The Chemical and Isotopic Differentiation of an Epizonal Magma Body: Organ Needle Pluton, New Mexico

Abstract

Major and trace element, and Nd and Sr isotopic compositions of INTRODUCTION whole rocks and mineral separates from the Oligocene, alkaline A fundamental problem of igneous petrogenesis is the Organ Needle pluton (ONP), southern New Mexico, constrain origin of the high-silica magmas that are tapped by largemodels for the differentiation of the magma body parental to this volume ash-flow tuffs (Hildreth, 1981). Many workers compositionally zoned and layered epizonal intrusive body. The agree that high-silica magmas are products of fractional data reveal that the pluton is rimmed by lower eNd (~ –5) and crystallization and crystal–liquid separation along the higher Sr/Sr (~0·7085) syenitic rocks than those in its interior side walls of large epizonal magma bodies, which together (eNd ~ –2, Sr/Sr ~0·7060) and that the bulk compositions create residual, differentiated magmas that buoyantly rise of the marginal rocks become more felsic with decreasing structural along the side walls and pool at the chamber roof (e.g. depth. At the deepest exposed levels of the pluton, the eNd ~ –5 Sparks et al., 1984; McBirney et al., 1985; Feeley & lithology is a compositionally heterogeneous inequigranular syenite. Davidson, 1994; Spera et al., 1995). However, except for Modal, compositional and isotopic data from separates of rare the studies of Sawka et al. (1990) and Mahood & Cornejo earth element (REE)-bearing major and accessory mineral phases (1992), little direct evidence of these processes has been (hornblende, titanite, apatite, zircon) demonstrate that this decoupling described from ash-flow tuffs or exposed epizonal plutons. of trace and major elements in the inequigranular syenite results The paucity of evidence from ash-flow tuffs reflects their from accumulation of light REE (LREE)-bearing minerals that selective and possibly spatially biased sampling of the were evidently separated from silicic magmas as the latter rose along entire parental magma body (Wilson & Hildreth, 1997). the sides of the magma chamber. Chemical and isotopic data for Also, although epizonal plutons may expose the crysmicrogranular mafic enclaves, as well as for restite xenoliths of tallized remnants of an original magma body, these Precambrian granite wall rock, indicate that the isotopic distinction commonly show evidence of subsolidus chemical rebetween the marginal and interior facies of the ONP probably equilibration (Hildreth, 1981). reflects assimilation of the wall rock by eNd ~ –2 mafic magmas A rare example of an epizonal intrusive rock that near the base of the magma system. Fractional crystallization and has preserved evidence of the processes involved in crystal–liquid separation of the crustally contaminated magma at the differentiation of shallow-level silicic magmas is the the base and along the margins of the chamber generated the highly reversely zoned Organ Needle pluton (ONP), one phase silicic magmas that ultimately pooled at the chamber top. of the Oligocene, alkalic Organ Mountain batholith of