Alkaline Magmatic Activity Research Articles

Tectono-magmatic evolution during the extensional phase of a Wilson Cycle: a review of the Alpine Tethys case and implications for Atlantic- type margins

Despite the recent progress in seismic imaging and dynamic modelling, the tectono-magmatic evolution during late orogenic extension and rifting is at present little understood. Answering questions such as when and where magma forms, how it percolates and how it is emplaced requires access to magmatic rocks from different crustal and mantle levels, which is impossible in modern rift systems. An alternative, presented in this study, is to use remnants of fossil extensional systems exposed in collisional orogens. Here we review the Permian to Early Cretaceous tectono-magmatic evolution recorded in the Alpine realm of Western Europe. While most of the previous studies reviewed the geochemical and petrological characteristics of the magmatic rocks, here we rely on a holistic approach to link the magmatic evolution to extensional processes on a large scale. We synthesize our observations on the timing and location of magma production in a Wheeler diagram. Our study shows that two major magmatic events can be defined: the first one took place during the Permian and was linked to post- orogenic extension of the Variscan belt. It affected a large portion of Western Europe, where it was responsible for significant sub-aerial volcanic activity, for the emplacement of massive magmatic additions at the base and within the crust, and for the depletion of the lower subcontinental lithospheric mantle. The second included several pulses of magmatic activity related to the multiphase rifting event that led to the successive opening of the Meliata-Vardar, Alpine Tethys and southern North Atlantic (proto-)oceanic domains. In the Alpine Tethys, the first rift-related magmatic activity was linked to melt infiltration within the subcontinental lithospheric mantle during the so-called necking stage. Basalts were emplaced only later, during and after mantle exhumation. Their chemical signature reflects the interaction between melts derived from Depleted Mid-oceanic ridge basalt Mantle (DMM) and the previously refertilized/infiltrated subcontinental mantle. Late alkaline magmatic activity post- dated breakup. A main result of this study is that the tectono-magmatic evolution of extensional systems cannot be explained by simple models using ß-factors or mantle temperatures alone but also needs to integrate the inheritance acquired during previous tectonic phases.

Timing constraints on alkaline magmatic activity and implications for lithospheric thinning beneath the North China Craton: evidence from zircon geochronology of syenite from the Huyanshan complex in the Lyuliangshan uplift belt, Shanxi Province

Timing constraints on alkaline magmatic activity and implications for lithospheric thinning beneath the North China Craton: evidence from zircon geochronology of syenite from the Huyanshan complex in the Lyuliangshan uplift belt, Shanxi Province

Zircon U-Pb ages, geochemistry, and Sr-Nd isotope ratios for early cretaceous magmatic rocks, southern Saqqez, northwestern Iran

Sanandaj-Sirjan Zone (SaSZ) is one of the most dynamic structural zones of Iran, which is divided into three main parts: Northern, Central and Southern. The northern SaSZ has been affected by deformation due to fault activities near the Zagros suture zone, and mylonitic structures have overprinted these rocks and was affected by three episodes of magma injection during the Permian-Carboniferous, Early Cretaceous and Cenozoic. In this study, the rock units investigated that have been considered Precambrian-Paleozoic basement on geological maps. This paper considers zircon U-Pb dating, whole-rock chemistry and Sr-Nd isotope ratios of Cretaceous magmatic rocks in the N-SaSZ to develop a new geodynamic model for the evolution of these magmatic rocks. The new zircon U-Pb ages obtained in this study show that the magmatic rocks crystallized at 115–107 Ma in the Early Cretaceous (Aptian-Albian) and are much younger than the supposed ages presented on geological maps. This complex classified into two main groups of basic-intermediate and acidic rocks based on SiO2 contents. The whole-rock chemistry of the basaltic and andesitic rocks, which are interbedded with marine shallow-water sedimentary deposits, shows their typical calc-alkaline affinity and subordinate tholeiitic series on an active margin. The positive εNd(t) of approximately +4 for some undifferentiated basalts with negative Ti and Nb anomalies shows the relation of these rocks to calc-alkaline magmatism and was generated by the partial melting of subcontinental lithospheric mantle (SCLM). Granitoid rocks with some affinity to the peraluminous group with a negative εNd(t) value (-3.2) mainly and negative Ti and Nb anomalies plot in an active margin tectonic setting. Simultaneous mafic calc-alkaline volcanism and the generation of granitic intrusions in the Early Cretaceous could have occurred on an active margin. Due to the absence of Jurassic arc related magmatic rocks in northern SaSZ and presence of Cretaceous calc alkaline magmatic activity, which are not observed in the central SaSZ, support the idea that the subduction of the Neotethys beneath the northern SaSZ started in the Early Cretaceous.

