滇西来利山花岗岩年代学、地球化学特征及其壳源部分熔融成因

与锡矿有关的花岗岩成因一直是地学界关注的焦点。来利山花岗岩位于云南省腾冲—梁河地区(花岗岩带上),构造上属于腾梁花岗岩带。该花岗岩带总体呈北北东向带状展布,构成冈底斯的南延部分。来利山岩体主要由似斑状黑云母二长花岗岩和中粗粒黑云母正长花岗岩组成。本次工作在前人研究的基础上对来利山花岗岩进行了岩石学、地球化学和年代学研究,获得二长花岗岩(14L-4)锆石LA-ICP-MS U-Pb年龄为(52.34±0.68)Ma(MSWD=1.4,n=26)。地球化学数据显示二长花岗岩相对富铝(Al2O3=14.70%~15.27%)、碱(K2O+Na2O=8.07%~8.50%)、钙(Ca O=1.64%~2.56%)、REE、Ba。铝饱和度(A/CNK)为0.97~1.12,平均为1.04,属偏铝质到弱过铝质岩石。正长花岗岩相对富硅(Si O2=74.57%~76.69%)、Rb,强烈亏损Ba、Sr、P、Ti、Eu,铝饱和度(A/CNK)为1.05~1.18,平均为1.09,属弱过铝质岩石。两类花岗岩系同源岩浆于不同阶段侵位形成,二者在矿物学、地球化学、年代学等方面表现出良好的演化关系,均显示S型花岗岩的特征。其形成与印度大陆和亚洲大陆碰撞过程中地壳物质部分熔融有关。     

Geochronology and geochemistry of the Early Cretaceous Jigongshan and Qijianfeng batholiths in the Tongbai orogen, central China: implications for lower crustal delamination

The Jigongshan and Qijianfeng batholiths in the Tongbai orogen consist mainly of porphyritic hornblende-biotite monzogranite, biotite monzogranite, and biotite syenogranite, which are variably intruded by lamprophyre, diorite, and syenogranite dykes. Mafic microgranular enclaves commonly occur in the hornblende-biotite monzogranite, whereas surmicaceous enclaves are found in the biotite monzogranite. Both batholiths have zircon U–Pb ages ranging from ca. 139 to 120 Ma, indicating their emplacement in the Early Cretaceous. The hornblende-biotite monzogranite has an adakitic affinity marked by relatively high Sr/Y and (La/Yb) _N ratios, lack of Eu anomalies, low MgO and Ni contents, and Na₂O > K₂O. Its chemical compositions, combined with enriched Sr–Nd isotopic signatures, suggest formation by dehydration melting of mafic rocks in a thickened lower crust. This thickened crust resulted from the Permo-Triassic subduction-collision between the North China and South China blocks and persisted until the Early Cretaceous. The biotite monzogranite and biotite syenogranite have low Al₂O₃, CaO, and Sr contents, low Rb/Sr, FeOt/MgO, and (Na₂O + K₂O)/CaO ratios, and flat HREE patterns with moderate to weak Eu anomalies. They were produced by partial melting of crustal materials under relatively low pressure. Partial melting at different crustal levels could have significantly contributed to mechanical weakening of the crust. The diorite and lamprophyre dykes show linear trends between SiO₂ and major or trace elements on Harker diagrams, with two lamprophyre samples containing normative nepheline and olivine. These rocks have high La/Yb and Dy/Yb ratios, both displaying co-variation with contents of Yb. They were originated from relatively deep lithospheric mantle followed by fractionation of olivine + clinopyroxene + apatite + Fe–Ti oxides. Extensive partial melting in the lithospheric mantle indicates relatively high temperatures at this level. We suggest that the presence of adakitic magmas, thickened but weakened crust and high temperatures in the lithosphere mantle point to lower crustal delamination in the Early Cretaceous in the Tongbai orogen.     

内蒙古赤峰地区早二叠世花岗岩LA-ICP-MS锆石U-Pb年龄及地球化学特征

以赤峰地区铭山复式岩体、尖山子岩体为研究对象,通过锆石U-Pb年龄、地球化学研究,确定了其形成时代,探讨了岩石成因和岩浆源区性质及其形成的构造背景。LA-ICP-MS锆石~(206)Pb/~(238)U测年结果表明,铭山复式岩体中灰白色斑状黑云二长花岗岩、尖山子岩体中浅肉红色细粒正长花岗岩分别形成于284.4±7.9Ma、294.7±8.5Ma,为早二叠世。根据地质体间接触关系可知,铭山复式岩体中的二长花岗岩岩体和尖山子岩体均为复式岩体,有待进一步解体。斑状黑云二长花岗岩SiO_2含量较高,K_2O+Na_2O含量较高,A/CNK1,属于高钾钙碱性I型花岗岩。细粒正长花岗岩Al_2O_3含量为14.32%~15.14%,Na_2O/K_2O=0.71~0.99,A/CNK=1.17~1.20,在标准矿物中出现刚玉分子。二者微量元素特征相似,富集大离子亲石元素,亏损高场强元素Nb、Ta,具Sr、P、Ti的负异常,表现出岛弧岩浆岩类特征。结合岩石地球化学、区域地质特征,认为赤峰地区早二叠世岩体形成于岛弧/活动大陆边缘构造背景,其形成与古亚洲洋向南俯冲有关。     

