Thermal Step Demagnetization Research Articles

Temporal constraints on genesis of the Caravia-Berbes fluorite deposits of Asturias, Spain, from paleomagnetism

The Caravia-Berbes district is a major fluorite producing area in Europe. The fluorite occurs as mantos replacing breccio-conglomerates of the Middle Permian Caravia Formation and as veins and irregular replacive bodies in the unconformably underlying limestone of the Late Carboniferous Caliza de Montaña Formation. Paleomagnetic analyses were done using alternating field and thermal step demagnetization and saturation remanence methods. Ten sites (82 specimens) in massive and disseminated fluorite ore from the Emilio manto yielded a stable chemical remanent magnetization (CRM) in hematite inclusions with a direction of Decl. 20.1°, Incl. 67.3° (α95=6.1°). After correction for Neogene Pyrenean tilt, the manto's paleoinclination gives a CRM acquisition age of 206±8Ma that dates a major hydrothermal and ore emplacement event. The age is coeval with the onset of Pangea's break up as the Iberian microplate began to split from the Armorican terrane of Eurasia as part of the ocean-forming CAMP (Central Atlantic Magmatic Province) event. Another 11 sites (109 specimens) of silicified dolostone of the Caliza de Montaña Formation just below the Cueto L'Aspa manto yielded a stable CRM that resides in single and pseudosingle domain magnetite inclusions that has a direction of Decl. 328.7°, Incl. 76.6° (α95=4.9°). After Neogene tilt correction, the altered zone's paleopole gives a CRM acquisition age of 115±3Ma. This age shows that the western Cantabrian basin also underwent a major hydrothermal alteration and remagnetization event by fluid flow through steep re-activated faults when Iberia rotated 35±2° away from Eurasia during Aptian-Albian time.

Paleomagnetic dating of magmatic phases at the Cantung tungsten deposit, Northwest Territories, Canada

The Cantung tungsten–copper (W–Cu) skarn orebodies are hosted by Proterozoic and Lower Cambrian metasedimentary rocks next to the Cretaceous “Mine Stock” monzogranite. Paleomagnetic analyses of 283 specimens from the Open Pit scheelite–chalcopyrite orebody (17 sites) and from adjacent host rocks including the aplite dikes (11 sites) isolated a stable characteristic remanent magnetization (ChRM), mostly by alternating field and then thermal step demagnetization. The step demagnetization results along with rock magnetic analyses of the W concentrate show that the main remanence carriers are single- or pseudosingle-domain pyrrhotite, titanomagnetite, and (or) magnetite. There is no statistically significant difference at 95% confidence between the site mean ChRM directions for the W–Cu ore, the host rock, or the aplite dikes populations. This result indicates that the intrusion of the Mine Stock is coeval with the genesis of the scheelite skarn ore and with dike emplacement to give an overall mean ChRM direction of declination 342.9°, inclination 82.0° (N = 22 sites, radius of cone of 95% confidence α95 = 4.2°, precision parameter k = 54.7) that defines a paleopole at 76.2°N latitude, 212.2°E longitude (radius of cone of 95% confidence A95 = 8.1°). This paleopole is concordant with the coeval 98 Ma North American paleopole at 92% confidence, which provides strong evidence that the eastern Selwyn Basin has been an autochthonous part of North America since the mid-Cretaceous.

Paleomagnetic Evidence for an Early Permian Age of the Lisheen Zn-Pb Deposit, Ireland