Subduction-related melt refertilisation and alkaline metasomatism in the Eastern Transylvanian Basin lithospheric mantle: Evidence from mineral chemistry and noble gases in fluid inclusions

Calc-alkaline and alkaline magmatic activity is generally separated in space and/or in time. The Eastern Transylvanian Basin in Romania is one of the few places where, during Pleistocene, alkaline eruptions occurred contemporaneously with the calc-alkaline activity. Mantle xenoliths entrained in Perşani Mts. alkaline volcanic products have been studied in order to investigate the interaction of metasomatic agents of different magmatic affinities with the mantle wedge. Based on mineral major and trace element and noble gases in fluid inclusions, two main events have been recognized. The first was a pervasive, complete re-fertilization of a previously depleted mantle by a calc-alkaline subduction-related melt, causing the formation of very fertile, amphibole-bearing lithotypes. This is shown by the a) increased amounts of modal clinopyroxene up to 21.9 % with Al2O3 contents up to 8.16 wt%, higher than what is expected for clinopyroxene in Primordial Mantle; b) 4He/40Ar* ratios up to 1.2, within the reported range for mantle production; c) 3He/4He in olivine, opx and cpx of 5.8 ± 0.2 Ra, among the most radiogenic values of European mantle, below the typical MORB mantle value (8 ± 1 Ra), reflecting recycling of crustal material in the local lithosphere. The second event is related to later interaction with an alkaline metasomatic agent similar to the host basalts that caused slight LREE enrichment in pyroxenes and disseminated amphiboles and precipitation of vein amphiboles with a composition similar to amphiboles megacrysts also found in the Perşani Mts. volcanic deposits. This is highlighted by the 4He/40Ar* and 3He/4He values found in some opx and cpx, up to 2.5 and 6.6 Ra, respectively, more typical of magmatic fluids.

Pyrite chemistry: A new window into Au-Te ore-forming processes in alkaline epithermal districts, Cripple Creek, Colorado

Tellurium has a wide variety of applications, most importantly in the solar energy industry and is eco-toxicologically significant; however, the magmatic-hydrothermal processes causing the pronounced Te enrichment together with Au in some epithermal districts are still poorly constrained. Hydrothermal and alkaline magmatic activity in post-subduction environments are suggested to be a critical component in the evolution of this Te-rich sub-class of low-sulfidation epithermal deposits. Cripple Creek represents an example for a world-class low-sulfidation epithermal Au-Te anomaly in the continental crust. This area represents a natural laboratory to investigate the processes of ore-formation and Au-Te enrichment in alkaline igneous rock-hosted epithermal systems.Here, we present the first micro-analytical approach that combines petrographic observations with in situ LA-ICP-MS analyses and trace element mapping to define the key ore-forming processes of Au and Te in the Cripple Creek epithermal complex. Two main styles of mineralization can be distinguished: (1) low-grade Au disseminated pyrite-rich ores in the permeable brecciated host rocks and (2) high-grade quartz-fluorite veins rich in calaverite (AuTe2), coloradoite (HgTe), petzite (Ag3AuTe2), altaite (PbTe) and native Au.Pyrite trace element mapping revealed distinct variations and decoupling of elements (Au-Te) and element pairs (Au-As vs. Co-Ni) within and between different sites in the epithermal district. This is reflected by the concentric/growth zoning of these elements in pyrite that were interpreted to be caused by fluid boiling associated with the deposition of the low-grade disseminated host rock ores at temperatures between 220 and 350 °C leading to the precipitation of Au, As, Co and Ni from the liquid phase and the preferential partitioning of Te into the vapor phase. The subsequent condensation of the Te-rich vapors in metal-bearing meteoric waters led to the Te precipitation being decoupled from Au in the low-grade ores.The high-grade Au-Te mineralization was likely initiated by the influx of magmatically derived oxidized fluids. Subsequently, fluid temperatures dropped due to mixing with meteoric waters resulting in fluid boiling under low pressure conditions at shallower crustal levels (<1000 m) compared to the low-grade ores. Elevated fO2 minimized the loss of Te to the vapor phase and the strong boiling conditions at lower fluid temperatures (105–200 °C) caused the contemporaneous precipitation of Au and Te in the high-grade veins.

The Hadamengou K-feldspar–quartz vein-type Au mineralization in the northern margin of the North China Craton (China): an Indosinian oxidized intrusion-related Au–(Mo) system?

