Studies of the Precambrian Herefoss granite. I. K-feldspar obliquity.

The Herefoss granitic pluton is a circular diapiric body cut by a fault that causes two different levels of the pluton to be exposed. The pluton has been the subject of a detailed petrochemical study based on samples taken at 2-km intervals. Five to ten grains of K-feldspar from each hand specimen used for the petrochemical study were hand-picked and X-rayed for this study of K-feldspar obliquity. Three modifications found to be present in the granite are orthoclase, near-maximum microcline, and RD K-feldspar, so the granite is neither an orthoclase nor a microcline granite. The east side of the pluton (higher level) predominantly contains high-Δ K-feldspar except for a quartzdioritic facies which contains low-Δ and RD K-feldspar. The west side of the pluton (lower level) mainly contains low-Δ and RD K-feldspar. The distribution of Δ-values is bimodal with peaks at high and low Δ-values and intermediate Δ-values represented only by RD K-feldspars. All variations from orthoclase to near-maximum microcline may be found in a single hand specimen, and single grains commonly show considerable variation in obliquity.

K-feldspar obliquity is compared with chemical composition of the rock in which the K-feldspar occurs. A strong positive correlation is found between presence of low-Δ K-feldspar and chemical concentration of CaO, Fe2O3, TiO2, and MgO; negative correlation is found for K2O. In this pluton, K-feldspar obliquity can be correlated with the chemical composition of the enclosing rock.

K-feldspar obliquity is discussed in terms of mineralogy, chemical composition of the rock, chemical composition of the K-feldspar perthite, type of perthite, deformation, contact effects, metamorphic facies, role of volatiles, occurrence of myrmekite, and cooling rate. The structural behavior of the pluton indicates that rocks containing high-Δ K-feldspar were richer in volatiles than rocks containing low-Δ K-feldspar. In addition, occurrence of low-Δ K-feldspars is linked to porphyroblastesis in rocks undergoing a chemical transformation.