Post-collisional Cenozoic extension in the northern Aegean: The high-K to shoshonitic intrusive rocks of the Maronia Magmatic Corridor, northeastern Greece

The Maronia Magmatic Corridor is a NE-trending belt of Oligocene plutons that intrudes the Kechros Dome of the northern Rhodope Core Complex in northeastern Greece. The post-collisional magmatism transitions from early high-K calc-alkaline magmatism in the NE to a younger, shoshonitic phase in the SW. We use a full suite of whole-rock geochemical analyses, including rare earth elements, to show a shared metasomatized mantle source of the magmatism. Evidence of plagioclase saturation from the onset of crystallization and amphibole-pyroxene-controlled fractionation in the high-K calc-alkaline magmatism suggest a drier (<4.75 wt% H 2 O) parental magma than is typical of subduction-related magmatism. Continued H 2 O depletion of the metasomatized source mantle resulted in the transition to a shoshonitic trend where deep crustal fractionation of an H 2 O-poor (< ~2 wt% H 2 O) magma in the absence of major olivine resulted in incompatible enrichment over a small range of SiO 2 . High-precision U-Pb zircon geochronology is presented here for the first time to provide chronological markers for the transition in the magmatic evolution of the Kechros dome. A 2.2 Myr break in magmatism separates the intrusion of the shoshonitic Maronia pluton at 29.8 Ma from the emplacement of the rest of the high-K calc-alkaline Maronia Magmatic Corridor between 32.9–32.0 Ma. The Maronia pluton is the hottest, driest, and youngest episode of post-collisional magmatism in the Kechros dome; we suggest that the emplacement of Maronia marks the cessation of magmatism in the northern Rhodope Core Complex as asthenospheric mantle upwelling migrated southward.


INTRODUCTION
Mountain-building events can generate thousands of kilometers of high elevation over periods of tens to hundreds of millions of years; the development of such high relief during orogenesis is supported by the development of a thick continental root (e.g., Dewey et al., 1993). In some orogens this crustal thickness is preserved long after collision, while in others post-collisional collapse of the orogen thins the crust and results in widespread extensional structures that dissect the region (e.g., Le Pichon and Angelier, 1979;Dewey, 1988;Wang et al., 2014). In extended orogens, strain localization can drive the formation of transcrustal normal detachment faults that transfer material from the ductile, lower crust to the surface and can aid the exhumation of metamorphic core complexes from >45 km depth (e.g., the North American Cordillera: Coney, 1987;Wernicke et al., 1987;Camp et al., 2015;and the Aegean: Lister et al., 1984;Jolivet and Brun, 2010;Burg, 2011;Kydonakis et al., 2015). Another feature of highly extended orogenic terranes is the ubiquitous presence of post-collisional magmatism. The conductive transfer of heat from upwelling hot, asthenospheric mantle beneath highly extended crust can generate small volume, partial melts that exploit crustal pathways developed during extensional deformation, leading to intrusive and extrusive post-collisional magmatism (e.g., McKenzie and Bickle 1988;von Blanckenburg and Davies 1995). Feedbacks between metamorphic core complex exhumation and magmatism are numerous and include magmatic volatiles facilitating crustal deformation and crustal thinning and resultant decompression and mantle upwelling, driving lithospheric melting (Teyssier and Whitney, 2002;Whitney et al., 2013;Platt et al., 2015). As a result, both metamorphic core complexes and post-collisional magmatic suites can be used to offer insights into the tectonic processes controlling crustal extension; here we focus on the petrogenesis of postcollisional magmatism.
In collisional orogenic belts, subduction-related calc-alkaline magmatism shows a trend of increasing K with maturity of the magmatic system (e.g., Lipman et al., 1972). The transition from high-K calc-alkaline to shoshonitic magmatism is common in post-collisional tectonic settings where the inherited metasomatized mantle wedge is depleted at the onset of magmatism (e.g., Aldanmaz et al., 2000;Seghedi et al., 2004). This high-K calc-alkaline to shoshonitic evolution in magma geochemistry is observed throughout the Aegean-for example, in the Eocene to middle-Miocene Biga orogenic rocks in NW Turkey and the Oligocene and Miocene magmatic rocks of NE and central Greece (e.g., Maronia, Samothraki, Lesbos and Limnos) (Del Moro et al., 1988;Pe-Piper et al., 2009;Ersoy et al., 2017).
The generation of potassic melts from the mantle is widely ascribed to be the result of anomalously hot mantle melting (Gill 1981). Source mantle geochemistry is also widely considered to be key in the generation PERKINS ET AL. | Chronological markers of post-collisional extension in the northern Aegean Timeline of Key Tectonic Events (see Table 1) Age (Ma) 0