K-feldspar–quartz vein-type Au deposits, represented by the Hadamengou super-large Au deposit, are widely developed in the Archean high-grade metamorphic terrane in the western segment of the northern margin of the North China Craton. However, the formation processes of these Au deposits have long been debated, and their genetic relationship with magmatic activity remains unclear. In this study, by examining the geological and geochemical characteristics of the Hadamengou Au deposit and several related intrusions, we find that the Au–(Mo) mineralization system may be related to Indosinian alkaline magmatic activity. The Dongshadegai and Xishadegai intrusions, which are related to the formation of the Hadamengou Au deposit, formed in an Indosinian extensional tectonic environment, and both are characterized by high silica and alkaline contents and strong oxidizing properties, indicating that the parent magmas of these intrusions were capable of dissolving and transporting metallic elements during magma formation and evolution. The Indosinian Hadamengou Au deposit contains minerals such as magnetite and barite, and the H, O, S and Pb isotopic characteristics of this deposit indicate that the ore-forming fluids originated from magmatic–hydrothermal fluids and had oxidizing properties. These observations suggest the presence of an oxidized intrusion-related Au–(Mo) mineralization system in the studied metamorphic terrane.

Isotopic and geochemical constraints on lead and fluid sources of the Pb[sbnd]Zn[sbnd]Ag mineralization in the polymetallic Tighza-Jbel Aouam district (central Morocco), and relationships with the geodynamic context

The WAu, PbZnAg, and SbBa mineralizations of the polymetallic Tighza-Jbel Aouam district (central Meseta, Morocco), are hosted in Paleozoic rocks surrounding late-Carboniferous granitic stocks. The PbZnAg Tighza deposit formed at 254 ± 16 Ma, and is clearly disconnected from the late-Variscan WAu deposit (295-280 Ma). The PbZnAg mineralization precipitated from a complex hydrothermal fluid. It displays air-normalized 3He/4He ratio (0.018–0.103) typical of the upper crust. This crustal component is confirmed by the oxygen and carbon isotope compositions (δ18O = +19 to +25‰; δ13C = −3.6 to −11.2‰) and the ɛNd values (−4.84 to −9.01) of gangue carbonates, which show mixing of (i) fluids that have interacted with late-Carboniferous magmatic rocks, and (ii) fluids in equilibrium with the Paleozoic metasediments. In addition, the PbZnAg mineralization has 40Ar/36Ar values in the range 284–315 typical of a meteoric fluid. The radiogenic Pb isotopic compositions (207Pb/204Pb = 15.70–15.80 and 206Pb/204Pb = 18.30–18.50) suggest leaching of Pb from the surrounding Paleozoic metasediments and late-Variscan granites, whereas the low radiogenic signatures (207Pb/204Pb = 15.40 and 206Pb/204Pb = 18.05) provide evidence of a deeper source attributed to the lower crust.Crustal thinning related to extensional tectonics in late-Permian and Early-Triassic lead to high-K calc-alkaline to alkaline magmatic activity, which is evidenced by a dense SW-NE-trending dike network that pre-dated the Atlantic Ocean opening (early Liassic times). This magmatic event induced a regional heat flux increase that triggered the circulation of a complex hydrothermal fluid, which has a strong crustal component, but also a meteoric and a lower crustal components. The polymetallic district of Tighza-Jbel Aouam thus results from superposition of an intrusion related porphyry-gold mineralization (WAu, 286 Ma) followed by a PbZnAg epithermal mineralization (254 Ma), during two distinct magmatic-hydrothermal events.The proposed metallogenic model for the PbZnAg Tighza-Jbel Aouam deposit provides new constraints for the PbZnAg exploration in the Moroccan Meseta. Exploration targets must take into account the following geological features: (i) Permo-triassic high-K calk-alkaline to alkaline dikes, (ii) extensional tectonics and reactivation of ancient crust-scale faults and shear zones, and (iii) Paleozoic series containing organic matter (e.g., black shales) subjected to low grade metamorphism (e.g., greenschist facies).

Mineral resources of high-tech metals in Russia: State of the art and outlook

Seven main ore-forming systems—porphyry and epithermal; orogenic related to granitic intrusions; magmatic ultramafic; volcanic-hosted massive sulfide and volcanic–sedimentary; sedimentary basins; related to alkaline magmatic activity; and placers and weathering mantles—are sources of high-tech critical metals. The following promising types of ore deposits containing high-tech critical metals as by-products are recognized: Cu–Mo porphyry, Fe–Cu–Au and Pb–Zn skarn, base-metal epithermal, volcanic-hosted massive sulfide, base-metal stratiform, various tin deposits, and placers containing rare metals including REE. The mineral resources of critical metals in Russia are compared with those known in other countries. The contents of high-tech critical metals in ores of some noble-metal deposits of the Russian Northeast are reported. It is shown that the subsurface of Russia possesses considerable mineral resource potential for hightech critical metals, which allows new enterprises to be created or production of operating enterprises to increase.

