M. Charles Gilbert

Magma traps and driving pressure: consequences for pluton shape and emplacement in an extensional regime

Abstract The level of emplacement and final form of felsic and mafic igneous rocks of the Wichita Mountains Igneous Province, southwestern Oklahoma, U.S.A. are discussed in light of magma driving pressure, lithostatic load, and crustal magma traps. Deposition of voluminous A-type rhyolites upon an eroded gabbroic substrate formed a subhorizontal strength anisotropy that acted as a crustal magma trap for subsequent rising felsic and mafic magma. Intruded along this crustal magma trap are the A-type sheet granites (length/thickness 100:1) of the Wichita Granite Group, of which the Mount Scott Granite sheet is typical, and smaller plutons of biotite bearing Roosevelt Gabbro. In marked contrast to the subhorizontal granite sheets, the gabbro plutons form more equant stocks with flat roofs and steep side walls. Late Diabase dikes cross-cut all other units, but accompanying basaltic flows are extremely rare in the volcanic pile. Based on magmastatic calculations, we draw the following conclusions concerning the level of emplacement and the shape of these intrusions. (1) Magma can rise to a depth at which the magma driving pressure becomes negligible. Magma that maintains a positive driving pressure at the surface has the potential to erupt. (2) Magma ascent may be arrested at a deeper level in the crust by a subhorizontal strength anisotropy (i.e. crustal magma trap) if the magma driving pressure is greater than or equal to the lithostatic load at the depth of the subhorizontal strength anisotropy. (3) Subhorizontal sheet-intrusions form along crustal magma traps when the magma driving pressure greatly exceeds the lithostatic load. Under such conditions, the magma driving pressure is sufficent to lift the overburden to create the necessary space for the intrusion. (4) Thicker steep-sided stocks or batholiths, with flat roofs, form at crustal magma traps when the magma driving pressure approximates that of the lithostatic load. Under these conditions, the necessary space for the intrusion must be created by other mechanisms (e.g. stoping). (5) Subvertical sheets (i.e. dikes) form when the magma driving pressure is less than the lithostatic load at the level of emplacement.

The A-type Mount Scott Granite sheet: Importance of crystal magma traps

The presence of rapakivi feldspar and of distinctive porphyritic texture of Mount Scott Granite indicates a period of crystallization prior to final emplacement beneath an extensive penecontemporaneous rhyolite volcanic pile. Final crystallization conditions are interpreted to have been <50 MPa at depths < <1.4 km based on stratigraphic constraints. However, geobarometry based on the Al content of amphibole phenocrysts and comparison of granite compositions with phase relations in the SiO2-NaAlSi3O8-KAlSi3O8 ternary system both yield pressure estimates of ≈200 MPa. These pressure estimates are interpreted as plumbing the depth of a temporary storage chamber at ≈7–8 km. This depth coincides, in this case, both with the probable Proterozoic basement-cover contact and with the calculated brittle-ductile transition at time of ascent of Mount Scott magma. Although rising magma that fed the preceeding voluminous Carlton Rhyolite apparently rose unimpeded past these horizontal anisotropies, rising magma that formed Mount Scott Granite temporarily paused at this depth. Based on magmastatic calculations, we suggest that horizontal anisotropies (e.g., brittle-ductile transition) become crustal magma traps where the magma driving pressure exceeds the lithostatic load when the anisotropy is encountered. During rifting, initial large influxes of magma may proceed passed crustal anisotropies but have the effect of increasing the relative magma driving pressure through reducing horizontal stress. Thus, magma driving pressure may eventually exceed the lithostatic load at the depth of the brittle-ductile transition thereby activating this crustal magma trap. Ponding of magma at the brittle-ductile transition chokes the eruption. Such a pause in magma supply rate may permit a return to initial stress conditions and deactivate the crustal magma trap. Once again magma will rise to the surface initiating a new magmatic cycle.

