TITANIUM ZONING IN QUARTZ: CONSTRAINTS ON THE CRYSTALLIZATION AND COOLING OF THE MOUNT SCOTT GRANITE PLUTON, SOUTHERN OKLAHOMA AULACOGEN, USA

The Mount Scott Granite pluton is a tabular body of alkali-feldspar granite exposed in the Wichita Mountains of southwestern Oklahoma. Eocambrian rift magmatism produced this and other subsequent shallow igneous bodies that exploited the base of contemporaneous volcanic strata. The Mount Scott Granite is porphyritic with phenocrysts of alkali feldspar and quartz. The former visibly exhibit rounded cores mantled by compositionally distinct rims surrounded by a matrix of variably granophyric texture. The quartz crystals are not optically zoned, but electron microprobe analysis and cathodoluminescence (CL) imaging reveal that these phenocrysts are also rounded and mantled. The rounded cores, their rims, and the surrounding matrix quartz all emit dominantly blue CL. Cores exhibit subtle fine oscillatory zoning in CL. The CL intensity of the cores is low, correlating with low Ti concentrations (40-60 ppm). A sharp (4 micron) boundary separates the cores from rims with stronger CL intensity and correspondingly elevated Ti concentrations (100-200 ppm). Many of the rims also exhibit zoning with rimward-decreasing CL intensity and Ti concentration. The CL intensities and Ti concentrations of the matrix quartz is comparable to that of the rims. Additionally, fractures and grain margins may exhibit strong, localized red CL signals, presumably the result of post-crystallization deformation and limited fluid alteration.

The rounded and mantled cores further corroborate prior evidence of pre-emplacement crystallization. However, previous estimates placed this event at a depth of 7-8 km; the Ti concentrations of these cores suggest higher initial crystallization pressures. Decompression during ascent resorbed and rounded the grains. Rims and then matrix quartz crystallized at the emplacement depth. The Ti concentrations of rim and matrix quartz produce reasonable estimates of temperatures at likely emplacement pressures (700-750 °C at 50 MPa).

The sharp boundary between the core and rim implies rapid cooling of the pluton following rim crystallization. Given the diffusivity of Ti in quartz, preliminary modeling indicates that a diffusion-modified step function replicates the core-rim boundary at 200 years at 700 °C, or 10 thousand years at 600 °C, or 1 million years at 500 °C.