Early Miocene post-collisional magmatism in NW Turkey: geochemical and geochronological constraints

The Alaçam region of NW Turkey lies within the Alpine collision zone between the Sakarya continent and the Menderes platform. Four different tectonic zones of these two continents form imbricated nappe packages (including the Afyon zone), intruded by the Alaçam granite. Newly determined U-Pb zircon ages of this granite are 20.0 ± 1.4 and 20.3 ± 3.3 Ma, indicating early Miocene emplacement. Rb-Sr biotite ages of the granite are 20.01 ± 0.20 and 20.17 ± 0.20 Ma, suggesting fast cooling at a shallow crustal level. Geochemical characteristics show that the Alaçam granite is similar to numerous EW-trending plutons in NW Anatolia.

Gneissic granites of the Afyon tectonic zone were intruded by the Miocene Alaçam granite and have been interpreted in earlier studies as sheared parts of the Alaçam granite, which formed along a crustal-scale detachment zone under an extensional regime. We determined a U-Pb zircon age of 314.9 ± 2.7 Ma for a gneissic granite sample of the Afyon zone, demonstrating that these rocks are unrelated to the Miocene Alaçam granite. The early Miocene granitic plutons bear post-collisional geochemical features and are interpreted as products of Alpine-type magmatism along the Izmir–Ankara suture zone in NW Turkey, and seem to have no genetic relation to the detachment zone.     

Triassic magmatism and Mo mineralization in Northeast China: geochronological and isotopic constraints from the Laojiagou porphyry Mo deposit

The Laojiagou Mo deposit is a newly discovered porphyry Mo deposit located in the Xilamulun Mo metallogenic belt, Northeast China. Mo mineralization mainly occurred within the monzogranite and monzogranite porphyry. Re–Os isochron dating of molybdenites indicate a mineralization age of 234.9 ± 3.1 Ma. Zircon LA–ICP–MS U–Pb analysis for monzogranite porphyry and monzogranite yield ²⁰⁶Pb/²³⁸U ages of 238.6 ± 1.8 and 241.3 ± 1.5 Ma, respectively, indicating that Laojiagou Mo mineralization is related to Middle Triassic magmatism. Hf isotopic compositions of zircons from both monzogranite porphyry and monzogranite are characterized by positive ε_Hf(t) values [ε_Hf(t) = 2.9–7.3 and 1.5–7.9, respectively] and young T_DM2 model ages, which implies that the magma was derived from juvenile crust created during accretion of the Central Asian Orogenic Belt (CAOB). Identification of the Laojiagou Mo deposit adds another important example of Triassic Mo mineralization in the Xilamulun Mo metallogenic belt where most Triassic Mo deposits in northeast China cluster around the northern margin of North China Craton. Based on the regional geological setting and geochronological and Hf isotope characteristics, we propose that Triassic Mo deposits and related magmatic rocks in northeast China formed during the last stages of evolution of the CAOB. These deposits formed during post-collisional extension after the closure of the Palaeo-Asian Ocean and amalgamation of the North China–Mongolian Block with the Siberian Craton.     

Tectonomagmatic evolution of a Jurassic Cordilleran flare-up along the Korean Peninsula: Geochronological and geochemical constraints from granitoid rocks

This study used new and published U-Pb geochronological, chemical, and Sr-Nd-Hf-O isotopic data (n > 2500) from Jurassic granite-granodiorite-diorite-monzonite-gabbro plutons in the southern part of the Korean Peninsula to assess the spatiotemporal evolution of a flare-up magmatism, its tectonic connection, and specific contributions of mantle and crustal reservoirs to the magmas generated. After a ~15 m.y. magmatic gap in the Late Triassic, calc-alkaline granitoids intruded into the outboard Yeongnam Massif, then magmatic activity migrated systematically toward the inboard Gyeonggi Massif. The early phase of the Jurassic magmatism is represented by relatively sodic plutons showing distinctly primitive isotopic signatures. The crustal signature of the plutons became increasingly prominent with decreasing age. Voluminous inboard plutons in the Gyeonggi Massif and the intervening Okcheon Belt are dominated by Middle Jurassic peraluminous granites that show isotopic compositions conspicuously shifted toward old crustal values. The Nd-Hf isotopic compositions of the inboard plutons are distinctly less radiogenic than those of Jurassic plutons in Southwest Japan and southeastern China, which corroborates the North China affinity of the Yeongnam and Gyeonggi massifs. The geochronological and geochemical data compiled in this study suggest a tectonomagmatic model consisting sequentially of (1) an extension-dominated arc system in the margin of the Yeongnam Massif (ca. 200–190 Ma); (2) low-angle subduction and the development of an advancing arc system (ca. 190–180 Ma); (3) continued low-angle subduction, extensive underthrusting of fertile crustal materials to the arc root, and consequent magmatic flare-up (ca. 180–170 Ma); and (4) flat subduction and the development of the Honam Shear Zone (ca. 170–160 Ma). The subsequent magmatic lull and previous dating results for synkinematic rocks and minerals indicate that the compressional arc system was maintained until the Early Cretaceous.     