Genetic relationships between skarn ore deposits and magmatic activity in the Ahar region, Western Alborz, NW Iran

Abstract Paleocene to Oligocene tectonic processes in northwest Iran resulted in extensive I-type calc-alkaline and alkaline magmatic activity in the Ahar region. Numerous skarn deposits formed in the contact between Upper Cretaceous impure carbonate rocks and Oligocene-Miocene plutonic rocks. This study presents new field observations of skarns in the western Alborz range and is based on geochemistry of igneous rocks, mineralogy of the important skarn deposits, and electron microprobe analyses of skarn minerals. These data are used to interpret the metasomatism during sequential skarn formation and the geotectonic setting of the skarn ore deposit related igneous rocks. The skarns were classified into exoskarn, endoskarn and ore skarn. Andraditic garnet is the main skarn mineral; the pyroxene belongs to the diopside-hedenbergite series. The skarnification started with pluton emplacement and metamorphism of carbonate rocks followed by prograde metasomatism and the formation of anhydrous minerals like garnet and pyroxene. The next stage resulted in retro gradation of anhydrous minerals along with the formation of oxide minerals (magnetite and hematite) followed by the formation of hydrosilicate minerals like epidote, actinolite, chlorite, quartz, sericite and sulfide mineralization. In addition to Fe, Si and Mg, substantial amounts of Cu, along with volatile components such as H2S and CO2 were added to the skarn system. Skarn mineralogy and geochemistry of the igneous rocks indicate an island arc or subduction-related origin of the Fe-Cu skarn deposit.

Palaeozoic Formations from Dobrogea and Pre-Dobrogea – An Overview

An overview of lithological, palaeontological and geochronological evidence existing for the Palaeozoic formations from Dobrogea and PrDobrogea has enabled a better understanding of the Palaeozoic history of these areas. The Lower Palaeozoic of Pre-Dobrogea, in places in continuity with the pelitic-silty facies of the underlying Vendian (Ediacaran) deposits, was one of the peri-Tornquist basins of Baltica, suggesting that the Scythian Platform in the Pre-Dobrogea basement represents the rifted margin of the East European Craton. In North Dobrogea two types of Palaeozoic succession have formed in different tectonic settings. Deep marine Ordovician-Devonian deposits, including pelagic cherts and shales, associated with turbidites, and facing Devonian carbonate platform deposits of the East European Craton, form northward-younging tectonic units of an accretionary wedge, tectonically accreted above a south-dipping subduction zone. South of the accretionary prism, the basinal to shallow marine Silurian-Devonian deposits of North Dobrogea, showing a similar lithology to the East Moesian successions, accumulated on top of low-grade Cambrian clastics with Avalonian affinity indicated by detrital zircons. Late Palaeozoic erosion was accompanied by deposition of continental alluvial, fluvial and volcano-sedimentary successions, overlying their basement above an imprecise Carboniferous gap. The low-grade metamorphic Boclugea terrane, showing Avalonian affinity, and the associated Lower Palaeozoic deposits represent East Moesian successions, docked to Baltica by the Lower Devonian and subsequently involved in the Hercynian orogeny, being affected by Late Carboniferous-Early Permian regional metamorphism and granite intrusion. The Late Carboniferous-Early Permian syn-tectonic sedimentation, regional metamorphism of Palaeozoic formations and development of a calc-alkaline volcano-plutonic arc indicate an active plate margin setting and an upper plate position of the Macin-type successions during the Variscan collision, when the Orliga terrane, with Cadomian affinity, was accreted to Laurussia along a north-dipping subduction zone of the Rheic Ocean. The East Moesian Lower Palaeozoic succession, overstepping its Ediacaran basement, represents an Avalonian terrane, docked to the Baltica margin in the Early Palaeozoic. A narrow terrane detached from the Trans-European Suture Zone (TESZ) margin of the Baltica palaeocontinent forms a tectonic wedge within the East Moesian basement. The Palaeozoic sedimentary record of East Moesia shows a quartzitic facies in the Ordovician, graptolite shales in Upper Ordovician-Wenlock, black argillites in the Ludlow-Pridoli and fine-grained clastics in the Lower Devonian. Eifelian continental sandstones are followed by a carbonate platform from Givetian to Tournaisian times and coal-bearing clastics in the Carboniferous, indicating a foredeep basin evolution. By the Eifelian both East Moesia and Pre-Dobrogea were part of Laurussia, sharing the same old red sandstone facies. The Permian is a time of rifting in Dobrogea and Pre-Dobrogea, although evidence for rifting in the East Moesian sedimentary record is very limited. In the eastern basins of Pre-Dobrogea, Permian rifting was accompanied by alkaline bimodal volcanism of the basalt-trachyte association, that affected also the northern margin of North Dobrogea. Late Permian within-plate alkaline magmatic activity emplaced plutonic and hypabyssal complexes along the south-western margin of North Dobrogea. The model proposed for the Palaeozoic history based on existing data for the north-western margin of the Black Sea records early Palaeozoic docking to Baltica of the Avalonian terrane of East Moesia, including the Boclugea terrane of North Dobrogea. Late Carboniferous-Early Permian accretion of the Cadomian Orliga terrane from North Dobrogea, accompanied by Hercynian metamorphism and granite intrusion, correlates with the closure of the Rheic Ocean. Subsequently, Avalonian and Cadomian terranes, together with a narrow terrane detached from the TESZ margin of Baltica palaeocontinent, were displaced southward along the strike-slip fault system of the TESZ.

