Karsli, Orhan; Caran, Şemsettin; Dokuz, Abdurrahman; Çoban, Hakan; Chen, Bin; Kandemir, Raif

The Eastern Pontides is characterized by the many of intrusive bodies formed throughout the late Mesozoic-early Cenozoic. Most of these are I-type granitoids, but here, we present for the first time an A-type pluton from the region to assess source characteristics and geodynamic implications. The A-type Pirnalli pluton has a SHRIMP zircon U-Pb age of 81.2±1Ma. It is composed of granite, syenite and quartz monzonite, and its enclaves are monzonitic in composition with elevated Ga/Al ratios and low Mg# (<42). The pluton is dominantly metaluminous (A/CNK=0.88 to 1.00) and belong to shoshonitic and ultra-potassic series. The samples are highly enriched in LREE and show significant negative Eu (Eu/Eu *=0.33 to 0.92), Ba, Nb, Sr and Ti anomalies in the spidergrams. The host rocks and their enclaves posses quite similar Sr-Nd isotopic compositions (I Sr=0.70693 to 0.70736, ε Nd (81Ma)=-2.6 to -2.0, T DM=0.94 to 1.12Ga), pointing to a lower crustal parental magma with a minor contribution of lithospheric mantle. We hypothesize that upwelling of asthenosphere triggered melting of chemically enriched upper mantle and basic magma formed. Melting of lower crust was provided by the underplating of this basic magma. Mixing between lower crust- and mantle-derived melts at depths of lower crust appears to be most reasonable petrogenetic process responsible for generation of the pluton. Sr-Nd isotope modeling suggests mixing of 82-90% of the lower crustal-derived melt with ~10-18% of the mantle-derived melt. Then the hybrid melt ascended to shallower crustal level and underwent a limited fractionation process to generate a variety of rock types. Our data also suggest that the A-type Pirnalli pluton likely formed at an extensional environment of active continental margin throughout the late Cretaceous. Ongoing extension then led to opening of Black Sea as a back-arc basin further north of the Eastern Pontides.

Uysal, Ibrahim; Ersoy, E. Yalcin; Karsli, Orhan; Dilek, Yildirim; Sadiklar, M. Burhan; Ottley, Chris J.; Tiepolo, Massimo; Meisel, Thomas

We present new, whole-rock major and trace element chemistry, including rare earth elements (REE), platinum-group elements (PGE), and Re-Os isotope data from the upper mantle peridotites of a Cretaceous Neo-Tethyan ophiolite in the Mugla area in SW Turkey. We also report extensive mineral chemistry data for these peridotites in order to better constrain their petrogenesis and tectonic environment of formation. The Mugla peridotites consist mainly of cpx-harzburgite, depleted harzburgite, and dunite. Cpx-harzburgites are characterized by their higher average CaO (2.27 wt.%), Al2O3 (2.07 wt.%), REE (53 ppb), and Os-187/Os-188((i)) ratios varying between 0.12497 and 0.12858. They contain Al-rich pyroxene with lower Cr content of coexisting spinel (Cr# = 13-22). In contrast, the depleted harzburgites and dunites are characterized by their lower average CaO (0.58 wt.%), Al2O3 (0.42 wt.%), and REE (1.24 ppb) values. Their clinopyroxenes are Al-poor and coexist with high-Cr spinel (Cr# = 33-83). The Os-187/Os-188((i)) ratios are in the range of 0.12078-0.12588 and are more unradiogenic compared to those of the cpx-harzburgites. Mineral chemistry and whole rock trace and PGE data indicate that formation of the Mugla peridotites cannot be explained by a single stage melting event; at least two-stages of melting and refertilization processes are needed to explain their geochemical characteristics. Trace element compositions of the cpx-harzburgites can be modeled by up to similar to 10-16% closed-system dynamic melting of a primitive mantle source, whereas those of the depleted harzburgites and dunites can be reproduced by similar to 10-16% open-system melting of an already depleted (similar to 16%) mantle. These models indicate that the cpx-harzburgites are the products of first-stage melting and low-degrees of melt-rock interaction that occurred in a mid-ocean ridge (MOR) environment. However, the depleted harzburgites and dunites are the product of second-stage melting and related refertilization which took place in a supra subduction zone (SSZ) environment. The Re-Os isotope systematics of the Mugla peridotites gives model age clusters of similar to 250 Ma, similar to 400 Ma and similar to 750 Ma that may record major tectonic events associated with the geodynamic evolution of the Neo-Tethyan. Rheic, and Proto-Tethyan oceans, respectively. Furthermore, >1000 Ma model ages can be interpreted as a result of an ancient melting event before the Proto-Tethys evolution. (C) 2011 Elsevier B.V. All rights reserved.

