The origin of a terrane: U/Pb zircon geochronology and tectonic evolution of the Xolapa complex (southern Mexico)

Abstract

Terrane analysis has established differences in lithology, style of deformation, metamorphism, and radiometric ages among several basement complexes in southern Mexico. In this paper, we identify similarities in the geologic history of the adjacent Acatlán, Oaxaca, and Xolapa complexes through a U/Pb zircon study on crustal sources and timing of magmatic, metamorphic, and tectonic activity in the Xolapa complex. Our regional tectonic scenario is based on both new chronologic and previously established structural data. Inherited zircon in the Xolapa complex indicates the presence of a Proterozoic (1.0–1.3 Ga) crustal component with an age range that overlaps Grenville crystallization dates (1.0–1.2 Ga) from the Oaxaca basement, and an inherited Grenville crustal component (1.0–1.1 Ga) identified in zircons from the Acatlán complex. The Proterozoic component in the Xolapa complex indicates either that it received sediments from a continental region of Grenvillean age, for example the Oaxaca basement, which is the closest exposed Grenville crust in southern Mexico, or that the Xolapa complex has a Grenville basement. In the latter case, the Xolapa complex has been modified by widespread high‐grade metamorphism and large‐scale migmatization. Metamorphism and migmatization occurred from 66 to 46 Ma and locally continued into Oligocene time. Magmatism in the Xolapa complex terminated with crustal growth by plutonism, which is characterized by a systematic pattern of eastward‐younging crystallization ages, from 35 Ma in the west (west of Acapulco) to 27 Ma in the east (east of Puerto Angel). Metamorphism and migmatization in the Xolapa crust may have originated from a contemporaneous change in several parameters of convergence between the Farallon and North American plates (e.g., rate and direction of subduction) and the early evolution of the Caribbean. This plate reorganization triggered sinistral transtension distributed across the southern Mexican continental margin and the eastward translation of the Chortis block. The eastward younging of the Xolapa plutons also is consistent with the motion of the Chortis block during eastward displacement of the Caribbean plate. This induced a shift of the magmatic arc from its Cretaceous to early Tertiary position along the Xolapa and Chortis blocks to its present mid‐Mexican position. We conclude that a single tectonic framework accounts for the Mesozoic and Cenozoic geologic history of the southern Mexican terranes.