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Paleomagnetic measurements indicate that a core dynamo probably existed on the Moon 4.2 billion years ago. However, the subsequent history of the lunar core dynamo is unknown. Here we report paleomagnetic, petrologic, and (40)Ar/(39)Ar thermochronometry measurements on the 3.7-billion-year-old mare basalt sample 10020. This sample contains a high-coercivity magnetization acquired in a stable field of at least ~12 microteslas. These data extend the known lifetime of the lunar dynamo by 500 million years. Such a long-lived lunar dynamo probably required a power source other than thermochemical convection from secular cooling of the lunar interior. The inferred strong intensity of the lunar paleofield presents a challenge to current dynamo theory.

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Studies of plutons indicate that they are the result of a complex interplay of magmatic processes occurring during magma generation, ascent, and emplacement. A critical tool for deciphering these processes is high-precision geochronology, which can help determine the timing and rates of magmatism in the crust. We conducted a field and U-Pb geochronological study of the Cretaceous Black Peak intrusive complex in the North Cascades of Washington State to investigate magmatism at a detailed scale and to refine estimates of plutonic construction rates. High-precision chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb geochronology was carried out on 31 samples from five mapped intrusive phases. Field relations in the Black Peak intrusive complex show intrusive contacts that vary from sharp to gradational. Whole-rock Sm/Nd, zircon oxygen isotopes, and zircon trace elements were obtained on subsets of representative samples. The U-Pb geochronology from the Black Peak intrusive complex documents batholith intrusion over 4.5 m.y. and suggests that magmatism was semicontinuous for a minimum of 3.5 m.y. Individual samples display age dispersion in single-zircon dates that ranges from ~10 5 yr to several 10 6 yr, with a general increase in the age range for younger samples. Whole-rock ε Nd and zircon δ 18 O for all Black Peak intrusive complex samples indicate that magmas were derived from mantle and crustal sources and that all magmas were isotopically homogenized prior to zircon saturation. Ti-in-zircon temperatures from zircon cores are generally above calculated zircon saturation temperatures, which suggests that most Black Peak intrusive complex magmas were zircon undersaturated in the melt source region. A range of thicknesses was considered, and a thickness of ~10 km for the Black Peak intrusive complex gives an average intrusion rate of ~1.1 ×10-3 km 3 /yr, which is high enough to sustain a magma reservoir in the shallow crust. The field evidence and long overall duration of intrusion are incompatible with the entire Black Peak intrusive complex being molten at any one time, but the larger, more compositionally homogeneous domains in the Black Peak intrusive complex are likely the solidified remnants of mushy magma bodies with ~10 5 yr durations. These data suggest that the Black Peak intrusive complex may have remained "mushy" for long periods of time (10 5 yr) and may indicate that the spread in dates within individual samples is best interpreted as either antecrystic recycling and/or protracted autocrystic growth.

The magmatic arc represented by the crystalline core of the North Cascades (Cascades core) reached a crustal thickness of >55 km in the mid-Cretaceous. Eocene collapse of the arc was marked by migmatization, magmatism, and exhumation of deep-crustal (9-12 kb) rocks at the same time as subsidence and rapid deposition in nearby transtensional nonmarine basins. The largest region of deeply exhumed rocks, the migmatitic Skagit Gneiss Complex, consists primarily of leucocratic, biotite tonalite orthogneiss intruded between ca. 76-59 Ma and 50-45 Ma. Well-layered biotite gneiss is also widespread. U-Pb (isotope dilution-thermal ionization mass spectrometry) dating of zircon and monazite from trondhjemitic leucosome and biotite gneiss mesosome indicates that metamorphism and melt generation/crystallization occurred at least intermittently from ca. 71 to 47 Ma, and the youngest U-Pb dates overlap Ar/Ar (biotite, muscovite) dates, compatible with rapid cooling. Mesoscopic to map-scale, gently plunging, upright folds have hinge lines subparallel to orogen-parallel (NW-SE) lineations in the Skagit Gneiss Complex, and are as young as 48 Ma. Eocene top-to-northwest flow occurred in parts of the complex. The gently to moderately dipping foliation, subhorizontal lineation, and constrictional domains are compatible with ductile transtension linked to dextral-normal displacement on the Ross Lake fault system, the northeastern boundary of the Cascades core. On the south flank of the core, sediments were deposited in part at ca. 51 Ma in the Swauk basin and shortly afterward folded, and then intruded by 47 Ma Teanaway basaltic dikes. Extension taken up by these dikes ranges from ~10% to 43%. Extension directions from Teanaway and other Eocene dikes are arc-parallel to arc-oblique. The shallow-crustal extension direction is counterclockwise (mostly 10°-30°) to the ductile flow direction, implying decoupling of brittle and ductile crust; however, some coupling is supported by the temporal coincidence between basin formation and partial melting and ductile flow, and the upright folding of both the Skagit Gneiss Complex and Swauk basin. Arc-oblique to arc-parallel flow probably resulted in part from dextral shear along the plate margin, along-strike gradients in crustal thickness, and thermally controlled rheology.