Spatial distribution and alignments of volcanic centers: Clues to the formation of monogenetic volcanic fields

http://www.mantleplumes.org/WebDocuments/Valentine2007.pdf

Tectonically controlled, time-predictable basaltic volcanism from a lithospheric mantle source (central Basin and Range Province, USA)

Volcanic rift zones and their intrusion swarms

Nd and Sr isotopic compositions were determined for the Tertiary volcanics from the back-arc side of the NE Japan arc. Sr isotopes show a linear trend through time from an enriched signature (87Sr86Sr = 0.705437) to a depleted signature (87Sr86Sr = 0.70270). In a complementary fashion, Nd isotopes start at low value (ϵNd = −0.80) and show a gradual increase (ϵNd=8.3) with decreasing age. Isotopic change of Nd and Sr from the enriched signature to the depleted one is synchronous with the opening of the Japan Sea at ∼ 15 Ma. This synchronism indicates that the opening of the Japan Sea was initiated by the injection of the asthenosphere. During the pre-opening stage, the mantle wedge was composed of a two-layered structure: the sub-continental lithosphere and the underlying asthenosphere. The volcanics of this stage characterized by an enriched isotopic signature were derived from a source with a higher proportion of sub-continental lithosphere. The sub-continental lithosphere of the back-arc side was thinned by the injection of the depleted asthenosphere, which accelerated the growth of the MORB source within the mantle wedge of the back-arc side and resulted in magma generation with the depleted isotopic signature of Nd and Sr at the post-opening stage.

/pdf/asthenospheric-injection-and-back-arc-opening-isotopic-4m8wh5z5i5.pdf

Asthenospheric injection and back-arc opening: Isotopic evidence from Northeast Japan

The drivers behind the inception of, and the variable, pulsatory eruption rates at distributed intraplate volcanic fields are not well understood. Such broad areas of monogenetic volcanism cover vast areas of the world and are often heavily populated. Reliable models to unravel their behaviour require robust spatio-temporal frameworks within the fields, but an analysis of the potential proximal and distal regional volcano-tectonic processes is also needed. Jeju Island (Republic of Korea) is a volcanic field that has been extensively drilled and dated. It is also located near one of the world’s best-studied tectonic plate boundaries: the subduction zone in southwestern Japan, which generates the Ryukyu and SW Japan arcs. A new set of 40Ar/39Ar ages collected from cores penetrating the entire Jeju eruptive pile, along with geochemical information, is used to construct a temporal and volumetric model for the volcano’s growth. The overall pattern indicates inception of volcanism at ~1.7 Ma, with an initial 1.2 Myr of low-rate activity, followed by over an order of magnitude rise over the last 0.5 Myr. The magma flux at Jeju correlates well with increased extension rates in the arc/backarc region. In particular, we infer that the increased trenchward mantle flow, caused by the greater rollback of the Philippine Sea Plate, activated pre-existing shear weaknesses in the mantle beneath Jeju, resulting in mantle upwelling and decompression melting that caused a change in compositions and an increase in eruption rates at Jeju. Thus, the volcanic activity of an intraplate field system can be modulated by regional subduction processes occurring more than 650 km away. This model may explain the frequent observation of pulsatory behaviour seen in many monogenetic volcanic fields worldwide that lie within 1,000 km of subduction zones.

Intraplate volcanism influenced by distal subduction tectonics at Jeju Island, Republic of Korea

Cenozoic lithospheric extension induced magmatism in Southwest Japan

An unspiked K–Ar dating method using a mass-fractionation correction procedure was applied to a Quaternary independent group of monogenetic volcanoes, Ojikajima Volcano Group, located in northwestern Kyushu in Southwest Japan, in order to clarify in detail secular variations in eruptive volume, locations of eruptive vents, and magma compositions in a single monogenetic volcano group. The major results were as follows: (a) K–Ar ages of monogenetic volcanoes distribute from 1.08 to 0.30 Ma, with voluminous peaks at approximately 1.0 and 0.6 Ma. (b) The volcanic activity commenced in the central part of the field, expanded to the whole field at approximately 0.6 Ma, and then shrank to the central area. (3) Concentrations of incompatible elements, such as Ba, K, and Nb, increase with decreasing age, whereas P, Y, and Zr concentrations remain constant. These concentrations suggest gradual decrease in the degree of partial melting from an identical mantle source with residual garnet.

Kazunori Matsui

Papers

Cenozoic lithospheric extension induced magmatism in Southwest Japan

K–Ar geochronology of a Quaternary monogenetic volcano group in Ojika Jima District, Southwest Japan