Alkaline Intrusions Research Articles

Petrology, geothermobarometry and geochemistry of granulite facies wall rocks and hosting gneiss of gemstone deposits from the Mogok area (Myanmar)

The Mogok Metamorphic Belt (MMB) of Myanmar formed during the Paleogene collision between the West Burma block and the Shan-Thai block. The MMB is mainly composed of medium to high-grade metamorphic marble, calc-silicate rocks, gneiss, quartzite, peridotite and igneous rocks such as granite, syenite and gabbro. The Mogok area in the central part of the MMB is well-known for magnificent quality ruby, spinel, sapphire, and peridot. To unravel the metamorphic PT-conditions prevailing during the formation of spinel and ruby from primary marble deposits in the Mogok area, three different types of high-grade quartz-garnet gneiss from the neighbourhood of gemstone mines were investigated by electron microprobe. Geothermobarometry reveals granulite facies PT-conditions of 756–792 °C at 7.4–7.6 kbar, which is reproduced by Theriak-Domino modelling within the error of both methods at water activities of 0.34–0.4. Shoshonitic and high-K calc-alkaline mafic dykes occur within marble forming conspicuous garnet-nepheline and clinopyroxene-clinoamphibole gneiss. Petrologic and geochemical investigations of these metadykes verify their granulite facies metamorphism and classify them as subduction-related magmatic rocks, which intruded the marble sequences. These investigations as well as previous studies show that spinel and ruby in marble of the Mogok area may have formed not only by metasomatism around alkaline intrusions, but also by granulite facies regional metamorphism.

REE and Fluorite Mineralization Characteristic in Ryongpho Alkaline Intrusion of the Sakju Complex, DPR Korea

REE and Fluorite Mineralization Characteristic in Ryongpho Alkaline Intrusion of the Sakju Complex, DPR Korea

Polybaric magmatic evolution of the eocene penticton group alkaline rocks, south-central British Columbia

Volcanic rocks of the Challis-Kamloops belt erupted during the Eocene (55–45 Ma) throughout south-central British Columbia, Idaho, and Wyoming of the North American Cordillera. The Eocene volcanic rocks of southern British Columbia can be subdivided into three distinct groups (Kamloops, Princeton, Penticton) based on geography and composition. The southernmost Penticton Group is unique as it is composed of volcanic and intrusive alkaline rocks (e.g., Marron Formation, Terrace Mountain outlier, Allendale Lake pluton) as opposed to calc-alkaline rocks that are typical of the Kamloops and Princeton groups to the north. The rocks of this study are from the Penticton Group, are compositionally variably (SiO 2 = 51.2–64.7 wt%), and have highly fractionated chondrite-normalized REE patterns (La N /Yb N = 71–104) without noticeable negative Eu-anomalies (Eu/Eu* = 0.81–1.00). The rocks have similar Sr-Nd isotopic ( 87 Sr/ 86 Sr i = 0.706422–0.706563; ε Nd ( t ) = −4.3–6.1) ratios and Neoproterozoic depleted mantle model ages (T DM = 794–873 Ma). Petrological modeling demonstrates that the primary process of chemical differentiation within the Penticton Group is fractional crystallization. However, it appears that the Marron, Allendale Lake, and Terrace rocks underwent fractional crystallization at different pressures (i.e., 4 GPa, 1.5 GPa, and 1 GPa). The polybaric differentiation of the Penticton Group indicates that the alkaline system evolved at different crustal depths in a similar manner as magmas related to the Yellowstone hotspot and markedly different from the Kamloops and Princeton groups. We suggest that the Penticton Group is underlain by ancient lithospheric mantle that was modified by Precambrian subduction processes and was isolated from the convecting mantle whereas the northern part of the Eocene volcanic belt is underlain by exotic accreted terranes (Intermontane Superterrane) and was derived from a different, younger lithospheric mantle. • The Penticton Group alkaline rocks were emplaced during the Eocene (∼50 Ma). • The primary process of chemical differentiation is fractional crystallization. • Differentiation occurred under variable pressure at a post-collisional setting. • The magmas were derived from the ancient lithospheric mantle of North America.

