AS and AV are grateful to the Director,CSIR-National Institute of Oceanography,Goa for his encouragement,support and permission to publish this work.AV sincerely acknowledges Dr.A.J.Locock for insightful discussion on the hydrous garnet.AS acknowledges the funding received from the Science and Engineering Research Board(SERB),Govt.of India under GAP 3291.The Secretary,MoES,Govt of India and Director,National Centre for Earth Science Studies,Thiruvananthapuram are thanked for providing analytical support to carry out the work.NS thanks Dr.Sneha Mukherjee for help during EPMA analyses.Authors are thankful to IUAC for extending LA-HR-ICPMS established under National Geochronology Facility funded by Ministry of Earth Science(MoES)with project reference number MoES/P.O.(Seismic)8(09)-Geochron/2012.We thankfully acknowledge the Editor-in-Chief and two anonymous reviewers for their constructive comments.This study is a part of Ph.D.work of AV.This is NIO's contribution no.7256. （SERB）