Subduction and Collision
Post-collisional Extension   Burg (2011) and Kydonakis et al. (2014), and timeline complied from Burg (2011). The northern Rhodope Core Complex (RCC) is separated from the southern RCC by the Nestos Thrust, as marked by the gray line. Within the northern RCC, domes of lower plate gneiss were exhumed along multiple detachment and thrust faults, which are poorly mapped and not marked on the map. The exception is the Kechros Detachment, local to the Maronia Magmatic Corridor (the study area) and is marked where field and ASTER data suggest the presence of a significant trans-crustal structure. Four gneiss domes make up the northern RCC; from west to east they are the Chepinska, Arda, Kardamos-Kesebir, and Biala-Rika-Kechros domes. The timeline summarizes the key tectonic events that affected the RCC from subduction through to continent collision and exhumation (using literature geochronology compiled in Table 1 convergence to subduction-related slab rollback and trans-crustal extension (Burchfiel et al., 2008).
Exhumation in the northern RCC ceased abruptly in the Oligocene, likely with the onset of post-collisional volcanism across the northern RCC (e.g., Rohrmeier et al., 2013), and resulted in an E-trending belt of small (~30 km diameter), unconnected gneiss domes (Kydonakis et al., 2015). This was contemporaneous with a change in extension style across the Aegean. Early Eocene-Oligocene extension was localized in the northern Aegean, restricted to the northern limit of the hinterland, the northern and southern RCC (Kydonakis et al., 2015). From the mid-Miocene onward, extension was distributed across the Aegean with the formation of the Central Cyclades Core Complex further south and continued exhumation and SW-rotation of the southern RCC (van Hinsbergen et al., 2008;Kydonakis et al., 2014Kydonakis et al., , 2015. The change in extension style is attributed to an acceleration in trench retreat related to the increased rate of convergence between Africa and Eurasia and slab tearing beneath western Turkey in the mid-Miocene (e.g., Jolivet and Brun, 2010;Jolivet et al., 2013;Brun et al., 2016).
Post-subduction magmatism was contemporaneous with trans-crustal extension, punctuating across the Rhodope Massif from the middle Eocene (42 Ma, Smiliyan and Yugovo Plutons; Table 2; Rohrmeier et al., 2013) to the mid-Miocene (11.32 Ma, Strymon Valley volcanics; Table 2; Georgiev et al., 2013). Post-subduction magmatism is closely linked to extension, with trans-crustal structures providing pathways for magmas, evidenced by the spatial association of plutonic rocks and detachment fault systems (e.g., Jones et al., 1992;Pe-Piper and Piper, 2006;Fig. 1). This is noticeable in the distinction between the northern RCC and southern RCC, where magmatism and concurrent volcanism cease in the late Oligocene in the northern RCC (e.g., Márton et al., 2010;Marchev et al., 2010) but continue into the Miocene in the southern RCC (Table 2; Georgiev et al., 2013).
Unconformably overlying the northern RCC metamorphic basement, syn-extensional sedimentation started in the mid-late Eocene as a transgressive sequence of conglomerates, limestones, sandstones, and marls filled the Thrace supra-detachment basin, formed in accommodation space to the SE of the exhuming northern RCC (Kilias et al., 2013). The switch from trans-crustal, ductile extension to upper-crustal, brittle extension in the northern RCC is recorded by the change in depositional environment from a supra-detachment basin in the Eocene-Oliogene, to a series of cross-cutting N-S-to NNE-SSW-trending fault-bound basins in the Oligocene-Miocene (Burchfield et al., 2003(Burchfield et al., , 2008Kilias et al., 2013).

The Maronia Magmatic Corridor
The MMC is a NE-trending magmatic belt, comprising at least six Oligocene gabbroic to granodioritic intrusions (Fig. 2): the Tris Visses, Halasmata, Leptokaria, Kirki, Kassiteres, and Maronia plutons. The MMC is hosted in the easternmost dome of the northern RCC, the Biala-Rika-Kechros dome (from here onward referred to as the Kechros Dome). First described by Del Moro et al. (1988), the MMC plutons all intrude the northern RCC and associated lithologies, hosted either within the Kechros gneiss dome to the NE (Halasmata and Tris Visses), straddling the Kechros basal detachment in the center of the corridor (Leptokaria),  Bonev et al. (2015). LA-ICP-MS-laser ablation inductively coupled plasma-mass spectrometry; WR-GRT-CPX-whole-rock-garnetclinopyroxene; SHRIMP-sensitive high-resolution ion microprobe.
or within the CRB greenschist basement to the SW (Kirki, Kassiteres, and Maronia; Fig. 2). Later Oligocene intercalated sedimentary and volcanic units onlap onto the MMC intrusions. Both the Kassiteres and Maronia plutons are cross-cut by younger porphyritic microgranite intrusions and host occurrences of porphyry-epithermal-style Au-Mo-Cu mineralization (e.g., Melfos et al., 2002;Voudouris et al., 2006). Rb-Sr biotite-wholerock isochron dating by Del Moro et al. (1988) suggests that the MMC plutons were emplaced between ca. 32 and 28 Ma, younging progressively from NE to SW along the belt.

Field Geology and Petrology
Field observations, with particular focus on relationships between lithologies, were made over multiple summer field seasons in northeastern Greece. We have prepared our geological map, Figure 2, from existing geological maps of the area supplemented by our own field observations. The spatial extent of the plutons, in particular Kirki, Leptokaria, Halasmata, and Tris Visses, are poorly constrained due to difficult field conditions. The lack of exposure, steep topography, dense vegetation, and poor road access bias the outcrop localities of both this work and previous geological mapping by Del Moro et al. (1988), with most mapped outcrops found along small river channels. However, based on our field observations, we consider it more likely that the observed plutons represent small outcrops of a larger magmatic system interconnected at depth that is poorly exposed, and that additional outcrops could be found in more inaccessible areas.
A suite of 36 samples was collected from the MMC, representing each of the observed intrusive lithologies and alteration styles. At least eight samples were taken from each of the Maronia and Kassiteres plutons, and from across the Kirki-Leptokaria-Halasmata-Tris Visses outcrops to reflect changes in mineralogy between samples. Standard polished thin sections and epoxy mounts were prepared for a representative selection of samples from each pluton, and detailed petrographic observations were  (2011), supplemented by more recent geochronology. TIMS-thermal ionization mass spectrometry; LA-ICP-MS-laser ablation-inductively coupled plasma-mass spectrometry. made under both transmitted and reflected light. Point counting of the least altered thin section(s) of each of the plutons was undertaken to obtain representative modal compositions of the samples. Scanning Electron Microscope (SEM) analysis was undertaken at the University of Bristol on a Hitachi S-3500 N instrument with 20 kV accelerating voltage and a 15 mm working distance. Phase identification was performed using energy dispersive spectra (EDS), and back-scatter electron (BSE) imagery was used to establish key textural relationships.

