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Petrology, of gabbro and associated dolerite dyke of Paharpur, Puruliya, West Bengal: Implication for petrogenetic process and tectonic setting

Aditi Mandal, Arijit Ray∗, Mayukhee Debnath and Sankar Prasad Paul Presidency University, Kolkata 700 073, India. ∗Corresponding author. e-mail: [email protected]

Paharpur gabbroic intrusive is an arcuate body running east–west paralleling the foliation of Chhota- nagpur Gneiss which acts as country . The main gabbroic body is intruded by a number of dolerite dykes running north–south. It is composed of clinopyroxene (Wo48En40Fs12–Wo51En40Fs09,mg no. 72–82), (An52–An90), hornblende (magnesian hornblende to ferro-tschermackite), orthopy- roxene (En76–En79) and . Hornblende occurs as large grain and constitutes around 60% of the rock. Both gabbro and associated dolerite dykes, show relatively primitive character (mg no. 65– 73). Primitive mantle-normalized and MORB-normalized spider diagrams indicate enrichment in Rb, Ba, Th, La, Sr and depletion in Nb, Zr, Y, Ti and Nd. The LILE enrichment and Nb, Ti, Zr, Y depletion suggest arc like geochemical signature for the gabbroic and doleritic rocks of Paharpur. Flat to slightly LREE fractionated pattern and variable degree of REE enrichment is observed. An early stage fraction- ation of clinopyroxene, plagioclase, orthopyroxene, ilmenite and late stage reaction of cumulate pile and evolved melt/hydrous fluid is suggested for magmatic evolution of gabbro. Associated dolerite dykes, which are geochemically similar to the gabbro, have tholeiitic with boninitic character. The mineralogi- cal and chemical compositions of intrusive rocks also have some similarity with mafic rocks of complex of subduction zone.

1. Introduction and Borgia 1989) or as a product of reaction of early crystallized cumulate (, and Hornblende-bearing gabbroic rocks ( and plagioclase) and water-rich evolved melt/aqueous plutons) are fairly common in subduction-related fluid (Prouteau et al 2001; Costa et al 2002). Horn- magmatic suite and have been considered to repre- blende and phlogopite (occasional) bearing gab- sent magmatic differentiation process in arc mag- broic xenoliths of Volcan San Pedro (Tartara San mas (Arculus and Wills 1980; Beard 1986; Yagi Pedro Volcanic Complex, Chilean Andes) are the and Takeshita 1987; De Bari 1994; Heliker 1995; result of multistage differentiation process involv- Hickey-Vargas et al 1995). The presence of horn- ing migration of evolved melt ± aqueous fluid blende as a dominant phase in gabbroic rocks through mafic cumulate pile (Costa et al 2002). of subduction zone has been considered either as This reaction produced high proportion of horn- an early crystallizing mineral from water bear- blende (up to 62%) in gabbroic rock with high ing mafic (Conrad and Kay 1984; Beard mg-no. and Cr2O3 content. This type of melt and

Keywords. Chhotanagpur Gneissic Complex; poikilitic hornblende; large-ion lithophile elements; Nb anomaly; subduction.

J. Syst. Sci. 121, No. 3, June 2012, pp. 793–812 c Indian Academy of Sciences 793 794 Aditi Mandal et al aqueous fluid migration, reaction and replacement to be an interesting problem. The present work is also reported from well-known Large Igneous attempts to characterize the gabbroic rocks and Province like Muscox (Irvine 1980), Skaergaard associated dykes of Paharpur and identify the (McBirney 1995), and Stillwater (Boudreau 1999). petrotectonic process. This will add knowledge on Besides, hornblende is found as a common min- the evolutionary history of CGC. eral in gabbroic rocks of ophiolite complexes all Northern part of Puruliya district is character- over the world (Beard 1986; Beccaluva et al ized by a number of mafic bodies intrusive within 2004 on Tethyan and Codilleran ophiolite; Bonev CGC. Some of these are associated with their ultra- and Stampfli 2005 on Evros ophiolite of Greece; mafic counterpart (Mandal et al 2007; Mandal Biggazzi et al 1989 on Petrota, W Thrace; Bonev and Ray 2009). The mafic rocks of Paharpur, and Stampfli 2008 on Rhodope Magmatic Com- are considered younger than CGC as evidenced plex, Bulgaria; Hebert and Laurent 1990 on by their intrusive nature and post-tectonic with petrology and geochemistry of arc-related Troodos respect to the shearing movement as evidenced ophiolite, Cyprus). Gabbroic rocks of subduction from their undeformed nature. The elliptical gab- zone having ophiolite affinity might be the result bro body only shows the effect of shearing at the of their formation in arc/back arc setting where marginal part along the contacts with the gran- extensional tectonism caused fracturing and dyke ite country rock which is the result of magmatic activity. Tholeiitic and boninitic dykes are com- flow during emplacement within CGC. The main mon in many of ophiolite complexes of arc set- east–west trending gabbroic body is intruded and ting where they are considered as supra-subduction cross-cut by a number of dolerite dykes running assemblage in marginal basin volcanic arc setting north–south (five in number). One dolerite dyke (Frass et al 1990; Magganas et al 1991; Magganas trends northeast–southwest and another trends 2002, 2007; Bonev and Stampfli 2005, 2008). northwest–southeast (figure 1). The gabbroic rocks of Paharpur are intrusive The present research work attempts to study into granite gneiss of Chhotanagpur Gneissic Com- the mode of occurrence, petrology and geochem- plex (CGC, 800–1600 Ma) of the eastern Indian istry of mafic intrusive rocks of Paharpur, Puruliya Shield. The gabbroic rocks of Paharpur are younger and West Bengal and understand the magmatic than the granite gneiss of CGC, but no radio- evolution process in an arc-setting. metric age is available for them. The high modal abundance of hornblende in gabbroic rocks of Paharpur imparts a peculiar petrographic status. 2. Geological setting Such hornblende-bearing gabbroic rocks have not been reported from CGC. The petrogenetic pro- The study area forms a part of the Chhotanag- cess and tectonic setting which favour formation pur Gneissic Complex (CGC) and is character- of abundant hornblende in gabbroic rocks appear ized by the occurrence of medium- to high-grade

o 96 38' 33''E Scale + + ++ + 400 m S02 v + v v + + v 75 v 72b 78 S01 S13-16C v + v v 73 S05 77A Paharpur v . v v S96/2D 281m S03 o . v v o 23 17' 77 84 + 23 17' 13''N + 15C v 13''N 85 v S08 v + + L5 v + + + + 87 80o E Index v + Gabbro + 10C + + v Delhi 20o v Dolerite Puruliya S07 Kolkata N + CGC-Granite gneiss + Mumbai 0 483 Sample location + + Scale in km o

