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GSA Bulletin; January 2009; v. 121; no. 1-2; p. 39-54; DOI: 10.1130/B26317.1
© 2009 Geological Society of America
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Postorogenic shoshonitic rocks and their origin by melting underplated basalts: The Miocene of Limnos, Greece

Georgia Pe-Piper1,*, David J.W. Piper2, Ioannis Koukouvelas3, Lila M. Dolansky4 and Sotiris Kokkalas5

1 Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada
2 Geological Survey of Canada (Atlantic), Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada
3 Department of Geology, University of Patras, Patras 26110, Greece
4 Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada
5 Department of Geology, University of Patras, Patras 26110, Greece

Correspondence: {dagger}E-mail: gpiper{at}smu.ca.


   FOOTNOTES
 
GSA Data Repository Item 2008167, petrographic descriptions and chemical analyses of minerals and glasses, is available at www.geosociety.org/pubs/ft2008.htm. Requests may also be sent to editing{at}geosociety.org. Back


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 REGIONAL GEOLOGICAL SETTING
 MATERIALS AND METHODS
 VOLCANIC STRATIGRAPHY OF LIMNOS
 PETROGRAPHY, GEOCHEMISTRY, AND...
 DISCUSSION
 BROADER IMPLICATIONS
 CONCLUSIONS
 REFERENCES CITED
 
Potassium-rich volcanic rocks of the shoshonite suite are common features of postorogenic extensional settings inboard from subduction zones. Various petrogenetic processes and tectonic settings have been proposed for their origin. Early Miocene volcanic rocks of Limnos, part of the northeast Aegean shoshonite belt, show distinctive geochemical features that allow their petrogenesis to be well constrained. The rocks are principally trachyandesites and dacites. Very strong fractionation of light and middle rare earth elements (REEs), similar to that found in adakites, is inconsistent with a mantle source, but it can be modeled by melting of meta-basalt enriched in incompatible elements. A comparison with experimental melting of metabasaltic amphibolite requires small degrees of dehydration melting of amphibole, plagioclase, clinopyroxene, and minor garnet at a temperature >950 °C. Melting was triggered by mantle-derived magma, evidenced by repetitive zoning in clinopyroxene with Cr-rich cores. Nd and Sm isotopes suggest that some of this magma was similar to lamproite found elsewhere in this shoshonite belt and some was of asthenospheric origin. The amphibolite source is inferred to be subduction-enriched metabasalt that underplated the crust during pre-Mesozoic subduction. The regional trigger for dehydration melting was upwelling of asthenosphere as a result of slab detachment. The geochemistry and radiogenic isotopes of other shoshonitic rocks in the northeastern Aegean suggest a similar origin, but with higher degrees of partial melting of base-of-crust metabasaltic amphibolite. Similar processes appear likely for shoshonitic magmatism in some postcollisional settings elsewhere.

Key Words: shoshonite • petrogenesis • lower crust • amphibolite • dehydration melting • REE


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
REGIONAL GEOLOGICAL SETTING
 MATERIALS AND METHODS
 VOLCANIC STRATIGRAPHY OF LIMNOS
 PETROGRAPHY, GEOCHEMISTRY, AND...
 DISCUSSION
 BROADER IMPLICATIONS
 CONCLUSIONS
 REFERENCES CITED
 
Significance of Shoshonitic Volcanism
Shoshonitic rocks are high-potassium trachyandesites, generally erupted in large stratovolcanoes that are coeval with or follow calc-alkaline volcanism during postorogenic extension in subduction-related orogens. The type shoshonite, in the Eocene Absaroka volcanic province of Wyoming (Feeley, 2003), lies inboard from calc-alkaline volcanism and was originally interpreted as demonstrating an increase in potassium content of volcanic rocks with increasing depth to a subducting slab (Lipman et al., 1971). Shoshonites have been described from many orogens, including the Andes (Kay and Kay, 1993), Sierra Nevada (Manley et al., 2000), central Mexican volcanic belt (Blatter et al., 2001), and the Carpathians (Seghedi et al., 2004), but their tectonic and petrogenetic significance remains disputed. Some have argued that shoshonites and associated calc-alkaline volcanism may result from active subduction (Blatter et al., 2001; Bonev and Beccaletto, 2007). Most authors interpret subcontinental lithospheric mantle or asthenospheric mantle, both previously enriched in incompatible elements by earlier subduction, as the principal source of both shoshonitic and associated calc-alkaline magmas (Aldanmaz et al., 2000; Seghedi et al., 2004). Others have emphasized magma fractionation and crustal assimilation as the dominant processes influencing the eruptive character of both shoshonites and associated calc-alkaline rocks (Meen, 1987; Feeley and Cosca, 2003). Shoshonitic volcanism generally has restricted spatial and temporal distribution, and the ultimate origin of the magmatism has been commonly related to thermal events in the mantle, particularly related to slab break-off or lower-crustal delamination (Kay and Kay, 1993; Aldanmaz et al., 2000; Pe-Piper and Piper, 2007).

The northeast Aegean shoshonite belt consists of large Lower Miocene volcanic centers in the Greek islands of Limnos, Lesbos, and Samothraki, and an extensive area of northwestern Anatolia (Turkey) (Fig. 1). The volcanic rocks are principally potassium-rich rocks that classify mostly as trachyandesite and trachyte in the International Union of Geological Sciences (IUGS) nomenclature (Fig. 2A) but fall in the potassium-rich shoshonite suite of Peccerillo and Taylor (1976) (Fig. 2B). The belt includes a wide range of other minor rock types, including lamproite, calc-alkaline andesites, and high-Sr-Ba granitoids. In the island of Limnos, however, despite a range of volcanological settings, the erupted products have a narrow range of compositions (Fig. 2). The purpose of this paper is to take advantage of the apparent simplicity and uniqueness of the Limnos rocks to establish the most important petrogenetic processes in their evolution. Other rocks of the northeast Aegean shoshonite belt are then reconsidered in light of these findings, and their significance is evaluated in relation to the general issue of the tectonic setting of shoshonites.


Figure 01
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  		Figure 1. Regional map showing distribution of the early Miocene shoshonitic volcanism of the northeastern Aegean and northwestern Anatolia, the surface trace of oceanic sutures that closed in the Cretaceous and Paleogene, and the main tectonic and volcanic features of the modern subduction zone (modified from Pe-Piper and Piper, 2002). AE—Agios Evstratios, I-A suture—Izmir–Ankara suture; PT—Patmos, SK—Skyros. Approximate line of section for Figure 12 is also shown.

