The Rio Grande Rift in Context of Regional Post-40 M.Y. Volcanic and Tectonics Events Wolfgang E. Elston, Wolfgang E. Elston Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this authorTheodore J. Bornhorst, Theodore J. Bornhorst Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this author Wolfgang E. Elston, Wolfgang E. Elston Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this authorTheodore J. Bornhorst, Theodore J. Bornhorst Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131Search for more papers by this author Book Editor(s):Robert E. Riecker, Robert E. RieckerSearch for more papers by this author First published: 01 January 1979 https://doi.org/10.1029/SP014p0416Citations: 27Book Series:Special Publications AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onEmailFacebookTwitterLinkedInRedditWechat Summary In context with other post-40 m.y. volcanic and tectonic events of southwestern North America (including regional extension, possibly by hundreds of km, and eruption of 106 km3 of calc-alkalic volcanic rocks in the Basin and Range province and Sierra Madre Occidental), formation of the Rio Grande graben system is relatively local and recent. Events in New Mexico and Colorado can be interpreted in three overlapping stages (dates indicate the minimum range of each stage) . (1) Modified Andean arc stage, 40 to 29 m.y., with calc-alkalic volcanism. (2) Modified back-arc extension stage, 30 to 18 m.y., with eruption of basaltic andesite and related rocks and high-silica rhyolite. (3) Intraplate block faulting, 21 m.y. to present, volcanism basaltic and locally rhyolitic. During calc-alkalic volcanism of stage (1) and rhyolitic volcanism of stage (2), large composite plutons were passively emplaced. Their cupolas blistered and burst, resulting in individual eruptions of as much as 103 km3 of ash-flow tuff. Modified back-arc extension made room for the plutons and also explains the present 1,000 km width of the volcanic zone, its thin crust, high heat flow and Sn-wave attenuation. Extension during stage (3) was less than in stage (2). In areas of large-scale volcanism during the preceding stages, the crust broke up into a broad zone of Basin and Range fault blocks; in regions of little or no earlier volcanism the cooler and more rigid crust broke into blocks that were larger and more distinctly defined. The Rio Grande graben system assumes the characteristics of a sharply-defined rift in northern New Mexico but merges into the broader Basin and Range province as it enters the region of mid-Tertiary volcanism south of Socorro. References D. J. Andrews, N. H. Sleep, Numerical modeling of tectonic flow behind island arcs, Geophys. J. Roy. Astr. Soc., 38, 237–251, 1974. 10.1111/j.1365-246X.1974.tb04118.x Web of Science®Google Scholar K.-I. Aoki, A. M. Kudo, Major-element variations of late Cenozoic basalts of New Mexico, N. Mex. Geol. Soc. Spec. Publ., 5, 82–87, 1976. Google Scholar Tanya Atwater, Implications of plate tectonics for the Cenozoic tectonic evolution of western North America, Geol. Soc. Am. Bull., 81, 3513–3536, 1970. 10.1130/0016-7606(1970)81[3513:IOPTFT]2.0.CO;2 Web of Science®Google Scholar G. O. Bachman, H. H. Mehnert, New K-Ar dates and late Pliocene to Holocene geomorphic history of the central Rio Grande region, New Mexico, Geol. Soc. Am. Bull., 89, 283–292, 1978. 10.1130/0016-7606(1978)89<283:NKDATL>2.0.CO;2 CASWeb of Science®Google Scholar R. A. Bailey, G. B. Dalrymple, M. A. Lanphere, Volcanism, structure, and geochronology of Long Valley, California, J. Geophys. Res., 81, 725–744, 1976. 10.1029/JB081i005p00725 CASWeb of Science®Google Scholar Muawia Barazangi, Bryan Isacks, Lateral variations of seismic-wave attenuation in the upper mantle above the inclined earthquake zone of the Tonga Island arc, J. Geophys. Res., 76, 8493–8516, 1971. 