96 38' 33''E o 84 E

Figure 1. Geological map of the study area around Paharpur. Sample locations are indicated in the map which are analysed for determination of modal and chemical composition of gabbro, dolerite and . Petrology and geochemistry of hornblende gabbro 795 metamorphic rocks. Intrusive rocks such as por- composition trending mostly N–S (average width phyritic granite, gabbro and dolerite are found 15 m). Thus, it is found that the general orienta- as concordant and discordant intrusions within tion of these younger dolerite dykes is at a high CGC. Available geochronological data based on angle (almost 90 degree) with the elongation of the K/Ar method of biotites from granitoid gabbroic body. gneiss and muscovite from leucogranitoid assign ages of 870 ± 40 Ma, and 810 ± 40 Ma, respec- tively (Baidya et al 1987a, 1987b). Rb/Sr method 3. Petrography and mineral composition assigns ages of 1071±64 Ma for porphyritic hyper- sthene granite near Raghunathpur, NE Puruliya The optical properties of minerals and their tex- and 1178 ± 61 Ma for migmatites from Murguma, tural relationships have been studied using a high SE Puruliya, Ray Barman et al 1994). U–Pb dating resolution NIKON polarizing microscope (model of zircon from the Bengal from further no. 064333) at the Department of Geology, Pres- east which is the part of a deformed and metamor- idency University. Mineral composition has been phosed gabbro-anothosite-layered complex assigns determined at the EPMA Lab, CPL, CHQ, Geolog- a Mesoproterozoic age (∼1550 Ma) – an early mag- ical Survey of India, Kolkata. Analyses were per- matic event in the CGC (Ghosh and Chatterjee formed with a 15 KV accelerating voltage, 15 × 2008). Gabbroic rocks of Paharpur are undeformed 10−10 A beam current, 5 μm spot size and 50 s live with no effect of and have no spa- time for each element. Mineral compositions were tial and temporal relationship with those of Ben- determined with CAMECA S ×100. Natural min- gal of Bankura which occur in the eral standards were used for all elements except Mn eastern part of CGC. The Bengal anorthosite of and Ti, for which synthetic standards were used. Mesoproterzoic age is intimately associated (often The analyses were standardized using natural stan- interlayered) with gabbroic rocks both of which dards supplied by SPI Suppliers, Structure Probe are deformed, metamorphosed and strongly foli- Inc., Canada. The replicate analyses showed identi- ated. The undeformed, unmetamorphosed massive cal results. Precision of analyses is within the error gabbroic rocks of Paharpur are definitely younger limit of ±3%. than Bengal anorthosite, but their actual age The main gabbroic rocks are greyish green, cannot be ascertained as no age data is available coarse-grained and porphyritic. A number of in existing literature. A general E–W strike with dolerite dykes, later formed, trending roughly N–S, northerly dip of the foliation is the dominant fabric intrude this main gabbroic body, outcrop in north- of the granitoid gneisses of the CGC (Sengupta and ern and southern flanks. Another E–W trending Sarkar 1964; Ghosh 1983; Mazumder 1988; Sarkar dyke is outcropping at the northernmost part of 1988; Baidya et al 1987a, 1987b). The monotonous the intrusive and two others running N–S at its northerly dips may be ascribed to isoclinal folds eastern and western sides (figure 1). The dykes are with overturned southern limb in the present area. medium-grained, porphyritic and at places with Precambrian rocks of the area have undergone mineral lineation and slickensides. three phases of folding (Baidya et al 1987a, 1987b). The huge gabbroic intrusive consists mainly of The last phase of deformation in the area is repre- plagioclase, hornblende, , apatite and sented by the development of a set of shear zones. some opaque minerals such as , hematite The most prominent of these is the southernmost and rutile. one and is known as the North Puruliya Shear Plagioclase (An51–An90, table 1) is coarse, tab- Zone (NPSZ). Other E–W trending shear zones are ular and lath-shaped together with medium-sized also observed in the southern part of the NPSZ. aggregates. They occupy up to 45% (by volume) One shear zone (E–W) is observed just to the of the rocks and range in composition from ande- south of the study area affecting granite gneiss. sine to bytonite. Relic laths and equant grains of The main gabbroic masses as well as the intrusive plagioclase (An51) are poikilitically enclosed within dykes are not affected by this E–W shearing as large plates of hornblende in gabbro. At places, these rocks do not have shear-related E–W folia- plagioclase is resorbed by hornblende. tion. They are dominantly massive. The northern Hornblende occupies around 60% of the rock margin of the gabbroic body shows crude E–W foli- and shows different schemes of pleochroism from ation. These foliations, primary in nature, possibly colourless to light green and green to greenish have developed at the time of its emplacement. brown. Compositionally, they are mostly magnetio- The main intrusive body of Paharpur is a gab- hornblende to ferro-tschermackite through tscher- broic mass (running E–W, paralleling the foliation mackite hornblende and tschermackite (mg no. 65– of granitic country rock) of more or less ellipti- 73, table 2, figure 2). Primary hornblende grains cal outline (1500 × 500 m) with two ends tapered are discrete, small, subhedral and fewer in num- and is intruded by a number of dykes of doleritic ber. Hornblende in Paharpur gabbro occurs mostly 796 Aditi Mandal et al 00.01000000000.02000.01 .3.4.6.6000000000.080.01 45.7 45.78 46.77 47.76 47.880.030.140.060.060000.030.010.040 47.26 55.36 56.11 47.53 53.47 48.16 46.8 58.19 57.8 34.83 34.71 34.23 33.65 33.61 33.87 28.75 28.32 34.02 29.74 33.58 34.01 26.08 26.79 Representative composition of plagioclase from intrusives of Paharpur. 3 3 2 O 1.11 1.21 1.73 2.15 2.2 1.86 5.64 5.67 1.84 4.71 2.15 1.7 6.91 7.16 2 O O O 0.02 0.02 0.04 0.05 0.06 0.02 0.16 0.12 0.02 0.13 0.03 0.02 0.29 0.26 2 O 00000000000000 2 2 2 2 Table 1. Sl.no.1234567891011121314 RockSiO G* G* G* G*Number of ions on the basisSi of 8 O* Al G*FeMn00000000000000.001 Mg00.00500.00100000000.00100 G*Ca 2.1 1.886Na 0.004K G* 2.098 1.875Ti00000000000.001000 0.007Cr0.0010.0050.0020.0020000.00100.001000.0030 0.916 2.136 1.843OH00000000000000 G* 0.099 0.005Albite 0.919 0.001 2.177 1.807Anorthite 0.108 G* 0.001 0.877 0.001 2.178 1.802 0.153 0.90 0.004 0.10 G* 0.834 0.002 2.163 1.827 0.19 0.002 0.89 0.11 0.838 0.003 2.474 1.514 G* 0.194 0.003 0.85 0.15 0.003 0.85 1.487 2.5 0.165 0.006 G* 0.001 0.81 0.528 0.19 1.823 2.161 0.488 0.002 D* 0.009 0.511 1.576 0.81 0.19 2.405 0.007 0.49 0.007 0.858 D* 0.84 0.16 1.8 0.001 2.19 0.162 0.001 0.608 1.836 0.52 0.002 2.144 0.48 0.411 0.007 0.821 1.373 0.005 2.598 0.189 0.51 0.49 0.002 0.879 0.002 2.564 0.151 1.4 0.84 0.001 0.16 0.427 0.598 0.017 0.60 0.453 0.40 0.615 0.015 0.81 0.19 0.85 0.15 0.42 0.58 0.42 0.58 Cr Al H FeOMnO00000000000000.03 MgO00.0700.0100000000.0200 CaONa 0.1 18.62 0.17 18.71 0.14 17.93 0.03 17.08 0.1 17.19 17.33 0.05 11.03 0.07 10.71 0.17 17.6 0.05 12.62 0.18 16.86 0.02 17.9 0.04 8.93 0.14 9.52 0.04 K TiO G*: gabbro, D*: dolerite; O*: oxygen. Petrology and geochemistry of hornblende gabbro 797 12 10.9 10.6 6.5 12.4 12.3 12.7 13.1 7.99 7.46 7.43 8.82 5.89 11.7 11.5 11.3 11.38 43.9 45.5 43.9 50.70.97 410.15 0.79 41.4 0.02 2.43 39.9 0.06 0.28 40 0.09 1.93 48.3 0.14 1.98 49.4 2.01 0 49.2 1.85 0.02 47.9 0.35 0.04 51.1 0.34 0.04 41 0.07 0.3 40.7 0.07 0.49 40.9 0.03 0.2 41.04 2.34 0 2.05 0.01 2.2 0 1.99 0.1 0.04 Representative composition of from intrusives of Paharpur. 3 3 2 O 1.71 1.65 1.5 0.63 1.64 1.58 1.63 1.75 0.79 0.77 0.76 0.82 0.6 1.51 1.33 1.55 1.41 2 O O O 0.74 0.6 0.54 0.47 1.76 1.63 1.72 1.8 0.67 0.61 0.63 0.77 0.41 2.14 2.15 2.02 2.08 2 O 2.05 2.07 2.05 2.1 1.98 1.97 1.94 1.96 2.06 2.08 2.07 2.06 2.09 1.99 1.95 1.97 1.97 2 2 2 2 Table 2. Al Cr H Sl.no.1234567891011121314151617 RockSiO G* G* G* G* G*Cations on the basis G* of 22Si O* Al G*FeMn 6.43Mg G* 2.07Ca 1.35 6.6 0.02Na 1.86 G*K 3.16 1.21 6.42 0.01Ti 1.83 1.97 2 G* 3.34Cr0.0200.010.010.02000.010.010.010.0100000.010.004 1.41 0.49 7.23 0.02 1.09OH2222222222222222 1.98 0.14 3.27 G* 1.38 0.46 0.11 0.03 6.2 2.21 1.93 0.11 3.32 2.21 0.43 G* 0.09 0.03 6.29 1.97 2.2 2.21 0.1 2.03 0.17 0.27 G* 0.02 6.17 1.95 2.31 2.28 0.09 2.35 0.48 0.03 0.02 6.12 1.94 D* 2.35 2.01 0.34 2.38 0.47 0.22 0.03 7.02 1.93 1.37 D* 2.03 0.32 1.51 0.49 0.23 0.03 7.12 1.93 1.27 3.09 0.34 D* 1.49 0.52 0.23 0.02 7.14 1.27 3.12 0.35 1.43 0.22 2 D* 0.21 0.03 6.97 1.51 3.14 0.13 1.48 0.21 1.98 0.04 0.01 7.31 0.99 2.99 0.11 1.31 0.21 1.98 0.04 0.02 6.17 2.07 3.38 0.12 2.42 0.23 2.01 0.03 0.02 6.25 2.08 2.07 0.14 2.32 0.17 2.01 0.05 0.02 6.24 2.03 2.05 0.08 2.28 0.44 2.09 0.02 0.03 6.255 2.043 2.14 0.41 2.325 2.12 0.27 0.4 0.021 2.111 0.42 2.09 0.24 0.46 0.39 2.095 0.25 0.416 0.404 0.229 FeOMnOMgOCaO 11 0.13Na 14.5 12.6 0.1 10 15.5 12.8 0.12 11.5 15 12.3 0.26 11.6 15.6 12.9 0.26 17.5 9.8 0.14 12 16 10.1 0.15 11.9 18.1 8.72 0.25 11.7 18.6 8.93 0.27 11.8 12.4 14.3 0.12 12.8 12.3 14.5 0.26 12.8 11.8 14.5 0.1 12.7 12.2 13.8 0.16 12.9 10.9 15.8 0.19 13.1 19.2 9.2 0.15 18.1 13 8.97 17.9 0.2 12.9 9.39 18.24 12.8 0.17 9.29 12.83 K TiO G*: gabbro, D*: dolerite, O*: oxygen. 798 Aditi Mandal et al

