Geochemistry and petrology of Mesozoic-Cenozoic magmatic complexes of the southern framing of the Aldan shield: Geodynamic problems

This paper summarizes the results of long-term geological, petrological, and geochemical investigations of the Mesozoic-Cenozoic complexes of the Stanovoy Range in order to determine the main reasons for their generation and evolution. The analysis of this material showed that the compositionally variable Late Mesozoic igneous complexes of the Stanovoy Range were formed in various depth facies, from abyssal to surficial. The majority of their salic complexes show minor compositional variations, whereas the mafic complexes are more variable, especially in the southeast of the region. The southeastern Stanovoy Range comprises comparable amounts of both subalkaline and low-alkali igneous rocks, whereas the central part is dominated by subalkaline rocks, and the northwestern part contains rocks only of the shoshonite-latite series. This zoning is fundamentally different from that of typical island arcs, which are characterized by the occurrence of volcanic rocks of similar alkalinity in each zone. Extrusive and intrusive rocks with similar alkali and silica contents (and schlieren-like inclusions in the granitoids of the region) were formed from common magmas of corresponding chemical compositions. In addition, the mafic and most of the salic magmas were formed as independent melting products, whereas the magmas of intermediate composition were formed mainly by mixing of chemically contrasting liquids (i.e., salic and basic). It was shown that the available information on the magmatism of the region is best interpreted in terms of the model of mantle diapirism. In particular, mantle diapirs ascended rather slowly during the Mesozoic and occurred over the whole territory of the Stanovoy Range during the Jurassic-Cretaceous stage (J₃-K₁), when alkaline and subalkaline basalts were formed. During the Early-Late Cretaceous stage, mantle diapirs produced alkali-poor basalts in the central and eastern parts. During the Cenozoic, the diapir ascended rather rapidly but only in a small area in the eastern part of the region forming alkali basalts. In contrast to the Cenozoic, the Earth’s crust was strongly affected by mantle diapirs and related mafic magmas in the Mesozoic. As a result, crustal sequences were reworked by fluids and subsequently yielded tremendous volumes of compositionally corresponding salic magmas, which interacted and mixed with mafic magmas producing the corresponding chemical zoning. The maximum generation of crustal magmas was confined to the axial zones of ascending diapirs, where the highest energy effects took place, whereas the role of autochthonous gneissic granites increased away from the axis at the expense of typical intrusive complexes.     

Variations in the Nd isotopic ratios and canonical ratios of concentrations of incompatible elements as an indication of mixing sources of alkali granitoids and basites in the Khaldzan-Buregtei massif and the Khaldzan-Buregtei rare-metal deposit in western Mongolia

