Abstract

Seven samples of biotite-bearing arkosic quartzite were collected along a traverse essentially normal to isograds in a series of regionally metamorphosed sedimentary rocks in the Peninsular Ranges, California. X-ray fluorescence, rapid chemical, and emission spectrographic analyses have been made of total rock, biotite, ilmenite, and K feldspar.

Going from lower to upper hornblende-hornfels facies, the following changes are observed: bulk composition of the total rock is unchanged except for decrease in Fe2O3/FeO ratio, slight increase in MnO, TiO2, Sr, Co, La, and Y, and possible decrease in V (some variation may be a relict from the parent sediment); in biotite, Fe2O3/FeO and CaO decrease, Ba, La, Ti, and Mn increase erratically, 2V and basal spacing decrease, and color reddens; in ilmenite, Ni and Mn increase and Fe2O3, Zr, Cr, V, and Co decrease; in plagioclase, An content increases; in K feldspar, Ab content increases slightly; O18/O16 fractionations between biotite and K feldspar decrease but suggest by comparison with values in other areas that temperature everywhere in the sequence was more than 550° C at the time of the last equilibration. Trace-element fractionation between ilmenite and biotite may serve as a geothermometer.

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