Isotopic evidence on the origin of compositional layering in an epizonal magma body

Details Isotopic evidence on the origin of compositional layering in an epizonal magma body Isotopic evidence on the origin of compositional layering in an epizonal magma body

Nov 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995e%26psl.136...31v&link_type=abstract

Earth and Planetary Science Letters, vol. 136, Issue 1-2, pp.31-41

Computer Science

3

Scientific paper

A detailed isotopic study of the Oligocene age (36 Ma), alkaline composition Organ Needle pluton in south-central New Mexico was undertaken to test models for the generation of compositional layering in silicic, epizonal magma bodies. The pluton is isotopically heterogeneous with its alkali feldspar granite composition cap (73-76% SiO 2 ) having lower initial Nd and higher ratio than the underlying main syenite (-5 vs. -2 and ~ 0.709 vs. ~ 0.706, respectively). Both lithologies have isotopic compositions significantly different from those of the Precambrian granite wall-rock ( Nd ~ -12.1 and ~ 0.784 at 36 Ma). The isotope data indicate that none of the lithologies of the pluton represent the products solely of roof or wall-rock melting, and that the capping granite could not have been derived in a closed-system from differentiation of the underlying syenitic magma. However, the capping granite has isotopic compositions similar to those of a chemically heterogeneous inequigranular syenite found along the margin of the pluton at its deepest exposed level (4-6 km paleodepth). Field observations, and new age determinations, confirm that this lithology was comagmatic with the remainder of the pluton. We conclude that the capping granitic magma was derived from buoyant Nd ~ -5 magma rising along the margins of the magma chamber and that the inequigranular syenite preserves a remnant of this sidewall magma. The main syenite body was encapsulated in this marginal, lower Nd , magma, while undergoing closed-system differentiation, most likely from a mafic progenitor. The Nd = -5 magma may represent mafic magma initially equivalent isotopically to the main syenite but which subsequently assimilated Precambrian wall-rock (~ 10% by mass) at the base of the magma system.

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