Petrogenesis of Mesoproterozoic K‐rich granitoids,southern Mt Angelay igneous complex,Cloncurry district,northwest Queensland

The K‐rich granitoids of the southern Mt Angelay igneous complex belong to the younger phases of the Williams and Naraku Batholiths (<1540 Ma) in the Cloncurry district. Granitoids of the complex form a series of I‐type, K‐rich, metaluminous monzodiorite to subaluminous syenogranite. These intrusions have geochemical affinities akin to ‘A‐type’ granites and contain plagioclase, alkali feldspar, quartz, biotite, hornblende and typically accessory magnetite, titanite, apatite and zircon. With increasing SiO₂ the granitoids vary from alkaline to subalkaline, and exhibit a decrease in TiO₂, Al₂O₃, Fe₂O₃*, MnO, MgO, CaO, P₂O₅, Cu, Sr, Zr, LREE and Eu, with an increase in Na₂O, K₂O, Rb, Pb, Th, U, Y and HREE. This suite of relatively oxidised granitoids (<1.0 log units above NNO) were emplaced after the peak of metamorphism and pre‐ to post‐D₃, a major east‐west horizontal‐shortening event. The synchronous emplacement of high‐temperature mafic (>960°C) and foliated felsic (>900°C) granitoids formed zones of mingled and mixed monzonite and quartz monzonite to monzogranite containing abundant rapakivi K‐feldspar. These intrusions are interpreted to have been derived from source rocks of different compositions, and probably by different degrees of partial melting. The unfoliated felsic granitoids are considered to represent the fractionated equivalents of older foliated felsic granitoids. All granitoids possess a Sr‐depleted and Y‐undepleted signature, which suggests that the source material probably contained plagioclase and no garnet, restricting magma production to <800–1000 MPa (～24–30 km). Underplating of mantle‐derived mafic material into mid‐crustal levels is considered the most viable mechanism to produce these high‐temperature K‐rich granitoids at these pressures. The composition of the felsic granitoids is consistent with derivation from a crustal source with a tonalitic to granodioritic composition. However, the mafic granitoids require a more mafic, possibly gabbroic source, which may have been supplemented with minor mantle‐derived material. These granitoids are also enriched in Th, U, LREE and depleted in Ba, Ti, Nb and Sr and compare closely to the Mesoproterozoic granitoids of the Gawler Craton. The economic significance of these styles of granitoids may also be highlighted by the close spatial relationship of hematitic K‐feldspar, magnetite, fluorite and pyrite‐rich veins, alteration and filled miarolitic cavities with the least‐evolved felsic intrusions. This style of veining has a probable magmatic origin and is similar to the gangue assemblage associated with Ernest Henry‐style Fe‐oxide‐(Cu–Au) mineralisation, which suggests that these granitoids represent prospective sources of fluids associated with Cu–Au mineralisation in the district.     

Geochemistry,geochronology and Hf isotope of granitoids in the Chinese Altai: Implications for Paleozoic tectonic evolution of the Central Asian Orogenic Belt