10.1029/JB076i035p08493 Web of Science®Google Scholar D. S. Barker, Northern Trans-Pecos magmatic province: Introduction and comparison with the Kenya rift, Geol. Soc. Am. Bull., 88, 1421–1427, 1977. 10.1130/0016-7606(1977)88<1421:NTMPIA>2.0.CO;2 CASWeb of Science®Google Scholar R. J. Bridwell, Lithospheric thinning and late Cenozoic thermal and tectonic regime of the northern Rio Grande rift, N. Mex. Geol. Soc. Guidebook of Vermejo Park, 27th Field Conf., 283–292, 1976. Google Scholar F. M. Byers Jr., W. J. Carr, P. B. Orkild, W. D. Quinlivan, K. A. Sargent, Volcanic suites and related cauldrons of Timber Mountain-Oasis Valley caldera complex, southern Nevada U.S. Geol. Surv. Prof. Paper , 919, 70p., 1976. Google Scholar R. M. Chamberlin, Structural development of the Lemitar Mountains, an intrarift fault-block uplift, central New Mexico (abstract), Program and Abstracts, 1978 International Symposium on the Rio Grande rift, LA-7487-C, 22–24, Los Alamos Scientific Laboratory1978. Google Scholar C. E. Chapin, The Rio Grande Rift, Part I: Modifications and additions, N. Mex. Geol. Soc. Guidebook of San Luis Basin, 22nd Field Conf., 191–202, 1971. Google Scholar C. E. Chapin, R. M. Chamberlin, G. R. Osburn, D. W. White, A. R. Sanford, Exploration framework of the Socorro geothermal area, New Mexico, N. Mex. Geol. Soc. Spec. Publ., 7, 115–129, 1978. Google Scholar Field guide to selected cauldrons and mining districts of the Datil-Mogollon volcanic field, New Mexico N. Mex. Geol. Soc. Spec. Publ., 7, C. E. Chapin W. E. Elston 149p., 1978. Google Scholar C. E. Chapin, W. R. Seager, Evolution of Rio Grande rift in the Socorro and Las Cruces areas, N. Mex. Geol. Soc. Guidebook of Las Cruces Country, 26th Field Conf., 297–322, 1975. Google Scholar S. E. Church, M. Tatsumoto, Lead isotope relations in oceanic ridge basalt from the Juan de Fuca-Gorda ridge area, N.E. Pacific Ocean, Contrib. Mineral. Petrol., 53, 253–279, 1975. 10.1007/BF00382443 CASWeb of Science®Google Scholar P. J. Coney, Cordilleran tectonics and North American plate motion, Am. J. Sci., 272, 603–628, 1972. 10.2475/ajs.272.7.603 Web of Science®Google Scholar P. J. Coney, S. J. Reynolds, Cordilleran Benioff zones, Nature, 270, 403–406, 1977. 10.1038/270403a0 Web of Science®Google Scholar Lindreth Cordell, Regional geophysical setting of the Rio Grande rift, Geol. Soc. Am. Bull., 89, 1073–1090, 1978. 10.1130/0016-7606(1978)89<1073:RGSOTR>2.0.CO;2 Web of Science®Google Scholar E. G. Deal, W. E. Elston, E. E. Erb, S. L. Peterson, D. E. Reiter, P. E. Damon, M. Shafiqullah, Cenozoic volcanic geology of the Basin and Range province in Hidalgo County, southwesternmost New Mexico: Progress report No. 1, N. Mex. Geol. Soc. Guidebook of Land of Cochise, 29th Field Conf., 219–229, 1978. Google Scholar E. R. Decker, S. B. Smithson, Heat flow and gravity interpretation in southern New Mexico and West Texas, J. Geophys. Res., 80, 2542–2552, 1975. 10.1029/JB080i017p02542 Web of Science®Google Scholar W. R. Dickinson, Potash-depth (K-h) relations in continental margin and intra-oceanic magmatic arcs, Geology, 3, 53–56, 1975. 10.1130/0091-7613(1975)3<53:PKRICM>2.0.CO;2 Web of Science®Google Scholar G. P. Eaton, R. L. Christiansen, H. M. Iyer, D. R. Mabey, H. R. Blank Jr., I. Zietz, M. L. Gettings, Magma beneath Yellowstone National Park, Science, 188, 787–796, 1975. 10.1126/science.188.4190.787 CASPubMedWeb of Science®Google Scholar W. E. Elston, Mid-Tertiary cauldrons and their relationship to mineral resources: a brief review, N. Mex. Geol. Soc. Spec. Publ., 7, 107–113, 1978. Google Scholar W. E. Elston, P. E. Damon, P. J. Coney, R. C. Rhodes, E. I. Smith, Michael Bikerman, Tertiary volcanic rocks, Mogollon-Datil province, New Mexico, and surrounding region: K-Ar dates, patterns of erup-tion and periods of mineralization, Geol. Soc. Am. Bull., 84, 2259–2274, 1973. 