Differential core-to-periphery emplacement of mafic magma in an Archean Complex and its implications: An example from Southern Granulite Terrain, India

An integrated study on the structure, petrology, mineral chemistry, bulk-rock geochemistry, in-situ garnet geochemistry and emplacement mechanisms of a mafic intrusion is presented from the southern granulite terrain of southern India.

Field study, modal mineralogy and bulk-rock geochemistry characterize the mafic intrusion as hornblende-gabbro to dioritic-gabbro. Structural relationships indicate that the intrusions followed the pre-existing foliations of the rocks of the Bhavani Gneissic Complex. Velocity modelling indicates that the outer zone (periphery), with relatively less viscosity, intruded at a faster rate than the more viscous inner zone (core) resulting in shearing along the core-periphery boundary that led to the development of foliation with appropriate shear sense indicators. The foliation gradually disappears when mapped from the periphery zone of the intrusion towards the core, indicating that the shearing is restricted to the peripheral zone of the intrusion. Absence of intracrystalline deformation of the mineral grains in the foliated periphery zone suggests melt-assisted (magmatic) origin for the foliation in the periphery during emplacement. The orientations of the foliations in the host rocks and along the periphery of the intrusion indicate that the emplacement was syn-to-post kinematic, in the form of an inclined plug within the rocks of the Bhavani Gneissic complex.

Geochemical data show compositional variations in the periphery and core of the intrusion. Bulk-rock geochemical data and in-situ mineral chemistry reveal that parental magma had a tholeiitic affinity in the arc-subduction zone tectonic setting with garnet in its source. The nearly flat chondrite-normalised REE pattern with a mild negative Eu-anomaly and low CaO, and Al₂O₃ content in the rock, suggests the removal of plagioclase from basic parental magma. Trace element compositions indicate that garnet contains variable amounts of Ni, Cr, Ti, and Co from core to periphery of the intrusion and shows a negative Europium anomaly. Such variations may be due to the initial fluid flow and appropriate fO₂ condition, leading to magnetite precipitation, which incorporated Ni, Ti, Cr and Co. In general, a higher concentration of Ni and Cr in the core of the intrusion than the periphery, suggests that the core may be explored for Ni-Cr and PGE potential.