Contrasting metamorphic evolution of metasedimentary rocks from the Çine and Selimiye nappes in the Anatolide belt, western Turkey

Abstract P–T conditions, mineral isograds, the relation of the latter to foliation planes and kinematic indicators are used to elucidate the tectonic nature and evolution of a shear zone in an orogen exhumed from mid‐crustal depths in western Turkey. Furthermore, we discuss whether simple monometamorphic fabrics of rock units from different nappes result from one single orogeny or are related to different orogenies. Metasedimentary rocks from the Çine and Selimiye nappes at the southern rim of the Anatolide belt of western Turkey record different metamorphic evolutions. The Eocene Selimiye shear zone separates both nappes. Metasedimentary rocks from the Çine nappe underneath the Selimiye shear zone record maximum P–T conditions of about 7 kbar and >550 °C. Metasedimentary rocks from the overlying Selimiye nappe have maximum P–T conditions of 4 kbar and c. 525 °C near the base of the nappe. Kinematic indicators in both nappes are related to movement on the Selimiye shear zone and consistently show a top‐S shear sense. Metamorphic grade in the Selimiye nappe decreases structurally upwards as indicated by mineral isograds defining the garnet‐chlorite zone at the base, the chloritoid‐biotite zone and the biotite‐chlorite zone at the top of the nappe. The mineral isograds in the Selimiye nappe run parallel to the regional S_R foliation, parallel the Selimiye shear zone and indicate that the Selimiye shear zone formed during this prograde greenschist to lower amphibolite facies metamorphic event but remained active after the peak of metamorphism. ⁴⁰Ar/³⁹Ar mica ages and the tectonometamorphic relationship with the Eocene Cyclades–Menderes thrust, which occurs above the Selimiye nappe in the study area, suggests an Eocene age of metamorphism in the Selimiye nappe. Metasedimentary rocks of the Çine nappe 20–30 km north of the Selimiye shear zone record maximum P–T conditions of 8–11 kbar and 600–650 °C. An age of about 550 Ma is indicated for amphibolite facies metamorphism and associated top‐N shear in the orthogneiss of the Çine nappe. Our study shows that simple monophase tectonometamorphic fabrics do not always indicate a simple orogenic development of a nappe stack. Preservation in some areas and complete overprinting of those fabrics in other areas apparently occur very heterogeneously.     

Tectonic significance of mafic volcanic rocks in a Mesozoic sequence of the Menderes Massif, West Turkey

The Mesozoic platform sequence of the Menderes Massif consists of thick succession of detrital and carbonate rocks. In this sequence there are mafic metavolcanic rocks at two different levels. The first level of mafic metavolcanic intercalations is in the Late Triassic detrital-rich series located in the ÇaltayL Formation, which is the lowermost unit of the Mesozoic platform. The second level of the mafic metavolcanic rocks is located in the Late Cretaceous-(?)Paleocene Selçuk Formation laying on top of the platform sequence. The ÇaltayL Formation, which is composed of mica-schists, thinly-bedded cherts, calc-schist and mafic volcanic intercalations unconformably overlie the BayLndLr Formation, which consists of mica-schists, phyllites, and white quartzites of Palaeozoic or probably older age. The mafic volcanic rocks in the ÇaltayL Formation are alkaline basalts with within plate characteristics and are formed during an intraplate extension. The ÇaltayL Formation is conformably overlain by the KayaaltL Formation represented by calc-schists, dolomitic marbles, and rudist- and emery-bearing massive marbles in ascending order. The Selçuk Formation overlies the KayaaltL Formation and consists of a mica-schist matrix with allochthonous blocks of mafic volcanic rocks, metaperidotites, metagabbros and massive marbles. The mafic volcanic rocks in the Selçuk Formation are tholeiitic basalts and are petrologically similar to mid-oceanic basalts. The geological and geochemical characteristics of the mafic metavolcanic rocks in the ÇaltayL Formation indicate that during the Late Triassic, the Menderes platform was segmented, probably by the opening of a branch of the Neotethyan Ocean. Between the Late Triassic and the Late Cretaceous, the Menderes carbonate platform was built up. During the Latest Cretaceous-Early Paleocene, a slab of oceanic crust obducted on this platform and provided slices of mafic metavolcanic rocks into the Selçuk Formation.     