Vendian-Early Paleozoic granitoid magmatism in Eastern Tuva

We summarize results of geological, geochronological, petrogeochemical, and isotope-geochemical (Sr-Nd) studies of Late Vendian-Early Paleozoic granitoid batholiths in Eastern Tuva (Kaa-Khem, East Tannu-Ola, Khamsara, etc.). Analysis of geochronological (U-Pb, Ar-Ar) data has shown that the Late Vendian-Early Paleozoic granitoids in Eastern Tuva formed in several stages in the time interval 562–450 Ma and at different geodynamic stages of the regional evolution: island-arc (562–518 Ma) and accretion-collision (500–450 Ma), with the latter stage characterized by more intense granitoid magmatism. Diorite-tonalite-plagiogranite associations with different petrogeochemical parameters are the most widespread in the region. Petrogeochemical studies of the Late Vendian-Early Paleozoic plagiogranitoid associations have revealed high- and low-alumina varieties reflecting different conditions of formation of parental melts. At the island-arc stage of the regional evolution, only low-alumina plagiogranites of tholeiitic (M-type) and calc-alkalic (I-type) series formed. Their parental melts were generated at 3–8 kbar through the partial melting of N-MORB-type metabasalts in equilibrium with amphibole restite. Isotope-geochemical studies have shown positive £_Nd values (6.9-6.3) and low Sr isotope ratios ((⁸⁷Sr/⁸⁶Sr)₀ = 0.7034-0.7046). The lower (as compared with the depleted mantle) e_Nd values and specific petrogeochemical composition (negative Nb-Ta and Ti anomalies) of the plagiogranites reflect the subduction nature of metabasic substratum and the subordinate role of ancient crustal material. At the accretion-collision stage of the regional evolution, high- and low-alumina plagiogranitoids of calc-alkalic series (I-type) formed. The high-alumina plagiogranitoids are products of melting of N-MORB-type metabasalts in equilibrium with garnet restite at > 15 kbar in the lower part of the collisional structures, and the low-alumina ones formed through the melting of metabasites in equilibrium with amphibole restite at < 8 kbar in the upper part of the same structures. The Sr-Nd isotope data for the high- and low-alumina plagiogranitoids generated at the accretion-collision stage show that the rejuvenation of rocks is accompanied by the decrease in e_Nd (from 6.2 to 3.4) and the increase in their model Nd age !_Nd(DM) (from 0.73 87 86 to 0.92 Ga) and ( Sr/ Sr)0 (0.7036-0.7048). This points to the essentially metabasic composition of the parental substratum, as in the case of the island-arc plagiogranitoids, and the progressive supply of ancient crustal material to the magma generation zone.     

Construction of the granitoid crust of an island arc part I: geochronological and geochemical constraints from the plutonic Kohistan (NW Pakistan)

We present major and trace element analyses and U–Pb zircon intrusion ages from I-type granitoids sampled along a crustal transect in the vicinity of the Chilas gabbronorite of the Kohistan paleo-arc. The aim is to investigate the roles of fractional crystallization of mantle-derived melts and partial melting of lower crustal amphibolites to produce the magmatic upper crust of an island arc. The analyzed samples span a wide calc-alkaline compositional range (diorite–tonalite–granodiorite–granite) and have typical subduction-related trace element signatures. Their intrusion ages (75.1 ± 4.5–42.1 ± 4.4 Ma) are younger than the Chilas Complex (~85 Ma). The new results indicate, in conjunction with literature data, that granitoid formation in the Kohistan arc was a continuous rather than punctuated process. Field observations and the presence of inherited zircons indicate the importance of assimilation processes. Field relations, petrographic observations and major and trace element compositions of the granitoid indicate the importance of amphibole fractionation for their origin. It is concluded that granitoids in the Kohistan arc are derivative products of mantle derived melts that evolved through amphibole-dominated fractionation and intra crustal assimilation.     

