High-Precision U-Pb Geochronology Links Magmatism in the Southwestern Laurentia Large Igneous Province and Midcontinent Rift

The Southwestern Laurentia large igneous province (SWLLIP) comprises voluminous, widespread ca 1.1 Ga magmatism in the southwestern United States and northern Mexico. The timing and tempo of SWLLIP magmatism and its relationship to other late Mesoproterozoic igneous provinces have been unclear due to difficulties in dating mafic rocks at high precision. New precise U-Pb zircon dates for comagmatic felsic segregations within mafic rocks reveal distinct magmatic episodes at ca. 1098 Ma (represented by massive sills in Death Valley, California, the Grand Canyon, and central Arizona) and ca. 1083 Ma (represented by the Cardenas Basalts in the Grand Canyon and a sill in the Dead Mountains, California). The ca. 1098 Ma magmatic pulse was short-lived, lasting 0.25^+0.67_-0.24 m.y., and voluminous and widespread, evidenced by the ≥100 m sills in Death Valley, the Grand Canyon, and central Arizona, consistent with decompression melting of an upwelling mantle plume. The ca. 1083 Ma magmatism may have been generated by a secondary plume pulse or post-plume lithosphere extension. The ca. 1098 Ma pulse of magmatism in southwestern Laurentia occurred ≁2 m.y. prior to an anomalous renewal of voluminous melt generation in the Midcontinent Rift of central Laurentia that is recorded by the ca. 1096 Ma Duluth Complex layered mafic intrusions. Rates of lateral plume spread predicted by mantle plume lubrication theory support a model where a plume derived from the deep mantle impinged near southwestern Laurentia, then spread to thinned Midcontinent Rift lithosphere over ~2 m.y. to elevate mantle temperatures and generate melt. This geodynamic hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for that anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift

The effects of 10 to >160 GPa shock on the magnetic properties of basalt and diabase

© 2016. American Geophysical Union. All Rights Reserved.Hypervelocity impacts within the solar system affect both the magnetic remanence and bulk magnetic properties of planetary materials. Spherical shock experiments are a novel way to simulate shock events that enable materials to reach high shock pressures with a variable pressure profile across a single sample (ranging between ∼10 and >160 GPa). Here we present spherical shock experiments on basaltic lava flow and diabase dike samples from the Osler Volcanic Group whose ferromagnetic mineralogy is dominated by pseudo-single-domain (titano)magnetite. Our experiments reveal shock-induced changes in rock magnetic properties including a significant increase in remanent coercivity. Electron and magnetic force microscopy support the interpretation that this coercivity increase is the result of grain fracturing and associated domain wall pinning in multidomain grains. We introduce a method to discriminate between mechanical and thermal effects of shock on magnetic properties. Our approach involves conducting vacuum-heating experiments on untreated specimens and comparing the hysteresis properties of heated and shocked specimens. First-order reversal curve (FORC) experiments on untreated, heated, and shocked specimens demonstrate that shock and heating effects are fundamentally different for these samples: shock has a magnetic hardening effect that does not alter the intrinsic shape of FORC distributions, while heating alters the magnetic mineralogy as evident from significant changes in the shape of FORC contours. These experiments contextualize paleomagnetic and rock magnetic data of naturally shocked materials from terrestrial and extraterrestrial impact craters

Magmatic activity and plate motion during the latent stage of Midcontinent Rift development