Tectonic transport directions of the Lycian nappes in southwest Turkey constrained by kinematic indicators

The orientation, asymmetry and cross-cutting relationships of the structures along the contact zone between the Lycian nappes and the Menderes Massif suggest the presence of three deformation phases in the Milas region of southwest Turkey. The first deformation phase (D1) is characterized by a ductile deformation with top-to-the-NE sense of shear. Structural data of the first deformation measured along the uppermost part of the Menderes Massif and the base of the Lycian nappes suggest that the lowermost unit of the Lycian nappes was emplaced initially from southwest to northeast onto the Menderes Massif during the Early Eocene. The second deformation phase (D2) is also ductile in nature and is characterized by an E–W-trending stretching lineation with a bivergent sense of shear, which is probably related to the load of the overlying nappes. A third deformation phase (D3) is characterized by south-dipping normal faults with top-to-the-S sense of movement. This third deformation phase can be related to southward movement of the Lycian nappes along a low-angle décollement zone. The tectonic contact between the Menderes Massif and the Lycian nappes and their strongly-deformed rocks are unconformably covered by approximately flat-lying, coal-bearing Early–Middle Miocene sedimentary rocks, which constrains the upper time limit for all three deformation phases.     

川东褶皱带构造发育深度层次与变形样式

地壳上层的结构特征和变形样式在垂直方向上的变化远远大于其在水平维度上的分异。川东褶皱带自晚古生代以来地史演化统一、地层展布稳定,之后新生代盆、山演化分异幅度较大,使得不同深度的地层和基底得以出露,不同地壳深度层次的构造样式得以展示,这为研究应力的垂向分异,提供了很好的条件。本文基于地壳垂直方向上变形几何的不守恒、构造脱耦以及构造层次的概念,通过野外构造现象的详细解析、野外脆性破裂产状统计、断裂之间交切关系以及活动性质观察等综合分析,结合遥感图像研究,对川东褶皱区隔挡式褶皱和隔槽式褶皱形成提出了新的解释模型。以华蓥山与齐岳山为界,川东褶皱带被分为3个呈叠置关系的区域。研究表明华蓥山以西(Ⅰ区)没有发生强烈的构造变形,变形深度最小(<2 km); 华蓥山与齐岳山之间(Ⅱ区)构造样式为在北北东向剪切作用下形成的陡立构造面理,变形深度为2～5 km; 而齐岳山以东(Ⅲ区)的构造样式是发育轴向北东的宽缓褶曲,变形深度为4～6.6 km。研究分析后得出,川东褶皱带在晚古生代以来,没有经历过大幅度的地壳垂向运动和明显的旋转运动,而白垩纪以后,发育了早期北北东向和晚期北东向的两期构造变形。Ⅱ、Ⅲ两区的构造样式发育于同一应力场(北西-南东主应力场),而晚期北东向断裂活动是形成上述3个区域呈现出断块并置的原因。另外,由于后期不同断块抬升和剥露的差异,使3个区域迥异的构造样式呈现在地表。这一认识对研究油气相关的构造圈闭、固体多金属矿产相关的矿床深度问题以及大地构造学等问题都有创新意义。     

坦桑尼亚西南部乌本迪带的构造演化特征

乌本迪带主要位于坦桑尼亚西南部,由8个地体构成,不同地体具有不同的构造特征、岩浆作用和变质作用年龄。乌本迪带不是单一构造-热事件的产物,而是经过不同阶段的俯冲造山作用和盆地内沉积作用形成。本文通过综合整理不同时期乌本迪带内地质事件和构造地质背景资料,将乌本迪带的演化分为9个阶段,分别为新太古代阶段(2740～2640Ma)、新太古代-古元古代沉积阶段(2640～2050Ma)、古元古代乌萨加蓝阶段(2050～1930Ma)和乌本迪阶段(1930～1800Ma)、古-中元古代沉积阶段(1800～1400Ma)、中元古代的基博拉阶段(1400～1080Ma)、中-新元古代伊鲁米德阶段(1080～850Ma)以及新元古代-寒武纪早泛非作用伸展阶段(850～600Ma)和晚泛非作用阶段(600～500Ma),其中形成于乌萨加蓝阶段的弧后洋盆之后可能继续扩张,在乌本迪阶段又发生俯冲造山作用。乌本迪带内金矿田的形成与构造演化密切相关,早期阶段形成的"导矿"和"容矿"构造在后期演化中又不同程度的再活化,形成多期次的矿化作用。     