 

Figure 02
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  		Figure 2. Nomenclature of analyzed samples from Limnos in (A) the International Union of Geological Sciences (IUGS) system (Le Bas et al., 1996) and (B) the shoshonite diagram of Peccerillo and Taylor (1976). B also shows fields for Lower Miocene rocks of Samothraki (Vlahou et al., 2006), western Anatolia (Aldanmaz et al., 2000), and Lesbos (Pe-Piper and Piper, 1992).

 

   REGIONAL GEOLOGICAL SETTING
TOP
 ABSTRACT
 INTRODUCTION
 REGIONAL GEOLOGICAL SETTING
MATERIALS AND METHODS
 VOLCANIC STRATIGRAPHY OF LIMNOS
 PETROGRAPHY, GEOCHEMISTRY, AND...
 DISCUSSION
 BROADER IMPLICATIONS
 CONCLUSIONS
 REFERENCES CITED
 
The Late Cretaceous to Paleogene Hellenide orogeny resulted in closure of the Pindos, Vardar-Izmir-Ankara, and Intra-Pontide oceans and the collision of intervening continental blocks of Apulia, Pelagonia, and Rhodope with continental Europe (Fig. 1). Subduction-related Mesozoic-Paleogene igneous activity was widespread in the Rhodope block but lacking in the Apulia and Pelagonia blocks. Postcollisional extension began in the early Paleogene in the Rhodope massif and became widespread in the Aegean Sea in the Miocene. The distribution of Cenozoic igneous rocks in Greece is summarized by Pe-Piper and Piper (2002), and their distribution in western Anatolia (Turkey) is given by Dilek and Altunkaynak (2007). Eocene calc-alkaline plutonism is widespread in Rhodope and extreme northwest Anatolia, together with minor extrusive counterparts. Oligocene I-type granites and calc-alkaline to shoshonitic volcanic rocks are widespread in Rhodope and northwest Anatolia, where this igneous activity was correlated by Dilek and Altunkaynak (2007) to compressional tectonics, partial melting of a keel of enriched subcontinental lithosphere, and substantial crustal contamination.

The early Miocene northeastern Aegean shoshonite belt is spatially and temporally distinct from Oligocene volcanism in Greece, but it partly overlaps with Oligocene volcanism in northwestern Anatolia. The belt extends farther south into central western Anatolia, where it spans the Izmir-Ankara suture (Fig. 1). This shoshonite belt lies inboard from the collisional suture between the Pelagonia and Apulia continental blocks, which resulted from the final northeastward subduction of the Mesozoic Pindos Ocean (Piper, 2006).

In the northeast Aegean, large early Miocene stratovolcanoes, or their remnants, are known from Limnos, Lesbos, Samothraki, offshore from Agios Evstratios, and over a large area of central western Anatolia (Pe-Piper and Piper, 2002; Vlahou et al., 2006; Altunkaynak and Dilek, 2006) (Fig. 1). Lamproite dikes on the adjacent island of Lesbos are synchronous with shoshonitic volcanism (Pe-Piper et al., 2003). Volcanism was synchronous with widespread postorogenic extension (Dinter, 1998; Bonev and Beccaletto, 2007). A belt of small middle Miocene adakite volcanoes extends from Evia to Chios, just outboard of the shoshonite belt (Pe-Piper and Piper, 1994, 2007). Two types of tectonic models have been proposed for the origin of the shoshonitic rocks. Dilek and Altunkaynak (2007) emphasized the importance of crustal thickening producing a keel of lithospheric mantle. Aldanmaz et al. (2000) proposed lower-crustal delamination as a means of promoting asthenospheric upwelling.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 REGIONAL GEOLOGICAL SETTING
 MATERIALS AND METHODS
VOLCANIC STRATIGRAPHY OF LIMNOS
 PETROGRAPHY, GEOCHEMISTRY, AND...
 DISCUSSION
 BROADER IMPLICATIONS
 CONCLUSIONS
 REFERENCES CITED
 
Our field work focused on confirming and refining the stratigraphic relationships proposed by Innocenti et al. (1994). Petrography was based on examination of 67 polished thin sections. Whole-rock chemical composition was determined for 44 samples (Fig. 3; Table 1) using lithium metaborat-tetraborate fusion inductively coupled plasma (ICP) major-element analysis with ICP mass spectrometry (MS) for trace elements. Chemical mineralogy and chemical analyses of glass were determined using a JEOL-733 electron microprobe. Backscattered electron images were also taken of selected minerals to characterize compositional zoning and growth patterns.


Figure 03
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  		Figure 3. Geological map of Limnos. Geological boundaries are from Innocenti et al. (1994), but assignment of rock units has been modified. Orientations of large elongate clasts on bedding planes of ignimbrite near Romanou are shown. At base, schematic cross section of Limnos shows inferred relationships between different volcanic units.

 

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  		TABLE 1. SELECTED WHOLE-ROCK GEOCHEMICAL ANALYSES OF LOWER MIOCENE VOLCANIC ROCKS OF LIMNOS

 

   VOLCANIC STRATIGRAPHY OF LIMNOS
TOP
 ABSTRACT
 INTRODUCTION
 REGIONAL GEOLOGICAL SETTING
 MATERIALS AND METHODS
 VOLCANIC STRATIGRAPHY OF LIMNOS
PETROGRAPHY, GEOCHEMISTRY, AND...
 DISCUSSION
 BROADER IMPLICATIONS
 CONCLUSIONS
 REFERENCES CITED
 
The oldest rocks on Limnos are a middle Eocene–Oligocene flysch succession, which was gently folded prior to early Miocene igneous activity. This flysch accumulated in postorogenic basins associated with northward postorogenic collapse of the Rhodope-Sakarya orogen, which formed by closure of the Vardar-Izmir-Ankara ocean (Bonev and Beccaletto, 2007). The Lower Miocene volcanic rocks cover about half of the island and consist of hypabyssal intrusions, lava domes, flows, and pyroclastic deposits, divided by Innocenti et al. (1994) into three volcanic units (Fig. 3). The oldest volcanic rocks are in the southern part of the Katalakon Unit, consisting principally of subvolcanic stocks, dikes, and sills of porphyritic trachyandesite and dacite that include the Fakos quartz monzonite.

