<b>Neoproterozoic Gabbro and Diorites of the Tianquan County</b> <i>,</i> <b>Longmenshan Orogenic Belt, SW China: U-Pb Ages, Petrogenesis and Tectonic Settings</b>

Abstract Petrological and isotope-geochemical studies of magmatic rocks are a necessary part of most tectonic and metallogenic reconstructions, in particular, in orogenic belts formed at active margins of cratonic blocks that experienced long-term and tectonically complicated evolution. Magmatic complexes exposed at the western margin of the Yangtze Block have been studied by many research groups; however, their petrogenesis and tectonic origins remain not fully understood and debatable that limits the validity of this or that tectonic model. In this paper, we present first U-Pb ages, whole-rock geochemical and Sm-Nd and Hf-in-zircon isotope data from gabbroids and diorites exposed in the Tianquan area of the Longmenshan orogenic belt in an attempt to provide more details on their petrogenesis, mantle sources, and tectonic origin. The rocks are calc-alkaline gabbro, gabbro-diorite, and hornblende diorite. There are several compositionally distinct groups: Group 1 gabbro and diorites, Group 2 gabbros, Group 3 diorites, high-Ti gabbro, and monzo-gabbro. Group 1 gabbro (816 Ma) is less enriched in LREE and has less differentiated heavy REE compared to Group 2 gabbros, monzo-gabbro (796 Ma), and high-Ti gabbro (790 Ma). All rocks are characterized by positive εNd(t) and εHf(t). Group 1 gabbros and high-Ti gabbro were derived by high-degree partial melting of depleted harzburgite and spinel lherzolite with participation of subduction-related fluids. Group 1 gabbros experienced 10%–80% fractionation of olivine and clinopyroxene to produce Group 1 diorites (813 and 804 Ma). Group 2 gabbros were derived by low-degree melting of an enriched mantle source affected by slab-derived melts. Group 3 diorites (808 Ma) formed with involvement of subducted sediments. The monzo-gabbro is similar to OIB and formed by a low-degree melted garnet lherzolite. Our data confirm the continuous Neoproterozoic magmatism in western Yangtze. The 860–800 Ma stage of supra-subduction magmatism was followed by a stage of back-arc rifting at ca. 796 Ma.