High-pressure–low-temperature metamorphism of metasedimentary rocks, southern Menderes Massif, western Turkey

Kilometer-scale lenses of quartz-rich metasedimentary rocks crop out in a discontinuous belt along the southern margin of the Menderes Massif, Turkey, and preserve evidence for high-pressure–low-temperature (HP–LT) metamorphism related to subduction of a continental margin during Alpine orogeny. Kyanite schist, quartzite, and quartz veins contain kyanite + phengite + Mg-chlorite, and the veins also contain magnesiocarpholite. A deformed carbonate metaconglomerate juxtaposed with the quartzite-dominated unit does not contain HP index minerals, and likely represents the tectonized boundary of the siliceous rocks with adjacent marble. The HP–LT rocks (10–12 kbar, 470–570 °C) record different pressure conditions than the adjacent, apparently lower pressure Menderes metasedimentary sequence. Despite this difference there is disagreement as to whether these HP–LT rocks are part of the Menderes sequence or are related to the tectonically overlying Cycladic blueschist unit. If the former, the entire southern Menderes Massif experienced HP–LT metamorphism but the evidence has been obliterated from most rocks; if the latter, rocks recording different metamorphic-kinematic conditions experienced different tectonic histories and were tectonically juxtaposed during thrusting. Based on observations and data in this study, the second model better accounts for the differences in P–T-deformation histories of the southern Menderes Massif rocks, and suggests that the HP–LT rocks are not part of the Menderes cover sequence.     

Homogenous deformation in precambrian supracrustal rocks of Kumlinge area,southwest Finland

Supracrustal rocks form a trough-like prolonged syncline surrounded by granitic rocks.They consist of quartz-feldspar schists (leptites) overlain by amphibolites. The stratigraphic succession in the syncline is as follows: undermost lie leptites followed by amphibolitic pillow lavas, which are gradually superseded by pillow breccias and agglomerates. A small area of polygonally jointed amphibolites lies above the pillow breccias. The succession gives the impression of volcanic activity in a gradually declining depth of water.The pillow lavas and agglomerates are commonly flattened or elongated and only locally undeformed. Elongation-type deformation is most pronounced in the agglomerates of the central parts of the syncline, where it now reaches its maximum breadth. The axial ratios of the elongated agglomerate fragments are approx. 2/3/75. Around the margins of the syncline the pillow lavas are strongly flattened. The most flattened pillows have axial ratios of approx 1/60/60.The flattening deformation in the adjacent leptitic rocks can be estimated from boudinage and folding in numerous pegmatite dikes. The estimated deformation in the leptites agrees reasonably well with the observed deformation in the adjacent pillow lavas.A gravitative sinking of the dense amphibolites into the underlying granitic material could possibly account for variations in the type of deformation within the syncline.     

Geology of the Bozdag area, central Menderes massif, SW Turkey: Pan-African basement and Alpine deformation

The Menderes massif consists of a Precambrian Core Series that preserves evidence for a polymetamorphic history and a Paleozoic/Mesozoic Cover Series that experienced only the Alpine tectonometamorphic evolution. Structural, petrographic, and geochronologic investigations in the central Menderes massif demonstrate that (a) part of the metamorphic and structural evolution of the Precambrian basement is older than the undeformed 551±1.4-Ma-old Birgi metagranite, and (b) inferred Alpine fabrics overprinting the Cover Series largely have the same attitudes as the old structures in the much older Core Series. The inferred Alpine fabrics include both contractional and extensional structures. Contraction under greenschist to amphibolite facies conditions resulted in the imbrication of the Core and Cover Series and generated an inverted metamorphic sequence by north-directed thrusting. During Alpine extension, most of the south-dipping thrust faults were reactivated as extensional shear zones under decreasing greenschist facies conditions.     

