*6.3.1 Dolerite*

As compare to basalts, the dolerite shows (wt %) high SiO2 (54.50), low Al2O3 (13.50), low total alkalies (4.27), (Na2O: 2.18, K2O: 2.09), low total iron (9.70), low TiO2 (2.83), low CaO (4.96) and high MgO (7.20).

On the basis of geochemical data, Nakora acidic volcanic and basic rocks are plotted in the TAS diagram (**Figure 5**) [23] in which rhyolites lie in the field of rhyolite and dacite however dacite is very close to rhyolite where basic rocks lie in the field of basalt, trachybasalt, basaltic trachy–andesite and basaltic andesite. In tectonic discrimination R1 – R2 diagram [24], the Nakora granites fall in the field of anorogenic (**Figure 6**). Usually, the intrusion of anorogenic felsic magma into upper crust follows a cycle of compressive tectonic activity and orogenic magmatism [25]. The clustering of the granites in the anorogenic field indicates the limited melting of a crustal source [24].

In the Harker diagram, 1909 [26], the SiO2 (wt %) is plotted along the X axis and other oxide concentrations are plotted along Y axis. These diagrams reveal that Nakora rocks show four different distribution trends (**Figure 7**). The Nakora rocks show a regular decrease in Al2O3, MgO, Fe2O3 and CaO with increasing silica and continuous increase in K2O with increasing value of silica. In the case of TiO2 and P2O5, the rocks show constant distribution trend. In case of Na2O, the rocks show totally constant Na2O concentration with increasing silica content. The Nakora basalts contain more Al2O3, K2O, Fe2O3, TiO2, CaO and P2O5 as compared to other basic rocks. MgO and Na2O are more in dolerite and gabbro respectively. Granites show high MgO, TiO2, K2O and P2O5 as compared to other acid volcanic rocks. Al2O3 and Fe2O3 are more in tuff and trachydacite respectively.

**Figure 6.** *R1-R2 diagram [24] of major granitoid association. Symbols: Grey granite ( ), Pink granite ( ).*

*Petrology and Geochemistry of Nakora Ring Complex with Emphasis on Tectonics… DOI: http://dx.doi.org/10.5772/intechopen.98609*

#### **Figure 7.**

*Harker variation diagram of SiO vs. oxide showing the variation of Nakora rocks. Symbols: Peralkaline Granite ( ), Metaluminous Granite ( ), Peraluminous granite ( ), Rhyolite ( ), Dacite ( ), Trachydacite ( ),Tuff ( ), Basalt ( ), Gabbro ( ), Dolerite ( ).*

On the basis of different tectonic discrimination diagrams [27], which are based on mineral assemblages, the tectonic environment of the Nakora rocks is clarified. In the various tectonic discriminating diagrams (Y+Nb vs. Rb), when Nakora granites are plotted, they fall within plate granites (**Figure 8**). The above diagrams therefore demonstrate that this phase of magmatism was anorogenic. The primitive mantle normalized multi-element pattern (normalization values from Sun and McDonough [28]) for three Nakora basic rocks (2 basalts and 1 dolerite) displays similar pattern (**Figure 9**). However, the dolerite dyke is showing highly Pb enrichment as compared to basalts. The Nakora basic rocks show LREE enriched nature and they have consistent negative Nb, Ta, Sr. and Zr anomalies. The HREE pattern of dolerite dyke is showing parallel arrangement with HREE pattern of

**Figure 8.** *Y + Nb vs. Rb diagram of Nakora acidic rocks.*

**Figure 9.** *Primitive mantle normalized trace element patterns for Nakora basic rocks [28].*

other basic rocks. Hence close resemblance is observed between the dolerite dyke and Nakora basalts in terms of their LREE enrichment and negative Nb, Ta and Zr anomalies. Thus based on the above observations we conclude that these rocks may be related to same source. The chondrite normalized REE patterns for all types of Nakora rocks are shown in **Figure 10**. REE patterns in the Nakora acid volcanics and basic rocks are characterized by sub-parallel patterns with strong negative Eu anomaly (Eu/Eu\* = 0.06 to 0.12, avg. 0.08). But in basic, positive Eu anomaly (Eu/ Eu\* = 0.24 to 0.44, avg. 0.34) is observed. The Nakora basic rocks are less enriched in LREE and HREE as compared to acid volcanics. On the other hand, rhyolites and granites are showing almost similar abundances of REE which is probably due to

*Petrology and Geochemistry of Nakora Ring Complex with Emphasis on Tectonics… DOI: http://dx.doi.org/10.5772/intechopen.98609*

**Figure 10.** *Chondrite normalized REE patterns for Nakora rocks [28].*

their comagmatic nature. In general, the fractionation is more in HREE as compared to LREE in Nakora acid volcanic rocks, whereas in basics, almost flat normalized patterns are observed. Thus the sub-parallel REE patterns of all Nakora rocks suggest a common magmatic source.

The Siwana magma is derived from the mantle and has A-type geochemical parameters [29]. With the assimilated Achaean crust, the Jalor complex suggests primary mantle derivation with a variable degree of crustal contamination [30]. The Jhunjhunu granites seem to have come from a granodioritic composition source [31].

In the **Figure 11**, the chondrite normalized pattern is derived for partial melt by 33% partial melting leaving a residue 48% plagioclase, 33% opx and 19% cpx which closely approaches the REE patterns of Nakora basalts. The Bhilwara mafic metavolcanic which is taken from outside the study area and mixed Nakora gabbros

#### **Figure 11.**

*Chondrite-normalized diagram showing the calculated REE patterns for melts produced by 33% batch partial melting of mafic metavolcanic from Bhilwara leaving a residue consisting of 48% plagioclase, 33% orthopyroxene and 19% clinopyroxene.*

taken from the study area are showing maximum similarities with the REE patterns of the Nakora basalts. Hence, the Nakora basalts could have been derived by different degrees of partial melting of source rock similar to Bhilwara mafic metavolcanic/Nakora gabbros composition.
