Open access peer-reviewed chapter - ONLINE FIRST

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

By Mohsin M. Ghazal, Ali I. Al-Juboury and Sabhan M. Jalal

Submitted: March 15th 2019Reviewed: September 23rd 2019Published: October 26th 2019

DOI: 10.5772/intechopen.89861

Downloaded: 21

Abstract

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, An51–61; andesine, An35 to An42; and oligoclase, An22). Moreover, there is sodic plagioclase (albite, An0.1 to An04) whose coexistence with the other more calcic plagioclase means that albitization had occurred. The other essential mineral is pyroxene (endiopside, en66–68 wo27–28 fs05–06; and subcalcic augite, en72 wo14 fs14). Olivine (Fo80–81) 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.

Keywords

  • petrography
  • mineral chemistry
  • electron microprobe
  • Chalki basalts
  • Iraq

1. 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 Chalki Formation was defined by Wetzel (unpublished report, 1952 in [1]) (which has no synonyms) who named it after the Chalki village (Figure 1).

Figure 1.

Geological map of northern Iraq showing the Paleozoic succession including Chalki volcanics within Pirispiki formation in Ora section (A) and the studied section (Chalki Nasara, section B) (modified after [8]).

Lithologically, they represent dull green and grayish green, red- and white-speckled, 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.

2. 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.

3. 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 wavelength-dispersive 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.

4. Results

4.1. Petrography

The volcanic rocks are olivine-basalt, sometimes doleritic, or even andesitic-basalt. They are of greenish to greenish gray color due to high chloritization after olivine. They are associated with little phyllites and pyroclasts of volcano-sedimentary 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.

Figure 2.

Representative photomicrograph for Chalki basalt showing (A) dumpy prismatic olivine with rim alteration to black iron oxide and reddish brown iddingsite (arrows), chlorite (Chl), and white fine laths of plagioclase. (B) Plagioclase laths (arrows), altered olivine to iddingsite, iron oxide. Under-crossed nicols. Scale bar is 0.2 mm.

4.2. Mineral chemistry

4.2.1. Plagioclase

The calcic plagioclase (labradorite, An51 to An61) is the essential mineral of Chalki basalt resembling the basaltic type. Some grains are less calcic (andesine, An35 to An42). Moreover, there is sodic plagioclase (albite, An0.1 to An04) as a result of albitization process (Table 1 and Figure 3).

