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Aluminous sapphirine granulites from the Eastern Ghats Belt (India): Phase relations and relevance to counterclockwise P-T history

¹ Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, India
² Mineralogisch-Petrologisches Institut, der Universität Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany

^* Corresponding author, e-mail: abba{at}gg.iitkgp.ernet.in; abbhat55{at}yahoo.com

In the Eastern Ghats Belt, 7-9-3 sapphirine in biotite-free Crd + Crn ± Sil ± Spl restitic domains occurs interlayered with Opx + Crd + K-feldspar assemblages extensively replaced by biotite. Natural settings and experimental phase relations indicate the restitic assemblages were derived by low pressure (< 5 kbar), high temperature (> 800 °C) anatexis of pelitic protoliths. In the restitic domain, sapphirine formed by the reaction Crn + Crd + Spl = Spr. Volumetrically minor amounts of sillimanite associated with sapphirine were produced by the ancillary reaction Crn + Crd = Spr + Sil. The decomposition of sapphirine + cordierite to En + Sil symplectites is attributed to the reaction Spr + Crd = En + Sil; occasional spinel in the symplectite may have formed by the reaction Spr = Spl + Sil + En. A quantitative activity-corrected M(F)AS grid constructed for Grt and Qtz-absent Mg, Al-rich metapelites suggests that sapphirine stabilization was induced by loading at 4.5 kbar, 900 °C. The decomposition of sapphirine occurred during cooling down to 700 °C at 8 Kbar in a thickened crust.

The record of low-P prograde heating that led to high T (> 800 °C, P < 5 kbar) anatexis and sapphirine stabilization at 900 °C at P 4.5 kbar differs from known counterclockwise P-T trajectories for Eastern Ghats granulites that assume simultaneous increase in P and T, with T_max attained broadly synchronous with P_max at 9 kbar. The phase petrological relations in the sapphirine granulites strongly indicate that near-peak T was attained prior to the attainment of P_max value. The initial high thermal anomaly is reconciled with asthenospheric heating below a thinned crust, and does not correspond with the thermal condition in the upper zone of a crust of normal thickness experiencing asthenospheric perturbation. Since loading induces temperature increase due to shallowing of geotherms, the cooling of the thickened crust can only be explained by conductive heat loss to "cooler" thrust slices above the hot crust, and/or asthenospheric depression. The post-T_max cooling in the CCW orogen suggests the continuance of thermal relaxation in the thickened crust in its attempt to attain isostatic stability with its environs.

Key-words: sapphirine granulites, counterclockwise P-T path, MAS grid, lithospheric thinning, prograde heating.