Approaches to the Low Grade Metamorphic History of the Karakaya Complex by Chlorite Mineralogy and Geochemistry

In this study, chlorite is used to investigate the diagenetic-metamorphic evolution and accurate geological history of the different units belonging to the Karakaya complex, Turkey. Primary and secondary chlorite minerals in the very low-grade metamorphic rocks display interference colors of blue and brown and an appearance of optical isotropy. Chlorites are present in the matrix, pores, and/or rocks units as platy/flaky and partly radial forms. X-ray diffraction (XRD) data indicate that Mg-Fe chlorites with entirely IIb polytype (trioctahedral) exhibit a variety of compositions, such as brunsvigite-diabantite-chamosite. The major element contents and structural formulas of chlorite also suggest these were derived from both felsic and metabasic source rocks. Trace and rare earth element (REE) concentrations of chlorites increase with increasing grade of metamorphism, and these geochemical changes can be related to the tectonic structures, formational mechanics, and environments present during their generation.

Chlorite occurs pervasively in the various units of the Karakaya Complex, one of the most debated tectonic settings within Turkey [19][20][21].In this study, the utility of chlorite is demonstrated relative to the interpretation of the diagenetic-metamorphic evolution of the different units within the Karakaya complex.

Geotectonic Setting and Lithology
The Karakaya Complex [22] within the Sakarya Tectonic Unit [23] consists of units representing the remnants of the subduction accretionary prism formed by the closure of the Pre-Jurassic Paleotethys Ocean [24,25].It is divided into two areas [26] (Figure 1) with the Lower Karakaya Complex (LKC), identified as the Nilüfer unit in northwestern Anatolia and Turhal Metamorphites in the central NE Anatolia [27,28]; this unit is structurally and stratigraphically located at the bottom, and consists of rocks subjected to metamorphism at the end of the Paleozoic or during the Triassic [24].The LKC units are divided into the upper and lower (LKC-LP and LKC-UP) parts in accordance with their petrographic properties and the mineralogy of phyllosilicates, which correspond to blueschist and greenschist facies, respectively [21].These are represented by metapsammite (metasandstone), metapelites (slate, phyllite, schist), metacarbonates (metalimestone, metadolomite, marble), and metabasic rocks (metavolcanic, metatuff) in NW Anatolia, and metapelitic (phyllite, slate, metasiltstone), metasandstone, metamagmatic (metabasalt, metatuff, metagabbro, metadiabase), and metalimestone in the central NE Anatolia region.