The paper reports data on the Nd isotopic composition and the evaluated composition of the sources of magmatism that produced massifs of alkali and basic rocks of the Khaldzan-Buregtei group. The massifs were emplaced in the terminal Devonian at 392–395 Ma in the Ozernaya zone of western Mongolia. The host rocks of the massifs are ophiolites of the early Caledonian Ozernaya zone, which were dated at 545–522 Ma. The massifs were emplaced in the following succession (listed in order from older to younger): (1) nordmarkites and dolerites syngenetic with them; (2) alkali granites and syngenetic dolerites; (3) dike ekerites; (4) dike pantellerites; (5) rare-metal granitoids; (6) alkali and intermediate basites and quartz syenites; and (7) miarolitic rare-metal alkali granites. Our data on the Nd isotopic composition [?_Nd(T)] and conventionally used (canonical) ratios of incompatible elements (Nb/U, Zr/Nb, and La/Yb) in rocks from the alkaline massifs and their host ophiolites indicate that all of these rocks were derived mostly from mantle and mantle-crustal enriched sources like OIB, E-MORB, and IAB with a subordinate contribution of N-MORB (DM) and upper continental crustal material. The variations in the ?_Nd(T) values in rocks of these massifs suggest multiple mixing of the sources or magmas derived from them when the massifs composing the Khaldzan-Buregtei group were produced. The OIB and E-MORB sources were mixed when the rocks with mantle signatures were formed. The occurrence of nordmarkites, alkali granites, and other rocks whose isotopic and geochemical signatures are intermediate between the values for mantle and crustal sources testifies to the mixing of mantle and crustal magmas. The crustal source itself, which consisted of rocks of the ophiolite complex, was obviously isotopically and geochemically heterogeneous, as also were the magmas derived from it. The model proposed for the genesis of alkali rocks of the Khaldzan-Buregtei massifs implies that the magmas were derived at two major depth levels: (1) mantle, at which the plume source mixed with an E-MORB source, and (2) crustal, at which the ophiolites were melted, and this gave rise to the parental magmas of the nordmarkites and alkali granites. The basites were derived immediately from the mantle. The mantle syenites, pantellerites, and rare-metal granitoids were produced either by the deep crystallization differentiation of basite magma or by the partial melting of the parental basites and the subsequent crystallization differentiation of the generated magmas. Differentiation likely took place in an intermediate chamber at depth levels close to the crustal (ophiolite) level of magma generation. Only such conditions could ensure the intense mixing of mantle and crustal magmas. The principal factor initiating magma generation in the region was the mantle plume that controlled within-plate magmatism in the Altai-Sayan area and the basite magmas related to this plume, which gave rise to small dikes and magmatic bodies in the group of intrusive massifs.     

Neoproterozoic alkaline magmatism and associated igneous rocks in the western framing of the Siberian craton: petrography,geochemistry, and geochronology

The formation and evolution conditions for alkaline magmatism and associated igneous rocks in the western framing of the Siberian craton are shown by the example of alkaline and subalkaline intrusive bodies of the Yenisei Ridge. Here we present petrographic, mineralogical, geochemical, and geochronological data for the rocks of the Srednetatarka and Yagodka plutons located within the Tatarka–Ishimba suture zone. Ferroan and metaluminous varieties enriched with rare elements (Nb, Ta, Zr, Hf, and REE) are making up most of the studied rocks. They formed at the stages of fractional crystallization of alkaline magma in a setting of active continental margin in the west of the Siberian craton in the Late Neoproterozoic (710–690 Ma). As differentiates of mantle magmas, these rocks associate with Nb-enriched rocks—A-type leucogranites and carbonatites. Sm/Nd and Rb/Sr isotopic data imply a predominance of the mantle component in the magmatic sources of the mafic and intermediate rocks as well as contamination processes of various volumes of continental crustal material by this magma.     

云南个旧杂岩体年代学与地球化学:岩石成因和幔源岩浆对锡成矿贡献

锡矿往往与长英质岩浆岩伴生,然而锡矿形成的热能源区尚不清楚,其可能与地幔物质相关。我国云南锡矿带中出露的中-酸性岩石及碱性岩杂岩体为研究锡矿及其周围岩浆成因提供了良好的物质条件。本文报道了云南个旧地区代表性的花岗岩、辉长-闪长岩和碱性岩类新的全岩地球化学、锆石U-Pb年代学和Hf同位素数据。LA-ICP-MS锆石U-Pb定年表明上述岩石分别形成于81.43±0.46Ma(82.89±0.58M)、81.35±0.22Ma和80.35±0.72Ma,指示它们为晚白垩世近同期岩浆活动的产物。其中闪长岩、碱性岩和花岗岩中锆石的Hf同位素组成不均一,ε_(Hf)(t)分别为-4.2~+0.8、-7.5~-1.9和-8.4~+0.4。尽管这些岩体的侵入时代一致,但它们的地球化学特征和同位素特征存在差异,表明这三类火成岩来自不同的岩浆源区,三者不是同一母岩浆相互演化的关系。个旧杂岩体中花岗岩为弱过铝质岩石,SiO_2与P_2O_5含量呈负相关的关系,排除S型花岗岩的可能。亏损Zr、Nb、Sr、Eu等大离子亲石元素的特征可能为锆石、磷灰石、长石类造岩矿物分离结晶作用的结果;Zr、Nb、Ce和Y总量较低,低的FeOT/MgO比值和低的锆石饱和温度表明,指示出个旧地区的花岗岩应为高分异I型花岗质岩石而非A型花岗岩。个旧地区形成于晚白垩世时期的中基性、碱性岩石可能为不同的幔源岩浆近同时侵入的产物,底侵的幔源熔体带来热量诱发中、下地壳岩石发生部分熔融形成含矿的花岗岩,幔源岩浆对于成矿至少在能量也可能在成矿物质上有重要的贡献。     

