3.25 0.14 13.84 26.14 0.10

3.02 0.19 14.64 25.45 0.09

100.56

100.04

6 (0) 1.734 0.022 0.366

6 (0) 1.777 0.019 0.309

0.099 0.004 0.750 1.018 0.007

0.092 0.006 0.796 0.994 0.006

4.000

3.999

In association I, leucite occurs as coarse aggregates, engulfing lobate to amoeboid diopside within it (Fig. 4). It shows decomposition to K-feldspar and kalsilite intergrowths (Fig. 5). Thin flakes of graphite, occupy the interstices of leucite as criss-cross flakes (Fig. 6) . In the absence of isotope composition, flakes of graphite can not as such be taken as juvenile carbon, but the absence of graphite in diopsidite, its occurrence in the interstices of leucites, and its occurrence as an included phase in late K-feldspar may not be incidental. There are two generations of K-feldspar in the rock. The early generation one (Kfs,) does not contain Ba (Anal no. 6, Table l), is symplectiticallyintergrown with kalsilite (partially altered to clay minerals), occurs within the confines of leucite. This suggests that the intergrowths represent subsolidus decompositionof leucite. Both leucite and kalsilite are nearly pure phases with minor Fe3+and Na contents (Table 1). The second generation K-feldspar (Kfs,) occurs as thin veins and patches in leucite (Fig. 71, and is enriched in Ba with the celsian component ranging from 10-15mole.% (Anal nos. 9and 10, Table 1).Diopside in the UP rock has core compositions (Al,03= 8.7 to 9.9 Gondwana Research, V: 3, No.1,2000

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Fig. 3. Thin section microphotograph in diopsidite, showing late barian K-feldsparengulfing relic scapolite within it. The lobate to amoeboid shape of the scapolite (Scp) blebs implies resorption by the K-feldspar (K-fs). Note also the smoothening of diopside (Di) grain boundary. Partially cross nicols.

Fig. 4. Back scatter image (BSI) photograph of the association I in the UP rock. The lobate to amoeboid diopside (Di) crystals are embedded in a mosaic of coarse K-feldspar + kalsilite intergrowths, decomposing after primary leucite. Note the crisscross flakes of graphite (dark) within the symplectite mass.

Fig. 5. Back scatter image (BSI) photograph of the association I in the UP rock, showing finger-print intergrowth of K-feldspar + kalsilite after primary leucite (Lc). A large graphite flake (dark) occurs at the margin of the symplectite mass.

Fig. 6. BSI photograph of the association I in the UP rock showing the occurrence of graphite (dark) as criss-cross flakes at the interstices of leucite (Lc). Note the highly amoeboid shape of engulfed diopside (Di).

Fig. 7. BSI photograph of the association I in the UP rock showing veins of second generation K-feldspar (Kfs,) within coarse aggregates of primary leucite (Lc). Kfs, engulfs iron-sulphide (white), graphite (dark) and fine relics of leucite within it.

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wt%, TiO, = 1.9 wt%) similar to that in wall rock diopsidite (Al,O, = 7.2 to 8.6 wt%, TiO, = 0.7 to 0.8 wt%). However, the former shows sharp enrichment in Al,O, and TiO, contents towards the rim (Al,O, = 11.2 to 11.7 wt%, TiO, = 2 to 2.1 wt%). In association 11, the ultrapotassic phases occur conspicuously in the interstices of diopside. The most common texture is a fine, finger-print intergrowth of K-feldspar and kalsilite, after primary leucite. Similar texture was earlier described by Sandiford and Santosh (1991) from Punalur area. Clusters of diopside are seen engulfed within the symplectite mass, still maintaining original mosaic texture of diopsidite. Only locally, one of the crystal faces, in direct contact with the symplectite shows curvature, implying resorption, possibly d u e t o melt-crystal i n t e r a c t i o n . The

symplectite is rimmed by a thin shell of Ba- rich Kfeldspar. The vein morphology of the UP rock, local presence of clusters of diopside within the UP phases, that can be traced to the original wall rock, and finally close compositional similarity of diopside core in association I with that in diopsidite, suggest that diopside in the UP rock is a wall rock component. This is in contrast to its occurrence as a liquidus phase, in a variety of K-rich lavas and plugs, lamprophyres and in nephelinites, basanites and alkali olivine basalt.

Bulk Chemistry The major, trace and REE composition of the studied UP rock, an average lamproite, diopsidite, calc silicate

Table 2. Major and trace element composition of UP and associated rocks and of an average lamproite. Sample No. Rock Name SiO, TiO, A1203 FeO *Fe,O, MnO MgO CaO Na,O K2O PZO,

so3 Total sc V

Cr

co Ni cu

Zn Ga As Rb Sr

Y Zr Nb Mo Ba La Ce Pr Nd Sm Pb Th U

1 UP

2 Lamproite

47.59 0.39 22.49

53.3 3.0 9.1 6.3

2.47 0.02 2.30 9.23 0.62 11.24 0.11 0.23 96.69

< LLD 63 26 1 21 7 22 21

< LLD a77 164 9 185 21 1 199 31 36 5 7 3 23 57 15

0.10

12.1 5.8 1.4 7.2 1.3 99.50 17 123 580 37 420

n.d. n.d. n.d. n.d. 272 1530 27 922 95

n.d. 5120 240 400

n.d. 207 24

n.d. 46 4.9

3

4

5

6

Diopsidite

Diopsidite"