1.0

Tccher- 0.9 makite horn- blende - Magnesio 0.8 Actinolite horn- hornblende Tschermakite blende 0.7

+) 0.6

2

0.5

Mg/(Mg+Fe 0.4 Ferro Ferro- Ferro- Ferro- actinolite tscher- actinolite hornblende Ferro- makite 0.3 horn- tschermakite blende horn- blende 0.2

0.1

0.0 8.0 7.5 7.25 6.5 6.25 5.5 Total Si

Figure 2. Representative amphibole composition of intrusives of Paharpur. Black-filled squares represent the amphibole compositions. as secondary grains as evident from their irregu- pyroxenes are only partly altered along grain lar, rim-like occurrences and large; anhedral char- boundary to hornblende which occurs as reaction acter. Secondary are thought to have rim. At places, alteration occurs along both bound- formed due to alteration of pyroxenes and pla- ary and cleavage planes, so that the remaining gioclases. In two samples of dolerite (77, 77A), pyroxene grains are almost completely replaced by hornblende. At places, hornblende is very large in size, forming large phenocryst embedded in a relatively fine ground mass of plagioclase and pyroxene. This type of texture attributes to a por- phyritic nature of the rock. Opaque minerals are also present as inclusion within hornblende. At some places, hornblende occurs as large enclosing grain within which small pyroxene, ilmenite and euhedral plagioclase grains are floating as discrete grains. This type of hornblende grain is defined as ‘poikilitic hornblende’ (figure 3). Pyroxenes are of two types. Clinopyroxene is more abundant (10% to 30%, mg no. 72–82). They occur as medium-to-coarse subhedral grains fre- quently replaced by hornblende. Some grains con- tain schiller plates. These are mostly diopsidic in composition (Wo48En40Fs12–Wo50En43Fs07,table 3). Orthopyroxenes (pleochroic from pale pink to pale green) are very small in abundance (0.5–2%) and frequently crowded with fine pinkish brown Figure 3. Poikilitic hornblende (brown) enclosing early dusts. These are mostly hypersthene and formed plagioclase and pyroxene grains. Hbl – Hornblende, in composition (En60–En79, table 4). At places, Pl – Plagioclase, Px – Pyroxene, Op – Opaque. Petrology and geochemistry of hornblende gabbro 799 53.21.95 52.9 2.13 54.2 1.01 53.6 0.79 52.10.21 2.730.15 0.35 52 2.69 0.08 0 51.6 3.71 0.03 0.04 52.51 2.3 0.36 0.09 54 0.91 0.32 54.3 0 0.73 0.41 53.36 0.08 1.15 0.22 53.8 0.03 1.13 0.05 51.61 1.66 0 51.6 0 1.65 0.01 51.5 0.11 1.73 51.68 0.1 0.08 1.51 0.14 0 0.15 0.04 0.13 0.08 0.08 0.04 0 0 Representative composition of clinopyroxene from intrusives of Paharpur. 3 3 2 O 0.22 0.21 0.39 0.28 0.47 0.43 0.59 0.38 0.37 0.34 0.4 0.32 0.46 0.44 0.45 0.43 2 O O 00.1.1 0000000 0 0000.020 O 00000.010.010 2 O 0000000 000 00 000 0 2 2 2 2 Table 3. ln.246 8 910111213141516 Sl.no.1234567 RockSiO G* G* G* G* 0 Number of 000 ions G* 0 on the basisSi of 6 O* Al G*FeMn G*MgCa 1.94Na G* 0.08K 0.16 1.93 0.01Ti 0.09 G*Cr 0.84 0.16 1.98 0000000 000 0.01OH 0.04 0000000 0.97 000.00200 000 0000000 00 0.84 0000.0010 G* 0.19 0.02 1.98 000Wollastonite 0 0.01 0.04 0.97Enstatite 0.78 0.19 0.02 1.93 0.01 G*Ferrosilite 0.01 0.12 50 0.98 0.27 0.03 1.94 0.8 0.01 0.43 0.01 0.12 0.99 G* 0.07 50 0.25 0.02 1.91 0.73 0.43 0.01 0.16 0.92 0.07 0 D* 0.27 0.03 1.945 0.73 50 0.41 0.01 0.92 0.09 0.1 0.261 0.03 1.98 D* 0 0.7 0.41 50 0.92 0.01 0.09 0.04 0.19 0.04 1.99 0.01 0.751 0.38 D* 0.49 0.912 0.01 0.13 0.03 0.18 0.027 1.968 0.01 0.79 0.39 0.49 0.99 0.01 D* 0.12 0.186 0.03 1.97 0.05 0.01 0.38 0.49 0.99 0.8 0.13 0.01 0.19 0.02 1.917 0.05 0.006 0.998 0.776 0.48 0.4 0.01 0.12 0.361 0.029 1.93 0.073 0.99 0.78 0.51 0.41 0 0.012 0.09 0.35 0.02 1.93 0.07 0.976 0.51 0.68 0.41 0.01 0.08 0.37 0.033 1.934 0.08 0 0.97 0.68 0.01 0.51 0.443 0.03 0.09 0.067 0.4 0.003 0.94 0.67 0.017 0.51 0.03 0.09 0.4 0.861 0.691 0 0.031 0.48 0.18 0.34 0.004 0.49 0.17 0.34 0.48 0 0.19 0.34 0.43 0.22 0.35 0 0.002 Al FeOMnOMgOCaONa 5.2 0.19 15.4 5.22Cr 0.22 24.9H 15.4 6.36 0.16 24.9 14.4 0.22 6.2 25 14.5 0.23 8.57 13.1 25 0.35 8.1 13.2 23.2 0.24 8.59 12.7 23 0.32 8.42 13.61 23.2 0.18 6.18 14.5 22.99 0.19 5.76 14.7 25.2 14.11 0.3 6.03 25.2 14.3 0.33 6.1 25.25 12.28 0.37 11.62 25.2 12.2 0.31 11.2 24.52 11.9 0.43 11.7 24.3 12.4 14.15 0.54 23.5 21.49 K TiO G*: gabbro, D*: dolerite, O*: oxygen. 800 Aditi Mandal et al On the basis of 22 O* On the basis of 32 O* Orthopyroxene Biotite Ilmenite Magnetite 0 0 0 0.19 0.1 0 0.03 0.01 0 0.03 0 0.1 0.03 0.08 55 46.03 55.32 52.75 52.93 52.1 50.9 50.4 50.8 51.2 36.03 35.53 0 0.04 1.72 13.29 0.81 0.90.03 0.87 0.38 0.97 0.01 0.81 0.01 0.74 0.03 0.82 0.06 0.86 0.1 13.6 0.02 13.57 0.01 0.04 0.05 5.52 5.18 0.86 45.6 0.19 Representative compositions of orthopyroxene, biotite, ilmenite and magnetite from intrusives of Paharpur. 