Late Cenozoic Magmatism Research Articles

3D seismic analysis of complex faulting patterns and fluid escape features and their relation to Late Cenozoic Magmatism in the Bass Basin, offshore SE Australia

Abstract Igneous intrusions in sedimentary petroleum basins are often perceived as having a negative impact on the elements of the petroleum system, though the impact of intrusion-related deformation features on petroleum systems and broader geoenergy applications is not well understood. In this study, we use 3D seismic reflection data to document a variety of deformation styles that are spatially and temporally associated late Cenozoic magmatic activity in the Bass Basin, offshore southeastern Australia; three types of normal fault systems (conjugate faults, concentric faults, radial faults) and fluid escape pipes. These deformation features occur in the overburden up to ∼600 m above underlying igneous intrusions, within the Eocene to Miocene Demons Bluff and Torquay formations. The conjugate faults bound graben and are interpreted to have formed in response to underlying dyke intrusions. The radial faults are interpreted to have formed in response to overburden uplift, though the link between these and associated igneous activity is less clear. We identify 101 fluid escape features that show variation in both the morphology of their surficial depressions and of the seismic reflection characteristics of their infilling deposits. These features are interpreted to be hydrothermal or volcanic vents with underlying pipe-like feeders, depending on their spatial association with adjacent or underlying igneous intrusions. The concentric fault systems are associated with surficial depressions, and quantitative analysis of reflection sags within these depressions suggest that they are a result of subsurface subsidence in response to formation of maar-craters. The intrusion-related deformation features documented in this study may have multiple effects on working petroleum systems, such as providing secondary fluid flow pathways that can either reduce seal integrity, or enabling migration of fluids into shallower reservoirs.

Hf isotope compositions of basaltic rocks from Jeju Island, South Korea: Implications for the origin of the dichotomous isotopic distribution of late Cenozoic magmatism in East Asia

The mantle sources of late Cenozoic intraplate basalts in East Asia can be divided into two distinct domains on the basis of Sr–Nd–Pb isotope systematics: DMM–EM1 in the north and DMM–EM2 in the south. However, the origin of this dichotomous isotopic distribution remains unclear. Here, we present new Hf isotope data for basaltic rocks from Jeju Island, South Korea, and compare the data with published data for coeval basalts from East Asia to understand the nature of the enriched components. The Jeju basalts have 176Hf/177Hf ratios of 0.282861–0.282965 and εHf values of +3.2 to +6.8. In an HfNd isotope plot, the basalts plot along the terrestrial array, without marked decoupling between Hf and Nd isotopes. Late Cenozoic basalts in East Asia fall mainly in the field of global ocean-island basalts. However, basalts in NE Asia tend to be much less radiogenic than those in SE Asia. In addition, some samples deviate from the terrestrial array, reflecting the involvement of various lithologies in the source. Major- and trace-element contents of the Asian basalts indicate the presence of subducted oceanic crust (eclogite/pyroxenite) in the mantle source. However, such crust is not a plausible source for the isotopically enriched components, given the less radiogenic Pb isotope compositions and higher 207Pb/206Pb ratios (0.816–0.917) of the late Cenozoic basalts compared with the HIMU end-member. Instead, subducted sediments could have been the enriched source. The representative EM1-type Wudalianchi basalts in NE Asia have high (Zr/Zr*)N ratios that show a negative correlation with εHf values, indicating the presence of ancient zircon-bearing sediments in the source. In contrast, SE Asian basalts have high Th/La ratios, suggesting the involvement of pelagic clay-rich sediments. A lack of meaningful correlations between Th/La ratios and εHf values indicates that the sediments are relatively young and were recycled with only a short residence time in the mantle source. PbNd isotope modeling shows that ancient (~2.2 Ga) sediments were involved in the mantle source of the NE Asian basalts, compared with relatively young sediments in the mantle source of the SE Asian basalts.

