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Geologic and isotopic investigations of the early Cretaceous Sierra Nevada Batholith, Tulare Co., CA, and the Ivrea Zone, NW Italian Alps: examples of interaction between mantle-derived magma and continental crust

Citation

Knott, Diane Clemens (1992) Geologic and isotopic investigations of the early Cretaceous Sierra Nevada Batholith, Tulare Co., CA, and the Ivrea Zone, NW Italian Alps: examples of interaction between mantle-derived magma and continental crust. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:04222011-143434002

Abstract

Two igneous suites containing layered ultramafic-mafic cumulates were investigated with the intent to characterize the parental magma and to identify processes significant to the petrogenesis of these rocks. In both study areas, the early Cretaceous Sierra Nevada batholith and the Ivrea Zone, isotopic systematics of the cumulates were found to preserve the characteristics of the mantle-derived parental magma and to record the effects of fractional crystallization and assimilation. Modeling the relative importance of these processes and characterization of the material derived from the mantle are necessary to understanding the growth of the continental crust.

Geologic mapping of 110 mi2 of the 125 to 110 Ma Stokes Mountain region reveals the presence of layered cumulate megaxenoliths and two coeval ring dike complexes. Petrographic analysis and geochemical modeling of 125 dominantly mafic and intermediate samples demonstrate the comagmatic nature of this suite. Combined oxygen, strontium and neodymium analysis of 22 samples indicates, however, that each ring complex was fed by an isotopically distinct parental magma (εNd(115) = +6.1, Sri = 0.70338, δ18O = 6.6‰ ; (εNd(115) = +5.7, Sri = 0.70372, δ18O = 6.7‰) both of which were derived from a variably contaminated, depleted mantle source. Minor assimilation of continentally-derived metasediments and mafic-ultramafic material of the Kings-Kaweah ophiolite further affected the isotopic evolution of the two subsuites. Hydrothermal alteration in the subvolcanic environment is recorded only by rare stoped xenoliths of 120 Ma hypabyssal intrusives.

Late Hercynian (≈300 - 270 Ma) magmatism produced the 10 km thick Mafic Complex lying at the base of the Ivrea-Strona-Ceneri crustal cross section. δ18O analysis of 237 whole rock samples and 26 mineral separates reveals that presumably early intrusions into the cool crust preserve the depleted mantle signature of the modeled parental magma (εNd(115) = +7, Sri = 0.703, δ18O = 6.5‰) while later intrusions assimilated significant amounts of the 10 - 12‰ metapelite. Subsequent intrusion of voluminous basaltic magma fonned a large, convecting magma chamber in which assimilation was concentrated within boundary layers. Such lower crustal production of high-18O (δ18O = 8 - 10‰) mafic magmas is suggested as contributing to the petrogenesis of upper crustal Permian granites.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:gabbro; Sierra Nevada; Ivrea Zone; oxygen, strontium, neodymium; isotope geochemistry
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geology
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Taylor, Hugh P. (advisor)
  • Saleeby, Jason B. (co-advisor)
Thesis Committee:
  • Wyllie, Peter J. (chair)
  • Stolper, Edward M.
  • Epstein, Samuel
  • Rossman, George Robert
  • Albee, Arden Leroy
  • Taylor, Hugh P.
  • Saleeby, Jason B.
Defense Date:6 April 1992
Author Email:dclemensknott (AT) fullerton.edu
Record Number:CaltechTHESIS:04222011-143434002
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:04222011-143434002
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:6347
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:22 Apr 2011 22:40
Last Modified:02 Apr 2014 22:51

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