CHAPTER 7 PROVENANCE AND TECTONICS

Transcription

1 CHAPTER 7 PROVENANCE AND TECTONICS

2 CHAPTER 7 PROVENANCE AND TECTONICS Provenance data plays an important role in identification of the processes of formation of sedimentary rocks which dominates the deposition of the sediments. The chemical signature of the source or the parent material helps in identification of the chemical-mineralogical modifications associated with the evolution of the sediments which serves as the source for the formation of different rocks (Taylor and McLennan, 985). It helps in characterizing the unexposed crust, assessing the palaeogeographic reconstruction, lateral displacement in orogens and reconstruction of evolutionary model of the reworked sediments. The provenance data has been used for centuries to provide important constraints on the source of sedimentation in different basins along with the period and environment of deposition. Thus the analysis of tectonic settings helps in understanding the depositional and evolutionary history of the sediments. 7. Chemical Discrimination Diagrams for Sakoli Group of rocks Major and trace elements can be subject to important mobilization and fractionation during weathering, mineral accumulation, diagenesis and metamorphism. Thus there chemical behaviour has to be studied to have to basic idea of the source resulted in the accumulation of these sediments. The analysed samples have lower alkali vs. silica ratio (Log SiO 2 /Al 2 O 3 =.8-2.9). Log K 2 O/Na 2 O ratios are also variable, ranging from -. to.66. Thus in the Pettijohn et al., (972) diagram based on of log (Na 2 O/K 2 O) vs. log (SiO 2 /Al 2 O 3 ) content the Sakoli Group of sediments can be classified as arkose, subarkose and lithic arenite with only three samples are classified as sublithic arenite and four as greywacke (Figure 7.). The petrological and geochemical classification of these sandstones shows similar features of the studied rock samples. Chapter 7 Page 67

3 2 Greywacke log (Na2O/K2O) - Arkose Lithic Arenite Sub Arkose Sublithic Arenite Qtz. Arenite log (SiO2/Al2O3) Figure 7.: Log Na 2 O/K 2 O vs. Log SiO 2 /Al 2 O 3 binary geochemical classification diagram for Sakoli Group of rocks {after Pettijohn et al., 972 (indicated by dashed lines); modified and boundaries redrawn by Heron, 988 (indicated by solid lines)}. TiO2(wt%) BASALT BASALT GRANITE GRANITE BASALT GRANITE K 2O (wt%) Al2O3(wt%) Na2O(wt%) Figure 7.2: Al 2 O 3 vs. TiO 2 and Na 2 O vs. K 2 O binary geochemical classification diagrams for Sakoli Group of rocks. Chapter 7 Page 68

4 Additionally, a shift of the chemical composition of the samples towards lower log(na 2 O/K 2 O) ratios suggests that their behaviour is mainly controlled by the high mobility of Na 2 O, especially during chemical weathering, diagenesis and secondary alteration processes (McLennan, 2; Zimmermann and Bahlburg, 23). Plagioclase.6 Basalt.2 Smectite Al2O3 Gibbsite Kaolinite Chlorite Illite.8 Muscovite.6 Biotite K-feldspar.8.2 CaO+Na2O K2O Trend =gabbro, 2=tonalite, 3=diorite, 4=granodiorite, 5=granite Figure 7.3: Ternary diagram of Al 2 O 3 -CaO+Na 2 O-K 2 O (A-CN-K) showing weathering trends at the source of Sakoli Group of sediments (after Nesbitt and Young, 982 and Nyakairu and Koeberl, 2). The high content of Na 2 O and K 2 O appears to be due to the predominance of albite and K-feldspars in the sediments. In the Al 2 O 3 -TiO 2 and Na 2 O-K 2 O plot (Figure 7.2), the composition of Sakoli Group of metasediments shows a progressive decrease of TiO 2 content w.r.t. Al 2 O 3 and Na 2 O w.r.t. K 2 O thereby attesting its derivation from a mafic to felsic source area. In the Al 2 O 3 -TiO 2 plot the arkose and quartzite have much lower concentration of clay minerals than the phyllite and schist samples as the Chapter 7 Page 69

5 concentration of Al 2 O 3 is lower in these samples comparative to the later ones. The samples in the Al 2 O 3 -TiO 2 diagram also plot in the granite to basaltic granite field suggesting a mixed source for the deposition of these sediments. Similarly Na 2 O-K 2 O plot also indicate the granitic source for the deposition of these sediments. The A-CN-K (Al 2 O 3 -CaO+Na 2 O-K 2 O) is useful for examining weathering histories and K-metasomatism (Nesbitt and Young, 984; Fedo et al., 995, 996). In Sakoli Group of sediments most of the Group-I (schist) and Group-IV (quartzite) samples lie to the line parallel to the A-CN join while other intersect it at an angle of 3º. All this may be because of the K-enrichment metasomatism of the rocks which is a widespread feature in the Precambrian sedimentary rocks (Fedo et al., 996). In the Figure 7.3 all the samples reveal the indications of strong chemical weathering. Fe2O3 t +MgO.2 Ferromagnesian Potassic Sandstone Sodic Sandstone Potassic Sandstone.8.2 Na2O K2O Figure 7.4: Fe 2 O t 3 +MgO-Na 2 O-K 2 O ternary diagram for chemical classification of the metasedimentary rocks of Sakoli Group (from Blatt et. al., 98). Chapter 7 Page 7