Whole-Rock Geochemistry
Powders of <25 µm grain size were prepared at the University of Bristol and analyzed in the Inorganic Geochemical Laboratories at the British Geological Survey (BGS), Keyworth. Powders were prepared for each of the igneous lithologies collected, with a total of 36 samples and eight repeats analyzed. Weathered outer rims of individual samples were removed such that only the freshest material from the cores of the samples were used for analysis. Samples were then crushed and milled in a ball mill using an agate jar with 12 agate balls. Due to the large grain size (>1 mm) and relative heterogeneity of the samples, >1 kg of rock was used to ensure a homogenous powder was produced; a greater weight of sample was used for coarser-grained samples from the Maronia pluton. Samples were analyzed following the UKAS Accreditation Schedule using wavelength dispersive-X-ray fluorescence spectrometry (WD-XRFS) on fused glass beads for major and minor element oxides, Na 2 O 2 fusion inductively coupled plasma-mass spectrometry (ICP-MS) for minor and trace element concentrations, and an aqua regia digest ICP-MS for Sc and Te.
New geochemical data presented in this study supplements previously published work by Del Moro et al. (1988), Papadopoulou et al. (2004), and Mavroudchiev et al. (1993), presenting for the first time a complete suite of trace element and REE analyses for the entire magmatic belt. The results have been filtered by loss on ignition (LOI) values such that any results with LOI >2 wt% have been omitted, removing any bias in the results from heavily clay-altered or hydrated samples.

U-Pb Zircon Geochronology
A subset of nine samples from the MMC sample suite were selected for high-precision U-Pb zircon geochronology (Maronia, n = 4; Kassiteres and Leptokaria n = 5). The MMC zircons have broadly similar euhedral morphologies with many inclusions of apatite, titanite, and melt. Zircons from Maronia were typically larger than those from Kassiteres and Leptokaria and were less abundant in the mineral separate. Inclusion-and defect-free crystals with well-preserved euhedral crystal faces were preferentially selected from the mineral separate for analysis (further details of sample preparation and zircon selection are included in GSA Data Repository File DR2 1 ). The remaining zircon crystals were mounted in epoxy, polished, and imaged at the University of Bristol using a Centaur Cathodoluminescence (CL) detector on a Hitachi S-3500N Scanning Electron Microscope at 10 kV and 23 mm working distance. CL imagery of the zircons was used to investigate the internal morphology of the crystals, with particular focus on growth and resorption features in the zircons.
A total of 42 zircon crystals were analyzed at the NERC Isotope Geosciences Laboratory (NIGL) using the CA-ID-TIMS technique. Three to seven individual zircon crystals from each sample were thermally annealed and chemically abraded to reduce Pb loss (Mattinson, 2005). Isotope dilution and mass spectrometry protocol followed the techniques and instrumentation described by Sahy et al. (2015), and further details are included in GSA Data Repository File DR2.

FIELD GEOLOGY AND PETROLOGY
We present primary field and petrographic observations of the MMC plutons. This work builds upon that of previous authors, in particular Del Moro et al. (1988), and focuses on the most voluminous intrusions along the belt: Maronia, Kassiteres, and Leptokaria (Fig. 2). Two additional, later phases of magmatism are found intruding each of the plutons: an array of aplitic dykes and small (~500 m 2 ), silicic porphyry intrusions. The felsic units are volumetrically insignificant compared to the host plutons and associated with post-pluton hydrothermal activity, including porphyry-style alteration and mineralization in the area, leading to intense alteration of these rocks (e.g., Melfos and Voudouris, 2016, and references therein). We consider the small-volume, evolved lithologies to be less important to the discussion on the changing geochemistry of the mantle magmatic system in the northern RCC, as they are highly fractionated and likely represent a mixed geochemical signature, reflecting both crustal and mantle melt sources.

The Leptokaria Complex
The Leptokaria, Kirki, Halasmata, and Tris Visses plutons were first described individually by Del Moro et al. (1988). However, in this paper, we group these plutons together on the basis of similar geochemistry and age, and we refer to them collectively as the Leptokaria complex, after the largest pluton in the group (Fig. 2).
The equigranular (0.2-1.0 mm) Leptokaria complex ranges from monzogabbro to granite in composition (Fig. 3A). A primary assemblage of phenocrystic plagioclase + magnetite + hornblende + biotite is found in all samples of the Leptokaria pluton; clino-and ortho-pyroxene are present in the dioritic Leptokaria samples, while quartz + K-feldspar and accessory zircon + apatite are also present in the more evolved samples. Interstitial, granophyric quartz and K-feldspar intergrowths are found throughout the more evolved samples. Weak alteration, principally chlorite + epidote ± biotite ± orthoclase, is pervasive across Leptokaria, with breakdown of mafic phases and sericitization of plagioclase. Small (0.5-1-cm-wide) veinlets of albite ± chlorite ± epidote are also common. Five of the Leptokaria samples have a cumulate-like texture with interlocking phenocrysts of plagioclase, amphibole (replacing relict clinopyroxene), and magnetite and the development of interstitial, poikolitic amphibole and plagioclase and visible plagioclase rims (Figs. 3B-3C). These cumulate-like rocks likely characterize the mineral phases present during earliest magmatic crystallization and differentiation. Alteration is continuous through the cumulate samples with chlorite-biotite-epidote alteration of mafic phases the most common reactions observed.
The Leptokaria plutons are intruded into ortho-and para-gneisses of the Kechros Dome within the northern RCC. Around Leptokaria village, the pluton is hosted in a two-mica pegmatitic leucogranite, although the contact between the leucogranite and Leptokaria complex is not seen. Contacts between the plutons and the gneisses elsewhere in the Leptokaria Complex are undulose, with mixing of globules of gneiss in the pluton and vice-versa, this suggests that the host gneisses were not completely solid during magma emplacement and accumulation.

