Description: Tunupa volcano is a composite cone in the central Andean arc of South America located ~115 km behind the arc front. We present new geochemical data and 40 Ar/ 39 Ar age determinations from Tunupa volcano and the nearby Huayrana lavas, and we discuss their petrogenesis within the context of the lithospheric dynamics and orogenic volcanism of the southern Altiplano region (~18.5°S–21°S). The Tunupa edifice was constructed between 1.55 ± 0.01 and 1.40 ± 0.04 Ma, and the lavas exhibit typical subduction signatures with positive large ion lithophile element (LILE) and negative high field strength element (HFSE) anomalies. Relative to composite centers of the frontal arc, the Tunupa lavas are enriched in HFSEs, particularly Nb, Ta, and Ti. Nb-Ta-Ti enrichments are also observed in Pliocene and younger monogenetic lavas in the Altiplano Basin to the east of Tunupa, as well as in rear arc lavas elsewhere on the central Andean Plateau. Nb concentrations show very little variation with silica content or other indices of differentiation at Tunupa and most other central Andean composite centers. We propose that this distinct compositional domain reflects an amphibole- and/or phlogopite-rich mantle lithospheric source. Breakdown of these minerals during lithospheric delamination may provide a melting trigger for Tunupa, as has been suggested for other rear arc plateau lavas of the central Andes, and for plateau regions globally. The ca. 11 Ma Huayrana lavas indicate that this process had begun in the central Altiplano Basin by this time. The enriched Nb-Ta-Ti signature of plateau lavas may be an important indicator of hydrous mineral breakdown within the mantle lithosphere, and it can be detected in lavas that that have likely experienced crustal contamination.

Description: Intraoceanic volcanic arcs have long been recognized as sites where continental crust is created. Yet, despite their importance to understanding magmatic systems and the evolution of our planet, very little is known about their long-term rates of magma production and crust formation. Constraining both crustal construction and destruction processes at intraoceanic arcs allows for improved estimates of magma production. Our revised magma production rates for active intraoceanic arcs are consistent with those calculated for mid-ocean ridge segments that have slow to moderate spreading rates. This is surprising because magma production at intraoceanic arcs has traditionally been assumed to be significantly less than that at mid-ocean ridges.

Description: Discovery of seafloor volcanism west of Buldir Volcano, the westernmost emergent volcano in the Aleutian arc, demonstrates that surface expression of active Aleutian volcanism falls below sea level just west of 175·9°E longitude, but is otherwise continuous from mainland Alaska to Kamchatka. Lavas dredged from newly discovered seafloor volcanoes up to 300 km west of Buldir have end-member geochemical characteristics that provide new insights into the role of subducted basalt as a source component in Aleutian magmas. Western Aleutian seafloor lavas define a highly calc-alkaline series with 50–70% SiO 2 . Most samples have Mg-numbers [Mg# = Mg/(Mg + Fe)] greater than 0·60, with higher MgO and lower FeO* compared with average Aleutian volcanic rocks at all silica contents. Common basalts and basaltic andesites in the series are primitive, with average Mg# values of 0·67 (±0·02, n = 99, 1SD), and have Sr concentrations (423 ± 29 ppm, n = 99) and La/Yb ratios (4·5 ± 0·4, n = 29) that are typical of island arc basaltic lavas. A smaller group of basaltic samples is more evolved and geochemically more enriched, with higher and more variable Sr and La/Yb (average Mg# = 0·61 ± 0·1, n = 31; Sr = 882 ± 333 ppm, n = 31; La/Yb = 9·1 ± 0·9, n = 16). None of the geochemically enriched basalts or basaltic andesites has low Y (〈15 ppm) or Yb (〈1·5 ppm), so none show the influence of residual or cumulate garnet. In contrast, most western seafloor andesites, dacites and rhyodacites have higher Sr (〉1000 ppm) and are adakitic, with strongly fractionated trace element patterns (Sr/Y = 50–350, La/Yb = 8–35, Dy/Yb = 2·0–3·5) with low relative abundances of Nb and Ta (La/Ta 〉 100), consistent with an enhanced role for residual or cumulate garnet + rutile. All western seafloor lavas have uniformly radiogenic Hf and Nd isotopes, with Nd = 9·1 ± 0·3 ( n = 31) and Hf = 14·5 ± 0·6 ( n = 27). Lead isotopes are variable and decrease with increasing SiO 2 from basalts with 206 Pb/ 204 Pb = 18·51 ± 0·05 ( n = 11) to dacites and rhyodacites with 206 Pb/ 204 Pb = 18·43 ± 0·04 ( n = 18). Western seafloor lavas form a steep trend in 207 Pb/ 204 Pb– 206 Pb/ 204 Pb space, and are collinear with lavas from emergent Aleutian volcanoes, which mostly have 206 Pb/ 204 Pb 〉 18·6 and 207 Pb/ 204 Pb 〉 15·52. High MgO and Mg# relative to silica, flat to decreasing abundances of incompatible elements, and decreasing Pb isotope ratios with increasing SiO 2 rule out an origin for the dacites and rhyodacites by fractional crystallization. The physical setting of some samples (erupted through Bering Sea oceanic lithosphere) rules out an origin for their garnet + rutile trace element signature by melting in the deep crust. Adakitic trace element patterns in the dacites and rhyodacites are therefore interpreted as the product of melting of mid-ocean ridge basalt (MORB) eclogite in the subducting oceanic crust. Western seafloor andesites, dacites and rhyodacites define a geochemical end-member that is isotopically like MORB, with strongly fractionated Ta/Hf, Ta/Nd, Ce/Pb, Yb/Nd and Sr/Y. This eclogite component appears to be present in lavas throughout the arc. Mass-balance modeling indicates that it may contribute 36–50% of the light rare earth elements and 18% of the Hf that is present in Aleutian volcanic rocks. Close juxtaposition of high-Mg# basalt, andesite and dacite implies widely variable temperatures in the western Aleutian mantle wedge. A conceptual model explaining this shows interaction of hydrous eclogite melts with mantle peridotite to produce buoyant diapirs of pyroxenite and pyroxenite melt. These diapirs reach the base of the crust and feed surface volcanism in the western Aleutians, but are diluted by extensive melting in a hotter mantle wedge in the eastern part of the arc.

