REE and Sr-Nd isotope characteristics of hydrothermal chimney at Jade area in the Okinawa Trough

Hydrothermal chimney is a product of hydrothermal activity on the seabed. Chimney samples dredged from Jade hydrothermal area in Izena depression of the Okinawa Trough, are characterized by relatively enriched light rare earth elements (LREE) and strongly positive Eu anomalies. 87Sr/86Sr and 143Nd/144Nd of these samples are exactly between those of seawater and of acidic pumice, averaged at 0.708928 and 0.512292, respectively. These characteristics imply that the main source of hydrothermal sulfide at Jade area is possibly the undersurface acidic rocks. The mineralizing mechanism can be summarized as follows: Large amount of mineralized material would be leached out and LREE-enriched hydrothermal solution would be subsequently produced as a result of thermo-chemical exchange reaction between acidic volcanic rocks and heated seawater that penetrated in advance from upper water mass. The spurting out from the seabed and quickly crystallizing in the seawater of hydrothermal solution are responsible for the formation of Cu-Zn sulfide and barite-amorphous SiO2 minerals that are characterized by enriched LREE and positively strong Eu anomalies.

Trace Elements and Sr, Nd Isotope Geochemistry of the Lajimiao Norite-Gabbro from the North Qinling Belt

The Lajimiao norite-gabbro complex, as a part of the ophiolites on the southern side of the North Qinling belt, consists of gabbro and norite-gabbro. They were derived from different magma series: the gabbro was derived from tholeiitic magma series with higher TiO2, REE abundance and Fe3+/Fe2+ ratio ; norite-gabbro was derived from calc-alkali magma series with lower TiO2, Fe3+/Fe2+ ratio and REE abundance and much lower HREE abundance, which suggests that the source of the norite-gabbro magma was deeper and controlled by eclogite facies. Geochemical characteristics of both plutonic rocks are similar to those of island-arc basalts, such as relatively high contents of Ba, Pb and Sr and relatively low contents of Nb, Zr and N j.The Sr, Nd isotopic characteristics of the Lajimiao norite-gabbro complex are similar to those of ophiolites. Its εNd values are constant, about+2; whereas εst values have wide variation from - 6.4 to +31.2 and positively correlate with Na2O, H2O+ and CO2 contents and the Fe3+/Fe2+ ratio. The εNd- Nd/Th, εNd- La/Nb and εNd-Ba/Nb diagrams clearly show that there were significant components of terrigenous sediments in the mantle source of the Lajimiao norite-gabbro complex. It suggests that large amount of sediments had been carried into the mantle by the subducted ancient Qinling sea plate during the Palaeozoic.

Basement Characteristics and Crustal Evolution of the Copper-Gold Metallogenic Belt in the Middle and Lower Reaches of the Yangtze River:Some Isotope Constraints

Studies of the Pb, Sr and Nd isotopic composition of Mesozoic intrusive rocks indicate that the basement of the copper-gold metallogenic belt of the middle and lower reaches of the Yangtze River has “two-layer structure” and partly has “multi-layered structure”, and is inhomogeneous and shows the distinct feature of E-W provincialism. The calculated model lead ages (t1) are mostly greater than 2600 Ma, and the model neodymium ages (TDM) vary from 953 to 2276 Ma and concentrate in two time intervals: 1800–2000 Ma and 1200–1600 Ma. It is concluded that the basement of the MBYR is composed of the Late Archaeozoic to Middle Proterozoic metamorphic series and that the crust was initiated in the Archaean and continued to grow in the Early and Middle Proterozoic, and the proportion of new crust formed by mantle differentiation during the Late Proterozoic is low.

