부산대학교 도서관

In modern plate tectonic regimes, continental crust is generated above subduction zones in magmatic island arcs. Models for continental growth-largely based on the modeling of geochemical processes that can transform mantle melts into the intermediate composition of bulk continental crust-have been hampered by a lack of definitive geochronology, which could clarify the temporal emplacement of igneous rocks at the base of island arc crust. The Kohistan Arc Complex (KAC) of Pakistan is a rare window into a nearly complete section of lower arc crust, revealing a sequence of igneous rocks representing discrete batches of magma underplated along the base of the arc crust. We present high-precision isotope dilution-thermal ionization mass spectrometry U-Pb geochronological data from zircon, and Lu-Hf and Sm-Nd isochron data from several mafic lower crustal cumulates within the KAC. These data establish a clear downward-younging age trend throughout this succession of cumulate complexes, demonstrating a total magmatic duration of ~20 m.y., corresponding to a rate of lower crust formation of up to 200-260 km³ km-1 Ma-1. These results provide the first direct evidence of a sequential process of underplating, revealing a fundamental mechanism responsible for building the root of island arc crust. [ABSTRACT FROM AUTHOR]