In-situ plagioclase geochemistry and Pb isotopic compositions from Mesozoic granitoids in the northeastern North China Block and its petrogenetic implications

Abstract

Plagioclase has been widely used to reflect the magmatic evolution and the crust-mantle interaction based on its compositional zoning. However, the abilities in recording the adakitic characteristics and the reworking of continental crust are not understood. This paper reports new in-situ plagioclase geochemistry and Pb isotopic compositions of eight Mesozoic granitoid plutons in the northeastern North China Block (NCB). Based on our previous whole-rock and in-situ apatite geochemistry, we divided them into two groups as adakitic rocks and non-adakitic rocks. Comparison and analysis of plagioclase geochemistry in this paper and previous data between these two groups show that plagioclase crystallized from adakitic rocks possesses higher Sr (735–1588 ppm, generally >1000 ppm) concentrations, δEu (δEu = 2EuN / (SmN + GdN; N represents after primitive mantle normalize) and Sr/Y (1159–81,344, generally >5000) values than those from non-adakitic rocks. Therefore, we believe that the geochemical index of plagioclase Sr–Y–δEu could reserve the adakitic characteristics and then propose two plagioclase geochemical diagrams (Sr/Y versus Y and δEu versus Sr/Y) to identify the adakitic characteristics. Furthermore, the in-situ plagioclase Pb isotopic compositions (206Pb/204Pb ranging from 16.948 to 18.146, 207Pb/204Pb ranging from 15.403 to 15.656 and 208Pb/204Pb ranging from 37.250 to 39.088), together with the rock-assemblages and spatial-temporal distributions of Mesozoic rocks as well as the variations of whole-rock Sr–Nd–Pb and zircon HfO isotopic compositions reveal the variation of crustal properties during Mesozoic. Late Triassic magmatism originated from reworking of juvenile lower crust in normal thickness. Early Jurassic magmatism were formed by partial melting of ancient thickened lower crust. Late Jurassic magmatism were derived from remelting of ancient lower crust in normal thickness. Early Cretaceous magmatism were derived from reworking of ancient lower crust in different depth.