A Geochemical Study Of Early Cretaceous Postcollisional Mafic Igneous Rocks From The Qinling-Hong’an-Dabie Orogens

The Qinling-Tongbai-Hong’an-Dabie-Sulu orogenic belt, also termed as the Central Orogenic Belt of China, marks the final amalgamation between the North China Block and the South China block from the Paleozoic to Mesozoic in central China. It is not only one of the largest and best-exposed high-pressure/ultrahigh-pressure metamorphic terranes on Earth, but also one of the regions with the most profound occurrence of postcollisional magmatism. Studies concerning the petrological and geochemical characteristics, mantle source and tectonic setting of postcollisional mafic-ultramafic rocks have important implications for understanding of the recycling of subducted crustal materials and the tectonic evolution of collisional orogens. This dissertation focuses on petrology and geochemistry of Early Cretaceous postcollisional mafic igneous rocks in the Qinling-Hong’an-Dabie orogens. The results provide new evidence for the recycling of subducted oceanic and continental crust, and new constraints on the mechanism of slab-mantle interaction, the nature of orogenic lithospheric mantle and the magmatic processes of postcollisional mafic magmatism. This has great bearing on chemical geodynamics of the recycling of subducted crustal materials at continental subduction zone.The Daoshichong and Zhujiapu postcollisional mafic igneous rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr-Nd-Pb isotopes in addition to zircon U-Pb ages and Hf-O isotopes. The results provide new constrains on the nature of orogenic lithospheric mantle and the recycling of subducted continental crust of the South China Block. Zircon U-Pb dating gives consistent ages of125±3to131±1Ma for magma crystallization. Residual zircon cores in the postcollisional mafic rocks were identified by the CL imaging and U-Pb dating for the first time. They exhibit Triassic ages of234±5Ma and Neoproterozoic ages of770±11Ma and739±9Ma, in agreement with ages of tectonothermal events for ultrahigh-pressure metamorphism and protolith formation respectively. These residual zircon cores provide the geochronological evidence for the involvement of the subducted South China Block in their mantle source. The zircon Hf-O isotope compositions of the postcollisional mafic rocks show systematic variations that can be categorized into three groups. Group I has the lowest δ18O values of2.0to2.9%o but the highest εHf(t) values of-3.5to-1.0with the youngest Hf model ages of1.2to1.4Ga. Group Ⅱ displays intermediate δ18O values of4.0to5.1‰andf(t) values of-22.5to-13.2with Hf model ages of2.0to2.6Ga. Group III exhibits the highest δ18O values of5.2to7.3‰but the lowest εHf(t) values of-29.1to-18.6with the oldest Hf model ages of2.4to3.0Ga. The three groups of Hf-O isotope compositions correspond to a three-layer Hf-O isotope structure in the subducted continental crust, suggesting their involvement in the mantle source. These mafic-ultramafic rocks also have high contents of MgO (up to18.0wt%), Cr (up to1546ppm) and Ni (up to349ppm), but low contents of SiO2(41.0-51.9wt%), and exhibit arc-like trace element distribution patterns and enriched Sr-Nd-Pb isotope compositions. These geochemical features indicate their derivation from partial melting of special mantle sources that are fertile in lithochemistry and enriched not only in LILE and LREE but also in Sr-Nd-Pb isotopes. The mantle sources are suggested to be part of the orogenic lithospheric mantle and they would be generated by melt-peridotite reaction in the continental subduction channel. The enriched signatures of incompatible trace elements and radiogenic isotopes in the mantle sources would be caused by metasomatic reaction of the overlying subcontinental lithospheric mantle (SCLM) peridotite with felsic melts derived from the subducted continental crust during the Triassic continental collision. Significant differences in element and isotope compositions between different mafic intrusions suggest that the orogenic lithospheric mantle is geochemically heterogeneous, with possible anatexis of hornblende-rich and pyroxene-rich lithologies in the mantle sources. The heterogeneity is primarily attributed to differences in the compositions of felsic melts derived from subducted crust with a tectonic affinity to the South China Block, whereas the same SCLM of the North China Block was involved in the crustal metasomatism in the subduction channel.The subduction of continental crust is hypothetically caused by gravitational pulling of subducting oceanic crust. Recycling of oceanic crustal materials is also expected to occur in the continental subduction zone. This is demonstrated by an integrated study of major-trace elements and stable-radiogenic isotopes in postcollisional mafic igneous rocks from the Hong’an orogen. Mafic dykes in the Hong’an orogen exhibit OIB-like trace element distribution patterns, high Nb/U and TiO2/Al2O3ratios, relatively depleted radiogenic isotope compositions with consistently high εNd(t) values of-1.8to4.5and low initial87Sr/86Sr ratios of0.7040to0.7050. They are significantly different from mafic intrusives in the Dabie orogen that exhibit arc-like trace element distribution patterns and relatively enriched radiogenic isotope compositions with low εNd(t) values of-2.3to-20.7and high initial87Sr/86Sr ratios of0.7061to0.7114. The Hong’an mafic dykes also display variable zircon Hf-O isotope compositions. These observations are interpreted as indicating significant differences in the nature of mantle sources between the Hong’an and Dabie mafic rocks. Two types of melt-peridotite reaction are assumed to occur in oceanic and continental subduction channels, respectively, during continental collision. Reaction of juvenile SCLM peridotite with felsic melt derived from subducting oceanic basalt