Contact Metamorphic Granophyres of Partial Melting Origin Adjacent to Dolerite Dykes in Karnataka

Abstract

Granophyric rocks produced at the contact of post-Dharwar dolerite dykes in the host gneisses and granites of Peninsular Gneissic Complex stand out conspicuously at the weathered dyke margins in the vicinity of Hole Narsipur, Karnataka. The degree of development of granophyric texture is variable, and with its progressive increase, there is a shift in the host rock composition from granodiorite to alkali granite. Since there is no possibility of addition of potash from the dolerite to the granodiorite (the reverse is commonly the case) to cause this potash enrichment in host rocks, the origin of granophyres in the contact zones is attributed to partial melting rather than metasomatism or assimilation. This view is consistent with experimental evidence that partial melting of sodie gneisses (tonalites, trondjhemites and even granodiorites) results in the production of more potassic rocks like adamellites and alkali granites. The production of granophyres in the present case is, therefore, taken as a textural evidence for the partial melting of quartzofeldspathic gneisses. Similar phenomena are observed at the contact of Skaergaard intrusion of Greenland and Newer Dolerite of Singhbhum in eastern India.