Plutonic record of a caldera-forming silicic eruption: The shatter zone of the Cadillac Mountain granite, coastal Maine

Although it is widely accepted that large silicic calderas are associated with voluminous synvol-canic intrusive complexes at depth, geological evidence for caldera-forming eruptions preserved in plutonic rocks has largely been elusive. Here, we document a rare example of such evidence in the Cadillac Mountain intrusive complex, Maine (USA), where erosion has revealed a remarkable marginal “shatter zone” that records evidence for a major caldera-forming eruption. This shatter zone, up to >1 km wide, is bounded by a steep ring fault at its outer margin, which grades inward into Cadillac Mountain granite. Its outer margins are characterized by intensely brecciated and deformed country rock injected by felsite veins, reflecting explosive fragmentation associated with eruptive decompression. This marginal facies grades inward to a chaotic mélange of variably rounded and remelted country rock blocks in granitic matrix, reflecting debris eroded from ring fault conduit walls and milled in an eruptive jet before collapsing onto crystal mush. Further inward, blocks up to 80 m in size were stoped from the collapsing chamber roof and settled onto strong mush. Textural and chemical variations in the shatter zone matrix reveal syneruptive ascent of distinct silicic and more mafic magma from depth, which was likely drawn through the highly permeable shatter zone toward areas of low pressure beneath active vents. The Cadillac Mountain shatter zone provides clear evidence for a major eruption preserved in the plutonic record and supports the origin of some granites as the cumulate roots of large silicic volcanic systems.