6 3 Quartzites log (SiO2/Al2O3) 2 Granites Sandstones Greywackes Carbonates Basalts -2-2 log {(CaO+Na2O)/K2O} 3 Siliceous log (SiO2/Al2O3) 2 Argillaceous Calcareous -2-2 log (Na2O+CaO/K2O) Figure 7.5: The logarithms plot of the weight ratios of SiO 2 /Al 2 O 3 against (Na 2 O+CaO)/K 2 O for Sakoli Group of rocks (Garrels and Mackenzie, 969). Chapter 7 Page 7

7 The Fe 2 O t 3 +MgO-Na 2 O-K 2 O discrimination diagram shows the ferromagnesian potassic nature of the studied rock samples (Figure 7.4). The plot also indicate that the low- Ca+Na, high K samples are arkosic to lithic arkosic in nature or they can also be classified as chemically mature sediments whereas a few that fall within the sodic and potassic field represents a greywacke and arenitic protolith respectively as their source. A convenient graphical representation for the studied metasediments involves the logarithms of the weight ratios of SiO 2 /Al 2 O 3 against (Na 2 O+CaO)/K 2 O suggesting the granitic source accompanied with some crustal influences responsible for the deposition of the Sakoli Group of sediments (Figure 7.5). The plot also suggests the strong chemical weathering of the source resulted in the formation of the clay minerals. 7.2 Tectonic Discrimination Diagrams for Sakoli Group of rocks A comparison of major and trace element characteristics of the Archean sediments of the present study through various discrimination diagrams has helped to constrain the possible tectonic environment that helped in the deposition of these sediments. As suggested by Bhatia and Crook (986) large variation in the discrimination diagrams might create difficulty in identification of any clear tectonic settings but in case of Sakoli Group of sediments no such wide variation has been observed. Discrimination of tectonic settings on the basis of major element data was also proposed by Bhatia (983); these include oceanic island arc, continental island arc, active continental margin, and passive margin settings. The sediments t from the Sakoli Group have TiO 2, Al 2 O 3 /SiO 2 ratio, Fe 2 O 3 and MgO contents similar to sediments from young continental-margin arcs or to continental rifts or back-arc basins developed in or near continental crust {Figure 7.6 (a) to (d)}. The relatively pure quartzite within the group, however, has low contents of these elements similar to the sediments from cratonic basins. The Al 2 O 3 /SiO 2 ratio indicates the proportion of quartz in the sediments. All these parameters, Fe 2 O 3 +MgO, Al 2 O 3 /SiO 2 and TiO 2 decrease from the oceanic setting to a continental setting. In general the samples fall in the area of active continental Chapter 7 Page 72

8 margin and passive margin fields of the Al 2 O 3 /SiO 2 versus Fe 2 O 3 +MgO; TiO 2 versus Fe 2 O 3 +MgO; Al 2 O 3 /(CaO+Na 2 O) versus Fe 2 O 3 +MgO but there is much scatter on plot the K 2 O/Na 2 O versus Fe 2 O 3 +MgO plot, with data spreading across three fields (active continental margin, continental island arc, passive margin). Thus it is apparent that the geochemical data cannot discriminate between passive and active continental margins synchronous with deposition only by these discrimination diagrams but still they provide a general idea about the tectonic settings in which these sediments have been deposited. Binary plot in figure 7.6 (e) and (f) indicate the non-marine and the intermediate between peralkaline and peraluminous nature of the sediments respectively. A number of heavy minerals (like Zircon, Allanite and Monazite) are dominated by trace elements, and thus their accumulation in high concentration may significantly influence the trace element concentrations in sedimentary rocks (McLennan et al., 993). In this study, Zircon enrichment in sediments has been reflected by relationships between Th/Sc and Zr/Sc (McLennan et al., 993). On this diagram the analyzed samples do not follow simply the general provenance-dependent compositional variation trend, infact maximum number of samples are falling in the high Zr/Sc range typical of zircon accumulation associated with sediment recycling and sorting {Figure 7.7 (a)}. Ratios such as La/Sc, Th/Sc, Th/Co, and Th/Cr are significantly different in felsic and basic rocks and may allow constraints on the average provenance composition (Wronkiewicz and Condie, 99; Cox et al., 995; Cullers, 995). Th/Sc, Th/Co, Th/Cr, Cr/Th, and La/Sc ratios of the Sakoli Group of sediments included in this study are compared with those of sediments derived from felsic and basic rocks (fine fraction) as well as to upper continental crust (UCC) and PAAS values. This comparison also suggests that these ratios are having a mixed source comprising of both mafic and felsic rocks. Chapter 7 Page 73

