Age and Origin of Paleogene Granitoids from Western Yunnan Province,China:Geochemistry,SHRIMP Zircon Ages,and Hf-in-Zircon Isotopic Compositions

We report geochemical data, SHRIMP zircon ages and Hf-in-zircon isotopic compositions for Cenozoic granitoids from major fault systems in the Tethyan belt in western Yunnan Province, southwestern China.Four magmatic pulses occurred in the Paleogene, namely at ca.57 Ma, ca.50 Ma, 45–40 Ma, and 38–34 Ma.Early magmatism of this episode（57–50 Ma） produced S-type granites whose zircons yielded εHf（t） values of-5.0 to-0.3.In contrast, late magmatism of this episode reflects heterogeneous sources.Zircons from a granite porphyry along the Ailaoshan-Red River fault system have slightly positive εHf（t） values suggesting derivation from relatively young crust and/or a juvenile source.However, zircons from a granite along the Gaoligong fault system have strongly negative εHf（t） values and suggest derivation from a Paleoproterozoic crustal source.The composition of the granitoids varies with age（from ca.57 Ma to ca.34 Ma） from peraluminous to metaluminous and also suggests a change from syn-collisional to late-orogenic tectonic setting.A new tectonic model, impacting lithospheric wedge（ILW） is shown for the origin of Paleogene granitoids in this paper.

Petrogenesis and Tectonic Significance of the Xiuwacu Two-Period Magmatism in Geza Arc of Yunnan Province： Constraints from Lithogeochemistry, Zircon U-Pb Geochronology and Hf isotopic Compositions

Objective The Late Cretaceous Xiuwacu ore-bearing porphyry is located in the Geza area of southern Yidun arc, SW China. In this area, the rock mass is mainly composed of three lithofacies： biotite granite porphyry, monzonitic granite and light alkali feldspar granite. As a part of the Yidun arc, the Geza arc has common structure and temporal- spatial evolution with the ~idun arc, which has experienced three stages of oceanic crust subduction, collision orogeny and intracontinent convergence stages. The molybdenite ores in the area are mainly hosted in monzonitic granite-porphyry and structural fracture zone, and the ore bodies are strictly controlled by faults. In recent years, great geological prospecting results have been achieved in Xiuwacu, and the deposit has reached a medium scale. However, there are few researches on the metallogenic porphyry. Based on the previous research, we determined the rock-forming and ore-forming age of the porphyry, and found that there were two stages of magmatism intrusion in Xiuwacu： Indosinian and Yanshanian. We also discussed the geochemical characteristics and source area of the rocks in the area.

New Discovery and Zircon Hf Isotopic Composition of the Early Yanshannian Monzogranite in the Yingyangguan Group of Eastern Guangxi, China

Objective The Yingyangguan Group is widely exposed in the southwestern segment of boundary belt between Yangtze and Cathaysian blocks and is mainly composed of a suit of lower-grade metamorphic marine volcanoclastic- sedimentary rocks （Zhou Hanwen et al., 2002）. Its forming age and petrogenesis are critical for better understanding the orogenic process and relationship of the Yangtze and Cathaysian blocks since the Neoproterozoic. However, few zircon U-Pb ages of samples from the Yingyangguan Group have been reported, which are much debated, such as the metamorphic spilite （819±11 Ma）, keratophyre （415.1±2.1 Ma） and ignimbrite （821.3±3.9 Ma） （Tian Yang et al., 2015 and references in）. In this study, we focus on the new discovered Early Yanshannian monzogranite and its mafic enclaves from the Yingyangguan Group, and conducted zircon U-Pb dating and in-situ Hf isotopic analyses to constrain their petrogenesis.

LA-ICP-MS Zircon U-Pb Age and Hf Isotopic Composition of Late Carboniferous Granodiorite in the Southern East Junggar,Xinjiang:Genesis and Tectonic Implication

A large area of Late Paleozoic intrusions occursalong the Kalamaili fault in North Xinjiang,which is divided into I-type and A-type granite(Liu et al.,2013),and are the ideal objects for revealing the geological evolution of this region.However,the study of the granodioritic pluton in East Junggar is particularly weak.

