Li-Qun Dai

Developing plate tectonics theory from oceanic subduction zones to collisional orogens

Crustal subduction and continental collision is the core of plate tectonics theory. Understanding the formation and evolution of continental collision orogens is a key to develop the theory of plate tectonics. Different types of subduction zones have been categorized based on the nature of subducted crust. Two types of collisional orogens, i.e. arc-continent and continent-continent collisional orogens, have been recognized based on the nature of collisional blocks and the composition of derivative rocks. Arc-continent collisional orogens contain both ancient and juvenile crustal rocks, and reworking of those rocks at the post-collisional stage generates magmatic rocks with different geochemical compositions. If an orogen is built by collision between two relatively old continental blocks, post-collisional magmatic rocks are only derived from reworking of the old crustal rocks. Collisional orogens undergo reactivation and reworking at action of lithosphere extension, with inheritance not only in the tectonic regime but also in the geochemical compositions of reworked products (i.e., magmatic rocks). In order to unravel basic principles for the evolution of continental tectonics at the post-collisional stages, it is necessary to investigate the reworking of orogenic belts in the post-collisional regime, to recognize physicochemical differences in deep continental collision zones, and to understand petrogenetic links between the nature of subducted crust and post-collisional magmatic rocks. Afterwards we are in a position to build the systematics of continental tectonics and thus to develop the plate tectonics theory.

Postcollisional mafic igneous rocks record crust-mantle interaction during continental deep subduction.

Findings of coesite and microdiamond in metamorphic rocks of supracrustal protolith led to the recognition of continental subduction to mantle depths. The crust-mantle interaction is expected to take place during subduction of the continental crust beneath the subcontinental lithospheric mantle wedge. This is recorded by postcollisional mafic igneous rocks in the Dabie-Sulu orogenic belt and its adjacent continental margin in the North China Block. These rocks exhibit the geochemical inheritance of whole-rock trace elements and Sr-Nd-Pb isotopes as well as zircon U-Pb ages and Hf-O isotopes from felsic melts derived from the subducted continental crust. Reaction of such melts with the overlying wedge peridotite would transfer the crustal signatures to the mantle sources for postcollisional mafic magmatism. Therefore, postcollisonal mafic igneous rocks above continental subduction zones are an analog to arc volcanics above oceanic subduction zones, providing an additional laboratory for the study of crust-mantle interaction at convergent plate margins.

Geochemical insights into the role of metasomatic hornblendite in generating alkali basalts

Experimental petrology suggested the role of hornblendite in generating alkali basalt. This mechanism is confirmed by an integrated study of major-trace elements and radiogenic isotopes for Mesozoic alkali basalts from the Qinling orogen in China. The alkali basalts have high contents of MgO (4.8–11.1 wt %, Mg# = 47–69), Na2O + K2O (2.9–5.4 wt %), TiO2 (2.0–3.1 wt %) but low content of SiO2 (41.4–49.6 wt %), which are generally silica-undersaturated with normative minerals of nepheline and olivine. They exhibit OIB-like trace element distribution patterns, with enrichment of LILE and LREE but no depletion of HFSE relative to the primitive mantle. They also show relatively depleted Sr-Nd-Hf isotope compositions, with low initial 87Sr/86Sr ratios of 0.7028–0.7058, positive eNd(t) values of 4.0–9.8 and eHf(t) values of 8.8–13.5 for whole-rock, and positive eHf(t) values of 5.2–16.4 for zircon. Such element and isotope features indicate their origination from the juvenile subcontinental lithospheric mantle (SCLM) source with involvement of crustal components. The alkali basalts generally have high K2O/Na2O ratios, and high K2O and TiO2 contents, suggesting their derivation from partial melting of hornblendite-rich mantle lithology. They also exhibit variable K/La and Ti/La ratios that are correlated with (La/Yb)N ratios, indicating a geochemical heterogeneity of the SCLM source. Taken together, all the above geochemical features can be accounted for by partial melting of a hornblendite-rich SCLM source. The hornblendite would be generated by reaction of the juvenile SCLM wedge peridotite with hydrous felsic melts derived from subducted Palaeotethyan oceanic crust at the slab-mantle interface in the subduction channel. Therefore, orogenic alkali basalts record recycling of the subducted fossil oceanic crust, and the metasomatic hornblendite is an important lithology in local SCLM domains above fossil subduction channels.

