17 Cordilleran Magmatic Forensics II—Insights from the Igneous, Metamorphic, and Sedimentary Record

Tuesday, 23 May 2017
1:30 PM - 4:50 PM
Room 318A

ZIRCON TRACE ELEMENT AND ISOTOPIC RECORDS OF SIERRA NEVADAN ARC MAGMATISM DURING AN ARCHETYPAL MAGMATIC FLARE-UP

Paper number: 17-2
Program: Theme Sessions
Time: 1:35 PM – 1:55 PM

Authors

Abstract

The Cretaceous Sierra Nevada batholith is recognized globally as an archetypal magmatic arc flare-up, and the large (several 103 km2,) normally zoned intrusions of Late Cretaceous age that occur along the crest of the range are the most spectacular manifestations of flare-up magmatism.  Although there is general consensus that the Sierra Crest suites must have been constructed from numerous increments of magma, there is little consensus on the longevity and size of possible magma bodies that might have existed during their construction. The origin of the zonation, from older tonalites and mafic granodiorites comprising the marginal units to younger, more felsic granodiorites and granites in the interiors, remains enigmatic, although to a first order it must reflect the large-scale geodynamics of the arc. 

Trace element and Hf and O isotopes from zircon provide complementary information that can together be used to probe the sources and assembly of the intrusions. Key trace element indicators (Ti-in-zircon model temperatures, REE patterns and ratios) show important time-dependent shifts that repeat along the arc, and thus reflect important arc-scale shifts in the environment of magma formation and also in subsequent higher-level storage conditions. The shifts record a pronounced change from overall hotter conditions of magma formation and initial storage (zircon undersaturation) near the "apex" of the flare-up to markedly cooler, more water-rich conditions of magma formation and storage (at or near zircon saturation) as arc-magmatism waned. 

Hf and O isotopes in zircon show important variations in crustal and mantle sources that are generally harder to discern from whole rock isotopic trends. Although there is a modest shift to more crustal isotopic values (lower εHf and higher δ18O) in the suites, there are equally important arc-scale and intrusion-scale spatial variations that must reflect lithosphere-scale variations and shifts in magma source and/or plumbing. 

The zircon data also can be used to evaluate/refine recent models for arc-scale differentiation that have been proposed for Cordilleran arcs.

Geological Society of America Abstracts with Programs. Vol. 49, No. 4

doi: 10.1130/abs/2017CD-292479

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