Zircon U-Pb Chronology and Whole-rock Sr-Nd Isotope Compositions of Granite Porphyry in the Kaladaban Area of the Northern Altyn

Objective The Altyn Tagh marks the northern margin of the Qinghai-Tibet Plateau and lies between the Tarim block to the north and the Qaidam block,Qilian Orogen,and Kunlun orogenic belt to the south.The Altyn Tagh region contains ophiolite,high-to ultrahigh-pressure metamorphic rocks,and igneous rocks.Previous research has virified the occurrence of continental rifting,subduction,slab roll-back,and collision between the Tarim block and Proto-Tethys oceanic plate.Moreover,Kaladaban volcanic rocks are mainly distributed in the north Altyn region.Studies of the magmatic evolution of this region have proposed that Altyn oceanic plate was subducted during the Ordovician(Han et al.,2012;Wang et al.,2017).However,the specific timing and other aspects of the subduction are debated,and an investigation of granite porphyry in the Kaladaban area would improve our understanding of this subduction event.In this study,we present new U-Pb zircon dating result and Sr-Nd isotope composition data for granite porphyry from the North Altyn region.The objective is to constrain the timing of subduction of the North Altyn oceanic plate and establish the petrogenesis and magma source of the granite porphyry.

Zircon U-Pb-Hf isotopes and mineral chemistry of Early Cretaceous granodiorite in the Lunggar iron deposit in central Lhasa, Tibet Y, China

The Lunggar iron deposit belongs to the Bangong-Nujiang metallogenic belt and is located in central Lhasa on the Tibetan Plateau.In the Lunggar deposit,iron mineralization formed in the skarnization contact zone between the Early Cretaceous granodiorite and the late Permian Xiala Formation limestone.In this study,we achieved detailed zircon U-Pb-Hf isotopes and mineral chemistry for the Early Cretaceous granodiorite.Zircon U-Pb dating results indicate that the Early Cretaceous granodiorite emplaced at ca.119 Ma.Based on the trace elements in zircons and the mineral chemical composition of amphibole and biotite,the Early Cretaceous granodiorite was believed to form under condition of high temperature(>700°C),low pressure(100400 MPa),and relatively high oxygen fugacity(lgfO2)(13.6 to 13.9)and H2O content(4%8%).Zircon trace elements,Hf isotope and biotite chemistry collectively reveal that significant juvenile mantle-derived magmas contributed to the source of the granodiorite.The relatively high logfO2 and shallow magma chamber are beneficial for skarn iron mineralization,implying remarkable potential for further prospecting in the Lunggar iron deposit.

A Deep Mantle Source for the Late Neoarchean Metamorphosed Basalts in Eastern Hebei,North China Craton:Insights from Whole-Rock Geochemistry and Sm-Nd Isotopes,and Zircon U-Pb-Hf Isotopes

The basalts within the greenstone belt worldwide serve as an ideal target to decipher the nature of Archean mantle sources and further to extend the understanding of the early stages of Earth's evolution.To provide important insights into the issues,we carried out a detailed investigation of whole-rock geochemistry and Sm-Nd isotopes,and zircon U-Pb-Hf isotopes for the Late Neoarchean metamorphosed basalts in eastern Hebei,North China Craton.U-Pb isotopic dating using the LA-ICPMS on zircons reveals that the basalts in eastern Hebei erupted at ca.2.48-2.51 Ga and subsequently experienced multiple regional metamorphic events at 2477 and 1798 Ma,respectively.The metamorphosed basalts are featured by low SiO_(2),MgO,K_(2)O+Na_(2)O,and high Fe O contents,endowed with the subalkaline and high-Fe tholeiitic affinities.The radiogenic initial Nd and Hf isotope values and correlations among V,Ni and Cr contents strongly imply that the basalts experienced significant clinopyroxene and olivine fractionation and minor crustal contamination during magma evolution.They are also characterized by the relatively low total REE contents and exhibit significant depletions to moderate enrichments in the LREE contents,indicating the derivation from a deep mantle source in an Archean proto-mantle plume setting.