Late paleozoic-Early Mesozoic within-plate magmatism in North Asia: traps, rifts, giant batholiths, and the geodynamics of their origin

A number of large areas of igneous provinces produced in North Asia in the Late Paleozoic and Early Mesozoic include Siberian and Tarim traps and giant rift systems. Among them, the Central Asian Rift System (CARS) has the most complicated structure, evolved during the longest time, and is a large (3000 × 600 km) latitudinally oriented belt of rift zones extending from Transbaikalia and Mongolia to Middle Asia and including the Tarim traps in western China. CARS was produced in the Late Carboniferous, and its further evolution was associated with the lateral migration of rifting zones; it ended in the Early Jurassic and lasted for approximately 110 Ma. CARS was produced on an active continental margin of the Siberian continent and is noted for largest batholiths, which were emplaced simultaneously with rifting. The batholiths are surrounded by rift zones and compose, together with them, concentrically zoned magmatic areas, with crustal (granitoid) magmatism focused within their central portions, whereas mantle (rift-related) magmatism is predominant in troughs and grabens in peripheral zones. The batholiths show geological and isotopic geochemical evidence that their granitoids were produced by the anatexis of the host rocks at active involvement of mantle magmas. Zonal magmatic areas of the type are viewed as analogues of large igneous provinces formed in the environments characteristic of active continental margins. Large within-plate magmatic provinces in North Asia are thought to have been generated in relation to the overlap of at least two mantle plumes by the Siberian continent during its movement above the hot mantle field. In the continental lithosphere, mantle plumes initiated within-plate magmatic activity and facilitated rifting and the generation of traps and alkaline basite and alkali-salic magmatic associations. Because of the stressed states during collision of various type in the continental margin, the mantle melts did not ascend higher than the lowest crustal levels. The thermal effect of these melts on the crustal rocks induced anatexis and eventually predetermined the generation of the batholiths.     

Composition,sources, and mechanisms of origin of rare-metal granitoids in the Late Paleozoic Eastern Sayan zone of alkaline magmatism: A case study of the Ulaan Tolgoi massif

The Ulaan Tolgoi massif of rare-metal (Ta, Nb, and Zr) granites was formed at approximately 300Ma in the Eastern Sayan zone of rare-metal alkaline magmatism. The massif consists of alkaline salic rocks of various composition (listed in chronologic order of their emplacement): alkaline syenite → alkaline syenite pegmatite → pantellerite → alkaline granite, including ore-bearing alkaline granite, whose Ta and Nb concentrations reach significant values. The evolution of the massif ended with the emplacement of trachybasaltic andesite. The rocks of the massif show systematic enrichment in incompatible elements in the final differentiation products of the alkaline salic magmas. The differentiation processes during the early evolution of the massif occurred in an open system, with influx of melts that contained various proportions of incompatible elements. The magma system was closed during the origin of the ore-bearing granites. Rare-metal granitoids in the Eastern Sayan zone were produced by magmas formed by interaction between mantle melts (which formed the mafic dikes) with crustal material. The mantle melts likely affected the lower parts of the crust and either induced its melting, with later mixing the anatectic and mantle magmas, or assimilated crustal material and generated melts with crustal–mantle characteristics. The origin of the Eastern Sayan zone of rare-metal alkaline magmatism was related to rifting, which was triggered by interaction between the Tarim and Barguzin mantle plumes. The Eastern Sayan zone was formed in the marginal part of the Barguzin magmatic province, and rare-metal magmas in it were likely generated in relation with the activity of the Barguzin plume.     

