عنوان

The Toba Tuffs and caldera complex, Sumatra, Indonesia:

TLpq303679617

انگلیسی

The Toba Tuffs and caldera complex, Sumatra, Indonesia:

[Thesis]

C. A. Chesner

Insights into magma bodies and eruptions

Michigan Technological University

1988

445

Ph.D.

Michigan Technological University

1988

During the past 1.2 m.y., a magma chamber of batholithic proportions has developed under the 100 by 30 km Toba Caldera Complex. Four separate eruptions have occurred from vents within the present collapse structure and document the growth of the laterally continuous magma body which eventually erupted the 2800 km Youngest Toba Tuff (YTT) at 75 ka. The calderas associated with the three oldest tuffs are obscured by caldera collapse resulting from eruption of the YTT. Prominent features indicative of resurgence include Samosir Island and the Uluan Block, two sides of single resurgent dome that has resurged since eruption of the YTT. In the past 75,000 years, lava domes have been extruded accompanying resurgence on Samosir Island and along the caldera's western ring fracture. A relatively homogeneous two-pyroxene dacite was erupted during the first phase of Toba activity. Magma erupted during the successive eruptions was compositionally zoned, generally from rhyodacite to rhyolite. Extensive fractional crystallization in double-diffusive convecting magma bodies, is suggested for creation of the compositional zonation. The zoned tuffs contain up to 40% crystals consisting of quartz, sanidine, plagioclase, biotite, and amphibole. Minor phases include magnetite, ilmenite, allanite zircon, fayalite, and orthopyroxene. Inclusions of apatite and pyrrhotite are common. Intensive parameter calculations suggest that extensive crystallization occurred at temperatures between 710 and 770C at depths of 9-12 km for the three quartz-bearing tuffs. Water pressure estimates imply that Pusd\rm\sb{tot}\gg P\sb{H2O}usd, thus volatile oversaturation did not initiate the eruptions. The tuffs have Sr/Sr ratios ranging from 0.7136 to 0.7149, and usd\delta\ \spusdO contents around +8.5. This characterizes the Toba magmas as crustal melts which formed in the peculiar tectonic regime of northern Sumatra. Strontium isotopic variations between the tuffs is small. Within units, slight Sr/Sr trends towards more radiogenic compositions with magmatic evolution, can be attributed to magmatic aging of high Rb/Sr melts in time periods of 0.5 to 1.0 m.y., and/or assimilation of radiogenic wall-rocks into the roof of the magma body. The young lava domes maintain similar Sr/Sr ratios, indicating that crustal melts still persist beneath Toba.

Earth sciences

Geochemistry

Geology

Geology

C. A. Chesner

p

[Thesis]

276903

a

Y