Generation of Nb-enriched mafic rocks and associated adakitic rocks from the southeastern Central Asian Orogenic Belt:Evidence of crust-mantle interaction

Melting of subducting oceanic lithosphere and associated melt-mantle interactions in convergent plate margins require specific geodynamic environment that allows the oceanic slab to be abnormally heated.Here we focus on the Early Mesozoic mafic rocks and granite porphyry,which provide insights into slab melting processes associated with final closure of the Paleo-Asian Ocean.The granite porphyry samples are calc-alkaline and distinguished by high Sr contents,strong depletion of heavy rare earth elements,resulting in high(La/Yb);and Sr/Y ratios,and negligible Eu anomalies.Based on their high Na_(2)O and Mg O,low K_(2)O contents,positiveε_(Hf)(t)andε_(Nd)(t)and low(^(87)Sr/^(86)Sr)ivalues,we propose that the granite porphyry was likely derived from partial melting of subducting Paleo-Asian oceanic crust.The Nb-enriched mafic rocks are enriched in Rb,Th,U,Pb and K,and depleted in Nb,Ta,Ba,P and Ti,corroborating a subduction-related origin.Their heterogeneous Sr-Nd-Hf-O isotopic compositions and other geochemical features suggest that they were likely derived from partial melting of peridotitic mantle wedge interacted with oceanic slab-derived adakitic melts.Trace element and isotope modeling results and elevated zirconδ^(18)O values suggest variable subducting sediments input into the mantle wedge,dominated by terrigenous sediments.Synthesizing the widely-developed bimodal rock associations,conjugated dikes,thermal metamorphism,tectonic characteristics,paleomagnetic constraints,and paleogeographical evidence along the Solonke-Changchun suture zone,we identify a slab window triggered by slab break-off,which accounts for slab melting and formation of the Nb-enriched mafic rocks and associated adakitic granite porphyry in southeastern Central Asian Orogenic Belt.

Late Jurassic adakitic ore-bearing granodiorite porphyry intrusions in the Xiaokele porphyry Cu(–Mo)deposit,Northeast China:implications for petrogenesis and tectonic setting

The Xiaokele Cu(–Mo)deposit is a recently discovered porphyry deposit in the northern Great Xing’an Range(GXR)of northeast China.The ore bodies in this deposit are mainly hosted within granodiorite porphyry intrusions.Potassic,phyllic,and propylitic alteration zones develop from center to edge.In this paper,we present zircon LA–ICP–MS U–Pb ages,zircon Hf isotopic compositions,and whole-rock geochemistry of the ore-bearing granodiorite porphyries from the Xiaokele Cu(–Mo)deposit.Zircon U–Pb dating suggests that the Xiaokele granodiorite porphyries were emplaced at 148.8±1.1 Ma(weighted-mean age;n=14).The Xiaokele granodiorite porphyries display high SiO2,Al2O3,Sr,and Sr/Y,low K2O/Na2O,MgO,Yb,and Y,belonging to high-SiO2 adakites produced by partial melting of the subducted oceanic slab.Marine sediments were involved in the magma source of the Xiaokele granodiorite porphyries,as indicated by enriched Sr–Nd isotopic compositions(eNd(-t)=-1.17–-0.27),low positive zircon eHf(t)values(0.4–2.2),and high Th contents(4.06–5.20).The adakitic magma subsequently interacted with the mantle peridotites during ascent through the mantle wedge.The Xiaokele granodiorite porphyries were derived from slab melting during the southward subduction of the Mongol–Okhotsk Ocean.

Geological characteristics of the Nankai Trough subduction zone and their tectonic significances

The Nankai Trough subduction zone is a typical subduction system characterized by subduction of multiple geological units of the Philippine Sea Plate(the Kyushu-Palau Ridge,the Shikoku Basin,the Kinan Seamount Chain,and the Izu-Bonin Arc)beneath the Eurasian Plate in the southwest of Japan.This study presents a geophysical and geochemical analysis of the Nankai Trough subduction zone in order to determine the features and subduction effects of each geological unit.The results show that the Nankai Trough is characterized by lowgravity anomalies(–20 mGal to–40 mGal)and high heat flow(60–200 mW/m2)in the middle part and low heat flow(20–80 mW/m2)in the western and eastern parts.The crust of the subducting plate is 5–20 km thick.The mantle composition of the subducting plate is progressively depleted from west to east.Subduction of aseismic ridges(e.g.,the Kyushu-Palau Ridge,the Kinan Seamount Chain,and the Zenisu Ridge)is a common process that leads to a series of subduction effects at the Nankai Trough.Firstly,aseismic ridge or seamount chain subduction may deform the overriding plate,resulting in irregular concave topography along the front edge of the accretionary wedge.Secondly,it may have served as a seismic barrier inhibiting rupture propagation in the 1944 Mw 8.1 and 1946 Mw 8.3 earthquakes.In addition,subduction of the Kyushu-Palau Ridge and hot and young Shikoku Basin lithosphere may induce slab melting,resulting in adakitic magmatism and the provision of ore-forming metals for the formation of porphyry copper and gold deposits in the overriding Japan Arc.Based on comparisons of their geophysical and geochemical characteristics,we suggest that,although the Izu-Bonin Arc has already collided with the Japan Arc,the Kyushu-Palau Ridge,which represents a remnant arc of the Izu-Bonin Arc,is still at the subduction stage characterized by a single-vergence system and a topographic boundary with the Japan Arc.

Decoupling between Oxygen and Radiogenic Isotopes: Evidence for Generation of Juvenile Continental Crust by Partial Melting of Subducted Oceanic Crust

There is increasing evidence indicating that melts derived from subducted oceanic crust and sediments may have played a key role in building continental crust. This mechanism predicts that juvenile arc crust should have oxygen isotope characteristics ranging from mantle-like to supracrustal, but consistent mantle-like radiogenic(Nd-Hf) isotopic signatures. Here we present in-situ zircon U-Pb dating, Hf-O isotope analyses, and whole rock major-trace element and Nd isotope analyses of a granitoid from NW India. In-situ secondary ion mass spectrometry(SIMS) zircon U-Pb dating yields a weighted mean ^(207)Pb/^(206)Pb age of 873±6 Ma for the granitoid. It displays mantle-like zircon εHf(εHf(873 Ma)= +9.3 to +10.9) and whole-rock Nd(εNd(873 Ma)= +3.5) values but supracrustal δ^(18)O values, the latter mostly varying between 9‰ and 10‰. The calculated whole-rock δ^(18)O value of 11.3‰±0.6‰ matches well with those of hydrothermally-altered pillow lavas and sheeted dykes from ophiolites. The major and trace element composition of the granitoid is similar to petrological experimental melts derived from a mixture of MORB+sediments. Thus, the granitoid most likely represents the product of partial melting of the uppermost oceanic crust(MORB+sediments). We propose that the decoupling between Hf-Nd and O isotopes as observed in this granitoid can be used as a powerful tool for the identification of slab melting contributing to juvenile continental crustal growth. Such isotopic decoupling can also account for high δ^(18)O values observed in ancient juvenile continental crust, such as Archean tonalitetrondhjemite-granodiorite suites.

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.