Glaucophane and barroisite eclogites from the Upper Kaghan nappe: implications for the metamorphic history of the NW Himalaya

Abstract

AbstractThis paper presents the results of a petrographical and mineral chemical study of glaucophane- and barroisite-bearing eclogites from the Upper Kaghan nappe in the Higher Himalayan Crystallines of the Pakistan Himalaya, and discusses the implications of the P-T path recorded in such rocks for the tectonic and metamorphic history of the NW Himalaya. The eclogites described here come from a previously undescribed outcrop at Gittidas, but belong to the same suite as the garnet-omphacite-phengite-quartz-rutile eclogites previously described elsewhere in the Upper Kaghan nappe. The metamorphic peak assemblage is garnet-omphacite-rutile-quartz in glaucophane eclogite and garnet-omphacite-zoisite-rutile±kyanite±phengite±ankerite in barroisite eclogite. Most samples contain a significant amount of amphibole, white mica and quartz. White mica may be present either as part of the peak assemblage (phengite) or as a retrogressive phase after kyanite (paragonite). Amphibole is later than the metamorphic peak assemblage and is barroisite in most samples, but in relatively Fe-rich eclogite it is glaucophane with significant Na in the A site, Ca in the M4 site and tetrahedral A1. Garnet displays strong prograde zoning in the barroisite eclogites, with Mg increasing and Fe decreasing from core to rim. The iron-rich core is crowded with mineral inclusions of the peak assemblage, but inclusions of earlier paragonite, green and blue-green amphibole were also found. Peak metamorphic conditions in the barroisite eclogites have been estimated at T = 610 ± 30 °C and P = 24 ± 2 kbar from Fe/Mg partition in garnet-omphacite pairs, and from the garnet-omphacite-phengite barometer. These values are close to the equilibration conditions estimated for the eclogites of the North Himalayan Tso-Morari Dome. Strong similarities between metamorphic evolution of the Upper Kaghan nappe and metamorphic evolution of the eclogite-bearing units of the Neelum valley just to the east of the Kaghan valley indicate that the eclogite occurrences of the Kaghan-Neelum area define an eclogite-bearing terrane of regional extent, which was subjected to high pressure metamorphism in middle Eocene times. A comparison of the metamorphic evolution recorded in the eclogites of the NW Himalaya with that of the granulitized eclogites recently discovered in the E Himalaya suggests the possibility of a Himalaya-wide eclogitic metamorphism of pre-Miocene age. Therefore, the main difference between the Higher Himalayan Crystalline nappes of the NW Himalaya and those of the E Himalaya appears to lie less in the early part of their metamorphic evolution than in the different P-T paths they followed during exhumation.