LAYERED STRUCTURES IN THE LUCERNE GRANITE, HANCOCK COUNTY, MAINE

The Lucerne pluton is predominantly comprised of a coarse-grained seriate biotite granite ± hornblende with megacrystic feldspar crystals. Mafic minerals are on average 10% of the granite’s modal composition. A road cut (UTM zone 19, 532319.0678, 4953066.0116) exposes granite characterized by multiple sub-horizontal layers. Each layer is comprised of a lower zone dominated by mafic minerals (schlieren) grading to an upper leucocratic zone of feldspar and quartz. The schlieren consists of biotite, apatite, amphibole, zircon, chevkinite, allanite, ilmenite, and fluorite. Within each layer the size of feldspar crystals coarsens upward. However, the maximum size of feldspar crystals decreases within each successive layer, producing an overall fining upwards sequence. The couplet layers exhibit textures similar to cross-bedding, ripple marks, and channel structures. A sub-horizontal aplite dike caps the entire section of layered granite. Similar occurrences of layered granite have been described by Clarke and Clarke (1998).

Electron microprobe analyses of biotite from the feldspar-cumulate and from the schlieren layers have similar chemical compositions, which are consistent with biotite from throughout the Lucerne pluton (Wones, 1980). This suggests that biotite crystallized prior to ‘deposition’ of the layers. The mineralogical layering reflects mechanical segregation of minerals and subsequent crystallization, rather than strictly in-situ crystallization from a compositionally zoned melt. Whole-rock analyses on schlieren and feldspar-cumulate samples have major- and trace-element abundances that correlate with the observed mineral segregation and are compatible with fractional crystallization. Elevated fluorine (fluorite) and boron (tourmaline) for the Lucerne magma (Wones, 1980) dramatically reduce melt viscosity and serve to enhance processes leading to mineral segregation (gravitational settling, flow sorting). These sedimentary textures indicate flow segregation and gravitational settling played a role in forming this distinctive layered granite. This implies more dynamic conditions during crystallization and potentially represents an important mechanism for crystal-melt fractionation within felsic magma chambers.