Tracing the 850-Ma continental flood basalts from a piece of subducted continental crust in the North Qaidam UHPM belt,NW China

A Paleozoic ultrahigh-pressure metamorphic (UHPM) belt extends along the northern margin of the Qaidam Basin, North Tibetan Plateau. Eclogites in the Yuka eclogite terrane, northwest part of this UHPM belt, occur as blocks or layers of varying size intercalated with granitic and pelitic gneisses. These eclogites have protoliths geochemically similar to enriched-type mid-ocean ridge basalts (E-MORB) and oceanic island basalts (OIB). On the basis of Ti/Y ratios, they can be divided into low-Ti and high-Ti groups. The low-Ti group (LTG) eclogites exhibit relatively low TiO₂ (most <2.5 wt%) and Ti/Y (<500) but comparatively high Mg# (48–55), whereas the high-Ti group (HTG) eclogites have high TiO₂ (most >2.5 wt%) and Ti/Y (>500) but lower Mg# (46–52). Zircons from two eclogite samples gave a magmatic crystallization (protolith) age of ∼850 Ma and a UHPM age of ∼433 Ma. The occurrence, geochemical features and age data of the Yuka eclogites suggest that their protoliths are segments of continental flood basalts (CFBs) with a mantle plume origin, similar to most typical CFBs. Our observation, together with the tectonic history and regional geologic context, lend support for the large scale onset of mantle plume within the Rodinia supercontinent at ∼850 Ma. The Qaidam block is probably one of the fragments of the Rodinia supercontinent with a volcanic-rifted passive margin. The latter may have been dragged to mantle depths by its subducting leading edge of the oceanic lithosphere in the Early Paleozoic.     

碧玺充填探讨

采用宝石显微镜、红外光谱仪和荧光能谱仪对广州市荔湾广场的105件碧玺样品进行了测试分析。结果表明,绝大多数碧玺样品都存在充填处理的现象,其内部充填物为黄色与白色絮状物,并具有蓝色闪光、流动构造等特征;其中,充填现象不明显的碧玺样品在红外吸收光谱仪和荧光能谱仪下也可检测到有机物及Pb。     

Geochemistry, Geochronology and Isotopic Evolution of the Chewore-Rufunsa Terrane, Southern Irumide Belt: a Mesoproterozoic Continental Margin Arc

The southern Irumide Belt (SIB) is an ENE–WSW-trending,late Mesoproterozoic orogenic belt located between the Congo–Tanzania–Bangweulu(CTB) and Kalahari cratons in central southern Africa. It isseparated from the late Mesoproterozoic Irumide Belt (IB) tothe north by Permo-Triassic graben, raising the possibilitythat the younger rifts reactivated a suture between the twobelts that has been rendered cryptic as a result of youngerKaroo cover. Both belts are dominated by calc-alkaline gneisses,but in addition the SIB contains abundant metavolcanic and metasedimentaryrocks. In this study we present detailed geochemical, isotopicand geochronological data for volcanic and plutonic lithologiesfrom the southernmost part of the SIB, the Chewore–RufunsaTerrane. This terrane comprises a wide variety of supracrustalto mid-crustal rocks that have major- and trace-element compositionssimilar to magmas formed in present-day subduction zones. Chondrite-normalizedrare earth element (REE) profiles and whole-rock Sm–Ndisotope compositions indicate that the parental supra-subductionmelts interacted with, and were contaminated by sialic continentalcrust, implying a continental-margin-arc setting. Secondaryionization mass spectrometry dating of magmatic zircon has yieldedcrystallization ages between c. 1095 and 1040 Ma, similar toelsewhere in the SIB. U–Pb dating and in situ Lu–Hfisotopic analyses of abundant xenocrystic zircon extracted fromthe late Mesoproterozoic granitoids indicate that the contaminantcontinental basement was principally Palaeoproterozoic in ageand had a juvenile isotopic signature at the time of its formation.These data are in contrast to those for the IB, which is characterizedby younger, c. 1020 Ma, calc-alkaline gneisses that formed bythe direct recycling of Archaean crust without significant additionof any juvenile material. We suggest that the SIB developedby the subduction of oceanic crust under the margin of an unnamedcontinental mass until ocean closure at c. 1040 Ma. Subsequentcollision between the SIB and the CTB margin led to the cessationof magmatism in the SIB and the initiation of compression andcrustal melting in the IB. KEY WORDS: geochemistry; Mesoproterozoic; SHRIMP zircon U–Pb dating; Sm–Nd isotopes; Southern Irumide Belt     