Kassiteres
Kassiteres is a medium-grained (1-3 mm), clinopyroxene-hornblende diorite and is the next pluton moving SW along the MMC from Leptokaria (Fig. 2). The smallest of the named intrusions, Kassiteres has an aerial extent of ca. 1.8 km 2 . Kassiteres intrudes into metapelitic greenschists of the CRB basement and is unconformably overlain by Oligocene volcaniclastic units. The primary mineralogy of the Kassiteres diorite is clinopyroxene + hornblende + plagioclase + biotite + magnetite phenocrysts and interstitial K-feldspar + quartz with accessory apatite + titanite + zircon (Fig. 3E). Mixing of mafic, clinopyroxene-rich, and felsic, biotite-rich, end members are visible on the macroscopic scale, suggesting periodic injection of new melt into the crystallizing magmatic system (Fig. 3D). Interstitial granophyric quartz-feldspar intergrowths suggest rapid cooling of interstitial melt during the later stages of pluton crystallization (Fig. 3F).
The Kassiteres pluton is variably altered due to hydrothermal activity related to the later intrusion of a porphyritic stock and aplite dykes. Breakdown of clinopyroxene and hornblende to biotite, chlorite, and plagioclase is characteristic of potassic alteration at Kassiteres, pervasive to varying degrees of intensity throughout the pluton. Advanced clay alteration is found in discrete fault zones within the diorite where the fault planes provide a pathway for enhanced fluid-wallrock interaction. This alteration grades from silicified, advanced argillic alteration (silica + low-temperature clays) proximal to the fault plane, to argillic alteration (high-temperature clays) over 5 m from the fault. A diffuse gradation into potassically altered diorite occurs over a greater distance of 15-20 m from the faults.

Maronia
Maronia is the southernmost pluton of the MMC, separated by ~30 km from its nearest neighbor, Kassiteres. Outcropping as a stock with an area of ~3.2 km 2 , it is intruded into metapelites and marbles of the CRB. Three intrusive lithologies were recognized at Maronia by Papadopoulou et al. (2004): a voluminous monzonite-quartz monzonite unit (Fig. 4A), a minor gabbroic unit, and a minor porphyritic granitic unit. The gabbro is texturally and mineralogically indistinguishable from the monzonite-quartz monzonite unit in all but whole-rock geochemistry, while the granite porphyry shows sharp, cross-cutting contacts with the rest of the pluton (Fig. 4F). In this paper, we consider the gabbro-monzonite-quartz monzonite as a single unit (referred to herein as the monzonite unit) with subdivisions arising from varying phenocryst proportions.
Additional igneous facies are observed along the western margin of Maronia where the pluton intrudes CRB marbles. In addition to a thin (~1 m) endo-skarn boundary along the marble-monzonite contact (Katerinopoulou et al., 2009;Fig. 4E), an orthocumulus facies is exposed in the northwest of the pluton extending for up to 10 m into the monzonite from endoskarn boundary (Fig. 4D). Cumulus pyroxene adacrysts and intercumulus oikocrystic orthoclase-albite suggest that the western margin of the pluton exposes close to the base of the Maronia plutonic system. This orthocumulus facies is not observed on the eastern margin of Maronia where a ~250-m-wide hornfels aureole extends into the host CRB metapelites.
The NNE-SSW trending Ktismata Fault cuts the pluton separating the eastern and western portions of the pluton (Melfos et al., 2002;Fig. 4F). Differing intrusive relationships between the western (deeper, orthocumulus facies) and eastern (shallow, baked aureole) margins of the pluton suggest an east-dipping normal fault. Minimal right-lateral displacement of < 100 m is observed where the fault cuts the northern pluton margin, which can be accounted for by a minor oblique component of motion.
An additional strike-slip fault, the Marmaritsa Fault, cuts the southern margin of the Maronia pluton subparallel to the coastline. This structure is part of a regional crustal trend, potentially related to the North Anatolian Fault Zone (Melfos et al., 2002). The brittle nature of the faulting, cutting Oligocene magmatic features within Maronia, suggest post-emplacement initiation of both structures.

