CHAPTER 8 SUMMARY AND CONCLUSIONS

Transcription

1 CHAPTER 8 SUMMARY AND CONCLUSIONS

2 The Aravalli Mountain Range (AMR) is the main edifice of NW Indian shield. It is about 800 km long and 200 km wide with NE-SW strike. The rocks of AMR are hosted in an Archean basement referred to as Banded Gneissic Complex or BGC (Heron, 1953), in the form of linear belts representing Paleoproterozoic Aravalli Supergroup and Mesoproterozoic Delhi Supergroup and a curvilinear sedimentary Vindhyan basin. The 3.3 Ga old BGC (Gopalan et al., 1990) represents the oldest cratonic nucleus of the northwestern Indian shield and form the basement for all the Proterozoic cover sequences. The main constituents of BGC are gneisses of different types including TTG gneisses, granitic bodies, amphibolites and metasediments. The latter two types constitute minor component and occur as enclaves. The metasedimentary sequence is represented by Naharmagra quartzite of ~2800 Ma age. The AMR has been divided into two segments occurring to the south and north of Ajmer city (Fig.1.2). The southern segment is broadly constituted by three sub- belts namely the Udaipur belt and the Jharol belt which constitute Aravalli Supergroup and SDFB of the Delhi Supergroup. In northern segment of AMR rocks of the Delhi Supergroup occur in three sub-parallel basins (Fig 2.3), viz. Bayana Basin, Alwar Basin and Khetri Basin. In the eastern side of AMR rocks of great Vindhyan basin are exposed. The Delhi fold belt runs for entire length of AMR in its axial region. On the basis of geological differences and geochronological data the Delhi fold belt has been divided into north and south Delhi fold belts. The SDFB is an ensemble of various metasedimentary rocks ranging from fine grained phyllite to coarse grained quartzite. Western margin of SDFB is characterized by a thick mafic ultramafic sequence referred to as Phulad ophiolite (Gupta et. al., 1980). The evolutionary history of crust in this part has been visualized from the magmatic rocks alone. A continuous sedimentary record ranging in age from Mesoarchean to end-proterozoic is available in Aravalli cratonic block, which makes it quite good study area for geochemical investigations on sedimentary rock records particularly of Precambrian age. However, these sedimentary rocks have not been studied for their geochemistry in the past. The present study is first attempt to present major, trace and rare earth element data of the sedimentary rocks of Phulad ophiolite belt (Kumbalgarh Group). The petrographic studies of quartzites of Kumbalgarh Group are also undertaken. The generated geochemical data in combination with petrographical data are utilized to 165

3 constrain and locate their provenance and also weathering conditions, paleoclimate, and tectonic setting prevailing at the time of their deposition. The geochemical data particularly elements of petrogenetic significance such as K 2 O/Na 2 O, Rb/Sr, La/Th, (La/Yb) N and (Gd/Yb) N ratios of Kumbalgarh quartzites have been compared with those of underlying Mesoproterozoic Gogunda quartzite and Archean Naharmagra quartzite (~2800 Ma) to constrain changes in crust building processes across A-P boundary. Some major observations and inferences adduced from present study are given below 1. The petrographic study of the Kumbalgarh quartzites reveal that, these quartzites may be broadly grouped into four categories viz. quartz arenites, feldspathic quartzite with considerable amount of feldspars apart from quartz, sericitic quartzite and amphibole feldspathic quartzite. The mineralogy of Kumbalgarh quartzite and that of associated metasediments is suggestive of their origin from a provenance(s) which had both felsic and mafic igneous rocks and high grade metamorphic supracrustals. 2. The plots of Kumbalgarh metasediments on Qt-F-L and Qm-F-Lt diagrams suggest that the detritus of the metasediments were derived from the granitegneisses exhumed in the craton interior and medium to high grade metamorphic supracrustals forming recycled orogen provenance. 3. The correlation matrix (Table 5.4) indicates certain associations in Kumbalgarh clastics. (i) SiO 2 of all lithologies except pelitic gneiss show moderate to strong negative correlation with Al 2 O 3 (r=0.5 to 0.9). Such correlation is expected, because in sedimentary rocks the Al 2 O 3 and SiO 2 contents are controlled by aluminous clay and quartz contents respectively. (ii) The slight illite control is also indicated by moderate correlation between Al 2 O 3 and K 2 O in phyllite (r=0.58) and quartzite (r=0.59). Na 2 O to some extent appears to be bounded in clay minerals as it is weakly to moderately correlated with Al 2 O 3 in pelitic schist (r=0.48), greywackes (r=0.41) and quartzite (r=0.64). (iii) Fe 2 O 3 shows moderate to high positive correlation with Al 2 O 3 in pelitic gneisses (r=0.63), greywackes (r=0.74) and quartzites (r=0.48) thereby, 166

