Resumo

ARANDA-GOMEZ, José Jorge et al. Late Cenozoic intraplate-type volcanism in central and northern Mexico: a revision. Bol. Soc. Geol. Mex [online]. 2005, vol.57, n.3, pp.187-225. ISSN 1405-3322.  https://doi.org/10.18268/bsgm2005v57n3a1.

Intraplate-type volcanism (late Oligocene-Quaternary) occurs in Mexico in regions that belong to the North American and Pacific plates. Magmas erupted through oceanic crust formed voluminous shield volcanoes (Socorro: ~2400 km3) located on or near fossil spreading ridges. Rocks in the subaerial part of the volcanoes form continuous and coherent rock series (Guadalupe) or bimodal sets (Socorro). Crystall fractionation of alkali basalt in shallow magma chambers and assimilation of hydrothermally altered rocks determined the elemental and isotopic composition of the mafic and intermediate rocks of Socorro. Trachytes from Socorro were originated by 5-10% partial melting of alkali basalt. Crystal fractionation of a parental trachytic magma originated the rhyolites. Felsic rocks also show the evidence of assimilation of hydrothermally altered rocks.

Continental mafic intraplate-type volcanism (late Oligocene - Quaternary) is scattered throughout the region located north of the Mexican Volcanic Belt. This phenomenon is independent of older volcanic provinces, boundaries between today s geologic/tectonic provinces, and limits among tectonostratigraphic terranes. Most intraplate-type volcanic rocks are alkalic [i. e. theyplot above the FeOt/MgO=(0.156 x SiO2) - 6.69 line in the SiO2 vs. FeOt/MgO diagram of Miyashiro (1974) andhave high TiO2, Nb, and Ta] andhostperidotite xenoliths from the upper mantle, and/or megacrysts, and/or feldspathic granulite xenoliths from deepportions of the crust. Most volcanicfields with mantle xenolith are in the southern part of the Basin and Range Province, but there are localities in regions where normal faulting is minor or absent in the geology exposed at the surface. Some of the most extensive and voluminous intraplate-type volcanic fields in northern México were contemporaneous with normal faulting. In some areas volcanoes are aligned along regional normal faults and/or their products are interlayered with graben-fill gravel deposits. However, in most areas the tie between extension and volcanism is not evident. The locations of some volcanic fields suggest that magma ascent was influenced by regional faults. These basement structures have complex evolutions and they separate large tectonic domains.

Dominant petrogenetic processes in magmas associated to the first pulses of extension (early Oligocene - Miocene), those immediately after the change from ENE compression to ENE extension, are different from those manifested in Plio-Quaternary magmas. Volcanic fields associated with the earlier extension may contain primitive rocks, similar to those erupted by Plio-Quaternary volcanoes. However, older rocks are commonly more evolved and show clear geochemical evidence for assimilation of crustal material, whereas contamination is subtle or absent inyounger intraplate-type suites. This is attributed to a gradual change in tectonic and thermal conditions in the crust in the region. During the early stages of extension (≤33-30 Ma) the crust was hot as a consequence of the subduction-related mid-Tertiary ignimbrite flare up. This thermal condition raised the brittle-ductile transition and brittle structures (i. e., normal faults) were not able to propagate to deeper parts of the crust, preventing the formation of conduits for rapid magma ascent. The result-ing slow magma ascent favored AFC processes and gravitational settling of dense xenoliths. As the crust cooled down, brittle structures caused by ENE to NE extension were able to propagate deeper and xenolith-bearing magmas reached the surface.

Nephelinites exposed in the southern part of San Luis Potosí are the most silica-undersaturated intraplate-type rocks in Mexico. Assuming a similar garnet-lherzolite mantle source, the difference between the nephelinites and hawaiites found a short distance north has been attributed to a larger degree of partial melting in the hawaiites. The most evolved samples in the studied area are pantellerites and comendites from Sonora, as well as trachytes from isolated occurrencees in Sonora, Chihuahua and Tamaulipas. Unfortunately, little is known about them. It is worth mentioning that highly evolved rocks are relatively common in the Eastern/Alkalic region. Compared to central and northwestern México, evidence of synvolcanic extension in this region is scarce or absent.

The chemical composition of some of the primitive rocks from San Quintín, B. C., suggests progressive fusion at rela-tively low pressures in the mantle (i. e., from spinelperidotite). Compared with samples from central México, the xenoliths from San Quintín yield significantly lower temperatures and pressures.

Hypersthene-bearing, unaltered mafic rocks occur in many intraplate type volcanic fields and apparently were coeval with mafic alkalic volcanic products. This phenomenon has not been analyzed and only for hyperstene-normative mafic rocks of Durango, it has been argued that they inherited some components from the mid Tertiary, subduction-related activity. This mantle component may be occasionally melted and contribute to the intraplate type-magmas.

The temporal change in mantle sources, from shallow lithospheric mantle to deeper asthenospheric mantle, proposed for the United States portion of the Basin and Range has not been demonstrated in central México. It seems that the AFC processes in the early magmas of México mask some of the geochemical characteristics inherited from the mantle sources.

Palavras-chave : Alkalic rocks; extension; Basin and Range; xenolith.