CV-32
Wt%12345678
SiO254.5956.2057.1566.9165.2867.7658.8255.16
Al2O327.4426.0626.2519.4120.8119.6525.0427.56
FeO*0.590.920.870.910.560.920.770.77
CaO10.328.568.490.340.880.157.2510.15
SrO0.000.000.000.000.000.000.000.03
BaO0.000.010.000.000.000.000.000.00
Na2O5.336.246.0710.7010.2011.236.635.16
K2O0.350.240.500.100.910.130.580.27
Total98.698.299.398.498.699.899.199.1
apfu
Si2.5012.5752.5872.9792.9142.9762.6572.511
Al1.4811.4071.4001.0181.0951.0171.3331.478
Fe2+0.0230.0350.0330.0340.0210.0340.0290.029
Ca0.5070.4200.4120.0160.0420.0070.3510.495
Sr0.0000.0000.0000.0000.0000.0000.0000.000
Ba0.0000.0000.0000.0000.0000.0000.0000.000
Na0.4730.5540.5330.9240.8830.9560.5800.456
K0.0210.0140.0290.0060.0520.0070.0340.016
Ab47.3056.0754.7597.6790.3498.5260.1647.17
An50.6342.4842.311.734.330.7136.3551.21
Or2.071.452.940.605.330.773.491.61
LabradoriteAndesineAndesineAlbiteAlbiteAlbiteAndesineLabradorite
CV-25
Wt%123456
SiO257.8053.2052.5462.3252.9157.66
Al2O325.4628.7729.1722.9729.2026.26
FeO*1.200.680.860.710.670.55
CaO7.5211.6812.084.7111.648.36
SrO0.000.020.000.060.000.00
BaO0.000.000.000.050.010.03
Na2O6.604.614.158.644.296.32
K2O0.560.190.330.630.520.52
Total99.199.199.1100.199.299.7
apfu
Si2.6212.4332.4082.7742.4192.597
Al1.3611.5511.5761.2051.5731.394
Fe2+0.0460.0260.0330.0260.0260.021
Ca0.3650.5720.5930.2250.5700.404
Sr0.0000.0000.0000.0000.0000.000
Ba0.0000.0000.0000.0000.0000.000
Na0.5800.4080.3690.7450.3800.552
K0.0320.0110.0190.0360.0300.030
Ab59.3541.1837.5874.1238.7656.01
An37.3557.7260.4522.3458.1340.94
Or3.301.101.973.543.113.05
AndesineLabradoriteLabradoriteOligoclaseLabradoriteAndesine
CV-37
Wt%12345
SiO257.3254.9658.7056.9355.49
Al2O325.8527.0824.9624.4027.90
FeO*1.001.190.871.650.98
CaO8.109.947.056.7610.06
SrO0.000.000.000.000.00
BaO0.000.000.030.020.01
Na2O6.135.286.796.255.46
K2O0.480.270.610.610.39
Total98.998.799.096.6100.3
apfu
Si2.6042.5162.6562.6472.502
Al1.3841.4611.3311.3371.482
Fe2+0.0380.0460.0330.0640.037
Ca0.3940.4870.3420.3370.486
Sr0.0000.0000.0000.0000.000
Ba0.0000.0000.0000.0000.000
Na0.5400.4690.5950.5630.477
K0.0280.0160.0350.0360.022
Ab56.1448.2361.2660.1648.43
An40.9950.1435.1435.9849.31
Or2.871.643.613.862.26
AndesineLabradoriteAndesineAndesineLabradorite
CV-41
Wt%123456789
SiO268.1367.8468.0267.7768.2066.1567.2467.3967.87
Al2O319.4519.3419.2519.5919.5620.0919.5119.4619.59
FeO*0.470.560.770.950.541.550.770.710.63
CaO0.040.040.030.100.070.410.320.080.12
SrO0.000.000.000.000.000.000.000.000.00
BaO0.000.010.000.000.000.370.000.000.00
Na2O11.1611.1311.2411.1811.4310.4211.0411.0911.14
K2O0.030.040.020.070.050.290.070.170.09
Total99.399.099.399.799.999.398.998.999.4
apfu
Si2.9962.9952.9962.9802.9882.9402.9772.9832.985
Al1.0081.0060.9991.0151.0101.0531.0181.0151.016
Fe2+0.0170.0210.0280.0350.0200.0580.0280.0260.023
Ca0.0020.0020.0010.0050.0030.0200.0150.0040.006
Sr0.0000.0000.0000.0000.0000.0000.0000.0000.000
Ba0.0000.0000.0000.0000.0000.0010.0000.0000.000
Na0.9510.9530.9590.9530.9710.8980.9480.9520.950
K0.0020.0020.0010.0040.0030.0160.0040.0100.005
Ab99.5999.5999.7299.1199.3996.1598.0398.6198.90
An0.210.190.140.500.322.101.590.370.60
Or0.210.220.130.390.301.750.391.020.51
AlbiteAlbiteAlbiteAlbiteAlbiteAlbiteAlbiteAlbiteAlbite

Table 1.

Chemical composition (wt%) and atom per formula unit (apfu) of plagioclase on the basis of 8 O (samples CV-32, CV-25, CV-37, CV-41).

Figure 3.

An-Ab-Or plot for Chalki basalt plagioclase showing the composition variation from labradorite, andesine, oligoclase, to albite (modified from [9]).

4.2.2. Olivine

Chalki olivine is forsteritic (Fo80 to Fo81) and consequently high Mg# (80–81) (Table 2). It contains NiO (0.11–0.12 wt%), which is plotted against (Fo%) revealing its tectonic origin (Figure 4).

Figure 4.

Fo vs. NiO for Chalki basalt olivines shown as type tectonic (adapted from [10]).

4.2.3. Pyroxene

The pyroxene of Chalki basalt is endiopside to subcalcic augite as shown in Table 3 and Figure 5.