The Keweenawan Midcontinent Rift of North America records significant continental rifting between ca. 1110 and 1085 Ma, and preserves the most detailed paleomagnetic record of plate motion of any continent in Precambrian time. U/Pb dates from extrusive and intrusive rocks of the western Lake Superior Basin suggest a latent stage of reduced magmatic activity from ca. 1106 to 1100 Ma that places constraints on the dynamics of rift development and the record of plate motion. However, it has remained unclear whether this stage is a feature of the entire >2500-km-long rift. The succession of picritic and basaltic lava flows at Mamainse Point in the eastern Lake Superior Basin may be the most continuous and best exposed record of rift-related volcanism and magnetic reversals, but its age and duration relative to the latent stage has been uncertain due to a lack of radioisotopic dates. We present a weighted mean [superscript 206]Pb/[superscript 238]U date of 1100.36 ± 0.25 Ma on zircon crystals isolated from a newly discovered tuff within the upper reversed polarity portion of the stratigraphy below the Great Conglomerate. This date indicates that eruptive activity at Mamainse Point continued during the interval of diminished magmatic activity in the western Lake Superior Basin. This result strengthens the chronostratigraphic framework of rift development while explaining the preservation of additional geomagnetic reversals at Mamainse Point and the record of progressively decreasing paleomagnetic inclination that is indicative of rapid paleogeographic change.University of Minnesota. Precambrian Research Center (Grant

Magmatic activity and plate motion during the latent stage of Midcontinent Rift development

The Keweenawan Midcontinent Rift of North America records significant continental rifting between ca. 1110 and 1085 Ma, and preserves the most detailed paleomagnetic record of plate motion of any continent in Precambrian time. U/Pb dates from extrusive and intrusive rocks of the western Lake Superior Basin suggest a latent stage of reduced magmatic activity from ca. 1106 to 1100 Ma that places constraints on the dynamics of rift development and the record of plate motion. However, it has remained unclear whether this stage is a feature of the entire >2500-km-long rift. The succession of picritic and basaltic lava flows at Mamainse Point in the eastern Lake Superior Basin may be the most continuous and best exposed record of rift-related volcanism and magnetic reversals, but its age and duration relative to the latent stage has been uncertain due to a lack of radioisotopic dates. We present a weighted mean 206Pb/238U date of 1100.36 ± 0.25 Ma on zircon crystals isolated from a newly discovered tuff within the upper reversed polarity portion of the stratigraphy below the Great Conglomerate. This date indicates that eruptive activity at Mamainse Point continued during the interval of diminished magmatic activity in the western Lake Superior Basin. This result strengthens the chronostratigraphic framework of rift development while explaining the preservation of additional geomagnetic reversals at Mamainse Point and the record of progressively decreasing paleomagnetic inclination that is indicative of rapid paleogeographic change

Paleomagnetism of the Teel Basalts from the Zavkhan Terrane: Implications for Paleozoic Paleogeography in Mongolia and the Growth of Continental Crust

A narrow extensional basin on the Zavkhan terrane of Mongolia exposes a \u3e1.8-km-thick succession of basalt flows within the Teel Formation, along with rhyolites and interflow sediments. We present new U-Pb zircon ages of 446.03 ± 0.21 Ma (chemical abrasion–isotope dilution–thermal ionization mass spectrometry) on a rhyolite in the Teel Formation and 286 ± 5 Ma (laser ablation–inductively coupled plasma–mass spectrometry) on a nearby granitic intrusion (Tonkhil Complex). New paleomagnetic data yield a magnetite remanence that is likely primary, acquired during cooling of flows. The mean direction is statistically improved after tilt corrections; however, the tilt test significance is limited given the low variation in tilt between flows. We interpret a second remanence, held by hematite, as an overprint that was likely acquired later in the Paleozoic Era. The tilt-corrected magnetite direction implies a paleolatitude of ∼20°, while the hematite overprint is equatorial in both geographic and tilt-corrected coordinates. The ca. 446 Ma Teel remanence is consistent with an Ordovician paleogeographic position near Siberia; however, the hematite direction requires subsequent drift to the equator, indicating that these Mongolian terranes were not continuously connected to Siberia, which moved away from the tropics during the Paleozoic Era. This result is consistent with biogeographic constraints and a previously proposed model wherein Amuria traveled with North China during the Permian Period and collided with Siberia during the Jurassic to Triassic closure of the Mongol-Okhotsk Ocean. In this model, continental growth occurred through the collision and oroclinal buckling of a ribbon continent rather than long-lived accretion on the margin of a major craton