Mafic dikes of the Mariinsky Taiga Alkaline Province, Kuznetsk Alatau terrane, southwestern Siberia: Intraplate alkaline magmatism in the Central Asian Orogenic Belt

The Mariinsky Taiga Alkaline Province (MTAP) is located in the Kuznetsk Alatau terrane of southwestern Siberia, which is part of Altai-Sayan orogenic system, of the Central Asian Orogenic Belt. The MTAP consists of a wide distribution of Paleozoic alkaline-basite magmatism (volcanic, subvolcanic and intrusive), which has been recently characterized with the recognition of at least two main timings, ca. 500 and ca. 400 Ma (herein labeled MTAP-1 and MTAP-2, respectively). We newly characterize the petrology, geochemistry, and age constraints of NNW and NNE trending alkaline dikes which cross-cut the alkaline intrusions and host volcanogenic and carbonate units and which are inferred to belong to MTAP-2. The dikes consist of ultrabasic foidolites (urtites, urtite-porphyry, ijolite-porphyry, microijolites), basic foidolites (plagioclase ijolites), alkaline basites (gabbro-diabase, labradorite porphyrites, camptonites and tamaraites), and nepheline, analcime and alkaline syenites. Their chemical composition is characterized by a wide range in silica (SiO 2 = 38.7–60.7 wt%), and alkalinity (Na 2 O + K 2 O = 4–18.7 wt%; Na 2 O/K 2 O = 0.5–8.3 wt%), low titanium (TiO 2 = 0.07–2.15 wt%) and high alumina content (Al 2 O 3 = 11.1–28.5 wt%), which corresponds to the K-Na differentiates of the basic alkaline magmatic series. The sum of rare-earth elements (ΣREE) increases from alkaline basite (ΣREE = 103.58–218.94 ppm), to the main foidolites (ΣREE = 137.74–217.61 ppm), and then to the ultrabasic foidolites (ΣREE = 55.56–268.88 ppm). Maximum REE content occurs in nepheline syenites and microsyenites (ΣREE = 73.24–1566.96 ppm) and they are the final product of the differentiation of foidite and basite melts. All dike varieties are characterized by low concentrations of the most highly charged elements (Th, Nb, Ta, Hf, and Y), which, according to their concentrations, plot between oceanic island basalt (OIB) and island arc basalt (IAB). The presence of a negative Nb-Ta anomaly and the relative enrichment of Rb, Ba, Sr and U indicate probable interaction of a mantle plume melts with lithospheric mantle metasomatized in an earlier subduction event. The primary isotope ratio 87 Sr/ 86 Sr(t) = 0.705388–0.706025 and the range of ɛNd(t) from +3.5 to +6.53 in the rocks indicate interaction of mantle derived melts with crustal components. Sm-Nd and U-Pb dating of the alkaline-basite dikes matches the boundary of the Early-Middle Devonian (406–389 Ma) and indicates membership in MTAP-2, which can be linked to the regional ca. 400 Ma Altai-Sayan Large Igneous Province/Rift System. Both the NNW and NNE trending dikes are associated with, and probably fed from, nearby alkaline intrusions such as i.e. Kiya-Shaltyr pluton which are also ca. 400 Ma (part of MTAP-2), but are independent of older ca. 500 Ma intrusions (which belong to MTAP-1) such as the University pluton and which are suggested to belong to a older plume-generated regional event. • Characterization of NNW and NNE trending dike belts in the MTAP, western Siberia. • They formed in the Middle Paleozoic (406–389 Ma) in a plume-rifting event. • Isotopic parameters ɛSr(t) indicate a strong contamination by crustal components. • The dike represent intraplate magmas emplaced in a shear setting.

Light on the origin of the verdete siltstone, Bambuí group, central Minas Gerais state, Brazil

The Verdete is a variety of a greenish siltstone that occurs occasionally at different stratigraphic levels of the Serra da Saudade Formation, Bambuí Group (Neoproterozoic to Cambrian age). The main rock types related to the Verdete are potassium rich and show the follow mineralogy: quartz, illite/glauconite, orthoclase, albite, ilmenite, fluorapatite and dawsonite. Conventionally, the Verdete has been considered as formed by diagenetic processes in transgressive conditions at calm water environments. However, the present research based on different data considers the Verdete as result of potassic metasomatism in preexisting detritic sedimentary rock (siltstone, fine sandstone and greywacke). The main arguments that support the potassic metasomatic alteration hypothesis are: i) coincidence of the Verdete main occurrences with geophysical anomalies interpreted as alkaline intrusions in depth; ii) presence of dawsonite which is one hydrothermal sodium carbonate; iii) local presence of brecciated facies of phosphorite cross cutting the Verdete facies; iv) lateral discontinuity of the beds and v) microscopic evidence of substitution in coarser facies. The potassic fluids percolated in fracture zone in a northeast trend in the main area of occurrence in central Minas Gerais State (Cedro do Abaeté Town), and besides this region there are also incidence in other areas. The Verdete is considered as a possible source of potassium to be applied to agromineral or industrial supply for fertilization production. In this way, the understanding of its origin must be important to develop new processes or improve the methods to the potassium recover (chemical, biological and thermal routes).

