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 xenoliths from Kilbourne Hole demonstrate the varying melting behavior of relatively dry crustal lithologies in the region. Granodioritic gneisses (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 source as the granodiorites at lower temperatures, but is outstripped in melt production by the granodiorite gneisses at high temperature, yielding only 26% melt by 1000-degrees-C. A pelitic granulite (KH-12) proved to be refractory (confirming earlier predictions based on geochemistry) and did not yield significant melt even at 1000-degrees-C. All melts have the composition of metaluminous to slightly peraluminous granites and are unlikely to be individually recognizable as magma contaminants on the basis of major element chemistry. However, the relative stability of K-feldspar during partial melting will produce recognizable signatures in Ba. Eu, K/Ba. and Ba/Rb. Melts of KH-11, which retains substantial K-feldspar throughout the melting interval, are generally low in Ba (< 500 800 ppm), have high K/Ba and low Ba/Rb (est.) (62-124 and 1-3, respectively). Melts of KH-9, in which all K-feldspar disappears with the onset of melting. are Ba-rich [2000-2600 ppm, K/Ba = 16-22, 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 melts coexisting with a K-feldspar-free restite. Although REE were not measured in either feldspar or melt, the high Kspar/melt Kds for Eu suggests that the melts coexisting with K-feldspar will have strong negative Eu anomalies. Isotopic and trace element models for magma contamination need to take into account the melting behavior of isotopic reservoirs. For example, the most radiogenic (and incompatible element-rich) sample examined here (the pelitic granulite, Sr-87/Sr-86 = 0.757) is refractory, while samples with far less radiogenic Sr (Sr-87/Sr-86 = 0.708-0.732) produced substantial melt. This suggests that, in this area, the isotopic signature of contamination may be more subtle than expected. The experimental results can be used to model the petrogenesis of Oligocene volcanic rocks exposed 150 km to the NW of Kilbourne Hole, in the Black Range in the Mogollon-Datil volcanic field. The experimental results suggest that a crustal melting origin for the Kneeling Nun and Caballo Blanco Tuffs is unlikely, even though such an interpretation 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 the 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 > 0.74 in the region is consistent with the refractory character of the pelitic granulite.