Lower crustal melting and the role of open-system processes in the genesis of syn-orogenic quartz diorite–granite–leucogranite associations: constraints from Sr–Nd–O isotopes from the Bandombaai Complex, Namibia     

Tina van de Flierdt  Stephan Hoernes  Stefan Jung  Peter Masberg  Edgar Hoffer  Urs Schaltegger  Hans Friedrichsen 《Lithos》2003,67(3-4):205-226

The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma.

Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (_{Nd (540 Ma)}=−6.3 to −19.8; δ¹⁸O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (⁸⁷Sr/⁸⁶Sr_initial=0.70794–0.70982). Two leucogranites and one aplite have higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.70828–0.71559), but similar initial _Nd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K₂O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (<0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material.  相似文献   

Carboniferous Post-collisional Rift Volcanism of the Tianshan Mountains, Northwestern China   总被引：22，自引：0，他引：22  

Xia Linqi  Xu Xueyi  Xia Zuchun  LI XiangMin  MA Zhongping  WANG Lishe 《《地质学报》英文版》2003,77(3):338-360

The Tianshan Carboniferous post-collisional rift volcanic rocks occur in northwestern China as a large igneous province. Based on petrogeochemical data, the Tianshan Carboniferous post-collisional rift basic lavas can be classified into two major magma types: (1) the low-Ti/Y type situated in the eastern-central Tianshan area, which exhibits low Ti/Y (<500), Ce/Yb (<15) and SiO2 (43-55%), and relatively high Fe2O3T (6.4-11.5%); (2) the high-Ti/Y type situated in the western Tianshan area, which has high Ti/Y (>500), Ce/Yb (>11) and SiO2 (49-55%), and relatively low Fe2O3T (5.8-7.8%). Elemental data suggest that chemical variations of the low-Ti/Y and high-Ti/Y lavas cannot be explained by fractional crystallization from a common parental magma. The Tianshan Carboniferous basic lavas originated most likely from an OIB-like asthenospheric mantle source (87Sr/86Sr(t) ≈ 0.703-0.705, εNd(t) ≈ +4 to +7). The crustal contamination and continental lithospheric mantle have also contributed significantly to the formation of the basic lavas of the Tianshan Carboniferous post-collisional rift. The silicic lavas were probably generated by partial melting of the crust. The data of this study show that spatial petrogeochemical variations exist in the Carboniferous post-collisional rift volcanics province in the Tianshan region. Occurrence of the thickest volcanics dominated by tholeiitic lavas may imply that the center of the mantle-melting anomaly (mantle plume) was in the eastern Tianshan area at that time. The basic volcanic magmas in the eastern Tianshan area were generated by a relatively high degree of partial melting of the mantle source around the spinel-garnet transition zone, whereas the alkaline basaltic lavas are of the dominant magma type in the western Tianshan area, which were generated by a low degree of partial melting of the mantle source within the stable garnet region, thus the basic lavas of the western Tianshan area might have resulted from relatively thick lithosphere and low geothermal gradient.  相似文献   

Provenance analysis and tectonic setting of the Ordovician clastic deposits in the southern Puna Basin, NW Argentina   总被引：9，自引：0，他引：9  

