Pergamon Jourttal of Southeast Asian Earth Sciences, Vol. 14, Nos 314, pp. 185-198. 1996 Copyright 0 1996 Elswiet Science Ltd Printed in Great Britain. All rights resewed PII: !30743-9547(%poo57-8 0743-9547196 $15.00 + 0.00 Magmatic evolution of mafic granulites from Anakapalle, Eastern Ghats, India: implications for tectonic setting of a Precambrian high-grade terrain Pulak Sengupta, * Somnath Dasgupta, * Uttam Kumar Bhui,* Jiirgen Ehlt and Masato Fukuokat *Department of Geological Sciences, Jadavpur University, Calcutta 700 032, India TMineralogisch-Petrologisches Institute, University of Bonn, Bonn, Germany IDepartment of Earth and Planetary Sciences, Hiroshima University, Hiroshima, Japan (Received 17 January 1996; accepted for publication 24 April 1996) Abstract-Mafic granulites showing intrusive relationships with enclosing pelitic, calcareous and quartzofeldspathic gneisses at Anakapalle, Eastern Ghats belt, share a common retrograde metamorphic history (decompression followed by near-isobaric cooling) and are, therefore, considered to be syn-metamorphic. Detail textural, phase chemical and bulk chemical analyses of the mafic granulites show that (a) these are melts derived through fractionation of a primary tholeiitic magma and (b) they crystallized at temperatures < 1000°C and were thus in thermal equilibrium with the country rock granulites during peak metamorphism. Comparison with experimental data on similar bulk compositions constrains the depth of emplacement of the magmas at 30-35 km. Geochemical characteristics indicate that the mafic magmas are essentially similar to continental flood basalts and have thus been generated in an extensional set-up. The apparent clockwise trajectory recorded in the Anakapalle granulites was produced by extension of the crust of near-normal thickness with concommitant basic magmatism. Copyright 0 1996 Elsevier Science Ltd Introduction The tectonic setting of granulite formation plays a key role in understanding crustal evolution through time and space (Bohlen, 1991; Harley, 1992; Brown, 1993, and references cited therein). The tectonic processes in high-grade terrains are usually inferred from P-T-t trajectories constrained from metapelites, high Mg-Al granulites, orthopyroxene-bearing quartzofeldspathic gneisses, talc-silicate rocks and mafic granulites (Harley, 1989, 1992; Brown, 1993). It has now become evident that tectonic interpretation of P-T-r trajec- tories is often equivocal and contrasting P-T trajec- tories are recorded in the same erogenic belt (cf. Brown, 1993), while a similar P-T trajectory can be a product of different tectonic settings (cf. Dasgupta et al., 1994). Mafic granulites form important constitu- ents of all high-grade terrains. Mineral and petrochem- ical attributes of many such mafic granulites indicate that these represent syntectonically emplaced quenched basic melts, preserve magmatic signatures and suffered very little compositional change subsequent to their magmatic stage (Bohlen and Essene, 1978; Sandiford and Powell, 1986a; Kuehner and Green, 199 1; Kuehner, 1992, and references cited therein). Thus, it is expected that geochemical attributes of these rocks, supported by experimental data on basaltic rocks, would provide independent information on the tec- tonic processes operative in the deep continental crust. Further, the depth of emplacement of these magmas can be constrained through the application of the experimental results of Kuehner and Green (1991) and Kuehner (1992). This would provide, in turn, further insight into the tectonic processes, parts of which may not be recorded in the enclosing granulites (cf. Kuehner and Green, 1991; Harley and Fitzsimons, 1995). Despite their importance, petrogenesis and tectonic discrimination of basic igneous rocks meta- morphosed to granulite facies have rarely been attempted. The Eastern Ghats granulite belt in India (Fig. 1) exposes a variety of syn-metamorphic two-pyroxene granulites (Sengupta et al., 1990; Dasgupta et al., 1991, 1993; Dasgupta, 1995) and, thus, are particularly suitable for such studies. These rocks largely preserve relict igneous textures because of fluid-absent gran- ulite-facies metamorphism (Dasgupta et al., 1991). In this communication we synthesize the phase and bulk chemical characteristics of mafic granulites from an area north of Anakapalle in the Eastern Ghats belt (Fig. 1) to decipher their magmatic evolutionary history and the possible tectonic setting of magmatism. The results are combined and compared with the metamorphic P-T trajectory deduced from the enclos- ing granulites (Dasgupta et al., 1994) for a better and comprehensive understanding of the paleotectonic processes during the Precambrian time. Geological background The Eastern Ghats belt along the eastern coast of India (Fig. 1) is one of the few well-studied granulite terrains in the world which records unequivocal evidence of ultrahigh temperature (2 1OCMYC) of metamorphism at 9-10 kbar pressure on an anticlockwise P-T trajectory (Sengupta et al., 1990; Dasgupta and Sengupta, 1995, and references cited therein). Sub- 185
