Potassium- Argon dates for Trachytic rocks on Sao Miguel, Azores E.H. McKee and R.B. Moore Isochron/West, Bulletin of Isotopic Geochronology, v. 58, pp. 9-11 Downloaded from: https://geoinfo.nmt.edu/publications/periodicals/isochronwest/home.cfml?Issue=58 Isochron/West was published at irregular intervals from 1971 to 1996. The journal was patterned after the journal Radiocarbon and covered isotopic age-dating (except carbon-14) on rocks and minerals from the Western Hemisphere. Initially, the geographic scope of papers was restricted to the western half of the United States, but was later expanded. The journal was sponsored and staffed by the New Mexico Bureau of Mines (now Geology) & Mineral Resources and the Nevada Bureau of Mines & Geology. All back-issue papers are available for free: https://geoinfo.nmt.edu/publications/periodicals/isochronwest This page is intentionally left blank to maintain order of facing pages. POTASSIUM-ARGON DATES FOR TRACHYTIC ROCKS ON SAO MIGUEL, AZORES EDWIN H. McKEE U.S. Geological Survey, Menio Park, CA 94025 RICHARD B. MOORE U.S. Geological Survey, Denver, CO 80225 Recent geological mapping (Moore, 1 991), augmented by stratigraphic studies and radiocarbon and K-Ar dating, is used by Moore (1 990)to evaluate the eruptive frequency of Sao Miguel, the largest island in the Azores. The radiocarbon ages are reported by Moore and Rubin (1 991); the K-Ar ages are reported here. All rocks dated for Moore's studies are Pleistocene in age. Sao Miguel is the largest (about 62 X 1 3 km) and most populous (about 150,000 inhabitants) of the nine Azores Islands (fig. 1). Most of the France \ J[taly Azores ^ Portugal islands have been volcanically active (Portugal) (? in historic time, and five eruptions Sao have taken place on Sao Miguel in Miguel Lisbon the past 500 years. Mediterranean The Azores lie on both sides of the Strait of mid-Atlantic ridge near the triple North Gibraltar Tunisia junction of the North America, Atlantic Morocco Eurasia, and Africa plates (fig. 2). An Ocean active spreading center, the NW- trending Terceira rift, cuts the Canary Islands ® western part of Sao Miguel (Searle, Algena (Spain) 1980). South of Sao Miguel, the E-trending inactive east Azores frac / ture zone may mark the pre-Azores, Eurasian-African plate boundary 500 1000 Kilometers (Krause and Watkins, 1 970). Sao Miguel is comprised of four major volcanoes and a number of FIGURE 1. Location of the Azores west of Portugal in the North small cinder cones and local lava Atlantic Ocean (after Moore, 1990) flows (fig. 3). The three western volcanoes (Sete Cidades, Agua de Pau, and Furnas) are stratovolcanoes and are mostly trachytic; the eastern volcano (Nordeste) is a shield of 30°W 25°W alkali basalt with minor trachyte. The cinder cones and local lava flows are O Eurasia plate alkali basalt with minor trachyte and Flores/S^ ^Graciosa tristanite. — 39°N The western stratovolcanoes are Faial QTerceira Quaternary in age and many of the lava and pyroclastic units have been Pico Jorge dated by radiocarbon techniques using roots and plant material asso Africa plate Miguel ciated with them (Shotton and '' \ others, 1 968; Shotton and Williams, Santa Maria ^ 1969, 1971; Moore and Rubin, — 37°N 1 991). The Nordeste shield volcano is deeply eroded and is Pliocene and Azores Fracture Zone Pleistocene in age (Moore, 1990). 100 Kilometers K-Ar ages as old as about 4 Ma and as young as about 1 Ma have been reported from this volcano (Abdel- FIGURE 2. Azores and the major tectonic elements in its vicinity Monem and others, 1975). (after Searie, 1980). [ISOCHRON/WEST, no. 58, March 1992] 10 ROCKS DATED 25 sew 25 10'W Two lava flows from each of the three western stratovolcanoes, Sete Atlantic Ocean Cidades, Agua de Pau, and Furnas, Sete Cidades Volcano were dated by K-Ar to establish the age of the older parts of these de Pan volcanoes. Radiocarbon dates were used to outline the young history of Volcano each volcano and to provide data to Nordeste interpret eruption frequency (Moore, Volcano 1990). One sample of a xenolith from the Furnas volcano was dated mbadas by K-Ar. This xenolith is assumed to -37 45'N be from the buried shield of the Nordeste volcano directly east of the Ponta Delgada Furnas volcano. The lava flows were Furnas Povoagao dated using specially prepared whole Volcano Caldera rock samples (see sample prepara tion); the xenolith was dated using a 0 10 Kilometers K-feldspar mineral separate. L SAMPLE PREPARATION FIGURE 3. Sao Miguel Island with Its four major volcanoes. Sample localities for K-Ar age analyses