Dokuz, İbrahim; Uysal, İbrahim; Karsli, Orhan; Kandemir, Raif

-

Uysal, İbrahim; Dokuz, Abdurrahman; Karsli, Orhan; Aydın, Faruk; Kandemir, Raif

-

Dokuz, Abdurrahman; Uysal, Ibrahim; Kaliwoda, Melanie; Karsli, Orhan; Ottley, Chris J.; Kandemir, Raif

The boundary between the Gondwana and Laurussia, or a terrane separated from it, is traced by a suture recording the closure of the Rheic ocean in the Pulur, Beyçam and Kurtoǧlu areas of the eastern Sakarya Zone of Turkey. Pulur lherzolite has a lower Cr# [100Cr/(Cr+Al)=5-37] of the spinel and slightly enriched flat medium-heavy rare earth element (M-HREE) whole-rock pattern typical for those of the fertile abyssal-type lherzolite. By contrast, the Beyçam harzburgite displays the mineralogical and geochemical features of a moderately depleted residue at a supra subduction zone (SSZ). Some of spinels from the Beyçam peridotites show TiO2 contents (0.15-0.48wt.%) identical to a later interaction with a melt of island-arc tholeiite (IAT) composition. Meta-basalt from the Beyçam area is the representative of such a melt and displays the whole-rock M-HREE pattern of a complementary product of the Beyçam harzburgite. Data presented in this study support the primarily northward directed closure of the Rheic ocean, but suggest that the Variscan basement in the eastern part of the Sakarya Zone consists of mantle peridotites formed in a south-vergent SSZ. This dual polarity of subduction in the Rheic ocean, at least in the second half of its closure, is also favored by the geological structure and lithological diversity of the Variscan rocks. These are: (1) spatial distribution of the peridotites, e.g., SSZ-type harzburgites to the north and abyssal-type lherzolites to the south, (2) lack of high-pressure metamorphic rocks, (3) decrease in the degree of Variscan metamorphism toward the north, and (4) emplacement of Variscan syn-collisional magmatism into the low grade metamorphic rocks to the north. Paleotethys was the other ocean of the region created in the Late Paleozoic. Northward subduction of the Rheic ocean from Early Devonian to Early Carboniferous appears to be the most plausible mechanism responsible for the opening of Paleotethys. In this scenario the Paleotethys is envisaged as a back-arc basin opened to the north in the wake of separated terranes from Laurussia. Onset of trench-Rheic mid-ocean ridge collision in the Early Devonian along the northern margin of Laurussia has been proposed to account for the initiation of SSZ-type subduction to the south. The short duration of this subduction appears insufficient to open a fully developed back-arc basin to the south because the elimination of Rheic mid-ocean ridge to the north accelerated the oceanic closure.

Karsli, Orhan; Ketenci, Murat; Uysal, Ibrahim; Dokuz, Abdurrahman; Aydin, Faruk; Chen, Bin; Kandemir, Raif; Wijbrans, Jan

The tectonic setting of the Eastern Pontides during the late Mesozoic to early Cenozoic remains a subject of debate. Petrogenesis of adakite-like granitoid porphyries plays a critical role in determining the nature of the lower continental crust and mantle dynamics during orogenic processes in the region. Here we describe, for the first time, the late Paleocene to early Eocene adakite-like granitoid porphyries from the northern part of the Eastern Pontides although their counterparts in the southern part have recently been found. The adakitic porphyries, which emplaced into the subduction-related Turonian-Santonian volcanics from northern part of the region, consist of I-type calc-alkaline quartz monzonite-tonalite (SiO2 = 62.89-65.07 wt.%) and high-k calc-alkaline granodiorite-granite associations (SiO2 = 69.06-70.43 wt.%). The former displays peraluminous to metaluminous signatures, whereas the latter shows peraluminous geochemical character. The granite-granodiorite porphyries have high K2O (3.32-3.84 wt.%), and low Na2O (3.48-4.61 wt.%) and MgO (0.91-1.04 wt.%) relative to the quartz monzonite-tonalite association (K2O = 1.50-1.92 wt.%; Na2O = 4.08-6.45 wt.%; MgO = 1.44-2.07 wt.%). Ar-Ar geochronology studies on the amphibole separates reveal that the adakite-like porphyries have a crystallization ages of 51.34 +/- 0.27 to 53.55 +/- 0.34 Ma. Here, we contend that these rocks were formed by partial fusion of a mafic lower continental crust in a collisional phase but not in a subduction setting. All the samples exhibit the typical geochemical characteristics of adakite, that is, high Sr (250-1141 ppm), high Sr/Y ratios (16-147), low Y (6.8-14.8 ppm) and low HREE concentrations; they are similar to adakites formed by slab melting associated with the subduction zone. However, the rocks exhibit heterogeneity in isotopic composition, with I-Sr ranging from 0.70554 to 0.70986, epsilon(Nd) (51 Ma) from -8.5 to -0.9 and Nd model ages from 0.72 to 1.26 Ga. The samples are characterized by relatively high Th, Th/U and no significant Eu anomalies, implying that garnet was stable in their source during partial melting. The compositional diversity between rock groups is probably related to partial melting of heterogeneous lower crustal source. All of the features are inconsistent with a slab-melting origin and slab-related petrogenetic model, but instead, they favor an origin by melting of heterogeneous lower continental crust due to a thermal anomaly that was induced by the upwelling of the asthenosphere through a slab break-off in a collisional setting. These interpretations argue against the evidence for the presence of an early Cenozoic arc setting in the Eastern Pontides. Instead, the early Cenozoic time in the region may be attributed to a geodynamic response to a post-collisional uplift phase that occurred along the continent-continent collision between the Pontide and the Anatolide-Tauride blocks. (C) 2011 Elsevier B.V. All rights reserved.

