Lead


Lay summary
This proposal aims to i) use innovative geochronological and thermochronological methods to investigate the response of continental crust to collision and accretion within geologically distinct regions of the circum-Maracaibo region, and ii) to test the validity of the multi-domain diffusion (MDD) model to derive accurate, continuous thermal history solutions for alkali feldspars using the 40Ar/39Ar method.The Maracaibo Triangular Block (MTB) provides a well exposed section of rocks that permit the processes of continent-continent and continent-ocean arc/plateau collision and accretion to be studied, and any concluding models will be useful analogues for interpreting geological observations in other locations where continental plates have grown via the collision and accretion of oceanic and continental crust. Geochronological and thermochronological data will be used to construct semi-continuous thermal history paths and a temporally constrained tectonic framework. Zircon U-Pb LA-ICP-MS ages will provide a baseline to interpret the thermochronological data. High temperature (>300°C; apatite and titanite U-Pb; hornblende, white mica and biotite 40Ar/39Ar) thermochronological methods will be used to extract thermal history paths from Precambrian and Palaeozoic granitoids. Lower temperature thermochronological methods (<300°C; alkali feldspar 40Ar/39Ar, zircon and apatite fission-track analysis, zircon and apatite (U-Th)/He analysis) will be used to provide quantitative information about i) the response of the north western South American Plate to Pacific active margin tectonics, and ii) the collision of the Caribbean Plate against the north-western South American Plate at 75-73 Ma. Our interpretations will be used to understand how the margins of continental plates respond to collision and accretion events, and to develop models for the Phanerozoic geological evolution of north-western South America.The long, protracted cooling history of basement sequences provides an excellent opportunity to test the MDD model via a detailed investigation of the distribution of radiogenic 40Ar within single alkali feldspar crystals. We intend to utilise a CO2-IR laser equipped with a 2-wavelength, co-axial optical pyrometer and state-of-the-art 193nm UV excimer to perform both laser heating and in-situ ablation of the same, chemically unaltered alkali feldspar crystal to obtain diffusion data. Cyclical step-heating and in-situ analyses of the same crystal would represent the most direct method of attempting to match radiogenic daughter isotope concentrations yielded by both gas extraction methods, and therefore represents a direct test of the MDD model.