Calc silicate granulite

Calc silicate granulite'

44.50 0.62 11.16

50.15 0.66 11.81

54.45 0.96 15.32

51.47 0.65 11.78

4.16

5.06 0.15 8.99 19.34 0.15 3.12 0.12 0.03 99.52

8.09 0.14 2.20 16.39 1.14 0.15 0.13 <0.04 99.01

5.03 0.15 5.36 16.89 0.21 5.34 0.10 0.04 96.99

0.11 13.16 24.25 0.21 0.11 0.37 0.03 98.6% 14 67 62 19 20 5 88 16 115 5 122 29 22 1 1 < LLD 124 4 14 3 9 4 15 21 7

7 69 59 25 36 44 100 15 7 125 82 24 162


< LLD

9 92 90 34 58 22 127 21 22 6 116 38 174 21

12 68 66 22 33 3 81 16 1 266 134 22 165 9

<1

CUD

19 42 75 <9 30 8 14 32 11

1770 27 48 5 23 5 23 16

1

Av. lamproite composition (n = 284) from Bergman (1987); *Total Fe as Fe,O, ; * : Potash feldspathised rocks; < LLD : Lower limit of detection; n.d. : not determined.

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ULTRAPOTASSIC ROCKS FROM EASTERN GHATS BELT

granulite and their K-metasomatised varieties are presented in Table 2. The sample locations are shown in Fig.1. The Borra UP rock from association I shows anomalously high Al,O,, but lower MgO, TiO, and FeO contents, compared to the average lamproite and other UP rocks. This, coupled with the presence of wall rock components, indicate that the UP rock is crustally contaminated. It would, therefore, be appropriate to estimate the crustal and liquidus components before classifying the UP rock. Consequently, the bulk composition is normalized against an average composition of diopside, meionite and Fe-sulphide in the host diopsidite. The calculated norm gives the following minerals: leucite (-55%), diopside (-20%), meionite (-20%), Fe-sulphides and graphite (together - 5%). The estimation of diopside and meionite in 1:1ratio is in close agreement to that in the diopsidite. This suggests incorporation of crustal components, derived from the enclosing diopsidite. Therefore, subtracting A1,0, partitioned in xenocrystic diopside and meionite (completely dissolved), the Al,O, content in UP magma is estimated to be -12 wt%. The recalculated K,O/Na,O ratio of -12 (molar) and K,O/Al,O, ratio of -1 (molar) indicates that the UP melt has a lamproitic affinity (cf. Bergman, 1987). However, as compared to the average lamproite, the Borra UP rock is significantly depleted in LREE, Sr, Ba, Zr and compatible elements, but shows enrichment in Rb, Th and U (Table 2). The depletion could be the result of (i) its highly evolved nature, and/or (ii) lack of incompatible element-enriched accessory phases. However, the K/Rb ratio in the Borra UP rock (-106) is very close to that in diamondiferous lamproite (-85). Interestingly, the diopsidite as well as calc silicate granulite, away from the shear zone are impoverished in incompatible elements (Sample Nos. 3 and 5, Table 2). This also signifies that the enrichment of Rb, Th and U in the UP rock is not related to granulite mixing at the site of magma emplacement. Diopsidite and calc-silicate granulite, containing late K-feldspar (K-fs,), are strongly enriched in Ba (611 to 1770 ppm) compared to its unmodified varieties (Sample Nos. 4 and 6, Table 2). The trace element data suggest that while Rb was preferentially partitioned in leucite, Ba was partitioned to K-feldspar.

Discussion Recent experimental results, combined with evidence from whole-rock chemistry (including volatiles), tectonic settings and the nature of xenoliths, indicate that lamproites and other ultrapotassic magmas are mantle derived melts and do not represent simple melting of recycled crust (Bergman, 1987). The Borra UP rock has Gondwana Research, K 3, No. 1,2000

