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.

Hydrous Juvenile Lower Crust at the Western Yangtze Craton Margin as the Main Source of the Beiya Porphyry-skarn Au Deposit

The Beiya porphyry-skarn Au deposit is one of the largest gold deposits in China,temporally and spatially associated with Eocene intrusions in a post-collisional setting in western Yunnan,China.In this study,we report new whole-rock geochemistry,Sr-Nd isotope,zircon U-Pb geochronology and in situ zircon Hf-O isotopes of quartz-monzonite and biotite-monzonite porphyries from the Beiya deposit.The porphyry-skarn mineralization at the Beiya deposit is mainly associated with the quartz monzonite porphyry(35.8±0.6 Ma),while the biotite-monzonite porphyry(34.3±0.5 Ma)represents a post-mineralization intrusion crosscutting the main orebodies and the quartz-monzonite porphyry.Both intrusions have high-K and adakitic composition and are characterized by high Sr/Y ratios,high SiO_(2)and Al_(2)O_(3)concentrations(SiO_(2)=69.80-73.86 wt%;Al_(2)O_(3)=14.11-15.19 wt%),and low MgO,Cr,and Ni concentrations(MgO=0.2-1.0 wt%;Cr=1.76-11.13 ppm;Ni=2.52-11.72 ppm).Their Sr-Nd isotope compositions(^(87)Sr/^(86)Sr=0.7066-0.7077;εNd(t)=−5.3 to−1.5)are consistent with the lower crustal-derived amphibolite xenoliths(^(87)Sr/^(86)Sr=0.7060-0.7100;εNd(t)=−10.0 to 0.0),indicating that they might be derived from a thickened juvenile lower crust beneath the Yangtze Craton.The biotite-monzonite porphyry has lower zirconδ^(18)O values of+5.3‰to+6.8‰and higherεHf(t)values of−2.3 to+5.5 than those of the quartz-monzonite porphyry withδ^(18)O values of+7.1‰to+8.2‰andεHf(t)values of−3.8 to+1.5,implying that they were derived from different parts of the lower crust.High Ba/La and Pb/Ce ratios suggest that the quartz-monzonite porphyry is derived from a volatiles-rich reservoir.Relatively higher La/Yb,Sm/Yb and Dy/Yb ratios of the biotite-monzonite porphyry indicate residual garnet in the source,indicating a deeper source than that of the quartz-monzonite porphyry.The hydrous components should be represented by the amphibole-rich lithologies,which has relatively shallower depth than that of the garnet-bearing mafic thickened lower crust.Our data suggest that the mineralized quartz-monzonite porphyry at the Beiya deposit is derived from partial melting of amphibole-rich lithologies in the upper part of the thickened juvenile lower crust beneath the Yangtze Craton,while the post-mineralization biotite-monzonite porphyry is derived from the basal,and volatiles-poor,part of the juvenile lower crust.