Transcript
Pergamon
Jourttal of Southeast Asian Earth Sciences, Vol. 14, Nos 314, pp. 185-198. 1996 Copyright 0 1996 Elswiet Science Ltd
Printed in Great Britain. All rights resewed PII: !30743-9547(%poo57-8 0743-9547196 $15.00 + 0.00
Magmatic evolution of mafic granulites from Anakapalle, Eastern Ghats, India: implications for tectonic setting of a
*Department of Geological Sciences, Jadavpur University, Calcutta 700 032, India TMineralogisch-Petrologisches Institute, University of Bonn, Bonn, Germany
IDepartment of Earth and Planetary Sciences, Hiroshima University, Hiroshima, Japan
(Received 17 January 1996; accepted for publication 24 April 1996)
Abstract-Mafic granulites showing intrusive relationships with enclosing pelitic, calcareous and quartzofeldspathic gneisses at Anakapalle, Eastern Ghats belt, share a common retrograde metamorphic history (decompression followed by near-isobaric cooling) and are, therefore, considered to be syn-metamorphic. Detail textural, phase chemical and bulk chemical analyses of the mafic granulites show that (a) these are melts derived through fractionation of a primary tholeiitic magma and (b) they crystallized at temperatures < 1000°C and were thus in thermal equilibrium with the country rock granulites during peak metamorphism. Comparison with experimental data on similar bulk compositions constrains the depth of emplacement of the magmas at 30-35 km. Geochemical characteristics indicate that the mafic magmas are essentially similar to continental flood basalts and have thus been generated in an extensional set-up. The apparent clockwise trajectory recorded in the Anakapalle granulites was produced by extension of the crust of near-normal thickness with concommitant basic magmatism. Copyright 0 1996 Elsevier Science Ltd
Introduction
The tectonic setting of granulite formation plays a key role in understanding crustal evolution through time and space (Bohlen, 1991; Harley, 1992; Brown, 1993, and references cited therein). The tectonic processes in high-grade terrains are usually inferred from P-T-t trajectories constrained from metapelites, high Mg-Al granulites, orthopyroxene-bearing quartzofeldspathic gneisses, talc-silicate rocks and mafic granulites (Harley, 1989, 1992; Brown, 1993). It has now become evident that tectonic interpretation of P-T-r trajec- tories is often equivocal and contrasting P-T trajec- tories are recorded in the same erogenic belt (cf. Brown, 1993), while a similar P-T trajectory can be a product of different tectonic settings (cf. Dasgupta et al., 1994). Mafic granulites form important constitu- ents of all high-grade terrains. Mineral and petrochem- ical attributes of many such mafic granulites indicate that these represent syntectonically emplaced quenched basic melts, preserve magmatic signatures and suffered very little compositional change subsequent to their magmatic stage (Bohlen and Essene, 1978; Sandiford and Powell, 1986a; Kuehner and Green, 199 1; Kuehner, 1992, and references cited therein). Thus, it is expected that geochemical attributes of these rocks, supported by experimental data on basaltic rocks, would provide independent information on the tec- tonic processes operative in the deep continental crust. Further, the depth of emplacement of these magmas can be constrained through the application of the experimental results of Kuehner and Green (1991) and Kuehner (1992). This would provide, in turn, further insight into the tectonic processes, parts of which may
not be recorded in the enclosing granulites (cf. Kuehner and Green, 1991; Harley and Fitzsimons, 1995). Despite their importance, petrogenesis and tectonic discrimination of basic igneous rocks meta- morphosed to granulite facies have rarely been attempted.
The Eastern Ghats granulite belt in India (Fig. 1) exposes a variety of syn-metamorphic two-pyroxene granulites (Sengupta et al., 1990; Dasgupta et al., 1991, 1993; Dasgupta, 1995) and, thus, are particularly suitable for such studies. These rocks largely preserve relict igneous textures because of fluid-absent gran- ulite-facies metamorphism (Dasgupta et al., 1991). In this communication we synthesize the phase and bulk chemical characteristics of mafic granulites from an area north of Anakapalle in the Eastern Ghats belt (Fig. 1) to decipher their magmatic evolutionary history and the possible tectonic setting of magmatism. The results are combined and compared with the metamorphic P-T trajectory deduced from the enclos- ing granulites (Dasgupta et al., 1994) for a better and comprehensive understanding of the paleotectonic processes during the Precambrian time.
Geological background
The Eastern Ghats belt along the eastern coast of India (Fig. 1) is one of the few well-studied granulite terrains in the world which records unequivocal evidence of ultrahigh temperature (2 1OCMYC) of metamorphism at 9-10 kbar pressure on an anticlockwise P-T trajectory (Sengupta et al., 1990; Dasgupta and Sengupta, 1995, and references cited therein). Sub-
