MINERALOGY OF SHEARED SERPENTINITE                                         NEAR MLAKAWA MOUNTAIN, PENJWIN DISTRICT, KURDISTAN REGION, NE IRAQ

O. Mousa, Irfan; O. Mohammad, Yousif

	MINERALOGY OF SHEARED SERPENTINITE NEAR MLAKAWA MOUNTAIN, PENJWIN DISTRICT, KURDISTAN REGION, NE IRAQ
Iraqi Bulletin of Geology and Mining
Article 1, Volume 5, Issue 2, 2009, Pages 101-117
Authors
Irfan O. Mousa; Yousif O. Mohammad
Abstract
Shear serpentinite represents two meters thick serpentinite, formed at the contact of the Penjween Ophiolite sequence and Tertiary molasses (Miocene Merga Red Beds). It is formed of metamorphism of massive serpentinite as a result of Tertiary post-collision tectonics. Mineralogically, shear serpentinite consists of serpentine polymorphs mainly antigorite, lizardite – chrysotile serpentines, bands of carbonate minerals, subordinate amount of magnetite. There are two types of the later: a) chemically zoned large grains with cores of primary Cr-spinel and/ or pre-serpentanization ferrichromites; and b) small, unzoned, pure syn-serpentanization magnetite grains. The first type of magnetite occupies the outermost of rimmed Cr-spinel. Lizardite – chrysotile fibers enclose strips within antigorite suggesting that it formed on expense of this mixture of chrysotile and lizardite. Antigorite is formed under shear stress condition during post-collision tectonics. Chemical analysis of serpentine polymorphs shows that the antigorite is richer in FeO and poorer in MgO and SiO2 than that in lizardite – chrysotile serpentines. The co-existing of high-Fe antigorite with lizardite – chrysotile assemblage may suggest that the earlier is stable in slightly higher temperature condition (>300° C) than that of lizardite and chrysotile serpentine. Back scattered images (BSE) show that antigorite is granular light gray, whereas the lizardite – chrysotile is fibrous dark gray. Chlorite is a Cr-rich clniochlorite (2.6 wt.% Cr2O3). It is formed through the reaction of ferrichromite with serpentine minerals during retrograde metamorphism. The occurrence of this shear serpentine along post-collision strike – slip fault zones suggests that they used these zones of weakness for protrusion.

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