The Chinese Altai in northwestern Xinjiang has numerous outcrops of granitoids which provide critical information on accretionary orogenic processes and crustal growth of the Central Asian Orogenic Belt.Zircon U-Pb ages, Hf-isotopic compositions and whole-rock geochemistry of monzogranite and granodiorites in the Qinghe County are employed to elucidate Paleozoic tectonics of the Chinese Altai. Granodiorites have crystallization ages of 424.6 ± 3.1 Ma(MSWD = 0.23) and 404.0 ± 3.4 Ma(MSWD = 0.18);monzogranite was emplaced in the early Permian with a crystallization age of 293.7 ± 4.6 Ma(MSWD = 1.06). Both granodiorites and monzogranite are I-type granites with A/CNK ratios of 0.92 -0.97 and 1.03 -1.06, respectively. They also show similar geochemical features of high HREE and Y contents, low Sr contents and Sr/Y ratios, as well as enrichment of Cs, Rb, Th and U, and depletion of Nb, Ta, P and Ti.These geochemical features indicate that the monzogranite and granodiorites were formed in an arc setting related to subduction. The gneissic monzogranites display high SiO_2 and K_2 O contents, and belong to the high-K calc-alkaline series. In the chondrite normalized REE distribution pattern, the monzogranite samples exhibit enrichment of LREE with strong negative Eu anomalies(σE u =0.44 -0.53), zircon εHf(t) values from +7.24 to +12.63 and two-stage Hf model ages of 463 -740 Ma. This suggests that the monzogranite was generated from the mixing of pelitic and mantle material. The granodiorite samples are calc-alkaline granites with lower contents of Si O_2 and Na_2 O + K_2 O, higher contents of TiO_2, Fe_2O_3~t, MgO and CaO compared to the monzogranite samples. They also show enrichment of LREE and moderate negative Eu anomalies(σE u= 0.54 =0.81), as well as slightly higher differentiation of LREE than that of HREE. The425 Ma granodiorite has zircon εHf(t) values from -0.51 to +1.98 and two-stage Hf model ages of 1133 -1240 Ma, whereas the 404 Ma granodiorite displays those of +2.52 to +7.50 and 816 -1071 Ma.Geochemistry and zircon Hf isotopic compositions indicate that granodiorites were formed by partial melting of juvenile lower crust. Together with regional geology and previous data, the geochemical and geochronological data of the monzogranite and granodiorites from this study suggest long-lived subduction and accretion along the Altai Orogen during ca. 425 -294 Ma.     

柴北缘超高压地体折返过程中地壳深熔的岩石学研究

宏观、微观岩石学、地球化学和年代学研究表明,柴北缘锡铁山和绿梁山单元富含斜长石的浅色体和富含钾长石的浅色体是超高压地体折返过程中榴辉岩和片麻岩部分熔融的产物。阴极发光图像显示富含斜长石的浅色体中锆石具有明显的核-边双层结构,锆石核部无明显分带特征,并呈现出重稀土平坦和无Eu异常的稀土配分模式,～450Ma的年龄结果与区域上榴辉岩峰期变质时代一致;发光较弱的锆石边部具不明显的环带结构和较低的Th/U比值,～426Ma年龄结果代表了熔体的结晶时代。富含钾长石的浅色体中的锆石U-Pb定年结果记录的～910Ma、～450Ma和～426Ma三组年龄分别代表了片麻岩原岩结晶时代、高压-超高压变质作用时代和熔体结晶时代。富含斜长石的浅色体具有高SiO_2、Al_2O_3、CaO、Na_2O、Sr和LREE,而低MgO、FeO~T、K_2O、Y、Yb和HREE的英云闪长岩-奥长花岗岩的地球化学特征;而富含钾长石的浅色体具有高的SiO_2、Al_2O_3和K_2O+Na_2O,而较低的CaO、MgO、REE的花岗岩地球化学特征。黝帘石和少量的多硅白云母的脱水分解是触发超高压榴辉岩发生部分熔融形成富含斜长石的浅色体的主要机制;而多硅白云母的脱水分解则是触发超高压片麻岩部分熔融形成富含钾长石浅色体的主要机制。这些浅色体显著的促进了柴北缘超高压地体的快速折返,并对大陆俯冲隧道中的元素迁移和壳-幔作用具有重要的影响。     

