New rock magnetic and paleomagnetic results for the 1.64 Ga Suomenniemi dyke swarm, SE Finland

Peer reviewe

Multi-source and multi-stage metal mobilization during the tectonic evolution of the Central Lapland Greenstone Belt, Finland: implications for the formation of orogenic Au deposits

Precambrian greenstone belts are prospective terrains for orogenic Au deposits worldwide, but the sources of Au, base metals, metalloids, and ligands enriched within the deposits are still debated. Metamorphic devolatilization is a key mechanism for generating Au-rich hydrothermal fluids, but the respective role of the metavolcanic and metasedimentary rocks present within these belts in releasing ore-forming elements is still not fully understood. The Central Lapland Greenstone Belt (CLGB), Finland, one of the largest Paleoproterozoic greenstone belts, hosts numerous orogenic Au deposits and is composed of variably metamorphosed volcanic and sedimentary rocks. Characterization of element behavior during prograde metamorphism highlights that (1) metavolcanic rocks release significant Au, As, Sn, Te, and possibly S; (2) metasedimentary rocks release significant S, C, Cu, As, Se, Mo, Sn, Sb, Te, and U, but limited Au; and (3) metakomatiite releases C and possibly Au. Throughout the CLGB metamorphic evolution, two main stages are identified for metal mobilization: (1) prograde metamorphism at ~ 1.92–1.86 Ga, promoting the formation of typical orogenic Au deposits and (2) late orogenic evolution between ~ 1.83 and 1.76 Ga, promoting the formation of both typical and atypical orogenic Au deposits. The complex lithologic diversity, tectonic evolution, and metamorphic history of the CLGB highlight that metal mobilization can occur at different stages of an orogenic cycle and from different sources, stressing the necessity to consider the complete dynamic and long-lasting evolution of orogenic belts when investigating the source of Au, ligands, metals, and metalloids in orogenic Au deposits

Infiltration of meteoric water in the South Tibetan Detachment (Mount Everest, Himalaya): When and why?

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Baltica during the Ediacaran and Cambrian : A paleomagnetic study of Hailuoto sediments in Finland

We present a new Late Neoproterozoic paleomagnetic pole for Baltica from an inclined 272 m deep oriented sedimentary drill core in Hailuoto, Western Finland. The depositional age of the Hailuoto sediments is poorly constrained at 570-600 Ma. Three components of magnetization were isolated with thermal and alternating field (AF) demagnetization treatments. The ChRM (characteristic remanence magnetization) component is a high coercivity/unblocking temperature dual polarity component that passes a reversal test. The combined observed ChRM component of the Hailuoto sediments (D = 334.2 degrees; I = 44.4 degrees; alpha(95) = 7 2; k = 16.5) yields a paleomagnetic pole of Plat = 48.7 degrees N and Plon = 241.1 degrees E with A95 = 8.1 degrees. The inclination corrected direction (f = 0.6) of D = 334.4 degrees; I = 57.7 degrees; alpha(95) = 5.8 degrees; k = 25.2 yields a paleomagnetic pole of Plat = 60.5 degrees N and Plon = 247.9 degrees E with A95 = 7.6 degrees. As it is a dual-polarity ChRM carried by both magnetite and hematite, with no resemblance to younger events, we interpret it as a primary component. A paleolatitude for Hailuoto of 383 was calculated from the ChRM. Two secondary components were identified. The first is a low coercivity/blocking temperature component with a remanent magnetization of D = 239.0 degrees; I = 67.3 degrees; alpha(95) = 8.7 degrees (N = 13 samples), which we interpret as drilling-induced remanent magnetization (DIRM). The second secondary component has a remanent magnetization of D = 49.4 degrees; I = 34.9 degrees; alpha(95) = 8.6 degrees (N = 5 samples) and is commonly seen in Fennoscandian formations. The ChRM Hailuoto pole adds to the scattered Ediacaran paleomagnetic data of Baltica and indicate large distances between other late Neoproterozoic and early Cambrian paleomagnetic poles. We present reconstructions of Baltica and Laurentia between 616 and 550 Ma which move Baltica from high latitudes (615 Ma), over the polar region, to low latitudes (550 Ma), and Laurentia from low latitudes (615 Ma) to a polar position (570 Ma) and back to an equatorial position (550 Ma). A low to mid latitude position of Baltica determined by the Hailuoto paleomagnetic pole, and the lack of glaciogenic sediments determined in an earlier study of Hailuoto sediments indicate a warm deposition environment. (C) 2015 Elsevier B.V. All rights reserved.Peer reviewe