Sr AND Nd ISOTOPIC COMPOSITION OF THE ULTRAMAFIC XENOLITHS FROM EASTERM CHINA:INFERENCES ABOUT THE STRUCTURE OF THE SUBCONTINENTAL LITHOSPHERE MANTLE AND THE ORIGIN OF BASALTIC MAGMAS

Uliramafie xenoliths provide an important constraint on the composition,strueture and evolution of the lithosphere mantle beneath eastern China. Most of the xenoliths entrained by the Cenzoic basalts have de pleted Sr and Nd isotopie composition. Interestingly evidences have been found for old ehriehed lithosphere mantle beneath thick Arehaean and post-Arehaean erust (＞35 km ) and for relatively homogeneous and depleted lithosphere mantle bencath thin erust (＜ 35 km ). Furthermore the chemieal composition of the lithosphere mantle is overall fertile regardless of the age of the overlying erust. The spatial variability in ehemieal and Sr and Nd isotopie eompositions of the ultramafie xenoliths in relation to the lithospherie ages and geometry implies a mixture strueture of the lithosphere mantle beheath easiern China. which consists of recently aecreted asthenospherie diapirs and old modified remnants. Isotopie eomposition of the Cenozoie basalts show obvious similarities with ocein island basalt (OIH ). We argue that enriehed lithosphere mantle delamineted during Mesozoic-Cenozoie tectonie-thermal events was generally involved in the Cenozoie basaltic voleanism.

Zircon SHRIMP U–Pb geochronology,geochemistry,and geological significance of dacite in the Zhesang gold district,southeast Yunnan Province,China

The Late Paleozoic and Mesozoic tectonic framework of the Nanpanjiang Basin has much been disputed.Herein,the middle-acid volcanic rock,dacite,exposed to the Zhesang gold district,southeast Yunnan Province,has been analyzed.The results show that the dacite belongs to a calc-alkaline series,SiO_(2)contents range from 62.79 to 76.66 wt%.Zircon SHRIMP U–Pb dating of dacite demonstrates that they were formed in the Early Triassic(247.8±1.7 Ma,MSWD=1.2).All samples exhibit enrichment in LILE(e.g.Rb,K,Th,and U),and depletion in HFSE(e.g.Nb,Ta,and Ti),which has the geochemical affinity of I-type granite.La–La/Sm and La–La/Yb discrimination diagrams show that the partial melting,mainly of the mafic lower crust,of rocks,plays a major role in the formation process.The dacite has low initial ^(87)Sr/^(86)Sr ratios(0.706954 to 0.708589)and negative ε_(Nd)(t)values(-11.77 to-10.88).Zircons in dacite have ε_(Hf)(t)values of-16.2 to-8.3,and the two-stage Hf model ages are 1799–2301 Ma,mostly concentrated between 1800 and 1900 Ma,indicating that the magma source area is the reconstructed ancient lower crust mixed with some mantle materials,and crystal fractionation process underwent in the late stage of magma migration.This study reveals that the arc-volcanic rocks of the Early Triassic in the southern margin of the Nanpanjiang Basin were formed by the subduction of the Late Paleozoic ocean basin within the border region between China and Vietnam.

Sr and Nd Isotopic Characteristics and Magma Genesis of Mesozoic Volcanic Rocks Along the Coastal Region of Southeastern China

The Mesozoic volcanic rocks in the coastal region of southeastern China were superimposed on some different basement tectonic elements. The volcanic rocks developed in these different basement tectonic elements have great differences in Sr and Nd isotopic compositions. The rocks in western Zhejiang and northeastern Jiangxi Provinces which belong to the Lower Yangtze subplate have lower initial 87Sr/ 86Sr ratios, but are higher in initial Nd isotopic ratios. The initial 143Nd / 144Nd values of the volcanic rocks developed in the Cathaysian subplate increase clearly from early to late in time, and from the core of the Wuyishan uplift coastwards constantly, but the initial 87Sr/86Sr values tend to decrease. The isotopic characteristics and their spatial variations in Mesozoic volcanic rocks in the study region are, to a great extent, manifestations of the isotopic characteristics in basement metamorphic complexes, and the generation of the Mesozoic acid magma in this region is attributed to the recycling of pre-existing crustal materials.

