Rapakivi Research Articles

Granitoid magmatism in the Qinling orogen, central China and its bearing on orogenic evolution

The Qinling orogen is a typical composite orogen for understanding multi-stages of magmatism and orogenic processes. Many studies have been carried out on the magmatic rocks in the Qinling orogen but their petrogenesis is still controversial. This paper presents a review of all granitoid rocks based on previous and new studies of geochronology and geochemistry. Four distinct periods of granitoid magmatism, Neoproterozoic (979–711 Ma), Paleozoic (507–400 Ma), Early Mesozoic (250–185 Ma) and Late Mesozoic (160–100 Ma), have been recognized from the Qinling orogen according to zircon U-Pb ages, intrusion associations and deformation, as well as regional geology. The Neoproterozoic granitic rocks consist of three stages at 979–911, 894–815 and 759–711 Ma, respectively, corresponding to strongly deformed S-type, weakly deformed I-type and A-type granitoids. They can be interpreted as magmatic occurrences in syn-collisional, post-collisional and extensional settings, respectively, in response to old continental terranes of the Neoproterozoic tectonomagmatic events in the old continents of China, such as South China and Tarim cratons. Although this continental terrane would be involved in the Phanerozoic Qinling orogeny, the Neoproterozoic magmatic rocks are not the products of the Qinling orogenic processes. The Paleozoic magmatic rocks can be classified into three stages at 507–470, 460–422 and 415–400 Ma, respectively. The first-stage magmatism is temporally associated with ultra-high pressure metamorphism in the North Qinling terrane. These magmatic rocks are interpreted as magmatic occurrences in subductional, syn-collisional and post-collisional settings, respectively. The Early Mesozoic magmatic rocks occur in two stages at 252–185 and 225–200 Ma, respectively. The first-stage granitoids are mainly represented by I-type quartz diorites and granodiorites, and the second stage by granodiorites and monzogranites with the I-to A-type characteristics and some with rapakivi textures. Their emplacement ages and geochemical parameters such as A/CNK, K2O/Na2O ratios and eNd(t) values do not show any polarity change in perpendicular to subduction/collision zone. Therefore, all these Early Mesozoic granitoids are unlikely the product of continental subduction as some researchers suggested. Instead, they are plausibly related to the subduction of the Mianlue Ocean and the subsequent collision between the South China Craton and the South Qinling terrane. The Late Mesozoic granitoids were emplaced mainly at two stages of 160–130 and 120–100 Ma, and characterized by the evolution from I- to I-A- and A-type granitoids. These characteristics are consistent with the granitoid magmatic evolution from contractional to extensional settings during the Jurassic/Cretaceous in eastern China. Accordingly, the Late Mesozoic granitoid rocks in the Qinling orogen probably have a similar petrogenetic mechanism to those of the huge magmatic belt along the western Pacific margin, i.e., intra-continent magmatism related to a far-field effect of the subduction of Paleo-Pacific plate.

REE and Lu-Hf systematics of zircons from rapakivi granites and associated rocks of supercontinent Nuna (Columbia)

The last of the rifting events at 1.37–1.45 Ga that is considered to have led to the final breakup of Nuna was recorded in most of its building blocks (Laurentia, Baltica, and Siberian craton). At the type locality for the Riphean on the western slope of the South Urals, this event was recognized as the Mashak igneous event (∼1380 Ma) and can be classified as a LIP based on the volumes of magma generated (total igneous volumes >0.05 × 106 km3). The emplacement of large volumes of A-type granitic plutons in Laurentia and northwestern Baltica (in the present coordinates) was almost synchronous with this event. In the South Urals, within the Bashkirian meganticlinorium, the processes of granite generation and emplacement during this time took place on a much smaller scale. Anorogenic granites of this region are represented by rapakivi granites of the Berdyaush massif. The first REE and Lu-Hf isotope data for zircons from rapakivi granites and associated rocks of the Berdyaush massif reveal almost identical Lu-Hf isotope ages (∼1383 Ma) of rapakivi granites, quartz syenitic diorites, and nepheline syenites. It was shown that these rocks, which were emplaced into the heterogeneous Proterozoic crust between the Early and Middle Riphean, have the Hf isotopic compositions and Lu-Hf model ages reflecting the contribution from crustal sources with ages of 2013–2219 Ma, whereas the associated gabbros indicate derivation from a source with the Lu-Hf ages of 1658 ± 1685 Ma and moderately depleted ɛHf(1383) of (4.1 ± 0.7–4.9 ± 0.6). The REE characteristics (ΣREE, Eu/Eu*, Ce/Ce*, Pr/Pr*, La/Lu) and Lu-Hf isotope systematics of zircons can be used to constrain the history and genetic evolution of supercontinents, e.g., supercontinent Nuna (Columbia) in particular.

