Petrochemical Characteristics Of Basic Dykes And Its Geological Significance In Gannan Area

The Gannan area,in the Cathaysian plate,near by the Wuyi-Yunkai fold system in the east and the Wugong-Zhuguang fold system in the west,is extensively developing numerous periods magmatic rocks.As an important tungsten-tin metallogenic concentration area in China,the Gannan area has been the subject of geologists.However,it has been lack of systematic discussion of the basic dyke in Gannan area.As we all know,the basic dykes derived from the mantle usually represent the continental break-up and it has special significance in continental dynamics.So,this paper selects the basic dyke which coming from the Taoxikeng tungsten-tin ore district in Gannan area as the object of analysis.By collating the previous research results of basic dyke in Gannan area,and discussing its spatial and temporal distribution,geochemical characteristics,magma evolution,source area properties and tectonic significance,this paper has obtained the following preliminary findings:?1?The basic dykes from the Taoxikeng tungsten-tin ore district are diabase and garganite,which has the following characteristics: The SiO₂ content,Mg^# value and ? value?Rittmann Index?of the sample were between 46.08% to 53.44%,61.67 to 69.9 and 0.66 to3.15,respectively,and we can find that the Mg^# value is higher,and the ? value indicates that it belonged to Calcium alkaline series;the REE patterns of samples shows a right-dip curve,in which the HREE patterns curve of the sample is relatively flat and the LREE has a certain degree of enrichment,and there is no obvious Eu negative anomaly;the samples are strongly depletion of Nb,K and other incompatible elements and general depletion of P,Ti,but they enrich LREE and LIEF such as Rb,Th,U,and Sr.In addition,the intrusion age of this group of basic dykes is 158.2¹59.6 Ma,which is similar to the time of diagenesis and mineralization of the Jiulongnao granite batholith,and its geotectonic background is the continental extension zone.As a result,it may be the product of the Late Mesozoic lithosphere stretching-extension geodynamic setting in southeastern China.?2?The basic dykes in The Gannan area are mainly distributed in the Zengkeng and Zhangkeng areas of Hengshi Town?422.8 Ma,421.7 Ma?,and the Taoxikeng tungsten-tin ore district in Chongyi County?158.2¹59.6 Ma?,and the Dajishan tungsten ore district in Quannan County?142.8 Ma?,and the southern part of the Zhuguang Mountains?¹40 Ma,¹05 Ma,⁹0 Ma?,and the Caotaobei?92 Ma?and the Aozibei uranium ore district both in Huichang County,and the Yanbei tin ore district in Xiefang Town of Ruijin City?91.7 Ma,78.6⁸4.5 Ma?,and the Pangushan tungsten ore district in Yudu County?76.93 Ma?.And its lithology are mainly diabase and lamprophyre,including vogesite,mica-plagioclase lamprophyre,and spessartine,which can be divided into Caledonian,Mid-Yanshanian and Late Yanshanian.The rock series of the basic dykes in Gannan area have different characteristics in different eras: the basic dykes in 159.6¹58.2 Ma and 84.5⁷6.93 Ma are the calc-alkaline series,while those in 142.8⁹0 Ma are alkaline series.However,the basic dykes in 422.8⁴21.7 Ma are both the calc-alkaline series and alkaline series.And they are characterized by enrichment of LILE and LREE,as well as depleted of incompatible elements and transition element Ti.Moreover,their LREE and HREE have obvious differentiation,so that their REE patterns show a right-dip curve.And its geochemical characteristics show that the formation process of basic dikes in different periods is different in Gannan area.Some of them were formed by partial melting of the mantle,and others were formed by the fractional crystallization of monoclinic pyroxene from basaltic magma.But the basic dykes of the Mid-Yanshanian and Late Yanshanian are derived from enriched mantle,which may be more composed of EMII.Finally,the geotectonic background of the basic dykes in Gannan area belongs to the continental extensional zone or continental rift.Therefore,they are the products of the extensional tectonic stress regime,indicating that has experienced a series of regional tectonic moment,including the extension after Caledonian fold orogeny period in South China,the joint effect of Palao-Asian oceanic tectonic domain and Circum Pacific tectonic domain in the Mid-Yanshanian,and the impact of the Circum Pacific tectonic domain in the Late Yanshanian.