Hybrid mafic dykes from Delwara Shear Zone, Mt. Abu, NW India

Abstract

Mafic dykes intrude the composite Mt. Abu granite batholith as a minor and the last phase of magmatism. The dykes are sub-vertical, variable in width and visibly compact, however, features of alteration and shearing can be seen. The dykes occurring within the recently identified and described, Delwara Shear Zone (DWSZ), from the western margin of the Mt. Abu batholith are intensely to moderately sheared and intricately mixed with the host granitoids. The mafic dykes occurring within the shear zone bear evidence of assimilating the host granitoids during their ascent, seen as relicts, streaks and sub-rounded K-feldspar clasts in mafic dykes. The hybridization has resulted in unusual geochemical signatures of the mafic dykes such as higher silica levels, erratic and high incompatible trace element abundances and lack of any systematic trends. Mixing line calculations on the mafic dyke samples reveal between 30 to 60% felsic input into the mafic dykes. Mafic dykes outside the shear zone in the Mt. Abu are meter scale in width and generally free of felsic inclusions owing to small volumes of mafic melts. Large volume of mafic melts are required for assimilating up to 60% felsic component which has been identified as approximately 100 m wide zone within the DWSZ. Shearing has played an important role in providing the channel ways and for sustained high temperatures to allow such hybridization.