similarities with lamproite in its major element composition, but is significantly modified as well. This is reflected in its difference with lamproite in trace element composition and content of mafic liquidus phases. Petrographic and geochemical data show that the Borra UP rock is metasomatised at its site of emplacement by dissolution of the diopsidite wall rocks. It is, therefore, difficult to precisely estimate its depth of origin. Nevertheless, the experimental findings that leucite normative melts can be derived by partial melting of phlogopite in peridotite at a pressure of -19 kb for volatile-free basis (Wendlandt and Eggler, 1980) and the possible primary nature of graphite suggest that the Borra UP rock owes its origin in the mantle and was possibly extracted from the graphite stability field, at a depth shallower than 150 km. The localization of the UP rock along shear zones suggests that the emplacement of mantle-derived melt was shear-zone controlled and was coeval with D, deformation and M, metamorphism (= M, of Dasgupta and Sengupta, 1998) at Borra (Bhowmik, 1997). That its emplacement is a post-peak phenomenon is also indicated by the presence of peak metamorphic assemblage as well as their breakdown products in the host rocks as xenocrysts in leucite and K-feldspar pools. The P-T estimate of M, metamorphism (- 5kb, -60065OoC) is also in agreement with the experimental constraint on the stability of leucite (Scarfe et al., 1966). At Borra, leucite and its breakdown product kalsilite + Kfeldspar additionally constrain M,. At -650°C, the experimentally determined, leucite =kalsilite + K-feldspar equilibrium (Scarfe et a1.,1966) is intersected at 5 kb. This is consistent with the idea of mid crustal emplacement of the UP melt at the Borra area. Both petrographic and field relation indicate that subsequent to its emplacement in the mid-crustal granulites, the UP melt because of its low viscosity (McKenzie, 1989), propagated in vein system, following weak planes in the host diopsidite. In the process, the melt interacted with the wall rock components with two consequences. Continued melt propagation led to destruction of the mosaic texture in the wall rock, so that diopside becomes separated from the original framework and floats in leucite aggregates as extensively resorbed wall rock component. Secondly, there was continuous chemical exchange between the two components, such that simultaneous with dissolution and melting of both diopside and meionite, the peripheral part of restitic diopside became high in alumina content. The excess CaTs could have been gained from dissolved meionite. This is supported by near absence of meionite in the UP rock, compared to its higher abundance in the unmodified

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S.K. BHOWMIK

diopsidite. Estimation of melt / wall rock ratio from 0.2 to 0.6 in the BCGC also signifies that the UP melt could incorporate at least 40% of crustal component, mainly as Si, Al, Mg and Ca. It has been increasingly realized that there is considerable mixing between crustal and mantle reservoirs (Vollmer, 1976, 1977; Hawkesworth and Vollmer, 1979) and it has important bearing in the petrogenesis of the UP rocks. However, there is intense debate rcgarding the site of mixing, whether within the upper mantle whereupon crustal material is brought to that level by subduction process (e.g. Thompson, 1977; Peccerillo, 1985, 1990; Beccaluva et al., 1991) or within the crust during the ascent of magma. The present study is, however, unique in this respect. It documents in situ crustal addition to the mantle-derived UP melt at midcrustal depths. Documentation of such deeper level interaction is, however, rather scanty because of general occurrence of ultrapotassic magmatism in volcanic and subvolcanic settings. In the latter, mixing between the two reservoirs is interpreted only through detailed major, trace, REE and isotopic characterisation. Recently, Sm-Nd garnet-whole rock (Shaw et al., 1997) and discordant U-Pb sphene and the 4oAr-39Ar hornblende ages (Mezger and Cosca, 1999) of 500-550 Ma, from the northern segment of the Eastern Ghats Belt indicate that the last major thermal overprint and attendant shear zone reworking was during the Pan-African orogeny In the Eastern Ghats belt, the Grenvillian orogeny was responsible for regional scale reworking of an earlier UHT granulites; particularly in its northern segment which ultimately culminated in a steep decompression. Since there is no difference between terminal M, and M, pressure, the decompressed granulites probably became part of a stable middle crust till the onset of Pan African orogeny. The shear-controlled emplacement of UP melt in the mid-crust and infiltration of C0,-rich fluid (Bhowmik et al., 1995), during this period, therefore requires a renewed spell of thermal perturbation in the mantle and a change in stress regime from compressional to extensional (Sykes, 1978). Significantly enough, extensive alkaline and felsic magmatism is a characteristic feature of Pan African event in Kerala (Santosh and Drury, 1988) and in other segments of the East Gondwanaland (Rajesh et al., 1996) and have been explained by taphrogenic (rift-related) magmatism in the region (Nair and Santosh, 1985). A similar tectonic setting can be visualised for the Borra rocks, too. Finally, the extensive K-feldspathisation in t h e Eastern Ghats belt (Bhattacharya, 1972; Sengupta et al., 1990) could have been related to the alkaline magmatism. This certainly holds good for Borra area, where formation of late phlogopite in shear zones transecting diopsidites can be correlated with the emplacement of the UP melt.

Acknowledgments The present contribution constitutes part of the author’s Ph.D. dissertation work carried out at the Dept. of Geological Sciences, Jadavpur University. The manuscript incorporates the suggestions and comments of Prof. Somnath Dasgupta and Prof. Prasanta Kumar Bhattacharyya who were kind enough to review an earlier draft. The manuscript has also been considerably improved by suggestions from an anonymous reviewer and comments of Dr. M. Santosh. Analytical facilities rendered by Shri J. K. Bhalla, Director, EPMA Lab., GSI, Faridabad and Shri P. K. Basu , Director, SEM Lab., GSI, Calcutta is gratefully acknowledged. Thc author is also thankful to Dr. N. C. Pant and Shri S. Shome for help provided during mineral chemical analyses and back scatter image photography. Dr. T. Pal is thanked for processing the geological map in the computer. The author also acknowledges the CSIR for providing the research grant during his tenure as JRF and SRF at the Jadavpur University.

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