Using Detrital Garnets to Determine Provenance: a Case Study from the Vertiskos Unit (Serbomacedonian Massif, N

Miner Petrol (2014) 108:187–206 DOI 10.1007/s00710-013-0295-1

ORIGINAL PAPER

Using detrital garnets to determine provenance: a case study from the Vertiskos Unit (Serbomacedonian Massif, N. Greece)

Ioannis K. Georgiadis & Antonios Koroneos & Lamprini Papadopoulou & Nikolaos Kantiranis & Alexios E. Tamparopoulos & Ananias Tsirambides

Received: 2 February 2013 /Accepted: 20 May 2013 /Published online: 9 June 2013 # Springer-Verlag Wien 2013

Abstract Garnet single crystals of several millimeters in composition, tectonics, climate and relief; these factors diameter were collected from the uppermost horizon of a grouped together are referred to as provenance (e.g. soil profile developing immediately on the gneissic rocks of Dickinson 1970; Suttner 1974). In a more descriptive way, the Vertiskos Unit of the Serbomacedonian Massif in north- the provenance includes the paleogeography of a region, the ern Greece. The garnets were analyzed for major elements identification of possible source areas for the clastic material by EDS analyzer mounted on a scanning electron micro- under study and the revealing of details concerning the scope, and the obtained data were utilized to determine their paleocurrents and the paleoslope (Pettijohn et al. 1973). source rocks. Bivariate diagrams, spider diagrams as well as Especially Quaternary sediments are considered very useful statistical analysis were used in order to correlate and com- in provenance studies, because the effects of diagenetic pare the garnet composition of the basement rocks of the dissolution are minor compared to ancient sandstones Vertiskos Unit with the existing reference data. This case (Garzanti and Ando 2007). study demonstrates the difficulty in assigning a source rock According to Blatt et al. (1972), the accessory or heavy to sediment, using only the chemical compositional of de- minerals are extremely useful constituents of sediments with trital garnet. Direct linking of the detrital garnets and the regard to determining provenance despite rarely being pres- outcropping rocks is not always possible despite well doc- ent in amounts greater than 1 % of the clastic rock. Heavy umented outcrop lithologies. This is largely due to a com- minerals have long been used as indices of provenance, plex metamorphic evolution that leads to overlapping com- though the processes of transportation and the environment positions between garnets originating from different lithol- of deposition may alter the composition of the detrital frac- ogies that have undergone similar metamorphic processes tion. Pettijohn et al. (1973) consider iron-poor garnet as a and alteration effects. stable detrital heavy mineral and iron-rich garnet as a moderately stable one. Since garnet (a) is the most common heavy mineral in Introduction most siliciclastic sandstones, (b) is resistant to alteration under weathering and common diagenetic conditions and The factors that combine to produce detrital assemblages (c) has compositions that may reflect the compositions of found in modern and ancient sediments include source-rock the source rocks, it is commonly selected for provenance studies (Morton 1987; Di Giulio et al. 1999). Especially in podzol-type soil profiles, garnet is considered moderately Editorial handling: J. Kosler stable and is present under acidic conditions but not under * : : : I. K. Georgiadis: ( ) A. Koroneos L. Papadopoulou strongly alkaline (e.g. Morton and Hallsworth 1999). N. Kantiranis A. Tsirambides The garnet group is the most widely used provenance School of Geology, Department of Mineralogy-Petrology-Economic Geology, Aristotle University of Thessaloniki, 541 24 indicator in NW European sediments, due to its abundance Thessaloniki, Greece and its wide range of potential compositions (Morton et al. e-mail: [email protected] 2004). In general, detrital garnet is considered to mostly originate from metamorphic rocks (Blatt et al. 2006). A. E. Tamparopoulos Institute for Structural Engineering, University of Natural Resources Caution is advised though, since garnet assemblages may and Applied Life Sciences, 1190 Vienna, Austria demonstrate significant changes with age (Win et al. 2007). 188 I.K. Georgiadis et al.

According to Morton et al. (2004), tracing garnet com- Geological setting positions directly back to their source rocks is more difficult than using them for discriminating sandstones of different The study area belongs to the Serbomacedonian Massif provenance; this is mainly due to the lack of a comprehen- (SMM) (Kockel and Walther 1965) and more specifically sive database on garnet compositions in basement rocks. comprises the Northern and major part of the Vertiskos Unit Therefore a multidisciplinary approach is advised (e.g. (Kockel et al. 1971, 1977), northern Greece. The SMM forms Morton and Hallsworth 1999; Win et al. 2007). a zone of rocks approximately 600 km long and 60–100 km Suggate and Hall (2013) have found that not all garnets wide, with N/NW (344o)-S/SE (132o) trend, in the central part can be definitely matched to a protolith; this issue espe- of the Balkan Peninsula. It is divided into three geomorphol- cially occurs when the source rocks are unknown, when ogic units, namely the Northern, the Central and the Southern compositional data from garnet-bearing rocks are not avail- (Psilovikos 1984). The Vertiskos (geotectonic) Unit studied able, or finally because the garnets have been removed by here coincides with the northern part of the Southern erosion. In these cases it is unclear whether compositional (geomorphologic) Unit of the SMM. It is an area of diverse data from other areas can be used to accurately determine relief, with mountain-horst and basin-graben structures provenance. (Psilovikos 1984). For the present work we utilized solely geochemical The maximum altitude of the Vertiskos Unit is approxi- compositional data acquired from detrital garnet pheno- mately 1,179 m and the Unit is drained from several rivers crysts, collected from a modern soil profile developing and torrents. This complex drainage system is generally of immediately on the basement rocks of the Vertiskos Unit dendritic pattern, discharging its detrital load to Gallikos and (Serbomacedonian Massif, N. Greece). It represents a case Strymon Rivers (west and east ends of the Massif, respec- study of determining the source rocks using only the chem- tively), to Doirani and Kerkini Lakes to the north and to ical composition of garnet. The problems that arose are Lagada and Volvi Lakes to the south (north and south ends documented and several methods for solving them are of the Unit, respectively), Fig. 1. The Unit generally has a employed. smooth relief, with smoothly sloping mountains and two