The Lisheen Zn-Pb deposit is one of the major Lower Carboniferous carbonate-hosted base metal deposits of the Irish Midlands. The timing of ore mineralization and, therefore, its genesis remains much debated because of a lack of absolute age determinations. Paleomagnetic analysis of 432 specimens from 12 sites in ore mineralization and 10 sites in host rocks at Lisheen isolated a well-defined stable characteristic remanent magnetization (ChRM) direction using AF step demagnetization, except in one site of massive sulfide mineralization and two sites of postore pink dolomite. Thermal step demagnetization and saturation remanence tests show that the ChRM is carried dominantly by single domain magnetite. A paleomagnetic fold test is negative, indicating that the ChRM postdates burial lithification and deformation. The ChRM directions from eight host-rock and 12 Zn-Pb mineralized sites are indistinguishable at 95 percent confidence. The combined ChRM directions give a mean paleopole at 41.6°S, 18.8°W (semiaxes of the oval of 95% confidence along and perpendicular to the site-pole great circle, dp = 1.7, dm = 3.3) that, when compared to the robust apparent polar wander path for Laurentia rotated into European coordinates, gives an age of 277 ± 7 (2σ) Ma for the ChRM. This 277 ± 7 Ma age is close to the paleomagnetic ages of 290 ± 9 (2σ) Ma for Zn-Pb mineralization at Galmoy and 269 ± 4 (2σ) Ma for the Ba mineralization at Silvermines, which are ~10 km northeast and ~30 km west of Lisheen, respectively. This Early Permian magnetization postdates peak Variscan orogenic heating that is observed in all Carboniferous strata in Ireland. Our preferred interpretation is that the paleomagnetic data, in conjunction with other thermal data, support an entirely epigenetic origin in contrast to the pre-Variscan syn-genetic or syndiagenetic models favored by previous authors. We propose a topographically driven fluid flow, either from the south of Ireland or from closer to the deposit, which was modified by localized convection during the Variscan orogeny.

Paleomagnetism of the HYC Zn-Pb SEDEX Deposit, Australia: Evidence of An Epigenetic Origin

The HYC Zn-Pb-Ag deposit of the McArthur River Mines Ltd. is in the late Paleoproterozoic HYC Pyritic Shale Member of the 1639 ± 2 Ma (2σ) Barney Creek Formation in the McArthur Basin of the Northern Territory, Australia. This deposit is often cited as the archtypical syngenetic-syndiagenetic McArthur-type sedimentary exhalative (SEDEX) deposit. Paleomagnetic analysis of 273 specimens from 27 sites in seven of eight ore zones (20 sites) and their host rocks (7 sites) using mostly alternating field step demagnetization isolated a well-defined stable characteristic remanent magnetization (ChRM) in all sites but one tuff site. Thermal step demagnetization, saturation remanence, and artificial mill-concentrate specimen tests show that about two-thirds of the ChRM is carried by single- and pseudosingle-domain pyrrhotite and about one-third by single-domain magnetite, which are interpreted to occur as submicroscopic inclusions in ore-stage minerals including sphalerite and galena. Paleomagnetic conglomerate and fold tests are overwhelmingly negative, showing that the ChRM postdates lithification, burial, and fold deformation. The ChRM of the host rocks and ore gives a paleopole at 77.0° S, 167.0° E ( n = 20, α95 = 3.6°). This pole is coincident with that of the 1636 ± 4 Ma (2σ) Linott Formation, providing a direct date for the ChRM. Relative dating using the apparent polar wander path for the southern McArthur Basin dates the ChRM at 1637 ± 1 Ma (1σ). Thus both dating methods show that the ChRM is ~2–3 m.y. younger than the primary age of the host rocks. Using available vitrinite reflectance values, time-temperature calculations indicate that the ore lenses have been heated to 310° ± 30°C for ~105 yr, whereas nearby and remote host rocks have been subjected to much lower temperatures. The results of this study suggest that primary sea-floor sediments, perhaps with syngenetic pyrite, were buried to a depth of ~800 m over 2–3 m.y., with attendant lithification and synclinal folding during rift fault tectonics. The preferred model discussed in this paper is that ascending 310° ± 30°C hydrothermal fluids in the Emu fault zone subsequently penetrated the pyritic zones or other metal-bearing sediments by dissolution and/or replacement, precipitating sphalerite, galena, secondary pyrite, and other ore-stage minerals with pyrrhotite and magnetite inclusions to form an epigenetic ore deposit.

Paleomagnetism of the Fort Knox Stock, Alaska, and rotation of the Yukon–Tanana terrane after 92.5 Ma