High‐Resolution Broadband Lg Attenuation Structure of the Anatolian Crust and Its Implications for Mantle Upwelling and Plateau Uplift

AbstractThe Anatolian Plateau, currently experiencing rapid uplift and westward escape, records both the termination of oceanic subduction and the conversion to continental collision. The crustal response to the transition of the subduction environment from eastern to western Anatolia can be inferred by the seismic velocity and attenuation structures. With this study, we construct a broadband Lg‐wave attenuation model for the Anatolian Plateau and use it to constrain lateral crust heterogeneities linked to this transition. Crustal Lg attenuation links late Cenozoic magmatism with asthenospheric upwelling by characterizing the lithospheric thermal structure. The widely distributed strong attenuation observed in eastern Anatolia may be related to the crustal partial melting due to mantle upwelling after the delamination and subsequent break‐off of the Bitlis slab. Lithospheric dripping in central Anatolia likely facilitates the mantle flows through the window between the Cyprus and Aegean slabs, which results in the piecemeal low anomaly in central Anatolia.

Late Miocene Exhumation of the Western Cordillera, Ecuador, Driven by Increased Coupling Between the Subducting Carnegie Ridge and the South American Continent

AbstractThe subduction of bathymetric highs, such as aseismic ridges, leads to far‐reaching changes in the dynamics of subduction zones with increased plate coupling and deformation in the upper plate. Subduction of the submarine Carnegie Ridge on the Nazca Plate has fundamentally impacted late Cenozoic magmatism and tectonic activity in the northern Andes. However, the timing of onset of Carnegie Ridge subduction has been a matter of debate. Time‐temperature inverse modeling of new thermochronological data from the Western Cordillera of Ecuador reveals two phases of cooling separated by isothermal conditions. The first cooling phase postdates early and middle Miocene magmatism in the Western Cordillera and is attributed to post‐magmatic thermal relaxation. The second cooling phase started after 6 Ma. Inferred to record the onset of tectonically controlled rock uplift and exhumation in the Western Cordillera, this phase is coeval with the last cooling phase recorded in the Eastern Cordillera. Based on these findings, we suggest that the onset of subduction of the Carnegie Ridge at ∼6–5 Ma increased plate coupling at the subduction interface, promoting shortening, regional rock uplift, and exhumation in the northern Andes. Overall, our results highlight the essential role of bathymetric highs in driving regional upper‐plate deformation at non‐collisional convergent plate margins.

The Origin of Late Cenozoic Magmatism in the South China Sea and Southeast Asia

AbstractBasaltic lavas sample recycled crustal materials from their mantle source. Constraining the location and residence time of these recycled materials in the mantle is critical to understand global mantle dynamics. In this study, we present new whole‐rock major and trace element abundances, Sr‐Nd‐Mo‐Os isotopes, water contents and He isotopes of volcanic glasses, U‐Pb ages of zircons, and compositions of melt inclusions, spinels and olivines from the South China Sea (SCS) seamounts lavas. These new data are compared with literature data from intraplate volcanism of similar age from Southeast (SE) Asia. The isotope data of late Cenozoic lavas from the SCS seamounts and SE Asia can be explained by mixing between enriched mantle 2 (EM2) and depleted mid‐ocean ridge basalt mantle components. Our data are consistent with the EM2 signature of late Cenozoic lavas derived from recycled young oceanic crust and sediments. The compositions of olivine phenocrysts indicate an olivine‐dominated (peridotitic) mantle source. There is currently no evidence for a high‐3He/4He mantle plume component beneath the SCS. Our results combined with geophysical data and plate reconstructions suggest that the late Cenozoic magmatism is related to the upwelling of instabilities from the mantle transition zone (MTZ) triggered by a stagnant slab. The SCS seamount lavas sample an enriched MTZ containing young recycled materials, consistent with regional past subduction. Our study provides additional evidence that storage and recycling of crustal materials in or near the MTZ is an important mechanism to develop global mantle heterogeneities sampled by intraplate volcanoes.

Cenozoic temporal variation of crustal thickness in the Urumieh-Dokhtar and Alborz magmatic belts, Iran

