8. METASOMATIC AND METAMORPHIC PECULIARITIES OF THE DIABASE DIKE ALTERATION IN HOLE 504B, LEG 140

ABSTRACT The diabase dikes recovered during Leg 140 display four types of mineralogical alteration: (1) chlorite + amphibole + titanite; (2) chlorite + amphibole + ilmenite; (3) epidote + prehnite + anhydrite; (4) zeolites + smectite. The first two types represent both background and vein alteration. The third and fourth types are locally overprinted on the previous types. The third type exhibits oxidative features, whereas the fourth formed with cooling of the dike complex. Some veins and patches of strong alteration are surrounded by infiltrate-metasomatic halos, which suggest a difference in fluid pressure in pores and fractures. The temperature of the main alteration types ranged between 350° and 400°C. Noticeable temperature fluctuations in space and time were caused by alternating heating and cooling resulting from intrusions of younger dikes within short time intervals