The Romanou Unit consists of stratified pyroclastic and volcaniclastic deposits up to 160 m thick. Volcaniclastic breccias and conglomerates with clasts up to boulder size occur near the base, and these are overlain by several welded ignimbrite horizons interbedded with flow tuffs that interfinger with five chert beds containing plant fossils. The cherts thicken toward meter-scale circular masses that appear to mark the sites of hot springs. Locally, domes and mono mictic breccias are present. Rocks of the Romanou Unit range from trachyandesite to rhyolite; this unit includes the most potassium-rich rocks of the island, occurring as glassy trachyandesite lavas and as blocks within conglomerate. The Therma Unit of Innocenti et al. (1994) (Fig. 3) resembles parts of the Romanou Unit and consists of interbedded marl and tuff with early Miocene plant fossils in tectonic contact with the apparently overlying Myrina Unit.

The Myrina Unit consists of domes and monomictic breccias (Innocenti et al., 1994). The rocks are all strongly porphyritic and are mainly dacite with subordinate trachydacite. The Agios Ioannis subunit is composed of hydrothermally altered hypabyssal rocks that are geochemically intermediate between the northern rocks of the Katalakon Unit and the Myrina Unit.

Previous K-Ar age determinations by Innocenti et al. (1994) and Fytikas et al. (1980, 1984) show considerable scatter and are not precisely located. The northern Katalakon subunit gave ages of 20–21 Ma; the Myrina Unit mostly gave ages of 18–20 Ma; and imprecise ages of ca. 20 Ma were obtained from the Romanou Unit. A new K-Ar date on phlogopite from a lava block in the Romanou Unit (sample 70B) yielded an age of 22.3 ± 0.7 Ma. The total age range from Limnos is similar to that on the nearby islands of Samothraki and Lesbos (Pe-Piper and Piper, 2002, their Fig. 199), and in central western Anatolia (Dilek and Altunkaynak, 2007, their Fig. 8).


Figure 08
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  		Figure 8. Variations in rare earth elements (REE) normalized to C1 chondrite (McDonough and Sun, 1995). For comparison, the lamproite and basanite from Figure 7 and two representative Lower Miocene trachyandesites from each of Western Anatolia (Aldanmaz et al., 2000) and Samothraki (Vlahou et al., 2006) are also shown.

 

   PETROGRAPHY, GEOCHEMISTRY, AND MINERAL CHEMISTRY
TOP
 ABSTRACT
 INTRODUCTION
 REGIONAL GEOLOGICAL SETTING
 MATERIALS AND METHODS
 VOLCANIC STRATIGRAPHY OF LIMNOS
 PETROGRAPHY, GEOCHEMISTRY, AND...
DISCUSSION
 BROADER IMPLICATIONS
 CONCLUSIONS
 REFERENCES CITED
 
Rock Classification and Stratigraphic Distribution
Based on chemical composition of representative fresh samples (Table 1), most rocks from the Romanou Unit and the Katalakon Unit in southern Limnos (including the Fakos quartz monzonite) are trachyandesite or trachyte (hereafter referred to generally as trachyandesite), whereas most rocks from the Katalakon Unit in northwestern Limnos and the Myrina Unit are of dacite to trachydacite composition (hereafter dacite), using the IUGS nomenclature (LeBas et al., 1986) (Fig. 2A). Interstitial glass from rocks of the Romanou Unit is mostly of trachytic composition (Fig. 2A). All the rocks fall within the shoshonite field of Peccerillo and Taylor (1976), whereas the glasses are ultrapotassic (Fig. 2B).

Petrography and Alteration
Details of petrography of analyzed samples and tables of mineral analyses are provided as an electronic supplement (see GSA Data Repository1). Hypabyssal trachyandesite to dacite of the Katalakon Unit is highly porphyritic (Fig. 4), with phenocrysts and microphenocrysts of feldspar and varying amounts of idiomorphic clinopyroxene, amphibole, and phlogopite. The Fakos quartz monzonite subunit is medium-grained, holocrystalline rock with inequigranular, interlocking crystals; major minerals are plagioclase, K-feldspar, and quartz, with subordinate altered clinopyroxene, brown-green amphibole, and phlogopite. Trachyandesite from the Romanou Unit has phenocrysts and microphenocrysts of clinopyroxene, subordinate plagioclase and phlogopite, ± rare K-feldspar, in a hyalopilitic groundmass. Myrina Unit dacite has phenocrysts of plagioclase, K-feldspar, rare phlogopitebiotite, and amphibole ± quartz in a hyalopilitic to trachytic groundmass.


Figure 04
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  		Figure 4. Backscattered electron images showing zoning in feldspar and clinopyrox-ene phenocrysts. (A) Sanidine (san, with wt% BaO) and plagioclase (plg, with wt% An), Agios Ioannis subunit. (B) Plagioclase (with wt% An), Agios Ioannis subunit. (C) Clinopyroxene (cpx, with wt% Cr2O3), Romanou Unit. (D) Clinopyroxene (with wt% Cr2O3), Fakos quartz monzonite subunit.

 
Zoning in phenocrystal minerals is pervasive. Feldspars commonly show oscillatory zoning that lacks any distinct pattern. Some plagioclase phenocrysts show corrosion relationships and evidence of growth discontinuities (Fig. 4B); mantled glomerophyric textures are also found. Clinopyroxene phenocrysts show up to three zoning cycles, where each shows a progression from high Mg (and commonly high Cr) to high Fe, generally terminated by a corrosion surface (Figs. 4C and 4D). Some amphibole phenocrysts show repetitive zoning patterns with intermittent corrosion surfaces, analogous to those seen in the clinopyroxenes.

Plagioclase phenocrysts from the Katalakon Unit are almost entirely of andesine (Fig. 5A); those from the Romanou Unit are principally andesine with rare labradorite rims; and in the Myrina Unit, they range from oligoclase to labradorite. Sanidine is present in the Myrina Unit and in some rocks of the Romanou Unit. Clinopyroxene has a narrow compositional range near the augitediopside boundary (Fig. 5B). Amphibole is magnesiohornblende to edenite to magnesio-hastingsite (Fig. 5C). Mica is principally phlogopite, with lesser biotite in younger rocks.


Figure 05
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  		Figure 5. Composition of (A) feldspars, (B) amphiboles, and (C) clinopyroxenes from the volcanic rocks of Limnos. Abbreviations: Ab—albite; An—anorthite; En—enstatite; Fs—ferrosilite; Or—orthoclase; Wo—wollastonite.

