Potassium Isotopic Composition Of Neoarchean TTG Rocks In The Western Shandong Province,North China Craton

Potassium(K)is one of the most important elements in the earth.The variation of its isotopic composition has high application potential in tracing crust-mantle differentiation,crustal evolution and supergene geological processes.In recent years,with the development of analytical and testing techniques,the variation of K isotope in earth system has become an international research focus.The upper continental crust is one of the important K reservoirs on earth.Preliminary research results show that the K isotope composition of the upper continental crust is significantly heterogeneous,while the K isotope composition of granitic rocks,as an important component of the upper continental crust,also shows strong heterogeneity.However,previous studies on the upper continental granitic rocks mainly focus on the Phanerozoic,and lack of research on the K isotopic composition of the early continental granitic rocks.The granitic rocks represented by Archean TTG series rocks are important components of the early crust.What is their K isotopic composition?The answer to this question is of great theoretical and practical significance not only to enrich the characteristics of K isotope reservoir in the Earth system,but also to explore the mechanism of crustal growth and evolution in the early Earth.1.The?⁴¹K values of whole rocks,K-feldspar and biotite are-0.50‰to-0.24‰,-0.54‰to-0.22‰,and-0.71‰to-0.34‰,respectively.All samples have low LOI values,and there is no correlation between?⁴¹K and LOI and CIA valuesrule out that late alteration is not the main factor for K isotope fractionation2.The?⁴¹K values of TTG rocks and potassic granites which with consistent composition in the source area respectively are positively correlated with variation parameters of major elements(Na+K)/(Ca+Mg)and K₂O/Na₂O ratio,which reflect the degree of magma evolution.In addition,the?⁴¹K values of whole rocks are positively correlated with the?⁴¹K values of K-rich minerals.The above characteristics reveal the existence of K isotope fractionation during the magma evolution,and the granitic rocks tend to enrich heavy K isotope with the increase of magma evolution.It is further speculated that the late crystallization of K-rich minerals is the main factor controlling the K isotope composition of rocks.3.The early Neoarchean TTG rocks are enriched with heavy K isotopes(?⁴¹K=-0.31±0.04‰),which are heavier than other samples and phanerozoic granites.The TTG rocks have a high Sr/Y ratio,which reflects the high pressure during the formation of their melts.In the diagram of?⁴¹K and Sr/Y relationship,TTG is positively correlated with phanerozoic I-type granites,indicating that the K isotopic composition of TTG source region is affected by pressure.The K isotopic composition of the potassic granite is relatively light(?⁴¹K=-0.50±0.07‰),and the U/K,Th/La and Th/Nd values of the it is relatively high,indicating that the addition of the sedimentary components in the source region.The K isotopic composition of the quartz syenite(?⁴¹K=-0.49±0.07‰)is similar to that of the potassic granite,but the sedimentary composition of the source area is lower,which is the result of inheriting the K isotopic composition of the mantle source area.The amphibole has heavy K isotopic composition(?⁴¹K=-0.34±0.09‰),and the relationship between?⁴¹K and Ba/Th indicates that the source region was subjected to subduction alteration of oceanic crust melt metasomatism.Geochemical mixing simulation shows that the K isotope composition of granodiorites is controlled by the magmatic mixing process of heavy K isotope enriched mantle-derived melt and light K isotope enriched crust-derived melt.4.The K isotopic composition of TTG series rocks in western Shandong indicates that the K isotopic composition of the early crust also shows significant heterogeneity.Compared with phanerozoic granites,the above K isotope composition characteristics indicate that the early crust was more enriched in heavy K isotopes.In addition,the K isotope characteristics of the early crust further suggest that there was a mature oceanic plate subduction system in the late Neoarchean in western Shandong,indicating that plate tectonics existed in this period.