Repeated crustal thickening and recycling during the Andean orogeny in north Chile (21°–26°S)

Abstract

Understanding Neogene arc crustal thickening in the central Andes requires (1) some estimate of initial pre‐Neogene (prior to 26 Ma) crustal thicknesses and (2) mechanisms that account for the remaining deficit in crustal thickening (10–30%). Mid‐Miocene horizontal crustal shortening can explain most but not all crustal thickening in the modern central Andean arc. Systematic changes in geochemical and Sr, Nd, and selected Pb isotopic data of Late Cretaceous–Eocene (∼78–37 Ma) and older arc magmatic episodes from north Chile provide new constraints on both. First, Andean crust may have been significantly thickened by long‐term underplating of mantle‐derived basalt from Jurassic to present. Second, estimated initial (late Eocene) crustal thicknesses of ∼45 km are consistent with (1) amphibole‐ and garnet‐bearing residual mineralogies for late Eocene syntectonic/posttectonic granitoids, (2) lower crustal P wave velocities of 7.3–7.7 km s−1 compatible with underplated mafic crust, and (3) results from recent experimental petrologic work showing garnet stability in mafic mineralogies ≥12 kbar (≥40 km crustal thickness). Analogous to older Andean magmatic episodes in north Chile, newly underplated basaltic crust may account for the remaining deficit in Neogene crustal thickening. Similar evolutionary patterns in geochemistry and initial Sr and Nd isotopic characteristics of Andean (200 Ma to present) magmatic rocks suggest that the Andean orogeny in this region evolved by a combination of processes of repeated arc migration, tectonic and magmatic crustal thickening, and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust.

Geochemical differences between subduction- and collision-related copper-bearing porphyries and implications for metallogenesis

“…deeper than those associated with the EPRIM samples (e.g., Haschke et al, 2002;Haschke and Günther, 2003;Chung et al, 2009). The magma source depths of MPST is probably related with the underplated mafic magmas and increasing crustal thickness during Eocene and Oligocene (e.g., Chung et al, 2009).…”

Episodic crustal growth in the southern segment of the Trans-North China Orogen across the Archean-Proterozoic boundary

“…The last two processes would produce dominantly juvenile crust, which contradicts the abundant inherited zircons and negative whole rock ε Nd (t) and zircon ε Hf (t) values recorded by the Dengfeng and Xiaoqinling TTGs. The thickening of crust by repeated underplating would be a long-term process and result in evolutionary changes in geochemical and isotopic characteristics (Haschke et al, 2002), which is different from the uniform age of inherited zircons in the Xiaoqinling TTGs (Fig. 3).…”

“…Therefore, the Dengfeng and Taihua TTGs at 2.30-2.57 Ga are all partial melts of thickened lower crust, which might have formed through orogeny (e.g., Burg and Ford, 1997), repeated underplating (e.g., Furlong and Fountain, 1986;Haschke et al, 2002), intraoceanic plate stacking (De Wit, 1998), and/or oceanic plateau accretion (e.g., De Wit et al, 1992;Desrochers et al, 1993;Condie, 1997). The last two processes would produce dominantly juvenile crust, which contradicts the abundant inherited zircons and negative whole rock ε Nd (t) and zircon ε Hf (t) values recorded by the Dengfeng and Xiaoqinling TTGs.…”

Petrogenesis of the Early Eocene adakitic rocks in the Napuri area, southern Lhasa: Partial melting of thickened lower crust during slab break-off and implications for crustal thickening in southern Tibet

“…Along the Andean convergent margin, good correlations have been documented between the REE contents of arc lavas and crustal thickness (e.g., Haschke et al, 2002). Thus, the maximum light/heavy REE ratios have been used to infer the maximum thickness of the Tibetan crust and constrain its evolution (e.g., Chung et al, 2009).…”