Age Of Volcanic Rocks Research Articles

Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico

Research Article| October 01, 1997 Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico Fred W. McDowell; Fred W. McDowell 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Jaime Roldán-Quintana; Jaime Roldán-Quintana 2Instituto de Geología, Universidad Nacional Autónoma de México, Apartado Postal 1039, Hermosillo, Sonora 83000, México Search for other works by this author on: GSW Google Scholar Ricardo Amaya-Martínez Ricardo Amaya-Martínez 3Departamento de Geología, Universidad de Sonora, Hermosillo, Sonora 83000, México Search for other works by this author on: GSW Google Scholar GSA Bulletin (1997) 109 (10): 1349–1360. https://doi.org/10.1130/0016-7606(1997)109<1349:IOSAVD>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Fred W. McDowell, Jaime Roldán-Quintana, Ricardo Amaya-Martínez; Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico. GSA Bulletin 1997;; 109 (10): 1349–1360. doi: https://doi.org/10.1130/0016-7606(1997)109<1349:IOSAVD>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Clastic sedimentary deposits and associated volcanic rocks record the progress of Tertiary extension in the Mexican state of Sonora. These deposits accumulated within basins located throughout the eastern two-thirds of the state. The rocks are slightly indurated conglomerates and sandstones; clast types reflect the local highland exposures. Volcanic units that bound and are interlayered with the sedimentary rocks provide stratigraphic and time markers of basin evolution and, by inference, of marginal fault development. This record has been examined in an east-west belt across south-central Sonora.The volcanic rocks occur in three distinct associations. At the base and interbedded within the lower portions of the sedimentary sections are lava flows of dominantly basaltic andesite composition. In most localities the overlying clastic sedimentary strata are conformable with these lava flows, and we conclude that the magmatism was triggered by early faulting along the basin margins. A second association present near the base of the sections comprises massive lava domes and flows of intermediate composition that contain distinctive phenocrysts of dark brown amphibole. The domes apparently formed when viscous lava welled up along developing marginal basin faults. In one case a transition was observed over a short distance from a structureless dome to a thick lava flow that is conformably interbedded with, and sheds clasts laterally into, the sedimentary section. A third volcanic association comprises layers of rhyolitic lava flows and ignimbrites that overlie the coarsest and thickest lower portions of the sedimentary sections.K-Ar ages for volcanic rocks in the two oldest of these settings indicate that each Tertiary basin had a distinct history with no apparent regional geographic pattern. In the Rio Yaqui basin, where exhumation by the major river of Sonora has exposed a full section of the volcanic and clastic sequences, extension was possibly as old as 27 Ma. Elsewhere, deposition of sediments was underway in most basins by 24 Ma, and in all basins by 20 Ma. K-Ar ages of the younger rhyolitic volcanic rocks fall within a narrow range from 12.8 to 10.5 Ma. These ages provide a younger time limit to deposition of the coarser and thicker portions of the clastic sedimentary sections.The mafic lava flows, dominantly basaltic andesite, are similar in major-element composition to mafic lava flows that cap felsic sections within the Sierra Madre Occidental volcanic field of western Mexico and the mid-Tertiary Datil-Mogollon volcanic field in southwestern New Mexico. They are higher in silica and lower in total alkalies than younger capping mafic lava flows within the Gulf of California extensional province of western Sonora and Neogene basaltic lava flows in the Datil-Mogollon field. They are similarly distinct from basaltic dikes and lava flows that are clearly associated with Basin and Range faulting in Trans-Pecos Texas and in northern Durango state.A convergent-margin magmatic arc was active in the present-day Gulf of California region between 24 and 11.5 Ma. The earliest extension in south-central Sonora was therefore occurring in a back-arc setting. Continued sedimentation and rotation of volcanic strata indicate that extension continued throughout the region after 10 Ma with little coeval volcanism. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

山陰西部山島火山岩の産状とＫ‐Ａｒ年代

山陰西部山島火山岩の産状とＫ‐Ａｒ年代

大阪東南部，二上層群の中新世火山岩の放射年代

大阪東南部，二上層群の中新世火山岩の放射年代

U–Pb zircon geochronology of Lower Jurassic and Paleozoic Stikinian strata and Tertiary intrusions, northwestern British Columbia

