**8. Discussion**

#### **8.1 Mineralogy of the source rocks**

At Badelyan locality, Kolosh and Tanjero formations are exposed at the suture boundary of the Iraqi thrust zone near Jabal Hassan Beg mountain. These are comprised of the accretionary prism lying directly beneath the emplacement sheets of thrust zone. Based on field observations, characteristic sedimentary structures and sedimentological features all together refer to deep marine sedimentation originated by turbidity currents.

Petrographic examination of the Kolosh sandstones reveals high percentages of fresh olivine, pyroxene, intrusive ultrabasic, and basic and extrusive basic igneous rock fragments, which are controlled by the provenance and tectonic evolution of the Zagros thrust belt and surrounding areas. The mineral assemblages reveal

#### *Abnormal Ophiolite (Olivine/Pyroxene Rich) Sandstone NE Iraq: An Approach to the Origin… DOI: http://dx.doi.org/10.5772/intechopen.108333*

various potential tectonically-controlled source rock complexes (cf. mantle, oceanic crust, continental crust, and volcanic arcs). The presence of olivine, diopside, and augite accompanied with ultrabasic rock fragments (cf. dunite, peridotite tectonite, heirzburgite, lherzolite, and pyroxenite) suggest ophiolitic mantle origin [21, 23–26, 28, 43].

The peridotites of the ophiolite belt are ranging in composition from lherzolite to dunite through heirzburgite. These rock types and mineral associations are similar to those of forearc peridotites, which are represented by fertile alpine mantel lherzolite to dunite and are depleted in tectonite heirzburgite [44]. It is suggested that these ultramafic bodies are huge fragments of supra subduction zone of residual mantle peridotites [45]. Serpentinization is observed in along cleavages and basal fractures of the Fe-Mg minerals [25], as complementary processes during the evolution of the ultramafic part of the ophiolite sequence adjacent to the foreland basin. Serpentinization of peridotite is thought to have taken place during the subduction stages before the collision of the Arabian plate with the Asian plate [21, 45].

Diopside-diallage is usually derived from coarse-grained gabbro and basalt and is usually accompanied by forstertic olivine [44]. Omphacite is similar to diopside, augite, and jadeite, which are found exclusively in eclogite cf. deep mantle of high T and P conditions. Hedenbergite is usually derived from peridotite and fayalite ferrogabbro [25].

Augite is essential mineral in peridotite, gabbro, basalt, and olivine gabbro. Pigeonite may be derived from basalt, diabase, and dolerite, whereas, aegirine-augite compose an essential member in the trachyte and basalt. Spodumene is a rare mineral, derived from Li-granite [25]. Enstatite occurs in all types of basic igneous rocks. Mg-rich orthopyroxene occurs in ultrabasic igneous rocks (cf. pyroxenite, heirzburgite, lherzolite, serpentinite, and picrate with Mg-olivine) [25]. Hypersthene is found in gabbro, norite, and andesite [21, 26, 27].

Mg-rich olivine (e.g., forsterite and monticellite) is an essential mineral in most ultrabasic igneous rocks (cf. dunites, peridotites, and picrites). Olivine forsterite and chrysolite are derived from peridotite, olivine gabbro, and basalt. More Fe-rich olivine (e.g., fayalite) is derived from alkali basalt, ferrogabbro, and trachyte [25]. Monticellite is derived from basalt, and chondrodite is derived from high-grade metamorphic rocks [21–23, 26, 27].

The altered grains show various degree of alteration and are found in relatively high percentages. It is composed mainly of uralite and iron oxides. Most of the altered grains are derived from basic and ultrabasic rock fragments as well as olivine and pyroxene.

Ca-plagioclase members An95-An55 cf. anorthite, bytownite, and labradorite, are characteristic minerals of ultrabasic and basic igneous rocks cf. peridotite, heirzburgite, lherzolite, gabbro, and basalt. Anorthite is derived from ultrabasic intrusive igneous rocks cf. peridotite, pyroxenite, heirzburgite, and lherzolite and from gabbro. Bytownite and labradorite are derived mainly from basalt and gabbro [23, 26, 43, 44]. The alkali-feldspar varieties are derived from acidic intrusive and extrusive igneous rocks. Orthoclase is derived from granites, while anorthoclase and sanidine with cristobalite are derived from acidic and intermediate extrusive igneous rocks, such as rhyolite, trachyite, and andesite [24–26, 43, 44, 46].

Varieties of rock fragments are important indicators of source rocks and provenance. The most important abundant types in the Kolosh sandstones were derived from intrusive ultrabasic and intrusive and extrusive basic igneous rocks (cf. pyroxenite, lherzolite, heirzburgite, peridotite, gabbro, and basalt). These rock types are

derived from mantle and oceanic crust as an ophiolite sequence [21, 25, 26, 44]. The basalt and andesite fragments are derived from volcanic arcs [26, 44].