Fig. 1 Petrographic sketch map of the study area according to IGME (1978a, b, c, 1979a, b, c, 1990) Using detrital garnets to determine provenance 189 extensive uplifted planation surfaces at 400–600 m and & Acid plutonic rocks. These rocks are generally known as 600–800 m (Psilovikos 1984). the Arnea Granite Suite (e.g. Dixon and Dimitriadis The Greek part of the SMM is divided into the upper 1987) and their age is Permian-Triassic (247–244 Ma, Vertiskos Unit and the lower Kerdillia Unit (Kockel et al. Poli et al. 2009). 1971, 1977). More recently, on the basis of lithological char- & Pegmatites. They are found as veins in the mica schists acteristics of the basement rocks, the SMM was further divid- and the amphibolites. Their age is Jurassic to Tertiary ed into several main Units, namely Pirgadikia, Vertiskos, (Marakis 1969; Zervas 1980). Arnea and Kerdillia (Himmerkus et al. 2007). The older & Small occurrences of Tertiary acid volcanic rocks. definition of Kockel et al. (1977) is used throughout this study. & Poorly sorted Tertiary massive graywackes along with The metamorphic basement of the Vertiskos Unit consists poorly sorted and massive arkoses (Chatzidimitriadis et of mainly Palaeozoic metasediments and metabasic rocks, al. 1993). along with Mesozoic granites. In general, the Unit has The principal rock types of the Vertiskos Unit are deformed undergone a metamorphosis during the Paleozoic to the orthogneisses, with their precursor rock being a medium- amphibolite facies, followed by a Cretaceous retrograde grained porphyritic biotite-granite, now preserved in strain- metamorphosis to the greenschist facies (Mountrakis free pockets in the orthogneisses (Himmerkus et al. 2009). 2002). In detail the Unit is composed of the following lith- The orthogneisses are associated with amphibolites and low- ologic types: grade metasediments, the latter present only at the western border of the SMM and at its central part (Himmerkus et al. & Gneisses (orthogneisses, garnetiferous migmatitic gneisses, 2007). To the West the meta-sediments are in tectonic contact mica schists, pelites, semi-pelites, meta-sandstones, meta- with the Circum Rhodope Belt, a low-grade metamorphic arkoses and marbles). These lithologies occupy the bulk of volcano-sedimentary succession (Kauffmann et al. 1976). the study area. Some marbles are rarely found as beds or Especially the meta-sediments of the western border of the lenses in schists, mainly with stratigraphic contacts. The SMM must have as protoliths potassic arkoses and gray- orthogneisses are considered to originally be volcanic-arc wackes of mixed provenance, igneous and sedimentary granites showing within-plate affinities, demonstrating an (Veranis et al. 1990). age of 490–405 Ma (Meinhold 2007). The meta-sediments Two garnet generations can be distinguished in the rocks are Palaeozoic or older and actually demonstrate a poly- comprising the basement (Kourou 1991; Sidiropoulos metamorphic character (I.G.M.E. 1979a, b, c; Dixon and 1991): The first includes the older, rounded garnets, which Dimitriadis 1987). Meinhold (2007) reported that the max- sometimes can be found only as relics in the host rocks. The imum age of deposition for the meta-sediments is Early second includes the younger, smaller and fairly idioblastic Ordovician and that the upper time limit is Late ones. Ordovician-Silurian. The poly-phase metamorphism has Outcropping migmatites, mica schists and meta- taken place during the Mesozoic (Himmerkus et al. 2009). sandstones host both generations of garnet. The second & Migmatites (Palaeozoic or older). They are found as generation garnets, contrary to the first, show growth zona- thick intercalations in mica schists, with gradual transi- tion, a nucleus rich in Mn and Ca and a rim rich in Fe and tions between these lithologies. Their protoliths must Mg. Garnets in amphibolites show no regular zonation. have been the previously mentioned meta-sediments However, towards the rim of the crystal a significant in- (Kourou 1991). crease in Mn and decrease in Mg is observed; this diffusion & Amphibolites (Palaeozoic or older). They are found as zonation is likely due to a later metamorphic event. Finally, thick intercalations in migmatites and mica schists and eclogites and amphibolitized eclogites contain only relics of with gradual transitions between these lithologies. These garnet crystals with no zonation, mainly almandinic in com- rocks are of igneous origin, probably from tholeiitic position, and rich in grossular and pyrope. Chemical analy- magma (Kassoli-Fournaraki 1982; Papadopoulos 1982). ses typical of the cores of garnets hosted in the rocks & Ultrabasic rocks (serpentinites and talc-chlorite schists). comprising the Vertiskos Unit, are presented in Table 1. These outcrop throughout the Unit as lenses tectonically Five are the metamorphic episodes that affected the emplaced into the mica schists. Their age is Palaeozoic or Vertiskos Unit (Kourou 1991; Sidiropoulos 1991): older. & Eclogites and amphibolitized eclogites (Paleozoic or & M1 episode. Hercynian age (Devonian or prior), older). They are found as lenses in the mica schists, with P>9 kb, 400 °C

b amphibolitized eclogites demonstrate T≤440 °C.

A detailed overview of the metamorphic evolution of the . amphibolite and

Am. Ecl. (1) SMM is given by Kilias et al. (1999), Himmerkus et al.

b (2009) and Meinhold et al. (2010). Amphib In the Vertiskos Unit strong evidence of neotectonic activity is recorded (Psilovikos 1984). The basin-grabens of the Unit have developed in two stages (Psilovikos

G.M.Gneiss (1) 1977): The first occurred during the Early-Mid Miocene b and the second during the Late Pliocene-Early Pleistocene, both due to the active extension in the central Aegean area. Neotectonic activity continues until today, through high . amphibolitized eclogite, M. Schist (1) seismicity and earthquakes of shallow focal depth Ecl . (<17 km) (Psilovikos 1984). The SMM as a whole is

Am characterised by an active tectonic lineament of NNW-SSE

a direction, with N-S extensional trend (Pavlides et al. 1988; Voidomatis et al. 1990; Zagorchev 2007). M-Ecl. The total thickness of the Neogene and Quaternary de- posits in the smaller grabens and the larger basins ranges from 500 m to 1,500 m and 1,500 m to 3,500 m, respec- a tively. This feature indicates very active exogenic processes, producing almost planar surfaces in the Vertiskos Unit Eclogite (Psilovikos 1984). garnetiferous migmatitic gneiss, a Gneiss . Materials and methods M . Amphib. (1) G Fourteen garnet single-crystals of various diameters (0.3 cm

a for G1 to 1.9 cm for G15) were hand-picked from the upper horizon of a soil profile developing directly upon the mica schist, Paleozoic basement (N: 40o59′86.3″,E:23o12′62.0″, ) S-Pelite (1) Fig. 2), along a 20 m traverse. The garnets were examined Schist . ). The number in parenthesis indicates the generation of the garnet 1991 by means of infrared spectroscopy, X-ray diffraction and M electron microscopy in order to reveal their structure, com- 1991 a position and provenance. The textural properties and the bulk mineralogical composition of the host soils, along with 0.020 0.1300.020 0.002 0.020 0.058 0.012 0.034 0.007 0.050 0.014 0.060 0.000 0.090 0.000 0.030 0.060 0.155 0.010 0.000 0.15 0.03 0.02 0.02 0.03 0.23 0.03 0.29 0.01 0.000 0.1 0.178 0.000 0.12 0.016 0.18 0.00 0.06 0.01 0.14 0.01 0.05 Representative chemical composition (wt. %) of the core of the garnets from the basement rocks 37.280 37.460 37.06921.430 38.056 20.860 38.003 21.163 37.13036.500 20.894 39.070 32.840 21.376 38.790 33.591 20.730 37.590 32.490 21.690 37.649 29.599 22.010 25.950 21.160 37.86 24.200 21.282 24.900 28.240 37.45 20.40 27.255 37.42 39.08 20.64 22.31 39.96 20.90 37.679 21.73 28.33 38.472 21.77 28.39 36.57 21.364 23.78 37.48 21.146 22.43 37.82 21.48 35.650 37.60 21.49 29.237 21.20 23.55 20.82 22.55 27.69 24.71 semi-pelite, the petrography of the underlying mica schist, have been 3 3 t 2 2 presented in Georgiadis et al. (2012); in general, they are O O 2 2 Pelite From Kourou ( From Sidiropoulos ( - Al FeO Table 1 Oxides Pelite (1) SiO a b TiO Cr MnOMgO 1.050CaO 3.890 0.810Total 3.180 1.090 2.719 101.280 100.740 2.743 5.440 1.238 100.429 101.014 2.291 3.130 101.137 1.256 99.190 2.280 3.256 5.979 100.390 100.650 1.650 0.810 7.600 99.260 11.400 100.233 6.810 0.670 7.750 1.410 5.570 100.27 4.660 8.560 0.692 100.73 6.160 1.876 100.97 11.323 100.71 8.98 101.88 101.459 0.75 102.027 9.80 100.59 8.01 101. 2.28 8.03 3.98 2.36 0.58 3.81 6.42 1.17 8.87 6.87 2.007 9.28 3.180 2.531 1.580 0.827 9.11 9.620 1.06 9.55 8.70 0.90 5.13 9.36 1.66 6.50 7.59 1.16 9.71 S very poorly sorted to poorly sorted, arkosic gravelly muddy Using detrital garnets to determine provenance 191

