Heterogeneous orogenic lithospheric mantle beneath the North Qaidam orogen: Geochemical evidence from syn-exhumation and post-collisional mafic magmatic rocks

Abstract

Syn-exhumation and post-collisional mafic magmatic rocks are generally exposed in continental collision orogens. Their geochemical compositions can not only reflect the nature of orogenic lithospheric mantle but also provide a window to trace crust-mantle interaction in continental collision zones. Herein, we present a combined study of whole-rock major-trace elements and Sr-Nd-Hf isotopes as well as zircon U Pb ages and Hf O isotopes for the Paleozoic mafic magmatic rocks from the North Qaidam orogen, northeastern Tibet. Zircon U Pb dating for the rocks yields two groups of ages of 420 ± 8 to 395 ± 2 Ma and 383 ± 5 to 368 ± 3 Ma, corresponding to the syn -exhumation and post-collisional stages, respectively. These mafic magmatic rocks are characterized by arc-like trace element distribution patterns, enriched to depleted radiogenic Sr-Nd-Hf isotope compositions and high zircon δ 18 O values, which are comparable to those of the orthogneiss and continental eclogite in this orogen. This indicates their derivation from the orogenic lithospheric mantle modified by the subducted continental crust during the continental collision. Significant differences in radiogenic isotope compositions between the syn -exhumation and post-collisional mafic magmatic rocks suggest their mantle sources were mainly metasomatized by the orthogneiss- and continental eclogite-derived melts, respectively. Quantitative modelling of trace element compositions shows that about 93.0:6.0:1.0 and 91.7:2.8:5.5 mixture of the mantle peridotite, orthogneiss- and continental eclogite-derived melts with 23% and 28% aggregated fractional melting can closely match the syn -exhumation and post-collisional mafic magmatic rocks, respectively. The reaction of the continental crust-derived melts with mantle peridotite would result in the heterogeneous orogenic lithospheric mantle. Therefore, the syn -exhumation and post-collisional mafic magmatic rocks provide a snapshot of the compositional variations in the orogenic lithospheric mantle and the crust-mantle interaction in the continental collision zone. • Syn-exhumation/post-collisional mafic magmatic rocks in the North Qiadam orogen. • They were derived from a heterogeneous orogenic lithospheric mantle. • The mantle source was metasomatized by melts from the subducted continental crust. • These rocks record crust-mantle interaction in the continental subduction channel.