OCCURRENCE OF HIGH-PRESSURE RODINGITES IN THE OPHIOLITES OF INDUS SUTURE ZONE, EASTERN LADAKH,THE HIMALAYA:TECTONIC AND METALLOGENIC SIGNIFICANCE

摘要

Recently,considerable attention is being paid in studying the high|pressure (rodingites and eclogites etc)crustal segments for understanding the architecture and evolution of collision orogens.This paper presents the geology,mineralogy and geochemistry of the rodingites,the first reported occurrence in eastern Ladakh,the Himalaya.Nidar ophiolite is one of the well exposed,nearly a complete ophiolite of the Indus Suture Zone present in the eastern Ladakh.Field studies across the Nidar ophiolite in the Nidar—Kyun Tso section unraveled the occurrence of relatively strongly developed rodingites.Rodingites are very hard and dense.They occur as layers and also as boudins within and at the contacts of the serpentinites.The rodingites are fine to medium grained.Grossular is the dominant rodingite mineral and occurs as well developed crystals.At places grossular has coronitic texture.Diopside,clinozosite,rutile and opaques are the other main Ca|rich minerals present in rodingites.The matrix of the rodingite minerals is highly birefringent.The rodingite mineral assemblage indicates the development of rodingite in the pressure and temperature range of 18～25 kbar and 700 to 800℃,respectively.Rodingites have high abundances (mass fraction) of CaO (10%～12%) and Al-2O-3 (12%) contents and generally low in SiO-2 (46%) and MgO (7%～8%) contents.They have chondrite|normalized Rare Earth Element (REE) abundances of 25 to 40× on the LREE and 37 to 50× on the MREE and 15 to 24× on the HREE.Overall the REE patterns tend to be concave|upwards,or relatively light|REE depleted with almost no Eu|anomaly.The geologic occurrence,the mineralogy and geochemical (major,trace and REE) data of the rodingites indicate that they were initially gabbros/basalt that have undergone Ca|metasomatism during serpentinization,followed by high|pressure recrystallization to rodingites.