QUARTZ GLOMEROCRYSTS: A TEXTURAL RECORD OF MAGMA ASCENT

Petrographic and cathodulminescence (CL) investigation of monomineralic quartz glomerocrysts in an Eocambrian alkali feldspar “rhyolite dike” in the Wichita Mountains of Oklahoma identifies textures consistent with glomerocryst formation during quartz dissolution rather than during quartz crystallization (i.e., synneusis). The dike is a porphryitic microgranite with subhedral to euhedral phenocrysts (18%) of variably exsolved salmon alkali feldspar and grey quartz set in a very fine-grained matrix (80%) of interlocking quartz and alkali feldspar, iron oxides, minor zircon, and miarolitic cavities (2%). Quartz glomerocrysts, up to 3.5 mm in size, are typically comprised of 2-6 individual quartz phenocrysts (0.08-1.7mm) that exhibit subhedral to embayed crystal forms. Euhedral quartz crystals are rare. All quartz CL images exhibit internal zonation in CL intensity attributed to variation in Ti concentration with brighter zones having higher Ti concentration. Zoning includes oscillatory zoning, normal, and reverse zoning within the same crystal. Oscillatory zones commonly define euhedral crystal forms with overall normal zonation consistent with magmatic growth zoning. Internal resorbed and embayed surfaces are prominent near cores and rims of quartz phenocrysts. Overgrowths on these dissolution surfaces are typically brighter indicating reverse zonation. Internal compositional zonation is abruptly truncated at high angles to the irregular external surface that defines the margins of quartz phenocrysts. External resorbed surfaces of quartz phenocrysts are shared in glomerocrysts. This indicates quartz phenocrysts, juxtaposed during magma movement, were in the process of being dissolved when glomerocrysts were being formed. Overlap of boundary layer melts formed during dissolution may promote phenocrysts to adhere together consistent with the monomineralic nature of glomerocrysts (Hogan, 1993). Dissolution of quartz is likely to have occurred as a result of decompression during magma ascent, although reverse zonation suggest introduction of hotter magma may also have contributed to quartz dissolution. The presence of texturally similar quartz glomerocrysts in felsic igneous rocks (e.g., granites) may provide a textural record of magma ascent prior to final emplacement.