A-type granites in space and time: Relationship to the supercontinent cycle and mantle events

Abstract

Field, petrologic and geochemical data indicate that post-Archean A-type granites were emplaced in extensional, compressional or transpressional tectonic regimes, mostly in accretionary orogen (>75%) and continental rift settings (10-20%), with only minor examples in hotspot (≤ 8%) and collisional orogen (≤ 5%) settings. Only hotspot/rift-related A-type granites should be considered characteristic of within-plate or intracratonic settings since most A-type granites are orogen-related. The greatest proportion of post-Archean A-type granites was produced in orogens of two ages: 1.7-1.0 Ga in the Great Proterozoic Accretionary Orogen in Laurentia and Baltica, and at 300-100 Ma in East Asian orogens. The widespread production of A-type granite magma in orogens between 1700 and 1100 Ma is not consistent with a single-lid tectonic regime during the Mesoproterozoic. 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), Arabian-Nubian orogen (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, orogen A-type granites dominate (chiefly in back-arc settings), 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 orogenic 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 orogen 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 such as global plume events or mantle overturn.