Solidification of the Mount Givens Granodiorite, Sierra Nevada, California

Abstract

Petrographic study and modal and chemical analyses of samples collected along a traverse from the margin to the interior of the bulbous head at the north end of the Mount Givens Granodiorite show that it is compositionally zoned. Relatively mafic rock in the margin, composed of higher temperature mineral assemblages, passes inward, with only one identifiable internal contact, to more felsic rock composed of lower temperature mineral assemblages. Hornblende and biotite decrease in amount inward from the external contact, plagioclase is rather constant, and quartz and K-feldspar increase inward. The anorthite content of plagioclase and the MgO content of hornblende decrease from the cores to the margins of crystals and from the margin to the interior of the pluton. We interpret this pattern to have resulted from crystal fractionation during inward solidification with falling temperature. Fractionation occurred because residual solid material (restite) carried upward in the magma from the place of origin of the magma, together with crystals precipitating from the melt phase, was progressively cleared from the magma by settling downward and/or by accretion to the solidifying margins of the magma chamber. The kind, composition, and proportion of crystals precipitating changed with falling temperature and changing composition of the melt phase. The Mount Givens Granodiorite differs from the Tuolumne Intrusive Series in the following ways: (1) more abundant mafic inclusions and mottled cores in plagioclase suggest greater abundance of residual solid material from the source region of the magma; (2) more calcic rims of plagioclase crystals in the marginal rock indicate that the Mount Givens magma was at a higher temperature when the exposed rocks began to solidify; (3) the presence of quartz in tiny sub-equant crystals as well as in intergranular stringers in the marginal rock of the Mount Givens indicates that the Mount Givens magma was saturated in quartz as well as in plagioclase, hornblende, biotite, and magnetite when the first rocks began to solidify, whereas the Tuolumne magma was not initially saturated in quartz; (4) decreasing grain size of the mafic minerals inward precludes chilling and suggests that the wallrocks were preheated before solidification began; (5) the presence of ilmenite, absent in the Tuolumne Intrusive Series, and smaller amounts of magnetite and higher , suggest generally lower fOi in the Mount Givens magma; (6) resurgence of magma containing higher temperature settled crystals appears \o have occurred during solidification of the Mount Givens Granodiorite whereas surges of Tuolumne magma apparently contained no settled, higher temperature, minerals; (7) the fact that is generally greater than one in the Mount Givens Granodiorite and less than one in the Tuolumne Intrusive Series probably accounts for plagioclase with larger An content crystallizing in the Mount Givens magma when the cotectic was intersected and K-feldspar began to crystallize.