 
Many rocks of the Katalakon Unit are substantially altered—they show field evidence for hydrothermally altered zones and a characteristic green color in outcrop, due to chlorite, along with calcite and hydrous iron oxides. Many in situ volcanic and hypabyssal rocks of the Romanou Unit also show field evidence for hydrothermal alteration, but this is lacking in the conglomerates that were sampled. Rocks with field or petrographic evidence for strong alteration were not considered further and showed one or more of the following geochemical criteria (1) high loss on ignition (LOI > 5%); (2) anomalously low CaO contents; and (3) SiO2 contents of >78%. In rocks considered fresh, there has been considerable alteration of clinopyroxene and amphibole to opaque oxides and chlorite, but mica shows only rims of opaque oxides, and, in some samples, glassy groundmass is preserved. Rocks with LOI of 2.5%–5% showed some scatter in large ion lithophile elements (LILE) (Fig. 6) but no systematic variability in other elements.


Figure 06
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  		Figure 6. Plots of major and selected trace elements against SiO2. Fresh samples only. Symbols are as in Figure 2. Well-developed trends are shown by dashed lines. LOI—loss on ignition.

 
Element Variation and Radiogenic Isotopes
Most fresh Limnos volcanic rocks have a range of SiO2 (on a volatil-free basis) from 60 wt% to 68 wt%. Element variation with SiO2 shows three major groups of rocks: the southern Katalakon (including Fakos) and Romanou Units, the northern Katalakon Unit, and the Myrina Unit. Most rocks of the southern Katalakon and Romanou Units have 60%–63% SiO2, and with increasing SiO2, they show a rapid decrease in MgO and CaO, an increase in Al2O3 and Na2O, and constant K2O and Rb for rocks with <2.5% LOI. Interstitial glass from lavas of the Romanou Unit contains exceptionally high K2O and low CaO compared with host rock.

Most rocks of the northern Katalakon and Myrina Units have 63.5%–67.5% SiO2. For most elements, the two units show the same trend with increasing SiO2, although there is considerable scatter: decreasing MgO, TiO2, FeOT, CaO (slight), and Zr; constant K2O; slightly increasing Rb for rocks with <2.5% LOI; and scatter in Al2O3 and Na2O. The element Y is much less abundant in the Myrina Unit than in the northern Katalakon Unit. In general, dacite is less enriched in LILE (e.g., Rb, Ba; Fig. 6) and high field strength elements (HFSE; e.g., Zr, Nb; Fig. 6) compared with trachyandesite.

Incompatible trac-element concentrations for the most primitive rocks (those with high Mg, Cr, and Ni, and low SiO2) from the main units, normalized to pyrolite (McDonough and Sun, 1995), are plotted as multi-element patterns in Figure 7. Most samples have similar profiles and are significantly enriched in all the large ion lithophile elements (LILE), such as Rb, Ba, Th, U, and K, relative to rare earth elements (REEs) and the high field strength elements (HFSEs), in particular Nb, Ta, and Ti. The greatest enrichment is found in the Romanou and southern Katalakon Units. The Myrina Unit differs from the northern Katalakon and Agios Ioannis Units principally in having relatively higher Rb, K, Sr, Ta, and Hf, and lower HFSE.


Figure 07
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  		Figure 7. Variations in selected elements normalized to pyrolite (McDonough and Sun, 1995). The most primitive (lowest SiO2, high MgO) sample from each unit is plotted. Also plotted are a representative Lesbos lamproite (LL639, G. Pe-Piper, 2007, personal commun.) and a basanite from Turkish Thrace (EA519: Aldanmaz et al., 2006).

 
Chondrite-normalized rare earth element (REE) patterns from all units are remarkable in showing a very steep slope from La to Ho and nearly flat REE patterns from Ho to Lu, with a small negative Eu anomaly (Fig. 8). Absolute REE concentrations decrease with increasing SiO2 for Romanou and Myrina Unit rocks. The northern Katalakon subunit rocks are less enriched in light (L) REEs than the southern Katalakon and Romanou rocks. The Myrina Unit rocks are less enriched in elements from La to Gd, but they are relatively more enriched in Tb to Lu, with HoN < LuN (Fig. 8).

Nd and Sr isotopes from the Katalakon and Romanou Units (Table 2) are very radiogenic: {epsilon}Nd from –6.4 to –7.7 and 87Sr/86Sr from 0.7090 to 0.7097 (Fig. 9A). The Myrina Unit is more primitive: {epsilon}Nd of –3.8 and 87Sr/86Sr of 0.7077.


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  		TABLE 2. Nd/Sm AND Rb/Sr ISOTOPES FROM LOWER MIOCENE VOLCANIC ROCKS OF LIMNOS

 

Figure 09
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  		Figure 9. (A) Plot of Nd versus Sr isotopes for Limnos. Symbols are as in Figure 2. Plot also shows samples of Lower Miocene volcanic rocks from Lesbos (Pe-Piper and Piper, 2001), Samothraki (Vlahou et al., 2006), and western Anatolia (Altunkaynak and Dilek, 2006), and fields for some other groups of Aegean-area rocks (from Pe-Piper and Piper, 2002). (B–C) Plot of Pb isotope compositions of NE Aegean shoshonites from Samothraki and Lesbos together with fields for some other groups of Aegean-area rocks (from data compiled in Pe-Piper and Piper, 2002). Crustal growth curve is from Stacey and Kramers (1975).

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 REGIONAL GEOLOGICAL SETTING
 MATERIALS AND METHODS
 VOLCANIC STRATIGRAPHY OF LIMNOS
 PETROGRAPHY, GEOCHEMISTRY, AND...
 DISCUSSION
BROADER IMPLICATIONS
 CONCLUSIONS
 REFERENCES CITED
 
Volcanic Stratigraphy of Limnos
The volcanic stratigraphy of Limnos is important for understanding the temporal variation represented by different lava units. Subvolcanic intrusions of the Katalakon Unit into Paleogene flysch are numerous in western Limnos, whereas such intrusions are lacking in the eastern part of the island, except for a few bodies in an E-W–trending zone between Moudros and Kaminia (Fig. 3). The 160-m-thick Romanou Unit in the eastern part of the island rests unconformably on Paleogene flysch, and its alternation of volcaniclastic conglomerate and pyroclastic flows suggests that it formed a low-relief flank deposit to a more elevated volcanic source, perhaps a stratovolcano, now largely removed by erosion either in western Limnos or in a present marine area. Trachyandesite volcaniclastic clasts and minor flows from the Romanou Unit are chemically similar to the intrusive rocks of the Katalakon Unit in southern Limnos, including the Fakos quartz monzonite, which probably represent the deeper part of the volcanic edifice from which the Romanou Unit volcaniclastic and pyroclastic rocks were derived. Strong hydrothermal circulation synchronous with the Romanou volcano is indicated by the cherts of the Romanou Unit, which resemble hot-spring deposits described by De Wet and Hubert (1989).

