Diverse Mesoproterozoic basaltic magmatism in west Texas

Abstract

TWO distinct basaltic suites characterize ~1165-to 1120-Ma magmatism in west Tbxas. The ~1 1 6 3 Ma Pecos mafic intrusive complex consists of layered mafic and ultramafic rocks. Megacyclic units record repeated influx and mixing of tholeiitic magmas to yield a predominantly noritic body. Basaltic magmatism also accompanied emplacement of the 1120 f 35-Ma Red Bluff granitic suite in the Franklin Mountains. In terms of their major elements, these basaltic rocks are transitional between tholeiitic and alkaline, but their low concentrations of Nb suggest a subduction-related origin. Except for their low Nb contents, the Franklin Mountains basaltic rocks are similar to coeval basaltic intrusive and volcanic rocks in the southwestern U.S.A. Of the numerous proposed tectonic settings, an extensional environment is preferred. Possible extensional associations include splays of the Midcontinent Rift, rifting caused by collision of a continental fragment during the Grenville orogeny, and extension related to collapse of over-thickened crust during the Grenville event. The low Nb contents of basalts associated with Franklin Mountains basaltic rocks are best explained as an artifact of older subduction, which modified the mantle source of the basalts hundreds of millions of years prior to their emplacement.

U-Pb geochronology of 1.1 Ga diabase in the southwestern United States: Testing models for the origin of a post-Grenville large igneous province

“…It is more ambiguous as to whether the mafi c and ultramafi c intrusions that make up the Pecos mafi c intrusive complex should be included as part of the Southwestern Laurentia large igneous province. We agree with Barnes et al (1999) that one of the ages from the Pecos mafi c intrusive complex, 1163 ± 4 Ma from a single zircon (Keller et al, 1989), places the intrusion well outside of the range of ages of the other diabase samples. However, Keller et al (1989) also obtained an age of 1095 ± 14 Ma from titanite in another sample.…”

“…1B) extends from southeastern California to southwestern Arizona (Howard, 1991). Barnes et al (1999) grouped the Pikes Peak batholith into the province, along with mafi c igneous rocks in west Texas. Our data confi rm that diabase intrusions from western New Mexico also belong to this igneous province, which we now refer to as the Southwestern Laurentia large igneous province (SWLLIP).…”

Tectonic inferences from the ca. 1255–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis

“…Mafi c igneous rocks span a broad interval in time, from ca. 1235 Ma in the Grenville of Ontario (Bethune, 1997), 1109-1087 Ma in the Midcontinent region (Van Schmus, 1992), to 1165-1120 Ma in west Texas (Keller et al, 1989;Barnes et al, 1999), and new data indicate 1115-1104 Ma in the Grand Canyon. Mafi c intrusions commonly occur as sills in both Mesoproterozoic sedimentary rocks and the crystalline basement in the southwestern United States, which led Howard (1991) to postulate that sills record subvertical σ 3 during NWdirected shortening.…”

Petrology and geochemistry of late-stage intrusions of the A-type, mid-Proterozoic Pikes Peak batholith (Central Colorado, USA): implications for petrogenetic models

“…the size of the Nb anomalies is similar). Barnes et al (1999) emphasized that assimilation of crustal rocks into a primitive basaltic magma will be accompanied by fractional crystallization (FC) and that Nb will not be depleted unless a significant amount of an oxide phase (s), with a large distribution coefficient for Nb, is included in the fractionating assemblage. High abundances of Fe and Ti (cf Tables 2 and 3) in Pikes Peak mafic rocks indicate that oxide minerals (e.g.…”

“…magnetite, ilmenite) were not removed from parental magmas. Hence, the Nb depletions in these rocks are difficult to explain by AFC processes (cf Barnes et al, 1999).…”