4 indicating that weathering in the source area controlled to some extent the abundance of these elements. (iv) The strong positive correlation between Fe 2 O 3 and MnO in all lithologies suggested by correlation co-efficient ranging from to 0.94 and also the positive correlation between Fe 2 O 3 and MgO in pelitic gneiss (r=0.57), pelitic schist (r=0.90) and quartzite (r=0.65) hint towards basic affinity for these detritus. (v) An oxygenated atmosphere is indicated by the positive correlation between Fe 2 O 3 and TiO 2 because in such conditions iron is retained in paleoweathering profile (Young and Nesbitt, 1998; Holland et al., 1989; Holland and Beukes, 1990). 4. The trace element correlation matrix (Table 5.5) reveals significant associations. (i) Cr-Ni show strong positive correlation in all lithologies barring quartzites thereby, indicating ultramafic/mafic signature in the provenance. (ii) The mafic affinity is also evidenced by the strong to moderate positive correlation between V-Co-Cr-Ni (V-Co r=0.65, Cr-Ni r=0.77) in pelitic schist (PSH2) and V-Ni in Q1 (r=0.46) and V-Cr in pelitic gneiss (r=0.78). 5. AUCC, PAAS, NASC normalized spiderplots of Kumbalgarh clastics suggest that the source as well as the conditions which formed the Kumbalgarh Group sediments were slightly different than those which produced these standard composites. 6. Based on their REE normalized patterns and REE abundances, the pelitic schists can be distinguished into two groups viz. PSH1 and PSH2. Both LREE and HREE are significantly fractionated in PSH1, which is evident from their La/Sm N ratio ranging from 1.15 to 5.19 (average=4.01) and Gd/Yb N ratio ranging from 0.88 to 7.58 (average=2.41). PSH2 is characterized by nearly flat REE pattern (La/Yb N : 1.31 to 1.36, average=1.33). 7. The greywackes are characterized by moderately fractionated LREE (La/Sm N : 1.30 to 3.41, average=2.55) and less fractionated HREE (Gd/Yb N : 0.58 to 1.29, average=0.77). They have distinct positive Eu anomaly (average Eu/Eu* = 1.37) and indicate effect of local plagioclase accumulation. 8. Based on their chondrite normalized REE patterns, quartzites can be divided into 3 subgroups. 167

5 9. The first group of quartzites (Q1) shows strongly fractionated REE patterns. The second group of quartzites (Q2) possesses moderately fractionated REE abundances (La/Yb N : 1.6 to 8.62, average=4.97). In this group of quartzites, LREE are significantly fractionated (La/Sm N : 1.83 to 4.06, average=3.42) than HREE, (Gd/Yb N : 0.64 to 1.62, average=1.05). The REE patterns as seen in the second group of quartzite are characteristic of mafic rocks, thereby, indicating mafic input from the provenance for the detritus of this group. The third group of quartzites (Q3) show enriched levels of HREE (La/Yb N : 1.1 to 2.54, average=1.97) and the normalized REE patterns of this group of quartzites bear signature of felsic rocks. 10. It may be interpreted that HREE abundances of the lithounits of Kumbalgarh were controlled by minor accessory minerals which were concentrated during weathering of the granodioritic to granitic source. And the detritus was brought from different source(s) having different accessory phases responsible for such REE abundances. 11. The REE concentrations and chondrite normalized patterns of phyllite and calc gneiss are nearly same and mimic to those of PAAS. 12. The geochemical synthesis of the data indicates that the petrographic divisions adopted during the course of thin section study of quartzites, is not completely replicated in their geochemical data. Only the sercitic quartzites belong to a single geochemical distinct group in quartzites (i.e.q2) whereas remaining petrographic divisions of quartzites do not belong to any geochemically distinct group of quartzites as discussed earlier. 13. The average CIA values of sediments of Kumbalgarh Group varying from to 65.15, lie between unweathered clastic rocks (CIA<50) and average standard shales (CIA=70-75; McLennan et al., 1993) suggesting less to moderate weathering condition in the source area of these sediments. 14. Generally the samples of pelitic gneisses, greywackes, phyllites, quartzites and pelitic schists plot along the linear trend of tonalite and granodiorite in the A- CN-K ternary diagram given by Nesbitt and Young (1984). 15. Few samples of Kumbalgarh clastic rocks plot below the feldspar join in the A-CN-K diagram, probably due to their low CIA values appeared to be sourced from a high Mg parent. 168

