Petrography and petrochemistry of some zagroside geosynclinal calcarenites from north-eastern Iraq

Abstract

This paper traces the general chrono-petrographic and -petrochemical trends in the evolution of the sand- and silt-sized carbonate clastic beds of the geosynclinal basins bordering the Zagros orogen in north-eastern Iraq. This orogen was repeatedly uplifted from at least the Late Cretaceous to the present. Clastics in older marginal basins, particularly those near the uplifted orogen, were themselves uplifted and contributed debris to younger basins. All successive basins were therefore filled mostly with carbonate lithoclasts and to a lesser extent by a debris of radiolarian chert. Clastic grains of feldspar, monocrystalline quartz as well as volcaniclastic debris were subordinate. Twenty-two samples collected from calcarenites of the most important Cretaceous to Pliocene Zagros marginal basins were chemically analysed for their major elements and for Cr, Cu, Ga, Ni, Rb, Sr, V, Zn and Zr. In clastics of all these ages, elements such as Al, Ti, Fe, P, Zr, Ga, V and K correlate fairly to crudely with each other as they are particularly concentrated in the detrital non-carbonate particles, viz. ilmenite, magnetite, apatite, zircon, feldspar and clay minerals. On a Ti-Cr-Zr triangular diagram, plots of calcarenites of the same formation tend to cluster together on a loop in an order corresponding to their age. Zr progressively increases in the younger clastics while Ti relatively increases in the older sediments irrespective of whether they were metamorphosed or not. The increase in Ti is suggestive of an earlier basaltic activity. Cr had its maximum relative concentration in the Early to Middle Miocene clastic formations, indicating that the ophiolitic slivers caught up in the initial Zagros collision suture had the maximum exposure to weathering during this interval.