Paper No. 14
Presentation Time: 12:15 PM


WIESENFELD, John A.1, SCHWARTZ, Joshua J.2, LACKEY, Jade Star3, MIRANDA, Elena A.1, ZEIDAN, Tina1, HOLGREN, Stephanie L.1 and COBLE, Matthew A.4, (1)Department of Geological Sciences, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330-8266, (2)Department of Geological Sciences, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91330, (3)Geology Department, Pomona College, Claremont, CA 91711, (4)Department of Geological Sciences, Stanford University, Stanford, CA 94305,

Convergent-margin arc magmatism is typically understood to be a product of partial melting of the mantle wedge; however the relative contribution of deep-crustal partial melts to arc magmas is poorly understood. The Cucamonga Terrane (CT), in the southeastern San Gabriel Mountains, exposes an exhumed deep-crustal cross section of a Cretaceous continental magmatic arc, which allows investigation of the links between granulite-facies metamorphism, partial melting, and mid-crustal pluton emplacement. Garnet granulites in the CT are interlayered with quartzo-feldspathic gneisses and lesser abundant marbles and calc-silicates. Stromatic migmatite textures are locally abundant and developed at 8-9 kbar and ca. 775–800°C. Mid-crustal tonalites structurally overlie mafic granulites and preserve mylonitic fabrics. Tonalites are intruded by a post-kinematic suite of granitic dikes that cut mylonitic fabrics.

We report 40 new whole rock geochemical analyses and 6 new SHRIMP-RG zircon ages to document time scales of metamorphism and magmatism in the CT. Undeformed metamorphic zircons from garnet granulites display high U cores surrounded by lower U and Th/U (<0.1) rims. Both cores and rims show depletions in heavy-rare-earth elements suggesting coeval growth of garnet during prograde metamorphism. 206Pb/238U error-weighted average ages for zircon cores and rims are 86.2 ± 0.7 Ma and 83.8 ± 1.5 Ma, respectively. Metamorphic zircons from a calc-silicate yielded a similar error-weighted average age of 86.1 ± 2.5 Ma. Zircons from a mid-crustal tonalite yielded an error-weighted average age of 85.8 ± 0.6 Ma indicating coeval granulite-facies metamorphism and tonalite emplacement. Low U, metamorphic rims on tonalite zircons yielded an age of 75.5 ± 5.3 Ma, which overlaps with ages from two granitic dikes in the area (76.2 ± 0.5 and 74.0 ± 0.7 Ma).

We performed whole rock geochemical modeling to determine possible geochemical links between granulites and tonalitic magmatism. We use several granulite compositions as sources and model various partial melting scenarios. Our results indicate that high Sr/Y tonalite compositions are consistent with 20-30% partial melting of garnet granulites, which supports field observations and temporal links from zircon geochronology.