New U–Pb zircon ages are reported from western Stikinia. Devonian and Pennsylvanian ages of volcanic rocks at Oweegee dome confirm the presence of pre-Permian strata, and with Paleozoic and Triassic detrital zircons from Lower Jurassic sandstone, they help to demonstrate pre-Lower Jurassic deformation and uplift. The absence of pre-Paleozoic inherited zircon from all samples is consistent with Nd–Sr isotopic data which suggest that Stikinia consists mainly of juvenile crust. U–Pb ages for posttectonic intrusions suggest that structures in Skeena Fold Belt in the Kinskuch area formed prior to Eocene time. Five ages for felsic volcanic rocks from stratigraphically well-constrained upper parts of the Hazelton arc are approximately 196–199 Ma and suggest near-contemporaneity for cessation of volcanism in the areas studied. The Sinemurian or late Sinemurian – early Pliensbachian ages are older than previously reported U–Pb and biostratigraphic ages for presumed correlative rocks to the west, and westward-migrating volcanism is implied. Together with Toarcian fossils from overlying sandstone, the new ages suggest that a hiatus of moderate duration preceded regionally extensive sedimentation.

Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge

Research Article| May 01, 1995 Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge G. M. Yogodzinski; G. M. Yogodzinski 1Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14850 Search for other works by this author on: GSW Google Scholar R. W. Kay; R. W. Kay 1Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14850 Search for other works by this author on: GSW Google Scholar O. N. Volynets; O. N. Volynets 2Institute of Volcanic Geology and Geochemistry, Petropavlovsk, Kamchatka 683006, Russia Search for other works by this author on: GSW Google Scholar A. V. Koloskov; A. V. Koloskov 2Institute of Volcanic Geology and Geochemistry, Petropavlovsk, Kamchatka 683006, Russia Search for other works by this author on: GSW Google Scholar S. M. Kay S. M. Kay 3Institute for the Study of the Continents and Department of Geological Sciences, Cornell University, Ithaca, New York 14850 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1995) 107 (5): 505–519. https://doi.org/10.1130/0016-7606(1995)107<0505:MAITWA>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation G. M. Yogodzinski, R. W. Kay, O. N. Volynets, A. V. Koloskov, S. M. Kay; Magnesian andesite in the western Aleutian Komandorsky region: Implications for slab melting and processes in the mantle wedge. GSA Bulletin 1995;; 107 (5): 505–519. doi: https://doi.org/10.1130/0016-7606(1995)107<0505:MAITWA>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The role of the subducting lithospheric slab in the genesis of mantle-derived (primitive) magmas is investigated through a study of volcanic rocks formed in the tectonically strike-slip–dominated western Aleutian arc. Two types of chemically and petrologically distinctive primitive andesites have been found among the Miocene–late Pleistocene–age volcanic rocks in the western Aleutians. These are termed the “Adak-type” and “Piip-type” magnesian andesites. Trace element and isotopic characteristics indicate that Adak-type magnesian andesites (adakites) formed principally as small percentage melts of the basaltic portion of the subducting oceanic crust, leaving a clinopyroxene-garnet-rutile residual mineralogy. The resulting slab melt signature (high La/Yb, Sr) distinguishes Adak-type magnesian andesites from all other Aleutian volcanic rocks. Primitive characteristics (high Mg#, Cr, Ni) and intermediate compositions (∼59% SiO2) of Adak-type magnesian andesites were acquired by interaction with peridotite and/or basalt in the mantle wedge. The absence of olivine phenocrysts from Adak-type magnesian andesites indicates that they were not equilibrated with peridotite and so are unlike Piip-type magnesian andesites, which appear to have equilibrated under low pressure and hydrous conditions in the subarc mantle. Piip-type magnesian andesites also contain a slab melt component, but reaction-equilibration with peridotite has lowered La/Yb and Sr to levels like those of common Aleutian volcanic rocks.Miocene-age calc-alkaline rocks of the Komandorsky Islands have chemical characteristics transitional between those of Adak-type magnesian andesites and common Aleutian volcanic rocks from the central and eastern arc. In a source mixture of depleted mantle wedge, slab melt, and sediment, the Komandorsky rocks have a relatively large contribution from the slab melt endmember. The strong slab melt signature among western Aleutian rocks is attributed to highly oblique convergence that produced a slow subduction path into the subarc mantle. Geochemically, the slab melt provided a high Sr, La/Yb, La/Ta, and low Ti/Hf endmember to the western Aleutian source mixture. The enhanced role for slab melting in the western Aleutians may be like that predicted for Archean systems and for modern systems where the subduction zone is warm. In this regard, Adak-type magnesian andesites are probably the appropriate analog to sanukitoids and other primitive andesitic rocks of Archean age. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Te- and Se-bearing epithermal Au-Ag mineralization, Prasolovskoye, Kunashir Island, Kuril island arc

