PROTRACTED PEGMATITE INTRUSION in SOUTHEASTERN CONNECTICUT?

Pegmatite is a common constituent of the gneiss outcrops exposed along the SE coast of Connecticut. A complex history of intrusion and deformation is recorded, and crosscutting relations between the pegmatite bodies are not always clear. However, some generalizations about observations of pegmatite intrusions at 5 sites in Groton, New London and Waterford can be made:

1) older pegmatite bodies (I) tend to exist in large, irregular-shaped intrusions with diffuse boundaries, sometimes following foliation patterns in the host rock

2) younger pegmatite bodies (II) appear to intrude along structures found in the older pegmatites (I)

3) deformation and low-grade metamorphism continued as younger pegmatites (II) were intruded

4) the youngest pegmatite bodies (III) tend to occupy relatively small, well-defined and mostly undeformed dikes that crosscut all other pegmatite features

5) the youngest pegmatite dikes (III) are associated with similarly-oriented quartz veins

6) mineralogy is simple in all observed pegmatites: albite, orthoclase, quartz and traces of biotite

7) inward zoning in all the pegmatite bodies is consistently albite-rich, orthoclase-rich and quartz-rich

8) older pegmatite bodies (I,II) experienced plastic deformation; the youngest pegmatite dikes (III) did not

9) pegmatites of intermediate age (II) exhibit both plastic and brittle deformation caused by an array of forces; boudins, ptygmatic folds, and meter scale shear bands occur most in this group of features

10) alternating ductile and brittle deformation conditions are apparent, mostly in older structures (I, II)

R.E. Zartman & Hermes have suggested a late Permian age for pegmatites in this area. We suggest that these pegmatites formed between the final stages of the Alleghenian orogeny and the initiation of Mesozoic rifting and would make excellent subjects for further examination (eg. geochronological study, petrographic study, fluid inclusion work).

R.E. Zartman & OD Hermes, 1987, “Archean inheritance in zircon from late Paleozoic granites from the Avalon zone of southeastern New England: an African connection” Earth Planet Science Letters 82, 305-315.