Source and magma mixing processes in continental subduction factory: Geochemical evidence from postcollisional mafic igneous rocks in the Dabie orogen

Abstract

Postcollisional mafic igneous rocks commonly exhibit petrological and geochemical heterogeneities, but their origin still remains enigmatic. While source mixing is substantial due to the crustmantle interaction during continental collision, magma mixing is also significant during postcollisional magmatism. The two processes are illustrated by Early Cretaceous mafic igneous rocks in the Dabie orogen. These mafic rocks show arc-like trace element distribution patterns and enriched Sr-Nd-Pb isotope compositions, indicating their origination from enriched mantle sources. They have variable whole-rock e Nd (t) values of 217.6 to 25.2 and zircon e Hf (t) values of 229.0 to 27.7, pointing to source heterogeneities. Such whole-rock geochemical features are interpreted by the source mixing through melt-peridotite reaction in the continental subduction channel. Clinopyroxene and plagioclase megacrystals show complex textural and compositional variations, recording three stages of mineral crystallization during magma evolution. Cpx-1 core has low Cr and Ni but high Ba, Rb and K, indicating its crystallization from a mafic melt (Melt 1) derived from partial melting of hydrous peridotite rich in phlogopite. Cpx-1 mantle and Cpx-2 exhibit significantly high Cr, Ni and Al 2 O 3 but low Rb and Ba, suggesting their crystallization from pyroxenite-derived mafic melt (Melt 2). Whole-rock initial 87 Sr/ 86 Sr ratios of gabbro lies between those of Pl-1core (crystallized from Melt 1) and Pl-1 mantle and Pl-2 core (crystallized from Melt 2), providing isotopic evidence for magma mixing between Melt 1 and Melt 2. Taken together, a heterogeneously enriched mantle source would be generated by the source mixing due to reaction of the overlying subcontinental lithospheric mantle wedge peridotite with felsic melts derived from partial melting of different rocks of the deeply subducted continental crust during the continental collision. The magma mixing would occur between mafic melts that were derived from partial melting of the heterogeneously metasomatic mantle domains in the postcollisional stage. As a consequence, the source and magma mixing processes in the continental subduction factory are responsible for the significant variations in the whole-rock and mineral geochemistries of postcollisional mafic igneous rocks.

Two types of the crust-mantle interaction in continental subduction zones

“…Partial melting of the mantle metasomatites gives rise to various mafic igneous rocks above subduction zones. As such, their petrogenesis can be accounted for by the slab-mantle interaction model (Zheng, 2012;Zhao et al, 2013;Chen et al, 2014;Dai et al, , 2015aDai et al, , 2015b.…”

“…Reaction of these melts with the overlying mantle wedge peridotite would generate pyroxene/amphibole-rich metasomatites. The metasomatites could be stored in the lithospheric mantle for a long time, and then become partially melted to give rise to mafic igneous rocks, which have OIB-like trace element distribution patterns, depleted radiogenic isotope compositions and H-O isotope compositions different from those of the normal mantle (Zheng, 2012;Dai et al, 2014Dai et al, , 2015aDai et al, , 2015bXu et al, 2014aXu et al, , 2014b.…”

“…In the continental subduction channel, these melts would react with the overlying SCLM wedge peridotite, forming pyroxene/amphibole-rich peridotite, olivine-poor pyroxenite and even hornblendite. These metasomatites can serve as the mantle sources of syn-exhumation and postcollisional mafic igneous rocks in continental collision orogens (Dai et al, , 2015aZhao et al, 2012Zhao et al, , 2013. Because the mantle metasomatites were generated by reaction of the ancient lithospheric mantle with felsic melts from the continental crust, the resulted mafic igneous rocks exhibit arc-like trace element distribution patterns and enriched radiogenic isotope compositions.…”

“…Coexisting UHP oceanic and continental crustal rocks have been found in many continental subduction zones (Beltrando et al, 2010;Wu and Zheng, 2013;Song et al, 2014;Zhou et al, 2015). In addition, postcollisional mafic igneous rocks have recorded the recycling of not only the oceanic crust but also the continental crust (Dai et al, 2011(Dai et al, , 2015a(Dai et al, , 2015bMerle et al, 2014). Therefore, it is necessary to decipher the petrological and geochemical records of the recycling of different types of crustal rocks and crust-mantle interaction during continental collision, to probe into similarities and differences in the slab-mantle interaction between continental subduction and fossil oceanic subduction, and to advance the understanding of chemical geodynamics for the recycling of crustal materials.…”

“…Previous studies of elemental and isotopic geochemistry have revealed that the postcollisional mafic igneous rocks in the Hong'an-Dabie-Sulu orogens and SE North China did not suffer significant crustal contamination during magma ascent, and their geochemical features thus can be used to determine the nature of their mantle sources Dai et al, 2012Dai et al, , 2015aDai et al, , 2015bZhao et al, 2013, and references therein). Although these mafic igneous rocks exhibit wide variations in lithochemical compositions and rock types due to crystal fractionation, the continental crustal signatures are prominent in these rocks in terms of trace element and isotope compositions, such as enrichment of LILE and LREE, depletion of HFSE (Figure 3), high whole-rock initial 87 Sr/ 86 Sr ratios, low whole-rock ε Nd (t) values ( Figure 4) and low zircon ε Hf (t) values ( Figure 5(b)).…”