April GEOLOGY and GSA TODAY media highlights

A Precambrian proximal ejecta blanket from Scotland

Kenneth Amor et al., Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK. Pages 299-302.

Scientists have found evidence of the biggest meteorite ever to hit the British Isles, in rock formations in northwest Scotland. Amor et al. believe a meteorite at least 500 meters in diameter hit the region about 1.2 billion years ago. Ejected material from the huge meteorite strike is scattered over an area about 50 kilometers across, roughly centered on the northern Scottish town of Ullapool. The evidence lies in a layer of rock that experts believe is the ejected material thrown out during the formation of a meteorite crater. Chemical testing of the rock found the characteristic signature of meteoritic material, which has high levels of the key element iridium, normally only found in low concentrations in surface rocks on Earth. The target rocks, analyzed under a microscope, also imply a meteorite strike, revealing tell-tale microscopic parallel fractures. These are formed during the very high pressures of a meteorite impact. This is the most spectacular evidence for a meteorite impact within the British Isles found to date, and the actual meteorite crater is thought to lie within the immediate vicinity, buried under younger rocks.

Porphyroblast rotation versus nonrotation: Conflict resolution!

T.H. Bell et al., School of Earth and Environmental Sciences James Cook University, Townsville, Queensland 4811, Australia. Pages 303-306.

Spiral shaped trails of inclusions in large garnet crystals, as well as other mineral phases with a similar large growth habit relative to the matrix (porphyroblasts), have always been thought of as a product of rotation of the crystal as it grew in a deforming rock. Two decades ago it was proposed that they formed without rotation of the porphyroblast and that the inclusion trails represented successively orthogonally overprinted planar structures (foliations). It was argued that these were overgrown by the garnet and could be used to access the full deformation history of the rock in the orientation in which it formed relative to the surrounding rock mass. Most geologists are highly skeptical of this, even though the data gathered on these structures in natural rocks supports this proposition. This skepticism is reinforced by the fact that over 99% of all theoretical, experimental, and computer modeling of this phenomenon resulted in rotation of the porphyroblast. The computer modeling presented by Bell et al. reveals, for the first time, that this nonrotation phenomenon does occur in the same geometric circumstances of deformation in which large porphyroblasts grow. This is a major scientific breakthrough and will eventually allow general acceptance of inclusion trails as a tool to enable understanding of many unresolved geologic phenomena.

Lithosphere erosion and crustal growth in subduction zones: Insights from initiation of the nascent East Philippine Arc

Colin G. Macpherson, Department of Earth Sciences, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, UK. Pages 307-310.