冀西北与偏碱性岩有关的金矿床地质特征及成因

与偏碱性岩有关的金矿是我国金矿床的一种新类型，本文从区域地质，矿床地质、稳定同位素，矿物流体包裹体，微量元素，稀土元素地球化学角度出发，研究了河北省西北部的该类型金矿的地质特征，探讨了这一新类型金矿床的控矿因素及形成机制，并把研究区内与偏碱性岩有关的金矿床分为“工坪式”及“后沟式”两种，研究结果证实，该类型金矿属于与偏碱性岩有成因联系的岩浆热液型金矿床。     

安徽沿江地区岩浆岩的深部构造信息

本区分布着十分广泛的中生代岩浆岩,根据岩浆岩的物质组成、岩石组合、地壳结构、成矿作用及Ｎｄ、Ｓｒ、Ｐｂ同位素组成与比值变化等特征,可划分出３个岩浆岩带。在内带发育幔源型碱性玄武岩浆演化系列的基性—中酸性侵入岩,伴生有矽卡岩型Ｃｕ、Ｆｅ、Ｓ、Ａｕ矿床。北外带以中酸性侵入岩为主,属于下地壳重熔产物。南外带以Ｓ型花岗岩为主,显示出由上地壳重熔的特点。南、北两带均为钙碱性演化系列,并伴生斑岩型铜矿床。     

Geochemistry and genesis of ultrapotassic and potassic magmatic rocks on the eastern shore of Chaun Bay in Chukotka and their role in the metallogenic specialization of tin-bearing granitoids

Basite magmatism preceding the intrusion of large volumes of felsic magmas takes place only during powerful tectonic rearrangements, which span both the continental crust and lithospheric mantle. The study of this magmatism makes it possible to solve many genetic problems and obtain important geological information on the sources and processes that are responsible for granitoid magmatism. This paper reports the results of the geochemical study of potassic and ultrapotassic magmatic rocks that predate the intrusion of the granitoid complex and belong to it. In terms of geochemistry, the studied magmatic rocks of Chukotka correspond to the derivatives of potassic and ultrapotassic magmas, which allows us to use the models of formation of ultrapotassic magmas for interpreting the genetic features of tin-bearing granites, in particular, for explaining the anomalous contents of incompatible elements in these rocks. Using modern genetic models in combination with geological, geophysical, and geochemical data, it is established that the source of this specialization was the lithospheric mantle domain. The domain was formed within a convergent geologic boundary owing to the metasomatic reworking of the mantle wedge by fluids that were released during dehydration of the oceanic lithosphere. Based on the obtained results, a new model was proposed for the formation of tin-bearing granitoids in the collisional orogens. This model is underlain by the concept of a particular lithospheric source, which acquired its geochemical and metallogenic signatures during intense tectonic transformation that involved the lithospheric mantle. These signatures were inherited by magmas formed during melting within this domain.     

Plume-related mantle source of super-large rare metal deposits from the Lovozero and Khibina massifs on the Kola Peninsula, Eastern part of Baltic Shield: Sr, Nd and Hf isotope systematics