Rock magnetic investigations constraining relative timing for gold deposits in Finland

Palaeomagnetic and anisotropy of magnetic susceptibility (AMS) studies were carried out on a orogenic gold deposit in Jokisivu, located in the western part of the Pirkanmaa Belt in the Svecofennian domain of southern Finland. These results are compared with previous studies obtained from Satulinmäki, belonging to the Forssa Group in the western part of the Häme Belt, southern Finland and also results from the Central Lapland Greenstone Belt in northern Finland. The main aim of the studies was to test the capability of palaeomagnetic and AMS methods to provide relative age constraints about the structurally controlled gold formation processes. Palaeomagnetic data were used to obtain timing for the emplacement of hydrothermal fluids relative to geological structures. AMS was used to delineate the magnetic fabric. Petrophysical measurements and rock magnetic tests were carried out to define the magnetic minerals and their magnetic domain states as they have importance in preservation of the ancient remanent magnetization. Both the magnetic carriers and the remanence directions of the gold deposits in southern Finland deviate from those previously reported from the Central Lapland Greenstone Belt. The main magnetic mineral in the southern Finland deposits is coarse- to fine-grained monoclinic pyrrhotite whereas in the Central Lapland Greenstone Belt the magnetization is carried by fine-grained magnetite/titanomagnetite. The remanence directions in the southern Finland deposits are rotated and deflected so that no Svecofennian directions have been preserved. This, coupled with correlations between the orientation and orientation distribution of the magnetic and the rock fabric elements imply that the hydrothermal fluids were injected pre/syntectonically during the late stages of Svecofennian orogeny. This is contrasting to the Central Lapland Greenstone Belt, where previous works inferred that the well-preserved 1.88–1.84 Ga Svecofennian palaeomagnetic directions indicate that the gold-bearing hydrothermal fluids were emplaced in existing fractures during the late- or post-deformational stage of the orogeny

Paleomagnetic constraints on an Archean–Paleoproterozoic Superior–Karelia connection: New evidence from Archean Karelia

Charno–enderbitic granitoids in the Karelia craton of the Fennoscandian shield have been studied paleomagnetically. The characteristic remanence component has a steep negative inclination and is interpreted to record magnetization at a maximum age of 2684 ± 2 Ma. Consistently stable results were obtained from 12 sites in the Koitere area, corresponding to regions with high positive magnetic anomalies and high remanence intensities. Petrographic studies, coupled with rock magnetic investigations, indicate that the remanence resides in fine SD/PSD magnetite grains formed during Neoarchean clinopyroxene alteration. Cross-cutting vertical/subvertical Paleoproterozoic dolerite dykes suggest that the Koitere granitoids are in their original orientations and were not affected by Svecofennian deformation at ca. 1.9–1.8 Ga.The Koitere granitoids have an opposite polarity compared to the steep positive inclination remanence direction of the previously studied ca. 2.63 Ga Varpaisjärvi enderbites and granulites. The data from Koitere and Varpaisjärvi imply that at ca. 2.7–2.6 Ga the Karelia craton was located at high latitudes of 80–60°, whereas previous paleomagnetic data from ca. 2.5 Ga formations in the Vodlozero terrane in NW Russia indicate a near-equatorial position.Comparison of paleomagnetic data from the Koitere and Varpaisjärvi granulite-grade rocks with rocks of similar age in the Superior craton shows that at ca. 2.7–2.6 Ga the Superior and Karelia cratons were located at high latitudes and in close proximity, although the present data cannot demonstrate that the cratons were amalgamated. However, during the Archean–Paleoproterozoic transition at ca. 2.50 Ga both cratons experienced significant rotation and drifting to near-equatorial paleolatitudes, suggesting that the Superior and Karelia cratons may have been attached at that time

