GENESIS AND METAMORPHISM OF THE TUTING-TIDDING SUTURE ZONE OPHIOLITES, NORTHEAST HIMALAYA IN AN ANCIENT SUBDUCTION ZONE: CONSTRAINTS FROM GEOCHEMISTRY AND P-T MODELLING

Tuting-Tidding Suture Zone (TTSZ) consists of a relatively lesser explored ophiolite complex. The ultramafic cumulates of this ophiolite have high SiO₂ (~42.51) and MgO (~41.61), low Cao (<0.07) and TiO₂ (<0.10), contain high forsterite number (Fo - 89.6-97.6) in olivine, high in Cr# (>0.90) in Cr-spinel and show saucer shaped REE patterns with enrichment in LREE indicating a boninite like magma source for them. The mafic rocks occurring as intrusive bodies have REE patterns similar to N-MORB (ƩREE = 23.34 to 59.12) with a slight enrichment (La_N/Yb_N = 1.49 to 2.58) indicating involvement of a crustal component in their formation. The amphibolites are geochemically similar to the mafic intrusives but with some island arc component. The whole section has been subject to metamorphism upto amphibolite facies as evident from the mineralogy. Conventional geothermobarometry and P-T pseudosections constructed for the garnet bearing amphibolites and mafic dykes reveal retrograde metamorphic temperature and pressure of 500-525°C and 7.5 to 9 kbar for the garnet bearing amphibolites and 525°C-600°C and 10.2 to 11.1 kbar for the mafic dykes. The adjacent ultramafic rocks also show moderate degree of metamorphism as observed from their mineral assemblages. These evidences suggest that the TTSZ ophiolites were formed in an intra-oceanic subduction zone where the depleted mantle present in the mantle wedge interacted with fluids from the subducting oceanic plate to give rise to the ultramafic cumulates. This was followed by the formation of the mafic intrusives and amphibolites during the forearc spreading by partial melting of deeper less depleted mantle with involvement of the subducting fluids. Further we interpret that prior to the Himalayan orogeny, during the ocean-continent collision, the ophiolite complex of the TTSZ underwent moderate degree of metamorphism which possibly was in line with their obduction.