Paper No. 6
Presentation Time: 2:45 PM
GENERATION OF FELSIC MAGMAS AT THE BASE OF THE CRUST: PARTIAL MELTING PARAMETERS FOR TRACE AND RARE EARTH ELEMENT SIGNATURES IN GRANITIC MAGMAS FROM THE FEHR GRANITE, NORTHERN SASKATCHEWAN, CANADA
KOTEAS, G. Christopher, Earth and Environmental Sciences, Norwich University, 158 Harmon Drive, Northfield, VT 05663, SEAMAN, Sheila J., Department of Geosciences, University of Massachusetts, Amherst, MA 01003 and WILLIAMS, Michael L., Department of Geosciences, University of Massachusetts, Amherst, 611 North Pleasant Street, Amherst, MA 01003, gkoteas@norwich.edu
The composition of felsic partial melts generated via migmatization of granitic rocks in deep crustal environments is determined by a series of temperature- and volatile-controlled petrogenetic processes. The generation, transport and geochemical evolution of felsic magmas at the base of the crust are essential to the development of geochemical signatures in granites at higher crustal levels. Studies of the Fehr granite migmatite from the Athabasca Granulite Terrane in northern Saskatchewan, Canada suggest that six mechanisms exert fundamental controls on trace element and rare earth element (REE) composition of mobile felsic partial melt. These processes are 1) the growth of peritectic phases in melanosome, 2) the degree of partial melting, 3) the kinetics of melting large alkali feldspar crystals versus a fine grained granitic matrix, 4) the rate of leucosome transport away from generation sites, 5) the position of accessory phases such as zircon, apatite, allanite, and monazite in the protolith (i.e. hosted by matrix vs. crystals) and 6) the ability of granitic leucosome to either break down or inherit accessory phases.
The shielding of accessory phases by relatively stable major phases, especially plagioclase feldspar, depends on the dominant mechanism for partial melting, the volume of partial melt produced, and the residence time of partial melt near the site of generation. Electron probe microanalysis studies of geochemical variation linked to microtextures in paleosome, melanosome, and leucosome in the Fehr granite migmatite suggest that there are multiple transport pathways for trace elements and REE. Modeling based on whole rock geochemistry suggests that variation in the degree of partial melting, from ~10 to 40 volume percent, plays a significant role in the partitioning of REE and of high field strength elements. However, microgeochemical analysis suggests that the ability of partial melt to efficiently break down and/or remobilize accessory phases has a first order control on leucosome composition. This potential variability of partial melt composition has direct implications for the composition of felsic magmas and the contamination signatures of mafic and felsic magmas that interacted in the lower crust, but may ultimately be emplaced in the middle and shallow crust.
