Geochronology of Bimodal Alkaline Volcanism in the Balcones Igneous Province, Texas: Implications for Cretaceous Intraplate Magmatism in the Northern Gulf of Mexico Magmatic Zone

Abstract

Small-volume Late Cretaceous monogenetic alkaline volcanism along the southern margin of North America resulted in a broad igneous belt more than 1200 km long from the Trans Pecos region of west Texas to central Mississippi, collectively forming a northern Gulf of Mexico magmatic zone (NGMMZ). The locus of igneous activity is associated with the discontinuity separating Mesoproterozoic cratonic lithosphere and Jurassic transitional lithosphere, a zone approximating the southern margin of Laurentia, the subsurface trend of the Pennsylvanian Ouachita orogenic belt, and the trace of the Miocene Balcones fault zone in Texas. Although previous studies have attempted to determine the ages of igneous activity in the region, few well-constrained geochronologic data using modern high-resolution techniques are available. We determined the age of eruption for the Balcones igneous province (BIP), a 400-km-long subsegment of the NGMMZ, using modern 40 Ar/ 39 Ar and U-Pb geochronology methods. Our results suggest that previously reported 40 K/ 40 Ar and 40 Ar/ 39 Ar ages underestimate the age of igneous activity by as much as 17%. New ages from this study, along with reevaluation of previous results, suggest that BIP igneous activity occurred in two discrete phases, each lasting only 2.6 m.yr. and separated by 2.7 m.yr.: older mafic volcanism occurred between 81.5 and 84.1 Ma and younger felsic volcanism between 76.2 and 78.8 Ma. The total interval of 8 m.yr. for BIP igneous activity is much shorter than had previously been inferred. The best geochronologic results are obtained from U-Pb dating of zircon mineral (phonolites) separates and 40 Ar/ 39 Ar dating of phlogopite (nephelinites and basanites) and amphibole (phonolites) separates.

Lithospheric structure of the Texas‐Gulf of Mexico passive margin from surface wave dispersion and migrated Ps receiver functions

“…The BIP extends 400 km along the juncture between Mesoproterozoic cratonic lithosphere and Jurassic transitional lithosphere (the Gulf Coastal Plain) from near Austin, north of the X4 profile, to southwest of San Antonio. Recent dating identifies two distinct periods of activity in the Balcones Igneous Province: mafic volcanism between 81.5 and 84.1 Ma and felsic volcanism between 76.2 and 78.8 Ma [Griffin et al, 2010]. The compositional differences and age distribution of the volcanic centers makes it unlikely that the BIP was produced by a mantle plume and the briefness of its activity suggests it was cut off from a fertile mantle that might have replenished its budgets of alkali and volatile elements [Griffin et al, 2010].…”

“…Recent dating identifies two distinct periods of activity in the Balcones Igneous Province: mafic volcanism between 81.5 and 84.1 Ma and felsic volcanism between 76.2 and 78.8 Ma [Griffin et al, 2010]. The compositional differences and age distribution of the volcanic centers makes it unlikely that the BIP was produced by a mantle plume and the briefness of its activity suggests it was cut off from a fertile mantle that might have replenished its budgets of alkali and volatile elements [Griffin et al, 2010]. Event ''B'' would then represent a low-velocity zone produced by the depletion of volatile elements through volcanism.…”

Composition of the mantle lithosphere beneath south-central Laurentia: Evidence from peridotite xenoliths, Knippa, Texas

“…Balcones Igneous Province basanites give 40 Ar/ 39 Ar ages of ~81.5-83.5 Ma (Griffin et al, 2010). The xenoliths are spinel peridotites.…”

“…The black groundmass contains finely dispersed clinopyroxene, plagioclase, olivine, nepheline, titaniferous magnetite, melilite, zeolite, amphibole, phlogopite, and apatite. Mg# ( = Mg/Mg + Fe) of the basanites range from 0.67 to 0.75, have low Ni and Cr abundances, and show strong, light rare-earth element (LREE) enrichment, with chondrite-normalized (La/Yb) N = 19-24 (Griffin et al, 2010).…”

Uppermost mantle anisotropy beneath the southern Laurentian margin: Evidence from Knippa peridotite xenoliths, Texas

“…This is the only known mantle peridotite locality in Texas [ Young and Lee , 2009]. Mantle xenoliths were carried up by Late Cretaceous (∼87 Ma) quite primitive nephelinites of the Balcones Igneous Province (BIP) [ Griffin et al , 2010]. BIP volcanoes approximate the boundary between the ∼1.1–1.4 Ga southernmost Laurentian (Texas) craton and Jurassic age transitional lithosphere along the GoM margin.…”