185
186 P. Sengupta et al.
sequent to peak metamorphism, the rocks cooled nearly (Sanyal and Fukuoka, 1995). Calculated positions of isobarically down to -750” (references as above). The mineral equilibria, deduced from textural criteria, and granulite complex was later subjected to a nearly geothermobarometric data define a retrograde P-T isothermal decompressive orogeny involving 3-4 kbar trajectory during which a steep decompression of consequent to unroofing of a crust of nearly 15 km w 1.5 kbar from 8 kbar at 900°C was followed by (Kamineni and Rao, 1988; Sengupta et al., 1990; near-isobaric cooling of -200°C (Dasgupta et al., 1994; Dasgupta et al., 1992). The area investigated in this Sanyal and Fukuoka, 1995). In the absence of any work is situated 5 km north of Anakapalle (Fig. 1) and documentation of the prograde path of metamorphism exposes linear patches of khondalite (garnet-sillimanite- in this area, the overall P-T trajectory remained perthite-quartz gneiss), leptynite (garnet-plagioclase- indeterminate. Thus, the deduced retrograde trajectory perthite-quartz gneiss) containing lenses of spinal can be explained in terms of extension of a crust of granulite, orthopyroxene-bearing quartzofeldspathic near-normal thickness (Sandiford and Powell, 1986b) or gneisses, mafic granulites, sapphirine granulites and as a part of an overall ACW (anticlockwise) trajectory talc-silicate granulites (Fig. 2). Mafic granulites mainly (Anovitz and Chase, 1990; Anovitz, 1991). Grew and occur as concordant bodies and are locally interdigi- Manton (1986) dated this metamorphic event at tated with orthopyroxene granulite (Fig. 2 in Dasgupta Anakapalle to be ca. 1000 Ma and later correlated it et al., 1994). In most places, a thin leucocratic band with the Rayner orogeny in East Antarctica (Grew consisting dominantly of quartz and K-feldspar with et al., 1988). The rocks subsequently suffered localized minor plagioclase separates mafic granulites from hydration. In this backdrop, deciphering the magmatic orthopyroxene granulites (Fig. 3 in Dasgupta et al., history of the mafic granulites of Anakapalle assumes 1994). The mafic granulites also include lenses and more significance because of their tectonic implication. blocks of orthopyroxene granulite and a variety of sapphirine granulite (Dasgupta et al., 1994; Sanyal and Fukuoka, 1995). Taken together, all these features Petrochemistry of the mafic granulites suggest that the basic magma (now mafic granulite) was intrusive into the orthopyroxene granulite and Petrography supracrustal rocks of the study area. The rocks have suffered two phases of deformation and four stages of The following mineral association is present in the metamorphic reconstitution have been recognized studied mafic granulites: clinopyroxene-orthopyroxene-
76*E 96’E _ I
14*N -
200km
M.R Mahanadi River GG Godavari Graben
Fig. 1. Location map of the Eastern Ghats belt shewing the study area.
Fig. 2. Geological map of the area north of Anakapalle.
187
plagioclase-ilmenit~uartz-K-feldspar-biotit~apatite. Quartz occurs only in trace amounts, particularly at clinopyroxene-plagioclase contacts. The most common occurrence of clinopyroxene is as subhedral polygonal to subrounded porphyroblasts forming a granoblastic mosaic with plagioclase, orthopyroxene and ilmenite (Fig. 3). Clinopyroxene porphyroblasts contain several sets of exsolution lamellae. The most common one is of orthopyroxene parallel to (100) cleavage in the host particularly concentrated in the core of the grains (see Fig. 7 in Dasgupta et al., 1994). Another set of exsolved blebs and spindles of pyroxene is oriented at high angles to the previous set (Fig. 4) and may be interpreted as “001” pigeonite (Olilla et al., 1988; Dasgupta et al.,
1991, 1993). In some grains of clinopyroxene a third set of widely spaced exsolved beads occur at low angles to (100) orthopyroxene lamellae (Fig. 4). The third set is similar to the “low-temperature pigeonite” inferred by Olilla et al. (1988) from the Adirondack granulites. Unlike the clinopyroxene porphyroblasts, orthopyrox- ene is free of any exsolved lamellae/blebs. Further, some smaller subhedral orthopyroxene grains girdle clinopy- roxene porphyroblasts containing (100) orthopyroxene lamellae (see Fig. 7 in Dasgupta et al., 1994). These smaller orthopyroxene grains could have originated through grain boundary diffusion of some of the exsolution lamellae (cf. Bohlen and Essene, 1978; Sandiford and Powell, 1986a; Dasgupta et a/., 1991).
P. Sengupta et al.
Fig. 3. Subhedral polygonal to rounded porphyroblasts of plagioclase (P), clinopyroxene (C), orthopyroxene (0) and ilmenite (11) form a mosaic of granoblastic texture (traced from
EPMA back-scattered image). Bar = 0.03 cm.
Plagioclase porphyroblasts are locally recrystallized along the peripheries to smaller strain-free grains. Ilmenite contains finely exsolved titanohematite lamel- lae. Besides some late biotite replacing the ferromagne- Sian phases, the rock is devoid of any hydrous phase (except ANK-66M). Exsolution textures in clinopyrox- ene indicate a magmatic origin for the grains and their subsequent re-equilibration during cooling through the pyroxene solvii (Bohlen and Essene, 1978; Olilla et al., 1988; Dasgupta et al., 1991, 1994) and discussed further in the following section.
Mineral chemistry
Chemical composition of the minerals was deter- mined by JEOL-JXA 733 and 8600 Electron Probe Microanalyzers using natural and synthetic standards. Raw data were corrected by ZAF. Representative mineral composition data have been given in Das- gupta et al. (1994). Re-integrated composition of
Fig. 4. (001) pigeonite spindles (1) are intersected by (100) lamellae (2) of orthopyroxene at high angles in clinopyroxene (C) host. Third set of exsolved stringers (3) are at low angle to (100) lamellae (traced from EPMA back-scattered image).