Wt%1234
SiO239.2940.0138.5239.34
TiO20.020.040.020.04
Al2O30.190.240.190.24
Cr2O30.020.060.020.06
FeO*17.2018.0116.8617.71
MnO0.370.480.360.47
NiO0.120.120.120.11
ZnO0.540.000.530.00
MgO43.0341.3542.1940.66
CaO0.010.640.010.63
Na2O0.040.010.040.01
F0.050.070.050.07
Cl0.000.000.000.00
Total100.88101.0398.9099.34
apfu
Si0.9921.0100.9921.010
Al0.0060.0070.0060.007
Ti0.0000.0010.0000.001
Cr0.0000.0010.0000.001
Fe0.3630.3800.3630.380
Mn0.0080.0100.0080.010
Mg1.6201.5561.6201.556
Ca0.0000.0170.0000.017
Na0.0020.0010.0020.001
K0.0000.0000.0000.000
Ni0.0010.0010.0010.001
Zn0.0040.0000.0040.000
Fo81.36179.94681.36179.946
Fa18.63920.05418.63920.054
Mg#81.68680.36781.68680.367

Table 2.

Chemical composition (wt%) and atom per formula unit (apfu) of olivine on the basis of 4 O (sample CV-67).

Figure 5.

En-Fs-Wo triangular diagram for Chalki basalt pyroxene shown as endiopside and subcalcic augite (after [11]).

The chemical formula of pyroxene is XYZ2O6 [11].

4.2.4. 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.

Wt%12345
SiO256.5456.4356.5357.3150.47
TiO20.080.070.030.060.05
Al2O31.922.051.581.110.88
Cr2O30.090.120.190.080.11
FeO*3.283.223.123.399.99
MnO0.130.030.080.090.21
NiO0.050.050.030.020.04
ZnO0.210.000.000.000.02
MgO22.2722.3322.9122.7529.19
CaO13.1713.0312.7912.758.18
Na2O0.230.290.190.100.12
F0.010.000.000.000.05
Cl0.000.010.010.010.01
Total98.097.697.597.799.3
apfu
Si2.0282.0272.0332.0551.850
Aliv−0.028−0.027−0.033−0.0550.150
Alvi0.1090.1140.1000.101−0.112
Ti0.0020.0020.0010.0010.001
Cr0.0020.0030.0060.0020.003
Fe2+−0.314−0.316−0.335−0.324−0.495
Mn0.0040.0010.0000.0030.006
Mg1.1911.1961.2291.2161.595
Ni0.0020.0020.0010.0000.001
Zn0.0060.0000.0000.0000.001
Fe2+0.4120.4130.4290.4260.801
Ca0.5060.5020.4930.4900.321
Na0.0160.0200.0140.0070.008
K0.0000.0000.0000.0000.000
Z2.0002.0002.0002.0002.000
Y1.0021.0021.0011.0001.001
X0.9340.9350.9360.9231.131
Xmg0.9240.9250.9290.9230.839
fs5.4785.3865.1775.62613.776
en66.32266.65967.67567.27771.777
wo28.19927.95527.14827.09714.447

Table 3.

Chemical composition (wt%) and atom per formula unit (apfu) of pyroxene on the basis of 6 O (sample CV-67).

Figure 6.

Si vs. Mg/(Mg + Fe2+) diagram for Chalki basalt amphiboles shown as magnesiohornblende (after [12]).

The general formula of amphiboles is W(0–1)X2Y5Z8O22(OH,F)2 [11].

4.2.5. 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 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).

Wt%123
SiO245.7744.945.8
Al2O311.0911.4511.45
TiO20.911.10.77
Cr2O31.080.960.67
FeO*7.037.998.02
MnO0.060.030.01
MgO17.0416.5516.87
CaO12.1412.412.66
Na2O1.450.880.6
Total96.696.396.9
(apfu)
Si6.6006.5226.592
Aliv1.4001.4781.408
Alvi0.4830.4820.534
Ti0.0990.1200.083
Cr0.1230.1100.076
Fe2+0.6270.7010.685
Mn0.0070.0040.001
Mg3.6623.5843.620
Fe2+0.2210.2700.280
Ca1.8741.9301.952
Na−0.095−0.199−0.232
K000
Z888
Y555
X222
W000
Mg#0.8120.7870.789

Table 4.

Chemical composition (wt%) and atom per formula unit (apfu) of hornblende on the basis of 23 O (sample CV-67).

Figure 7.

Chalki basalt chlorite shown as mainly diabantite and less penninite (after [13]).

4.2.6. 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 low-grade metamorphism and of alpine type.