3 3 2 O 0 1.84 0.01 0 0.03 0.03 0 0.04 0.02 0 0.1 0.19 0 0 2 O O O 0 0.24 0 0.03 0 0 0 0 0 0 9.98 9.85 0.01 0 2 O 0 0 0 0 0 0 0 0 0 0 3.93 3.88 0 0 2 2 2 2 Table 4. FeOMnOMgOCaONa 15 0.19 27.5 7.55 0.93 0.03Cr 15.56 16.03 12.6 0.41 27.84 26.82 19.03 0.21 0.71 24.34 20.97 0.39 0.89 25 20.6 0.89 0.42 16.9 30 0.79 0.43 16.7 29.8 1.07 0.78 16.6 30.1 0.93 0.67 16.7 29.9 1.05 0.46 11.31 19.74 0.57 0.07 0.01 11.5 18.68 0.06 0.01 49.94 0.49 0.78 0.04 92.55 0.04 0 0.03 Al ln. 2 3 4 5 67891012 1 1 Sl.no.1 RockSiO GN* GN* GN* GN* G*Number of ions on the basisSi G* of 6 O* AlFe D*MnMg D* 1.97Ca 0.07Na 0.45 1.714 D*K 0.01 0.583 4Ti 1.46 0.235 1.981Cr D* 0.001 0.034 0.04 0OH00000000004 0.864 1.989 0.48 0.012 0.503 0 0Enstatite D* 0.04 1.486Ferrosilite 0.845 1.98 0 0.023 0.008 0.133 0.038 0 0.76 1.07 0.24 0.761 0 1.97 0.011 0.028 0.016 0.001 0.04 D* 0.011 0.79 1.169 0.79 1.97 0.03 0.21 0.017 0.04 0 0 1.16 0.97 0.76 1.97 0 0.03 0 0.02 0.24 0.03 0.001 D* 0.97 0.98 1.98 0.002 0.04 0.56 0.03 0 0.04 0.44 0.97 0 0.96 1.98 0.03 0.04 0.61 0.006 0.04 0 0 0.39 0.96 0.001 0.96 5.493 D* 0.03 0.02 0.60 0.003 2.444 0.96 2.516 0.4 0 0 0.009 0.02 0.50 0 5.495 2.572 0 2.474 0.5 0.001 0.50 0 2.416 0 0.008 0 0.5 0.50 0 2.652 0.001 0 0.018 0 0 11.942 0.50 0 0.5 0.189 0 0.209 0 0 0.5 0.013 0 31.165 0.41 0.016 0 0.03 0 0 0.024 0 0.012 0 1.941 0.056 0.633 1.944 0 0.603 0 0.012 0.003 9.804 0.004 0.007 0.057 0 0.025 K TiO H GN*: gabbronorite, G*:gabbro, D*: dolerite, O*: oxygen. Petrology and geochemistry of hornblende gabbro 801 pyroxene looks greenish, resembling hornblende. At other places, pyroxenes are totally replaced by hornblende, so that there are no pyroxene grains; only occurrence of hornblende and their nature suggest that they are alteration products. Apatites are rare and occur as small to medium- sized, elliptical grains generally associated with pyroxene and opaque minerals. Phlogopite is less-abundant in these rocks, only a few grains are observed. Rutile occurs as patchy grains with brilliant red- dish brown colour associated with plagioclase and hornblende. Opaque minerals occur in the form of ilmenite (subhedral) and a few magnetite (euhedral) asso- ciated with pyroxene and hornblende. Texturally, the main gabbroic mass is character- ized by porphyritic and poikilitic texture. Dolerite dykes are composed mainly of plagio- clase–pyroxene–hornblende with subordinate amount of olivine, rutile and opaque. Plagioclase (An42, table 1) occurs as criss-cross medium-sized laths forming triangular and poly- gonal spaces filled up by anhedral pyroxenes and opaques giving rise to intergranular texture. Pyroxenes occur both in the form of orthopy- roxene (hypersthene) and clinopyroxene (diopside- salite). Orthopyroxene is En50Fs50 (table 4) and clinopyroxene varies from Wo43En35Fs22 to Wo49En34Fs17 in composition (table 3). They occur as elongate (generally orthopyroxene) and subhe- dral equant (generally clinopyroxene) grains. The elongate grains follow the longer contact of pla- gioclase laths. Pyroxene occurs as thin reaction rims around olivine. Individual grains of hypers- thene (pleochroic from pale green to pale pink) are crowded with numerous tiny pinkish-brown dusts. Some pyroxene grains contain schiller plates and also extremely fine needles of opaque along the prismatic cleavage giving turbidity to the grains. occur as medium-sized rounded grains successively surrounded by coronas of orthopyrox- ene, brownish hornblende and spongy actinolite; the latter being in contact with the lath-shaped plagioclase. The rimmed olivine grain, in the core of the above corona is at many places repre- sented by black dusts and highly irregular grains of magnetite, some of which have filled up the cracks. Hornblende (X = yellowish green, Y = dirty green,  ◦ Z = brownish green; ZC =20 ) is medium-grained, anhedral to subhedral. They are mostly magnesio- hornblende (figure 2, table 2) in composition. Brownish hornblende frequently has replaced coarser pyroxenes either forming corona along the

contact with plagioclase laths or forming irregular Modal (volume %) composition of Paharpur intrusives. patches within pyroxenes. In many cases of such replacement, minute opaque grains are intimately

associated with hornblende–pyroxene aggregate. Table 5. Sl. no. Sample no.1 Rock type23 Olivine4 L5 Orthopyroxene5 72B Clinopyroxene6 757 Plagioclase Gabbro 77A Gabbro8 78 K-feldspar9 10C Gabbro Dolerite Quartz10 15C – –11 Hornblende 77 Gabbro Gabbro 84 Actinolite – Gabbro – 87 73 Biotite Dolerite – – 1.4 15.3 Opaque Dolerite Dolerite – Basalt 8.6 9.0 – 20.2 – 3.9 11.9 – 19.9 – 2.0 22.3 2.3 14.9 22.0 8.2 10.2 26.6 25.9 11.9 – – 38.0 36.5 11.2 – 2.5 2.3 34.0 7.4 28.0 5.0 32.0 2.4 26.7 – 6.0 2.8 28.9 3.9 37.1 3.0 5.0 2.2 2.3 50.0 33.4 – 3.0 – 40.5 30.0 2.5 26.8 – 2.0 – 42.3 – 2.0 0.2 – 42.5 61.7 – 2.1 – 30.2 – – – – 8.8 – – – – – 23.2 1.6 0.6 – 35.4 23.9 0.2 0.5 1.2 – – – 2.7 – 7.8 – 2.1 4.4 – 4.5 – 10.2 15.3 802 Aditi Mandal et al

Gabbro Dolerite Basalt Pl Pl Anorthosite

Norite

Gabbro/ Hbl gabbro Gabbro Px-hbl gabbro/norite

Gabbronorite

Plag-bearing Plag-bearing hbl px hornblendite Plag-bearing websterite Px Opx Hbl Cpx

Figure 4. Modal classification of intrusives of Paharpur after Streckeisen (1973).

