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25 pages, 10016 KiB

Open AccessFeature PaperArticle

Quantitative Data Extraction from Orthopyroxene Trace Element Maps and Its Potential to Examine the Formation of the UG2 Unit, Bushveld Complex

by Maurice Brodbeck, Teresa Ubide, Ilka C. Kleinhanns, Ronny Schoenberg and Balz S. Kamber

Minerals 2019, 9(2), 73; https://doi.org/10.3390/min9020073 - 27 Jan 2019

Viewed by 5056

Abstract

We present a mineral chemical stratigraphy across the UG2 Unit, situated in the Upper Critical Zone of the Bushveld Complex. The magmatic evolution of the rocks hosting this world-class ore deposit for chromium and platinum group elements (PGEs) is still under debate. Orthopyroxene [...] Read more.

We present a mineral chemical stratigraphy across the UG2 Unit, situated in the Upper Critical Zone of the Bushveld Complex. The magmatic evolution of the rocks hosting this world-class ore deposit for chromium and platinum group elements (PGEs) is still under debate. Orthopyroxene is a common phase across the magmatic stratigraphy of the unit. It hosts trace metals, whose relative contents can record information about magmatic processes. This study determined laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) trace element data for orthopyroxene and spot LA multi-collector ICP-MS Sr-isotope data for plagioclase. Mapping of trace element distributions revealed internal zoning in Cr, V, Y, and Th. Unlike in conventional spot analysis, quantitative trace element concentrations were extracted post-analysis from two-dimensional LA-ICP-MS maps. This approach offered advantages over spot analysis. The maps revealed internal complexities (e.g., inclusions, exsolved phases, zonations and diffusion halos) that could be avoided with suitable thresholds for chemical filter elements (i.e., 100 ppm < Cr < 4000 ppm, Y < 6 ppm), permitting fast, automated extraction of pure orthopyroxene data pixels from elemental maps covering noritic, troctolitic, anorthositic and chromititic mineral assemblages. Diffusive equilibration in orthopyroxene was evaluated by comparing zonations in elements with different known diffusivities. Magnesium and Ni are homogenously distributed, indicative of rapid diffusive processes, while Cr and V show zonations, interpreted to largely reflect primary distribution. Thorium and Y apparently diffused along discrete pathways, indicated by concentration anomalies following orthopyroxene lamellae. A systematic study of the trace element inventory of orthopyroxene across the succession of the UG2 Unit, complemented with Sr isotope signatures of plagioclase, revealed clear evidence for open-system processes: compatible elements (Cr, Ni) are decoupled from Mg# variations in orthopyroxene from specific UG2 subunits; distinct Mg# peaks coincide with negative V excursions in orthopyroxene from chromite-bearing subunits; towards the top of the unit, plagioclase shows a distinct ⁸⁷Sr/⁸⁶Sr_initial peak and orthopyroxene shows Cr-depleted and Th-enriched rims. These chemical features cannot be explained by crystal mixing alone and argue for the involvement of at least two chemically distinct melts in the formation of the studied UG2 section. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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21 pages, 7556 KiB

Open AccessArticle

Rutile from Panasqueira (Central Portugal): An Excellent Pathfinder for Wolframite Deposition

by Eleonora Carocci, Christian Marignac, Michel Cathelineau, Laurent Truche, Andreï Lecomte and Filipe Pinto

Minerals 2019, 9(1), 9; https://doi.org/10.3390/min9010009 - 24 Dec 2018

Cited by 28 | Viewed by 4900

Abstract

Abundant W-rich rutile in the tourmalinized wall-rocks from the Panasqueira W-deposit appears to be a marker of the onset of the main wolframite depositing event. Rutile displays spectacular zoning, both sector (SZ) and oscillatory (OZ). An extensive set of compositional data obtained on [...] Read more.