Our new date of 22.3±0.7 Ma confirms that the Romanou Unit is older than the northern Katalakon Unit, which was dated at ca. 20–21 Ma by Innocenti et al. (1994). The predominance of subvolcanic intrusions and the higher degree of alteration suggest that the northern Katalakon Unit is older than the littl-eroded domes of the Myrina Unit, consistent with the ca. 18–20 Ma radiometric ages of the Myrina Unit (Innocenti et al., 1994).

Source of Magma
The Role of Fractionation
The SiO2 range of voluminous magmatic rock types is small, from 60% to 68%. No rocks or enclaves with <59% SiO2 (anhydrous basis) are known from Limnos. Systematic variation in compatible elements such as Mg, Cr, Ni, and Ca with increasing SiO2 content (Fig. 6) implies fractionation of olivine, clinopyroxene, and plagioclase. Some incompatible elements, notably Rb, Ba, and Zr, also decrease with increasing SiO2, probably as a result of fractionation of accessory biotite and zircon. The most primitive rocks have 3%–4% MgO and high abundances of both compatible (e.g., ~80 ppm Cr) and incompatible (e.g., ~5% K2O) elements.

Radiogenic Isotopes
The very evolved Nd and Sr isotopes from the Katalakon and Romanou Units, which give {epsilon}Nd from –6.4 to –7.7 and 87Sr/86Sr from 0.7090 to 0.7097 (Fig. 9A), resemble values found in other Neogene volcanic rocks elsewhere in the central Aegean, including Miocene adakite from Evia, lamproite from Lesbos, and granite from Samothraki, as well as other shoshonitic rocks (Figs. 1 and 9A; Pe-Piper and Piper, 2001; Altunkaynak and Dilek, 2006). Altunkaynak and Dilek (2006) recognized that these evolved isotopic values could represent either subduction-enriched source or contamination within the crust, and they preferred the crustal contamination hypothesis because of the change in isotopic character through time from rocks that were otherwise similar. In contrast, there is no variation in Nd or Sr isotopes in Limnos with indicators of crustal assimilation such as Pb, Th/Yb, or Zr/Nb.

The very evolved Nd and Sr isotope ratios in trachyte, adakite, lamproite, and shoshonite of the central and north Aegean correspond to unusually high 207Pb/204Pb and 208Pb/204Pb ratios (Figs. 9B and 9C), which result from an increase in the Th/Pb ratio as a result of subduction followed by sufficient time for evolution of high 207Pb and 208Pb values (Pe-Piper, 1994). This result contrasts with less evolved Nd and Sr isotopes of the Miocene of the southeast Aegean and the Oligocene of Rhodope (Fig. 9), both of which have 208Pb/204Pb isotope ratios closer to the normal growth curve of Stacey and Kramers (1975). These areas experienced Cretaceous to Cenozoic subduction of the African plate and the Vardar Ocean, respectively. In the Apulian and Pelagonian continental blocks, there is no evidence for subduction-related igneous activity since at least the Paleozoic.

Mineralogical Evidence for Mantle Derivation
Mineralogical data suggest an important role for mantle-derived magma. The Mg- and Cr-rich zones in clinopyroxene phenocrysts indicate crystallization from a primitive basaltic magma. Mg-rich clinopyroxene is similar to that from Serbian lamproites (Prelevic et al., 2004), and phlogopite is similar to that in Lesbos lamproite (Pe-Piper et al., 2003). The repetitive zoning with corrosion surfaces in clinopyroxene and amphibole and the resorption zones in plagioclase phenocrysts could have resulted from periodic input of basalt into a magma chamber. Such features are common elsewhere in the northeast Aegean shoshonite belt (Pe-Piper, 1984).

The Significance of the REE Enrichment
Geochemical data, however, show that the bulk of the erupted magma was not of mantle origin. The unusually steep light (L) to medium (M) REE patterns, with LaN reaching >400 times chondrite (Fig. 8), are not a normal product of melting of mantle peridotite. The degree of REE fractionation in the rocks from Limnos, using the modeling of La/Sm and Sm/Yb applied to shoshonites from western Anatolia by Aldanmaz et al. (2000), would require ~1% partial melting of strongly enriched garnet lherzolite (with garnet:spinel ~4:1) to attain the observed La/Sm and Sm/Yb ratios, yet the heavy (H) REE patterns from Limnos show no evidence for major involvement of garnet. Somewhat similar steep patterns are known from adakites derived from partial melting of oceanic crust (e.g., Kay, 1978), except that adakites also show HREE fractionation. In contrast to conditions inferred for the generation of adakite melts, the lack of strong Sr/Y fractionation in the Limnos rocks indicates the involvement of plagioclase, and the flat HREE pattern indicates little involvement of garnet (Martin, 1999). The degree of fractionation of the LREEs and MREEs in the Limnos rocks shows no systematic relationship to indicators of fractionation and/or crustal contamination such as MgO, SiO2, and Th (e.g., Fig. 6J), and extreme special manipulation would be needed to produce such REE patterns by contamination with continental crust (Fig. 8A). The very consistent and distinctively fractionated REE patterns in the Limnos rocks must reflect the dominant petrogenetic process.

Partial melting of lower-crustal amphibolite of metabasaltic composition enriched in incompatible elements could account for many of the distinctive features of the Limnos trachyandesite. An analogous process, but with a tholeiitic source, is interpreted for the origin of Archean TTG (tonalit-trondhjemite-granodiorite) suites (Rapp et al., 1991). Most experimental work on melting of metabasalt has used mid-ocean-ridge basalt (MORB) tholeiite or Na-alkalic basalt as starting materials (e.g., Rapp and Watson, 1995), producing the Na-rich melts characteristic of the tonalit-trondhjemite suite. Mafic underplating of continental crust provides a mechanism for uniform isotopic and trac-element characteristics over a large area (Rudnick, 1990).

Experimental dehydration melting experiments on slightly enriched basaltic amphibolite (0.8% K2O) by Sen and Dunn (1994) produced melts with 60%–70% SiO2 and ~2.5% K2O at 1.5 GPa and ~4% K2O at 2 GPa. Melting took place in two stages: the first involved almost modal melting of original amphibolite minerals amphibole, plagioclase ± quartz, leaving an eclogitic restite, which melted nonmodally in the second phase (85% clinopyroxene [cpx], 15% garnet [grt]) at ~10 wt% melting at 1.5 GPa.

