On the causes of 18O-depletion and 18O/16O homogenization during regional metamorphism; the East Humboldt Range core complex, Nevada

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doi: 10.1007/s004100050028
Authors:Peters, M. T.; Wickham, S. M.
Author Affiliations:Primary:
University of Chicago, Department of Geophysical Sciences, Chicago, IL, United States
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 119(1), p.68-82. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:1995
Note:In English. 58 refs.; illus., incl. 2 tables, geol. sketch maps
Summary:Previous stable isotope studies at Lizzies Basin, Nevada, showed that metasedimentary rocks are 18O-depleted relative to protolith values, particularly in the lower parts of the section (Lower Zone) where the rocks are also isotopically homogeneous on a scale of hundreds of metres (quartz Δ18O = +9.0 to +9.60/00). In contrast, metasedimentary rocks at higher levels of Lizzies Basin (Upper Zone) are less 18O-depleted and more heterogeneous in Δ18O. In order to understand more fully the isotopic evolution of this terrain, a series of metre-scale traverses across various metamorphic and igneous lithologies were completed at Lizzies Basin and at the structurally higher Angel Lake locality. Traverses in the Lizzies Lower Zone and in the lower parts of Angel Lake (Angel Lake Lower Sequence) across various silicate lithologies, including abundant granitic rocks, reveal similar degrees of homogeneity, although the average Δ18O values are higher at Angel Lake. In contrast, traverses which include substantial thicknesses of marble and calc-silicate gneiss and very little granitic rock have more heterogeneous quartz Δ18O values (+11.9 to +13.40/00), and also have a higher average Δ18O (+12.90/00), than observed elsewhere. The scale of 18O/16O homogeneity in quartz observed at Lizzies Basin and Angel Lake (metres to hundreds of metres) requires fluid-mediated isotope exchange, which accompanied Tertiary metamorphism. There is a correlation between the degree of 18O-depletion in metasedimentary rocks, 18O/16O homogenization between lithologies and the proportion of granitic rocks (leucogranites in particular) within any part of the section, and a corresponding anticorrelation with the proportion of marble. This points to a causal relationship, whereby the leucogranites (as well as the Tertiary hornblende diorite and biotite monzogranite) acted as both a relatively low-18O reservoir and a source of fluids to enhance exchange, while the marbles hindered isotope depletion and homogenization by acting as relatively high-18O reservoirs and impermeable layers. Material balance calculations help delineate the plausible mechanisms of exchange between granitic rocks and metasediments. Fluid-mediated exchange by convective recirculation of magmatic fluids on a scale of metres is the mechanism which explains all the observations. The generalized model for the evolution of the East Humboldt Range core complex provides an excellent opportunity to establish the main causes and controlling factors of 18O-depletion and 18O/16O homogenization during regional metamorphism. [P.Br.]
Subjects:C-13/C-12; Carbon; Geotraverses; Granites; Homogeneity; Igneous rocks; Isotopes; Metamorphic core complexes; Metamorphic rocks; Metamorphism; Metasedimentary rocks; O-18/O-16; Oxygen; Plutonic rocks; Regional metamorphism; Silicates; Stable isotopes; East Humboldt Range; Elko County Nevada; Nevada; United States; Lizzies Basin
Abstract Numbers:95M/4524
Record ID:1995037552
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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