Devonian to Permian plate tectonic cycle of the Paleo-Tethys Orogen in southwest China (II): Insights from zircon ages of ophiolites, arc/back-arc assemblages and within-plate igneous rocks and generation of the Emeishan CFB province

摘要

New SHRIMP zircon ages for ophiolites, arc/back-arc assemblages and within-plate igneous rocks constrain the evolution of the Paleo-Tethys Orogen of southwest China. Ophiolites represent remnants of the Paleo-Tethys ocean, which was divided by the Simao terrene into two tracts, the main ocean in the west (current position) and a branch in the east The main ocean has been inferred to originate from Gondwana break-up in the Early Devonian, and the heritage is preserved as the Changning-Menglian ophiolite (SSZ-type). The oceanic branch is represented by the Ailaoshan (NMORB-type)-Jinshajinag (EMORB-type) ophiolites. Zircons from a diabase and a plagiogranite of the Ailaoshan ophiolite yielded ~(206)Pb/~(238)U ages of 382.9 ±3.9 Ma and 375.9 ± 4.2 Ma. The age of a cumulate gabbro from the Jinshajiang ophiolite is 343.5 ± 2.7 Ma. A metagabbro of the Changning-Menglian ophiolite yielded a crystallization age of 267.1 ±3.1 Ma. We interpret the ages of the Ailaoshan and Jinshajiang ophiolites to represent sea-floor spreading in the branch ocean, whereas the age of the Changning-Menglian ophiolite reflects closure of the main ocean. Arc/back-arc assemblages (zoned mafic-ultramafic complexes and arc volcanic rocks) are records of convergent plate boundary magmatic activity. Rocks of the Gicha complex (back-arc rift) yielded zircon ages of 306.2 ±4.6 Ma for a tonalite, 301.0 ± 2.9 Ma for a microgabbro, 297.1 ± 2.0 Ma for a plagioclase hornblendite, and 281.3 ± 1.7 Ma for a diabase. A gabbro and a tonalite of the Banpo complex (plutonic arc) yielded identical ages of 285.6 ± 1.7 Ma and 285.8 ±2.0 Ma. A basalt of the Yaxianqiao volcanic arc was dated at 266.2 ± 2.2 Ma. Within-plate igneous rocks were also dated. Two amphbolite xenoliths in the Jinshajiang ophiolite melange, derived from low-Ti continental flood basalts, have zircon ages of 439.3 ± 3.5 Ma and 403.7 ± 3.1 Ma, which we interpret to represent the inception of continental rifting prior to the opening of the branch ocean. The small Dalongkai differentiated mafic-ultramafic intrusion reflects post-orogenic extension and is characterized by superimposed arc and within-plate geochemical signatures. Zircons from a plagioclase pyroxenite of this intrusion yielded an age of 245.6 ± 1.4 Ma.These new results, combined with geological and geochemical data, allow the reconstruction of a Devonian to Permian plate tectonic cycle in which generation of the Emeishan CFB province correlates with Paleo-Tethys orogeny in time. This study provides evidence for cyclic continent-ocean-continent development via multiple orogenesis, as well as temporal constraints on the dynamic relationship between plate tectonics and generation of a large igneous province.