9 2 (a).8 (b) TiO2 (wt%).6 OIA.2 CIA.8 PM ACM Fe2O3(T)+MgO (wt%) 6 (c) Al2O3/SiO2.6.2 CIA ACM OIA PM Fe2O3(T)+MgO (wt%) (d) K2O/Na2O PM ACM CIA OIA Fe2O3(T)+MgO (wt%) Al2O3/ (CaO+Na2O) PM ACM CIA OIA Fe2O3(T)+MgO (wt%) (e) 4 (f) K2O/Al2O3... Al2O3 (wt%) MgO/Al2O Al2O3+CaO+Na2O+K2O (wt%) Figure 7.6: Tectonic discrimination binary plot of FeO+MgO vs. (a) TiO 2, (b) Al 2 O 3 /SiO 2, (c) K 2 O/Na 2 O and (d) Al 2 O 3 / (CaO+Na 2 O) (after Bhatia, 983) (e) MgO/Al 2 O 3 vs. K 2 O/Al 2 O 3 and (f) Al 2 O 3 +CaO+ Na 2 O+ K 2 O (ACNK) vs. Al 2 O 3 for the samples of Sakoli Group. Chapter 7 Page 74

10 (a) (b) Th/Sc Co/Th Co/Th.... Zr/Sc.... Sc/Th (c) (e) La (ppm). Mixing Line... La/Sc 2 (d) Yb (ppm) (f).3.3 Sm/Nd.2 Sm/Nd Th/Sc Th/Cr Figure 7.7: Trace element ratio plots showing the properties of different groups of the studied rocks samples of Sakoli Group. Chapter 7 Page 75

11 The Th/Sc, Th/Co, Th/Cr, Cr/Th, and La/Sc ratios for the studied metasediments are 2.96 (felsic), 4.44 (felsic),. (mafic), 83.8 (mafic) and 5.74 (felsic) respectively which are falling in the range of felsic and mafic source values provided by Cullers (994, 2). In addition, the La/Th and Th/Sc ratios are fairly constant in sedimentary rocks (2.4 and.9, respectively; Taylor and McLennan, 985). The Sakoli group of sediments broadly define an array on provenance trace element ratio diagrams as Cr/Th versus Th/Sc {Figure 7.7 (c)} suggestive of mixing of source end members (Taylor and McLennan, 985; Cullers et al., 987). On this diagram, the Sakoli Group sediments can be produced by mixing average early Proterozoic basalt and granitoids, although other components that lie on or near the array may also contribute to the sediment mixture. Th La ACM.6 ACM, PM CIA OIA.2 CIA PM OIA Co Zr/ Th Sc Figure 7.8: Ternary diagram Th-Co-Zr/ and La-Th-Sc for Sakoli Group of sediments (after Bhatia and Crook, 986). OIA-Ocean Island Arc, CIA-Continental Island Arc, ACM-Active Continental Margin and PM-Passive Margin. Bhatia and Crook (986) have devised a scheme for distinguishing among the tectonic settings of sedimentary basins using immobile trace elements such as La, Th, Sc, Zr, Y and Co. On the La-Th-Sc and Th-Co-Zr/ discriminating plots of Bhatia Chapter 7 Page 76

12 and Crook (986) the samples of Sakoli Group of sediments plot in the passive margin and in between continental island arc and active continental margin fields (Figure 7.8), confirming the passive settings/provenance along with some later active continental settings for the Sakoli Group of sediments. The inferred provenance settings suggest a localized source(s) for these sediments. This inferred provenance together with the identification of several likely source components (i.e., mafic rocks, andesitic rocks, TTG granites) suggests that the sediments were derived from adjacent penecontemporaneous volcanic belt which might be the basement rocks of Amgaon Gneissic Complex (AGC) and Sakoli volcanic suite. K2O/Na2O. ACM PM ARC SiO2 (wt%) K2O (wt%).. Qtz. Rich Qtz. Poor... Na2O(wt%) Figure 7.9: K 2 O/Na 2 O vs. SiO 2 and K 2 O vs. Na 2 O binary diagram for Sakoli Group of sediments (after Roser and Korsch, 986). ARC is the oceanic island-arc margin field; ACM is the active-continental margin field; PM is the passive margin field. The passive margin (PM) settings suggest the mineralogically mature (quartz rich) sediments deposited in plate interiors at stable continental margins or intracratonic basins (equivalent to the trailing-edge tectonic settings of Marynard et al., 982; Kepp et al., 983) while the active continental margin (ACM) settings suggest quartz-intermediate sediments derived from tectonically active continental margins on or adjacent to active plate boundaries (e.g. trench, fore-arc and back-arc settings). The Sakoli Group of sediments thus represents the mixing of average early Chapter 7 Page 77

