The chemical and isotopic differentiation of an epizonal magma body: OrganNeedle Pluton, New Mexico

Major and trace element, and Nd and Sr isotopic compositions of whole rocks and mineral separates from the Oligocene, alkaline Organ Needle pluton (ON P), southern New Mexico, constrain models for the differentiation of the ma gma body parental to this compositionally zoned and layered epizonal intrus ive body. The data reveal that the pluton is rimmed by lower epsilon(Nd) (s imilar to -5) and higher Sr-87/Sr-86 (similar to 0.7085) syenitic rocks tha n those in its interior (epsilon(Nd) similar to -2, Sr-87/Sr-86 -0.7060) an d that the bulk compositions of the marginal rocks become more felsic with decreasing structural depth. At the deepest exposed levels of the pluton, t he epsilon(Nd) similar to -5 lithology is a compositionally heterogeneous i nequigranular syenite. Modal, compositional and isotopic data from separate s of ran earth element (REE)-bearing major and accessory mineral phases (ho rnblende, titanite, apatite, zircon) demonstrate that this decoupling of tr ace and major elements in the inequigranular syenite results from accumulat ion of light REE (LREE)-bearing minerals that were evidently separated from silicic magmas as the latter rose along the sides of the magma chamber Che mical and isotopic data for microgranular mafic enclaves, as well as for re stite xenoliths of Precambrian granite wall rock, indicate that the isotopi c distinction between the marginal and interior facies of the ONP probably reflects assimilation of the wall rock by epsilon(Nd) similar to -2 mafic m agmas near the base of the magma system. Fractional crystallization and cry stal-liquid separation of the crustally contaminated magma at the bare and along the margins of the chamber generated the highly silicic magmas that u ltimately pooled at the chamber top.