THE ORIGIN OF A TERRANCE - U PB ZIRCON GEOCHRONOLOGY AND TECTONIC EVOLUTION OF THE XOLAPA COMPLEX (SOUTHERN MEXICO)/

Terrane analysis has established differences in lithology, style of de formation, metamorphism, and radiometric ages among several basement c omplexes in southern Mexico. In this paper, we idenify similarities in the geologic history of the adjacent Actalan, Oaxaca, and Xolapa comp lexes through a U/Pb zircon study on crustal sources and timing of mag matic, metamorphic, and tectonic activity in the Xolapa complex. Our r egional tectonic scenario is based on both new chronologic and previou sly established structural data. Inherited zircon in the Xolapa comple x indicates the presence of a Proterozoic (1.0-1.3 Ga) crustal compone nt indicates the presence range that overlaps Grenville crystallizatio n dates (1.0-1.2 Ga) from the Oaxaca basement, and an inherited Grenvi lle crustal component (1.0-1.1 Ga) identified in zircons from the Acat lan complex. The Proterozoic component in the Xolapa complex indicates either that it received sediments from a continental region of Grenvi llean age, for example the Oaxaca basement, which is the closest expos ed 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 migmati zation. Metamorphism and migmatization occurred from 66 to 46 Ma and l ocally 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 Acapulo) to 27 Ma in the east (east of Puer to Angel). Metamorphism and migmatization in the Xolapa crust may have originated from a contemporaneous change in several parameters of con vergence between the Farallon and North American plates (e.g., rate an d direction of subduction) and the early evolution of the Caribbean. T his plate reorganization triggered sinistral transtension distributed across the southern Mexican continental margin and the eastward transl ation of the Chortis block. The eastward younging of the Xolapa pluton s also is consistent with the motion of the Chortis block during eastw ard displacement of the Caribbean plate. This induced a shift of the m agmatic arc from its Cretaceous to early Tertiary position along the X olapa and Chortis blocks to its present mid-Mexican position. We concl ude that a single tectonic framework accounts for the Mesozoic and Cen ozoic geologic history of the southern Mexican terranes.