Xenoliths From Isla Isabel, Nayarit, Mexico: The Nature of the Upper Mantle Underneath the Western Part of the Mexican Volcanic Belt

Isla Isabel is located ~65 km NW of San Blas (Nayarit), off the Pacific coast of central Mexico. The island is a Quaternary (Ar/Ar < 0.7 Ma) volcanic complex built atop attenuated continental crust. Isabel lies on the east side of the mouth of the Gulf of California, near the area previously occupied (early Pliocene) by Los Cabos Block. Southeast of Isabel, on the mainland, is the NW-trending Tepic-Zacoalco rift, a major volcano-tectonic structure in the western part of the Mexican Volcanic Belt. On land, the rift is the boundary between the Jalisco and Sierra Madre Occidental blocks, and Isabel lies along its projection. Immediately S of Isabel is the San Blas Trough, a swale that trends NW-SE, co-linear with a gravity lineation parallel to the Tamayo and San Blas fault zones, which are the transform boundaries between the northern Rivera and North American plates. Plio-Quaternary alkaline and calc-alkaline lavas have erupted contemporaneously in the Tepic-Zacoalco rift, but so far no mantle xenoliths have been reported in them. Isabel's rocks are intra-plate type alkaline basalts to trachybasalts, with 5-6%\ normative Ne. Primary paragenesis in the lavas is: Ol + Pl + Cpx + TMt. Small (< 5 cm) peridotite xenoliths, and xenocrysts derived from them, are ubiquitous in the rocks. Eleven xenoliths were studied comprising 3 dunites, 7 harzburgites (one Pl-bearing), and 1 gabbro. Compared to other Mexican xenolith localities N of the MVB, they are refractory as they are depleted in, or lack, Cpx. Ol crystals in xenoliths are homogenous and their Mg#\ s range as follows: peridotites (92-88), gabbro (84), and Pl-harzburgite (80). Cores of primary Ol phenocrysts (90.5-86.5) in Isabel's volcanic rocks are significantly higher in Mg#\ s than in Ol from other Mexican xenolith localities (max. 86) and overlap with the associated peridotite xenoliths. Such overlap has not been reported for other Mexican xenolith localities. Xenolith equilibration temperatures for 5 peridotites were calculated with the TBKN model. Assuming P=15 kb, temperatures are 1021-1112°C. Pressure estimates, based on Ca-exchange model between co-existing Ol and Cpx, assuming the TBKN calculated at 15 kb, range 4.2-14.8 kb, which lies at the shallow range of pressures for other Mexican xenoliths. Only two samples from the coastal locality of San Quintin match the lowest pressures obtained for Isabel. Although the 5 Isabel's xenoliths are Sp- peridotites, one Pl-harzburgite is present in the suite. The only other Mexican Pl-bearing peridotite xenolith reported is from Pinacate, which lies a short distance from the northern end of the Gulf of California. The unusual nature of the mantle beneath Isla Isabel is attributed to the combination of volcano-tectonic features in the area: a protracted history of subduction-related volcanism associated first to the Sierra Madre Occidental and later to the MVB, attenuation of the continental crust related to the opening of the Gulf of California, and inception of sea floor spreading in the nearby Alarcon Basin.