Au-Ag-Te-Se-bearing epithermal mineralization at Prasolovskoye, northwest Kunashir Island Kurils, occurs along fault zones developed within a circular intrusive zone in Miocene to Quaternary age volcanic rocks. The ores show complex textures and mineralogy and can be divided into three stages based on tectonic fracturing and mineral associations: pyritic (I), polymetallic (II), and gold-silver (III). Stages I and II occur as hydrothermal replacement zones, veins, and breccias and are base metal sulfide rich without economic gold and silver grades. Fluid inclusion and sulfur isotope data indicate that stages I and II formed from relatively high temperature (up to 340'C) fluids with salinities of 1.2 to 5.5 wt percent NaCl equiv. Following intrusion of rhyolitic dikes after stages I and II, gold-silver mineralization (stage III) formed during the late Pliocene (based on a K-Ar age of 1.94 + or - 0.10 Ma for a gold-bearing, adularia- rich vein sample). Vein-related, wall-rock alteration zones (0.5-2.5 m thick) occur along stage III veins, overprinting pervasive propylitie assemblages, and consist mainly of silicification and sericitization. Stage III ore mineralization is composed of four substages which show a progressive change in mineralogy with increasing paragenetic time: substage IIIA, bismuthinite-cassiterite-quartz; IIIB, polysulfide quartz; IIIC, telluride quartz; IIID, calcite-adularia-quartz. Each substage contains economic concentrations of gold and silver and shows distinct mineral assemblages with specific Au/Ag ratios and gold fineness values. Telluride quartz veins (usually 1-2 m thick) of substage IIIC are most typical of economic veins at Prasolovskoye and show characteristics of adularia-sericite-type deposits. Ore minerals in this substage consist of sulfides, tellurides, selenides, sulfosalts, native metals, sulfates oxides hydroxides, and tellurates. Fluid inclusion data indicate that the main Au-Ag-Te-Se mineralization during substage IIIC occurred at temperatures between 150 degrees and 245 degrees C from low-salinity (0.4-1.6 wt % NaCl equiv) fluids. Gold deposition was most likely the result of boiling of hydrothermal fluids. Measured and calculated O and H isotope compositions of stage III fluids (delta 18 O = -4.7 to -9.49%; delta D = -80 to -83%) indicate meteoric water dominance during the Au-Ag-Te-Se mineralizing event.

K-Ar Ages of Volcanic Rocks from the Taradake Area

K-Ar Ages of Volcanic Rocks from the Taradake Area

Ｋ‐Ａｒ年代から見た信越高原地域の火山活動

In order to clarify the temporal and spatial patterns in arc volcanism, 55 K-Ar ages of volcanic rocks from 17 volcanoes in Shin-etsu Highland, central Japan were determined. In addition, life spans, volume of erupted materials and eruption rates of each volcano were estimated. Graphical analysis demonstrates that volume of ejecta varies proportionately with both life span and eruption rate, and that there is no significant correlation between eruption rate and distance from the volcanic front. The life span of each volcano in this Highland is less than 0.6 m. y. In the central Shiga and southern Asama area, the volcanism started at 1 Ma and is still active. However the former had a peak in the activity at around 0.5 Ma, while the latter is apparently most intense at present. Northern Kenashi area has the volcanism without peak in 1.7-0.2 Ma, though the activity within a volcanic cluster or chain in central Japan lasts generally for 1 m. y. or less with a peak.

Paleomagnetism and K‐Ar ages of volcanic rocks from Long Valley Caldera, California

Paleomagnetic measurements and K‐Ar age determinations on volcanic rocks from Long Valley caldera, California, have enabled further refinement of eruptive activity within this large silicic volcanic center. K‐Ar age determinations show that postcaldera volcanic eruptions began 0.73 m.y. ago and continued periodically until about 50,000 years ago. The eruptions were not temporally random but tended to occur in distinct episodes separated by periods of quiescence. Volcanism in the western half of the caldera was particularly intense between 0.15 and 0.50 m.y. ago, when many units ranging in composition from basalt to rhyolite were erupted. An average of the virtual geomagnetic poles for 33 units from the caldera yields a paleomagnetic pole at 89.7°N, 138.4°E, (α95 = 5.1°), which is indistinguishable from the earth's rotational axis. The ancient geomagnetic field dispersion about this mean pole is 16.0°, with upper and lower limits of 19.3° and 13.6°, respectively.

