The stability and origin of sodicgedrite in ultrah

Authors: Tomohito Kanazawa Toshiaki Tsunogae Kei Sato M Santosh

Publish Date: 2008/07/24

Volume: 157, Issue: 1, Pages: 95-

PDF Link

Abstract

MgAlrich rocks from the PalghatCauvery Shear Zone System PCSZ within the Gondwana suture zone in southern India contain sodicgedrite as one of the prograde to peak phases stable during T = 900–990°C ultrahightemperature metamorphism Gedrite in these samples is Mgrich Mg/Fe + Mg = X Mg = 069–080 and shows wide variation in Na2O content 14–23 wt NaA = 033–061 pfu Gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite The gedrite proximal to garnet shows an increase in NaA and AlIV from the core NaA = 040–051 pfu AlIV = 16–19 pfu to the rim NaA = 049–061 pfu AlIV = 20–22 pfu suggesting the progress of the following dehydration reaction Ged + Ky → NaGed + Grt + Crd + H2O This reaction suggests that as the reactants broke down during the prograde stage the remaining gedrite became enriched in Na to form sodicgedrite which is regarded as a unique feature of highgrade rocks with MgAlrich and K–Sipoor bulk chemistry We carried out highPT experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar and 1000°C However the product gedrite is Napoor with only 013 wt Na2O NaA = 0015–0034 pfu In contrast the matrix glass contains up to 85 wt Na2O suggesting that with the progressive melting of the starting material Na was partitioned into the melt rather than gedrite The results therefore imply that the occurrence of sodicgedrite in the UHT rocks of the PCSZ is probably due to the low H2O activity during peak PT conditions that restricted extensive partial melting in these rocks leaving Na partitioned into the solid phase gedrite The occurrence of abundant primary CO2rich fluid inclusions in this rock which possibly infiltrated along the collisional suture during the final amalgamation of the Gondwana supercontinent strengthens the inference of low water activityWe thank the staff at Gondwana Research Office in Trivandrum and Ms Preetha Warrier for their helpful support and Dr N Nishida for his assistance on microprobe analyses Kanazawa thanks his colleagues Messrs I Hossain H Ohyama H Tadokoro Ms N Kondou and Dr Y Miyagi for their helpful suggestions Prof Christian Nicollet and an anonymous reviewer provided helpful comments on the earlier version of this manuscript We thank these referees as well as Prof JLR Touret for his valuable editorial suggestions This is a contribution to the GrantinAid from the Japanese Ministry of Education Sports Culture Science and Technology to Tsunogae no 17340158 20340148 and Santosh no 17403013 as well as JSPSINSA Joint Research Program

Keywords: