A PLUTONIC RECORD OF A PYROCLASTIC ERUPTION: THE INTRUSIVE BRECCIA (“SHATTER ZONE”) OF THE CADILLAC MOUNTAIN GRANITE, MAINE (Invited Presentation)

The tilted Cadillac Mountain granite (CMG) is ~ 15 km in diameter, up to 3 km thick and underlain on its deeper western base by 2+ km of layered gabbro-diorite (GD). It has, for 12 km along its shallow eastern margin, an intrusive breccia (IB) up to 1 km wide, within a steep ring fault, which, along with the breccia, continues for a further 18 km along the westward-deepening southern CMG margin, ending at the underlying GD.

The shallow eastern IB has 3 distinct zones: A) an outer zone against the ring fault of broken up, deformed sedimentary Bar Harbor Fm (BHF) invaded by thin veins of aphanitic felsite and blocks of diabase with < 15% matrix, which coarsens inward; B) a central zone with blocks of BHF (< 1 m), diabase and scarce rhyolite (< 1 to 10s of m) in fg to mg matrix (20-60%) of biotite granite with digested fragments of BHF and common flow fabrics, and C) an inner zone with blocks (10-80 m) of BHF, diabase and rhyolite in a more mafic granitic matrix (>60%) of hypersolvus feldspar, quartz, Fe-cpx, Fe-hbl, and fayalite. Clast size distribution of CR fragments in zones A and B indicates a high-energy environment consistent with a major pyroclastic eruption (Roy et al., 2012).

In C the feldspar has a consistent sequence of zones: 1) a core of ~ An₁₀Ab₈₀Or₁₀; 2) a transition ~ 0.5 mm wide of 10-15 Or-Ab oscillations (~ Or₁₀to Or₃₀- each ~ 30 microns wide), and 3) a rim of An₃Ab₇₀Or₂₇. These feldspars are largely restricted to zone C matrix and occur for >20 km along the IB. This great extent suggests oscillations were caused by an intensive parameter, likely sudden pressure (P) variation.

Prior to a large eruption, the magma chamber likely had episodic, sudden drops in P due to degassing and minor eruptive events. Since the lower part of the chamber was relatively dry, a drop in P would have caused the alkali feldspar solidus to shift to higher T, causing the melt to fall below the liquidus and shift the equilibrium feldspar to higher Or values, matching the initial oscillatory zone observed. Based on the mineralogy, the apparent large inward increase in the T of the matrix from zone A to C probably records initial eruption of a cooler hydrous felsic cap (matrix in zone A) and subsequent upwelling of deep, hot hybrid magma (frombelow CMG crystal mush and above the GD) with hypersolvus feldspar that had felt a sequence of P fluctuations (matrix in zone C) prior to the eruption.