| The sill is an igneous intrusion consistent with the occurrence of wall-rocks.The emplacement and differentiation mechanisms of the sill are an important part of understanding the magmatic channel system,which has been paying increasing attention in recent years.The emplacement of long-distance layered sills can promote the lateral transport of magma for thousands of kilometers,and can also be used as a huge magma storage unit.The formation of sill can have an important influence on the evolution of sedimentary basins and the formation of ore deposits.The study of sill in the tectonic setting related to mantle plume shows that its formation is controlled by wall-rock or magmatic conditions.There are great differences between arc magma and magma related to mantle plumes in composition,temperature and flux,and whether the controlling factors of sill formation play a role in arc magma has not been fully verified.The emplacement mechanism of sill has always been one of the frontier scientific problems in igneous rock research.Although sill is common in sedimentary basins,little is known about the emplacement mechanism of the sill.Seismic interpretation and experimental simulation can reveal whether there is a contact relationship between sill and sill,and between sill and wall-rock.However,the existing geochemical evidence shows that the interconnected sills do not necessarily come from the same magmatism,and these indirect methods cannot objectively reflect the complex emplacement mechanism of the sill.In addition,under normal circumstances,the differentiation of magma in mafic-ultramafic sill will lead to the gradual change of magma from ultramafic to mafic and then to more evolved felsic composition,and the most evolved composition remains in the center of the sill.However,it has been reported that in some mafic-ultramafic sills,there is a marginal reversal,that is,the central component of the batholith is the most primitive,and the transition to both sides is a more evolved component.This complex geological phenomenon has made it difficult for researchers to understand the differentiation process of mafic-ultramafic sills.The Jiangnan orogen is an accretionary orogen developed in the Neoproterozoic in South China.The Fanjingshan area in northeast Guizhou Province is located in the western section of the Jiangnan orogen.Due to the uplift of the crust,the Neoproterozoic arc magma channel system in this area was lifted to the surface.Specifically,a large number of mafic-ultramafic sills were intruded into the strata from the Taojinhe Formation to the Huixiangping Formation in the lower part of the Neoproterozoic Fanjingshan Group,a small number of mafic rocks were exposed in the Tongchang Formation,and the basalts were only in the Huixiang Formation.appears in the ping group.These mafic-ultramafic sills are consistent with sedimentary bedding,and within a limited outcrop range,the lateral extension can reach tens of kilometers,and its long-distance transport mechanism is currently unknown.It is worth noting that previous reports suggest that the mafic-ultramafic sills in the Huixiangping Formation show more primitive reverse differentiation in the center of the sills on the vertical profile.At present,the understanding of the formation of the long-distance sill,emplacement model and differentiation process of the Fanjingshan is relatively weak.The main findings and conclusions are as follows:(1)The wall-rock assemblage of mafic-ultramafic sill emplacement in the Fanjingshan area is complex,with a higher rigidity in the upper layer than in the lower layer,and a higher rigidity in the lower layer than in the upper layer.This suggests that the formation of the sills in the Fanjingshan area is not fully controlled by the rigidity difference between the upper and lower wall-rocks.The calculation results show that the dimensionless temperature(θ)of the Fanjingshan magma is about0.66.Under the conditions of lower wall-rocks being sandstone,siltstone,mudstone and tuff,the range of dimensionless flux(φ)is 0.02～17.38,0.01～0.75,0.04～3.86 and 0.02～76.42 respectively.Most of these values are within the range of sill formation,indicating that the formation of the Fanjingshan sills is also controlled by magma solidification effects.In the Fanjingshan area,30 sills are present in the Fanjingshan Group,with 21 of them occurring between clastic rocks.A comparison of global long-distance sill formation settings indicates that clastic rocks in sedimentary basins mainly control sill formation.(2)LA-ICP-MS zircon U-Pb dating indicates that the mafic sills in the lower Taojinhe Formation of the Fanjingshan Group were emplaced at ～832 Ma,8 Ma later than the eruption age(840 Ma)of the overlying Huixiangping Formation basalts.Minor mafic sills are also present in the Tongchang Formation above the basalts,suggesting that two episodes of mafic magmatism may have occurred within a short time interval in the Fanjingshan area.Principal component analysis(PCA)results show that the mafic sills in the Taojinhe Formation are separated from the maficultramafic sills and basalts in the overlying strata(Huixiangping,Xiaojiahe and Yujiahe Formations).The Huixiangping basalts and the mafic sills in the Huixiangping,Xiaojiahe and Yujiahe Formations overlap,indicating that the Taojinhe Formation mafic sills were supplied by an independent magma source,whereas the Huixiangping Formation basalts and the mafic-ultramafic sills in the Huixiangping,Xiaojiahe and Yujiahe Formations may have been supplied by interconnected magmas.The Huixiangping Formation basalts and the mafic-ultramafic sills in the Huixiangping,Xiaojiahe and Yujiahe Formations have negative εNd(t)values(-4.9 to-0.2)and negative anomalies of Ni and Ti,whereas the mafic sills in the Taoyinhe Formation have positive εNd(t)values(+0.9 to +1.6)and no obvious negative anomalies of Ni and Ti,indicating that the former experienced significant crustal contamination while the latter did not.The Fanjingshan basalts and mafic-ultramafic rocks have IPGE depletion relative to PPGE and high Pd/Ir ratios(4.75–184),indicating that they are products of low degree partial melting of the mantle.The mafic rocks of the Taojinhe Formation have PPGE depletion relative to the basalts of the Huixiangping Formation and the mafic-ultramafic rocks of the Huixiangping,Xiaojiahe,and Yujiahe Formations,while IPGE contents are similar,indicating that they are products of low degree partial melting of the mantle after the basalts of the Huixiangping Formation and the mafic-ultramafic rocks of the Huixiangping,Xiaojiahe,and Yujiahe Formations.In summary,there may have been two episodes of mafic magmatism in the Fanjingshan area in a short period.The first episode occurred at ac.840 Ma and was produced by low degree partial melting of the mantle,affected by obvious crustal contamination,and supplied by interconnected magma conduits,resulting in magma eruption and emplacement of mafic-ultramafic sills of the Huixiangping,Xiaojiahe,and Yujiahe Formations.The second episode of magmatism occurred at ca.832 Ma and was derived from a similar mantle source with low degrees of partial melting.It utilized the first episode’s magmatic channels and was not significantly crustal contaminate,resulting in the emplacement of mafic sills in the Taojinhe Formation.(3)This study investigated the mafic-ultramafic sill with the phenomenon of reversed compositional in the Huixiangping Formation of the Fanjingshan Group.Through detailed sampling,it was found that the sill exhibits an M-shaped geochemical profile.Combining the characteristics of mineral grain size,composition,whole-rock MgO,REE,Sr,and Y variations,it was proposed that its formation involves two steps of in situ crystallization.In the first step,there is a gradual increase in Mg# and incompatible element contents from the sill margin towards the interior.The crystallization trend is opposite to the evolution trend of normal basaltic magma,and the mineral grain size shows an increasing trend.This indicates that the edge inversion is formed by in situ crystallization of magma continuously flowing from the bottom and top of the sill in an open system.In the second step,the contents of Mg# and incompatible elements gradually decrease towards the middle of the sill,which is consistent with the evolution trend of normal basaltic magma.Additionally,the mineral grain size shows a decreasing trend.This suggests that the magma crystallized in situ within a closed system,leading to the inward evolution of components in the middle of the sill.Eventually,this resulted in the formation of a sill with an M-shaped geochemical profile. |
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