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

Plate subduction is an important mechanism for exchanging the mass and energy between the mantle and the crust, and the igneous rocks in subduction zones are the important carriers for studying the recycling of crustal materials and the crust-mantle interaction. This study presents a review of geochronology and geochemistry for postcollisional mafic igneous rocks from the Hong’an-Dabie-Sulu orogens and the southeastern edge of the North China Block. The available results indicate two types of the crust-mantle interaction in the continental subduction zone, which are represented by two types of mafic igneous rocks with distinct geochemical compositions. The first type of rocks exhibit arc-like trace element distribution patterns (i.e. enrichment of LILE, LREE and Pb, but depletion of HFSE) and enriched radiogenic Sr-Nd isotope compositions, whereas the second type of rocks show OIB-like trace element distribution patterns (i.e. enrichment of LILE and LREE, but no depletion of HFSE) and depleted radiogenic Sr-Nd isotope compositions. Both of them have variable zircon O isotope compositions, which are different from those of the normal mantle zircon, and contain residual crustal zircons. These geochemical features indicate that the two types of mafic igneous rocks were originated from the different natures of mantle sources. The mantle source for the second type of rocks would be generated by reaction of the overlying juvenile lithospheric mantle with felsic melts originated from previously subducted oceanic crust, whereas the mantle source for the first type of rocks would be generated by reaction of the overlying ancient lithospheric mantle of the North China Block with felsic melts from subsequently subducted continental crust of the South China Block. Therefore, there exist two types of the crust-mantle interaction in the continental subduction zone, and the postcollisional mafic igneous rocks provide petrological and geochemical records of the slab-mantle interactions in continental collision orogens.

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

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. 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 eNd(t) values of −17.6 to −5.2 and zircon eHf(t) values of −29.0 to −7.7, 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 Al2O3 but low Rb and Ba, suggesting their crystallization from pyroxenite-derived mafic melt (Melt 2). Whole-rock initial 87Sr/86Sr 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.

Recycling of Paleotethyan oceanic crust; geochemical record from postcollisional mafic igneous rocks in the Tongbai-Hong'an Orogens

Plate tectonics can develop from oceanic subduction to continental collision, with disappearance of presubducted oceanic crust in continental subduction zones. This is typical for collisional orogens where Tethyan oceanic crust was consumed during continental convergence, with recycling of the Tethyan oceanic crust into the mantle. However, it is difficult to trace the geochemical signature of the previously subducted oceanic crust in collisional orogens. Postcollisional mafic igneous rocks in the Tongbai-Hong’an orogens provide a geochemical record of crust-mantle interaction during Paleotethyan oceanic subduction preceding the Triassic continental collision. These mafic igneous rocks were emplaced in the Cretaceous after continental collision and exhibit oceanic-island basalt (OIB)−like trace-element distribution patterns, with enrichment of large ion lithophile elements (LILEs) and light rare earth elements (LREEs) but no depletion of high field strength elements (HFSEs). They also show relatively depleted whole-rock Sr-Nd-Hf isotope compositions, with initial 87 Sr/ 86 Sr ratios of 0.7043−0.7050, e Nd ( t ) values of −1.8−4.5, and e Hf ( t ) values of 4.7−10.3. Such geochemical features indicate their origin from a juvenile mantle source that contained subducted oceanic crust−derived material. Thus, we suggest that the mantle source was generated by a melt-peridotite reaction at the slab-mantle interface in the Paleotethyan oceanic subduction channel during continental collision. The original peridotite would have been characterized by depletion of melt-mobile incompatible trace elements and radiogenic Sr-Nd-Hf isotopes, corresponding to the juvenile subarc lithospheric mantle. In contrast, the subducted crust−derived material would have been characterized by enrichment of LILEs and LREEs but no depletion of HFSEs, corresponding to felsic melts that were derived from partial melting of the subducting Paleotethyan oceanic basalt and sediment outside the rutile stability field. As such, the melt-peridotite reaction would generate pyroxenite-rich ultramafic metasomatites in the lower part of the mantle wedge, with relative enrichment of LILEs and LREEs but no depletion of HFSEs. In this model, partial melting of the metasomatites gave rise to the postcollisional mafic igneous rocks in the Tongbai-Hong’an orogens. Therefore, the postcollisional mafic igneous rocks record the recycling of Paleotethyan oceanic crust in the collisional orogen.

Geochemical Distinction between Carbonate and Silicate Metasomatism in Generating the Mantle Sources of Alkali Basalts

Crustal metasomatism by subduction is considered as an important mechanism for generating mantle heterogeneity through infiltration of different metasomatic agents into the mantle. As a consequence of the subduction of oceanic crust (including oceanic basaltic rocks and seafloor sediments), both carbonate and silicate metasomatism are expected to occur at the slab–mantle interface in an oceanic subduction channel. This is demonstrated by an integrated study of major and trace elements, stable Mg-isotopes and radiogenic Sr–Nd–Hf isotopes in Cenozoic and Mesozoic alkali basalts from the West Qinling orogen, China. Although the two series of continental basalts show ocean island basalt (OIB)-like trace element distribution patterns and relatively depleted Sr– Nd–Hf isotope compositions, they exhibit differences in other geochemical variables. The Cenozoic basalts have low SiO2, but high CaO and MgO concentrations, and high CaO/Al2O3 ratios but low d Mg values of –0 54 to –0 32ø. They have relatively high abundances of melt-mobile incompatible trace elements such as the light rare earth elements (LREE) and most large ion lithophile elements (LILE), but low abundances of K, Pb, Zr, Hf and Ti, with high (La/Yb)N values but low Ti/Eu and Hf/Sm ratios. In contrast, the Mesozoic basalts have relatively high SiO2, but low CaO and MgO concentrations, and low CaO/Al2O3 ratios, but high d 26 Mg values of –0 35 to –0 21ø. They have relatively low abundances of melt-mobile incompatible trace elements such as the LILE and LREE, but are relatively enriched in Zr, Hf and Ti, with low (La/Yb)N values but high Ti/Eu and Hf/Sm ratios. These observations indicate a significant difference in the composition of the mantle sources between the two periods of alkali basalt magmatism. Whereas the low d Mg values of the Cenozoic basalts indicate the involvement of sedimentary carbonate in their mantle source, the high d Mg values of the Mesozoic basalts point to a primary contribution from a silicate component. Metasomatic reaction of depleted mid-ocean ridge basalt (MORB)-source mantle peridotite with carbonate and silicate melts, respectively, at the slab–mantle interface in the Paleotethyan oceanic subduction channel are responsible for the generation of their mantle sources. Both carbonate and silicate melts have been incorporated into the depleted MORB-source mantle wedge producing ‘crustal metasomatism’ in an oceanic subduction channel. We suggest that the type of metasomatic agent in the mantle sources is the key to the composition of the resultant alkali basalts.