The REE patterns in the Limnos rocks can be modeled following the experiments of Sen and Dunn (1994) by assuming a metabasaltic amphibolite parent rock that had been enriched and underplated at the base of the crust during earlier subduction. In the absence of direct information on the composition of this underplated metaba-salt from xenoliths, we explored the implications of a trac-element composition assuming a composition that is the mean of mid-crustal Upper Miocene gabbro from Samos (Mezger et al., 1985) and Delos (Pe-Piper et al., 2002), which formed in the Neogene South Aegean subduction system. Mineral-melt partition coefficients for andesite were used after Rollinson (1993): use of more detailed analyses of partition coefficients (e.g., Green et al., 2000) was not warranted given uncertainties in starting composition.

With this starting material, in which La is enriched 100-fold relative to chondrite and HREEs are enriched 20-fold (Fig. 10), partial melting of amphibole produces a characteristic relative depletion in MREEs and relative enrichment in both LREEs and HREEs. The observed flat HREE pattern can be modeled only by permitting an addition of a small amount of partial melting of garnet. Differential fractionation of Eu between plagioclase and amphibole depends on the amount of each mineral that is melted. To obtain the strong enrichment in LREEs, melting of substantial proportion of clinopyroxene is required. The modeled proportions in Figure 10 are similar to those inferred by Sen and Dunn (1994) in their 1.5 GPa experiments, except that the nonmodal melting of eclogite restite involves less garnet than they proposed. In addition to producing the strong fractionation of LREEs, melting of enriched amphibolitic metabasalt would also result in the strong enrichment in LILE and HFSE, including a high K/Na ratio. The negative spikes in Nb, Ta, and Ti are consistent with retention of oxide phases during partial melting. There is no correlation between these three elements and the degree of fractionation of REEs.


Figure 10
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  		Figure 10. Modeled rare earth element (REE) compositions derived from partial melting of enriched metabasalt and mixing with lamproite magma, also showing REE compositions of lamproite and basanite from Figure 7. Starting material (assumed metabasalt) is a 50:50 mix of Samos and Delos subduction-related diorites; batch partial melting uses mineral-melt partition coefficients for andesite from Rollinson (1993). For further explanation, see text.

 
The similarity in LREE and MREE patterns in the northern Katalakon and Myrina Units implies that the same metabasaltic amphibolite was the predominant source for the dacites. The lesser enrichment in LREEs and MREEs compared with the trachyandesite could result from partial melting at a lower temperature, when the proportion of amphibole to clinopyroxene melting is higher (Table 3), resulting in the higher LuN/HoN ratio and lower Y found in the Myrina Unit (Fig. 8).


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  		TABLE 3. SUMMARY OF MAJOR-ELEMENT COMPOSITION OBTAINED BY MIXING LAMPROITE OR BASANITE MANTLE-DERIVED MAGMA WITH MELTS FROM AMPHIBOLITIC METABASALT

 
The Geochemical Role of Mantle Melts
Partial melting of lower-crustal metabasaltic amphibolite enriched in incompatible elements thus accounts for distinctive trac-element features of the Limnos rocks, in particular, the unusual fractionation of the REEs. However, mantle-derived melts were also important, as indicated by the mineralogical data. Temperatures of >950 °C are likely necessary to partially melt metabasalt to yield magmas with <65% SiO2 (e.g., Rapp et al., 1991; Sen and Dunn, 1994), which suggests advection of hot, mantle-derived basaltic magma as a heat source. The small range of eruptive products and absence of basalt, despite evidence for the presence of basaltic magma from mineralogy, are interpreted to be analogous to processes at Arenal Volcano (Streck et al., 2002). Here, lavas of rather constant basaltic andesite composition have been erupted over 30 yr, despite mineralogic evidence for repeated influx of basalt to the magma chamber.

Although no basaltic rocks or enclaves are known from Limnos, lamproite dikes on the adjacent island of Lesbos were emplaced coeval with shoshonitic volcanism (Pe-Piper et al., 2003; G. Pe-Piper, 2007, personal commun.). The trac-element composition of Lesbos lamproite is rather similar to that of both the Lesbos and Limnos trachyandesite, except that LREEs are much less enriched (Figs. 7 and 8).

A major-element mass balance for the most primitive trachyandesite on Limnos can be achieved with ~30% Lesbos lamproite mixed with a 64% SiO2 melt derived from 15% partial melting of enriched metabasaltic amphibolite. A crustal thickness of 40–50 km seems appropriate, so we used the major-element melting proportions of the 1.5 GPa experiments of Sen and Dunn (1994) (Table 3). At 15% partial melting, a 64% SiO2 melt would be produced at ~1000 °C, with melting of subequal amounts of amphibole and clinopyroxene. The major-element composition of this melt in Table 3 is derived directly from the experimental results of Sen and Dunn (1994), using their starting composition. The principal anomalies are in CaO and K2O (estimated too low) and Na2O and Al2O3 (too high). Higher K2O in the trachyandesite could be a consequence of higher LILE in the enriched amphibolite compared to the 0.8% K2O used by Sen and Dunn (1994). Variance in CaO, Na2O, and Al2O3 suggests a difference in the plagioclase content of the amphibolite source compared with Sen and Dunn's (1994) starting composition, or minor plagioclase fractionation during magma mixing and ascent.

The Nd and Sr isotopes from the Myrina Unit are less radiogenic than all the other rocks, suggesting that formation of the Myrina Unit may also have involved an asthenospheric mantle component. It may indicate the role of a rising asthenospheric diapir that later led to late Miocene alkalic volcanism with positive {epsilon}Nd in western Anatolia and Turkish Thrace (e.g., Aldanmaz et al., 2006). Mixing of 20% asthenospheric basanite magma (based on Aldanmaz et al., 2006) with the inferred metabasalt melt is required to produce the observed isotopic composition. Most calculated element abundances are relatively insensitive to the mantle component of the erupted products, whether lamproite or basanite. Compared to lamproite, basanite has much lower K2O (~0.8%, cf. 3.7%) and higher TiO2 (2.7%, cf. 1.1%): this is consistent with the systematically lower K2O values of the dacite compared to trachyandesite (Fig. 6G), whereas TiO2 appears to be influenced by fractionation in both rock types (Fig. 6B).

Comparison with Shoshonitic Rocks Elsewhere in the Northeast Aegean Belt
Elsewhere in the northeast Aegean shoshonite belt, a wider range of rock compositions is present, including some rocks with greater calc-alkaline affinity (Fig. 2). Nevertheless, most of the eruptive products are trachyandesite, andesite, trachyte, and dacite. In most of these rocks, the LREE and MREE patterns are not as distinctively steep as in the Limnos volcanic rocks (Fig. 11), although a few rocks in all the other volcanic centers overlap with the Limnos rocks with less-fractionated REEs. In Lesbos, complexly zoned plagioclase and clinopyroxene with high Al and Cr are common in shoshonitic lavas but rare in more calc-alkaline lavas (Pe-Piper, 1984). The more calc-alkaline lavas resemble the Myrina Unit in having less radiogenic Nd and Sm isotopes, and to a lesser extent, less radiogenic Pb isotope compositions, all consistent with a greater asthenospheric component (Fig. 9).


