Volcanic Rocks Of The Pingshui Group And Plagiogranite Of This Period In Jiangshan-Shaoxing Fault: Petrogenesis And Its Tectonic

Late Mesoproterozoic-early Neoproterozoic magmatic rocks are widespread in southeastern margin of the Yangtze block, and their origin and genesis have significant implications for understanding the process of amalgamation between the Yangtze and Cathaysia Blocks. Therefore, the thesis takes the spilite and keratophyre of the Pingshui Group , the Nb-enriched basalts and the Pingshui plagiogranite pluton as examples, reports their chronology, mineralogy, petrology , geochemistry,and discuss the nature of magma sources and its tectonic background.Although the spilite of Pingshui Group that posses geochemical affinity to the MORB basalts with the same pattern of Chondrite- normalized REE distribution diagram and primitive mantle- normalized spider diagram,We are intended to take for the spilite as products of slab window in the arc-island or in an active continental margin but not as in the mid-ocean ridge combined with the regional tectonic history and present study. The keratophyre shares lots of geochemical characters with Taohong pluton and Xiqiu pluton in Pingshui region, maybe they are cogenetic rocks in different occurrence in the same tectonic setting.Some geochemical characters between the spilite and keratophyre imply they arenot share the primary magma.Nb-enriched basalts of this area have some major-element similarities of the typical Nb-enriched basalts(ie,the contents of P2O5 and TiO₂),but their contents of HREE (ie., Yb=1.80～3.17ppm) are higher than the Nb-enriched basalts of Cenozoic period(ie., Yb=1.32～3.17ppm), The transitional element contents of Nb-enriched basalts of Pingshui area(ie.,Cr=22.57～44.45ppm,Ni=17.31～25.93ppm)are lower than the Nb-enriched basalts of Cenozoic period(Defant et al.,1992;Sajona et al.,1993,1996;Robles et al.,2001).Moreover,these Nb-enriched basalts are most likely as those of Archaean(ie., Yb=2.01～5.01ppm,Cr<50ppm,Ni<100ppm)(Sajona et al.,1993,1996;Wang Qiang et al.,2006).As mentioned above,the Nb-enriched basalts of this area may have the same petrogenesis of Archaean Nb-enriched basalts.However,a few of points should be addressed here.Some samples have higher rations of(La/Nb)pm because of its position of the margin wein that were contaminated by crust-derived materials,or affected by fractional crystallization apatite ,titanite and rutile at the early stage of magmatic evlution. 3.Petrogenesis of the Pingshui plagiogranite pluton.SHRIMP U–Pb zircon data indicate that the Pingshui pluton was emplaced at 901±4.8 Ma. Though the Pingshui plagiogranite plution posses geochemical affinity to the I-type granites,field observations suggests that it was mantle-derived plagiogranite formed in an active continental margin or island-arc region similar with that of the Mesozoic arc-island in Chukotka Peninsula Ridge(ie.,the contents of REE , Rb<10ppm,ratio of HFES and sprimitive mantle- normalized spider diagram ).The primary magma of the Pingshui plution was derived from intensive fractional crystallization of the basaltic primary magma but related to the slab of ocean.