Pan-African magmatism in the Menderes Massif: geochronological data from leucocratic tourmaline orthogneisses in western Turkey

The Menderes Massif, exposed in western Anatolia, is a metamorphic complex cropping out in the Alpine orogenic belt. The metamorphic rock succession of the Massif is made up of a Precambrian basement and overlying Paleozoic-early Tertiary cover series. The Pan-African basement is composed of late Proterozoic metasedimentary rocks consisting of partially migmatized paragneisses and conformably overlying medium- to high-grade mica schists, intruded by orthogneisses and metagabbros. Along the southern flank of the southern submassif, we recognized well-preserved primary contact relationship between biotite and leucocratic tourmaline orthogneisses and country rocks as the orthogneisses represent numerous large plutons, stocks and vein rocks intruded into a basement of garnet mica schists. Based on the radiometric data, the primary deposition age of the precursors of the country rocks, garnet mica schist, can be constrained between 600 and 550?Ma (latest Neoproterozoic). The North Africa–Arabian-Nubian Shield in the Mozambique Belt can be suggested as the possible provenance of these metaclastics. The intrusion ages of the leucocratic tourmaline orthogneisses and biotite orthogneisses were dated at 550–540?Ma (latest Neoproterozoic–earliest Cambrian) by zircon U/Pb and Pb/Pb geochronology. These granitoids represent the products of the widespread Pan-African acidic magmatic activity, which can be attributed to the closure of the Mozambique Ocean during the final collision of East and West Gondwana. Detrital zircon ages at about 550?Ma in the Paleozoic muscovite-quartz schists show that these Pan-African granitoids in the basement form the source rocks of the cover series of the Menderes Massif.     

辽宁丹东地区侏罗纪花岗岩的变形时代：^40Ar／^39Ar年代学制约

辽东半岛出露大量片麻状花岗岩,其锆石U-Pb年龄为180～157Ma,该套岩石的最大特点表现在岩石明显经历了早期上盘向NW推覆和后期近EW向伸展作用的韧性改造,然而其变形时代却一直未得到确定。本文以丹东市西南部黑沟二云母二长花岗岩岩体为例,通过激光^40Ar/^39Ar年代学研究探讨该岩体的变形时代。激光^40Ar/^39Ar定年结果表明,黑沟岩体经历的早期推覆、挤压事件发生在～143Ma,而后期地壳伸展作用则发生在121～113Ma,并且该区在早白垩纪期间经历了快速冷却、抬升过程。从而表明在晚侏罗纪-早白垩纪期间(143～113Ma)辽东半岛经历了区域构造体制变革。结合前人大地构造研究成果,本文认为辽东半岛晚侏罗纪-早白垩纪NW向推覆、挤压事件是古太平洋板块向欧亚大陆俯冲作用的结果,而晚期(早白垩纪)的地壳伸展事件是古太平洋板块俯冲作用的转向和变速、华北东部岩石圈较薄以及挤压后地壳松弛等综合作用的结果。     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

Pre-Tertiary blueschist terrains in the Hellenides: evidence from detrital minerals of flysch successions

Detrital blue amphiboles detected in flysch sandstones of the Hellenides of mainland Greece give important information on uplift and exhumation history of blueschist terrains. The occurrence of these HP/LT minerals in the terminal flysch of the Pindos zone as well as in the Othrys flysch of the Subpelagonian zone from late Maastrichtian/ Palaeocene time onwards proves a Pre-Tertiary (Eohellenic) stage of blueschist formation. Detrital blue amphiboles from flysch sequences of the Ionian zone indicate a further pulse of uplift of high-pressure rocks during the Palaeocene/Eocene. Blueschist rocks of this latter time range are radiometrically well documented in the Hellenides.     