Materials and Methods
In total, 253 rock samples were taken from various units within the area of investigation.The weathered surface and soil on the samples were removed by crushing and then washing with ultrapure water.Subsequently, samples underwent various processes such as thin sectioning, crushing-grinding-screening, clay separation, X-ray diffraction (XRD), and optical microscopy (OM) at Cumhuriyet University (CU), Department of Geology and Mineralogy-Petrography and Geochemistry Research Laboratories (MİPJAL).Scanning electron microscopy (SEM) was conducted at the Turkish Petroleum Corporation Research Center (Ankara).
The samples investigated by XRD were first crushed in pieces of 3-5 cm in diameter with a hammer and then into fine particles (<5 mm) in a Fritisch rock crusher.Depending on their hardness, samples were then ground for 10-30 min in a silicon carbide bowl grinder and/or agate mortar.The powder material was then placed in plastic bags and labeled and ready for analysis.The XRD analyses were conducted with a DMAX III X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) (Anode = Cu (Cu Kα = 1.541871Å), Filter = Ni, Voltage = 35 kV, Current = 15 mA, Gonimeter speed = 2°/min, Paper speed = 2 cm/min, Time constant = 1 s, Slits = 1°-0.15mm-1°-0.30mm, Paper interval 2θ = 5°-35°).
As a result of the XRD analyses, all rock and clay-sized components (<2 μm) of the samples were defined and their semi-quantitative percentages were calculated using the external standard method [33].Mineral intensity factors were used in all rock and clay fraction calculations and the reflections were measured in mm.In this method, kaolinite was taken as a reference from the glycolylated preparations for clay fraction and dolomite for whole rock [34].Quartz was used as the internal standard in the measurement of d-spaces.The definitions of clay minerals were based mainly according to their (001) basal reflections.
The separation of phyllosilicate/clay minerals was conducted at the Clay Separation Laboratory, Department of Geological Engineering, Cumhuriyet University.The clay separation process necessary for the XRD-CF analyses essentially consists of chemical solving (removal of non-clay fraction), centrifugation-decantation/resting and washing, suspending-sedimentation-siphoning-centrifugation, and bottling.In the absence of suspended particles, the separation process was accelerated by the addition of Calgon.Centrifugation was performed using a Sepatech Varifuge 3.2S (Heraeus Group, Frankfurt, Germany) model centrifuger at 5600 cycles/min speed and 200 cc capacities with stainless steel and/or plastic bowls.Three oriented slide preparations from each clay mud were prepared as plastering, or in suspension and these were dried at room temperature.Clay fraction diffractograms were obtained by putting through the processes of normal-AD (air-dried), glycolylated-G (leaving in ethylene glycol vapour in a dessicator for 16 h at 60 °C) and heated-H (heating in the oven for 4 h at 490 °C).The goniometer speed at the intervals was set at 1°/min and the recording interval was set as 2θ = 2-30° (±0.04°).
In the measurement of the illite crystallinity, the width at the half-height of the 10-Å illite peak Δ°2θ (Kübler index- [35,36]) was used.The peak widths were calibrated according to Kisch [37] and Warr and Rice [38] standards.WINFIT [39] programme (http://xray.geol.uni-erlangen.de/html/software/soft.html) was used to distinguish the single and two illite phases with the asymmetric and symmetric peaks as well as crystalline illite-WCI and poor crystalline illite-PCI.For the lower and upper limits of the anchizone, 0.21 and 0.37 Δ°2θ, and 0.25 and 0.42 Δ°2θ were taken respectively for Kisch [37,40] and CIS [38] standards.
The following method was used in order to determine the chemical components of the chlorites: d(001) reflection values were found as from d(005) peaks and the amount of tetrahedral Al was established according to d(001) = 14.55 Å − 0.29Al IV formula [42].The amount of octahedral Fe 2+ was obtained from the diagram using the [I(002) + I(004)]/[I(001) + I(003)] [43] and I(002)/I(001) and I(004)/I(003) [44] ratios.In this method, the Fe contents in the talc and brucite layers can be determined with the help of the I(004)/I(003) ve I(002)/I(001) basal peak ratios of the chlorite minerals.Mg contents were determined in accordance with the Fe + Al VI + Mg = 6 equation by presuming that Al IV = Al VI .
The major, trace, and rare earth element (REE) analyses of four pure (or nearly pure) chlorite fractions were performed at Activation Laboratories Ltd. (Actlabs, Ancaster, ON, Canada).Fusion inductively coupled plasma (ICP) was used for major element analysis, while inductively coupled plasma-mass spectrometry (ICP-MS) was employed for the analyses of trace and REEs.Details of the analysis method and the instrument detection limits are provided on Actlabs' website (http://www.actlabs.com/).

Optical Microscope Examinations
Plagioclase microliths are present in microlithic porphyritic metavolcanic rocks representing the Nilüfer Unit within the LKC-LP in NW Anatolia.The groundmass material consists mainly of volcanic glass, phyllosilicates, and amphiboles.The volcanic glass within the matrix has been subjected to mainly chloritization, silicification and partly Fe-oxidation, and carbonization.Chlorite minerals show blue and brown interference colors, but are mainly green in plane polarized light.Chlorite plates oriented parallel to the crystallographic c-axis and quartz-rich zones that are sequenced in the direction of c-crystallographic axis in the blue schists {001} of the Nilüfer Unit (LKC-LP), and are typical textures indicative of metamorphic differentiation (Figure 2a).Isotropiclike chlorite minerals in the matrix are observed in porphyroblasts that form in tremolite-rich metabasalts of the LKC-UP, which formed under greenschist facies conditions (Figure 2b).
Platy chlorite minerals (displaying blue interference color) result from the neoformation within greenschist facies metasandstones defined as Turhal Metamorphites of LKC-UP in the central NE Anatolia.Muscovite, biotite, and chlorite stack (pod) structures showing micro-orientation and bending in certain metasandstones are typical.The stacks look platy/flaky, and partly radial, and these have three different mineralogical compositions that are chlorite-muscovite (CMS), chlorite-biotite (CBS), and biotite-muscovite (BMS) stacks (Figure 2c).In metavolcanic rocks, green chlorite minerals are characterized by blue and brown interference colours.The chlorites in the matrix and pores may also show optical isotropic-like features as in corresponding rocks in NW Anatolia (Figure 2d).
The chlorites in the pores of sandstones in the Orhanlar Unit among UKC units in NW Anatolia display a blue interference colour (Figure 3a,b).Chlorite has a distinctive reaction rim that indicates retrograde metamorphism.Furthermore, calcite and rare epidote minerals are other components in the pores.Volcanic rocks constituting the main lithology of the Çal Unit have hypocrystaline porphyritic, vitrophyric-microlitic porphyritic, amygdaloidal, and partly glomeroporfiric textures.The groundmass consists of plagioclase microlites and mostly chloritized volcanic glass (Figure 3c).
The groundmass for the metasandstones from the Devecidağ Melange among UKC units in the central NE Anatolia consists of Fe oxidation and phyllosilicates matrix in addition to calcite and/or dolomite and silica cement.Chlorite minerals are typically observed with blue interference color developed within pores and matrix (Figure 3d).