U–Pb and Re–Os geochronology and geodynamic setting of the Dabaoshan polymetallic deposit, northern Guangdong Province, South China

The Dabaoshan polymetallic deposit in northern Guangdong Province contains iron, copper, lead, zinc, molybdenum, tungsten and sulfur mineral resources. Porphyry-type Mo(W) and skarn-type Mo-W mineralization occurs along the internal and external contact zones of the granodioritic porphyry, respectively. LA-ICP-MS U–Pb dating of zircons from two granodioritic porphyry samples yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 175.8 ± 1.5 Ma (MSWD = 0.037) and 175.0 ± 1.7 Ma (MSWD = 0.41). They can be pooled together to yield a combined weighted age of 175.4 ± 1.6 Ma (MSWD = 0.26), which is interpreted as the emplacement age of the granodioritic porphyry. Re–Os dating of three molybdenite samples from porphyry and skarn ores yielded consistent model ages of 163.2 ± 2.3 Ma to 165.2 ± 2.4 Ma, with a weighted mean of 163.9 ± 1.3 Ma (MSWD = 0.81), which is the age of Mo–W mineralization. These ages are consistent with the molybdenite Re–Os model age (164.7 ± 3 Ma) measured by Mao et al. (2004a) for the stratiform Cu–Pb–Zn orebody, and they can yield a weighted mean of 164.0 ± 2.5 Ma (MSWD = 0.16). This implies that Mo–W and Cu–Pb–Zn mineralization in the Dabaoshan polymetallic deposit are the products of one mineralization event. The mineralization in the deposit coincides closely with that of Mo-polymetallic mineralization (164–149 Ma) elsewhere in the Nanling region, comprising an important polymetallic metallogenic belt of south China, and corresponds to the second episode of Mesozoic metallogenesis in South China. Combined with previous studies, we suggest that the Dabaoshan polymetallic deposit is related to post-collisional lithosphere extension in the Nanling region of South China. Geological data and Pb isotopic evolution diagrams, together with stable isotopic data of fluid inclusions (δ¹⁸O = − 3.75–7.0‰, δD = − 50.7 to − 56.1‰) and ore sulfides (δ³⁴S = − 2–3‰), suggest a genetic relationship between the Dabaoshan polymetallic deposit, the granodioritic porphyry and the dacitic porphyry. These data, combined with the Re content (64.7 to 102.4 ppm) of molybdenite, indicate that the ore-forming components were derived from mixed crustal and mantle sources.     

Geochemical and Geochronological Constraints on the Origin of the Meta-basic Volcanic Rocks in the Tengtiaohe Zone,Southeast Yunnan

The meta-basic volcanic rocks in the Tengtiaohe Zone yield zircon U–Pb ages of 258.8±2.5 Ma and 259.2±1.8 Ma, respectively which agree with the ages of flood basalts of ELIP and are similar to the basaltic rocks and komatiites from the Song Da Zone in northern Vietnam. The results suggest that the age of meta-basic volcanic rocks is Late Permian, rather than the Early Permian or Early Carboniferous ages as previously inferred. Most meta-basic volcanic rocks are strongly enriched in LREEs relative to HREEs and display trace element patterns similar to the ELIP high-Ti basalts, and are enriched in LILEs with negative Sr anomalies. Their initial ~(87)Sr/~(86) Sr ratios range from 0.705974 to 0.706188 and εNd(t) from-0.82 to-2.11. Their magmas were derived from an enriched and deep mantle source without significant crustal contamination. These meta-basic volcanic rocks formed in ELIP. Therefore, the Tengtiaohe Zone is not an ophiolite zone and can link to the Song Da Zone in northern Vietnam.     

