Isotopic investigation of metasomatism in subduction zones: the Franciscan Complex, California

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6458683

Nelson, B K; Depaolo, D J

Metasomatizing fluids associated with metamorphism in subduction zones may be an important agent for the chemical modification of subducted rocks and of the overlying mantle wedge, both of which may be sources for island arc magmas. To characterize the timing of metasomatism and the source and nature of the fluids, the authors measured Sm-Nd and Rb-Sr isotope ratios of vein-filling minerals and actinolitic rinds cross-cutting meta-basaltic blocks from the Franciscan Central belt. 12 of 14 unmetasomatized samples of greenstone, blueschist and eclogite blocks have measured element of/sub ND/=+8 to +11, indicating MOR protoliths. Measured element of/sub Sr/ values scatter, but initial element of/sub Sr/ for whole rocks and garnets at 150 m.y. are +1 to +16. In element of/sub Nd=element of/sub Sr/ space the initial ratios are shifted to higher element of/sub Sr/ values with respect to unaltered MORB, suggesting seawater-hydrothermal alteration of the protolith. Foliated actinolite +/- white mica rinds from an eclogite and blueschist, both with element of/sub Nd/=+9, have element of/sub Nd/ of +6 and +1 respectively. The Rb-Sr age of the aragonite-glaucophane pair is 153.6 +/- 2.4 m.y., with initial /sup 87/Sr//sup 86/Sr = 0.7052 and element of/sub Nd/ = +6.3. The Rb-Sr age of a white mica-zoisite pair from a vein in a Tiburon eclogite is 151.9 +/- 0.4 m.y., initial /sup 87/Sr//sup 86/Sr = 0.7050 and element of/sub Nd/=+6.7. The vein fluids had similar isotopic compositions, but some rind-forming fluids had element of/sub Nd/<+1 suggesting a component derived from continental crust. Jadeitic metagreywacke, such as that from Pacheco Pass may be a source for low element of/sub Nd/ fluids. Beyond alteration at a MOR, subduction-related metasomatism may further modify eclogite returned to the mantle.

• Research Organization: Univ. of California, Los Angeles (USA)
• OSTI ID: 6458683
• Report Number(s): CONF-8510489-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580201 -- Geophysics-- Seismology &amp; Tectonics-- (1980-1989)
580400* -- Geochemistry-- (-1989)
AGE ESTIMATION
ALKALI METAL ISOTOPES
ALKALINE EARTH ISOTOPES
BASALT
CALIFORNIA
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
FEDERAL REGION IX
FLUIDS
GEOTHERMAL FLUIDS
HYDROTHERMAL ALTERATION
IGNEOUS ROCKS
ISOTOPE DATING
ISOTOPE RATIO
ISOTOPES
MAGMATISM
METAMORPHIC ROCKS
METAMORPHISM
NEODYMIUM ISOTOPES
NORTH AMERICA
PETROGENESIS
PLATE TECTONICS
QUANTITATIVE CHEMICAL ANALYSIS
RARE EARTH ISOTOPES
ROCK-FLUID INTERACTIONS
ROCKS
RUBIDIUM ISOTOPES
SAMARIUM ISOTOPES
STRONTIUM ISOTOPES
SUBDUCTION ZONES
TECTONICS
USA
VOLCANIC ROCKS