PETROGENESIS OF THE EAST FORK MEMBER RHYOLITES, VALLES CALDERA, NEW MEXICO, USA

The most recent volcanism in the Valles caldera is represented by the El Cajete Pyroclastic Beds (ECPB), Battleship Rock Ignimbrite (BRI), and Banco Bonito Flow (BBF) as well as the VC-1 rhyolite lava, which are collectively known as the East Fork Member of the Valles Rhyolite. The triggering event that caused these recent eruptions at 55 to 40 ka (after a 460 ka lull in volcanism) has been suggested to be the intrusion of mafic magma at depth, thus causing the re-melting of a silicic intrusive body.

Crystal chemistry and detailed petrography indicates that two distinct crystal populations are present in the ECPB, BRI, and BBF. The VC-1 rhyolite was not sampled in this study. Large (≥ 1 mm) resorbed or subhedral crystals represent one population, and small (≤ 0.5 mm) euhedral crystals represent the other. Almost all crystals present in the East Fork Member can be divided into these two populations.

A majority of the resorbed plagioclase and sanidine crystals have rim overgrowths. The overgrowths are of the same mineral phase (i.e. sanidine or plagioclase) as the interior in most cases. Electron microprobe transects reveal three types of zonation. Reverse zonation is defined by a high sodic core with a high calcic rim. Normal has a high calcic core with a high sodic rim. Patchy is defined by no specific pattern in the change in chemistry. In contrast, transects across biotite and amphibole reveal no zoning. The zonation in the plagioclase and sanidine and the lack thereof in the biotite and amphibole suggests different sources for the crystals and of magma mixing.

XRF and ICPMS data show trends between major elements and increasing SiO₂. A nearly linear decrease is observed in Al₂O₃. A general decrease is shown by FeO, MgO and P₂O₅.The plots of trace elements versus Nb can be divided into two groups based on their trends with increasing Nb. The following show a general increase: Y, Yb, U, Rb, Th, Ta, Ce, Nd, and Lu. A general decrease is seen in Sr, Zn, and Ba. These trends suggest that fractional crystallization was taking place.

TIMS data shows that these units are rather primitive in nature based on their isotopic ratios. The units all yielded isotopic ⁸⁷Sr/⁸⁶Sr ratios of about 0.0747 and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.5124. The values are similar to those for other basalts erupted in the Jemez volcanic field, but do not reflect the values of the Precambrian crust.