http://argonlab.gl.ntu.edu.tw/papers/Jahn-ChemGeol1999-Dabie.pdf

Crust-mantle interaction induced by deep subduction of the continental crust: geochemical and Sr-Nd isotopic evidence from post-collisional mafic-ultramafic intrusions of the northern Dabie complex, central China

Stable isotope geochemistry of ultrahigh pressure metamorphic rocks from the Dabie–Sulu orogen in China: implications for geodynamics and fluid regime

Slab breakoff is the buoyancy-driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision. In a recent paper Davies and von Blanckenburg [1994] have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts. Breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere, melting of its enriched layers, and thus to bimodal magmatism. Breakoff will also lead to thermal weakening of the subducted crustal lithosphere, thereby allowing buoyant rise of released crustal slices from mantle depths. In this paper we present a test of this model in the Tertiary evolution of the European Alps. In the Alps, both basaltic and granitoid magmatism occur between 42 and 25 Ma, following the closure of oceanic basins by subduction and continental collision. The granitoids are now well established to result from mixing of basalt with assimilated continental crust. To identify the tectonically crucial origin of the partial mantle melts, we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole Alpine arc. Their trace element and isotopic composition suggests that they have been formed by low-degree melting of the mechanically stable lithospheric mantle. We see no evidence for melting of asthenospheric mantle. It was thus not decompressed to depths shallower than 50 km. Once initiated, rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust. This explains why all Alpine magmatic rocks intruded almost synchronously along a strike-slip fault, the Periadriatic Lineament. A compilation of ages from Penninic high-pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55–40 Ma, followed by uplift at 40–35 Ma. From the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff. The slab breakoff model fulfills its predictions in the case of the Alps and therefore supports the assumptions made in the theoretical model on a geological basis. We believe that the characteristic association of magmatic activity with the return of high-pressure rocks to the surface allows the identification of this process in the Earth's mountain belts.

/pdf/slab-breakoff-a-model-for-syncollisional-magmatism-and-2g6udfx1c7.pdf

Slab breakoff: A model for syncollisional magmatism and tectonics in the Alps

The collision zone between the Sino-Korean and Yangtze cratons in central China is marked by subduction-related metamorphic rocks composed of crustal protoliths. These range in metamorphic grade from ultrahigh-pressure eclogite to low-greenschist facies. The paragenesis of the ultrahigh-pressure rocks (coesite and diamond-bearing) indicates subduction to depths greater than ∼120 km as a result of continental collision. Despite extreme metamorphic conditions, zircons have not undergone Pb loss but rather illustrate two periods of growth: crystallization between 700 and 800 Ma and ultrahigh-pressure metamorphism at 218 ± 2.5 Ma. Previous studies have predicted a time-transgressive collision, but age data presented in this study indicate that the collision was approximately coeval along the length of the suture. U-Pb zircon ages of these ultrahigh-pressure metamorphic rocks are 218.4 ± 2.5 Ma and 217.1 ± 8.7 Ma for the Dabie Mountains and Shandong peninsula, respectively. The similar ages imply that the Tan-Lu fault, which offsets the ultrahigh-pressure metamorphic rocks, is a secondary feature not related to collision or subduction. Whole rock isotopic data show that both Rb-Sr and U-Pb were open systems during metamorphism but that Sm-Nd probably remained closed. Nd initial isotopes display crustal signatures, indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth. Sm-Nd data in conjunction with U-Pb zircon dating indicate that the ultrahigh-pressure rocks are not typical Yangtze craton basement (2.9 Ga) but that they originally crystallized in a rift environment between ∼700 and 800 Ma. Discrepancies between 40Ar/39Ar plateaus, Sm-Nd isochrons, and U-Pb zircon ages render calculation of a P-T-t path unrealistic.

Geochronology and isotopic character of ultrahigh‐pressure metamorphism with implications for collision of the Sino‐Korean and Yangtze cratons, central China

Deliang Liu

Papers

Excess argon in phengite from eclogite: Evidence from dating of eclogite minerals by SmNd, RbSr and 40Ar39Ar methods