|Petrogenesis of the protolith for the Tirodi gneiss by A-type granite magmatism: The geochemical evidence|
M. V. Subba Rao, B. L. Narayana, V. Divakara Rao and G. L. N. Reddy
National Geophysical Research Institute, Hyderabad 500 007, India
In the Central Indian region, the Tirodi gneisses occur intimately associated with the Sausar Group metasediments. The protoliths for these gneisses might have originated by rift-related intracontinental felsic A-type (anorogenic) magmatism. The geochemical signatures in them like high Zr + Nb + Y + Ce, Ga/Al, Zr/Ni, Rb/Sr and Y/Sc, are quite similar to the A-type granite characteristics and point to the derivation of the protoliths by the melting of lower crustal dehydrated assemblages at high temperatures attained as a result of possible ponding of basaltic magma/magma underplating. The occurrence of mafic rocks related to subduction zone magmatism and back-arc environment in the vicinity of these gneisses in the Central Indian region attest to such an origin for the protoliths of the Tirodi gneisses.IN the Central Indian region, the Tirodi gneiss is a major lithological unit (Figure 1). These quartzofeldspathic granitic gneisses are intimately associated with the metasediments of the Sausar Group. The basementcover relationships of the granitic gneisses and the associated metasediments are obscured by the extensive migmatization at several places especially at the peripheries. There exist two schools of thought regarding the mutual relationships of the Tirodi gneisses and the metasediments of the Sausar Group. One school subscribes to the theory that the Tirodi gneisses constitute the basement on which the sediments of the Sausar Group have been deposited, while the other group of workers are of the opinion that the protoliths of the Tirodi gneisses intruded into the sediments of the Sausar Group13. RbSr isotopic studies4 could not help much in resolving this issue. In the absence of reliable and quality trace and rare earth element (REE) data, the limited studies so far on the Tirodi gneisses, could not focus on the petrogenetic aspects of the protoliths of these gneisses.
In this study, an attempt is made to throw some light on the nature of the magmatism that led to the formation of the protoliths of the Tirodi gneisses, based on a comprehensive new set of major and trace element (including REE) data of 20 representative samples of the Tirodi gneisses. Further the possible petrogenesis of the protoliths is evaluated.
Tirodi gneiss is a part of the Central Indian gneissic complex (Figure 1) and is exposed to the north of the supposed Central Indian suture (CIS). The samples studied are mainly from areas surrounding the mining
Figure 1. Geological map of the Central Indian region, showing the location of the Tirodi gneiss. 1, CIS/Mylonite zone; 2, Deccan basalt and Lameta formation; 3, Gondwana group; 4, Granites; 5, Tirodi gneiss; 6, Granulites; 7, Sausar metasediments; 8, Chattisgarh group; 9, Khairagarh group; 10, Chilpi group; 11, Nandagaon group; 12, Dongargarh granitoid; 13. Malanjkhand granitoid; 14, Sakoli group; 15, Amgaon gneisses; 16, Attitude of bedding.
town of Tirodi. These granitic gneisses are divided into grey and pink granitic gneisses which grade imperceptibly into each other. Pink gneisses cut across the grey gneisses and both are traversed by aplites and pegmatites. These gneisses show EW trending foliation defined by parallelism of the micas and elongate feldspar and quartz grains. Migmatites of pink and grey colour are banded in appearance and are always associated with granitic gneisses but are less frequent. Ptygmatic folds of quartzofeldspathic veins are common.
The essential mineral assemblages in these gneisses are quartz, microclinemicroperthite and plagioclase. Deformation is not significant and no deformation textures are noticed. Antiperthite, chess-board albite and albite rims around plagioclase at the contact of k-feldspar are present. The plagioclase in these rocks varies from albite to andesine. The twin laws observed in plagioclase suggest magmatic origin for the protoliths of these gneisses while those of migmatites favour a metamorphic origin in the upper amphibolite facies. K-feldspars show ex-solution perthites indicating the high temperature condition of formation. Presence of antiperthites also suggests high grade metamorphic or anatectic conditions. Both biotite and muscovite form the micas. The accessories include zircon, apatite, sphene, tourmaline and magnetite.
Twenty representative samples of the Tirodi gneiss
were analysed for major, trace and rare earth element compositions. Selected analyses are
given in Tables 1 and 2. These gneisses are characterized by a wide range of compositions
with SiO2 ranging from 59.07% to 75.69 wt% (Av. 70.59%); with moderate to high
Al2O3 (13.37 to 16.56%; Av. 14.86%); low to moderate concentrations
of TiO2 (0.03 to 1.62%; Av. 0.53%); low CaO (0.53 to 2.88%; Av. 1.44%); low MgO
(except one sample which has high MgO concentration of 5.23%; 0.03 to 5.23%; Av. 0.82%);
variable but moderate to high levels of alkalies (Na2O range from 1.79 to
4.80%, Av. 3.12%; K2O varies from 1.51 to 8.14%; Av. 5.61%) with low to
moderate levels of Fe2O3T (0.85 to 9.43%; Av. 3.57%). The
average composition of these gneisses are remarkably similar to the average A-type granite
(Table 3). Thus, these compositions reflect on the petrogenetic aspects of these gneisses.