The Kivalliq Igneous Suite: Anorogenic bimodal magmatism at 1.75 Ga in the western Churchill Province, Canada

The Kivalliq Igneous Suite (1.77–1.73Ga) includes the Nueltin Granite intrusions; gabbro and anorthosite intrusions; three swarms of mafic dykes; and basalt, rhyolite and pyroclastic rocks of the Pitz Formation (Wharton Group, middle Dubawnt Supergroup). It was emplaced in the reworked Archaean hinterland of the trans-Hudson orogen, in the central western Churchill Province (WCP) of the Canadian Shield. The Nueltin granites have field relations, mineral assemblages, and compositions typical of metaluminous to peraluminous, Proterozoic to Recent rapakivi granites, and the suite comprises a classic bimodal anorogenic province. Kivalliq rhyolites/granites were erupted/emplaced in a N–S band (the “Nueltin corridor”) about 700km long and 300km wide, with volcanic rocks and dykes concentrated at the north end and coarse plutonic rocks to the south. The McRae Lake dyke swarm originates among three prominent gabbro/anorthosite intrusions inside the corridor, and forms a narrow array extending a minimum of 600km to the NE along the central axis of the WCP. Early mafic rocks (intrusions and some lavas, 1.77Ga) had an alkali basalt parent, with subalkaline basalts (sills, dykes, and mafic enclaves) present at the peak time of granite emplacement (1.75Ga). Average initial ɛNd of basaltic rocks at 1.75Ga is −4.0, indicating a significant crustal component. Sm–Nd depleted mantle model ages of granites and rhyolites (average initial ɛNd=−8.7) range from 2.2 to 3Ga, consistent with melted Archaean crust variably mixed with juvenile basalt. Metallogenic prospects of the Kivalliq Suite include U–Th–Zr–Y–REE enriched granite and syenite, epithermal Ag/Au at silicic centres (including mafic/felsic ring complexes), and Pb–Cu sulphides associated with mafic intrusions. Topaz rhyolites are present, but Sn-enriched rocks have not been observed. The Kivalliq Suite is one of about 15 prominent anorogenic provinces and dyke swarms which erupted within the Nuna supercontinent at ca. 1.75Ga, and the thermal effects of the Kivalliq event are recorded in U–Pb and K–Ar data throughout most of the western Churchill Province.

REE-bearing minerals in the Rapakavi Grante, Southern Finland

Abstract The studied rock samples belong to the classic1615–1645 Ma Wiborg Rapakivi granite terrane of southeasternFinland. Geochemical studies show that the rapakivigranites and associated rocks form metaluminousto peraluminous A-type granites and plot in the "withinplate granites (WPG)" field on the tectonic discriminate diagramsfrom the Onkamaa, Suomenniemi and Luumäki.The rapakivi granite displays enrichment of light overheavy REE (LREE/HREE = 2-34) and usually negative Euanomalies (Eu/Eu* = 0.01 - 1.4). Enrichment in REE in somestudied samples is confined to highly fractionated portionsof the Rapakivi granite. Fractional crystallization ofthe evolving fluorite-rich peraluminous granitic magmawas accompanied, particularly at later stages by fluid fractionation,which plays an important role in the genesis ofthe REE-mineralization.The studied rapakivi granites host REE-minerals includingmonazite-(Ce), allanite (Ce), bastnäsite (Ce), xenotime,thorite and REE-bearing mineral apatite. Monazite and allaniteare the most important REE carriers in the studiedgranites and these minerals are strongly enriched in theLREE.Monazites are hosted in apatite, quartz, plagioclase,K-feldspar, and biotite. Grain size of monazite is variableranging from 50 to >100 μm.Monazite contains 48-68 wt%REE2O3, 24.3-29.3 wt% P2O5 and low Th<1.5 wt%ThO2. TheY, REE, U, Th-bearing minerals are not commonly associatedwith the primary minerals except for Th-bearingminerals, which occur as silicates (e.g. thorite, ThSiO4);and/or replace other elements in the structure of some accessorymineral, especially xenotime, brabantite, zircon,and apatite.Electron probe microanalysis (EPMA) provides an indicationof solid solution series between thorite-xenotimezircon,which are related to hydrothermal solutions enrichedin REE, Y, P, U, F, and Zr.

