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Whole-Rock Geochemistry of Schists from Syros, Cyclades, Greece

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Whole-Rock Geochemistry of Schists from Syros, Cyclades, Greece WHOLE-ROCK GEOCHEMISTRY OF SCHISTS FROM SYROS, CYCLADES, GREECE STACEY KEPLER Amherst College Sponsor: John T. Cheney INTRODUCTION METHODS The island of Syros is located within the Attic- Thirty-five schists were collected from ten Cycladic Metamorphic Belt and preserves localities on Syros. Although the emphasis rocks with eclogite and epidote-blueschist- was felsic schists, intermediate and mafic facies mineral assemblages (Okrusch and schists were also collected where interlayered Brocker, 1990). Locally, these high-pressure, with the felsic rocks. Four locations (Lia low temperature rocks are overprinted by Beach Trail, Palm Beach, San Michalis, and greenschist-facies mineral assemblages. Two Katergaki) were extensively sampled due to lithotectonic units have been identified on complex cross-cutting and structural relations Syros: enclaves of mafic rocks and volcano- of felsic and mafic rocks in these outcrops. In sedimentary rocks consisting of widespread this way, samples were collected representing schists and marbles. Mafic rocks occur in a wide range of mineral assemblages and discrete, fault-bounded units around the island geographical locations. All thirty-five samples and are juxtaposed with the volumetrically were petrographically described, and the dominant felsic schists and marbles. The freshest twenty-five rocks of these were schists are interpreted as metamorphosed selected for chemical analyses. Major oxide volcanics and volcaniclastic sediments, and trace element compositions were whereas the marbles are metamorphosed determined by ICP-MS at Acme Analytical limestones (Putlitz et al., 2000). Fine-grained Labs in Vancouver, B.C. and coarse-grained mafic rocks are interpreted to be meta-basalts from an ocean floor setting PETROGRAPHY and meta-gabbros, respectively (Dixon and Each location contains a variety of rock types Ridley, 1987). and most of the samples contain a strong Preliminary chemical analyses of the schists of foliation and/or lineation of quartz+white Syros (Prinkey, 2001; Sinitsin, 2001) and the mica. Only a few samples have massive neighboring island of Sifnos (Mocek, 2001), fabrics and these are either greenschist-facies have suggested a range of possible protoliths, schists or rocks dominated by jadeitic from MORB to island arc-affinities. This pyroxene. study focuses on quartz-white mica schists Based on mineral assemblages, the rocks can that encompass a wide range in composition be subdivided into four groups. Group 1 from mafic to felsic. The goal is to determine consists of felsic rocks that contain greater the tectonic setting of rock protoliths to better than 70 percent quartz+white mica with minor characterize the types of material that entered amounts of garnet±albite±chlorite±epidote. the relic subduction zone. Major oxide and Group 2 consists of blue amphibole-bearing trace element geochemical analyses are used mineral rocks that range from mafic to to determine protoliths and to characterize the intermediate and also contain quartz+white tectonic setting of these protoliths. mica±garnet. Variable amounts of greenschist- facies minerals (±epidote±chlorite±albite) are Mineral compositions from nine selected present. Foliation is defined by blue samples were obtained using the SEM/EDS at amphibole+quartz+white mica and wraps Amherst. Blue amphibole ranges from around garnet where present. Group 3 consists glaucophane to ferroglaucophane, and white of rocks with the greenschist-facies mineral micas are paragonite and phengite. Garnets assemblage quartz+white are almandine rich (60-80 percent) with lesser mica+garnet±chlorite±epidote±albite. Group 4 and varying grossular content (10-40 percent). consists of rocks composed principally of They are typically zoned with almandine jadeite+quartz, with lesser amounts of white component decreasing and grossular and mica±albite±garnet±blue spessartine increasing by ten to twenty percent amphibole±omphacite±acmite. They have towards the core. Clinopyroxene compositions massive fabrics and contain 0.5-1.0 centimeter include jadeite, aegerine-augite, acmite and rims of albite that surround jadeite. The rocks omphacite. Both omphacite and jadeite occur also contain minor amounts of rutile, titanite, in the groundmass of some samples whereas apatite, calcite, and/or opaques. aegerine-augite and acmite only occur as thin rims around albite. WHOLE-ROCK GEOCHEMISTRY Table 1: Major oxides and Rare Earth Element data for analyzed rocks, ordered by the mineral assemblage groupings described above. The analyzed rocks have chemical SiO2 (figure not shown), indicating that they compositions consistent with igneous may have been relatively mobile during protoliths ranging from basalt to rhyolite, with metamorphism. Group 1 felsic rocks mostly a greater concentration of samples in the plot as rhyolite, with one sample in the dacite dacite and rhyolite fields, as shown in Figure field. Group 2 blue amphibole-bearing rocks 1. Felsic rocks with silica contents greater than are basalts, basaltic-andesites, and andesites. 