Abstract

Magmatic crystallization of the White Tank quartz monzonite, California, has produced two sequences of differentiation. One sequence is marked by an increase in the size of potassium feldspar crystals and an increase in the Ab/An ratio of the plagioclase from the older toward the younger rocks. The second sequence is marked by an increase in the Ab/An ratio of the plagioclase from the older toward the younger rocks and by the presence of primary muscovite and garnet in the younger rocks.

Textures vary with position in the intrusions. Quartz forms aggregates of several grains except near the margins of the intrusions, where it occurs as individual grains. Large potassium feldspar crystals that formed late in the first differentiation sequence are gray, grid-twinned, and commonly intergrown with sodic plagioclase; the earlier-crystallizing grains are pink, poorly twinned, and exhibit no such intergrowth. The more calcic plagioclase grains are zoned and imperfectly twinned, whereas the more sodic grains are twinned and poorly zoned. Biotite and most accessory minerals occur in aggregates, but sphene is scattered through the rock.

In the first differentiation sequence the amounts of lead and possibly strontium in the potassium feldspar decrease from older to younger rocks, and the amounts of barium and rubidium remain constant. In the second sequence, the amounts of barium, lead, and strontium in the potassium feldspar decrease during differentiation, and the amount of rubidium is constant.

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