PEGMATITE-CONTAINING SHEAR ZONES: A MECHANISM FOR THE FORMATION OF LARGE ISOLATED FELDSPAR AUGEN

Large, isolated feldspar augen (8.0 to 0.5 cm) in quartzo-feldspathic shear zones can be the product of intense disaggregation of pegmatites. Such augen are commonly misinterpreted as original porphyroclasts/blasts. This mechanism of forming isolated feldspar augen is documented by pegmatites that underwent extension and thinning under amphibolite facies conditions in a low angle ductile normal fault at the basement-cover contact of the Willimantic dome, eastern Connecticut.

Field relations illustrate a laterally progressive disaggregation and boudinage of pegmatite layers. Intact pegmatites containing large feldspar augen surrounded by recrystallized pegmatite gradually disassociate forming strands of floating feldspar augen, connected by very thin layers of recrystallized quartzo-feldspathic material parallel to the dominant foliation. The final separation and attenuation of these thin connective layers results in completely isolated, individual feldspars that mimic the geometry of a porphyroblast or porphyroclast with a recrystallized tail. Thin section observations indicate that these “isolated” feldspars are still connected by slip surfaces and/or micro-scale lenses of recrystallized quartzo-feldspathic material.

Feldspar grains within single sheared pegmatite strands vary dramatically in their degree of deformation and recrystallization. Largely undeformed, intact crystals with little to no recrystallization occur along side oblate, completely recrystallized grains. Less deformed augen commonly contain kink bands or bent lattices that have localized partial to complete rotational recrystallization. Other augen are rimmed by rotationally recrystallized grains and deformation-induced myrmekite. The degree of internal deformation and recrystallization of each feldspar directly relates to its original orientation and the amount of locally present quartz.

This mechanism for the formation of large, isolated feldspar augen through pegmatite disaggregation may be common and should be considered when studying other shear zones. Such augen can be distinguished from porphyroclasts/blasts by their large size, their internal structure (graphic and perthite intergrowths, etc.), and the presence of relict connections between grains.