Figure 11
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  		Figure 11. (A) Variation in degree of light (L) and middle (M) rare earth element (REE) enrichment in Limnos rocks; symbols are as in Figure 2. Plot also shows fields for Lower Miocene volcanic rocks from Lesbos (gray tone, from Pe-Piper and Piper, 1992), Samothraki (Vlahou et al., 2006), and western Anatolia (Aldanmaz et al., 2000), lamproite from Lesbos (cf. Fig. 7), and average continental crust (avg. cont. crust, from Wedepohl, 1995). Selected high-La trachyandesite from other shoshonites: S—Sunlight volcano (Feeley and Cosca, 2003), A—central Andes (Kontak et al., 1986). (B) Modeled enrichment in LREE and MREE resulting from partial melting of enriched metabasalt (cf. Fig. 10), for various modal mineralogies (%) and proportions of partial melting (0.01, 0.1, 0.2, 0.5).

 
We have argued that the distinctive LREE and MREE fractionation in Limnos can be explained only by partial melting of metabasalt in the absence of significant garnet. The presence of similarly fractionated REE patterns in some rocks from the other shoshonite centers suggests that this process of partial melting of under-plated metabasalt was also active there. The fact that these centers contain rocks with geochemistry unlike those of Limnos indicates the role of other petrogenetic processes, which may include: (1) A greater degree of partial melting of underplated metabasalt, which would result in less LREE and MREE fractionation (Fig. 11). This process is suggested by the similar Nd, Sr, and Pb isotope composition of felsic volcanic rocks in Lesbos and in the Samothraki granite (Fig. 9). (2) More hydrous melting conditions also may have existed, so that the involvement of clinopyroxene was less, again resulting in less LREE and MREE fractionation (Fig. 11). (3) A greater component of melt from enriched lithospheric mantle in the erupted magmas could provide the observed enrichment in incompatible elements without the extreme fractionation of LREE–MREE (Fig. 8). (4) A greater component of melt from asthenospheric mantle may be responsible for more calc-alkaline character and less radiogenic isotopic compositions. (5) The role of intermediate to felsic crustal rocks and greater amounts of fractionation, perhaps through assimilation with fractional crystallization (AFC) processes, cannot be excluded, particularly for more felsic eruptive products. Because of crustal heterogeneity, such processes are difficult to model and, therefore, test. The extreme concentrations of some elements in trachytic glass at Limnos (Fig. 6) suggest crustal AFC processes. Elsewhere, Feeley and Cosca (2003) have argued that the distinction between calc-alkali and shoshonitic magmas in the type locality is a result of crustal processes. On the other hand, the Pb, Nd, and Sr isotopic data for felsic volcanic rocks in Lesbos and Samothraki (Fig. 9) do not support significant assimilation in the crust, unless that crust had isotopic composition similar to the underplated metabasalt.

A General Tectonic-Petrogenetic Model for Shoshonitic Volcanism of the Northeast Aegean
In the northeast Aegean, crustal thickness following Apulia-Pelagonia collision was greatest in the Oligocene to early Miocene, before the effects of widespread crustal extension that began in the early Miocene. The geochemical features reviewed here indicate that partial melting of subduction-enriched metabasaltic amphibolite took place with dehydration melting of amphibole and plagioclase, followed by predominant clinopyroxene melting from the eclogitic restite. The lack of adakite characteristics, including low Sr/Y ratios and Y > 15 ppm, argues that this source was not the same as eclogitic metabasalt that has been identified as the source of adakite magma by numerous authors (Martin, 1999; Wang et al., 2005).

Dehydration melting is most readily achieved by a thermal anomaly resulting from advection of asthenosphere (Fig. 12). Petrologic evidence for such asthenospheric upwelling is found in the late Miocene to Quaternary alkali basalts of western Anatolia (e.g., Güleç, 1991; Aldanmaz et al., 2006), the melting of slab ocean crust to form the adakites of the central Aegean (Pe-Piper and Piper, 1994), and the scattered occurrence of lamproites and lamprophyres in the Aegean region (Altherr and Siebel, 2002; Pe-Piper et al., 2003). Furthermore, we have shown that the Nd and Sr isotopic composition in the Myrina Unit suggests that asthenospheric magmas had risen to the base of crust, thus providing an effective means of heat transfer.


Figure 12
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  		Figure 12. Tectonomagmatic model for the evolution of the trachyandesites of Limnos and adjacent areas of the northeastern Aegean shoshonite belt. Pl-out indicates pressure at which plagioclase is unstable. For explanation, see text. Schematic location is shown in Figure 1.

 
Two hypotheses might account for the upwelling: (1) Seismic tomography shows evidence for Neogene slab breakoff in the Hellenic subduction system (Wortel and Spakman, 2000), and a similar process in the Vardar-Izmir-Ankara subduction system has been proposed as the cause of early Cenozoic magmatism in NW Anatolia (Dilek and Altunkaynak, 2007). (2) Lithospheric delamination in an area adjacent to the shoshonite belt might have been the trigger for asthenospheric upwelling, as argued on geophysical grounds for potassic magmatism of the Andes (Kay and Kay, 1993) and Sierra Nevada (Manley et al., 2000).

Neither the volume of Oligocene to Neogene detrital sediment in the Hellenides nor a pressur-temperatur-time (P-T-t) analysis of exhumed metamorphic core complexes supports the concept of major crustal thickening on the scale inferred for Tibet, the Andes, and the Sierra Nevada. The persistence of an oceanic subduction zone from the eastern Mediterranean ocean area of the African plate throughout the Hellenide orogeny (van Hinsbergen et al., 2005) means that subduction rollback (Royden, 1993) precluded large-scale crustal thickening. For these reasons, lithospheric delamination is not favored as the triggering mechanism for asthenospheric upwelling.

Slab breakoff from the Vardar-Izmir-Ankara subduction system has been proposed as the cause of Paleogene orogenic collapse and high-potassium magmatism in the Rhodope block and the Biga Peninsula of NW Anatolia (Altunkaynak and Dilek, 2006). However, the early Miocene northeast Aegean shoshonite belt is not only younger, but it lies south of the Paleogene volcanism, overlapping only in Samothraki and in parts of NW Anatolia. Furthermore, it extends south of the Vardar-Izmir-Ankara suture zone in western Anatolia, suggesting that it is unlikely to be related to slab breakoff from a closing Vardar Ocean. Seismic tomography shows no appropriately located slab breakoff from the African plate (van Hinsbergen et al., 2005).

