RI0183D. Geology and Tertiary Igneous Activity of the Hen Egg Mountain and Christmas Mountains Quadrangles, Big Bend Region, Trans-Pecos Texas, by C. D. Henry, J. G. Price, and D. E. Miser. 105 p., 35 figs., 6 tables, 1 appendix, 1989. doi.org/10.23867/RI0183D.
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Cretaceous sedimentary rocks and Tertiary intrusive, extrusive, and volcaniclastic rocks dominate the geology of the Hen Egg Mountain and Christmas Mountains quadrangles in the Big Bend region of Texas. The Cretaceous rocks consist of limestones, shales, mudstones, and sandstones that are host rocks for the Tertiary intrusions.
This report focuses on the Tertiary intrusive and extrusive rocks. The intrusions were emplaced in several distinct intervals between 47 and 20 Ma ago (Eocene to Miocene) but are dominantly 44 to 40 Ma old. All intrusions are small (less than a few kilometers in diameter) and were shallowly emplaced as laccoliths, sills, and dikes. The Eocene intrusions fall into two petrographically and chemically distinct suites: (1) a silica-undersaturated series from nepheline-normative hawaiite to syenite and (2) a silica-oversaturated series from trachyandesite to high-silica, peralkaline rhyolite. The silica-undersaturated rocks dominantly form sills or dikes and, more rarely, laccoliths having little structural relief. The silicic rocks form laccoliths having considerable structural relief, including trap-door laccoliths, and, less commonly, sills or dikes. The undersaturated rocks occur primarily in the central part of the Hen Egg Mountain quadrangle; the oversaturated intrusions occur throughout both quadrangles.
A single nepheline-normative hawaiite sill was emplaced 28 Ma ago (late Oligocene) in the southeastern corner of the Christmas Mountains quadrangle. Abundant north-northwest-trending dikes of strongly nepheline-normative hawaiites and basalts were emplaced at 24 to 20 Ma (early Miocene), contemporaneous with initial Basin and Range faulting. These latter rocks commonly host mantle xenoliths.
Silicic volcanism accompanying late Eocene intrusion occurred in two areas. A caldera complex in the Christmas Mountains represents an unusual eruptive laccolith, a caldera type not previously recognized. These calderas developed on the Christmas Mountains dome, a large (8 x 5 km), elliptical laccolithic uplift that resulted from emplacement of the caldera magma body. The complex produced an extensive series of rhyolitic to quartz trachytic airfall and ash-flow tuffs, silicic lavas and domes, and coarse debris-avalanche deposits; these rocks are divided into five stratigraphic sequences. At least four caldera collapses accompanied eruption, and the fill in the two youngest calderas dips steeply inward, indicating post-eruption subsidence. More minor volcanism consisted of massive air-fall or ash-flow tuff associated with high-silica, peralkaline rhyolite domes in the northwest corner of the Hen Egg Mountain quadrangle.
The major structural features of the quadrangles are domes produced by intrusive emplacement. Laramide folding either did not occur or was a minor influence. Three north-northwest-trending Basin and Range fault zones transect the quadrangles. Displacement is mostly down to the west and less than 1 km.
The only significant mineral production in the area was of fluorite formed at the contacts between alkalic, silicic intrusions and Cretaceous carbonate host rocks. The presence of numerous prospects, the unusual trace-element enrichment in several of the fluorite deposits, and the overall geologic setting and types of igneous rocks suggest significant potential for deposits of rare metals.
Keywords: alkalic rocks, ash-flow tuffs, calderas, K-Ar, laccoliths, ore deposits, Tertiary, Trans-Pecos Texas
Regional Geologic Setting
Precambrian and Paleozoic Rocks
Del Carmen Limestone
Sue Peaks Formation
Santa Elena Limestone
Del Rio Clay
Boquillas and Pen Formations undifferentiated
Tertiary Volcanic and Sedimentary Rocks
Alamo Creek Basalt Member
Undifferentiated, dominantly reworked tuffs
Devil's Graveyard Formation
Undifferentiated Devil's Graveyard Formation
Tuff at Hen Egg Mountain
Colluvium and landslide blocks
Upper Middle Eocene.
Hawaiitic sills and dikes
Christmas Mountains gabbro
High-silica peralkaline rhyolite
Low-silica peralkaline rhyolite
Sparsely porphyritic, peralkaline quartz trachyte
Abundantly porphyritic, peralkaline quartz trachyte
Metaluminous quartz trachyte
Altered quartz trachyte
Dikes and small sills
Igneous Petrology and Geochemistry
Silica Saturation and Chemical Classification of Igneous Rocks
Rocks of Contrasting Tectonic Settings.
The Christmas Mountains Caldera Complex
Stratigraphy of Caldera-Related Rocks
Sequence I: initial tuffs and lavas
Lower quartz trachyte lavas
Debris flows and tuffs
Ash-flow tuff at Little Christmas Mountain.
