Four billion years of secular compositional change in granitoids

Condie, Kent C.; Puetz, Stephen J.; Spencer, Christopher J.; Roberts, Nick M.W.. 2024 Four billion years of secular compositional change in granitoids. Chemical Geology, 644, 121868.

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From a new carefully filtered geochemical database, kernel density and bootstrap resampling time series show the existence of three compositional time intervals (> 200 Myr duration) in granitoids: Interval 1 the Archean (4.0–2.5 Ga), Interval 2 the Mesoproterozoic (1.8–1.0 Ga), and Interval 3 the Neoproterozoic-Phanerozoic (1.0–0 Ma). During these intervals, sample-based and site-based geochemical averages show minimal differences, which together with the widespread geographic distribution of sites suggests they are global in extent. Most incompatible elements in granitoids are low in Interval 1, high in Interval 2 and intermediate in Interval 3. In contrast to compositional intervals, compositional anomalies (≤ 200 Myr) appear to be of only local geographic significance (except for the Rodinia anomaly at 1.0 Ga). Tectonic settings in Intervals 2 and 3 granitoids are similar, the major difference is a smaller fraction of within-plate types during Interval 2. After 2.5 Ga, almost all granitoid sites are in orogens. The major differences between our secular geochemical results and those previously reported using other databases are due to 1) lack of thorough filtering, 2) variable bin widths in compiled databases and 3) local geographic biases. Our results indicate that secular geochemical trends and anomalies should not be considered to be of global extent without careful filtering and thorough examination of the geographic distribution of samples. Our results also show that caution must be exercised to make sure that sites with strongly deviant compositions do not overly influence mean values. Although heating of the upper mantle may be required to explain the Mesoproterozoic granitoid interval, continental lithosphere insulation is not supported as a major cause of this heating because granitoids and large igneous provinces are chiefly in peripheral orogens and not in cratons as predicted by the insulation model. There are two transitions in granitoid composition: the first at 2.5–2.0 Ga may reflect a decreasing depth of magma generation and crustal thickness in response to propagation of sustained subduction around the globe, and the second at 1–0.5 Ga, continued cooling of the mantle, lithosphere strengthening, and perhaps subduction of continental lithosphere.

Item Type: Publication - Article
Digital Object Identifier (DOI):
ISSN: 00092541
Date made live: 26 Jan 2024 14:47 +0 (UTC)

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