Potential for critical mineral deposits in Maine, USA

An analysis of the potential for deposits of critical minerals and elements in Maine presented here includes data and discussions for antimony, beryllium, cesium, chromium, cobalt, graphite, lithium, manganese, niobium, platinum group elements, rhenium, rare earth elements, tin, tantalum, tellurium, titanium, uranium, vanadium, tungsten, and zirconium. Deposits are divided into two groups based on geological settings and common ore-deposit terminology. One group consists of known deposits (sediment-hosted manganese, volcanogenic massive sulphide, porphyry copper-molybdenum, mafic- and ultramafic-hosted nickel-copper [-cobalt-platinum group elements], pegmatitic lithium-cesium-tantalum) that are in most cases relatively large, well-documented, and have been explored extensively in the past. The second, and much larger group of different minerals and elements, comprises small deposits, prospects, and occurrences that are minimally explored or unexplored. The qualitative assessment used in this study relies on three key criteria: (1) the presence of known deposits, prospects, or mineral occurrences; (2) favourable geologic settings for having certain deposit types based on current ore deposit models; and (3) geochemical anomalies in rocks or stream sediments, including panned concentrates. Among 20 different deposit types considered herein, a high resource potential is assigned only to three: (1) sediment-hosted manganese, (2) mafic- and ultramafic-hosted nickel-copper(-cobalt-platinum group elements), and (3) pegmatitic lithium-cesium-tantalum. Moderate potential is assigned to 11 other deposit types, including: (1) porphyry copper-molybdenum (-rhenium, selenium, tellurium, bismuth, platinum group elements); (2) chromium in ophiolites; (3) platinum group elements in ophiolitic ultramafic rocks; (4) granite-hosted uranium-thorium; (5) tin in granitic plutons and veins; (6) niobium, tantalum, and rare earth elements in alkaline intrusions; (7) tungsten and bismuth in polymetallic veins; (8) vanadium in black shales; (9) antimony in orogenic veins and replacements; (10) tellurium in epithermal deposits; and (11) uranium in peat.

Evidence of gold related to Neoarchean alkaline magmatism in the Abitibi greenstone belt (Canada) from mineral parageneses and microscale trace element geochemistry on pyrite

Most of Archean gold deposits are classified as orogenic gold systems (OGS), precipitated by metamorphic fluids during compressional deformation at convergent plate margins. However, Neoarchean gold deposits are also found spatially related to post-collisional alkaline intrusions such as syenites, suggesting a genetic link between gold and magmas. To bring novel evidence on this relation, ore parageneses related to Bachelor/O’Brien, Beattie, Douay and Golden Arrow gold deposits were studied together with trace elements distribution at micron-scale in Au-bearing pyrites. Further, the δ34Spyrite signature in the Beattie deposit were compared to epithermal Phanerozoic analogues in post-collisional tectonic settings. Except for Golden Arrow, results show a paragenetic assemblage of K-feldspar-albite-anhydrite-barite-celestite-apatite-rutile-calcite-dolomite which is also common in Phanerozoic magmatic-hydrothermal systems. Gold-bearing pyrites are characterized by a higher abundance of most trace elements compared to OGS. Positive Au-Ag-Bi-Te-Tl-W-anomalies – which are not observed in OGS – suggest deposition from highly oxidized fluids, while Au precipitation in OGS is mainly controlled by reduced fluids. The presence of highly oxidized environments is also confirmed by very low δ34Spyrite values ranging from −17.3‰ to −13.9‰ VCDT found at Beattie, in the range of those found in Phanerozoic deposits of Tuvatu and Emperor in Fiji. It is proposed that Archean gold related with late alkaline continental magmatism is deposited both at the top of porphyritic system and in epithermal environment, before the acme of deformation and metamorphism.

A Study on Apatite from Mesozoic Alkaline Intrusive Complexes, Central High Atlas, Morocco

There are abundant phosphate mines in the High Atlas Mountains of Morocco. Gem-quality apatite is produced at the Anemzi deposit, but its associated gem mineralogy is relatively poorly studied. In this study, apatite from the Anemzi mine in Morocco was analysed using standard gemmological characterisation methods, including basic tests (hardness, relative density, microscopy, etc.), spectroscopic tests (infrared, Raman, and ultraviolet–visible spectroscopy), and chemical analyses (electron probe and laser ablation inductively coupled plasma mass spectrometry). This paper explores the gemmological characteristics of Moroccan apatites and the information on diagenesis recorded for apatites by comparing them with apatites from other sources. Apatite from the Anemzi deposit is an igneous fluorapatite. The relatively high Cl content of the apatite suggests that the magma in the area has a high Cl content, whereas the high Cl/F ratio may indicate that the deposit formed in a slab subduction environment. The characteristics of the major and trace elements indicate high oxygen and sulphur escapes in the apatite-forming magma.