Similarly the trace element compositions exhibit significant variations, particularly in
the case of Rb, Sr, Y, Zr and to certain extent in Nb, Cs, Ba, Hf and Ta as also U and Th
(Table 1). The Na2OK2O relationship in these gneisses assigns
a predominantly granite- to quartz-monzonite character to them (Figure 2). On the AFM (Na2O + K2OFe2O3TMgO)
plot they depict a calc-alkaline character
LaN/SmN which indicates LREE fractionation has a positive correlation with SiO2. However, at higher SiO2 levels the trend is not evident, with large variation in LaN/SmN. On the other hand CeN/YbN tends to be low at higher SiO2 levels (Figure 4). Similarly GdN/YbN which is indicative of HREE fractionation has a broad negative correlation.
The chondrite-normalized rare earth element patterns
of these gneisses suggest overall fractionation of REE (Figure 5); LREE enrichment and
evident (CeN/YbN = 0.79 to 49.98; Av. 14.77); significant negative Eu anomalies are discernible except in
Figure 2. Na2OK2O relationship in the Tirodi gneiss depicting a granitic- to quartz-monzonite character except one sample which shows tonalite character.Figure 3. Alkalies total iron as Fe2O3MgO (AFM) ternary diagram showing mainly calk-alkaline fractionation trend for the protoliths of the Tirodi gneiss.
one sample with significant positive anomaly (Eu/Eu* = 0.041.78; Av. 0.43). å REE varies from 49.89 to 897.07 ppm with an average of 305.89 ppm.
Figure 4. Variation diagram showing the relationship of SiO2 with Th, Nb, U and Hf and REE ratios. Figure 5. Chondrite normalized REE patterns of the Tirodi gneiss with all the three groups showing negative Eu anomalies of varying degrees.
K/Rb in these gneisses varies from 82 to 441, with an average K/Rb of 224, which is quite similar to the reported upper crustal K/Rb of 230. These gneisses are characterized by relatively enriched large ion lithophile elements (LILE) like K, Rb, Ba, Cs, U and Th, similar to the acid igneous rocks of the upper crustal regions.
These variations suggest that the LILE geochemistry is probably not significantly affected by post-formational changes and reflect their original nature, since these gneisses are not characterized by metamorphism of any significantly high grade. Moreover, the variations in the high field strength (HFS) elements as also the REE compositions suggest derivation from mixed and evolved source regions.
These gneisses can be divided into three groups with respect to their total REE contents and chondrite-normalized rare earth element patterns, i.e. a high REE group, a moderate REE group and a low REE group (Figure 5). Almost all the groups exhibit negative Eu
anomalies (with the exception of one sample), though of different extent in the three groups, and varying degrees of LREE and HREE fractionation.
Based on the relationship of 10000*Ga/Al vs Nb, Ce, Zr and Y, most of the samples fall in the A-type granite field, with only a few samples tending to be I- and S-type granites (Figure 6). Similarly the plot of Zr + Nb + Ce + Y vs 10000*Ga/Al suggest an A-type character for these gneisses (Figure 7). On the tectonic discrimination plots, these gneisses indicate a within- plate granite (WPG) character5, with only a few samples straddling the volcanic arc granites (VAG) and syn-collision granite (syn-COLG) fields (Figure 8). On the ternary plot involving Y-Nb-3Ga, these gneisses overwhelmingly fall in the A2 group of the A-type granites (figure not shown). They are characterized by high SiO2, Na2O, K2O and are relatively low in MgO, CaO and P2O5. Similarly, among trace elements Rb, Zr, Y, Nb, LREE and Ga are enriched in these gneisses, while Sr, Eu, Sc, V, Ni and Cr are relatively low (Tables 1 and 2). Negative Eu anomalies, high ratios of Rb/Sr, K/Rb, Ga/Al, Zr/Ni, Y/Sc and high Zr + Nb + Y + Ce are recorded, which distinguish these gneisses from I- and S-type magmas613. All the samples have consistently high Ga values (ranging from 11 to 89 ppm) and usually high ratios of 10000*Ga/Al (1.4810.46). The average composition of the Tirodi gneisses resembles the average A-type granite in several respects (Table 3), apart from most of the samples falling in the A-type granite field
Figure 6. 10000*Ga/Al vs Nb, Ce, Y and Zr in the Tirodi gneiss suggesting their mainly A-type granite character. I, I-type granite; S, S-type granite.
