New constraints on the metamorphic evolution of the Eastern Ghats Belt, India, based on relict composite inclusions in garnet from ultrahigh‐temperature sapphirine granulites

Abstract

The sapphirine granulites from Gangaraju Madugula, Eastern Ghats Belt (EGB), India preserve a rich variety of mineral assemblages and unique isolated and composite mineral inclusions within garnet that provide robust evidence for extreme crustal metamorphism at ultrahigh temperature (UHT) conditions (>9008C). Diagnostic UHT assemblages in these rocks include sapphirine þ quartz, spinel þ quartz and high alumina orthopyroxene þ sillimanite þ quartz. The stability of spinel þ quartz, sapphirine þ quartz and orthopyroxene þ sillimanite þ quartz assemblages provides evidence for temperatures exceeding 9608C at moderate pressures. The mineral association of garnet-orthopyroxene is indicative of a subsequent high P-UHT metamorphic event as indicated by the high alumina contents of orthopyroxene (>10 wt% Al 2 O 3 ) coexisting with garnet. Peak P-T conditions of $9708C and 9.5 kbars are calculated from conventional garnet-orthopyroxene geothermobarometry. Calculated isochemical sections constructed in the model system Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -Fe 2 O 3 (NCKFMASHTO) for the sapphirine granulites and garnet-orthopyroxene granulites adequately predict phase relationships that are consistent with those observed in the rocks. An evaluation of the assemblages and textures and P-T estimates indicate a three-stage evolution of the sapphirine granulites and associated garnet-orthopyroxene granulites: (1) inclusion assemblages with sapphirine, spinel and quartz on the low-pressure prograde path (M 1 stage); (2) a peak UHT assemblage of porphyroblastic garnet-orthopyroxene (M 2 stage) and (3) a retrogression that resulted in orthopyroxene-cordierite intergrowths and biotite rims on Grt (M 3 stage). The sapphirine granulites and associated garnet-orthopyroxene granulites indicate that they grew during the prograde and retrograde stage. The thermo-barometric estimates from mineral compositions and the complimentary isochemical sections approached from bulk rock compositions allow tighter constraints to be placed on the P-T evolution of this sector of the EGB.

Sapphirine granulites from Panasapattu, Eastern Ghats belt, India: Ultrahigh-temperature metamorphism in a Proterozoic convergent plate margin

“…The isochemical sections have been calculated in the model system Na 2 OeCaOeK 2 OeFeOeMgOe Al 2 O 3 eSiO 2 eH 2 O (NCKFMASH). Several recent studies have demonstrated the potential of the application of isochemical sections ("pseudosections") to evaluate the stability of mineral assemblages and their evolution in peT space during prograde, peak and retrograde metamorphism (e.g., Tinkham et al, 2001;White et al, 2001;White and Powell, 2002;Zeh and Holness, 2003;Kelsey, 2008;Nasipuri et al, 2009;Dharma Rao and Chmielowski, 2010). The results in this study thus provide new insights on UHT conditions in a deeply buried crustal segment of the Eastern Ghats belt with implications for the thermal and tectonic evolution of this collisional orogen.…”