The Influence of the Ailaoshan‐Red River Shear Zone on the Mineralization of the Beiya Deposit on the Southeastern Margin of the Tibetan Plateau Revealed by a 3‐D Magnetotelluric Survey

AbstractThe Beiya gold polymetallic ore deposit is one of the largest gold deposit in the world. Previous studies have suggested the formation of the Beiya deposit is closely related to the porphyritic alkaline intrusions. Different tectonic models have been proposed by geologists to explain the alkaline intrusions. However, no consensus has been reached and data for imaging of the deep crustal structure is lacking. In this study, we use magnetotelluric data to image the 3‐D crustal structure beneath the Beiya deposit and adjacent areas. The 3‐D electrical resistivity model is characterized by two conductors and three resistors, which depicts the magmatic system and fluids migration path related to the Beiya deposit. One shallow conductor beneath the Beiya deposit is suggested to represent a cooled magma chamber, which provided essential heat and metal‐rich fluids to the Beiya deposit. The shallow conductor is connected to a deep conductor located beneath the Ailaoshan‐Red River shear zone. Thus, magmas originated from mantle upwelling in an extensional tectonic background is believed to provide the heat that melted the crust and generated intermediate melts, the Ailaoshan‐Red River shear zone is proposed to facilitate ascending of partial melts from the base of the crust. Moreover, two resistors encircles the shallow conductor at upper crustal depths could have acted as an impermeable barrier that helped further subtraction of metal‐rich fluids. The Beiya gold polymetallic deposit is believed to be a typical porphyry‐skarn deposit formed closely related to the Ailaoshan‐Red River shear zone in an extensional tectonic setting.

Mesozoic to Cenozoic alkaline and tholeiitic magmatism related to West Gondwana break-up and dispersal

For over 50 years, Mesozoic tholeiites, kimberlites and carbonatites from the South American platform have been enabled the understanding of melting processes in the Earth’s upper mantle. However, the genetic relationship between alkaline and tholeiitic magmatism remains unknown. In this context, an extensive review, based on a compilation of published geochemical and isotopic data, shows an integrated evolution for mantle-derived magmatism in South America. The K-rich alkaline-carbonatite intrusions occur widespread through time at 255–209 Ma, 146–106 Ma and 91–71 Ma. Moreover, the Na-rich magmatic episodes are also documented at 130–120 Ma and 66–32 Ma. Tholeiitic basaltic lavas and dikes are recorded at ∼200 Ma in Northern Brazil and mainly between 134 and 131 Ma in the Paraná Magmatic Province. Simultaneous tholeiitic lavas and carbonatitic complexes are related to near isothermal decompression of enriched asthenospheric and lithospheric mantle sources at different depths (80–200 km). Likewise, the 267–226 Ma kimberlites in the Amazonian Craton, ∼128 Ma Rosário kimberlite in the Rio de la Plata Craton and 88–80 Ma Alto Paranaíba kimberlites in the western edge of the São Francisco Craton provide evidence for deeper (>200 km) metasomatized mantle sources. Compiled numerical, geophysical and geological data support the proposal that the thickness of the lithosphere, extension rates and the presence of previous weak crustal structures contributed to the generation and emplacement of multiple tholeiitic and alkaline intrusions from 250 to 30 Ma. We propose that several crustal extension events induced repeated thermal convection cells in the metasomatized asthenosphere, which triggered partial melting in the previously enriched and heterogeneous lithospheric mantle. The local mantle composition, depth and crustal extension ratios controlled the magma composition. Thus, West Gondwana break-up and dispersal played a crucial role in the Mesozoic to Paleogene melting processes of the metasomatized mantle in South America.

The geology of the late-Cretaceous Salpeterkop volcano near Sutherland: A geomorphologic benchmark

The Salpeterkop volcano is spatially part of the Sutherland Suite of alkaline rocks in the Northern Cape. It is one of the best preserved volcanoes in South Africa with part of the tuff ring still intact, remnants of the ejecta mantle outside the crater still preserved, and is host to epiclastic rocks, including ash and lapillistone and water lain sediments, inside the crater. New dates from apatites and phlogopites from Salpeterkop suggest an age close to 70 Ma. This Upper Cretaceous age is supported by the silicified wood found within the epiclastic sediments. Its relationship to the alkaline rocks in this cluster is not entirely clear but field evidence suggests that initial olivine melilitites and ultramafic bodies were followed by the main eruption that produced Salpeterkop. Clear igneous components in the associated breccias and pyroclastics are rare but indicate that this volcano was linked to an alkaline (trachytic) intrusion driven by phreatic magmatism. This is further highlighted by the presence of (nepheline?) syenite xenoliths in some carbonatite breccias and dykes that are part of the later carbonatites with its associated hydrothermal alteration products. Although the carbonatites are largely late-stage, there is evidence of earlier carbonatite activity from a precursor carbonatite dyke that has off-set an olivine melilitite dyke. Relatively unaltered pyroclastics, associated with two vents, northeast and northwest of the crater respectively, represent the final phase of this volcanic centre. Finally, the preservation of the Salpeterkop crater and its associated volcaniclastics highlights the change of intense landscape denudation, that ensued from Gondwana break-up to the end of the Cretaceous, to a period of drastically reduced erosion rates during the Cenozoic Era.

