Petrology of Mafic Plutons Associated with Calc-Alkaline Granitoids, Chilliwack Batholith, North Cascades, Washington |
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| Authors: | TEPPER JEFFREY H |
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| Institution: | DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF WASHINGTON, SEATTLE, WA 98195, USA, AND GEOLOGY DEPARTMENT, UNION COLLEGE SCHENECTADY, NY 12308, USA |
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| Abstract: | Small (<5 km2), lithologically diverse gabbro and dioritestocks make up  2% of the 34 to 2 Ma Chilliwack batholith, andoverlap in age with associated calc-alkaline granitoids. Thesemafic plutons are similar to those in other I-type bath-oliths,and represent basaltic magmas present during batholith formation.Objectives of this study are: (1) to examine the origins ofboth interpluton and intrapluton petrologic diversity, and (2)to compare chemical and Sr-Nd isotopic traits of these gabbroswith those of Cascade are basalts. Mafic rocks in the Chilliwackare divided into a medium-K series (MKS) and a low-K series(LKS). The former contain 0.7–2.4 wt% K20 and are similarin composition to calc-alkaline basalts and basaltic andesites.Inverse REE modeling supports derivation of the MKS by 9–27%melting of a garnet-free, LREE-enriched source (La/ bN 2). ChilliwackLKS gabbros have chemical characteristics of low-K olivine tho-eiites,including low K2O (0.3–0.5 wt%) and La/bN (1.7–3.4),and high CaO (8.8–11.3 wt%) and Na2O/K2O (6–22).These traits suggest a source with more clinopyroxene and lowerLa/bN than the MKS source. Differences in  Nd(O) between MKSand LKS gabbros suggest that lower Nd/Sm is a long-lived LKSsource characteristic. Lithologic variation within compositeplutons of both series resulted primarily from multiple intrusionof related magmas, in some cases differentiates of a commonparent. Two contrasting examples were studied in detail. AtMt Sefrit, MKS variation (gabbronorite-quartz diorite) is modeledby low-pressure fractionation (ol + plag + cpx), accompaniedby 10% wallrock assimilation. In contrast, chemical and Sr-Ndisotopic variation among LKS gabbro-quartz diorite at CopperLake points to crystallization dominated by clinopyroxene+plagioclaseCr-spinel,indicative of differentiation at pressures  10 kbar, althoughthe assimilant in this case is poorly constrained. Chemicaland isotopic similarities between these mafic plutons and QuaternaryCascade lavas indicate that mafic magmas present during theproduction of Chilli-wack granitoids were low-and medium-K arebasalts. KEY WORDS: are magmatism; Cascades; gabbro; granitoid; trace element
*Present address: Department of Physics, Astronomy, & Geology, Valdosta State University, Valdosta, GA 31698, USA. Telephone: (912) 249–4847 or 333–5752 Fax: (912) 333–7389. e-mail: jtepper{at}valdosta.peachnet.edu |
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