Late Alpine evolution of the central Menderes Massif, western Turkey

The central Menderes Massif (western Turkey) is characterized by an overall dome-shaped Alpine foliation pattern and a N-NNE-trending stretching lineation. A section through the southern flank of the central submassif along the northern margin of Büyük Menderes graben has been studied. There, asymmetric non-coaxial fabrics indicate that the submassif has experienced two distinct phases of Alpine deformation: a top-to-the N-NNE contractional phase and a top-to-the S-SSW extensional event. The former fabrics are coeval with a regional prograde Barrovian-type metamorphism at greenschist to upper-amphibolite facies conditions. This event, known as the main Menderes metamorphism, is thought to be the result of internal imbrication of the Menderes Massif rocks along south-verging thrust sheets during the collision of the Sakarya continent in the north and the Anatolide-Tauride platform in the south across the Gzmir-Ankara suture during the (?)Palaeocene-Eocene. Top-to-the S-SSW fabrics, represented by a well-developed ductile shear band foliation associated with inclined and/or curved foliation, asymmetric boudins, and cataclasites, were clearly superimposed on earlier contractional fabrics. These fabrics are interpreted to be related to a low-grade (greenschist?) retrogressive metamorphism and a continuum of deformation from ductile to brittle in the footwall rocks of a south-dipping, presently low-angle normal fault that accompanied Early Miocene orogenic collapse and continental extension in western Turkey. A similar tectono-metamorphic history has been documented for the northern flank of the dome along the southern margin of the Gediz graben with top-to-the N-NNE extensional fabrics. The exhumation of the central Menderes Massif can therefore be attributed to a model of symmetric gravity collapse of the previously thickened crust in the submassif area. The central submassif is thus interpreted as a piece of ductile lower-middle crust that was exhumed along two normal-sense shear zones with opposing vergence and may be regarded as a typical symmetrical metamorphic core complex. These relationships are consistent with previous models that the Miocene exhumation of the Menderes Massif and Cycladic Massif in the Aegean Sea was a result of bivergent extension.     

Subduction related antigorite CPO patterns from forearc mantle in the Sanbagawa belt,southwest Japan

Antigorite (Atg) is stable throughout large parts of the wedge mantle of most subduction zones. Atg shows strong acoustic anisotropy and crystallographic preferred orientation (CPO) patterns of this mineral may contribute significantly to seismic anisotropy in convergent margins. Atg CPO patterns from the Higashi-Akaishi (HA) forearc mantle body of southwest Japan adds to the data set suggesting the most common Atg CPO pattern has a c-axis perpendicular to the foliation and a b-axis parallel to the stretching lineation. Statistical analysis using the eigenvector method of Atg CPO from two mutually perpendicular directions in the same sample (YZ-section and XZ-section) shows no significant differences implying sample preparation has no significant affect on the resulting Atg CPO. Reuss (uniform stress) averages of anisotropy for the Higashi-Akaishi samples are approximately treble the values for Voigt (uniform strain) averages. When comparing calculated anisotropy of hydrated mantle peridotite samples—such as the Higashi-Akaishi unit—with observed S-wave delay times in convergent margins, the appropriate averaging method needs to be considered.     

Monazite ages and the evolution of the Menderes Massif, western Turkey

The Menderes Massif experienced polyphase deformation, but distinguishing Pan-African events from Alpine tectono-metamorphic evolution, and discriminating Eocene–Oligocene shortening from recent extension remain controversial. To address this, monazite in garnet-bearing rocks from the massifs Gordes, Central, and Cine sections were dated in thin section (in situ) using the Th–Pb ion microprobe method. Cambro–Ordovician monazite inclusions in Cine and Central Menderes Massif garnets are ~450 m.y. older than matrix grains. Monazites in reaction with allanite from the Kuzey Detachment, which bounds the northern edge of the Central Menderes Massif, are 17±5 Ma and 4.5±1.0 Ma. The Pliocene result shows that dating of monazite can record the time of extension. The Kuzey Detachment might have exhumed rocks a lateral distance of ~53 km at a rapid rate of ~12 mm/year assuming the present ~20° ramp dip, Pliocene monazite crystallization at ~450°C, and a geothermal gradient of ~25°C/km. Assuming an angle of 60°, the rate decreases to ~5 mm/year, with the detachment surface at ~21 km depth in the Pliocene. Two Gordes Massif monazites show a similar allanite–monazite reaction relationship and are 29.6±1.1 Ma and 27.9±1.0 Ma, suggesting that the Cenozoic extension in the Gordes Massif, and possibly the entire Menderes Massif, might have begun in the Late Oligocene.     

