DIKE INTRUSION INTO UNCONSOLIDATED SANDSTONE AND THE DEVELOPMENT OF QUARTZITE CONTACT ZONES

Shallow, near-surface magma emplacement into a porous, unlithified san dstone occurred by forceful dike dilation and was accompanied by a loc alized hydrothermal event. Petrographic fabric, authigenic mineralogy and petrophysical data, grade smoothly through the quartzite contact z one outward into the friable Inmar sandstone of Makhtesh Ramon, Israel . Quartz grain deformation, fracturing and rehealing of grains, and pr essure solution constitute overwhelming evidence of a compressive envi ronment adjacent to dike margins, albeit with temperatures and pressur es insufficient to fully recrystallize detrital grains. Dikes were acc ommodated primarily by the repacking of the sandstone with both brittl e and elastic grain interaction. Post emplacement heating together wit h elastic compressive stress dissipation, lead to fracture healing and pressure solution. The occasional columnar-jointing must be related t o thermal contraction of the quartzite during later cooling and result ing mode I cracks. The alteration of both contact zone and dike materi al occurred via a transitory and confined hydrothermal seepage up dike margins. Elsewhere around the world highly altered intrusions bordere d by indurated sometimes uniformly jointed sediments, may also be the result of forceful magma emplacement and hydrothermal alteration at ne ar-surface levels.