13 Proterozoic basalt and granitoids responsible for the deposition of these sediments, although other components (like mafic source) that lie on or near the array may also contribute as the source for these sediments. Al2O3.2.8 Ms Kfs A.6 Sm.6.8 C B.2 Chl Hb Bt CaO+Na2O+K2O FeO t +MgO Figure 7.: Ternary diagram of Al 2 O 3 -CaO+Na 2 O+K 2 O-FeO t +MgO (A-CNK-FM) showing weathering trends of (a) granitic source (b) basaltic source according to Nesbitt and Young, 989. The two arrows by the side of (a) and (b) represent the diagenetic/metasomatic transformation of kaolinite into illite and chlorite for Sakoli Group of rocks. (Abrv. Kfs-feldspar, Ms-muscovite, Sm-smectite, Hb-hornblende, Btbiotite, adapted from Camiré et al, 993). The K 2 O/Na 2 O vs. SiO 2 plot (Figure 7.9) is very useful in separating data into broad provenance groups. This plot was first suggested by Middleton (96), who used it to define eugeosynclinal provenances, and later used by Roser and Korsch (986) to define modern tectonic settings: oceanic arc, active continental margin Chapter 7 Page 78

14 (including continental margin arcs), and passive margins. The data presented here shows that all the samples fall within the passive margin (PM) and active continental margin (ACM) field. Only two samples fall in oceanic island arc margin (ARC) field. As deduced from the other diagrams the plot also confirms the mineralogically mature sediments took part in the deposition of the Sakoli Group of sediments. Crook (974) subdivided the sediments into three types on the basis of SiO 2 contents and the relative K 2 O/Na 2 O ratios. All the Sakoli Group of sediment samples under study are classified as quartz rich with K 2 O/Na 2 O> with a few falling within the field of quartz intermediate and two samples with quartz poor nature. The quartz rich samples suggest the passive margin settings of their deposition while the quartz intermediate samples indicate the active continental margin settings. CaO.2.8 OIA CIA ACM.2 PM Na2O K2O Figure 7.: CaO-Na 2 O-K 2 O ternary plot indicating the sediments of Sakoli Groups from OIA, CIA, ACM and PM tectonic settings (after Bhatia, 983). OIA-Ocean Island Arc, CIA-Continental Island Arc, ACM-Active Continental Margin and PM- Passive Margin. Chapter 7 Page 79

15 The A-CNK-FM ternary plot (Figure 7.) suggest that the Sakoli Group of sediments have been derived from a non-homogenous source and are plotted away the igneous trend near the Al 2 O 3 field between the weathering trends of basaltic and granitic rocks. Thus, indicating that the source material has been significantly affected by the chemical weathering (Camiré et al, 993). The CaO-Na 2 O-K 2 O (C-N-K) ternary plot (Figure 7.) displays a clear and distinctive differentiation of tectonic settings. The studied samples of Sakoli Group of sediments seem to represent the PM material. A few samples fall under the OIA field while 3 samples of arkosic sandstone fall outside the marked fields. The samples that fall in the PM Field have lower CaO content. The mineralogical maturity, enrichment of Zr and heavy minerals in the sediments, the high Na 2 O/K 2 O ratio, positive correlation with the REEs and high content of clay minerals represent a granitic source along with some phases of crustal contamination for the deposition of these sediments which have undergone strong chemical weathering. Thus the geochemical parameters and functions used by different authors were applied to identify the provenance and depositional history in the study area. 7.3 Chemical Discrimination Diagrams for Sausar Group of rocks The sediments deposited in any sedimentary basin provide an excellent opportunity to understand the nature of the rock. The major and trace elements data can be used to examine the geochemical characteristics of the sediments. It also helps in verifying the stratigraphic relation and identification of the provenance and protolith of the meta-sedimentary rocks. Thus there chemical behaviour has to be studied to have to basic idea of the source resulted in the accumulation of these sediments. The analysed samples have lower alkali vs. silica ratio (Log SiO 2 /Al 2 O 3 = ). Log K 2 O/Na 2 O ratios are also variable, ranging from -.3 to.2. Thus in the Pettijohn et al., (972) diagram based on of log (Na 2 O/K 2 O) vs. log (SiO 2 /Al 2 O 3 ) content the Sausar Group of sediments fall within lithic arenite to sub-lithic arenite Chapter 7 Page 8

16 with only two samples one falling in greywacke field and other in quartz arenite field. (Figure 7.2). The petrological and geochemical classification of these sandstones shows similar features of the studied rock samples. Greywacke log (Na2O/K2O) Arkose Lithic Arenite Sub Arkose Sublithic Arenite Qtz. Arenite log (SiO2/Al2O3) Figure 7.2: Log Na 2 O/K 2 O vs. Log SiO 2 /Al 2 O 3 binary geochemical classification diagram {after Pettijohn et al., 972 (indicated by dashed lines); modified and boundaries redrawn by Heron, 988 (indicated by solid lines)}. Additionally, a shift of the chemical composition of the samples towards lower log(na 2 O/K 2 O) ratios suggests that the Na 2 O is highly mobile and moves away from the system, especially during chemical weathering, diagenesis and secondary alteration processes (McLennan, 2; Zimmermann and Bahlburg, 23). The high content of K 2 O appears to be due to the predominance of K-feldspars in the sediments. In the Al 2 O 3 -TiO 2 and Na 2 O-K 2 O plot (Figure 7.3), the composition of Sausar Group of metasediments shows a positive correlation between TiO 2 and Al 2 O 3 while in Na 2 O vs. K 2 O plot the samples are clustered at a single point. In Na 2 O vs. K 2 O plot Group-I and Group-II show positive correlation. In the Al 2 O 3 - Chapter 7 Page 8

