In southern Mexico, discrete ultramafic intrusive bodies larger than 4 km2 are genetically related to their enclosing volcano-sedimentary terranes. These terranes are the Cuicateco and Guerrero, which include the Cuicateco and Tierra Caliente metamorphic complexes, respectively. Their basement is largely unknown, and the ultramafic masses previously have been interpreted as allochthonous dismembered ophiolites. To constrain the age of these accreted terranes, the geologic setting and 40Ar/39Ar ages are presented from the localities of Loma Baya – El Tamarindo, Guerrero, and San Pedro Limón, State of México, in the Tierra Caliente Complex, and Teotitlán – Conceptión Pápalo, Oaxaca, in the Cuicateco Complex. The Teotitlán – Conceptión Pápalo area is characterized by a sequence of metamorphosed intermediate granitoid rocks, andesitic lavas, tuffs, and psammites. The intrusive rocks and lavas are pervasively mylonitized and show abundant ultramafic segregations. The largest ultramafic bodies were emplaced into metatuffs, following the regional east-verging structural trend. Three hornblende separates from a hornblende diorite and one from a hornblende-rich clinopyroxenite yielded plateau ages of ca. 130 Ma. The ultramafic rocks in the Tierra Caliente Complex lack orthopyroxene and transitionally grade outward into hornblende diorites. The El Tamarindo ultramafic body (4.6 km2) rose as a diapir into an Albian volcano-sedimentary sequence. The best estimate for the age of this body is ca. 112 Ma from a hornblende separated from a pegmatite dike sample. This age is similar to the Albian fauna reported at Zihuatanejo and other K–Ar ages from ultramafic rocks at Camalotitos. The rigid and poorly serpentinized San Pedro Limon Stock (15 km2) was vertically emplaced into the Xochipala Formation (Cenomanian) during a regional Neogene transpressive disturbance. Four hornblende samples from hornblende clinopyroxenites and hornblendites yielded remarkably flat age spectra, indicating an age of ca. 105 Ma for the San Pedro Limon Stock. Based on the good correlation between the 40Ar/39Ar ages reported here and earlier faunistic data, it is concluded that the ultramafic rocks cooled penecontemporaneously with the deposition of the enclosing volcano-sedimentary sequences.

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