The 92.5 Ma Fort Knox granodiorite stock, near the western end of the Fairbanks Belt in the Yukon–Tanana terrane (YTT) of central Alaska, hosts a world-class gold mine. The stock has been analysed paleomagnetically using thermal and alternating-field step demagnetization and isothermal remanence methods. This pluton retains a primary thermoremanent magnetization at 18 sites (232 specimens) that resides mainly in single-to pseudosingle-domain magnetite with a direction of D = 228.8°, I = 84.3° ( N = 18, k = 130, α 95 = 3.0°), giving a paleopole at 56.5°N, 197.1°E ( dp = 5.9°, dm = 5.8°). The pluton's host rock, the Fairbanks schist, does not retain a stable coherent remanence. Relative to the North American craton, the stock's paleoinclination indicates that the Fairbanks Belt has undergone nonsignificant poleward (northwesterly) translation of 25 ± 750 km only. Analysed in concert with the few available paleoinclinations available for the YTT in Yukon, the paleoinclination suggests further that the YTT has undergone only ∼ 250 to 450 km of dextral displacement along the Tintina fault in the past ∼ 100 Ma and, therefore, is parautocthonous since the mid-Cretaceous. The stock's paleodeclination records 121 ± 35° of counterclockwise rotation relative to the North American craton. Consideration of models published for Alaska's tectonic evolution suggests that this paleodeclination discordance is caused by rotations associated with the opening of the Canada Basin, with dextral displacement on the Tintina fault, and with development of the western Alaskan orocline. Thus the paleomagnetic results for the Fort Knox stock support a thin-skin tectonic model for the accretion of the YTT and Intermontane Belt terranes to the northern Cordillera.

New 42 Ma cratonic North American paleomagnetic pole from the Yukon underscores another Cordilleran paleomagnetism-geology conundrum

Eocene posttectonic plutons of the Beaver River alkalic complex in southeastern Yukon intruded Devonian–Mississippian and Triassic sandstones in the Foothills of the Canadian Cordillera. A paleomagnetic collection of 27 sites from three separate plutons produced 326 specimens that were analyzed using alternating field and thermal step demagnetization methods. The A component characteristic remanent magnetization (ChRM) resides in magnetite with normal polarity in the 42.6 ± 0.8 Ma Beaver River pluton, reversed polarity in the 42.1 ± 0.7 Ma Larson Creek East pluton, and both polarities in the 41.3 ± 0.4 Ma Larson Creek West pluton, corresponding with magnetic polarity chrons 20n, 19r, and the boundary between chron 19r and 18n, respectively. The ChRMs of the plutons are indistinguishable (2σ) with a mean for the 42.0 ± 0.5 Ma complex of D = 158.8°, I = –73.1° (N = 21 sites, α95 = 3.0°, k = 116.8). A positive paleomagnetic contact test shows the A component to be primary, and the poorly isolated B component suggests the host rocks for Larson Creek West are Early to Middle Devonian. The paleopole for the Beaver River complex at 79.2°N, 145.8°E (N = 21, dp = 4.8°, dm = 5.4°; Q = 7) is concordant with interpolated 42 Ma reference poles for the North American craton. In contrast, paleopoles from the accreted Intermontane and eastern Coast Belt terranes record clockwise rotations of 24° ± 10° (Eocene) and 13° ± 5° (Oligocene–Pliocene), indicating that the allochthonous Intermontane terranes have been progressively driven ~240 ± 120 km eastwards up and over pericratonic and cratonic North American lower crust by Pacific plate subduction since the mid-Eocene.

Paleomagnetism of the Navan Zn-Pb Deposit, Ireland

For the past quarter century the Navan mine has exploited Europe’s premier Zn-Pb deposit. Its genesis is controversial, in part because it has not proved amenable to radiometric dating. The ore is epigenetic and is hosted in Lower Carboniferous platform carbonates of the early Courceyan Navan Group (~351 ± 4 Ma) and of the late Chadian-early Arundian boulder conglomerate (~338 ± 1 Ma). Paleomagnetic analyses were done on 282 specimens from 26 sites in the mine using alternating-field and thermal step demagnetization plus isothermal remanence analyses. Paleomagnetic fold tests on Navan Group ore and Arundian carbonate strata and conglomerate tests on mineralized and unmineralized clasts were negative, meaning that the clasts and adjacent rocks were magnetized at some time after clast deposition and that the magnetization postdates folding. Further, a contact test using a Tertiary dike that cut ore was positive, meaning that the remanence predates intrusion of the dike. Both the ore and host rocks retain a stable characteristic remanence direction in single- to pseudosingle-domain magnetite at D = 196.0°, I = 6.0°, (N = 21, α 95 = 2.6°, k = 147) that defines a pole position of 25.6° W, 31.8° S ( δ p = 1.3°, δ m = 2.6°). Comparison of the pole to the rotated Laurentian and unrotated eastern Avalonian apparent polar wander paths yields a late Arundian to early Asbian age of 333 ± 4 Ma for the chemical remanent magnetization that is primary in the ore minerals and a secondary remagnetization in the host rocks. The result implies that the ore mineralization event followed deposition and tilting of the overlying Arundian-Holkerian Lucan Formation at Navan.