Zircon LA-ICP-MS U-Pb dating and Sr-Nd isotope study of the Guposhan granite complex, Guangxi, China

Zircon U-Pb dating by the LA-ICP-MS method was applied to determining the ages of different units of the Guposhan granite complex, among which the East Guposhan unit is 160.8±1.6 Ma, the West Guposhan unit is 165.0±1.9 Ma, and the Lisong unit is 163.0±1.3 Ma in age. Much similarity in ages of the three units has thus proved that the whole Guposhan granite complex was formed in the same period of time. They were the products of large-scale granitic magmatism through crust-remelting in the first stage of the Middle Yanshanian in South China. However, the three units have differences both in petrology and in geochemistry. Besides the differences in major, trace and rare-earth elements, they are distinct in their Rb-Sr and Sm-Nd isotopic compositions. The East Guposhan unit and Lisong unit and its enclaves have a similar (87Sr/86Sr)i value of 0.7064 with an average of εNd(t)=-3.03, indicating that more mantle material was evolved in the magma derivation; whereas the West Guposhan unit has a higher (87Sr/86Sr)i value of 0.7173 but a lower εNd(t) value of -5.00, and is characterized by strong negative Eu anomalies and higher Rb/Sr ratios, suggesting that its source materials were composed of relatively old crust components and new mantle-derived components. In addition, an inherited zircon grain in the East Guposhan unit (GP-1) yielded a 206Pb/238U age of 806.4 Ma, which is similar to the ages of the Jiulin cordierite granite in northern Jiangxi and of the Yinqiao migmatic granite in Guangxi in the HZH granite zone. All this may provide new evidence for Late Proterozoic magmatism in the HZH granite zone.

Detrital Zircon U-Pb Geochronology and Lu-Hf Isotopic Compositions of the Wuliangshan Metasediment Rocks in SW Yunnan(China)and Its Provenance Implications

The Wuliangshan Group occurs to the east of the Lancang giant igneous zone in SW Yunnan, and is mainly composed of low-grade metamorphosed sedimentary rocks. The group has been considered as the syn-orogenic product of the Baoshan with Simao-Indochina blocks. However, its depositional time and provenance remain to be poorly constrained. This paper presents zircon U-Pb dating and Lu-Hf-isotopic data for five representative sandstone samples from the Wuliangshan Group. The detrital grains yield a major age-peak at ~259 Ma, and four subordinary age-peaks at ~1 859, ~941, ~788, and ~447 Ma, respectively. Our results suggest that the Wuliangshan metasedimentary sequence was deposited after Middle Triassic rather than previously-thought Cambrian. The detrital zircon age spectrum, along with in-situ Lu-Hf isotopic data suggest that the Wuliangshan Group might be a syncollisional sedimentary product related to the collision of Baoshan with Simao-Indochina blocks. It is inferred that the provenance of the Wuliangshan Group is mainly from the Simao/Yangtze blocks to the east rather than the Baoshan Block or Lancang igneous zone to the west.

Petrogenesis of the concealed Daqiling intrusion in Guangxi and its tectonic significance: Constraints from geochemistry, zircon U-Pb dating and Nd-Hf isotopic compositions

