Dynamic control on serpentine crystallization in veins within within southern Zagros suture zone Serpentinites, Penjween complex, NE Iraq

Abstract EN

The Penjween serpentinized peridotites occupy parts of the southern Zagros Suture Zone displaying pseudomorphic and non-pseudomorphic micro-textures, in addition to different types of vein serpentine formation.

Serpentinization is accompanied by abundant veining marked by generations of vein-filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation.

The author have selected a representative set of veins from serpentinized peridotite of Penjween ophiolite from different localities and studied them in detail for their microstructures by coupling optical and scanning electron microscopy (SEM) with Back-Scattered electron (BSE) images.

Four main veining episodes (V1 to V4) accompany the serpentinization of the oceanic lithosphere of the Penjween serpentinite peridotites.

Serpentine microstructures and mineral composition of veins provide information on the relative rates of crack opening, vein mineral precipitation, the crystallization conditions, and the temporal evolution of alteration.

The first episode represents vein generation V1 and is interpreted as tectonically controlled penetration of early seawater-dominated fluid within peridotites, enhancing thermal cracking and mesh texture formation.

The second episode marks two subsequent vein types (V2 and V3) formed in a closed, diffusive system and accommodated volume expansion required to reach serpentinization of the protolith (Stage 1).

The last vein episode (V4) records instead an open hydrothermal system, where brittle fracturing and advective transfer dominated and enabled the completion of serpentinization (Stage 2).

These results show a complete history of alteration, with the crystallization of different types of serpentine recording different tectonic events and modes of hydrothermal alteration of the Tethys oceanic lithosphere.