ALBERT

Description: The fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late Paleozoic Variscan belt in continental Europe. In particular, the Bohemian Massif at its eastern extremity preserves well the evolution of an Andean-type orogen involved in continental collision. This process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate. This led to gravity-driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials. In the studied example, this zone of interaction repeatedly reappears within the orogen, forming a so-called "diffuse cryptic suture zone."

Description: 〈span〉New geochemical and geochronological data are used to characterize the geodynamic setting of metasediments, felsic orthogneisses, and eclogite and amphibolite lenses forming the Beishan complex, NW China, at the southern part of the Central Asian Orogenic Belt. The metasediments correspond compositionally to immature greywackes receiving detritus from a heterogeneous source involving a magmatic arc and a Precambrian continental crust. Metagranitoids, represented by felsic orthogneisses, show both composition of greywacke-derived granitic melt with incompatible trace element patterns similar to the host metasediments. The eclogite lenses are characterized by high Nb contents (5.34−27.3 ppm), high (Nb/La)〈sub〉N〈/sub〉 (〉1), and low Zr/Nb ratios (〈4.5), which together with variable and negative whole-rock ε〈sub〉Nd〈/sub〉(t) (−4.3 to −10.3) and zircon ε〈sub〉Hf〈/sub〉(t) (−5.0 to +2.3) values indicate an origin of enriched mantle source as commonly manifested by back-arc basalts at stretched continental margins. Combined with monazite rare earth element analysis, the in situ monazite U-Pb dating of metagraywacke (880.7 ± 7.9) suggests garnet growth during a high-temperature (HT) metamorphic event. Together with U-Pb dating of zircon metamorphic rims in amphibolite (910.9 ± 3.0 Ma), this indicates that the whole crustal edifice underwent a Grenvillian-age metamorphic event. The protolith ages of the eclogite (889.3 ± 4.8 Ma) and orthogneiss (867.5 ± 1.9 Ma) suggest that basalt underplating and sediment melting were nearly coeval with this HT metamorphism. Altogether, the new data allow placing the Beishan Orogen into a Grenvillean geodynamic scenario where: (1) The late Mesoproterozoic to early Neoproterozoic was marked by deposition of the greywacke sequence coeval with formation of an early arc. (2) Subsequently, an asthenospheric upwelling generated basaltic magma underneath the thinned subcontinental mantle lithosphere that was responsible for HT metamorphism, melting of the back-arc basin greywackes and intrusion of granitic magmas. These events correspond to a Peri-Rodinian supra-subduction system that differs substantially from the Neoproterozoic ophiolite sequences described in the Mongolian part of the Central Asian Orogenic Belt, thus indicating important lateral variability of supra-subduction processes along the Rodinian margin.〈/span〉