Biotites are rare (table 4). In the following section, the major, minor Rutile is patchy and its rare occurrence is seen and trace element content of mafic intrusives of to be associated with hornblende and plagioclase. Paharpur are discussed with the help of suitable dia- Opaques are mainly ilmenite with occasional grams and some of the chemical parameters of these magnetite (table 4). In most of the cases, it occurs rocks are compared with those of Mid-oceanic as fine flakes and fills the fractures in rimmed Ridge Basalt (MORB), island-arc tholeiite (IAT) olivine. and calc-alkaline basalt (CAB) (Wilson 1989). In most of the cases, dolerites show intergranular Porphyritic gabbro shows SiO2 content ranging texture. Some finer-grained dolerites show mineral between 44.57% and 52.28%. MgO content is fairly lineation. high and ranges from 8.55% to 15.65%. On the Modal analysis of gabbroic intrusives and younger other hand, Al2O3 content varies from 6.79% to dolerite dykes of Paharpur has been carried out 18.92%. CaO content is high (10.15% to 17.26%). using automatic point counter and the modal K2O content of gabbroic rock ranges from 0.07% composition is given in table 5. Using the modal to 0.63%. TiO2 content is fairly low and varies from proportion of plagioclase, orthopyroxene, clinopy- 0.38% to 1.88%. P2O5 ranges from 0.04% to 1.0%. roxene and hornblende, gabbroic and doleritic High MgO (>8%), SiO2 (>50%) and low TiO2 rocks have been classified (figure 4) using the (0.4%) content imparts a boninitic character with IUGS scheme (Streckeisen 1973). Intrusive rocks high Ca content. have been identified as gabbronorite/hornblende Trace element abundances (including compatible gabbronorite. and incompatible elements) are variable. Among the incompatible elements, Zr abundances are highly variable. Gabbroic intrusive shows much 4. Geochemistry lower abundances of Zr value (14.5–56.8 ppm). Y shows restricted range of distribution from 10 Major trace (including rare earth) element com- to 23.6 ppm. Gabbroic intrusive has Rb content position of eight samples of gabbroic rocks were of 6.1–37.6 ppm. Sr shows a very wide range of determined at the Chemical Division, Geological variation from 111.2 to 582.1 ppm. Incompatible Survey of India, Kolkata using XRF and another elements (Zr, Y, Rb) show low abundance. The eight from National Geophysical Research Insti- abundance of compatible elements (Ni, Co, Cr) is tute, Hyderabad using XRF and ICP-MS. Inter- variable in different parts of the gabbroic intrusive. national rock standards such as BHVO, JB 2 for Ni content varies from 19.1 to 165.5 ppm and Cr major element and trace elements respectively, content ranges from 32.3 to 570.5 ppm. Overall, the were also run along with the samples to check pre- gabbroic body shows enrichment in Rb, Th, Ba, Sr cision and accuracy of measurement. The certified and depletion in Nb and Zr compared to average and analysed values of BHVO and JB 2 are given MORB (Wilson 1989), this chemical characteristic in the tables (tables 6 and 7) along with major and is very much similar to the Dhanjori mafic volcanic trace element abundances of samples to check the rocks from adjacent Singbhum region (Roy et al error limits of measurement. 2002). Petrology and geochemistry of hornblende gabbro 803 0.430.12 0.38 0.12 0.43 0.13 1.88 0.04 0.43 0.04 0.57 0.22 0.45 0.1 1.1 0.42 0.16 0.07 0.9 0.13 0.72 0.75 0.83 0.37 1 1.14 0.05 1.55 0.13 2.09 0.2 1.15 0.32 0.2 2.71 2.67 47.25 52.2811.22 51.87 44.950.831 14.6 0.576 13.75 48.3 0.664 14.11 1.798 51.02 7.12 2.79 6.79 0.62 44.57 51.35 18.92 49.05 0.99 46.93 14.23 46.24 14.97 1.239 46.96 11.87 1.011 52.18 1.496 15.6 46.04 15.24 4.46 46.98 48.16 3.85 7.3 48.71 16.32 1.83 12.86 2.86 12.05 11.21 49.63 1.749 18.72 1.14 49.89 13.8 1.83 13.71 10.39 Major oxide composition and CIPW norm from intrusives of Paharpur. 3 3 5 2 O 0.7 1.11 1.12 1.52 0.54 0.84 1.72 2.37 1.68 1.92 2.16 2.1 0.88 1.91 2.56 2.33 2.97 2.26 2.21 2 O O O 0.19 0.26 0.33 0.07 0.11 0.63 0.15 0.58 0.14 0.26 0.14 0.25 0.22 0.22 0.45 0.37 1.04 0.52 0.011 2 O 2 2 2 2 Table 6. P mg no. 63.66 62.25 61.18 30.85 69.74 73.72 57.06 36.93 54.25 34.53 54.17 43.82 67.43 47.34 42.64 27.26 47.00 Al Fe FeOMnO 7.479MgO 5.184 0.14CaO 5.976 16.18 0.11 13.1Na 16.84 6.64 0.12 8.55 15.75 0.15 14.54 9.42 5.58 10.15 7.22 0.29 17.3 15.3 0.1 17.3 15.65 8.91 11.15 9.099 10.45 11.84 0.11 13.46 7.04 9.58 6.53 0.16 10.41 10.78 10.79 0.15 10.88 6.28 7.1 13.8 0.2 8.82 8.32 11.54 0.29 6.82 16.35 8.12 15.74 0.32 11.82 11.51 10.23 13 0.3 9.71 7.93 0.25 10.86 5.07 0.34 1.83 0.21 5.9 11.41 10.36 0.19 5.05 11.33 0.16 7.23 0.11 7.12 ln. 111111111 19 2 3 4 5 6 7 8 9101112131415161718 Sl.no.1 RockSample G* 72bSiO G* 75 G* 85 G* 78 G* S05 S96/2D G* S13–16C 77 G* 84CIPW norm D* 87Qz D* S01Or D* S02AbAn D* S03 0 1.14Di En 6.02 D* 4.08 S08 1.55Di En 27.34 23.78 9.48 3.31 1.98 34.42Di S07 18.32 En D* 32.01 15.88 9.62 16.95 0.42Hy 32.05 En 12.1 73 13.1 0 7.96 D*Hy 0.66 11.07 Fs 6.1 16.9 4.62 29.1Ol 0.1 3.33 Fo 4.87 basalt D* 12.96 3.75 0.04 0Ol 30.13 Fa 4.67 7.15 9.5 12.14 3.83 21Mt B* 12.87 4.67 BHVO* 5.42 5.15Ilm 8.31 22.13 Aleutian 0 44.2 11.68 5.33 0 0.9Ap 3.35 BHVO* 4.42 14.8 1.23 1.12 Certified 0 26.86 5.07 0.83 0 0.84 33.47 3.48 3.38 20.35 2.09 8.55 Measured 0.77 24.14 0.73 4.74 14.46 0.27 0.98 0.84 0 8.61 32.57 17.05 0 0.83 0.26 2.66 13.28 1.62 7.35 1.06 18.34 4.03 9.3 31.5 13.28 3.64 0.29 1.07 17.82 5.18 0.83 15.49 0.54 4.1 1.69 2.6 0.09 9.72 8.39 0.83 36.19 4.42 5.86 7.53 1.48 12.56 13.77 0.91 22.34 0.09 16.57 27.7 2.96 15.21 2.46 8.73 1.09 0 1.32 21.73 21.6 7.37 8.37 8.7 18.89 19.97 0.48 4.62 9.7 1.34 10.53 1.84 0 19.39 14.14 3.6 10.55 1.46 2.67 10.59 0.76 0.87 6.95 5.67 3.45 2.22 5.18 0 13.52 0.22 11.29 0 1.82 5.94 4.96 2.12 4.05 4.14 6.47 0 1.49 0.81 3.55 0.35 3.1 3.13 4.05 0 2.28 1.44 0.16 1.18 1.8 8.94 7.23 0.15 1.47 4.3 6.5 14.77 0.3 6.7 1.37 6.17 3.29 5.6 1.65 1.58 0 5.07 2.69 2.19 0 0.71 5.33 4.26 0.11 5.74 2.22 0 16.33 0 0.29 2.96 3.96 2.57 4.03 0 0.44 1.37 0.71 0 2.5 K TiO G*: gabbro; D*: dolerite; B*: basalt; BHVO: Basalt, Hawaian Volcanic Observatory. 804 Aditi Mandal et al Trace and REE composition of Paharpur intrusives. Table 7. SampleRockSc 72V G*CrCo 72BNi 67.22 192.86Cu G* 524.33Zn 65.082 77 54.681Ga 174.75 148.02 570.54Rb G* 20.97 61.469 65.07Sr 166.4 77D 86.012 165.95 22.69Y 20.85 46.74 8.455 G* 60.649Zr 36.514 171 18.36 32.26Nb 78 6.14 35.652 48.12 332.77Cs 85.56 8.565 300.03 19.05 10.589 164.51Ba G* 87.7 43.912 25.434 5.148 318.17 66.883La 88.36 12.84 85 165.91Ce 10.564 239.88 116.7 0.68 128.1 27.847 13.54 109.4 87.526Pr 0.265 97.977 G* 12.94 23.341 39.4 156.005Nd 13.52 185.512 116.74 147.62 0.831 13.83 111.2Sm 7.829 75 35.5 113.34 80.718 0.152 17.085 18.684 28.655 19.49Eu 103.3 13.77 37.08 59.193 233.613 D* 37.598Gd 181.53 2.607 2.77 131.292 31.88 80.54 7.833 11.503 19.117 49.38 34.18 0.89Tb 23.587 17.91 14.9 120.339 41.34 84 219 3.522Dy 10.582 582.11 2.771 68.16 56.821 68.78 2.628 14.87 51.16 D* 40.9 6.85 0.919Ho 0.905 11.689 11.071 23.3 57.64 10.62 15.13 2.852 383Er 618.045 14.521 60.2 3.49 87 5.076 15.31 73.5 41.877 8.549 0.354 1.58Tm 6.952 9.772 0.946 1.711 78.85 D* 13.8 36.7 2.036Yb 8.972 195.5 2.901 8.6 23.545 0.391 10.056 34.552 2.743Lu 1.619 11.531 388 2.16 103 0.933 7 12.74 0.367 73 320 0.52 0.292Hf 0.819 36 6.164 126.3 B* 8.373 0.109 1.99 3.46 27.259Ta 12.96 5.726 11.758 2.219 14 17.5 0.332 30.146 54.4 2.686 0.48 0.462 Aleutian 0.551Pb 17.45 1.34 0.881 135 13.917 0.244 3.472 27.4 0.096 578Th 12.24 8.15 basalt 0.102 2.26 19.726 23.48 3.485 39.8 0.46 0.468 3.367 8.646U 0.895 40.4 0.517 0.978 0.862 JB/2 52.796 17.601 0.411 14.2 0.118 3.375 1.6 0.095 4.854 49.72 0.22 17.43 standard 2.37 14 561.001 576 0.468 0.898 2.104 3.575 0.486 26.47 1.079 1.681 2.559 1.036 0.856 1.4 13.853 21.7 0.29 227 37.92 0.151 1.534 10.16 68.5 analysed 3.321 JB/2 110 0.22 13.976 3.783 4.455 0.2 0.191 0.765 0.529 5.37 69.6 0.374 0.559 3.019 0.99 1.373 3.348 0.879 0.29 25.7 178 6.2 0.882 0.39 17 1.697 226.219 10.6 0.198 0.246 1.962 2.572 372 0.177 106.786 0.368 1.03 24.9 1.159 0.858 0.909 6.529 1.89 0.297 0.271 1.29 0.402 1.19 51.4 0.923 174.347 19.1 1.347 0.2 0.258 1.123 0.121 2.94 16.558 1.787 7.91 6.121 0.21 1.816 0.283 0.191 1.65 0.548 1.283 17 24.311 0.506 0.527 208 2.978 4.62 0.715 0.107 0.212 51.198 0.8 0.208 1.67 0.482 1.09 2.91 0.689 0.09 6.77 1.02 2.176 0.9 2.37 7.37 0.444 14.1 208.136 0.493 0.572 1.16 0.7 3.72 0.086 0.4 0.626 0.101 2.637 1.452 0.547 0.753 1.2 0.96 1.069 3.75 6.519 1.32 2.014 0.093 0.253 2.341 6.24 0.66 0.91 0.31 0.59 0.355 6.7 1.162 0.839 2.25 0.931 4.41 0.282 1.877 0.77 0.86 0.419 0.212 0.838 0.919 0.7 3.55 3.28 0.241 0.93 0.59 0.62 6.456 4.01 0.66 2.13 0.096 1.123 2.248 0.538 0.73 0.854 0.81 3.342 3.66 0.114 0.29 1.46 1.95 1.17 1.98 0.626 2.63 0.11 0.24 0.779 3.28 2.21 3.68 0.39 0.45 2.51 1.42 0.93 2.642 0.2 5.4 0.33 0.379 0.454 1.455 2.55 0.16 0.2 5.072 0.303 0.155 G*: gabbro; D*: dolerite; B*: basalt. Petrology and geochemistry of hornblende gabbro 805