Mesozoic mafic magmatism in North China: Implications for thinning and destruction of cratonic lithosphere

The North China Craton (NCC) has been thinned from >200 km to <100 km in its eastern part. The ancient subcontinental lithospheric mantle (SCLM) has been replaced by the juvenile SCLM in the Meoszoic. During this period, the NCC was destructed as indicated by extensive magmatism in the Early Cretaceous. While there is a consensus on the thinning and destruction of cratonic lithosphere in North China, it has been hotly debated about the mechanism of cartonic destruction. This study attempts to provide a resolution to current debates in the view of Mesozoic mafic magmatism in North China. We made a compilation of geochemical data available for Mesozoic mafic igneous rocks in the NCC. The results indicate that these mafic igneous rocks can be categorized into two series, manifesting a dramatic change in the nature of mantle sources at ~121 Ma. Mafic igneous rocks emplaced at this age start to show both oceanic island basalts (OIB)-like trace element distribution patterns and depleted to weakly enriched Sr-Nd isotope compositions. In contrast, mafic igneous rocks emplaced before and after this age exhibit both island arc basalts (IAB)-like trace element distribution patterns and enriched Sr-Nd isotope compositions. This difference indicates a geochemical mutation in the SCLM of North China at ~121 Ma. Although mafic magmatism also took place in the Late Triassic, it was related to exhumation of the deeply subducted South China continental crust because the subduction of Paleo-Pacific slab was not operated at that time. Paleo-Pacific slab started to subduct beneath the eastern margin of Eruasian continent since the Jurrasic. The subducting slab and its overlying SCLM wedge were coupled in the Jurassic, and slab dehydration resulted in hydration and weakening of the cratonic mantle. The mantle sources of ancient IAB-like mafic igneous rocks are a kind of ultramafic metasomatites that were generated by reaction of the cratonic mantle wedge peridotite not only with aqueous solutions derived from dehydration of the subducting Paleo-Pacific oceanic crust in the Jurassic but also with hydrous melts derived from partial melting of the subducting South China continental crust in the Triassic. On the other hand, the mantle sources of juvenile OIB-like mafic igneous rocks are also a kind of ultramafic metasomatites that were generated by reaction of the asthenospheric mantle underneath the North China lithosphere with hydrous felsic melts derived from partial melting of the subducting Paleo-Pacific oceanic crust. The subducting Paleo-Pacific slab became rollback at ~144 Ma. Afterwards the SCLM base was heated by laterally filled asthenospheric mantle, leading to thinning of the hydrated and weakened cratonic mantle. There was extensive bimodal magmatism at 130 to 120 Ma, marking intensive destruction of the cratonic lithosphere. Not only the ultramafic metasomatites in the lower part of the cratonic mantle wedge underwent partial melting to produce mafic igneous rocks showing negative εNd(t) values, depletion in Nb and Ta but enrichment in Pb, but also the lower continent crust overlying the cratonic mantle wedge was heated for extensive felsic magmatism. At the same time, the rollback slab surface was heated by the laterally filled asthenospheric mantle, resulting in partial melting of the previously dehydrated rocks beyond rutile stability on the slab surface. This produce still hydrous felsic melts, which metasomatized the overlying asthenospheric mantle peridotite to generate the ultramafic metasomatites that show positive εNd(t) values, no depletion or even enrichment in Nb and Ta but depletion in Pb. Partial melting of such metasomatites started at ~121 Ma, giving rise to the mafic igneous rocks with juvenile OIB-like geochemical signatures. In this context, the age of ~121 Ma may terminate replacement of the ancient SCLM by the juvenile SCLM in North China. Paleo-Pacific slab was not subducted to the mantle transition zone in the Mesozoic as revealed by modern seismic tomography, and it was subducted at a low angle since the Jurassic, like the subduction of Nazca Plate beneath American continent. This flat subduction would not only chemically metasomatize the cratonic mantle but also physically erode the cratonic mantle. Therefore, the interaction between Paleo-Pacific slab and the cratonic mantle is the first-order geodynamic mechanism for the thinning and destruction of cratonic lithosphere in North China.