Showing papers by "Paul W. Layer published in 2001"

Miocene high-pressure metamorphism in the Cyclades and Crete, Aegean Sea, Greece: Evidence for large-magnitude displacement on the Cretan detachment

Abstract: The Cyclades in the backarc region of the present Hellenic subduction zone are known for widespread Late Cretaceous to Eocene high-pressure metamorphism in the Cycladic blueschist unit. We report 40 Ar/ 39 Ar and Rb/Sr phengite ages of 24–21 Ma for high- pressure metamorphism (8–10 kbar, 350–400 °C) in the lowest tectonic unit in the Cyclades, the Basal unit, which structurally underlies the Cycladic blueschist unit. The Basal unit is correlated with the Tripolitza unit of the External Hellenides in the forearc region of the Hellenic subduction zone. The Tripolitza unit is unmetamorphosed on Crete, where it is separated from the underlying high-pressure (8–10 kbar, 300–400 °C) Plattenkalk and Phyllite-Quartzite units by the extensional Cretan detachment. The age for high- pressure metamorphism in the latter units is similar to our age for the Basal unit in the Cyclades. Because pressure-temperature conditions in the Plattenkalk and Phyllite- Quartzite units on Crete and the Basal unit in the Cyclades are also similar, they must have been in close proximity in the early Miocene Hellenic subduction zone. A palinspastic reconstruction suggests a subsequent displacement of >100 km on the Cretan detachment. This is one of the greatest displacement magnitudes ever reported from detachment faults. Because of this large offset, the Cretan detachment was an efficient agent for exhuming high-pressure rocks.

Tectonic setting of the plutonic belts of Yakutia, northeast Russia, based on 40Ar/39Ar geochronology and trace element geochemistry

Abstract: Samples from plutonic belts and individual intrusions in the Sakha Republic (Yakutia), northeastern Russia, were analyzed using 40Ar/39Ar geochronology and trace element geochemistry. Four phases of activity were identified: a subduction event along the southwest margin of the Kolyma-Omolon superterrane with apparent ages of ca. 160–140 Ma, a superposed collisional event dated at 143–138 Ma, a subduction event along the northern margin of the superterrane dated at 130–123 Ma with a slightly older to contemporaneous extensional event to the west; and intrusions associated with east-west extension between 106 and 92 Ma. These phases are interpreted to represent (1) subduction under and (2) accretion to Asia of Kolyma-Omolon followed by local extension, (3) subduction along the northern edge of this superterrane, and (4) regional extension following closure of the South Anyui suture and/or the start of subduction associated with the Okhotsk-Chukotka volcanic belt.

The Giant Muruntau Gold Deposit: Geologic, Geochronologic, and Fluid Inclusion Constraints on Ore Genesis

Abstract: We reinterpret the regional geologic setting of the giant Muruntau gold deposit and report new 40 Ar/ 39 Ar isotope age determinations and a laser Raman microprobe analysis of fluid inclusions. New 40 Ar/ 39 Ar isotope age determinations of hydrothermal sericite selvages to gold-stage quartz veins are in excellent agreement with older Rb-Sr dates on auriferous quartz veins. They suggest Triassic sericite formation at 245 and 220 Ma, some 30 m.y. later than subjacent felsic intrusions as defined by Rb-Sr data. These dates call into question the role of magmatic fluid, metal, and heat input. Indeed, the Rb-Sr pluton ages may be a reflection of hydrothermal activity rather than cooling from magmatic conditions. The Muruntau deposit is situated adjacent to a major rift, which has been periodically active from the Devonian to the Tertiary and has accumulated a substantial thickness of chemical, volcaniclastic, and clastic sediments, including red beds and evaporites. We infer that the Muruntau deposit is so large because of the presence of a major source of reduced sulfur in these nearby evaporitic sediments, via thermochemical sulfate reduction. Some support for the role of reduced sulfur is provided by the presence of detectable H2S in fluid inclusions from late-stage auriferous quartz veins. Reduced sulfur-rich fluids could have been focused into the depositional site along major northeast-trending structures, which have influenced rift-basin architecture as recently as the Jurassic. Oxidation could, therefore, be an important ore-precipitating mechanism. A change from early CH4- to later syngold CO2dominant fluid inclusions may be an indication of this process.

Excess argon in Mount St. Helens plagioclase as a recorder of magmatic processes

Abstract: Excess argon in plagioclase crystals from young (<4000 ybp) tephra layers from Mount St. Helens, Washington, illustrates the importance of argon isotopes in understanding magmatic processes. 40Ar/39Ar step-heating identifies two distinct argon reservoirs in the plagioclase; a high Cl/K, low Ca/K reservoir with atmospheric 40Ar/36Ar and a low Cl/K, higher Ca/K reservoir with variable 40Ar/36Ar. The first is probably glass, whereas the second is “true” plagioclase. Felsic dacite plagioclases have little or no excess argon, but those from more mafic layers have significantly higher 40Ar/36Ar, indicating a non-atmospheric source. These variations are seen within a layer that contains both mafic and felsic dacite, and are inconsistent with either xenocrystic or restitic origin for plagioclase. The magma chamber exhibits long-term open-system degassing behavior punctuated by short-term fluctuations from influx of high 40Ar/36Ar basalt. The relative timing between mixing and eruption can affect the amount of excess argon recorded in plagioclase.