Abundant W-rich rutile in the tourmalinized wall-rocks from the Panasqueira W-deposit appears to be a marker of the onset of the main wolframite depositing event. Rutile displays spectacular zoning, both sector (SZ) and oscillatory (OZ). An extensive set of compositional data obtained on crystals, beforehand studied using back-scattered electron images and X-ray maps, was used to address (i) the effects of SZ on differential trapping of minor elements, and (ii) the significance of the OZ in deciphering fluid sources and fluid circulation dynamics. Particular attention was paid to Sn, W (Nb, Ta) concentrations in rutile as pathfinders of the W deposition. Concerning the sector zoning, W is more incorporated than (Nb, Ta) onto more efficient faces, whereas Sn contents are nearly not impacted. The net effect of the sector zoning is thus a progressive increase of the relative weight of Sn from pyramid to prism faces, in combination with a less significant increase in the relative weight of Nb + Ta. The oscillatory zoning concerns most minor elements: W, Nb (Ta), Fe, V, Cr and Sn. In the frequent doublets, the clear bands are in general enriched in W relatively to the dark ones, whereas the inverse is true for Nb and Ta. The doublets may be viewed as the result of the successive influx of (i) a W-rich, Nb + Ta poor fluid, abruptly replaced by (or mixed to) (ii) a Nb + Ta-rich and W-poor fluid. The Nb + Ta-rich fluid could be in turn related to a rare-metal granite layer observed atop of the Panasqueira granite. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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30 pages, 5451 KiB

Open AccessArticle

Apatite Chemical Compositions from Acadian-Related Granitoids of New Brunswick, Canada: Implications for Petrogenesis and Metallogenesis

by Zeinab Azadbakht, David R. Lentz and Christopher R.M. McFarlane

Minerals 2018, 8(12), 598; https://doi.org/10.3390/min8120598 - 17 Dec 2018

Cited by 35 | Viewed by 9247

Abstract

The geochemistry of apatite crystals from fifteen fertile and infertile Acadian-related granitoids of New Brunswick (Canada) was studied in situ, using electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry to further investigate petrogenesis and fertility index among these intrusions. The results indicate [...] Read more.

The geochemistry of apatite crystals from fifteen fertile and infertile Acadian-related granitoids of New Brunswick (Canada) was studied in situ, using electron microprobe and laser ablation-inductively coupled plasma-mass spectrometry to further investigate petrogenesis and fertility index among these intrusions. The results indicate a clear geochemical contrast between barren and mineralized samples where apatite grains from barren intrusions are the most hydrous (OH > 0.3 wt. %), with lowest Mn (<1700 ppm), Fe (<800 ppm), and Sn (<0.01 ppm). In contrast, apatite grains from Cu-Mo related intrusions are distinguished by higher Cl (>0.1 wt. %), (La/Yb)_N ratios of 21.17, (Eu/Eu*)_N ratios of 0.30, and LREE/HREE ratios of 6.03. Apatites from Sn-W related magmatic suites have the highest F (>3 wt. %), Mn (>5350 ppm), Fe (>2200 ppm), Y (>4900 ppm), Sn (>2 ppm), and the lowest Cl (<0.01 wt. %), Sr (<60 ppm), U (<18 ppm), Th (<29 ppm), (Eu/Eu*)_N ratios (<0.01), and (La/Yb)_N ratios (<0.88). Lastly, apatite grains from Mo-bearing systems have the lowest SiO₂ (<0.4 wt. %), Sr (<33 ppm), Th (<28 ppm), a moderate Mn (~3800 ppm), Y (~3500 ppm), and highest FeOt (<0.9 wt. %). However, the results indicated apatite Mn, Sr, LREE/HREE, and (Eu/Eu*)_N ratios as the best fertility indices used for discriminating barren from fertile granite intrusions. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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31 pages, 16024 KiB

Open AccessArticle

Occurrence of Felsic Rocks in Oceanic Gabbros from IODP Hole U1473A: Implications for Evolved Melt Migration in the Lower Oceanic Crust

by Du Khac Nguyen, Tomoaki Morishita, Yusuke Soda, Akihiro Tamura, Biswajit Ghosh, Yumiko Harigane, Lydéric France, Chuanzhou Liu, James H. Natland, Alessio Sanfilippo, Christopher J. MacLeod, Peter Blum and Henry J. B. Dick

Minerals 2018, 8(12), 583; https://doi.org/10.3390/min8120583 - 10 Dec 2018

Cited by 27 | Viewed by 8219

Abstract

Felsic rocks are minor in abundance but occur ubiquitously in International Ocean Discovery Program Hole U1473A, Southwest Indian Ridge. The trace element abundances of high-Ti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied [...] Read more.