Strike-slip faulting and block rotation in the Lake Mead fault system

Research Article| December 01, 1986 Strike-slip faulting and block rotation in the Lake Mead fault system Hagai Ron; Hagai Ron 1Department of Geophysics, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar Atilla Aydin; Atilla Aydin 2Department of Geosciences, Purdue University, West Lafayette, Indiana 47907 Search for other works by this author on: GSW Google Scholar Amos Nur Amos Nur 3Department of Geophysics, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar Author and Article Information Hagai Ron 1Department of Geophysics, Stanford University, Stanford, California 94305 Atilla Aydin 2Department of Geosciences, Purdue University, West Lafayette, Indiana 47907 Amos Nur 3Department of Geophysics, Stanford University, Stanford, California 94305 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1986) 14 (12): 1020–1023. https://doi.org/10.1130/0091-7613(1986)14<1020:SFABRI>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Hagai Ron, Atilla Aydin, Amos Nur; Strike-slip faulting and block rotation in the Lake Mead fault system. Geology 1986;; 14 (12): 1020–1023. doi: https://doi.org/10.1130/0091-7613(1986)14<1020:SFABRI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Strike-slip faults in the Basin and Range province have often been considered passive boundaries between differentially extended domains of tilted normal faults and are thus considered secondary in accommodating regional horizontal deformation. Paleomagnotic investigation of late Miocene age volcanic rocks, displaced by the left-lateral fault system of Lake Mead, Nevada, shows: (1) that these rocks have not been affected by significant structural tilt, the difference between observed and expected inclinations being only −0.6° ± 14.9° and (2) a significant horizontal counterclockwise rotation of −29.4° ± 8.5° about a vertical axis. This rotation was accommodated by slip on northwest-trending, right-lateral strike-slip faults; this implies significant west-northwest elongation. Results of the investigation indicate that strike-slip faulting is the primary process accommodating crustal deformation along the Lake Mead fault system and that tilting in response to normal faulting is secondary. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

K-Ar Ages of Volcanic Rocks from Easter Island

K-Ar Ages of Volcanic Rocks from Easter Island

Paleomagnetic Study and K-Ar Age of Volcanic Rocks from Kayo-dake, Akita Prefecture, Northeast Japan

Paleomagnetic Study and K-Ar Age of Volcanic Rocks from Kayo-dake, Akita Prefecture, Northeast Japan

K-Ar ages of volcanic rocks from the northern area of the Yatsugatake volcanic chain,central Japan.

K-Ar ages of volcanic rocks from the northern area of the Yatsugatake volcanic chain,central Japan.

能登半島新第三紀火山岩のＫ‐Ａｒ年代

K-Ar age determinations were carried out on 12 volcanic rocks from the Noto Peninsula. Ages range from 8.64 to 27.9 Ma and are generally consistent with paleontological and stratigraphical data. Ages of volcanic rocks from the Anamizu Formation are grouped into two: 24-28 Ma in the northeast part and 16-17 Ma in the northwest part. Rocks from the Yanagida Formation give ages of 15.2 and 19.7 Ma, the former is slightly younger than expected by micropaleontological data. The Iwakurayama Rhyolite and the Kurosaki Volcanics are dated at 16.1 and 8.64 Ma, respectively.

87Rb- 87Sr constraints on the genesis and evolution of the Cantal continental volcanic system (France)

We have investigated 24 whole rocks and mineral separates of five different rock types from the Cantal shield volcano in France, applying high-precision Rb-Sr techniques. The chemical and isotopic systematics suggest the distinction of two series throughout the different rock classes, one practically uncontaminated, the other seriously influenced by wall rock assimilation. The first group comprises basalts and intermediate rocks with 87Sr/ 86Sr= 0.70340–0.70382. The second group in addition includes rhyolites and the corresponding 87Sr/ 86Sr ratios vary between 0.70421 and 0.71270. The data of mineral separates support the hybridization hypothesis and possibly suggest an original 87Sr/ 86Sr ratio of 0.7028 for the magma source region. Moreover they provide internal isochron ages which place a period of extensive volcanic activity at 8.1–8.8 m.y. ago in accord with K-Ar ages of volcanic rocks from the center of the Cantal volcano.

Subduction, Buoyant Braking, Flipping, and Strike-Slip Faulting in the Northern Caribbean

The distribution of metamorphic rocks and their division into continental and oceanic types, the distribution of ophiolites and alpine ultramafic rocks, and the distribution and age of volcanic rocks in the Greater Antilles indicates that a northward-dipping subduction zone may have been the first orogenic event in the northern Caribbean. Buoyant continental crust entering the zone from the south stopped that subduction in Late Jurassic or Early Cretaceous time (120-130 m.y.b.p.), as evidenced by lithologic types and metamorphic rock ages in Cuba, Dominican Republic, and Puerto Rico. Continued compression resulted in a new, "flipped" subduction zone dipping southward on the north side of the old island arc. The second episode of subduction began about 110 m.y. ago and formed most of the exposed island arc. Two alternative models are proposed: 1) continuous subduction on a variably-dipping plane such that the western segment dipped too gently to permit volcanism after 85 m.y.b.p.; 2) subduction converted about 85 m.y.b.p. into strike-slip motion with anatectic volcanism. In both models, intense deformation ended at 45 m.y.b.p., volcanism ended in the west at 85 m.y.b.p. and in the east at 45 m.y.b.p. Still-continued compression of the Caribbean microplate since 45 m.y.b.p. has resolved itself into an eastward transform motion which continues to the present.

