petrology of ultramafic and related rocks along iraqi zagros thrust zone

1-Field observation and Petrography
From Jun to August 2007, 200 rock samples were collected from
three different localities within the Iraqi Zagros Thrust Zone, including
Penjwin ophiolite, Mawat ophiolite and Pauza ultramafic body. Samples
were extracted using Estwing hammers. All 200 samples were prepared
for thin section using a standard rock saw, and 50 representative samples
were chosen to be made into polished thin sections. Samples to be made
into polished thin sections were chosen on the basis of least weathering
and least apparent degree of serpentinization to represent the freshest
and most undeformed rocks across the entire rock outcrops.
3-1 Penjwin ultramafic body
3-1-1 Dunite
Dunite occurs as lenses (30 cm × 30cm in dimension) (Fig.3-1) or as
envelopes surrounding chromitite ore bodies. The contact between the
dunite lenses and their host peridotite are relatively sharp but the dunite
envelopes around the chromitites grade outward into harzburgite. In
dunite, olivine forms anhedral grians, 0.1- 0.5 mm across, many of which
have typical kink band texture, similar to deformational fabric seen in the
mantle tectonites (Raymond, 2002; Coshi and Gardner, 2004). Chromian
spinel forms euhedral grians, 0.5 mm across, and is nearly opaque to dark
reddish brown in thin section. It is commonly inhomogeneous, displaying a
zoning pattern of Cr-rich core compositions to Cr-poor rim compositions.
The chromite crystals often associate thin rims of magnetite and ferrian
chromite or chromian magnetite (nomenclature after Evans and Frost,
1975; Barnes, 2000), which developed during hydrothermal alteration or
serpentinization.
29