Mineral chemistry and thermobarometry of the mafic igneous rocks of Yamne Valley, Arunachal Pradesh, India

The Yamne Valley is a part of the Eastern Himalayan syntaxial band and is located in East Siang district of Arunachal Pradesh. The mafic igneous rocks exposed along Pasighat-Mariyang road section of Yamne valley have been considered for the present study. The area under study lies in the East Siang district and is bound by latitudes 28°06º17¹N to 28°14º25¹N and longitudes 95°10^º30¹E 95°18¹00^ºE. This present study is aimed at elucidating the chemical composition of plagioclase, clinopyroxene, ilmenite and chlorite present in the mafic igneous rocks of Yamne Valley in order to understand the geotectonic and geothermobarometric conditions. The studied basaltic rocks are albitized to spilites suggesting late magmatic metasomatic process. The mineralogical assemblage of these rocks is plagioclase (Ab_52–98), clinopyroxene (En₁₈Wo₃₂Fs₁₅ to En₄₉Wo₄₇Fs₃₇), Fe-Ti oxides and chlorite (Ca < 0.10 apfu). Clinopyroxenes are Ca-rich (Wo_32–47) and Na poor (Na₂O<0.44). The estimated pressure and water content of the magma by using clinopyroxene compositions are <5 Kbar and 10% H₂O for basalt and gabbro respectively which suggests that the magma chamber was located <“16–23 km below the earth's surface. Crystallization temperatures yield 1080ÚC-1109ÚC and 1032ÚC-1070ÚC for basalt and gabbro, respectively. The mineral compositions of the studied rocks have indicated mostly an Ocean Floor Basalt (OFB) tectonic environment which was supposed to have been associated with Mid Oceanic Ridge Basalts (MORB) formed in an extensional environment.