Wijbrans, Jan; Uysal, İbrahim; Chen, Bın; Karsli, Orhan; Erduran, Murat; Boztuğ, Durmuş; Kandemir, Raif

-

Karsli, Orhan; Dokuz, Abdurrahman; Uysal, Ibrahim; Aydin, Faruk; Kandemir, Raif; Wijbrans, Jan

Early Cenozoic (48-50 Ma) adakitic volcanic rocks from the Eastern Pontides, NE Turkey, consist of calc-alkaline and high-K calc-alkaline andesite and dacite, with SiO2 contents ranging from 56.01 to 65.44 wt.%. This is the first time that Early Eocene volcanism and adakites have been reported from the region. The rocks are composed of plagioclase, amphibole, quartz, and Mg-rich biotite. They have high and low-Mg# values ranging from 55 to 62 and 13 to 42, respectively. High-Mg# rocks have higher Ni and Co contents than low-Mg# samples. The rocks exhibit enrichments in large ion lithophile elements including the light rare earth elements, depletions in Nb, Ta and Ti and have high La/Yb and Sr/Y ratios. Their relative high ISr (0.70474-0.70640) and low εNd (50 Ma) values (- 2.3 to 0.8) are inconsistent with an origin as partial melts of a subducted oceanic slab. Combined major- and trace element and Sr-Nd isotope data suggest that the adakitic magmas are related to the unique tectonic setting of this region, where a transition from a collision to an extension stage has created thickening and delamination of the Pontide mafic lower crust at 50 Ma. The high-Mg adakitic magmas resulted from partial melting of the delaminated eclogitic mafic lower crust that sank into the relatively hot subcrustal mantle, and its subsequent interaction with the mantle peridotite during upward transport, leaving garnet as the residual phase, elevates the MgO content and Mg# of the magmas, whereas low-Mg# magmas formed by the melting of newly exposed lower crustal rocks caused by asthenospheric upwelling, which supplies heat flux to the lower crust. The data also suggest that the mafic lower continental crust beneath the region was thickened between the Late Cretaceous and the Late Paleocene and delaminated during Late Paleocene to Early Eocene time, which coincides with the initial stage of crustal thinning caused by crustal extensional events in the Eastern Pontides and rules out the possibility of an extensional regime before ~ 50 Ma in the region during the Late Mesozoic to Early Cenozoic.

Karsli, Orhan; Dokuz, Abdurrahman; Uysal, Ibrahim; Aydin, Faruk; Chen, Bin; Kandemir, Raif; Wijbrans, Jan

We present elemental and Sr-Nd-Pb isotopic data for the magmatic suite (~79 Ma) of the Harşit pluton, from the Eastern Pontides (NE Turkey), with the aim of determining its magma source and geodynamic evolution. The pluton comprises granite, granodiorite, tonalite and minor diorite (SiO2 = 59.43-76.95 wt%), with only minor gabbroic diorite mafic microgranular enclaves in composition (SiO2 = 54.95-56.32 wt%), and exhibits low Mg# (<46). All samples show a high-K calc-alkaline differentiation trend and I-type features. The chondrite-normalized REE patterns are fractionated [(La/Yb)n = 2.40-12.44] and display weak Eu anomalies (Eu/Eu* = 0.30-0.76). The rocks are characterized by enrichment of LILE and depletion of HFSE. The Harşit host rocks have weak concave-upward REE patterns, suggesting that amphibole and garnet played a significant role in their generation during magma segregation. The host rocks and their enclaves are isotopically indistinguishable. Sr-Nd isotopic data for all of the samples display ISr = 0.70676-0.70708, εNd(79 Ma) = -4.4 to -3.3, with TDM = 1.09-1.36 Ga. The lead isotopic ratios are (206Pb/204Pb) = 18.79-18.87, (207Pb/204Pb) = 15.59-15.61 and (208Pb/204Pb) = 38.71-38.83. These geochemical data rule out pure crustal-derived magma genesis in a post-collision extensional stage and suggest mixed-origin magma generation in a subduction setting. The melting that generated these high-K granitoidic rocks may have resulted from the upper Cretaceous subduction of the Izmir-Ankara-Erzincan oceanic slab beneath the Eurasian block in the region. The back-arc extensional events would have caused melting of the enriched subcontinental lithospheric mantle and formed mafic magma. The underplating of the lower crust by mafic magmas would have played a significant role in the generation of high-K magma. Thus, a thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower part of the crust. In this scenario, the lithospheric mantle-derived basaltic melt first mixed with granitic magma of crustal origin at depth. Then, the melts, which subsequently underwent a fractional crystallization and crustal assimilation processes, could ascend to shallower crustal levels to generate a variety of rock types ranging from diorite to granite. Sr-Nd isotope modeling shows that the generation of these magmas involved ~65-75% of the lower crustal-derived melt and ~25-35% of subcontinental lithospheric mantle. Further, geochemical data and the Ar-Ar plateau age on hornblende, combined with regional studies, imply that the Harşit pluton formed in a subduction setting and that the back-arc extensional period started by least ~79 Ma in the Eastern Pontides.

