New Mexico Geological Society Annual Spring Meeting — Abstracts

We will explore in this presentation some observations pertaining to the genesis of the two different suites of volcanic rocks which have been erupted in recent times over large areas of western North America, including New Mexico, in response to tensional, block-faulted tectonic regimes. The two types are tholeiitic (or hypersthene normative) and alkaline (or nepheline normative); we will lock first at the spatial distribution of the two and then at their major- and trace-element characteristics.

The majority of tholeiitic examples are found in the old, interior or cratonic portions of North America, as shown in Figure 1 of Livaccari and Perry (1993). Examples of this type of field in New Mexico are the Taos Plateau volcanic field and much of the volcanism which fringes the Colorado Plateau. The preponderance of magmatism to the south and west of this interior portion is alkaline. Excellent examples of this type are Geronimo, Potrillo, Cima, San Quintin (in Baja California), and the Pinacate volcanic fields. There are exceptions to this generalization. The alkaline lavas of "cratonic" North America, such as Hopi Buttes and the volcanism in northeastern New Mexico tend, however, to be highly silica-undersaturated and thus are distinct from the main lineage of alkaline lavas to the south and west; the examples of tholeiitic lavas in this latter environment are few and far between. There is, for example, one tholeiitic flow reported for the entire Pinacate field, one for the San Quintin field, and none in the Potrillo or Geronimo fields. Fields that lie within the interior region, whether they are
tholeiitic or alkaline, have non-asthenospheric isotopic (Sr, Nd, and He) character, whereas, alkaline examples in the second environment are uniformly asthenospheric in character. We know of no isotopic data for tholeiitic lavas from this environment.

The two types of lavas lie on opposite sides of the low-pressure plane of critical saturation. Very few have Mg numbers appropriate to primary magmas, and in the Potrillo volcanic we have documented evidence of low-pressure fractionation. Thus, initial differences in magma composition between the two types may have been accentuated by crystallization towards the two separate eutectic compositions rather than having been fundamentally different as primary magmas. Alkaline lavas are systematically enriched in incompatible elements (e.g. Zr, Nb, LREE, Rb, and Sr) relative to tholeiitic ones. The correlation between silica undersaturation and incompatible element enrichment is consistent with degree of partial melting exerting a fundamental control on geochemistry. The two groups converge on similar heavy rare-earth element concentrations, implying that they are generated from similar source materials and not, for instance, from the garnet and spiI)el peridotite fields respectively. Thus, the distribution of magma types may have more to do with state of stress in the lithosphere and steepened geotherms from upwelling asthenosphere, which may allow small degrees of partial melt to reach the surface without modification, than to derivation from different depths or different types of material within the mantle.