CONSTRUCTION OF PLUTONS BY HORIZONTAL DEPOSITIONAL AND INTRUSIVE SHEETS

Pluton assembly entails 3 processes: #1 feeding (by ascending magma); #2 emplacement (ponding of magma); #3 accumulation of "rock" (creating coherent, ~solid material). In a general sense #1->#2->#3 represents a time sequence, but small and large scale repetitions and reversals abound. 3-D architecture of plutons exposed in steep tilt blocks in southern Nevada provides insights into processes #2 and #3. Aztec Wash and Searchlight plutons (AWP, SLP) and Spirit Mountain batholith (SMB) reveal features that attest to the importance of horizontal sheets in their construction:

EMPLACEMENT SHEETS: Felsic magma that replenished AWP and SMB late in their histories formed numerous fine-grained, sill-like bodies from ~m to 100 m thick. The compositions of these sheets suggest that they solidified from unfractionated, melt-rich magma (low-SiO2 rhyolite). Recharging mafic magma formed sills and thick composite sheets in SLP, SMB, and possibly AWP, and lava flow-like sheets that spread on the extant bases of felsic magma lenses in AWP. Distinct km-scale lenses of granitoid rocks with relatively large length/height ratios appear to mark zones of magma emplacement and mush accumulation within SMB. ACCUMULATION SHEETS: The sills and "lava flows" described above mark accumulation of rock as well as magma emplacement. In addition, sills of highly fractionated granite (high-SiO2 rhyolite), extracted from accumulating mushes, are common in SLP and SMB. Depositional accumulations of crystals and displaced semi-solid rock fragments (cumulates/breccias/debris flows) are the most voluminous components of all 3 intrusions. Mafic cumulates are thin (<~10 m), but felsic cumulate sequences reach km scale. Timescales over which these plutons grew and solidified (SLP, SMB : >1 m.y.; AWP: several hundred k.a. [SHRIMP U-Pb]) require replenishment and waxing and waning abundance of melt, consistent with observed field features. The stubby shapes of the intrusions (l/h ~2) do not reflect the high aspect ratios of increments or accumulation features. Plutons accumulated upward (by deposition), by interleaving of quenched intrusive sheets, and less commonly downward (as chilled margins and solidification fronts). Exposures of feeder dikes are rare and/or obscure, probably because they collapsed after termination of magma flux.