Geodynamic Background of Intracontinental Cenozoic Alkaline Volcanic Rocks in Laojiezi, Western Yangtze Craton: Constraints from Sr-Nd-Hf-O Isotopes

The Laojiezi alkaline volcanic rocks, which are located in the intraplate region of the Yangtze craton, coincide with the formation of the Jinshajiang-Ailaoshan-Red River alkaline rock belt. Although this belt has been widely studied by geologists because of its porphyry-related Pb-Ag-Au polymetallic deposit and geotectonic location, the material sources of this belt are still debate. Whole-rock analyses show that these rocks have high total alkali contents(3.73–11.08 wt%), and their aluminum saturation index(ASI) values widely vary from 0.82 to 3.07, which comprise a metaluminous-peraluminous magma series. These rocks are characterized by high K(K2 O/Na2 O>1) and low Ti and Mg contents; enrichment in large-ion lithophile elements, such as Rb, Ba, K and light rare earth elements; and depletion in high field strength elements, such as Ta, Nb, P, and Ti. These rocks exhibit moderate Eu(Eu/Eu*=0.86–1.04) and Ce(Ce/Ce*=0.63–0.96) anomalies. Their(87 Sr/86 Sr)i, εNd(t), zircon εHf(t) and δ18 O values range from 0.70839 to 0.71013, from-10.16 to-12.45, from-19.6 to-5.8, and from 5.69‰ to 8.54‰, respectively, and their Nd and Hf two-stage model ages(TDM2) are 1.67–1.86 Ga and 1.27–2.02 Ga, respectively. These data reflect the primary partial melting of Paleoproterozoic to Mesoproterozoic lower crust with minor residual continental lithospheric mantle and supracrustal metasediments. The lithosphere was likely thickened along the southeastern margin of the Tibetan Plateau following the Indian-Asian continentcontinent collision(65–41 Ma). During the post-collision phase(36–16 Ma), the transition from a compressional to extensional setting triggered the convective removal of the over-thickened CLM beneath the Yangtze craton, which led to the upwelling of asthenospheric materials. This process created alkali-rich and high-K magma through the partial melting of the thickened lower crust. Magma that carried Cu-Au-Pb-Ag minerals was emplaced by strike-slip motion along the E-to W-or ENE-to WSWtrending tectonically weak zone, finally forming an alkaline porphyry Cu-Au-Pb-Ag polymetallic deposit.

A New Type of Rare Metal Deposit:The Yushishan Leptynite-Type Nb-Ta Deposit in Eastern Altun,Gansu Province,NW China

The Yusishan deposit is a newly found leptynite-type niobium and tantalum(Nb-Ta)deposit,which is located in the Eastern Altun of Gansu Province.The leptynite of the Neoproterozoic Aoyougou Formation occurs more than 10 km long and 3 km wide.In this paper,we carried out a detailed study of the leptynite and related mineralization.The U-Pb dating of zircon reveals ages of 831±5 Ma and 790±5 Ma for the unmineralized leptynite and 491±4 Ma and 455±4 Ma for the mineralized leptynite.The petrographic and geochemical evidence identified the protolith of the leptynite as alkalin e volcanic rocks that erupted during Neoproterozoic at ca.830 Ma.At ca.490 Ma,the tectono-magmatic and metamorphic event triggered remobilization and enrichment of Nb-Ta as well as other critical metals(REE,Zr,Hf etc)with the formation of industry orebodies in the leptynite strata.The Yushishan deposit presents many similarities with the alkaline volcanic rocks related rare metal deposits in the south Qinling of China and the Brochman,Toogni and Southern Peak Range deposits in Australian.We therefore propose that the Yushishan deposit is a new type(sub-type)of Nb-Ta deposit and termed as leptynite type that represents the metamorphic counterpart of the alkaline volcanic rocks related rare metal deposits.