Mobilization of Cu in the continental lower crust:A perspective from Cu isotopes

Recent studies have shown that Cu-rich sulfide accumulates in the lower continental crust and serves as a critical reservoir to balance Cu depletion in the upper crust.Recycling of Cu in the lower crust is also assumed to be a major metal source for non-arc setting porphyry Cu deposits.To test this hypothesis and further explore the behavior of Cu in the lower crust,we analyzed the elemental and Cu isotopic compositions of lower crustal rocks from different geological domains.The collected samples include hornblendites from the Kohistan arc,granulite xenoliths and hornblendites from the Gangdese arc,hornblendites and gabbros from the Laiyuan complex in the North China Craton,and hornblendite xenoliths from the western margin of the Yangtze Craton.These lower crustal rocks have experienced varying degrees of primary or secondary sulfide accumulation,with significantly varied Cu contents(11.2 to 145 ppm)andδδ^(65)(1.05‰to 1.40‰).Petrography and geochemistry reveal varying degrees of metasomatism and fluid interaction in these rocks,and based on this,they can be further divided into three groups:Group I includes the Gangdese granulites and Yunnan hornblendites,which perhaps experienced significant metasomatism.This suite of rocks shows enrichment ofδ^(65)(dδ^(65)=0.01‰to 1.40‰),positively correlated with metasomatism(dδ^(65)vs.Ce/Pb).We suggest the secondary sulfides which transformed from sulfates during the interaction between lower crust and arc magma are dominant in these rocks,so the feature of heavy isotope enrichment is inherited.Group II includes Laiyuan hornblendites and gabbros,derived from the same parental magma and emplaced at different depths(hornblendites,23.3–28.1 km;gabbros 8.4–11.1 km).The Cu isotopic compositions are strongly fractionated between these two kinds of rocks,with low dδ^(65)in the hornblendites(0.00‰to 0.28‰)and highly polarized dδ^(65)in the gabbros(1.05‰to 0.81‰).Geochemical indicators and mineral assemblages reveal that fluid interaction is most likely responsible for this feature.Primary sulfides were decomposed by fluids and reprecipitated at shallower depths.Since this process involves multiple redox reactions,the Cu isotopic composition in the shallowed emplaced gabbros was large fractionated.Group III includes the Gangdese hornblendites and Kohistan hornblendites which show negligible impacts of subduction-like metasomatism and fluid interaction.The Gangdese hornblendites show a homogeneous and unfractionated Cu isotopic composition(-0.09‰to 0.18‰)and Cu content(83.4 to 128 ppm),suggesting insignificant Cu migration and isotope fractionation.In contrast,the Cu isotopic composition of the Kohistan hornblendites is strongly fractionated(-0.36‰to 1.27‰).Geochemistry and modeling results suggest partial melting plays a role in the Cu isotope fractionation.The light Cu isotopes are preferentially distributed into sulfide melts and removed from the source region during partial melting of the lower crust,resulting in a decrease in Cu content and enrichment of heavy Cu isotopes in residues.Results suggest that partial melting and fluid interaction are two efficient mechanisms that encourage Cu migration in the lower crust.

Effects of Water on the Rheology of Dominant Minerals and Rocks in the Continental Lower Crust: A Review

As an important part of the continental lithosphere, the continental lower crust can influence and control many important geodynamic processes, which are of great significance to the evolution of the lithosphere. Extensive plastic deformation is common in continental lower crust. There have been many studies focusing on the rheology of the continental lower crust in the past few decades. This paper provides a review on the effects of water on the rheology of dominant minerals(clinopyroxene, plagioclase and garnet) and rocks in the continental lower crust. The water contents in continental lower crustal minerals and rocks are in general rich and very heterogenous from sample to sample and region to region. Water can significantly reduce the strength of clinopyroxene, plagioclase, garnet and lower crustal rocks. Water can also have a profound influence on fabric development and slip systems in lower crustal minerals. Quantitative experimental investigations and extensive natural studies of water effect on rheology are necessary to refine the classic lithosphere strength profile models and to address the existing controversy on strength of the continental lower crust.