Bar = 0.01 cm.
lamellae-rich portions of clinopyroxene grains was obtained by defocused electron beam of 25 pm diameter. The re-integrated compositions fall within the augite field in the pyroxene quadrilateral within 40 mol% wollastonite. However, spot analysis of clinopyroxene grains away from the lamellae reveal compositions close to the salite-augite boundary with 43 mol% wollastonite. Clinopyroxene contains 0.04 0.8 p.f.u. of Fe3+ calculated after Bohlen and Essene (1978). The grains are zoned with respect to AltO3 (3.5 wt% at the core to 2.4 wt% in the rim), NarO (0.5 wt% at the core and 0.3 wt% at the rim) and CaO (20.84 wt% to 22.37 wt%, respectively) against plagioclase. Against orthopyroxene, clinopyroxene shows Fe-Mg re-equilibration, whereby Xug increases from 0.69 to 0.72 from core to rim. Compositions of the exsolved lamellae could not be determined because of their fine size. Orthopyroxene shows rimward increase in X, against clinopyroxene. Porphyroblastic orthopyroxenes are essentially enstatite-ferrosilite solid solution with low Al”‘. Plagioclase shows rimward depletion in An (59 to 54 mol%) against clinopyrox- ene.
Exsolution history in clinopyroxene and its implication in the magmatic and metamorphic evolution of the mafic granulite
The equilibrated granoblastic texture displayed by clinopyroxene, orthopyroxene, plagioclase and ilmenite in the studied rocks, in conjunction with the fact that the minerals do not show any reaction relation, suggest that these phases attained stable configuration prior to the onset of metamorphism. Metamorphic effects are limited to exsolution of Ca-poor pyroxenes in calcic pyroxene host including the granular exsolu- tion (discussed below), Fe-Mg resetting between pyroxenes and Al redistribution between clinopyroxene and plagioclase. Clinopyroxene (having the bulk, composition of augite) orthopyroxene, plagioclase and ilmenite crystallized early from the magma. Exsolution of “001” pigeonite from augite occurred at high temperatures during magmatic cooling due to the widening of the augite-pigeonite solvus, and this was later inverted to (100) orthopyroxene either during the later stages of magmatic cooling or during metamor- phic overprint (cf. Olilla et al., 1988). Low-tempera- ture pigeonite occurring at low angles to (100) orthopyroxene exsolved out possibly during metamor- phic cooling (Olilla et al., 1988). It will be argued later, on the basis of phase equilibrium experimental data on rocks having a similar bulk composition as the studied rocks, that the mafic magma was emplaced at a depth corresponding to 9 & 1 kbar. At this pressure, stabilization of orthopyroxene-augite instead of pigeonite-augite for the given composition restricts the temperature of magmatic crystallization to < 1000°C (Lindsley, 1983; Davidson, 1988; Davidson and Lindsley, 1985, 1989). Using the core compo- sitions, clinopyroxene plagioclase-quartz assemblage yields 8 kbar, 9OO”C, while the rim compositions of the same phases give 6.5 kbar, 870°C (method after Anovitz, 1991; discussed in Dasgupta et al., 1994). The deduced decompressive retrograde trajectory tal- lies perfectly with that obtained from associated rocks (Fig. 14 in Dasgupta et al., 1994). Thus, the mafic
Mafic granulites, Anakapalle 189
Table 1. Whole-rock chemical analysis and CIPW norm of representative mafic granulites of Anakapalle
granulites record identical metamorphic history as the associated rocks. Two-pyroxene thermometry (Kretz, 1982; Davidson and Lindsley, 1989) shows wide scatter but distinctly higher temperatures (780- 800°C) at the core and lower (600-65O”C) at the rims (Dasgupta et al., 1994). The scatter could be due to Ca-Fe-Mg exchange during subsolidus cooling either at the magmatic stage or in the metamorphic stage.
Summarizing, the petrographic and phase chemical characteristics of the studied mafic granulites constrain the crystallization temperature of the magma at T < 1000°C. Clinopyroxene records evidence of the multi-stage exsolution of pigeonite in the course of
magmatic and metamorphic cooling. The mafic gran- ulites show identical retrograde P-T trajectory with steep decompression as the associated rocks.
Geochemistry
Selection of samples. Petrogenetic and tectonic discrimination using whole-rock chemistry of meta- morphosed basic rocks of continental origin are beset with the problems of contamination either during melt-wall rock interaction or due to metamorphism or both. This problem has been succinctly addressed in a number of publications, e.g. Seymour and Kumarapeli (1995), Blackburn and Srivastava (1994) and references
190 P. Sengupta et al.
Calc-Alkaline
h@W NO Fig. 5. Anakapalle mafic .granulites plotted in the alkali (NazO + K20)-FeO*-MgO diagram (after Irvine and Bara- gar, 1971). The plots show the iron enrichment trend
characteristic of tholeiites.
cited therein. To circumvent these problems, at least to a large extent, the following screening tests were employed on the collected samples.
(a)
@I
Samples are collected from freshly excavated quarries of basic granulites and away from contacts with the other lithologies. Samples with > 1 ~01% retrograde amphibole, biotite and/or carbonates are eliminated. An exception is sample ANK-66M, which contained retrograde amphibole, but was considered in view of high Mg# (discussed later). The chosen samples frequently contain magmatic pyroxenes and display relict igneous textures (described earlier). Thus, the chosen samples are expected to have been metamorphosed under high rock-fluid ratio. The samples that pass the first screening are then put to the geochemical test. The molecular
Total WTi
Al WJ
Fig. 6. Atomic plots of the Anakapalle mafic gram&es (shown as dots) on the Mg-Al-(total Fe + Ti) diagram after Jensen (1976). The field bounding the dots encompasses Vestfold block tholeiites (data from Collerson and Sheraton,
1986).
