Zeolites in Burial Diagenesis and Low-grade Metamorphic rocks

Abstract

Many zeolite assemblages originated during burial and subsequent low-grade metamorphism of rocks, particularly those of volcaniclastic origin. Eskola (1939) first suggested this type of zeolitization and applied the term “zeolite facies” to assemblages formed under such low-temperature and low-pressure conditions. Although several definitions of the “zeolite facies” have been proposed over the years (e.g. Fyfe et al. 1958), the phrase as used here includes both low-grade metamorphic assemblages and zeolite assemblages formed successively during burial. The regional occurrence of zeolites in sedimentary rocks was first recognized by Coombs (1954), who made detailed petrographic studies of Triassic tuffaceous greywackes and tuffs of andesitic to rhyolitic in composition in the Taringatura area of New Zealand. Coombs recognized a zonal arrangement of heulandite and analcime near the top of the sequence and heulandite and laumontite near the bottom. He also recognized a similar progressive zeolitization with depth in clastic sediments in other orogenic belts. The zeolite assemblages described by Coombs represent a rather advanced stage of alteration, as opposed to an earlier stage of zeolitization that was subsequently described in Neogene silicic volcaniclastic rocks from Japan. In the latter rocks, zones (from top to bottom) of fresh glass, clinoptilolite + mordenite, analcime + heulandite, laumontite, and albite were recognized (Utada 1965, 1970). Progressive zeolitization has also been reported as a result of the thermal effects of intrusive masses (see Seki et al. 1969) and around volcanic calderas (Utada and Ito 1989). Recently, Boles (1991) described the low-temperature formation of laumontite and stilbite in fractures and faults related to post-Jurassic folding and uplift of the Southland Syncline, New Zealand. The physico-chemical conditions of zeolite crystallization in burial diagenesis and low-grade metamorphic environments can vary widely. Both borehole data (see Iijima 1995) and experimental results (Liou 1970; 1971a,b,c …