Scanning Electron Microscopy
The greenschist facies sample of shale (NKK-93) from the Nilüfer Unit, LKC-UP in NW Anatolia consists of chlorite, illite, quartz, feldspar, and calcite, with chlorite minerals forming clusters in the form of plates (Figure 4a).The illites are mainly in the form of parallel, partly radially sequenced long and thin filaments (2-30 μm) (Figure 4b).

Polytype
The results of polytype examinations of pure chlorite fractions for the Nilüfer unit in NW Anatolia and Turhal Metamorphites in the central NE Anatolia of the LKC are given in Table 2.It was detected that the chlorite associations of the LKC-LP and LKC-UP, which correspond to blueschist and greenschist facies in metavolcanic and metapelitic rocks of the Nilüfer Unit and Turhal Metamorphites are entirely of IIb polytype (Figure 6).The chemical compositions of the chlorites for the LKC units determined in accordance with basal peak values and intensity ratios are listed in Tables 3 and 4 and shown in Figure 7.The reflection values of the (001) surface in the chlorites in NW Anatolia vary between 14.189-14.338Å (14.249Å on average) for LKC-LP and 14.172-14.392Å (14.262Å on average) for LKC-UP; and 14.202-14.366Å (14.275Å on average) for the Orhanlar Unit.For the Çal Unit, the average reflection value is 14.269 Å.

Figure 7.
Fe content within the talc layer and distribution of talc and brucite layers of chlorite minerals according to basal peak ratios [44] in the Karakaya Complex units.Table 3. X-ray diffraction (XRD) basal reflections and chemical compositions of chlorite minerals in the Lower Karakaya Complex units.B: Brindley [42], B&B: Brindley and Brown [43], C&D: Chagnon and Desjardins [44]), Al IV = Al VI and Mg = 6 − (Fe + Al IV ).According to the methods outlined by Brindley and Brown [43] and Chagnon and Desjardins [44], the Fe content in talc and brucite layers of the chlorites are similar, and the Chagnon and Desjardins [44] method was taken as a basis for determining octahedral Fe (Figure 7).According to the diagram, the Fe contents in the talc layer of the chlorites range between 1.0 and 2.4.Therefore, 2.10-3.50(2.83 on average) was established for the octahedral Fe contents of the chlorites for LKC-UP in NW Anatolia, 2.80-4.11(3.41 on average) for LKC-UP, and 2.11-4.50(3.44 on average) and 2.44 for UKC Orhanlar and Çal units, respectively.Octahedral Al contents are 0.73-1.24(1.04 on average) for LKC-LP, 0.54-1.21(0.99 on average) for LKC-UP, and 0.63-1.14(0.94 on average) and 0.98 for UKC (Orhanlar and Çal units), respectively.Octahedral Mg contents are 1.26-3.17(2.13 on average) for LKC-LP, 0.40-2.31(1.59 on average) for LKC-UP, and 0.56-2.75(1.62 on average) and 2.58 for UKC (Orhanlar and Çal units), respectively.Total amounts of octahedral cations for LKC-L, LKC-U and UKC (Orhanlar and Çal units) is 6.00 and it was noted that the chlorites are of trioctahedral component and the average structural formulae are provided below:  The substitution of Al in the place of tetrahedral Si leads to a reduction in layer thickness and thus a reduction in d( 001) value as a result of the strengthening of the layer and the bond between the layers [45].
On the basis of the diagram using Al IV − Fe/(Fe + Mg) ratios [46] the, LKC-LP chlorites examined here fall in brunsvigit-diabantite zone, whereas LKC-UP chlorites cluster in brunsvigit zone; UKC chlorites plot within the diabandite-chamosite zone and show variable compositions.According to the AIPEA classification [47], the chlorites investigated here classify mainly as chamosite and partly clinochlore compositions (Figure 8).In contrast, it is observed that only one (NKK-113) of the two samples defined as IIb polytype falls within the area defined by Foster [46].Distributions of the first three basal reflections (I001, I002 and I003) of the chlorites [16] indicate that the LKC and UKC units are distinct (Figure 9).The intensities of the chlorites shift in a way such that they get closer to the I002-I003 line as their Fe/(Fe + Mg) contents increase.The difference in intensities for the LKC-UP and LKC-LP units relative to their I002 and I003 can be attributed to their relatively higher iron and magnesium contents, respectively.Moreover, the high intensities related to 003 peaks in LKC-LP are the result of higher Fe contents in brucite layers rather than talc layers (Figure 7).
Combined AI and KI data indicate that crystallinity decreases for samples from LKC units compared to the UKC units and the units diverge (Figure 10).There is a weak correlation relation between KI and AI values and it is different than (epizone: 0.21 Δ°2θ, anchizone: 0.53 Δ°2θ) the AI values corresponding to the epizone (AI < 0.26 Δ°2θ) and anchizone (AI = 0.26-0.37Δ°2θ) as suggested by Árkai [15].

