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[1] Seismic tomography studies indicate that the Earth's mantle structure is characterized by African and Pacific seismically slow velocity anomalies (i.e., superplumes) and circumPacific seismically fast anomalies (i.e., a globally spherical harmonic degree 2 structure). However, the cause for and time evolution of the African and Pacific superplumes and the degree 2 mantle structure remain poorly understood with two competing proposals. First, the African and Pacific superplumes have remained largely unchanged for at least the last 300 Myr and possibly much longer. Second, the African superplume is formed sometime after the formation of Pangea (i.e., at 330 Ma) and the mantle in the African hemisphere is predominated by cold downwelling structures before and during the assembly of Pangea, while the Pacific superplume has been stable for the Pangea supercontinent cycle (i.e., globally a degree 1 structure before the Pangea formation). Here, we construct a proxy model of plate motions for the African hemisphere for the last 450 Myr since the Early Paleozoic using the paleogeographic reconstruction of continents constrained by paleomagnetic and geological observations. Coupled with assumed oceanic plate motions for the Pacific hemisphere, this proxy model for the plate motion history is used as time-dependent surface boundary condition in three-dimensional spherical models of thermochemical mantle convection to study the evolution of mantle structure, particularly the African mantle structure, since the Early Paleozoic. Our model calculations reproduce well the present-day mantle structure including the African and Pacific superplumes and generally support the second proposal with a dynamic cause for the superplume structure. Our results suggest that while the mantle in the African hemisphere before the assembly of Pangea is predominated by the cold downwelling structure resulting from plate convergence between Gondwana and Laurussia, it is unlikely that the bulk of the African superplume structure can be formed before ∼230 Ma (i.e., ∼100 Myr after the assembly of Pangea). Particularly, the last 120 Myr plate motion plays an important role in generating the African superplume. Our models have implications for understanding the global-scale magmatism, tectonics, mantle dynamics, and thermal evolution history for the Earth since the Early Paleozoic.

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The Central Asian Orogenic Belt (CAOB) is a natural laboratory for the study of accretionary tectonics and crustal growth owing to its massive generation of juvenile crust in the Paleozoic. There is a debate, however, on the mechanism of this growth. In the Baogutu area of the western Junggar region, northern Xinjiang (west China), diorite-granodiorite porphyry plutons and dikes are widely associated with Cu-Au mineralization. In this study, we present new results of zircon U-Pb geochronology, major and trace elements, and Sr-Nd-Pb-Hf isotope analyses for two diorite-granodiorite porphyry plutons and two dikes from this area. LA-ICPMS zircon U-Pb analyses of four plutonic and dike samples yield Late Carboniferous ages of 315 -310 Ma. The Baogutu diorite-granodiorite porphyries exhibit low-Fe and calc-alkaline compositions. They are also geochemically characterized by high Sr (346-841 ppm) contents, low Y (9.18-16.5 ppm) and Yb (0.95-1.60 ppm) contents, and relatively high Sr/Y (31-67) ratios, which are similar to those of typical adakites. In addition, some samples have relatively high MgO (2.35-8.32 wt.%) and Mg # (48-75) values, and Cr (22.7-291 ppm) and Ni (32.0-132 ppm) contents, which are similar to those of high-Mg andesites. All rock samples exhibit mid-oceanic ridge basalt (MORB)-like Nd-Sr-Pb-Hf isotope features: high ε Nd (t) (+5.8-+8.3) and ε Hf (t) (+13.1-+15.7) values, and relatively low ( 87 Sr/ 86 Sr) i (0.7033 to 0.7054) and ( 206 Pb/ 204 Pb) i (17.842-18.055). The Baogutu adakitic rocks also contain reversely zoned clinopyroxene phenocrysts, which have low MgO cores and relatively high MgO rims. Geochemical modeling indicates that the Baogutu adakitic rocks could have been derived by mixing ~95% altered oceanic crust-derived melts with ~5%sediment-derived melts. Taking into account the regional geology, I-and A-type granitoids and Cu-Au mineralization, and the presence of Carboniferous ophiolite mé langes in northern Xinjiang, we suggest that the Baogutu adakitic rocks were most probably generated by partial melting of a slab edge close to a subducting spreading ridge 3 in the Late Carboniferous. Ridge subduction and the resultant slab window probably caused strong extension in the overlying lithosphere, extensive melting of subducting oceanic crust, mantle and juvenile lower crust, and interaction between slab-derived melts and the mantle. Thus, events associated with ridge subduction are likely to have played an important role in crustal growth in the CAOB in addition to previously recognized accretion of subduction and arc complexes and post-collisional crustal melting.

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