??
-1s ’ I I I I 1 0.6 0.7 0.6 0.5 0.4 0.3
mg number Fig. 7. Sympathetic variation between Mg (atomic Mg/total Fe + Mg) and degree of under-saturation expressed in negative values of normative quartz (which is equivalent to normative
olivine) for the Anakapalle mafic granulites.
proportion plots of Beswick and Soucie (1978) have been used to evaluate the characteristics of major-element alteration accompanying meta- morphism. The samples that show chemical effects of metamorphism are eliminated. Further samples with K-Rb ratios ~700 are also discarded.
The screened samples have been selected for the purpose of petrogenetic and tectonic discrimination. Table 1 shows the representative analyses of the selected samples. Samples were analyzed using a PHILIPS XRF at the University of Bonn.
Major-element characteristics. The metabasic rocks of the study area have a small range of SiOz (50 + 1 wt%) but fairly large range of Mg # ( * 41-59). However, most of the data fall close to 50. On the AFM diagram the rocks define a distinct iron enrichment trend character- istic of tholeiite (Fig. 5). Corroboration of tholeiitic affiliation is also obtained from the plot of atomic Mg-Al-(Fe + Ti) (Fig. 6) used in Collerson and
lo’ p Alkaline / !6- , / / 0 /
s
/
/’ /’
&4- / /’ Subalkaline 8 , z /’
2- /’ s
/’ / /
0 (’ I I I
35 40 45 50 55 60
Fig. 8. The alkali (Na20 + K20) silica distribution diagram for the study mafic granulites. The discriminant line is from
Irvine and Baragar (1971).
Mafic granulites, Anakapalle
Table 2. Compositional ranges of mafic rocks from different areas with the same range of Mg# as the study mafic granulites*
* 1. Mafic granulites, Orissa (Bowes and Dash, 1992). 2. Deccan basalts (Krishnamurthy and Cox, 1977; Mahoney et al., 1985; Bodas et al., 1988). 3. Columbia river basalt (Swanson and Wright, 1981). 4. Two-pyroxene granulite, Delegate pipes (sample R698) (Irving, 1974). 5, 6. Mafic dykes; Napier Complex, East Antarctica (49590 and 3793, respectively) (Kuehner, 1992). 7, 8. Mafic dykes; Vestfold hills, East Antarctica (206 and 060, respectively) (Kuehner, 1992). FeO* = Fe0 + 0.89981 x Fe*O,.
Sheraton (1986). All the samples contain normative 01 and Hy and hence are best classified as olivine tholeiites. There exists a sympathetic variation between Mg# and the degree of under-saturation (expressed in proportion of normative 01; Fig. 7). On the total alkali (Na20 + K20 wt%)-Si02 diagram all the data plot within the field of sub-alkaline basalt (Fig. 8). The average TiOl and KzO values (1.13-l .54 and 0.31-0.75, respectively) fall closer to MORB than the continental tholeiitic basalt (BVSP, 1981). The major- element abundances of the study rocks show remark- able similarity with the Proterozoic metabasic dykes of East Antarctica, the continental flood basalts and the xenoliths of two-pyroxene granulite from the Delegate pipes (Table 2). In all these cases, comparison was made for the same range of Mg# (Table 2). This enables direct application of the results of the available experiments carried out on these samples to understand the petrogenesis of the study rocks.
Trace-element characteristics. The Anakapalle mafic granulites have modest Cr and Ni contents (Cr = 75-
270 ppm; Ni = 54-107 ppm; Table 1). Both these elements show sympathetic variation with the Mg# . The trace-element distribution of the study rocks including the incompatible trace elements show remark- able similarities with the continental flood basalts and the Grenville dyke rocks (see the spidergrams in Figs 9a and 9b). However, La and Ce concentration in some of the samples are even lower than the MORB concen- tration (Fig. 9a). Although the concentration of these elements are within the uncertainty of the instrument, also early fractionation of magnetite which (I?- D(La) = 1.5-3, Kp(Ce) = 1.3-3; Rollinson, 1993) can also produce this depletion.
The average Zr and Ti02 concentrations (55-92 ppm and 1.13-1.54 wt%, respectively) are both low as compared to N-MORB but similar to the Archean tholeiite (90 ppm and 0.92 wt%, respectively) and those from the Proterozoic East Indian Shield (Blackburn and Srivastava, 1994). On the Zr-Y vs Zr-Nb diagram ANK-66M plots within the field stipulated for transitional MORB (Fig. 10). Other data fall below but close to this field.
192 P. Sengupta et al.
Elements
Fig. 9a. MORB-normalized spidergrams for Anakapalle mafic granulites. The shaded area indicates the range for continental flood basalts and the dotted area for Grenville dyke rocks. The compositions used for normalization are taken from Clarke
(1990).
