Age and origin of earliest adakitic-like magmatism in Panama: Implications for the tectonic evolution of the Panamanian magmatic arc system

40-20Ma marks a fundamental interval in the evolution of the 70-0Ma Panamanian magmatic arc system. During this period, there is no evidence of Panamanian magmatic arc activity to the east of the Panama Canal Basin while to the west and in localized regions to the east of the Panama Canal Basin a phase of intrusive-only activity is recorded. Fundamentally, geochemical and geochronological evidence presented herein indicate that this intrusive activity was predominantly 'adakitic-like' and becomes younger from west to east along an approximately W-E striking lineament. Granodiorites of the Petaquilla batholith, western Panama yield LAM-ICP-MS ²⁰⁶Pb/ ²³⁸U zircon ages of 29.0+0.7, -0.6Ma, 28.5+0.7, -0.5Ma, 28.3+0.5, -0.4Ma and 26.2+0.5, -0.9Ma. To the east of the Panama Canal Basin zircons from a hypabyssal diorite of the mainly intermediate Majé subvolcanic suite, cedes a mean ²⁰⁶Pb/ ²³⁸U age of 18.9+0.4Ma. Relative to other 70-5Ma Panamanian magmatic arc lavas and intrusives, Majé and Petaquilla intrusives yield adakitic-like major and trace element abundances (e.g., >15wt.% Al ₂O3, generally >3.5wt.% Na ₂O, >400ppm Sr, <1.9ppm Yb, <18ppm Y, Sr/Y that ranges to >120) and strongly fractionated HREE patterns. These 30-26Ma (Petaquilla) and 19Ma (Majé) suites are also compositionally similar to a subvolcanic suite of rare, circa 25Ma adakitic-like, andesitic intrusives which occur within the Panama Canal Basin midway between Petaquilla and Majé and at the same approximate latitude as Petaquilla and Majé. Collectively, the geochemical and geochronological data for the adakitic-like intrusives arc consistent with formation via partial melting of lowermost, mafic crust above a sub-horizontal slab tear that propagated from the west (Petaquilla) to the east (Majé) between 30 and 19Ma. Our new tectonic model postulates that collision between the Panamanian magmatic arc system and an 'indentor' (e.g., a tract of thickened buoyant, oceanic crust or plateau) occurred at about 40Ma, a time of which coincides with the initiation of left-lateral offset of the Early (i.e., 70-40Ma) Arc system. This collision resulted in the shutdown of the Early Arc system, possible steepening of the subducting Farallon slab and ultimately slab break-off and the phase of mainly adakitic-like intrusive activity. Subsequent to slab removal by ~20Ma, NE-dipping subduction jumped to the south and initiated production of the Later (i.e., post 20Ma) Arc system soon thereafter.