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

Conference · Tue Jan 01 00:00:00 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-
• Journal Information: Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
• Country of Publication: United States
• Language: English

Related Subjects

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