Udo Zimmermann  Heinrich Bahlburg† 《Sedimentology》2003,50(6):1079-1104

Provenance studies on Early to Middle Ordovician clastic formations of the southern Puna basin in north-western Argentina indicate that the sedimentary detritus is generally composed of reworked crustal material. Tremadoc quartz-rich turbidites (Tolar Chico Formation, mean composition Q_t89 F7 L4) are followed by volcaniclastic rocks and greywackes (Tolillar Formation, mean Q_t33 F42 L25). These are in turn overlain by volcaniclastic deposits (mean Q_t24 F30 L46) of the Diablo Formation (late Arenig–early Llanvirn) that are intercalated by lava flows. All units were deformed in the Oclóyic Orogeny during the Middle and Late Ordovician. Sandstones of the Tolar Chico Formation are characterized by Th/Sc ratios > 1, La/Sc ratios ≈ 10, whereas associated fine-grained wackes show slightly lower values for both ratios. LREE (light rare earth elements) enrichment of the arenites is ≈ 50× chondrite, Eu/Eu* values are between 0·72 and 0·92, and flat HREE (heavy rare earth elements) patterns indicate a derivation from mostly felsic rocks of typical upper crustal composition. The ε_{Nd(t = sed)} values scatter around −11 to −9. The calculated Nd-T_DM residence ages vary between 1·8 and 2·0 Ga indicating contribution by a Palaeoproterozoic crustal component. The Th/Sc and La/Sc ratios of the Tolillar Formation are lower than those of the Tolar Chico Formation. Normalized REE (rare earth elements) patterns display a similar shape to PAAS (post-Archaean average Australian shale) but with higher abundances of HREEs. Eu/Eu* values range between 0·44 and 1·17, where the higher values reflect the abundance of plagioclase and feldspar-bearing volcanic lithoclasts. Average ε_{Nd(t = sed)} values are less negative at −5·1, and Nd-T_DM are lower at 1·6 Ga. This is consistent with characteristics of regional rocks of upper continental crust composition, which most probably represent the sources of the studied detritus. The rocks of the Diablo Formation have the lowest Th/Sc and La/Sc ratios, lower LREE abundances than the average continental crust and are slightly enriched in HREEs. Eu/Eu* values are between 0·63 and 1·17. The Nd isotopes (ε_{Nd(t = sed)} = −3 to −1; T_DM = 1·2 Ga) indicate that one source component was less fractionated than both the underlying Early Ordovician and the overlying Middle Ordovician units. Synsedimentary vulcanites in the Diablo Formation show the same isotopic composition. Our data indicate that the sedimentary detritus is generally composed of reworked crustal material, but that the Diablo Formation appears to contain ≈ 80% of a less fractionated component, derived from a contemporaneous continental volcanic arc. There are no data indicating an exotic detrital source or the accretion of an exotic block at this part of the Gondwana margin during the Ordovician.  相似文献   

A Reappraisal of Rb, Y, Zr, Pb and Th Values in Geochemical Reference Material BHVO-1     

William J. Chazey III  Clive R. Neal  Jinesh C. Jain  William S. Kinman 《Geostandards and Geoanalytical Research》2003,27(2):181-192

The geochemical reference material BHVO-1 was analysed by a variety of techniques over a six year period. These techniques included inductively coupled plasma-mass spectrometry and atomic emission spectroscopy (ICP-MS and ICP-AES, respectively), laser ablation ICP-MS and spark source mass spectroscopy. Inconsistencies between the published consensus values reported by Gladney and Roelandts (1988, Geostandards Newsletter) and the results of our study are noted for Rb, Y, Zr, Pb and Th. The values reported here for Rb, Y, Zr and Pb are generally lower, while Th is higher than the consensus value. This is not an analytical artefact unique to the University of Notre Dame ICP-MS facility, as most of the BHVO-1 analyses reported over the last ten to twenty years are in agreement with our results. We propose new consensus values for each of these elements as follows: Rb = 9.3 ± 0.2 μg g-1 (compared to 11 ± 2 μg g-1), Y = 24.4 ± 1.3 μg g-1 (compared to 27.6 ± 1.7 μg g-1), Zr = 172 ± 10 μg g-1 (compared to 179 ± 21 μg g-1), Pb = 2.2 ± 0.2 μg g-1 (compared to 2.6 ± 0.9 μg g-1) and Th = 1.22 ± 0.02 μg g-1 (compared to 1.08 ± 0.15 μg g-1).  相似文献   

