Ultrahigh-pressure metamorphism has recently been reported from various crustal rocks in the Seve Nappe Complex(SNC)in which microdiamonds were found.However,in gneiss from the Lower Seve Nappe(LSN),neither any direct...Ultrahigh-pressure metamorphism has recently been reported from various crustal rocks in the Seve Nappe Complex(SNC)in which microdiamonds were found.However,in gneiss from the Lower Seve Nappe(LSN),neither any direct petrographic indication for UHP was reported nor the metamorphic evolution was well constrained.We studied a mylonitic gneiss from the Handol area of the LSN and applied phase-diagram modeling and Ti-in-biotite thermometry.Based on the compositions of garnet and biotite and observed mineral assemblages,a path was reconstructed passing through about 8 kbar and 730℃at prograde metamorphism.Peak-pressure and initial retrograde stages occurred at 9.0–10.2 kbar at 745-775℃,and 7–9 kbar at<750℃,respectively.No ultrahigh-pressure evidence was recognized compatible with medium-pressure metamorphism deduced in earlier studies of gneiss from the SNC.As higher peak pressures were reported recently for metamorphic rocks of the LSN,a possible interpretation is that slices or erased blocks were subducted,metamorphosed at different depths,and exhumed in a subduction channel.However,the dominant gneiss of the SNC experienced only a medium-pressure metamorphism in the upper part of the downgoing Baltica Plate.Rocks from different depth levels were brought together in an exhumation channel located between Baltica and the overlying plate.展开更多
The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia.The Seve Nappe Complex(SNC)of the ...The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia.The Seve Nappe Complex(SNC)of the Scandinavian Caledonides includes relics of the outermost Baltoscandian passive margin that were subducted to mantle depths.The earliest of the deep subduction events has been dated to ca.500-480 Ma.Evidence of this event has been reported from the northern exposures of the SNC.Farther south in the central and southern segments of the SNC,(ultra)high-pressure rocks have yielded younger ages in the range of ca.470-440 Ma.This study provides the first record of the early Caledonian event in the southern SNC.The evidence has been obtained by depth profiling of zircon grains that were extracted from the Tv?r?klumparna microdiamond-bearing gneiss.These zircon grains preserve eclogite facies overgrowths that crystallized at 482.6±3.8 Ma.A second,chemically-distinct zircon overgrowth records granulite facies metamorphism at 439.3±3.6 Ma,which corroborates previous geochronological evidence for granulite facies metamorphism at this time.Based on these results,we propose that the entire outer margin of Baltica was subducted in the late Cambrian to early Ordovician,but the record of this event may be almost entirely eradicated in the vast majority of lithologies by pervasive late Ordovician to early Silurian metamorphism.展开更多
基金partially financially supported by the National Natural Science Foundation of China(No.42002068)the MOST Special Fund from the State Key Laboratory of GPMR,China University of Geosciences(Wuhan)the 111 Project(No.BP0719022)。
文摘Ultrahigh-pressure metamorphism has recently been reported from various crustal rocks in the Seve Nappe Complex(SNC)in which microdiamonds were found.However,in gneiss from the Lower Seve Nappe(LSN),neither any direct petrographic indication for UHP was reported nor the metamorphic evolution was well constrained.We studied a mylonitic gneiss from the Handol area of the LSN and applied phase-diagram modeling and Ti-in-biotite thermometry.Based on the compositions of garnet and biotite and observed mineral assemblages,a path was reconstructed passing through about 8 kbar and 730℃at prograde metamorphism.Peak-pressure and initial retrograde stages occurred at 9.0–10.2 kbar at 745-775℃,and 7–9 kbar at<750℃,respectively.No ultrahigh-pressure evidence was recognized compatible with medium-pressure metamorphism deduced in earlier studies of gneiss from the SNC.As higher peak pressures were reported recently for metamorphic rocks of the LSN,a possible interpretation is that slices or erased blocks were subducted,metamorphosed at different depths,and exhumed in a subduction channel.However,the dominant gneiss of the SNC experienced only a medium-pressure metamorphism in the upper part of the downgoing Baltica Plate.Rocks from different depth levels were brought together in an exhumation channel located between Baltica and the overlying plate.
基金funded by the National Science Centre,Poland CALSUB project no.2014/14/E/ST10/00321financial support has been provided by the Polish National Agency for Academic Exchange Scholarship PPN/IWA/2018/1/00030/U/00001 granted to C.Barnes。
文摘The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia.The Seve Nappe Complex(SNC)of the Scandinavian Caledonides includes relics of the outermost Baltoscandian passive margin that were subducted to mantle depths.The earliest of the deep subduction events has been dated to ca.500-480 Ma.Evidence of this event has been reported from the northern exposures of the SNC.Farther south in the central and southern segments of the SNC,(ultra)high-pressure rocks have yielded younger ages in the range of ca.470-440 Ma.This study provides the first record of the early Caledonian event in the southern SNC.The evidence has been obtained by depth profiling of zircon grains that were extracted from the Tv?r?klumparna microdiamond-bearing gneiss.These zircon grains preserve eclogite facies overgrowths that crystallized at 482.6±3.8 Ma.A second,chemically-distinct zircon overgrowth records granulite facies metamorphism at 439.3±3.6 Ma,which corroborates previous geochronological evidence for granulite facies metamorphism at this time.Based on these results,we propose that the entire outer margin of Baltica was subducted in the late Cambrian to early Ordovician,but the record of this event may be almost entirely eradicated in the vast majority of lithologies by pervasive late Ordovician to early Silurian metamorphism.
