EVIDENCE FOR PROGRADE OLIVINE PORPHYROBLAST GROWTH USING CALCITE-DOLOMITE THERMOMETRY IN THE UBEHEBE PEAK CONTACT AUREOLE

Calcite-dolomite thermometry from metamorphic calcite halos formed during olivine porphyroblasts growth in siliceous dolomites of the Ubehebe Peak contact aureole were used to constrain the growth history of olivine. A general trend of higher temperatures with increasing proximity towards the contact is observed for samples collected at the aureole scale (1000m). There is a significant scatter about the mean temperature obtained for each single rock sample. Resulting histograms of X_Mg exhibit a near Gaussian-shape. On a thin section scale, domains of higher X_Mg-content of calcites are found where olivine is present, while lower values are typical for regions were tremolite is found. This suggests that calcite maintains at least a partial memory of its formation temperature. Multiple analyses within single grains show lower X_Mg-values towards the grain boundaries. We interpret this to reflect the samples T-t-path. Originally, a grain of calcite formed as product of a metamorphic reaction records the T-conditions during its formation. After that it will be affected by partial diffusional re-equilibration along grain boundaries. Hence, the single grain zoning is attributed to partial re-equilibration during cooling, while its center approximates the formation temperature. 3-D distributions of platy olivine porphyroblasts were determined using µC-Tomography and central cuts through 5 porphyroblasts were made. Mg-content of calcite for 200-300 spot analyses surrounding each olivine crystal were obtained. Analyses of calcite halos surrounding each porphyroblast report the lowest temperatures next to the center of platy olivines. The presence of higher Mg-content in calcite towards the ends of the plates indicates higher formation temperatures and hence higher temperatures for the growth of the distal parts of the olivine. Assuming that olivine grew from the center towards the rim of the plates, this indicates that growth of the olivine took place during heating and was complete by the time maximum T was reached for the samples. This information can be used with calculated T-t data for the aureole to refine parameters used in kinetic models of nucleation and growth.