Upper quartz trachyte lavas
Undifferentiated sequence I
Related rocks in Big Bend National Park
Sequence II: debris-flow deposits, rheomorphic tuff, and lava dome
Lower debris-flow deposits
Quartz trachyte porphyry
Upper debris flows
Sequence III: massive debris-avalanche deposits
Sequence IV: caldera-fill tuff and breccia.
Limestone collapse breccia
Sequence V: quartz trachyte porphyry intrusions
Structure of the Christmas Mountains Dome and Calderas
The Christmas Mountains dome
First and second calderas
Main western caldera
Main eastern caldera
Evolution of the Christmas Mountains Caldera Complex
Fluorspar, Beryllium,and Molybdenum
Other Mineral Commodities
Structures Related to Igneous Activity
Metaluminous quartz trachyte laccolith
Adobe Walls dome
Wildhorse Mountain intrusion
Paynes Waterhole anticline
Hen Egg Mountain intrusions
Basin and Range Faults
Terlingua Creek graben.
Wildhorse - Hen Egg fault zone
Christmas Mountains fault zone
Implications of Dike Trends for Paleostress Orientations
Appendix: Chemical Analyses of Igneous Rocks, Christmas Mountains and Hen Egg Mountain Quadrangles
1. Location of the Hen Egg Mountain and Christmas Mountains quadrangles and other selected features of Trans-Pecos Texas and northern Mexico
2. Pre-mid-Tertiary tectonic elements of Trans-Pecos Texas and adjacent Mexico
3. Approximate distribution of Tertiary volcanic rocks of Trans-Pecos Texas and adjacent regions of Mexico, New Mexico, and Arizona
4. Published geologic maps that depict areas in and adjacent to the Hen Egg Mountain and Christmas Mountains quadrangles
5. Stratigraphy and thickness of Cretaceous rocks in the Christmas Mountains area
6. Photographs of Cretaceous rocks
7. Stratigraphy of volcanic and volcaniclastic rocks in and adjacent to the Hen Egg Mountain and Christmas Mountains quadrangles
8. Photographs of Tertiary rocks
9. Typical formation of Quaternary colluvium
10. Distribution of major mapped intrusive rock types
11. Photomicrographs of intrusive rocks
12. Flow-banded rhyolite in sill 1 km northwest of Adobe Walls dome, Hen Egg Mountain quadrangle
13. View to south of dikes of lower Miocene hawaiite in southwest corner of Hen Egg Mountain quadrangle
14. Igneous rocks in the Hen Egg Mountain and Christmas Mountains quadrangles in the chemical classification of alkalic rocks devised by Barker (1979)
15. Histogram of silica content of all samples listed in the appendix
16. Silicic igneous rocks of the Hen Egg Mountain and Christmas Mountains quadrangles
17. Plot of alkalinity, molar (Na + K)/A1, versus differentiation index
18. AFM diagram for nepheline-normative rocks 19. Plots of MgO and TiO, versus differentiation index
20. Alkali-silica and lime--s ilica plots of rocks from the Hen Egg Mountain and Christmas Mountains quadrangles
21. Index map of calderas in the Christmas Mountains caldera complex
22. Stratigraphy of initial tuffs and lavas west and south of the Christmas Mountains
23. General distribution of volcanic rocks of sequence I in the northwestern part of the Tule Mountain quadrangle.
24. Photographs of rocks of sequences I, II, and III
25. Stratigraphy of sequence II
26. Stratigraphy and thicknesses of caldera fill in the main western and eastern calderas
27. Photographs of caldera-fill rocks of sequence IV
28. Photographs of quartz trachyte intrusions of sequence V
29. Photographs illustrating structure of Christmas Mountains calderas
30. Relative enrichment of various trace elements in fluorspar associated with igneous rocks of Trans-Pecos Texas and adjacent New Mexico
31. Photographs of intrusive structures
32. Photographs of Basin and Range fault structures
33. Generalized distribution and orientation of dikes in the Christmas Mountains area
34. Histogram of K-Ar ages of igneous rocks of the Christmas Mountains area
35. Diagram showing evolution of the Christmas Mountains area
1. Magmatic-tectonic phases in Trans-Pecos Texas
2. K-Ar isotopic data,Christmas Mountains rocks
3. General characteristics of intrusive rock types
4. General stratigraphy of the Christmas Mountains caldera complex
5. Chemical analyses of Christmas Mountains area fluorspar
6. Chemical analyses of samples from mercury prospects on the south side of the Christmas Mountains
Plate (in inside back pocket of book)
Geologic map of the Hen Egg Mountain and Christmas Mountains quadrangles
Henry, C. D., Price, J. G., and Miser, D. E., 1989, Geology and Tertiary Igneous Activity of the Hen Egg Mountain and Christmas Mountains Quadrangles, Big Bend Region, Trans-Pecos Texas: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 183, 105 p.