Origin of arc-like magmatism at fossil convergent plate boundaries: Geochemical insights from Mesozoic igneous rocks in the Middle to Lower Yangtze Valley, South China

A lithological association comprising high-K calc-alkaline (HKCA) intrusions and shoshonitic volcanics, sometimes with A-type granites, is common along fossil convergent plate boundaries. Although igneous rocks in this association show arc-like geochemical signatures, their genetic link to subduction of oceanic slabs has been generally overlooked because their present occurrences are far away from modern subduction zones. This issue is addressed by an integrated study of Mesozoic igneous rocks from the Middle to Lower Yangtze Valley (MLYV) in South China. These igneous rocks have their lithologies starting from gabbroic to granitic HKCA intrusions with rare lamprophyres, followed by mafic to felsic shoshonitic volcanics/subvolcanics with slightly younger A-type granites. All of the mafic igneous rocks exhibit typical arc-like trace element distribution patterns with significant enrichment in LILE and LREE but depletion in HFSE. The HKCA intrusions are generally more enriched in Sr-NdHf isotope compositions than the shoshonitic volcanics/subvolcanics and A-type granites with some overlap. Whereas the less enriched Sr-Nd isotope compositions for these Mesozoic arc-like igneous rocks in the MLYV are comparable with the continental arc igneous rocks of Neoproterozoic age in the Jiangnan orogen between the Cathaysia terrane and the Yangtze craton, the more enriched ones suggest involvement of the ancient continental crust in their magma sources. As such, the mantle sources for the Mesozoic mafic igneous rocks would be generated by previous metasomatism of the mantle wedge, which is most likely related to subduction of the Cathaysia oceanic slab beneath the Yangtze craton in the Early Neoproterozoic. The shoshonitic mafic rocks become less and less enriched in potassium relative to sodium with time, which is associated with a decrease in their Rb/Sr ratios and an increase in their epsilon Nd(t) values. Such compositional changes indicate a progressive transition of their mantle sources from a more enriched phlogopite-bearing one to a less enriched amphibolebearing one. In association with the gradually less enriched Nd isotope composition, furthermore, there are systematical decreases in both Sr/Y and (La/Yb)N ratios and increases in both Y and Yb concentrations from the felsic HKCA rocks through the felsic shoshonitic rocks to the A-type granites. This indicates that the crustal components in the source rocks for the Mesozoic felsic igneous rocks in the MLYV have changed from the more enriched ancient deep crust to the less enriched juvenile shallow crust. While these two aspects of transition in source lithology and geochemistry are recorded by the different compositions of igneous rocks, partial melting of the source rocks is dictated by thinning and extension of the Neoproterozoic orogenic lithosphere for destruction in the Mesozoic. The systematical younging of the mafic magmatism from the southwest to the northeast indicates that the orogenic destruction was triggered by asthenospheric upwelling in response to northeastward rollback of the subducting Paleo-Pacific (Izanagi) slab. As such, it is superimposition of the lithospheric thinning and extension on the fossil convergent plate margin that resulted in the generation of the unique rock association with the arc affinity in the continental interior.