Proterozoic rapakivi granites from the North Qaidam orogen, NW China: Implications for basement attribution

Voluminous Proterozoic (~1700Ma) rapakivi granites occur in several cratons, especially in the northern hemisphere. Similar Proterozoic rapakivi granites have recently been recognized in the Paleozoic North Qaidam orogen, western segment of the China Central Orogenic System (CCOS). SHRIMP zircon U–Pb dating of these granites yielded ages of 1778±17 and 1778±12Ma. These granites exhibit typical rapakivi textures. They are ferroan, alkalic to alkalic-calc, metaluminous to peraluminous and characterized by high Ga/Al ratios, Na2O+K2O and rare earth elements (apart from Eu) contents, but low MgO, CaO and Sr contents. These are typical A-type granite features. Whole-rock εNd(t) values of the granites range from −6.09 to −5.74 with Nd model ages of 2762 to 2733Ma, and their zircon εHf(t) values are from −8.3 to −5.2 with two-stage Hf model ages of 2944 to 2800Ma, suggesting that these rocks were derived from old continental crust. The ages, rapakivi texture and geochemical features suggest that these granites are very close to typical Proterozoic (~1700Ma) rapakivi granites within the North China Craton (NCC) and belong to the group of Proterozoic rapakivi granites of the northern hemisphere. These indicate that part of the basement of the North Qaidam orogen in the western CCOS is similar to that of the NCC or was probably derived from it, and then became involved in the CCOS. This provides new data to solve the dispute on the basement origin in this orogen.

The source of Proterozoic anorthosite and rapakivi granite magmatism: evidence from combined in situ Hf–O isotopes of zircon in the Ahvenisto complex, southeastern Finland

The isotope compositions of massif-type anorthosites in Proterozoic anorthosite–mangerite–charnockite–granite (AMCG) complexes are commonly dominated by crustal values. Olivine-bearing anorthositic rocks in several AMCG suites have, however, been shown to display juvenile character, suggesting that variably depleted mantle reservoirs were involved in their genesis. A coupled in situ zircon Hf–O isotope dataset from the 1.64 Ga Ahvenisto AMCG complex in the 1.54–1.65 Ga Fennoscandian rapakivi granite–massif-type anorthosite province reveals correlated juvenile isotope signals (δ 18 O zrn = 5.4–6.6‰; initial ϵ Hf = −1.1 to +3.4) in the most primitive gabbroic rock type of the suite suggesting a depleted mantle origin for the anorthositic rocks. This signal is not as prominent in the more evolved co-magmatic anorthositic rocks (δ 18 O zrn = 6.3–7.8‰; initial ϵ Hf = −0.8 to +2.0), most probably owing to contamination of the mantle-derived primary magma by crustal material. A rapakivi granite associated with the anorthositic rocks has different isotope composition (δ 18 O zrn = 7.4–8.6‰; initial ϵ Hf = −2.1 to +0.5) that points to a crustal source. Supplementary material: Full analytical results and details and SEM images of the analysed grains with spatial information on both methods are available at: http://www.geolsoc.org.uk/SUP18780 .

Rapakivi granites within Phanerozoic collisional orogens as a possible consequence of continental subduction and following exhumation of the Precambrian crust: Evidences from the Permian Jangart rapakivi in South Tien Shan collisional belt, Eastern Kyrgyzstan

All worldwide known rare occurrences of Phanerozoic A-type granites with typical rapakivi texture do not usually have any significant volume of contemporaneous basic rocks in magmatic association and occur only in those collisional orogens, which border ancient cratons and contain HP–UHP metamorphic terranes, suggesting connection of their petrogenesis with a deep continental subduction and subsequent exhumation of the ancient crust. Early Permian Jangart rapakivis and genetically related leucogranites and Li-mica granites widely spread within the Saryjaz syntaxis, which is distinguished by the shortest distance between the Tarim craton and Caledonian Kazakh continent, collided in the Late Carboniferous with the possibly deepest continental subduction of the Tarim under the South Tien Shan collisional belt. Rapakivi plutons occur within the remarkable huge (20×120km) mega-mélange (mega-breccia) terrane of diapiric (not tectonic) origin, which was identified by the latest geological data and deciphering of satellite imagery. According to geophysical data the most of subducted Tarim continental crust was trapped within lithospheric mantle and never exhumed. But in the Saryjaz syntaxis, where the collisional suture has been subjected to flecsure-like oroclinal bending due to sinistral strike-slip tectonics during oblique collision between the Tarim and Kazakh plates, the subduction channel was weakened and exhumation occurred. The suggested P–T–t path of exhumed slices of the subducted Tarim continental crust presumes a relatively long (around 20Ma) residence time on a mantle depth as well as additional internal radiogenic heating and rapid (around 2Ma) isothermal exhumation, which were favorable for extensive melting and subsequent rapakivi formation. Phanerozoic rapakivis originated from the same ancient lower crust granulite source rocks as their abundant Proterozoic counterparts but mechanism of heating and melting of those source rocks was radically different.