Multistage crystallization of tonalitic enclaves in granitoid rocks (Hercynian belt,Spain): implications for magma mixing

The crystallization sequence, nucleation density and crystal index (n = nucleation density/mode) of tonalitic enclaves from different granitoids of the Hercynian orogeny indicate that they are pieces of magma partially crystallized in the interior of synplutonic bodies of basic magma, which was injected into a silicic magma chamber. A rapid cooling stage can be identified from the high nucleation density and high n-values of minerals. A final stage of slow cooling, is identified by a low nucleation density and low n-values. At this stage a residual melt crystallized in thermal equilibrium to the cooling rate of the whole magma chamber. Thermal equilibrium and convection are necessary conditions for mixing. Enclaves can be interpreted as the remaining, non-disaggregated portions of an early injected mafic magma, which in turn underwent hybridization during injection. Their presence indicates the existence of magma mixing processes and the possibility that the host granitoid was a hybrid rock.

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Zusammenfassung Die Kristallisationsabfolge, Keimdichte und der Kristallindex (n = Keimdichte/Modus) von tonalitischen Einschlüssen verschiedener Granitoide des variszischen Orogens zeigen, daß sie Teile eines Magmas sind, das teilweise im Inneren von basischen synplutonischen Magmenkörpern auskristallisierte und in eine silikatische Magmenkammer injiziert wurde. Eine Zeit schnellen Abkühlens kann an Hand der hohen Keimdichte und der niedrigen n-Werte festgestellt werden. Zu diesem Zeitpunkt kristallisierte eine Restschmelze unter thermischen Gleichgewichtsbedingungen und der Abkühlungsgeschwindigkeit der ganzen Magmenkammer aus. Thermisches Gleichgewicht und Konvektion sind für das Mischen notwendig. Einschlüsse können als nichtzerfallene Reste eines früh injizierten mafischen Magmas angesehen werden, welches während der Injizierung eine Hybridisierung erfuhr. Ihre Gegenwart zeigen das Vorhandensein von Magmenmischungsvorgängen und die Möglichkeit, daß der Muttergranitoid ein Hybridgestein war, an.

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Resumen La secuencia de cristalización, densidad de nucleación e índice cristalino (n = densidad de nucleación/moda) de enclaves tonalíticos de diferentes granitoides hercínicos, indican que dichos enclaves representan fragmentas de magma parcialmente cristalizados en el interior de cuerpos simplutónicos de magma básico, que originalmente fue inyectado en una cámara magmática félsica. Se puede identificar una etapa de enfriamiento rápido a partir de los altos valores de n y alta densidad de nucleación de los minerales. Una etapa final de enfriamiento lento es detectada por una baja densidad de nucleación y bajos valores de n de determinados minerales. Un líquido residual cristalizó en esta etapa lenta en equilibrio térmico y a la misma tasa de enfriamiento que toda la cámara magmática en conjunto. Equilibrio térmico y convección son condiciones necesarias para que se produzca mezcla. Los enclaves pueden ser interpretados como porciones no hibridadas de magma que han sido homogéneamente distribuidos dentro del granitoide encajante. Su presencia indicaría la existencia de procesos de mezcla de magmas y la posibilidad de que el granitoide encajante sea una roca híbrida.

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秦岭商丹断裂带的构造样式与变形分析

商丹断裂带作为扬子板块与华北板块的缝合带,在秦岭造山带中占据重要的地位。对其进行精细的构造研究,是了解扬子板块与华北板块相互作用过程的关键。研究工作在商丹断裂带东段展开,对该带进行了详细的野外观察、显微构造观察、糜棱岩化过程中温度的估算、有限应变测量、差异应力的计算以及糜棱岩化过程中运动学涡度的计算。变形温压条件指示糜棱岩形成于中-高绿片岩相到低角闪岩相变质条件。差异应力计算结果指示商丹带具备大型构造动力特征。涡度分析表明该断裂带的变形以纯剪切作用占据主导作用,但包含有简单剪切的分量,从而证明了扬子板块与华北板块之间具有以碰撞挤压为主的斜向汇聚方式。     