Tourism, Scientific, and Didactic Potential of the Ultrabasic-Alkaline Intrusion in Afrikanda with Perovskite Mineral (Kola Peninsula, N Russia) and of the Related Built Heritage

The present article is an example of research on the tourism, scientific, and teaching potential of the ultrabasic-alkaline intrusion with perovskite minerals in Afrikanda (Kola Peninsula, North Russia) and the possibility of its development as part of a local geopark site. A proposal to create a protected zone in this site, to expose sites of geological, natural, historical, and tourist interest is presented. Didactic routes and locations of geo-point visitors are proposed. Currently, there are numerous quarries and other mining objects of high historical, technical, and geological value. In the vicinity, there is a trough lake and post-glacial formations that can supplement knowledge about this region and add interest to would-be visitors. The remains of a historical camp for workers and miners are also visible in the intrusion area. In the vicinity of Afrikanda village, there is an international railroad line to Murmansk and St. Petersburg with a working railroad station. The place has high historical and geological potential for tourists and very rare and interesting mineralization. The last is a key value. The location is associated with 30 alkaline intrusions in NE Fennoskandia. There are opportunities to create a museum with a park where, with relatively little funding (most of the existing infrastructure buildings are in good condition), the intrusions can be shown to tourists. We proposed a conservation area with exposures, quarries, mine infrastructure, and historical sites, along with the location of geosites, hiking trails, and a geo-point for visitors. These sites are prospective for the economic revival of Afrikanda village and can protect the unique exposures.

Source diversity of Late Neoproterozoic granitoid magmatism across an orogen-scale lineament in southern Brazil and Uruguay: Whole-rock geochemistry, zircon geochronology and Sr-Nd-Hf isotope evidence

Granitoids of similar Brasiliano age (638–540 Ma) are widespread in southern Brazil and Uruguay on both sides of an orogen-parallel lineament comprising from south to north, the Sierra Ballena, Dorsal de Canguçu and Major Gercino shear zones. We reviewed mineralogy, geochemistry and geochronology of granitoids that occur in the two domains located east and west of the mentioned lineament. Most of the granitoid magmatism is calc-alkalic to alkali-calcic and magnesian to ferroan, but variations are observed (aluminosity increases in eastern relative to western domain granitoids, and alkaline intrusions are more frequent in the western domain). Considering mineralogy and geochemistry the K-rich calc-alkaline granitoids (KCG) type of Barbarin's predominate, but amphibole-rich calc-alkaline granitoids (ACG), muscovite-bearing peraluminous granitoids (MPG) and peralkaline and alkaline granitoids (PAG) type affinities (although subordinate) do also occur. High-barium-strontium (HiBaSr) granitoids (with and without shoshonitic affinity) are frequent in the western but sporadic in the eastern domain. Granitoids intruding in the western domains of Santa Catarina, Rio Grande do Sul and Uruguay generally display crystallization ages between 638 and 540 Ma, Archaean to Neoproterozoic inherited zircon ages (3365–640 Ma) and strongly evolved isotopic signatures and model ages (ϵNd600 of −8.5 to −32; Nd TDM2 ages of 1.9–3.7 Ga; ϵHf (t) of −0.46 to −35.7 and Hf TDMCrustal ages of 1.5–3.6 Ga). Granitoids from the eastern domain, while showing a similar range of crystallization ages (630–543 Ma), generally reveal contrastingly younger inherited zircon ages (774–642 Ma) and less evolved isotope characteristics and model ages (ϵNd600 of −2.3 to – 11.8, Nd TDM2 of 1.4–2.1 Ga; ϵHf(t) of −0.21 to −12.5 and Hf TDMCrustal ages of 1.5–2.4 Ga).The assessment of the data compiled for this contribution indicate that: a) in southern Brazil and Uruguay, the Sierra Ballena-Dorsal de Canguçu-Major Gercino Lineament, act as a remarkable tectonic regional boundary between domains whose granitoids generally (but not always) show a pattern of contrasting whole-rock key chemical indicators, zircon inheritance, isotope signatures and model ages reflecting different granitoid sources; b) granitic melt production took place from the interaction of sources of different age (e.g. old Archaean to Paleoproterozoic crust and/or of Tonian age arc-materials) and composition (e.g. crustal sources: metaigneous rocks, metasediments vs. mantle-derived sources: juvenile basaltic melts and/or metasomatized mantle); c) crustal components strongly outweigh mantle ones in the source of the granitoids, suggesting that recycling was the chief mechanism; d) however, the spatial association of plutonic intermediate to basic rocks and felsic granitoids is sometimes significant in some sectors of southern Brazil and may be indicative of the adding up of new material from the mantle (i.e “crustal growth” processes) to the crust within a post-collisional environment.