We present regional variations of whole-rock Sr/Y and (La/Yb)N ratios of magmatic rocks along the Cenozoic Urumieh-Dokhtar and Alborz magmatic belts, Iran. Both the magmatic belts are located at the north of the main Zagros Neo-Tethyan suture. The Urumieh-Dokhtar magmatic belt (UDMB), which trends NW-SE for 1000 km across Iran, was characterized by the intensive volcanism and plutonism, and defined the magmatic front (MF) of the Zagros orogenic belt. The Alborz magmatic belt (AMB) is situated to the north, and characterized by less intense magmatic activity. The Alborz magmatic belt was formed behind it in the rear-arc (RA) domain. A striking feature of the both magmatic belts is the transition from normal calc-alkaline arc magmatism during the Eocene–Oligocene to adakite-like calc-alkaline magmatism during the Middle to Late Miocene–Pliocene. The late-Cenozoic magmatism of the UDMB and AMB shows higher Sr/Y and (La/Yb)N. However, it should be noted that crustal thickening event is intensive in the UDMB than AMB during Late Cenozoic. Using the composition of the Lale-Zar zircons from the SE UDMB we determined the oxygen fugacity (fO2) during zircon crystallization to be between FMQ (fayalite–magnetite–quartz buffer) -0.69 to +2.41, whereas those of the Hashroud-Teckmdash-Gormolla zircons from NW AMB range from −1.22 to +5.99. The fO2 estimates suggest relatively more oxidized conditions for the Late Cenozoic igneous rocks of the AMB. Compiled data from the UDMB and AMB intrusions show an increase in average zircon crystallization temperatures with decreasing age. These outcomes have been interpreted in terms of variation of the crustal thickness, from 30 to 35 km during Eocene-Oligocene to 40–55 km during the middle-late Miocene. We propose the increase in crustal thickness is associated with the collision between the Arabian plate and Iran and subsequent convergence during the middle-late Miocene.

Petrogenetic and tectonic controls on magma fertility and the formation of post-subduction porphyry and epithermal mineralization along the late Cenozoic Anatolian Metallogenic Trend, Turkey

The late Cenozoic Anatolian Metallogenic Trend in Turkey is a manifestation of westerly Aegean subduction and easterly Arabian continental collision along the Western Tethyan Orogenic Belt. The 1500 × 200 km late Cenozoic Anatolian magmatism hosts many gold-rich porphyry and epithermal deposits and prospects (~ 27 Moz Au) that formed during the tearing of the Aegean slab and Arabian slab break-off events that initiated at 15 Ma in western Anatolia and 25 Ma in eastern Anatolia. Although the temporal and spatial correlations between slab segmentation, late Cenozoic magmatism, and associated gold-rich porphyry and epithermal mineralization has been intuitively proposed, the genetic relationships between asthenospheric upwelling, subcontinental lithospheric mantle instability, and magma fertility have not been well established. Herein, new geochemical and radiogenic Sr, Nd, and Pb isotope data from Miocene to Pliocene magmatic rocks that are genetically related to porphyry, epithermal, and skarn systems from western, central, and eastern Anatolia are presented and interpreted with previously published geochemical data from late Cenozoic intrusion-related mineral systems throughout Anatolia. The magma fertility of the late Cenozoic post-subduction igneous suites along the late Cenozoic Anatolian Metallogenic Trend is assessed and is best indicated by the SiO2 (61.8 ± 5.5 wt%) content, Sr/Y (> 20) and Ba/Ta ratios (> 428), and (eNd)i values (< 0). Fertile magmatism dominantly resulted from the melting of the metasomatized Anatolian SCLM and was accompanied by amphibole ± clinopyroxene and plagioclase fractionation in the absence of garnet residue as indicated by low (Dy/Yb)N and Eun/Eu* ratios. Magma fertility indicators evolved through time along the late Cenozoic Anatolian Metallogenic Trend. In western Anatolia, fertile magmatism peaked in the middle to late Miocene (15–9 Ma) and was preceded by less-fertile early Miocene magmatism (21–18 Ma) and followed by barren Pliocene to Quaternary, alkaline lavas. The geochemical signatures of mineralized igneous units in central and eastern Anatolia indicate that fertile magmas formed early as regional magmatism evolved towards less hydrous, less oxidized, shallower, and mantle-dominant signature. The reduction of magmatic fertility since the late Miocene throughout Anatolia was caused by the upwelling of hot, dry asthenosphere through the Aegean slab tear and Arabian slab break-off, SCLM instability, and subsequent removal in absence of an over-thickened crust. The spatial migration of slab ruptures and slab gap opening modified mantle fluxes, reduced the SCLM thickness and volume of fusible material, and caused the migration of melting source. Thus, the segmentation of the Southern Neotethyan oceanic slab triggered the metallogenic events along the Anatolian Trend but subsequently contributed to the cessation of production of fertile magma.

