U-PB AND K-AR GEOCHRONOLOGY IN PALEOZOIC AND MESOZOIC INTRUSIVE ROCKSOF THE COASTAL CORDILLERA, VALPARAISO, CHILE

The U-Pb and K-Ar geochronology applied to intrusive rocks from the Co astal Batholith of Central Chile, demonstrates the existence of a base ment block of the Mirasol Unit, with a crystallization age of 299 +/- 10 Ma, exposed in the northern block of the Melipilla Fault. The age o f 214 +/- 1 Ma obtained in the 'Dioritas Gneisicas de Cartagena Unit', indicates that a Late Triassic magmatism took place in this region; i t coincides with the end of an extensive crustal melting period, propo sed for northern Chile. The ages of the Jurassic plutonic units (Lagun a Verde, Sauce, Penuelas and Limache) are restricted to the 156-161 Ma interval, showing in certain cases, inherited zircons from an unknown source. The difference between ages obtained using both chronological methods is a few million years, indicating that a short time passed b etween the crystallization and the cooling of the plutonic bodies, as well as a fast magmatic differentiation process. The Laguna Verde and Sauce Units, experienced a fast uplift, probably as a result of an ext ensional tectonic process in the magmatic are, or induced by the magma tic pressure through fracture zones during Middle Jurassic.