Figure 7. Zr + Nb + Ce + Y vs 10000*Ga/Al in the Tirodi gneisses indicating the predominantly A-type character of these gneisses. FG, Fractionated Granites; OGT, Other granite types.
Figure 8. Tectonic discrimination diagrams (A = Y + Nb vs Rb; B = Yb vs Ta) for the Tirodi gneisses, indicating their predominantly within plate granite nature. VAG, Volcanic arc granite; syn-COLG, Syn-collision granites.
and the within plate granite side in the tectonic discrimination diagrams5. Enrichment in high field strength elements (HFSE) is a characteristic feature of these gneisses. The concentrations and ratios of several HFS elements which are considered relatively immobile during most alteration and metamorphic processes may be more reliable14.
As A-type granites occur in non-orogenic settings, both within plate and along margins during the waning stages of subduction related magmatism, the possibility of a petrogenetic link between these granitic gneisses and the accretionary tectonics of the Central Indian region cannot be ruled out. The Dongargarh Super Group metavolcanics, particularly the Khairagarh volcanics are stratigraphically equivalent to the Sausars. They are considered to be the products of subduction related volcanism15,16. The emplacement of the volcanics might have supplied the necessary heat required for melting the lower crustal rocks. The occurrence of granulites in close proximity to these gneisses also supports the origin of the protoliths for the Tirodi gneisses from lower crustal source region. The protoliths of the Tirodi gneisses could be products of lower crustal melting. The necessary fluids required for enabling the resultant melt get enriched in the incompatible and HFS elements might have been provided by the basic magma. The Nb-Y-3Ga relationship as also the Nb-Y-Ce relationship in these gneisses suggest that they possibly belong to the A2-type granites which can form by the involvement of magma generated by continental margin and island-arc basalts, possibly during the waning stages of subduction or non-orogenic setting in a marginal basin environment. Such felsic magmas are normally produced during the rapid development of island arc/back-arc basin system via extension within the margin of a pre-existing older continental crust. Sedimentation and magmatic activity within the basin ceases on closure of the back-arc basin and accretion of the crust in the Proterozoic14.
A-type granite plutonism usually, though not exclusively, occurs in rift related environments in continental crustal regions. The production of anorogenic granite magma requires: (i) a source of dehydrated, reasonably salic crust; (ii) a source of heat sufficient to partially melt this crust; and (iii) a conduit to lead the magma to a higher level9,10. Such a scenario could be envisaged for the petrogenesis of the protoliths of the Tirodi gneisses. A plausible petrogenetic model for the origin of the protoliths for these granitic gneisses could be their derivation by extended fractional crystallization of a melt generated at lower crustal levels, contemporaneous with basalt emplacement in a rift-related extensional environment. Initial deposition of the sedimentary sequence of the Sausar Group might have started prior to the emplacement of the granitic melt, leading to a limited assimilation of the sediments already in place. That is possibly why some samples of the Tirodi gneiss unit tend towards the syn-collision and volcanic arc granite fields on the tectonic discrimination diagrams (Figure 8) and show a limited affinity towards the I- and S-type granites (Figure 6), though the overall A-type character is evident. There is a general high degree of concordance between Y/Nb and Yb/Ta ratios, which is the case in several A-type granite suites. Similarly Y-Yb and Nb-Ta show geochemically similar behaviour and these pairs of elements show concordance, as is also the case in several A-type granites. A detailed look at the overall and average compositions of the Tirodi gneiss unit brings out the remarkable similarities in several respects to the A-type granite (Table 3). The similarities are more striking in the case of CaO, Iron, Na2O, K2O, Ga, Rb, Y, Zr, La, Ce and Nd. Enrichment of HFS elements is a characteristic feature of these gneisses like in the A-type granites. The geochemical fingerprints in the Tirodi gneisses as mentioned above are consistent with the mixing of a newly mantle-derived component with an older crustal component through lower crustal assimilation and possibly the sediments of the Sausar Group on a limited scale, as suggested by the somewhat lower levels of Ba, U and Th and the moderate-to-high Al2O3 levels in these gneisses compared to the average A-type granite and the fact that some samples of these gneisses fall outside the A-type granite and within plate granite fields (Figure 6). The lithological configuration of the Central Indian craton suggests such a tectonic environment as can be seen from the occurrence of the subduction-related basic volcanism and the amphibolites which exhibit formation in a continental margin tectonic setting and a back-arc basin tectonic environment respectively16,17.
ACKNOWLEDGEMENTS. We are grateful to the Director, NGRI for encouragement and kind permission to publish the paper. This work was carried out under a project funded by DST. The help of Drs P. K. Govil and V. Balaram in major and trace element analyses respectively is thankfully acknowledged.
Received 18 May 1998; revised accepted 18 February 1998