Geochemistry
Structural formulas [48] of the chlorite minerals for the KC units were calculated based on the major and trace element contents and 14 oxygen atoms (per formula unit) and are listed in Tables 6 and 7.The structural formulas of two samples were recalculated by removing Al2O3 and SiO2 percentages depending on K2O, Na2O and CaO with 1:1:6 and 1:1:2 ratios of theoretical feldspar composition (K-feldspar K2O•Al2O3•6SiO2, albite Na2O•Al2O3•6SiO2, anortite CaO•Al2O3•2SiO2) in the clay fraction.The charge deficiency resulting from the introduction of Al from feldspar within chlorite is balanced by K, Na, and Ca from the feldspars.[49].According to the Zane and Weiss [49] classification, the source for the LKC-LP chlorites generally correspond to metabasic rocks, whereas those for the LKC-UP chlorites are felsic rocks, and those for the UKC chlorites correspond to both felsic and metabasic sources, with felsic being predominant (Figure 11).
The trace element concentrations of chlorite minerals are compared in Figure 12, and these are extremely variable.The substitution of elements with similar geochemical behaviour is mostly observed in the metatuff sample of Nilüfer unit.The highest trace element enrichment for the chlorites is observed in the LKC-LP unit (TKK-36) and the lowest in the LKC-UP unit (NKK-98).Cr, Ni, V, and Cu (transition elements), Ga (low field strength elements-LFSEs), and Nb, Zr and U (high field strength elements-HFSEs) record the highest enrichment in LKC-LP chlorites.As for LKC-UP chlorites, Zn (transition elements), W (lithophile elements); and Rb, Ba and Sr (LFSEs) are present at high concentrations.Hence, trace element concentrations are a function of the protoliths, i.e., igneous versus metamorphic parent materials.
Chondrite-normalized trace element patterns for chlorite are shown in Figure 13.Trace element concentrations for the North American Shale Composite (NASC) are from Gromet et al. [50], with the exception of Nb and Y (from Condie, [51]).The chlorite minerals exhibit marked variation when compared to chondrite values, and generally have lower concentrations relative to NASC, except for Ti and U.The latter shows an enrichment of up to 5600 times in the volcanogenic chlorite for sample LKC-LP (TKK-36) and therefore exhibits a strong positive anomaly (Figure 13).In all chlorites, K, Sr, and P display significant negative anomalies.The chlorite from sample TKK-30 within the LKC-LP exhibits the lowest chondrite normalized values.The chlorite chondrite normalized REE abundances are shown in Figure 14.NASC values (with the exception of Ho and Tm; Haskin et al. [52]) are taken from Gromet et al. [50].REE contents of all chlorite minerals are lower than NASC and exhibit varying degrees of enrichment compared to chondrite [53].The highest chondrite normalized values are displayed by the volcanogenic chlorite (NKK-98) from the LKC-UP unit, whereas the lowest values are recorded in the volcanogenic chlorite (TKK-36) from the LKC-LP unit.Furthermore, light REE (LREEs; La-Gd) concentrations of chlorite minerals show a slight decrease when compared to those for the heavy REEs (HREEs; Tb-Lu).In the chlorite for the LKC-LP unit within the NW-Anatolia, Pr defines a negative anomaly, while Sm yields a slight positive anomaly.The Mg content of chlorite is higher in the LKC-LP blueschist facies, while the Fe content in the LKC-UP greenschist is related to the mineralogical composition of the rocks as a result of the increasing heat/pressure due to burial depth.The difference in metamorphic degree, in particular between the LKC-LP (blueschist) and LKC-UP (greenschist) units, seems to have also affected the chemical compositions of the chlorites since the latter can be related to lithological differences and the nature of the protolith.
The features of transition to semi-regular or regular stacking are observed in chlorite layers [47,63,64], and these structural (polytype) changes may be partly attributed to heating, which then serves as a qualitative method for geothermometric assessment [65].It has been established that the transformation from Ib → IIb type chlorites occurs at temperatures <200 °C [10]; the chlorites investigated here are IIb and those for Turhal Metamorphites within the UKC units are of 2M1 polytype [20], both of which indicate a similar degree of metamorphism.
The combined optical, mineralogical, morphological, and geochemical evidence reported here for chlorite are shown to be related to the structures, formation mechanisms, and tectonic environments present during their formation.Consequently, chlorite plays a key role in distinguishing units with distinct geological evolutions within the Karakaya Complex.