The samples from the hidden Daqiling muscovite monzonite granite, which has recently been recognized within the Limu Sn-polymetallic ore field, have been analyzed for zircon U-Pb ages and whole rock geochemical and Nd-Hf isotopic compositions to discuss its genesis, source, and tectonic setting. LA-ICP-MS zircon U-Pb dating indicates that the granite crystallized in the late Indosinian（224.8±1.6 Ma）. The granite is enriched in SiO2 and K2 O and low in CaO and Na2 O. It is strongly peraluminous with the A/CNK values of 1.09–1.20 and 1.4 vol%–2.7 vol% normal corundum. Chondrite-normalized REE patterns show slightly right-dipping shape with strongly negative Eu anomalies（δEu =0.08–0.17）. All samples show enrichment of LILEs（Cs, Rb and K） and HFSEs（U, Pb, Ce and Hf）, but have relatively low contents of Ba, Sr and Ti. The zircon saturation temperatures（Tzr） are from 711 to 740°C, which are slightly lower than the average value of typical S-type granite（764°C）. The granite has negative εNd（t） and εHf（t） values, which change from ？9.1 to ？10.1 with the peak values of ？9.2 to ？9.0 and from ？3.7 to ？12.6 with the peak values of ？6 to ？5, respectively. The C DMT（Nd） and C DMT（Hf） values are 1.74–1.82 Ga with the peak values of 1.73–1.75 Ga and 1.49–2.04 Ga with the peak values of 1.5–1.6 Ga, respectively. These characteristics reveal that the source region of the granite is dominantly late Paleoproterozoic to early Mesoproterozoic crustal materials. Seven inherited magmatic zircons are dated at the age of 248.6±4.3 Ma, which suggests the existence of the early Indosinian granite in Limu area. These zircons have the εHf（t） values of ？6.7– ？2.3, similar to those of the Daqiling granite, implying the involvement of the early Indosinian granite during the formation of the Daqiling granite. Inherited zircon of 945±11 Ma has the εHf（t） and TDM（Hf） values of 8.7 and 1.14 Ga, respectively, compatible with those of the Neoproterozoic arc magmatic rocks in the eastern Jiangnan orogenic belt. Therefore we inferred that Neoproterozoic arc magma might have been involved in the formation of the Daqiling granite, and that the Neoproterozoic arc magma belt and continent-arc collision belt between the Yangtze and Cathaysia Blocks might have extended westsouthward to Limu region. It is proposed that the underplating of mantle materials triggered by crustal extension and thinning resulted in partial melting of crustal materials to form the Daqiling granite in the late Indosinian under post-collisional tectonic setting.

U-Pb Zircon Age,Geochemical and Sr-Nd-Hf Isotopic Compositions of Neoproterozoic Granitoids in Northwestern Margin of Yangtze Block (South China):Implications for Neoproterozoic Tectonic Evolution

ABSTRACT： The widespread Neoproterozoic magmatism along the Yangtze block carries critical in- formation for understanding the Neoproterozoic evolution of the Yangtze block. In the northwestern margin of the Yangtze block, the Hannan （汉南） intrusive complex includes the Wudumen （五堵门）, Erliba （二里坝） and Zushidian （祖师殿） granitoids. Using LA-ICP-MS U-Pb zircon dating method, the Wudumen and Erliba granitoids yielded magma crystallization ages of 785±4 and 778±3 Ma, respectively. Samples from these three granitoids show variable SiO2 contents ranging from 58.8% to 72.6%. They are characterized by enrichment of Al2O3（14.97%-17.87%）, Na2O（3.80%-5.33%） and Sr （504ppm-741 ppm）, and depletion of Y （〈19 ppm） and HREE （e.g., Yb〈1.6 ppm）, resulting in high Sr/Y （29-161） and （La/Yb）N （7.3-27.8） ratios. The geochemical features of the granitoids are comparable with those of adakite. The granitoids have zircon εHdt） values of ＋3.65 to ＋10.05, whole-rock εNd（t） values of -0.09 to ＋2.98 and whole-rock initial ^87Sr/^86Sr ratios of 0.7034-0.7039, indicating that their magma was derived from a juvenile crustal source. Together with geochemical and Hf-Sr-Nd isotopic compositions, it is suggested that the granitoids formed in island-arc setting and originated from partial melting of a subducted oceanic slab. The results support a model that the Yangtze block was surrounded by ocean and arc magmatism in its northern and northwestern margins in Neoproterozoic.

