Derek Wongus

The magmatic growth of K-feldspar megacrysts from the Halifax Pluton has been investigated.  The study consisted of detailed textural characterization of two separate samples of granitic rocks from Peggy’s Cove and Prospect, Nova Scotia.  The whole-rock textures, megacryst concentrations, and crystal shapes suggest a different magmatic history for these samples.  Detailed petrographic studies further suggest different crystallization histories for these two areas.  Specifically, the sample from Peggy’s Cove shows far fewer megacrysts and also less euhedral grain shapes.  The megacrysts are also less strongly zoned and contain fewer large inclusions.  The megacrysts from Prospect display oscillatory zoning and abundant, aligned inclusions, including inclusions of oscillatory zoned plagioclase.  Barium and other elements define zoning in megacrysts which suggests that magma mixing and resorption of the megacrysts has occurred.  This is particularly pronounced in the Peggy’s Cove sample.  Using two-feldspar (plagioclase and alkali feldspar) geothermometry to determine temperature variability within the megacrysts, the temperatures recorded for the high temperature phase of megecryst growth are 832^oC to 913^oC in Peggy’s Cove and 670^oC to 770^oC at Prospect.  This, along with the textural differences and the chemical zonation in the megacrysts strongly suggests that the sample from Peggy’s Cove underwent a period of resorption and regrowth due to an increase in the magma temperature.  The most likely explanation for this is a magma mixing event.  The relationship between Ba in the megacrysts and the differences in local temperatures recorded in these two samples suggests a difference in the effects of magma mixing and that the increase in temperature caused by this event was focused in the Peggy’s Cove area.  The temperature ranges recorded by perthite exsolutions are also different.  Peggy’s Cove records a temperature range of between 483^oC and 520^oC, with a higher temperature range of 530^oC to 586^oC being recorded at Prospect.  This lends further support to the suggestion that the megacrysts at Peggy’s Cove were re-equilibrated and partially resorbed at high temperatures where they also experienced some Ca-loss.  On the other hand, the megacrysts at Prospect have not experienced resorption and thus, did not undergo significant Ca-loss.  Thus, subsequent perthite exsolution occurred at higher temperatures in the more Ca-rich megacrysts at Prospect and at  lower temperatures in megacrysts from Peggy’s Cove which were already lower in Ca.  The results confirm the observations from an increasing number of studies which have documented the dynamic and heterogeneous nature of magmatic crystallization mechanisms in granitic rocks.

Keywords: megacrysts; zonation; magmatic; granitoid; Halifax; pluton; magma mixing; cathodoluminescence; perthite; exsolutions; plagioclase; alkali feldspar; barium; geothermometry and heterogeneity
Pages: 89
Supervisor: Richard Cox