Lamprophyres from Southern Karnataka, Dharwar Craton, India: Insight on the Rodinia Break-up and Addition of Juvenile Crust

The late Archean Dharwar Craton is an important part of the Archean and Proterozoic terrains in Peninsular India.Dharwar Craton consists of Western and Eastern Blocks,separated by the Chitradurga Shear Zone.Eastern

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.

Zircon U-Pb ages and O-Hf isotopes of Quaternary trachytes from the East Sea:Implications for the genesis of low-δ^(18)O magmas

Quaternary intraplate magmatism formed several volcanic islands and seamounts,including Dokdo(DD),Ulleungdo(UD),Simheungtack(ST),Anyongbok,and Isabu in the southwest of the East Sea back-arc basin.In this study,we present whole-rock geochemical,zircon U–Pb age,and in situ O–Hf isotope data for the submerged volcanic rocks from DD,UD,and ST to provide new insights into the eruption timing of these volcanoes and constrain the magma evolution processes.All samples used in this study were trachytes and exhibited ferroan,alkalic,and metaluminous to weakly peraluminous characteristics.They showed light rare earth element(REE)-enriched patterns with(La/Yb)N ratios of 25.3–31.7 and mostly negative Eu anomalies in a chondrite-normalized REE plot.In addition,they were enriched in large-ion lithophile elements and high field strength elements;they exhibited positive Pb anomalies and strongly negative Ba,Sr,P,and Ti anomalies.The zircons yielded a weighted-mean 206Pb/238U age of 2.61,0.348–0.704,and 2.76–2.94 Ma for the DD,UD,and ST trachytes,respectively.All zircons exhibited lowerδ^(18)O values than normal depleted mantle values,regardless of the crystallization age and spatial distribution of volcanoes.Theδ^(18)O values showed no correlation with U contents or Th/U ratios,indicating that the lowδ^(18)O signatures were of primary magmatic origin.The Hf isotopic compositions of the zircons were relatively heterogeneous but predominately characterized by positive eHf values.Binary O–Hf mixing modeling revealed that low-δ^(18)O rocks with positive eHf values from the UD and ST volcanoes were derived from a hybrid source of recycled juvenile crustal materials with low-δ^(18)O and positive eHf signatures and an enriched mantle source with normalδ^(18)O and negative eHf values.The juvenile oceanic crust in the source was likely metasomatized by seawater at high temperatures prior to melting.In contrast,the felsic magma that formed the DD volcanoes may have assimilated with regional basement rocks(Triassic–Jurassic granitoids),resulting in increasedδ^(18)O values and decreased eHf values relative to those of the UD and ST volcanoes.Our study highlights the significant contribution of recycled oceanic crust materials to the generation of the Quaternary magmas.

Do Supercontinent-Superplume Cycles Control the Growth and Evolution of Continental Crust?

The evolution of continental crust can be directly linked to the first-order supercontinent-superplume cycles.We demonstrate that:(1)a mantle-like oxygen isotopic signature is not a diagnostic feature for distinguishing crustal addition from the reworking of pre-existing continental crust;(2)juvenile continental crust shows a wide range of whole-rock Hf isotopic compositions throughout Earth's history;and(3)detrital zircon Hf model ages cannot reliably determine the growth of continental crust.Thus,the wide use of zircon Hf model ages,based on zircon grains with mantle-like oxygen isotopes,is inappropriate for estimating the timing of continental crustal generation.Based on an analysis of global Hf and O isotope and zircon age databases,we argue that the actual U-Pb crystallization ages of juvenile zircon grains provide the best opportunity to unravel crustal growth through time and to test its relationship with supercontinent-superplume cycles.Furthermore,when the Hf isotopes of these juvenile grains plot within the field of juvenile continental crust,they correlate well with times of global mantle depletion as recorded by Os and He isotopes,plume activity as recorded by LIP events,and periods of crustal growth and the breakup of supercontinents.In contrast,zircon grains crystallized from magmas that were produced by partial melting of pre-existing continental crust show U-Pb age peaks that correspond mainly to times of supercontinent assembly and crustal reworking.Detailed analysis shows the key role played by recycling of mafic crustal components in the generation of juvenile continental crust.