10.1130/0016-7606(1973)84<2259:TVRMPN>2.0.CO;2 Web of Science®Google Scholar Cenozoic volcanism in southwestern New Mexico, a volume in memory of Rodney C. Rhodes, 1943–1975 N. Mex. Geol. Soc. Spec. Publ., 5, W. E. Elston S. A. Northrop 151., 1976. Google Scholar R. C. Epis, C. E. Chapin, Geomorphic and tectonic implications of the post-Laramide, late Eocene erosion surface in the southern Rocky Mountains, Geol. Soc. Am. Mem., 144, 45–74, 1975. 10.1130/MEM144-p45 Google Scholar D. B. Eppler, The geology of the San Antonio Mountain area near Tres Piedras, Taos County, New Mexico, M.S. Thesis, Univ. N. Mex., Albuquerque, 77, p., 1976. Google Scholar W. S. Fyfe, A. R. McBirney, Subduction and the structure of andesitic volcanic belts, Am. J. Sci., 272-A, 285–297, 1975. Google Scholar James Gilluly, Tectonics involved in the evolution of mountain ranges, The Nature of the Solid Earth E. C. Robertson 406–439, McGraw-Hill, New York 1970. Google Scholar T. H. Green, A. E. Ringwood, Genesis of the calc-alkaline igneous rock suite, Contrib. Mineral. Petrol., 18, 105–162, 1968. 10.1007/BF00371806 CASGoogle Scholar Warren Hamilton, Neogene extension of the western United States (abstract), Geol. Soc. Am. Abstr. w. Programs, 7, 1098p., 1975. Google Scholar J. W. Hawkins Jr., Petrologic and geochemical characteristics of marginal basin basalts, Am. Geophys. Union Maurice Ewing Ser., 1, 355–365, 1977. 10.1029/ME001p0355 CASGoogle Scholar T. N. Irvine, W. R. A. Baragar, A guide to chemical classification of the common volcanic rocks, Canadian Jour. of Earth Sciences, 8, 523–548, 1971. 10.1139/e71-055 CASWeb of Science®Google Scholar B. L. Isacks, Muawia Barazangi, Geometry of Benioff zones: Lateral segmentation and downward bending of the subducted lithosphere, Am. Geophys. Union Maurice Ewing Ser., 1, 99–111, 1977. 10.1029/ME001p0099 Google Scholar P. Jakes, A. J. R. White, Major and trace element abundances in volcanic rocks of orogenic areas, Geol. Soc. Am. Bull., 83, 29–39, 1972. 10.1130/0016-7606(1972)83[29:MATEAI]2.0.CO;2 CASWeb of Science®Google Scholar D. E. Karig, Origin and development of marginal basins in the western Pacific, J. Geophys. Res., 76, 2542–2561, 1971. 10.1029/JB076i011p02542 Web of Science®Google Scholar S. B. Keith, Paleosubduction geometries inferred from Cretaceous and Tertiary magmatic patterns in southwestern North America, Geology, 9, 516–521, 1978. 10.1130/0091-7613(1978)6<516:PGIFCA>2.0.CO;2 Web of Science®Google Scholar D. H. Krohn, Gravity survey of the Mogollon Plateau volcanic province, southwestern New Mexico, N. Mex. Geol. Soc. Spec. Publ., 5, 113–116, 1976. Google Scholar Ikuo Kushiro, Melting of hydrous upper mantle and possible generation of andesitic magma: An approach from synthetic systems, Earth Planet. Sci. Letters, 22, 294–299, 1974. 10.1016/0012-821X(74)90138-1 CASWeb of Science®Google Scholar Ikuo Kushiro, H. S. Yoder, Melting of forsterite and enstatite at high pressures under hydrous conditions, Carnegie Inst. Washington Yearbook, 67, 153–158, 1969. Google Scholar W. P. Leeman, J. J. W. Rogers, Late Cenozoic alkali-olivine province U.S.A., Contrib. Mineral. Petrol., 25, 1–24, 1970. 10.1007/BF00383059 CASWeb of Science®Google Scholar Waldemar Lindgren, L. C. Graton, C. H. Gordon, The ore depos s of New Mexico U.S. Geol. Surv. Prof. Paper , 68, 361., 1910. Google Scholar P. W. Lipman, B. R. Doe, C. E. Hedge, T. A. Steven, Petrologic evolution of the San Juan volcanic field, southwestern Colorado: Pb and Sr isotopic evidence, Geol. Soc. Am. Bull., 89, 59–82, 1978. CASWeb of Science®Google Scholar P. W. Lipman, H. H. Mehnert, Late Cenozoic basaltic volcanism and development of the Rio Grande depression in the southern Rocky Mountains, Geol. Soc. Am. Mem., 144, 119–154, 1975. 