Experimental study of titanite-fluorite equilibria in the A-type Mount Scott Granite: Implications for assessing F contents of felsic magma

Titanite and fluorite stability in melt were experimentally evaluated at 850 °C, 200 MPa, f(O 2 ) ≈ NNO (nickel-nickel oxide oxygen buffer) as functions of total F and H 2 O content. Experiments employed the metaluminous Mount Scott Granite of the Wichita igneous province, Oklahoma. Over a large range of added H 2 O (∼1–7 wt%), melts containing 1 wt% F precipitated fluorite without titanite. In addition, at high F (≥ 1.2 wt%) plagioclase and hornblende reacted to form biotite. Thus, an increase in F during crystallization may explain the observed higher modal abundance of plagioclase and hornblende in titanite-dominant samples vs. higher modal biotite in fluorite-dominant samples within the Mount Scott Granite pluton. Coexistence of magmatic titanite and fluorite in the Mount Scott Granite pluton implies F m of ∼1 wt% at the point in its crystallization history where these minerals coprecipitated. We suggest that the presence of primary fluorite within high-temperature, shallowly emplaced, moderate f (O 2 ), subaluminous felsic rocks indicates high magmatic fluorine, whereas titanite without fluorite in such rocks indicates low initial fluorine.

Load-induced subsidence of the Ancestral Rocky Mountains recorded by preservation of Permian landscapes

The Ancestral Rocky Mountains (ARM) formed a system of highlands and adjacent basins that developed during Pennsylvanian–earliest Permian deformation of interior western North America. The cause of this intracratonic deformation remains debated, although many have linked it to far-field compression associated with the Carboniferous–Permian Ouachita-Marathon orogeny of southern North America. The ultimate disappearance of the ARM uplifts has long been attributed to erosional beveling presumed to have prevailed into the Triassic–Jurassic. New observations, however, indicate an abrupt and unusual termination for the largest of the ARM uplifts. Field evidence from paleohighlands in the central ARM of Oklahoma and Colorado indicates that Lower Permian strata onlap Pennsylvanian-aged faults and bury as much as 1000 m of relief atop the paleohighlands. In parts of Oklahoma and Colorado, late Cenozoic partial exhumation of these paleohighlands has revealed landscapes dating from Permian time. These relationships suggest cessation of uplift followed by active subsidence of a broad region that encompassed both basins and uplifted crustal blocks and that commenced in Early Permian time, directly following the Pennsylvanian tectonic apogee of the ARM. Independent from these geological observations, geophysical data reveal a regional-scale mafic load underpinning these paleohighlands, emplaced during Cambrian rifting associated with the southern Oklahoma aulacogen. Geophysical modeling of the effects of such a load in the presence of a horizontal stress field, such as that implied by ARM orogenesis, indicates that the amplitude of flexurally supported features is modulated nonlinearly. This leads to buckling and thrust formation with the application of sufficient compressive stress, and subsidence of topography formed by buckling upon relaxation of the high compressional stresses. We therefore infer that the core ARM highlands subsided owing to the presence of a high-density upper crustal root, and that this subsidence began in the Early Permian owing to relaxation of the in-plane compressional stresses that had accompanied the last phase of the Ouachita-Marathon orogeny of southern and southwestern Laurentia. Our results highlight the importance of tectonic inheritance in intraplate orogenesis and epeirogenesis, including its potential role in hastening the reduction of regional elevation, and enabling the ultimate preservation of paleolandscapes.

The Southern Oklahoma Aulacogen: A Cambrian analog for Mid-Proterozoic AMCG (Anorthosite-Mangerite-Charnockite-Granite) complexes?

Comparison of the Cambrian Southern Oklahoma Aulacogen and the distinctive anorthosite-mangerite-charnockite-granite (AMCG) complexes of the Mid-Proterozoic reveal striking similarities in the temporal and spatial association of igneous rock types in these provinces that suggests a commonality in their petrogenesis. Igneous rocks that comprise the Cambrian Southern Oklahoma Aulacogen include: 1) voluminous anorthositic gabbros, 2) Fe-rich, Ti-rich, and P-rich, biotite-bearing gabbros, 3) A-type leucocratic alkali-feldspar rhyolite and granites (some with rapakivi textures, see Price et al., 1996a), and 4) a suite of “late” diabase dikes. Nearly all of these igneous units have counterparts that occur within AMCG complexes. A noticeable exception in the Southern Oklahoma Aulacogen is the absence of coarse grained massif-type anorthosites, with their characteristic high-Al orthopyroxene megacrysts. This important distinction is interpreted to reflect differences in the conditions of crystallization and emplacement of magmas giving rise to these two provinces. AMCG complexes develop at significantly deeper levels in the crust where assimilation of aluminous continental crust may operate more efficiently due to higher ambient temperatures. In contrast, the epizonal to volcanic conditions of crystallization, and lack of evidence for significant contributions of ancient crust in either mafic or felsic igneous rocks, suggest that parent magmas to the igneous rocks of the Southern Oklahoma Aulacogen were rapidly transported to the emplacement level, where they cooled quickly, thus inhibiting opportunities for large scale crustal assimilation. We further speculate that, if exposed, the large mid-crustal mafic root beneath the Southern Oklahoma Aulacogen, inferred from geophysical and petrologic arguments, would have the appearance of a typical Mid-Proterozoic AMCG complex. Conversely, penecontemporaneous tectono-magmatic provinces, such as the Southern Oklahoma Aulacogen, may have overlain Mid-Proterozoic AMCG complexes and have been subsequently removed by erosion.

