Spanish Crown grant of 1534 to adelantado Pedro de Mendoza to explore, pacify, and settle the Río de la Plata basin along the Atlantic; founded the first Buenos Aires (1536, later abandoned) and Asunción (1537) before Mendoza's death at sea.
Superseded by the Governorate of the Río de la Plata after the 1542 creation of the Viceroyalty of Peru brought the region under Lima's authority.
Hakluyt Society volume of primary accounts (Ulrich Schmidt et al.) covering exactly the Mendoza/Ayolas/Irala governorship years of this polity — polity-specific, not a broad survey.