mapping of matrix phases; implications for mass transfer during
crenulation cleavage development in the Moretown Formation, western
M L, Scheltema
K E, Jercinovic
M J, 2001
of Structural Geology. 23; 6-7, Pages 923-939.
Fig. 3. Photomicrograph
(acrossed polars; buncrossed polars) of
a single crenulation microfold from hinge of sample MT-1. From left
to right, sample shows four alternating domains: QFMQFM.
Box shows location of Fig. 4. (c) Mg K compositional map of same
area as (a), (b) (orange=chlorite; RED=biotite). (d) Ca K compositional
map of same area as (a), (b) (yellow-orange=plagioclase; PURPLE=albite).
See text for discussion.
compositional maps provide a new tool for investigating mass transfer
during cleavage formation. The Moretown Formation of western Massachusetts
contains a well-developed crenulation cleavage with alternating mica-rich
crenulation limbs and mica-poor crenulation hinges. Compositional
mapping shows two generations of plagioclase, the second of which
was synchronous with the crenulation cleavage. A significant amount
of the syn-crenulation plagioclase (10-20% modally) grew in hinge
domains. A small amount of syn-crenulation plagioclase ( approximately
1%) and a large amount of phengitic muscovite grew in limb domains.
The maps also show that uncrenulated domains experienced mass transfer
and reactivation of older cleavages, and thus cannot be used as "undeformed"
reference domains for comparison with crenulated regions. Compositional
mapping facilitates a new degree of integration between petrologic
and structural analysis. Knowledge of the structural context of compositional
domains allows better selection of phases and compositions for interpreting
metamorphic reactions and linking metamorphism to deformational stages.
Knowledge of syntectonic reactions provides new insights into mass
transfer and volume change during deformation. In the Moretown Formation,
plagioclase- and phengite-producing reactions play a large role in
controlling the nature and magnitude of mass transfer, but microstructures
control the location of reactants and products within the evolving
to Appalachian Research