Cretaceous K‐Ar ages of volcanic rocks from the Musicians Seamounts and the Hawaiian Ridge

K‐Ar conventional and 40Ar/39Ar ages on plagioclase from altered basalt samples show that Khatchaturian and Rachmaninoff Seamounts in the Musicians Seamounts, north‐central Pacific, are 65.2 ± 2.6 and 86.6 ± 5.2 m.y. old, respectively. The minimum age of Wentworth Seamount, located on the Hawaiian Ridge near Midway, is 71 ± 5 m.y. Wentworth appears to be a Cretaceous volcano that was incorporated into the Hawaiian volcanic chain. A single boulder of rhyolite dredged from the northern slope of the seamount underlying Necker Island has an age of 77.6 ± 1.7 m.y. Apparently Necker is a composite seamount constructed of both Cretaceous and late Tertiary volcanoes. With one possible exception, the ages of these and other Cretaceous seamounts in the north‐central Pacific are less than or equal to the age of the adjacent sea floor indicating that the seamounts formed at or near the crest of the East Pacific Rise.

Geochronology of Archean and Proterozoic Rocks in the Southern District of Keewatin

Rb–Sr and U–Pb dating techniques have been utilized to identify and date Archean supracrustal rocks within the Churchill structural province in regions where K–Ar age determinations have recorded only the effects of younger Hudsonian orogeny. The age of emplacement of Archean granodiorite has been established at 2550 m.y., a determination that also provides a minimum age for volcanic rocks intruded by the granodiorite.The overlying Proterozoic Hurwitz Group volcanic rocks have been dated for the first time at 1808 ± 35 m.y. (Upper Aphebian). A post-Hurwitz Group quartz monzonite pluton intruded the granodiorite gneiss at 1772 ± 22 m.y. and the age of the post-tectonic Nueltin Lake Granite has been established at 1700 ± 16 m.y. (Paleohelikian).It is concluded that the Hurwitz Group cannot be correlated with the Huronian succession in Ontario as the Hurwitz Group rocks are 300 to 400 m.y. younger than the Huronian strata.

Potassium‐argon ages on the Cainozoic Volcanic rocks of Eastern Victoria, Australia

Abstract The Cainozoic volcanic rocks of eastern Victoria comprise most of the Older Volcanic Series. K‐Ar ages of volcanic rocks in South Gippsland range from middle Palaeocene to middle Eocene; the older ages are to the east. In the Eastern Highlands the majority of lavas are of late Eocene to middle Oligocene age, with some lavas of late Oligocene to Pliocene age. The Cainozoic volcanic activity in Victoria is thought to have been almost continuous since it started in the early Palaeocene, with peaks of activity from Palaeocene to Eocene and from Pliocene to Pleistocene. The uplift of the Eastern Highlands is thought to have taken place in three sub‐equal stages, starting in the middle Mesozoic. The last uplift is inferred to have begun some time after the Oligocene, and to have resulted in less than 300 m of uplift in the central and southern parts of the highlands, and negligible uplift in the northern. In the Oligocene the highlands had a relief of over 1000 m, and a drainage system similar to that ...

The effect of hydration on the K/Ar ages of volcanic rocks

To evaluate the effect of alteration on the K/Ar ages of volcanic rocks, the relation between the apparent K/Ar age and the H 2O(+) content was examined. For obsidian, there is a clear indication that hydrated samples lost some of their radiogenic argon, while fresh obsidians show more reliable K/Ar ages. For dredged submarine basalts, a similar tendency is observed: rocks with higher H 2O(=) contents show K/Ar ages younger than the rocks with lower H 2O(=) contents. Similar phenomena are also observed for subaerial basalts and andesites, although the tendency is sometimes less clear than in the obsidians and the submarine basalts. From these results, it is suggested that H 2O(=) content of volcanic rocks might be used as a criterion to check the validity of K/Ar ages: incomplete retention of radiogenic argon may occur in volcanic rocks containing more than 1% H 2O(=).