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1.
With a few exceptions, shales from the Archean Witwatersrand Supergroup (~2800 Ma) in South Africa are depleted in Na, Ca, LILE, REE and HFSE compared to Phanerozoic shales. Cr, Co and Ni are enriched in all Witwatersrand shales and Fe and Mg are high in shales from the West Rand Groups (WRG) and lower Central Rand Group (CRG). Shales from the CRG and uppermost WRG are enriched in Na, Al, LILE, REE, HFSE and transition metals relative to shales from the lower WRG. Chondrite-normalized REE patterns for all Witwatersrand shales are enriched in light-REE and exhibit small to moderate negative Eu anomalies. A positive correlation of REE and Al2O3 contents in the shales suggests that REE are contained principally in clay minerals.Relative to shales from the CRG, shales from the WRG exhibit depletions of Na, Ca and Sr, a feature probably reflecting intense chemical weathering of their source rocks. CIA indices in Witwatersrand shales are variable (chiefly 70–98), even within the same shale unit. Such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism.Compared to present-day upper continental crust, all but the Orange Grove, Roodepoort, and K8 shales appear to have been derived from continental sources depleted in LILE, REE, and HFSE and enriched in transition metals. Computer mixing models based on six relatively immobile elements (Th, Hf, Yb, La, Sc, Co) and four source rocks indicate that the relative proportions of granite, basalt and komatiite increased with time in sediment source areas at the expense of tonalite. The contributions of basalt and komatiite appear to reach a maximum during deposition of the Booysens shale, and granite during deposition of the K8 shales and possibly during deposition of the Orange Grove shales.  相似文献   

2.
The major and trace element characteristics of black shales from the Lower Cretaceous Paja Formation of Colombia are broadly comparable with those of the average upper continental crust. Among the exceptions are marked enrichments in V, Cr, and Ni. These enrichments are associated with high organic carbon contents. CaO and Na2O are strongly depleted, leading to high values for both the Chemical Index of Alteration (77–96) and the Plagioclase Index of Alteration (86–99), which indicates derivation from a stable, intensely weathered felsic source terrane. The REE abundances and patterns vary considerably but can be divided into three main groups according to their characteristics and stratigraphic position. Four samples from the lower part of the Paja Formation (Group 1) are characterized by LREE-enriched chondrite-normalized patterns (average LaN/YbN = 8.41) and significant negative Eu anomalies (average Eu/Eu1 = 0.63). A second group of five samples (Group 2), also from the lower part, have relatively flat REE patterns (average LaN/YbN = 1.84) and only slightly smaller Eu anomalies (average Eu/Eu1 = 0.69). Six samples from the middle and upper parts (Group 3) have highly fractionated patterns (average LaN/YbN = 15.35), resembling those of Group 1, and an identical average Eu/Eu1 of 0.63. The fractionated REE patterns and significant negative Eu anomalies in Groups 1 and 3 are consistent with derivation from an evolved felsic source. The flatter patterns of Group 2 shale and strongly concave MREE-depleted patterns in two additional shales likely were produced during diagenesis, rather than reflecting more mafic detrital inputs. An analysis of a single sandstone suggests diagenetic modification of the REE, because its REE pattern is identical to that of the upper continental crust except for the presence of a significant positive Eu anomaly (Eu/Eu1 = 1.15). Felsic provenance for all samples is suggested by the clustering on the Th/Sc–Zr/Sc and GdN/YbN–Eu/Eu1 diagrams. Averages of unmodified Groups 1 and 3 REE patterns compare well with cratonic sediments from the Roraima Formation in the Guyana Shield, suggesting derivation from a continental source of similar composition. In comparison with modern sediments, the geochemical parameters (K2O/Na2O, LaN/YbN, LaN/SmN, Eu/Eu1, La/Sc, La/Y, Ce/Sc) suggest the Paja Formation was deposited at a passive margin. The Paja shales thus represent highly mature sediments recycled from deeply weathered, older, sedimentary/metasedimentary rocks, possibly in the Guyana Shield, though Na-rich volcanic/granitic rocks may have contributed to some extent.  相似文献   

3.
General trends of the formation of Middle Riphean fine-grained aluminosiliciclastic rocks in the Bashkir Meganticlinorium are considered. It is shown that Yurmatinian shales do not contain any significant pyroclastic admixture. Judging from the relatively constant Th/Cr ratio throughout the Yurmatinian section, the tectonic regime in the study territory during the early Middle Riphean is suggested to be rather stable. The main paleoclimatic indices and indicators of the pelitic material maturity (CIA, CIW, IVC, PIA, and Ce/Y) suggest that paleodrainage systems in the early Middle Riphean were dominated by humid climate that gave way to the arid or semiarid type in the middle Yurmatinian. The low Mo/Mn ratio and some other indicators of redox conditions in shales from all Yurmatinian lithostratigraphic units show that no explicit reducing conditions existed in the basin during the early Middle Riphean. The shales were characterized by the increase in K2O/Al2O3 ratio, gradual enrichment in REE, and growth of LREE/HREE and LaN/YbN ratios toward the middle Yurmatinian, indicating the gain of an appreciable amount of slightly weathered arkosic aluminosiliciclastic material in the sedimentary basin about 1220–1200 Ma ago. The REE distribution and the UCC- and AUC-normalized shale compositions suggest that the eroded upper crust was compositionally close to the UCC. The occurrence of mafic and ultramafic rocks is also inferred. Data points of Yurmatinian shales plotted in the Cr–Ni, Eu/Eu*–GdN/YbN, and (La/YB)N–YbN diagrams are localized between the fields of Upper Archean and post-Archean rocks or within the latter field. Hence, post-Archean igneous and metamorphic complexes prevailed in paleodrainage systems of the early Middle Riphean. This is also confirmed by the model Nd ages.  相似文献   

