Donald L. Turner

K-Ar dating of the cook-austral island chain: A test of the hot-spot hypothesis

Abstract 40 K- 40 Ar dating indicates that the Cook-Austral chain contains the island with the oldest exposed volcanic rocks on the Pacific plate (Mangaia, 19.3 ± 0.6 Ma B.P.). We have also determined ages from the previously undated islands of Atiu, Mitiaro, Mauke, and Rimatara and from Mangaia, Aitutaki and Rarotonga, for which only unlocated samples had been previously dated. Dated volcanism on Aitutaki spans an interval of at least 7 Ma. The ages from Aitutaki, Atiu, Mauke, Mitiaro, Rarotonga, and Rarutu are much younger than ages predicted by the “hot-spot” hypothesis, and ages from Rimatara may be older than predicted by the hot-spot hypothesis. However, both new and previously reported age data from Mangaia are consistent with the predicted age for this island. Virtually all age data from island and seamount chains on the Pacific plate are consistent with a “hot-line” hypothesis, which makes less specific age predictions than does the hotspot hypothesis.

The Nevados de Payachata volcanic region (18°S/69°W, N. Chile)

Subduction-related volcanism in the Nevados de Payachata region of the Central Andes at 18°S comprises two temporally and geochemically distinct phases. An older period of magmatism is represented by glaciated stratocones and ignimbrite sheets of late Miocene age. The Pleistocene to Recent phase (≤0.3 Ma) includes the twin stratovolcanoes Volcan Pomerape and Volcan Parinacota (the Nevados de Payachata volcanic group) and two small centers to the west (i. e., Caquena and Vilacollo). Both stratovolcanoes consist of an older dome-and-flow series capped by an andesitic cone. The younger cone, i. e., V. Parinacota, suffered a postglacial cone collapse producing a widespread debris-avalanche deposit. Subsequently, the cone reformed during a brief, second volcanic episode. A number of small, relatively mafic, satellitic cinder cones and associated flows were produced during the most recent activity at V. Parinacota. At the older cone, i. e., V. Pomerape, an early dome sequence with an overlying isolated mafic spatter cone and the cone-forming andesitic-dacitic phase (mostly flows) have been recognized. The two Nevados de Payachata stratovolcanoes display continuous major- and trace-element trends from high-K2O basaltic andesites through rhyolites (53%–76% SiO2) that are well defined and distinct from those of the older volcanic centers. Petrography, chemical composition, and eruptive styles at V. Parinacota differ between pre- and post-debris-avalanche lavas. Precollapse flows have abundant amphibole (at SiO2 > 59 wt%) and lower Mg numbers than postcollapse lavas, which are generally less silicic and more restricted in composition. Compositional variations indicate that the magmas of the Nevados de Payachata volcanic group evolved through a combination of fractional crystallization, crustal assimilation, and intratrend magma mixing. Isotope compositions exhibit only minor variations. Pb-isotope ratios are relatively low (206Pb/204Pb = 17.95–18.20 and208Pb/204Pb = 38.2–38.5);87Sr/86Sr ratios range 0.70612–0.70707,143Nd/144Nd ratios range 0.51238–0.51230, andγ18OSMOW values range from + 6.8%o to + 7.6%o SMOW. A comparison with other Central Volcanic Zone centers shows that the Nevados de Payachata magmas are unusually rich in Ba (up to 1800 ppm) and Sr (up to 1700 ppm) and thus represent an unusual chemical signature in the Andean arc. These chemical and isotope variations suggest a complex petrogenetic evolution involving at least three distinct components. Primary mantle-derived melts, which are similar to those generated by subduction processes throughout the Andean arc, are modified by deep crustal interactions to produce magmas that are parental to those erupted at the surface. These magmas subsequently evolve at shallower levels through assimilation-crystallization processes involving upper crust and intratrend magma mixing which in both cases were restricted to end members of low isotopic contrast.