扬子克拉通西缘康定杂岩中花岗质岩石的成因及其构造意义

扬子克拉通西缘康定杂岩中的片麻状花岗岩主要由英云闪长岩、花岗闪长岩、灰白色细粒二长花岗岩和少量的粉红色粗粒二长花岗岩组成.其中英云闪长岩和花岗闪长岩形成于797～795Ma,灰白色细粒二长花岗岩SHRIMP锆石定年产生一个206Pb/238U权重平均767±24Ma,被解释为该期花岗质岩浆的结晶年龄.英云闪长岩、花岗闪长岩和灰白色细粒二长花岗岩和其中的闪长岩包体表现为右斜式稀土配分模式,具有很高的(La/Yb)N比值,无Eu异常,在原始地幔标准化的多元素蜘蛛网状图上表现了明显的Nh、Ta、P和Ti负异常.而粉红色粗粒二长花岗岩却表现了平坦的稀土配分模式,具有强烈的负Eu异常和强烈的Nh、Ta、sr、P和Ti负异常,但是富集大离子亲石元素.所有这些片麻状花岗岩具有εNd(t)=-0.57～+5.67,绝大部分样品εNd(t)>0.结合地质学、岩石学、地球化学和Sm-Nd同位素特征,康定杂岩中英云闪长岩、花岗闪长岩和灰白色细粒二长花岗岩形成于来自亏损地幔的初生地壳玄武质岩石和相关的杂砂岩在高压条件下的部分熔融,而粉红色粗粒二长花岗岩的岩浆导源于表壳岩低压条件下的部分熔融.结合这些片麻状花岗岩的岩石成因和构造鉴别,表明扬子克拉通西缘康定杂岩中新元古代片麻状花岗岩形成于安第斯型活动大陆边缘.     

松潘- 甘孜造山带南部江浪穹隆中侏罗世花岗岩及构造意义

新火山和乌拉溪花岗岩体侵位于松潘-甘孜造山带南部的江浪穹隆中,是理解松潘-甘孜造山带的构造演化的重要窗口.锆石U-Pb定年结果表明新火山和乌拉溪花岗岩体的结晶年龄为162±1～170±0.5 Ma.岩体主要由石英、斜长石、钾长石和少量黑云母组成,具有低A/CNK(1.10～0.99)、TFeO/MgO(8.55～2.83)和K2O/Na2O(1.34～0.51),较低的Zr+Nb+Ce+Y浓度(平均258×10-6),Al2O3、P2O5与SiO2呈负相关,相当于I型花岗岩的特征.一些主微量元素具有明显的关联性,说明在岩浆演化过程中发生了分离结晶(例如:角闪石和钛铁矿).富集轻稀土和大离子亲石元素,亏损高场强元素,并具有较低的Mg#值(38.7～17.3)和Y/Nb值(0.45～0.16)以及多数为负的εHf(t)值(-24.8～-7.6),证明岩浆可能起源于古老地壳的部分熔融并受到幔源或者新生地壳熔融混染.松潘-甘孜地块的中侏罗世岩浆活动是在造山运动向陆内伸展这一构造体制转化背景下,由于地壳伸展造成了古老中基性地壳的部分熔融.     

Assessment of artificial aquifer recharge potential in the Kucuk Menderes River Basin, Turkey

The Kucuk Menderes River Basin in western Turkey has been facing continuous groundwater-level decline for decades. Previous studies have suggested that, to avoid aquifer depletion in the basin, artificial recharge structures should be constructed. To assess artificial aquifer recharge potential in one of the subbasins, a two-dimensional (2-D) groundwater model was set up using SEEP/W software. The material functions and parameters used in the model for both saturated and unsaturated conditions were taken from previous studies. The model has been calibrated under transient conditions. The excess runoff volume that could be collected in the recharge basins was estimated from flood frequency analysis. Various scenarios were simulated to observe the change in groundwater level and storage with respect to different exceedance probabilities. Simulation results suggest that a significant increase in groundwater storage is achieved by applying surface artificial-recharge methods. In addition to the recharge basins, to reinforce the effect of artificial recharge, simulations are repeated with underground dam construction at the downstream side of the basin. Although groundwater storage is increased with the addition of the dam, the increase in groundwater storage was not sufficient to warrant the construction.     