17 TiO 2 plot the Group-III and Group-IV have much lower concentration of clay minerals as they have lower concentration of Al 2 O 3. The samples in the Al 2 O 3 -TiO 2 diagram also plot in the basalt to granitic basalt field suggesting a mixed source for the deposition of these sediments. Similarly Na 2 O-K 2 O plot also indicate the granitic to granodioritic source of these sediments. TiO2(wt%).2.8 BASALT BASALT GRANITE GRANITE BASALT GRANITE K2O (wt%) Granite Al2O3(wt%) Tonalite Na2O (wt%) Figure 7.3: Al 2 O 3 vs. TiO 2 and Na 2 O vs. K 2 O binary geochemical classification diagrams for Sausar Group of rocks. The weathering history of clastic sedimentary rocks can also be constrained by examining the relationship among several major elements. On the A-CN-K (Al 2 O 3 - CaO+Na 2 O-K 2 O, Figure 7.4) chemical discrimination diagram is useful for understanding the weathering histories and K-metasomatism (Nesbitt and Young, 984; Fedo et al., 995, 996). In Sausar Group of sediments the samples of all the four different groups lie to the line parallel to the A-K join suggesting a uniformly weathered source and intersect at an angle of 3º to 5º defining the ideal trend of weathering of a primary source which is tonalitic to granitic in composition. The samples of Sausar Group of rocks plot in between the plagioclase-muscovite and plagioclase-kaolinite tie lines suggesting that the relative abundances of these elements are influenced by the alteration of plagioclase in the source area. The Chapter 7 Page 82

18 variations along the feldspar to muscovite-kaolinite tie line indicate that the suite includes a spectrum of rocks ranging from samples affected only by incipient chemical weathering (Fedo et al., 995), where the process resulted only in the simple breakdown of plagioclase, to rocks where the weathering led to the removal of alkali and alkali earth elements from the clay minerals (Taylor and McLennan, 985). Plagioclase.6 Basalt.2 Smectite Al2O3 Gibbsite Kaolinite Chlorite Illite.8 Muscovite.6 Biotite K-feldspar.8.2 CaO+Na2O K2O Trend =gabbro, 2=tonalite, 3=diorite, 4=granodiorite, 5=granite Figure 7.4: Ternary diagram of Al 2 O 3 -CaO+Na 2 O-K 2 O (A-CN-K) showing weathering trends at the source of Sausar Group of sediments (after Nesbitt and Young, 982 and Nyakairu and Koeberl, 2). On Fe 2 O t 3 +MgO-Na 2 O-K 2 O (Figure 7.5) triangular diagram of Blatt et. al. (98) Group-II samples of the Sausar Group of sediments are classified as potassic whereas the quartz rich samples are classified as ferromagnesian potassic. They Chapter 7 Page 83

19 represent the enrichment of both feroomagnesian elements and potassium indicating the chemically mature nature of the sediments. Fe2O3 t +MgO Ferromagnesian Potassic Sandstone Sodic Sandstone Potassic Sandstone.8.2 Na2O K2O Figure 7.5: Fe 2 O t 3 +MgO-Na 2 O-K 2 O ternary diagram for chemical classification of the metasedimentary rocks of Sausar Group of rocks (Blatt et. al., 98). The weight ratios of SiO 2 /Al 2 O 3 against (Na 2 O+CaO)/K 2 O show positive correlation and suggest the granitic to quartzitic source for the deposition of Sausar Group of sediments except three samples of Group-I and one sample of Group-II which show a basaltic source for the deposition of them. The data also exhibit some crustal influences responsible for the deposition these sediments (Figure 7.6). Chapter 7 Page 84

20 2 Quartzites (a).6 Sandstones Carbonates log (SiO2/Al2O3).2 Granites Greywackes.8 Basalts log {(CaO+Na2O)/K2O} 2.6 Siliceous (b) log (SiO2/Al2O3).2.8 Argillaceous Calcareous log (Na2O+CaO/K2O) Figure 7.6: The logarithms plot of the weight ratios of SiO 2 /Al 2 O 3 against (Na 2 O+CaO)/K 2 O for Sausar Group of rocks representing the chemical behavior of the samples (Garrels and Mackenzie, 969). Chapter 7 Page 85