Dokuz, Abdurrahman; Karsli, Orhan; Chen, Bin; Uysal, Ibrahim

Small granitoids emplaced into the early Jurassic volcani-clastic succession in the Yusufeli area, northeastern Turkey, can be temporally and geochemically classified into two groups: early Jurassic low-K and late Jurassic high-K. 40Ar-39Ar hornblende analyses yielded 188.0 ± 4.3 Ma for the Dutlupi{dotless}nar intrusion, dating the subduction related rifting in the region. It comprises metaluminous to weakly peraluminous (ASI = 0.94-1.11) granodiorite and, to a lesser extent, tonalite whose K2O-poor (< 2.04 wt.%) nature and weak negative Eu anomalies (Eu/Eu* = 0.9-0.7) preclude derivation by fractional crystallization from a K-rich melt. Sr, Nd and Pb isotopic data reveal derivation by partial melting from an already cooled tholeiitic basic rocks which had mantle-like isotope signature. The Sumbated intrusion formed in the late Jurassic (153.0 ± 3.4 Ma) and consists chiefly of metaluminous (ASI = 0.84-0.99) quartz monzodiorite. Medium to high-K2O, relatively high MgO and Sr contents, flat HREE patterns without prominent Eu anomalies, slightly positive εNd(t) values (+ 1.5 to + 2.5) and low ISr ratios (0.7046-0.7056) are consistent with an origin by dehydration melting of a juvenile source, above the garnet stability field, dominated by likely K-amphibole bearing calc-alkaline mafic rocks. Geochemical data show that fractional crystallization from a Sumbated-like quartz monzodioritic magma is the fundamental process responsible for the evolved compositional range of the Keçikaya intrusion. The geochemical and geochronological data presented here indicate that the late Jurassic magmatism occurred in a post-collisional setting. Slab-breakoff, which was followed by shortly after collision, seems to be the most plausible mechanism for the generation of medium to high-K calc-alkaline rocks of the Sumbated and the Keçikaya intrusions, indicating a switch in the geodynamic setting, e.g., from pre-collision to post-collision in the middle Jurassic in the eastern Pontides.

Aydin, Faruk; Karsli, Orhan; Sadiklar, M. Burhan

Clinopyroxene phenocrysts and microphenocrysts in different series of the Neogene alkaline volcanic rocks from the eastern Pontides (NE Turkey) record various stages in the crystallization conditions and evolution history of the alkaline melt as well as its origin. Crystal chemical studies reveal that the clinopyroxenes in each rock series show strong textural and compositional similarities, which all reflect a common petrogenetic affinity. They have relatively high Mg-numbers (0.68&amp;#8211;0.95), variable Al2O3 (1.3&amp;#8211; 9.6 wt%), low TiO2 (&lt;2.7 wt%) and Na2O (&lt;0.9 wt%) contents and low Al[6]/Al[4] ratios (mostly &lt;0.25), suggesting relatively lowpressure crystallization conditions of the magma in the storage region. The pressures calculated for the clinopyroxenes in each series are nearly similar and vary in the range of 2.4&amp;#8211;4.6 &plusmn; 0.9 kbars, which approximately corresponds to a crystallization depth of 7&amp;#8211; 14 &plusmn; 3 km. The analyses of the compositional trends of the clinopyroxenes indicate the following types of zoning: (i) oscillatory and sectorial zoning related to melt crystallization (i.e. rapid cooling and crystallization), (ii) oscillatory, reverse zoning related to the different crystallization paths under a variable fluid regime, (iii) normal zoning related to the differentiation and fractional crystallization of the magma. Based on the primitive mantle- and chondrite-normalized trace and rare earth element patterns, all clinopyroxenes have high abundances of incompatible elements (i.e. La, Ce) with negative high field strength element anomalies (i.e. Zr, Ti) and low Nb/Y (0.1&amp;#8211;0.2), Th/Y (&lt; 0.1) and Rb/Y (&lt; 0.03) ratios, suggesting derivation from a similar source. Obtained textural and mineral chemical data, as well as whole-rock compositions, thus suggest that the clinopyroxenes may have started to crystallize from alkaline basaltic magma derived from a homogeneous lithospheric mantle enriched by an earlier subduction event. After this process, the alkaline magma, from which early clinopyroxenes crystallized, underwent a relatively low-pressure fractional crystallization process. This was in closed magma chambers at different levels of the crust (or within a volcanic conduit system devoid of interaction processes), shown by variations in the different crystallization paths and in the fluid regime of the melt during differentiation and ascent of the magma, in a post-collisional extensional tectonic regime which affected the eastern Pontides during the Neogene.

