STRUCTURAL, PETROLOGICAL, AND GEOCHRONOLOGICAL CHARACTERIZATION OF THE AGOYAN UNIT IN THE NORTHERN CORDILLERA REAL, ECUADOR

The Cordillera Real (CR), the eastern range of the Andes orogenic belt in Ecuador and has been proposed to have formed by the amalgamation of multiple terranes in the Mesozoic, or to be autochthonous, and record a single collision event, during the late Cretaceous. The Loja terrane (LT), a proposed accreted terrane, forms the main topographic backbone of the CR and trends relatively continuously through the entire length of Ecuador (~955km). The Agoyan (AG, proposed Paleozoic age), a primary unit of the LT, is described as schists and gneisses of pelitic origin. Petrological studies throughout the CR yield eclogite-facies conditions primarily in the Agoyan whose metamorphic age and tectonic significance are poorly understood.

A 6-week field season was conducted in 2022 over a ~115 km² field area east of Pimampiro, Imbabura Province of Ecuador. Fieldwork focused on characterizing the structure of the northern AG outcrops and its surrounding units, and sample collection for petrographic, petrological, petrochronological, and geochronological analyses. AG structural data suggests sub-vertical tight isoclinal folding, formed by both pure and simple shear, which strikes NE-SW, sub-parallel to the northern CR belt. Structural analysis of the foliation data shows folding to be upright with sub-horizontal hinges and an eastward vergence.

A field sample of quartz white mica paragneiss from the AG yielded a population of possible metamorphic zircons of ~225-245 Ma and minimum depositional age of ~270 Ma; younger than originally proposed. Sm-Nd isochrons obtained from garnets of two AG samples yield an age of ~79 Ma and two monazite analyses from the same rocks yield an age of 65 Ma. Two interpretations for the AG could be 1) it represents the coeval subduction zone that produced the early Mesozoic batholiths to the east or 2) it represents a metamorphic assemblage experiencing crustal thickening associated with the late Cretaceous collision.

Work in progress includes analysis of oriented thin sections for microstructures and shear indicators, CPO analysis of quartz fabrics to assess their strength throughout the unit, and calculation of pseudo-sections and various thermometers and barometers to constrain P-T paths to enhance our understanding of the AG and its tectonic significance.