Petrology and geochronology of Xuejiashiliang igneous complex and their genetic link to the lithospheric thinning during the Yanshanian orogenesis in eastern China

The Xuejiashiliang igneous complex, similar to 150 km north of the City of Beijing, is an important member of the Mesozoic Yanshanian orogen in eastern China. This complex consists of gabbro, monzogabbro, monzonite, syenite and granite. In situ zircon U/Pb dating shows that the Xuejiashiliang complex was emplaced at similar to 128.8-123.7 Ma (i.e., Ki). Field and petrographic observations together with bulk-rock major element, trace element and Sr-Nd-Pb isotopic data suggest that the gabbro represent remnants of a mafic intrusion formed from cooling of a mantle derived melt that underplated beneath or intruded into the lower crust. The monzogabbro may be the product of deep crustal assimilation of this mantle derived melt. The syenite may have precipitated from a melt produced by deep crustal melting caused by the mantle derived melt. The monzonite may have formed from mixing between melts parental to the syenite and monzogabbro. That is, all these diverse lithologies may have resulted from varying degrees of mantle melt induced crustal melting, melt assimilation, differentiation and mixing. The granite is best interpreted as resulting from upper crustal melting and advanced degrees of differentiation. The remarkably similar Nb-Sr-Pb isotopes of all these lithologies (except for Sr-87/Sr-86 of the granite) with an EMI-like signature point to a common source they share. This common source could be ancient lithospheric mantle, but we consider the Archean lower crust to be the more likely candidate. The high Sr-87/Sr-86 (0.8955) of the granite resulted from radiogenic ingrowth of Sr-87 due to the elevated Rb/Sr ratio (similar to 22.4). The high [La/Yb](CN) and Sr/Y ratios of all these lithologies (except the granite) are consistent with magma genes at depths where garnet is a stable phase. This is consistent with the condition of syenite genes that requires pressures equivalent to depths in excess of 50 km. All these constrain that the complex may have formed at the base of the thickened crust, perhaps genetically associated with episodes of compressional tectonics prior to lithosphere thinning. The widespread early Cretaceous (K-1(1)) granitoid magmatism in eastern China, including the Xuejiashiliang complex, requires volumetrically significant basaltic magmas underplating at the base of or intruding within the Archean lower crust. Possible eclogitization of these underplated basaltic magmas/rocks at the base of the thickened crust would raise the bulk density of the lithosphere, thus allowing portions of the lithosphere to sink into the asthenosphere. If eclogite foundering is indeed an immediate cause of the lithosphere thinning, then it is critical to understand the origin of volumetrically significant basaltic magmatism beneath eastern China in the Mesozoic for a better understanding of the Yanshanian orogenesis. (C) 2006 Elsevier B.V All rights reserved.