21 7.4 Tectonic Discrimination Diagrams for Sausar Group of rocks Geochemical and provenance studies of clastic sedimentary rocks aim to decipher the composition and geological evolution of the sediment source areas and to constrain the tectonic setting of the depositional basin. The bulk geochemical analysis of sandstones is a powerful tool for provenance studies (Götze, 998; Dinelli et al., 999; Cullers, 2; Varga et al., 23; Zimmermann and Bahlburg, 23). Due to their relatively immobile nature, the distribution of selected trace elements, such as the high field strength elements Th, Sc and Zr and REE (rare earth elements), are particularly useful indicators of geological processes. Additionally, some key trace element ratios (La/Sc, La/Th, Co/Th, Th/Sc, Cr/Th, Cr/Zr) are widely used to discriminate the provenance and tectonic setting of sandstones (Taylor and McLennan, 985; Bhatia and Crook, 986; McLennan and Taylor, 99; Bauluz et al., 2). The geochemical composition of clastic sedimentary rocks was shown by Bhatia (983) and McLennan et al. (993) to reflect complex factors that include provenance, transport, weathering, and depositional setting. Geochemical techniques and characteristics give insights to the tectonic conditions that control these factors. A comparison of major and trace element characteristics of the Archean sediments of the present study through various discrimination diagrams has helped to constrain the possible tectonic environment that helped in the deposition of these sediments. As suggested by Bhatia and Crook (986) large variation in the discrimination diagrams might create difficulty in identification of any clear tectonic settings. In this part the results of chemical analysis obtained for Sausar Group of sediments are plotted to decipher the tectonic environment in which these sediments have been deposited. Discrimination of tectonic settings on the basis of major element data as proposed by Bhatia (983) includes oceanic island arc, continental island arc, active continental margin, and passive margin settings. The geochemical concept behind these discrimination diagrams {Figure 7.7 (a to d)} was based on a general decrease in Fe 2 O t 3 +MgO, TiO 2 and Al 2 O 3 /SiO 2 and increase in Al 2 O 3 /(CaO+Na 2 O) and K 2 O/Na 2 O as the tectonic settings changes from Chapter 7 Page 86

22 OIA-CIA-ACM-PM. The sediments from the Sausar Group have TiO 2, Al 2 O 3 /SiO 2, Al 2 O 3 /(CaO+Na 2 O), K 2 O/Na 2 O ratios and Fe 2 O t 3 + MgO contents similar to sediments from young active continental-margin or back-arc basins developed in or near continental crust {Figure 7.7 (a) to (d)}. The relatively pure quartzite within the group has lower concentration of these elements. In general the samples fall in the area of active continental margin and passive margin fields in all the four different plots, with only a few schist samples falling within OIA fields. Thus it is apparent that the geochemical data cannot discriminate between passive and active continental margins synchronous with deposition only by these discrimination diagrams but still they provide a basic idea about the tectonic settings in which these sediments have been deposited. Binary plot in figure 7.7 (e) shows the non-marine nature of the sediments. The binary diagram in figure 7.7 (f) shows that the samples of Group-III and Group- IV have higher values of both the ratios while Group-I and Group-II have lower Al 2 O 3 /SiO 2 ratio with variably low K 2 O/Na 2 O ratios. Heavy minerals like Zircon and Monazite are dominated by trace elements therefore their accumulation in high concentration may significantly affect the concentration of trace element in sedimentary rocks (McLennan et al., 993). Large variation in Th, Sc, La and their ratios in Sausar Group of rocks raises the possibility that composition of these rocks might have been influenced by these heavy minerals (McLennan, 989). The Th/Sc vs. Zr/Sc variation diagram is a useful measure to assess the contribution of pre-existing sources. A Th/Sc ratio > of sedimentary rocks reflects input from fairly evolved crustal igneous rocks (Taylor and McLennan, 985). Th/Sc ratio <.8 is an indication of source other than the typical continental crust, probably a mafic source or input from mature or recycled source if coupled with higher ratio of Zr/Sc (>). In this study, the Th/Sc ratio varies between.8-4. and Zr/Sc ratio is > (except a few schist and arkose samples) indicating a mixed source between evolved crustal igneous rocks and mafic rocks from a mature or recycled orogen. Zircon enrichment in sediments has been reflected by relationships between Th/Sc and Zr/Sc (McLennan et al., 993). On this diagram the analyzed samples do not follow simply the general provenance-dependent compositional variation trend, Chapter 7 Page 87

23 but the maximum number of samples are falling in the zone of zircon accumulation associated with sediment recycling and sorting {Figure 7.8 (a)}..2 (a) (b) TiO2 (wt%).8 ACM Fe2O3(T)+MgO (wt%) 6 PM CIA OIA (c) Al2O3/SiO ACM PM CIA OIA Fe2O3(T)+MgO (wt%) (d) K2O/Na2O Fe2O3(T)+MgO (wt%) PM ACM CIA OIA (e) Al2O3/ (CaO+Na2O) 3 2 PM ACM CIA OIA Fe2O3(T)+MgO (wt%) (f) K2O/Al2O3. SiO2/Al2O3... MgO/Al2O3. K2O/Na2O Figure 7.7: Tectonic discrimination binary plot of FeO+MgO vs. (a) TiO 2, (b) Al 2 O 3 /SiO 2, (c) K 2 O/Na 2 O and (d) Al 2 O 3 / (CaO+Na 2 O) (after Bhatia, 983) (e) MgO/Al 2 O 3 vs. K 2 O/Al 2 O 3 and (f) SiO2/Al2O3 vs. K 2 O/Na 2 O for the Sausar Group of sediments. Chapter 7 Page 88