Eocene (51 Ma) end to northward translation of the Coast Plutonic Complex: paleomagnetism and K–Ar dating of the White Pass Dikes

The White Pass mafic dikes are located in the Coast Belt, inboard of the Coast Shear Zone–Denali Fault System, east of Skagway, Alaska, astride the Alaska–British Columbia border. The dikes intrude Eocene felsic plutons in the Coast Plutonic Complex. Three K–Ar analyses date the dikes at 51.9±0.8 Ma (biotite), 50.1±1.0 Ma and 45.0±0.8 Ma (both whole rock). Structural analysis on these vertical dikes indicates that the study area has not been tilted significantly since dike emplacement. Conventional alternating field and thermal step-demagnetization and saturation-remanence paleomagnetic methods were used to analyse 407 specimens from 27 dike and five host rock sites. Three contact tests show that the specimens retain a primary thermoremanent magnetization (TRM) carried in pseudosingle to multidomain magnetite, although some magnetization in a few dikes is carried by pyrrhotite. The dual-polarity TRM is mostly reversed and the 24 accepted dike sites give a pole position of 245.7° E, 76.3° N ( d p=4.2°, d m=4.4°), indicating a paleolatitude discordance of 8±4° south and a clockwise rotation of 40±9° with respect to the North American craton. These data, along with other paleomagnetic data on Eocene rocks, indicate that the allochthonous eastern Coast and Intermontane Belt terranes had halted their northward translation by about 51 Ma but continued to be rotated clockwise by an average of 24±10° thereafter, probably on underlying sole faults.

Paleomagnetism of the Society Cliffs dolostone and the age of the Nanisivik zinc deposits, Baffin Island, Canada

Dolostones of the ∼1200 Ma Society Cliffs Formation within the hydrothermal zone surrounding the Nanisivik zinc deposits retain a stable characteristic remanent magnetization (ChRM) on alternating field and thermal step demagnetization. Based on the thermal data and saturation isothermal remanence analyses, the ChRM resides in pseudosingle domain magnetite and hematite. A paleomagnetic fold test favours a post-folding ChRM, and a paleomagnetic contact test, using a Franklin gabbro dike, indicates that the ChRM predates ∼720 Ma. The pole position calculated from the ChRM direction is at 168.2°E, 42.8°N (δp=4.9°, δm=6.8°), giving an age of 1095 ± 10 Ma on the well-defined “Logan Loop” portion of the North American apparent polar wander path. This age is considered to date recrystallization of the dolostone host rocks in the halo around the hydrothermal sulfide deposits. No evidence is found for a postulated Cretaceous remagnetization event in the region.

The 1830 Ma Trans-Hudson hairpin from paleomagnetism of the Wapisu gneiss dome, Kisseynew Domain, Manitoba

The Wapisu gneiss dome is located in the northeastern part of the Kisseynew Domain of the Trans-Hudson Orogen (THO) in north-central Manitoba. The dome is circular, about 6 km in diameter, with steeply-dipping flanks. It is composed of upper amphibolite-facies gneisses derived from turbiditic sediments, with leucogranitic sill-like intrusions that were metamorphosed, starting at about 1830 Ma. Alternating field and thermal step demagnetization of 153 specimens from 17 sites around the perimeter of the dome isolated a characteristic remanent magnetization (ChRM) direction of D = 346.6°, I = 78.9° (α95 = 5.3°, k = 46, N = 17) that gives a pole of 119.5° W, 75.8° N (dp = 9.5°, dm = 10.0°). Unblocking temperatures and saturation isothermal remanence analyses show that the ChRM resides mostly in single to pseudosingle domain magnetite or titanomagnetite, with minor hematite commonly present and with minor pyrrhotite present in the leucogranites. The ChRM is found to be postfolding with >>99.9% confidence, indicating acquisition on cooling from peak metamorphism at ~1810 ± 10 Ma. This Wapisu gneiss dome paleopole is the first from the Kisseynew Domain and the first from the 1830 to 1770 Ma interval in the THO. It indicates an ~90° bend or hairpin in the apparent polar wander path for the THO juvenile terranes and Superior craton. It is speculated that the hairpin marks the collisional impact of the Archean Sask craton and (or) Hearne craton, which drove the Paleoproterozoic Kisseynew Domain into the Archean Superior craton.

