New Ideas on the Regional Trends and Implications of Middle to Late Tertiary Mantle Magmatism in the Four Corners Region, USA
Abstract
Widespread emplacement of alkaline to subalkaline mantle magmas in the Four Corners region in the middle to late Tertiary gave rise to mafic to ultramafic dikes and diatremes that are exposed along the northern and western edges of the San Juan Basin. The close temporal and spatial relationship of this pulse of mafic magmatism with larger-scale events in the adjacent San Juan volcanic field and Rio Grande Rift hints at a regional connection between all of these magmatic events. This presentation highlights the outcomes of research done over the past decade that give new insight into the history of mantle magmatism in the Four Corners area, and provides a greater understanding of the regional magmatic record. Potassic mafic rocks of the Navajo volcanic field (NVF) were mostly emplaced between 29 and 23 Ma. New Ar-Ar age analyses yield ages from 24 to 20 Ma for mafic rocks exposed in diatreme-dike complexes and dike swarms in southwestern Colorado. A later pulse of potassic magmatism is also recorded by 8 Ma to 5 Ma mafic dikes exposed further north of the NVF. Taken together, existing and new ages constrain a longer-lived period of mafic magmatism, similar in extent to what is documented in the Rio Grande Rift. Past and recent studies establish that Tertiary ultramafic to mafic rocks exposed on the margins of the San Juan Basin range from alkaline-ultrapotassic to mildly alkaline or subalkaline. Chemical signatures and Cl-isotope data from Oligocene to Miocene mafic rocks in the region indicate that magmatism was dominated by partial melting of metasomatized lithospheric mantle with contributions from deeper mantle sources that had signatures akin to oceanic-island basalts. Mineral and bulk-rock chemical data indicate that potassic to ultrapotassic mafic to ultramafic rocks in the region have high F/Cl ratios. The ubiquitous presence of late-stage carbonate in these rocks also suggests that magmas were CO2 rich. Phlogopite in rock samples from different suites exhibit complex trends in F and Cl versus major element oxides. These data record multiple stages of crystallization under variable magmatic conditions during the rise and emplacement of melts. New stable C and O isotope data are consistent with magmatic gases being a dominant and driving force of diatreme formation. This contrasts with previous ideas that have focused on external sources of volatiles (e.g., groundwater) to generate the "fuel" to cause these eruptions. The trends of Tertiary mafic dike swarms in the region fall mostly between 350° and 45° with subordinate sets of dikes having roughly east-west trends. In most locations the dikes either cut or were emplaced into pre-existing faults and regional joint sets, but in some areas the dikes are displaced by conjugate sets of normal faults. Dike trends and fracture patterns are consistent with the onset of incipient NW-SE to E-W regional crustal extension, possibly by reactivation of pre-existing crustal-scale fractures in response to compressive stresses. This provided avenues for the invasion of mantle magmas that could have triggered crustal melting and extensive volcanism in the adjacent San Juan Mountains.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.V31F2586G
- Keywords:
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- 1038 GEOCHEMISTRY / Mantle processes;
- 3641 MINERALOGY AND PETROLOGY / Extrusive structures and rocks;
- 8499 VOLCANOLOGY / General or miscellaneous;
- 9604 INFORMATION RELATED TO GEOLOGIC TIME / Cenozoic