GUERRERO TERRANE OF MEXICO - ITS ROLE IN THE SOUTHERN CORDILLERA FROMNEW GEOCHEMICAL DATA

The Guerrero terrane makes up most,of the western part of Mexico, is o ne of the largest terranes of the North American Cordillera, and is ch aracterized by an Upper Jurassic-Lower Cretaceous volcanic-sedimentary sequence of arc affinity. Metamorphic rocks that crop out in the west ern area of the terrane (Arteaga complex) may represent its basement. They are mostly composed of terrigenous sediments (Varales Formation) with minor basaltic pillow lavas, chert, tuff, and limestone. Initial epsilon(Nd) values (+13) and rare earth element (REE) values for pillo w lavas of the Arteaga complex are characteristic of mid-ocean ridge b asalts (MORB). In contrast, the Varales Formation sedimentary rocks fr om the Arteaga complex have negative initial epsilon(Nd)(-6.2 and -7.2 ) and are enriched in light REEs. These data indicate that the sedimen ts of the Varales Formation were supplied from an evolved continental crust. The overlying Jurassic(?)-Cretaceous arc-related rocks have ini tial epsilon(Nd) (+7.9 to +3.9) and REE patterns similar to those of e volved intraoceanic island arcs. These data show that the evolution of the Guerrero terrane had an early pre-Cretaceous(?) stage, which cons isted of an oceanic crust receiving sediments from a continental sourc e, and a Cretaceous stage, which was the development of an island arc. The oceanic-continental isotopic signature of the Arteaga complex is different from other western North American Cordilleran terranes (e.g. , Alexander, Wrangellia) that are more completely ''oceanic'' in affin ity. Nevertheless, the extensive Jurassic(?)-Cretaceous arc represents additions of juvenile material to the western North American Cordille ra.