Fig. 2 a Macroscopic sample of the mica schist immediately below the host soil of the garnets studied. b Photomicrograph of the mica schist; protectonic fractured garnet porphyroblasts among white mica and biotite flakes. Porphyroblastic texture. The horizontal dimension of the picture equals to about 2 mm. Nicol-. c Photomicrograph of the mica schist; garnet porphyroblast with inclusions among white mica flakes. The horizontal dimension of the picture equals to about 2 mm. Nicol-. d Photomicrograph of the host soil; fractured garnet idioblast. The horizontal dimension of the picture equals to about 2 mm. Nicol-

sands and gravelly muds, containing abundant detrital gar- The intact half-crystals were examined with FTIR mi- net and amphibole. croscope i-series Perkin-Elmer, connected with spectrom- Each garnet phenocryst was cut equatorially in two eter Spectrum 1000, Perkin-Elmer (Fig. 3b) and chemi- pieces: The first half of the crystal was crushed and pulver- cally with scanning electron microscopy (SEM-EDS); the ized in an agate mortar with the use of acetone to prevent air apparatus used was JEOL JSM-840A (SEM) and ISIS 300 access and Fe oxidation (Goncharov and Saltykova 2008), (ΕDS), with accelerating voltage 20 keV, intensity 0.4 mA whereas the other half was left intact. and pure Co as standard. A total of 108 chemical analyses The pulverized portion of the crystal, after homogeniza- were obtained. The average chemical composition of the tion was used for powder X-ray diffraction (PXRD) study garnets studied is presented in Table 2,whereasFig.4 (Fig. 3a). Powder X-ray diffraction was performed on a demonstrates representative rim-to-core compositions. In Philips PW 1710 diffractometer with Ni-filtered CuKα ra- the latter the CaO, MnO, MgO and FeOt wt.% of garnet diation; 0.1 g of garnet powder from each crystal were single crystals is shown. For the cation allocations in the scanned over the interval of 5–65o 2θ, with a step of crystal structure, the Fe2+/Fe3+ ratio was calculated as- 0.02o/5 s and at a scanning speed of 0.24o per minute. suming full occupancy.

Fig. 3 a X-ray pattern of garnet G11. b FTIR spectra of garnet G11 192 Table 2 Average chemical composition of the detrital garnets analyzed

Oxides (wt. %) G1 SD G2 SD G3 SD G4 SD G5 SD G6 SD G7 SD n6575888 SiO2 38.02 0.12 38.40 0.45 37.94 0.30 36.82 0.18 38.86 0.72 37.64 0.37 37.50 0.67 TiO2 nd – nd – nd – nd – nd – 0.08 0.22 0.18 0.35 Al2O3 20.84 0.54 21.56 0.59 21.43 0.19 15.34 1.15 22.01 0.39 21.21 0.35 19.95 4.07 Cr2O3 nd – nd – nd – nd – nd – nd – nd – FeOt 27.56 2.61 20.37 3.47 29.89 1.00 30.52 2.52 18.42 2.44 25.48 3.77 24.62 8.71 MnO 1.08 0.90 2.78 2.82 0.26 0.45 5.45 2.18 3.10 2.41 4.39 3.47 4.26 3.01 MgO 4.10 1.17 3.70 3.03 4.39 1.11 2.03 0.55 5.89 3.99 2.23 1.35 2.04 0.55 CaO 8.16 2.76 12.88 4.25 6.03 0.63 9.46 1.59 11.46 4.69 8.76 1.66 11.28 2.55 Total 99.77 99.69 99.94 99.62 99.72 99.80 99.82 Members (%) Schorlomite-Al 0.09 0.54 Morimotoite 0.29 Spessartine 2.41 6.12 0.57 12.53 6.74 9.87 9.58 Pyrope 16.07 14.35 17.19 8.23 22.53 8.83 8.07 Almandine 58.51 43.52 65.02 51.70 38.97 56.38 50.16 Grossular 19.88 34.99 16.50 1.15 31.27 24.38 25.09 Andradite 3.12 0.91 0.50 26.37 0.24 0.15 6.45 Cation allocation Si 3.000 2.997 2.993 2.999 2.993 3.011 2.986 Al 0.000 0.003 0.007 0.001 0.007 0.000 0.014 T (IV) 3.000 3.000 3.000 3.000 3.000 3.011 3.000 Al 1.937 1.980 1.985 1.472 1.990 2.000 1.858 Fe3+ 0.063 0.024 0.022 0.528 0.017 0.000 0.135 Ti 0.000 0.000 0.000 0.000 0.000 0.005 0.011 O (VI) 2.000 2.004 2.007 2.001 2.007 2.005 2.003 Mg 0.482 0.430 0.516 0.247 0.676 0.149 0.242 Fe2+ 1.755 1.306 1.950 1.551 1.169 1.689 1.505 Mn 0.072 0.184 0.017 0.376 0.202 0.241 0.287 Ca 0.690 1.077 0.510 0.826 0.945 0.889 0.963 C (VIII) 2.999 2.997 2.993 2.999 2.993 2.968 2.997

Oxides (wt. %) G8 SD G9 SD G10 SD G11 SD G13 SD G14 SD G15 SD n86 101565 5

SiO2 40.86 1.50 38.86 0.27 37.73 0.34 37.42 0.38 36.32 0.30 37.54 0.48 38.23 0.18 ..Gogai tal. et Georgiadis I.K. TiO2 0.35 0.98 nd – nd – nd – nd – 0.16 0.36 0.06 0.13 Al2O3 23.29 1.29 21.63 0.31 21.16 0.81 21.40 0.51 12.14 1.09 21.06 0.53 21.32 0.25 Cr2O3 nd – nd – nd – nd – 0.10 0.25 nd – nd – FeOt 4.54 5.65 19.94 0.82 22.93 3.49 23.44 6.44 32.80 4.50 24.27 2.21 18.49 2.74 MnO 1.29 3.66 1.34 1.08 4.71 3.12 5.16 4.63 6.37 4.27 6.02 2.39 4.04 2.40 MgO 12.42 3.74 5.23 1.58 2.13 1.67 1.73 1.01 2.00 1.36 2.59 1.18 2.43 1.47 sn ertlgrest eemn provenance determine to garnets detrital Using Table 2 (continued)