The two world’s largest complexes of highly alkaline nepheline syenites and related rare metal loparite and eudialyte deposits, the Khibina and Lovozero massifs, occur in the central part of the Kola Peninsula. We measured for the first time in situ the trace element concentrations and the Sr, Nd and Hf isotope ratios by LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometer) in loparite, eudialyte an in some other pegmatitic minerals. The results are in aggreement with the whole rock Sr and Nd isotope which suggests the formation of these superlarge rare metal deposits in a magmatic closed system. The initial Hf, Sr, Nd isotope ratios are similar to the isotopic signatures of OIB indicating depleted mantle as a source. This leads to the suggestion that the origin of these gigantic alkaline intrusions is connected to a deep seated mantle source—possibly to a lower mantle plume. The required combination of a depleted mantle and high rare metal enrichment in the source can be explained by the input of incompatible elements by metasomatising melts/fluids into the zones of alkaline magma generation shortly before the partial melting event (to avoid ingrowth of radiogenic isotopes). The minerals belovite and pyrochlore from the pegmatites are abnormally high in ⁸⁷Sr /⁸⁶Sr ratios. This may be explained by closed system isotope evolution as a result of a significant increase in Rb/Sr during the evolution of the peralkaline magma.     

Lead isotope evolution of the Central European upper mantle:Constraints from the Bohemian Massif

The Pb isotope composition of the upper mantle beneath Central Europe is heterogeneous due to the subduction of regionally contrasting material during the Variscan and Alpine orogenies.Late Variscan to Cenozoic mantlederived melts allow mapping this heterogeneity on a regional scale for the last ca.340 Myr.Late Cretaceous and Cenozoic anorogenic magmatic rocks of the Bohemian Massif(lamprophyres,volcanic rocks of basanite/tephrite and trachyte/phonolite series) concentrate mostly in the Eger Rift.Cretaceous ultramafic lamprophyres yielded the most radiogenic Pb isotope signatures reflecting a maximum contribution from metasomatised lithospheric mantle,whereas Tertiary alkaline lamprophyres originated from mantle with less radiogenic ~(206)Pb/~(204)b ratios suggesting a more substantial modification of lithospheric source by interaction with asthenosphericderived melts.Cenozoic volcanic rocks of the basanite/tephrite and trachyte/phonolite series define a linear mixing trend between these components,indicating dilution of the initial lithospheric mantle signature by upwelling asthenosphere during rifting.The Pb isotope composition of Late Cretaceous and Cenozoic magmatic rocks of the Bohemian Massif follows the same Pb growth curve as Variscan orogenic lamprophyres and lamproites that formed during the collision between Laurussia,Gondwana,and associated terranes.This implies that the crustal Pb signature in the post-Variscan mantle is repeatedly sampled by younger anorogenic melts.Most Cenozoic mantle-derived rocks of Central Europe show similar Pb isotope ranges as the Bohemian Massif.     

A gradual transition to plate tectonics on Earth between 3.2 to 2.7 billion years ago

Zircon crystals precipitated from granitoid magmas contain a robust record of the age and chemistry of continental magmatism spanning some 4.375 Ga of Earth history, a record that charts initiation of plate tectonics. However, constraining when exactly plate tectonics began to dominate crustal growth processes is challenging as the geochemical signatures of individual rocks may reflect local subduction processes rather than global plate tectonics. Here we apply counting statistics to a global database of coupled U–Pb and Hf isotope analyses on magmatic zircon grains from continental igneous and sedimentary rocks to quantify changes in the compositions of their source rocks. The analysis reveals a globally significant change in the sources of granitoid magmas between 3.2 and 2.7 Ga. These secular changes in zircon chemistry are driven by a coupling of the deep (depleted mantle) and shallow (crustal) Earth reservoirs, consistent with a geodynamic regime dominated by Wilson cycle style plate tectonics.     

Composition,sources, and geodynamic nature of giant batholiths in Central Asia: Evidence from the geochemistry and Nd isotopic characteristics of granitoids in the Khangai zonal magmatic area

