The Van Horn Mountains Caldera, Trans-Pecos Texas: Geology and Development of a Small...Ash-Flow Caldera

RI0151. The Van Horn Mountains Caldera, Trans-Pecos Texas: Geology and Development of a Small (10-km²) Ash-Flow Caldera, by C. D. Henry and J. G. Price. 46 p., 31 figs., 3 tables, 1 plate, 1 appendix, 1986. ISSN: 0082335X: Print.


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ABSTRACT
The Van Horn Mountains caldera is a small (~10-km²) igneous center in the Trans-Pecos volcanic province. The caldera formed 37 to 38 mya during eruption of the first of two ash-flow tuffs related to the caldera. Part of the first tuff ponded within the caldera; the lower marker horizon of the Chambers Tuff in the Sierra Vieja south of the Van Horn Mountains is probably the correlative outflow tuff. Following collapse, the caldera was partly filled by a heterogeneous assemblage of air-fall tuff, tuffaceous sediment, and possible ash-flow tuff. Contemporaneous with this filling, a rhyolite porphyry was emplaced in the middle of the caldera. The porphyry is comparable to the resurgent dome of other calderas but does not uplift the intruded rocks. An intrusive-extrusive complex of basalt, trachyte, and minor rhyolite was emplaced just outside the eastern margin of the caldera; flows from this complex spilled over the rim and into the caldera. Eruption of a second major ash-flow tuff, herein named the High Lonesome Tuff, may have led to additional collapse.

Collapse occurred dominantly along a single, major, nearly circular fracture zone. Embayments in the fracture zone were influenced by precaldera, largely Laramide structures. Large, commonly brecciated blocks of wall rocks slumped into the caldera and were incorporated into caldera fill. Precaldera topography channeled most of the ash-flow tuff to the south, through a low area in the caldera wall.

The 10-km² area of the caldera and the no more than 10-km total volume of erupted ash-flow tuff make it one of the smallest ash-flow calderas worldwide and more comparable to collapse features associated with stratovolcanoes. Nevertheless, it shows all the features typical of much larger volcanic centers.

No known ore deposits are associated with the caldera, but the rhyolite porphyry is hydrothermally altered, exhibiting silicification and sericitization typical of many porphyry molybdenum deposits. The apparent lack of deposits may be a function of the small size of the magmatic system, lack of sufficient trace element enrichment by differentiation, or lack of exposure resulting from minimal erosion.

Keywords: ash-flow tuff, caldera, igneous rocks, K-Ar, Trans-Pecos Texas

CONTENTS

ABSTRACT

INTRODUCTION

REGIONAL SETTING

PURPOSE

STRATIGRAPHY

PRECAMBRIAN ROCKS

PERMIAN ROCKS

CRETACEOUS ROCKS

TERTIARY ROCKS

Colmena Formation

Buckshot lgnimbrite

Rocks of the Van Horn Mountains caldera

Garden of the Gods intrusion

Caldera-forming ash-'flow tuff

Lower marker horizon of the Chambers Tuff.

Rhyolite porphyry intrusion

Tuff-breccia

Hogeye Tuff

Intrusive-extrusive complex

Field relations

Petrography

High Lonesome Tuff

Tuffaceous sediments above the High Lonesome Tuff

Trachyte of High Lonesome Peak

Miocene to Recent basin fill

QUATERNARY DEPOSITS WITHIN THE VAN HORN MOUNTAINS CALDERA

GEOCHEMISTRY

STRUCTURE

PRECAMBRIAN DEFORMATION

LATE PENNSYLVANIAN - EARLY PERMIAN UPLIFT

LARAMIDE FOLDING AND THRUSTING

CALDERA STRUCTURE

BASIN AND RANGE DEFORMATION.

