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The mafic magmatism is mainly manifested in South Indian shield as dyke instrusions are episodic in discrete phases. The mafic dyke swarms of Tirupati and its surroundings are E-W trending mainly with subordinate ENE-WSW directions. Normative mineral composition of these dykes range from gabbro-norite, monzogabbro monzodiorite and diorite. The chemical characteristics of these dykes show that they are Fe and Fe-Mg rich tholeiites with more differentiated to less differentiated.
Based on major, trace and REE Geochemistry the dykes may be categorised into three groups. First group of dykes has high TiO2 (2.81%) Nb/La, (0.8 compared to chondrite value of 1.0) and Nb/Th (4.4 as compared to typical intraplate alkali basalts 10-15) indicate the characteristics of post colliosional basalt has combined features of intraplate and subduction related basalts. Such features could also be imposed by strong crustal contamination. High Li (66-88ppm), K2O (2.86-3.66%), Rb (125-168ppm) may indicate the secondary processes or involvement of sediment. These dykes have distinct negative Sr, Eu, Nb and Ta anomalies possibly reflecting sediment subduction which can superimpose a crustal signature. The Geochemistry suggests that the dykes may be intraplate basalts with crustal contamination or involvement of subduction modified lithospheric mantle. The second group have low TiO2 (∼2%) and Al203 (∼î10%) high Mgo (∼7-12.5%), LREE enrichment, HREE depletion suggest large degree of mantle melting under high pressure with garnet in the residue during magma generation. The high degree of melting and high temperature of the mantle indicates mantle plume connection. Mantle normalized patterns indicate the effects of sediment addition and/arc subduction. The third group characterised by the presence of low concentration of Rb, Ba and K than the rest of other two groups. The chemical characteristics and elemental ratios supports lesser degree of upper crustal contamination. There is close similarity in the concentrations of Th, U, Nb, Ti, Sr, P, Nd, Zr in the dykes with upper crustal rocks.
Major, trace, REE, and REE inversion studies on these dykes are indicative of deep mantle plume source. These rocks also demonstrate crustal fractionation with progressive changes from intense crustal contamination (sediment and granitic material) to less contamination. REE inversion studies indicate melting (?î20%) of metasomatically enriched lithosphere. The close similarity between the present dykes composition and the Deccan Bushe flows indicative of the presence of similar magma sources. The emplacement of present dyke swarms may be considered as unsuccessful attempts to break up the already cratonized continental crust.
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