Formation mechanism and geological implications of co-existing pegmatite veins-calc-silicate rocks in the Weihai area,northern Sulu orogenic belt
Situated at the northeastern terminus of the Sulu orogenic belt,Weihai boasts a diverse array of ultrahigh-pressure(UHP)metamorphic rocks,including gneiss and eclogite,rendering it an ideal locale for investigating metamorphism,anatexis,and the ramifications of melting in continental subduction zones.Based on detailed field observation,this paper presents the results of an integrated study of petrology,geochemistry and zircon geochronology on a suite of calc-silicate rocks,pegmatite and the host gneisses in Weihai.The primary aim is to advance our comprehension of multistage partial melting processes of UHP rocks and the associated geological phenomena.Cathodoluminescence imaging reveals a core-rim structure in the analyzed rocks.The inherited zircon cores have 206Pb/238U dates ranging from 859±20Ma to 272±6Ma,consistent with the ages of relict igneous zircons found in UHP rocks within the Sulu belt.On the other hand,the newly formed zircon domains display three distinct metamorphic age groups:237±4Ma,224± 7Ma～218±2Ma,and 177±5Ma～166±3Ma.Based on the rare earth element characteristics of these zircon domains,the age of 237 ±4Ma is linked to peak UHP metamorphism,while the zircon domains dated at 224±7Ma～218±2Ma and 177±5Ma～166±3Ma align with typical anatectic zircons,signifying the periods of partial melting of the subducted continental crust during high-pressure eclogite-facies exhumation and initial orogenic collapse,respectively.Distinct Hf isotope compositions in inherited zircon domains within the pegmatite compared to those in the host gneiss suggest a non-direct origin of the melts responsible for pegmatite formation from the host rocks.Moreover,both clinopyroxene and the calc-silicate rocks exhibit low contents of compatible elements such as Cr and Ni,negative Eu anomalies,and enrichment in large ion lithophile elements alongside depletion in high field strength elements.The calc-silicate rocks display enrichment of Sr-Nd isotopic compositions akin to UHP gneisses in the Sulu belt,and the Hf isotope compositions of inherited/newly formed zircon domains resemble those of the pegmatite.By synthesizing these findings with previous research,we posit that the calc-silicate rocks originated from the interaction of felsic melts derived from the continental crust during exhumation with small marble blocks enclosed within UHP gneisses.Overall,this research provides further insights into the timing and sources of hydrous melt during two-stage melting processes associated with exhumation and collapse of the subducted continental crust,shedding light on melt effects and mass cycling at depth in continental subduction zones.
Calc-silicate rockPegmatiteMigmatitic gneissPartial meltingSulu orogenic belt
万方数据
维普
