DIFFUSION TO DISSOLUTION PRECIPITATION - EVIDENCE OF TRANSITION OF THE TWO PROCESSES DURING TWO STAGE CORONA GROWTH: A CASE STUDY FROM NAMAKKAL, TAMIL NADU, INDIA

Dolerite dykes from SGT exhibit development of disequilibrium reaction textures like corona, symplectite and pseudomorph. They form at the contact of thermodynamically incompatible earlier igneous mineral phases. Depending on composition two distinct types of dyke were identified – olivine (Ol) absent (Type-I) and olivine bearing (Type-II). In Type-I garnet (Grt) +quartz (Qtz) or amphibole (Am) + Qtz form at clinopyroxene (Cpx) and plagioclase (Pl) boundary. In Type-II orthopyroxene (Opx) + magnetite (Mag) and Am form in between Ol and Pl. Mineral zoning within corona in Type-I dyke indicates a control of composition. Al rich Grt/Am always develops at the contact of Pl and Qtz develops near Cpx. Grt and Qtz in some places show symplectitic texture implying corona growth in a single stage. In Type-II epitaxially grown opx contains a thin layer of Mag within it. Opx is optically continuous across the Mag layer and Opx+Mag mimic the Ol shape with a constant thickness. Opx+Mag layer on Ol is independent of associate minerals and is found to develop at the contact of Pl as well as Cpx. Am layer in the Type-II corona contain inclusions of Opx within it and develops only when Opx+Mag develop on Ol near Pl. Am is found intruding the Opx and Pl suggesting its formation in expense of both. Hence, in contrast to single stage corona in Type-I, Type-II dyke shows evidence of two stage corona growth. Opx+Mag formed from Ol in the 1^st stage followed by Am formation from coronal Opx and primary Pl. Conventional geothermobarometry and pseudosection study (NCFMASH system) yielded 6.5 to 7.5 kb pressure for corona growth and ~800˚C as peak metamorphic temperature (development of Opx+Mag layer). Am formation in Type-II dyke indicates temperature drop at ~ 650-750˚C also supporting the formation temperature of Grt/Am+Qtz in Type-I. Optical continuity of Opx through Mag layer, uniform thickness of corona (Type-II) along with preferential growth of aluminous phase near Pl (Type-I) suggest that diffusive mass transfer as a mechanism of formation, whereas Opx and Pl boundary encroachment by Am clearly suggest dissolution precipitation process. This transition of two processes in Type-II can be related to isobaric cooling or availability of fluid as reflected by formation of hydrous mineral(Am) in 2^nd stage of coronal growth compared to anhydrous mineralogy of 1^st stage.