Late Laramide dolomite recrystallization of the Husky Rainbow "A" hydrocarbon Devonian reservoir, northwestern Alberta, Canada: paleomagnetic and geochemical evidence

The Rainbow Field is in reefal carbonates of the Middle Devonian Keg River Formation in the Western Canada Sedimentary Basin. Alternating field and thermal step demagnetization was done on specimens from unoriented core from a vertical, an inclined, and a horizontal well core in the dolomite reservoir. Although they had no viscous remanent magnetization component to use for orientation, most specimens had a well-defined characteristic remanent magnetization (ChRM) that resides in single to pseudosingle domain magnetite. By rotating the mean ChRM direction around its core axis, a small circle can be generated for each core and the small circles intersect in the true ChRM direction of D = 168°, I = -73.5o (α95 = 5.8°, k = 32.2). Its paleopole of 164°E, 83°N (A95 = 10°) defines a Tertiary age with one sigma limits of Middle Eocene to Middle Miocene age. Petrographic examination defines four generations of dolomite. Matrix dolomite has 60-100 µm diameter crystals that were later recrystallized to 200-400 µm. Dolomite cements are represented by vug-filling coarse dolomite (100-200 µm) and saddle dolomite (1000 µm). All four generations of dolomite give similar δ18O values of -10.7 to -16.5‰ (Peedee Belemnite, PDB), δ13C values of +0.7 to +3.2‰ (PDB), and Sr isotopic ratios of 0.70826 to 0.70846 that do not match the expected Middle Devonian carbonate or seawater values. We interpret these data to indicate that mixed pre-Laramide basinal fluids, heated by burial during the Laramide Orogeny, were present during late Laramide time when the dolomites were recrystallized and (or) precipitated prior to petroleum migration and accumulation in the Rainbow "A" reservoir. Thus the combined use of paleomagnetism, geochemistry, and petrography has been proven to be a useful technique to date and identify dolomitization events and pathways for the migration of hydrocarbons.

Paleomagnetism of the Deschambault pegmatites: Stillstand and hairpin at the end of the Paleoproterozoic Trans-Hudson Orogeny, Canada

The 1766 ± 5 Ma Deschambault pegmatites are anorogenic intrusions emplaced into the Glennie domain at the end of the Trans-Hudson Orogeny (THO) in north-central Saskatchewan. They are composed mainly of orthoclase and quartz with minor biotite, muscovite, tourmaline and beryl. A coherent primary characteristic remanence is retained in all 18 sites (170 specimens) that resides in magnetite, hematite and minor pyrrhotite, giving a direction of Dec. = 28.3°, Inc. = 82.1°, α95 = 4.0°, and k = 77.5 based on alternating field and thermal step demagnetization and saturation remanence analyses. The pegmatites' pole position, along with recently published ∼1810 ± 10 Ma and ∼1795 ± 15 Ma poles for the THO, define a stillstand and hairpin in the apparent polar wander path for the THO that marks continent-continent collision of the Archean Superior, Sask and Hearne (?) cratons.

Laramide orogenic fluid flow into the Western Canada sedimentary basin; evidence from paleomagnetic dating of the Kicking Horse mississippi valley-type ore deposit

The Kicking Horse and Monarch Mississippi Valley-type zinc-lead ore deposits are in the southern Rocky Mountains of the foreland belt in the Canadian Cordillera of British Columbia. These epigenetic sphalerite-galena deposits are hosted by a massive sparry dolomite sheet in Middle Cambrian dolostones of the Cathedral Formation on the southwestern margin of the western Canada sedimentary basin. These strata were folded, thrust faulted, and uplifted during the Laramide orogeny. Paleomagnetic analysis was done on 316 specimens from 32 mineralized and host-rock sites, using alternating field and thermal step demagnetization and isothermal remanence methods. The results show that the characteristic remanent magnetization is carried by both pyrrhotite and magnetite. The increase in characteristic remanent magnetization intensity near Mississippi Valley-type mineralization ties the magnetization to the mineralization event. Fold tests on a small scale ( approximately 10 m) and a large scale ( approximately 10 km) show that the characteristic remanent magnetization postdates the main Laramide deformation event. Comparison of the characteristic remanent magnetization directions shows that the host rocks were remagnetized when the mineralization was emplaced with a primary characteristic remanent magnetization. The Kicking Horse mineralization and adjacent host rocks form the bulk of the collection and give a Late Cretaceous pole position of 64.0 degrees N, 152.8 degrees W (semi-axes of oval of 95% confidence: delta p = 5.4 degrees , delta m = 5.8 degrees ; number of sites (specimens) = 19 (180)). The age of this pole coincides with the Cretaceous normal superchron and conforms to the finding of all normal characteristic remanent magnetization directions except for one sample with an antipodal direction. No site retains a characteristic remanent magnetization that predates the Laramide orogeny. The evidence for and against a pre-Laramide Mississippi Valley-type mineralizing and dolomitizing event in the western Canada sedimentary basin is reviewed, and it is concluded that the main fluid flow event in the basin occurred during the Late Cretaceous-Paleocene Laramide orogeny.