Aydin, Faruk; Thompson, Richard M.; Karsli, Orhan; Uchida, Hinako; Burt, Jason B.; Downs, Robert T.

Chemical and structural data are reported for C2/c pyroxene phenocrysts collected from three potassic series (Group A: basanite-tephrite, Group B: tephrite-phonolitic tephrite, Group C: alkaline basalt-trachybasalt) of the Neogene alkaline volcanics (NAVs) in northeastern Turkey, in order to investigate the evolution of the magmatic plumbing system and the location of magma chamber(s) with crystallization conditions. The rock series hosting the clinopyroxene phenocrysts show generally porphyritic texture and have a variable phenocryst-rich nature (20-58%), with phenocryst assemblages characterized by cpx ± ol ± plag ± foid ± amp ± bio. The clinopyroxene phenocrysts can be chemically classified as Ti- and Fe3-rich Al-diopsides for Groups A and B (ABcpxs) and Ti- and Fe3-poor Al-diopsides for Group C (Ccpxs). They have poorly variable composition, clustering in the diopside field. Structurally, the diopside groups have nearly similar a (ranging from 9.73 to 9.75 Å), Vcell (437.2-440.9 Å3), and hbetai angle values (106.01°-106.23°), but some differences in polyhedral parameters and geometries of the AB-cpxs and C-cpxs have been observed. For example, the AB-cpxs are characterized by. larger c (5.27-5.30 vs. 5.25-5.28 Å), VT (2.27-2.30 vs. 2.23-2.28 Å3), and VM2 (25.53-25.72 vs. 25.41-25.59 Å3) values and smaller b (8.87-8.88 vs. 8.88-8.91 Å) and VM1(11.49-1.63 vs. 11.64-11.83 Å3) values with respect to the C-cpxs. In addition, the AB-cpxs show higher values of VM2/VM1 (2.20-2.23) due to large VM2 and small VM1 compared to the VM2/ VM1 ratios of the C-cpxs (<2.19). Such differences in the crystal structure of the AB-cpxs and C-cpxs from the NAVs are partly related to different crystallization pressures, but mostly related to variation in melt composition and, possibly, the influence of other crystallizing mineral phases. In particular, R(M2-O1) and R(M1-O2) (i.e. bond lengths) differences in the clinopyroxenes of different groups support the presence of evolved host rocks with different alkaline character (i.e. silicaundersaturated Groups A-B and silica-saturated Group C). Based on the cpx-geothermobarometry, the crystallization pressures for the C-cpxs are lower than 4.5 kbars, but the AB-cpxs have relatively high-pressure values (5.6- 10.6 kbars), suggesting that the AB-cpxs crystallized in higher pressure environments. The relatively higher crystallization temperatures of the AB-cpxs also indicate higher cooling rates. The P-T estimates suggest that the source regions of the clinopyroxene phenocrysts from the NAVs were crustal magma chambers in a closed plumbing system at a moderate- to low-pressure regime.

Aydin, Faruk; Karsli, Orhan; Sadiklar, M. Burhan

Clinopyroxene phenocrysts and microphenocrysts in different series of the Neogene alkaline volcanic rocks from the eastern Pontides (NE Turkey) record various stages in the crystallization conditions and evolution history of the alkaline melt as well as its origin. Crystal chemical studies reveal that the clinopyroxenes in each rock series show strong textural and compositional similarities, which all reflect a common petrogenetic affinity. They have relatively high Mg-numbers (0.68-0.95), variable Al2O3 (1.3-9.6 wt%), low TiO2 (<2.7 wt%) and Na2O (<0.9 wt%) contents and low Al[6]/Al[4] ratios (mostly <0.25), suggesting relatively low-pressure crystallization conditions of the magma in the storage region. The pressures calculated for the clinopyroxenes in each series are nearly similar and vary in the range of 2.4-4.6 ± 0.9 kbars, which approximately corresponds to a crystallization depth of 7-14 ± 3 km. The analyses of the compositional trends of the clinopyroxenes indicate the following types of zoning: (i) oscillatory and sectorial zoning related to melt crystallization (i.e. rapid cooling and crystallization), (ii) oscillatory, reverse zoning related to the different crystallization paths under a variable fluid regime, (iii) normal zoning related to the differentiation and fractional crystallization of the magma. Based on the primitive mantle- and chondrite-normalized trace and rare earth element patterns, all clinopyroxenes have high abundances of incompatible elements (i.e. La, Ce) with negative high field strength element anomalies (i.e. Zr, Ti) and low Nb/Y (0.1-0.2), Th/Y (< 0.1) and Rb/Y (< 0.03) ratios, suggesting derivation from a similar source. Obtained textural and mineral chemical data, as well as whole-rock compositions, thus suggest that the clinopyroxenes may have started to crystallize from alkaline basaltic magma derived from a homogeneous lithospheric mantle enriched by an earlier subduction event. After this process, the alkaline magma, from which early clinopyroxenes crystallized, underwent a relatively low-pressure fractional crystallization process. This was in closed magma chambers at different levels of the crust (or within a volcanic conduit system devoid of interaction processes), shown by variations in the different crystallization paths and in the fluid regime of the melt during differentiation and ascent of the magma, in a post-collisional extensional tectonic regime which affected the eastern Pontides during the Neogene.