CV-32CV-67
Wt%12345612
SiO232.0232.7633.2634.0534.8835.4732.2430.38
TiO20.090.000.040.050.000.020.020.03
Al2O313.5814.8013.4413.8113.7513.3213.9018.68
Cr2O30.030.010.040.010.030.001.140.16
FeO*17.2514.7516.2514.5213.1813.467.176.04
MnO0.110.060.050.020.030.030.030.06
NiO0.080.090.090.070.090.070.050.10
ZnO0.000.230.070.370.240.110.060.00
MgO21.6621.6422.4822.7623.7623.5431.0330.76
CaO0.480.470.380.390.310.320.260.03
Na2O0.020.050.060.040.070.120.010.05
F0.070.040.060.040.080.100.080.03
Cl0.010.000.010.000.010.030.020.01
Total85.484.986.286.186.486.686.086.3
Apfu
Si6.6116.6896.7426.8366.9167.0206.3095.861
Aliv1.3891.3111.2581.1641.0840.9801.6912.139
Alvi1.9142.2511.9542.1032.1282.1261.5132.108
Ti0.0130.0000.0070.0080.0000.0030.0030.005
Cr0.0040.0020.0060.0020.0040.0000.1770.024
Fe2.9792.5182.7552.4372.1842.2271.1740.974
Mn0.0200.0100.0080.0040.0060.0040.0060.010
Ni0.0130.0150.0150.0120.0150.0120.0070.016
Zn0.0000.0350.0100.0540.0350.0170.0080.000
Mg6.6676.5876.7946.8117.0246.9459.0548.848
Ca0.1070.1030.0840.0830.0660.0680.0540.005
Na0.0080.0180.0220.0150.0260.0460.0030.019
CV-25
Wt%1234567
SiO223.4425.9627.7631.4233.2436.6937.34
TiO20.720.240.330.190.050.000.04
Al2O311.4213.1513.8415.3116.1919.5821.16
Cr2O30.010.110.040.010.000.030.08
FeO*34.6930.3826.2717.2914.3212.7911.40
MnO0.030.040.040.070.090.010.00
NiO0.080.080.100.110.090.080.06
ZnO0.120.000.100.110.000.340.33
MgO17.1618.7319.8622.7123.5220.3218.84
CaO0.110.130.120.110.240.270.26
Na2O0.040.020.040.010.040.070.06
F0.020.040.070.110.110.100.10
Cl0.020.020.010.020.010.010.01
Total87.988.988.687.587.990.389.7
apfu
Si5.3325.6245.8686.3346.5246.8626.947
Aliv2.6682.3762.1321.6661.4761.1381.053
Alvi0.3940.9811.3151.9722.2693.1793.587
Ti0.1240.0390.0530.0290.0070.0000.005
Cr0.0020.0180.0070.0020.0010.0050.012
Fe6.5985.5044.6422.9142.3502.0001.773
Mn0.0050.0070.0070.0130.0140.0020.000
Ni0.0140.0140.0170.0180.0130.0110.009
Zn0.0200.0000.0160.0170.0000.0480.046
Mg5.8186.0506.2606.8266.8835.6665.226
Ca0.0270.0300.0270.0250.0490.0530.051
Na0.0180.0100.0170.0050.0170.0260.023
CV-37
Wt%12345678
SiO232.1832.6832.6932.8532.8732.9532.9633.09
TiO20.020.040.000.050.090.010.060.00
Al2O315.6815.5515.5615.6613.9215.6215.4815.16
Cr2O30.130.180.090.080.000.150.020.04
FeO*14.7514.2914.4714.7717.1914.9615.0814.87
MnO0.010.030.040.030.090.040.040.04
NiO0.050.110.060.110.070.060.060.12
ZnO0.000.180.000.000.430.390.260.18
MgO23.8124.6724.6124.2122.2024.2023.5523.70
CaO0.120.200.120.150.390.160.180.20
Na2O0.040.020.030.010.010.030.050.03
F0.060.100.040.110.050.090.120.10
Cl0.010.010.010.010.000.010.010.01
Total86.988.187.788.087.388.787.987.5
apfu
Si6.4216.4296.4296.4656.6296.4596.5166.558
