SECULAR AGE PATTERNS IN A-TYPE GRANITES: A POSSIBLE MANTLE EVENT CONNECTION

Field, petrologic and geochemical data indicate that post-Archean A-type granites were emplaced in extensional, compressional or transpressional tectonic regimes, mostly in back arc (74%) and continental rift settings (20%), with only minor examples in hotspot (4%) and collisional orogen (2%) settings. Only hotspot/rift-related A-type granites should be considered characteristic of within-plate or intracratonic settings, since most A-type granites appear to be arc-related. The highest fraction of A/Non-A type granites formed at 1.7-1.0 Ga in the Great Proterozoic Accretionary Orogen of Laurentia and Baltica, although the database is biased by large numbers of samples from Central/Southwest USA. Age frequency peaks in A-type granites occur at 2.7, 1.85, 1.45, 1.1, 0.6, 0.3 and 0.1 Ga. The geographic age distribution of these peaks is heterogeneous as recorded in Eastern Asia (300 and 100 Ma), Gondwana terranes (600 Ma), the Laurentian Grenville orogen (1100 Ma), Central/Southwest Laurentian orogens (1450 Ma), and in Brazilian orogens (1850 and 2700 Ma). Thus, none of these age peaks should be considered global based on our current database. During supercontinent assembly, back arc A-type granites dominate, and during breakup, hotspot/rift types increase in abundance, but do not dominate until the breakup of Pangea in the last 200 Myr. A-type granite frequency peaks at 1.85, 1.1, 0.6 and 0.3 Ga comprise mostly back arc types and correspond to a high frequency of orogens at these times and those at 1.85, 1.45, 1.1 and 0.6 Ga also coincide with peaks in plate speed. A remarkable correlation of six back arc A-type granite frequency peaks at 2.7, 1.85, 1.1, 0.6, 0.3 and 0.1 Ga with zircon and LIP age peaks suggests that back arc A-type granite frequency is related to secular mantle events.