Palaeomagnetic dating of sub-Torridon Group weathering profiles, NW Scotland: verification of Neoproterozoic palaeosols

A palaeoplain that cuts across Lewisian Gneiss in the Cape Wrath area, northwest Scotland, is weathered to depths of 1–6 m beneath fluvial red conglomerates and sandstones of the Applecross Formation of the early Neoproterozoic Torridon Group. Weathered Lewisian rocks are enriched in K 2 O and Fe 2 O 3 , and contain hematite as an alteration product of biotite and magnetite. Palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration. Thermal step demagnetization of 87 oriented core specimens of biotite-gneiss and amphibolite spanning four profiles at Poll a’ Mhurain and Sheigra reveal a stable, high-temperature component with a mean direction of remanence of D= 149.1°, I =57.2° (α 95 = 5.0°) (corrected for dip of the overlying Applecross strata) that gives a pole at 9.8°S, 199.2°E (dp = 5.3°, dm=7.3°). This direction differs significantly from that for little-weathered Lewisian at those two localities and also from directions for the late Mesoproterozoic Stoer Group. The remanence direction for the weathered rocks is, however, near the dip-corrected high-temperature remanence direction (D= 122.3°, I =50.5°, α 95 =10.8°) obtained by us from the thermal step demagnetization of 39 sandstone specimens of the Applecross Formation in the Cape Wrath area. Furthermore, the direction for the weathered rocks agrees closely with other dip-corrected results for the Torridon Group, such as the detrital remanent magnetization (DRM) mean direction (D= 148.3°, I =59.9°, α 95 =13.8°) for the Applecross Formation on Rum and that for the main outcrop of the Aultbea Formation ( D =139.7°, I =52.6°, α 95 = 14.5°). The concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the Cape Wrath area acquired their remanence penecontemporaneously with deposition of the Applecross and Aultbea formations. The palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of Applecross age ( c. 980 Ma) that formed at a palaeolatitude of 38° and constitute the oldest known palaeosols in the British Isles—views that the weathering is substantially older or younger than Applecross deposition cannot be sustained. Our findings carry implications for Applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early Neoproterozoic time.

A Tertiary age from paleomagnetism for mississippi valley-type zinc-lead mineralization in Upper Silesia, Poland

Ore and host rocks of the Middle Triassic Muschelkalk carbonates were sampled at 35 sites in three operating mines and several quarries in the Cracow-Silesia Mississippi Valley-type district of southern Poland. Palcomagnetic analysis was done using alternating field and thermal step demagnetization and saturation isothermal remanence tests. Specimens from three limestone and two early dolomite sites retain a dual-polarity A remanent magnetization component (D = 43 degrees , I = 51 degrees , k = 40, alpha 95 = 12 degrees ), which yields a Middle to Upper Triassic pole. A resides in single domain to pseudosingle domain magnetite in limestone and is probably primary, and in single domain-pseudosingle domain magnetite and hematite in early dolomite and is probably diagenetic. Two limestone sites carry a Tertiary remagnetization and most early dolomite sites carry a partial Tertiary remagnetization, or hybrid remanence, identified by palcomagnetic fold and breccia tests. Late dolomite and Mississippi Valley-type mineralization from 14 sites retain a dual-polarity C remanence component (D = 3 degrees , I = 66 degrees , k = 34, alpha 95 = 7 degrees ), which defines a Tertiary pole position. Conglomerate and breccia tests confirm that the clasts in late dolomite were remagnetized. The Tertiary age for the Mississippi Valley-type ore deposits is within the post-mid-Jurassic to pre-Miocene window permitted by the geologic evidence for mineralization. The age also supports gravity-driven fluid flow models for ore genesis that are associated with the Alpine orogeny. The dual polarities and elongate distribution of site mean directions suggest a 1 to 20 m.y. duration for the late dolomitization and mineralization event.