Description: Prior to the ad 1902 Plinian eruption of 8 km 3 of dacite and subsequent growth of the 〉1 km 3 Santiaguito dacite dome complex, Santa María volcano grew into an 8 km 3 composite cone over ~75 kyr in four phases (at 103–72, 72, 60–46, and 35–25 ka). The 1902 eruption occurred after an ~25 kyr period of repose in growth of the composite cone. To provide context for processes that ultimately led to the 1902 eruption, we present geochemical and isotopic (Sr, Nd, Pb, U-series) data from lavas of the composite cone for which ages are constrained by 40 Ar/ 39 Ar dating. The four cone-building phases comprise basaltic to basaltic-andesite lava (51·4–56·1% SiO 2 ) whose major- and trace-element compositions suggest that crystallization was important in differentiation. Relative to other Central American arc volcanoes, these lavas also have large 238 U excesses and high 207 Pb/ 204 Pb ratios that imply melting of a mantle wedge modified to an unusual extent by fluid from subducted crust and sediment of the Cocos plate. Major- and trace-element and isotopic variations over time imply that mafic recharge and magma mixing were prevalent during early phases of cone-building, whereas assimilation processes were more dominant during the latest stage of cone growth. Indeed, some early erupted basalts have lower 143 Nd/ 144 Nd and higher 87 Sr/ 86 Sr ratios than more SiO 2 -rich basaltic andesites that erupted during the final phase of cone-building. These features point to an assimilant that is not typical continental crust and instead may be more like mid-ocean ridge basalt with respect to major- and trace-element composition and Sr, Nd, Pb, and U–Th isotope ratios. Energy-constrained modeling of a parental basalt that undergoes crystal fractionation, assimilation and periodic recharge with basalt in the lower crust can reproduce lava compositions erupted during phases I–III and the early part of phase IV. Modeling further indicates that assimilation within the lower crust of partially melted garnet-amphibolite metabasalt, without basaltic recharge, may produce the youngest cone-forming lavas in phase IV. These models link the 8 km 3 of cone growth over 75 kyr to the mass flux of magma into the crust. Our findings suggest an along-arc magma flux into the lower crust beneath Santa María of 〉20 km 3 km – 1 Myr – 1 , which is higher than anticipated in recent numerical–thermal approaches to basalt–crust interaction. Consequently, the thermal incubation period needed to produce hybrid basaltic-andesite magma may be only a few tens of thousand years.

Description: The development of integrated astronomical and radioisotopic time scales from rhythmic strata of the Western Interior Basin (WIB) has played a fundamental role in the refinement of Late Cretaceous chronostratigraphy. In this study, X-ray fluorescence (XRF) core scanning is utilized to develop a new elemental data set for cyclostratigraphic investigation of Cenomanian-Turonian strata in the WIB, using material from the Aristocrat-Angus-12-8 core (north-central Colorado). The XRF data set yields the first continuous 5-mm-resolution analysis of lithogenic, biogenic, and syngenetic-authigenic proxies through the uppermost Lincoln Limestone Member, the Hartland Shale Member, and the Bridge Creek Limestone Member, including oceanic anoxic event 2 (OAE 2). The 40 Ar/ 39 Ar ages from ashes in three biozones, including a new age from the Dunveganoceras pondi biozone (uppermost Lincoln Limestone Member), provide geochronologic constraints for the cyclostratigraphic analysis. Astrochronologic testing of the 5-mm-resolution XRF weight percent CaCO 3 data via average spectral misfit analysis yields strong evidence for astronomical influence on climate and sedimentation. Results from the Bridge Creek Limestone Member are consistent with the previously published astrochronology from the U.S. Geological Survey #1 Portland core (central Colorado), and identification of an astronomical signal in the underlying Hartland Shale Member now permits extension of the WIB astrochronology into the earlier Cenomanian, prior to OAE 2. High rates of sedimentation in the Angus core during the interval of OAE 2 initiation, as compared to the Portland core, allow recognition of a strong precessional control on bedding development. As a consequence, the new results provide a rare high-resolution chronometer for the onset of OAE 2, and the timing of proposed hydrothermal trace metal enrichment as observed in the 5 mm XRF data.