Temporal, Spatial and Geochemical Discriminations of Granitoids in South Korea

Abstract: Five groups of the Phanerozoic granitoids in South Korea can be deduced from their temporal and spatial distributions: (1) Jurassic granitoids in the Gyeonggi massif, (2) Permo-Jurassic granitoids in the Ogcheon belt, (3) Permo-Jurassic granitoids in the Yeongnam massif, (4) Cretaceous granitoids in the Ogcheon belt, and (5) Cretaceous granitoids in the Gyeongsang basin. Though the granitoids of each group generally show calc-alkaline and orogenic natures, the petrological, geochemical and genetical features are different with each other. The Permo-Jurassic granitoids in the Ogcheon belt have lower contents of Al₂O₃, Fe₂O₃, CaO, P₂O₅, but higher of FeO, FeOT, MgO, K₂O than those in the Yeongnam massif. From higher feature of K₂O, Na₂O+K₂O and K₂O/Na₂O, the Ogcheon belt seems to have been located at closer continent side relative to the Yeongnam massif during Permo-Jurassic time. From lower values of Fe₂O₃/FeO and magnetic susceptibility the granitoids of the Ogcheon belt had been solidified under more reducing environment than those of the Yeongnam massif. The Cretaceous granitoids in the Ogcheon belt have lower contents of TiO₂, Fe₂O₃, FeO, FeOT, CaO and P₂O₅, but higher of MgO, K₂O, Na₂O+K₂O and K₂O/Na₂O than those in the Gyeongsang basin. This feature indicates that the Ogcheon belt would correspond to the continental environment of magma genesis during Cretaceous time. Higher values of Fe₂O₃/FeO and magnetic susceptibility in the Cretaceous granitoids in the Gyeongsang basin suggest that the granitoids had been solidified under highly oxidizing environment. From the particular chemical features of K₂O, Na₂O+K₂O and K₂O/Na₂O, the Permo-Jurassic granitoids in the Ogcheon belt, the Yeongnam massif as well as the Cretaceous ones in the Gyeongsang basin can be categorized to the continental margin type granite. The Jurassic granitoids in the Gyeonggi massif are possibly of collision type, and the Cretaceous granitoids in the Ogcheon belt of post–orogenic, intra–conti–nent type. The Jurassic granitoids in the Gyeonggi massif had been possibly generated by crustal melting during the collision of Gyeonggi massif to the northern Pyeongnam basin block. The Cretaceous granitoids in the Ogcheon belt had been emplaced at the hinterland of the continental margin during post-orogenic stage of the Honam Shear Zone. The Cretaceous granitoids in the Gyeongsang basin are often compared with Japanese Cretaceous?Paleogene granitoids in their geochemical and genetical natures. For the granitoid composition, the granitoids in the Gyeongsang basin are higher in Fe₂O₃, Fe₂O₃/FeO, Na₂O, K₂O, Na₂O+K₂O and K₂O/Na₂O, but lower in Al₂O₃, FeO, MnO, CaO and P₂O₅ than the Japanese granitoids. The contents of TiO₂, FeOT and MgO are similar in both granitoids. This geochemical contrast would imply that the Cretaceous granitoid magmas in the Gyeongsang basin had been originated at closer place to the continent side under more tensional field, and solidified under more oxidizing environment than the coeval Japanese granitoid magmas.     

The Geochemical and Zircon Trace Elements Characteristics of A-type Granitoids in Boziguoer,Baicheng County,Xinjiang

The Boziguoer A-type granitoids in Baicheng County,Xinjiang,belong to the northern margin of the Tarim platform as well as the neighboring EW-oriented alkaline intrusive rocks.The rocks comprise an aegirine or arfvedsonite quartz alkali feldspar syenite,an aegirine or arfvedsonite alkali feldspar granite,and a biotite alkali feldspar syenite.The major rock-forming minerals are albite,K-feldspar,quartz,arfvedsonite,aegirine,and siderophyllite.The accessory minerals are mainly zircon,pyrochlore,thorite,fluorite,monazite,bastnaesite,xenotime,and astrophyllite.The chemical composition of the alkaline granitoids show that SiO2 varies from 64.55％ to 72.29％ with a mean value of 67.32％,Na2O＋K2O is high （9.85％-11.87％） with a mean of 11.14％,K2O is 2.39％-5.47％ （mean =4.73％）,the K2O/Na2O ratios are 0.31-0.96,Al2O3 ranges from 12.58％ to 15.44％,and total FeOT is between 2.35％ and 5.65％.CaO,MgO,MnO,and TiO2 are low.The REE content is high and the total SREE is （263-1219） ppm （mean =776 ppm）,showing LREE enrichment and HREE depletion with strong negative Eu anomalies.In addition,the chondrite-normalized REE patterns of the alkaline granitoids belong to the ＂seagull＂ pattern of the right-type.The Zr content is （113-1246） ppm （mean =594 ppm）,Zr＋Nb＋Ce＋Y is between （478-2203） ppm with a mean of 1362 ppm.Furthermore,the alkaline granitoids have high HFSE （Ga,Nb,Ta,Zr,and Hf） content and low LILE （Ba,K,and Sr） content.The Nb/Ta ratio varies from 7.23 to 32.59 （mean =16.59） and the Zr/Hf ratio is 16.69-58.04 （mean =36.80）.The zircons are depleted in LREE and enriched in HREE.The chondrite-normalized REE patterns of the zircons are of the ＂seagull＂ pattern of the left-inclined type with strong negative Eu anomaly and without a Ce anomaly.The Boziguoer A-type granitoids share similar features with A1-type granites.The average temperature of the granitic magma was estimated at 832-839℃.The Boziguoer A-type granitoids show crust-mantle mixing and may have formed in an anorogenic intraplate tectonic setting under high-temperature,anhydrous,and low oxygen fugacity conditions.     

