William P. Meurer

The effect of remanence anisotropy on paleointensity estimates: a case study from the Archean Stillwater Complex

Paleomagnetism of Archean rocks potentially provides information about the early development of the Earth and of the geodynamo. Precambrian layered intrusive rocks are good candidates for paleomagnetic studies: such complexes are commonly relatively unaltered and may contain some single-domain magnetite ‘armored’ by silicate mineral grains. However, layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy. Magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected. Here we examine the magnetic anisotropy of an anorthosite sample with a well-developed magmatic foliation. The effect of the sample’s remanence fabric on paleointensity determinations is significant: paleointensities estimated by the method of Thellier and Thellier range from 17 to 55 μT for specimens magnetized in a field of 25 μT. We describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the Stillwater Complex (MT, USA) of ∼32 μT (adjusted for the effects of slow cooling).

Evidence for the protracted construction of slow-spread oceanic crust by small magmatic injections

Abstract Gabbroic cumulates drilled south of the Kane Transform Fault on the slow-spread Mid-Atlantic Ridge preserve up to three discrete magnetization components. Here we use absolute age constraints derived from the paleomagnetic data to develop a model for the magmatic construction of this section of the lower oceanic crust. By comparing the paleomagnetic data with mineral compositions, and based on thermal models of local reheating, we infer that magmas that began crystallizing in the upper mantle intruded into the lower oceanic crust and formed meter-scale sills. Some of these magmas were crystal-laden and the subsequent expulsion of interstitial liquid from them produced ‘cumulus’ sills. These small-scale magmatic injections took place over at least 210 000 years and at distances of ∼3 km from the ridge axis and may have formed much of the lower crust. This model explains many of the complexities described in this area and can be used to help understand the general formation of oceanic crust at slow-spread ridges.

The Ulvö Gabbro Complex of the 1.27-1.25 Ga Central Scandinavian Dolerite Group (CSDG): Intrusive age, magmatic setting and metamorphic history

Abstract The Ulvö Gabbro Complex (UGC) belongs to the 1.27-1.25 Ga Central Scandinavian Dolerite Group (CSDG), which manifests a major event of intra-cratonic mafic magmatism possibly related to break-up between Laurentia and Baltica. Lopolithic, layered intrusions of the UGC outcrop in east central Sweden and west central Finland. Fresh baddeleyite grains in a pegmatoidal gabbro were dated by the TIMS U-Pb method. The age of 1256.2±1.1 Ma is interpreted to date crystallisation of the UGC, and is consistent with the age of other mafic intrusions of the CSDG in Västerbotten and south-western Finland. Previously reported K-Ar biotite data confirm that the UGC was not heated above the blocking temperature for Ar diffusion in biotite (∼250[ddot]C) after ∼1.2 Ga. This suggests that UGC was virtually unaffected by later geological events and that magmatic textures and mineralogy have not been modified after solidification.

The Stillwater Complex: Integrating Zircon Geochronological and Geochemical Constraints on the Age, Emplacement History and Crystallization of a Large, Open-System Layered Intrusion

The Neoarchean Stillwater Complex, one of the world’s largest known layered intrusions and host to a rich platinum-group element deposit known as the J-M Reef, represents one of the cornerstones for the study of magmatic processes in the Earth’s crust. A complete framework for crystallization of the Stillwater Complex is presented based on the trace element geochemistry of zircon and comprehensive U–Pb zircon–baddeleyite–titanite–rutile geochronology of 22 samples through the magmatic stratigraphy. Trace element concentrations and ratios in zircon are highly variable and support crystallization of zircon from fractionated interstitial melt at near-solidus temperatures in the ultramafic and mafic cumulates (Ti-in-zircon thermometry1⁄4 980–720 C). U–Pb geochronological results indicate that the Stillwater Complex crystallized over a 3 million-year interval from 2712 Ma (Basal series) to 2709 Ma (Banded series); late-stage granophyres and at least one phase of post-emplacement mafic dikes also crystallized at 2709 Ma. The dates reveal that the intrusion was not constructed in a strictly sequential stratigraphic order from the base (oldest) to the top (youngest) such that the cumulate succession in the complex does not follow the stratigraphic law of superposition. Two distinct age groups are recognized in the Ultramafic series. The lowermost Peridotite zone, up to and including the G chromitite, crystallized at 2710 Ma from magmas emplaced below the overlying uppermost Peridotite and Bronzitite zones that crystallized earlier at 2711 Ma. Based on the age and locally discordant nature of the J-M Reef, the base of this sequence likely represents an intrusion-wide magmatic unconformity that formed during the onset of renewed and voluminous magmatism at 2709 Ma. The thick anorthosite units in the Middle Banded series are older (2710 Ma) than the rest of the Banded series, a feature consistent with a flotation cumulate or ‘rockberg’ model. The anorthosites are related to crystallization of mafic and ultramafic rocks now preserved in the Ultramafic series and in the lower part of the Lower Banded series below the J-M Reef. The Stillwater Complex was constructed by repeated injections of magma that crystallized to produce a stack of amalgamated sills, some out-of-sequence, consequently it does not constitute the crystallized products of a progressively filled and cooled magma chamber. This calls into question current concepts regarding the intrusive and crystallization histories of major open-system layered intrusions and challenges us to rethink our understanding of the timescales of magma processes and emplacement in these large and petrologically significant and remarkable complexes. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 153 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 1, 153–190 doi: 10.1093/petrology/egy024 Advance Access Publication Date: 28 February 2018