The sedimentary rock fragments were derived from various sources. Intrabasinal mudstone grains are most probably derived from mudstone beds by the effect of turbidity currents. The limestone fragments are extrabasinal and derived from Cretaceous carbonate formations. The chert and chalcedony fragments are suggested to derive from acidic volcanic rocks of arc and/or deep marine radiolarian beds of the ophiolite sequence [17, 18, 20, 44].

#### **8.2 Potential ophiolites provenance**

Anomalous concentrations of pyroxene, olivine, and ultrabasic/basic igneous grains in the Kolosh sandstones are suggested to derive from two sources: thrusted ophiolite sequences and lava flow basalt from island arc in the foreland basin. The emplacement of the Neotethyan oceanic crust took place during late Cretaceous (118–97 Ma), while the emplacement of Hasan Beg ophiolite complex was (106– 92 Ma) ago [47]. Along the southern Neotethys suture, the outer Zagros orogenic belt (OZOB) crops out, including Rayat, Piranshahr, Kermanshah, Neyriz, and Haji-Abad Ophiolites resulted, of the late Cretaceous collision between the Sanandaj-Sirjan and the Arabian shield [47, 48].

Sedimentation of the qulqula group cf. deep-marine radiolarian chert, and carbonates of Arabian passive-margin is dated as Valanginian-Turonian (140–89 Ma), which constrains initial orogenesis and early subsidence of the Zagros foreland basin to about 90 Ma [49–51]. The provenance data of the clastics in the Tanjero formation (Maastrichtian), Kolosh formation, and Suwais red beds (Paleocene-Eocene) reveal partial derivation from basic and ultrabasic sources related to ophiolite emplacement [49–51]. In the studied area, several pre-tertiary tectonically-derived igneous complexes may have served as source rocks for the sandstones of the Kolosh formation.

Hasan Beg mountain successions is situated beside the Badelyan area and very near to Sidekhan province. Hasan Beg igneous complex comprises a late Cretaceous remnant of the ophiolite-arc system that developed within the Neotethys ocean. This igneous complex was subsequently accreted to the Arabian plate during the late Cretaceous to Paleocene [50]. It is predominantly consisting of calc-alkaline basaltic andesite to andesite cutting across by micro gabbro and diorite dikes indicating Albian-Cenomanian age (106–92 Ma) [47, 50]. Hasan Beg mountain rock units start at the bottom by pillow lavas with sheared and highly weathered intense deformed chlorite slate, which forms the contact between the lower pyroclastic metavolcanic rocks and the upper part of metasediments cf. slate, shale, and sandstone. The overlying metasediments are composed of highly fossiliferous black shale interbedded with 20 m thick sandstone lens. These rocks are overlain by radiolarian chert with a total thickness of about 1000–2000 m, and this variable is due to thrusting [52].

Qalander-sidekhan mountain sequence is located about 20 km from Badelyan locality and composed from bottom to top of Tanjero and Aqra formations (late Cretaceous) separated by tectonic contact from the overlain govanda limestone (early-middle Miocene age), succeeded by red bed sandstones (middle-late Miocene), which overlies by a tectonic slice of naopurdan metavolcanic rocks [52]. In the Qalander locality the naopurdan rocks are divided into three parts: 1) The lower metavolcanic unit is composed mainly of basalt flows, 2) The middle unit is variable in thicknesses averaging about 50 m and composed of pillow lavas, brecciated lavas, and inter-pillow ash. Many dolerite dykes are cutting across the Qalander successions,

*Abnormal Ophiolite (Olivine/Pyroxene Rich) Sandstone NE Iraq: An Approach to the Origin… DOI: http://dx.doi.org/10.5772/intechopen.108333*

in which pillow and flow lavas are about 450 m thick. The upper unit contains sequences of sandstones grades upward to conglomerates, containing pebbles of basalt, serpentinite, and marble, which have been derived from erosion and rapid deposition of the igneous rocks, probably from naopurdan and walash metavolcanic successions [45, 53].

The Sidekhan mountain, located NW of Qalander mountain has the same successions as the Qalander mountain with govanda formation and naopurdan successions, with tectonic breccia.

Rayat ophiolite mantle sequence lies in the NE corner of Iraq (30 Km from Badelyan area) and consists of serpentinized peridotites, serpentinite, tectonite heirzburgite, lherzolite, and dunite with Cr-spinel [27]. They are mantle residues with distinct geochemical signatures of affinities of both the mid-ocean ridge and supra subduction zone. The crustal succession includes gabbro, dibasic dikes, rare pillow basalts, and radiolarite overlain by late Cretaceous pelagic limestone. Rayat ophiolite is extended to Piranshahr area in NW Iran and to the Cilo ophiolite in SE Turkey [44, 46]. The Rayat ophiolitic mélange is mainly composed of peridotite and sheared serpentinite with heirzburgite [45]. The chromitite in the Rayat peridotite is similar to the mantle chromitite and Moho transition zone chromitite (upper mantle zone) of the Tethyan ophiolites [45].

