ZEOLITE DISTRIBUTION ALONG VATNSDALFJALL, SKAGI PENINSULA, NORTHWEST ICELAND

In northwest Iceland, on the Skagi Peninsula, Vatnsdalfjall is a glacially carved ridge that exposes the upper ~700 m of crust formed about 7 Ma ago at the extinct Hunafloi-Skagi rift zone. The lava flows form a flexure zone, dipping gently (4^◦-10^◦) west toward the abandoned rift axis, with the dip of lava flows increasing with depth. This regional flexure zone is interrupted by a local area of steeply dipping (45^◦-60^◦) lavas known as the Monocline.

A previous study in southeast Iceland determined zeolites are sensitive indicators of temperatures and pressures of formation. This same conclusion will be used to determine the depth of burial of the Vatnsdalfjall lavas. The distribution of zeolites are mapped from a series of 11 transects across approximately 21.5 km of the length of the ridge. Zeolites are found in vesiculated flow tops, ranging in size from mm to a few cm, although cavities up to 20 cm wide are observed. 47 samples were selected for analysis by XRD. Based on the spatial distribution of zeolites, two relatively flat lying zeolite zones are identified (assumed to parallel the paleosurface); the Heulandite+Stilbite zone and the Laumontite zone. The Heulandite+Stilbite zone was first identified in Eastern Greenland, where it occurs over the Laumontite zone at a depth of ~2500m below the paleosurface. The Vatnsdalfjall zones are disrupted in the Monocline sequence, which could result from several scenarios: (1) the Monocline was zeolitized prior to folding so that low-temperature zeolites are preserved at depth; (2) rapid burial of Monocoline lavas prevented zeolites from equilibrating, forming zoned amygdules where low-temperature zeolites surround high-tempreature zeolites; (3) some of the Monocline lavas are not tholeiitic, so that zeolite assemblage changes with the chemistry of the host rock. Some combination of scenarios (2) and (3) seems most likely. Horizontally layered chalcedony in filled vesicles suggests that alteration took place after the formation of the Monocline. Zeolites with low Si/Al ratios (e.g., mesolite) only appear in the Monocline, suggesting some control of the host rock chemistry on zeolite distribution. Further geochemical and thin section analyses, and estimates of the geothermal gradient will be used to better understand the depth of burial of the monocline.