Late Devonian to early Carboniferous arc-related magmatism in the Baolidao arc, Inner Mongolia, China: Significance for southward accretion of the eastern Central Asian orogenic belt

Abstract

The Central Asian orogenic belt, formed in response to consumption of the Paleo–Asian Ocean, is one of the largest and most complex accretionary collages in the world and was responsible for considerable Phanerozoic juvenile crustal growth in Central Asia. The timing of subduction-accretion processes and closure of the Paleo–Asian Ocean is controversial. The Xilingol complex, composed of deformed quartzofeldspathic rocks and lenticular or quasi-lamellar amphibolites, is located on the northern section of the eastern Central Asian orogenic belt in Inner Mongolia, China. In this paper, we present a systematic study of the petrology, whole-rock geochemistry, and geochronology of an amphibolite and an epidote amphibolite from the complex. The protolith of the amphibolite is a gabbro or gabbroic diorite with a laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) zircon U-Pb age of 382 ± 4 Ma and uniform eHf(t) values (–1.06 to +1.25). The protolith of the epidote amphibolite is a quartz diorite with a LA-ICP-MS zircon U-Pb age of 327 ± 5 Ma and uniform positive eHf(t) values (+0.78 to +4.11). The primitive magma of the Devonian gabbroic dike was generated by partial melting of a spinel lherzolite lithospheric mantle that was modified by fluids and melts from subducted slab components. A newly enriched lithospheric mantle is a possible source region for the Devonian mafic rocks. Fractionation of olivine and hornblende played a dominant role in magma differentiation with little or no crustal contamination. Amphibolite-facies metamorphism affected the Devonian gabbroic rocks at 321.6 ± 3.1 Ma, and the quartz diorite underwent epidote amphibolite-facies metamorphism at 279.4 ± 5.3 Ma, based on hornblende 40Ar/39Ar dating. The Devonian to Carboniferous intrusive rocks in the eastern Central Asian orogenic belt likely formed during the collapse of a mature arc at the southern margin of the South Mongolian microcontinent. Combining our results with previous data, we identify an initial phase of postcollisional extension (382–340 Ma) that occurred after earlier compression related to intra-oceanic subduction (484–469 Ma), ridge subduction (440–434 Ma), and arc-continent collision (427–383 Ma). We also constrain increasing extension accompanying extensive collapse of the mature arc (340–309 Ma), northward subduction of the forearc oceanic crust (322–274 Ma) coeval with development of the Hegenshan back-arc basin (354–269 Ma), and final collision (246–228 Ma). The presence of an accretionary belt along the southern margin of the South Mongolian microcontinent reflects the importance of continental growth by accretion of an arc chain during the Late Cambrian–Middle Triassic in the eastern Central Asian orogenic belt.