Mineral Chemistry of Chalki Basalts in Northern Iraq and Their Petrological Significance

Chalki basalts as a small body of volcanic rocks have green to grayish green color due to their nearly complete alteration to chlorite. The essential minerals of Chalki basalt to andesitic basalts are plagioclase (labradorite, An 51 – 61 ; andesine, An 35 to An 42 ; and oligoclase, An 22 ). Moreover, there is sodic plagioclase (albite, An 0.1 to An 04 ) whose coexistence with the other more calcic plagioclase means that albitization had occurred. The other essential mineral is pyroxene (endiopside, en 66 – 68 wo 27 – 28 fs 05 – 06 ; and subcalcic augite, en 72 wo 14 fs 14 ). Olivine (Fo 80 – 81 ) is also present. According to the NiO content (0.11 – 0.12 wt%) in olivine grains, they are interpreted to be originated tectonically. The prevalent chlorite in all the samples is mainly diabantite and penninite, indicating chloritization after the ferromagnesian olivine and pyroxene. Serpentine (type lizardite and chrysotile) is also recorded as lesser alteration product after the forsteritic olivine. Rare secondary hornblende (type magnesiohornblende) is also found. The spinel group as accessory minerals is defined as magnetite, chromian magnetite, and chromian spinel giving the imprints of their metamorphic origin due to low temperature sub-sea metamorphism and also of alpine type. ), plagioclase (Ca-K α ), barite (Ba-L α ), celestite (Sr-L α ), chrome diopside (Mg-K α ), hematite (Fe-K α ), rutile (Ti-K α ), rhodonite (Mn-K α ), chromite (Cr-K α ), nickel silicide (Ni-K α ), and zinc sulfide (Zn-K β ). Rounds of standard analyses were performed prior to and after the suite of thin sections. Concentrations are calculated using the PAP matrix correction factors. Correction for “ excess ” F by interference of the Fe-L α peak with F-K α peak was accomplished by measuring an F-free Fe-bearing standard (hematite) and calculating a correction factor of 0.029.


Introduction
The Paleozoic Chalki volcanic rocks crop out in a restricted part of the northern Thrust Zone of Iraq close to Iraqi-Turkish border and are considered as integral part of the upper part of Pirispiki Red Beds (late Devonian) Formation [1][2][3]. Chalki Formation was defined by Wetzel (unpublished report, 1952 in [1]) (which has no synonyms) who named it after the Chalki village ( Figure 1).
Lithologically, they represent dull green and grayish green, red-and whitespeckled, altered olivine-rich basaltic rocks (flow or intrusions) alternating with intercalations of bright red, ash-containing soft siltstones and shale.
They are undated and their origin is uncertain. Outcrop is typically not diagnostic. They are considered to be extrusive by [4] based on their identification of ash layers (not observed in this study). Petrographically, the bulk of the material consists of olivine basalts or fine-grained dolerites, with hematite-magnetite-rimmed pseudomorphs, in chlorite, replacing the olivine. There are albitized plagioclase laths and considerable amounts of chlorite and ankeritic carbonates in the groundmass. Locally the basalts are crossed by numerous veins of white ankerite with fibrous chalcedony.
The Chalki volcanic rocks probably had suffered severe sub-sea alterations. Therefore, this work aims to study mineral chemistry using electron probe microanalyzer (EPMA) in order to distinguish the various phases of minerals resulted from the alteration on basaltic rocks of the Chalki volcanics and interpret their petrologic significance.

Geologic setting
The Chalki volcanics located near Kaista (Khabour valley, Amadia district, N. Iraq) occur as basalt intercalations, of 2-5 m in thickness within the Pirispiki Formation. The type section lies at Chalki village ( Figure 1) in which the basaltic beds are associated with ash-containing shales and siltstones occupying most of the uppermost 20 m of the section. The type section is 16 m thick in aggregate [4]. Sharland et al. [5] interpreted the Chalki volcanics to represent back-arc rift volcanics associated with the initiation of subduction along the Tethyan margin of the Arabian Plate. They interpreted the initiation of subduction to have caused the so-called Hercynian orogeny in the late Devonian times. The age of the "Hercynian orogeny" in the Arabian Plate has been reported to range from pre-Late Devonian to middle Carboniferous [6].
Subduction along the southern margin of the Palaeo-Tethys is supported by the occurrence of Devonian-Carboniferous volcanic and metamorphic rocks found in the Kuh-Sefid area of the Sanandaj-Sirjan Zone [7]. However, the central part of the Arabian plate was probably not significantly affected by this subduction.