Temporal, spatial and geochemical evolution of late Cenozoic post-subduction magmatism in central and eastern Anatolia, Turkey

Indentation of the Arabian platform into the eastern domain of the Anatolide-Tauride Block (ATB) led to sub-horizontal rupture and break-off of the Arabian segment of the subducting southern Neotethyan oceanic slab beneath ATB in the late Cenozoic. Although this rupture has been well-defined by seismic tomography and numerical modeling, its relationship with concomitant magmatism in central and eastern Anatolia has not been firmly demonstrated. To address this issue, we compiled, integrated and interpreted an extensive database of previously-published age and geochemical data. Our analysis shows that late Cenozoic magmatism in both eastern and central Anatolia was nearly continuous over a distance of ~1000 km, and is herein named the Eastern Anatolian Magmatic Belt. Magmatism initiated at 23 Ma in response to rupturing of the north-dipping subducting oceanic slab below eastern Anatolia and upwelling of hot asthenospheric mantle. The onset of initial rupture-related magmatism is ~12 Ma older than previously proposed. The magmatic belt migrated westward from eastern Anatolia (23-15 Ma) to central Anatolia (12 Ma-present) and is a proxy for the westward progression of the slab rupture and related asthenospheric infiltration. Magmatism generated from the shallow melting of the previously-metasomatized Anatolian subcontinental lithospheric mantle and asthenosphere by decompression due to the impingement of the Arabian sub-slab asthenospheric mantle. In eastern Anatolia, slab-rupture magmatism was followed by a second phase of magmatism that formed in response to destabilization and convective removal of the thick Anatolian lithospheric root (13 Ma-present). This new magmatic model emphasizes that the geochemical signature of post-subduction magmatism, as recorded by primitive basalts that constantly evolved through time and space. Magmatic sources recorded the shifting of mantle melt source domains that were controlled by slab segmentation and induced mantle flow.

Miocene–Pliocene Volcanism of Central Armenia: Geochronology and the Role of AFC Processes in Magma Petrogenesis

This paper presents the results from an isotope-geochronological and geochemical-petrological study of young volcanic rocks sampled in central Armenia, which were formed during the first stage of the Late Cenozoic magmatism in the Lesser Caucasus at the Miocene/Pliocene boundary. We have found the boundary of the area where the Miocene to Pliocene volcanism was evolving, determined its scale and total duration (~1 Ma, between 5.7 and 4.7 Ma), and identified phases of magmatic activity: I during 5.56 ± 0.18, II during 5.39 ± 0.04, and III during 4.86 ± 0.20 Ma. Petrologic data show that the volcanic rocks of central Armenia studied here compose a continuous compositional series from potassium trachybasalt to mugearite to (trachy)andesite to trachyte to rhyolite, while the leading process in the petrogenesis of their parent magmas had been fractional crystallization during the entire period of magmatic activity from the Late Miocene to the Early Pliocene. Crustal assimilation was of limited importance: some small influence of this process was only found for some late lavas that were erupted during phase III. The geochemical evolution of lavas during the earlier phases of activity was occurring unidirectionally from basic to acid compositions; the resumed eruptions of trachyandesites and andesites during phase III after a long intermission was caused by replenishment of mantle melts into magma chambers beneath the region. The entire data set for the area of central Armenia was analyzed to identify one volcanic unit that includes all Miocene to Pliocene magmatic rocks that are found there. The mantle source that was responsible for melt generation beneath the better known area of the Lesser Caucasus during the time period of interest was an OIB-type asthenospheric mantle that had been metasomatized by the subduction of the oceanic lithosphere from the Neotethys Basin.

Early Pleistocene magmatism in the central part of the Greater Caucasus

An isotope-geochronological study of young magmatism in the central part of the Greater Caucasus (Kazbek neovolcanic area) on the territory of Russia and Georgia has been carried out. It was proved for the first time that, in the Early Pleistocene, there was a separate impulse of magmatic activity in this area. The area of endogenic activity for the period identified was contoured on the basis of the integrated isotope-geochronological, petrological-geochemical, and geological data. It has been shown that the Early Pleistocene volcanism inherits the area of Neogene volcanism in the Kazbek region and, therefore, presents the final impulse of the second (Pliocene) stage of the Late Cenozoic magmatism. Thus, Early Pleistocene volcanism was not a precursor of Late Quaternary magmatism as the latter has other spatial patterns of the location of volcanic centers.

