Abstract Spatially resolved argon isotope measurements have been performed on neutron-irradiated samples of NW Africa (NWA) 4797. Shock heating of NWA 4797 completely melted and vesiculated precursor igneous plagioclase, which cooled to an assemblage of plagioclase crystals with interstitial glasses of variable composition (Ca/K ratios). Using a focused ultraviolet laser beam, is has been possible to distinguish between argon isotopic signatures from groundmass minerals (igneous olivine + pyroxene), plagioclase and a shock vein. This study focuses on the potential for this meteorite to shed light on shock ages of shergottites. Apparent 40 Ar/ 39 Ar ages of groundmass minerals show that there are large amounts of excess argon in this phase, yielding a wide range of calculated ages from 690 ± 30 Ma to several apparent ages older than 4.5 Ga. A traverse of laser-probe extractions across the 1 mm-diameter shock vein in NWA 4797 yielded apparent 40 Ar/ 39 Ar ages younger than the groundmass. A signature of the Martian atmosphere, identified by 40 Ar/ 36 Ar ratios of 1600–1900, was not found in the NWA 4797 shock vein. This is distinct from other shergottites where the products of shock melting contain a nearly pure sample of Martian atmosphere. We attribute this to a distinct formation mechanism, and hence gas-trapping mechanism, of the NWA 4797 shock vein. We undertook 44 analyses of plagioclase areas identified by SEM analysis. Ages ranged from 45 ± 27 to 3771 ± 109 Ma and yield an average age of 375 ± 77 Ma, considerably younger than ages obtained in this study from either the groundmass or the shock vein. A plot of age v. 37 Ar/ 39 Ar for plagioclase showed a continuum of ages from the oldest to youngest ages measured. Older ages are correlated with higher Ca/K ratios of plagioclase, indicating contamination from groundmass minerals rich in excess argon. The youngest ages correlate to plagioclase extractions with the lowest Ca/K ratios, interpreted to have crystallized from a nearly pure plagioclase melt with contributions from a K-rich mesostasis. We see no evidence for multiple shock events in NWA 4797. Rather, we favour the interpretation that the cosmic-ray exposure (CRE) age of 3.0±0.5 Ma, obtained on NWA 4797 in this study using cosmogenic 38 Ar, approximates the timing of shock melting in this meteorite. Supplementary material: Laser probe argon isotopic data for NWA 4797 obtained in this study are available at http://www.geolsoc.org.uk/SUP18602 .

Abstract This chapter describes and gives elemental abundances of many of the accreted volcanic rocks and of a few hypabyssal rocks of Alaska. These rocks range from early Paleozoic (or perhaps late Precambrian) to Eocene age. All formed prior to accretion of the terrane containing them and thus were generated either as primary features in the ancestral Pacific Ocean or on terranes or superterranes carried by plates underlying that ocean. These accreted volcanic rocks are important in terms of continental growth by accretion of oceanic rocks. Various workers have asserted that such growth is by accretion of intraoceanic island arcs. This assertion, however, must be appreciably modified for the ca. 400,000-km 2 region of southern and central Alaska that is underlain by accreted rocks. Though these rocks are not known in sufficient detail to yield a precise figure, I estimate that no more than 70 to 75 percent of this newly formed crust consists of former island arcs and arc-derived epiclastic sedimentary rocks. Most of the tectonostratigraphic (lithotectonic) terranes of Alaska have minor exposures of volcanic rocks. Accounts of local and regional geology of the state contain cursory to extensive descriptions of such rocks. However, a catalog of such occurrences is not considered appropriate for this volume, and we discuss here only rocks studied by modern methods. The particular terranes containing these rocks are shown on Plate 13 (Barker and others, this volume), whereas all tectonostratigraphic terranes of Alaska are shown on Plate 3 (Silberling and others, this volume). Though virtually all