New monazite U-Pb age constraints on the evolution of the Paleoproterozoic Vaasa granitoid batholith, western Finland

The Vaasa batholith, western Finland, is a large, peraluminous granitoid pluton that crystallized at 1.88-1.87 Ga during the culmination of the Svecofennian orogeny. The batholith has gradual contacts, through metatexites and diatexites, with the enveloping metasedimentary rocks of the Bothnian Belt. We present ID-TIMS U-Pb age data on monazite from granitoids and xenoliths of the Vaasa batholith and combine these with published U-Pb zircon ages in order to shed further light on the evolution of the Vaasa batholith. The apparent monazite ages for seven of the examined samples are 1870-1863 Ma, and 1855+/-3 Ma for one further sample from the southern part of the batholith. Combined with pre-existing data, the monazite ages of the granitoids are 9 to 18 Ma (face values) or 3 to 9 Ma (external errors considered) younger than the U-Pb zircon crystallization ages from respective samples. Our new data suggest slow cooling for the Vaasa batholith - the closure/saturation temperature of the monazite U-Pb system was probably reached in similar to 10 m.y. after the crystallization of magmatic zircon in the examined rocks.The Vaasa batholith, western Finland, is a large, peraluminous granitoid pluton that crystallized at 1.88-1.87 Ga during the culmination of the Svecofennian orogeny. The batholith has gradual contacts, through metatexites and diatexites, with the enveloping metasedimentary rocks of the Bothnian Belt. We present ID-TIMS U-Pb age data on monazite from granitoids and xenoliths of the Vaasa batholith and combine these with published U-Pb zircon ages in order to shed further light on the evolution of the Vaasa batholith. The apparent monazite ages for seven of the examined samples are 1870-1863 Ma, and 1855+/-3 Ma for one further sample from the southern part of the batholith. Combined with pre-existing data, the monazite ages of the granitoids are 9 to 18 Ma (face values) or 3 to 9 Ma (external errors considered) younger than the U-Pb zircon crystallization ages from respective samples. Our new data suggest slow cooling for the Vaasa batholith - the closure/saturation temperature of the monazite U-Pb system was probably reached in similar to 10 m.y. after the crystallization of magmatic zircon in the examined rocks.The Vaasa batholith, western Finland, is a large, peraluminous granitoid pluton that crystallized at 1.88-1.87 Ga during the culmination of the Svecofennian orogeny. The batholith has gradual contacts, through metatexites and diatexites, with the enveloping metasedimentary rocks of the Bothnian Belt. We present ID-TIMS U-Pb age data on monazite from granitoids and xenoliths of the Vaasa batholith and combine these with published U-Pb zircon ages in order to shed further light on the evolution of the Vaasa batholith. The apparent monazite ages for seven of the examined samples are 1870-1863 Ma, and 1855+/-3 Ma for one further sample from the southern part of the batholith. Combined with pre-existing data, the monazite ages of the granitoids are 9 to 18 Ma (face values) or 3 to 9 Ma (external errors considered) younger than the U-Pb zircon crystallization ages from respective samples. Our new data suggest slow cooling for the Vaasa batholith - the closure/saturation temperature of the monazite U-Pb system was probably reached in similar to 10 m.y. after the crystallization of magmatic zircon in the examined rocks.Peer reviewe

Formation of segregation structures in Hafnarhraun pāhoehoe lobe, SW Iceland: a window into crystal–melt separation in basaltic magma