Aliv1.5791.5711.5711.5351.3711.5411.4841.442
Alvi2.1102.0332.0332.0971.9382.0672.1232.098
Ti0.0030.0060.0060.0080.0130.0010.0080.000
Cr0.0200.0290.0290.0130.0000.0230.0030.007
Fe2.4622.3512.3512.4302.8982.4512.4932.464
Mn0.0010.0050.0050.0050.0150.0060.0070.006
Ni0.0080.0180.0180.0170.0110.0100.0100.019
Zn0.0000.0270.0270.0000.0650.0560.0390.027
Mg7.0847.2357.2357.1026.6757.0706.9417.000
Ca0.0260.0420.0420.0320.0850.0330.0370.043
Na0.0160.0060.0060.0040.0050.0120.0180.010
CV-41
Wt%12345678
SiO233.0632.5432.3932.6232.5832.0631.8233.02
TiO20.050.040.040.040.010.010.000.00
Al2O314.9815.0015.0215.0415.2615.4015.4415.45
Cr2O30.090.210.200.080.080.030.020.05
FeO*16.3816.6616.6216.9615.8416.3817.1116.05
MnO0.020.060.010.020.050.090.110.06
NiO0.020.030.050.110.090.110.060.10
ZnO0.000.000.060.170.370.020.050.61
MgO22.7722.6722.5722.4522.4622.2821.6322.80
CaO0.200.190.200.210.220.290.610.19
Na2O0.030.020.030.030.050.020.000.01
F0.060.070.090.070.000.090.090.06
Cl0.010.020.020.010.000.010.020.00
Total87.787.587.387.887.086.887.088.4
apfu
Si6.5796.5126.5016.5196.5326.4656.4406.529
Aliv1.4211.4881.4991.4811.4681.5351.5601.471
Alvi2.0922.0492.0522.0612.1392.1262.1222.130
Ti0.0070.0060.0060.0060.0020.0010.0000.000
Cr0.0140.0330.0320.0120.0130.0050.0040.008
Fe2.7262.7872.7892.8342.6552.7632.8942.653
Mn0.0040.0110.0020.0030.0080.0160.0200.010
Ni0.0020.0040.0090.0180.0140.0180.0100.016
Zn0.0000.0000.0080.0250.0540.0030.0070.088
Mg6.7556.7636.7546.6886.7136.7016.5276.719
Ca0.0420.0400.0430.0440.0470.0630.1320.041
Na0.0100.0070.0120.0130.0200.0090.0010.005
CV-41
Wt%910111213141516
SiO232.6231.9732.5331.6231.3534.4232.1930.87
TiO21.220.080.000.040.010.050.010.01
Al2O315.5015.5616.1316.1516.3116.4616.7117.19
Cr2O30.140.130.110.120.150.100.170.18
FeO*15.2516.6714.4714.1814.5913.9514.8714.53
MnO0.040.050.040.040.020.030.030.07
NiO0.050.110.190.090.110.110.110.06
ZnO0.250.010.100.000.140.020.280.32
MgO23.1721.6124.0621.9422.7420.2523.3522.42
CaO0.170.400.110.210.130.600.130.12
Na2O0.010.020.050.010.030.930.050.04
F0.070.110.100.000.080.080.080.01
Cl0.020.010.000.020.000.010.020.01
Total88.586.787.984.485.787.088.085.8
apfu
Si6.4146.4676.4106.4696.3526.8036.3536.243
Aliv1.5861.5331.5901.5311.6481.1971.6471.757
Alvi2.0062.1752.1562.3622.2462.6372.2392.342
Ti0.1810.0120.0000.0060.0010.0080.0020.001
Cr0.0210.0200.0170.0190.0230.0160.0260.029
Fe2.5082.8192.3842.4252.4712.3062.4542.458
Mn0.0070.0080.0070.0060.0040.0040.0040.012
Ni0.0080.0170.0300.0140.0180.0180.0180.010
Zn0.0360.0020.0150.0000.0210.0030.0400.048
Mg6.7926.5147.0676.6896.8705.9676.8696.761
Ca0.0360.0860.0240.0450.0290.1260.0270.025
Na0.0050.0080.0180.0040.0130.3570.0190.015

