Ti-rich Amphibole of Igneous Origin in MOR-Type Gabbroic Rocks (Northern Apennine Ophiolites, Italy): An Ion Microprobe Study

Abstract

Two major hypotheses have been formulated for the origin of Ti-rich amphibole in MOR-type gabbroic rocks (e.g. Otten, 1984). In the first, amphibole derives from an igneous trapped liquid. This hypothesis implies that the parental magma had a relatively high volatile content, thus contrasting with the nearly anhydrous nature of MORB. The second hypothesis assumes a metamorphic origin, i.e. amphibole was originated by solid state reactions of igneous minerals with volatile-rich seawater-derived fluids. In the gabbroic rocks from Northern Apennine ophiolites, Ti-rich amphibole occurs both with igneous and metamorphic texture. The gabbroic rocks were formed by low-pressure fractional crystallization of N-MORB type liquids in a slowspreading ridge system (Tiepolo et al., 1997). Accessory Ti-rich amphibole with igneous texture is found as the last crystallizing mineral, in interstices between plagioclase and clinopyroxene and as rim around Fe-Ti-oxides. Morever, Ti-rich amphibole occurs as a major mineral in the most evolved, volumetrically subordinate, intrusive lithologies (albitites). These rocks consist of albitic plagioclase and Ti-rich amphibole (up to 30 % by volume), both showing euhedral to subhedral habitus, plus accessory Fe-Ti-oxides, apatite and zircon. This work aims to unravel the origin of Ti-rich amphibole with igneous texture. Amphibole from Mgand Fe-rich gabbroic rocks, as well as from albitite, has been analysed for major elements by electron microprobe, and for halogen (F, CI and H) and trace elements (REE, Y, Sr, Zr, Cr, V, Sc, Ti, Ba, K, Nb) by ion microprobe.