Formation of the Late Cenozoic volcanic complexes of the Lesser Caucasus

The paper considers the role of the lithospheric mantle and asthenosphere during the Late Cenozoic collision volcanism of the Lesser Caucasus. The results of petrogeochemical studies show that the products of volcanism of the West Volcanic Zone of Armenia and the calc-alkaline andesite–dacite–rhyodacite complex of the Neogene Kelbadzhar and Karabakh plateaus were formed from an enriched source in a suprasubduction setting. Late Pliocene–Quaternary moderately alkaline and alkaline volcanic rocks of the Lesser Caucasus differ in petrogeochemistry from suprasubduction volcanic rocks. In trace element contents and patterns, they are similar to rocks formed from an enriched mantle source. Comparative analysis of the geological and geophysical data suggests the model of lithospheric slab break-off of the thickened lithosphere as the triggering mechanism for Late Cenozoic magmatism of the Lesser Caucasus.

九州地方の後期新生代の火成活動と地質構造発達史の諸問題( 地球科学200号を記念して,200号記念号)

九州地方の後期新生代の火成活動と地質構造発達史の諸問題( 地球科学200号を記念して,200号記念号)

東北本州弧における後期新生代の火成活動史( 島弧の深部構造-地質・地震・地震波トモグラフィによる解析(II))

東北本州弧における後期新生代の火成活動史( 島弧の深部構造-地質・地震・地震波トモグラフィによる解析(II))

Petrogenesis of OIB-like basaltic volcanic rocks in a continental collision zone: Late Cenozoic magmatism of Eastern Iran

Hundreds of compositionally diverse volcanoes have erupted on the Turkish–Iranian Plateau in the last 15Myr, attributed to one or more of the processes of Middle Miocene Tethyan slab break-off, localised lithospheric delamination and small-scale convection. Magmatism post-dates the initial Late Eocene or Early Oligocene Arabia–Eurasia collision by >10Myr. By contrast, in the east of the plateau in Eastern Iran there has been intermittent magmatism from the Late Oligocene to the Quaternary. Magma types include alkali basalt flows and scoria cones and adakite-like intermediate–felsic lavas and domes. New elemental and Nd–Sr–Pb–Hf isotope data from Quaternary mafic alkaline flows near Quchan in the Meshkan area in the NE of the country are combined with existing data from Miocene–Quaternary volcanic centres in the Binalud Range and the Nayband/Neh Fault zones. These mafic, incompatible element-enriched rocks, with positive Nb–Ta anomalies, are OIB-like and are argued to have formed by low-degree melting of a heterogeneous mantle source which contained DMM- and EMII-like components. At least some of the melting took place in the garnet stability field. Significant crustal contamination during magma ascent is recognised in the Nayband/Neh samples. Penecontemporaneous adakite-like rocks are argued to be high-pressure fractionates of basaltic melts which may have a separate source to the OIB-like basalts. Late Cenozoic mantle melting was aided by (1) localised extension and (2) enhanced small-scale asthenospheric convection related to Makran subduction, low mantle viscosity owing to the prior Tethyan subduction history of the region, and possibly edge-driven convection on the margin of the thickened Zagros lithosphere.

Evolution of late Cenozoic magmatism and the crust–mantle structure in the NE Japan Arc

Abstract We review the evolution of late Cenozoic magmatism in the NE Japan arc, and examine the relationship between the magmatism and the crust–mantle structure. Recent studies reveal secular changes in the mode of magmatic activity, the magma plumbing system, erupted volumes and magmatic composition associated with the evolution of crust–mantle structures related to the tectonic evolution of the arc. The evolution of Cenozoic magmatism in the arc can be divided into three periods: the continental margin (66–21 Ma), the back-arc basin (21–13.5 Ma) and the island-arc period (13.5–0 Ma). Magmatic evolution in the back-arc basin and the island-arc periods appears to be related to the 2D to 3D change in the convection pattern of the mantle wedge related to the asthenosphere upwelling and subsequent cooling of the mantle. Geodynamic changes in the mantle caused back-arc basin basalt eruptions during the back-arc basin opening (basalt phase) followed by crustal heating and re-melting, which generated many felsic plutons and calderas (rhyolite/granite phase) in the early stage of the island-arc period. This was followed by crustal cooling and strong compression, which ensured vent connections and mixing between deeper mafic and shallower felsic magmas, erupting large volumes of Quaternary andesites (andesite phase).