TIME CONSTRAINTS ON THE FORMATION OF THE KANDALAKSHA AND KERETSK GRABENS OF THE WHITE SEA PALEO-RIFT SYSTEM FROM NEW ISOTOPIC GEOCHRONOLOGICAL DATA

Initially, the age and stratigraphic position of the Tersk formation were determined with respect to the fact that this formation overlaps the Early Proterozoic granitoids. Its top was marked by the rocks penetrated by the Late Devonian alkaline intrusions, including explosion pipes.This article presents the U-Pb isotopic dating of detrital zircon grains (dZr) from sandstones of the Tersk formation. It describes the geochemical compositions of the rocks and the Sm-Nd study results. In our study, the weighted average age of four youngest dZr grains from the sandstones of the Tersk formation is 1145±20 Ma, which suggests that the rocks above the studied rock layer (see the Tersk formation cross-section) are is not older than the end of the Middle Riphean. The U-Pb isotopic ages of dZr grains (provenance signals) from the sandstones of the Tersk formation were compared to the ages of other Upper Precambrian clastic strata in the northeastern East European platform (EEP) and adjacent areas. Our comparative analysis shows that these rocks significantly differ in age. This conclusion is in good agreement with the idea that at the end of the Middle and during the Late Riphean, several small (mainly closed) basins separated by uplifts dominated in the paleogeographic setting of the area wherein the White Sea rift system (WSRS) formed and developed. Temporal connections of these basins with the ocean were possible. Such paleogeographic setting does not favour the development of large rivers; this is why the grabens are mainly filled with local rock materials. The Keretsk and Kandalaksha grabens (WSRS) are filled with marine sediments eroded from the grabens walls. The local sediment sources include eclogite complexes (~1.9 Ga), which basic magmatism is dated at ~2.4–2.5 and ~2.7–2.9 Ga. Any potential primary sources for dZr grains are lacking in the area near the Keretsk graben. We suggest that such grains occurred due to recycling of the secondary sources of zircon, i.e. originated from ancient local sedimentary formations.

Petrogenesis and geodynamics of Eocene alkaline intrusions in the pre-salt sedimentary sequence of Santos Basin, Brazil

Lamprophyres and phonolite intrusions were sampled in the so-called Pre-Salt sequence of well 1-BRSA-905-RJS in northern Santos Basin nearby the Cabo Frio Structural High in SE Brazil. These alkaline rocks indicate the existence of a strongly silica-undersaturated trend. However, fractional crystallization modeling as well as Sr-Nd-Pb-Hf isotope data shows that the alkaline lamprophyres are not suitable parental compositions to the evolved felsic phonolitic magmas. The Sr-Nd-Pb-Hf isotope compositions of the lamprophyric magmas represent mixing of EM1 and Trindade plume mantle components. The EM1 is possibly related with the melt- and/or fluid-induced metasomatic processes on the subcontinental lithospheric mantle (SCLM) that occurred beneath the South American Platform during the Neo-Proterozoic-Cambrian subduction processes associated with the final stages of the Gondwana amalgamation, as recorded by the TDM ages (601-499 Ma) of the studied intrusions in Santos. 40Ar/39Ar analysis presented integrated age of 41.06±0.02Ma and 38.62±0.02Ma for phonolite and lamprophyre samples, respectively. A “plateau” age assignment for the true crystallization age of the sample could not be obtained due to the discordance in the ages of individual steps. This discordance was attributed to loss of radiogenic 40Ar that can happen in nature due to reheating events or metasomatism. An interaction between the Trindade plume head and SCLM is proposed for the geodynamics of Santos Basin at 40 Ma. The presence of an anomalously hot mantle represented by the deflected Trindade plume head beneath Santos at the Eocene is supported by the petrological and geochronological data. Therefore, it may be relevant to consider high geothemal flow regimes in basin evolution models proposed for Santos during the Eocene.