Data on the composition, inner structure, and magma sources of giant batholith in the Central Asian Orogenic Belt are analyzed with reference to the Khangai batholith. The Khangai batholith was emplaced in the Late Permian–Early Triassic (270–240 Ma) and is the largest accumulations (>150000 km²) of granite plutons in central Mongolia. The plutons are dominated by granites of normal alkalinity and contain subalkaline granites and more rare alkaline granites. The batholith is hosted in the Khangai zonal magmatic area, which consists of the batholith itself and surrounding rift zones. The zones are made up of bimodal basalt–trachyte–comendite (pantellerite) or basalt-dominated (alkaline basalt) volcanic associations, whose intrusive rocks are dominated by syenite and granite, granosyenite, and leucogranite. Both the batholith and the rift zones were produced within the time span of 270–240 Ma. Although the rocks composing the batholith and its rift surroundings are different, they are related through a broad spectrum of transitional varieties, which suggests that that the mantle and crustal melts could interact at various scale when the magmatic area was produced. A model is suggested to explain how the geological structure of the magmatic area and the composition of the magmatic associations that make up its various zones were controlled by the interaction between a mantle plume and the lithospheric folded area. The mantle melts emplaced into the lower crust are thought to not only have been heat sources and thus induced melting but also have predetermined the variable geochemical and isotopic characteristics of the granitoids. In the marginal portions of the zonal area, the activity of the mantle plume triggered rifting associated with bimodal and alkaline granite magmatism. The formation of giant batholiths was typical of the evolution of the active continental margin of the Siberian paleocontinent in the Late Paleozoic and Early Mesozoic: the Khangai, Angara–Vitim, and Khentei batholiths were formed in this area within a relatively brief time span between 300 and 190Ma. The batholiths share certain features: they consist of granitoids of a broad compositional range, from tonalite and plagiogranite to granosyenite and rare-metal granites; and the batholiths were produced in relation to rifting processes that also formed rift magmatic zones in the surroundings of the batholiths. The large-scale and unusual batholith-forming processes are thought to have occurred when the active continental margin of the Late Paleozoic Siberian continent overlapped a number of hotspots in the Paleo- Asian Ocean. This resulted in the origin of a giant anorogenic magmatic province, which included batholiths, flood-basalt areas in Tarim and Junggar, and the Central Asian Rift System. The batholiths are structural elements of the latter and components of the zonal magmatic areas.     

Petrology,zircon U–Pb geochronology and tectonic implications of the A1-type intrusions: Keban region,eastern Turkey

The Southeast Anatolian Orogenic Belt (SAOB), the most extensive segment of the Alpine-Himalayan Orogenic Belt, resulted from the northward subduction of the southern branch of the Neotethys oceanic crust beneath the Anatolian micro-plate. We present new whole-rock geochemistry, zircon U–Pb ages, and Lu–Hf isotope data from the stocks and dykes with a length of up to tens of meters belonging to the Keban magmatic rocks, eastern Turkey. These rocks are represented by syenite and quartz monzonite intruded into the Keban metamorphic complex. The geochemistry data indicates that the samples bear mostly metaluminous, variably high alkalines (K₂O + Na₂O), Ga/Al ratios and zircon saturation temperature, and typically the A-type granite characters. According to the Y/Nb vs Yb/Ta diagram, the Keban magmatic rocks show A₁-type geochemical signatures modified by crustal melts. Syenite and quartz monzonite samples from Keban magmatic rocks give zircon U–Pb ages of 77.4 ± 0.34 Ma, 76.3 ± 0.3 Ma and 76.36 ± 0.34 Ma, respectively. On the primitive mantle-normalised trace element patterns, the Keban magmatic rocks show enrichment in large-ion lithophile elements (LILEs) relative to high field strength elements (HFSEs). They are coupled with slightly negative Nb–Ta anomalies. Chondrite-normalised rare earth-element patterns show strong enrichment in LREEs relative to HREEs, a typical A-type granites feature. The zircons have negative εHf_(t) values that vary from ?2.68 to ?0.41, and Hf model ages (T_DM2) range from 1171.54 to 1329.26 Ma, indicating the enriched lithospheric mantle sources and crustal contribution. The sources and evolution of the alkaline magmas might be related to the post-collisional tectonic setting.     

