Tectonic significance of Cretaceous-Tertiary magmatic and structural evolution of the northern margin of the Xolapa Complex,Tierra Colorada area,southern Mexico

摘要

The Tierra Colorada area sits along the northern limit of the Xolapa Complex,where it is juxtaposed against the Mixteco (Paleozoic) and Guerrero (Mesozoic) terranes of southern Mexico,just north of Acapulco.This paper presents combined structural and geochronological data from Tierra Colorada area that show evidence of four deformational events and several episodes of arc magmatism during Mesozoic and Cenozoic time.The oldest magmatism is represented by ca.165 Ma granitoids and was followed by intrusion of the foliated El Pozuelo granite (129 ± 0.5 Ma;concordant U-Pb zircon analysis).This intrusion postdates D_1 metamorphism and migmatization in the Xolapa Complex.The next magmatic episode is represented by the peraluminous,foliated El Salitre granite (55.3 ±3.3 Ma;mineral-whole-rock Rb-Sr isochron) and the protomylonitic Las Pinas I-type granite (54.2 ± 5.8 Ma;lower intercept U-Pb zircon).Las Pifias granite is characterized by D_2 ductile fabric with normal,top-to-the north-northwest sense of shear,deformed at 45-50 Ma (Rb-Sr and K-Ar ages).The ca.34 Ma undeformed granites correspond to the last intrusive pulse in the area,postdating both D_3 south-southwest-verging thrusting of the Cretaceous Morelos Formation over sheared granites and Lower Cretaceous volcanic rocks,and open folding during D_4.These four pulses of subduction-related magmatism in the Tierra Colorada area indicate a regular northeastward subduction at the Mesoamerican trench since Jurassic time,and alternate with contractile and/or exten-sional tectonic events.The gap in magmatic activity ca.90-100 Ma roughly coincides with deposition of platformal limestones of the Morelos Formation during the middle Cretaceous.The stable conditions during deposition of the Morelos Formation may have resulted from a combination of back-arc extension and development of a passive margin during the Early-middle Cretaceous,which postdated the accretion of an exotic block,either the Guerrero terrane or the Chortis block.Following the Laramide orogeny in southern Mexico (roughly during the Late Cretaceous) the Paleocene-Miocene tectonic evolution in the Tierra Colorada area involved an alternation of magmatic pulses with extensional and contractile events.This was the result of a combination of several factors,including the geometry of the subducted slab,convergence rate,stress transmission between the subducting and overlying plates,and the rate of subduction erosion.