Voluminous intermediates at three Tertiary central volcanoes in northwestern Iceland: Implications for crustal magmatic processes during tectonic evolution of Iceland
Abstract
Preliminary studies have been conducted on three Tertiary central volcanoes in northwest Iceland erupted in the now-abandoned Skagi-Snaefellsnes rift zone between 14 and 8 Ma: Hrafnfjordur (~14 Ma); Arnes (~11 Ma); and "southern Skagi" (7.8 Ma). Intermediate lavas are a significant component of the volcanic output of all three central volcanoes, though the processes by which intermediate magmas formed vary from system to system. The most voluminous unit exposed in the southern portion of the Hrafnfjordur central volcano is a ~300- m-thick homogeneous andesite exposed in Seljafall; this is immediately overlain by a basaltic andesite. Variation diagrams of these units exhibit well-defined inflected trends which project back to trends in associated basalts, indicating a relationship by fractional crystallization. Nearby, thinner andesite and dacite lavas south of Leirufjordur, including the Trollafell dacite dome, lie on distinct compositional trends. Some of these lavas lie on fractionation trends while others clearly reflect magma mixing, and the dacite dome cannot be related to any of the other lavas by fractionation. The Arnes central volcano is composed of a continuous spectrum of lavas from basalt to high-silica rhyolite. The core of the system consists of an extensive >300-m-thick sequence of andesite lavas. Basaltic andesites, dacites and rhyolites overlie this unit in the north. The southern portion of the central volcano is dominated by rhyolites. All of the intermediate lavas lie on trends in variation diagrams that reflect continuous fractionation from a basaltic parent magma. These trends continue to the dacites and rhyolites suggesting the interpretation that most rhyolites in this system were produced by fractionation-dominated crustal magmatic processes rather than crustal melting. The "southern Skagi" central volcano erupted a lower sequence of minor basaltic andesites, dacites, and rhyolites, a >3 km3 intermediate lava, an extensive high-silica rhyolite, and dacite lavas intercalated with the overlying basalts. The main intermediate lava is hybrid lava, andesite to rhyolite in composition, whose hybrid character is readily traced in the field because of coarse plagioclase phenocrysts in the basaltic end-member. A triangular array in variation diagrams reflects mingling/mixing of the basaltic magma with a continuous spectrum of silicic end-members from dacite to high-silica rhyolite. The compositional spectrum of the lower sequence clearly defines a mixing trend between moderately evolved basalt and high- silica rhyolite. The upper dacites define a trend that could be interpreted as either an intermediate on a continuous fraction trend through all of the other non-mixed units, or the product of mixing of evolved basalt and low-silica rhyolite. Most Recent and active volcanic centers in Iceland are bimodal with only minor intermediates, Hekla being a notable exception. The Tertiary centers discussed here represent a clear departure from this association. Silicic lavas in volcanic centers in the modern rift zones appear to have originated largely by crustal melting, while those in flank zones may be the result of fractionation-dominated processes. In this regard, at least some of the Tertiary centers erupted in the Skagi-Snaefellsnes rift zone appear to have evolved through processes more akin to flank zones. Together, these observations highlight fundamental differences in crustal magmatic processes between the Tertiary rift zones and the modern rifts of Iceland.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V41D2127J
- Keywords:
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- 3618 Magma chamber processes (1036);
- 3640 Igneous petrology;
- 8137 Hotspots;
- large igneous provinces;
- and flood basalt volcanism;
- 8400 VOLCANOLOGY