Paleomagnetism and the Late Jurassic genesis of the Illinois-Kentucky fluorspar deposits

The Illinois-Kentucky fluorspar district has yielded most of the United States fluorite production. The fluorspar ore occurs as epigenetic replacement deposits in Mississippian limestone and as vein deposits. Paleomagnetic analysis was done on 324 specimens from 33 sites in fluorspar ore and in host limestones and cap-rock sandstones both adjacent to and remote from ore using alternating field and thermal step demagnetization. Saturation isothermal remanence methods were used to identify magnetization carriers in typical specimens and mineral crystals. An A characteristic remanent magnetization (RM) component was found outside the district in stratigraphically equivalent limestones that record a Pennsylvanian Kiaman remagnetization, and a C characteristic RM was found in surface weathered specimens that records the Recent earth9s magnetic field. At 28 sites in ore and nearby host rocks a B characteristic RM was found that gives a Late Jurassic pole position of 152.4 degrees E, 78.0 degrees N (dp = 3.0 degrees , dm = 4.4 degrees ). Late Jurassic genesis for the fluorspar fits within the permissible geologic window from Early Permian to Late Cretaceous and agrees with fission track and strontium isotope dating on the fluorites but does not agree with a recent Sm/Nd date. It is suggested that a Late Jurassic hydrothermal here termed the Rosiclare event, formed the fluorspar deposits, probably remagnetized Kiaman-remagnetized host rocks, probably was controlled geographically by faults of the Reelfoot-Wabash rift system, and probably coincided with the substantial uplift in the region that formed the Pascola arch.

Are post-Middle Miocene tilts present in the southern Coast Belt of British Columbia? A case study of present Earth's magnetic field overprints

Tectonic tilt is a possible explanation for the discordant Cretaceous paleomagnetic poles obtained from intrusions in the Coast Belt of the Canadian Cordillera. If tilting of these intrusions has taken place, post-Cretaceous intrusions may have been similarly tilted. Paleomagnetism of the Middle Miocene Rogers Creek pluton (16 Ma) in the central-southern Coast Belt was utilized to test the tilt hypothesis. Well-grouped stable primary magnetizations have been isolated in 11 sites with both normal and reverse polarities. The mean remanence component, directed at D = 000.7°, I = 67.9° (α95 = 3.9°, paleolatitude 50.9 ± 9°N, paleopole 88.3°N, 257.5°E, A95 = 5.7°), was likely acquired upon emplacement at 16 Ma. This direction agrees with the Middle Miocene reference direction (D = 357°, I = 68°) calculated from the updated Miocene (10–20 Ma) cratonic reference pole for North America. This concordant result, when combined with concordant data from other mid-Tertiary intrusions to the south, defines a southern Coast Belt domain that shows no evidence of post-Middle Miocene tectonic tilt. This result does not preclude tilting of the mid-Cretaceous Spuzzum batholith within this domain; however, if tilting took place it must have occurred prior to the Middle Miocene. The concordant data from the Rogers Creek pluton provide a Middle Miocene paleohorizontal datum for future work investigating crustal tilts in the Coast Belt.A comparison of Tertiary Coast Belt paleopoles demonstrates that they are systematically rotated clockwise and are near-sided relative to the cratonic reference poles. This implies that the Coast Belt has been displaced 300 km southward and rotated 8° clockwise, or regionally tilted downward to the northwest, or some combination of these two end members, since the Late Miocene. These scenarios appear untenable. Evidence is given that the directional bias of the paleopoles is caused by unremoved present Earth's magnetic field (PEMF) overprint magnetizations, which comprise 10–25% of the total remanence in most cases. The implication is that demagnetization and analysis techniques used in previous Tertiary Coast Belt studies, to varying degrees, may have insufficiently removed the PEMF overprints. The case is made for the importance of thorough alternating field and thermal step demagnetization, of all specimens, in order to maximize magnetic component data from which the principal magnetic components can be best isolated and PEMF components removed.

