Incremental assembly and prolonged consolidation of Cordilleran magma chambers: Evidence from the Southern Rocky Mountain volcanic field

Abstract

Recent inference that Mesozoic Cordilleran plutons grew incrementally during >10 6 yr intervals, without the presence of voluminous eruptible magma at any stage, minimizes close associations with large ignimbrite calderas. Alternatively, Tertiary ignimbrites in the Rocky Mountains and elsewhere, with volumes of 1-5 × 10 3 km 3 , record multistage histories of magma accumulation, fractionation, and solidifi cation in upper parts of large subvolcanic plutons that were sufficiently liquid to erupt. Individual calderas, up to 75 km across with 2-5 km subsidence, are direct evidence for shallow magma bodies comparable to the largest granitic plutons. As exemplifi ed by the composite Southern Rocky Mountain volcanic fi eld (here summarized comprehensively for the fi rst time), which is comparable in areal extent, magma composition, eruptive volume, and duration to continental-margin volcanism of the central Andes, nested calderas that erupted compositionally diverse tuffs document deep composite subsidence and rapid evolution in subvolcanic magma bodies. Spacing of Tertiary calderas at distances of tens to hundreds of kilometers is comparable to Mesozoic Cordilleran pluton spacing. Downwind ash in eastern Cordilleran sediments records large-scale explosive volcanism concurrent with Mesozoic batholith growth. Mineral fabrics and gradients indicate unifi ed fl owage of many pluton interiors before complete solidifi cation, and some plutons contain ring dikes or other textural evidence for roof subsidence. Geophysical data show that low-density upper-crustal rocks, inferred to be plutons, are 10 km or more thick beneath many calderas. Most ignimbrites are more evolved than associated plutons; evidence that the subcaldera chambers retained voluminous residua from fractionation. Initial incre-mental pluton growth in the upper crust was likely recorded by modest eruptions from central volcanoes; preparation for calderascale ignimbrite eruption involved recurrent magma input and homogenization high in the chamber. Some eroded calderas expose shallow granites of similar age and composition to tuffs, recording sustained postcaldera magmatism.Plutons thus provide an integrated record of prolonged magmatic evolution, while volcanism offers snapshots of conditions at early stages. Growth of subvolcanic batholiths involved sustained multistage opensystem processes. These commonly involved ignimbrite eruptions at times of peak power input, but assembly and consolidation processes continued at diminishing rates long after peak volcanism. Some evidence cited for early incremental pluton assembly more likely records late events during or after volcanism. Contrasts between relatively primitive arc systems dominated by andesitic compositions and small upper-crustal plutons versus more silicic volcanic fi elds and associated batholiths probably refl ect intertwined contrasts in crustal thickness and magmatic power input. Lower power input would lead to a Cascade-or Aleutian-type arc system, where intermediate-composition magma erupts dire...

Deep crustal anatexis, magma mixing, and the generation of epizonal plutons in the Southern Rocky Mountains, Colorado

“…later with the eruption of larger volume topaz rhyolites (Christiansen et al 1983;Knox 2005). The progression from early intermediate composition volcanism to later silicic ignimbrite deposition is similar to that observed through multiple cycles in the southern Rocky Mountain volcanic field in southern Colorado (Lipman 2007). The latter is generally considered to be the easternmost expression of the mid-Tertiary "ignimbrite flareup", a period of intense magmatic activity, primarily centered in the western USA, that took place ~25 to 40 Ma (Coney and Reynolds 1977;Armstrong and Ward 1991).…”

Quickening the Pulse: Fractal Tempos in Continental Arc Magmatism

“…By examining the space-timevolume record of volcanic activity, the episodic nature and variability of intensity of eruptions can be identified. Particularly in young arcs, or in environments where erosion is minimal and the volcanic record is well preserved, relatively robust extrusive fluxes (if the area through which a magma flows is known) can be delineated through mapping, stratigraphic correlation, and extensive geochronology (McIntosh et al 1992;de Silva and Gosnold 2007;Lipman 2007;Best et al 2013). Magmatic addition rates, or fluxes, are determined by assuming the commonly quoted volcanic-to-plutonic ratios of 3:1, 5:1, and 10:1.…”

“…McIntosh et al 1992;Bryan et al 2007;Lipman 2007;Best et al 2013). The primary feature of the time-volume pattern of these ignimbrite flare-ups is that the eruptive history is episodic and in three distinct stages: first, a waxing period of relatively low eruptive flux; second, a climactic stage (the peak of the flare-up) when the largest eruptions happen and the eruptive flux is at its highest; third, a waning stage that returns the arc to a steady state (de Silva and Gosnold 2007;Lipman 2007). …”

Natural equivalents of thermal gradient experiments

“…8b). The contexts of application for these crystallization experiments at 500 MPa are representative of typical dimensions of batholith-like, large silicic magma bodies (Lipman, 2007;Scandone and Acocella, 2007;Petford et al, 2000) and small silicic plutons.…”