The late Early-Paleozoic granitic magmatism in Northwestern Fujian, China: constraints on intraplate orogeny in the South China block

The Early Paleozoic tectono-thermal event was a significant orogenic activity during the Phanerozoic era,which had a profound impact on the early crust of the South China Block(SCB) and established the foundation for later tectonic activity.The Wuyi-Yunkai orogenic belt in Southeastern China was extensively exposed to Early Paleozoic magmatism,the genetic mechanism of which remains controversial.To shed light on this issue,detailed petrological,geochemical,and zircon U-Pb-Hf isotopic studies were carried out on two granitoids,namely the Yuntongshan pluton and the Gaoqiao pluton,identified in the central Wuyishan.Zircon U-Pb chronology of the Yuntongshan and Gaoqiao bodies yielded ages of437±4 Ma(MSWD=2.2) and 404±2 Ma(MSWD=12),respectively,indicating that they were emplaced during the Early Silurian and Early Devonian periods.These granitoids are primarily composed of biotite-granite and biotite-monzonitic-granites,with high concentrations of S_(i)O_(2)(73.59-75.91 wt%),K_(2)O+Na_(2)O(8.31-8.73wt%),and low contents of MgO,CaO,Cr,Ni.They are classified as high-K calc-alkaline and weakly metaluminous-strongly peraluminous S-type granites.These granitoids are enriched in light rare earth elements(LREEs) and large ion lithophile elements(LILEs) and depleted in heavy rare earth elements(HREEs) and high field strength elements(HFSEs) with arc affinity.The εHf(t) values of-3.3 to-15.4 with two-stage Hf model ages ranging from 2829 to 1644 Ma,combined with the presence of Neoproterozoic inherited zircons,suggest that the primary magma of these granitoids was derived from the partial melting of Neoproterozoic crust with a Paleoproterozoic crustal model age.These findings,combined with the spatio-temporal distribution of regional magmatism,reveal that the late Early-Paleozoic granitoids formed in the intraplate orogenic background originating from the subduction of the proto-Tethys Ocean and proto-Pacific Ocean around the margin of the east Gondwana supercontinent.

Robust Timing Constraints for Granitic Magmatism and Hydrothermal Mineralization in the Tieshanlong W-Sn Ore Field,Eastern Nanling Range,South China

The Tieshanlong ore field is an important part of the Nanling Range,which is famous worldwide for its W-Sn mineralization.Notably,the mineralization age of the Tieshanlong ore field is not well constrained,and our field investigation reveals that granitic emplacement occurred at different stages.However,previous studies have not distinguished these multiple stages of magmatism.The Tieshanlong granite complex is closely related to the Huangsha quartz vein-type W-Sn deposit and Tongling skarn-type Cu-W-Sn deposit in this field.Through field investigations and isotopic age analyses,this work studies the relationship between multistage magmatic activity and mineralization in the Tieshanlong ore field.LA-ICP-MS zircon U-Pb isotope analyses revealed that the first-and second-staged granites formed at 154.2±0.6 Ma(MSDW=1.4)and 151.2±0.4 Ma(MSDW=1.5),with zirconε_(Hf)(t)values ranging from-13.1 to-10.5 and from-14.7 to-11.1,respectively.These data suggest that the Tieshanlong granite complex was derived from the partial melting of ancient crustal material.LA-ICP-MS U-Pb dating of wolframite and cassiterite reveals that W-Sn mineralization occurred at 160-150 Ma,which agrees well with the U-Pb dating results of the second-staged granite within analytical errors.The magmatic activity in this ore field can be divided into three stages:175-154 Ma,154-150 Ma and 150-145 Ma.The quartz vein-and skarn-type W-Sn mineralization is closely related to second-staged fine-grained twomica granite,and formed earlier than skarn-type Cu-mineralization.This study establishes a metallogenic model for the Tieshanlong ore field,and this model has important practical significance for identifying concealed W-Sn(-Cu)deposits around other granitic complexes in the Nanling Range.