KRb PtBalMlNbWSrRNdNM Ti Y SC V ZnCuNiCr
Elements
Fig. 9b. Chondrite-normalized extended rare-earth element (REE) patterns for Anakapalle mafic granulites. Other
explanations same as in Fig. 9a.
Petrogenesis
Any modeling pertaining to petrogenesis of magmatic rocks using bulk compositional data is primarily pivoted on the assumption that the study rocks represent melt compositions at the time of emplacement. The mineral assemblages and the geochemical attributes of many deep-seated gabbroic rocks can be best explained in terms of a cumulate origin (cf. Sen, 1988). Needless to say that geochemical modeling of these rocks assumed to be quenched melts would lead to erroneous conclusion about their petrogenesis. However, remarkable compo- sitional similarity (including the incompatible elements) of the study rocks with the unequivocal melt compositions (Table 2; Figs 9a and 9b) clearly demonstrate that the study rocks are quenched liquids. This is further corroborated by the presence of pelitic xenoliths in it and the intrusive field relations (discussed earlier).
10 -
P-MORB
o! ’ I I1 I
0 10 20 Zr/Nb
30
Fig. 10. Zr-Y vs Zr-Nb distributions of the study mafic granulites. Fields are reproduced from Seymour and Kumarapeli (1995). The dot falling on the boundary of
T-MORB is of ANK-66M.
It has been shown earlier that the composition, particularly the distribution of the HFSE of the Anakapalle mafic granulites, overlaps with the MORB and continental flood basalts. However, the low Mg# and the low abundance of the compatible elements (Table 1) in the study rocks indicate that these quenched liquids have been subjected to fractionation since segregating from the mantle. Even the most magnesian sample ANK-66M has Mg# 59 and contains only 176 ppm Ni. These figures are well below the stipulated values for the primary magma of the MORB (Mg# 70 and Ni=250-300 ppm; BVSP, 1981). The composition of the study rocks are projected on the planes 01-S-PI and Ol-Cpx-S (Figs 11 and 12). Figure 11 also incorporates the fields of Deccan basalts and MORB. It is evident that the compositions spread over these two fields, implying similarities in petrogenesis. The Anakapalle rocks when plotted on oxide-oxide diagrams of Sen (1988) (Fig. 13) form a small cluster overlapping the fields of Deccan basalts, suggesting petrogenetic similarities between these and the study rocks.
Sen (1988) showed that the composition of the Deccan basalt can be generated from a primary magma through fractionation of olivine, clinopyroxene and plagioclase in different proportions to produce magmas with only 5-8 wt% MgO. A similar argument can also be made in the case of the study rocks in view of their compositional overlap with the Deccan basalts together with the sympathetic variation of Cr and Ni with MgO content of the rocks (Fig. 14).
Further insight into the batch melting of the mantle and subsequent crystal fractionation of the primary melts can be readily obtained by plotting Zr against Ni (Rajamani et al., 1985; Blackburn and Srivastava, 1994). Blackburn and Srivastava (1994) computed two curves for a lherzolite source (2000 ppm Ni and 2.8 ppm Sr; data taken from Taylor and McLennan, 1981) one showing batch melting at different percent- age (curve A of Fig. 15) and the other showing crystal fractionation beginning with a 30% batch melt (curve B of Fig. 15). The crystallization sequence
Mafic granulites, Anakapalle
CLINOPYROXENE
(4 (b)
Fig. 11. Molecular plots of Anakapalle mafic granulites onto the planes. (a) olivine-plagioclase-silica projected from clinopyroxene and (b) olivine-clinopyroxene and silica projected from plagioclase (after Walker et al., 1979). Fields square indicate the CMAS haploperiodotite solidus melts from 1 atmosphere to 20 kbar (after Presnall ef al., 1979). Solid numbered lines indicate the high pressure pseudoinvariant curves (after Takahashi and Kushiro, 1983). The fields shown are of Deccan basalts (solid line) and of MORB
(dashed line). Data from Sen (1988).
in this calculation was assumed to be as follows: up to 25% of crystallization olivine is the sole phase to fractionate, followed by crystallization of olivine + clinopyroxene + plagioclase + magnetite. The Anakapalle mafic granulites, except the sample ANK- 66M, plot close to the calculated fractionation curve suggesting an analogous fractionation process (Fig. 15). The higher Zr of ANK-66M could be related either to the stabilization of retrograde amphibole in these rocks (discussed earlier) or crustal contamination (cf. Black- bum and Srivastava, 1994).
Depth of magma emplacement
It has been mentioned earlier that the compositional spread of the study rocks (ANK-66M and ANK-288) show resemblance with the experimental run compo-
PLAGIOCLASE
193
sitions of Irving (1974) and Kuehner (1992) (see Table 2). Since, the petrochemical data support the fact that the samples under investigation are quenched liquids and that the post-magmatic chemical and mineral changes are minimal, the emplacement depth of the Anakapalle mafic granulite can be constrained by comparing the phase assemblage of the study samples with that of the run products of the aforesaid experimental work. Figure 20 reproduces the stability fields of different subsolidus assemblages of Irving (1974, R698). Non-ap- pearance of olivine as liquidus phase suggest crystalliza- tion depth of the study mafic granulite to be > 5.5 kbar at 800-1000°C (Fig. 20). Similarly, non-appearance of garnet constrains the maximum pressure c 10 kbar at the same temperature range (Fig. 20). The actual bounding pressure would shift slightly to lower values in view of the slightly Fe-rich composition of the
CLINW’YROXENE
(a) (b) Fig. 12. Anakapalle mafic granulites plotted on the isomolar pseudoliquidus phase diagrams of Elthon (1983): (a) projected from clinopyroxene on olivine-plagioclase-SiO* plane and (b) projected from plagioclase on olivine-clinopyroxene-SiO? plane. One atmosphere phase equilibria are from Walker et al. (1979) and 10, 15
and 20 kbar phase equilibria are from Elthon and Scarfe (1984).
