Tectonic Significance and Age of Doleritic Sill Near Bandhalimal in the Singhora Protobasin of Chhattisgarh Basin, Central India

Abstract

Dolerites traversing Raipur Group of Chhattisgarh Basin have been reported by various workers but their radiometric age is not determined for ascertaining the absolute age of Raipur Group. Recent works on magmatic zircon and monazite from tuffs of Raipur and Singhora Groups have added significant radiometric and authentic data to accept the age of Chhattisgarh Supergroup. Present study reports Rb/Sr whole rock age from dolerite intruded into basal Rehatikhol Formation, of Singhora Group. It is exposed over a length of ~2 km with thickness of 2.0 m near Bandhalimal (21^°22'20":83^°15'15") village of Chhattisgarh State. Dolerite shows chilled margin and concordant relationship with overlying and underlying shales, and induration effects on the shale of contact zones. The sill has several cavities and vugs filled with altered material. Mineralogical, textural and chemical characters with ophitic to subophitic texture and varying degree of alterations confirm doleritic composition of the sill. The lower Mg number (5.6–35.3) of these rocks suggests the evolved nature of magma and extensive crustal contamination. Major and trace elemental concentration indicate continental tholeiite, which is further supported by various other chemical diagrams such as alkaline index {A.I.= Na₂O + K₂O/(SiO₂–43) × 0.17} against Al₂O₃, SiO₂ vs. FeO^t/MgO, and total alkali vs. silica diagram. In the TiO₂-K₂O-P₂O₅ trilinear diagram most of the samples plot in the continental field. Thus continental environment is suggested for the intrusion of dolerite. Rb-Sr whole rock estimation carried out at Geochronology Laboratory AMD, Hyderabad, on 8 representative rock samples of dolerite, provide 1,100 Ma reference line with initial ratio = 0.7095 ± 0.002. The ⁸⁷Rb/⁸⁶Sr and ⁸⁷Sr/⁸⁶Sr ratio vary from 0.11 to 0.29 and 0.71166 to 0.71583 respectively. In addition to above galena along with pyrite have also been observed in the fracture zones of Rehatikhol Formation near west of Juba and are located ~2.5 km south of Bandhalimal. The orientation of these fracture zones is sub-parallel to the sill. On this assumption hydrothermal activity in the fracture zone is considered coeval with the intrusion of sill and studied for any age relationship. Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 19–20, ²⁰⁷Pb/²⁰⁴Pb = 16.4–16.7 and ²⁰⁸Pb/²⁰⁴Pb = 38.2–38.8) studied for eight galena samples indicate model ages varying from 660 to 1,100 Ma with “μ₂” (²³⁸U/²⁰⁴Pb) values of 13.2 to 13.6 for second stage, based on two stage Pb growth model. This observation confirms the existence of igneous/hydrothermal activity in the basin and is consistent with the Rb/Sr age of sill as mentioned above. Therefore it is concluded that the rocks of Rehatikhol formation are older than 1,100 Ma considering two different sample population and methodology. A concomitant thermal event in the basin at ~1,100 Ma with related hydrothermal activity is thus proved by this communication.