Table 5.

Chemical composition (wt%) and atom per formula unit (apfu) of chlorite on the basis of 28 O (samples CV-32, CV-67, CV-25, CV-37, CV-41).

Figure 8.

Cr-Al-Fe plot of Chalki basalt spinels shown as magnetite, chromian magnetite, and chromian spinel (after [14]).

Figure 9.

Cr# vs. Mg# of Chalki basalt spinels indicating their metamorphic origin as a result of low-grade greenschist metamorphism and alpine type (after [15]).

4.2.7. 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.

S 25S 41S 67
Wt%123112
SiO20.250.081.080.120.110.19
TiO210.930.611.720.500.810.97
Al2O34.1936.7420.1241.110.420.26
Cr2O321.0722.9630.2623.032.032.50
FeO*55.6828.0236.8417.6689.9189.06
MnO0.010.000.000.000.090.08
NiO0.100.130.080.200.190.17
ZnO1.810.000.290.030.440.35
MgO0.1710.875.4915.450.430.38
CaO0.020.000.020.520.020.01
Na2O0.030.030.000.020.010.00
F0.000.020.000.100.000.00
Cl0.000.000.010.010.000.00
Total94.399.595.998.894.594.0
apfu
Si0.01010.00240.03740.00350.00530.0095
Al0.20221.30190.82131.38890.02420.0154
Ti0.33670.01380.04480.01080.03010.0362
Cr0.68270.54570.82870.52190.07930.0978
1.23171.86391.73211.92500.13880.1590
Fe1.90830.70451.06700.42333.71363.6864
Mn0.00040.00000.00000.00000.00370.0035
Ni0.00340.00310.00220.00470.00750.0069
Zn0.05470.00000.00740.00070.01600.0127
Mg0.01060.48750.28340.66010.03150.0284
Ca0.00070.00010.00080.01600.00130.0007
Na0.00200.00170.00000.00100.00120.0000
1.98011.19691.36071.10583.77473.7387
Fe2+*0.63610.23480.35570.14111.23791.2288
Fe3+*1.41210.52130.78960.31322.74812.7279
Cr#0.7710.2950.5020.2730.7660.864
Mg#0.0170.6750.4430.8240.0250.0226
Cr-magnetiteCr-spinelCr-spinelCr-spinelmagnetitemagnetite

Table 6.

Chemical composition (wt%) and atom per formula unit (apfu) of spinel group on the basis of 4 O (samples CV-25, CV-41, CV-67).

Calculated stoichometrically.


Wt%1234567
SiO247.3646.7540.6441.8041.8442.5241.36
TiO20.030.000.000.000.060.000.03
Al2O30.480.630.170.350.860.600.38
Cr2O30.010.000.010.020.010.030.00
FeO*9.8811.2210.5015.1413.8413.7111.09
MnO0.200.100.190.210.250.180.26
NiO0.270.130.230.140.100.190.01
ZnO0.140.240.580.210.270.000.30
MgO33.2130.5036.3628.7729.5329.3531.96
CaO0.270.420.110.980.320.370.14
Na2O0.070.050.030.030.040.000.03
F0.010.090.090.090.050.090.09
Cl0.020.020.020.020.020.010.00
Total92.090.288.987.887.287.185.6
apfu
Si4.2984.3543.9114.1414.1324.1904.107
Ti0.0020.0000.0000.0000.0040.0000.002
Al0.0520.0690.0190.0410.1000.0700.044
Cr0.0010.0000.0010.0010.0010.0020.000
Fe0.7500.8740.8451.2551.1431.1300.921
Mn0.0160.0080.0150.0180.0210.0150.022
Ni0.0200.0100.0180.0110.0080.0150.001
Zn0.0090.0170.0410.0150.0190.0000.022
Mg4.4934.2345.2164.2494.3474.3124.732
Ca0.0270.0420.0110.1040.0340.0390.015
Na0.0120.0090.0060.0050.0070.0000.005
9.6799.61610.0829.8409.8179.7749.871

Table 7.

Chemical composition (wt%) and atom per formula unit (apfu) of serpentines on the basis of 14 O (CV-67).

Figure 10.

Chalki basalt serpentines shown as type lizardite and chrysotile (after [16]).

5. 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 (An51–61) to andesine (An35–41) to andesine (An22) reflects the andesitic variety of these volcanic rocks in addition to basalt, while the coexisting albite composition (An0.1-04) indicates clearly that the rocks had suffered albitization due to low-grade sub-sea metamorphism.

  3. The NiO content (0.11–0.12 wt%) in the Chalki forsteritic olivine (Fo80–81) reflects the tectonic origin of this extrusion.

  4. The enrichment of Chalki pyroxene in MgO causes its shifting from diopside-augite trend of the layered igneous rocks to endiopside (en66–68 wo27–28 fs05–06) and subcalcic augite (en72 wo14 fs14).

  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).

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Mohsin M. Ghazal, Ali I. Al-Juboury and Sabhan M. Jalal (October 26th 2019). Mineral Chemistry of Chalki Basalts in Northern Iraq and Their Petrological Significance [Online First], IntechOpen, DOI: 10.5772/intechopen.89861. Available from:

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