Publisher's version (útgefin grein).To gain insights into crystal–melt separation processes during basalt differentiation, we have studied an 8-m-thick pāhoehoe lava lobe from the Hafnarhraun lava flow field in SW Iceland. The lobe has abundant melt segregations, porous cylindrical and sheet-like structures, generally interpreted as separated residual melts of a lava lobe. We divide these melt segregations into three types based on morphology and composition: vesicle cylinders (VC), type 1 horizontal vesicle sheets (HVS1), and type 2 horizontal vesicle sheets (HVS2). Remarkably, the studied VC are not simple residual melts generated by fractional crystallization, but their composition points to removal of plagioclase from the parental lava. HVS1 resemble VC, but have fractionated more olivine (ol) + plagioclase (plg) ± augite and have lost most, if not all, of their olivine phenocrysts. HVS2 are Fe-rich and evolved, corresponding to residual melts after 50–60% fractional crystallization of the lobe. We suggest that the Hafnarhraun VC formed in a two-stage process. Firstly, VC forming residual melt and vapor detached as rising diapirs from ol+plg+melt+vapor mush near the lava base, and later, these VC diapirs accumulated ol phenocrysts and minor plg microphenocrysts in the lava core. HVS1 represent accumulations of VC to the viscous base of the solidifying upper crust of the lobe, and HVS2 formed as evolved vapor-saturated residual melts seeped into voids within the upper crust. Such vapor-aided differentiation, here documented for the Hafnarhraun lava, may also apply to shallow crustal magma storage zones, contributing to the formation of evolved basalts.The Nordic Volcanological Center funded this work 2015–2017. We thank Robert A. Askew and Leó Kristjánsson for the aid in sampling, Atli Hjartarson and Guðmundur H. Guðfinnsson for the help in sample preparation and microprobe analyses, and Richard J. Brown for the editorial handling. Comments from Kaisa Nikkilä, Scott Rowland, and anonymous reviewer significantly improved the manuscript. Funding information: Open access funding provided by University of Helsinki including Helsinki University Central Hospital.Peer Reviewe

1.88 Ga granitoids at Sorsakoski, Central Finland: A-type magmatism within the Raahe-Ladoga suture zone

Four quartz monzonite – granite intrusions forming the Sorsakoski granite lithodeme, are found within the Raahe-Ladoga suture zone in Central Finland. The prevailing potassium feldspar megacrystic quartz monzonites and granites form a bimodal association with diorites and gabbros. The granitoids are mainly calc-alkaline, ferroan, per- to metaluminous, and have high Zr and REE contents. Dominant mafic minerals are biotite and hornblende, clinopyroxene and orthopyroxene are locally present. The mafic units display effects of fractionation of clinopyroxene. In our interpretation, these intrusions were emplaced during regional late stages of deformation and postcrystallisation deformation partitioned into major shear zones leaving bulk of the intrusions relatively undeformed. Based on one new (1876 ± 6 Ma) and one pre-existing (1882 ± 5 Ma) U-Pb zircon age determination, the crystallisation age of the granitoids can be assumed at ca. 1880 Ma. Based on mineralogy, petrography, geochemistry, bimodal nature of magmatism and age, we correlate these intrusions to the A-type rocks of the previously described Saarijärvi suite. This shows that the syn-orogenic A-type magmatism extended eastwards beyond the Central Finland Granitoid Complex

Pohjois-Suomen varhaisproterotsooisen Näränkävaaran kerrosintruusion petrogeneesi, osa I: pohjoisen peridotiitit ja niiden suhde kerrossarjaan ja uusiin magmapulsseihin

The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, has a surface area of 25 km x 5 km and a stratigraphic thickness of ~3 km. The main body of the intrusion includes a 1.5–2 km thick basal dunite series and a 1.3 km thick peridotitic-dioritic layered series, the latter with two peridotitic reversals related to magma recharge. In addition, a series of poorly known elongate poikilitic harzburgitic intrusions (the northern peridotites) are found along the NE contact between the intrusion and the granite-gneiss basement complex. We investigate new mineral and whole-rock geochemical data from the northern peridotites, with the aim of clarifying their petrogenetic relationship to the main layered body of the intrusion. The northern peridotites form a 200–400 m thick cumulate series grading from olivine orthocumulates (OC) at the northern basement complex contact to olivine-orthopyroxene heteradcumulates (HAC) towards the main intrusion body in the south. The OC show whole-rock and mineral chemical trends consistent with origin as rapidly cooled olivine-melt mixtures. The HAC have crystallized in situ from a relatively Cr- and SiO2-rich magma. Based on lithological and stratigraphical correlations, the northern peridotites are linked to the emplacement of the magma that caused the first reversal in the layered series: marginal orthocumulates were formed at the initial emplacement of a new pulse of LREE-enriched siliceous high-MgO basaltic (SHMB) magma into the Näränkävaara chamber, followed by heteradcumulate formation from a fractionating magma with added external SiO2 and fluid. Ubiquitous granite-gneiss xenoliths and felsic veins in drill core suggest assimilation may have been a local process. The northern peridotite parental magma shows undepleted metal ratios suggesting no sulfide saturation occurred prior to emplacement.Peer reviewe

