| The study area is located in the Nixiong area of Cuoqin County in the middle of the Lhasa block.It is located in the Longer-Gombujiangda complexed magmatic arc and the Cuoqin-Shenza magmatic arc,and on a larger scale,in the middle of the Yarlung Zangbo River suture zone and the Bangong Lake-Nujiang suture zone.During the work,we've redefined the sandstone of the Lower Jurassic Quluo Formation and the Lower Cretaceous Zelong Group for the strata in the study area through the chronological characteristics indicated by paleontology fossils in the strata,stratigraphic correlation with the same lithologic and fossil characteristics as those in the adjacent areas,and zircon U-Pb dating analysis method.Due to the lack of sedimentary records between the two strata in the area,this paper takes these two sets of terrigenous clastic rocks as the research objects by the field profiles investigating,combined with analyses of sandstone debris components and geochemical methods.The purpose is to study their petrological and sedimentological characteristics,summarize and compare the similarities and differences of their provenance and tectonic background,and analyze the evolution of their sedimentary records.Thus,some certain basis and constraints from petrology and stratigraphy can be provided to the sedimentary record evolution during the early Jurassic to early Cretaceous period in the study area.However,the deltas in the Quluo Formation are braided river deltas,while the Zelong Group are normal deltas.The Quluo Formation is mainly composed a set of gray-yellow lithic sandstone,lithic-quartz sandstone with interbeded dark siltstone.The Zelong Group,which contains a great number of tuffaceous components,is composed of a cyclic set of light gray tuffaceous feldspar-quartz sandstone and feldspar-lithic sandstone,lithic-quartz graywacke with some conglomerate and grey black tuffaceous siltstone/mudstone.The objects are both from deltaic-barrier coastal sedimentary system under sea-land transition environment.After microscopic observation and statistics on 21 samples of the Quluo Formation and 10 for the Zelong Group,it can be concluded that both are mainly composed of quartz and lithic grains.But the quartz and sedimentary-lithic grains are relatively more enriched in the Quluo Formation,while the Zelong Group has more volcanic material in and between the grains.The triangle diagram of the sandstone debris component from Dickinson shows that the sediments in the Quluo Formation are mostly from the transition-recycled orogenic belt source region with a small number in the lithic-recycled orogeny.And in the Zelong Group,the sediments are mainly from transition-recycled orogeny and several magmatic arc source.The major,trace and rare earth element characteristics and related diagrams from the Lower Jurassic Quluo formation and Lower Cretaceous Zelong Group indicate that both of them are enriched with felsic composition with little femic component,which came from the acid provenance region of upper continental crust.In contrast,sandstones from the Quluo Formation has a higher maturity of composition and a higher degree of sedimentary recycling,while the sandstone of the Zelong Group has more input of volcanic material and femic composition,indicating the shorter distance to the source of the magmatic arc.The discriminant graphs of geochemical structure show that the tectonic setting of Quluo Formation is passive continental margin which is relatively stable,while the Zelong Group is from active continental margin which is closer to the magmatic arc.Combined with the sedimentary records and the regional tectonic background,it can be assumed that the study area was influenced by the subduction of the oceanic crust and the plate motion during the Early Jurassic to the Early Cretaceous,changing the tectonic settings and sources the objects,which gives a certain indication and geological significance for the tectonic evolution analysis of Lhasa block from the early Jurassic to early Cretaceous period. |
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