Whole-Rock Geochemistry
The MMC samples vary in composition from 48 to 68 wt% SiO 2 ( Fig. 5; Table 3; GSA Data Repository Table DR1). The Maronia pluton has a more restricted SiO 2 compositional range (50-56 wt% SiO 2 ), while the Kassiteres and Leptokaria samples cover the full range (48-68 wt% SiO 2 ). Within the Leptokaria sample suite, a difference in SiO 2 content separates cumulate-like samples (48-53 wt% SiO 2 ) from the intermediate to felsic plutonic samples (55-68 wt% SiO 2 ), with all the Leptokaria samples falling on a linear fractionation trend. Samples from the MMC follow a calc-alkaline differentiation trend; however, their similarity to "typical" calc-alkaline magmatism is limited. As SiO 2 increases with differentiation, compatible elements, including MgO, with fractionation (Fig. 6H). The MMC shows a spatial and geochemical migration from purely calc-alkaline magmatism in the Leptokaria samples in the north, to high-K calc alkaline magmatism in the central Kassiteres samples, to shoshonitic magmatism in the southern Maronia pluton (Fig. 5A). In the Maronia samples, the absolute concentration of the incompatible elements is 1-1.5 times greater than in Kassiteres and Leptokaria samples, in line with shoshonitic magmatism (Figs. 6G-6H).
Irregular trace element patterns are observed from all plutons along the MMC (Fig. 7A). The MMC samples are enriched in fluid mobile, largeion lithophile elements (LILEs; e.g., Ba, Rb, Pb) and depleted in fluid immobile, high field-strength elements (HFSEs; e.g., Nb, Ta, Zr) compared to the primitive mantle, in a trace signature typical of a metasomatized, fertile mantle source (Elliott et al., 1997). The plutons are enriched in the light REEs relative the heavy REEs (Fig. 7B) and show decreasing Dy/ Yb (middle REE to heavy REE fractionation) with differentiation (Fig.  6J). The mildly concave REE pattern is flatter than the highly convex, amphibole-controlled REE fractionation trend, and is most similar to that of a clinopyroxene-hornblende-controlled REE pattern (Pearce and Peate, 1995;Tatsumi, 1989;Davidson et al., 2013). A pronounced, negative Eu anomaly, which decreases with increasing SiO 2 , is observed in all of the MMC samples (Fig. 6I).

U-Pb Zircon Geochronology
CL imaging revealed simple zoning patterns in most all of the zircons from the MMC (further descriptions are included in GSA Data Repository File DR2). Oscillatory and sector zoning in the crystals reflects a simple magmatic history of the zircons with little evidence of zircon inheritance suggesting the zircons experienced a single period of crystallization. Therefore, dissolution of full zircon crystals was justified to use the largest sample volume and reduce analytical uncertainty, offering the highestprecision dates. Two individual zircon analyses were excluded from our interpretations as high common Pb (Pb* > 2.5 pg) was taken as an indicator of a low precision analysis (see GSA Data Repository Table DR3).
Our results were interpreted within a magmatic framework in which it is recognized that zircon crystallization is protracted over time scales of 10 5 -10 6 yr (e.g., von Quadt et al., 2011). The youngest statistically significant population of zircons was sought to represent the timing of the final stages of crystallization. Our preferred age interpretations are presented in Table 4; alternative interpretations and a discussion of the sample discordance are included in GSA Data Repository File DR2. All ages reported are 206 Pb/ 238 U to minimize the influence of excess 207 Pb, the result of initial U/Pa disequilibrium, on our age interpretations (Crowley et al., 2007). Dates are reported in millions of years ago (Ma) to 2σ uncertainty.
The 206 Pb/ 238 U ages form two distinct groups (Fig. 8). We can interpret two geological windows in time for pluton emplacement and crystallization, (a) intrusion of the Kassiteres and Leptokaria plutons over 0.9 Myrs (from 32.96 to 32.02 Ma); and 2.2 Myrs later (b) intrusion of the Maronia pluton over 0.2 Myrs (from 29.78 to 29.57 Ma). Antecrystic zircons, those crystallized prior to final pluton crystallization either during an earlier phase of magmatism or deeper in the magmatic plumbing system, are recognized in the MMC analyses (e.g., z2 RT14_022; Table  DR3 [see footnote 1]). Zircons that are much older that the interpreted ages of crystallization (>5 Myrs) are interpreted as xenocrystic zircons, entrained from the wallrock during magma ascent (e.g., z2 RT14_024; File DR2 [see footnote 1]). The autocrystic zircons, those crystallized in situ in the magmatic system, show good clustering in seven of the nine samples from the MMC, allowing interpretation of a youngest population of zircons (Table 4). In Leptokaria samples RT14_022 and RT14_024, the youngest zircon is taken as a maximum age of crystallization of the pluton, allowing the possibility of younger zircons in the sample.

DISCUSSION
Our geochemical and isotopic data strongly suggest heterogeneity between Maronia and the remainder of the MMC. Therefore, the petrogenesis of the Maronia pluton will be discussed separately to the Kassiteres-Leptokaria magmatic suite before the magmatism along the MMC as a whole is discussed in relation to the extensional tectonics in the Kechros dome.    SiO 2 (wt %) SiO 2 (wt %)     Note: x uncertainty-analytical only; y uncertainty-analytical + tracer calibration + 238 U decay constant. MSWD-mean square weighted deviation.