The Ediacaran Ferani and Rutig volcano-sedimentary successions of the northernmost Arabian-Nubian Shield (ANS): New insights from zircon U–Pb geochronology, geochemistry and O–Nd isotope ratios

Late Neoproterozoic (Ediacaran) evolution of the northernmost segment of the Arabian-Nubian Shield (ANS) included vast intrusion of granitoids and development of volcano-sedimentary basins. Here we describe two of such volcano-sedimentary successions (Rutig and Ferani) exposed in southern Sinai, Egypt. The successions are dominated by intermediate to silicic volcanics of mostly high-K calc-alkaline affinity, interbedded with immature sediments. Minor andesites form the basal parts of both successions, whereas the dominant volcanics are dacites to rhyolites. The latter are highly fractionated and include A-type granite affinities. In the Rutig succession, the basal andesites have characteristics of low-silica adakitic (LSA) rocks. New zircon U–Pb dating by SIMS of dacites and rhyolites from both successions yielded ages between ca. 627 and 593 Ma. Combined with field evidence and previous age data from interbedded conglomerates, the Rutig succession is divided into a lower and an upper part recording distinct volcano-sedimentary cycles. The lower Rutig volcanic rocks of ca. 630–615 Ma and the associated sediments were folded and then unconformably overlain by the upper Rutig succession (615–590 Ma). Dacites and rhyolites from the lower and upper parts of the Ferani succession record ages of 607–593 Ma overlapping with the end of volcanic activity in the upper Rutig. SIMS δ 18O of dated zircons from dacites in the Rutig and Ferani are all within the mantle range for zircon, and show no consistent trends associated to cores and rim domains. Coupled with minor inheritance in the Rutig and Ferani volcanics this indicates little crustal contamination during evolution of the magmatic suites. Whole-rock ɛNd( t) for the volcanics of the Rutig range between +2.7 and +3.2, whereas slightly but significantly higher values of +3.5 to +4.7 are recorded in the Ferani volcanics. The values are consistent with the juvenile character of the ANS magmas, and also further corroborate models of geographical isotope provinciality in this segment of the shield. Detailed comparison of geochemical patterns and timing of volcanic activity confirms that Rutig and Ferani, as well as several other volcano-sedimentary successions of Sinai correlate with the Dokhan Volcanics of the Eastern Desert of Egypt. The Dokhan Volcanics including those exposed in the Rutig and Ferani successions compare well in terms of age and geochemical patterns with the local 635–590 Ma calc-alkaline plutonic suite, but some volcanic rocks display geochemical similarities with a younger (608–580 Ma) alkaline plutonic suite. This variance may in part reflect a change in the tectonic regime at ca. 610–600 Ma with a change to more alkaline compositions, as reflected in the plutonic suites. However, the occurrence of early (ca. 620 Ma) volcanic rocks of highly fractionated A-type granite affinity in the Rutig succession has not been documented in the calc-alkaline plutonic suite and requires a different explanation. We interpret the Ediacaran magmatic activity in the northernmost ANS to have occurred in a post-collisional regime and note for the occurrence of ca. 635–630 Ma adakitic rocks, including the andesites of the lower Rutig. These may record early manifestations of lithospheric delamination involving parts of mafic lower crust that melted in the process to form adakitic magmas. However, major asthenospheric upwelling, which heated the lithospheric mantle and the crust and triggered vast plutonic and volcanic activity, peaked much later, between 610 and 590 Ma. This later event was also associated with the transition between high-K calc-alkaline and alkaline magmatic activity in this region.

In-situ chemical, U–Pb dating, and Hf isotope investigation of megacrystic zircons, Malaita (Solomon Islands): Evidence for multi-stage alkaline magmatic activity beneath the Ontong Java Plateau

