PETROLOGICAL CHARACTERISTICS OF THE PUNTA MITA BASALTS, NAYARIT, MEXICO

The Punta Mita peninsula (Nayarit) is located in the NW border of the Jalisco Block, in the western limit of the Transmexican Volcanic Belt. Within the Punta Mita Peninsula, diverse outcrops of volcanic rocks occur that include rhyolite and basalt. The rhyolitic rocks outcrop in the northwestern area of Punta Mita, and their age has been determined to be 11.1 Ma. The basaltic rocks are present at several locations: a) Punta Negra, and b) to the southeast of the peninsula, between Punta Pantoque and Punta Las Cargadas. The Punta Negra basaltic rocks have an age of 10.2 Ma, and their chemical characteristics indicate a calc-alkaline to transitional affinity.

The basaltic rocks from outcrops located to the southeast have not been studied. This work includes the mineralogical and chemical study of the southeast Punta Mita basalts. These rocks are vesicular lava flows that outcrop on the shore and the sea bottom, and are interbedded with pyroclastic flows. The pyroclastic deposits contain vesicular basaltic and sandstone clasts. Pillow lavas can be identified in some outcrops on the shore. The basalts present a vesicular aphanitic texture. The phenocrystals are plagioclase (andesine-labradorite) and clinopyroxene (Ca-rich augite).

All the outcrops present an intensity of alteration from 50% to 60% that affects mostly the matrix and has prevented accurate dating of these rocks. The alteration is mainly composed of celadonite, pyrite, carbonates and iron oxides and hydroxides. The vugs are filled with zeolites (heulandite and analcime), barite and calcite.

The chemical characteristics of the basalts indicate that they can be classified as intra-plate basalts according to their trace element contents. They are possibly associated to the recent extensional tectonic activity in the western part of Mexico, and can be related with the basalts that outcrop at the north of the volcanic district of Nayarit that were dated to be 1Ma old. Therefore, volcanism, represented by the Punta Mita southeast basalts, may be one the most recent volcanic activity in the westernmost section of the Jalisco Block.