Reverse zoning in the resurgent intrusions of the Grizzly Peak cauldron, Sawatch Range, Colorado

The deeply eroded Grizzly Peak cauldron, 17 by 23 km and source of the 34-m.y.-old Grizzly Peak Tuff, is located in the Sawatch Range in west-central Colorado. The Lincoln Gulch composite stock is exposed in the core of a resurgent dome in the northern part of the cauldron and consists of three units. Their forceful emplacement was a cause of cauldron resurgence. The first intrusion lifted its roof along a vertical fault that surrounds the stock. The second resurgent intrusion, a complex body with broad sill-like overhangs, was emplaced within the first intrusion by further piston uplift of the roof. The third intrusion, emplaced in the second, is a small funnel-shaped body. Normal faults in surrounding inter-layered intracaldera tuff and caldera-collapse breccia are radial to the intrusion-cored piston block. Most of the tuff dips away from the resurgent intrusions. Pyroclastic dikes cutting tuff that roofs the stock may have been emplaced by venting of the intrusions.

The three resurgent intrusions are concentrically zoned, with the most differentiated material at the margins. From margin to core, compositions range from biotite granodiorite to hornblende-bearing biotite quartz monzodiorite. The intrusions are porphyroaphanitic throughout, but both phenocrysts and groundmass become coarser toward the cores. The compositional patterns are termed reverse zonations because most documented concentrically zoned intrusions become more silicic toward their cores. Crystal settling and marginal accretion of crystalline phases can be eliminated as means of forming reverse zoning because mafic-coreward gradients in element and mineral content are developed inward from contacts of all attitudes. Diffusion-based mechanisms of in situ differentiation are too slow, given the short solidification times for these sill-like bodies. We interpret reverse zonation to result from rearrangement of a vertically graded magma column during emplacement of the intrusions. Progressively more mafic magma from successively deeper levels in a subjacent chamber rose into the cores of the intrusions, displacing more silicic magma toward the margins. This emplacement pattern was preserved by rapid solidification, perhaps due to volatile loss on venting.