Felsic rocks are minor in abundance but occur ubiquitously in International Ocean Discovery Program Hole U1473A, Southwest Indian Ridge. The trace element abundances of high-Ti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied felsic rocks support crystallization origin from highly-evolved melts, probably controlled by fractional crystallization. Based on geochemical criteria and texture of the mineral assemblage in felsic rocks and their relationship with host gabbros, they can be divided into three types: (1) Felsic rock with sharp boundaries is formed when felsic melt intrudes into fractures of host gabbros, resulting in minimal interaction between the melt and the wall minerals. (2) Replacive felsic rock, which is characterized by a pseudomorphic replacement of minerals in the host gabbro. This vein type is caused by the replacement of the host mineralogy by minerals in equilibrium with the felsic melts. (3) Felsic rock with diffused boundaries is formed either by infiltration of felsic melt into the solidifying gabbro body or crystallization of interstitial melts. Infiltration modes of felsic melts are likely controlled by the temperature condition of the cooling host gabbros. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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18 pages, 4387 KiB

Open AccessArticle

LA-ICP-MS Analysis of Clinopyroxenes in Basaltic Pyroclastic Rocks from the Xisha Islands, Northwestern South China Sea

by Yu Zhang, Kefu Yu and Handong Qian

Minerals 2018, 8(12), 575; https://doi.org/10.3390/min8120575 - 7 Dec 2018

Cited by 12 | Viewed by 4721

Abstract

Cenozoic volcanic rocks were recently discovered during full-coring kilometer-scale major scientific drilling in the Xisha Islands, northwestern South China Sea. A systematic mineralogical study of these samples was performed for this paper. The results show that the volcanic rock samples are basaltic pyroclastic. [...] Read more.

Cenozoic volcanic rocks were recently discovered during full-coring kilometer-scale major scientific drilling in the Xisha Islands, northwestern South China Sea. A systematic mineralogical study of these samples was performed for this paper. The results show that the volcanic rock samples are basaltic pyroclastic. The major elements demonstrate that the clinopyroxenes are diopside and fassaite, which contain high Al₂O₃ (5.33–11.2 wt. %), TiO₂ (2.13–4.78 wt. %) and CaO (22.5–23.7 wt. %). Clinopyroxenes have high REE abundances (104–215 ppm) and are strongly enriched in LREE (LREE/HREE = 3.56–5.14, La/Yb_N = 2.61–5.1). Large-ion lithophile elements show depleted characteristics. Nb/Ta shows obvious fractionation features: Nb is lightly enriched, relative to primitive mantle, but Ta is heavily depleted, relative to primitive mantle. The parental magma of the basaltic pyroclastic rocks belongs to a silica-undersaturated alkaline series, characterized by a high temperature, low pressure, and low oxygen fugacity. The Al^IV content increases with decreasing Si concentration. The Si-unsaturated state causes Si-Al isomorphic replacement during the formation of clinopyroxene. The electric charge imbalance caused by the replacement of Si by Al is mainly compensated by Fe³⁺. The clinopyroxene discrimination diagrams show that the parental magma formed in an intraplate tectonic setting environment. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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14 pages, 3895 KiB

Open AccessArticle

Geodynamic Significance of the Mesoproterozoic Magmatism of the Udzha Paleo-Rift (Northern Siberian Craton) Based on U-Pb Geochronology and Paleomagnetic Data

by Sergey V. Malyshev, Aleksander M. Pasenko, Alexei V. Ivanov, Dmitrii P. Gladkochub, Valery M. Savatenkov, Sebastien Meffre, Adam Abersteiner, Vadim S. Kamenetsky and Vasiliy. D. Shcherbakov

Minerals 2018, 8(12), 555; https://doi.org/10.3390/min8120555 - 29 Nov 2018

Cited by 14 | Viewed by 3963

Abstract

The emplacement age of the Great Udzha Dyke (northern Siberian Craton) was determined by the U-Pb dating of apatite using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). This produced an age of 1386 ± 30 Ma. This dyke along with two other [...] Read more.

