CRUSTAL CONTAMINATION IN EARLY BASIN AND RANGE HAWAIITES OF THE LOS ENCINOS VOLCANIC FIELD, CENTRAL MEXICO

The Los Encinos Volcanic Field (LEVF) consists of Miocene (10.6-13.6 M a) hawaiitic volcanic necks and lava-capped mesas that crop out sparse ly over an area of 11,500 km(2) at the eastern margin of the Mexican B asin and Range Province (BRP). The LEVF rocks are similar to other ear ly extensional hawaiites from the southern BRP, and provide numerous c ontrasts with younger basanites and alkali basalts that erupted during the Quaternary at the Ventura and Santo Domingo Volcanic Fields about 100 km to the south. A suite of 18 LEVF hawaiites was studied in thin section, and analyzed for mineral compositions, whole-rock major and trace element compositions, and Sr, Nd, and Pb isotopic ratios. All sa mples contain the stable minerals plagioclase (An(53-64)), olivine (Fo (61-88)), clinopyroxene, titanomagnetite, and minor biotite. Most samp les also contain a complex assemblage of resorbed and reacted xenocrys ts and megacrysts. Some of these minerals appear to have crystallized slowly from related, but more differentiated magmas, but other xenocry sts were clearly derived from lower-crustal, high-grade orthogneisses and paragneisses that are found as large xenoliths in the nearby Quate rnary volcanic fields. Quartz xenocrysts are especially common in many hawaiites (up to 3.9 vol%) and show a wide range of reaction styles. One sample contains micro-xenoliths of sillimanite- and quartz-bearing paragneiss with fine-grained domains that are interpreted as coronal structures related to original garnet xenocrysts. The geochemical effe cts of crustal contamination in the LEVF hawaiites vary widely. Five s amples appear to be essentially uncontaminated (type U). Aside from be ing somewhat differentiated from a more primitive parent, the type-U s amples can be used to infer the geochemistry of the mantle that was me lting during the early stages of basin-and-range rifting. Type-U sampl es range up to epsilon(Nd)=+7.6 and down to Sr-87/Sr-86 = 0.70286 and Pb-206/Pb-204 = 18.74, compositions that are more extreme than any of the nearby Quaternary volcanic rocks. The other 13 samples are divided into two contamination types, A and B. Both types show trends toward higher Sr-87/Sr-86 (to 0.7040) and Pb-206/Pb-2O4 (to 18.98), lower eps ilon(Nd) (to +3.1), and elevated Yb, which appear to reflect bulk or A FC-style contamination by granulites, particularly garnet-bearing para gneisses. Type-A hawaiites also show selective enrichments in Cs, Rb, Th: Sb, U, Pb, K, and Si. These elements were probably transferred int o the type-A hawaiitic magmas through mixing with low-degree partial m elts from deep-crustal granulites. The enrichments of these elements i n type-A hawaiites complement the depletions of many of these same ele ments in high-grade granulites worldwide and provide insight into the origin of those depletions. Mixing models between type-U hawaiites and paragneiss xenoliths indicate that up to 45% of the Pb found in type- A hawaiites is crustally derived. In comparison with more mafic Quater nary basanitic rocks from the volcanic fields to the south, which carr ied large peridotite and granulite xenoliths to the surface, the LEVF hawaiites are relatively differentiated and megacryst rich, but free o f large xenoliths, and show a wide variety of petrographic and geochem ical evidence for crustal contamination. These differences probably re flect the slow and interrupted ascent of the LEVF hawaiites during ear ly stages of basin-and-range extension in the Miocene, when the crust had a somewhat lower density and the entire lithosphere was relatively thick and cool. We argue that Quaternary basanites were able to ascen d significantly faster through the thinner, hotter, and more fractured and extended lithosphere, whose crust was made denser by mafic intrus ions during the preceding magmatic episode. Consequently the Quaternar y basanites rose without stagnating and interacting with crustal litho logies, and without losing their entrained peridotite xenoliths.