24 (a) (b) Th/Sc. Co/Th La/Sc. Zr/Sc M G T (c) La (ppm) Mixing Line.. La/Sc (d).24. Th/Sc (e) Yb (ppm) (f).2 BBV Sm/Nd.6 Cr/Th Th/Sc. Th/Sc Figure 7.8: Trace element ratio plots showing the properties of different groups of the studied rocks samples of Sausar Group. Chapter 7 Page 89

25 Ratios of transition trace elements (like Cr, Co, Ni, Sc and V etc.) are highly variable in Sausar Group of rocks. For example, Cr=86-83ppm (avg. 549ppm), Co=2.-29.ppm (avg. 8.6ppm), Ni=.-59.ppm (avg ppm), Sc=4.-8.ppm (avg. 8.83ppm) and V=.-6.ppm (avg ppm). Here the average Sc, Ni and Co concentrations are comparable to the average upper crust (values from Taylor and McLennan, 98) but significantly depleted to PAAS and NASC (values from Taylor and McLennan, 98 & Gormet et al., 984 respectively). Th/Sc, Co/Th, Cr/Th and La/Sc ratios of the Sausar Group of rocks are compared with the sediments derived from felsic and basic rocks as well as from the upper continental crust (UCC) and PAAS. This comparison also suggests that these ratios are having a mixed source comprising of both mafic and felsic rocks. The average Th/Sc, Co/Th, Cr/Th and La/Sc ratios for the Sausar Group of metasediments are.6 (felsic),.52 (mafic), (mafic), and 2.79 (felsic) respectively which are falling in the range of felsic and mafic source values provided by Cullers (994, 2). In addition, the average La/Th and Th/Sc ratios are fairly constant i.e., 3.39 and.6 respectively in Sausar Group of sediments which is closely equivalent to the Archean sedimentary rocks (2.4 and.9, respectively; Taylor and McLennan, 985). The Sausar Group of sediments broadly define an array on provenance trace element ratio suggestive of mixing of source end members. In Figure 7.8 (f), the sediments can be produced by mixing of average early Proterozoic basalt and granitoids, although other components that lie on or near the array may also contribute to the source. Immobile trace elements in detrital sediments have always been used successfully in discrimination diagrams of paleotectonic settings. Bhatia and Crook (986) have devised a scheme for distinguishing among the tectonic settings of sedimentary basins using immobile trace elements. The La-Sc-Th and Th-Sc-Zr/ ternary diagrams (Bhatia and Crook, 986) have been used to differentiate between island arc (oceanic or continental) and continental margin (active or passive) settings. In the present study, the La-Th-Sc discrimination diagram (Figure 7.9) which was basically constructed for the discrimination of greywackes (Bhatia and Crook, 986) Chapter 7 Page 9

26 is used here to represent the tectonic discrimination history of the Sausar Group of sediments. The distribution of all the four different groups of sediments congregates in the fields of continental arcs and continental margins with three samples falling within ocean island arc field. The La-Th-Sc ternary diagram also discriminate between felsic and mafic provenance of clastic sedimentary rocks (Taylor and McLennan, 985), the samples of Sausar Group are almost indistinguishable and cluster in the field for mixed sources close to PAAS- and PS-like provenance (as defined by Taylor and McLennan, 985). Th La ACM PM.6 ACM, PM CIA.8 CIA.2.8 OIA.2 OIA Sc Zr/ Th Sc Figure 7.9: Ternary diagram Th-Co-Zr/ and La-Th-Sc for Sausar Group of sediments (after Bhatia and Crook, 986). OIA-Ocean Island Arc, CIA-Continental Island Arc, ACM-Active Continental Margin and PM-Passive Margin. On the Th-Co-Zr/ discrimination diagram the samples of Sausar Group of rocks plot in the continental arc field (also scattered around it) and in between continental island arc and active continental margin fields (Figure 7.9), only three samples from Group-II are falling within ocean island arc field and two samples from Group-II and one sample form Group-III are falling in passive margin field. However, slightly high Zirconium content in these rocks also indicates Zr-enrichment in the samples because of which the samples have been shifted towards the higher Sc-Zr/ axis. Thus the inferred Chapter 7 Page 9