The emplacement age of the Great Udzha Dyke (northern Siberian Craton) was determined by the U-Pb dating of apatite using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). This produced an age of 1386 ± 30 Ma. This dyke along with two other adjacent intrusions, which cross-cut the sedimentary units of the Udzha paleo-rift, were subjected to paleomagnetic investigation. The paleomagnetic poles for the Udzha paleo-rift intrusions are consistent with previous results published for the Chieress dyke in the Anabar shield of the Siberian Craton (1384 ± 2 Ma). Our results suggest that there was a period of intense volcanism in the northern Siberian Craton, as well as allow us to reconstruct the apparent migration of the Siberian Craton during the Mesoproterozoic. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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15 pages, 3294 KiB

Open AccessArticle

An Isotope Study of the Dzhida Mo–W Ore Field (Western Transbaikalia, Russia)

by German S. Ripp, Olga K. Smirnova, Ivan A. Izbrodin, Eugeny I. Lastochkin, Mikhail O. Rampilov and Viktor F. Posokhov

Minerals 2018, 8(12), 546; https://doi.org/10.3390/min8120546 - 24 Nov 2018

Cited by 4 | Viewed by 3078

Abstract

The Dzhida ore field includes Pervomaika (Mo), Inkur (W) and Kholtoson (W) deposits. This article presents stable and radiogenic isotopic data (O, C, D, S, Sr and Nd) in an attempt to better understand the petrogenetic processes and the problem concerning the sources [...] Read more.

The Dzhida ore field includes Pervomaika (Mo), Inkur (W) and Kholtoson (W) deposits. This article presents stable and radiogenic isotopic data (O, C, D, S, Sr and Nd) in an attempt to better understand the petrogenetic processes and the problem concerning the sources of ore-forming fluids. Granites from the Pervomaika deposit, which includes Mo-ores, as well as the syenite dikes that precede W-mineralization, have low δ¹⁸O values (about 5‰ and 4‰ respectively), and low initial ratios ⁸⁷Sr/⁸⁶Sr (0.704–0.705). The εNd (T) values (+0.9–−1.1) in granites and syenites are close to the evolution trend of the mantle-derived source. It was determined that a mantle-derived source was involved in ore-forming processes. It was also confirmed that δ³⁴S values in sulfide minerals (molybdenite, pyrite, sphalerite, galena, and chalcopyrite) were close to the meteoric standard (from −2‰ to +2‰). The δ¹³C and δ¹⁸O values in carbonate minerals (rhodochrosite and ankerite) of the Kholtoson deposit are located within the primary igneous carbonatite (PIC)-square, as a possible juvenile source of CO₂. This was also confirmed by the δ¹⁸O and δD values in muscovite from greisens (4.2‰–6.5‰ δ¹⁸O, –78.8‰ … –84.0‰ δD). The δ¹⁸O values calculated in a fluid equilibrated with hydrothermal minerals indicated a meteoric origin. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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17 pages, 7244 KiB

Open AccessArticle

Geochemistry and Petrogenesis of Mesoproterozoic Dykes of the Irkutsk Promontory, Southern Part of the Siberian Craton

by T. V. Donskaya, D. P. Gladkochub, R. E. Ernst, S. A. Pisarevsky, A. M. Mazukabzov and E. I. Demonterova

Minerals 2018, 8(12), 545; https://doi.org/10.3390/min8120545 - 23 Nov 2018

Cited by 13 | Viewed by 3618

Abstract

We present new geochemical and Nd isotopic data on two Mesoproterozoic Listvyanka (1350 ± 6 Ma) and Goloustnaya (1338 ± 3 Ma) mafic dyke swarms located in the Irkutsk Promontory of the southern part of the Siberian craton. Listvyanka dykes are sub-vertical with [...] Read more.

We present new geochemical and Nd isotopic data on two Mesoproterozoic Listvyanka (1350 ± 6 Ma) and Goloustnaya (1338 ± 3 Ma) mafic dyke swarms located in the Irkutsk Promontory of the southern part of the Siberian craton. Listvyanka dykes are sub-vertical with NNE trend, while Goloustnaya dykes are characterized by prevailing W trend. Listvyanka and Goloustnaya dykes are composed of medium to coarse grained dolerites. All dolerites correspond to sub-alkaline tholeiitic basalts according to their major-element compositions with lower to moderate mg#, varying from 36 to 54. The trace and rare earth element abundances in Listvyanka and Goloustnaya dolerites are generally close to basalts of the oceanic island basalts (OIB) type. The Listvyanka dolerites demonstrate slightly positive εNd(t) values varying from +1.1 to +1.5, while the Goloustnaya dolerites are characterized by lower εNd(t) values ranging from −0.9 to +0.1. Geochemical and isotopic affinities of the Listvyanka dolerites suggest their enrichment by a mantle plume related source. For the Goloustnaya dolerites, we assume also some additional lithospheric input to their mantle plume-related source. The emplacement of both studied dolerites took place in intracontinental extensional setting, caused by a single rising mantle plume. Listvyanka and Goloustnaya dolerites are coeval to several mafic magmatic events in northern Laurentia and likely represent part of the Mesoproterozoic plumbing system of a Siberian–Laurentian Large Igneous Province. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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22 pages, 12557 KiB