New constraints on 1.7 Gyr of brittle tectonic evolution in southwestern Finland derived from a structural study at the site of a potential nuclear waste repository (Olkiluoto Island)

The brittle deformation history of southwestern Finland was reconstructed through paleostress inversion of fault-slip data from outcrops and drill cores at the site of a planned nuclear waste repository. Seven distinct paleostress states have been resolved spanning 1.7 Ga of geological history. (1) NW–SE to NNW–SSE transpression acted soon after 1.75 Ga, when conditions leading to brittle deformation were first attained. (2) N–S to NE–SW transpression caused partial reactivation of (1). (3) NW–SE extension followed during the Gothian event and at the time of rapakivi magmatism. (4) Renewed NE–SW transtension occurred between 1.60 and 1.30 Ga, forming the NW–SE-elongated Satakunta graben. (5) A significant and so far unrecognised phase of NE–SW compression has been found to postdate both rapakivi granites and 1.27 Ga olivine diabase sills. (6) E–W transpression followed during the early stages of the Mesoproterozoic Sveconorwegian orogeny and predated (7) almost coaxial E–W extension of the late Sveconorwegian orogenic collapse. Our results prove that the shield's ‘crust fragmentation’ (i.e. generation of new fractures) was short lived in comparison to its ‘jostling phase’ (i.e. reactivation of old fractures). The shield thus reached structural maturity with respect to its saturation with brittle structures already in the late Mesoproterozoic. We predict that that future stress changes will most probably be accommodated by reactivation of the existing and inherited brittle structures rather than by formation of new deformation zones.

Did a proto-ocean basin form along the southeastern Rae cratonic margin? Evidence from U-Pb geochronology, geochemistry (Sm-Nd and whole-rock), and stratigraphy of the Paleoproterozoic Piling Group, northern Canada

The Paleoproterozoic Piling Group along the southeastern Rae margin, northern Canada, is characterized by thick, deep marine turbidite deposits not observed in time-equivalent, intracratonic basin units further southwest. Models invoked to explain this feature include development of a full-ocean, back-arc, or proto-ocean basin followed by turbidite sedimentation. We present new and existing U-Pb geochronological, Nd isotope, geochemical, and stratigraphic evidence that support a proto-ocean basin model, and we explore the events leading to the formation and closure of such a rift basin during the middle Paleoproterozoic. Sedimentation initiated largely after ca. 2160 Ma with deposition of craton-derived, shallow marine siliciclastic strata (Dewar Lakes formation). Continued extension resulted in accumulation of south-facing carbonate beds (Flint Lake formation) and likely concomitant, arc-like tholeiitic to picritic volcanism and voluminous volcaniclastic sedimentation (lower Bravo Lake formation) farther outboard at ca. 1980 Ma. Accumulation of intrabasinal siliciclastic strata above lower Bravo Lake formation rocks may mark a hiatus in mafic-ultramafic magmatism. By ca. 1923 Ma, upper Bravo Lake formation, within-plate–type alkaline sill emplacement and volcanism occurred within highly extended crust. The oceanic island basalt–like signatures of the Bravo Lake formation rocks (but lack of depleted, mid-ocean-ridge basalt–type compositions) suggest that by this time the thinned Rae continental lithosphere had fragmented into small crustal block(s) and narrow zone(s) of incipient oceanic crust farther outboard of the Piling Group basin. Rapid subsidence of the southeastern Rae margin ensued, leading to deposition of euxinic (Astarte River formation) and overlying turbiditic strata (Longstaff Bluff formation). The post–ca. 1915 Ma northern turbiditic sedimentary units were likely derived from a thoroughly mixed, two-component source with possible input from the Snowbird tectonic zone and Bravo Lake formation, whereas the post–ca. 1930 Ma southern turbidite unit may have been sourced from the Meta Incognita microcontinent, presently exposed further south. We favor a rift margin over a foreland basin setting for the deposition of the northern turbidite deposits. Subsequent mantle upwelling associated with incipient ocean formation may have triggered melting of highly thinned continental crust resulting in emplacement of late-stage, ca. 1897 Ma, contaminated rapakivi granite and highly differentiated mafic sills. Our results are most consistent, albeit not exclusively, with the much debated model of asthenospheric upwelling and incipient rifting along the Rae-Hearne boundary farther southwest at ca. 1.9 Ga. Later accretion of the Meta Incognita microcontinent led to basin closure and development of a north-verging fold-and-thrust belt after ca. 1883 Ma.

Emplacement and magnetic fabrics of rapakivi granite intrusions within Wiborg and Åland rapakivi granite batholiths in Finland

