Chapter 5: Cumulate nodules in the Thera pyroclastics

Abstract

Gabbroic and dioritic nodules are common in some horizons of the Thera pyroclastics. These nodules contain pyroxene as the main ferromagnesian phase in contrast to the early Akrotiri centre, which contain nodules with abundant hornblende (Nicholls 1971a; Pichler & Kussmaul 1972). Calcsilicate nodules and gabbroic nodules have also been recognized in Santorini lavas (Fouqu6 1879; Nicholls 1971a). Previous studies of Santorini volcanic rocks indicate that crystal fractionation has played an important role in generating the compositional range basalt-andesite-dacite-rhyodacite (Nicholls 1971a; Mann 1983; Huijsmans 1985; Barton & Huijsmans 1986). This interpretation is also consistent with much of the petrological data presented in Chapter 4 and the geochemical data and models presented in Chapter 6. However, disequilibrium phenocryst assemblages in some lavas (Huijsmans 1985; Chapter 4), banded pumice clasts in many pyroclastic deposits (Druitt et al. 1989; Chapters 4), isotopic disequilibria in phenocrysts (Pyle et al. 1988; Chapters 6 and 7) and complex zoning patterns in plagioclase crystals (Stamatelopoulou-Seymour et al. 1990) indicate that magma mixing has also occurred. In addition, trace element and isotopic abundances can be interpreted in terms of assimilation of continental crust during fractionation (Barton et al. 1983; Stamatelopoulou-Seymour et al. 1990; Chapter 6). The nodules provide information on petrogenetic processes that have influenced the evolution of Santorini magmas. Their petrology is documented in this chapter and compared with that of associated juvenile ejecta. The nodules are identified as cumulates from andesitic and dacitic magmas, con- firming the importance of fractional crystallization in the petrogenesis of Santorini magmas. They also demonstrate that mafic plutonic rocks can be formed from intermediate to silicic magmas.