DIVERSE MINERAL CLUMPS IN LATE PALEOZOIC MINETTE DIKES, CHARLOTTE AREA, NC; A LEGACY OF EXPLOSIVE, STEPWISE ASCENT FROM THE UPPER MANTLE

The North Carolina minettes occur as thin dikes less than 1 meter in width. Kinks and other crystal deformations in phlogopite would be annealed within a few hours at low-pressure liquidus temperatures (1100°-1200° C) of minette magma. Thus the deformations originated shortly before dike emplacement and the ascent and in situ cooling rates were fast enough to prevent annealing. Minettes are known to rise from depths of 100 to 150 kms. In the upper mantle or lower crust, catastrophic implosion of a small magma chamber could provide a sudden energy pulse to shock the contents of the chamber, generate the kinks, and explosively propel the chamber contents upward. Spherulitic textures in dike feldspars attest to very rapid in situ cooling. Ascent from the upper mantle probably occurs in steps. After each ascent spurt, the magma is temporarily stored in small chambers in the upper mantle or lower crust above the source region. Crystallization proceeds with gradually lowered Mg* in the liquid and accumulation of a fluid phase, probably dominated by CO2. A new spurt of hotter, more primitive magma injected from below triggers failure of the chamber and the melt, crystals, and fluid are propelled upward in the next step of the ascent. Sieve-textured cpx clumps up to 1 cm in dia with minor phlogopite and similarly large clumps of intimately intergrown coarse phlogopite and cpx crystallized in a deep chamber from melt blobs generated by decompression melting of the same minerals following an ascent spurt. They were solid when emplaced in the dike. Coarse sand to small lapilli-size blobs of fine grained cpx, phlogopite, and minor apatite represent quenched melt drops. Spherical to elliptical clumps of fine grained alkali feldspar and phlogopite with minor rutile and hercynite crystallized in a deeper crustal chamber and were probably solid when emplaced in the dike. Peridotite, vein quartz, and quartzofeldspathic clumps represent entrained wallrock and pyritic clusters and talc-calcite pseudomorphs after olivine probably were entrained from the upper tips of earlier, slightly deeper dikes. The magma emplaced as the dike was a highly mobile, compositionally heterogeneous mixture of melt, solid cognate and wallrock fragments, and a fluid phase.