A TALE OF TWO APLITES – A COMPARATIVE STUDY

We have undertaken a detailed study of aplites using scanning electron microscopy and x-ray microanalysis. We started out with an aplite dike (Aplite-1) from a Cenozoic intrusion in the north Cascade Range of Washington State and have determined and reported on its principal characteristics. We now have analyzed a second aplite dike rock (Aplite-2) from the same intrusive body and discovered that it shows some distinct differences in composition and texture from Aplite-1. Aplite-1 is a fine-grained equigranular rock consisting almost entirely of quartz and K-spar. Scattered throughout this matrix are high-Z (atomic number) domains that are composed mainly of Fe/Ti oxides but also include trace minerals such as zircon, pyrophanite, thorite, and uraninite, often with two or three species that crystallized together in the same domain. The dike rock, besides being much finer grained, has a composition different from that of the wall rock and along the borders between them there does not appear to have been any interaction (no contact zone).

Aplite-2 is also a fine-grained equigranular rock and consists mainly of quartz and k-spar with high-Z oxide domains that include exotic trace minerals. However, unlike the first rock, there are small amounts of plagioclase and there are common occurrences of quartz/K-spar intergrowths that resemble “graphic granite” type intergrowths found in some pegmatites and other coarse-grained rocks.

Based on what we have observed thus far we offer the following provisional conclusions: (1) Aplites are late-phase phenomena (post plutonic rather than synplutonic); (2) All aplite dikes are not texturally and/or compositionally the same, even within the same intrusive body; (3) Aplites can provide some insight into the history of the pluton in which they occur. To attempt to explain why these aplites are different we offer the following hypotheses: (1) The two dikes were emplaced at different times; (2) The two were emplaced in different parts of the pluton - top to bottom, border to core; (3) The sizes (total volume) of the two dikes are different; (4) Two different sources provided the raw materials that crystallized into the two dikes.