Pre-Cenozoic intra-plate magmatism along the Central Andes (17–34°S): Composition of the mantle at an active margin

Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with rather uniform initial ¹⁴³Nd/¹⁴⁴Nd ( 0.5127–0.5128) and ⁸⁷Sr/⁸⁶Sr ( 0.7032–0.7040). The initial ²⁰⁶Pb/²⁰⁴Pb ratios are variable (18.5–19.7) at uniform ²⁰⁷Pb/²⁰⁴Pb ratios (15.60 ± 0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late Palaeozoic age) intra-plate magmatic rocks (¹⁴³Nd/¹⁴⁴Nd  0.5123, ⁸⁷Sr/⁸⁶Sr  0.704, ²⁰⁶Pb/²⁰⁴Pb  17.5–18.5, and ²⁰⁷Pb/²⁰⁴Pb  15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield.

Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene) intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle did not contribute to Mesozoic to Cenozoic arc magmatism.     

Origin and interaction of mafic and felsic magmas in an evolving late orogenic setting: the Early Paleozoic Terra Nova Intrusive Complex, Antarctica

The Abbott Unit (∼508 Ma) and the Vegetation Unit (∼475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land, Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement styles and depths, ranging from forcible at 0.4–0.5 GPa for the Abbott Unit to passive at ∼0.2 GPa for the Vegetation Unit. Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly enriched incompatible element distribution, similar to common continental arc basalts and (2) the modelled primary Vegetation Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite and Vegetation leucogranite, respectively. Received: 31 March 1998 / Accepted: 3 May 1999     

Early Cretaceous trachybasalt-trachyte-trachyrhyolitic volcanism in the Nyalga basin (Central Mongolia): sources and evolution of continental rift magmas

As shown by geological, mineralogical, and isotope geochemical data, trachybasaltic-trachytic-trachyrhyolitic (TTT) rocks from the Nyalga basin in Central Mongolia result from several eruptions of fractionated magmas within a short time span at about 120 Ma. Their parental basaltic melts formed by partial melting of mantle peridotite which was metasomatized and hydrated during previous subduction events. Basaltic trachyandesites have high TiO₂ and K₂O, relatively high P₂O₅, and low MgO contents, medium ⁸⁷Sr/⁸⁶Sr(₀) ratios (0.70526-0.70567), and almost zero or slightly negative ε_Nd(T) values. The isotope geochemical signatures of TTT rocks are typical of Late Mesozoic basaltic rocks from rift zones of Mongolia and Transbaikalia. The sources of basaltic magma at volcanic centers of Northern and Central Asia apparently moved from a shallower and more hydrous region to deeper and less hydrated lithospheric mantle (from spinel to garnet-bearing peridotite) between the Late Paleozoic and the latest Mesozoic. The geochemistry and mineralogy of TTT rocks fit the best models implying fractional crystallization of basaltic trachyandesitic, trachytic, and trachyrhyodacitic magmas. Mass balance calculations indicate that trachytic and trachydacitic magmas formed after crystallization of labradorite-andesine, Ti-augite, Sr-apatite, Ti-magnetite, and ilmenite from basaltic trachyandesitic melts. The melts evolved from trachytic to trachyrhyodacitic and trachyrhyolitic compositions as a result of prevalent crystallization of K-Na feldspar, with zircon, chevkinite-Ce, and LREE-enriched apatite involved in fractionation. Trachytic, trachyrhyodacitic, and trachyrhyolitic residual melts were produced by the evolution of compositionally different parental melts (basaltic trachyandesitic, trachytic, and trachyrhyodacitic, respectively), which moved to shallower continental crust and accumulated in isolated chambers. Judging by their isotopic signatures, the melts assimilated some crustal material, according to the assimilation and fractional crystallization (AFC) model.     