Open AccessArticle

Petrogenesis and Metallogenic Implications of Neoproterozoic Granodiorite in the Super-Large Shimensi Tungsten-Copper Deposit in Northern Jiangxi, South China

by Wenfeng Wei, Chunkit Lai, Bing Yan, Xiaoxi Zhu, Shengqiong Song and Lei Liu

Minerals 2018, 8(10), 429; https://doi.org/10.3390/min8100429 - 29 Sep 2018

Cited by 8 | Viewed by 3435

Abstract

The newly discovered Shimensi deposit is a super-large tungsten-copper (W–Cu) deposit with a metal reserve of 742.55 thousand tonnes (kt) W and 403.6 kt Cu. The orebodies are hosted in Mesozoic granites, which intruded the poorly documented Shimensi granodiorite belonging to the Jiuling [...] Read more.

The newly discovered Shimensi deposit is a super-large tungsten-copper (W–Cu) deposit with a metal reserve of 742.55 thousand tonnes (kt) W and 403.6 kt Cu. The orebodies are hosted in Mesozoic granites, which intruded the poorly documented Shimensi granodiorite belonging to the Jiuling batholith, the largest intrusion (outcrop > 2500 km²) in South China. Our new SHRIMP (Sensitive High Resolution Ion MicroProbe) zircon dating revealed that the granodiorite at Shimensi (ca. 830–827 Ma) was formed coeval (within analytical uncertainty) or slightly earlier than those in many other places (ca. 819–807 Ma) of the Jiuling batholith. The Neoproterozoic Shimensi granodiorite is peraluminous and high-K calc-alkaline, and contains low P content with no S-type trend (positive P₂O₅ vs. SiO₂ correlation) displayed, thus best classified as peraluminous I-type. The I-type classification is also supported by the zircon REE patterns, largely (93%) positive εHf(t) (−0.87 to 6.60) and relatively low δ¹⁸O (5.8–7.7‰). The Neoproterozoic Shimensi granodiorite was formed after the continental arc magmatism (ca. 845–835 Ma), but before the post-collisional S-type granite emplacement (ca. 825–815 Ma) in the Jiangnan Orogen. Therefore, we propose that the Shimensi granodiorite was formed in a collisional/early post-collisional setting. The δ¹⁸O increase from the Shimensi granodiorite to many younger (ca. 819–807 Ma) granodiorites (6.0–8.5‰) in the Jiuling batholith probably reflects an increase of supracrustal rock-derived melts with the progress of collision. The Shimensi granodiorite contains low zircon Ce⁴⁺/Ce³⁺ and Eu/Eu*, suggesting a relatively reducing magma that does not favor porphyry Cu–Au mineralization. This left a high background Cu concentration (avg. 196 ppm) in the Neoproterozoic granodiorite, which may have contributed to the Mesozoic W–Cu mineralization, when the granodiorite is intruded and assimilated by the Mesozoic granites. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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27 pages, 8462 KiB

Open AccessArticle

Paleoproterozoic Multiple Tectonothermal Events in the Longshoushan Area, Western North China Craton and Their Geological Implication: Evidence from Geochemistry, Zircon U–Pb Geochronology and Hf Isotopes

by Renyu Zeng, Jianqing Lai, Xiancheng Mao, Bin Li, Jiandong Zhang, Richard C. BAYLESS and Lizhi Yang

Minerals 2018, 8(9), 361; https://doi.org/10.3390/min8090361 - 21 Aug 2018

Cited by 21 | Viewed by 4620

Abstract

The Alxa block is located in the southwestern margin of the North China Craton. The Paleoproterozoic tectonic evolution, crustal growth and tectonic affinity of the block remain unknown or controversial. The Longshoushan (LS) area is one of the few areas that outcrop Paleoproterozoic [...] Read more.