Zircon U–Pb dating,geochemical,and Sr–Nd–Pb–Hf isotopic constraints on the age and origin of intermediate to felsic igneous rocks at South Altyn,Xinjiang,China

As a part of a giant trending fault system in the Asian continent and one where a strong zone of left strikeslip fault is present,the Altyn Orogenic belt(AOB)has become an important focus for research.Magmatic rocks are widely distributed across the AOB.However,many investigations have focused primarily on Paleozoic igneous rocks;discussion of Mesozoic related igneous activity is often ignored.Here we present the result of studies of representative diorite and granite rocks outcropping in the AOB,within the Xinjiang Uygur Autonomous Region,South Altyn,China.We present new zircon LA-ICP-MS U-Pb age,geochemical,and Sr-Nd-Pb-Hf isotopic data for these sample suites,identifying them as typical igneous rocks formed between 238±1.5 and 238.8±1.1 Ma.The rocks that we studied fall into the alkaline series,also enriched in light rare earth elements(LREE),some large ion lithophile elements(LILE;e.g.,Rb,Ba,Sr,and K),Pb,Th and U,and depleted in heavy rare earth elements(HREE),Nb,Ta,Hf,and Ti.The granite and diorite have high initial 87Sr/86Sr ratios(0.7062-0.7114),negativeεNd(t)values(-8.8 to-11.3),εHf(t)values(-8.7 to-18.7),and relatively constant Pb isotopic ratios((206-Pb/204Pb)i=6.74-17.884,(207Pb/204Pb)i=15.51-15.58,and(208Pb/204Pb)i=35.36-38.04),respectively.This suggests that the magmas parental to these rocks were generated from the partial melting of the ancient crust.The parental magmas to these rocks experienced a degree of fractionation of plagioclase,K-feldspar,and hornblende,possibly during rapid magma ascent.Based on these studies,we propose a reasonable model for the origin of the investigated rocks from the Xinjiang Uygur Autonomous Region of South Altyn,which involves crustal thickening,lithospheric extension,and asthenosphere upwelling,that induced crustal melting.

A geochronological and geochemical study on the granodiorite porphyry and its implication for the mineralization in the Dayaoshan metallogenic belt,Southeastern China

A systematic study combining U-Pb zircon dating,lithogeochemical and Sr-Nd isotopic analyses was carried out upon the Xinping granodiorite porphyry in the Dayaoshan metallogenic belt to understand its petrogenesis and tectonic significance.LA-ICP-MS U-Pb zircon dating yielded a 442.7±5.8 Ma age,indicating that the granodiorite porphyry was emplaced during the Llandovery Silurian of the Early Paleozoic.The granodiorite porphyry shares the same geochemical characteristics such as Eu negative anomaly as other syn-tectonic granite plutons in the region,including the granodiorite porphyry in Dawangding and granite porphyries in the Dali Cu-Mo deposit and Longtoushang old deposit,indicating a similar magma evolution process.The Xinping granodiorite porphyry has high contents of SiO2(67.871.8%)and K2O(1.78-3.42%)and is metaluminous-peraluminous with A/CNK ratios ranging from 0.97 to 1.06,indicative of high-potassium calc-alkaline to calc-alkaline affinity.It is a I-type granite enriched in large ion lithophile elements Rb,Sr,while depleted in Ba and high field-strength element Nb.Tectonically,a collision between the Yunkai Block from the south and the Guangxi Yunnan-North Vietnam Block from the north during the Early Paleozoic was followed by uplifting of the Dayaoshan terrane.The Xinping granodiorite porphyry was likely emplaced during the collision.Sr-Nd isotopic analyses show that the granodiorite porphyry has initial 87Sr/86Sr ratios(Isr)of 0.7080-0.7104,εNd(t)range from-0.08 to-4.09,and t2DM between 1.19 and 1.51 Ga,well within the north-east low-value zone of the Cathaysia block,indicating a Paleoproterozoic Cathaysia basement source and an involvement of under plating mantle magma.Field observations,geochronological data,and 3D spatial distribution all lead to the conclusion that the Early Paleozoic Xinping granodiorite porphyry does not have any metallogenic and temporal relationships with the Xinping gold deposit(which has a Jurassic-Early Cretaceous age based on previous studies)but a close metallogenic relation to W-Mo mineralization.