Paleomagnetism of the Keweenawan Chipman Lake and Seabrook Lake carbonatite complexes, Ontario

The Chipman Lake complex crops out as a series of carbonatite and related alkalic mafic dikes in the Wabigoon Subprovince of the Superior Province, whereas the Seabrook Lake complex crops out as an alkalic syenite – carbonatite stock in the Abitibi Subprovince. Paleomagnetic analysis was done on specimens from 23 and 19 sites located in and around the Chipman Lake and Seabrook Lake complexes, respectively, using detailed alternating-field and thermal step demagnetization and isothermal remanent magnetization tests. Contact tests with adjacent Archean host rocks show that both complexes retain a primary characteristic remanence (ChRM). The Chipman Lake's ChRM is retained in 11 dikes with normal polarity and one dike with reversed polarity and at one site with normal polarity and one site with reversed polarity from the fenite alteration zone. Its ChRM gives a pole position at 186°E, 38°N (dp = 7°, dm = 11°), which corresponds to a Keweenawan age of 1098 ± 10 Ma, suggesting that younger K–Ar amphibole ages do not date emplacement. The ChRM of the host rock, the Chipman Lake diorite stock, gives a pole at 49°E, 51°N (dp = 8°, dm = 13°), showing that it is not part of the Keweenawan complex but may be a 2.45 Ga Matachewan intrusive. The Seabrook Lake complex's ChRM is found at six normal polarity sites from within the complex and at four normal and three reversed polarity sites from within the fenitized Archean granite and Matachewan diabase of the contact aureole. It gives a pole position at 180°E, 46°N (dp = 11°, dm = 17°), which corresponds to a Keweenawan age of 1103 ± 10 Ma, agreeing with K/Ar biotite ages. The paleomagnetic data indicate that no significant motion on the Kapuskasing Structural Zone occurred after emplacement of the complexes excluding minor vertical uplift of less than about 4 km, and that there were multiple polarity transitions of a symmetric Earth's magnetic field during Keweenawan time.

Late Devonian paleomagnetic age for the Polaris Mississippi Valley-type Zn–Pb deposit, Canadian Arctic Archipelago

The Polaris Mississippi Valley-type (MVT) zinc–lead deposit is located in the Middle to Upper Ordovician Thumb Mountain Formation of the Cornwallis Fold Belt in the Canadian Arctic Archipelago. Paleomagnetic analysis was done on samples from 29 sites in the sphalerite–galena ore, the surrounding dolomitic envelope, and the adjacent host-rock limestones, using alternating-field and thermal step demagnetization, scanning electron microscopy with energy-dispersive analysis, and saturation isothermal remanente analyses. Excluding the modern viscous remanence component, the ore, dolomite alteration, and host limestones carry only a prefolding A remanent magnetization component that resides in single- and pseudosingle-domain magnetite, both as discrete grains and as inclusions in the MVT ore minerals. All evidence indicates that the mineralization and magnetization events were coeval. The mean magnetization direction of D = 145.8°, I = −31.1° (α95 = 5.9°, k = 26.2, N = 24) after tilt and a minor plunge correction gives a Late Devonian pole position of 28.6°N, 121.2°E (dp = 3.7°, dm = 6.6°, N = 24). The results indicate that the ore deposit was formed during the onset of the Ellesmerian Orogeny and tilted later in the orogeny.

Paleomagnetism of the Proterozoic Wathaman batholith and the suturing of the Trans-Hudson orogen in Saskatchewan

The Wathaman (Wathaman–Chipewyan) batholith is an 1854 ± 11 Ma, northeast-trending, homogenous, felsic pluton that is over 900 km long. It is thought to be a magmatic arc, with the Archean Hearne craton on its northwestern side as the hinterland and the remains of the Early Proterozoic Manikewan oceanic crust of the Trans-Hudson orogen on its southeastern side. Alternating-field and thermal step demagnetization methods isolate an A remanence component with a mean direction of D = 134.6°, I = 54.1° (α95 = 3.5°, k = 94, N = 19). Isothermal remanent magnetization tests confirm that this A magnetization component is preserved in pseudosingle to multidomain magnetite and in hematite. Contact tests with intruded older rocks of the Peter Lake domain, with younger crosscutting mafic dikes and with younger crosscutting shear zones, indicate that A is a primary remanence. Its pole position of 67°W, 9°N (dp = 3°, dm = 5°) confirms that it was formed along the margin of the Slave–Rae–Hearne craton, supporting tectonic models that it records a suture zone. It also indicates that the LaRonge – Lynn Lake domain, Flin Flon domain, and Superior Province were translated relatively northwestward into the suture by at least 11 ± 11°, 27 ± 12°, and 49° ± 16°, respectively, as the Manikewan Ocean closed.