壳幔混染作用——下扬子区早中生代玄武岩同位素地质特征     

方中  陶仙聪  李惠民  张根娣 《地球学报》1997,18(Z1):12-14

下扬子区中生代（231Ma）玄武质火山岩全岩εNd（t）值-8.62～1.92,87Sr/86Sr 值0.7074～0.7179，206Pb/204Pb值为17.786～18.323。岩石中含有前寒武纪捕虏晶锆石。研究表明玄武质岩石可能是由亏损地幔与元古宙地壳岩石混染作用的结果。  相似文献   

The origin and distribution of HAPs elements in relation to maceral composition of the A1 lignite bed (Paleocene, Calvert Bluff Formation, Wilcox Group), Calvert mine area, east-central Texas     

Sharon S. Crowley  Peter D. Warwick  Leslie F. Ruppert  James Pontolillo 《International Journal of Coal Geology》1997,34(3-4)

The origin and distribution of twelve potentially Hazardous Air Pollutants (HAPs; As, Be, Cd, Cr, Co, Hg, Mn, Ni, Pb, Sb, Se, and U) identified in the 1990 Clean Air Act Amendments were examined in relation to the maceral composition of the A1 bed (Paleocene, Calvert Bluff Formation, Wilcox Group) of the Calvert mine in east-central Texas. The 3.2 m-thick A1 bed was divided into nine incremental channel samples (7 lignite samples and 2 shaley coal samples) on the basis of megascopic characteristics. Results indicate that As, Cd, Cr, Ni, Pb, Sb, and U are strongly correlated with ash yield and are enriched in the shaley coal samples. We infer that these elements are associated with inorganic constituents in the coal bed and may be derived from a penecontemporaneous stream channel located several kilometers southeast of the mining block. Of the HAPs elements studied, Mn and Hg are the most poorly correlated to ash yield. We infer an organic association for Mn; Hg may be associated with pyrite. The rest of the trace elements (Be, Co, and Se) are weakly correlated with ash yield. Further analytical work is necessary to determine the mode of occurrence for these elements. Overall, concentrations of the HAPs elements are generally similar to or less than those reported in previous studies of lignites of the Wilcox Group, east-central region, Texas. Petrographic analysis indicates the following ranges in composition for the seven lignite samples: liptinites (5–8%), huminites (88–95%), and inertinites (trace amounts to 7%). Samples from the middle portion of the A1 bed contain abundant crypto-eugelinite compared to the rest of the samples; this relationship suggests that the degradation of plant material was an important process during the development of the peat mire. With the exception of Hg and Mn, relatively low levels of the HAPs elements studied are found in the samples containing abundant crypto-eugelinite. We infer that the peat-forming environment for this portion of the coal bed was very wet with minimal detrital input. Relatively high concentrations of crypto-humotelinite were found in samples from the top and base of the coal bed. The presence of abundant crypto-humotefinite in this part of the coal bed suggests the accumulation of wood-rich peat under conditions conducive to a high degree of tissue preservation in the peat mire. Although several of the trace elements (Be, Co, Ni, and Sb) exhibit enrichment in these samples, they are not necessarily chemically associated with humotelinite. We infer that these elements, with the exception of Be, are possibly associated with deposition of the roof and floor rock of the coal bed; however, further analytical work would be necessary to confirm this hypothesis. Beryllium may have an organic origin.  相似文献   

贵州大厂锑矿有机质与有机成矿作用   总被引：6，自引：0，他引：6  

叶造军  施继锡 《矿物学报》1997,17(3):310-315

用有机地球化学方法对矿区与右江盆地有机质的分析研究表明，容矿的大厂层有机碳与锑含量相对较高；有机包裹体主要是气态烃包裹体与沥青包裹体，在气态烃包裹体中含0％～30％的液态烃，发暗黄色荧光；有机气相成分中四种烷烃甲烷、乙烷、丙烷、丁烷均有显示；从主成矿期萤石中的沥青及右江盆地再沉积火山碎屑岩中的干酪根的生物标志物测定结果来看，二者在正烷烃、类异戊二烯烷烃、萜烷与甾烷标志物特征上较为接近，表明右江盆地沉积物中的有机质与锑矿中的有机质有亲缘关系。模拟实验证明原油对锑有较强的萃取迁移能力；而有机质在锑成矿过程中主要起三方面的作用：淋滤、萃取迁移及还原沉淀。  相似文献   

