| Longshan complex is located at the junction of Qinling and Qilian Orogen, and mainly distributed in Longshan Mountain, the west of Longxian County, which is separated from the Huluhe complex by the Baoping-Shanmen Fault to south, and connected with the Liupanshan Basin by the sub-fault to north. The upper magma is so extensive in this area that only Longshan complex is left as lentoid. Longshang complex is a metamorphic assemblage of volcanics and debris sediments, consisting of felsic gneisses, amphibolites, aluminous gneisses and marbles, and it is intensively intruded by basaltic dykes. Because of its tectonic complexity and obscure relationship with other nearby complex, the nature of Longshan complex is still under dispute. There is two contrasting views that Longshan complex should belong to Qinling complex or Kuanping complex. However, some geologists suggest that the position of Longshan complex is comparable with the Lesser Qinling zone. Due to the presence of ophiolites and volcanic arcs from Neoproterozoic to early Paleozoic along North Qinling orogen and North Qilian orogen, those two orogen is regarded as an identical system. However, the junction part of the Qinling-Qilian orogen experienced multistage magmatic and tectonic modification so that the regional research is difficult before detailed study on Longshan complex is performed.Based on the existing information of the regional investigation, geochemistry and chronology are applied to the igneous gneisses and basalts from Longshan complex. The achievements is following:(1) The igneous granitic gneisses from Tiefousi-Xiwali section is fallen into the area oftonalite-granodiorite in the An-Ab-Or diagram, which is geochemically high in SiO2 and A12O3 contents, and Na2O/K2O, Sr/Y and La/Yb(N) ratios, resembling the geochemical character of Archean TTG And the ratio of Sm/Nd, Th/Ta and Rb/Zr is comparable with TTG gneisses from Taihua complex. The zircon dating separated by LA-ICP-MS indicates that Longshan complex is formed in 2.5Ga, and reconstructed by the metamorphic events of 1.9Ga, which, together with Taihua complex, Sushui complex, Dengfeng complex and so on, makes up the basement of southern margin of the North China Craton (NCC). However, the contact relationship between above basements is covered by the Cenozoic Weihe Basin.(2) The granitic intrusions and basaltic dykes from Longshan complex are all enriched inLILE and LREE, and depleted in HSFE and HREE, which is obviously the result of subduction. And the dating of zircon separated by LA-ICP-MS are identical, yeilding age at 450Ma, consistent with the time of the early-Paleozoic island arc volcanics in Qinling-Qilian orogen.(3) Xijie complex has been regarded as supracrustal sequences in Longshan complex or Qinling complex. In regional investigation report of 1996, Xiejie complex is redefined as metamorphic intrusion, subdivided into two units, Dianzigou gneiss and Longkouwan gneiss. Longkouwan granitic gneiss is dated by LA-ICP-MS in this work, which offers firstly reliable chronological data for Xinjie complex. |
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