Magmatic lobes as "snapshots" of magma chamber growth and evolution in large, composite batholiths: An example from the Tuolumne intrusion, Sierra Nevada, California

Abstract

Precise chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages in combination with detailed fi eld mapping, 40 Ar/ 39 Ar thermochronology, and fi nite difference thermal modeling in the magmatic lobes of the Tuolumne batholith characterize these 10-60 km 2 bodies as shorter-lived, simpler magmatic systems that represent increments of batholith growth. Lobes provide shorter-term records of internal and external processes that are potentially obliterated in the main body of long-lived, composite batholiths. Zircon ages complemented by thermal modeling indicate that lobe-sized magma chambers were present between ~0.2 and 1 m.y., representing only a small fraction of the total duration of melt presence in the main body. During these shorter intervals, a concentric pattern of normal compositional zoning formed during inward crystallization and widespread zircon recycling in the lobes. Lobes largely evolved as individual magma bodies that did not interact signifi cantly with the main, more complex magma chamber(s). Antecrystic zircons and the range of autocrysts, used to track the extent of interconnected melt, record only a limited range of ages and have contrasting zircon populations to those found in the same units in the main batholith. We consider lobes to either be single batches formed during continuous magma fl ow or multiple, quickly coalescing pulses that in either case formed separate magma chambers that failed to amalgamate with other compositionally distinct pulses such as those occurring in the central batholith. Zircon age comparisons between all four lobes and the main body imply that growth of the Tuolumne intrusion was not stationary, but that the locus of magmatism shifted both inward and northwestward.

Textures and melt-crystal-gas interactions in granites

“…This anisotropic and discontinuous aspect of flow and growth is at present generally admitted. It has been confirmed by field observations (de Saint Blanquat et al, 2006), structural discontinuities (Vigneresse and Bouchez, 1997;Memeti et al, 2010) and age determinations (Miller and Paterson, 2001;Coleman et al, 2003;Leuthold et al, 2012). It has been verified by analogous (Román Berdiel et al, 1995;Arbaret et al, 1996) and numerical experiments (Rabinowicz and Vigneresse, 2004).…”

Structure of volcano plumbing systems: A review of multi-parametric effects

“…Chiarabba et al (2004a), on the basis of a shallow seismic tomography of Vulcano Island (Aeolian Arc, Italy) observe that at a depth >0.5 km, the rise of magma is controlled by NW-SE fractures associated with the activity of the NW-SE-striking, rightlateral strike-slip to oblique-slip, Tindari-Letojanni fault system (Fig. 32C) (Mazzuoli et al, 1995).…”

Formation and transfer of stoped blocks into magma chambers: The high-temperature interplay between focused porous flow, cracking, channel flow, host-rock anisotropy, and regional deformation

“…This batholith consists of three inward-nested, inward-and northward-younging, and progressively more evolved intrusive units called the Kuna Crest, Half Dome, and Cathedral Peak, and a scattering of leucogranite bodies in all of these units, such as the Johnson Granite Porphyry (Bateman, 1992;Memeti et al, 2010b). Geochronologic studies (Kistler and Fleck, 1994;Coleman and Glazner, 1997;Coleman et al, 2004;Matzel et al, 2005Matzel et al, , 2006Miller et al, 2007;Memeti et al, 2010b) indicate that the batholith was constructed over an ~10 m.y.…”

“…Geochronologic studies (Kistler and Fleck, 1994;Coleman and Glazner, 1997;Coleman et al, 2004;Matzel et al, 2005Matzel et al, , 2006Miller et al, 2007;Memeti et al, 2010b) indicate that the batholith was constructed over an ~10 m.y. duration between ca.…”

“…85 Ma, and youngs both inward and to the northeast. Several large lobes or fi ngers of magma that are normally zoned and young inward (Memeti et al, 2010b) extend outward from the main Tuolumne Batholith body. One of these lobes, consisting of porphyritic Half Dome Granodiorite, extends southward from the main batholith and intrudes into slightly older granitic units, the Red Devil Lake and Turner Lake granites.…”