6 16. It is inferred from the A-CNK-FM plot that the Kumbalgarh clastic sediments are derived from a mixed source. The plot patterns of pelitic schists, pelitic gneiss, greywackes and phyllites are closely related which indicate similar source rocks for these rocks. 17. On the basis of CIW, the Kumbalgarh clastic rocks maybe interpreted to have suffered low to moderate weathering. 18. The lower values of ICV (<1) in case of Kumbalgarh clastic rocks indicate that they are compositionally mature sediments, poor in non clay silicates or dominated by minerals such as those of the kandite family that have low values and possibly derived from a tectonically quiescent or cratonic environments (Weaver, 1989). 19. All the Kumbalgarh clastic rocks have K 2 O/Al 2 O 3 ratio around 0.2 (< 0.4), thus indicating minimal alkali feldspar in the source rock of these sediments and in turn suggests presence of plagioclase oriented rocks in the catchment region. 20. The Th/U average ratios of quartzites, pelitic gneiss and phyllite are comparable with the Th/U ratio of upper crustal rocks (about 3.5 to 4.0).While the low ratios of calc gneiss and greywackes can be interpreted as dominantly reflecting a low ratio in the source rocks (McLennan, 1989; McLennan and Taylor, 1991). The Th and U content show high positive correlation (Table 5.6) in the Kumbalgarh clastic rocks (expect in PSH2 and phyllites) which is suggestive of the fact that the U content of these rocks is not due to weathering processes, but inherited from the source and thus low Th/U ratio of these rocs is also primary. Barring two samples of quartzites, the Th/U ratios of Kumbalgarh rocks <10 is well in accordance with their metamorphic grade, from lower greenschist to amphibolite facies. 21. Various geochemical parameters such as CIA, CIW, PIA, Th/U ratio, ICV values and K 2 O/Al 2 O 3 as described above suggest moderate tropical climatic conditions during deposition of Kumbalgarh sedimentary fill and indicate that extreme weathering conditions were probably negligible in the source area. 22. In the clastic rocks of Kumbalgarh the overall average K 2 O/Na 2 O ratio ranges from 0.49 to K 2 O/Na 2 O values >1 (except in greywackes), classify the Kumbalgarh metasediments as quartz rich type (Crook, 1974). In the K 2 O/Na 2 O SiO 2 diagram (Roser and Korsch, 1986), though majority of 169

7 samples plot in the passive margin tectonic setting field good number of samples fall in the oceanic island arc margin field (Fig. 7.1). Close scrutiny of the variations in K 2 O/Na 2 O ratios with respect to SiO 2 (Fig.7.1), and SiO 2 /Al 2 O 3 ratio in the various members of Kumbalgarh Group indicate their derivation from a terrain possessing felsic and mafic representatives. 23. Large range of variation in average Al 2 O 3 /TiO 2 ratio from to is an indication of complex source terrain having a spectrum of igneous, metaigneous and metasedimentary rocks. However, these variations in Al 2 O 3 /TiO 2 ratio may also be a result of hydraulic sorting or grain size sorting (McLennan et al., 1990). 24. From major element based diagram of Roser and Korsch (1988), it is clear that samples of Kumbalgarh metasediments plot in P4-P3-P2 fields in descending numbers indicating thereby that they were derived from a variety of sources. 25. There is large variation among ratios of Th/Sc and La/Sc and thus probably indicates that Kumbalgarh metasediments received inputs from both felsic as well as mafic rock. 26. Bivariant log-log plots like Th/Sc versus Sc, La/Sc versus Th/Sc, La/Th versus Hf and Co/Th versus La/Sc, Cr/Th versus Th/Sc ratio-ratio plot of Totten et al. (2000), Th/Sc vs. Zr/Sc plot indicate the derivation of detritus for these rocks from a mixed source consisting predominantly of igneous rocks with variable but minor amount of pre-existing sedimentary sequences. 27. The La-Th-Sc and Th-Sc-Zr/10 ternary plots indicate procurement of detritus for the Kumbalgarh Group rocks from a bimodal igneous source. 28. The modeling based on REE concentrations of different end members suggests that average Kumbalgarh clastics represent mixture of sediments derived from a provenance consisting of 20% Berach granite, 30% TTG gneisses and 50% mafic rocks of BGC basement. 29. It may be visualized that the Kumbalgarh basin in SDFB evolved as rifted basin which received initial cratonic detritus mainly from Archean BGC basement from east and relatively younger detritus from Phulad ophiolite (Delhi arc) from the west (Khan et al., 2005; Ahmad et al., 2008). 30. Therefore, the SDFB can be interpreted as a successor sedimentary sequence deposited in an extensional back arc basin closely around 1.8 Ga in a continental margin arc terrain. It can therefore, be suggested that SDFB 170

8 evolved from an active continental margin (back arc basin), through extensional rifting, to passive continental margin and that Phulad ophiolite originated in arc setting in response to the closure of Jharol ocean. Furthermore, an eastward subduction along the western margin of Delhi arc resulted in the accretion of Delhi arc, with the metasediments of SDFB and the Aravalli Supergroup rocks. This observation is in close agreement with the model proposed by Sudgen et al. (1990), Khan et al. (2005) and Ahmad et al. (2008) for the evolution of Precambrian crust in this part of NW Indian shield. 31. Important geochemical changes across A-P boundary observed by the comparison of geochemical data of Kumbalgarh Group quartzites, Gogunda quartzites and Archean Naharmagra quartzites as they represent a long part of earth history are as follows: (i) K 2 O/Na 2 O ratio changed from 2.12 during Archean to 5.41 during Proterozoic. As the granitic rocks formed during Archean were commonly Na rich whereas in post Archean they became K rich. (ii) There is an increase in the concentration of LILE and HFSE from Archean to Proterozoic suggesting change in the site(s) of mantle magma production. (iii) There is a significant decrease in (Gd/Yb) N ratio from 2.35 during Archean to 0.55 during Proterozoic as revealed by this study in this part of Indian shield. Consequently, HREE flattened with time and role of garnet in the generation of felsic magma decreased through Precambrian ages. 171