| Mountain root removal in the Dabie orogen is inferred to have occurred in early Cretaceous. Adakitic rocks, regarded as partial melts from thickened/delaminated eclogitic crust, can provide key constraints on the mountain root removal processes and the composition and the architecture of the crustal root. Previous studies on granitoids in the Dabie orogen mainly focused on the North Dabie Zone. In this study, a systematic study on the geochronology, major and trace elemental chemistry, and radiogenic isotope composition of granitoids from other units of the Dabie orogen was conducted. The purposes of this study include:(i) as high Sr/Y and La/Yb, based on which adakitic rocks are generally identified, can be produced by multiple processes, additional criteria are presented that can identify adakitic rocks as partial melts of thickened crust; (ii) to reveal the geochemical differences between high-Mg and low-Mg adakitic rocks and the relevant mechanism during melt/mantle interaction, providing a geochemical basis to identify partial melts from delaminated lower crust; (iii) to reveal the spatial and temporal distribution of low-Mg and high-Mg adakitic rocks, providing constraints on the triggering mechanism of the mountain root removal process; and (iv) to trace the crustal architecture and constitution of the Dabie orogen and its mountain root, giving constraints on crustal thickening in collisional zones. Major conclusions are summarized as following.1. Geochemical characteristics and identification of low-Mg adakitic rocksLow-Mg high Sr/Y granites (HSG) from the Dabie orogen, relative to normal granitoids, display, besides high Sr/Y and low Y contents, the following distinct chemical features:(1) separate trends with higher Sr contents in Sr versus SiO2, and versus CaO diagrams; (2) positively correlated and significantly elevated Sr/Y, (La/Yb)N, (Dy/Yb)N and Nb/Ta (up to 225,153,3.1 and 19.5, respectively). These chemical features are best explained by deep melting of a thickened lower continental crust (LCC) with garnet-dominant, plagioclase-poor, and rutile-present residual phases. Pseudo-adakites reported in the literature do not show these features. Therefore, partial melts from thickened LCC can be identified by the chemical features found in this study.2. Geochemical characteristics of high-Mg adakitic rocks:how melt/mantle interaction influences the melt compositionHigh-Mg HSG from the Dabie orogen have high Sr/Y (31～100) and (La/Yb)N (16～48), high SiO2 (57.2～68.9 wt.%) and Mg# (44-63). Their Sr-Nd-Pb isotope compositions are characterized by lowεNd(t) (-24.9～-14.3), slightly enriched 87Sr/86Sr(i) (0.7057～0.7077), and low 206Pb/204Pb (15.59～16.60). Similar rocks are found widespread along the southern part of the Tan-Lu fault. These rocks belong to high-Mg adakitic rocks produced by delamination and foundering of mafic lower crust. Besides high MgO, Cr and Ni contents and high Mg#, the high-Mg adakitic rocks also show, compared to low-Mg adakitic rocks, the following features:(1) relatively low (La/Yb)N, (Dy/Yb)N, Sr/CaO and Sr/Y; (2) at a given SiO2, lower Al, Na, La and Sr contents; (3) at given MgO, MgO/FeOt, Ni and Ni/Co higher than normal basaltic magmas. Thus, the high-Mg adakitic rocks can be explained by derivation from delaminated lower continental crust and interaction with the mantle. Features (1) and (2) suggest that Opx is the major resulting phase without garnet and that the mass of the metasomatic melt may increase during the melt/mantle interaction process. Feature (3) suggests that the solid phase during melt/mantle interaction is an Opx-rich assemblage which has lower Kd (MgO) (bulk) and higher DNi (bulk) and DNi/Dco (bulk) partition coefficients than an olivine-rich assemblage during partial melting of the mantle.3. Spatial and temporal distribution of high-Mg adakitic rocks:Implications for triggering mechanism of the mountain root removalU-Pb age data suggest that although partial melting of thickened crust of the mountain root (formation of low-Mg adakitic rocks) occurred during the time period of 143-130 Ma, high-Mg adakitic rocks in the Dabie orogen emplaced within the short period from 131-130 Ma, indicating removal of the mountain root after 130-131 Ma. Low-Mg adakitic rocks occur in all units of the Dabie orogen, indicating a thickened crust exists beneath the whole Dabie orogen prior to the early Cretaceous. High-Mg adakitic rocks seem to be restricted to the eastern margin of the orogen, close