Petrology, geochemistry, and age of the Spurr volcanic complex, eastern Aleutian arc

The Spurr volcanic complex (SVC) is a calc-alkaline, medium-K, sequence of andesites erupted over the last 250000 years by the eastern-most currently active volcanic center in the Aleutian arc. The ancestral Mt. Spurr was built mostly of andesites of uniform composition (58%–60% SiO2), although andesite production was episodically interrupted by the introduction of new batches of more mafic magma. Near the end of the Pleistocene the ancestral Mt. Spurr underwent avalanche caldera formation, resulting in the production of a volcanic debris avalanche with overlying ashflows. Immediately afterward, a large dome (the present Mt. Spurr) formed in the caldera. Both the ash flows and dome are made of acid andesite more silicic (60%–63% SiO2) than any analyzed lavas from the ancestral Mt. Spurr, yet contain olivine and amphibole xenocrysts derived from more mafic magma. The mafic magma (53%–57% SiO2) erupted during and after dome emplacement from a separate vent only 3 km away. Hybrid block-and-ash flows and lavas were also produced. The vents for the silicic and mafic lavas are in the center and in the breach of the 5-by-6-km horseshoe-shaped caldera, respectively, and are less than 4 km apart. Late Holocene eruptive activity is restricted to Crater Peak, and magmas continue to be relatively mafic. SVC lavas are plag ±ol+cpx±opx+mt bearing. All postcaldera units contain small amounts of high-Al2O3, high-alkali amphibole, and proto-Crater Peak and Crater Peak lavas contain abundant pyroxenite and anorthosite clots presumably derived from an immediately preexisting magma chamber. Ranges of mineral chemistries within individual samples are often nearly as large as ranges of mineral chemistries throughout the SVC suite, suggesting that magma mixing is common. Elevated Sr, Pb, and O isotope ratios and trace-element systematics incompatible with fractional crystallization suggest that a significant amount of continental crust from the upper plate has been assimilated by SVC magmas during their evolution.

Nature and timing of movement on Hines Creek strand of Denali fault system, Alaska

The Hines Creek strand of the Denali fault system in the central Alaska Range juxtaposes continental basement rocks on the north with younger Paleozoic and Mesozoic rocks more characteristic of an oceanic environment on the south. A pluton that intrudes the Hines Creek strand has a 95-m.y. cooling age indicated by K-Ar dating and establishes a maximum age for strata that overlie the pluton unconformably. The Hines Creek fault may have been the locus of major strike-slip movement prior to 95 m.y. ago. Subsequently, strike-slip movement along the Denali system has taken place on the McKinley strand 32 km to the south.

Radiometric Ages of Kodiak Seamount and Giacomini Guyot, Gulf of Alaska: Implications for Circum-Pacific Tectonics

Kodiak Seamount and Giacomini Guyot have been dated at 22.6 � 1.1 and 19.9 � 1.0 [2σ (standard deviation)] x 106 years, respectively. Concordant whole-rock and plagioclase potassium-argon dates and fission-track apatite ages demonstrate that significant quantities of excess radiogenic 40Ar are not present in the dated samples. These seamounts are the northwesternmost edifices of the Pratt-Welker chain, which cuts obliquely across magnetic anomaly patterns in an older northeastern Pacific sea floor. The older of the two dated seamounts is in the Aleutian Trench, apparently about to be subducted. If one assumes that seamounts are generated by plate motion over a fixed hot spot in the mantle, a Pacific-plate motion of 6.6 centimeters per year during early Miocene time may be calculated.

Blueschists of the Kodiak Islands, Alaska: An extension of the Seldovia schist terrane

The Kodiak Islands are composed of a series of northeast-trending belts of schists and deep-sea rock types that are interpreted as having been accreted to the continental margin during several discrete phases of subduction since early Mesozoic time. The Kodiak Islands schist terrane is the oldest of these accretionary belts and crops out discontinuously along the northwest side of the islands. Metamorphic rocks in this belt include quartz-mica schist, marble, metachert, greenschist, blueschist, and epidote amphibolite; the rocks yield Early Jurassic K-Ar mineral ages. These ages apparently provide a measure for the age of emplacement of the Kodiak Islands schists and are consistent with independently determined age estimates based on (1) biostratigraphy of the associated forearc basin deposits and (2) K-Ar ages from the associated plutonic arc on the Alaska Peninsula. Similarities in rock types, mineral ages, and tectonic setting indicate that the Kodiak Islands schist terrane is the southwestern extension of the Seldovia blueschists of the Kenai Peninsula.