中天山卡瓦布拉克杂岩带中闪长岩的锆石U-Pb年龄及Hf同位素特征

卡瓦布拉克杂岩带出露于中天山地块东段卡瓦布拉克—阿克塔格地区,沿卡瓦布拉克断裂呈东西向展布。笔者选择构成该杂岩带的中—基性岩石主体闪长岩,开展了LA-ICP-MS锆石U-Pb年代学和LA-MC-ICP-MS锆石Hf同位素研究。结果表明:闪长岩中锆石呈自形—半自形,发育典型的岩浆锆石振荡生长环带,Th/U值较高(均大于0.40),且Th、U含量呈现较好的正相关关系,为典型的岩浆成因锆石;这些锆石的206Pb/238 U年龄加权平均值为(375±1)Ma,MSWD=0.081,属晚泥盆世,可代表其结晶年龄;锆石具有较均一的Hf同位素组成,初始比值为0.282 655~0.282 747、εHf(t)值为4.0~7.2,其对应的亏损地幔模式年龄为714~842Ma。结合区域地质资料认为,卡瓦布拉克杂岩带中的闪长岩由亏损岩石圈地幔发生部分熔融而形成。     

Two Periods of Skarn Mineralization in the Baizhangyan W–Mo Deposit,Southern Anhui Province,Southeast China: Evidence from Zircon U–Pb and Molybdenite Re–Os and Sulfur Isotope Data

The recently discovered Baizhangyan skarn‐porphyry type W–Mo deposit in southern Anhui Province in SE China occurs near the Middle–Lower Yangtze Valley polymetallic metallogenic belt. The deposit is closely temporally‐spatially associated with the Mesozoic Qingyang granitic complex composed of g ranodiorite, monzonitic g ranite, and alkaline g ranite. Orebodies of the deposit occur as horizons, veins, and lenses within the limestones of Sinian Lantian Formation contacting with buried fine‐grained granite, and diorite dykes. There are two types of W mineralization: major skarn W–Mo mineralization and minor granite‐hosted disseminated Mo mineralization. Among skarn mineralization, mineral assemblages and cross‐cutting relationships within both skarn ores and intrusions reveal two distinct periods of mineralization, i.e. the first W–Au period related to the intrusion of diorite dykes, and the subsequent W–Mo period related to the intrusion of the fine‐grained granite. In this paper, we report new zircon U–Pb and molybdenite Re–Os ages with the aim of constraining the relationships among the monzonitic granite, fine‐grained granite, diorite dykes, and W mineralization. Zircons of the monzonitic granite, the fine‐grained granite, and diorite dykes yield weighted mean U–Pb ages of 129.0 ± 1.2 Ma, 135.34 ± 0.92 Ma and 145.3 ± 1.7 Ma, respectively. Ten molybdenite Re–Os age determinations yield an isochron age of 136.9 ± 4.5 Ma and a weighted mean age of 135.0 ± 1.2 Ma. The molybdenites have δ³⁴S values of 3.6‰–6.6‰ and their Re contents ranging from 7.23 ppm to 15.23 ppm. A second group of two molybdenite samples yield ages of 143.8 ± 2.1 and 146.3 ± 2.0 Ma, containing Re concentrations of 50.5–50.9 ppm, and with δ³⁴S values of 1.6‰–4.8‰. The molybdenites from these two distinct groups of samples contain moderate concentrations of Re (7.23–50.48 ppm), suggesting that metals within the deposit have a mixed crust–mantle provenance. Field observation and new age and isotope data obtained in this study indicate that the first diorite dyke‐related skarn W–Au mineralization took place in the Early Cretaceous peaking at 143.0–146.3 Ma, and was associated with a mixed crust–mantle system. The second fine‐grained granite‐related skarn W–Mo mineralization took place a little later at 135.0–136.9 Ma, and was crust‐dominated. The fine‐grained granite was not formed by fractionation of the Qingyang monzonitic granite. This finding suggests that the first period of skarn W–Au mineralization in the Baizhangyan deposit resulted from interaction between basaltic magmas derived from the upper lithospheric mantle and crustal material at 143.0–146.3 and the subsequent period of W–Mo mineralization derived from the crust at 135.0–136.9 Ma.     