Geochronology and geochemistry of the W-Mo-ore-related granitic rocks from eastern Ningzhen,lower Yangtze river belt,eastern China

Here we present zircon U–Pb–Hf and wholerock major and trace element studies of eastern Ningzhen W-Mo-ore-related magmatic rocks,Yushan and Longwangshan granitic rocks,to constrain their form timing,magma sources,and tectonic settings.The results showed that the two plutons were formed in the Early Cretaceous with;Pb/;U ages of 107.8±1.2 and 105.2±1.5 Ma(;Pb/;U),respectively.The trapped/residual zircons are mainly distributed in 2.0–2.5 Ga.The two intrusions are characterized by high silicon(68.60–73.99%),high aluminum(13.56–15.02%),high Mg#(47–55),high Sr,Sr/Y,LaN/YN,and low Yb,falling into high Mg#adakitic rock region.The zirconεHf(t)values of the two intrusions range from-24.8 to-13.2,indicating an ancient continental crust in their magma sources.The average Ti-inzircon temperature is 689°C,slightly higher than those of other high-Mg adakitic rocks in the lower Yangtze River belt,but lower than those of high-Mg adakitic rocks in the Southern Tanlu Fault(STLF).Zircon Ce;/Ce;show low oxygen fugacity(LWS-1:3–400,average 92;sample ZYS-4:9–382,average 93).These geochemical features indicate a thickened lower continental crust in the Eastern Ningzhen region in a subduction setting.Comparing the geochemical characteristics of the eastern Ningzhen to the western Ningzhen and other areas in the Lower Yangtze River Metallogenic Belt(LYRMB)and the high-Mg ore-barren adakitic rocks of the STLF,we propose that the magmatic rocks from eastern Ningzhen may be mainly from a thickened lower continental crust that hybridized with a very small part of mantle sources,while the west Ningzhen magmatic rocks may have experienced a higher degree of mantle contaminations in their source.The metallogenic differences between the eastern(W–Mo)and western(Cu–Fe–Pb–Zn)parts of Ningzhen also indicate different proportions of crustal materials in their magma source.

Magnetic Petrology of Archean High-Grade Terrains: Window into Deep Crustal Structure and Geodynamic Processes

The major results about magnetic petrology of Archearn high grade terrains in the world are reviewed in this article, focusing on the relationship between rock magnetism and deformation, and metamorphism and intensity of magnetization of the lower continental crust. The important problems about the magnetic study of rocks for high grade terrains are advanced.

U-Pb Geochronology, Elemental and Sr-Nd Isotopic Geochemistry of the Houyaoyu Granite Porphyries： Implication for the Genesis of Early Cretaceous Felsic Intrusions in East Qinling

The Early Cretaceous Houyaoyu granite porphyries are located in the south margin of the North China Craton. Field observations, petrography, geochronology, major and trace elemental and Sr-Nd isotopic compositions are reported to elucidate the genesis of the Houyaoyu granite porphyries. SIMS zircon U-Pb analyses for the Houyaoyu granite porphyries yield two concordant ages of 133.2±2.3 （2σ） and 131±1.1 （2σ） Ma, respectively. Major and trace elemental compositions indicate that these porphyries are high-K I-type granites with high contents of SiO2, K20, Rb, U, Pb, low Nb, Ta, Ti, and P. Initial S：Sr/S6Sr ratios range from 0.708 3 to 0.709 7, and εNd（t） values range from -9.13 to -12.3, with corresponding two-stage depleted-mantie Nd model ages （T2DM） varying from 1.57 to 1.91 Ga. This suggests that the Houyaoyu granite porphyries were predominantly derived from ancient lower conti- nental crust, with minor involvement of mantie-derived components. On the basis of the tectonic evolution of the Qinling Orogen and geochemical characteristics of the Houyaoyu granite porphyries, it is proposed that they were formed in an extensional tectonic setting related to lithospheric destruction of the North China Craton, and produced Mo and Pb-Zn mineralization in East Qinling Orogen. KEY WORDS： East Qinling, granite porphyries, ancient lower continental crust, destruction of North China Craton.