New palaeoproterozoic palaeomagnetic data from Central and Northern Finland indicate a long-lived stable position for Fennoscandia

The Svecofennian gabbro intrusions coincide temporally with the global 2100-1800 Ma orogens related to the amalgamation of the Mesoproterozoic supercontinent Nuna. We provide a new reliable 1891-1875 Ma palaeomagnetic pole for Fennoscandia based on rock magnetic and palaeomagnetic studies on the Svecofennian intrusions in central Finland to fill gaps in the Palaeoproterozoic palaeomagnetic record. By using the new pole together with other global high-quality data, we propose a new palaeogeographic reconstruction at 1885 Ma. This, together with previous data, supports a long-lived relatively stable position of Fennoscandia at low to moderate latitudes at 1890-1790 Ma. Similar stable pole positions have also been obtained for Kalahari at 1880-1830 Ma, Siberia at 1880-1850 Ma, and possibly India at 1980-1775 Ma. A new reconstruction at the beginning of this period indicates the convergence of several cratons at 1885 Ma in the initial stages of the amalgamation of the Nuna supercontinent at low to moderate latitudes. The close proximity of cratons at low to moderate latitudes is further supported by global and regional palaeoclimatic indicators. Stable position of several cratons could indicate a global period of minimal apparent drift at ca. 1880-1830 Ma. Before this period, the global palaeomagnetic record indicates large back-and-forth swings, most prominently seen in the high-resolution 2020-1870 Ma Coronation loops of the Slave craton. These large back-and-forth movements have been explained as resulting from an unstable geomagnetic field or basin- or local-scale vertical-axis rotations. However, the most likely explanation is inertial interchange true polar wander (IITPW) events, which is in line with the suggestion of large amplitude true polar wander events during the formation of the supercontinent.Validerad;2023;Nivå 2;2023-10-11 (joosat);CC BY 4.0 LicenseFunder: University of Helsinki; Academy of Finland (#288277); Petter and Margit Forsström Foundation</p

Paleomagnetic and geochronological studies on Paleoproterozoic diabase dykes of Karelia, East Finland-Key for testing the Superia supercraton

Paleomagnetic results are presented for two Paleoproterozoic mafic dykes in the Taivalkoski area in northern Karelia Province of the Fennoscandian shield where, based on K-Ar data, the crust has seen minimal effects of the otherwise pervasive 1.8-1.9 Ga Svecofennian orogeny. Within this study a new U-Pb baddeleyite age of 2339 +/- 18 Ma has been determined for one of the E-W trending dykes (dyke AD13). The paleomagnetic results show that a strong Svecofennian overprinting is pervasive in the area. Upon thermal or AF demagnetization four remanence directions were obtained. Most typical are the secondary Svecofennian remanence direction A (intermediate down to the NNW) and remanence direction B (intermediate down to the NNE). Component D (D = 115.4 degrees, 1=50.5 degrees, alpha(95) =2.6 degrees) yielding a virtual geomagnetic pole (VGP) D (Plat= -19.5 degrees N, Plon= 263.3 degrees, A95 = 3.1 degrees) is obtained from baked rocks for dyke WD, and based on a positive baked contact test is interpreted to represent the primary magnetization dating from about 2.4 Ga. Dyke AD13 carries only secondary A and B components, its unbaked host migmatites carry reversed A (A(R)) component, and the baked host rock carries a component D' (D = 134.5 degrees, 1= -7.3 degrees, alpha(95) = 8.8 degrees), which yields a VGP pole D' (Plat= -20.4 degrees N, Plon = 257.3 degrees, A(95) = 7.6 degrees), possibly representing magnetization at 2.3 Ga. The new paleomagnetic data from the Karelia Province compared to similar-aged paleomagnetic data from the Superior Province does not support the recently proposed Superia configuration, based upon dyke swarm trajectories. (c) 2013 Elsevier B.V. All rights reserved