Aydin, Faruk; Karsli, Orhan; Chen, Bin

Whole-rock geochemistry, Sr-Nd-Pb isotopes and K-Ar data are reported for alkaline samples collected from the Neogene alkaline volcanics (NAVs) in the Eastern Pontides, northeastern Turkey, in order to investigate their source and petrogenesis and geodynamic evaluation of the region. The NAVs were made of three groups that comprise of basanite-tephrite (feldspar-free; Group A), tephrite-tephriphonolite (feldspar and feldspathoid-bearing; Group B) and alkaline basalt-rhyolite (feldspathoid-free; Group C) series. These rocks cover a broad compositional range from silica-undersaturated to silica-oversaturated types, almost all of which are potassic in character. They show enrichment of LREE and LILE and depletion of HFSE, without a Eu anomaly in most of the mafic samples. Textural features and calculated pressures based on the Cpx-barometer in each series indicate that the alkaline magma equilibrated at shallow crustal depths under a pressure of about 3-4.5 kbar and approximating a crystallization depth of 9-14 km. The NAVs are slightly depleted in isotopic composition, with respect to 87Sr/86Sr (ranging from 0.705018 to 0.705643) and 143Nd/144Nd (ranging from 0.512662 to 0.512714) that indicate young Nd model ages (0.51-059 Ga). This may indicate that the parent melts tapped a homogeneous and young lithospheric mantle source which was metasomatized by subduction-derived sediments during the Late Mesozoic. Pb isotopic compositions (206Pb/204Pb = 18.85-18.95; 207Pb/204Pb = 15.60-15.74; 208Pb/204Pb = 38.82-39.25) may also be consistent with a model for an enriched subcontinental lithospheric mantle source. Lithospheric thinning and resultant upwelling of asthenosphere induced by lithospheric delamination may have favoured partial melting of chemically enriched, young lithospheric mantle beneath the Eastern Pontides. Then, the melt subsequently underwent a fractional crystallization process along with or without minor amounts of crustal assimilation, generating a wide variety of rock types in a post-collision extensional regime in the Eastern Pontides during the Neogene.

Karsli, Orhan; Chen, Bin; Uysal, Ibrahim; Aydin, Faruk; Wijbrans, Jan R.; Kandemir, Raif

Whole-rock geochemical and Sr, Nd and Pb isotope data are presented for a representative suite of the Quaternary Erzincan Volcanics (QEV) from the Erzincan basin (EB) along the North Anatolian Fault Zone, aiming to understand their origin and implications for basalt-lower crust interaction. Unspiked K-Ar and 40Ar/39Ar dating of dome lavas from the QEVs yielded ages of 102 ± 2 to 1061 ± 88 ka. The QEVs range from high-K low silica trachy-andesite to rhyolite in composition, with rhyolite volumetrically the most abundant. All rocks show high-K calc-alkaline affinity, a geochemical signature common to many post-collisional magmas. They are characterized by enrichment in LILE (Rb, Ba, K, Th) and LREE ((La/Yb)CN = 3-33), with pronounced depletion of HFSE. All lavas show negligible or small negative Eu anomalies in the andesitic to dacitic and strong Eu anomalies in the rhyolitic samples. These rocks have relatively low 87Sr/86Sr = 0.70404-0.70587 and slightly depleted Nd isotopic compositions (εNd from - 0.9 to 2.8), with significantly varied Mg# ranging from 2 to 53. Pb isotopic compositions [206Pb/204Pb = 18.90-19.02, 207Pb/204Pb = 15.64-15.70, 208Pb/204Pb = 38.91-39.97] reveal an enriched source signature, which implies that some portions of metasomatized lithospheric mantle could have contributed to their genesis. The isotopic ratios and chemical features along with the textural and compositional disequilibrium of the plagioclases and amphiboles suggest that mixing of mafic and felsic magmas played an important role in the magma genesis. A possible scenario for the genesis of these volcanic rocks is that basaltic magma formed as a result of partial melting of a subcontinental lithospheric mantle source responding to a possible upwelling of asthenospheric mantle which was caused by the extension produced by strike-slip tectonics. Underplating of these high-temperature basaltic magmas sparked partial fusion of a juvenile lower continental crust producing felsic melts; then magma mixing between basaltic and the felsic magmas followed. Fractional crystallization, with minor amounts crustal contamination could have played an important role in the evolution of magma. Modelling based on Sr and Nd isotope data shows that less than 10% of a basic magma and about 90% of juvenile lower continental crustal material was involved in the generation of the QEVs in a pull-apart basin along the North Anatolian Fault Zone.

