Geochronology And Geochemistry Of The Miaoya Carbonatite Complex(Hubei Province):Implications For Petrogenesis And Metallogenesis

Carbonatite magma is believed to generate from the upper mantle,which is volatile rich withlow viscosity,,and highly enriched in incompatible elements.Carbonatite in generalcould buffer the crustal contamination and provide important information for the long-term evolution of the mantle.Carbonatites and alkaline silicate rocks are related in space.They are closely associated with Nb and REE resourcesand carbonatite-related REE deposits contains high REE grade and large amount.The genetic relationship between carbonatite and alkaline silicate rocks remains controversial,and the REE enrichment mechanism and metallogenic process are still not clear.The Miaoya carbonatite complex locates in the southwestern margin of the Wudang Terrane along the southern edge of the South Qinling belt,which is close to the Mianlue suture.The complex composes of carbonatite and syenite,both rock suites host abundant REE and Nb mineralization,which is the second largest REE-Nb deposit in China.The emplacement age of the Miaoya carbonatite complex and the geochronological relationship between carbonatite and syenite have long been debated.In this study,we present new in-situ geochronological data and combined by chemical and isotopic analysis determined from accessory minerals zircon,monazite,apatite and columbite from both carbonatite and syenite at Miaoya,to accurately define the metallogenic age and time-space relationship with the tectonic ecolution of Qinling belt.Syenite is mainly composed of K-feldspar and microcline with minor amounts of albite,biotite,plagioclase,quartz,and calcite.Accessory minerals include zircon,monazite,columbite,fluorapatite,Nb-rich rutile and sulfides.Carbonatite mainly consists of medium-to fine-grained calcite,along with minor K-feldspar,albite,biotite,fluorapatite,monazite,columbite,bastnasite,aeschynite,quartz,Nb-rich rutile and sulfides.Zircon crystals in syenite are larger,but they are all broken.Growth zonation and alteration can be identified in SEM-CL images for zircon from both carbonatite and syenite.The SEM-BSE images and energy spectrum analysis indicated some of zircon grains contain mineral inclusions,including calcite,apatite and monazite.The trace element compositions of zircon from the Miaoya complex show the characteristics of Th-rich and U-poor and a steep increasing trend from LREE to HREE without Eu anomaly,which is typical for magmatic zircon from carbonatite and alkaline rocks.These characteristics are obviously different from the captured zircons in the REE distribution patterns.Geochronology research show that zircon from syenite and carbonatite yield a 208Pb/232 Th ages of 442.6±4 Ma and 426.5±8 Ma,respectively.A few apatite grains in carbonatite yield a 206Pb/238 U ages of 433±11 Ma,indicating that carbonatite is ~15 Myr younger than syenite.These ages represents the emplacement age of syenite and carbonatite respectively.Combining with the regional geological background,we believe that genesis of the Miaoya syenite and carbonatite are related to the opening of the Mianlue ocean in the Silurian.,and a large number of mafic dyke outcropped in the region belong to the products of rifting volcanic activities in the Silurian.Monazite grains in carbonatite and syenite are small but widespread.Monazite occurs as a vein of ovoid aggregate extending a long distance in the matrix of syenite,whereas in carbonatite it is either enclosed within apatite or occurs in the rim of the apatite.The results of the analysis show that the Th-Pb age of monazite in syenite and carbonatite are basically the uniformity,giving a weighted average age of 240±4.8 Ma,which is basically consistent with the previous analysis.Combined with regional geological background,the closure of the Mianlue ocean might have induced the Triassic tectonic-thermal events,which not only leads to an intracrystalline dissolution-reprecipitation reaction of apatite,but also leads to a large number of monazite in the metasomatism of the syenite.In addition,there are two ages of columbite in carbonatite.Columbite dispersed within carbonatite giving a U-Pb ages of 230 Ma,whereas these associated with apatite banded giving highly variable ages from 380 to 440 Ma.The age of the former is basically the same as that of emplacement age of carbonatite;the latter is in accordance with the age of hydrothermal monazite.These features indicating that there are a strong hydrothermal event during the Triassic leading to REE and Nb enrichment and mineralization.The detailed C-O-Sr isotope and trace element compositions have been obtained for the calcite from carbonatite in order to track the magmatic and hydrothermal source for the generation of the Miaoya carbonatite complex.The carbon and oxygen isotope of calcite from carbonatite show a large range of variation,?13C(-6.65~-3.32‰),?18O(7.71~12.02).This indicate that carbonatite suffered contamination from country rocks,or hydrothermal alteration.The sovite carbonatite in the early crystallized is higher Sr contents(9000~ 12000 ppm),and the Sr isotope of calcite and apatite are relatively homogeneous(0.7035 ± 0.0001),which representing the initial Sr isotopes of carbonatite.Some areas undergone tectonic activation and hydrothermal metasomatism could be observed variation characteristics of trace elements and Sr isotope from a carbonatite lens center to the edge.In the central position,calcite has the highest contents of Sr and REE,but the lowest Sr isotope(0.7035~0.7042).However,in the edge position,mylonite zone,the decrease of Sr and REE but the increase of Sr isotope of calcite(0.7043~0.7046),indicating that the late hydrothermal fluids probably mixed with crustal composition.The hydrothermal fluids migrated out REE from calcite carbonatite to ankerite carbonatite veins and precipitated crystallization LREE minerals(monazite,bastnasite,parisite and allanite).