Geochronology and geochemistry of magmatic rocks in the Dongzi–Changhanboluo Pb–Zn ore district in Chifeng,Inner Mongolia,and their relationship with metallogenesis

Bulk-rock elements,isotopes,and zircon U–Pb ages are reported for magmatic rocks in the Dongzi–Changhanboluo Pb–Zn ore district in Chifeng,Inner Mongolia,China.Zircon U–Pb dating identified four stages of magmatism:Late Silurian gabbroic diorite(*420 Ma),Middle Permian monzonite(*274 Ma),Late Jurassic quartz porphyry and ignimbrite,breccia tuff(153–158 Ma)and Early Cretaceous andesitic porphyrite(*127 Ma).Integrating field observations,geochronology,and element and isotope geochemistry indicated a complex petrogenetic history of the magmatic rocks.The gabbroic diorite may have been sourced from EM1-type mantle.The source of the monzonite may have been mantle metasomatized by melt from the subducting plate.The Jurassic volcaniclastic rocks formed in a medium-pressure,high-temperature environment,possibly in the background of crustal thickening in a syncollisional stage and an early postcollisional stage.During this process,shaly sedimentary rocks were brought into the deep crust and heated,followed by the rapid isostatic uplift of the crust,which caused partial melting of the sedimentary rocks.Quartz monzonite porphyry and quartz porphyry formed by partial melting of mantle metasomatized by subducted sediments,but the quartz porphyry experienced high-degree differentiation and evolution.The andesitic porphyrite has characteristics similar to those of Permian monzonite,indicating that its source area was also the zone of mantle metasomatized by subducted sediment.The late Silurian and Permian magmatic rocks in this area most likely formed against a continental arc background related to the subduction of the Paleo-Asian Ocean Plate beneath the North China Plate.The Late Jurassic magmatic rocks suggest that the northern margin of the North China Craton may have been in a postcollisional setting during the Late Jurassic,with no obvious crustal thinning.The Cretaceous andesitic porphyrite may have formed against the background of lithospheric extension and thinning.According to the comprehensive analysis of geological characteristics,diagenetic and metallogenic epochs,and Pb isotope data,the formation of ore bodies in the Dongzi–Changhanboluo ore district was closely related to the Jurassic quartz porphyry.

The giant tin polymetallic mineralization in southwest China:Integrated geochemical and isotopic constraints and implications for Cretaceous tectonomagmatic event

