Late Cretaceous-Oligocene magmatic record in southern Mexico: The case for a temporal slab window along the evolving Caribbean-North America-Farallon triple boundary

Continental magmatism in southern Mexico is expected to record the eastward displacement of the Farallon‐North America‐Caribbean triple junction. However, a trench‐transversal belt of magmatism in the central western Guerrero State does not fit into a regular pattern of arc migration and reorientation following the formation of the WSW trending Acapulco trench in the Cenozoic. We revised the magmatic pattern of southern Mexico using an updated database and new laser ablation inductively coupled plasma–mass spectrometry sensitive high‐resolution ion microprobe, Ar‐Ar ages, and geochemical and geologic data for the coastal part of the anomalous Guerrero belt. Our data reveal a persistent magmatic activity between ~75 and 35 Ma, with a changing character at the Paleocene‐Eocene boundary (circa 56 Ma). Late Cretaceous‐Paleocene granitoids have an adakitic signature imprinted by stable garnet in the source and show no plagioclase fractionation, indicative of wet and oxidized magmas. Eocene rocks consist of an almost bimodal suite of plutonic bodies covered by a succession of mafic lavas. Granitic plutons show plagioclase fractionation and flat middle rare earth element‐heavy rare earth element; gabbros have a tholeiitic character, indicative of dryer and more reduced magmas. They appear later and were emplaced at shallow depth in an extensional sedimentary basin. We interpret the magmatic record of the Guerrero belt as the response to two concurrent processes: (1) a temporal window in the Farallon slab induced by the concurrent subduction along the two noncollinear trench segments of southern Mexico (WNW trending) and of the Chortís block (NNW trending) and (2) a scissor‐like transtensional rifting associated to counterclockwise rotation and eastward motion of the Chortís block. Key Points Trench‐transversal magmatism in Guerrero (southern Mexico) lasted ~30 MaAdakitic and ferroan geochemistry suggest temporal slab windowSlab window produced by change in trench geometry inducing diverging slab pull