Anisotropy of magnetic susceptibility (AMS) studies were carried out in two areas in Finland: the Ruotsinpyhtää intrusion within the Wiborg rapakivi granite batholith and the Saltvik intrusions within the Åland rapakivi granite batholith. The main aim of this study was to understand the emplacement history of these mid-Proterozoic rapakivi granites. Aeromagnetic images reveal structures of ca. 5–10 km intrusions that build up the large rapakivi granite batholiths of Åland and Wiborg. Magnetic susceptibility data from the database of the Geological Survey of Finland, including more than 1700 samples from the Wiborg rapakivi batholith and almost 900 samples from the Åland rapakivi batholith, were compared with measurements from the present study. The mean susceptibility is ca. 1500 μSI for the Wiborg batholith and ca. 10,000 μSI for the Åland batholith. Samples taken for this study demonstrate that the mean value for the Ruotsinpyhtää intrusion is ca. 1200 μSI and for the Saltvik intrusions ca. 24,000 μSI. Thermomagnetic measurements reveal that the magnetic susceptibility is mainly derived either from paramagnetic minerals or from magnetite. The absence of solid-state deformation features such as breccia or contact deformation indicates a cauldron-type subsidence emplacement. The AMS measurements from Ruotsinpyhtää confirm these proposals, with concentric gently dipping magnetic foliations that support a ring complex structure above a piston-type subsidence system. The Saltvik area consists of a number of smaller elliptical intrusions of different rapakivi granites forming a multiple intrusive complex. The magnetic fabric shows a general westward dipping of the pyterlite and eastward dipping of the contiguous even-grained rapakivi granite, which indicates a central inflow of magma batches towards the east and west resulting from a laccolitic emplacement of magma batches, while the main mechanism for space creation was derived from subsidence. • Aeromagnetic images reveal structures within rapakivi granite batholiths in Finland • The rapakivi granites show both paramagnetic and ferromagnetic susceptibilities • The magnetic fabric results indicate a laccolitic emplacement • The main mechanism for space creation was derived from subsidence

Maakivi ehitusmaterjalina: töötlemine ja kasutus

Fieldstone has been used as a building material for many centuries. In Estonia, two large groups of stone suitable for building can be distinguished in terms of architectural outcome and construction technique: calcareous stones (mostly carbonate flagstone and dolomite) and granite boulders, or field stones. The latter cover a broad spectrum of natural igneous and metamorphic rocks (rapakivi granite, diabases, gneiss etc.). In this article, we focus on natural stone belonging to the second group: in particular how it can be processed – split with wedges or a sledgehammer – and historical means of transporting stones. We also take a look at the ethical, aesthetic and cultural aspects of fieldstone use.Field stones were carried to Estonia by the continental glacier. In general, it can be said that fieldstone is a hard and strong stone suitable for use as a building material, which due to its density and low porosity can be used in underground and other supporting structures. These properties mean that fieldstone’s thermal conductivity is high, which is why it is not suitable for building dwellings. It has most commonly been used to build stables, cellars, barns and, to a lesser extent, taverns and other public buildings.Fieldstone is a material with a small ecological footprint which does not participate directly in the carbon cycle, as does timber. Fieldstone processing and transport is relatively energy-intensive, but this is compensated for by the longevity of the structures made. Visually, fieldstone is a very strong and eye-catching material. The surface of each stone is different, making each structure unique. The strong visual message and the long tradition of using fieldstone as a building material are the main factors that guarantee that there are people in Estonia who still commission fieldstone buildings and smaller items.

Mesoproterozoic arc magmatism in SE India: Petrology, zircon U–Pb geochronology and Hf isotopes of the Bopudi felsic suite from Eastern Ghats Belt

The southern segment of the Eastern Ghats Mobile Belt (EGMB) in India was an active convergent margin during Mesoproterozoic, prior to the final collision in Neoproterozoic during the assembly of the Rodinia supercontinent. Here we present mineralogical, whole-rock geochemical, zircon U–Pb and Hf isotopic data from a granitoid suite in the Bopudi region in the EGGB. The granitoid complex comprises quartz monzodiorite with small stocks of rapakivi granites. The monzodiorite, locally porphyritic, contains K-feldspar megacrysts, plagioclase, quartz, biotite and ortho-amphibole. The presence of mantled ovoid megacrysts of alkali feldspar embaying early-formed quartz, and the presence of two generations of the phenocrystic phases in the rapakivi granites indicate features typical of rapakivi granites. The K-feldspar phenocrysts in the rapakivi granite are mantled by medium-grained aggregates of microcline (Ab7 Or93), which is compositionally equivalent to the rim of Kfs phenocryst and Pl (An23–24 Ab75). The geochemistry of both the granitoids shows arc-like features for REE and trace elements. LA-ICP-MS zircon analyses reveal 207Pb/206Pb ages of 1582 (MSWD=1.4) for the rapakivi granite 1605±3Ma (MSWD=3.9) for the monzodiorite. The zircons from all the granitoid samples show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly, suggesting a common melt source. The zircons from the monzodiorite have a limited variation in initial 176Hf/177Hf ratios of 0.28171–0.28188, with εHf(t) values of −2.2 to +2.8. Correspondingly, their two-stage Hf isotope model ages (TDM2) ranging from 2.15 to 2.47Ga probably suggest a mixed source for the magma involving melting of the Paleoproterozoic basement and injection of subduction-related juvenile magmas. The prominent Mesoproterozoic ages of these granitoids suggest subduction-related arc magmatism in a convergent margin setting associated with the amalgamation of the Columbia-derived fragments within the Neoproterozoic Rodinia assembly.

