CONTROLS ON THE TRANSITION FROM SUBVOLCANIC TO PLUTONIC CONDITIONS IN A MAGMATIC SYSTEM: THE ELBA EXAMPLE

Western and central Elba are dominated by magmatic rocks that formed at a common center over a period of about 1 Ma. Three episodes of laccolith growth produced distinctive porphyritic rocks prior to emplacement of the Monte Capanne pluton. These porphyries have subvolcanic textures with euhedral sanidine, quartz, plagioclase and biotite suspended in a very fine-grained matrix. Geochemical characteristics indicate evolving character, first with anatectic melts (Capo Bianco aplite), then initiation of hybridization of melt derived by both muscovite and biotite dehydration melting (Portoferraio porphyry), and finally more extensive hybridization (megacrystic San Martino porphyry). This evolution involved changes in the character of source area melting, crystal growth of early phases, and hybridization by interaction with evolving mantle-derived magmas.

Transition from emplacement of San Martino porphyry (2-3 km) to the Monte Capanne pluton immediately below (3-4 km) included profound change in texture while phenocryst character remained essentially unchanged. Sanidine megacrysts in the porphyry exhibit smooth euhedral crystals while orthoclase megacrysts in the pluton are euhedral but with rough surfaces resulting from poikilitic overgrowths. Monte Capanne K-feldspar megacrysts commonly exhibit well-developed hourglass zoning defined by minute, oriented euhedral biotite inclusions with Fe# = 0.45 compared to higher Fe# values for biotite of the matrix. Also present as inclusions of similar size are quartz, apatite, zircon and monazite, as well as crystallographically aligned plagioclase (An₃₉-An₂₁) that exhibit concentric oscillatory zoning. These inclusions indicate that K-feldspar was not the initial liquidus phase. Sanidine megacrysts in the older San Martino porphyry have essentially identical characteristics. This case study suggests that subvolcanic laccoliths and plutons may represent different outcomes of the same geological process. Transition from laccolithic to plutonic form can be regarded as a combination of factors such as variations in magma supply rates and transient variations in horizontal extensional tectonic stress, as well as downward migration of both the source region and the magma traps as a result of the addition of magma layers at higher levels in the crust.