are numbered (after Moore, 1990). Because of the known young age of the lava flows from the three stratovolcanoes (less than 250,000 yrs B.P.), special care based on experience with hundreds of replicated analyses the amount of radlogenically pro duced Ar from the sample related to ""Ar from atmos in the Menio Park laboratories. The constants used in age pheric contamination in the sample. Geologically young determination are those from the Subcommission on samples normally yield very low ratios of sample-produced Geochronology (Steiger and Jager, 1977). radiogenic Ar ( Ar*) to atmospheric *°Ar This small percentage of sample radiogenic *0Ar leads to a very laTge SAMPLE DESCRIPTIONS ° ! eliminate as much atmospheric Ar as Sete Cidades volcano "lO "eeted 60 100 mpch ® was crushed, sieved to K-Ar 1. MF-81-43mr-o / piLd iSa i" f Wds Tristanite flow (from base of the sea cliff on the west which it was washpH acid for 30 minutes after ern edge of Sao Miguel Island, Azores; locality 1 on thorough, washing in waS,"he samr"'®'-J^^®'' figure 3). Analytical data: K2O = 4.24%; Ar - heat lamp for several under a 4.49203 X 10-'3 mol/gm; *''Ar*/E''°Ar = 8.82/o. Collected by: R. B. Moore. sample was then weighed an^c (whole rock) 74,000 ± 6,000 yrs B.P. crucible and boiled in distlllpH f ^ molybdenum cruciblevacuum extractionwithsampte^rdedl!',"- line after a 's ultra-high- 2. MF-81-75 „ , Trachyte flow (base of the northern caldera wall, locali outStill wetof the and extraSn still warm lines than andTf n Penod but while ty 2 on figure 3, Sao Miguel Island, Azores). Analytical done In the usual maler as data: K2O = 4.57%; ""Ar* = 1.3599 x 10 Lanphere (1 969). The *°Ar*/E«oAr'an by Dalrymple and mol/gm; *°l\x*l'L*°kr = 30.1%. Collected by: R. B. 4.79%, which yielded ± val.iaa n/o n 39.9% to Moore. calculated age. f 2.9 and 9.7% of the (whole rock) 210,000 ± 8,000 yrs B.P. Aqua de Pau volcano analytical methods 3. MF-a3-15 Peralkallne trachyte flow (from the upper part of the laboratories^ of^the^U S^'^Geo'? In the western flank of the stratovolcano, locality 3 on 3- ceduresCalifornia. as Analyses described w!re?rsta^H'^®"'^®^ by Dalrvmnle^anH S3o Miguel Island, Azores). Analytical data: K2O - Potassium analyses were oerf^rmoAitv i- ® 969). 5.38%; "OAr* = 9.3769 x 10"'^ mol/gm; flux fusion flame-photometrv terhn- ^ ''^bium-metaborate *°Ar*/E''OAr = 13 01%. Collected by: R. B. Moore. ing as an Intern'al (whole rock) 121,000 ± 5,000 yrs B.P. analyses were performed using a five-collector r^Iss spec" 4. MF-83-79 ^-Ar trometer (Stacey and others, 1981). The precision of the Trachyte flow (Lombadas, locality 4 on figure 3, Sao data, indicated by the ± value. Is the estimated analytical Miguel Island, Azores). Analytical data: K2O = 6.02%; uncertainty at one standard deviation. It represents uncer ^°Ar* = 3.96551 x 10-^ mol/gm; "Ar'/E^oAr = tainties in the measurement of radiogenic ""Ar and K^O 16.67%. Collected by: R. B. Moore. (whole rock)46,000 ± 6,000 yrs B.P. [ISOCHRON/WEST,no. 58, March 1992] 11 Furnas volcano Ingamells, C. O. (1970) Lithium metaborate flux in silicate 5. MF-81-153 K-Ar analysis: Anal. Chim. Acta, v. 52, p. 32. Trachyte flow (from the western wall of the caldera, Krause, D. C., and Watkins, N. D.(1970) North Atlantic crustai locality 5 on figure 3, Sao Miguel Island, Azores). genesis in the vicinity of the Azores: Geophy. Jour. Royal. Analytical data: K2O = 6.31%;^oAr* = 4.30988 x Astron. Society, v. 19, p. 261-283. 10-^3 mol/gm; ^0Ar*/L^0Ar = 9.45%. Collected by: Moore, R. B. (1990) Volcanic geology and eruption frequency, R. B. Moore. Sao Miguel, Azores: Bull. Volcanology, v. 52, p. 602-614. (whole rock)48,000 ± 4,000 yrs B.P. (1991) Geologic map of Sao Miguel, Azores: U.S. Geological Survey Misc. Investigations Series, Map 1-2007. 6. MF-81-14 K-Ar Moore, R. B., and Rubin, Meyer(1991) Radiocarbon dates for lava Tristanite flow (from base of the sea cliff on the flows and pyroclastic deposits on Sao Miguel, Azores: Radio southern edge of Sao Miguel Island, Azores; locality 6 carbon, v. 33, no. 1, p. 151-164. on figure 3)- Analytical data: K2O = 5.0%; ^°Ar* = Searle, R.(1980) Tectonic pattern of the Azores spreading center 6.69486 X 10-^3 mol/gm; ^oAr*/E^°Ar = 4.79%. and triple junction: Earth and Planetary Science Letters, v. 51, Collected by: R. B. Moore. p. 415. (whole rock)93,000 ± 9,000 yrs B.P. Shotton, F. W., Blundell, D. J., and Williams, R. E. G. (1968) Birmingham University radiocarbon dates II: Radiocarbon, v. 10, 7. MF'81-162 K-Ar p. 200-206. Syenite xenolith from the Furnas volcano, probably Shotton, F.