Previous investigations of pipe-like intrusions of alnöite within northern Malaita (Solomon Islands) have detailed the chemical and isotopic nature of the alnöite and entrained megacrysts/xenoliths. Alnöite emplacement is poorly constrained since available ages include an Ar–Ar date of 34Ma (phlogopite) from a mantle xenolith, and a 206Pb/238U date of 33.9Ma for a zircon megacryst. Hence, we report chemical data, in-situ U–Pb age determinations and Hf isotope compositions for megacrystic zircons recovered from alnöite-derived, ilmenite-rich gravels in the Auluta, Kwainale, and Faufaumela rivers of Malaita.The Zr/Hf ratio (39 to 50) is variable for zircons from Auluta and Faufaumela, whereas it is relatively uniform (40 to 42) in most zircons from Kwainale. Chemical imaging reveals the homogeneous nature for all of the 16 grains analyzed. Trace element compositions obtained by LA-ICP-MS indicate parallel chondrite-normalized REE patterns at variable levels of enrichment; these patterns combined with their low abundances (<1 to 10ppm) of U, Th, and Pb confirm their mantle origin. In-situ U–Pb dating conducted by LA-ICP-MS (n=94 analyses) define a total range in weighted mean (WM) 206Pb/238U ages between ∼35 and ∼52Ma. The zircons from Auluta define a range of WM 206Pb/238U ages between 34.9±2.0Ma and 45.1±2.5Ma (2σ) that correlate negatively with Zr/Hf ratios and total REE contents. Conversely, the chemically homogeneous zircons from Kwainale define a uniform age spectrum yielding a WM 206Pb/238U age of 36.7±0.5Ma (2σ). In-situ Hf isotope analyses (n=30) are uniform and define a WM 176Hf/177Hf value of 0.282933±0.000013 (2σ), which is identical to the previously reported whole rock value for the Malaitan alnöite (0.282939±0.000007). Correlations between ages and chemical compositions (i.e., Auluta zircons), and the uniform Hf isotope compositions are consistent with zircon formation from a common Ontong Java Plateau (OJP)-like mantle undergoing progressive CO2-dominated metasomatism over a ∼17Ma interval. This unique example of prolonged highly alkaline magmatism within an intraplate oceanic setting mimics that defined by cratonic kimberlite provinces and suggests that the Malaitan upper mantle section of the OJP represents an analogy to continental lithosphere.

Multi-stage emplacement of alkaline and peralkaline syenite–granite suites in the Mongolian–Transbaikalian Belt, Russia: Evidence from U–Pb geochronology and whole rock geochemistry

The Mongolian–Transbaikalian Belt (MTB) is the largest and most representative component of the Central-Asian network of Late Paleozoic and Mesozoic granites and syenites and associated volcanic rocks. The MTB incorporates over 350 individual plutons and numerous volcanic fields, predominantly of alkaline and peralkaline composition. We present new U–Pb zircon and titanite geochronological data from five plutonic complexes located in the central part of the MTB. Four of them (Bryansky, Khorinsk, Kharitonovo and Malo-Kunalei) are peralkaline and alkali-feldspar granitoids, and one plutonic complex (Ust-Khilok) represents a shoshonitic series (K-rich gabbro, monzonite and syenite). The new data confirm the presence of three distinct stages of highly alkaline magmatic activity within the MTB: Early Permian (280–275 Ma), Late Triassic (230–219 Ma) and Late Jurassic (∼ 152 Ma). The dated shoshonitic plutonic complex formed between 280 and 278 Ma, overlapping with the formation of the Early Permian alkali-feldspar–peralkaline granitoid rocks. The ages of the alkali-feldspar and peralkaline magmatic suites are indistinguishable within a single magmatic complex. This suggests that several episodes of syenite and granite magma emplacement occurred within < 2 myr intervals. The close association in space and time of geochemically distinct igneous rocks points to co-existence of discrete magma chambers comprising magma from different sources. Early Permian magmatic activity in the MTB completed a continuous post-collisional cycle of magmatism that started in the Carboniferous (at ∼ 330 Ma) and lasted for about 50 myr. The Late Triassic stage marks the beginning of anorogenic tectonic setting in the MTB evolution. Late Jurassic peralkaline magmatism in the territory of the MTB is related to the formation of Mesozoic metamorphic core complexes, which are abundant in northern Mongolia and Transbaikalia and formed in an anorogenic within-plate environment.

The Cretaceous Peri-Atlantic Alkaline Pulse (PAAP): Deep mantle plume origin or shallow lithospheric break-up?

Although the Early Jurassic initial phases of Atlantic rifting were marked by the tholeiitic basalt flows and dikes of the Central Atlantic Magmatic Province (CAMP), the Cretaceous Era was characterised by widespread alkaline igneous activity on widely separated peri-Atlantic continental margins. Nearly half the peri-Atlantic alkaline rocks dated between 250 to 50 Ma fall within the 125–80 Ma range. This period of intense alkaline activity during the mid-Cretaceous formed the Peri-Atlantic Alkaline Pulse (PAAP). The mechanisms for the opening of the Atlantic Ocean, as well as for the CAMP and later PAAP magmatic events, have been attributed either to deep mantle plumes or to a combination of tensional forces, lithospheric rifting and structural controls. We describe the PAAP event for the first time and test the plume and structural models. The PAAP displays evidence of strong structural control and appears to be related to reactivation of pre-existing lithospheric structures throughout the peri-Atlantic zone. Early stages of continental separation and abrupt changes in plate motion were the most likely mechanisms for the reactivation of favourably oriented pre-existing structures. Mid-Cretaceous pulses in alkaline magmatic activity were closely associated with major Atlantic tectonic events. The opening of the South Atlantic, the change in intensity and direction of both seafloor spreading and plate motion, and the consequent fluctuation in intra-plate stresses were responsible for two spikes in peri-Atlantic alkaline magmatism at ~ 125 Ma and ~ 85–80 Ma. The general lack of evidence for a deep hotspot associated with the peri-Atlantic zone seems incompatible with a mantle plume origin. On the other hand, the spatial distribution of the PAAP along passive continental margins suggests an ideal setting for the development of edge-driven convection. We propose that the periodic reactivation of deep-seated pre-existing zones of weakness during the major stages of Atlantic tectonic evolution, combined with coeval asthenospheric upwelling due to edge-driven convection and continental insulation flow, enhanced the ascent of alkaline magmas. This mechanism of shallow, small-scale upwelling during periodic structural reactivation provides a more likely explanation for the PAAP than the commonly proposed deep mantle plume scenario.

