HR: 1330h
AN: V32C-1036    [PDF]
TI: Timing of Magmatism and Metamorphism in the Lower Crust Beneath the Colorado Plateau From Coupled U-Pb and Lu-Hf Isotopic Analysis of Zircon From Xenoliths
AU: * Crowley, J L
EM: jlcrowle@mit.edu
AF: Dept of EAPS, MIT, Cambridge, MA 02139 United States
AU: Schmitz, M D
EM: schmitz@dtm.ciw.edu
AF: DTM, Carnegie Instit, Washington, DC 20015
AU: Bowring, S A
EM: sbowring@mit.edu
AF: Dept of EAPS, MIT, Cambridge, MA 02139 United States
AU: Williams, M L
EM: mlw@geo.umass.edu
AF: Dept of Geosci, UMass, Amherst, MA 01003 United States
AU: Karlstrom, K E
EM: kek1@unm.edu
AF: Dept of EPS, UNM, Albuquerque, NM 87131 United States
AB: A major uncertainty in models for Proterozoic evolution of the southwestern U.S. is whether lower crust that formed and accreted at 1.8-1.6 Ga was substantially modified ca. 1.4 Ga by mafic underplating that produced voluminous granitic magmatism and associated low P - high T metamorphism (0.35-0.6 GPa, 500-700 C) preserved in middle and upper crustal basement rocks. A regionally extensive high-velocity (7.xx) lower crust has been interpreted as evidence of underplated magmas. We undertook a U-Pb and Lu-Hf isotopic study of zircons from lower crustal xenoliths to constrain the timing of magmatism and metamorphism beneath the Colorado Plateau. Xenoliths of mafic and felsic granulite from the Navajo volcanic field are interpreted as lower crustal samples based on the primary mineralogy of Cpx-Grt-Pl +/- Qtz and P-T estimates of ca. 1.3 GPa and 800 C. CL imaging of the internal zoning in zircon guided our attempts to isolate homogeneous, moderate sized (50-100 microns) domains through grain fragmentation and air-abrasion. Fragments of grains from eight xenoliths were dated by the ID-TIMS U-Pb method and the solutions from many of these fragments were analyzed for Lu-Hf isotopes by MC-ICPMS. Weak, curved, and sector zoning that is typical of metamorphic zircon is prominent in the mafic xenoliths yet sparse in the felsic ones. Fragments from these zoned domains yielded U-Pb dates that define groups at 1420-1414, 1410-1395 (most prevalent dates), 1385, and 1360 Ma. There is substantial variability in the initial Hf isotope compositions of these grains (Epsilon Hf = -0.7 to +13.6; TDM = 1.8-1.3 Ga), from the same xenolith as well as between xenoliths. This dispersion suggests growth of metamorphic zircon from a variety of introduced fluid compositions or from protolith minerals with diverse time-integrated Lu/Hf ratios. Many of the felsic and mafic xenoliths have zircon cores or whole grains with zoning that may indicate magmatic crystallization. Four felsic granulites have oscillatory-zoned grains with U-Pb dates of 1710 and 1640-1620 Ma, and unradiogenic Hf isotope compositions with depleted mantle model ages of 1.8-1.6 Ga; this concordance of crystallization and source extraction ages supports an igneous protolith interpretation. Three mafic xenoliths have sector- and oscillatory-zoned grains that crystallized at ca. 1435 Ma, however their unradiogenic Hf isotope compositions and model ages of 1.7-1.6 Ga complicate interpretation of the ca. 1435 Ma dates as representing mafic magmatic crystallization, unless these melts were substantially contaminated by older crust. Further petrologic and geochemical studies of the mafic xenoliths are necessary to evaluate this hypothesis. The results suggest a complex lower crust beneath the Colorado Plateau with ca. 1435 Ma mafic magmatism in ca. 1.7-1.6 Ga felsic rocks followed by episodic metamorphism that coincided with regional granitic magmatism and metamorphism in exposed basement rocks. This coincidence suggests a link between ca. 1.4 Ga mafic magmatism and metamorphism in the lower crust and tectonothermal events in the middle crust. The 1.7-1.6 Ga Hf model ages from many metamorphic zircons and all igneous zircons in the mafic xenoliths indicate that they are crustally contaminated ca. 1.4 Ga mafic magmas or restites from ca. 1.4 Ga granite extraction.
DE: 1020 Composition of the crust
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 3660 Metamorphic petrology
DE: 8110 Continental tectonics--general (0905)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting