Emplacement of a xenolith-rich sill, Lajitas, Texas

Abstract

A 2–15m thick hawaiite sill in the Cretaceous Boquillas Formation west of the Big Bend National Park, Texas, locally contains a zone 1–12m thick containing 25vol% xenoliths. The inclusions, ranging in size from 2cm to 1m, are dominated by cumulate gabbros and white quartzite. The mafic inclusions broke from the sidewall cumulates in a reservoir that fed the hawaiite sill. Many xenoliths show contacts between quartzite and igneous rock, marked by clinopyroxene crystals perpendicular to the contacts. The quartzite xenoliths are permeated variably by quenched mafic liquid (now devitrified glass with plagioclase+clinopyroxene+smectite), and some quartz grains are fringed by inverted tridymite. Stratigraphic reconstruction indicates that the sill was emplaced at a depth shallower than 0.5km, but the source of the Paleozoic quartzite and sidewall cumulate xenoliths was at least 1km below the sill.The high concentration of xenoliths, their lack of imbrication, orientation, or sorting, the preservation of unbroken thin slabs of limestone, and the absence of xenoliths at the top and bottom of the sill all support emplacement by plug flow of xenolith-charged magma in the center, lubricated by less viscous xenolith-free magma at the margins. Crystal-size distribution of plagioclase indicates that mean crystal-residence time for cumulates was about 75years, and crystallization time at the aphyric upper margin of the sill was about 2years.