Temporal and Petrogenetic Links Between Mesoproterozoic Alkaline and Carbonatite Magmas at Mountain Pass, California

Abstract Mountain Pass is the site of the most economically important rare earth element (REE) deposit in the United States. Mesoproterozoic alkaline intrusions are spatiotemporally associated with a composite carbonatite stock that hosts REE ore. Understanding the genesis of the alkaline and carbonatite magmas is an essential scientific goal for a society in which critical minerals are in high demand and will continue to be so for the foreseeable future. We present an ion microprobe study of zircon crystals in shonkinite and syenite intrusions to establish geochronological and geochemical constraints on the igneous underpinnings of the Mountain Pass REE deposit. Silicate whole-rock compositions occupy a broad spectrum (50–72 wt % SiO2), are ultrapotassic (6–9 wt % K2O; K2O/Na2O = 2–9), and have highly elevated concentrations of REEs (La 500–1,100× chondritic). Zircon concordia 206Pb/238U-207Pb/235U ages determined for shonkinite and syenite units are 1409 ± 8, 1409 ± 12, 1410 ± 8, and 1415 ± 6 Ma (2σ). Most shonkinite dikes are dominated by inherited Paleoproterozoic xenocrysts, but there are sparse primary zircons with 207Pb/206Pb ages of 1390–1380 ± 15 Ma for the youngest grains. Our new zircon U-Pb ages for shonkinite and syenite units overlap published monazite Th-Pb ages for the carbonatite orebody and a smaller carbonatite dike. Inherited zircons in shonkinite and syenite units are ubiquitous and have a multimodal distribution of 207Pb/206Pb ages that cluster in the range of 1785–1600 ± 10–30 Ma. Primary zircons have generally lower Hf (<11,000 ppm) and higher Eu/Eu* (>0.6), Th (>300 ppm), Th/U (>1), and Ti-in-zircon temperatures (>800°C) than inherited zircons. Oxygen isotope data reveals a large range in δ18O values for primary zircons, from mantle (5–5.5‰) to crustal and supracrustal (7–9‰). A couple of low-δ18O outliers (2‰) point to a component of shallow crust altered by meteoric water. The δ18O range of inherited zircons (5–10‰) overlaps that of the primary zircons. Our study supports a model in which alkaline and carbonatite magmatism occurred over tens of millions of years, repeatedly tapping a metasomatized mantle source, which endowed magmas with elevated REEs and other diagnostic components (e.g., F, Ba). Though this metasomatized mantle region existed for the duration of Mountain Pass magmatism, it probably did not predate magmatism by substantial geologic time (>100 m.y.), based on the similarity of 1500 Ma zircons with the dominantly 1800–1600 Ma inherited zircons, as opposed to the 1450–1350 Ma primary zircons. Mountain Pass magmas had diverse crustal inputs from assimilation of Paleoproterozoic and Mesoproterozoic igneous, metaigneous, and metasedimentary rocks. Crustal assimilation is only apparent from high spatial resolution zircon analyses and underscores the need for mineral-scale approaches in understanding the genesis of the Mountain Pass system.

Influence of Host Marble Rocks on the Formation of Intrusive Alkaline Rocks and Carbonatites of Sangilen (E. Siberia, Russia)

The study of the O and C isotope composition of calcite from nepheline syenites, ijolites and carbonatites of the Chik intrusion and the intrusions of the Erzin–Tarbagatay group of Sangilen (Eastern Siberia, Russia) showed derivation from alkaline melts enriched with a carbonate component from the host marbleized sedimentary rocks. The calculations showed that about 40% of the initial mass of carbonates involved in the interaction with silicate melts have remained after decarbonation. During the assimilation of the carbonate, an oxygen isotope exchange took place between the residual carbonate material and the silicate phase. Crystallization products of such hybrid magmas are carbonatite veins, calcite-rich nepheline rocks and their pegmatites with a calcite core.

Petrological and geochemical constraints on the origin of apatite ores from Mesozoic alkaline intrusive complexes, Central High-Atlas, Morocco

Five main apatite deposits from the Moroccan Central High-Atlas —including Anemzi, Tirrhist-Inouzane, Tassent, Ait-Daoud-Toumliline and Tasraft— have been investigated through integrated field, petrographic, mineralogical, and geochemical studies. The apatite ore occurs as veins, few mm to several cm thick, spatially associated with syenite-quartz monzonite (dominant) to monzodiorite-gabbrodiorite bodies emplaced during the Atlas Jurassic-Cretaceous alkaline magmatic event. Gem-quality apatite crystals, up to 15 cm in size, are associated with albite, clinopyroxene (hedenbergite–augite–diopside), amphibole (hornblende–edenite–actinolite), K-feldspar, quartz, magnetite, titanite, epidote, prehnite, and calcite. Two types of apatite have been distinguished according to their halogen contents: F-rich and Cl-rich apatites. Both types are enriched in REE (up to 2 wt% ΣREE), and have major and trace element contents consistent with a magmatic origin, corroborated by the chemical similarity with accessory apatite in the host alkaline intrusions, as well as with other reference igneous apatites. However, hydrothermal fluids percolating through country sedimentary rocks were also involved, as indicated by the striking Na enrichment observed in the Cl-rich apatites, likely inherited from Triassic evaporites, showing close spatial relationships with the alkaline intrusions. These fluids would also account for pervasive Na-metasomatism of alkaline magmatic rocks, as observed in the wall rocks of the apatite veins.