Rare earth exploration potential in Finland

Exploration for economic deposits of rare earth elements (REEs) is currently active all over the world. In this case the REE potential of Finland was evaluated by analysing existing relevant data (literature, databases, and drill cores) combined with new geochemical surveys and drillings of new targets. Yttrium and lanthanum contents in the regional till and rock geochemical databases of the GTK show regional anomalies for the Paleoproterozoic Tana Belt and the Central Lapland granitoid complex in northern Finland and the Mesoproterozoic rapakivi granite areas in southern Finland. Local lanthanum anomalies are observed also for the Devonian Sokli carbonatite complex and the Paleoproterozoic Korsnäs and Otanmäki areas that seem to be the most promising of the current REE prospects in Finland. The highest REE concentrations are in the carbonatite veins of the Korsnäs Pb–REE-mine (0.9%) and the alkaline gneiss in Otanmäki, central Finland (2.4%). Among the new prospects the highest concentrations of REE are in the fenite zone of the Sokli carbonatite complex (1–2%) and in the Kovela monazite–granite in southern Finland (0.5–4.3%). Other studied prospects and small showings with anomalous REE-contents are associated with carbonatites, alkaline rocks, appinites, arkosic gneisses, albitites, rapakivi granites and granite pegmatites. Typical REE-bearing minerals in the targets include monazite, ancylite, bastnaesite, allanite, xenotime, apatite and zircon.

The Suursaari conglomerate (SE Fennoscandian shield; Russia)—Indication of cratonic conditions and rapid reworking of quartz arenitic cover at the outset of the emplacement of the rapakivi granites at ca. 1.65 Ga

Ultramature, quartz arenitic conglomerate of the Suursaari Island, Gulf of Finland, Russia is one of the oldest known post-Svecofennian sedimentary rocks on the Fennoscandian shield and provides an important sedimentary record from the period between the collapse and resultant stabilization of the Svecofennian orogen (at ca. 1.8Ga) and the emplacement of the first anorogenic rapakivi granites (at ca. 1.65Ga). The Suursaari conglomerate is considered a remnant from a previously unknown depositional stage involving formation of an ultramature mid-Proterozoic sedimentary cover in purely cratonic conditions. U–Pb detrital zircon ages from the conglomerate peak at 1.88Ga and 1.90Ga implying Svecofennian synorogenic granitoids as the dominant source. Svecofennian metasedimentary rocks are the most likely source for the observed Paleoproterozoic (2.05–1.90Ga) and Archean (3.3–2.5Ga) zircon ages. Moreover, ca. 7% (ten grains) of the zircons recovered from the quartz arenitic matrix of the Suursaari conglomerate are of rapakivi age (ca. 1.65Ga). These constrain a maximum depositional age for the conglomerate and imply that the Svecofennian orogen had reached continental peneplane prior to the rapakivi stage. The Suursaari conglomerate is thus one of the earliest marks of the initiation of a major intracratonic rifting stage. Rapid recycling of lithified sediment resulting in the deposition of the conglomerate was probably the result of accelerated erosion caused by block movements at the rapakivi stage (incipient, aborted rifting). The current exposure of the rapakivi plutons suggests rejuvenation of the topography of the peneplane at ca. 1.65Ga and significant localized erosion of the Svecofennian roof of the rapakivi plutons. This eventually led to enhanced erosion that tapped lithologic units beneath the ultramature sedimentary cover and to the deposition of the immature Mesoproterozoic ‘Jotnian’ arkosic redbeds, preserved in fault-bounded blocks in the vicinity of the rapakivi granite intrusions. The Suursaari conglomerate is compositionally comparable to the quartzites of the extensive cratonic cover sequences of midcontinental North America but it was not deposited in a subsiding, transient extensional basin setting.

Age, origin, and tectonic implications of Palaeozoic rapakivi granites in the North Qaidam orogen, Northwest China

Palaeozoic rapakivi granites occur in the western segment of the China Central Orogenic System. Exhibiting typical rapakivi texture, these granites contain magmatic microgranular enclaves of intermediate compositions. SHRIMP zircon U–Pb ages for the granites and enclaves are 433 ± 5 Ma and 433 ± 3 Ma, respectively. The rapakivi granites are magnesian to ferroan, calc-alkalic to alkalic, and are characterized by high FeOt/(FeOt + MgO) (0.74–0.91) and Ga/Al ratios, and SiO2, Na2O + K2O and rare earth element (apart from Eu) contents, but low CaO, Ba, and Sr contents. These are typical A-type granite geochemical features. The granites and enclaves exhibit a uniform decrease in TiO2, CaO, Na2O, K2O, FeO, and MgO with increasing SiO2, and both lithologies have similar trace element patterns. Whole-rock ϵNd(t) values vary from −9.2 to −8.7 for the granites and from −9.0 to −8.4 for the enclaves, but zircon ϵHf(t) values vary more widely from −5.8 to −0.2 and −4.6 to +5.1, respectively. Our data suggest that the granites and enclaves have crystallized from different magmas. The granites appear to have been derived from old continental crust, whereas the enclaves required a source having a juvenile component. The spherical shape and undeformed nature of the granites and their geochemical characteristics, coupled with the (ultra)-high pressure metamorphism and evolution of Palaeozoic granitoid magmatism in the North Qaidam orogen, indicate that the rapakivi granites were generated in a post-collisional setting. These rocks are therefore an example of Palaeozoic rapakivi granites emplaced in a post-collisional, extensional orogenic setting.

