Sediment-derived fluids in subduction zones; isotopic evidence from veins in blueschist and eclogite of the Franciscan Complex, California

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doi: 10.1130/0091-7613(1991)019<1033:SDFISZ>2.3.CO;2
Authors:Nelson, Bruce K.
Author Affiliations:Primary:
Univ. Wash., Dep. Geol. Sci., Seattle, WA, United States
Volume Title:Geology (Boulder)
Source:Geology (Boulder), 19(10), p.1033-1036. Publisher: Geological Society of America (GSA), Boulder, CO, United States. ISSN: 0091-7613
Publication Date:1991
Note:In English. 36 refs.; illus. incl. 1 table
Summary:Isotopic analyses of minerals from veins that cut high-grade blueschist and eclogite blocks in the central belt of the Franciscan Complex provide constraints on the chronology of metamorphic events and on the origin and movement of fluids within the subduction zone. A Rb-Sr age of 153 ±1 Ma obtained for minerals from veins and open cavities that formed contemporaneously with retrograde blueschist facies metamorphism is a minimum age for the prograde metamorphism. The veining precedes the last episode of sedimentary-matrix melange formation by a minimum of 15 to 20 Ma, during which time the blocks must have been stored within the subduction complex at low temperatures and without undergoing penetrative deformation. Initial Nd-isotope compositions (εNd) of the vein minerals range from +10.8 to -2.4, indicating that some fluids were derived predominantly from dehydration of subducted mid-ocean ridge basalt, but that other fluids had a component derived from subducted sediment. The provenance of the subducted sediment was within old continental crust, thus associating the Franciscan paleo-subduction complex with a continental craton by the time of vein formation. [L.C.H.]
Subjects:Absolute age; Alkaline earth metals; Blueschist facies; Continental crust; Crust; Dates; Eclogite; Facies; Fluid phase; Franciscan Complex; Geochemistry; High-grade metamorphism; Isotopes; Melange; Mesozoic; Metabasalt; Metaigneous rocks; Metals; Metamorphic rocks; Metamorphism; Metasomatism; Minerals; Nd-144/Nd-143; Neodymium; Prograde metamorphism; Rare earths; Rb/Sr; Retrograde metamorphism; Sr-87/Sr-86; Stable isotopes; Strontium; Subduction zones; Trace elements; Veins; Water of dehydration; California; Central California; Diablo Range; Oregon; United States; Environment; Evolution; Mount Hamilton Eclogite; Tiburon California
Abstract Numbers:92M/3110
Record ID:1991047215
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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