Constraining the prograde and retrograde P-T paths of granulites using decomposition of initially zoned garnets: an example from the Central Indian Tectonic Zone

Abstract

The present study from the Sausar Mobile Belt (SMB) in the southern part of the Central Indian Tectonic Zone (CITZ) demonstrates how microdomainal compositional variation of a single garnet porphyroblast in a metapelite granulite sample records the different segments of a near complete P-T path of metamorphic evolution. The microdomainal variation is ascribed to the preservation of growth zoning and heterogeneous distribution of diverse inclusion mineral assemblages. Subsequent mineral reactions under changing P-T conditions were controlled by this compositional heterogeneity. Four stages of metamorphic evolution have been deciphered. An early prograde stage (M_o) is implied by the rare presence of staurolite-biotite-quartz and in places of kyanite inclusion assemblages in other metapelite samples, together with the growth zoning preserved in garnet. The peak metamorphism (M₁) at ∼9.5 kbar, ∼850 °C is consistent with the biotite dehydration melting that produced garnet-K-feldspar and granitic leucosomes. This was followed by near isothermal decompression (M₂) at ∼6 kbar, ∼825 °C, during which different garnet segments behaved as separate microscale bulk compositions and decomposed both internally and externally to produce different retrograde mineral assemblages. In the quartz-bearing domain of almandine-rich and grossular-rich garnet core, grossular components in garnet reacted with included sillimanite and quartz to produce coronal plagioclase (X_An = 0.90). By contrast, grossular-rich garnet in quartz-absent domain reacted with included sillimanite to produce layered spinel_ss {X_Mg (Mg/Mg+Fe²⁺) = 0.23–0.26}, X_Al(Al/Al+Fe³⁺) = 0.71–0.81}-plagioclase (X_An = 0.91)-cordierite {X_Mg (Mg/Mg+Fe²⁺) = 0.80–0.83} coronas both in the core and inner rim region of garnet. During post-decompression cooling, reactions occurred at about 600°C (M₃), whereby quartz-bearing, sillimanite-absent microdomains of pyrope-rich, grossular-poor garnet outer rim decomposed to form relatively magnesian assemblages of cordierite-anthophyllite and cordierite-biotite-quartz. M₂ spinel_ss decomposed to polyphase domains of spinel-magnetite ± hogbomite at this stage. Collating the textural and geothermobarometric results, a clockwise P-T path has been deduced. The deduced P-T loop is consistent with a model of crustal thickening due to continental collision, followed by rapid vertical thinning, which appears to be the general feature of the Sausar Mobile Belt. Using model calculations of the preserved growth and diffusion zoning in garnet, we demonstrate rather short-lived nature of this collision orogeny (in the order of 40–60 Ma).