1. SOUTHERN RIO GRANDE RIFT ONSET BY 32 MA: NEW PETROLOGIC EVIDENCE FROM THE SOUTH RIM FORMATION
Benker, S. Christian 1, White, John C. 2, Ren, M. 3, and Anthony, Elizabeth, Y. 4
1 Arizona Western College, 2 Eastern Kentucky University, 3 University of Texas El Paso
extensional or near- extensional tectonic settings.
Avanzinelli et al. (2004) provide a good description of peralkaline tectonic environments as generally found in continental intraplate settings affected by rifting, however examples from oceanic islands close to active mid-ocean ridges have also been found. These authors note that more rarely peralkaline felsic rocks and alkali basalts are associated with orogenic magmatic suites. A number of other authors have also noted peralkaline magmatism in orogenic associated settings.
Despite the presence of peralkaline rocks in orogenic settings, McDonald (1974) notes these are typically minor members of orogenic suites whose existence can be attributed to complicated tectonic interactions involving zones of relatively small localized extension. Furthermore, it becomes important to note that all examples of peralkaline rocks associated with orogenic magmatism listed by Avanzinelli et al. (2004) have been subsequently related to some degree of, or association with, extension [see Benker, 2010].
While structural geologic evidence is certainly more prevalent in Trans-Pecos Texas, emplacement of peralkaline magmatism should not be overlooked in assessing crustal extension and determining timing of tectonic transition and southern Rio Grande rift onset.
INTRODUCTION
A-TYPE MAGMATISM: CLARIFICATION
The South Rim Formation is a series of Ma (Miggins, et al. 2007) comenditic quartz trachytic-rhyolitic volcanics and associated intrusives erupted and emplaced in Big Bend National Park, Texas. Magmatic parameters have been calculated for one of its two petrogenetic suites.
Previous studies based on the South Rim Formation’s Pine Canyon Suite indicated low temperatures (< 750 °C), reduced conditions (generally below the FMQ buffer), and low pressures (≤ 100 MPa) associated with magmatism. Newly discovered fayalite microphenocrysts in the South Rim Formation’s Emory Peak Suite have now allowed calculation of oxygen fugacity values (between and ΔFMQ over temperature ranges of °C) via mineral equilibria based QUILF95 calculations, for Emory Peak Suite. Thus, the two South Rim Formation suites display slightly different magmatic conditions.
Petrologic information from the South Rim Formation is correlated here with structural evidence from Trans-Pecos Texas and adjacent regions to evaluate debated timing of tectonic transition (Laramide shortening to Basin and Range extension) and onset of the southern Rio Grande Rift during the mid-Tertiary. The A-type and peralkaline characteristics of the South Rim Formation and other pre-31 Ma magmatism in Trans-Pecos Texas, in addition to evidence implying earlier Rio Grande Rift onset in Colorado and New Mexico, promotes a near-neutral setting in Trans-Pecos Texas by 32 Ma. This idea contrasts with interpretations of tectonic compression and arc-related magmatism until 31 Ma as suggested by some authors.
Due to ambiguity with the term in the past, it becomes important to clarify A-type magmatism and its implications. Bonin (2007) provided a detailed review from numerous sources and noted the following conclusions concerning A-type magmatism:
emplacement ages for these magmas are apparently unrelated to proximal major orogenic events
Fe-rich mafic mineralogy observed is predominantly resultant of reducing condition
Geochemically distinct due to high alkali, LILE, and HFSE contents, high Fe/Mg ratios, and OIB-type compositions
Additionally, Martin (2006) concluded A-type granitic magmatism is intimately tied to nepheline syenite and carbonatite origination; all of which are manifestations of anorogenic magmatism and are linked to processes occurring in the upper mantle and lower crust.
On the basis of these conclusions, rocks in the South Rim Formation and other rocks (see Table 1) emplaced prior to ~31 Ma can be characterized as A-type.
PETROLOGIC CONCLUSIONS
SIGNIFICANCE OF PERALKALINITY
Our research confirms both members of the South Rim Formation as forming under reducing conditions. Combined with other geochemical and mineralogic characteristics (see White et al., 2006; Benker, 2010) it is evident that South Rim Formation rocks demonstrate A-type (as clarified here) and peralkaline geochemistry.
Presence of A-type magmatism in Big Bend National Park, other portions of Trans-Pecos Texas (Table 1), and likely adjacent portions of Mexico suggests an anorogenic (approximately neutrally to perhaps initial transtensionally stressed) tectonic setting from Ma. While such magmatism pre-dates regional normal faulting, examples of magmatism preceding faulting by up to ~20 million years have been documented in similar tectonic settings in East Africa by Ebinger et al. (2000). Our results dispute the ~31 Ma tectonic transition and southern Rio Grande rift onset age interpreted by some authors.
Furthermore, A-type magmatism coupled with the presence of peralkaline magmatism could be interpreted (see above section) as reflective of some degree of extension—perhaps at increased lithospheric depth—prior to illustration as major Basin & Range faulting at the surface in southern Trans-Pecos Texas. Lesser degrees of extension in the upper crust should not be depreciated as they may represent distant and downplayed expressions of extensional activity in the lower crust and upper mantle. Particularly for segments of the Rio Grande Rift in New Mexico, findings by Wilson et al. (2005) concluded that lower crustal extension is distributed over an area four times the width of the rift’s surface expression. Therefore it should not be inferred that peralkaline magmatism at the surface, even if minor, is void of expressing some degree of tectonic tension.