ECONOMIC GEOLOGY

HYDROTHERMAL ALTERATION ASSOCIATED WITH THE RHYOLITE PORPHYRY OF

THE VAN HORN MOUNTAINS CALDERA

MICA

BUILDING STONE AND CRUSHED ROCK

MANGANESE AND BARITE

SILVER AND OTHER METALS

LACK OF MAJOR MINERAL DEPOSITS ASSOCIATED WITH THE VAN HORN MOUNTAINS CALDERA

GEOLOGIC HISTORY OF TERTIARY VOLCANISM

CALDERA SIZE AND ASH-FLOW TUFF VOLUMES: IMPLICATIONS FOR ERUPTION AND SUBSIDENCE

COMPARISON WITH PUBLISHED CALDERA MODELS

ACKNOWLEDGMENTS

REFERENCES

APPENDIX

Figures

1. Location of the Van Horn Mountains caldera in the volcanic field of Trans-Pecos Texas

2. Generalized geologic mop of the Van Horn Mountains caldera

3. Pre-Cenozoic stratigraphic column, Van HornMountains, Texas

4. Correlation of regional volcanic units, Van Horn Mountains and northern Sierra Vieja

5. Buckshot lgnimbrite overlying conglomerate in Colmeno Formation

6. Blister cone on upper surface of the Buckshot lgnimbrite

7. Intrusive contact of Gorden of the Gods rhyolite with the Buckshot lgnimbrite

8. Photomicrograph of quartz phenocrysts in groundmass of quartz, alkali feldspar, and opaque minerals, Garden of the Gods rhyolite

9a. Outcrop of caldera-forming ash-flow tuff showing abundant small rock fragments in a gray, devitrified but nonwelded matrix

9b. Photomicrograph of caldera-forming ash-flow tuff

10a. Possible flow of rhyolite porphyry extending as a flat lobe west of the main moss of the intrusion

10b. Photomicrograph of rhyolite porphyry with quartz and altered alkali feldspar phenocrysts

11. Outcrop of rhyolite porphyry overlain by tuff-breccia

12. Dike of flow-bonded rhyolite porphyry intruded into tuff·breccia

13. Tuff-breccia consisting of rhyolite porphyry, pumice, and pre-Tertiary rock fragments in a tuffaceous matrix

14a. Crudely bedded tuff-breccia

14b. Channel filled with water-laid tuff

15. Extremely coarse tuff-breccia with randomly oriented blocks of Cretaceous rocks

16. Internal brecciation of a 50-m block of Cox Sandstone within tuff-breccia

17. Volcanic stratigraphy within and near the Van Horn Mountains caldera

18. Fluvial tuffaceous sediments in the Hogeye Tuff

19a. Hill of intrusive trachyte cut into gently dipping Cretaceous rocks

19b. Concentric, vertical bonds of resistant trachyte and nonresistant tuff within the intrusive-extrusive complex

20. Photomicrograph of basalt with olivine and clinopyroxene phenocrysts

21. Photomicrograph of trachyte with clinopyroxene and plagioclase phenocrysts

22. Thick High Lonesome Tuff overlying the Hogeye Tuff

23. Known distribution of High Lonesome Tuff

24. Photomicrograph of High Lonesome Tuff vitrophyre with phenocrysts of alkali feldspar, clinopyroxene, and various opaque minerals

25. AFM plot of rocks from the Van Horn Mountains caldera

26. Harker variation diagram of total alkalis and CaO plotted against SiO₂

27. Harker variation diagram of TiO₂ plotted against SiO₂

28. Harker variation diagram of Sr plotted against SiO₂

29. Western topographic wall of caldera

30. Large slump block of Cretaceous rocks along western caldera wall

31. Paleotopography of the Van Horn Mountains caldera area at the time of caldera formation

Tables

1. Chemical analyses of rocks from the Van Horn Mountains caldera

2. Chemical analyses of rhyolite porphyry with stockwork quartz veinlets, Van Horn Mountains

3. K-Ar ages of rocks of the Van Horn Mountains caldera

Plate

Geologic map of Van Horn Mountains caldera



Citation
Henry, C. D., and Price, J. G., 1986, The Van Horn Mountains Caldera, Trans-Pecos Texas: Geology and Development of a Small (10-km²) Ash-Flow Caldera: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 151, 46 p.