西天山艾肯达坂二叠纪钾质火山岩的地球化学特征及岩石成因

西天山艾肯达坂组中粗安岩和粗面岩富碱(Na2O+K2O8%)、高铝(Al2O313.5%)和低钛(TiO21.31%),具有典型橄榄安粗岩系火山岩的岩石化学特征。两类钾质火山岩的微量元素和同位素地球化学特征差异显著:粗安岩明显富集大离子亲石元素和轻稀土元素,具有较高的Sr/Y(29～49)比值和相对较低的(87Sr/86Sr)i(0.7044～0.7045);而粗面岩大离子亲石元素和轻稀土元素的富集程度明显减弱,甚至出现了Sr强烈亏损的现象,它具有较低的Sr/Y(1.74～2.50)比值和相对较高的(87Sr/86Sr)i(0.7060～0.7080)。研究结果显示,艾肯达坂组中粗安岩和粗面岩均是后俯冲岩浆作用的产物,但它们分别形成于俯冲演化的不同阶段:粗安岩是后俯冲伸展阶段形成的典型橄榄安粗质碱性基性火山岩,它是富集地幔低程度(熔融程度低于5%)部分熔融的产物,其熔融相以角闪石为主,在残留相中存在着大量的石榴石,因此它具有较高Sr/Y比值;而粗面岩则是后俯冲挤压阶段形成的长英质的碱性火山岩,它是年青下地壳部分熔融的产物,其形成时间早于粗安岩。     

Quantitative phase petrology of cordierite–orthoamphibole gneisses and related rocks

Cordierite–orthoamphibole gneisses and rocks of similar composition commonly contain low‐variance mineral assemblages that can provide useful information about the metamorphic evolution of a terrane. New calculated petrogenetic grids and pseudosections are presented in the FeO–MgO–Al₂O₃–SiO₂–H₂O (FMASH), Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) and Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–Fe₂O₃ (NCKFMASHTO) chemical systems to investigate quantitatively the phase relations in these rocks. Although the bulk compositions of cordierite–orthoamphibole gneisses are close to FMASH, calculations in this system do not adequately account for the observed range of mineral assemblages. Calculations in NCKFMASH and NCKFMASHTO highlight the role of minor constituents such as Ca, Na and Fe³⁺ in the mineral assemblage evolution of such rocks and these systems are more appropriate for interpreting the evolution of natural examples.     

Petrogenesis and tectonic implications of early Palaeozoic igneous rocks of the western South Qilian Belt,central China

Early Palaeozoic granitoids in the South Qilian Belt, central China, record details of the tectonic evolution and crustal growth of the Qilian orogenic belt. Five representative granitoids from the western South Qilian Belt were sampled for zircon LA-ICPMS U–Pb dating, Lu–Hf isotopes, and whole-rock geochemical analyses. Zircon U–Pb dating of two porphyritic granodiorites and a porphyritic monzogranite yielded ages of 442.7 ± 3.5, 441.8 ± 4.3, and 435.4 ± 3.5 Ma, respectively. These granitoids exhibit a geochemical affinity to I-type granite, are metaluminous with a low aluminium saturation index (A/CNK = 0.75–1.15), have moderate Al₂O₃ and low MgO contents, high La/Yb and low Sr/Y ratios, and are depleted in Nb, Ta, P, and Ti, which suggests a subduction zone magmatic arc affinity, with mixing between a primary mantle-derived magma with lesser continental crustal material. The syenogranite and monzogranite from the South Qilian Belt, which yield U–Pb zircon ages of 440.4 ± 9.0 and 442.3 ± 1.2 Ma, respectively, have pronounced S-type geochemical affinities, are peraluminous with A/CNK values of 1.07–1.16, have relatively high SiO₂, Al₂O₃, K₂O, and Rb contents, low Y and Yb, low Sr/Y and La/Yb ratios, positive Th, U, and light Rare Earth Element (REE) anomalies, and depletions in Nb, Ta, Sr, and Ti. Their geochemical signature suggests derivation from partial melting of continental crust in a syn-collisional setting. The Hf isotopic data of zircons from the granitoids show a significant input of Paleoproterozoic crust in the crustal formation of the western South Qilian Belt in Palaeozoic. Compare the ε_Hf(t) value of S-type granite with that of I-type granite, the former may have a comparatively homogeneous source. Together with regional evidence, it is proposed that a collisional event occurred between the South Qilian Belt and the Central Qilian Belt at ca. 442–435 Ma.     

