Composition of ODP Leg 126 sandstones

Early diagenesis in Leg 126 forearc and backarc sands/sandstones is characterized by the dissolution of intermediate to mafic brown glass, the alteration of colorless rhyolitic glass to clay minerals, precipitation of thin clay-mineral rim cements, and minor precipitation of clinoptilolite cements. Later, more intense diagenesis is restricted to Oligocene forearc basin sediments at Sites 787,792, and 793. In these sections, the effects of early diagenesis have been intensified and overprinted by later diagenetic effects including (1) large-scale dissolution of feldspar and pyroxene crystals, (2) further dissolution of vitric components, (3) precipitation of minor carbonate cements, and (4) pervasive, multiple-staged zeolite cementation. Zeolite minerals present include analcite, mordenite, natrolite, heulandite, wairakite, chabazite, erionite, herschelite, and phillipsite. The latest diagenetic events appear to be the minor dissolution of zeolite cements and the precipitation of minor carbonate and potassium feldspar(?) cements. Observed porosity types include primary interparticles; primary intraparticles in vesicular glass and foraminifers; primary interparticles reduced by compaction and cementation; secondary intraparticles produced by dissolution of feldspar, nonopaque heavy minerals, volcanic glass, and foraminifer tests; and secondary interparticles produced by the dissolution of zeolite cements. Within forearc Oligocene sections at Sites 787 and 792, diagenetic effects appear to decrease with depth in the Oligocene section; however, at Site 793 the majority of samples are intensely altered.大洋钻探计划第126航次弧前与弧后砂及砂岩的早期成岩作用，以中性至基性棕色火山玻璃的溶蚀、无色流纹质火山玻璃蚀变为黏土矿物、薄层黏土矿物环边胶结物沉淀，以及少量斜发沸石胶结物析出为特征。后续更强的成岩作用仅发育于站位787、792与793的渐新世弧前盆地沉积物中。在这些剖面内，早期成岩作用的效应被后续成岩作用强化并叠加，后续成岩作用包括：(1) 长石与辉石晶体的大规模溶蚀；(2) 玻质组分的进一步溶蚀；(3) 少量碳酸盐胶结物的沉淀；(4) 广泛发育的多期沸石胶结作用。产出的沸石矿物包括方沸石、丝光沸石、钠沸石、辉沸石、歪沸石、菱沸石、毛沸石、赫歇尔沸石与钙十字沸石。最晚的成岩事件表现为沸石胶结物的少量溶蚀，以及少量碳酸盐与钾长石（？）胶结物的沉淀。本次研究观测到的孔隙类型包括：原生粒间孔隙；多孔状火山玻璃及有孔虫内的原生粒内孔隙；因压实与胶结作用缩减的原生粒间孔隙；由长石、非不透明重矿物、火山玻璃及有孔虫壳体溶蚀形成的次生粒内孔隙；以及由沸石胶结物溶蚀形成的次生粒间孔隙。在站位787与792的渐新世弧前剖面中，成岩作用效应随渐新统深度增加呈现减弱趋势；但在站位793，绝大多数样品均发生了强烈蚀变。