Contrasting geochemistry of the Cretaceous volcanic suites in Shandong province and its implications for the Mesozoic lower crust delamination in the eastern North China craton

摘要

The Qingshan volcanic sequences occur in the Mengyin and Jiaozhou basins, west and east of the Tan-Lu fault zone, respectively, were formed at 128 ± 2 and 106-98 Ma. Lithologically the Mengyin succession comprises unimodal andesite (53-65 wt% SiO2), whereas the Jiaozhou succession consists of bimodal andesite (56-59wt% SiO2) and rhyolite (69-77wt.% SiO2). High-Mg (Mg# >60) and low-Mg (Mg# <60) andesitic suites are recognized in the Mengyin volcanic rocks. They show tholeiitic and calc-alkaline trends, respectively. Though the two suites share common trace element features of LILE enrichment and HFSE depletion, higher incompatible element contents in the high-Mg andesite relative to the low-Mg andesite rule out their genetic connection by magmatic differentiation. Similarly, the Jiaozhou andesites also possess higher incompatible element contents compared to their interbedded rhyolites, suggesting their distinct source rocks. The Qingshan mafic volcanic suites are characterized by radiogenic Sr and unradiogenic Nd isotopic compositions, of which the Mengyin high-Mg andesitic rocks display the lowest ε_(Nd)(t) and highest initial Sr ratio. By contrast the Jiaozhou rhyolites possess a positive correlation between Sr-Nd isotopes and relatively unradiogenic Sr isotopic ratios. The Qingshan mafic rocks exhibit geochemical characteristics distinct from those of mantle peridotite- or pyroxenite-derived melt as well as from Fangcheng basalts, the only basalts documented in the Cretaceous volcanic suites in Shandong province. They are inferred to be crust-derived melts. However, a clear decreasing trend in ACNK with Mg# increasing and the elevated MgO, Cr and Ni contents relative to basalt-derived melts indicate assimilation with mantle peridotite during their pristine magmatic evolution. Such a two-step process can be best explained by the Archean lower crust delamination of the North China craton with a ~35% partial melting followed by a variable extent of metasomatic interaction with converting mantle peridotite, which thus resulted in the low-Mg and high-Mg andesitic suites. Nd-Sr isotopic and HFSE features imply that the converting mantle was mixed with the foundered lithospheric peridotite, which previously experienced subdurtion-related melt metasomatism during the collision of the Yangtze-North China cratons in the early Triassic The Qingshan volcanic suites in western Shandong are synchronous with the Tan-Lu strike-slip fault inferring that the lower crust delamination was initially triggered by sinistral motion of the Tan-Lu fault at ~ 130 Ma. Due to an eastward development of regional lithospheric delamination, the Jiaozhou bimodal volcanic succession was formed owing to the regional geothermal gradient increase and lithosphere thinning. The recognition of contrasting andesitic suites in the Qingshan volcanic succession and their temporal trend provide additional evidence for lower crust delamination during the early Cretaceous in the North China craton and new constraints on its lithosphere thinning model.