Oxides (wt. %) G8 SD G9 SD G10 SD G11 SD G13 SD G14 SD G15 SD

CaO 17.04 3.38 12.79 1.89 11.05 3.17 10.51 2.88 9.86 1.86 8.16 1.36 15.17 2.12 Total 99.79 99.79 99.71 99.65 99.58 99.81 99.75 Members (%) Schorlomite-Al 0.96 0.48 0.18 Morimotoite 2.92 11.68 11.60 14.83 13.53 8.93 Spessartine 2.64 20.06 8.36 6.85 8.20 10.25 9.46 Pyrope 44.79 41.74 48.11 50.99 35.74 52.96 39.09 Almandine 7.88 33.60 29.31 29.89 21.50 40.58 Grossular 42.55 1.65 1.86 29.04 1.22 1.67 Andradite 0.66 12.03 Cation allocation Si 2.965 2.999 2.993 2.980 2.964 2.988 2.994 Al 0.035 0.001 0.007 0.020 0.036 0.012 0.006 T (IV) 3.000 3.000 3.000 3.000 3.000 3.000 3.000 Al 1.957 1.966 1.971 1.989 1.131 1.964 1.961 Fe3+ 0.040 0.035 0.036 0.031 0.990 0.028 0.038 Ti 0.019 0.000 0.000 0.000 0.000 0.010 0.003 O (VI) 2.016 2.001 2.007 2.020 2.121 2.002 2.003 Mg 1.344 0.602 0.252 0.205 0.243 0.308 0.284 Fe2+ 0.236 1.252 1.485 1.530 1.248 1.588 1.172 Mn 0.079 0.087 0.317 0.348 0.440 0.406 0.268 Ca 1.325 1.057 0.939 0.897 0.862 0.696 1.273 C (VIII) 2.984 2.999 2.993 2.980 2.879 2.998 2.997

2+ FeOt total iron as Fe , n number of analyses, nd not detected and IV, VI, VIII coordination numbers 193 194 I.K. Georgiadis et al.

Fig. 4 Representative rim to core oxide compositions (wt. %) of the detrital garnet crystals

Results and discussion show the sharp absorbance at approximately 3,500 cm−1, designating them as not hydroxyl-ones. This observation is The a(Å) values of the garnets vary between spessartine also consistent with their unit cell length along (420) which is (11.621 Å) and almandine (11.526 Å) (Deer et al. 1982). always <2.65 Å (Meagher 1980). The vibrational spectra were

The same is true for their d(420) values (2.5891 Å to not further used for band assignment (e.g. Moore et al. 1971; 2.5974 Å). The FTIR spectra of the analyzed garnets do not Hunt et al. 1973). Using detrital garnets to determine provenance 195

Under the electron microscope some garnets revealed pyrope≤15 %. For the amphibolites of the Vertiskos Unit, fractures filled with secondary minerals. Inclusions were the same author found 59 %≤almandine≤66 %, 16 %≤ also found, namely ilmenite (in garnets G3, G4, G6, G13 grossular≤33 % and 6 %≤pyrope≤17 %. and G14), quartz (in G4), rutile (in G4, G6 and G14), According to these criteria and Table 2, crystal G1 may be plagioclase (in G4, G6 and G13), sphene (in G4) and zircon derived from the amphibolites of the basement, crystal G3 (in garnet G14), Fig. 5. may belong to the first garnet generation from the mica schist, The structural formula of the analyzed garnets was calcu- and crystal G6 belongs to the second garnet generation from lated according to Locock (2008) on the basis of 12 oxygen the mica schist. The rest of the crystals cannot be assigned to a atoms (Table 2). Based on the chemical composition, the protolith. It is obvious that, from the end-member composition crystals studied are mainly of almandinic composition, where- of the garnets based on their average chemical composition, as compositionally pyrope and spessartine are present too, only general assumptions can be made about their protoliths. along with some rare garnet members, namely morimotoite, In general, it seems that the use of average chemical compo- 2+ 2+ 3+ Ca3(TiFe )Si3O12, skiagite, (Fe3 )(Fe2 )Si3O12 and alumi- sition of a notably zoned mineral such as garnet should be num skorlomite, Ca3Ti2(SiAl2)O12. Chromium was not used only as a qualitative tool. detected in their composition, with the exception of crystal Due to the complex metamorphic evolution of the studied G13 (wt. 0.60 % in the outer rim). According to Deer et al. area, only the cores of the studied garnets (Table 3) were (1998), almandine garnet is typical of garnet bearing schists compared to the previous mentioned criteria (i.e. the core produced by the regional progressive metamorphism of argil- composition in FeOt, MgO, CaO, almandine, grossular and laceous sediments. Also, the same authors note that almandine pyrope). The chemical composition of the cores of zoned or more rarely almandine-pyrope garnets are found in rocks garnets are considered to be as close as possible to the peak metamorphosed to the granulite facies. metamorphic conditions, whereas rims normally record Kourou (1991) studied the basement rocks of the SMM post-peak metamorphic processes (e.g. Harlov et al. 2006). and found that the first garnet generation has FeOt>27 %, According to the chemical criteria of Kourou (1991)and MgO>2 % and CaO<6 %, whereas the second generation Table 3,garnetsG2,G6,G7,G10,G11andG13canbe shows FeOt<27 %, MgO<1.5 % and CaO>7 %. considered to belong to the second garnet generation from the Sidiropoulos (1991) distinguished the garnets based on their basement rocks. For the remaining ones, none definitely meet end member content (in wt.%), as follows; the first genera- the chemical criteria that would have allowed them to be attrib- tion garnets of the mica schist has 63 %≤almandine≤85 % uted to the first garnet generation. Based on the end member (with mode≥75 %), 1 %≤grossular≤27 % (with mode≤ composition according to Sidiropoulos (1991)andTable3, 15 %) and 6 %≤pyrope≤16 %. The second garnet genera- garnet G3 may belong to the first garnet generation of the mica tion of the mica schist contains 49 %≤almandine≤72 % schist or it is derived from the amphibolites and garnet G14 (with mode≤75 %), 22 %≤grossular≤25 % and 3 %≤ belongs to the second garnet generation of the mica schist.

Fig. 5 SEM photomicrographs of the garnets studied. a, b inclusions of feldspar (f), ilmenite (ilm), quartz (q) and amphibole (am) in crystal G13. c, d inclusions of zoisite (zo), zircon (zr), rutile (rt), ilmenite (ilm) and sphene (ti) in crystal G14 196 I.K. Georgiadis et al.

Table 3 Chemical composition of the core of the detrital garnets analyzed

Oxides (wt. %) G1 G2 G3 G4 G5 G6 G7 G8 G9 G10 G11 G13 G14 G15

SiO2 37.94 38.15 37.92 36.67 37.97 37.22 37.57 38.38 38.72 37.47 37.41 36.37 36.77 38.07

TiO2 nd nd nd nd nd 0.63 nd nd nd nd nd nd 0.81 0.29

Al2O3 21.20 20.65 21.43 14.69 21.71 21.03 21.19 21.95 21.48 21.56 21.39 12.88 21.03 21.21