Karsli, Orhan; Aydin, Faruk; Uysal, Ibrahim; Sadiklar, M. Burhan

Magnetite-ulvöspinel and ilmenite-hematite solid solution intergrowths from the high-K calc-alkaline Dölek and Sarιçiçek plutons, Eastern Turkey, were investigated using microprobe analyses. Compositions of twenty-eight samples from the host rocks and their enclaves' in the plutons were used to estimate the oxygen fugacity and temperature. The ilmenite and ulvöspinel component exsolves out along certain preferred crystallographic planes in the titanomagnetite of the host rocks, while they are always absent in those of the mafic-microgranular enclaves. The titanomagnetite and ilmenite show variations as Mt98-70Usp02-30 and Ilm99-65Hm01-35 in composition, respectively. Estimations of oxygen fugacity and temperature using the titanomagnetite-ilmenite thermometry/oxygen barometry range from log/O2 of -15.30 to -20.48 in host rocks, log/O2 of -15.39 to -20.80 in the mafic microgranular enclaves and 617±6 to 758±23 °C in host rocks, 622±6 to 735±24 °C in the mafic microgranular enclaves, possibly indicating crystallisation temperature. Applying magnetite-ilmenite thermometry/ oxygen barometry to the granitoid rocks also involves microprobe analyses of ilmenite lamellae in titanomagnetite and this method yielded mean temperatures of 679±18 °C. The specific forms and chemical properties of Fe-Ti oxides, and similarities in crystallization temperature and oxygen fugacity of the host rocks and the mafic microgranular enclaves (MME) obtained from the Fe-Ti oxide pairs imply that thermal equilibrium probably occurred between two contrasted magmas, which mixed invarious proportion so that possibly a felsic and a more mafic magma interaction occurred in a convectively dynamic magma chamber during crystallization of the plutons. Probably, underplating may be responsible for genesis of the hybrid plutons. Thus, for mixing of coeval magmas derived from a lithospheric upper mantle (mafic end-member) and lower crust (felsic end-member), a thermal anomaly should be supplied. Upwelling of hot asthenospheric material result in thermal perturbation and melting of lithospheric mantle. Intrusion of hot lithospheric mantle-derived mafic magma then induced lower crustal melting, producing felsic melt. Mixing of the lower crust-derived melt and lithospheric mantle-derived magma formed the hybrid pluton. This process requires a post-collisional extensional tectonic setting during the Eocene in the Eastern Pontides.

Uysal, Ibrahim; Kaliwoda, Melanie; Karsli, Orhan; Tarkian, Mahmud; Sadiklar, M. Burhan; Ottley, Chris J.

The bulk composition (major and trace elements) and the composition of minerals in peridotites of the Ortaca area, south-western Turkey, have been investigated to understand the processes controlling lithological and compositional variations in the dismembered mantle section, containing clinopyroxene-bearing harzburgite, harzburgite and dunite. The Ortaca peridotites vary from almost fertile clinopyroxene-bearing harzburgite (1.92-2.42 wt% Al2O3, 2.00-2.64% CaO) with a lower Cr# of the spinel phase (<24) to depleted harzburgite and dunite (0.17-1.60% Al2O3, 0.32-1.71% CaO) with a higher Cr# of the spinel phase (31-76). Systematic variations of major and trace elements of whole rocks and mineral phases are related to varying degrees of partial melting (up to 30%). The depleted harzburgite and dunite display LREE-enriched primitive-mantle-normalized patterns, suggesting mantle metasomatism, probably by fluid-bearing melt derived from the subducted slab, due to the melt-rock interaction process, whereas clinopyroxene-bearing harzburgite show HREE-enriched patterns. The Ortaca peridotites have low temperatures of equilibration relative to abyssal peridotites, because of H2O-assisted diffusional equilibration in the suprasubduction zone. They are characterized by oxygen fugacities Δlog FMQ between -1.61 and +0.74. Clearly, the clinopyroxene-bearing harzburgites define an increasing oxygen fugacity with almost constant Cr#, whereas the refractory harzburgite and dunite show a strong positive correlation between oxygen fugacity and Cr#. In view of the different measurements on minerals and whole rocks, and as a result of calculated temperatures and oxygen fugacities, we contend that the peridotites were subsequently affected by percolating hydrous boninitic melt, from which the high-Cr, low-Ti chromitites were formed, surrounded by a dunite envelope. It is likely that such an envelope of dunite resulted from interaction between melt and harzburgite, which caused the local removal of orthopyroxene within the mantle wedge above the suprasubduction zone.