Geology and geochemistry of the Redrock Granite and anorthosite xenoliths (Proterozoic) in the northern Burro Mountains, Grant County, New Mexico, USA

Mineral ages from the A-type granites and anorthosite xenoliths in the Redrock area in the northwestern Burro Mountains in southwestern New Mexico cluster around ~1220–1225 Ma and provide yet another example of bimodal igneous activity during this time period in the southwestern United States. The metaluminous to peraluminous, marginally alkaline to subalkaline Redrock Granite exhibits the textural, mineralogical, and geochemical features of A-type granitethat was emplaced at a relatively high crustal level. Field relationships, whole rock and mineral geochemical and isotopic trends suggest that the four phases of the Redrock Granite are genetically related, with the miarolitic biotite/alkali feldspar granite being the youngest phase. Spatial relationships and geochemical data suggest that the anorthosite xenoliths were coeval with the RedrockGranite, which is consistent with the anorthosite being derived from the upper mantle, possibly due to deep mantle upwellings, and the Redrock Granite from the lower crust. The process involved melting in the upper mantle, emplacement of anorthosite in the crust resulting in partial crustal melting and thinning, and, finally, intrusion of shallow silicic plutons, the Redrock Granite. The Redrock Granite and anorthosite were presumably derived from sources characterized by subtle, long-term LREE depletion, with εNd (at 1220 Ma) values on theorder of +1 to +2

Pohjois-Suomen varhaisproterotsooisen Näränkävaaran kerrosintruusion petrogeneesi, osa II: U-Pb ID-TIMS -ikä ja Sm-Nd-isotooppisystematiikka

Several mafic-ultramafic layered intrusions were emplaced in the NE Fennoscandian Shield during a magmatic episode at 2.44 Ga. The Paleoproterozoic Näränkävaara layered intrusion, northern Finland, is one of the largest ultramafic bodies in the Fennoscandian Shield, with a surface area of 25 km x 5 km and a magmatic stratigraphic thickness of ~3 km. The intrusion comprises a 1.3 km-thick peridotitic–dioritic layered series (2436 ± 5 Ma) with two peridotitic reversals, and a 1.5–2 km thick basal dunite series mainly composed of olivine adcumulates (dated here). The intrusion has been studied since the 1960’s, but several questions regarding its structure and petrogenesis remain. The basal dunite shows several lithological features typical of komatiitic rather than intrusive olivine cumulates; namely, >1 km-thick “extreme” olivine adcumulates, some showing textures with bimodal grain sizes, oscillating variations in Mg# with stratigraphic height, and poikilitic chromite. With Archean greenstone belts nearby, it was previously hypothesized that the basal dunite series could represent an Archean komatiitic wall rock to the Paleoproterozoic layered series. However, our new U-Pb ID-TIMS baddeleyite age of 2441.7 ± 0.9 Ma for the basal dunite series shows that the basal dunite and layered series of the Näränkävaara intrusion are co-genetic. New whole-rock Sm-Nd isotope data from key stratigraphic units (initial εNd at 2440 Ma of -3.5 to -1.7) indicate that the intrusion was constructed from repeated emplacement of LREE-enriched high-MgO basaltic magmas that were mantle-derived and contaminated by crust, similarly to other Fennoscandian 2.44 Ga intrusions. The parental magmas show similar compositions regardless of stratigraphic position, suggesting that most wall rock contamination and homogenization had occurred before emplacement, with in situ contamination being a relatively minor process. The open-system features of the basal dunite suggest that it may have formed (at least partly) as a feeder channel cumulate, possibly related to the ~100 km long Koillismaa-Näränkävaara Layered Igneous Complex. The Näränkävaara parental magmas show variably depleted metal ratios and could have potential for orthomagmatic mineral deposits, given the availability of S-rich wall rocks.Peer reviewe

