CUMULATE STRUCTURES WITHIN THE MT.WALDO PLUTON, MAINE

The Mt. Waldo granite (160 km²) crops out at the northern end of Penobscot Bay, Maine. It is a coarse-grained, seriate to porphyritic, sphene-bearing, biotite (± hornblende) granite in which mafic to felsic enclaves are abundant.

Planar mafic layers (schlieren) are observed in an abandoned quarry on Mosquito Mountain and trend N-S with variable dips to the east (10^o– 40^o). The schlieren layers have the same mineralogy as typical Waldo granite, however they are characterized by high concentrations of biotite and accessory minerals (magnetite, titanite, allanite, apatite, zircon, ± hornblende) such that the color index is £ 70. Accessory mineral concentrations may reach 10%. Layering occurs on a centimeter scale and commonly consists of 4-6 individual layers in one zone. Some layers can be traced for distances of £ 70 m as outcrop permits. Each layer has a sharp basal contact but grades upward into typical Mt. Waldo granite. Large euhedral feldspars (£ 6 cm) are oriented similarly to the schlieren layers in which they are found. Detailed mapping of the schlieren (1:40) reveals cross cutting relationships in which the eastern (upper) unit is always younger, consistent with sequential deposition of the layers. Enclave-rich zones parallel the schlieren. A foliation within the granite, composed of large (£ 7 cm), euhedral K-feldspar and plagioclase, is sub-parallel to the planar mafic layers. This would appear to be a magmatic foliation as evidence for solid state deformation beyond undulose extinction in quartz has not been observed.

We interpret these “layered” zones to be crystal cumulates deposited from density currents on the aggrading floor of the chamber. The sharp lower contact of these zones and cross cutting relationships demonstrate these currents are capable of eroding the chamber floor. Mineralogically similar layers in the Sierra Nevada Batholith have been chemically documented as cumulates (Reid et al. 1983). The granite foliation was also probably produced by deposition on the chamber floor.