On the origin of mafic magmatic enclaves (MMEs) in syn-collisional granitoids: evidence from the Baojishan pluton in the North Qilian Orogen, China

摘要

Mafic magmatic enclaves (MMEs) are abundant in Baojishan syn-collisional granitoids located in the eastern section of the North Qilian Orogen. Zircon U-Pb ages of the host granodiorite (433.7 +/- 3.4 Ma) and their MMEs (431.6 +/- 2.8 Ma) are the same as the time of the Qilian ocean closing and continental collision at similar to 440-420 Ma, indicating that the granitoids represent a magmatic response to the collision between the Qilian-Qaidam block and the Alashan block, The MMEs have the same mineralogy as the host granodioritc except that they are more abundant in mafic phases (e.g., amphibole and biotite) and thus have higher heavy rare earth element (HREE) abundances. Both the host granodiorite and the MMEs have light REE-enriched patterns and relatively flat HREE patterns (i.e., [Dy/Yb](N) = 1-1.1), They are enriched in large ion lithophile elements (LILEs; e.g., Rb, K, Pb) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta. Ti) and show a varying Sr anomaly (i.e., Sr/Sr*=0.9-2.2) for the host and a negative Sr anomaly (i.e., Sr/Sr*=0.4-0.6) for the MMEs. Both the host granodiorite and the MMEs have overlapping and indistinguishable Sr-Nd-Hf isotopic compositions (Sr-87/Sr-86((i)))=0.7067-0.7082, epsilon(Nd)(t)= -3.9--3.2, epsilon(Hf)(t)=1.0-14.7). The extremely high epsilon(Hf)(t)= 14.7 of sample BIS12-06MME likely results from the calculation due to nugget effect of zircons because of the unexpectedly high Hf (3.53 ppm) and too high Zr (128 ppm). All these characteristics are fully consistent with the MMEs being of cumulate origin formed at earlier stages of the same magmatic systems rather than representing mantle melt required by the popular and alleged magma mixing model. The radiogenic Sr and unradiogcnic Nd (epsilon(Nd)(t) <0) indicate the contribution of mature continental crust, while variably radiogenic Hf (epsilon(Hf)(t)>0) for both the MMEs and their host granodiorite manifest the significant mantle input. The apparent decoupling between Nd and Hf isotopes are likely caused by partial melting of recycled terrigenous sediments and the remaining part of the North Qilian ocean crust under the amphibolite facies conditions during the collision.