to the Tan-Lu fault, indicating the key role of the fault for mountain root removal. Nd-Pb isotopic composition indicates that adakitic rocks from the southern part of the Dabie orogen may be related to the contemporaneous activity of the Yangtze River fault.This study points out that adakitic rocks and post-collisional magmatism mainly distribute in the eastern Dabie region and the northern part of the orogen (North Huaiyang and North Dabie). Based on the evolution of the Tan-Lu fault and a new physical modelling experiment, it is suggested that large-scale sinistral strike-slip movements of the Tan-Lu fault during the early Cretaceous can lead to pull-apart extension, which may induce initial melting of the mountain root. The following transition from transtension to extension at c.131 Ma may trigger the foundering of some fragments of the eclogitic crust in the mountain root near the Tan-Lu fault.4. Sr-Nd-Pb isotopic compositions and constraints on the constitution and thickening mechanism of the mountain rootThe range of 87Sr/86Sr(i),d (t) and 206Pb/204Pb(i) for normal granitoids from the Dabie orogen are 0.7062～0.7105 (except one sample),-25.5～-12.7, and 15.51～16.85, respectively, whereas low-Mg adakitic rocks range from 0.7055～0.7087,-27.8～-13.8, and 15.69～17.16, respectively. Except normal granitoids that have slightly higher 87Sr/86Sr(i), normal granitoids, LMA and HMA show similar Sr-Nd-Pb isotopic compositions with low radiogenic Sr and Pb ratios and lowεNd (t). Comparison of Dabie granitoids with high Sr/Y granitoids from the North China Block (NCB) indicates that the lower crust of the South China Block (SCB) has higher Th/U relative to the NCB, and thus higher 208Pb/204Pb at a given 206Pb/204Pb ratio. observation indicates that the crustal mountain root is composed of ancient lower SCB crust.Adakitic rocks from North Dabie have unusually high Th/U (up to 51) with low U contents, indicating that their sources have experienced dehydration. No correlation between Th/U and 208Pb/206Pb suggests that the dehydration is related to the Triassic deep subduction. The low-Mg feature of adakitic rocks from North Dabie indicates that in early Cretaceous the dehydrated (deep subducted) mafic LCC is not covered by a lithospheric mantle, and may have rolled-back. The dehydrated mafic LCC has remained in the lower part of the crustal root underneath North Dabie till early Cretaceous.Adakitic rocks from the other three units, including North Huaiyang to the north of North Dabie, have Th/U (6.0±1.8,1SD) similar to the average composition of LCC. This suggests that parts of mafic lower SCB crust have been thrusted to the north of the deep suture and injected into the NCB crust.It is proposed that the crustal mountain root of the Dabie orogen has been thickened by preservation of the deep subducted mafic lower crust that rolled-back after slab break-off beneath the orogen (North Dabie), injection of the succeeding subducted SCB crust into the NCB crust (North Huaiyang), and shortening of the subducted SCB crust (South Dabie and Susong complex) during the late Triassic and Jrassic.5. Decoupling of Nd isotopic composition between granitoids and exhumed UHP blocks: Constraints on the crustal architecture in collisional zonesGranitoids from the Dabie-Sulu belt haveεNd(130 Ma) significantly lower than exhumed UHP eclogites and gneisses. Thus the former have T2DM older than the latter, indicating decoupling of the Nd isotopic system between exhumed UHP rocks and thickened mafic crustal root. This suggests that the protolith of exhumed UHP rocks mainly is the Neoproterozoic crust in the northern margin of SCB that had been subducted at an early stage of continental collision, while the mountain root from the inner part of SCB was subducted at a later stage. In continental collisional zones where deep subduction or UHP metamorphism of continental crust has happened (e.g. Dabie orogen), this process results in a much larger crustal shortening than in collisional zones without UHP metamorphism (e.g. Qinling orogen).Post-collisional mafic rocks from North Dabie have Sr-Nd-Pb isotopic compositions similar to those of adakitic rocks, but different from those of exhumed UHP rocks. This indicates that post-collisional mafic rocks originate from an enriched mantle hybridized or metasomatised by delaminated mountain root crust. removal, constitution and thickening mechanism of the crustal mountain root... |
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