阿尔金造山带南缘中―晚奥陶世正长花岗岩的发现及其地质意义

出露于阿尔金造山带帕夏拉依档沟一带的正长花岗岩,LA-ICP-MS锆石U-Pb测年结果显示其形成年龄为455.1±3.6Ma,属中―晚奥陶世。地球化学结果显示,主量元素具有富硅、富铝、富钾,低钛、贫钙、贫镁的特点,为强过铝质花岗岩系列,具高钾钙碱性特征。稀土元素总量较高,轻稀土元素富集、重稀土元素亏损,稀土元素球粒陨石标准化配分曲线有右倾型特征和明显的负Eu异常,与典型壳源花岗岩配分曲线一致。Ba、Sr、Ti等具负异常,Rb、Th、K等大离子亲石元素具正异常,显示S型花岗岩特征。结合原岩判别图解,推断其源区物质主要来源于上地壳变泥质沉积岩类。结合区域资料,认为正长花岗岩形成于挤压体制向拉张体制转换的构造环境,属后碰撞花岗岩类,表明在中―晚奥陶世阿中地块和柴达木地块已由挤压碰撞阶段转为伸展后碰撞阶段。     

滇西双江县勐库地区临沧花岗岩中暗色包体锆石U-Pb年代学及其地质意义

位于三江南段双江勐库地区的临沧花岗岩,主体岩性为黑云母二长花岗岩,其次为黑云母花岗闪长岩和碱长花岗岩。通过野外地质调查,在勐库热水塘附近黑云二长花岗岩中发现了与其紧密共生的暗色镁铁质微粒包体(MME)—闪长岩。本文对暗色闪长岩包体进行了岩相学观察、LA-ICP-MS锆石U-Pb定年和全岩主量元素分析。暗色闪长岩包体LA-ICP-MS锆石U-Pb年龄为230.9±1.2Ma,与中细粒黑云二长花岗岩LA-ICP-MS锆石U-Pb年龄(229.2±0.8Ma)基本一致。岩石地球化学特征表明,黑云二长花岗岩富K2O和Na2O,Na2OK2O,富Al2O3,铝饱和指数A/CNK平均为1.17;闪长岩K2O和Na2O含量中等,Na2OK2O,富Al2O3和Mg O,铝饱和指数A/CNK平均为0.77。二者稀土配分曲线为右倾的轻稀土富集型,二者均富集大离子亲石元素而亏损高场强元素Nb、Ta等,均具有相对较高的Mg#(黑云二长花岗岩30.50~61.41;闪长岩58.58~67.34)。中细粒黑云二长花岗岩Cr、Ni含量(平均值分别为46.96×10-6和11.21×10-6)小于闪长岩Cr、Ni含量(平均值分别为197.62×10-6和75.68×10-6)。综合研究表明,该地区花岗质岩浆的形成很可能与地幔流体作用引发的地壳部分熔融与壳幔岩浆混合作用密切相关,形成于陆陆碰撞-后碰撞的构造背景,暗示保山地块与思茅地块在230Ma已经进入了陆陆碰撞-后碰撞的地质时期。     

Arc magmatism in the Delhi Fold Belt: SHRIMP U–Pb zircon ages of granitoids and implications for Neoproterozoic convergent margin tectonics in NW India

The northwestern region of Peninsular India preserves important records of Precambrian plate tectonics and the role of Indian continent within Proterozoic supercontinents. In this study, we report precise SHRIMP zircon U–Pb ages from granitoids from the Sirohi terrane located along the western fringe of the Delhi Fold Belt in Rajasthan, NW India. The data reveal a range of Neoproterozoic ages from plagiogranite of Peshua, foliated granite of Devala, and porphyritic granite of Sai with zircon crystallization from magmas at 1015 ± 4.4 Ma, 966.5 ± 3.5 and 808 ± 3.1 respectively. The plagiogranite shows high SiO₂, Na₂O and extremely low K₂O, Rb, Ba, comparable with typical oceanic plagiogranites. These rocks possess low LREE and HREE concentrations and a relatively flat LREE–HREE slope, a well-developed negative Eu-anomaly and conspicuous Nb and Ti anomalies. Compared to the plagiogranite, the foliated Devala granite shows higher SiO₂ and moderate Na₂O, together with high K₂O and comparatively higher Rb, Ba, Sr and REE, with steep REE profiles and a weak positive Eu anomaly. In contrast to the plagiogranite and foliated granite, the porphrytic Sai granite has comparatively lower SiO₂ moderately higher Na₂O, extremely high Y, Zr, Nb and elevated REE. The geochemical features of the granitoids [HFSE depletion and LILE enrichment, Nb- and Ta-negative anomalies], and their plots in the fields of Volcanic Arc Granites and those from active continental margins in tectonic discrimination diagrams suggest widespread Neoproterozoic arc magmatism with changing magma chemistry in a protracted subduction realm. Our results offer important insights into a long-lived active continental margin in NW India during early and mid Neoproterozoic, consistent with recent similar observations on Cryogenian magmatic arcs widely distributed along the margins of the East African Orogen, and challenge some of the alternate models which link the magmatism to extensional tectonics associated with Rodinia supercontinent breakup.     

