EXPERIMENTAL MELTING OF CRUSTAL XENOLITHS FROM KILBOURNE HOLE, NEW-MEXICO AND IMPLICATIONS FOR THE CONTAMINATION AND GENESIS OF MAGMAS

Experiments (P = 6.9 kb; T = 900-1000-degrees-C) on four crustal xenol iths from Kilbourne Hole demonstrate the varying melting behavior of r elatively dry crustal lithologies in the region. Granodioritic gneisse s (samples KH-8 and KH-11) yield little melt (< 5-25%) by 925-degrees- C, but undergo extensive (30-50%) melting between 950 and 1000-degrees -C. A dioritic charnockite (KH-9) begins to melt, with the consumption of all modal K-feldspar, by 900-degrees-C. It is as fertile a melt so urce as the granodiorites at lower temperatures, but is outstripped in melt production by the granodiorite gneisses at high temperature, yie lding only 26% melt by 1000-degrees-C. A pelitic granulite (KH-12) pro ved to be refractory (confirming earlier predictions based on geochemi stry) and did not yield significant melt even at 1000-degrees-C. All m elts have the composition of metaluminous to slightly peraluminous gra nites and are unlikely to be individually recognizable as magma contam inants on the basis of major element chemistry. However, the relative stability of K-feldspar during partial melting will produce recognizab le signatures in Ba. Eu, K/Ba. and Ba/Rb. Melts of KH-11, which retain s substantial K-feldspar throughout the melting interval, are generall y low in Ba (< 500 800 ppm), have high K/Ba and low Ba/Rb (est.) (62-1 24 and 1-3, respectively). Melts of KH-9, in which all K-feldspar disa ppears with the onset of melting. are Ba-rich [2000-2600 ppm, K/Ba = 1 6-22, Ba/Rb (est.) = 25-47]. Melts of KH-8 have variable Ba contents; < 500 ppm Ba at low temperature but > 900 ppm Ba in high-temperature m elts coexisting with a K-feldspar-free restite. Although REE were not measured in either feldspar or melt, the high Kspar/melt Kds for Eu su ggests that the melts coexisting with K-feldspar will have strong nega tive Eu anomalies. Isotopic and trace element models for magma contami nation need to take into account the melting behavior of isotopic rese rvoirs. For example, the most radiogenic (and incompatible element-ric h) sample examined here (the pelitic granulite, Sr-87/Sr-86 = 0.757) i s refractory, while samples with far less radiogenic Sr (Sr-87/Sr-86 = 0.708-0.732) produced substantial melt. This suggests that, in this a rea, the isotopic signature of contamination may be more subtle than e xpected. The experimental results can be used to model the petrogenesi s of Oligocene volcanic rocks exposed 150 km to the NW of Kilbourne Ho le, in the Black Range in the Mogollon-Datil volcanic field. The exper imental results suggest that a crustal melting origin for the Kneeling Nun and Caballo Blanco Tuffs is unlikely, even though such an interpr etation is permitted by Sr isotopes. Crustal contamination of a mantle -derived magma best explains the chemical and isotopic characteristics of these tuffs. Both experimental and geochemical data suggest that t he rhyolites of Moccasin John Canyon and Diamond Creek could represent direct melts of granodiorite basement similar, but not identical, to the Kilbourne Hole granodiorites, perhaps slightly modified by crystal fractionation. The absence of volcanic rocks having Sr-87/Sr-86 > 0.7 4 in the region is consistent with the refractory character of the pel itic granulite.