Growth of complex sheeted zones during recycling of older magmaticunits into younger; Sawmill Canyon area, Tuolumne Batholith, SierraNevada, California

Abstract

In Sawmill Canyon, located near the eastern margin of the Tuolumnebatholith, central Sierra Nevada, California, a series ofpetrologically and structurally complex, magmatic sheeted zonesintrude older granodioritic units (Kuna Crest and equigranular HalfDome) and in one case truncate these units along a sharp contact.These sheeted zones (a) consist of numerous batches of (now frozen)magma, (b) display clear outward growth directions, (c) were activelydeforming during and after emplacement resulting in magmatic folds,faults and multiple magmatic mineral fabrics, and (d) are the locationof numerous, but localized magma flow structures (schlieren-boundedtubes, troughs, megacryst-rich pipes) and instabilities (load casts,flame structures, slumps, diapirs, ridge and pillar structures).Geochemical data indicate that the sheeted zones largely consist ofmagmas derived from the Half Dome granodiorite with some lateCathedral Peak granodiorite pulses, and with fractionation and flowsorting forming widespread layering in the above structures. Weinterpret these sheeted zones to record the pulsing of magma duringpropagation and expansion of opening-mode (Mode I), submagmaticfractures at the margins of large blocks of older, fairly solidifiedmagmatic pulses that were subsequently removed from the presentcrustal level. Elsewhere in the Tuolumne batholith we see similarfeatures suggesting that a ""recycling"" process, i.e., the breaking offof older parts of the magma chamber and incorporation into youngerintrusive units, occurred in this batholith. This recycling removed asignificant portion of older units and resulted in the formation ofsheeted zones and local instabilities in this batholith. Finally thisrecycling is one process responsible for transfer of zircon crystalsbetween units and for obscuring whole-rock geochemical signatures.

13 samples from this reference