阿尔金造山带青白口纪亚干布阳片麻岩年龄、地球化学特征及其地质意义

出露于阿中地块库木塔什萨依一带的亚干布阳片麻岩主要岩性为黑云斜长片麻岩、黑云二长片麻岩。利用LA-ICPMS方法进行锆石微区U-Pb同位素定年,得到206Pb/238U年龄加权平均值为900.2±2.9Ma,表明亚干布阳片麻岩原岩形成于新元古代早期青白口纪;地球化学结果显示,主量元素具有高SiO_2、Al_2O_3、K_2O+Na_2O含量,低Na_2O、MgO、CaO和TiO_2含量的特征,A/CNK值介于0.95～1.22之间,属于高钾钙碱性系列的过铝质花岗岩。岩石富集Rb、Th、K等大离子亲石元素,亏损Nb、Sr、P、Hf、Ti等高场强元素;岩石轻稀土元素分馏较强而重稀土元素分馏较弱,具有明显的负Eu异常,总体呈右倾的"V"字形稀土元素配分模式,显示典型的地壳重熔型花岗岩特征。亚干布阳片麻岩的源岩主要为地壳中沉积岩类的部分熔融,形成于俯冲-同碰撞构造环境。综上说明亚干布阳片麻岩是新元古代早期俯冲碰撞热事件的产物,反映阿中地块和柴达木地块青白口纪处于汇聚碰撞阶段,构造岩浆活动强烈,与Rodinia超大陆汇聚事件具有一致性。     

青海南山地区加里东期强过铝质花岗岩锆石U-Pb年龄、地球化学特征及其地质意义

青海南山地区位于南祁连构造带和西秦岭造山带的交接部位,在该地区元古宇变质地层中新厘定出一套含石榴子石白云母二长花岗岩,并对其进行了详细的岩石学、岩石地球化学和LA-ICP-MS锆石U-Pb定年研究。结果表明,浪日娘含石榴子石白云母二长花岗岩结晶年龄为438.7±4.2Ma,形成于早志留世早期。岩石含石榴子石、白云母、电气石等高铝矿物,同时具高SiO_2、富Al_2O_3特征,高铝饱和指数A/CNK=1.09～1.28,属高钾钙碱性强过铝质S型花岗岩;微量元素富集大离子亲石元素Cs、Rb、U、K和Pb,亏损高场强元素Nb、Ti、Zr、P和Ba、Sr;稀土元素总量低,配分曲线为轻稀土元素富集的右倾模式,具有弱-中等负Eu异常。高Rb/Sr值(1.83～3.95)、低CaO/Na_2O值(0.11～0.19),伴随有Pb正异常和Ba负异常,暗示源区物质成分为泥质岩并经历了缺水熔融条件下的白云母脱水熔融。结合岩体年龄及区域地质资料,推断其可能形成于原特提斯洋闭合碰撞造山过程。     

Petrogenesis and tectonic implications of Indosinian granitoids from Western Qinling Orogen,China:Products of magma-mixing and fractionation

The Western Qinling Orogen(WQO) is characterized by voluminous distribution of Indosinian granitoids,the formation of which provides an important window to unravel the geochemical and geodynamic evolution and associated metallogeny.Here we investigate a group of intrusions termed "Five Golden Flowers" based on petrological,geochemical,zircon U-Pb geochronological and Lu-Hf isotopic studies on the granitoids and their mafic microgranular enclaves(MMEs).Our results show that these intrusions are genetically divided into two types,namely,magma-mixing and highly fractionated.The Jiaochangba,Lujing,Zhongchuan,and Luchuba granitoids are biotite monzogranites(220±0.8 Ma to 217±2.6 Ma) with abundant coeval MMEs(220±.1 Ma to 217±2.7 Ma).The rocks contain moderate to high SiO_2,high MgO,Rb,Sr,Ba,and Th contents,but low TiO_2,P_2 O_5,and Sc values,A/CNK of 1.1,and a range of ε_(Hf)(t) values of-11.7 to +2.23 with corresponding T_(DM2)values of 1967-1228 Ma.The MMEs possess K-feldspar megacrysts,abundant acicular apatites,and show lopsided textures.They have lower SiO_2,Al_2 O_3,and Th contents,but higher MgO,TiO_2,and Sc,with ε_(Hf)(t) values of-18.0 to +3.18 and T_(DM1) of 849-720 Ma.The data indicate that the MMEs were derived from a magma sourced from the enriched lithospheric mantle.We suggest that these host granitoids were produced by partial melting of latePaleoproterozoic to early-Mesoproterozoic lower crust with the involvement of Neoproterozoic SCLM-derived mafic magmas.The Baijiazhuang pluton is dominantly composed of leucogranite(muscovite granite and twomica monzogranite,216±1.5 Ma) without MMEs.The rocks are peraluminous with high A/CNK(1.06-1.27).Compared with the other four granitoids,the Baijiazhuang leucogranite shows higher SiO_2 content,markedly lower concentrations of TiO_2,MgO,Al_2 O_3,CaO,and Fe_2 O_3~T,and lower LREE/HREE and(La/Yb)N values.These leucogranites are also rich in Rb,Th,and U,and display marked depletions in Ba,Sr,Ti,and Eu,indicating that they experienced significant fractionation.Zircon ε_(Hf)(t) values(-10.2 to-3.27) and T_(DM2)(1868-1424 Ma),as well as the Nb/Ta and K_2 O/Na_2 O values are similar to the other four granitoids,indicating that they are likely to have been derived from a similar source;with sediments playing only a minor role in the magma generation.The low contents of Yb and Y suggest that their partial melting was controlled by garnets and micrographic texture of K-feldspar reflects high-temperature melting through undercooling.Based on the above features,we infer that the Baijiazhuang leucogranite likely represents the product of high degree fractionation of the I-type biotite monzogranite magma which generated the other four granitoids at relatively high temperatures,within magma chambers at mid-crust depths.We propose that the granitoid suite was formed in the transitional setting from synto post-collision during the collisional orogeny between the SCB and NCB,following break-off of the subducted South China Block lithosphere during 220-216 Ma.     