The Gejiu-Bozushan-Laojunshan W-Sn polymetallic metallogenic belt(GBLB)in southeast Yunnan Province is an important part of the southwestern Yangtze Block in South China.Tin polymetallic mineralization in this belt includes the Niusipo,Malage,Songshujiao,Laochang and Kafang ore fields in the Gejiu area which are spatially and temporally associated with the Kafang-Laochang and Songshujiao granite plutons.These granites are characterized by variable A/CNK values(mostly>1.1,except for two samples with 1.09),high contents of SiO2(74.38-76.84 wt.%)and Al2 O3(12.46-14.05 wt.%)and variable CaO/Na2 O ratios(0.2-0.65)as well as high zirconδ18O values(7.74‰-9.86‰),indicative of S-type affinities.These rocks are depleted in Rb,Th,U,Ti,LREE[(La/Yb)N=1.4-20.51],Ba,Nb,Sr,and Ti and display strong negative Eu and Ba anomalies.The rocks possess high Rb/Sr and Rb/Ba ratios,relatively low initial 87Sr/86Sr ratios(0.6917-0.7101),and less radiogenicεNd(t)values(-8.0 to-9.1).The zircon grains from these rocks show negativeεHf(t)values in the range of-3.7 to-9.9 with mean TDM2(Nd)and TDM2(Hf)values of 1.57 Ga and 1.55 Ga.They show initial 207Pb/204Pb ranging from15.69 to 15.71 and 206Pb/204Pb from 18.36 to 18.70.Monazite from Songshujiao granites exhibits higher U and lower Th/U ratios,lowerδ18O values and higherεHf(t)values than those of the zircon grains in the KafangLaochang granites.The geochemical and isotopic features indicate that the Laochang-Kafang granites originated by partial melting of Mesoproterozoic crustal components including biotite-rich metapelite and metagraywacke,whereas the Songshujiao granites were derived from Mesoproterozoic muscovite-rich metapelite crustal source.Most zircon grains from the Songshujiao,Laochang and Kafang granites have high-U concentrations and their SIMS U-Pb ages show age scatter from 81.6 Ma to 88.6 Ma,80.7 Ma to 86.1 Ma and 82.3 Ma to 87.0 Ma,suggesting formation earlier than the monazite and cassiterite.Monazite SIMS U-Pb ages and Th-Pb ages of three same granite samples are consistent and show yielded 206 Pb/238U ages of 83.7±0.6 Ma,83.7±0.6 Ma,and 83.4±0.6 Ma,and 208Pb/232Th ages of 83.2±0.5 Ma,83.8±0.4 Ma,and 83.5±0.9 Ma,which are within the range of the SIMS zircon U-Pb ages from these rocks.The data constrain the crystallization of the granites at ca.83 Ma.In situ U-Pb dating of two cassiterite samples from the cassiterite-sulfide ore in the Songshujiao ore field and Kafang ore field,and two from the cassiterite-oxide+cassiterite bearing dolomite in the Laochang ore field yielded weighted mean 206 Pb/238U ages of 83.5±0.4 Ma(MSWD=0.6),83.5±0.4 Ma(MSWD=0.5),83.6±0.4 Ma(MSWD=0.6)and 83.2±0.7 Ma(MSWD=0.6),respectively.Combined with geological characteristics,the new geochronological data indicate that the formation of the granites and Sn polymetallic deposits are coeval.We correlate the magmatic and metallogenic event with lithospheric thinning and asthenosphere upwelling in continental extension setting in relation to the eastward subduction of the Neo-Tethys beneath the Sanjiang tectonic domain during Late Cretaceous.

Nd and Sr isotopic compositions of basalts from the Alatao Pass, Xinjiang

THE studied region is near the Alatao Pass in the northwestern Xinjiang Uygur AutonomousRegion.It is near the northeastern margin of the Gongneis Plate tectonically.The Aibihufault,situated in the northeastern border of the plate,striking NW,is a component of theBoloholo suture zone which separates the Gongneis Plate from the Junggar Plate.The Pre-cambrian basement crops out in the Wenquan County,about 100 km west of the studied re-

Crust–mantle interaction recorded by Early Cretaceous volcanic rocks within the southern margin of the North China Craton

