The Neoproterozoic Magamtism And Tectonic Implcations, The Fanjingshan Mt., Northeast Guizhou Province

Voluminous Precambrian rocks expose along the margin of, or within the SouthChina Block (SCB), which have become a hot study-object to reveal the Precambrianhistory of the Earth. The Jiangnan orogenic belt that separates the Yangtze andCathaysian blocks, is one of the hottest topics concerning the SCB geology, regardlessthe fact that this belt has not been well defined. A growing body of geological andgeochronological data led to below definition for the Jiangnan orogenic belt:"a largebelt of middle Neoproterozoic rocks between the Yangtze and Cathaysian blocks, thatcompose the major portion of the previously defined Jiangnan ancient land."In spiteof numerous previously-published high quality data, no consensus appears on thetectonic evolution of the Jiangnan orogenic belt, especially concerning the westernsegment of this belt. Reasons may include: (1) Field relationship has not beenpositively taken into account in interpreting most geochemical and geochronologicaldata; (2) Some key geochronological data are still absent, which obscured making areasonable tectonic model; and (3) Detail petrological information is commonlyunavailable for most geochemical studies.Accordingly, this paper tries to report a detailed case study of the Fanjingshan Mt.region, northeast Guizhou Province, South China, in order to: (1) reveal the fieldrelationship between the different geologic units; (2) make a reasonablegeochronological scheme on the basis of much more detail sampling; (3) select typicalrock samples for geochemistry study; (4) integrate all available data from field works,petrological observations, geochronological studies, and geochemical analyses tosuggest possible tectonic facies, and/then, (5) synthesis a tectonic evolution model forthe western segment of the Jiangnan orogenic belt.Two sedimentary sequences in the Fanjingshan region have been identified,which are separated by an angular unconformity: (1) below the unconformity is theFanjingshan Group, consisting mainly of meta-volcanics, meta-volcaniclastics andmeta-fine clastics. The content of volcanics and the grain-size of clastic decreaseupward, defining a complete basin sedimentary circle. (2) Resting upon the unconformity is another volcaniclastic sequence, the Xiajiang Group.The volcanics in the Fanjingshan Group include: spilite-keratophyre sequence(mainly meta-basalt with pillow), quartz spilite (silicic extrusion), and diabase dykesthat has a gradual boundary with the basalt. Besides, geological mapping has revealednumerous mafic-ultramafic and minor granitic plutons intruded the FanjingshanGroup. In-site LA-ICP-MS zircon U/Pb analyses demonstrate that the volcanism andsimultaneous sedimentation of the Fanjingshan Group took place during 860 to 815Ma, rather than previously-thought 870-800 Ma (Zhou et al., 2009) or after 800 Ma(Wang et al., 2010).On the basis of detail geochronological studies, two stages mafic-ultramaficmagmatism were revealed that gave rise two suites of mafic to ultramafic igneousrock in Fanjingshan region: (1) The earlier stage magmatism has resulted in basalthorizons and bed-paralleled diabase or gabbroic dykes composing the FanjingshanGroup. In-site LA-ICP-MS zircon U/Pb results indicate that the mafic magmatism andsedimentation were simultaneous during 856-820 Ma. Geochemistry revealed anE-MORB-like Chondrite-normalized REE pattern for the earlier stage mafic rocks,while their primitive-mantle normalized trace element spider diagrams are generallyE-MORB like but with obvious Nb, Ta, Sr, P, and Ti negative anomalies. Suchgeochemical feature suggests an intensive crustal contamination and magmadifferentiation during the mafic magmatism. (2) The later stage mafic magmatismoccurred during 815 to 745 Ma according to new in-site LA-ICP-MS zircon U/Pbresults, which gave rise voluminous diabase, gabbro, and minor ultramafic plutonsintruding the Fanjingshan Group. The later stage mafic rocks have similargeochemical characteristic as those of the earlier stage rocks: both have anE-MORB-like Chondrite-normalized REE pattern and E-MORB like primitive-mantlenormalized trace element spider diagrams with obvious Nb, Ta, Sr, P, and Ti negativeanomalies, suggesting they have a similar petrogenesis process. However, subtledifference in geochemistry suggests a higher degree mantle partial melting for theearlier mafic magma and a higher degree magma differentiation for the later stagemagmatism. The quartz spilite and basalt of the Fanjingshan Group define bi-modelvolcanism. The quartz spilite has high SiO₂-content up to 72 wt. %, and is highlycomparable with the silicic volcanics in a continental rift featured by strongdifferentiation of light and heavy rare earth elements (LREE and HREE), and traceelements including high-field-strength elements. The silicic volcanics have obviousnegative Ti-anomaly and medium Nb, Ta, Sr, and P negative anomalies, suggesting,again, an intensive crustal contamination.During the earlier stage mafic magmatism, some high-SiO₂, stronglyperaluminous leucogranites developed in Fanjingshan region. Geochemical studiesdemonstrate that the leucogranite is strongly peraluminous with very highSiO₂-content (up to 72 wt. %) and high A/CNK value > 1.1. The chondrite-normalizedREE patterns are eagle-like, exhibiting strong negative Eu-anomaly and obvioustetrahedral effect, suggesting they likely have resulted from a high temperature(>875oC)partial melting of clay-enriched psammitic rocks.Synthesizing available sedimentary petrologic, igneous petrologic, geochemical,and geochronological data, the Neoproterozoic tectonic evolution of the westernsegment of the Jiangnan orogenic belt may be divided into two stages: 860-815 Ma isthe Jinning-I stage, and 815-750 Ma is the Jinning-II stage, between them is theangular unconformity separating the Fanjingshan and Xiajiang Groups. Both theJinning-â… and -â…¡movements are displayed as continental-rifting-related volcanismand sedimentation. According to geologic data from other areas, the Jinningâ… stagerifting may have resulted in an aulacogen, likely representing the western tip of anocean between the Yangtze and Cathaysian blocks, while the Jinningâ…¡stage rifting iscommonly thought as the represent of the mantle plume activity that has led tobreakup of the Rodinia super-continent.