Jagoutz et al. (2007) argue for a long history of alkaline to mid-oceanic ridge basalt (MORB)–type magmatism occurring during the transition from amagmatic to magmatic rifting at the Iberia-Newfoundland conjugate margin. U-Pb zircon ages suggest that magmatism spanned the range of 110–127 Ma. Ar-Ar ages suggest a similar range for magmatism, along with a complex cooling and alteration history. The general premise put forward by Jagoutz and others, that magmatism at the Iberian margin began slowly during asthenospheric upwelling and progressed, probably with fits and starts, over a period of 15 m.y. or so to a full-blown mid-ocean ridge, is widely accepted (Schärer et al., 2000; Beard et al., 2002; Chazot et al., 2005).

This Comment arises from a disagreement with the authors concerning their petrologic and geologic interpretation of the albitites and kaersutite pegmatites at Ocean Drilling Program (ODP) site 1070 (ODP leg 173; Iberia Abyssal Plain) and the detailed regional models that follow from this interpretation. Key to the authors' arguments regarding an alkaline to enriched MORB (EMORB) transition at the Iberia margin is the interpretation of the albitites from site 1070 (zircon age 127 ± 4 Ma [U-Pb, SIMS]) as a suite of magmas different from and unrelated to the physically proximal, chemically similar and, within error, age-equivalent kaersutite pegmatites (hornblende Ar-Ar ages 124 ± 0.7 Ma).

First, there are problems with the suggested distinction between EMORB and alkaline magmatism at site 1070. Certainly, there is no chemical or petrographic evidence that the site 1070 albitites are more “alkaline ” than the pegmatites or, for that matter, alkaline at all. Jagoutz et al. base this distinction on mineralogical characteristics—e.g., the presence of sodic feldspar, biotite, zircon, and other trace phases in alkaline rocks versus pyroxenes, andesine, ilmenite, and kaersutite in MORB-derived rocks—that are easily attributed to fractionation.

There is no petrological or geological reason to doubt a genetic relationship between the site 1070 pegmatites and albitites. Albitite and kaersutite pegmatite occur together not only in clasts in the overlying breccia but also as intrusions in basement at site 1070 (Whitmarsh et al., 1998; Beard et al., 2002). Both the sodic character of the plagioclase and the high Zr content of the albitite are consistent with derivation from the Na- and Zr-rich pegmatites. The plagioclase in the pegmatites is zoned andesine (An37–51), which shows consistent Na enrichment toward the rims of igneous plagioclase (Beard et al., 2002). Na enrichment in interstitial melts in the pegmatites is also recorded by zoning in igneous amphiboles (Beard et al., 2002).The high Zr content of the pegmatite is manifested by Zr concentrations of 100 ppm in the amphiboles and nearly 200 ppm in the ilmenite, some grains of which contain exsolution lamellae of baddeleyite (Beard et al., 2002). Finally, while Jagoutz et al. emphasize the biotite content of the albitite, biotite is less abundant than amphibole in these rocks. Although no pristine igneous amphibole (or biotite) was found in the albitites, analysis of metamorphosed amphiboles shows very high TiO2 content (e.g., 1.5%) that clearly suggests derivation from a high-Ti (e.g., kaersutite) precursor.

The available data favor a genetic relationship between the pegmatite and albitite, over the correlation postulated by Jagoutz et al.—i.e., that the albitite is related to a biotite gabbro that was drilled 5000 km away in Newfoundland and, even at the time of formation, was likely hundreds of kilometers away (Jagoutz et al.). This is particularly true since (given its stated mineralogy; no chemical or detailed mineralogical data are provided) the Newfoundland gabbro is evidently a potassic rock, while the site 1070 albitite and pegmatite (like many Iberia margin mafic rocks; e.g., Chazot et al., 2005) are strongly sodic.

With respect to the age of magmatism, the Ar-Ar ages on kaersutite must be interpreted cautiously. The assumption that the 600 °C closure for amphibole approximates an igneous age may not be justified. This is not a normal oceanic environment. Beard et al. (2002) argue that the ambient temperature at the time of pegmatite emplacement may have exceeded 1000 °C. At the very least, the amount of time separating pegmatite emplacement and exhumation at site 1070 is poorly constrained. In the final analysis, however, this is moot: the zircon age of the albitite, and the hornblende cooling age of the pegmatite lie within analytical error of one another. Given that the cooling age is at least nominally postmagmatic, arguments that the two ages represent a real difference in age of magmatism are open to question.

Jagoutz et al. agree with previous studies that concluded that the onset of melting at the Iberia margin is related to the increasing thermal influence of hot ascending asthenospheric mantle, which culminates (eventually and outboard) in the formation of a full-blown spreading center. However, the suggested link between denudation faulting, variation in depth of melting, and magmatic variability between 128 and 124 Ma (as depicted in their Fig. 3), while technically possible, is speculative and does not appear to be supported by the available evidence from the Iberian margin.

At the latitude of interest, there is little hard evidence for any magmatism at either Iberia or Newfoundland between emplacement of the Iberian pegmatite/albitite suite at ca. 127 Ma and the injection of a gabbroic dike at the Newfoundland margin at ca. 113 Ma (see Schärer et al. [2000] for a discussion of the variation in age of magmatism during south-to-north propagation of the rift). In the absence of a credible distinction between the site 1070 albitites and pegmatites, the argument for chemically variable magmatism related to the details of denudation, uplift, and asthenospheric upwelling prior to the inception of MOR magmatism is severely compromised. There is little doubt that igneous rocks were emplaced at the Iberia-Newfoundland margin during this interval. Until these rocks are sampled, however, the nature of geochemical variability prior to the inception of MOR magmatism must remain an open question.

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