Arkansas crustal xenoliths: Implications for basement rocks of the northern Gulf Coast, USA

Abstract

Cretaceous diamond-bearing olivine lamproite occurrences of southwestern Arkansas straddle the geologic and physiographic boundary between the Gulf Coastal Plain and the Ouachita Mountains (Fig. 1). The Gulf Coastal Plain here is characterized by gently south-dipping Cretaceous sedimentary rocks that onlap and lie unconformably on intensely folded and faulted, east-west-trending, Paleozoic sedimentary rocks of the Ouachita Mountains. Rocks of this region are widely believed to lie near the southern margin of the midcontinent craton. XENOLITH PETROLOGYApproximately 2 kg of crustal xenoliths, 1-3 cm in size, were recovered from heavy media concentrates with a density of >2.82 g/cm 3 . Representative xenoliths of each group were selected for thin section analysis based on their abundance and lack of visible alteration.Approximately half of the crustal xenoliths are composed of nearsurface sedimentary rocks, which include conglomerate, sandstone, and chert. These rocks are similar in appearance to Cretaceous and Paleozoic Ouachita facies sedimentary rocks, which are known to exist in the area. Two pink oolitic dolomite xenoliths with pellets and intraclasts are anomalous with respect to local lithologies.Crystalline xenoliths are ~50% amphibolite, consisting dominantly of amphibole and lesser amounts of plagioclase with accessory biotite, chlorite, microcline, quartz, titanite, ilmenite, and apatite. The amphibolite specifi c gravity as measured in water ranges from 3.01 to 3.16, with an average value of ~3.08. Felsic composition xenoliths, consisting of granitic and rhyolitic rocks, are relatively rare, making up only ~5% of the deeper crystalline xenoliths. The average specifi c gravity of the felsic xenoliths is ~2.90. This high density is attributed to an abundance of iron oxides and epidote alteration, making them more susceptible to heavy media concentration.Approximately 40% of the recovered deep crystalline xenoliths are classifi ed as epidote-rich rocks. The specifi c gravity of epidote-rich rocks is variable, but it typically ranges from 3.1 to 3.2. Some fi ne-grained felsic xenoliths have been almost completely replaced by epidote. Epidote alteration of amphibolite xenoliths is observed but is less common. Relatively rare (~5%) dark-colored xenoliths consist almost exclusively of coarse-grained (~7 mm), weakly aligned micas defi ning a foliation. Specifi c gravities of the micaceous xenoliths average ~3.10. Selected crustal xenolith data are presented in Table 1.

Archean lithospheric mantle beneath Arkansas: Continental growth by microcontinent accretion

“…The 1.3-1.5 Ga population coincides with the main development of the crust in the Granite-Rhyolite province that underlies the area. K-Ar ages averaging 1.42 Ga on four amphibolite xenoliths from Twin Knobs are believed to represent the formation of the lower crust (Dunn, 2009), consistent with possible modification of the 1.4 Ga Moho via underplating beneath much of the southern Mazatzal and Granite-Rhyolite provinces (Keller et al, 2005).…”

“…1; see Dunn, 2009). Most of the sediments in these rifts were deposited in Cambrian time, and syn-rifting igneous rocks (gabbros , basalts, granites, and rhyolites) range in age from >580 Ma to 525 Ma (Lambert et al, 1988;Hogan et al, 1996).…”

“…Regional gravity and magnetic data indicate the three rifts are joined, forming a continuous rift structure more than 1000 km long. Crustal xenoliths from the Prairie Creek lamproites indicate a crystalline lower crust composed of granite and/or rhyolite and amphibolite (with K-Ar ages of 1.32-1.47 Ga), rare possibly Cambrian-Ordovician carbonate rocks, probably from the middle crust, and an upper crust composed of Paleozoic Ouachita sandstone and chert, overlain by Late Cretaceous sediments (Dunn, 2009). These rifts most likely were initiated in Neoproterozoic time (780-685 Ma), during the breakup of eastern Rodinia (Li et al, 2008).…”

“…This model is not universally accepted (e.g., Dalziel, 1997). An alternative possibility is that the deformation in the Ouachita Mountains reflects intracratonic reactivation (extension and sedimentation, followed by shortening and inversion) along an older (Proterozoic) suture (Dunn, 2009). This scenario is consistent with the observed spatial connection to adjacent failed rifts (aulacogens), the low metamorphic grades of the Paleozoic sediments, and the minor shortening ascribed to the Ouachita orogeny (Keller et al, 1989).…”

“…600 Ma Neoproterozoic breakup of the Rodinia supercontinent (Li et al, 2008), and inverted during the Ouachita orogeny (cf. Dunn, 2009).…”