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ABSTRACT

A new and simple integrated approach is proposed for qualitatively unravelling the crustal thickness of fossil magmatic systems based on the chemical and thermal records in amphibole-bearing magmatic rocks. Statistical analyses applied to a large multidimensional amphibole database show that Ti-rich and Si-poor magmatic amphiboles, which formed at high-temperature (T) conditions (>950 °C), were dominantly developed in basaltic to basaltic-andesitic (SiO2-poor, i.e., <55 wt%) magma within relatively thin crust (5–10 km). We find that for crustal thicknesses larger than 10 km, the occurrence of high-T amphiboles and basaltic magma decreases with increasing crustal thickness. This is because of mineral filtering in “mature” deep crustal hot zones that occur at the crust-mantle boundary (Moho). Given that subducting plates exert a direct control on the structural evolution (shortening or extension) of the overriding plates, probing crustal thickness in the past provides first-order information on the geodynamic processes that took place at plate margins. Using this approach, we document the progressive buildup of a thick (>40 km) Jurassic to Cretaceous accretionary belt along the circum-Pacific orogenic belts that bounded the Panthalassa Ocean. The destruction of this thick belt started at ca. 125 Ma and was initially recorded by the thinnest magmatic systems hosting amphibole-bearing magma. Thinning of the circum-Pacific orogenic belts became widespread in the northern regions of western America and in the western Pacific after ca. 75 Ma, possibly in response to oceanic plate segmentation, which triggered slab rollback and overriding plate extension.

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