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

Abstract

We report equilibrium sapphirine þ quartz assemblage in biotiteeorthopyroxeneegarnet granulites from a new locality in Panasapattu of Paderu region in the Eastern Ghats granulite belt, which provide new evidence for ultrahigh-temperature (UHT) metamorphism at 1030e1050 C and 10 kbar in this region. The development of migmatitic texture, stabilization of the garneteorthopyroxenee plagioclaseeK-feldspar association, prograde biotite inclusions within garnet and sapphirine as well as sapphirine and cordierite inclusions within garnet in these granulites indicate that the observed peak assemblages probably formed during prograde dehydration melting of a BteSilleQtz assemblage, and constrain the prograde stage of the peT path. The core domains of orthopyroxene porphyroblasts have up to w(Al 2 O 3 ) 9.6%, which suggest that the temperatures reached up to 1150 C suggesting extreme crustal metamorphism. These conditions were also confirmed by the garneteorthopyroxene thermobarometery, which yields a peT range of 1012e960 C and 9.4 kbar. The peT phase topologies computed using isochemical sections calculated in the model system Na 2 OeCaOeK 2 OeFeOeMgOeAl 2 O 3 eSiO 2 eH 2 O (NCKFMASH) for metapelites, garnet-free sapphirine granulites and garnet-bearing sapphirine granulites match the melt-bearing assemblages observed in these rocks. Isochemical sections constructed GEOSCIENCE FRONTIERS journal homepage: www.elsevier.com/locate/gsf GEOSCIENCE FRONTIERS 3(1) (2012) 9e31 in the NCKFMASH system for an average sub-aluminous metapelite bulk composition, and contoured for modal proportions of melt and garnet, as well as for the compositional isopleths of garnet, predict phase and reaction relations that are consistent with those observed in the rocks. Garnet and orthopyroxene contain Ti-rich phlogopite inclusions, suggesting formation by prograde melting reactions at the expense of phlogopite during ultrahigh-temperature conditions. These peT results underestimate 'peak' conditions, in part as a result of the modification of garnet compositions in the domains where some melt was retained. The post-peak evolution is constrained by a succession of melt-present reactions that occur at p < 10 kbar, inferred from micro-structural relations among various minerals. After high-temperature decompression from the metamorphic peak, the peT path followed a near isobaric cooling stage to T < 900 C. The UHT rocks investigated in this study occur within a continental collision suture which witnessed prolonged subductioneaccretion history prior to the final collision. We correlate the extreme metamorphism and the stabilization of UHT mineral assemblages to heat and volatile input from an upwelled asthenosphere during subductionecollision tectonics in a Proterozoic convergent plate margin. ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. All rights reserved.

Significance of post‐peak metamorphic reaction microstructures in the ultrahigh temperature Eastern Ghats Province, India

“…1,000–950 Ma (Bose et al., ; Dasgupta & Sengupta, ; Simmat & Raith, ), thus overlapping considerably with interpreted M1. A protracted single cycle evolution involving M1 and M2 was advocated by other studies (Dharma Rao et al., ; Gupta, ; Korhonen et al., ), where M2 occurred in the interval c . 1,000–930 Ma (Das et al., ; Dasgupta & Sengupta, ; Dasgupta et al., , ; Grew & Manton, ; Mezger & Cosca, ; Shaw et al., ) and M1 is older than c .…”

“…Late cordierite is a common feature of many (62%) of the studies listed in Table . In evaluating photomicrographs and accompanying descriptions in the published studies, the late cordierite is commonly not developed to great extent, typically being confined to narrow coronas or symplectites on higher pressure post‐peak M1 assemblages such as orthopyroxene–sillimanite, garnet–sillimanite and sapphirine–orthopyroxene, amongst others (Table ; Bose et al., , ; Das, Bose, et al., ; Dasgupta et al., ; Dharma Rao et al., ; Grew, ; Grew et al., ; Kamineni & Rao, ; Mohan et al., ; Pal & Bose, ; Prakash et al., ; Sarkar et al., ; Sengupta et al., ; Shaw & Arima, ). Late cordierite was interpreted in numerous studies to reflect the down‐pressure segment (i.e.…”

“…Bose et al., ; Dasgupta & Sengupta, ; Simmat & Raith, ), although M1 and M2 were argued to be temporally distinct and separated by a phase of enderbite–charnockite magmatism (Dasgupta & Sengupta, ; Dasgupta, Sengupta, Ehl, Raith, & Bardhan, ; Dasgupta et al., ; Dobmeier & Raith, ; Rickers, Raith, et al., ; Sengupta et al., ; Simmat & Raith, ). Recently, the majority of the spread of isotopic Grenvillian ages were interpreted to date a single, long‐lived, high‐temperature (including UHT) metamorphic event (Dharma Rao et al., ; Gupta, ; Korhonen, Clark, et al., ). Ages in the interval 1,130–970 Ma were interpreted to date the prograde, peak and initial isobaric cooling (M1) whereas weighted average ages at 970–930 Ma were interpreted to represent M1 melt crystallization near an elevated solidus along continued near‐isobaric cooling paths (Korhonen, Clark, et al., ) in M1.…”

“…The M1 event in the EGP was interpreted to involve UHT metamorphism followed by near‐isobaric cooling (e.g. Bhowmik, ; Bose & Das, ; Bose, Das, Dasgupta, Miura, & Fukuoka, ; Bose, Fukuoka, Sengupta, & Dasgupta, ; Das, Bhattacharya, Raith, Bhadra, and Banerjee, ; Das, Bose, Ohnishi, & Dasgupta, ; Dasgupta & Ehl, ; Dasgupta & Sengupta, , ; Dasgupta, Sengupta, Fukuoka, & Bhattacharya, ; Dasgupta et al., ; Dharma Rao et al., ; Grew, ; Kamineni & Rao, , 1988b; Korhonen, Brown, et al., ; Korhonen et al., ; Lal, Ackermand, & Upadhyay, ; Mohan, Tripathi, & Motoyoshi, ; Mukhopadhyay & Bhattacharya, ; Nasipuri, Bhattacharya, & Das, ; Pal & Bose, ; Prakash et al., ; Sarkar, Dasgupta, & Fukuoka, ; Sengupta, Karmakar, Dasgupta, & Fukuoka, ; Sengupta, Raith, & Dasgupta, ; Sengupta et al., ; Shaw & Arima, , ). Most researchers consider M1 to be characterized by an overall counter‐clockwise P–T path (see Table and preceding references).…”

“…Whereas M1 has been quantified using modern metamorphic analysis over a large part of the charnockite–migmatite zone (see below; Korhonen, Brown, et al., ; Korhonen et al., , ) the M2 part of the P–T record has only been quantified using modern metamorphic techniques for one locality (Dharma Rao, Santosh, & Chmielowski, ), and despite contentions to the contrary it is not clear whether there is a temporal relationship between M1 and M2 (see Bose et al., ; for summary). In order to quantify M2, we focus on the metamorphic evolution of metasedimentary rocks from the Anakapalle locality in the Eastern Khondalite Zone of the EGP (Figure ; Dasgupta et al., ; Ramakrishnan, Nanda, & Augustine, ; Rickers, Raith, & Dasgupta, ; Sanyal & Fukuoka, ).…”