Detrital zircon geochronology of lower to mid-crustal rocks associated with a microplate-continent collision, NW Argentina

New LA-ICPMS U-Pb detrital zircon data give insight into the age and origin of complexly deformed metaigneous and metasedimentary rocks in an ancient collision zone, and help clarify the structural relationships of lower to mid-crustal structural packages in the Sierra de Pie de Palo, northwest Argentina. The Pie de Palo range contains complexly deformed Precambrian crystalline rocks that have experienced single- or multi-phase metamorphism, the latest of which is associated with convergence and collision of the Precordillera microplate with the Famatina arc of Gondwana. The two main metamorphic events occurred at ~1070 Ma and ~470 Ma, based on isotopic ages from several samples. The regional extent of both events, and thus the geometry of the collisional system, is not well defined. West-vergent shear zones separate two Precambrian basement complexes, the Pie de Palo Complex and Central Complex, from the Caucete Group, a suite of intensely deformed, medium grade Cambrian quartzite and marble. Detrital zircons from the hanging wall of the ductile Duraznos shear zone within the Pie de Palo range constrain the timing of cessation of Precambrian deformation and reflect no significant regional metamorphism between 1070 Ma and 470 Ma. Garnets with one clear phase of growth in the hanging wall are juxtaposed with two-phase garnets in the footwall. ~25% of the detrital zircon grains analyzed from the immediate hanging wall are younger than 1050 Ma, precluding the existence of the hanging wall package and the Duraznos shear zone in the 1070 Ma event. Detrital zircon ages also constrain the geometry of the Las Pirquitas Thrust, and support the interpretation of a structural window within the Pie de Palo Complex into the younger Caucete Group. Age distribution peaks correspond to previous detrital zircon analyses of Caucete Group metasediments. The presence of a window into Caucete Group rocks amidst the Pie de Palo Complex implies a very low-angle thrust and a thin thrust sheet of Pie de Palo Complex above the Caucete Group. Furthermore, detrital zircon peaks also correspond to the hanging wall of the Duraznos, suggesting a possible source area for Caucete Group metasediments. The shallow Bajo Pequeño shear zone within the Central Complex, above the Duraznos shear zone, records an episode of intense flattening and metamorphism. Zircons from the hanging wall of this shear zone reflect an igneous source with an age between 1200 Ma and 1300 Ma, giving insight into one of the Central Complex protoliths and constraining the age of intrusion. The detrital zircon ages overall support the metamorphic evidence that the major lower to mid-crustal shear zones occur along boundaries between lithologic units that have fundamentally different metamorphic histories and/or protoliths. The Duraznos shear zone separates two basement complexes of differing age, metamorphic history, and protolith, and the Las Pirquitas thrust juxtaposes older basement with younger metasediments with similar detrital zircon peaks. Similarly, the Bajo Pequeño shear zone occurs along a lithologic boundary, placing metaigneous orthogneiss atop metasediments.