10.1130/MEM144-p119 CASGoogle Scholar P. W. Lipman, H. J. Prostka, R. L. Christiansen, Cenozoic volcanism and plate-tectonic evolution of the western United States, Part 1, Early and middle Cenozoic, Roy. Soc. London Phil. Trans., A, 271, 217–248, 1972. 10.1098/rsta.1972.0008 Web of Science®Google Scholar G. A. Macdonald, T. Katsura, Chemical composition of Hawaiian lavas, J. Petrol., 5, 82–133, 1964. 10.1093/petrology/5.1.82 CASWeb of Science®Google Scholar F. W. McDowell, Potassium-argon dating in the Trans-Pecos volcanic field, Cenozoic Geology of the Trans-Pecos Volcanic Field A. W. Walton 9–18, Univ. Kansas, 1978. Google Scholar T. R. McGetchin, and L. T. Silver, A crustal-upper mantle model for the Colorado Plateau based on observations of crystalline rock fragments in the Moses Rock dike, J. Geophys. Res., 77, 7022–7037. Google Scholar E. A. K. Middlemost, A simple classification of volcanic rocks, Bull. Volcanol., 36-2, 382–397, 1972. 10.1007/BF02596878 Google Scholar B. O. Mysen, A. L. Boettcher, Melting of a hydrous mantle II, J. Petrol., 16, 549–590, 1975. CASWeb of Science®Google Scholar I. A. Nicholls, A. E. Ringwood, Effect of water on olivine stability in tholeiites and the production of silica-saturated magmas in the island-arc environment, J. Geol., 81, 285–300, 1973. 10.1086/627871 Web of Science®Google Scholar I. A. Nicholls, Liquids in equilibrium with peridotitic mineral assemblages at high water pressures, Contrib. Mineral. Petrol., 45, 289–316, 1974. 10.1007/BF00371749 CASWeb of Science®Google Scholar J. Nicholls, I. S. E. Carmichael, J. C. Stormer, Silica activity and the Ptotal in igneous rocks, Contrib. Mineral. Petrol., 33, 1–20, 1971. 10.1007/BF00373791 CASWeb of Science®Google Scholar J. E. Oliver, Sidney Kaufman, Seismic reflection profiling of the deep basement: The Rio Grande rift, Geotimes, 21, 20–23, 1976. Google Scholar L. C. Pakiser, The basalt-eclogite transformation and crustal structure in the Western United States, U.S. Geol. Surv. Prof. Paper, 525-B, B1–B8, 1965. Google Scholar Z. E. Peterman, B. R. Doe, H. J. Prostka, Lead and uranium isotopes in rocks of the Absaroka volcanic field, Wyoming, Contrib. Mineral. Petrol., 27, 121–130, 1970. 10.1007/BF00371979 CASWeb of Science®Google Scholar Donald Plouff, L. C. Pakiser, Gravity study of San Juan Mountains, Colorado, U.S. Geol. Surv. Prof. Paper, 800-B, B183–B190, 1972. Google Scholar J. M. Proffett Jr., Cenozoic geology of the Yerington district, Nevada, and implications for the origin of Basin and Range faulting, Geol. Soc. Am. Bull., 88, 247–266. Google Scholar H. J. Prostka, S. S. Oriel, Genetic models for Snake River Plain, Idaho (abstract), Geol. Soc. Am. Abstr. w. Programs, 7, 1236., 1975. Google Scholar I. B. Ramberg, F. A. Cook, S. B. Smithson, Structure of the Rio Grande rift in southern New Mexico and West Texas based on gravity interpretation, Geol. Soc. Am. Bull., 89, 107–123, 1978. 10.1130/0016-7606(1978)89<107:SOTRGR>2.0.CO;2 Web of Science®Google Scholar Robert Reilinger, Jack Oliver, Modern uplift associated with a proposed magma body in the vicinity of Socorro, New Mexico, Geology, 4, 583–586, 1976. 10.1130/0091-7613(1976)4<583:MUAWAP>2.0.CO;2 Web of Science®Google Scholar Marshall Reiter, C. L. Edwards, H. Hartman, C. Weidman, Terrestrial heat flow along the Rio Grande rift, New Mexico and southern Colorado, Geol. Soc. Am. Bull., 86, 811–818, 1975. 