Little Chief Granite Porphyry: Feldspar Crystallization History

The Little Chief Granite originated as an almost completely liquid magma with a water content of at least 3 wt %. The initial crystallizing phases were oscillatorilly zoned calcic plagioclase (An_45–65), magnetite, and rare augite. Upward movement and decrease of load pressure caused resorption of the calcic plagioclase followed by crystallization of intermediate plagioclase (An_31–44), hornblende, biotite, magnetite, and ilmenite as the magma stabilized at depth. After less than 5% crystallization, the magma again moved upward to form a laccolithic magma chamber whose roof was near a depth of 4 km. This magma chamber was zoned from a core with bulk compositions near 68 wt % SiO₂ at temperatures near 750 °C outward to a margin with bulk compositions near 77 wt % SiO₂ and temperatures near 700 °C, in a load pressure range of 1.2 to 1.7 kb. Crystallization of sanidine, sodic plagioclase (An_17–23), hornblende, biotite, magnetite, sphene, and ilmenite occurred from a melt that was water rich but probably not water saturated. In the core of the magma chamber, sanidine crystallized briefly and was then rimmed by oligoclase, whereas in the outer parts sanidine and oligoclase continued to coexist.

After the magma was 30% to 50% crystallized, extensive assimilation of dolomitic wall rocks produced a contaminated, diopside-bearing marginal phase of the magma. Sufficient CO₂ vapor was generated that P_fiuid > P_load, causing extensive fracturing of the roof rocks, allowing a major dike swarm to be emplaced along the fractures, and initiating ascent of the magma. The over-pressure was recorded throughout the stock by single or paired macro-oscillations on the outer edges of the plagioclase phenocrysts. The magma moved upward and finally stabilized in the presently observed stock, which existed in a load pressure range of 0.3 to 0.9 kb and probably locally vented to the ground surface. During upward movement, sanidine was extensively replaced by sodic oligoclase (An_10–15) through a complex mechanism that included metastable patch perthite ex-solution. The groundmass feldspar composition varied as a function of depth in the magma chamber and apparently crystallized metastably under subsolidus “pressure quench” conditions at temperatures near 650 °C.