广西柳东地区火山岩锆石U-Pb-Hf同位素特征及其对岩浆源区的限定

【研究目的】柳东地区位于右江盆地东缘,近年来该地区早三叠世石炮组中首次发现一套火山岩夹层,为研究盆地东缘的构造演化特征提供了重要依据。【研究方法】本文以新发现的柳东地区火山岩为研究对象,通过LA-ICP-MS锆石U-Pb年龄及Hf同位素分析,研究右江盆地东缘火山岩的形成时代及岩浆源区特征。【研究结果】结果表明,火山岩的岩性为凝灰岩-凝灰质熔岩-凝灰质火山角砾岩组合,指示了典型的火山岩特征。其2件凝灰岩样品中锆石LA-ICP-MS U-Pb加权平均年龄分别为(244.0±1.9)Ma和(247.1±0.9)Ma,锆石微量元素(U、Th、Hf、Nb)表现出大陆岩浆弧属性特征。锆石Hf同位素结果显示ε_(Hf)(t)为-13.2~-1.2,二阶段模式年龄(T_(DM)^(C))为1.35~2.06 Ga,其岩浆源区主要为地壳物质,平均地壳存留年龄为1.7~2.06 Ga。【结论】该火山岩夹层与其所出露的地层时代基本相符,属于早三叠世末期—中三叠世早期火山喷发产物,火山岩形成于火山弧-同碰撞的构造环境,代表了弧后盆地的沉积环境,与印支期古特提斯洋的闭合相关。该成果为古特提斯构造域的演化研究提供了新的基础资料,具有重要意义。

Proto-Adamastor ocean crust(920 Ma)described in Brasiliano Orogen from coetaneous zircon and tourmaline

Proto-Adamastor ocean bathed Rodinia and successor continental fragments from 1.0-0.9 Ga up to0.75 Ga,and evolved into world Adamastor Ocean at 0.75-0.60 Ga.Mesoproterozoic oceanic crust is poorly preserved on continents,only indirect evidence registered in Brasiliano Orogen.We report first evidence of ophiolite originated in proto-Adamastor.We use multi-technique U-Pb-Hf zircon andδ^11B tourmaline isotopic and elemental compositions.The host tourmalinite is enclosed in metaserpentinite,both belonging to the Bossoroca ophiolite.Zircon is 920 Ma-old,εHf(920 Ma)=+12,HfTDM=1.0 Ga and has’oceanic’composition(e.g.,U/Yb<0.1).Tourmaline is dravite withδ^11B=+1.8‰(Tur 1),0‰(Tur 2),-8.5‰(Tur 3).These characteristics are a novel contribution to Rodinia and associated world ocean,because a fragment of proto-Adamastor oceanic crust and mantle evolved at the beginning of the Brasiliano Orogen.

东秦岭官坡地区火炎沟伟晶岩型锡铌钽矿床矿物学和年代学特征

东秦岭地区是我国重要的伟晶岩分布区和锂、铍、铌钽、铀成矿区,火炎沟伟晶岩型锡铌钽矿床发育有东秦岭地区独特的锡矿化。文章通过对火炎沟矿床的矿物学、锆石和锡石U-Pb年代学及锆石Hf同位素进行研究,查明火炎沟伟晶岩型锡铌钽矿床中稀有金属赋存状态、形成时代,并探讨含矿伟晶岩与区域内古生代花岗岩之间的成因联系,以期为研究东秦岭伟晶岩型稀有金属成矿特征和成矿机制提供依据。矿物成分显示火炎沟伟晶岩中长石主要是更长石和正长石,云母为富铁黑云母和白云母,磷灰石为氟磷灰石,电气石主要是黑电石气。伟晶岩弱分带性及云母、铌钽铁矿的成分显示相对于东秦岭地区伟晶岩型锂铍矿床,火炎沟伟晶岩具有相对低的分异程度。锆石U-Pb年龄为(424±2.5) Ma,锡石U-Pb年龄为(420.1±2.4) Ma和(420.6±3.2) Ma,显示火炎沟伟晶岩型锡铌钽矿床的成岩、成矿时代是近同期的,为424~420 Ma,与区域内伟晶岩型稀有金属矿床和铀矿床形成时代一致,是东秦岭晚志留世—早泥盆世伟晶岩型稀有金属和铀成矿作用的产物。锆石ε_(Hf)(t)值为-5.8~-4.16,二阶段模式年龄为2 341~2 194 Ma,显示源区为元古宙的壳源物质,与东秦岭伟晶岩及区域晚志留世过铝质岩浆岩具有相同的源区特征。火炎沟伟晶岩与东秦岭地区古生代灰池子岩体具有较大的源区差异,显示两者可能并无直接成因联系。火炎沟伟晶岩型锡铌钽矿床的发现充实了东秦岭伟晶岩型稀有金属矿化类型,并显示东秦岭地区具有寻找伟晶岩型锡矿的巨大潜力。