10.1130/0016-7606(1975)86<811:THFATR>2.0.CO;2 Web of Science®Google Scholar Jacques Renault, Major-element variations in the Potrillo, Carrizozo, and McCartys basalt fields, New Mexico N. Mex. Bur. Mines Min. Res. Circ. , 113, 22p., 1970. Google Scholar R. C. Rhodes, Petrologic framework of the Mogollon Plateau volcanic ring complex, New Mexico — surface expression of a major batholith, N. Mex. Geol. Soc. Spec. Publ., 5, 103–112, 1976. Google Scholar A. E. Ringwood, Petrogenesis of island arc systems, Am. Geophys. Union Maurice Ewing Ser., 1, 311–324, 1977. 10.1029/ME001p0311 CASGoogle Scholar A. R. Sanford, O. S. Alptekin, T. R. Toppozada, Use of reflection phases on microearthquake seismograms to map an unusual discontinuity beneath the Rio Grande rift, Seismol. Soc. Am. Bull., 63, 2021–2034, 1973. Web of Science®Google Scholar C. H. Scholz, Muawia Barazangi, and M. L. Sbar, Late Cenozoic evolution of the Great Basin, Western United States, as an ensialic interarc basin, Geol. Soc. Am. Bull., 82, 2979–2990, Google Scholar W. R. Seager, Resurgent volcano-tectonic depression of Oligocene age, south-central New Mexico, Geol. Soc. Am. Bull., 84, 3611–3626, 1973. 10.1130/0016-7606(1973)84<3611:RVDOOA>2.0.CO;2 Web of Science®Google Scholar R. L. Smith, R. A. Bailey, and C. S. Ross, Geologic map of the Jemez Mountains, New Mexico, U.S. Geol. Surv. Misc. Inv. Map I-571, 1970. Google Scholar T. A. Steven, P. W. Lipman, Calderas of the San Juan volcanic field, southwestern Colorado U.S. Geol. Surv. Prof. Paper , 958, 35p., 1976. Google Scholar J. H. Stewart, Origin of Basin and Range structures — a review (abstract), Geol. Soc. Am. Abstr. w. Programs, 7, 1284p., 1975. Google Scholar J. H. Stewart, and J. E. Carlson, Cenozoic rocks of Nevada, Nevada Bur. Mines Geol. Map 52, 1976. Google Scholar J. W. Stinnett Jr., A. M. Stueber, A strontium isotopic and geochemical study of volcanic rocks from the Datil-Mogollon field southwestern New Mexico (abstract), Geol. Soc. Am. Abstr. w. Programs, 8, 636–637, 1976. Google Scholar A. Sugimura, S. Uyeda, Island Arcs: Japan and Its Environs 247, Elsevier, Amsterdam, London, New York 1973. Google Scholar H. P. Taylor Jr., Oxygen and hydrogen isotope evidence for large-scale circulation and interaction between groundwaters and igneous intrusions, with special reference to the San Juan volcanic field, Colorado, Carnegie Inst. Washington Publ., 634, 299–324, 1974. CASGoogle Scholar M. M. Toksöz, Peter Bird, Formation and evolution of marginal basins and continental plateaus, Am. Geophys. Union Maurice Ewing Ser., 1, 379–393, 1977. 10.1029/ME001p0379 Google Scholar T. J. Ulrych, Oceanic basalt leads: a new interpretation and independent age for the Earth, Science, 158, 252–256, 1967. 10.1126/science.158.3798.252 CASPubMedWeb of Science®Google Scholar Seiya Uyeda, Some basic problems in the trench-arc-back arc system, Am. Geophys. Union Maurice Ewing Ser., 1, 1–14, 1977. 10.1029/ME001p0001 Google Scholar D. U. Wise, An outrageous hypothesis for the tectonic pattern of the North America Cordillera, Geol. Soc. Am. Bull., 74, 357–362, 1963. 10.1130/0016-7606(1963)74[357:AOHFTT]2.0.CO;2 Web of Science®Google Scholar P. J. Wyllie, Experimental petrology and global tectonics — a preview, Tectonophys., 17, 189–209, 1973. 10.1016/0040-1951(73)90002-4 CASWeb of Science®Google Scholar R. E. Zartman, Lead isotope provinces in the Cordillera of the western United States and their geologic significance, Econ. Geol., 69, 792–805, 1974. 10.2113/gsecongeo.69.6.792 CASWeb of Science®Google Scholar R. A. Zielinski, P. W. Lipman, Trace-element variations at Summer Coon volcano, San Juan Mountains, Colorado, and the origin of continental-interior andesites, Geol. Soc. Am. Bull., 87, 1477–1485, 1976. 10.1130/0016-7606(1976)87<1477:TVASCV>2.0.CO;2 CASWeb of Science®Google Scholar Citing Literature Rio Grande Rift: Tectonics and Magmatism, Volume 14 ReferencesRelatedInformation