Manifestations of miocene acid intrusive magmatism on the southern slope of the Greater Caucasus: First evidence from isotope geochronology

New isotope-geochronological data (K-Ar, Rb-Sr) provide tight geochronological constraints on the history of Late Cenozoic magmatism on the southern slope of the Greater Caucasus. Several previously unknown, rhyodacite intrusive bodies with an emplacement age of 6.9 ± 0.3 Ma (Late Miocene) are reported from the Kakheti-Lechkhumi regional fault zone in the Kvemo Svaneti-Racha area. Therefore, a pulse of acid intrusive magmatism took place in a period previously considered amagmatic in the Greater Caucasus. The petrological, geochemical, and isotopic data suggest that these rhyodacites are produced by crystallization differentiation of mantle-derived magmas, which are similar in composition to Miocene mafic lavas that erupted a few hundred thousand years later in the adjacent Central Georgian neovolcanic area. The presented results allow the conclusion that the volcanic activity within the Central Georgian neovolcanic area occurred at 7.2–6.0 Ma in two discrete pulses: (1) the emplacement of acid intrusions and (2) the eruption of trachybasaltic lavas. The emplacement of rhyodacite intrusions in the Kvemo Svaneti-Racha area marked the first pulse of young magmatism on the southern slope of the Main Caucasus range and simultaneously represented the second magmatic pulse (after granitoid magmatism of the Caucasian Mineral Waters region) within the entire Greater Caucasus.

Geochronological scale and evolution of late Cenozoic magmatism within the Caucasian segment of the alpine belt

Results of the isotope-geochronological studies of the Late Cenozoic magmatism of Caucasus have been considered. The Neogene-Quaternary volcanic activity is found to have evolved during the last 15 m. y. being most intensive in the Middle-Late Pliocene. Within separate neovolcanic areas of the Caucasus region, magmatism was of a clearly discrete character when intense eruption periods interchanged with prolonged (up to several million years) times of quiet conditions. Four stages of young magmatism of the Caucasus are recognized: the Middle Miocene (15–13 Ma), the Late Miocene (9–5 Ma), the Pliocene (4.5–1.6 Ma), and the Quaternary (less than 1.5 Ma). However, for certain areas the time limits of these stages were shifted relative to each other and overlap the whole age range from the mid-Miocene to the end of the Quaternary period. Therefore, within the collision zone, the Neogene-Quaternary magmatism evolved almost continuously during almost the last 9 m. y., but in the time interval of 13–9 Ma in the Caucasian segment, volcanic activity was possibly low. No evidence of directed lateral migration of volcanic activity within the entire Caucasus region was found. At the same time, in the Lesser Caucasus the young magmatism commenced earlier (∼15 Ma), compared to the Greater Caucasus (∼8 Ma).

Geochemical evolution of Middle—Late Cenozoic magmatism in the northern part of the Rio Grande Rift, Western United States

Geochemical studies of the Middle—Late Cenozoic succession of volcanic rocks from the northern part of the Rio Grande Rift were conducted. The initial activation of the rift structure was coeval with voluminous eruptions of lava and pyroclastic material of mainly intermediate and acid compositions in the San Juan volcanic field 35–27 Ma. The composition of the volcanic products after the rifting was dominated by basic and intermediate lavas. It is shown that the basanites and alkali basalts of the territory had geochemical characteristics of sublithospheric slab and above–sl ab sources. The processes of the riftogenic thinning of lithosphere are expressed by geochemical parameters that reflect the interaction between the liquids from the sublithospheric mantle and the rocks from different levels of both the lithospheric mantle and lower crust. In the 35–18 Ma interval, melts of different–depth sublithospheric and lithospheric sources erupted simultaneously in the northern part of the rift. However, the products of interaction between the sublithospheric and lithospheric materials dominated later in the past 15 Ma, although the sublithospheric magmatic liquids erupted at the northern structural termination of the rift within the Yampa volcanic field at about 10 Ma.

