| Knowledge of the chemical composition of the continental crust (CC) and its secular change is essential for understanding its origin. Understanding the Archean evolutionary history of the CC can also provide significant insights to its origin. Focusing on these two aspects, two research projects were conducted:1) estimating the Platinum-group element (PGE), Re and Au abundances in the upper continental crust (UCC) through time; 2) deciphering the Archean magmatic history of the Kongling terrain, Yangtze craton. The major achievements are:1) The correlations between PGE-Au and Cr, Ni in globally-distributed tillites from the Mesoarchean, Paleoproterozoic, Neoproterozoic and Paleozoic eras were used to estimate the PGE-Au abundances in the present-day UCC; 2) PGE-Au concentrations decrease in the tillite composites progressing from the Mesoarchean to the Neoproterozoic, coupling with the secular decreased concentrations in the first-row transition metals, suggesting that there was a greater proportion of mafic to ultramafic lithologies in the Archean UCC compared to younger UCC; 3) The present-day UCC PGE-Au abundance estimates from the averages of post-Paleoproterozoic composites are similar to those from loess samples, both of which are systematically lower than those obtained by the regression approach using all diamictite composites; we suggest the reason for the difference is that loess and our Neoproterozoic and Paleozoic diamictites preferentially sample younger UCC rocks, which have lower PGE-Au concentrations than older crust; 4) The present thesis reports, for the first time, A-type granitic gneisses with ages of 2.62-2.67 Ga in the Kongling terrain of the Yangtze craton; 5) Combined with the data in the literature, the Kongling terrain has experienced at least five Archean magmatic events; These episodes of magmatism were dominated by tonalite-trondhjemite-granodiorite before-2.8 Ga and afterwards by granite, reflecting a transition from subduction-to collision-related tectonics. |
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