Zircon Hf isotope of Yingfeng Rapakivi granites from the Quanji Massif and ∼2.7 Ga crustal growth

The Quanji (全吉) Massif is located in the Northwest China, which is interpreted as a micro-continent that is composed of metamorphic basement and stable cover strata. There are some controversies of genetic relationship between the Quanji Massif and the major cratons in China. In this study, we obtained in situ zircon U-Pb and Hf isotopic compositions of the Yingfeng (鹰峰) rapakivi granites from the northwest Quanji Massif by application of LA-MC-ICP-MS technique. Twenty U-Pb age measurements points are concordant or near concordant, and their weighted mean 207Pb/206Pb age is 1 793.9±6.4 Ma (MSWD= 1.09), yields an upper intercept age of 1 800±17 Ma (MSWD=0.41); 19 Hf isotope measurements yield a two-stage Hf model ages (T DM2) of 2.63 to 2.81 Ga, with a weighted average age of about 2.70±0.02 Ga and ɛHf(t) values variate between -8.91 to -5.35. This indicates that magma source of the Yingfeng rapakivi granites were produced from partial melting of late stage of Neoarchean juvenile crust, and suggests a significant crustal growth event occurred in the Quanji Massif at that time. The Quanji Massif might be an ancient continental segment detached from the Tarim Craton based on the crustal growth history and other geological records. The Tarim Craton (including the Quanji Massif) and the North China Craton had a similar or homological early crustal evolution around ∼2.7 Ga, which implies that Tarim Craton might be one of the component parts of North China Craton.

Implications of sulfur isotope fractionation in fracture-filling sulfides in crystalline bedrock, Olkiluoto, Finland

A detailed investigation of 34S/32S ratios in fracture-bound pyrite and marcasite was conducted at Olkiluoto, western Finland, in order to gain information on paleohydrogeochemical conditions in a deep groundwater (0–400m b.s.l.) environment. The bedrock at Olkiluoto is being considered for a final repository of spent nuclear fuel. The 34S/32S ratios were measured in situ on individual sulfide grains, using secondary ion mass spectrometry (SIMS) supplemented by analyses from bulk sulfide material. The δ34S values of pyrite and marcasite show considerable variations, from −50‰ to +82‰ VCDT. Using published records and new data from fracture filling minerals, the pyrite fillings were classified into groups representing distinct episodes of fracture mineral formation. The δ34S values of these groups record a transition from hydrothermal activity to a low temperature groundwater environment. The δ34S values of hydrothermal pyrite vary between −18‰ and +34‰. However, hydrothermal pyrite that definitively precipitated at 128–245°C, evidenced by fluid inclusion data from co-genetic calcite, is characterized by a more limited variation in δ34S from −8.6‰ to +3.9‰. The high variability of δ34S within the hydrothermal pyrite precipitates indicates that biogenic fractionation effects cannot be completely excluded. Hydrothermal pyrite precipitation is likely related to hydrothermal systems produced by Mesoproterozoic rapakivi granite and diabase intrusions. Subsequent pyrite generations show a wider spread of δ34S values. Pyrite types representing formation temperatures from ca. 50 to 90°C have δ34S values ranging from −40‰ to +82‰, and the latest low temperature (<80°C) pyrite precipitates are characterized by δ34S from −50‰ to +78‰. The wide range of δ34S values is attributed to bacterial SO42- reduction, affected by Rayleigh fractionation under restricted SO42- input. It can be calculated that the highest δ34S values in pyrite represent consumption of ca. 90% of the original SO42- in the deep groundwater in a fracture section. Furthermore, in situ variations of the δ34S values in pyrite grains indicate that pulses of fresh SO42- with relatively non modified S-isotopic composition have periodically entered the fractures. The latest low temperature pyrite formation event most likely occurred during the Holocene period when brackish, SO42--rich waters of the Litorina Sea infiltrated the fracture network at 8000–4000 BP, providing substrate for SO42--reducing bacteria. Due to the complex Quaternary history of the site and low temperature conditions existing for millions of years, latest fracture filling calcite and pyrite may represent a compilation of several low temperature precipitation events. The records of δ13C in calcite and δ34S in coexisting pyrite indicate that paleogroundwaters have been influenced by the migration of a redox front between methanic and sulfidic environments. These findings can be used to consider the possible geochemical changes that may affect the materials used to contain spent nuclear fuel.