Timescales of Magmatic Emplacement
Previous geochronological data from the MMC comprises nine Rb-Sr biotite-whole-rock pairs ranging from 34.9 ± 1.5-28.9 ± 1.5 Ma, which are interpreted to reflect an ~8 Myr window of magmatism along the MMC, younging from NE to SW (Del Moro et al., 1988). However, the Rb-Sr geochronometer is highly susceptible to disturbance by hydrothermal overprinting (Jenkin et al., 2001), and at least three of the MMC plutons are associated with significant hydrothermal activity (Melfos and Voudouris, 2017, and references therein). Our high-precision zircon CA-ID-TIMS U-Pb geochronological data show two tightly constrained magmatic windows: (i) 0.9 Myrs (from 32.045 ± 0.027-32.932 ± 0.039 Ma) for the Kassiteres-Leptokaria magmatic suite, and (ii) 0.2 Myrs (from 29.596 ± 0.027-29.762 ± 0.024) for the Maronia magmatic suite (Table  4; Fig. 8). These two discrete time periods are consistent with the distinct geochemical signature of Maronia compared to Kassiteres-Leptokaria.
Within the Kassiteres-Leptokaria magmatic suite, our data do not duplicate the younging of magmatic activity from NE to SW concluded by Del Moro et al. (1988). While the youngest and oldest samples, RT14_028 (32.045 ± 0.027 Ma) and RT14_010 (32.932 ± 0.039 Ma), respectively, do follow this SW-trend, the remaining three samples show ongoing crystallization in the intervening time period, consistent with pulsed magma accumulation across the pluton. This agrees with our earlier interpretation that there is potential for the Leptokaria outcrops to be connected at depth by a large, mostly unexposed magmatic system. The inter-sample age range (0.887 ± 0.039 Myrs at Leptokaria and 0.166 ± 0.028 Myrs at Maronia) reflects the time scale of pluton cooling. As the larger intrusive body, the duration of cooling of the Kassiteres-Leptokaria complex is nearly an order of magnitude greater than the Maronia pluton.
We emphasize that the high-precision U-Pb zircon geochronology presented in this paper shows a clear temporal break in exposed, intrusive magmatism in the MMC, reflecting two discrete magmatic periods. The first magmatic period produced the Kassiteres-Leptokaria magmatic suite and was followed, after an apparent 2.2 Myr break in recorded magmatism, by the Maronia plutonic complex.

Petrogenesis of the Kassiteres-Leptokaria Magmatic Suite
Equigranular, plutonic samples from the Kassiteres-Leptokaria magmatic suite record the first stage of post-collisional magmatism hosted in the southern Kechros dome. The high-K calc alkaline magmatic suite has trace element geochemistry that is consistent with a sub-continental lithospheric mantle source (Del Moro et al., 1988;Pe-Piper and Piper, 2001). The presence of hornblende phenocrysts in textural equilibrium with surrounding mineral phases (Fig. 3E) indicates crystallization of the magmatic suite at a depth of >~3 km (Lledo and Jenkins, 2007;Mutch et al., 2016). The intrusions show granophyric interstitial textures reflecting fast cooling of the last vestiges of melt during rapid final crystallization.
The major and minor element geochemistry of the Kassiteres-Leptokaria suite, most notably the steadily depleting Al 2 O 3 with fractionation, the consistently negative Eu anomaly, and the compatible behavior of Sr, suggest plagioclase was an important phase during magmatic differentiation. The presence of abundant plagioclase in the Leptokaria samples with a cumulate-like texture further supports the early appearance of plagioclase on the liquidus (Table 3). This is atypical of arc-type magmatism where partial mantle melts with elevated water contents suppress plagioclase saturation (Gaetani et al., 1993). Decreasing Dy/Yb with fractionation ( Fig. 6J) suggest clinopyroxene and/or amphibole are the predominant ferromagnesian phases controlling fractionation (Davidson et al., 2013). Amphibole is the most abundant phase in the cumulate-like samples (Table 3); however, relict subhedral clinopyroxene in the core of many of the amphibole phenocrysts (Fig. 3B) suggests clinopyroxene was a precursor to amphibole during fractionation. This can account for the more primitive REE pattern ( Fig. 7B; Smith, 2014).
Experimental studies on the effect of intensive parameters: temperature (T), pressure (P), H 2 O content, and oxygen fugacity (fO 2 ), on phase equilibria are commonly used to investigate the physiochemical conditions of magma differentiation, crystallization, and storage (e.g., Scaillet et al., 2016). Naney (1983) produced phase assemblage diagrams, at both 200 MPa (~6 km) and 800 MPa (~24 km), of fO 2 controlled granodiorite compositions as a function of T and H 2 O content. The synthetic starting composition used by Naney (1983), "R5 + 10MI," and the natural analogue hornblende, biotite, granodiorite R5 (Nockolds, 1954), are similar in composition to the Leptokaria tonalities that make up the more evolved end member of Kassiteres-Leptokaria magmatism (GSA Data Repository Table DR1). Using the major fractionating assemblage of plagioclase + pyroxene + amphibole and the phase diagrams of Naney (1983), the T and H 2 O field of lower crustal fractionation (at P = 800 MPa, 24 km) can be constrained to 880 < T < 980 °C and 3 < H 2 O content < 4.75 wt% (Fig. 9). The drier nature of the magmas suggests that the fertile mantle source has been tapped by multiple different melting events during which H 2 O has been mobilized, prior to the generation and emplacement of the Kassiteres-Leptokaria magmas between 32.9 and 32.0 Ma. We suggest that the elevated temperature necessary to melt a drier mantle wedge (>880 °C, Fig. 9) was provided by the accumulation of heat during prolonged (more than ~10 Myr) asthenospheric upwelling and mantle melting in the northern Aegean, which began in the mid-Eocene (e.g., Rohrmeier et al., 2013).