High radiogenic heat production in the Kerala Khondalite Block, Southern Granulite Province, India

In situ radioelemental (K, U and Th) analysis and heat production estimates have been made at 59 sites in the Kerala Khondalite Block (KKB) of the Southern Granulite Province (SGP) of India. Together with the in situ analyses on granulites and gneisses previously reported from 28 sites, and heat production estimated from the published geochemical analyses on granites and syenites of the KKB, the new data set allows good characterization of heat production for the major granulite facies rocks and granitoids of the KKB. Garnet biotite gneisses are characterized by high levels of Th and U, with mean values of 60 and 3 ppm, respectively. Khondalites, leptynites and charnockites have slightly lower levels of Th (23, 20 and 22 ppm, respectively) and U (2.9, 2.4 and 0.9 ppm, respectively). The mean K, U, Th abundances for the granites, leucogranites and granitic gneisses ranges from 3.9 to 4.3%, 2.6 to 4.3 ppm, 22 to 50 ppm respectively, and for the syenites 4.8%, 2 ppm and 5.7 ppm. Mean radiogenic heat production values for garnet–biotite gneiss, khondalite, leptynite and charnockite are 5.5, 2.7, 2.4 and 2.2 μW m−3, respectively. For the granites, leucogranites, granitic gneisses and syenites it is 2.6, 3.4, 4.6 and 1.4 μW m−3, respectively. Heat production of granulite facies rocks, which are the most abundant rocks in KKB, correlate well with Th, but less with U, suggesting that variation is caused by Th and U bearing accessory minerals such as monazite and zircon. The high heat production of the KKB granulites are in contrast to the low heat production of the Late Archaean granulites of the Northern Block (NB) of the SGP which are highly depleted in radioelements and also the granulites of Madurai Block (MB) that have higher radioelemental abundances than in the granulites of the NB. The high heat production of the KKB granulites could be due to the nature of protoliths and/or metasomatism associated with Neoproteroic- to- Pan African alkaline magmatic activity represented by alkali granite and syenite–carbonatite emplacements and emplacement of pegmatites.

Cretaceous alkaline intra-plate magmatism in the Ecuadorian Oriente Basin: Geochemical, geochronological and tectonic evidence

Small volumes of Cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the Ecuadorian Oriente foreland basin. They are characterized by a restricted range of compositional variation, low LILE/HFSE ratios and Sr–Nd isotope values within the range of oceanic island basalts (OIB). Reflection seismic data show that a pre-existing NNE–SSW Triassic and Jurassic rift controls the location and occurrence of these alkaline eruptive sites. Radiometric ages ( 40Ar– 39Ar, incremental heating method) and the biostratigraphic record of their surrounding sediments indicate a NNE–SSW systematic age variation for the emplacement of this alkaline volcanism: from Albian (110 ± 5.2 Ma) in the northern part of the Oriente Basin, to Campanian (82.2 ± 2.0 Ma) in the west-central part. The geochemical, geochronological and tectonic evidences suggest that asthenospheric mantle has upwelled and migrated to the SSW, into the region underlying the pre-existing Triassic and Jurassic rift (thin-spot?). We propose that subduction was abandoned, subsequent to the accretion of allochthonous terranes onto the Ecuadorian and Colombian margin in the latest Jurassic–earliest Cretaceous, causing the relict slab material, corresponding to the eastwards-directed leading plate, to roll-back. Unmodified asthenospheric mantle migrated into the region previously occupied by the slab. This resulted in partial melting and the release of magmatic material to the surface in the northern part of the Oriente Basin since at least Aptian times. Then, magmatism migrated along the SSW-trending Central Wrench Corridor of the Oriente Basin during the Upper Cretaceous, probably as a consequence of the lateral propagation of the transpressive inversion of the Triassic–Jurassic rift. Eventually, the Late Cretaceous east-dipping Andean subduction system was renewed farther west, and the development of the compressional retro-foreland Oriente Basin system halted the Cretaceous alkaline magmatic activity.