Figure 1: Distribution of the South Rim Formation: 1) Pulliam Peak; 2) Lost Mine Peak; 3) Casa Grande; 4) Crown Mountain; 5) Emory Peak; 6) South Rim; 7) Burro Mesa; 8) Kit Mountain; 9) Goat Mountain; 10) Brazos de Maria; 11) El Fuego de Ricardo; 12) Trap Mountain; 13) Round Mountain; 14) Horseshoe Canyon; 15) Cerro Castellan. Dashed circle in Chisos Mountains approximately delineates Pine Canyon caldera. Inset map shows extent of mid- to late Tertiary basins of the Rio Grande Rift (from Dickerson and Muehlberger, 1994; Chapin et al., 2004; White et al., 2006).
SOUTHERN TRANS-PECOS TRANSITIONAL DEBATE
While peralkalinity can be geochemically defined as molar excess of alkali relative to molar alumina, there remains uncertainty as to whether or not peralkaline magmatism can be defined as exclusively representing
The timing of transition from Laramide shortening to Basin and Range extension and initiation of the southern Rio Grande rift during the mid-Tertiary is debatable. Tectonic interpretations generally involve subduction-related arc magmatism with varying degrees of compression approaching the mid-Tertiary, a period of transition, and then tectonic extension (associated with southern Rio Grande rift onset) either prior to 36 Ma or circa 31 Ma.
Tectonic transition and southern rift onset at or prior to 36 Ma has been suggested by numerous authors (Lawton and McMillan, 1999; McMillan et al., 2000; Chapin et al., 2004; McIntosh and Chapin, 2004) on the basis of evidence including:
Global plate circuit analysis showing abrupt decrease, from cm/yr to 6-9 cm/yr, in Farallon-North American plate convergence rates after ~ 45 Ma (Stock and Molnar, 1988) along with presence of several low relief erosion surfaces underlying later Eocene to Oligocene sedimentary rocks and potentially extension-related volcanism
Structural and petrologic evidence from New Mexico and central Colorado including A) suggestions by Parker et al. (2005) that rocks of the San Juan volcanic field inherited arc-like orogenic geochemical signatures through extensive assimilation of ancient orogenic and anorogenic continental crust and B) emplacement of a porphyry copper system (formation of which is typical in zones where the magmatic arc is in a nearly neutral stress regime—usually during transitions in or out of subduction (Chapin et al., 2004)) in the Jarilla Mountains between 48 and 42 Ma (Beane et al., 1975).
Tectonic transition and southern rift onset at ~31 Ma has also been suggested by other authors (Price and Henry, 1984; Henry and McDowell, 1986; Henry and Price, 1986; Henry at al., 1991; James and Henry, 1991) due to:
Change in regional paleostress direction from north-northwesterly minimum principal stress (σ3) and east-northeasterly maximum principal stress (σ1)—characterizing compression until 32 Ma—to east-northeasterly σ3 and vertical σ1—definitive of extension afterward (Price and Henry, 1984; Henry and McDowell, 1986)
Differences in Zr/Nb, Y/Nb, and Ba/Nb in pre- and post-31 Ma basalts (James and Henry, 1991)
Onset of magmatism prior to < 28 Ma normal faulting in Trans-Pecos Texas (Dickerson and Muehlberger, 1994; Henry et al., 1991; James and Henry, 1991).
PRE-31 Ma ALTERNATIVE TECTONIC STRESS REGIME CONSIDERATIONS
Location and Evidence for Potential Alternative Stress
Age (Ma)
Suggestive Paleostress
Petrologic Notes
References
Structural and Magmatic Evidence in Trans-Pecos Texas
Considerable dike and vein orientation scatter suggests small magnitude difference between σ1 and σ3
48-32
neutral --
Delaney et al., 1986; Henry et al., 1991
Intrusions in the Christmas Mountains area
44-40
some degree of extension
peralkaline
Cameron et al., 1986; Henry et al., 1989
Infiernito tuffs north of the Chinati Mountains
~37
Henry et al., 1992
Bracks Rhyolite, Star Mountain Formation, and Crossen Trachyte in the Davis Mountains
36.8
near-neutral, neutral, some degree of extension
A-type, mildly peralkaline
Henry et al., 1993
Gomez tuff and other volcanism in the Davis Mountains
peralkaline-very peralkaline
Parker, 1983; Parker and White, 2008
Domes north and west of the Solitario laccocaldera
~35.4
Henry et al., 1997
South Rim Formation in central Big Bend National Park
A-type, peralkaline
Benker, 2005; White et al., 2006
Alternative σ1 interpretation of dike and vein orientation (disregarded as faulting was negligible)
pre-32
σ1 = east-northeast to vertical; some degree of extension
Henry et al., 1991
Structural and Magmatic Evidence in Adjacent Regions
Emplacement of porphyry copper system near the Jarilla Mountains (NM)
48-42
near-neutral
Beane et al., 1975
Regional stress field characterization for New Mexico and Colorado (NM, CO)
45-36
Chapin et al., 2004
Intercalation of half graben deposited conglomerate and Bell Top Formation (NM)
~36
McMillan et al., 2000
Geochemical signature of Rubio Peak Formation basalts and Bell Top Formation (NM)
~38-36
same protolith as extensional Uvas lavas
Villa Ahumada carbonatite complex in northern Chihuahua
near-neutral, neutral
likely A-type
Nandigam, 2000
Creation of Goodsight-Cedar Hills half graben (NM)
Mack, 2001; Mack et al., 1994
Dikes striking N40°W in the Organ batholith (NM)
32.5
some degree of northeast extension
Newcomer et al., 1983
7 km long dike generally striking northerly near San Carlos-Santana caldera complex (MEX) 32
Chuchla, 1981; Gregory, 1981
Table 1:
The Window
Chisos Mountains
Big Bend National Park