Tectonic implications of paleomagnetic and geochronologic data from the Yukon-Koyukuk province, Alaska

The paleomagnetic and geochronologic record of Alaska is complicated by overprints that make it difficult to relate the terranes of northern and southern Alaska to each other and to North America. To better understand these relationships and the overprinting, paleomagnetic and K-Ar dating samples were analyzed from the three major geologic units of the Yukon-Koyukuk province (YKP) of central Alaska. These units consist of a basal Jurassic-Early Cretaceous volcanic (island-arc) assemblage, Albian-Cenomanian clastic sedimentary rocks, and bimodal Eocene volcanic rocks. K-Ar age determinations of the early Tertiary volcanic rocks form two distinct groups. The older group (65–49 Ma) is mostly felsic and probably represents the maximum inboard penetration of low-angle subduction-related volcanism. These older rocks were deformed during the regional deformation of the YKP. The younger group (44−43 Ma) is post-deformational and composed dominantly of basalt. These results bracket a time (49−44 Ma) when deformation ceased in the province. This time interval is coincident with a major transition from convergence to strike-slip and extensional tectonic style throughout Alaska. The geologic units of the YKP display two characteristic paleomagnetic signatures. Primary directions (based on positive reversal, conglomerate, and/or bedding tilt stability tests) were obtained from the Albian-Cenomanian sedimentary rocks and Eocene volcanic rocks. The mid-Cretaceous sedimentary rocks show about 15° of poleward motion between 90−56 Ma (a time of high rates of convergence between Alaska and the Pacific and Eurasian plates). Age-equivalent rocks from neighboring regions (St. Matthew Island and the Alaskan North Slope) also indicate similar amounts of poleward (northward) motion, which suggests that the whole of western and northern Alaska may have moved northward about 10° relative to North America as a more or less coherent block. Paleomagnetic results from the Eocene volcanic rocks indicate that the YKP was in place by 56 Ma and formed part of the accretionary nucleus that served as a backstop for the accreted terranes of southern Alaska. Secondary (remagnetized) directions are recorded in the basal island-arc assemblage and in Albian-Cenomanian sediments near younger igneous occurrences. The clustering of secondary directions in the geographic reference frame indicates that they were acquired after deformation ceased (⩽49−44 Ma), which is consistent with the coincidence of the mean of these directions and the 54−44 Ma North American reference direction.

Radiometric dating of ash partings in Alaskan coal beds and upper Tertiary paleobotanical stages

New K-Ar and fission-track ages from volcanic ash partings in coal beds on the Kenai Peninsula, Alaska, substantiate an 8-m.y. age for the paleobotanical Homerian-Clamgulchian Stage boundary. An age estimate for the Seldovian-Homerian Stage boundary indicates that the Clamgulchian Stage spans an interval of at least 3.3 m.y. Direct radiometric correlations can now be made between these paleobotanical stages and radiometrically dated time-stratigraphic units in other parts of the world. Ash partings in coals can provide radiometric age control for stratigraphic correlations in terrestrial sections that may otherwise be difficult, if not impossible, to date or correlate by other means. The possibility of detrital contamination in ash-parting samples for dating purposes exists, but careful use of this coal-bed correlation technique can aid in the assessment of coal and petroleum resources in coal-bearing, terrestrial sedimentary sequences.

Radiometric age of the Chickaloon Formation of south-central Alaska: Location of the Paleocene-Eocene boundary

K-Ar and fission-track ages from two volcanic ash partings in upper Chickaloon Formation coals range from 53.3 ± 1.5 m.y. to 55.8 ± 1.7 m.y. These ages establish that the Paleocene-Eocene boundary (55 m.y.) occurs within the dated upper part of this formation.

Radiometric dating of ash partings in coal of the Eocene Puget Group, Washington: Implications for paleobotanical stages

New 40 K- 40 Ar and fission-track ages from volcanic ash partings in coal beds of the Eocene Puget Group of western Washington indicate a time span of about 41.2 ± 1.8 to 45.0 ± 2.1 (2σ) m.y. for the 1,890-m section of sediments exposed in the Green River area. These age data do not entirely support the previous early Eocene through early Oligocene paleobotanical age estimates for this section and for the four paleobotanical stages defined within it. Radiometric dating of floras assigned to the same stages outside the type section appears to be partially inconsistent with radiometric ages from the type section.