Slab breakoff from the Pindos Ocean is our favored explanation for the northeast Aegean shoshonite belt (Fig. 12). Estimates of the width of the Pindos Ocean range from 300 to 500 km (Stampfli and Borel, 2004; van Hinsbergen et al., 2005). The lack of Mesozoic–Paleogene igneous activity in the Pelagonia block suggests that most of the closure of the Pindos Ocean took place by ocean-ocean subduction (Pe-Piper and Piper, 1991). The youngest pelagic sediments of the Pindos Ocean are of early Paleocene age, and foredeep flysch deposition continued to the middle Eocene, with diachronous closure from northwest to southeast (Piper, 2006). Continental collision with Apulia and crustal thickening took place during the late Eocene and Oligocene, based on stratigraphic evidence from syn- and postorogenic basins (e.g., Zelilidis et al., 2002) and P-T-t paths in metamorphic core complexes (e.g., Buick and Holland, 1989). Any slab detachment would thus have initiated in the late Eocene. A high-velocity anomaly in sections B4 and B7 of Spakman et al. (1993) at ~230 km depth, located south of the Sea of Marmara, might represent such a slab or might be related to Vardar-Izmir-Ankara subduction.

The shoshonitic volcanism in Limnos is thus the result of three tectonic processes: (1) A major subduction event that resulted in enrichment of subcontinental lithospheric mantle and underplating of enriched gabbro (metabasalt) at the base of the crust; (2) Mesozoic microcontinental collision that led to crustal thickening, promoting partial melting following r-establishment of a stable geotherm; and (3) detachment of the subducting slab from the Pindos Ocean following continental collision that set up asthenospheric upwelling.


   BROADER IMPLICATIONS
TOP
 ABSTRACT
 INTRODUCTION
 REGIONAL GEOLOGICAL SETTING
 MATERIALS AND METHODS
 VOLCANIC STRATIGRAPHY OF LIMNOS
 PETROGRAPHY, GEOCHEMISTRY, AND...
 DISCUSSION
 BROADER IMPLICATIONS
CONCLUSIONS
 REFERENCES CITED
 
Potassium-rich shoshonitic volcanic rocks probably have different petrogenetic origins in different settings. Generally, the mineralogical and geochemical features of these rocks are insufficient to discriminate among the roles of melting of enriched mantle, enriched mafic lower crust, and crustal assimilation and fractional crystallization. However, the distinctive highly fractionated REE patterns of the Limnos shoshonites can be accounted for only by partial melting of mafic lower crust enriched in incompatible elements. Other shoshonitic rocks of the northeastern Aegean shoshonite belt show close similarities to Limnos rocks in their general geochemistry, radiogenic isotopes, and phenocryst composition and zoning, suggesting that for these rocks, similar petrogenetic processes were involved. Melting of enriched subcontinental lithospheric mantle played an important role in the petrogenesis of the rocks, whereas the impact of fractionation and crustal assimilation appears to have been relatively minor. Some shoshonites from other areas include trachyandesites with enriched LREE and MREE patterns similar to those of Limnos (e.g., in the type area; Feeley and Cosca, 2003; Fig. 11). The particular petrogenetic processes identified for Limnos and the northeast Aegean shoshonite belt are therefore probably active elsewhere.

It is also unlikely that any single tectonic process is responsible for shoshonite volcanism. In the particular setting of the Lower Miocene volcanoes of the northeastern Aegean, slab breakoff appears to have permitted rise of asthenospheric mantle. However, in other settings, processes such as lower-crustal delamination (Kay and Kay, 1993) or creation of a keel of lithospheric mantle (Dilek and Altunkaynak, 2007) may lead to potassium-rich shoshonitic volcanism.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 REGIONAL GEOLOGICAL SETTING
 MATERIALS AND METHODS
 VOLCANIC STRATIGRAPHY OF LIMNOS
 PETROGRAPHY, GEOCHEMISTRY, AND...
 DISCUSSION
 BROADER IMPLICATIONS
 CONCLUSIONS
REFERENCES CITED
 

  1. The shoshonitic volcanic rocks of Limnos consist of remnants of an early trachyandesite stratovolcano and subvolcanic dacitic intrusions on the flanks, followed by late dacitic domes.
  2. All the Limnos igneous rocks are characterized by very steep and enriched LREE–MREE patterns that cannot be reasonably accounted for either by crustal fractionation and assimilation processes or by mantle melting.
  3. The Limnos igneous rocks originated principally from dehydration melting of enriched metabasaltic amphibolite, involving melting of amphibole, plagioclase, clinopyroxene, and minor garnet, to produce the highly enriched LREE–MREE patterns. This base-of-crust melting was triggered by rising mantle melts, both from the asthenosphere and from the enriched subcontinental lithosphere. Complexly zoned clinopyroxene, amphibole, and plagioclase imply episodic supply of mantle melts to the base-of-crust magma chamber, and mass-balance calculations suggest that mantle melts make up 20%–30% of the erupted magma.
  4. Enriched metabasaltic amphibolite under-plated the crust during pre-Mesozoic subduction, which also resulted in the enrichment of lithospheric mantle in incompatible elements.
  5. The ultimate heat source for the magmatism was rising asthenosphere resulting from detachment of the subducting Pindos Ocean slab following continental collision.
  6. Rare trachyandesite with steep LREE–MREE patterns in other shoshonitic volcanic belts is diagnostic of the importance of lower-crustal metabasalt melting. Several tectonic scenarios may be responsible for the development of shoshonitic volcanism.


   ACKNOWLEDGMENTS
 
Pe-Piper's work in Greece is largely supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). Ben Moulton assisted with data compilation. Reviews by J.S. Beard, Dejan Prelevic, Paul Robinson, Yildirim Dilek, and an anonymous reviewer were extremely helpful in sharpening our interpretation.


   REFERENCES CITED
TOP
 ABSTRACT
 INTRODUCTION
 REGIONAL GEOLOGICAL SETTING
 MATERIALS AND METHODS
 VOLCANIC STRATIGRAPHY OF LIMNOS
 PETROGRAPHY, GEOCHEMISTRY, AND...
 DISCUSSION
 BROADER IMPLICATIONS
 CONCLUSIONS
 REFERENCES CITED
 

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