Petrogenesis of the Maronia Pluton
Maronia shows a different petrogenetic evolution to Kassiteres-Leptokaria, coincident with a 2.2 Myr temporal, and a 30 km spatial, break in recorded magmatism. The Maronia monzonite has a shoshonitic affinity with significant enrichment in incompatible and volatile elements (e.g., P 2 O 5 , Pb, Sr, Rb; Figs. 5B, 6G, 6H). Geochemical and isotopic evidence advocate a heterogeneous, highly metasomatized source mantle as the foundation for the generation of ultra-potassic shoshonitic to lamproitic magmas (e.g., Foley et al., 1987;Beccaluva et al., 2013;Condamine and Médard, 2014;Müller and Groves, 2016). Sr isotopic data from the MMC reported by Del Moro et al. (1988;  This is unlike the Sr isotopic data reported from the Eocene-Oligocene Biga orogenic volcanic rocks, NW Turkey, where the magmatic suite becomes increasingly radiogenic and potassic with time, which is interpreted to reflect an increasing crustal contribution (Ersoy et al., 2017). Consequently, we suggest that the MMC source mantle was enriched by a radiogenic source, likely crustal-derived metasomatic fluids, prior to the start of high-K calc-alkaline magmatism at Kassiteres-Leptokaria. Therefore, processes involved in magma generation and fractionation were responsible for the evolution of a shoshonitic trend at Maronia. Further to this, experimental data for the contamination and mixing of mantle and sedimentary melts in the production of a K-rich starting melt generate evolved, peraluminous shoshonitic magmas (Wang et al., 2017) while the Maronia magmatic suite has a highly restricted SiO 2 (51-56 wt% SiO 2 ) range and is metaluminous.
Melting of a phlogopite-rich veinlet in the highly metasomatized source mantle could be a potential source of an anomalously K-rich melt from which to fractionate a shoshonitic trend (e.g., Condamine and Médard, 2014). Such melts would inherit a diagnostic trace element pattern due to the compatible nature of LILEs (e.g., Rb and Ba) and incompatible nature of HFSEs (e.g., Zr and Y) in phlogopite (LaTourette et al., 1995). However, the Maronia samples show a relative enrichment in Rb and Ba over the Kassiteres-Leptokaria suite and similar values of Zr and Y (Fig.  7A). Direct mantle melts would be expected to have a Mg# = 80 -85 (Mg# = Mg/(Mg + Fe 2+ ) * 100; molar), which will decrease with fractionation (Herzberg et al., 2014). The Mg# of the Maronia shoshonites ranges from 49.4 < Mg # < 60.2 indicating that significant fractionation from direct lherzolitic or harzburgitic mantle would be required to produce the potassic fractionation trend observed at Maronia. Therefore we favor the model of pyroxene + An-rich plagioclase + magnetite fractionation in the absence of major olivine, in an H 2 O-poor magma, as a mechanism to produce a shoshonitic LLD (Meen, 1987;Freise et al., 2009;Lanzo et al., 2016;Beermann et al., 2017).
We have modeled fractional crystallization by subtracting varying proportions of the proposed orthopyroxene + clinopyroxene + An-rich plagioclase + magnetite fractionating assemblage (Meen, 1987;Beermann et al., 2017), from the most primitive known sample from Maronia (MP 81; Mavroudchiev et al., 1993). We do not include olivine in our fractionating assemblage as we consider it to be a minor phase in the generation of a LLD (Meen, 1987) and we assume K 2 O to be incompatible during fractionation (Freise et al., 2009). The observed fractionation trend from the parent magma can be closely matched by a modeled fractionation assemblage of 15% orthopyroxene (En 0.8), 25% clinopyroxene (Di 0.8), 52% plagioclase (An 75) and 8% magnetite (Mag 0.7; Figure 10A; Table 5). The close agreement between the modeled and observed LLDs suggests that fractionation of pyroxene + plagioclase (+ minor magnetite), which have higher silica content than olivine (OL ~30 wt% SiO 2 ; OPX ~50 wt% SiO 2 ; CPX ~45 wt% SiO 2 ; PLAG ~50 wt% SiO 2 ), can sufficiently enrich the residual magma in incompatible elements with small changes in melt SiO 2 , to produce a shoshonitic LLD (Meen, 1987;Beermann et al., 2017). Using the predicted fractionating phase assemblage and estimated bulk partition coefficients (Table 6), the observed trace element fractionation trends can be reproduced and are shown in Figure 10B for Ba, Cr, Rb, Sr, Zr, and Y.
A pyroxene + plagioclase + magnetite fractionating assemblage can be produced by an H 2 O-poor (<2-3 wt% H 2 O) magma at high-T (950-1050 °C) and a range of pressures (200-1,000 MPa; Baker and Eggler, 1987;Meen, 1987;Beermann et al., 2017). This suggests that fractionation of the same parental mantle source region as Kassiteres-Leptokaria could produce a shoshonitic trend with a similar trace element signature as the  Figure 10. Modeled major (A) and trace element (B) distribution during clinopyroxene + orthopyroxene + plagioclase + magnetite fractionation in the absence of olivine (after Meen, 1987). The distribution of Maronia samples can be closely reproduced by the model; see Table 5 for details of fractionating phase compositions and proportions and melting conditions between the intrusion of the Kassiteres-Leptokaria magmatic complex and intrusion of the Maronia pluton. We suggest that the temporal break (2.2 Myrs) in recorded magmatism in the MMC is the result of H 2 O loss in the source mantle driving up the solidus temperature such that, between intrusion of Kassiteres-Leptokaria and Maronia, the source mantle was at sub-solidus conditions. We propose that prolonged heat conduction from the upwelling thermal anomaly beneath the Kechros dome in the intervening 2.2 Myrs is a potential source for the additional heat input required. Resultant melting of the H 2 O-poor source mantle stabilized a pyroxene + plagioclase-dominated fractionating assemblage and generated a shoshonitic LLD at Maronia. A similar mechanism of restricted H 2 O flux into a metasomatized mantle source is suggested for the generation of the shoshonitic volcanics in the Aeolian arc, Italy (e.g., Peccerillo et al., 2013;Beermann et al., 2017).