Galalah-choman ultramafic/mafic rocks are situated about 25–28 km from Badelyan area. Galalah area consists of 50 m brecciated and pillow lavas alternating with ultramafic/mafic volcanoclastic rocks, and above with 40 m massive serpentinite and mélange marble of heterogeneous composition (mixture of calcite, chlorite, and serpentinite). This succession is overlain by a thin layer of radiolarian chert and red beds, which are tectonically separated by crush zone [54]. Choman successions are variable in thicknesses of about 200 m of pillow lava, interbedded with volcanic ash, tuff, and breccia [45]. Generally, galalah and choman rocks consist of basaltic flow lava interbedded with ultramafic/mafic rocks, overlying serpentinite rocks. They can be suggested as other sources of pyroxene, olivine and ultrabasic, and basic intrusive and extrusive basaltic igneous grains in the sandstone units.

The kata rash igneous complex comprises Cretaceous remnant arc and is broadly similar in age to those of late Cretaceous peri-Arabian ophiolite belt in other countries as well as other late Cretaceous Zagros supra subduction zone assemblages [53, 55]. This refers to the great lateral extent of late Cretaceous arc systems in the consumption of the Neotethys ocean.

#### **8.3 Sedimentary mechanism of olivine/pyroxene concentration**

Ophiolite successions are suggested as the main source of pyroxene and olivine grains in the Kolosh sandstones. It consists of rocks of various pyroxene/olivine-rich assemblages, including peridotite tectonite, lherzolite, serpentinized spinel peridotite, pyroxenite, serpentinite chromitite, heirzburgite, gabbro, and basalt lavas.

Later, slumps of these pyroxene/olivine-rich sands by turbidity currents are transported deeper in the basin to deposit finally as a turbidity sequence. Tectonic activity and seismicity in the foreland margin usually create turbidity currents and slumping of accumulated heavy sarends in the shallow zone to deeper margins [7, 16, 56].

Zagros Cretaceous ophiolite complexes are exposed in the early tertiary and are amenable to subaerial and shallow submarine erosion along the shorelines. Intense subaerial erosion enhanced the rapid mechanical disintegration of the ophiolites evidenced by the fresh olivine and pyroxene grains in the studied sandstones.

#### **Figure 5.**

*A model discusses the Late Cretaceous geodynamic evolution of subduction Arabian and Asian plates to form Zagros foreland basin, in Late Cretaceous-Paleocene age A) Various intrusive magmatic bodies with arc-related basaltic flows, B) continuous subduction and subsides of the Kolosh basin (trench) and subsequent erosion of ophiolitic; ultrabasic, gabbro, to deposit the Kolosh Formation in the continental slope and/or deep marine trench (modified after Calvo [57]).*

Submarine wave action in shallow margins induced concentration and accumulation of the heavier fraction of the detrital rich in pyroxene and olivine grains according to the model discussed by [57] (**Figure 5**).

The Kolosh formation is suggested to deposit in the deeper Zagros trench subduction margin. Intense tectonic activity and/or earthquake seismicity are responsible for successive episodic turbidity currents that deposited the Kolosh successions.

#### **8.4 Depositional environment and evolution**

Grain sizes, sedimentary structures, and facies types indicate tectonosedimentary evolution of deep marine environment (**Figure 3**). The facies types and associations refer to slope submarine channels advanced to submarine fans in deeper margins. The facies types accompanied by sedimentary structures refer to a debris flow in the inner fan advanced to outer fan and later to basin plane pelagic/hemipelagic margins (**Figures 4** and **5**). Submarine fans include lobe-fans and lobe-fringe produced by advanced turbidity currents toward the basin plane. Thin-bedded successions of the lobe-fringe are products of waning and relatively dilute turbidity currents.

The field observations show that the vertical and lateral extent of thin-bedded lobe-fringe sediments are the distal equivalents in both down-current and cross-current directions of the fine-grained and thin-bedded sandstone horizons that comprise the depositional lobes of the end of the outer fan margin.

Thus, the heavily concentrated turbidity currents, which reached the outer fan are dropped to the coarsest suspended load firstly in the inner fan margins. Through successive events, sediments are formed lobs of the coarse-grained and thick-bedded sandstone bodies, which are most probably deposited in and around the channel levee in inner fans. Farther downslope, the same current became more dilute suspensions carrying the finer-grained material to accumulate as peripheral fringes around
