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.

Incrementally constructed magma systems have been recognized from studies of the resulting plutons for more than three decades. However, magma addition rates, fl uxes, growth durations, sizes of increments, and sizes and durations of the resulting magma chambers have been diffi cult to ascertain, emphasizing the need for a better understanding of how magmatic systems evolve. Our results from studies of plutons and arc sections in the North American Cordillera indicate that a large range exists in all of these values. Although arc sections and individual plutons clearly have dramatic temporal changes in volumetric magma additions, true volumetric fl ux calculations are particularly diffi cult to determine. Thus, although subduction beneath arcs may have active durations of hundreds of millions of years, volumetrically most magmatism is emplaced during magmatic fl are-ups of ~10-30 m.y. duration. Individual plutons and batholiths in these arcs can grow in <0.5 m.y. to 10 m.y. Pulse sizes moving through these magma plumbing systems vary from small dike-like to large diapir-like pulses, both of which may form from earlier amalgamation of poorly defi ned pulses. Our thermal modeling, using a range of incremental growth scenarios, concludes that focused incremental growth with greater than a certain volumetric fl ux results in magma chambers that are much larger than individual pulses but less than the size of the fi nal batholith, and with hypersolidus durations of hundreds of thousands to millions of years. The volumetric magma fl ux and the spatial distribution of volumetric addition rates of magma, rather than size or shape of individual pulses, are the dominant controlling factors on growth scenarios and chamber sizes and durations.

In this study, structures in plutons and host rocks are coupled with geochronology to track paleodeformation fields from the late Paleozoic to Late Cretaceous in the central Sierra Nevada. Regional NW-striking host-rock foliation, NE-or SW-vergent thrust faults, and associated folds developed from the early Mesozoic to Early Cretaceous. Dextral transpressional shear zones developed in the Late Cretaceous. Strikes of steep-dipping magmatic foliations in Mesozoic plutons temporally vary from approximately NW (Triassic-Jurassic) to WNW (Late Cretaceous), displaying a progressive counterclockwise rotation. Joint interpretation based on combining host-rock and magmatic structures suggests that intra-arc paleodeformation fields were dominated by coaxial and arc-perpendicular contraction from the early Mesozoic to Early Cretaceous, becoming increasingly dextral transpressive in the Late Cretaceous. The switch from contraction to transpression was likely caused by oblique convergence between the Farallon and North American plates. Based on observations in the study area and other host-rock pendants in the central Sierra Nevada, we propose that the intensity of intra-arc deformation is cyclic. To some extent, it mimics the episodic pattern of arc magmatism: Stronger deformation coincides with magmatic flare-ups. Magmatism promotes intra-arc deformation, which in turn causes crustal thickening during transfer of materials downward to the magma source regions, potentially fertilizing source regions with supracrustal materials and resulting in increased magma generation. Thus, models addressing continental arc tempos should include intra-arc processes. Evolution of continental arcs may be influenced by linked cyclic processes within the arcs accompanied by noncyclic processes driven by events external to the arcs.

No abstract

No abstract