CHEMICAL VARIATION ALONG STRIKE IN FELDSPATHOIDAL ROCKS OF THE EASTERN ALKALIC BELT, TRANS-PECOS MAGMATIC PROVINCE, TEXAS AND NEW MEXICO
LEE S. POTTER*
Department of Geological Sciences C1140, The University of Texas at Austin, Austin, Texas 78712, U.S.A.
* Present address: Geology Department, Phelps Dodge Morenci, Inc.
4521 Highway 191, Morenci, Arizona 85540, U.S.A.
Abstract
Subduction-induced magmatism produced a NW-SE-trending narrow alignment of phonolite and nepheline trachyte intrusions in the eastern
Trans-Pecos region of Texas and New Mexico, known as the Eastern Alkalic Belt (EAB). This 400 km by 20 km belt of mantle-derived,
low-volume, highly fractionated intrusions crosses the boundaries of two distinct basement provinces, the Grenville and Ouachita fronts.
Intrusions within this belt display little variation in major-element compositions and isotopic ages (35 ± 2 Ma). Two distinct segments,
labeled the NW and SE segments, can be delineated based on changes in chemical variation at the Grenville Front. Increased chemical variation
also occurs within the lead isotope transition zone (PbTZ) of James & Henry (1993a, b) that is roughly coincident with the Ouachita Front. In
terms of the chemistry of the EAB rocks, it is unclear if the PbTZ marks a distinct discontinuity in the basement. One tephrite intrusion is
present in the SE segment and is a possible representative of parental melts. These rocks show no evidence of significant modification
by subduction processes, nor of significant involvement with continental crust despite their genesis beneath thick continental crust and
high degrees of differentiation. The normalized trace-element diagram of the EAB tephrite has a pattern like that of an ocean-island basalt,
and it resembles patterns of the leucocratic EAB rocks. The chemical similarity to OIB is striking; it persists despite extensive
differentiation. Heterogeneities within the asthenospheric source of the EAB are relatively minor in the area studied.