Cr2O3 nd nd nd nd nd nd nd nd nd nd nd nd nd nd

FeOt 25.95 16.12 29.83 30.21 16.37 18.71 20.63 14.69 20.09 21.17 14.34 26.73 22.40 16.22 MnO 1.98 5.72 1.05 6.30 5.82 10.45 8.60 10.34 2.03 6.65 13.50 10.44 7.09 5.89 MgO 2.83 1.48 3.53 1.99 1.92 1.49 1.09 3.68 4.20 1.13 nd 1.50 2.18 1.71 CaO 9.89 17.49 6.37 9.62 15.34 10.57 11.05 11.13 13.53 11.64 13.05 11.47 8.94 16.53 Total 99.80 99.62 100.12 99.47 99.13 100.10 100.14 100.17 100.05 99.61 99.69 99.38 99.22 99.93 Members (%) Schorlomite-Al 1.88 2.44 0.85 Majorite 0.09 0.06 Spessartine 4.43 12.69 2.34 14.54 12.95 23.49 19.33 22.75 4.44 14.96 30.49 24.23 16.06 13.03 Pyrope 11.02 5.80 13.80 8.07 7.51 5.91 4.32 14.23 16.15 4.47 6.12 8.69 6.66 Almandine 56.52 32.37 65.80 48.06 35.99 40.54 44.58 31.72 41.92 46.99 31.97 31.65 49.62 34.03 Grossular 26.87 44.75 17.99 43.19 26.76 30.55 30.96 35.40 33.13 37.27 22.55 43.34 Andradite 1.07 4.36 0.01 28.09 1.39 0.86 2.02 33.69 0.61 2.07 Skiagite 1.18 4.02 Cation allocation Si 3.002 2.999 3.001 2.998 2.993 2.962 2.989 2.990 2.998 2.986 2.992 2.991 2.950 2.983 Al 0.000 0.001 0.000 0.002 0.007 0.038 0.011 0.010 0.002 0.014 0.008 0.009 0.050 0.017 T (IV) 3.002 3.000 3.001 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 Al 1.977 1.913 1.999 1.413 2.010 1.935 1.976 2.005 1.959 2.012 2.008 1.240 1.938 1.942 Fe3+ 0.020 0.088 0.000 0.589 0.000 0.028 0.034 0.006 0.043 0.002 0.000 0.769 0.015 0.041 Ti 0.000 0.000 0.000 0.000 0.000 0.038 0.000 0.000 0.000 0.000 0.000 0.000 0.049 0.017 O (VI) 1.997 2.001 1.999 2.002 2.010 2.000 2.011 2.010 2.002 2.014 2.008 2.009 2.002 2.000 Mg 0.334 0.173 0.416 0.243 0.226 0.177 0.129 0.427 0.485 0.134 0.000 0.184 0.261 0.200 Fe2+ 1.697 0.972 1.974 1.476 1.079 1.217 1.338 0.951 1.258 1.409 0.959 1.069 1.488 1.022 Mn 0.133 0.381 0.070 0.436 0.389 0.704 0.580 0.682 0.133 0.449 0.915 0.727 0.482 0.391 Ca 0.838 1.473 0.540 0.843 1.296 0.901 0.942 0.929 1.123 0.994 1.118 1.011 0.768 1.388 C (VIII) 3.002 2.999 3.001 2.998 2.989 3.000 2.989 2.990 2.998 2.986 2.992 2.991 2.998 3.000

2+ FeOt Total iron as Fe , nd not detected and IV, VI, VIII coordination numbers

In a more precise way, we can select and compare only iron and very high calcium concentration in their core the chemical composition and end member composition of (namely G2, G5, G6, G8, G9, G11 and G15) were not the cores of the garnets studied (Table 3) with the core attributed to any protolith. The use of their core end member composition of the garnets from the basement, by compiling composition is of great interest; the origin assigned previ- data from Kourou (1991) and Sidiropoulos (1991). The end ously to garnets G1, G3, G4, G7, G10, G13 and G14 is members from the latter sources were recalculated according verified, plus the remaining garnets can be fairly assigned to a to Locock (2008), Table 4. According to the chemical protolith mainly by the use of their spessartine and pyrope criteria, garnets G1 and G4 belong to the second garnet content, aided by their core’s almandine and grossular content. generation of the semi-pelites, garnets G7, G10 and G13 In this way, garnet G2 probably belongs to the second garnet belong to the second garnet generation of the garnetiferous generation of the semi-pelites or the garnetiferous migmatitic migmatitic gneisses, garnet G3 belongs to the first garnet gneisses, garnets G5, G11 and G15 belong to the second generation of the pelites and garnet G14 belongs either to generation of the garnetiferous migmatitic gneisses, G6 prob- the second garnet generation of the semi-pelites, or to the ably belongs to the second garnet generation of the semi- second garnet generation of the garnetiferous migmatitic pelites, G8 probably belongs to the second garnet generation gneisses. The rest of the garnets that demonstrate very low of the mica schist and G9 can be attributed to meta-eclogites. sn ertlgrest eemn provenance determine to garnets detrital Using

Table 4 Chemical (wt. %) and end member composition of the core of the garnets from the basement rocks of the study area

S-Pelite (1)a Pelite (1)a M. Schist (1)b G.M.Gneiss (1)b S-Pelite (2)a M. Schist (2)b

Min Max Min Max Min Max Min Max Min Max Min Max

FeOt 32.49 33.591 29.2 36.62 22.31 26.78 34.52 22.369 35.65 22.55 23.55 MgO 2.291 2.743 1.655 3.89 0.75 1.26 3.79 0.571 3.75 0.90 1.06 CaO 3.13 5.979 0.92 9.39 9.80 1.38 9.00 1.58 10.526 8.70 9.36 MnO 1.238 2.719 0.81 3.03 8.98 1.31 10.09 1.10 9.36 9.11 9.55 Spessartine 2.77 % 6.15 % 1.81 % 6.86 % 20.30 % 2.96 % 22.66 % 2.47 % 21.30 % 20.52 % 21.30 % Pyrope 6.71 % 10.92 % 5.11 % 15.32 % 4.00 % 14.93 % 2.24 % 14.81 % 3.53 % 4.20 % Almandine 71.90 % 72.97 % 62.25 % 79.37 % 49.73 % 58.20 % 74.31 % 49.24 % 78.16 % 47.81 % 47.97 % Grossular 8.35 % 16.35 % 1.24 % 26.05 % 26.21 % 3.20 % 23.43 % 4.23 % 29.68 % 24.14 % 25.44 % n2 6 1 138 2

G.M.Gneiss (2)b Am. Ecl. (1)b Amphib. (1)a Eclogitea M-Ecl.a

Min Max Min Max Min Max Min Max Min Max

FeOt 20.32 28.09 22.43 23.78 25.95 30.829 23.326 24.9 27.255 28.936 MgO 0.85 2.72 6.42 6.87 1.65 4.042 5.302 6.81 1.876 4.66 CaO 4.43 9.90 8.87 9.28 5.973 11.40 7.75 10.308 6.16 11.323 MnO 4.08 11.36 0.58 1.17 1.227 4.957 0.67 1.056 0.154 1.41 Spessartine 9.03 % 25.54 % 1.26 % 2.50 % 2.70 % 11.07 % 1.46 % 2.29 % 0.34 % 3.16 % Pyrope 3.36 % 10.60 % 24.46 % 25.02 % 6.57 % 14.61 % 19.30 % 25.53 % 7.37 % 18.36 % Almandine 43.85 % 60.51 % 47.26 % 49.99 % 54.88 % 65.06 % 49.47 % 53.52 % 58.66 % 62.79 % Grossular 11.73 % 26.59 % 21.93 % 22.33 % 15.05 % 29.87 % 19.91 % 27.29 % 15.89 % 30.48 % n153 6 3 4 a From Sidiropoulos (1991) b From Kourou (1991) 2+ FeOt Total iron as Fe , n number of cores analyzed. Abbreviations as in Table 1 197 198 I.K. Georgiadis et al.