Crystallisation of fine- and coarse-grained A-type granite sheets of the Southern Oklahoma Aulacogen, U.S.A.

A-type felsic magmatism associated with the Cambrian Southern Oklahoma Aulacogen began with eruption of voluminous rhyolite to form a thick volcanic carapace on top of an eroded layered mafic complex. This angular unconformity became a crustal magma trap and was the locus for emplacement of later subvolcanic plutons. Rising felsic magma batches ponding along this crustal magma trap crystallised first as fine-grained granite sheets and then subsequently as coarser-grained granite sheets. Aplite dykes, pegmatite dykes and porphyries are common within the younger coarser-grained granite sheets but rare to absent within the older fine-grained granite sheets. The older fine-grained granite sheets typically contain abundant granophyre. The differences between fine-grained and coarse-grained granite sheets can largely be attributed to a progressive increase in the depth of the crustal magma trap as the aulacogen evolved. At low pressures (<200MPa) a small increase in the depth of emplacement results in a dramatic increase in the solubility of H 2 O in felsic magmas. This is a direct consequence of the shape of the H 2 O-saturated granite solidus. The effect of this slight increase in total pressure on the crystallisation of felsic magmas is to delay vapour saturation, increase the H 2 O content of the residual melt fractions and further depress the solidus temperature. Higher melt H 2 O contents, and an extended temperature range over which crystallisation can proceed, both favour crystallisation of coarser-grained granites. In addition, the potential for the development of late, H 2 O-rich, melt fractions is significantly enhanced. Upon reaching vapour saturation, these late melt fractions are likely to form porphyries, aplite dykes and pegmatite dykes. For the Southern Oklahoma Aulacogen, the progressive increase in the depth of the crustal magma trap at the base of the volcanic pile appears to reflect thickening of the volcanic pile during rifting, but may also reflect emplacement of earlier granite sheets. Thus, the change in textural characteristics of granite sheets of the Wichita Granite Group may hold considerable promise as an avenue for further investigation in interpreting the history of this rifting event.

Surface and Near-Surface Investigation of the Alteration of the Mount Scott Granite and Geometry of the Sandy Creek Gabbro Pluton, Hale Spring Area, Wichita Mountains, Oklahoma

Surface and subsurface investigation of the spatial relationships between the Cambrian-age Glen Mountains Layered Complex, Mount Scott Granite, and Sandy Creek Gabbro in the Hale Spring Area of the Wichita Mountains, shows that the upper portion of the Sandy Creek gabbro pluton is largely steep-walled, with a blunt irregular subhorizontal roof capped by Mount Scott Granite. In the subsurface, the near vertical sidewalls of the gabbro are presumed to truncate an older subhorizontal contact of regional extent, between the Glen Mountains Layered Complex substrate and the Mount Scott Granite cap. This subhorizontal contact is interpreted as an angular unconformity that developed on the layered complex and was subsequently buried by volcanic deposits of the Carlton Rhyolite Formation prior to intrusion of the Mount Scott Granite sheet. The Sandy Creek Gabbro does contain xenoliths of Glen Mountains Layered Complex and Meers Quartzite, a metasedimentary unit associated with this unconformity. Locally, a thin ledge of gabbro, with an irregular floor, protrudes more than 0.5 km to the south from the main body of the intrusion presumably exploiting this subhorizontal contact. Thus, the Sandy Creek Gabbro is a stock, capped by the floor of the Mount Scott Granite sheet, and only locally spreads laterally along the older unconformity, the contact between the Mount Scott Granite sheet and the underlying Glen Mountains Layered Complex.