Discrepancies between predicted and observed mineral reactions in staurolite schist, Pecos Baldy, New Mexico

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

OSTI ID:6576508

Grambling, J A

Staurolite-bearing rocks near Pecos Baldy contain minerals which define the continuous reaction STAUR+BI+Q=GAR+MUSC+H/sub 2/O. The reaction should proceed to the right as metamorphic grade increases northward. Mineral compositions become more magnesian to the north, consistent with the continuous reaction. However, contrary to the predicted reaction, staurolite becomes coarser and much more abundant from south to north. Mn depletion-zoning is more developed in isolated garnet than in garnet enclosed in staurolite or biotite, requiring that staurolite and biotite grew while garnet was growing. The simultaneous growth of all three porphyroblasts suggests that rock bulk compositions may have changed while minerals grew. The celadonite content of muscovite decreases across the staurolite zone, with loss of FeO at constant MgO. Such a change could have freed FeO for porphyroblast growth, but the addition of FeO would cause growth of garnet, not staurolite. As ilmenite is the only Fe-oxide, and is present in only trace amounts, ilmenite growth did not significantly affect FeO. Growth zoning in garnet must have been removed FeO and MnO from the rocks, shifting the bulk composition of the reactive minerals toward the staurolite-biotite join, but the low modal abundance of garnet (4%) seems insufficient to account for the large amount (12%) of unpredicted staurolite growth. The data can best be explained in two ways: either premetamorphic bulk compositions were more aluminous and magnesian in rocks farther north, and staurolite crystals grew larger as they broke down; or the rocks lost FeO, perhaps into a fluid phase, as metamorphism proceeded.

• Research Organization: Univ. of New Mexico, Albuquerque (USA)
• OSTI ID: 6576508
• 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
580400* -- Geochemistry-- (-1989)
CHEMICAL COMPOSITION
CHEMICAL REACTIONS
CHEMISTRY
FEDERAL REGION VI
GEOCHEMISTRY
METAMORPHIC ROCKS
METAMORPHISM
MINERALIZATION
MINERALOGY
MINERALS
NEW MEXICO
NORTH AMERICA
PETROGENESIS
ROCKS
SCHISTS
SILICATE MINERALS
USA