The Deep Tectonics And Metallogenesis In Lu-Zong Volcanic Basin, Anhui, China

The Lujiang-Zongyang(Lu-Zong for short) continental volcanic basin,one of the famous iron and copper ore concentrated areas in polymetallic metallogenic belt in the Middle to Lower Reaches of Yangtze River,formed during the Late Mesozoic.Through the research on metallogenic rules of typical deposits,systemic measurements of geometry,kinematics and dynamics of tectonic deformation symbols in the field,and the inversion of Mesozoic paleo-stress fields,combined with the comprehensive interpretation of main structural interfaces and structural assemblages in the deep on high resolution reflection seismic profiles,magnetotelluric sounding profiles,gravity and magnetic anomaly profiles,and geological cross sections across the Lu-Zong basin,the coupling processes of deep tectonism and near-surface metallogenic responses are established in the Lu-Zong ore concentrated area successfully.Based on the identification of the Mesozoic structural deformations and the recovery of the Mesozoic paleo-stress fields in Lu-Zong area,it is concluded that the Lu-Zong basin has suffered from two periods of tectonic movements and three stages of paleo-stress fields:(1) approximately N-S compression from Middle Triassic to Middle Jurassic(240～180 Ma) during Indosinian movement,represented by the stress field in Dabieshan foreland belt forming after the collision between North China and Yangtze plates;(2) The NW-SE compression from Late Jurassic to Early Cretaceous(170～140 Ma) during the main episode of Yanshan movement,which is the tectonic response of multi-direction compressive orogeny.Then,the structural direction was changed for the sinistral strike-slip of Tanlu fault;(3) the NNW-SSE continental extension(equal NEE-SWW compression) during middle Early Cretaceous(140～110Ma),which is the after effect of the Yanshan intracontinental orogeny. According to the geochronological statistics in Lu-Zong region and the Middle to Lower Reaches of Yangtze River,it is suggested that Lu-Zong ore concentrated area formed due to the huge magnitude of metallogenic materials brought from the deep through multi-cycle volcanic eruptions and multi-phase intrusions in 135～130 Ma,with the forming of iron ore-bearing strata of Longmenyuan,Zhuanqiao and Shuangmiao formations,and weak metallogenic information in Fushan formation. The comprehensive interpretation of seismic reflection characteristics,electric differences, gravity and magnetic anomaly shows that:(1) The MOHO interface occurs at ca.9.6～11 s TWT in the seismic profiles across the Lu-Zong volcanic basin,with the depth range of ca.33～30 kin and 3 km difference in depth.The MOHO interface,dipping in northwest with small angle,becomes shallower gradually from Dabie orogenic belt to Changjiang structural belt.The MOHO was truncated to be imbricate by the Tanlu fault belt and Luohe fault for two times.The seismic profiles show that the Tanlu fault belt has a width of ca.8～10 kin,dipping steeply in north-west.The Luohe fault,located at the northwestern margin of the Luzong basin,is dipping in southeast and cut through the MOHO in the deep, controlling the distribution of volcanic rocks in Early Cretaceous.The Luohe fault becomes to be a normal fault in the later period and dips in northwest in the shallow,controlling the distribution of the Upper Cretaceous red beds;(2) The NNE to NE striking Tanlu fault system,including Tanlu and Luohe faults etc,is considered to be the basin-controlling structure;the Lujiang-Huangguzha fault,located in the north side of the basin,may be the ore-controlling fault of Longqiao iron deposit and Yueshan lead-zinc deposit.The sedimentary characteristics in the shallow and some intrusive bodies under the sedimentary layers in the Changjiang tectonic belt are significant on the seismic profile.The multi-group arc-shaped strong reflection in the upper crust could be plutonic bodies under the volcanic basin,indicating the hypothesis of huge intrusive body with 800 km~2 could be right.The structural interface with ca.3～4.2 s TWT on the seismic profile,with depth of ca.6～8 km,and the lower resistance belt displayed on magnetotelluric sounding profile,may be the detachment surface between the shallow cover and the basement.The other structural surface with ca.5～7 s TWT and depth of ca.18～20 km on the seismic profile,may be the interface between the Middle and the Upper Crust,could be either the top or the bottom of magma chamber;(3) The volcanic rocks,with the thickness of ca.2～3 km,distribute only to the east of the Luohe fault with an ear-sharp,evidently identified by the seismic,gravity and magnetic anomaly, and magnetoteUuric sounding profiles.There is not another half of the ear-sharp basin under the red basin to the west of the Luohe fault;(4) The SE-dipping Luohe fault cut through the MOHO interface with a displacement of ca.3 km in the deep.The MOHO interface uplifted at the east side of the Luohe fault.The discontinuous arc-shaped strong reflections truncated by the Luohe fault in the Middle and Lower Crust shown on the seismic profile,suggest that the Luohe fault was the channel of mantle fluid migration and magma upwelling in the deep of the Lu-Zong volcanic basin.The volcanic rocks of Longmenyuan,Zhuanqiao and Shuangmiao cycles are mainly belonging to the eastward asymmetric fissure-type eruptions,though those of Fushan cycle are belonging to the central-type eruption.The Lu-Zong ore concentrated area formed in the tectonic setting of East Asia multi-direction convergent tectonic system in the Late Jurassic(ca.170～140 Ma),when the Pacific Plate subducted obliquely underneath the Eurasian Plate,the East China Plateau formed due to crustal thickening,the Tanlu sinistral strike-slip fault and the Luohe fault formed simultaneously,and the orogenic adacite series occurred.At ca.135～130 Ma,the tectonic regime transformed from compression to extension in eastern China,when the lithosphere delamination and thinning occurred,the melting point of mantle rocks decreased for the extension and decompression of the Tanlu fault zone,the enrichment mantle suffered partial melting and upwelling along the Luohe fault,the magma chambers formed at the interface of the Middle-Lower Crust,the mantle-derived magma interacted with crustal rocks,the adakite-alkaline magma formed and erupted or intruded, and the Lu-Zong Fe-S-Cu ore concentrated area formed due to the enrichment of huge metallogenic material brought from the deep through the Tan-Lu and Luohe faulting.Corresponding to the NNW-SSE extensional stress field during middle Early Cretaceous,the NW striking Luohe fault and its secondly faults and the cracks of volcanic apparatus controlled the formation and distribution of the Luohe,Dabaozhuang,Nihe,and Yangshan iron deposits at the western margin of the Lu-Zong volcanic basin and Zhongshan and Xiaoling pyrite deposits etc insider the basin.The unconformity between the Cretaceous volcanic rocks and the Jurassic sandstone is also the favorable metallogenic space in the northern and eastern parts of the basin. The Longqiao and Mabianshan iron deposits and Yueshan lead-zinc deposit formed in the unconformity and located at the intersections of the faults.