The nature and origin of Fe-Ti-P-rich rocks in the Qareaghaj mafic-ultramafic intrusion, NW Iran

Fe-Ti-P-rich rocks (FTP) are unusual with respect to their mineralogy and bulk composition. Varieties of these rocks are mostly related to Proterozoic massif-type anorthosites and to a lesser extent to the upper parts of mafic-ultramafic intracratonic layered complexes and other igneous rock suites. We present results on the geology, mineralogy and geochemistry of a new occurrence of FTP, associated with mafic rocks in the northwestern part of Iran. The Qareaghaj mafic-ultramafic intrusion (QMUI) is a small igneous body situated between Palaeozoic sedimentary rocks and a Precambrian low grade metamorphic complex. The QMUI is composed mainly of non-mineralized mafic and apatite- and Fe-Ti oxide-rich ultramafic rocks. The mafic rocks, mainly coarse-grained gabbro, microgabbro and amphibolite, have a simple mineral assemblage (plagioclase + clinopyroxene + ilmenite) and based on field observations, mineralogy and chemical composition are comagmatic. The ultramafic rocks with high proportion of olivine (similar to 40-66 vol.%), apatite (similar to 0.1-16 vol.%), ilmenite (similar to 11-19 vol.%) and magnetite (similar to 2-13vol.%), have unusual bulk compositions (e.g., SiO2 similar to 21-30wt.%, total iron expressed as Fe2O3 (tot) similar to 26-42wt.%, TiO2 similar to 5-11wt.%, MgO similar to 9-20wt.%, P2O5 up to 5.1wt.%, Cr similar to 40-160 ppm, Ni similar to 7-73 ppm). The FTP forms numerous sill-like layers, ranging in thickness from similar to 5cm to few meters. These rocks, totally enclosed in mafic rocks with sharp and concordant contacts, show a magmatic lamination and follow the general NW-SE trend of QMUI. The apatite-rich ultramafic rocks makes up 90-95% of the total ultramafic outcrops and contain Mg-poor olivine (Mg# similar to 40-58) and low-Mg spinel (Mg# similar to 30-44) in contrast to apatite-poor ones (similar to 60-63 and similar to 43-46, respectively). Field relationships, mineral compositions and geochemical data suggested that the FTP are not related to the mafic host rocks. On the contrary, they intruded latter into the gabbros during plastic, high temperature deformation in local shear zones. Fractional crystallization of P-rich ferrobasaltic parental magma at depth, probably in an open magmatic system, not far from the QMUI magma chamber, is considered as responsible for the formation of the evolved FTP in QMUI.