It is worth noting that even when using abundant refer- Most of the garnets studied also demonstrate zoned compo- ence data concerning exclusively the study area, the assign- sition from rim to core (Fig. 4). In general, Mn is depleted from ment of the studied detrital garnets to a protolith did not lead the core to the rim. The general compositional pattern observed, to definite results by the methods employed above. is that Fe and Mg concentrations increase towards the rim of the Suggate and Hall (2013) using a multi-stages procedure crystals, whereas Ca and Mn concentrations increase towards and two triangular plots on over 2500 garnet analyses gleaned the core. This suggests a growth zoning and not a diffusion one. from the literature, have shown that the garnet composition This kind of zonation also suggests that the garnet crystal formed can be linked to both a protolith and metamorphic grade. during one metamorphic episode (Tracy 1982). The presence of According to their methodology, the end-member composi- growth zonation in the garnet crystals indicates that these were tion of the cores of the detrital garnets in the present study formed below 650 °C, or never heated over this temperature implies an origination from metapelites or metabasites of after their formation (Woodsworth 1977; Sidiropoulos 1991). amphibolite facies (Fig. 6). The crystals G2, G4, G5, G11, Also present are garnets with a rather homogenous chemical G13 and G15 show anomalous behaviour, failing to cluster composition, as well as garnets with abrupt differentiations in consistently in both triangular plots. This feature, in conjunc- their composition; the former may have derived from higher tion with the morphological observations already made grade metamorphic rocks or they were homogenized during (Figs. 2 and 5), indicates possible alteration effects. retrograde metamorphism (e.g. the first garnet generation), The end member composition of the crystals studied was whereas the latter must record multiple metamorphic events. not utilized furthermore, since this paper is concerned main- The reader is referred to the works of Meagher (1980)and ly with the raw data provided by SEM chemical analysis Tracy (1982) for a discussion on this topic. Finally, the crystal (e.g. Rickwood 1968; Knowles 1987; Locock 2008). size does not necessarily correlate with their degree of zonation, i.e. the bigger the crystal, more zones are present. The scheme for determining the metamorphic grade of the garnets according to Nandi (1967)ispresentedinFig.7.This scheme is based on the use of the values (CaO + MnO) and

(FeOt + MgO); with increasing pressure and temperature, the garnet crystal is unable to accommodate in its structure cations with larger radius (i.e. Ca2+ and Mn2+), accommodating instead smaller ones (i.e. Fe2+ and Mg2+).Thedataforthegarnetsofthe basement are incorporated from Kourou (1991), Sidiropoulos (1991), Kilias et al. (1999) and Meinhold (2007), whereas those of the garnet cores studied here are incorporated from Table 3. Kilias et al. (1999) provided three garnet analyses from the eastern border of the Vertiskos Unit, two from mica schist (plotting both in the kyanite field, Fig. 7a) and one from a two-mica gneiss (plotting in the sillimanite field, Fig. 7a). The data from Meinhold (2007) refer to four different garnet crystals in orthogneiss (garnet-biotite gneiss). In Fig. 7a the core composition of the garnets studied is plotted against all the compositions of the garnets from the basement rocks. It can be deduced that the garnets G1, G3, G4 and G13 began crystallizing at higher metamorphic conditions, contrary to all others who began crystallizing at lower metamorphic conditions. In Fig. 7b and c the garnet cores studied are plotted against the bulk composition of the garnets from the basement rocks. Again, since the latter are poly- metamorphosed, no definite assumption can be made about the origin of these garnets. It is obvious that there is an overlap between the compositions of first and second garnet generation of the basement. Fig. 6 Two triangular plot of the cores of the garnets studied according In contrast, when we plot the rim-to-core analyses of garnets to Suggate and Hall (2013). 1=granites, 2=calc-silicates, skarns and they immediately are divided into five groups (namely I to V, rodingites, 3=amphibolites, granulites, blueschists, ultrabasic rocks with <55 % pyrope and sub-ophiolitic rocks, 4=Ultrabasic rocks with Fig. 7d to h respectively): The group I garnet (namely G15, >55 % pyrope and 5=blueschists Fig. 7d) represent low metamorphic prograde conditions. From Using detrital garnets to determine provenance 199

Fig. 7 Rim-to-core analyses of the garnets studied plotted against (1991), Kilias et al. (1999) and Meinhold (2007). See text for those of the basement rocks, according to Nandi (1967). Data for details. Abbreviations as in Table 1 the garnets of the basement are from Kourou (1991), Sidiropoulos 200 I.K. Georgiadis et al.

Fig. 7c it is seen that the second garnet generation of the mica conditions and their compositional variation resembles that schist and the garnet from garnet-biotite gneiss plot in the same of the second garnet generation from the garnetiferous field and with the same trend. By correlating these two features migmatitic gneisses (Fig. 7c). we can attribute as a protolith to these two garnets the mica In a similar manner, group III garnets (namely G1, G3, schist of the basement or the orthogneiss. G4, and G13, Fig. 7f) represents medium to high metamor- The group II garnets (namely G2, G5, G9, G10 and G14, phic prograde conditions and resembles the first garnet Fig. 7e) represents low to medium metamorphic prograde generation of the pelites.

Fig. 8 Spider diagrams of the garnets studied. Abbreviations as in Table 1 Using detrital garnets to determine provenance 201

Group IV garnets (namely G11, Fig. 7g) plots throughout the grade diagram and represents prograde metamorphic c conditions, resembling mostly the second garnet generation of the semi-pelites and to a lesser extent the first garnet generation of the mica schists and the garnetiferous Orthogneiss migmatitic gneisses (Figs. 7b and 8c). Finally, the group V garnets demonstrates an anomalous metamorphic evolution and thus special care is taken (namely b G6, G7 and G8, Fig. 7h). This is probably due to the influence G.M. Gneiss (2) of multiple metamorphic events. Of these, G6 resembles the path of the garnets from the meta-eclogites, G7 resembles the

path mostly of the second garnet generation of the semi-pelites b

(Fig. 7c) and G8 plots only in the field of garnet, as do the M. Schist (2) second garnet generation of the mica schist. The method of comparison just outlined seems promising, since it is descriptive in unfolding the metamorphic history of a Pelite – (2) a garnet crystal, utilizing rim-to-core chemical analyses, pro- S vided that adequate data from the potential protoliths are readily at hand. By applying it, uncertainty on their origin remain only for garnets G6, G7, G8, G11 and G15. b

