WAVES-ROPES-LOBES, FLUIDIZATION AND DEFORMATION AT LACCOLITH-HOST CONTACTS (ELBA ISLAND, ITALY)

Felsic porphyritic multilayer laccoliths of the Elba Island Miocene igneous complex offer a wide range of examples of contact features, many of which constraints magma flow direction. The Portoferraio laccolith intruded ~8 Ma at an average depth of about 2.6 km into Jurassic serpentinites and overlying Cretaceous flysch (dark shale, feldspathic sandstone, marly limestone) that make up the top two complexes of a thrust stack assembled by about 20 Ma. The three main layers of the megacrystic San Martino laccolith were emplaced in the Cretaceous host ~7.4 Ma at an average depth of 1.9 km.

Two sets of contact features are seen, particularly where the laccoliths intruded shales and serpentinite. Features in the first set are defined by the external morphology of intrusive surfaces. Wave structures occur ranging from rounded crests and V-shaped troughs, through sinusoidal, to “fully-developed” with breaking crests. Pahoehoe” rope structures represent a unique style of wave form. Decimetric lobate structures are reminiscent of load casts or lava pillows, seen with superimposed fine-scale waves and weak lineations on their surfaces. The second set of features is characterized by deformation and/or disruption of materials on one or both sides of the contact. Most common are solid-state stretching lineations made from quartz and feldspar phenocrysts. Deformation style is related to distance from contacts, with ductile strain giving way to brittle strain within the first few mm of the contact. These linear features occur on wave surfaces and oriented perpendicular to wave crests, independent of the regularity of wave patterns. Disruption structures at contacts progress from laminar structures in the igneous rock, involving only small amounts of fluidized host material, to increasing degrees of incorporation of such material. Fluidization of the flysch assemblage commonly forms chaotic structures with blocks of rigid host and angular porphyry fragments “swimming” throughout. Locally, decimetric-scale apophyses of porphyry occur within fluidized flysch. Disrupted breccias along contacts are seen incorporated within the intrusive rock, appearing as breccia “dikes” containing porphyry, flysch and ophiolitic clasts. Similar assemblages occur within the host rock as sharply defined breccia dikes up to 10 m in length.