New geochronological data on the Late Cretaceous alkaline magmatism of the northeast Iberian Peninsula

Alkaline magmatic activity took place in the Iberian Peninsula during the Late Cretaceous, between c. 100 and c. 65 Ma. This magmatism is well known only in southern Portugal and the Pyrenees. We present 40Ar/ 39Ar and K/Ar ages from three alkaline lamprophyres (camptonites) located in the Catalan Coastal Ranges. The ages obtained (80, 76 and 69 Ma) indicate that the intrusives concerned represent the youngest Mesozoic alkaline magmatism dated so far in the northeast Iberian Peninsula, establishing a link between the Pyrenean and Portuguese alkaline magmatism. Their position at the margin of the Iberian Plate is noteworthy and indicates the presence of zones of cortical relaxation (and perhaps thinning) after Iberian Plate rotation and during ocean-floor spreading in the North Atlantic.

New way to map old sutures using deformed alkaline rocks and carbonatites

By using a recent compilation of African alkaline igneous rocks and carbonatites, we show that nearly 90 % (28 of 32 occurrences) of nepheline syenite gneisses and deformed carbonatites are concentrated within known or inferred Proterozoic suture zones. Given the well-established intracontinental rift setting for these rocks and the likely continental collisional setting for their subsequent deformation, we suggest that deformed alkaline rocks and carbonatites (DARCs) represent the products of two well-defined parts of the Wilson cycle. DARCs mark the places where vanished oceans have opened and then closed. We further postulate that DARCs taken into the mantle lithosphere to ∼100 km depths at collision could provide source material for later alkaline magmatism. This possibility could account for the observation of recurrent alkaline magmatic activity over hundreds of million years in provinces such as that of southern Malawi.

SHRIMP U–Pb age for the emplacement of the Santana Granite and reactivation of the Porto Alegre Suture, southern Brazil

SHRIMP U–Pb zircon ages are reported from a sample of the Santana Granite that represents the beginning of alkaline magmatic activity in the Neoproterozoic of the Sul-Rio-Grandense Shield, Brazil. This granite outcrops in the city of Porto Alegre and has a NE–SW elongated shape (20km long and 5km wide). It is a medium- to coarse-grained syenogranitic rock with subvertical magmatic foliation and subhorizontal lineation with NE–SW trends, overprinted by a weak solid-state fabric. Zircons of this rock with 206Pb/238U ages between ca. 575 and ca. 617Ma are interpreted as having formed during magmatic crystallisation; the best fit for this age using five analyses provided a mean 206Pb/238U age of 600±10Ma. This age is also interpreted as the age of the reactivation of the Porto Alegre Suture, a crustal discontinuity along which the Santana Granite was emplaced. Younger isotopic data (<575Ma) for zircons from this sample are interpreted as due to Pb loss from metamictization. The 600-Ma age is consistent with previous results and marks a magmatic episode, probably related to an extensional tectonic episode that affected the NE of the Sul-Rio-Grandense Shield at the end of the Brasiliano Cycle.

Incompatible trace element and isotopic (D/H) characteristics of amphibole- and phlogopite-bearing ultramafic to mafic xenoliths from Kerguelen Islands (TAAF, South Indian Ocean)

Alkali basalts from the Kerguelen Islands have entrained numerous phlogopite- and amphibole-bearing ultramafic to mafic xenoliths. These are subdivided into mantle harzburgites, dunites and associated composite xenoliths that represent mantle wall-rock (Type-I) and high pressure (10–15 kbar) segregates (Type II). A lamprophyric dyke containing phlogopite megacrysts has been also studied. Chemical compositions of amphiboles and phlogopites from both xenolith types are similar to those recognized in many ultramafic and mafic volatile-bearing xenoliths from kimberlites and alkali basalts and in peridotites and pyroxenites from orogenic lherzolite massifs. Interstitial amphibole and phlogopite in harzburgites and dunites probably formed during diffuse percolation of highly alkaline basic silicate melt within the upper mantle (porous flow). Evidence from composite xenoliths suggest that similar mantle melts migrated through a network of dykes generated by hydraulic fracturing in the Kerguelen upper mantle. The lamprophyre is the surface expression of this highly alkaline magmatic activity. The δD values of −92 to −61‰ SMOW for mica and amphibole of Type I and Type II xenoliths and of the phlogopite megacrysts are within the accepted mantle range. Calculated δD-H 2 O values in equilibrium with amphiboles and micas have a bimodal distribution (- 65 ± 5‰ and −83 ± 5‰) indicating that the percolating fluids were isotopically heterogeneous. The ubiquity of the highly alkaline magmatic activity is probably related to the late intraplate activity of the Kerguelen mantle plume.