The Origin of the Early Proterozoic Kuandian Complex: Evidence from Geochemistry   总被引：3，自引：0，他引：3  

Zhang Lifei  Sun Min  Wu Jiahong Department of Geology  Peking University  Beijing Department of Earth Sciences  University of Hong Kong  Hong Kong Shenyang Institute of Geology  Mineral Resources  CAGS  Shenyang Liaoning Liu Xinzhu 《《地质学报》英文版》1997,71(1):15-32

The Kuandian Complex is scarcely preserved Early Proterozoic volcanic suite, formed2.3-2.4 Ga ago. It is located in an Early Proterozoic mobile belt bounded by the ArchaeanRangrim and Ryonggang Blocks of the northeastern Sino-Korean Craton. The Complex ismainly made up of amphibolites, gneisses, leucoleptite, leptite and layered granite. Petrologicaland geochemical studies show that the protoliths of the Complex are mainly assoctations ofbimodal volcanics and anorogenic granites. The Kuandian amphibolites are depleted in Nb, Ta,P and Ti, and enriched in LILE, e.g. K, Rb and Cs, with pronounced depletion of Sr relative toNd and Pb; La/Nb ratios are higher than 1(1.75 to 5.18). The trace element patterns of theamphibolites are similar to continental flood basalts formed by the Gondwana break-up, suchas those in South Karoo and Tasmania, which shows continental contamination. ε_(Nd) valuesranging from 0.70 to 1.94 of the Kuandian amphibolites and the relationships between Nb/Yband La/Yb suggest that contamination of basaltic magma happened in the mantle, rather thanalong the conduit. Isotope ratios of ~(208)Pb/~(204)Pb, ~(207)Pb/ ~(204)Pb, ~(206)Pb/~(204)Pb, ~(143)Nd/~(144)Ndand ~(87)Sr/~(86)Sr indicate that the magma was derived from a contaminated mantle source likeDMM or a mixture of DMM and EM2. The Kuandian Complex has Dupal anomaly, as is thecase with some continental basalts in the south hemisphere, e.g. in South Karoo and Tasmania.Petrochemical modelling proposes that the Kuandian gneiss, granite, and amphibolite camefrom the same parental magma, being products of strong fractional crystallization. Protoliths ofthe Kuandian Complex were formed in extensional tectonic setting during the transition fromcontinental crust to oceanic crust. The formation of the Kuandian Complex indicates that 2.3or 2.4 Ga ago tectonic evolution of the Sino-Korean craton was different from that of otherwell-studied Precambrian cratons, e.g. the North American shield, European platform andAustralian continent in that strong volcanic eruption resulted in its accretion. Besides, the con-taminated magma source with a Dupal anomaly for the Complex indicates that crust-mantleconvection whose scale was similar to that of the present plate tectonics had occurred at leastbefore the formation of the Kuandian Complex (2.3-2.4 Ga B.P).  相似文献   

金山韧性剪切带大型金矿床稀土元素地球化学   总被引：1，自引：0，他引：1  

杨子江 《黄金地质》1997,3(2):17-23

根据ＲＥＥ含量及其组成和∑ＲＥＥ、∑Ｃｅ／∑ｙ、δＥｕ、（Ｌａ／Ｙｂ）Ｎ……参征参数，以金山金矿床为例，探讨“金山式”矿床的形成、演倾与成矿的ＲＥＥ地球化学特征。  相似文献   

云南老王寨金矿煌斑岩地球化学研究中的某些问题──与宋新宇等同志商榷   总被引：2，自引：0，他引：2  

黄智龙  王联魁 《矿物岩石》1997,(2)

本文对“云南老王寨金矿煌斑岩地球化学研究”一文中的部分结论及其解释，提出一些不同意见与原文作者宋新宇等先生商榷  相似文献