194 P. Sengupta et al.
.‘I: 5 lo 15 20
MgO(wtW ’ (a) \
20\
I 1 I I s 10 15 20
MgOWW
09
Fig. 13. Oxide-oxide (wt%) plots of the study mafic granulites. The shaded area indicates the compositional range
of the Deccan tholeiites (data from Sen, 1988).
Anakapalle samples (Table 1). Following the approach of Kuehner and Green (1991) and Kuehner (1992), the compositions of the Anakapalle samples have been projected on the plane (Jd + CaTs)-Gl-Qtz and Di-Gl-Qtz (Fig. 21). The olivine-orthopyroxene satu- ration lines in Fig. 21 are taken from Kuehner (1992). Also plotted in these diagrams are compositions of mafic dykes from East Antarctica (including those used in Kuehner’s experiment). The majority of the data fall close to the 8 kbar olivine-orthopyroxene cotectic line (Fig. 21). Compositional similarity of the study rocks with the mafic dykes of East Antarctica (Table 2) together with non-appearance of olivine in the study rocks constrains the pressure of crystallization to be > 8 kbar. The deep crustal emplacement of the magmas of the study area is further corroborated when the compositions are projected on the isomolar pseudoliq- uidus phase diagrams of Elthon (1983) (Fig. 12).
I * 60
t
O6- ugobtu
(a)
-_I '5 6 6 l0
0) Fig. 14. Sympathetic variation of (a) Cr and (b) Ni with Fig. 16. Zr-Y vs Zr plots of the Anakapalle mafic granulites.
MgO wt% for Anakapalle mafic granulites. Field boundaries after Pearce and Norry (1979).
Nibwm)
Fig. 15. Diagram showing Zr (ppm) vs Ni (ppm) variation of the Anakapalle mafic granulites. For details, see text.
Collectively, all these evidences tightly constrain the depth of emplacement of Anakapalle mafic granulite at 9 + 1 kbar. The pressure value matches perfectly with the clinopyroxene-plagioclase-quartz barometry (8 kbar at 900°C discussed earlier).
Tectonic setting
Ever since the pioneering work of Pearce and Cann (1971, 1973), much effort has been made to ascribe magmas to diverse tectonic pigeonholes based on geochemical signatures. This is especially done with elements of low mobility, such as HFSE and REE. Although these tectono-magmatic discriminant dia- grams were initially constructed for Phanerozoic lavas, a good number of geochemists are of the opinion that the chemical-tectonic relationships have not changed with time and, hence, these discrimination schemes can be extended for the interpretation of the ancient magmatic rocks (cf. Rollinson, 1993; Bowes and Dash, 1992; Smith, 1992, Blackburn and Srivastava, 1994; Seymour and Kumarapeli, 1995; and references cited therein).
On the Zr vs Zr-Y diagram, the Anakapalle mafic granulites concentrated in the MORB field with some tendency towards the ‘Within Plate’ field (Fig. 16). A similar conclusion can also be reached using the ternary diagrams, such as Ti-Zr-Y and Ti-Zr-Sr (Fig. 17).
c r f tWithin plate basalts )- Island arc basalts
( .- Mid-ocean ridsc basalts .
20
$ 10
- z - ,B - t
lG
loo Zrbpm 1
Mafic granulites, Anakapalle 195
Ti/lOO A= LKT B=CAB C=OFB
(a) (b) Fig. 17. (a) Ti-Zr-Y and (b) Ti-Zr-Sr plots of the study granulites. Field boundaries after Pearce and Cann
(1973).
However, these diagrams cannot discriminate the tholeiites generated in ARC and MORB settings. TO resolve this ambiguity, the data are plotted on the Cr-Y diagram (Fig. 18). All the samples are separated from ARC but straddle the overlapping fields of ‘Within Plate Basalts’ and MORB (Fig. 18). The continental affinity of the studied rocks is also corroborated by the plots on the TiOrK@-P20s plane (Fig. 19). Further, Smith (1992) showed that the Proterozoic subduction-related basalts typically have La-Nb > 2.0 and Y < 20 ppm. In contrast, the basalts generated in an extensional set-up have La-Nb < 1.0 and Y > 20 ppm. The La-Nb ratios of the study rocks ranges between 0.09 and 2.83, with a mean of N 1.0. The Y content of all samples exceeds 20 ppm (Table 1). Summarizing the discriminating chemical-tectonic relationships discussed above all point towards the fact that the study rocks originated in an extensional set-up with a strong continental affinity.