Late Paleozoic rare-metal granitoid magmatism of the Southern Baikal region

The Late Paleozoic intraplate magmatism of the Selenga-Vitim structural zone of the Baikal region (Khamar-Daban Range) produced granitoids of different geochemical types: palingenic calc-alkaline granitoids, subalkaline monzogranites, and rare-metal Li-F granitoids and their subvolcanic analogues. Subalkaline and rare-metal granitoids occur in the periphery of the Late Paleozoic magmatic zone. Rare metal granite magmatism is manifested in this region as nearly N-S trending intrusive-dike belts comprising multiphase intrusions (Kharagul, Urugudei, and Bitu-Dzhida massifs) with an exposed area of ∼10 km² and an age of formation from 311 to 321 Ma and series of accompanying dikes. The early phases of the intrusions are made up of biotite granites usually with fluorite, which are changed during the late stage by typical topazbearing rare-metal amazonite-albite granites. In the subvolcanic facies, thicker subalkaline dikes of monzonite porphyry, granite porphyry, and elvan are changed by ongonites, topaz rhyolites, and topazites, which occasionally serve as cement in eruptive and fluid-explosive breccias. The development of multiphase intrusions from early biotite granites to late amazonite-albite granites with Li-F mica was accompanied by an increase in SiO₂ and, especially, Na₂O contents, whereas the level of (FeO + Fe₂O₃), CaO, and K₂O declined. Geochemical evolution includes an increase in the same direction in the contents of F, Li, Rb, Cs, Sn, Be, Ta, and Pb and a decrease in Ba, Sr, Zn, Zr, Th, and U. Similar evolution is also characteristic of the subvolcanic rocks, which emphasizes the genetic relation of the whole intrusive-dike complex of the Khamar-Daban province. Significant differences were detected in the distribution of K, Ba, Sr, and Zr between the calc-alkaline granitoids and rare-metal Li-F granites. The continental crust-normalized patterns of the raremetal granites show positive anomalies for Li, Rb, Nb, and Pb. The rare-metal Li-F granites could not be produced by palingenesis only, and their formation required specific conditions causing extensive accumulation of characteristic trace elements. During the evolution of granite melts, Li, Rb, Ta, Nb, Sn, W, and F are extensively accumulated in late intrusive phases, which indicates an important role of the processes of magmatic and fluid-magmatic differentiation during their formation. The composition and isotope geochemical characteristics of the supposed magma source material correspond to the ancient Precambrian continental crust with a mean model age of more than 1200 Ma.     

Petrogenesis and significance of late Caledonian granitoid magmatism in western Norway

Obduction of the late Ordovician Solund-Stavfjord Ophiolite Complex (443±3 Ma), west Norwegian Caledonides, involved generation and high-level emplacement of granitic and granodioritic dikes and plutons. Initial ⁸⁷Sr/⁸⁶Sr ratios in the granites are low (0.7042–0.7059), suggesting either a mantle component or a Rb-poor crustal source. Initial Nd (Nd(t)) ranges from-0.8 to-8.8, indicating that the granites represent recycling of old crustal rocks, which is supported by Precambrian inheritance in zircons from two of the studied granites. I argue that the Rb-Sr and the Sm-Nd isotope systems are decoupled in the sense that the Sr-and the Nd-isotopes derive their dominant signals from two different sources, a mantle source and a crustal source respectively. The granites are metaluminous to peraluminous and typically have high Sr, Ba and Na₂O/K₂O ratios. SiO₂ contents range from 66 to 74 wt%. REE abundances are highly variable; the La contents range from 80 to 200 times chondrite, and are inversely correlated with the contents of SiO₂. The concentration of Nd in the granites decreases asymptotically with decreasing Nd(t) suggesting fractional crystallization of accessory phases and assimilation of continental crust. This argument is supported by the presence of partly dismembered xenoliths in the granites with Nd(t)-values that are significantly lower than Nd(t)-values in the host granite. The following models are suggested for the granites. When the ophiolite complex obducted, an outboard subduction zone approached the continental margin, and subduction-related magmas accumulated beneath the continental margin, and probably intruded the overlying eugeosynclinal deposits. The mantle-derived magmas most likely evolved to granitoid composition by assimilation of these eugeosynclinal sediments and by fractional crystallization of amphibole, feldspar, sphene, and allanite. Alternatively, but less likely, the heat content of the mantle-derived magmas caused extensive melting of immature graywackes and calc-alkaline volcaniclastic rocks in the deepest portions of the eugeosyncline. Either way, during ascent, the compositions of the granitic melts were modified by fractional crystallization of LREE-rich phases and by assimilation of continental metasediments.     

Evolution of granitoid magmatism of the Kolyvan’-Tomsk folded zone: Evidence from the composition of Mg-Fe micas

Doklady Earth Sciences -