Geology and geochemistry of the synextensional Salihli granitoid in the Menderes core complex, western Anatolia, Turkey

The Miocene Salihli granitoid (SG) in the footwall of the Alasehir detachment in the Menderes core complex is a synextensional intrusion, providing important structural, geochronological, and geochemical constraints on the nature of late Cenozoic magmatism associated with crustal extension in the Aegean province. The NW–SE elongated pluton crosscuts the extensional mylonitic fabric in the metamorphic host rocks but is foliated, mylonitized, and cataclastically deformed in a ∼100‐m‐thick shear zone along the detachment. Mylonitic granitoids range from ultramylonites to protomylonites, and exhibit ductile deformation structures showing consistently top‐to‐the north shearing. Grain‐size reduction and dynamic recrystallization were most common deformational mechanisms producing the structural fabric of the mylonitic granitoid rocks. This earlier ductile fabric is overprinted by north‐dipping cataclastic foliation that developed sub‐parallel to the Alasehir detachment and its shear zone. Crystallization and cooling ages of the SG are nearly coeval with the documented early Miocene ages of metamorphism and deformation in its host rocks. The SG pluton ranges in composition from granite, granodiorite, and alkali‐feldspar granite, to monzogranite, and syenogranite; it consists of metaluminous to slightly peraluminous, high‐K calc‐alkaline rocks with silica contents between 62.36 and 73.95 wt‐%. The rocks are enriched in LILE and depleted in HFSE, and show strong negative anomalies in Ba, Nb, Sr, P, and Ti. These geochemical features suggest derivation of the SG melts from a subduction‐metasomatized, subcontinental lithospheric mantle. Invasion of the lower and middle crust by mantle‐derived melts triggered MASH‐type processes (melting, assimilation, storage, homogenization), resulting in the production of hybrid SG magmas. Asthenospheric upwelling caused by lithospheric delamination was a likely heat source that triggered the inferred partial melting of the mantle lithosphere. Continued extensional deformation and unroofing of the Menderes core complex and the synextensional SG in the mid to late Miocene was accompanied by the formation of an asymmetric supradetachment basin in the hanging wall of the Alasehir detachment.

Syn-extensional granitoids in the Menderes core complex and the late Cenozoic extensional tectonics of the Aegean province

Abstract The Miocene granitoid plutons exposed in the footwalls of major detachment faults in the Menderes core complex in western Anatolia represent syn-extensional intrusions, providing important geochronological and geochemical constraints on the nature of the late Cenozoic magmatism associated with crustal extension in the Aegean province. Ranging in composition from granite, granodiorite to monzonite, these plutons crosscut the extensional deformation fabrics in their metamorphic host rocks but are foliated, mylonitized and cataclastically deformed in shear zones along the detachment faults structurally upward near the surface. Crystallization and cooling ages of the granitoid rocks are nearly coeval with the documented ages of metamorphism and deformation dating back to the latest Oligocene–early Miocene that record tectonic extension and exhumation in the Menderes massif. The Menderes granitoids (MEG) are represented by mainly metaluminous-slightly peraluminous, high-K calc-alkaline and partly shoshonitic rocks with their silica contents ranging from 62.5 to 78.2 wt%. They display similar major and trace element characteristics and overlapping inter-element ratios (Zr/Nb, La/Nb, Rb/Nb, Ce/Y) suggesting common melt sources. Their enrichment in LILE, strong negative anomalies in Ba, Ta, Nb, Sr and Ti and high incompatible element abundances are consistent with derivation of their magmas from a subduction-metasomatized, heterogeneous sub-continental lithospheric mantle source. Fractional crystalization processes and lower to middle crustal contamination also affected the evolution of the MEG magmas. These geochemical characteristics of the MEG are similar to those of the granitoids in the Cyclades to the west and the Rhodope massif to the north. Partial melting of the subduction-metasomatized lithospheric mantle and the overlying lower-middle crust produced the MEG magmas starting in the late Oligocene–early Miocene. The heat and the basaltic material to induce this partial melting were provided by asthenospheric upwelling caused by lithospheric delamination. Rapid slab rollback of the post-Eocene Hellenic subduction zone may have peeled off the base of the subcontinental lithosphere, triggering the inferred lithospheric delamination. Both slab retreat-generated upper plate deformation and magmatically induced crustal weakening led to the onset of the Aegean extension, which has migrated southward through time.