Materials and methodology
The petrography of 15 samples collected from Chalki volcanics section at Chalki Nasara section (Figure 1) was determined using petrographic microscope at the Earth Science Department of Mosul University, Iraq. A Swift Polarized microscope is used in the petrographic description.
Electron microprobe analyses of minerals were performed using a Cameca SX-50 in the Department of Geology and Geophysics at the University of Utah, USA. Analyses were conducted using PC1, TAP, PET, and LiF crystals on four wavelengthdispersive spectrometers, with an accelerating voltage of 15 keV, a beam current of 30 nA, and a spot size of 10 mm. Peak intensities were measured for 20 s and background intensities for 10 s on both sides of the analytical peaks. Na was measured first, and analytical times were reduced by half in order to minimize sodium loss under the electron beam. The analytical routine for feldspars included Si, Al, Fe, Ca, Sr, Ba, Na, and K, and a separate analytical routine for mafic and other minerals added Ti, Cr, Mn, Ni, Zn, Mg, F, and Cl (K, Sr and Ba excluded). Mineral standards include fluorite (F-Kα), tugtupite (Cl-Kα), sanidine (Si-Kα, Al-Kα, K-Kα), albite (Na-Kα), plagioclase (Ca-Kα), barite (Ba-Lα), celestite (Sr-Lα), chrome diopside (Mg-Kα), hematite (Fe-Kα), rutile (Ti-Kα), rhodonite (Mn-Kα), chromite (Cr-Kα), nickel silicide (Ni-Kα), and zinc sulfide (Zn-Kβ). Rounds of standard analyses were performed prior to and after the suite of thin sections. Concentrations are calculated using the PAP matrix correction factors. Correction for "excess" F by interference of the Fe-Lα peak with F-Kα peak was accomplished by measuring an F-free Fe-bearing standard (hematite) and calculating a correction factor of 0.029.

Petrography
The volcanic rocks are olivine-basalt, sometimes doleritic, or even andesiticbasalt. They are of greenish to greenish gray color due to high chloritization after olivine. They are associated with little phyllites and pyroclasts of volcanosedimentary rocks. They contain also veins of carbonate minerals and quartz.
Texturally, the rocks of Chalki volcanics are porphyritic basaltic in general, cut by microscopic veins of carbonate minerals (calcite) in addition to minute quartz veins.
Mineralogically, the pseudomorphs after olivine phenocrysts are identified microscopically, such as chlorite, iddingsite, and iron oxides ( Figure 2). Another essential mineral is plagioclase (sometimes albitized), which sometimes slightly altered to sericite and kaolinite. No individual pyroxene grains have been identified, but they probably present as fine grains in the groundmass.

Plagioclase
The calcic plagioclase (labradorite, An 51 to An 61 ) is the essential mineral of Chalki basalt resembling the basaltic type. Some grains are less calcic (andesine, An 35 to An 42 ). Moreover, there is sodic plagioclase (albite, An 0.1 to An 04 ) as a result of albitization process (Table 1 and Figure 3).

Pyroxene
The pyroxene of Chalki basalt is endiopside to subcalcic augite as shown in Table 3 and Figure 5.
The chemical formula of pyroxene is XYZ 2 O 6 [11].

Hornblende
The rare mineral in Chalki basalt is hornblende, whose composition resembles the magnnesiohornblende as shown in Figure 6 after representation of its chemical composition given in Table 4.

Chlorite
The chlorite of Chalki basalt is the predominant secondary mineral in all samples giving the greenish color of the hand specimen samples. The huge number of   Table 3.
Chemical composition (wt%) and atom per formula unit (apfu) of pyroxene on the basis of 6 O (sample CV-67).   analysis for many samples as given in Table 5 reveals the prevalence of diabantite with less abundant penninite, in addition to few grains of clinochlorite, pyenochlorite, and ripidolite ( Figure 7).

Spinel group
The chemical data ( Table 6) and their representation (Figure 8) for the few measured points of spinel group display that they are magnetite, chromian magnetite, and chromian spinel. Moreover, the plot of their Cr# against Mg# (Figure 9) gives an important and clear sign for their metamorphic origin as a result of lowgrade metamorphism and of alpine type.  Table 5.  Chalki basalt chlorite shown as mainly diabantite and less penninite (after [13]).   Cr# vs. Mg# of Chalki basalt spinels indicating their metamorphic origin as a result of low-grade greenschist metamorphism and alpine type (after [15]).  Table 7.
Chemical composition (wt%) and atom per formula unit (apfu) of serpentines on the basis of 14 O (CV-67).

Serpentine
Another less abundant secondary mineral after olivine in Chalki basalt is the serpentine. Their chemistry ( Table 7) and plot ( Figure 10) reveal clearly that they are mostly of lizardite and chrysotile types.

Petrological significance
1. Olivine alteration to chlorite, serpentine, iddingsite, amphibole, and iron oxide as shown petrographically is a good evidence to low-grade metamorphism.
2. The variation of the plagioclase in Chalki basalt from labradorite (An 51-61 ) to andesine (An 35-41 ) to andesine (An 22 ) reflects the andesitic variety of these volcanic rocks in addition to basalt, while the coexisting albite composition (An 0.1-04 ) indicates clearly that the rocks had suffered albitization due to lowgrade sub-sea metamorphism.
5. The prevalent chlorite (mainly type diabantite and penninite) in all the Chalki samples represents its suffering from chloritization process after the ferromagnesian olivine and pyroxene.
6. Serpentine (type lizardite and chrysotile) is recorded as an alteration product after the forsteritic olivine, in addition to another rare amphibole secondary mineral of type magnesiohornblende.
7. The spinel group as accessory minerals in Chalki basalt is classified as magnetite, chromian magnetite, and chromian spinel confirming the metamorphic origin (low-temperature sub-sea metamorphism and also of alpine type).