Geochronological evidence and tectonic significance of Carboniferous magmatism in the southwest Trabzon area,eastern Pontides,Turkey

The northern and southern zones of the eastern Pontides (northeast Turkey) contain numerous plutons of varying ages and compositions. Geochemical and isotopic results on two Hercynian granitoid bodies located in the northern zone of the eastern Pontides allow a proper reconstruction of their origin for the first time. The intrusive rocks comprise four distinct bodies, two of which we investigated in detail. Based on LA–ICP–MS U–Pb zircon dating, the Derinoba and Kayadibi granites have similar ²⁰⁶Pb/²³⁸U versus ²⁰⁷Pb/²³⁵U Concordia ages of 311.1 ± 2.0 and 317.2 ± 3.5 million years for the former and 303.8 ± 1.5 million years for the latter. Aluminium saturation index values of both granites are between 0.95 and 1.35, indicating dominant peraluminous melt compositions. Both intrusions have high SiO₂ (74–77 wt.%) contents and show high-K calc-alkaline and I- to S-type characteristics. Primitive mantle-normalized element diagrams display enrichment in K, Rb, Th, and U, and depletion in Ba, Nb, Ta, Sr, P, and Ti. Chondrite-normalized rare earth element patterns are characterized by concave-upward shapes and pronounced negative Eu anomalies with La_cn/Yb_cn?=?4.6–9.7 and Eu_cn/Eu*?=?0.11–0.59 (Derinoba), and La_cn/Yb_cn?=?2.7–5.5 and Eu_cn/Eu*?=?0.31–0.37 (Kayadibi). These features imply crystal-melt fractionation of plagioclase and K-feldspar without significant involvement of garnet. The Derinoba samples have initial ?Nd values between –6.1 and –7.1 with Nd model ages and T _DM between 1.56 and 2.15 thousand million years. The Kayadibi samples show higher initial ?Nd_(I) values, –4.5 to –6.2, with Nd model ages between 1.50 and 1.72 thousand million years. This study demonstrates that the Sr isotope ratios generally display negative correlation with Nd isotopes; Sr isotope ratios were lowered in some samples by hydrothermal interaction or alteration. Isotopic and petrological data suggest that both granites were produced by the partial melting of early Palaeozoic lower crustal rocks, with minor contribution from the mantle. Collectively, these rocks represent a late stage of Hercynian magmatism in the eastern Pontides.     

Tectonic evolution of the South Tianshan orogen and adjacent regions,NW China: geochemical and age constraints of granitoid rocks

Geochemical and geochronological evidence was obtained from granitoids of the South Tianshan orogen and adjacent regions, which consist of three individual tectonic domains, the Kazakhstan–Yili plate, the Central Tianshan Terrane and the Tarim plate from north to south. The Central Tianshan Terrane is structurally bounded by the Early Paleozoic ‘Nikolaev Line–North Nalati Fault’ and Late Paleozoic ‘Atbashy–Inyl’chek–South Nalati–Qawabulak Fault’ zones against the Kazakhstan–Yili and Tarim plates, respectively. The meta-aluminous to weakly peraluminous granitic rocks, which are exposed along the Kekesu River and the Bikai River across the Central Tianshan Terrane, have a tholeiitic, calc-alkaline or high-potassium calc-alkaline composition (I-type). Geochemical trace element characteristics and the Y versus Rb–Nb or Y versus Nb discrimination diagrams favor a continental arc setting for these granitoid rocks. SHRIMP U–Pb and LA-ICP-MS U–Pb zircon age data indicate that the magmatism started at about 480 Ma, continued from 460 to 330 Ma and ended at about 275 Ma. The earlier magmatism (>470 Ma) is considered to be the result of a simultaneous southward and northward subduction of the Terskey Ocean beneath the northern margin of the Tarim plate and the Kazakhstan–Yili plate, respectively. The later magmatism (460–330 Ma) is related to the northward subduction of the South Tianshan Ocean beneath the southern margin of the Kazakhstan–Yili–Central Tianshan plate. The dataset presented here in conjunction with previously published data support a Late Paleozoic tectonic evolution of the South Tianshan orogen, not a Triassic one, as recently suggested by SHRIMP U–Pb zircon dating for eclogites.