70-75 weight percent either have sedimentary Group 3 greenschist-facies rocks plot in the protoliths or are meta-volcanic rocks that have dacite field. Group 4 jadeite rocks contain the undergone alkali leaching or silica enrichment. highest concentration of alkalis and with one Alkali leaching may have occurred because exception, plot as dacite and rhyolite. Na and K are scattered when plotted versus Figure 1: Total Alkalis vs. Silica plot. Three “rhyolitic” samples are not shown because they contain greater than 77 percent SiO2. Symbols are representative of REE slope versus chondrite (see Figure 3). Rare Earth Element (REE) data are >100 (Figure 2). Most of these rocks are from used to characterize the rocks because they are the felsic category, but a greenschist and typically immobile during metamorphism blueschist are also present. Type 2 and Type 3 (Rollinson, 1993). Rocks from the four rocks have slightly negative slopes and flat petrographic groupings do not neatly separate slopes, respectively with values 8-100 times into groups on the REE plots. When plotted on chondrite (Figure 2). Type 2 contains mostly chondrite-normalized REE plots, the rocks can felsic rocks, with three greenschist-facies be split into three types based on their slope. rocks. Type 3 contains rocks from all of the Type 1 rocks have highly negative slopes, petrographic categories, and the bulk are blue- with La ~100 times chondrite and La/Yb ratios amphibole-bearing rocks and jadeite-rocks. FeO* Figure 2. Rocks plotted relative to chondrite. Symbols represent slope of REE. Squares are the Na2O+K2O MgO most LREE enriched, filled triangles are slightly LREE enriched, and open triangles have flat slope. Figure 3. AFM Diagram showing the majority of rocks plotting in the calc-alkaline field, with a few plotting as tholeiitic. After Irvine and Barager (1971). Most of the samples have negative the neighboring Cycladic island of Sifnos europium anomalies, depletion of Ti, Nb, Sr, (Mocek, 2001). and Ba, and enrichment of Zr and Th. An AFM diagram (Figure 3) shows Depletion of Nb and enrichment of Th are most of the samples plotting in the calc- commonly associated with arc rocks (Mocek, alkaline field. Mafic samples with low 2001). Similar depletions and enrichments are amounts of K 0 are tholeiitic. found in jadeite-gneisses and blueschists on 2 SUMMARY A range of protoliths appear to be from Sifnos (Mocek, 2001). Jadeite-bearing represented in the schists of Syros. A subset rocks on Syros have flat REE patterns similar of blueschist- and greenschist-facies rocks that to those of jadeite-gneisses on Sifnos, but are plot as tholeiites on an AFM diagram and have more enriched relative to chondrite (Mocek, low amounts of K2O may represent ocean 2001). floor basalts (Brady, 2000). Many of the calc- Additional analysis is needed to relate alkaline rocks show chemical similarities to mineral assemblages and bulk chemistry in the Sifnos rocks that may have arc protoliths schists of Syros and to compare and results (Mocek, 2001). Felsic rocks with minor with previous analyses from Syros and Sifnos. amounts of greenschist-facies minerals have We continue to explore chemical indicators of highly LREE-enriched patterns similar to sedimentary versus volcanic protolith to chlorite-actinolite rocks on Sifnos (Mocek, evaluate the extent of possible mixing of 2001). Blue-amphibole bearing rocks with flat sedimentary and igneous material in these slopes relative to chondrite, Nb troughs, and rocks. Th enrichment are similar to that in blueschists REFERENCES CITED Dixon, J.E., and Ridley, J.R., 1987, Syros. In Helgeson, Prinkey, Debra R., 2001. Geochemical Analysis of H.C. (ed.), Chemical Transport in Metasomatic Mafic and Felsic Schists from South Point and Processes, 489-501, Reidel Publishing Company. Katergaki, Syros,Greece. In, Fourteenth Annual Keck Research in Geology Proceedings, p. 133- Irvine, T.N., and Barager, W.R.A., 1971. A guide to 136. the chemical classification of the common volcanic rocks: Canadian Journal of Earth Sciences, V. 8, p. Putlitz, B., Matthews, A., and Valley, John W., 2000. 523-548 Oxygen and hydrogen isotope study of high- pressure metagabbros and metabasalts (Cyclades, Mocek, B., 2001. Geochemical evidence for arc-type Greece): implications for the subduction of oceanic volcanism in the Aegean Sea: the blueschist unit of crust: Contributions toMineralogy and Petrology, v. Sifnos, Cyclades (Greece): Lithos, v. 57, p. 263- 138, p. 114-126. 289. Rollinson, H., 1993. Using Geochemical Data: Nakamura, N., 1974. Determination of REE, Ba, Fe, evaluation, presentation, interpretation: Longman Mg, Na, and K in carbonaceous and ordinary Singapore Publishers (Pte) Ltd., Singapore. chondrites. Geocheim. Cosmochim. Acta, v. 39, p. 757-775. Sinitsin, A., 2001. Origin and Evolution of the High- Pressure Meta-Igneous Assemblage near St. Okrusch, M., and Brocker, M., 1990. Eclogites Michalis, Syros, Greece. In, Fourteenth Annual associated with high-grade blueschists in the Keck Research in Geology Proceedings, p. 142- Cyclades archipelago, Greece: A Review: 146. European Journal of Mineralogy, v.2, p.451-478..
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