Structural, petrological, and tectonic constraints on the Loch Borralan and Loch Ailsh alkaline intrusions, Moine thrust zone, northwestern Scotland

During the Caledonian orogeny, the Moine thrust zone in northwestern Scotland (UK) emplaced Neoproterozoic Moine Supergroup rocks, metamorphosed during the Ordovician (Grampian) and Silurian (Scandian) orogenic periods, westward over the Laurentian passive margin in the northern highlands of Scotland. The Laurentian margin comprises Archean–Paleoproterozoic granulite and amphibolite facies basement (Scourian and Laxfordian complexes, Lewisian gneiss), Proterozoic sedimentary rocks (Stoer and Torridon Groups), and Cambrian–Ordovician passive-margin sediments. Four major thrusts, the Moine, Ben More, Glencoul, and Sole thrusts, are well exposed in the Assynt window. Two highly alkaline syenite intrusions crop out within the Moine thrust zone in the southern Assynt window. The Loch Ailsh and Loch Borralan intrusions range from ultramafic melanite-biotite pyroxenite and pseudoleucite-bearing biotite nepheline syenite (borolanite) to alkali-feldspar–bearing and quartz-bearing syenites. Within the thrust zone, syenites intrude up to the Ordovician Durness Group limestones and dolomites, forming a high-temperature contact metamorphic aureole with diopside-forsterite-phlogopite-brucite marbles exposed at Ledbeg quarry. Controversy remains as to whether the Loch Ailsh and Loch Borralan syenites were intruded prior to thrusting or intruded syn- or post-thrusting. Borolanites contain large white leucite crystals pseudomorphed by alkali feldspar, muscovite, and nepheline (pseudoleucite) that have been flattened and elongated during ductile shearing. The minerals pseudomorphing leucites show signs of ductile deformation indicating that high-temperature (~500 °C) deformation acted upon pseudomorphed leucite crystals that had previously undergone subsolidus breakdown. New detailed field mapping and structural and petrological observations are used to constrain the geological evolution of both the Loch Ailsh and the Loch Borralan intrusions and the chronology of the Moine thrust zone. The data supports the interpretation that both syenite bodies were intruded immediately prior to thrusting along the Moine, Ben More, and Borralan thrusts.

Evolution of Syenite Magmas: Insights from the Geology, Geochemistry and O-Nd Isotopic Characteristics of the Ordovician Saibar Intrusion, Altai-Sayan Area, Russia

In this paper, we provide insight into the evolution of syenite magmas based on geological data and petrographic, geochemical, and O-Nd isotope parameters of rocks of the Saibar intrusion located within the Minusinsk Trough, Altay-Sayan area. The intrusive suite includes predominant syenites, few bodies of melanocratic and leucocratic nepheline syenites (foyaites), and granites. In addition, dykes of granites and mafic rocks are present. The U-Pb zircon age from the melanocratic foyaites was determined to be 457 ± 10 Ma? Examined rocks show fractionated light rare earth element patterns, normalized to chondrite, with (La/Sm)n varying from 4 to 9, and a weakly fractionated distribution of medium and heavy rare elements, with (Dy/Yb)n from 0.35 to 1.23 and (Sm/Yb)n from 0.63 to 2.62. The spidergram normalized to the primitive mantle shows negative Ba, Sr, Nb, Ta, Ti, and Eu anomalies (Eu* = 0.48–0.60) and positive Rb, Th, and U anomalies. The δ18O values vary within 6.3 to 10.2‰, and εNd(t) from +4.1 to +5.0. We observe gradual transitions from syenites to foyaites. Assimilation by syenite magma of the host carbonate rocks was followed to transition from silica-saturated to silica-undersaturated conditions and removal of anorthite from the melt, which then led to nepheline. Granites of the main phase show depleted lithophile incompatible elements in comparison with syenites and foyaites. They originate via interaction of magmas at the marginal part (endocontact zone) of the intrusion, corresponding to north contact of the granites with the host felsic rocks. In comparison, the rock composition of granite dykes is enriched in lithophile incompatible elements, except for Zr, Hf, and Ti. These rocks are formed due to the differentiation of syenite magma without a significant effect of host rock assimilation. Mantle magmas must be used as parent magmas for syenites based on analysis of the formation model of other alkaline intrusions, which are similar in age to the Saibar intrusion. In the line of syenite intrusions of the Altai-Sayan province, the Saibar intrusion is no exception, and its origin is related to the evolution of mafic magmas that arose during the melting of the mantle under the influence of a mantle plume.