U-Pb zircon ages of the host rocks of the Juomasuo Au-Co-Cu deposit, northeastern Finland

U-Pb zircon single grain dating using the LA-MC-ICP-MS technique was utilized to determine the age of the host rocks of the Juomasuo Au-Co-Cu deposit located in the late Archean Kuusamo supracrustal belt. Even though the dated samples have diverse geochemical signatures that imply felsic volcanic and sedimentary precursors, the U-Pb data revealed heterogeneous detrital zircon populations for all samples. The host rocks are thus considered to belong to reworked sedimentary/volcano-sedimentary sequences. The maximun depositional ages of the samples fall in the 2.75-2.65 Ga age window, and no Paleoproterozoic ages were recovered in the examined zircon grains (151 analytical spots in total). In addition, a younger population (2.65-2.60 Ga) of internally featureless, BSE-pale/CL-dark zircon and zircon domains, was found in the mineralized sequences. These homogenized zircon grains resemble zircon formed in postmagmatic solid-state processes, in which zircon is recrystallized in metamorphic-hydrothermal conditions. This metamorphic-hydrothermal event most probably occurred in the original provenance area of the metasedimentary rocks.Peer reviewe

2.44 miljardia vuotta vanhan Pohjois-Suomessa sijaitsevan mafisen-ultramafisen Näränkävaaran kerrosintruusion kantamagma, magmaattinen stratigrafia ja reef-tyylinen PGE rikastuminen

About 20 mafic-ultramafic layered intrusions in the northern Fennoscandian shield were emplaced during a widespread magmatic event at 2.5-2.4 Ga. The intrusions host orthomagmatic Ni-Cu-PGE and Cr-V-Ti-Fe deposits. We update the magmatic stratigraphy of the 2.44-Ga Näränkävaara mafic-ultramafic body, northeastern Finland, on the basis of new drill core and outcrop observations. The Näränkävaara body consists of an extensive basal dunite (1700 m thick), and a layered series comprising a peridotitic-pyroxenitic ultramafic zone (600 m thick) and a gabbronoritic-dioritic mafic zone (700 m thick). Two reversals are found in the layered series. The composition of the layered series parental magma was approximated using a previously unidentified marginal series gabbronorite. The parental magma was siliceous high-Mg basalt with high MgO, Ni, and Cr, but also high SiO2 and Zr, which suggests primary magma contamination by felsic crust. Cu/Pd ratio below that of primitive mantle implies PGE-fertility. The structural position of the marginal series indicates that the thick basal dunite represents an older wallrock for the layered intrusion. A subeconomic reef-type PGE-enriched zone is found in the border zone between the ultramafic and mafic zones and has an average thickness of 25 m with 150-250 ppb of Pt + Pd + Au. Offset-type metal distribution and high sulfide tenor (50-300 ppm Pd) and R-factor (105) suggest reef formation by sulfide saturation induced by fractional crystallization. The reef-forming process was probably interrupted by influx of magma related to the first reversal. Metal ratios suggest that this replenishing magma was PGE-depleted before emplacement.Peer reviewe

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

<p>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 <em>in situ</em> 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 (δ¹⁸O_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 (δ¹⁸O_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 (δ¹⁸O_zrn = 7.4–8.6‰; initial ϵ_Hf = −2.1 to +0.5) that points to a crustal source. </p