ORIGIN OF QUARTZ CLUSTERS IN THE VINALHAVEN GRANITE, MAINE

The Vinalhaven intrusion in Maine consists mostly of coarse-grained granite commingled with smaller bodies of porphyry generated by remelting granite when coeval basalt was emplaced (Wiebe et al. 2004). A study of CL and Ti zoning in quartz confirmed this interpretation (Wiebe et al. 2007) and recognized that quartz clusters with multiple cores surrounded by continuous, complexly zoned mantles were common in granite and porphyry. Here we apply EBSD methods to determine crystal orientations in order to distinguish possible mechanisms of cluster formation - twinning, epitaxy, attachment of preformed crystals (synneusis), or skeletal growth.

In the granite, most quartz pairs have c-axes with random orientations; about 10% have nearly parallel or Esterel twin orientations, where the 2-D lattices of their dipyramidal faces match. The porphyry has relatively few quartz clusters per thin-section, all of which approximate parallel or Esterel twin orientations. Ti zoning of quartz pairs in porphyry indicates that they were attached prior to a major remelting event. In both granite and porphyry, Ti zoning suggests that most quartz clusters formed by synneusis, although some other clusters may have formed as twins or by epitaxy. Quartz habits rule out skeletal growth.

Field evidence suggests that much of the coarse-grained granite at Vinalhaven formed by accumulation of crystals on the floor of a magma chamber. Equant quartz crystals have Ti-zoning patterns that suggest they grew early as subhedral to euhedral crystals dominated by the dipyramidal form. If these quartz crystals were tightly packed in a cumulate mush, some crystals should have come into contact along their dipyramidal faces. Once in contact, continued compaction of the mush could have rotated the crystals until some pairs acquired orientations where their lattices matched on the dipyramidal faces - such as with parallel or Esterel twin orientations – creating strong bonds on these faces. Likely only a small proportion of pairs with matched dipyramidal faces initially formed in the coarse-grained granite, but based on the pairs analyzed in the porphyry, only these pairs survived the rejuvenation of the granite. These results suggest that synneusis of quartz crystals occurred within a cumulate crystal mush.