Detrital zircon geochronology from the Cambrian-Ordovician Bliss Sandstone, New Mexico: Evidence for contrasting Grenville-age and Cambrian sources on opposite sides of the Transcontinental Arch

Cambrian sandstones of the western United States and northern Mexico record deposition above a major unconformity on the rift margin of Laurentia. Detrital zircon ages from the Cambrian-Ordovician Bliss Sandstone in southern New Mexico are used to test models for the influence and significance of the Transcontinental Arch during the early Paleozoic. We obtained U-Pb zircon ages of basement rocks to test potential local source rocks in New Mexico. An orthogneiss in the Florida Mountains has an age of 1627 ± 18 Ma (all errors 2σ). The San Diego Mountain granite underlying the Bliss Sandstone is ∼1400 Ma. A rapakivi granite in the Little Hatchet Mountains has a U-Pb Concordia age of 1077 ± 4 Ma, coeval with the Pikes Peak granite and several granites in Sonora, Mexico. Three samples of the Florida Mountains granite underlying the Bliss Sandstone yield an average age of 510 ± 5 Ma; this is a rare and distinctive age of magmatism in western Laurentia. Zircons with these ages can be used as tracers for understanding transportation paths of sediment. The Bliss Sandstone was studied from eight localities in southern New Mexico; one overlies the Cambrian Florida Mountains granite, and three lie to the west. The sandstones are quartzarenite, subarkose, and arkose. The most abundant detrital zircon population included Grenville ages with a main peak at 1250 Ma. Other significant peaks are at 1465 Ma, 1625 Ma, 1710 Ma, and 1850 Ma, and these populations generally decrease in abundance with increasing age. Only five Archean zircons (out of 845) were discovered. Cambrian detrital zircons are present in the Florida Mountains Bliss Sandstone locality, directly overlying the Cambrian granite. They are also present in two localities 60–80 km southwest and northwest of the Florida Mountains. Maximum depositional ages from sandstones with Cambrian zircons range from 509 ± 7 Ma to 504 ± 12 Ma, overlapping with the age of the Florida Mountains granite. The four localities east of the Florida Mountains did not have any Cambrian detrital zircons. We interpret these data as indicating that the Cambrian intrusions of the Oklahoma-Colorado rift were not a source for the Cambrian zircons in the Bliss Sandstone. The Tapeats Sandstone in the Grand Canyon does not have Cambrian zircons and has a minor Grenville peak at 1070 Ma (Gehrels et al., 2011), significantly younger than the 1250 Ma Grenville peak in the Bliss Sandstone. We interpret these differences as being related to different source areas on opposite sides of the Transcontinental Arch. The differences in the specific ages of Cambrian plutons between the Oklahoma rift and the Florida Mountains granite can be used to test depositional models for other Cambrian sandstones of the southwest U.S. and in Mexico, where the presence of Cambrian zircons in the Caborca Block has implications for the Mojave-Sonora megashear hypothesis.

The first find of rapakivi granite in the Yenisei ridge: Age, PT conditions, and tectonic settings

The first find of rapakivi granite in the Yenisei ridge: Age, PT conditions, and tectonic settings

Tectonic significance of Late Precambrian calc-alkaline and alkaline magmatism in Saint Katherina Area, Southern Sinai, Egypt

Two magmatic rock suites are distinguished in the Late Precambrian basement of the Southern Sinai (NW Arabian-Nubian Shield, Egypt), namely (a) a calc-alkaline suite, and (b) an alkaline suite. The former includes Rutig Volcanics, quartz-diorite, quartz-monzonite and monzogranite, while the latter includes Katherina Volcanics and A-type granites. The minerals and textural features (kink, decussate and foliated textures) of the quartz-diorite reveal that it was subjected to deformation and thermal metamorphism. The Rutig Volcanics and quartz-diorite represent subduction related I-type magma, generated from anatexis of older crust with contribution of mantle-wedge magma. The quartz-monzonite and monzogranite are evolved from hybrid magma due to injection of the partly crystallized granitic magma by more basic melts. The mineralogical and geochemical characteristics of these granites indicate a mildly alkaline character and belonging to transitional magma type on the borderline between the calc-alkaline and alkaline magma. The mafic microgranular enclaves (MME) in the quartz-monzonite and monzogranite represent cooled globules from a dioritic magma mingled with the partly crystalline granitic magma. The association of rapakivi texture with the MME confirms the genetic link between the rapakivi texture and magma mixing. The overall characteristics of the Katherina Volcanics and Atype granite are consistent with within-plate tectonic setting. The Katherina Volcanics are derived from a crustal source with a mantle contribution, whereas A-type granites represent residual melts derived from deeper magma chamber through extreme fractional crystallization process.