湘东南新元古界大江边组和埃岐岭组的形成时代和物源—来自碎屑锆石U-Pb年代学的证据

运用LA-ICP-MS方法对湘东南泗洲山地区南华系大江边组上部泥质岩和震旦系(埃迪卡拉系)埃岐岭组底部变泥铁质砂岩碎屑锆石进行U-Pb测年,分别获得了28组和92组U-Pb有效年龄.前者锆石年龄主要集中分布于693 ～ 897 Ma(占67.9％),1810 ～1916Ma(占17.9％),1974～2177Ma(占14.2％)三个区间,最年轻的谐和错石年龄为734 ±4Ma(MSWD=1.07,n=4);后者锆石年龄主要集中分布于570～699Ma(占9.8％),715～ 774Ma(占7.6％),882～1272Ma(占67.4％),1300～ 1681Ma(占7.6％),1899 ～2490Ma(占7.6％)五个区间,最年轻的谐和锆石年龄为634±7Ma(MSWD=1.09,n=4).研究认为,大江边组上部地层的形成时代＜734±4Ma,与澄江组顶部、牛牯坪组、岩门寨组、拱洞组及隆里组等地层形成时代相近,揭示“板溪期”地层的顶界年龄及江口冰期开启年龄＜ 734±4Ma;埃岐岭组形成时代＜634 ±7Ma,与陡山沱组及蓝田组等地层形成时代相近,支持南沱冰期结束于635Ma左右的认识.综合研究认为,大江边组和埃岐岭组的物源可能分别主要来源于武夷地块南东部和南岭—云开地块南部的一个Grenville期造山带.推测物源的转换可能与南岭—云开地块、武夷地块在两组沉积时期之间聚合或紧邻有关,其时限约为734～634Ma.     

滇中禄丰地区中元古代早期球颗玄武岩的锆石U-Pb年龄、地球化学特征及其大地构造意义

在1∶5万区域地质调查的基础上,在禄丰地区东川群黑山组新发现了球颗玄武岩及凝灰岩夹层。采用LA-ICP-MS技术测得球颗玄武岩和凝灰质板岩中锆石U-Pb加权平均年龄分别为1678±10 Ma和1572±19 Ma,表明黑山组沉积时代为中元古代早期;其中球颗玄武岩SiO_2含量介于40.83%～52.56%,δEu正异常,大离子亲石元素Rb、Ba和高场强元素Nb、Ta、Th等强不相容元素相对富集,全岩地球化学显示为大陆板内低钛拉斑海相玄武岩,形成于伸展的构造环境,可能为Columbia超大陆裂解的产物。本次的发现,为准确地标定东川群时代、层序及大地构造属性提供新的参考依据,也为重塑整个滇中前寒武纪地层格架提供时代约束。     

Geological and C–O–S–Pb–Sr isotopic constraints on the origin of the Qingshan carbonate-hosted Pb–Zn deposit,Southwest China

Located in the western Yangtze Block, the Qingshan Pb–Zn deposit, part of the Sichuan–Yunnan–Guizhou Pb–Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining–Shuicheng anticline and its intraformational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.

The C–O isotope compositions of hydrothermal calcite and S–Pb–Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb–Zn deposit. δ¹³C_PDB and δ¹⁸O_SMOW values of calcite range from –5.0‰ to –3.4‰ and +18.9‰ to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO₂ in the hydrothermal fluid had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ³⁴S values of sulphide minerals range from +10.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20‰ to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (²⁰⁶Pb/²⁰⁴Pb = 18.561 to 18.768, ²⁰⁷Pb/²⁰⁴Pb = 15.701 to 15.920, and ²⁰⁸Pb/²⁰⁴Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. ⁸⁷Sr/⁸⁶Sr ratios of sphalerite range from 0.7107 to 0.7136 and (⁸⁷Sr/⁸⁶Sr)_200Ma ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C–O–S–Pb–Sr isotope compositions of the Qingshan Pb–Zn deposit indicate a mixed origin of the ore-forming fluids and metals.