Summary of Magmatism and Extension in the Northern RCC
Post-collisional magmatism in the northern RCC began in the mid-Eocene, contemporaneous with a period of trans-crustal extension and core complex exhumation, and continued into the Oligocene (Table 2; e.g., Marchev et al., 2005;Rohrmeier et al., 2013). This phase of postcollisional extension is attributed to the collapse of thickened orogenic lithosphere induced by slab rollback of the N-ward subducting African plate (e.g., Jolivet and Brun, 2010;Ring et al., 2010;Burg, 2011;Kydonakis et al., 2015). Eocene-Oligocene magmatism from across the northern RCC has a strong enriched metasomatic mantle signature, the result of asthenospheric mantle upwelling beneath thinned lithosphere remobilizing and re-melting the subcontinental lithospheric mantle (e.g., Pe-Piper et al., 2009).
In the Kechros dome, Eocene crustal extension was accommodated by tectonic unroofing of the lower-crustal gneisses along the Kechros detachment fault, a trans-crustal structure that would be later exploited by post-collisional magmatism (Marchev et al., 2010;Márton et al., 2010). An Oligocene, high-K calc-alkaline magmatic event at 32.9-32.0 Ma in the southern Kechros dome resulted in the intrusion of the Kassiteres-Leptokaria magmatic suite, a high-K calc-alkaline series of intrusions with a strong subduction-related sub-continental lithospheric mantle signature. Following a 2.2 Myr break in magmatism, the intrusion of the shoshonitic Maronia pluton marks the last magmatic activity in the Kechros dome prior to tectonic change. The potassic intrusion, enriched in incompatible and volatile elements, marks an anomalously hot, short period of magmatism in the Kechros dome. Melting of a now H 2 O-poor (<~2 wt% H 2 O) mantle source fractionated plagioclase + pyroxene to produce the Maronia intrusion and marked the culmination of asthenospheric heating of a mature, Cretaceous metasomatized lithospheric mantle. We suggest that the termination of magmatism in the Kechros dome following the intrusion of the Maronia pluton at 29.6 Ma reflects the southward migration of the asthenospheric upwelling, switching off the heat source for mantle melting beneath the Kechros dome. We hypothesize that the asthenospheric upwelling migration is co-incident with initiation of SW-rotation of the southern RCC as described by van Hinsbergen et al. (2008) and others. The continuation of post-collisional magmatism in the southern RCC, such as the Miocene shoshonitic magmatism in Samothraki (e.g., Christofides et al., 2000) and Skouries (e.g., Siron et al., 2016) reflects the persistence of asthenospheric upwelling beneath the southern RCC and melting of the mature, H 2 O-poor (<~2 wt% H 2 O) mantle wedge.

CONCLUSIONS
The MMC intrusions provide chronological markers and geochemical tracers that record the petrogenetic evolution of magmatism in the Kechros dome, northern RCC. Late Cretaceous to Oligocene extension of the northern Aegean resulted in asthenospheric mantle upwelling beneath the northern RCC, which provided a heat source for re-melting of the Cretaceous metasomatized mantle wedge. High-precision geochronology coupled with a full suite of whole-rock geochemical analyses, detailed field and petrographic observations, and petrological modeling demonstrate that: 1. The MMC does not represent a continuous magmatic period between 35 and 28 Ma (Del Moro et al. 1988) but two, temporally and geochemically distinct, magmatic complexes, the Kassiteres-Leptokaria magmatic suite and the Maronia plutonic complex. Intrusion of Kassiteres-Leptokaria was between 32.05 ± 0.02 and 32.93 ± 0.02 Ma, while magmatism at Maronia lasted from 29.60 ± 0.02-29.76 ± 0.02 Ma. In addition, it is recommended that the MMC zircon U-Pb data presented here are used in preference to Del Moro et al. (1988) to constrain the timing of magmatic activity in the MMC; 2. Extended hydrous (3 < H 2 O content < 4.75 wt%) partial mantle melting beneath the northern RCC allowed for the intrusion of batches Note: MP-81 was chosen as the starting composition as it had the most primitive mantle signature (a combination of low SiO 2 , high Mg# and high Ni, Cr concentrations). The predicted liquid lines of descent can be found in Figure 10 plotted alongside the whole-rock analyses.  of lithospheric mantle-source magma over an ~900,000 year time scale at Kassiteres-Leptokaria; 3. The shoshonitic affinity of the Maronia samples reflects incompatible element enrichment produced by plagioclase + pyroxene-controlled mantle fractionation from a source mantle highly depleted in H 2 O (<~2 wt% H 2 O); and therefore, 4. The temperature of mantle melting beneath the Kechros dome increased between intrusion of the Kassiteres-Leptokaria magmatic suite (at 32.9-32.0 Ma) and the Maronia pluton (29.8-29.6 Ma). Consequently, we suggest that the apparent 2.2 Myr break in recorded magmatism could be the result of a sub-solidus lithospheric mantle, and that heat accumulation in the intervening time period provided the heat required for the renewal of mantle melting.