10

Gabbro Dolerite Basalt

O (wt%) 5 Picro- 22 Foidite basalt Basaltic NaO+K Basalt andesite

0 35 45 55 65 75 79 SiO2

Figure 5. Total alkali vs. silica diagram of intrusives of Paharpur, after LeBas and Streckeisen (1991).

Gabbro Dolerite Basalt

140 700 120 600 100 500 80 400 Sr 60 300 Total REE Total 40 200 100 20 0 0 20 40 60 80 20 30 40 50 60 70 80

20 20

3 15 15 O 2 10 10 CaO

FeO+Fe 5 5

0 0 20 30 40 50 60 70 80 20 40 60 80

2.5 12

2 10 8 1.5 2 6 Nb

TiO 1 4 0.5 2 0 0 20 30 40 50 60 70 80 20 40 60 80 Mg No.

Figure 6. Mg no. variation diagram of intrusives of Paharpur (major element oxides in percent and trace element in ppm).

On the other hand, the later formed dolerite TiO2 0.37% and CaO 16.35%) appears to show dykes show silica range from 46.24% to 52.18%. a boninitic character while other samples are of MgO content is highly variable and ranges from tholeiitic affinity. 5.9% to 13%. CaO content is high (9.71–16.35%). Trace element abundances of the dolerite dykes Within these dykes, one sample with high SiO2, are also variable similar to the main gabbroic mass. MgO, CaO and low TiO2 (SiO2 52.18%, MgO 13%, Zr ranges from 10.1 to 14 ppm. Rb ranges between 806 Aditi Mandal et al

2.7 and 8.5 ppm. Sr shows a high range of 126.3 all the intrusives including gabbro and dolerite to 618.1 ppm. Within the compatible elements, Ni show flat to slightly LREE-fractionated pattern ranges between 10.6 and 233.6 ppm, whereas Cr and variable degree of overall REE enrichment content ranges from 17.9 to 185.5 ppm. with respect to the abundance in chondrite. Gab- Basaltic component of the mafic clan shows low bro has higher overall abundance of REE and SiO2 content (48.16%). All the other oxides are more pronounced LREE-fractionated REE pattern comparable with associated gabbro and dolerite. than dolerite. In basaltic dyke, no fractionation is This basaltic rocks show unusually high FeO observed. Most of the samples of dolerite and basalt (15.74%) and low MgO (5.9%). show distinct Eu anomaly, whereas gabbro do not Similar pattern of abundance (such as that of show appreciable Eu anomaly. gabbro) of trace elements is shown both by dolerite Primitive mantle-normalized spider diagram and basalt. (figure 9, primitive mantle data after Sun and The Paharpur intrusives when classified accord- McDonough 1989) indicates enrichment in Rb, Th, ing to TAS (total alkali vs. silica) diagram (figure La, Sr and Ba and depletion in Ti, Nb, Zr, Y, 5, after LeBas and Streckeisen 1991), are found to Hf, Sm and P. The LILE enrichment and Nb, Ti, lie mostly (14 out of 16) within the field of basalt Zr depletion suggest arc-like geochemical signature with one in picro-basalt field and one in basaltic of the parent for the main gabbroic intru- andesite field. sive and the younger dolerite dykes. In the N-type In mg no. variation diagram (figure 6), the intru- MORB-normalized spider diagram (figure 10, N- sives show a clear positive correlation with CaO, type MORB data after Sun and McDonough 1989), Ni and Sr, whereas total iron, total REE and elements such as Rb, Ba, Th, K, La, Ce, Sr and TiO2 show a good negative correlation. In AFM (A=Na2O+K2O; F=FeO+Fe2O3; M=MgO;) dia- 40 gram (figure 7, after Brown 1982), Paharpur intru- sives follow precisely (15 out of 16) the tholeiitic trend confirming a well-differentiated magmatic character which forms under low oxygen fugacity 10 condition. It prevents iron in the magma to pre- cipitate as oxide phase and its entry into silicate structure is prolonged to define an iron-enrichment trend or tholeiite trend (Bose 1997). In chondrite-normalized REE diagram (figure 8, Sample/Chondrite Basalt Dolerite chondrite data after Sun and McDonough 1989), Gabbro 1 La Pr Eu Tb Ho Tm Lu Ce Nd Sm Gd Dy Er Yb Gabbro Figure 8. Chondrite normalized REE diagram (chondrite Dolerite data after Sun and McDonough 1989) of intrusives of Basalt Paharpur.

