Formation of the Sierra Nevada Batholith:

magmatic and tectonic processes and their tempos

Preliminary announcement of a

Geological Society of America Field Forum

September 1-8, 2012

Conveners:  Paterson, Lackey, Memeti, Miller, Miller, Mundil, and Putirka

Overview:  The evolution of continental margin orogens and magmatic arcs involve non-steady state processes of subduction, orogeny, magmatism, exhumation, and erosion/redeposition. Recent studies have begun to examine the tempo of cyclic volumetric addition rates to arcs of plutonic and volcanic materials, driven by feedbacks between these processes both at the scale of arcs and single magmatic or volcanic systems. A full evaluation of these arc tempos requires the development and syntheses of large databases with high precision temporal control. One arc, where a number of research groups have been actively developing and synthesizing high-precision databases, is the Sierra Nevada, California. This Field Forum will bring together different researchers to share and discuss various data sets while examining components of the Mesozoic arc. Our goal is to foster cross-disciplinary discussions leading to a better understanding of the components of batholith formation, the tectonic controls on the tempo of arc development, and the significance of important new field, geochronologic, and geochemical databases.

Field Excursions:  The field program will focus on a west to east transect through the central Sierra Nevada to examine both volcanic and plutonic components of the Mesozoic arc, regional tectonics, and the characteristics of the underlying basement units.  Specific targets will include the following:

(1) ~151 Ma Guadalupe Igneous complex, a tilted plutonic section that exposes from top to bottom a volcanic section, granophyres, a mingled granite to dioritic section, a large sequence of high Mg, layered gabbros, and a potential underlying, vertically sheeted feeder zone.

(2) The ~124-105 Ma, ~ 80 X 40 km, tonalitic-granodioritic Fine Gold intrusive complex. This incrementally constructed complex intrudes across a fundamental basement suture (Foothills suture) between oceanic and displaced passive margin basement in the Sierras and thus allows us to examine issues of incremental chamber construction, magma-wall rock interactions, and the isotopic and geochemical systematics across a major basement suture.

(3) Plutons west of the Tuolumne batholith: The ~ 102-100 Ma Yosemite Valley Intrusive Suite (El Capitan/Mt. Hoffman granodiorite, Taft Granite and isolated mafic bodies), and ~98-95 Ma Yosemite Creek-Sentinel plutons.  New mapping, structural analysis, and a growing database of geochemistry (elemental and isotopic whole rock, minerals) and U/Pb geochronology, suggest that these plutons grew via numerous increments that produced superficially very different plutons with differing scales of heterogeneity.  The new data should allow us to speculate on why/how the resulting differences arise.

(4) The ~95-85 Ma Tuolumne intrusive complex, an ~1100 km², incrementally constructed, internally zoned complex that has been extensively studied over the last 10 years by a number of research groups. New datasets abound, including (a) extensive 1:10,000 scale mapping, (b) detailed structural studies and strain analyses; (c) whole rock and single mineral element geochemistry and a range of isotopic studies, (d) high precision CA-TIMS U/Pb zircon and ⁴⁰Ar-³⁹Ar geochronology, (e) and thermobarometry of plutons and host rocks. Models for the construction of this intrusive complex and interpretation of these datasets remain controversial and our focus will be on the presentation of new datasets and discussion of the proposed growth models.

(5) A nearly vertically tilted section through the Triassic to Cretaceous volcanic sequence that was constructed above the Mesozoic plutons and is now is exposed in the Saddlebag pendant located along the eastern margin of the Tuolumne Intrusive complex. New results will also be presented from studies of this volcanic section to the north (Virginia Canyon area) and south (Ritter Range pendant).

(6) There are also a number of new arc-scale datasets being developed and synthesized including (a) new mapping in a number of pendants and plutons, (b) extensive structural data; (c) strain analyses in both the central and southern Sierras; (d) CA-TIMS and LA-ICPMS ages of plutons and volcanic rocks plus LA-ICPMS detrital zircon ages from metasedimentary and volcaniclastic units; (e) geochemical and isotopic (Sr_i, e_Nd, Pb, O, Hf) analyses and regional syntheses; and (f) syntheses attempting to combine all of the above with regional tectonics. Where appropriate, we will present these new syntheses with the goal of discussing tempos at various scales.

Logistics: Maximum participants 54. Registration fees for students and recent postdoctoral researchers will be partially subsidized through NSF funds. The daily field excursions will involve road stops and short (<3 km) hikes at high elevations.

Forthcoming Details: watch for future announcements through GSA-MGPV and AGU-VGP sections and through the GSA website (http://www.geosociety.org/penrose/). Please feel free to contact Scott Paterson ([log in to unmask]) if questions arise.