In order to determine the origin of these crystals, their core Am. Ecl. (1) chemical composition was compared to the average core chemical composition of the garnets from the basement, Tables 3 and 5, using spider diagrams (Fig. 8). The amount b G.M. Gneiss (1) 37.450.04 0.30 39.4221.04 0.04 0.47 0.270.01 0.35 37.657 21.6530.48 0.571 0.03 0.01 37.025 0.17 0.070 0.05 2.55 21.191 0.643 23.13 37.64 0.067 0.233 0.05 0.150 21.485 0.68 0.007 0.66 29.769 0.007 38.15 0.042 21.10 0.008 4.273 0.09 0.005 23.050 0.25 0.707 0.25 21.65 0.03 0.007 25.58 0.06 0.01 2.31 0.04 20.30 0.02 0.05 0.03 1.12 0.02 5.30 2.46 0.89 0.30 4.626 3.242 9.330 0.311 6.56 2.27 2.95 1.32 2.26 0.73 6.64 0.23 1.875 1.106 0.980 0.113 1.31 0.46 1.21 0.25 of Cr2O3 was omitted from the ratios, since it was not detected 4.29 2.40 9.04 0.21 5.798 3.721 9.030 0.467 8.62 1.33 14.01 0.30 in the core of the detrital garnets under study (Table 3). In this – – – – – – – way, G7 and G11 seem to belong to the second garnet gener- – ation from the mica schists and G8 to the second garnet b generation from the semi-pelites. Garnets G6 and G15 are M. Schist (1) more problematic with respect to their attribution to a protolith; their TiO2 content is closer to that of the garnets from amphibolitized eclogites, but their MnO content resembles that a of the second garnet generation from the mica schist, the M-Ecl. orthogneiss and the first garnet generation from the garnetiferous migmatitic gneiss. Considering only MnO, G6 belongs to the mica schist and G15 to the garnetiferous migmatitic gneiss. a Müntener et al. (2000) utilized a binary plot of Mg vs. Ca atoms in the garnet formula, to demonstrate the metamorphic Eclogite path it underwent. In a similar manner, the distribution of the 1 garnets from the basement is presented in Fig. 9, along with the composition of the cores of the garnets studied. Only the Ca, Mg and Fe2+ atoms that occupy the distorted cube positions of a Amphib. (1) the garnet structure are utilized. From Fig. 9a and b it is shown that there is a major compositional overlap among the garnets of the semi-pelites, pelites, mica schists, garnetiferous ) migmatitic gneisses, orthogneisses, amphibolites and meta- a S-Pelite (1) ). Abbreviations as in Table ) eclogites of both generations. Also, the garnets from eclogites 1991

and amphibolitized eclogites plot together. Due to this fact, the 2007 garnets from the semi-pelites, pelites, mica schists, garnetifer- 1991

ous migmatitic gneisses, amphibolites and meta-eclogites are a Average chemical composition (wt. %) of the core of the garnets from the basement rocks grouped together in this study (cluster i in Fig. 9). In the same Pelite (1) 37.367 0.1130.069 37.56321.143 0.048 0.698 0.030 38.037 0.4150.009 21.029 0.539 0.04033.496 39.014 0.009 0.190 0.052 0.010 21.360 3.505 0.202 33.041 0.036 0.560 37.924 0.004 0.076 21.816 0.779 0.015 0.454 28.605 37.86 0.171 0.015 0.015 1.748 21.295 0.084 0.030 24.142 0.207 20.40 0.055 0.789 0.030 0.15 28.292 0.009 0.751 22.31 0.008 0.03 3 3 t 2 way, group ii is comprised of the garnets from the eclogites and 2 O O 2 2 From Kourou ( From Sidiropoulos ( From Meinhold ( Table 5 4113382154 Oxides Average SDn6263 Average SDSiO Average SD Average SD Average SD Average SD Average SD Average SDa Average SDb Averagec SD Average SD Average SD TiO Al Cr FeO MnO 1.405 0.822 1.979 1.047 2.194 1.433 0.845 0.195 0.685 0.532 8.98 MgO 2.980 1.029 2.517 0.320 3.061 0.878 5.894 0.805 3.290 1.274 0.75 CaO 4.255 3.770 4.555 2.015 7.723 2.000 8.873 1.307 8.621 2.249 9.80 amphibolitized eclogites (cluster ii in Fig. 9). Total 100.723 100.721 101.047 100.690 100.199 100.28 100.88 101.09 100.993 101.055 100.92 98.37 202 I.K. Georgiadis et al.

Fig. 9 Binary diagrams of the garnets studied against the garnets from the basement. Fe2+, Ca and Mg are atoms per formula unit, occupying only the distorted cube positions, calculated on the basis of 12 oxygens. i=pelitic and amphibolitic garnets and, ii=eclogitic garnets. Symbols as in Fig. 7

The approach adopted lacks the ability to distinguish be- eclogitic grade metamorphism in their composition are ad- tween the two garnet generations (Fig. 9). Also, due to the equately distinguished. On the same diagram four garnet complex metamorphic evolution of Vertiskos Unit, great com- fields are marked in relation to their weathering protoliths positional overlap of the garnets exists, among the various (Morton et al. 2003, 2004). In field A (high-Mg and low-Ca) lithologic types. Only the garnets hosted in eclogites are plot the garnets that originate from high-grade (granulite adequately distinguished. As a result, only broad garnet com- facies) metasedimentary or charnockitic rocks; field B positional fields may be outlined, representing metamorphic (low-Mg) represents the composition of garnets that origi- conditions rather than potential protoliths. Given also that the nate from low to moderate grade (amphibolite facies) basement of the Unit has undergone both prograde and retro- metasediments and gneisses; field C (high-Mg and high- grade metamorphism, this finding is of rather limited value. Ca) represents the garnets originating from orthogneisses, When using the ternary diagram of (Fe2++Mn)-Mg-Ca basic gneisses and eclogites and in field D (Fe3+-Ca garnets) (e.g. Di Giulio et al. 1999), Fig. 10, the garnets that recorded plot the andraditic garnets from skarns and low-grade matabasic rocks. This classification scheme (Fig. 10), in general, coincides with the garnet compositions from the basement rocks; indeed, the garnets of the semi-pelites, pelites, mica schists and gneisses plot smoothly into field B. The same is also true for the eclogitic garnets in field C. As it was expected, no garnet fell into field D, further validating this classification scheme. Yet, it was not possible to distinguish between the amphibolitic and pelitic garnets from the basement of the Unit. The multiple stages of metamorphism leaving their prints on their composition certainly did not help. Nevertheless, the four general groups of protoliths are fully functional and this ternary diagram could easily be the first step to disclose the provenance of garnet, Fig. 10 (Fe2++Mn)-Mg-Ca ternary diagram. Fe2+, Mn, Ca and Mg are atoms per formula unit, occupying only the distorted cube positions, especially when no adequate data exist for the garnet calculated on the basis of 12 oxygens. i=pelitic garnets, ii= composition in the basement rocks. amphibolitic garnets, iii=eclogitic garnets and iv=garnets from the In Fig. 10,onlysamplesG8andG9plotinfieldC; orthogneiss. A=high-Mg and low-Ca garnets, B=low-Mg garnets, garnet G8 presents an anomalous core composition and C=high-Mg and high-Ca garnets and D=Ca-garnets with Fe3+ substi- tution in the trivalent site (Morton et al. 2003). Symbols as in Fig. 7. only in conjunction with Figs. 7 and 8, was possible to See text for discussion match it to a protolith, namely the mica schist. The core Using detrital garnets to determine provenance 203

In the same manner, crystal G9 was previously associated to the garnetiferous migmatitic gneiss. This rock never underwent eclogitic facies metamorphism, as well as the mica schist. Again, this core composition (high-Mg and high-Ca) must be attributed to the chemical composition of the original material consumed for the formation of this garnet crystal. But if we accept that garnet G9 is related to the garnetiferous migmatitic gneiss, another possibility also occurs; Sidiropoulos (1991) mentioned that some of the rock samples, demonstrate ortho-character (i.e. igneous protoliths). If this is the case, then garnet G9 correctly fell in the field C (Fig. 9), since it also incorporates orthogneisses. As a final attempt, the statistical method of principal component analysis was utilized: The rock type for a sample is determined by a vector of length 8, containing the con-

centrations in the measured oxides (namely SiO2,TiO2, Fig. 11 The principal components and their capturing of sample’s Al2O3,Cr2O3, FeOt, MnO, MgO and CaO). However, the variance relative influence of those parameters, and their correspond- ing classification power, can be largely different. To that composition most likely does not represent the peak end, a Principal Component analysis (PCA) was performed, metamorphic event: It rather demonstrates that the nu- in order to identify a set of new parameters, namely the cleation of this crystal commenced by consuming a principal components, which can serve as a better basis for mineral phase or assemblage rich in Ca and Mg. This characterisation and classification of unknown samples is further supported by the fact that these mica schists (Jolliffe 2010). The principal components are linear combina- are found to be of para-origin, probably derived by the tions of the original (initial) parameters, ordered by their metamorphism of muds and arkoses; these are generally significance. Moreover, the transformed parameters are line- minerallogically immature rocks, since they contain arly uncorrelated. Figure 11 indeed shows that the first two feldspars and ferromagnesian minerals (Pettijohn et al principal components (PC1, PC2) can capture most of the 1973;Folk1974). sample’s variance. Firstly, the measured concentrations for

Fig. 12 Representation of the samples on the (PC1, PC2) plane 204 I.K. Georgiadis et al.