60 Gabbro Fe trend Dolerite O 22 Basalt Na O+K O

Tholeiitic 10

Calc-alkaline trend Sample/Primitive mantle

1 0.8 Rb Th Ta La Sr Sm Hf Tb P Ba Nb K Ce Nd Zr Ti Y MgO Figure 9. Primitive mantle normalized spider diagram Figure 7. AFM diagram of intrusives of Paharpur, after (primitive mantle data after Sun and McDonough 1989) of Brown (1982). intrusives of Paharpur. Petrology and geochemistry of hornblende gabbro 807

70 gabbroic and doleritic intrusives of Paharpur. Gabbro These chemical characteristics are correlatable Dolerite with the chemical compostion of hornblende- 10 Basalt bearing gabbroic rocks of ophiolitic suite and cross- cutting dykes of tholeiitic to boninitic affinity in subduction zone setting. The enrichment in incom- type MORB

− patible elements of low ionic potential or LILE 1 (compared to MORB and primitive mantle) in the gabbroic and doleritic rocks of Paharpur is con- sidered to have been derived from mantle source Sample/N region which has been metasomatized by sub- 0.1 ducted sediment and subduction zone fluid. On the Rb Th Ta La Sr Sm Hf Tb P Ba Nb K Ce Nd Zr Ti Y other hand, low abundance of elements of high ionic potential or HFSE might be the result of stabil- Figure 10. N-type MORB normalized spider diagram (N- ity of minor residual phases (e.g., rutile, zircon and type MORB data after Sun and McDonough 1989) of sphene) in the mantle source which preferentially intrusives of Paharpur. concentrate a range of HFSE. The SiO2 content of the magmatic rocks of Paharpur is comparable with IAT and average Nd show variable amount of enrichment while ele- MORB, MgO with arc and average Atlantic ments such as Nb, Zr, Ti, Hf, Y, Tb and P show MORB, Al2O3 is lower than the average IAT conspicuous depletion with respect to abundances and MORB, CaO corresponds with average of these elements in MORB. Atlantic MORB and IAB. Within the minor oxides, K2O is comparable with average Atlantic MORB and IAT, TiO2 with island-arc basalts, P2O5 matches well with MORB and island-arc basalts. 5. Petrogenesis Among the trace elements, Zr, Y and Rb match well with those of the IAT (elemental abundances Wide ranges of SiO2 (44.6–52.3%) and mg no. of average MORB, IAT, island-arc basalt, average (27.3–69.7) in gabbroic intrusives of Paharpur is Atlantic MORB are taken from Wilson 1989). considered to be the result of magmatic differen- Enrichment and wide range of Sr, Rb, La and Th tiation. K2O shows positive correlation with SiO2 found in the magmatic rocks of Paharpur might be due to its incompatible nature. MgO, TiO2 and the result of metasomatism of mantle source by sub- FeO+Fe2O3 show good linear negative correlation ducted sediment and subduction zone fluid. Low/ with SiO2 suggesting fractionation of early min- abundances of incompatible elements (Zr, Y, Nb, Ti, erals such as olivine, pyroxenes, plagioclase, mag- Ta, etc.) may be attributed to either high degree netite and ilmenite during differentiation. This is partial melting of mantle source or stability of mi- supported by clear positive correlation of CaO, nor residual phases (zircon, rutile and sphene) in the Ni, Sr and negative correlation of total REE, mantle source which preferentially concentrate these FeO+Fe2O3 and TiO2 with mg no. The negative trace elements. Although a few incompatible ele- Eu anomaly, observed in majority of the samples, ments have restricted range, most of them show suggest plagioclase fractionation as a dominant highly variable abundances, ranging from much mode of differentiation. The anorthite content of higher to lower than those of island-arc basalts some of the grains of plagioclase is high (An83–90). which possibly indicates the process other than min- It may indicate possibility of contamination or eral accumulation (migration of interstitial melt magma mixing. Paharpur intrusives show tholei- and aqueous fluid) have contributed bulk composi- itic and iron enrichment trend in AFM diagram tional variations among the intrusives as observed suggesting progressive iron enrichment during dif- from hornblende-bearing gabbro from Volcan San ferentiation which suggests crystallization under Pedro of Chilean Andes (Costa et al 2002). low fO2 condition. Plots in TAS diagram sug- Rare earth element composition of the rocks gest parent magma was of primitive in nature low when plotted in the Lu/Hf vs. La/Sm diagram abundances of Ni content suggest that the par- (figure 11, after Regelous et al 2003) lie in the ent magma is not a primary one, but an evolved fields of spinel and garnet peridotite and magma (olivine-fractionated). indicate 5% to 20% partial melting of the man- Enrichment in LILE (Rb, Th, La and Sr) and tle source. Similar observations have been made depletion in elements of high ionic potential or on the mafic intrusives of Sichuan Province, China High Field Strength Element (HFSE) like Nb, Zr, where the source was considered as garnet–spinel Hf, Sm, Tb, Ti and Y is characteristic of the lherzolite (Zhao and Fu 2007). The temperature 808 Aditi Mandal et al

0.20 20 Gabbro 10 Dolerite

5 5 0.15 3 3 Spinel peridotite 2 1 0.5 0.3 20 0.1 0.01 0.10

Lu/Hf

0.05 1 0.5 0.2

Garnet peridotite

0.00 0 246810 La/Sm

Figure 11. Lu/Hf vs. La/Sm plot for mafic intrusives of Paharpur, after Regelous et al (2003). for the crystallization of the mother magma for the temperature range of 805◦–1077◦C and pressure gabbroic intrusive of Paharpur is estimated from range of 5.9–7.9 kbar. Fractional crystallization of coexisting orthopyroxene–clinopyroxene pair to be early formed olivine, orthopyroxene, clinopyrox- 805◦–1077◦C using two pyroxene thermometer ene, plagioclase and ilmenite has been considered (after Kretz 1992). The pressure range over which as main process of differentiation accompanied by the parent magma had crystallized is estimated the reaction of early cumulate pile with extrane- from coexisting plagioclase–clinopyroxene pair to ous aqueous fluid/evolved melt. Large poikilitic be around 5.9 to 7.9 kbar using clinopyroxene– hornblende, which is a dominant mineral phase plagioclase barometer (after McCarthy and Douce and encloses all the early minerals, is consid- 1998). The estimated range of pressure and tem- ered as product of such fluid/evolved melt-induced perature for the crystallization of parent magma of reaction during late magmatic stage. Paharpur gabbroic rock is indicative of polybaric crystallization over the entire crustal column and corresponds well with mafic intrusions from the Cascadian Arc System (Ulmer 2007) and Mexican 6. Tectonic setting province of South America (Guttirez et al 2008) and the P–T condition corroborates well with Each specific tectonic setting is more or less typ- spinel peridotite as the mantle source rock for the ified by geochemical characteristics of associated mother magma. The pressure range (5.9–7.9 kbar) volcanic products. It is possible therefore that such is consistent with deep level (crust–mantle bound- geochemical characterization of basaltic rocks can ary) and intermediate crustal level (15–25 km) be used to recognize unknown tectonic setting. magma storage areas where fractionation-induced However, the major element chemistry is not so differentiation takes place. Lesser abundance of useful in tectonic discrimination. On the other HREE and flat HREE pattern in chondrite normal- hand, immobile, incompatible minor and trace ele- ized diagram possibly suggest stability of garnet in ments are very useful in tectonic discrimination as the residual phase during mantle melting. they have restricted range for individual tectonic In summary, it is considered that the mantle setting (Rollinson 1993). Abundances of trace ele- source region beneath Paharpur is considered to ments (Nb, Zr, Ti, P, Cr, Y, Mn, Sr and Ce) are have metasomatized by subducted sediment and used to understand the unknown tectonic setting subduction zone fluid leading to enrichment in of magmatic rocks of Paharpur using some tectonic LILE (Rb, Th, K, La and Sr) and relative deple- discrimination diagrams. tion in HFSE (Nb, Ti, Zr, Y and Tb) prior to melt- In the 2Nb–Zr/4–Y diagram (figure 12, after ing. Partial melting (5–20%) of such an enriched Meschede 1986), most (7 out of 9) of the samples mantle source gave rise to the parent magma are plotted within field D, which is the field for for the magmatic rocks of Paharpur. Magmatic N-type MORB or volcanic-arc basalts (VAB). crystallization and differentiation continued in the Similar observation can be obtained from MnO– deep to intermediate level crustal magma over a TiO2–P2O5 diagram (after Mullen 1983). Petrology and geochemistry of hornblende gabbro 809