PmagPy: Software package for paleomagnetic data analysis and a bridge to the Magnetics Information Consortium (MagIC) Database

The Magnetics Information Consortium (MagIC) database provides an archive with a flexible data model for paleomagnetic and rock magnetic data. The PmagPy software package is a cross-platform and open-source set of tools written in Python for the analysis of paleomagnetic data that serves as one interface to MagIC, accommodating various levels of user expertise. PmagPy facilitates thorough documentation of sampling, measurements, data sets, visualization, and interpretation of paleomagnetic and rock magnetic experimental data. Although not the only route into the MagIC database, PmagPy makes preparation of newly published data sets for contribution to MagIC as a byproduct of normal data analysis and allows manipulation as well as reanalysis of data sets downloaded from MagIC with a single software package. The graphical user interface (GUI), Pmag GUI enables use of much of PmagPy's functionality, but the full capabilities of PmagPy extend well beyond that. Over 400 programs and functions can be called from the command line interface mode, or from within the interactive Jupyter notebooks. Use of PmagPy within a notebook allows for documentation of the workflow from the laboratory to the production of each published figure or data table, making research results fully reproducible. The PmagPy design and its development using GitHub accommodates extensions to its capabilities through development of new tools by the user community. Here we describe the PmagPy software package and illustrate the power of data discovery and reuse through a reanalysis of published paleointensity data which illustrates how the effectiveness of selection criteria can be tested

Constraints on Neoproterozoic paleogeography and Paleozoic orogenesis from paleomagnetic records of the Bitter Springs Formation, Amadeus Basin, central Australia

The supercontinent Rodinia is hypothesized to have been assembled and positioned in tropical latitudes by the early Neoproterozoic Era. Paleomagnetic data from limestones of Svalbard and basaltic dikes of South China have been interpreted to record rapid changes in paleogeography driven by true polar wander that may have rotated the supercontinent in association with the ~800 Ma Bitter Springs carbon isotope event. To further constrain early Neoproterozoic paleogeography and to test proposed rapid rotations, we have developed sequence- and chemostratigraphically constrained paleomagnetic data from the Bitter Springs Formation of the Amadeus Basin of central Australia. A new paleomagnetic pole for the post–Bitter Springs stage ~770 Ma Johnny’s Creek Member (Bitter Springs Formation) provides a positive test for a long-lived history of Australia and Laurentia in a single supercontinent as its similar position to late Mesoproterozoic north Australia poles reproduces the closure of the Laurentian “Grenville Loop.” This new pole also provides support for the hypothesis that there was significant rotation between north and south+west Australia at the end of the Neoproterozoic as this rotation brings the south+west Australia ~755 Ma Mundine Well pole into much closer proximity to the north Australia Johnny’s Creek pole. Syn–Bitter Springs stage carbonates of the Love’s Creek Member of the formation contain a well-behaved remanence held by magnetite. The direction of this remanent magnetization falls on the Cambrian portion of Australia’s apparent polar wander path suggesting that the magnetite may have formed authigenically at that time. If primary, the Love’s Creek direction is consistent with the true polar wander hypothesis for the Bitter Springs stage, is internally consistent with the relative sea level changes inferred from the formation, and can constrain Australia to a SouthWest North America East AnTarctica (SWEAT) fit. A remanence held by pyrrhotite in carbonates of the Bitter Springs Formation corresponds to the apparent polar wander path of Australia at ~350 Ma. This component can be used to constrain the history of the Devonian-Carboniferous Alice Springs Orogeny as it demonstrates that regional folding of basinal sediments occurred prior to ~350 Ma, but that the latest stages of tectonism in the hinterland drove fluids through the sediments that altered redox conditions to favor pyrrhotite precipitation