The articles in this special issue (Vol. 89, no. 8) have been placed in the context of mineralization related to a volcano-plutonic continuum of siliceous magma bodies that rise through the crust under tectonic extension and progressively leak fluids, pyroclastic ejecta, and lava domes; explode with catastrophic evolution of ignimbrites lash-now tuffs) during concurrent caldera collapse; and continue to be a source of heat, fluids, and minor pyroclastic and lava dome eruptions. Long after original volcanism has ceased, faults related to caldera collapse and/or regional extension map control later magmatic and hydrothermal episodes. The degree to which meteoric waters are heated and mixed with magmatic fluids is the prime determinant for hydrothermal ore deposition and alteration; other factors include magma type, wall-rock composition, and stress regime. There are three principal tectono-petrographic-metallogenic associations:1. Calc-alkalic: in continental arcs and, especially, back arcs. Chalcophile base metals (including some Cu porphyries) below the zone of boiling, precious metals in the epithermal zone above; acid sulfate alteration of lava domes or near-surface intrusions if fluids are dominantly magmatic in early stages (Summitville, Colorado; Julcani, Peru); and adularia-sericite alteration in the more common case where magmatic fluids are diluted by deeply convecting meteoric waters (Creede, Colorado).2. High SiO2 and high F: in inboard continental zones of maximum extension and rifting. Mo and lithophile metals (Sn, W, U, Li) in porphyries and aplites (Questa, New Mexico) or in pyroclastic deposits and lava domes under conditions generally (but not exclusively) favoring high-temperature magmatic fluids (Taylor Creek, New Mexico).3. Alkalic affinity: ''anorogenic'' in plate interiors, Fe oxides-apatite (Kiruna, Sweden; Bafq, Iran) and Fe-Cu-U-Au-rare earth element (Olympic Dam) types; roles of magmatic and hydrothermal processes have been the subject of long-standing debates.Large (diameter = tens of kilometers; volume > 100 km(3)) resurgent ignimbrite cauldrons are the predominantly magmatic end members of a spectrum of volcano-tectonic phenomena; the tectonic end member consists of rifts and broad zones of extension with only minor leakage of magma. In the development of large ignimbrite cauldrons, the early caldera collapse stage disperses ore-forming trace elements but a later stage of ring fracture volcanism may develop conditions favoring their concentration by convecting geothermal-hydro thermal fluids in the caldera moat zone. especially if hydrologic conditions provide recharge of meteoric water. If a caldera lake is present, changes in lake level can critically affect the position of the boiling zone (Valles caldera, New Mexico), saline lake waters are especially potent for mineralization.The slow evolution since 1950 of concepts linking ignimbrites, cauldrons, and ore deposits is treated historically; it explains why these concepts have only recently been applied to world-class districts like Guanajuato, Mexico. The very size of the largest cauldrons has precluded their recognition by geologists working within the confines of a mining district. As these districts are commonly located in a caldera moat widened by postcollapse erosion, the principal ignimbrite is missing or patchy; in many instances, all volcanic rocks have been stripped from the mineralized part of the moat. In the Emery and Juniper cauldrons, New Mexico, for example, mineralization is hosted in Paleozoic carbonate rocks; the significance of distant barren volcanic rocks map not be apparent. Adjacent to the moat, caldera fill ignimbrite may be more than 1 km thick and has commonly been mistaken for granitoid rock, especially in pre-Cenozoic terranes. The distribution and nature of various breccia types may offer clues to a caldera setting.