Table 6 Presentation of protoliths assigned to each garnet crystal following the different methods described in the text

Garnet 1 2 3 4 5 6 7

G1 Amphib. (1) ? S-Pelite (2) S-Pelite (2) Pelite (1) – Gneiss G2 ? (2) ? S-Pelite (2) or G.M.Gneiss (2) – ? G.M.Gneiss (2) G3 M. Schist (1) M. Schist (1) or Pelite (1) Pelite (1) Pelite (1) – Gneiss Amphib. (1) G4 ? ? S-Pelite (2) S-Pelite (2) Pelite (1) – ? G5 ? ? ? G.M.Gneiss (2) G.M.Gneiss (2) – Orthogneiss G6 M. Schist (2) (2) ? S-Pelite (2) M-Ecl. ? M. Schist ? G7 ? (2) G.M.Gneiss (2) G.M.Gneiss (2) S-Pelite (2) ? M. Schist (2) Gneiss G8 ? ? ? M. Schist (2) M. Schist (2) ? S-Pelite (2) Orthogneiss G9 ? ? ? M-Ecl. G.M.Gneiss (2) – ? G10 ? (2) G.M.Gneiss (2) G.M.Gneiss (2) G.M.Gneiss (2) – Gneiss G11 ? (2) ? G.M.Gneiss (2) S-Pelite (2) ? M. Schist (2) ? G13 ? (2) G.M.Gneiss (2) G.M.Gneiss (2) Pelite (1) – ? G14 ? M. Schist (2) S-Pelite (2) or S-Pelite (2) or G.M.Gneiss (2) – ? G.M.Gneiss (2) G.M.Gneiss (2) G15 ? ? ? G.M.Gneiss (2) M. Schist or G.M.Gneiss ? Orthogneiss ?

1=according to chemical and end member criteria from Kourou (1991) and Sidiropoulos (1991) using the average composition of the detrital garnet, 2=according to chemical and end member criteria from Kourou (1991) and Sidiropoulos (1991) using the core composition of the detrital garnet, 3=comparison of core composition between the detrital garnets and the garnets from the basement rocks, 4=comparison of core end member composition between the detrital garnets and the garnets from the basement rocks, 5=utilizing Nandi (1967), 6=using spider diagrams and 7=using PCA; here the term “gneiss” includes all the pelites, schists, migmatitic gneisses and amphibolitic rocks. Abbreviations as in Table 1 each oxide are scaled by subtracting the mean value and Conclusions dividing by the standard deviation. This scaling is conducted in order to eliminate the imbalance among From all the methods employed above, it is obvious that by the 8 individual variances. Then, the PCA algorithm is the use only of the major oxides composition of the detrital performed (R Core Team 2012), and the principal com- garnet crystals, no conclusive results could be reached with ponents are obtained. The first two components capture respect to their provenance. This is especially true about the 58 % of the total variance; this percentage can be generation of garnet. But by combining all the previous viewed as the extent to which a representation of the methods, the amphibolites and eclogites were easily distinct samples on the (PC1, PC2) plane can depict the differ- and readily excluded as sources of the garnets. This is not ent rock types as separate groups of points. This repre- the case for the pelitic and gneissic rocks, which demon- sentation is shown in Fig. 12. Furthermore, the fourteen strate great compositional overlapping probably due to the samples of unknown type are also placed on the graph. complex metamorphic history they have undergone. From Fig. 12 it can be seen that the chemical affinities We can then minimize the source of the garnets studied to documented in the composition of the cores of the garnets the pelites, schists and gneisses comprising the basement (Figs. 9 and 10) continue to exist; again all garnets from the rocks of the Unit. The rocks mentioned also comprise the pelites, mica schists, garnetiferous migmatitic gneisses, am- bulk of the exposed basement, with amphibolites and phibolites and meta-eclogites, plot together. These from eclogites being scarce. The reference data along with the eclogites and amphibolitized eclogites continue to plot dis- newly obtained on garnets, also suggest this conclusion. tinctly. What this approach manages to do though, is to plot The assignment of a weathered protolith to a detrital separately those from the orthogneisses. Again, this ap- garnet solely from the latter’s chemical composition, de- proach is not capable of distinguishing between the two mands abundant data on the garnet compositions of the generations of garnets. basement. Even if this is the case, special care is needed All the above described methods are presented in regarding the metamorphic evolution of the probable weath- Table 6. The methods that resulted in Figs. 9 and 10 ered area-source of the detritus. Another factor of crucial are not incorporated in this table, since they probably importance, especially when dealing with detrital garnets, is indicate degree of metamorphism rather than definite their alteration due to erosion and transportation. The use of lithologic types. the two triangular diagram of almandine, pyrope, spessartine Using detrital garnets to determine provenance 205 and grossular + andradite + schorlomite was found to be of Goncharov AG, Saltykova AK (2008) Iron valency in minerals of great importance in order such an effect to be inferred. xenoliths and redox state of the upper mantle (by Mössbauer spectroscopy data). Hyperfine Interact 186:187–192 Overlaps and baffling results in the chemical composition Harlov DE, Johansson L, van den Kerkhof A, Förster H-J (2006) The of the garnets must be expected. role of advective fluid flow and diffusion during localized, solid- Ultimately, this study demonstrates the difficulty of state dehydration: Söndrum Stenhuggeriet, Halmstad, SW – assigning a source to sediment solely from detrital garnet Sweden. J Petrol 47:3 33 Himmerkus F, Anders B, Reischmann T, Kostopoulos DK (2007) composition, even with abundant reference data available. Gondwana-derived terranes in the northern Hellenides. In: In general, a multidisciplinary approach is strongly sug- Hatcher RD Jr, Carlson MP, McBride JH, Martínez-Catalán JR gested. If this is not easily accomplished, then the chemical (eds) 4-D Framework of Continental Crust. Geological Society of – composition of the cores of the garnets should be compared America Memoir, Boulder, pp 379 390 Himmerkus F, Reischmann T, Kostopoulos D (2009) Serbo- against all available compositional data from the garnets of Macedonian revisited: a Silurian basement terrane from northern the basement. The route followed in this paper was found Gondwana in the Internal Hellenides, Greece. Tectonophysics useful: Use of the ternary (Fe + Mn)-Mg-Ca diagram, use of 471:20–35 the (FeOt-MgO)-(CaO + MnO) binary diagram projecting Hunt GR, Salisbury JW, Lenhoff CJ (1973) Visible and near-infrared spectra of minerals and rocks: VI. Additional silicates. 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