伊犁地块北缘早志留世辉绿岩脉岩石成因与构造启示:年代学与地球化学制约

伊犁地块北缘发育新元古代、石炭纪等多期基性岩脉,记录了区域构造体制转换过程的重要信息。本文报道了在伊犁地块北缘温泉县乌斯图一带出露的前寒武纪变质岩系中新识别出早志留世辉绿岩脉及其围岩锆石U-Pb年龄与Hf同位素、岩石地球化学及全岩Sr-Nd同位素组成,并探讨了岩石成因及其构造意义。辉绿岩锆石U-Pb年龄为442±7Ma,形成于早志留世;围岩(淡色花岗岩)锆石U-Pb年龄为901±3Ma,形成于新元古代早期。辉绿岩样品具有较高的Fe_(2)O_(3)T(8.34%~9.81%)、全碱含量(K_(2)O+Na_(2)O=5.72%~6.86%)和较低的MgO(3.69%~4.38%)和TiO2(1.69%~2.00%)含量,属于高钾钙碱性系列岩石。岩石富集大离子亲石元素(LILEs,如Rb、Th、U、K),亏损高场强元素(HFSEs,如Nb、Ta、Ti),具有较低的Nb/Th比值(0.13~1.16)、Nb/La比值(0.42~0.45)及较高的Zr/Hf比值(39.6~42.2),锆石εHf(t)值介于+3.0~+9.1,全岩εNd(t)值介于+1.9~+3.6,暗示其来源于受俯冲流体改造的岩石圈地幔,源区岩石主要为尖晶石二辉橄榄岩和石榴石二辉橄榄岩。同时,岩石具有较低的Mg#值(49.6~52.5)和Cr(39.8×10^(-6))、Ni(17.1×10^(-6))含量,表明岩浆的演化过程中经历了镁铁质矿物的分离结晶,受地壳混染作用不明显。岩石Zr(212×10^(-6)~242×10^(-6))、Hf(5.16×10^(-6)~6.02×10^(-6))、Nb(6.69×10^(-6)~9.24×10^(-6))、Ta(0.60×10^(-6)~0.81×10^(-6))元素含量及Zr/Y(5.21~6.82)、Hf/Th(0.69~0.91)比值等指示辉绿岩脉形成于伸展环境,可能与奥陶纪末阿克陶-温泉微地块拼贴到伊犁地块后诱发的俯冲板片角度变化或构造体制转换而导致的伸展构造背景有关。