Figure 4 .
Figure 4. Scanning electron microscopy (SEM) photos of chlorite and associated minerals in the Lower Karakaya Complex units (Chl = Chlorite, I = Illite), (a) Platy chlorites in the shale sample of the Nilüfer Unit; (b) Thin-long filaments of illites in the shale sample of the Nilüfer Unit; (c) Radial needle-like chlorites and surrounding illites within the pores of metasandstones in the Turhal Metamorphites; (d) Fibrous illites in the metasandstones of the Turhal Metamorphites.

Figure 5 .
Figure 5. Oriented X-ray diffraction (XRD) patterns of chlorite minerals in the units of Lower Karakaya Complex-Lower Part, (a) Slate sample of blueschist facies in the Nilüfer Unit; (b) Metatuffite sample of greenschist facies in the Turhal Metamorphites.

Figure 6 .
Figure 6.Unoriented XRD patterns of chlorite minerals in the units of Lower Karakaya Complex-Lower Part, (a) Slate sample of blueschist facies in the Nilüfer Unit; (b) Metagabbro sample of greenschist facies in the Turhal Metamorphites.

Figure 9 .
Figure 9.The distribution of chlorite minerals in the Karakaya Complex units in the triangular diagram according to the first three basal reflections (Chlorite area: Árkai and Ghabrial [16]).
Tetrahedral Si-Al (1.00-0.83)and octahedral Mg-Fe-Al (4.64-4.79)substitutions yield total calculated octahedral cations in excess of 4 (4.77-5.33) in samples of chlorite from slate of the Nilüfer unit and the metatuffite of Turhal Metamorphites, which indicate their di-trioctahedral composition and is verified by the XRD data.Chlorites plot within the Type I zone represented by Mg-Fe chlorites (trioctahedral) in Al + □ − Mg − Fe diagram

Figure 11 .
Figure 11.The distribution of chlorites in the Karakaya Complex units in the Al+ □ − Mg − Fe triangular diagram according to octahedral compositions (□: Octahedral vacancy).

Figure 12 .
Figure 12.The correlation of chlorite minerals in the Karakaya Complex units according to the contents of trace elements (Arrows show values below the detection limits, M = Miscellaneous elements, H = Halogen elements, PM = Precious metals, LFSEs = Low field strength elements, HFSEs = High field strength elements).

Table 1 .
The dominant mineralogy of samples from the Karakaya Complex (KC) units.

Table 2 .
The results of polytype examination of chlorite minerals in the Karakaya Complex units.

Table 5 .
Crystallinity index values of chlorite (AI) and illite (KI) in the Karakaya Complex units.

Table 6 .
Major element composition and structural formulas of chlorite minerals in the Karakaya Complex units (*: The values show that theoretical feldspar composition is removed, ∑Fe2O3: Total iron as Fe2O3, LOI: Loss on ignition at 1000 °C, TC: Tetrahedral charge, TOC: Total octahedral charge, OC: Octahedral charge, ILC: Interlayer charge, TLC: Total layer charge).

Table 7 .
Trace element composition of chlorite minerals in the Karakaya Complex units (<: Concentrations are below the detection limit).