and overlying sediment before continental subduction would generate the mantle source for the Hong’an mafic dykes. Since partial melting of the subducting oceanic crust with the rutile breakdown is capable of generating the felsic melts with OIB-like trace element distribution patterns, high Nb/U and TiO2/Al2O3ratios, and relatively depleted radiogenic isotopes. Whereas reaction of the ancient SCLM peridotite with felsic melts from subducting continental crust would generate the mantle source for the Dabie mafic intrusives. Such contrasting types of melt-peridotite reaction at the slab-mantle interface are responsible for the systematic differences in the geochemical compositions of mantle sources, generating non-peridotite lithology such as pyroxenite and hornblendite. Therefore, postcollisoinal mafic rocks record the tectonic transition from oceanic subduction to continental collision.An integrated study of major-trace elements and radiogenic isotopes was performed for Early Cretaceous alkali basalts from West Qinling. The results not only highlight the hornblendite+peridotite mantle lithology for alkali basalts, but also record the recycling of subducted Paleotethyan oceanic crust in continental collision zone. The alkali basalts have high contents of MgO (7.18-11.1wt.%), Na2O+K2O (2.9-4.8wt.%), TiO2(2.2-2.9wt.%) but low contents of SiO2(41.4-45.7wt.%). They exhibit OIB-like trace element distribution patterns, with enrichment of LILE and LREE but no depletion of HFSE. They also show relatively depleted Sr-Nd-Hf isotope compositions, with low initial87Sr/86Sr ratios of0.7035to0.7058, positive εNd(t) values of4.6to7.7and εHf(t) values of8.8to13.5for whole-rock, and positive εHf(t) values of5.9to12.1for zircon. Such element and isotope characteristics indicate their origination from the juvenile SCLM source with involvement of oceanic crustal component. The variable Ba/Th, Sr/Y and (La/Yb)N ratios indicate the crustal materials are composed of oceanic basalt and overlying sediments. The alkali basalts generally display high K2O/Na2O ratios, and high K2O and TiO2contents, suggesting their derivation from partial melting of hornblendite-rich mantle lithology. Taken together, all the above geochemical characteristics can be accounted for by partial melting of hornblendite+peridotite SCLM source. The hornblendite would be generated by reaction of the mantle wedge peridotite with felsic melts derived from subducting oceanic crust at the slab-mantle interface in oceanic subduction channel. Therefore, the West Qinling alkaline basalts record the recycling of subducted Paleotethyan oceanic crust. The metasomatic hornblendite is an important lithology in the orogenic lithospheric mantle that was transformed from the mantle wedge overlying the oceanic subduction channel.Postcollisional mafic igneous rocks commonly exhibit petrological and geochemical heterogeneities, but their origin still remains enigmatic. While source mixing is substantial due to the crust-mantle interaction during continental collision, magma mixing is also significant during postcollisional magmatism. This is illustrated by the mafic igneous rocks of Early Cretaceous age in the Dabie orogen. These mafic rocks exhibit arc-like trace element distribution patterns and enriched Sr-Nd-Pb isotope compositions, indicating their derivation from enriched mantle sources. They have variable whole-rock εNd(t) values of-17.6to-5.2and zircon εHf(t) values of-29.0to-7.7, pointing to source heterogeneities. A relict zircon core yields a Neoproterozoic U-Pb age of765±8Ma, in agreement with the protolith age of ultrahigh-pressure metaigneous rocks in the Dabie orogen. Clinopyroxene and plagioclase megacrystals exhibit complex textural and compositional variations, recording three stages of magma evolution. Cpx-1core has low Cr and Ni but high Ba, Rb and K contents, indicating its crystallization from a mafic melt derived from partial melting of peridotite rich in phlogopite (Melt1). Cpx-1mantle and Cpx-2exhibit significantly high Cr, Ni and Al2O3but low Rb and Ba contents, suggesting their crystallization from a mafic melt derived from partial melting of pyroxenite (Melt1). Whole-rock initial87Sr/86Sr ratios of gabbro lies between those of Pl-1core (crystallized from Melt1) and Pl-1mantle and Pl-2core (crystallized from Melt2), providing isotopic evidence for magma mixing between Meltl and Melt2. With magma mixing and subsequent ascent, the mixed and evolved mafic magmas are produced to crystallize Cpx-1rim, Amp-2, Pl-1rim, Pl-2rim and matrix minerals. Thus, different ultramafic lithologies were involved in partial melting for the mafic magmatism. Taken together, the heterogeneously enriched mantle source would be generated by the source mixing due to the reaction of the overlying SCLM wedge peridotite with felsic melts derived from different layers 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 heterogeneous metasomatic mantle domains in the postcollisional stage. As a consequence, the source and magma mixing processes are responsible for the significant variations in the whole-rock and mineral geochemistry of postcollisional mafic igneous rocks.In summary, the postcollisoinal mafic rocks in the continental collision zone record the tectonic transition from oceanic subduction to continental collision. The fertile and enriched mantle lithologies (such as hornblendite and pyroxenite) would be generated by the reaction of the overlying SCLM peridotite with felsic melts of different compositionsderived from different subducted crustal materials during continental collision. The nature of slab-mantle interaction in subduction channel is a key to the origin of mantle sources for postcollisional mafic igneous rocks. Both source and magma mixing processes are responsible for the petrological and geochemical heterogeneities of postcollisional mafic igneous rocks.