Geochronology, petrology and geochemistry of the Nanlinshan and Banpo mafic-ultramafic intrusions: implications for subduction initiation in the eastern Paleo-Tethys

The Nanlinshan and Banpo mafic-ultramafic intrusions belong to the prominent Yunxian-Jinghong magmatic belt in the western part of the Simao Block, one of several Gondwana-derived continental fragments assembled by the closure of multiple Tethyan oceans. Different petrogenic models including ophiolites, MORB-type cumulates and Alaskan-type complexes have been proposed for these intrusions. In order to better constrain possible origins, we have undertaken an integrated geochronological, petrological and geochemical study of both intrusions. Precise CA-ID-TIMS zircon U-Pb dating indicates that the Nanlinshan and Banpo intrusions have similar ages of similar to 298 and 295 Ma, respectively, confirming that they are the oldest intrusive rocks in the Yunxian-Jinghong magmatic belt. A comparison between whole rock compositions and the compositions of major silicate minerals, including trace elements in clinopyroxene, reveals that (1) the ultramafic rocks of these intrusions are crystal cumulates of a relatively primitive magma, (2) the associated gabbroic and dioritic rocks are the products of more fractionated liquids, and (3) the parental magmas of these rocks were all depleted in some high field strength trace elements including Nb, Zr and Hf. Both intrusions are also characterized by elevated epsilon(Nd) values between +3.4 and +6.6. The positive epsilon(Nd) values coupled with negative Nb and Zr-Hf anomalies are consistent with the interpretation that these two intrusions are the products of subduction-related basaltic magmatism. The results from this study suggest that subduction of the Paleo-Tethys Ocean along the western margin of the Simao Block initiated as early as similar to 298 Ma and that the Simao Block and the Northern Qiangtang Block of the Tibet Plateau are separate Gondwana-derived continental fragments instead of a single fragment as previously reported.