Start |Author Index | View Uploaded Presentations | Meeting Information

2006 Philadelphia Annual Meeting (22–25 October 2006)

Paper No. 17
Presentation Time: 1:30 PM-5:30 PM

FRACTIONATION TRENDS AND ASSIMILATION IN CRETACEOUS AND TRIASSIC INTRUSIVES IN THE CENTRAL SAN BERNARDINO MOUNTAINS, CA


ALLEN, Ethan1, CLAUSEN, Benjamin L.2, NICK, Kevin E.1, MORTON, Douglas M.3 and KISTLER, Ronald W.4, (1)Earth and Biological Sciences, Loma Linda University, Griggs Hall, Loma Linda, CA 92350, (2)Geoscience Research Institute and Loma Linda University, 11060 Campus Street, Loma Linda, CA 92350, (3)US Geol Survey, Dept of Geol Sci, Univ of California, Riverside, CA 92521, (4)US Geol Survey, 345 Middlefield Road, Menlo Park, CA 94025, eallen06g@llu.edu

Cretaceous and Triassic granitoids in the central San Bernardino Mountains have been collected by Baird (1984), C.F. Miller (1977), and in this study. Chemical data from these rocks suggest 1) a common source, 2) fractionation in Cretaceous units, and 3) limited assimilation at the Cretaceous/Triassic contact.  

The Cretaceous Keller Peak (Kk) and Butler Peak (Kbp) cover at least 200 km2. These monzogranites, with 70-73% SiO2, also show tight groupings of major and trace elements and Rb/Sr isotope values. Chemical similarity suggests they are comagmatic while gradational changes in texture and mineralogy distinguish the units in the field.  

Kao, the Cretaceous granodiorite of Angeles Oaks, is south of Kk and is 63-67% SiO2. Linear fractionation trends and Rb/Sr isotope values suggest a common fractionation series for Kao-Kk-Kbp. The lower silica content of Kao indicates less fractionation and suggests earlier emplacement.  

Southwest of Kk, the City Creek monzogranite (Kcc) is 73-75% SiO2. Kcc also falls on a fractionation trend with Kk and Kbp in both major and trace elements, but elevated and highly variable 87Sr/86Sr ratios indicate contamination.  

Fawnskin monzonite (Trf) bounds Cretaceous units on the northeast, and is 55-62% SiO2. Mg and other compatible element trends suggest that Triassic and Cretaceous rocks in this study may share a common source. Trf shows greater variation in most elements than the Cretaceous units and plots in the alkaline field of a TAS diagram.  

Cretaceous Hanna Flat granodiorite (Kh) is 63-70% SiO2 and crops out in a gradational band up to 1.5 km wide separating Kk from Trf. F.K. Miller (1998) proposed that Kh is the result of contamination of Kk where it intruded Trf. Identifiable blocks of incorporated Trf are found near the margin. Further from the contact, biotite rich clots may represent partially disaggregated Trf. Geochemically, Kh is typically intermediate in composition between Kk and Trf for all elements, and is above the Cretaceous fractionation trend for K and Rb. Geochemical data support that the Kh composition is due to assimilation of Trf even when textural evidence is limited.

Session No. 231--Booth# 95

Petrology, Igneous (Posters)
Wednesday, 25 October 2006: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol. 38, No. 7, p.560
© Copyright 2006 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.
Back to: Petrology, Igneous (Posters)
<< Previous Abstract | Next Abstract >>