2Nb 0.03

Gabbro Calc-alkali basalt 4 Dolerite Ti 0.02 Basalt

0.01 I Island-arc tholeiite 7 0.1 0.2 6 9 Al II

A 8 Figure 14. Ti–Al plot of clinopyroxene of the mafic intru- 3 10 B sives of Paharpur, after Leterrier et al (1982). Black-filled 8 circles represent clinopyroxene compositions. 13 11 14 C Petrological and geochemical data give evidence 2 that the gabbroic rocks and associated dolerite D 12 dykes of Paharpur was formed in a subduction zone Zr/4 1 5 Y setting.

Figure 12. 2Nb–Zr/4–Y diagram of the mafic intrusives of Paharpur, after Meschede (1986). Black-filled squares repre- 7. Discussion sent the mafic intrusive compositions. Field AI: within-plate alkali basalt; AII: within-plate alkali basalt and within-plate Subduction zone is considered as the most com- tholeiite; B: E-type MORB; C: within-plate tholeiite and volcanic arc basalts; D: N-type MORB and volcanic arc plex area of magma generation. This is because of basalts. involvement of multiple source materials namely subducted , overriding mantle wedge, subducted sediments and subduction zone fluid Gabbro in melting episode and magma generation pro- Dolerite cess (Wilson 1989). The magmatic rocks of ophi- 1400 Basalt olitic assemblage in a subduction zone clearly dis- 1000 900 play a strong IAT and boninitic character (Winter 2001). Fluid phases usually play an important role Mid- in the magmatic process of subduction zone; as a oceanic ridge result hydrous minerals (hornblende±phlogopite) basalt are very common in the intrusives. Hornblende Cr in the gabbroic rocks of subduction zone occurs 100 either as early formed smaller crystals as seen in 68 Volcanic Adamello batholith, Italy (Ulmer et al 1983); horn- arc blende gabbro , California (Sisson et al 1996) basalt Within plate or as poikilitic crystals enclosing earlier-formed basalt minerals showing reaction relation as observed in peninsular ranges batholith, California (Smith et 10 al 1983); coastal batholith of Peru (Regan 1985); 10 27 47 100 Y hornblende gabbro from Chilean Andes (Costa et al 2002) and Evros from NE Greece (Bonev Figure 13. Cr–Y diagram for mafic intrusives of Paharpur, and Stampfli 2009). after Pearce (1982). The high modal proportion of poikilitic horn- blende (up to 60%) in the gabbroic intrusives of Paharpur is very significant. The mg no. of In the Cr–Y diagram (figure 13, after Pearce these hornblendes is also very high. Such high val- 1982), most (7 out of 9) of the intrusives plot within ues may indicate primitive characters, and pos- the field of VAB. sibly suggest that large proportion of hornblende Similar nature of the intrusives is observed in the are produced by reactions between early crystal- Cr–Ce/Sr diagram (after Pearce 1982). lized minerals (clinopyroxene orthopyroxene and Ti–Al plot of the clinopyroxenes (figure 14, after plagioclase) and evolved melt±aqueous fluids that Leterrier et al 1982) suggest tholeiitic nature of the percolate through the mafic cumulate triggered mafic intrusives of Paharpur. by protracted closed system crystallization during 810 Aditi Mandal et al late magmatic stage. Interactions between mafic study area have also been reported to show subduc- cumulate piles and invading water-rich silicic mag- tion zone geochemical signature (Mandal and Ray mas may provide a means for stabilizing substan- 2009). The orientation of fossil intra-plate subduc- tially larger quantities (nearly 62 volume%) of tion zone is likely to be E–W and a possible north- hornblende. ward subduction of the southern block might have The dolerite dykes, which intrude the main gab- resulted in formation of mafic rocks of Paharpur broic rock of Paharpur, show largely tholeiitic char- area with subduction zone component. acter with one sample showing high Mg–Si and low Ti abundance and appear to show a boninitic affin- ity. Arc tholeiites and boninites are known from 8. Conclusion the fore arcs of the modern arc/back arc environ- ment and points to the early stages of arc sys- Field and laboratory observations on gabbroic tem evolution (Taylor 1995). The magmatic assem- rocks and associated dolerite dykes of the study blage showing tholeiitic and boninitic affinities area suggest that these rocks were formed and is observed in ancient arc systems and ophiolite emplaced in subduction zone setting. The field suites. occurrence, rock-types, mineralogy, texture and Geochemical studies on the mafic intrusive rocks geochemistry of these rocks have striking similar- of Paharpur indicate an HFSE-depleted and LILE- ities with gabbroic rocks of subduction zone and enriched signature with conspicuous negative Nb, with basic rocks of ophiolite complex developed Ti and Zr anomaly which is considered charac- in supra subduction zone of arc-marginal basin teristic of basaltic rocks of subduction zone (Wil- setting. Paharpur intrusive is characterized by its son 1989; Bektas et al 2007). High values of Cr more basic, hydrous and evolved nature. Processes (18–670 ppm), Ni (14–234 ppm), La/Nb (2.5– other than mineral accumulation such as, migra- 12.6) and Zr/Nb (2.6–37.4) ratio, rule out the tion of interstitial melt and aqueous fluid may possibility of crustal contamination (average crust contribute bulk compositional variation among has La/Nb = 2.2, Zr/Nb = 16.2, after Weaver the intrusives indicated by abundances of trace 1991). Ba/Nb ratio is also very high in this rock elements. The variation in major elements and compared to the crust. In subduction zone, fluid ratios of trace elements suggest variable crustal phase and subducted sediment are two chief source contamination in mantle source prior to melting. materials which contribute significantly in mag- mas and impart characteristic geochemical signa- ture (Guram et al 2007; Shigenori and Kazuaki Acknowledgements 2007). In the present case, the subduction zone fluid which is enriched in LILE (Rb, Sr) is consid- Authors would like to thank the Head of the ered to have played a dominant role in metasoma- Department of Geology, Presidency University for tism of the mantle source along with the sediment using laboratory facility and DST-FIST for infras- which is rich in Th and LREE. Tectonic discrimi- tructural support. They also thank Dr Basab nation diagrams using chemical attributes suggest Chattopadhyay, Senior Geologist, Geological Sur- a calc-alkaline affinity for the mafic intrusives (gab- vey of India for EPMA in the Chemical Laboratory bro and associated dolerite dykes) of the study area of GSI, Kolkata; Mr P Mondal, Director, Chemical and a possible arc-setting. On the basis of the field Division, Geological Survey of India, Kolkata for occurrence, mineralogy, major and trace element major, trace and rare earth element analyses and behaviour of the mafic intrusives of the study area, Dr V Balaram, Head, Geochemistry, National Geo- it appears that the intrusive bodies have some char- physical Research Institute for the same. Thanks acters similar to gabbroic and doleritic rocks of are also due to the Department of Science and subduction zones. Similar observations were made Technology, Govt. of India, New Delhi for pro- from mafic rocks of Chilean Andes (Costa et al viding financial support for field work chemical 2002); Duke Island, southeastern Alaska (Himmel- analyses. berg and Loney 1994), northeastern Turkey (Bek- tas et al 2007); Sichuan province, southwestern References China (Zhao and Fu 2007); Evros ophiolite, NE Greece (Bonev and Stampfli 2009) and many other Arculus R J and Wills K J A 1980 The petrology of plutonic places. 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MS received 7 September 2011; revised 16 January 2012; accepted 18 January 2012