The Alxa block is located in the southwestern margin of the North China Craton. The Paleoproterozoic tectonic evolution, crustal growth and tectonic affinity of the block remain unknown or controversial. The Longshoushan (LS) area is one of the few areas that outcrop Paleoproterozoic to crystalline basement rocks in the Alxa Block. In this study, we preset whole-rock geochemistry, zircon U–Pb geochronology and Lu–Hf isotope data from metagabbro, metadiorite, quartz syenite, granitic leucosome and pegmatoid leucosome in the LS area. These rocks all are enriched in LREE and LILE, and depleted in HREE and HFSE. Eight new LA-ICP-MS zircon U–Pb ages yielded three magmatic ages of 2044 Ma, 2029 Ma and 1940 Ma, and three metamorphic ages of 1891 Ma, 1848 Ma and 1812 Ma. Lu–Hf analyses reveal that the magmatic zircons and anatectic/metamorphic zircons from all the rock types are characterized by positive εHf(t) (−0.16 to 10.89) and variable εHf(t) (−11.21 to 6.24), respectively. Based on the previous studies and our new data, we conclude that the LS area experienced three magmatic events (2.5–2.45 Ga, ~2.1–2.0 Ga and ~1.95–1.91 Ga) and three regional metamorphism/anataxis events (~1.93–1.89 Ga, ~1.86–1.84 Ga and ~1.81 Ga) in Paleoproterozoic. The age–Hf isotope data establishes two main crustal growth events at ~2.9–2.5 Ga and ~2.2–2.0 Ga in the LS area. These data indicate that the LS area experienced intraplate extensional setting in the middle Paleoproterozoic, and continental subduction, collision and exhumation in the late Paleoproterozoic. Combining the geochronological framework and tectonic evolution, we suggest that the Alxa Block is part of the Khondalite Belt. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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28 pages, 16558 KiB

Open AccessArticle

Mineralogical, Rock-Magnetic and Palaeomagnetic Properties of Metadolerites from Central Western Svalbard

by Mariusz Burzyński, Krzysztof Michalski, Geoffrey Manby and Krzysztof Nejbert

Minerals 2018, 8(7), 279; https://doi.org/10.3390/min8070279 - 29 Jun 2018

Cited by 2 | Viewed by 3670

Abstract

A combination of mineralogical, rock-magnetic and palaeomagnetic methods were employed in an attempt to shed a new light on the tectonism and paleogeography of Central Western Svalbard. The focus is on six metadolerite sites from the metamorphic Proterozoic–Lower Palaeozoic complex of south-western Oscar [...] Read more.

A combination of mineralogical, rock-magnetic and palaeomagnetic methods were employed in an attempt to shed a new light on the tectonism and paleogeography of Central Western Svalbard. The focus is on six metadolerite sites from the metamorphic Proterozoic–Lower Palaeozoic complex of south-western Oscar II Land (Western Spitsbergen). The primary mineral compositions of the metadolerites were strongly remineralized during Caledonian (sensu lato) greenschist-facies metamorphism although some younger tectonothermal modification is also apparent from the rock-magnetic studies. Rock-magnetic experiments supported by thin-section mineral identification and separation of Fe-containing fractions indicate that the main ferromagnetic carriers of the Natural Remanent Magnetization are represented by low-coercivity pyrrhotite and magnetite/maghemite. The investigated metadolerites are characterized by complex pattern of magnetization. The low-temperature palaeomagnetic components which demagnetized up to 250 °C, are characterized by high inclinations (~70–80°) potentially representing Mesozoic–Cenozoic remagnetization. The most stable middle-high temperature directions which demagnetized from 250 °C, were obtained from only two of six sites. Two Virtual Geomagnetic Poles calculated from two of the middle-high temperature site means do not correlate with the Laurussia reference path for syn- to post-Caledonian time. Two possible explanations of observed inconsistency are discussed. These are a modification of the Oscar II Land Caledonian basement geometry by listric faulting and/or tectonic rotations related to Daudmannsdalen–Protectorbreen high-strain (shear) zone. The results presented here suggest that post-Caledonian tectonic modification of the palaeomagnetic directions may be more a widespread feature of Western Svalbard. Full article

(This article belongs to the Special Issue Igneous Rocks: Minerals, Geochemistry and Ore Potential)

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