Geochemical signature of the granitoids in the Chalukou giant porphyry Mo deposit in the Heilongjiang Province,NE China

The Chalukou porphyry Mo deposit, located in the northern Great Xing'an Range, is the largest Mo deposit in the Xing'an–Mongolia orogenic belt. Its ore bodies are mainly hosted in the intermediate-felsic complex and Jurassic volcanic sedimentary rocks, of which Late Jurassic granite porphyry, quartz porphyry and fine-grained granite are closely associated with Mo mineralization. The Middle Jurassic monzogranite belongs to shoshonite series, with SiO₂ and Al₂O₃ contents of 69.48 to 74.98% and 12.35 to 14.48%, respectively. The total alkali (K₂O + Na₂O) content ranges from 7.67 to 10.42%, with K₂O/Na₂O ratios between 1.07 and 2.81. These rocks are strongly enriched in Rb and K but are depleted in Ta, Nb, P and Ti, with negative Eu anomalies and positive ε_Hf(t). The Late Jurassic granite porphyry, quartz porphyry and fine-grained granite are shoshonite or high-K calc-alkaline series. Their SiO₂ and Al₂O₃ contents range from 73.87 to 78.95% and 10.35 to 13.47%, respectively. The total alkali (K₂O + Na₂O) contents range from 8.06 to 10.02%, with K₂O/Na₂O ratios from 1.03 to 8.20. These rocks are strongly enriched in Rb, K and Th, but are depleted in P, Ti, Ba and Sr, indicating clear negative Eu anomalies and positive ε_Hf(t). The Middle–Late Jurassic granitoids in the Chalukou deposit are highly fractionated I-type granitoids, and derived from juvenile lower crust materials that originated from the Neoproterozoic depleted mantle. These granitoids formed in the extension tectonic setting of the post-collision stage of the Mongol–Okhotsk orogenic belt, probably superposed by the back-arc extension related to the subduction of the Paleo-Pacific Plate.     

Petrology and geochemistry of metapelites and basic granulites from Sonapahar region of Shillong Meghalaya gneissic complex, North East India

The geochemistry of the metapelites and basic granulite of Sonapahar has been studied in order to assess the nature of the protolith and their likely tectonic environment. Metapelites contains SiO₂ (av. 62.16wt %), K₂O+Na₂O (av. 4.57 wt%), Al₂O₃ (av. 16.69 wt%), MgO (av. 5.31 wt%), Mg# av. 0.60 and low TiO₂ (av. 0.68 wt%) with low Y (av. 23.48 ppm) and Yb (av. 1.6 ppm) which point to their derivation from andesite source. Metapelites show prominent negative europium anomaly (Eu_N/Eu* = 0.38–0.54) and fractionated LREE to HREE pattern (La_N/Lu_N) 17.23 to 56.69. Basic granulites demonstrate low content of SiO₂ (av50.95 wt%), Al₂O₃ (av13.67 wt%), K₂O (av 0.58 wt%) and enriched in Fe₂O₃ ^t (av 11.42 wt%), MgO (av 7.29 wt%), CaO (av 10.60 wt %), Na₂O (av 2.26 wt%), Mg# av. 0.56 and range from calc-alkaline to tholeiitic in nature. The geochemistry of the metapelites and basic granulites advocate that metapelites are derived from arc related andesitic source due to subduction whereas protoliths of basic granulites was originated in rift related volcanism through diapiric movement of hot rising mantle derived basaltic magmas.