Abstract Basic to intermediate volcanism of the Tertiary Mogollon-Datil volcanic field can be divided petrologically and geochemically into three temporal groups; Pre-30 Ma, 30–20, Ma and Post-20 Ma. The Pre-30 Ma and 30–20 Ma groups are dominated by high-K calc-alkaline andesites and mildly alkaline basaltic andesites respectively. These both have major and trace element characteristics typical of an orogenic origin. In contrast, the late Tertiary, Post-20 Ma lavas are typically alkaline basalts and have geochemical characteristics more consistent with a within-plate setting. The 30–20 Ma basic lavas have high Ba and Sr, and low Nb contents, resulting in high LIL/HFS element ratios (Ba/Nb c. 80). 87 Sr/ 86 Sr ratios are > 0.7065. These features are inferred to have been derived from continental mantle lithosphere modified by subduction-related processes in the Proterozoic. The Pre-30 Ma lavas have many similar characteristics but with Rb and Th enriched relative to Ba and Sr, lower 87 Sr/ 86 Sr, and more subalkalic parental magmas. In contrast, the Post-20 Ma lavas show a tendency to higher Nb contents (low LIL/HFS ratios), and lower 87 Sr/ 86 Sr ratios similar to OIB-like magmas derived from partial melting of the convecting asthenospheric mantle. The overall shift from predominantly lithosphere to asthenosphere-derived magmas with time in the evolution of the Mogollon-Datil volcanic field is consistent with models in which magmatism was triggered by lithosphere extension. It is concluded that calc-alkaline magmas with minor and trace element features similar to those from destructive plate margins were generated in an extensional tectonic setting. In detail, the marked change to within-plate style magmatism took place c. 10 Ma after the period of maximum extension.

Bushveld Complex and Vredefort Dome are unique features, formed in close proximity during the same time interval, approximately 2 Ga. Both show evidence of catastrophic events in the shallow marine environment of the otherwise stable Kaapvaal Craton. Explanation by multiple impacts of an asteroid, brecciated by an inter-asteroidal collision and disintegrating in Earth's gravity field is supported by pseudotachylite, shatter cones, coesite, and stishovite at Vredefort but these shock phenomena were not found in the Bushveld Complex. The Bushveld Complex was formerly interpreted as a lopolith, a view incompatible with gravity, electrical resistivity, magnetic, and seismic-reflection data. It is outlined by five inward-dipping lobes of layered ultramafic-mafic plutonic rocks that partly coalesce to form a basin-like feature 400 km in diameter and 65,000 sq. km. in area, equivalent to a small lunar mare. The Bushveld Complex is orders of magnitudes larger than other proposed terrestrial impact structures and differs from them in important ways. Its principal members, in order of age, are Rooiberg Felsite, RLS, and Lebowa Granite. The Bushveld-Vredefort events occurred during the interval from neutral or reducing atmosphere to oxidizing atmosphere. This transition is usually related to the evolution of photosynthesizing organisms. If the impact hypothesis for Bushveld-Vredefort can be confirmed, it may represent a global catastrophe sufficient to contribute to environmental changes favoring aerobic photosynthesizing eukaryotes over anaerobic prokaryotes.

Between 1217 and 1620 hours (PDT), on May 18, 1980, the magmatic eruption column of Mount St. Helens formed an ash fountain and pyroclastic flows dominated the eruption process over tephra ejection. Eurption-rate pulsations generally increased to a maximum at 1600 to 1700 hrs. After 1620 hrs, the eruption assumed an open-vent discharge with strong, vertical ejection of tephra. Relative eruption rates (relative mass flux rates) of the pyroclastic flows were determined by correlating sequential photographs and SLAR images, obtained during the eruption, with stratigraphy and surface morphology of the deposits.