DETRITAL GEOCHRONOLOGY AND LOW-TEMPERATURE THERMOCHRONOLOGY IN THE ISTHMUS OF PANAMA

The uplift history of the K-Paleocene Panamanian arc complex prior to the collision with South America is poorly understood and information on the degree of exhumation is lacking. Recent studies on the basement rocks located in the vicinity of the Canal Basin report U/Pb zircon ages of ca. 58-54 Ma for the Cerro Azul gabbro, ca. 49-39 Ma for the Mamoni quartzdiorite and ca. 29-26 Ma for the Petaquilla quartzdiorite. Apatite fission-track analyses yield cooling ages of ca. 50-44 Ma in Cerro Azul and ca. 45-35 Ma in Mamoni plutons. Zircon (U-Th)/He ages range between ca. 51 and 41 Ma in Cerro Azul; and apatite (U-Th)/He ages range from ca. 23-6 Ma, with predominant ages at ~12 Ma, in the Cerro Azul; ca. 29-19 Ma in Mamoni and ca. 22-19 Ma in Petaquilla. The Gatuncillo Formation (Late Eocene) is the oldest Tertiary sedimentary unit deposited in central Panama. In this study, U/Pb LA-ICP-MS analyses on detrital zircons reveal distinct age populations of 57–50 Ma, 42–39 Ma and 29 Ma, with the oldest group being the most predominant range in the samples. These data suggest that the source of the detritus was from igneous rocks similar in age to the Cerro Azul, Mamoni and Petaquilla plutonic complexes. Detrital apatite grains from the same samples yield single-grain (U-Th)/He ages ranging from 37 to 15 Ma. These ages are older and more scattered than the AHe ages from samples of the intrusive rocks. Based on the U-Pb zircon age population, most of the detritus was derived from Cerro Azul pluton. If the apatite (U-Th)/He system was closed since deposition, the detrital AHe data indicate relatively slow cooling of the parent pluton since between 37 and 15 Ma. This interpretation is consistent with the two-pulses of exhumation at ~47-42 Ma and ~12-9 Ma previously suggested by the pluton data. An alternative interpretation is that the detrital apatites are derived from multiple parent bodies yielding mixed AHe ages. The zircon U/Pb data indicate that the maximum depositional age of Gatuncillo Fm. is Oligocene or younger, suggesting that similar carbonate/detrital facies reported in nearby sedimentary basins are not time-equivalent to these strata in central Panama. The young AHe ages could indicate an even younger depositional age (<15 Ma), although partial ⁴He degassing from some of the apatites during burial may have caused the apparently younger ages.