Origin of magmatic components in the Diamante Tuff.

Burkert, Cosima, Freundt, Armin , Gilbert, David and Kutterolf, Steffen (2010) Origin of magmatic components in the Diamante Tuff. [Poster] In: SFB 574 Subduction Workshop. , 04.-07.11.2010, Pucon, Chile .

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The Diamante Ignimbrite, which covers an area of 23.000km2 in Chile and Argentina, was emplaced 0.45 Ma ago by a catastophic eruption that caused subsidence of the Diamante Caldera. The ignimbrite is underlain by two fallout deposits, one immediately beneath the ignimbrite, the other separated by sediments. All three deposits have rhyolitic compositions but the older fallout contains higher concentrations of incompatible elements at slightly lower silica contents, reflecting a somewhat different crystal fractionation history. Five juvenile components can be identified in the ignimbrite and its underlying fallout:
(1) The dominant white to pink rhyolitic pumice that contains plagioclase, biotite, quartz and ore phenocrysts; alkalifeldspar crystals occur in the matrix. (2) Light-gray rhyolitic pumice, also with plagioclase, biotite, quartz and ore phenocrysts, contains significant amounts of broken crystals and lithic fragments, and mostly occurs mixed with the white component in banded pumice clasts. (3) A highly vesicular, glassy rhyolitic pumice with very few phenocrysts. (4) Foliated, silky shinning white to grey pumice with strongly elongated vesicles and parallel-aligned phenocrysts. (5) A minor component of brown pumice distinct in both its dacitic bulk composition and the presence of amphibole phenocrysts next to plagioclase,
biotite and ore. Dark-brown regions in these pumice clasts contain higher amounts of amphibole and plagioclase microlites. There is no systematic vertical compositional zonation through the fallout and ignimbrite but pumice type (5) only occurs in the ignimbrite. Pumice types (1) to (4) share the same mineral and glass compositions. We interpret type (2) as magma that ascended along the conduit walls where high shear stresses caused crystal fragmentation and entrainment of lithic fragments. The dominant type (1) pumice, in contrast, reflects the main, less sheared magma ascending near conduit center. Foliated type (4) pumice may
have been magma from in-between these conduit regions. The higher vesicularity of type (3) relative to type (1) indicates that vesiculation in the magma was spatially heterogeneous. The ignimbrite also contains abundant plutonic lithic fragments mostly of andesitic to dacitic bulk compositions. While some of these lie on a fractionation trend leading to the rhyolitic pumice compositions, others have distinct chemical compositions. A fraction of the lithics contains amphibole, and these plutonic rocks have the same dacitic composition as the amphibolebearing pumice (5), which is distinct from the other pumice and lies off the main fractionation trend. We conclude that magma (5) represents a molten crustal component that was added to the rhyolitic magma reservoir.

Document Type: Conference or Workshop Item (Poster)
Keywords: Geodynamics; Geology; Diamante Ignimbrite;
Research affiliation: OceanRep > SFB 574 > C4
OceanRep > GEOMAR > FB4 Dynamics of the Ocean Floor > FB4-MUHS Magmatic and Hydrothermal Systems
OceanRep > SFB 574
Date Deposited: 13 Dec 2010 12:12
Last Modified: 23 Feb 2012 05:29
URI: http://oceanrep.geomar.de/id/eprint/10474

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