Karsli, Orhan; Chen, Bin; Aydin, Faruk; Şen, Cüneyt

The major and trace elements and Sr-Nd-Pb isotopes of the host rocks and the mafic microgranular enclaves (MME) gathered from the Dölek and Sariçiçek plutons, Eastern Turkey, were studied to understand the underlying petrogenesis and geodynamic setting. The plutons were emplaced at ∼ 43 Ma at shallow depths (∼ 5 to 9 km) as estimated from Al-in hornblende geobarometry. The host rocks consist of a variety of rock types ranging from diorite to granite (SiO2 = 56.98-72.67 wt.%; Mg# = 36.8-50.0) populated by MMEs of gabbroic diorite to monzodiorite in composition (SiO2 = 53.21-60.94 wt.%; Mg# = 44.4-53.5). All the rocks show a high-K calc-alkaline differentiation trend. Chondrite-normalized REE patterns are moderately fractionated and relatively flat [(La/Yb)N = 5.11 to 8.51]. They display small negative Eu anomalies (Eu/Eu* = 0.62 to 0.88), with enrichment of LILE and depletion of HFSE. Initial Nd-Sr isotopic compositions for the host rocks are εNd(43 Ma) = - 0.6 to 0.8 and mostly ISr = 0.70482-0.70548. The Nd model ages (TDM) vary from 0.84 to 0.99 Ga. The Pb isotopic ratios are (206Pb/204Pb) = 18.60-18.65, (207Pb/204Pb) = 15.61-15.66 and (208Pb/204Pb) = 38.69-38.85. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition, with ISr ranging from 0.70485 to 0.70517, εNd(43 Ma) - 0.1 to 0.8 and with Pb isotopic ratios of (206Pb/204Pb) = 18.58-18.64, (207Pb/204Pb) = 15.60-15.66 and (208Pb/204Pb) = 38.64-38.77. The MMEs have TDM ranging from 0.86 to 1.36 Ga. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data, in conjunction with the geodynamic evidence, suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, probably triggered by the upwelling of the asthenophere, and interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on the Sr-Nd isotope data indicates that ∼ 77-83% of the subcontinental lithospheric mantle involved in the genesis. Consequently, the interaction process played an important role in the genesis of the hybrid granitoid bodies, which subsequently underwent a fractional crystallization process along with minor amounts of crustal assimilation, en route to the upper crustal levels generating a wide variety of rock types ranging from diorite to granite in an extensional regime.

Karsli, Orhan

Quaternary Erzincan Volcanics (QEVs) from the Erzincan Basin consist of mega- and pheno-cryst-bearing high-K calc-alkaline dome lavas. Fourteen nearly phenocrystic domes, with a range of basaltic-andesite, andesite, dacite and rhyolite compositions, were emplaced in the North Anatolian Fault Zone. The emplacement ages yielded by the unspiked K-Ar technique range from 102 to 140 ka. The andesitic domes (each less than 3 km in diameter) contain amphibole megacrysts. Amphibole compositions show a linear variation from ferro-edenite, edenite to pargasite from rhyolite to andesite. Pargasitic amphibole megacrysts scattered into the groundmass are very similar in composition to the microlites. All plagioclases are <An53 mol%. Oscillation types are An32-50 whose variations range from 10 to 16 mol% An and have 10-150 μm in thickness. Pre-eruptive conditions, calculated from mega- and pheno-cryst composition, using pyroxene and two oxide thermometers and the Al-in-hornblende barometer, ranged from 918 to 837 °C and 6.6 to 4.3 kbar for andesitic magma, 824-755 °C and 4.6-4.2 kbar for dacitic magma to 803-692 °C and 4.3-3.9 kbar for rhyolitic magma, which correspond to a depth of >10 km for storage region of the crust. The fO2 values vary from -14.25 to -15.35 log units which are plotted just below nickel-nickel oxide (NNO) buffers. The systematic decrease in thermobarometric results from andesite to rhyolite is consistent with a single magma reservoir moving upward through the crust followed by fractional crystallization. Textural and compositional relationships of mega- and pheno-crystic phases suggest that magma mixing, fluid input to the reservoir and fractional crystallization processes, with a small amount crustal contamination play key role in evolution of the QEVs.

Karsli, Orhan; Aydin, Faruk; Sadiklar, M. Burhan

Plagioclase crystals characterized by compositional zoning are typical of the Zigana Granitoid (ZG), NE Turkey. The zoned crystals, which show textural equilibrium with the assemblage quartz + plagioclase + K-feldspar + hornblende + biotite + pyroxene, + magnetite + ilmenite, exhibit oscillatory zoning. The zoned plagioclase crystals, ranging in size from 3 to 6 mm, are oval and larger than normal lath-shaped crystals. The zoned crystals are divided petrographically into two main types: (i) crystals zones only at their rims, with spongy cores; and (ii) completely zoned ones. Composition of the zoned crystals ranges from An23 (oligoclase) to An70 (labradorite). Plagioclase crystals with oscillatory zoning are governed by strong substitution mechanisms of Ca2+ ⌊ Na+, Al 3+ ⌊ Sl4+ and weak substitution of Fe3+ for Sl4+. The Fe2O3 (t) (wt %) contents of the subject plagioclases increase with increasing An content. The petrographic and compositional changes from rim to rim (oscillatory zoning), and positive correlation between Fe2O3(t) (wt %) and An (mol %) contents of the zoned plagioclases, suggest recurrent mafic magma injection into a felsic magma chamber, and thus magma interaction as a dominant magmatic process during the evolution of the Zigana Granitoid (ZG).