This study presents new geochronological and geochemical data for Early Cretaceous volcanic rocks in the southern margin of the North China Craton(NCC),to discuss the crust–mantle interaction.The studied rocks include pyroxene andesites from Daying Formation,hornblende andesites and andesites from Jiudian Formation,and rhyolites from a hitherto unnamed Formation.These rocks formed in Early Cretaceous(138–120 Ma),with enrichment in light rare earth elements(REE),depletion in heavy REE and arc-like trace elements characteristics.Pyroxene andesites show low SiO_(2) contents and enriched Sr–Nd–Pb–Hf isotopic compositions,with orthopyroxene phenocryst and Paleoproterozoic(2320–1829 Ma)inherited zircons,suggesting that they originated from lithospheric mantle after metasomatism with NCC lower crustal materials.Hornblende andesites have low SiO_(2) contents and high Mg#(Mg#=100 Mg/(Mg+Fe^(2+)))values,indicating a lithospheric-mantle origin.Considering the distinct wholerock Sr isotopic compositions we divide them into two groups.Among them,the low(^(87)Sr/^(86)Sr)iandesites possess amount inherited Neoarchean to Neoproterozoic(2548–845 Ma)zircons,indicating the origin of lithospheric mantle with addition of Yangtze Craton(YC)and NCC materials.In comparison,the high(^(87)Sr/^(86)Sr)iandesites,with abundant Neoarchean–Paleozoic inherited zircons(3499–261 Ma),are formed by partial melting of lithospheric mantle with incorporation of NCC supracrustal rocks and YC materials.Rhyolites have extremely high SiO_(2)(77.63–82.52 wt.%)and low total Fe_(2)O_(3),Cr,Ni contents and Mg#values,combined with ancient inherited zircon ages(2316 and 2251 Ma),suggesting an origin of NCC lower continental crust.Considering the presence of resorption texture of quartz phenocryst,we propose a petrogenetic model of’crystal mushes’for rhyolites prior to their eruption.These constraints record the intense crust–mantle interaction in the southern margin of the NCC.Given the regional data and spatial distribution of Early Cretaceous rocks within NCC,we believe that the formation of these rocks is related to the contemporaneous far-field effect of the Paleo-Pacific Plate.

Garnet-Rich Granulite Xenoliths from the Hannuoba Basalts, North China: Petrogenesis and Implications for the Mesozoic Crust-Mantle Interaction

Garnet-rich granulite xenoliths collected from the Hannuoba basalts, the North China craton （NCC）, were studied to reveal the Mesozoic crnst-mantle interaction. These xenoliths are characterized by low SiO2 （37.7 wt.%-46.0 wt.%） and high Al2O3 （10.8 wt.%-17.9 wt.%） contents. Their Mg# （60-75, Mg#=100×Mg/（Mg＋Fe）, atomic number） are relatively low for their low SiO2 contents. They have low rare-earth element （REE） contents and LREE-rich REE patterns, and show remarkable enrichments in Sr relative to the adjacent REE. Some of them exhibit convex RISE patterns with a maximum at Nd and remarkably positive Eu anomalies. Taking into account their high garnet mode （generally 〉30%）, these features suggest that they are high-pressure metamorphic products of lowpressure cumulates （e.g., gabbro） after it had been depressed into the garnet stability field. They have evolved Nd and Sr isotopic compositions （143Nd/144Nd=0.511 763-0.512 173, STSr/86Sr=0.705 34-0.706 99） and fall in the trend defined by the 〉110 Ma Mesozoic basalts and high-Mg# andesites from the NCC. Zircon U-Pb dating by LA-ICP-MS shows a wide age range from 83 to 2 581 Ma, most of which cluster in 83-134 Ma. CL images of some Mesozoic zircons from the granulites show typical features of igneous zircons, providing direct evidence for the Mesozoic underplating event in this area. Neither peridotite-derived basaltic underplating model nor residue model of ancient lower crust after lithospheric thinning alone can reasonably explain the above features of the garnet-Hch granulite xenoliths. Combined with the previous research, we propose that most of the granulite xenoliths from the Hannuoba basalts are products of the Mesozoic magmatie underplating and mixing with the pre-existing lower crust （i.e., AFC process）. However, the melts could be mostly derived from partial melting of basaltic layers that were previously subducted （a fossil oceanic slab） or underplated into the base of the lithospheric mantle, or from partial melting of Archean lithospheric mantle that was variably hybridised by melts derived from foundered lower crustal edogite, although it cannot be excluded that some of the melts were derived from depleted man. tie peridotite. In other words, parent melts of most granulite xenoliths could share the same petrogenesis as the〉110 Ma Mesozoic basalts from the NCC.