Example of trondhjemite genesis by means of alkali metasomatism: Rockford Granite, Alabama Appalachians

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

OSTI ID:6610788

Drummond, M S; Wesolowski, D; Ragland, P C; Ragland, J

An alternative model for trondhjemite genesis is proposed where granite is transformed to trondhjemite via infiltration by a Na-rich metamorphic fluid. The Rockford Granite serves as the case example for this process and is characterized as a synmetamorphic, peraluminous trondhjemite-granite suite. The major process operative in the conversion of granite to trondhjemite involves cation exchange of Na for K in the feldspar and mica phases through a volatile fluid medium. Whole-rock delta/sup 18/O values for the trondhjemites are negatively correlated with the atomic prop. K/Na ratio indicating a partial reequilibration of the altered granitoids with a Na- and /sup 18/O-rich metamorphically derived fluid. Biotite decomposition to an Al-epidote-paragonitic muscovite-quartz assemblage is also associated with the Na-metasomatism, as are apatite replacement by Al-epidote and secondary zircon crystallization. The replacement of magmatic phases by metasomatic phases exemplifies the chemical changes produced during infiltration metasomatism where the trondhjemites are depleted in all REE's. The timing of the infiltration metasomatism is thought to have occurred during regional metamorphism, producing a discrete fluid phase in the surrounding amphibolite-grade metasediments. Foliation planes in the granitoid apparently served as conduts for fluid flow with reaction enhanced permeability accompanying the 8% molar volume reduction during Na for K exchange in the feldspars. A model is proposed where metamorphic fluids migrated updip and along strike from their source and were buffered by the presence or absence of two feldspars in the metasediments.

• Research Organization: Univ. of Alabama, Birmingham (USA)
• OSTI ID: 6610788
• 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
ALABAMA
GRANITES
CHEMICAL REACTIONS
HYDROTHERMAL ALTERATION
METAMORPHISM
APPALACHIAN MOUNTAINS
CHEMICAL REACTION KINETICS
CRYSTALLIZATION
FELDSPARS
GEOCHEMISTRY
GEOLOGIC MODELS
ION EXCHANGE
MINERALOGY
PETROGENESIS
CHEMISTRY
FEDERAL REGION IV
IGNEOUS ROCKS
KINETICS
MINERALS
MOUNTAINS
NORTH AMERICA
PHASE TRANSFORMATIONS
PLUTONIC ROCKS
REACTION KINETICS
ROCKS
USA
580400* - Geochemistry- (-1989)
580300 - Mineralogy
Petrology
& Rock Mechanics- (-1989)