El Capitan Geologic Mapping Project

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RP3

Big Wall climber
Twain Harte
Jul 21, 2015 - 05:08pm PT
It is funny this thread just got bumped because our paper about the petrology of El Capitan was just published. We used El Cap's ~1 km tall exposure to test hypotheses regarding the emplacement and evolution of plutons.

Thanks to the MANY people on this site who contributed to this project over the years.

-Roger Putnam

Check it out here.

Here is the abstract:
Detailed geologic mapping on the ∼1-km-tall, vertical southeast face of El Capitan was completed to determine the chronology and geometry of emplacement. Field relations reveal a complex intrusive history at the boundary between two intrusive suites involving interaction between several granitic units. No resolvable faulting or other postemplacement deformation was observed. New U-Pb zircon geochronologic data (laser ablation and isotope dilution) demonstrate assembly of the El Capitan Granite and diorites of the Rockslides and North America between ca. 106 and 103 Ma. New ages for the Taft (106.6 0.7 Ma), Leaning Tower (104.1 0.10 Ma), and Bridalveil (103.4 0.4 Ma) plutons reveal that they intruded over the same interval as the other plutonic rocks exposed on the face of El Capitan, although field relations and geochronology suggest a distinct order of emplacement.

Two sets of aplite dikes are also exposed. Their whole-rock compositions suggest segregation at depths of 56 km and derivation from the intrusive suites of Yosemite Valley or Buena Vista Crest. Chemical analysis of samples collected along ∼1-km-tall vertical transects through the El Capitan and Taft Granites reveals no systematic variations in major or trace elements. Analysis of 78 photographs within the El Capitan Granite also shows no systematic variations in texture or mineralogy with elevation. Lack of resolvable vertical variations in both field and petrologic observations is consistent with incremental assembly, and is hard to reconcile with models that envision magma chambers as large fractionating bodies.
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