It has been argued by many (reviewed in Rollinson, 1993) that the tectono-magmatic classification of basic rocks particularly for the very old rocks using only geochemical means is often fraught with problems such
Fig. 18. Cr-Y plots from the study granulites. Field boundaries after Gale and Pearce (1982).
as (a) uncertainties associated with initial mantle concentrations, (b) thermal structure in the Precambrian time and percentage of melt fraction, (c) fractional crystallization and (d) crustal contamination. Most of these factors are very difEcult to evaluate. This leaves serious doubts about the validity of these discriminant diagrams (Rollinson, 1993, p. 212). However, compo- sitional overlaps of the study samples with the magmatic rocks of that formed in undoubted extensional regime, such as MORB and continental flood basalts, and a remarkable similarity in the petrogenetic history of the two support the conclusion derived from the tectono- magmatic discriminant diagrams.
Discussion and conclusion
Phase relations and geochemical characteristics indicate that the liquids, now represented as mafic
TiOq
Continental
w p2°5 Fig. 19. TiOrK20-P,05 plots of the study granulites. Field boundary after Pearce et al. (1975). Shaded area is the field of
Deccan basalts.
196 P. Sengupta et al.
20 1
Temperature ( C) - Fig. 20. Stability fields of different assemblages produced during crystallization of basic magma having identical
composition as the study rocks (after Irving, 1974).
granulites at Anakapalle, show marked similarities both in chemical composition and petrogenesis with those of continental rift origin, e.g. flood basalts. It follows that the magmas originated in an extensional set-up. Petrochemical attributes indicate that the primary magmas of these liquids generated through extensive melting (-30%) of upper mantle source. Fractional crystallization of olivine, clinopyroxene, plagioclase and magnetite in different proportions produced melts having 5-8 wt% MgO. In-situ crystallization of these melts followed by cooling of the subsolidus assemblages converted these rocks to mafic granulites (Cox, 1980; Frost and Frost, 1987).
Notwithstanding the uncertainties of calculation, the estimated temperature (< 1000°C) and pressure (9 + 1 kb) at the time of emplacement of these magmas perfectly tally with the peak metamorphic conditions (cf. Dasgupta et a/., 1994) in the enclosing granulites. This follows that the basic magmas were in thermal equilibrium with the enclosing granulites and were emplaced at the same depth. Thus, the present analysis contradicts the contention of Bowes and Dash (1992),
Jd-CaTs
who postulated that the basic granulites of the Eastern Ghats belt are metamorphosed volcanics.
Dasgupta et al. (1994) identified a retrograde P-T trajectory for the enclosing granulites characterized by a steep decompression arm (AP = 1.5 kb), followed by a near-isobaric cooling arm. The mafic granulites share the same P-T history. Based on stability of mineral assemblages together with absence of characteristic felsic magmas, the authors ruled out the possibility of “collisional tectonics” as the cause of this apparent clockwise P-T trajectory. Thus, the magmatic history of the basic rocks developed in the present work supports the contention of Dasgupta et al. (1994). However, the authors argued that the apparent clockwise P-T trajectory of the Anakapalle granulites could have originated in one of the following situations: (a) during symmetric thinning of the lithosphere associated with extension of a continental crust of near-normal thickness, (b) as a part of the overall anticlockwise trajectory involving rapid tectonic denudation of a slightly over-thickened crust (due to addition of voluminous basic magmas) at or close to the thermal maxima.
In view of the petrochemical signatures of the study rocks, it is tempting to support the first alternative. This is also consistent with the re-working of an earlier granulite-facies rocks in this area by late Proterozoic granulite-facies event with an apparent clockwise trajectory (Grew and Manton, 1986; Dasgupta et al., 1994; Sanyal and Fukuoka, 1995).
Acknowledgements-Two of the authors (PS and SD) acknowledge support from the Department of Science and Technology, Government of India and Centre of Advanced Studies in Petrology from the Department of Geological Sciences, Jadavpur University. One of them (SD) further acknowledges support from the Alexander von Humboldt Stiftung. Another author (UKB) acknowledges support from the Council of Scientific and Industrial Research, Government of India, and another (JE) is thankful to the DFG, Germany, for a research grant. The authors are thankful to Prof. Gautam’ Sen for enlightening discussions during the preparation of the manuscript, to Sanjoy Sanyal for help during the field work and laboratory studies and to Prof. S. Hoernes for providing XRF facilities. They thank Dr M. Santosh for inviting them
L \
.\
\ \
‘1 / _.
01 01 OPX ob: 00 @I
Fig. 21. Anakapalle mafic granulite compositions (@) projected from (a) Di onto the plane (Jd + CaTs)-Gl-Qtz and (b) (An + Ab) onto the plane Di-Ol-Qtz; Green (1970) cited in Kuehner (1992). Pressure contours as in Kuehner (1992). 0 = 849590, A = #3790, Cl = #206, 0 = #060. In Fig. 21a # 49590 and # 206 falls on the study rock compositions and hence are not shown. For references see Table 2.
Mafic granulites, Anakapalle 197
to write this paper and for considering a very late submission, Davidson P. M. and Lindsley D. H. (1985) Thermodynamic analysis and they are thankful to Dr M. Arima and an anonymous of quadrilateral pyroxenes. Part II. Model calibration from reviewer for constructive suggestions. This paper is a experiments and application to geothermometry. Contrib. Mineral. contribution to IGCP Project 368. Petrol. 91, 390-404.
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