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The Cerro Mencenares volcanic center, Baja California Sur: Source and tectonic control on postsubduction magmatism within the Gulf Rift

¹ Università degli Studi di Milano, Dipartimento di Scienze della Terra, via Mangiagalli 34, 20133 Milano, Italy
² C.N.R., Centro di Studio per la Geodinamica Alpina e Quaternaria, via Mangiagalli 34, 20133 Milano, Italy

At the end of active subduction along the North American plate (12.5 Ma), in the Baja California peninsula arc magmatism was replaced mainly by alkali-rich high-Mg basaltic andesites and andesites with peculiar and unusually high K/Rb, Sr/Rb, and LREE/HREE ratios defined "bajaite" by Rogers et al. (1985) and "Baja alkalic suite" by Sawlan (1991). Two interesting features of this magmatic change are represented by (i) emplacement of transitional, high-K calc-alkaline andesites that preceded, in places, the Baja alkalic suite magmatism and (ii) subduction unrelated medium-K calc-alkaline magmatism in Baja California Sur, active in areas within the Gulf of California Rift, for example, Tres Virgenes–Reforma caldera, Isla Coronado. A previously unknown example of this calc-alkaline activity was studied in the area of Cerro Mencenares located north of Loreto, Baja California Sur. The Mencenares volcanic complex is located along the Gulf coast, east of the main Gulf escarpment. Activity of the Mencenares volcanic complex occurred in three stages: (i) an initial phase of rhyolitic dome emplacement; (ii) andesitic to dacitic volcanism that produced the Mencenares stratovolcano; (iii) late-stage emplacement of silicic domes and flows, mostly dacite and rhyolite, along north-south–trending normal faults. Tephra and lava flows interfinger with marine sediments of the Pliocene Loreto Basin and the youngest volcanic products were probably emplaced during Quaternary times. This volcanism is coeval with the Baja alkalic suite magmatism west of the Gulf escarpment. In the latter areas, Baja alkalic suite magmas ascended rapidly with only limited differentiation. In the Mencenares volcanic complex, a complex tectonic framework favored the formation of crustal magma chambers where more complex phenomena such as extensive differentiation and mixing could take place. A change from medium-K calc-alkaline toward Baja alkalic suite–like chemical characteristics is present in the younger products of the Mencenares volcanic complex and mixing possibly occurred between medium-K calc-alkaline and Baja alkalic suite magmas. Magma evolution in the Loreto area suggests that (i) Baja alkalic suite and medium-K calc-alkaline magmas can coexist in time and interact in the same volcanic field; (ii) if a relic, subduction-related component is the main cause of medium-K or high-K calc-alkaline volcanism then this component appears to decrease from east to west; and (iii) although prior calc-alkaline magmatism may not be unquestionably considered a necessary condition for the generation of Baja alkalic suite magmas, episodes of medium-K or high-K calc-alkaline melt extraction from mantle sources precede or accompany Baja alkalic suite magmatism in many instances.

This article has been cited by other articles:

				P. J. Umhoefer, L. Mayer, and R. J. Dorsey Evolution of the margin of the Gulf of California near Loreto, Baja California Peninsula, Mexico Geological Society of America Bulletin, July 1, 2002; 114(7): 849 - 868. [Abstract] [Full Text] [PDF]

				R. J. Dorsey and P. J. Umhoefer Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico Geological Society of America Bulletin, February 1, 2000; 112(2): 177 - 199. [Abstract] [Full Text] [PDF]