东天山晚古生代构造转折:红山南-天木东地区岩浆岩年代学与地球化学的约束

东天山是中亚造山带的重要组成部分,区内晚古生代岩浆活动与成矿作用强烈,是理解中亚造山带构造演化与成矿作用的关键地区。然而,前人对东天山构造带由俯冲向碰撞转变的时间和过程仍存在较大争议。本文对东天山红山南-天木东地区广泛出露的晚古生代岩浆岩开展了野外考察和岩相学鉴定,进行了年代学和岩石地球化学分析,以限定其形成时代、岩石成因和构造背景,进而探讨晚古生代构造演化过程。红山南-天木东地区岩浆岩主要为早石炭世火山岩和侵入岩,次为早二叠世侵入岩。早石炭世火山岩主要为安山岩(328.8±2.0Ma)和英安岩,侵入岩主要为辉长闪长岩(328.7±1.8Ma);早二叠世侵入岩为黑云母二长花岗岩(290.3±2.1Ma)和石英闪长岩(290.0±1.6Ma)。早石炭世岩浆岩富含角闪石和斜长石,为钙碱性,准铝质系列,富Rb、Ba、Th、U和Pb等大离子亲石元素,亏损Nb、Ta和Ti等高场强元素和Sr-Nd-Hf同位素,具有岛弧岩浆岩特征,是交代地幔楔部分熔融的产物。相对于早石炭世岩浆岩,早二叠世侵入岩富含黑云母和碱性长石,富集SiO_(2)、Na_(2)O和K 2O,贫Al_(2)O_(3)、MgO、Fe_(2)O T_(3)及CaO,同位素更亏损,为碰撞后背景下新生加厚地壳部分熔融的产物。总之,红山南-天木东地区的早石炭世与早二叠世岩浆岩地球化学差异显著,指示东天山构造背景从早石炭世大洋俯冲体制转变为早二叠世碰撞后造山体制,即其构造转折时间为晚石炭世-早二叠世。

The oldest basement rock in the Yangtze Craton revealed by zircon U-Pb age and Hf isotope composition

Here we report an integrated study of zircon U-Pb age and Hf isotope composition for a gneiss sample from the Kongling terrain in the Yangtze Craton. CL imaging reveals that most zircons are magmatic, and a few of them have thin metamorphic rims. The magmatic zircons gave a weighted mean U-Pb age of 3218±13 Ma, indicating the gneiss is the oldest basement rock in the Yangtze Craton found to date. They have εHf(t) value of -2.33±0.51,and two-stage Hf model age of 3679±49 Ma,indicating that the gneiss was derived from partial melting of >3.6 Ga crustal rock. The metamorphic rims yielded an age of 2732±16 Ma, implying that the metamorphic event occurred in the Neoarchean era, which may be also a major tectono-thermal event in the Yangtze Craton.

Petrogenesis of mafic enclaves from the north Taihang Yanshanian intermediate to felsic plutons: Evidence from petrological, geochemical, and zircon Hf-O isotopic data

We report the zircon Hf-O isotopic data for mafic enclaves from the Taihang Yanshanian intermediate to felsic plutons, and use them, along with the petrological, whole-rock chemical and Nd-Sr isotopic data, to reveal the petrogenesis of mafic enclaves. Mafic enclaves show magmatic textures and are finer-grained than host rocks. In places they are highly elongated due to stretching within the partially crystallized, convective felsic magma, but show no solid-state deformation. These data suggest that mafic enclaves and host rocks were co-existing, but compositionally distinct magmas. The mafic enclaves contain abundant hydrous minerals such as hornblende and biotite, with pyroxene relict being surrounded by hornblende reaction rim. Plagioclase xenocrysts from mafic enclaves show a complicated compositional and textural disequilibrium. Comparison between mafic enclaves and the immediate host rocks suggests that the two rock units are compositionally correlated. The εNd values of mafic enclaves are generally higher than the host rocks, though the Sr isotopic ratios of the two rock units are indistinguishable. Zircons from a single enclave sample show a significant variation in Hf isotopic compositions, with εHf = -10―-22, suggesting an origin through magma mixing between mafic and felsic magmas. This is supported by the relatively large variation of zircon O isotopic ratios (δ 18O = 5.5‰- 7.8‰) of the mafic enclaves. The petrogenesis of mafic enclaves could be described as below. Evolved basaltic magma (via fractionation of olivine and pyroxene) first mixed with crustally derived granitic melts in depths, forming a hybrid magma; then the hybrid magma broke up into discrete lumps upon entering the above felsic magma. Subsequently, the enclave-forming magma experienced a double mechanical transfer of plagioclase, and inward chemical transfer of fluid and Na, P, Y, Nb and Pb at the contacts with host felsic magmas.