Geochronology and phase equilibria modelling of ultra‐high temperature sapphirine + quartz‐bearing granulite at Usilampatti, Madurai Block, Southern India

Abstract

Sapphirine-bearing granulite from Usilampatti in the Madurai block of southern India preserves a variety of mineral textures and reactions that help in reconstructing a three-stage metamorphic evolution. Corroded biotite, sillimanite and quartz inclusions within garnet represent relics from the prograde history. Peak metamorphic conditions were attained with the development of sapphirine + quartz in textural equilibrium (Stage 1). This was followed by nearly isothermal decompression, leading to the formation of sapphirine + cordierite at Stage 2. Subsequent retrograde hydration (Stage 3) is only locally evident. Using the Perple_X software and the model system NCKFMASH, the peak P-T conditions were estimated from core compositions, and the retrograde evolution was deduced from rim or symplectite compositions of different minerals as computed by isopleths of X Mg garnet, X Ca garnet, X Mg orthopyroxene, X Mg sapphirine and X Mg biotite. The P-T conditions for Stage 1 thus obtained, and supported by thermodynamic modelling using the winTWQ programme, is approximately 9 kbar and 940°C. Stage 2 conditions were constrained as 6.7 kbar and 900°C. Dating of zircon and monazite in the sapphirine-bearing granulite and associated gneisses by the U-Pb method using LA-ICP-MS indicates metamorphic overprint of zircon (lower intercept ages of discordant data arrays) at 546 ± 8 and 547 ± 11 Ma and metamorphic growth of monazite between 542 ± 3 and 551 ± 2 Ma. Upper intercept ages for zircon point to zircon growth at approximately 2514 ± 66 Ma. Although it remains unclear whether the metamorphic age data refer to Stage 1 or Stage 2 or, most likely, a continuum between both, they clearly document a late Ediacaran age for ultra-high temperature (UHT) metamorphism in the area, which, based on the obtained P-T path, was most likely the result of crustal thickening followed by uplift and erosion. Thus, it is concluded that the sapphirine-bearing granulites formed in response to Pan-African orogeny that led to the collision of the western and eastern Madurai domains, whereas initial zircon growth probably took place during late Neoarchaean arc magmatism that formed much of the western domain.

Orthopyroxene – Cordierite ± Sapphirine Symplectite Replacing Garnet: Diagnostic Evidence for Isothermal Decompression (ITD) in the Usilampatti, South India

“…Isothermal decompression is also evident from symplectites of orthopyroxene-cordierite and orthopyroxene-sapphirine-cordierite. Institute of Science, BHU Varanasi, India DISCUSSION AND CONCLUSION Usilampatti granulites thermal peak condition suggests they are formed under the UHT regime (Prakash et al 2018). These granulite rocks preserve a record of their P-T evolution because of the development of spectacular reaction textures, strong refractory nature of minerals and varied mineralogy.…”

“…The Southern Granulite Terrane (SGT) forms a collage of several crustal blocks separated by extensive Proterozoic shear zones. The Madurai Block lies between the Palghat-Cauvery and the Achankovil shear zones (Prakash et al, 2010(Prakash et al, , 2018. Granulites are the high-grade metamorphic rocks that originate in the lower to mid-continental crust (John et al, 2005).…”

Ultrahigh‐temperature mafic granulites from the Madurai Block, southern India: Constraints from conventional thermobarometry, pseudosection analysis, and rare earth element‐based thermometry

“…In contrast, the Southern Granulite Terrane (SGT) experienced the last high-grade metamorphism during the Late Neoproterozoic to Early Cambrian (550-500 Ma) (e.g. Brandt et al, 2011Brandt et al, , 2014Braun, Montel, & Nicollet, 1998;Clark et al, 2015;Kooijman et al, 2011;Plavsa et al, 2012;Prakash et al, 2017;Raith, Srikantappa, Buhl, & Koehler, 1999;Sengupta et al, 2015;Tiwari & Sarkar, 2020). The Northern and the Southern Granulite Terranes are separated by a >100 km wide zone of E-W trending shear zones, known as the Palghat Cauvery Shear System (PCSS) (Collins et al, 2014;Drury et al, 1984).…”

“…The last phase of metamorphism is UHT where the inferred peak conditions are in the range of 950-1,050 C at 10-12 kbar. The UHT conditions are primarily reported from sapphirine-bearing Mg-Al rich metapelitic granulites in Perumalmalai (Prakash & Arima, 2003;Raith et al, 1997), Palani hills (Brandt et al, 2011), Karur (Sato & Santosh, 2007) and Komiloothu (Sato et al, 2009), Komateri (Tsunogae et al, 2008), Malapatty (Tsunogae & Santosh, 2003), Lachmanapatti (Tsunogae & Santosh, 2003), Sengal , Kiranur (Lal, Ackermand, Raith, Raase, & Seifert, 1984), Sakkarakottai , Usilampatti (Prakash et al, 2017), Ganguvarpatti (Kondou, Tsunogae, Santosh, & Shimizu, 2009), Vadakkampatti (Tiwari & Sarkar, 2020) and Rajapalyam. In comparison, only a few UHT conditions are reported from mafic granulites (Prakash et al, 2007(Prakash et al, , 2010.…”

“…Evidence for UHT metamorphism is primarily described from sapphirine-bearing metapelitic granulites preserving diagnostic mineral assemblages (e.g. Brandt et al, 2011;Prakash et al, 2017;Prakash & Arima, 2003;Sato & Santosh, 2007;Sato, Santosh, & Tsunogae, 2009;Tateishi, Tsunogae, Santosh, & Janardhan, 2004;Tsunogae & Santosh, 2003;Tsunogae, Santosh, Ohyama, & Sato, 2008). Although the occurrence of mafic granulites, in association with UHT metapelites, have been reported by many studies (e.g.…”