ROOF-ROCK CONTAMINATION OF TAYLOR-CREEK RHYOLITE, NEW-MEXICO, AS RECORDED IN HORNBLENDE PHENOCRYSTS AND BIOTITE XENOCRYSTS

The Taylor Creek Rhyolite, a group of coeval, mid-Tertiary, silica-ric h rhyolite lava domes in southwestern New Mexico, is notable for recor ding bulk-rock evidence of minor, yet easily measurable, contamination of its source magma reservoir resulting from assimilation of Proteroz oic roof rock. Most of the evidence is recorded in trace element conce ntrations and Sr-87/Sr-86(i) ratios, which are far different in uncont aminated magma and roof rocks. Hornblende phenocrysts and biotite xeno crysts also record the effects of contamination. Electron microprobe a nalyses show that all hornblende grains are zoned to Mg-rich and Fe- a nd Mn-poor rims. Rim MgO content is typically about 10 wt% greater tha n core MgO content. Other hornblende constituents are not measurably v ariable. Biotite xenocrysts, trace mineral constituents, are present o nly in the domes that are most contaminated, as judged by bulk-rock va riations in trace element concentrations and Sr-87/Sr-86(i). Biotite g rains are invariably partly to almost completely altered. Microprobe a nalyses of the cores of the least-altered grains show that large varia tions in Fe and Mg and that biotite contains 2-20 times as much Mg as fresh biotite phenocrysts in other silica-rich rhyolite lavas. Fe and Mg are negatively correlated in hornblende and biotite, consistent wit h mixing two end-member compositions. The mass ratio of contaminant to magma was probably less than 1:100, and major constituents, including Al, were not measurably affected in hornblende. Al-in-hornblende baro metry yields essentially a constant calculated pressure of about 1.5 k bar, which is consistent with the interpretation that all contaminatio n occurred in a boundary zone about 300 m thick at the top of the magm a reservoir.