27 provenance settings for Sausar Group of sediments suggest a localized mixed source(s) for their deposition. This inferred provenance together with the different source suggests that these sediments were derived from adjacent penecontemporaneous volcanic belt. K2O/Na2O Log (Fe 2 O 3 /K 2 O) ACM PM ARC SiO2 (wt%) 2 Fe-mudstone Mudstone Arkose Fe-sand Subarkose (a) (c) Sublitharenite Quartzarenite Log (SiO 2 /Al 2 O 3 ) K2O (wt%) Na2O+K2O (wt%) Qtz. Rich Qtz. Poor.. Na2O(wt%) Alkaline (d) Sub-Alkaline (b) SiO2 (wt%) Figure 7.2: (a) K 2 O/Na 2 O vs. SiO 2, (b) K 2 O vs. Na 2 O, (c) Log (Fe 2 O 3 /K 2 O) vs. Log (SiO 2 /Al 2 O 3 ) and (d) Na 2 O+K 2 O vs. SiO 2 binary diagrams for Sausar Group of sediments (after Roser and Korsch, 986). ARC is the oceanic island-arc margin field; ACM is the active-continental margin field; PM is the passive margin field. The Sausar Group of sediments are especially rich in silica and have high K 2 O/Na 2 O ratio. The K 2 O/Na 2 O vs. SiO 2 plot is strong discriminant of tectonic setting (Roser and Korsch, 986). In this diagram {Figure 7.2 (a)} the Sausar Group of Chapter 7 Page 92

28 clastic sediments data invariably fall in passive margin field of the discriminant plot, indicating their passive margin tectonic setting with only four Group-I and two Group-II samples falling within active continental margin field. Interestingly, most of Sausar Group of sediments fall on area of quartzose sedimentary provenance or a quartz rich horizon {Figure 7.2 (b)} having K 2 O/Na 2 O>, although they are in fact first cycle sediments derived from felsic igneous provenance. This is because of extreme reworking (sorting) of sediment in a stable cratonic regime that resulted in enrichment of SiO 2 and abnormally high SiO 2 /Al 2 O 3 ratios. Al2O3.2.8 Ms Kfs A.6 Sm.6.8 C B.2 Chl Hb Bt CaO+Na2O+K2O FeOt +MgO Figure 7.2: Ternary diagram of Al 2 O 3 -CaO+Na 2 O+K 2 O-FeO t +MgO (A-CNK-FM) showing weathering trends of (a) granitic source (b) basaltic source according to Nesbitt and Young, 989. The two arrows by the side of (a) and (b) represent the diagenetic/metasomatic transformation of kaolinite into illite and chlorite respectively for Sausar Group of rocks. (Abrv. Kfs-feldspar, Ms-muscovite, Sm-smectite, Hbhornblende, Bt-biotite, adapted from Camiré et al, 993). Chapter 7 Page 93

29 The mudrock samples are classified according to the scheme of Herron (988) which has been proven to be useful when applied to clastic sedimentary rocks (Roddaz et al., 26). In this classification scheme, the Sausar Group of sediments plot consistently in the mudstone to sublith-arenite field {Figure 7.2 (c)}. In Figure 7.2 (d) the samples are equally divided within the alkaline and sub-alkaline fields. The Group-I and Group-II samples show alkaline nature of the rocks whereas Group-III and Group-IV samples show sub-alkaline nature of the sediments. The A-CNK-FM ternary plot for Sausar Group of sediments (Figure 7.2) suggests that these sediments have been derived from a non-homogenous source and are plotted away from the igneous trend near the Al 2 O 3 axis between the weathering trends of basaltic and granitic rocks (nearer to the granite weathering trend). Thus, indicating that the source material has been significantly affected by the strong chemical weathering (Camiré et al, 993). The geochemical results are presented in the form of ternary diagrams with CaO Na 2 O K 2 O poles. The CaO-Na 2 O-K 2 O (C-N-K) ternary diagram (Figure 7.22) shows that the Sausar Group of samples are dispersed in the direction toward CaO K 2 O. This dispersal may be caused by variable concentrations of calcium and K- feldspar. The tendency of these sediments towards calcium and potassium poles may be due to the calc-alkaline properties of the igneous source rocks. This plot also displays a clear and distinctive differentiation of tectonic settings. The studied samples of Sausar Group of rocks seem to represent the nearly active margins to passive margin settings with some influence of arc settings. The mineralogical maturity, enrichment of Zr and heavy minerals in the sediments, the high Na 2 O/K 2 O ratio, positive correlation with the REEs and high content of clay minerals represent a granitic source along with some influence of crustal material responsible for the deposition of these sediments which have undergone strong chemical weathering. The geochemical parameters and functions used by different authors were applied to identify the provenance and depositional history in the study area. Chapter 7 Page 94

30 Thus provenance lithotype characterization provides a blueprint or framework with which assumptions can be made to determine the nature and type of the source, processes involved in the formation of the rocks and evolutionary history etc. which actively participate in the sedimentation processes. CaO.2.8 OIA CIA ACM.2 PM Na2O K2O Figure 7.22: CaO-Na 2 O-K 2 O ternary plot indicating the sediments of Sausar Groups from OIA, CIA, ACM and PM tectonic settings (after Bhatia, 983). OIA-Ocean Island Arc, CIA-Continental Island Arc, ACM-Active Continental Margin and PM- Passive Margin. Chapter 7 Page 95