Description: This study revises and improves the chronostratigraphic framework for late Turonian through early Campanian time based on work in the Western Interior U.S. and introduces new methods to better quantify uncertainties associated with the development of such time scales. Building on the unique attributes of the Western Interior Basin, which contains abundant volcanic ash beds and rhythmic strata interpreted to record orbital cycles, we integrate new radioisotopic data of improved accuracy with a recently published astrochronologic framework for the Niobrara Formation. New 40 Ar/ 39 Ar laser fusion ages corresponding to eight different ammonite biozones are determined by analysis of legacy samples, as well as newly collected material. These results are complemented by new U-Pb (zircon) chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages from four biozones in the study interval. When combined with published radioisotopic data from the Cenomanian-Turonian boundary, paired 206 Pb/ 238 U and 40 Ar/ 39 Ar ages spanning Cenomanian to Campanian time support an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. Stage boundary ages are estimated via integration of new radioisotopic data with the floating astrochronology for the Niobrara Formation. The ages are determined by anchoring the long eccentricity bandpass from spectral analysis of the Niobrara Formation to radioisotopic ages with the lowest uncertainty proximal to the boundary, and adding or subtracting time by parsing the 405 k.y. cycles. The new stage boundary age determinations are: 89.75 ± 0.38 Ma for the Turonian-Coniacian, 86.49 ± 0.44 Ma for the Coniacian-Santonian, and 84.19 ± 0.38 Ma for the Santonian-Campanian boundary. The 2 uncertainties for these estimates include systematic contributions from the radioisotopic measurements, astrochronologic methods, and geologic uncertainties (related to stratigraphic correlation and the presence of hiatuses). The latter geologic uncertainties have not been directly addressed in prior time scale studies and their determination was made possible by critical biostratigraphic observations. Each methodological approach employed in this study—new radioisotopic analysis, stratigraphic correlation, astrochronology, and ammonite and inoceramid biostratigraphy—was critical for achieving the final result.

Description: 40 Ar/ 39 Ar geochronology of basaltic to rhyolitic lavas, domes, and pyroclastic deposits from Ascension Island indicates that the maximum age of subaerially exposed samples is 1094 ka. Thirty-eight 40 Ar/ 39 Ar ages, coupled with new and existing geochemical data, constrain the eruptive histories of the four distinct mafic magma types (high Zr/Nb, low Zr/Nb, intermediate Zr/Nb, and Dark Slope Crater) and document temporal variations in magma sources. Lavas from the eastern felsic complex, previously assumed to be as old as or slightly younger than the 602–1094 ka Middleton Ridge complex, are as young as 52 ka. Basaltic to benmoreitic scoria cones and associated flows of the intermediate Zr/Nb magma type have been inferred to be the most recent eruptive products, yet their eruptive histories extend back to 705 ka. These intermediate Zr/Nb magmas are likely to be parental to the abundant trachytic to rhyolitic lavas and domes, which contradicts previous interpretations that call upon a high Zr/Nb parental basalt. Two distinct fractionation trends are observed in the trace element variations of Ascension trachytes and rhyolites. 40 Ar/ 39 Ar ages of the samples defining the two trends suggest that ilmenite fractionation dominated the Nb budget and thus controlled Zr/Nb ratios in early (〉931 ka) Ascension evolved magmas, whereas zircon–titanite fractionation was predominant in younger felsic magmas. The eruptive sequence and compositions of the subaerial lavas and domes at Ascension Island are unique in comparison with other ocean island volcanoes because of its on-axis location and eruptions of high-SiO 2 trachyte and rhyolite.

Description: Eocene terrestrial strata in western North America are now dated with sufficient precision to examine the hypothesis that rollback of the Farallon flat-slab created a trenchward-migrating wave of dynamic and thermal topography. We use patterns of accumulation and unconformity, lake-type stratigraphy, and physical and isotopic provenance tools to track its migration across the Laramide foreland between 53 Ma and 47 Ma. Hydrologic ponding and coincident unconformity development began at 53 Ma, culminating at 51.8 Ma in the hydrologic closure of an ~900,000 km 2 area between the Cordilleran and Rocky Mountain divides. Four subsequent stream diversions and a southwest-expanding regional unconformity record trenchward migration of uplift. From 50 to 47 Ma, volcaniclastic detritus from magmatic centers coincident with the uplifted region progressively filled the Green River Formation lakes from north to south. These findings are consistent with recent numerical and conceptual models of slab rollback that predict initial dynamic subsidence above the slab hinge, followed by uplift and volcanism triggered by influx of asthenosphere beneath the overriding plate. Based on the surface record, rollback proceeded systematically at a rate of ~6 cm/yr across the Wyoming craton from 53 to 47 Ma, resulting in removal of the entire Shatsky-conjugate oceanic plateau from the base of the North American lithosphere.