Granite on the margins
Magmas intrude the shallow crust by opening fractures, known as dikes. The photo shows a granitic dike cutting gabbroic rocks, in the foreground. Its likely source, the Quanah Granite pluton, is represented by the continuous exposure of granite at Elk Mountain, in the background.
In terms of igneous activity, volcanoes get all of the press. But there is much to magmatism under the Earth's surface. Plutons are masses of igneous rock though to arise from a magmatic emplacement event (and underground eruption, of sorts). Whereas volcanic flows merely displace the atmosphere during emplacement, intrusions displace solid rock. Intuitively, this is a more complicated process. Assessment of intrusive features informs our understanding of magmatism, as well as fluid-induced crustal deformation (earthquakes-inducing mechanisms, in some settings).
The Wichita Mountains in southwestern Oklahoma contain roughly a dozen granite plutons. These rocks were once magmas that pulsed into the shallow crust, at depths no greater than 2 km (1.2 mi). This magmatism resulted from continental rifting during the Cambrian Period (540 million year ago). The Quanah Granite is thought to be the youngest pluton. The bulk of its 35 square kilometers (13 sq. mi) of exposure is homogenous alkali-feldspar granite, exhibiting a hypidiomorphic granular texture with a grain size that is coarser than all but one of the other Wichita plutons. It is cut throughout by pegmatite and aplite dikes that may have originated from late-stage liquid fractions. However, the northern margin includes rocks that exhibit a finer irregular (allotriomorphic) texture, with both continuously variable grain-size (seriate) and bimodal grain-size (porphyritic) examples. Furthermore, the gabbroic rocks to the north of the pluton are transected by numerous granitic dikes.
Undergraduate researchers Amber Quevy and Alex Weiskircher Stevenson investigated the northern margin of Quanah pluton as part of Midwestern State's EURECA program. Amber located, mapped, and measured the dikes immediately adjacent to the pluton. Alex evaluated and mapped the distribution of the two finer facies in the pluton's northern margin. Both characterized representative samples through X-ray diffractometry and petrography. Alex's samples lent themselves to further evaluation through electron probe micro-analysis.
The Quanah Granite is dominated by a relatively coarse-grained texture, but hosts finer-grained intrusions on its margin. These magmas diked into the surrounding rocks.
Alex discovered that the finer seriate and porphyritic facies exhibit dike-like distributions that demonstrably cut across the coarser Quanah. The relative age relationship of the two finer facies (seriate and prophyritic) remains unresolved. All of the facies are dominated by perthitic alkali feldspar. But the coarser facies distinctively contains the sodic amphibole arfvedsonite, a mineral that becomes more prominent in pegmatite dikes that cut the coarse facies. This previously-noted mineral was confirmed by Alex's work. She found calcic amphibole and biotite in the prophyritic facies, and biotite only in the finer seriate facies. Alex also documented a weakly granophyric texture in the porpyhyriic facies.
Amber found that dike orientations vary, but three principal strike lines emerge, one of which is parallel to rift boundaries, presumably the direction of regional intermediate stress. She found that the dikes exhibit textures that include seriate, porphyritic intergrown-quartz and feldspar (granophyre), coarser hypidiomorphic granular, and pegamatitic. The rocks are dominated by perthitic alkali feldspar, typically pink, but one exposure contains amazonite (blue-green variety). The mafic minerals can be altered to chamosite or other chlorite-group phases. Unaltered dike samples generally contain biotite only, or biotite with hornblende, or biotite with clinopyroxene (presumably from the gabbroic wall rock).
The work implies that the coarse-facies Quanah magma and pegmatitic liquids intruded first and may have diked into the adjacent rocks to the north. The finer facies followed, intruding the pluton margins and diking into the surrounding rocks; these latter liquids seem to dominate dike compositions based on our sampling. Finer-facies liquids most likely exploited cooling-related contraction stresses within and adjacent to the pluton. The difference in mafic assemblages (arfvedsonite-bearing coarse facies versus the finer facies' biotite with or without hornblende) suggests the finer facies may be products of melts unrelated to the coarse-Quanah magma. Given the difference in texture, significant cooling followed the coarse grained facies prior to the intrusion of the two finer grained facies.