Geodynamic settings and magma sources of the Late Cretaceous-Early Paleocene magmatic complexes of northern Kamchatka

Geochemical and isotopic studies showed that the Late Cretaceous-Early Paleocene magmatic rocks of northern Kamchatka were formed in different geodynamic zones of a Late Cretaceous-Early Paleocene suprasubduction system: from a volcanic front to a back-arc rift basin. Suprasubduction magmas were derived from upper mantle garnet or spinel lherzolites variably depleted in terms of Nd isotopic composition or enriched in HFSE and showing varying Th/La, Th/Ta, Zr/Nb, and Nb/U ratios. Subduction-related fluids played an active role in this process. The suprasubduction mantle melts were not contaminated by crustal materials enriched in radiogenic Nd. A weak imprint of contamination was identified only in the lavas of Karaginsky Island.     

Collision-related slab break-off volcanism in the Eastern Anatolia,Kepez volcanic complex (TURKEY)

The Neogene–Quaternary volcanic products, related to Arabian and Anatolian Plate collision along the Bitlis Suture Zone, cover wide areas on both plates. One of these volcanic exposures on the Arabian Plate is the Kepez volcanic complex (KVC). This study aims explain to petrogenesis of KVC. Although some examples display alkaline affinities, the majority of the volcanic rock is calc-alkaline and can be defined in three main groups. ⁴⁰Ar/³⁹Ar data obtained from dacite, basalt and andesite rock groups within the KVC yield ages of between 13.5 and 15.5 Ma. Geochemical and petrographical data show that the andesitic rocks are products of homogeneous mixing between basic end-member magmas and dacitic magmas which are the products of partial melting of lower crustal compositions. Basaltic products of KVC are asthenospheric mantle derived, while dacitic and andesitic volcanic rocks are crustal origin. High Sr and Nd isotope ratios may indicate that andesitic and dacitic rocks originated from continental crust. The lithospheric mantle, which is subducting underneath the Anatolian plate, must have experienced slab break-off processes 13–15 million years ago and sunk into the asthenosphere. KVC were produced with the collision between Arabian and Anatolian Plates and related uplift of the East Anatolia region.     

Magmatic sources of dikes and veins in the Moncha Tundra Massif,Baltic Shield: Isotopic-geochronologic and geochemical evidence

The dike-vein complex of the Moncha Tundra Massif comprises dolerites, gabbro-pegmatites, and aplites. The dolerite dikes are classified into three groups: high-Ti ferrodolerites, ferrodolerites, low-Ti and low-Fe gabbro-dolerites. The U-Pb age of the ferrodolerites is 2505 ± 8 Ma, and the amphibole-plagioclase metagabbroids hosting a ferrodolerite dike are dated at 2516 ± 12 Ma. Data on the U-Pb isotopic system of zircon from the gabbro-pegmatites and titanite from the aplites indicate that the late magmatic evolution of the Moncha Tundra Massif proceeded at 2445 ± 1.7 Ma, and the youngest magmatic events in the massif related to the Svecofennian orogeny occurred at 1900 ± 9 Ma. The data obtained on the Sm-Nd and Rb-Sr isotopic systems and the distribution of trace elements and REE in rocks of the dike-vein complex of the massifs provide insight into the composition of the sources from which the parental magmas were derived. The high-Ti ferrodolerites were melted out of a deep-sitting plume source that contained an asthenospheric component. The ferrodolerites were derived from a mantle MORB-type source that contained a crustal component. The parental melts of the gabbro-dolerites were melted out of the lithospheric mantle depleted in incompatible elements after Archean crust-forming processes above an ascending mantle plume, with the participation of a crustal component. The gabbro-dolerites and the rocks of the layered complex of the Moncha Tundra Massif exhibit similar geochemical characteristics, which suggest that their parental melts could be derived from similar sources but with more clearly pronounced crustal contamination of the parental melts of the rocks of the massif itself. The geochemical traits of the gabbro-pegmatites are thought to be explained not only by the enrichment of the residual magmas in trace elements and a contribution of a crustal component but also by the uneven effect of sublithospheric mantle sources. The aplites were derived from a sialic crustal source.