26. TEXTURAL AND FLUID INCLUSION EVIDENCE FOR HYDROTHERMAL ACTIVITY IN THE VOLCANICLASTIC APRON OF GRAN CANARIA 1Sten Lindblom 2 and Martine Gérard 3 |
ABSTRACTDrilling in the volcaniclastic apron of Gran Canaria during Leg 157 has provided material for extending land-based studies of volcanism back to >15 Ma. Earlier shield stage and earlier eastern island pyroclastic rocks were penetrated, making possible a variety of studies. The one presented here focuses on hydrothermal activity and vein/vug deposition in those sections. The results indicate that the fluid activities at Sites 953, 954, and 956 were different. Site 954 is closest to the island and shows strong visible alteration of the host basaltic rock along veins. It provides the only measurable fluid inclusion data (from arago-nite). The results indicate a hot, saline fluid environment, acting in the shield stage with temperatures from 185° to 284°C (without pressure correction) and salinity between 4.6 and 15 wt% NaCl equivalent. Sites 953 and 956 exhibit less alteration. Single-phase inclusions indicate formation at lower temperatures from solutions of lower salinity. Textures and Raman spectrometry analyses further reveal a complex evolution of vein and vug fillings in the shield stage at Sites 954 and 956 and during the pre-shield (eastern islands) stage at Site 953. Site 953 give a paragenesis of prehnite, apophyllite, and phillipsite over a large section, with natrolite, analcime, and gmelinite occurring at distinct levels. Site 954 gives a paragenesis of carbonate, calcite, aragonite, siderite, phillipsite, and smectite. Site 956 gives a paragenesis of smectite, zeolite, siderite, calcite, aragonite, phillipsite, and analcime. Mineral stability and fluid inclusions together indicate an environment with relatively high, but varying salinity at all three sites and formation temperatures at 100°–150°C (Site 953), 200°–300°C (Site 954), and 100°–150°C (Site 956). |
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