Genesis of wollastonite- and grandite-rich skarns in a suite of marble-calc-silicate rocks from Sittampundi, Tamil Nadu: constraints on the P-T-fluid regime in parts of the Pan-African mobile belt of south India

Abstract

The Pan-African tectonothermal activities in areas near Sittampundi, south India, are characterized by metamorphic changes in an interlayered sequence of migmatitic metapelites, marble and calc-silicate rocks. This rock sequence underwent multiple episodes of folding, and was intruded by granite batholiths during and subsequent to these folding events. The marble and the calc-silicate rocks develop a variety of skarns, which on the basis of mineralogy; can be divided into the following types: Type I: wollastonite?+?clinopyroxene (mg# = 71-73) + grandite (16-21 mol% Adr) + quartz ±calcite, Type II: grandite (25-29 mol% Adr ) + clinopyroxene (mg# = 70) + calcite + quartz, and Type III: grandite (36-38 mol% Adr) + clinopyroxene (mg#=55-65) + epidote + scapolite + calcite + quartz. Type I skarn is 2-10 cm thick, and is dominated by wollastonite (>70 vol%) and commonly occurs as boudinaged layers parallel to the regional foliation Sn₁ related to the Fn₁ folds. Locally, thin discontinuous lenses and stringers of this skarn develop along the axial planes of Fn₂ folds. The Type II skarn, on the other hand, is devoid of wollastonite, rich in grandite garnet (40-70 vol%) and developed preferentially at the interface of clinopyroxene-rich calc-silicates layers and host marble during the later folding event. Reaction textures and the phase compositional data suggest the following reactions in the skarns: 1. calcite + SiO₂→wollastonite + V, 2. calcite + clinopyroxene + O₂→grandite + SiO₂ + V, 3. scapolite + calcite +quartz + clinopyroxene + O₂→grandite + V and 4. epidote + calcite + quartz + clinopyroxene + O₂→grandite + V Textural relations and composition of phases demonstrate that (a) silica metasomatism of the host marble by infiltration of aqueous fluids (X_CO2 < 0.15) led to production of large volumes of wollastonite in the wollastonite-rich skarn whereas mobility of FeO, SiO₂ and CaO across the interface of marble and calc-silicate and infiltration of aqueous fluids (X_CO2 < 0.35) were instrumental for the formation of grandite skarns. Composition of minerals in type II skarn indicates that Al₂O₃ was introduced in the host marble by the infiltrating fluid. Interpretation of mineral assemblages observed in the interlayered metapelites and the calcareous rocks in pseudosections, isothermal P-X_CO2 and isobaric T-X_CO2 diagrams tightly bracket the "peak" metamorphic conditions at c.9 ± 1 kbar and 750°± 30°C. Subsequent to 'peak' metamorphic conditions, the rocks were exhumed on a steeply decompressive P-T path. The estimated 'peak' P-T estimates are inconsistent with the "extreme" metamorphic conditions (>11 kbar and >950 ° C) inferred for the Pan-African tectonothermal events from the neighboring areas. Field and petrological attributes of these skarn rocks are consistent with the infiltration of aqueous fluid predominantly during the Fn₁ folding event at or close to the 'peak' metamorphic conditions. Petrological features indicate that the buffering capacity of the rocks was lost during the formation of type I and II skarns. However, the host rock could buffer the composition of the permeated fluids during the formation of type III skarn. Aqueous fluids derived from prograde metamorphism of the metapelites seem to be the likely source for the metasomatic fluids that led to the formation of the skarn rocks.