Pluton-Wallrock Interactions in the Sequoia Region: Evaluating Crustal Contamination in the Early Sierran Arc

What: The Sierra Nevada batholith is a collage of plutons interspersed with remnant pendants and septa of pre-batholith metamorphic wallrocks (Fig. 1). This Keck Project—the first Keck project to be done in the Sierra Nevada—will engage student participants in a detailed study of the record of the interplay between magmatism and metamorphism that is preserved in the geologically, geographically, biologically, and historically diverse Sequoia region in the west-central Sierra Nevada. During the project, students will become familiar with the geology of Sierra Nevada as a whole and its significance in the geologic evolution of California as well as the Cordillera as a whole. Extensive field and laboratory investigation are planned, including a session dedicated to dating of igneous and metamorphic rocks from the project at the Stanford/USGS SHRIMP RG Ion Microprobe.

When: July 8–August 4

Where: Sequoia National Park & National Forest, Stanford University SHRIMP Lab, Pomona College, CSU–Bakersfield

Who: Drs. Jade Star Lackey (Pomona College) & Staci Loewy (CSU—Bakersfield)

Project description and goals

In the Sequoia region, Cretaceous granitoid (gabbro to granite) plutons abut biotite schists, marbles, and other lithologies of the Kings Sequence (Fig. 1). The area has a number of high-silica granites, with associated gabbro and diorite stocks, which commonly abut steeply dipping pendants and septa of the Kings Sequence. Several of the high-silica granite are peraluminous and have evidence of contamination at their margins, and localized development of migmatites complexes in of biotite schists adjacent to some plutons.

The principle hypothesis to be tested in this project is that magmas (both felsic and mafic) ascended from source regions, they locally drove partial melting of Kings Sequence pendants. In this scenario, mafic magmas associated with the high-silica plutons are largely coeval and potentially supplied heat to drive partial melting of the metamorphic rocks. Migmatite complexes in the region may contain quenched former partial melts and therefore can be evaluated for melting reactions and potentially the “budget” of melt that they could produce. While the granites exposed at the current crustal levels are complex integrations of different source and contamination histories, crystal cargoes of inherited xenocrysts (zircon, garnet, aluminosilicate minerals, spinel, monazite) can be employed to begin to deconvolute this history.

Testing this hypothesis directly informs our understanding of the tempo and mode by which convergent margin batholiths recycle pre-existing crust. Such constraints allow estimation of mass and thermal transport in such settings, which lends to understanding of the crustal growth and evolution. The findings can also shed light on the mechanisms by which plutons are assembled and crystallize.

Specific goals of the Keck project include the following:

  • Refining the geochronologic framework of the region, including dating of the plutons and metamorphic framework rocks.
  • Establishing the P-T and melting history of the Kings Sequence in the Sequoia region
  • Evaluating the magmatic diversity among plutons in the area, with particular attention to the relationship of mafic stocks to the voluminous granodiorites in the area.

Student projects

Students will test the overarching hypothesis by completing a number of related petrology and geochemistry projects. Each student project will include components of fine-scale mapping of key field relations, sample collection (students will be sent home with fist-sized hand samples for further analysis). Students will leave the project with a large body of geochronology and trace element data in hand and we anticipate additional petrographic and geochemical analyses to be conducted in the ensuing year leading up to the completion of the project. Additional funding is budgeted for home campus laboratory expenses. Potential projects are listed below and can be refined depending on student and research advisor interest and expertise.

  • Partial Melting in Kings Sequence Schists (2 students). Migmatitic rocks are exposed in several areas of the field area and present numerous opportunities to combine detailed petrography, phase equilibria, and geochronology to evaluate the conditions of melting and composition of melts. Monazite and detrital zircon dating in the schists would provide temporal constraints to test linkages of the migmatites with surrounding plutons. These migmatites have diverse mineralogy including index minerals such as garnet, cordierite, andalusite, gahnite-hercynite spinel, etc.
  • Grant Grove Plutons (1 student). Prior study of the Grant Grove Pluton indicates significant localized contamination by metamorphic wallrocks. Several areas of the pluton have not been studied, particular northern areas where mafic stocks are associated with the pluton (Fig 1). This study would focus on the northern areas of the Grant Grove and Eshom Point pluton to explore the relationship between the granite, mafic stocks, and Kings Sequence.
  • Mineral King Plutons (1 student). The Mineral King, Castle Creek (Fig. 1) and other unnamed plutons in the southern part of the field area have not been studied in any detail. One student can investigate these plutons to evaluate their temporal and geochemical relation to the magmatism in the region.
  • Fry’s Point Pluton (1 student). The Fry’s point pluton is a complex body with multiple mafic intrusions and associated migmatite complexes. A student can document the diversity of this pluton and work with the migmatites team to evaluate for connectivity within the complex.
  • P-T-fluid conditions in marbles (1 student). Marbles and associated skarns in the area record useful information about magmatic fluid and volatile fluxes in the arc. Thus, another potential project is a petrologic or stable isotope study of these rocks constrain peak metamorphic P-T-XCO2 conditions. Some marbles contain crystalline graphite which may allows evaluation of peak P-T conditions through calcite-graphite carbon-isotope thermometry.

Field conditions

Field work will be conducted at altitudes ranging from 2000 to 7500 feet. Some investigations will occur at higher altitudes in the Mineral King area. Terrain varies from scrub oak, to chaparral to sub-alpine exposure, so participants should be prepared to work under a range of weather conditions. Most field sites are accessed on foot from roads and established trails, therefore extensive hiking expected. Field conditions can be challenging, including working in steep, densely vegetated (including poison oak) terrain. Students will be trained on the various natural and man-made hazards of the field work up front.

Course Preparation

Students must have taken Mineralogy and Petrology; Structural Geology is strongly encouraged. Additional coursework in tectonics, stratigraphy, and geochemistry is helpful. Strong field skills are a plus.

 

 

 

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