4.
The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting.

Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ?Nd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.  相似文献   

5.
The Hegenshan ophiolite in the Inner Mongolian-Daxinganling Orogenic Belt (IMDOB), northern China, consists of several discontinuous blocks composed dominantly of serpentinized ultramafic rocks with subordinate cumulate gabbros, mafic lavas and dikes, intruded by younger granodiorite dikes. The ultramafic rocks are highly depleted, serpentinized harzburgites with minor dunite, characterized by relative enrichment in large ion lithophile elements (LILE, e.g., Ba and Rb) and light rare earth elements (LREE). They are interpreted to be oceanic mantle that has undergone extensive melt extraction and variable degrees of metasomatism. The cumulate rocks consist mainly of gabbro and troctolite with LREE-depleted chondrite-normalized REE patterns showing significant positive Eu anomalies. They are enriched in LILE, depleted in Nb, and have high positive εNd(t) (+8 to +11), suggesting derivation from a subduction-modified N-MORB-like source. The gabbros and mafic dikes have essentially the same age (295 ± 15 and 298 ± 9 Ma, respectively). The mafic dikes have flat to LREE-depleted, chondrite-normalized REE patterns, are depleted in Nb, enriched in LILE and have N-MORB-type Nd isotopic signatures (εNd(t) = +8.1 to +10). The mafic lavas, erupted at 293 ± 1 Ma, can be divided into two groups; one composed of strongly deformed metabasalts similar in chemical and Nd–Sr isotopic compositions to the mafic dikes, and the other composed of undeformed and unmetamorphosed basalts with oceanic island basalt (OIB)-like trace element signatures and Nd isotopic compositions. The granodiorite dikes, which were intruded at 244 ± 4 Ma, have LREE-enriched, chondrite-normalized REE patterns with no Eu anomalies. Their abnormally high εNd(t) values (+6.3 to +6.8) and low ISr (0.70412 to 0.70450) suggest formation from melts derived from thickened oceanic crust during or shortly after closure of the Paleo-Asian Ocean. The structure, lithology and geochemistry of the Hegenshan ophiolite suggest that it is a Cordilleran-type body formed in a supra-subduction zone (SSZ) environment and amalgamated by collision of several fragments of Paleo-Asian lithosphere. Final emplacement and amalgamation occurred in the latest Permian or earliest Triassic.  相似文献   

6.
《International Geology Review》2012,54(10):1113-1144
The Kumbalgarh Group of the south Delhi fold belt are the main bedrock series exposed in the axial region of the Aravalli craton. Quartzites and greywackes, the chief clastic constituents of this group, are well exposed. Petrographic and bulk-rock analyses of these rocks permit determination of their provenance, tectonic setting of the basin, and the Archaean to Proterozoic crustal evolution. Greywackes comprise quartz, plagioclase, amphiboles, K-feldspar, and rock fragments. Based on mineralogy, we divided the quartzites into three categories: QTZ1 is chiefly composed of quartz with a silty matrix and a minor quantity of feldspars and QTZ2 contains significant mafic minerals as well as quartz and feldspars, whereas QTZ3 is more feldspathic than the other groups. All the lithounits have SiO2/Al2O3 ratios <～10 suggesting textural immaturity consistent with their sedimentary petrography. Greywackes display the least fractionated rare earth elements (REEs) (La/Yb N : avg. 2.55) with positive Eu anomalies (avg. Eu/Eu* = 1.34). QTZ1 contains strongly fractionated REE patterns (avg. La/Yb N : 13.56, avg. Eu/Eu* = 0.60), QTZ2 shows moderate REE fractionation (avg. La/Yb N : 4.97, avg. Eu/Eu* = 0.61), and QTZ3 possesses the least fractionated V-shaped REE patterns (avg. La/Yb N : 1.97, avg. Eu/Eu* = 0.51). Weathering attributes including chemical index of alteration (CIA), plagioclase index of alteration (PIA), chemical index of weathering (CIW), and A–CN–K plots assign a low to moderate degree of weathering to the Kumbalgarh sediments under a subtropical climate. Based on our synthesis of the petrographic and geochemical data, we suggest a provenance comprising basalts, tonalite–trondhjemite–granodiorite (TTG), and granite. Geochemical attributes indicate deposition of the detritus in an extensional backarc basin receiving sedimentary input from opposite directions. The opening and then closure of the South Delhi Basin was the last phase of the break-up of the supercontinent, columbia, which began by abortive rifting of the Udaipur belt and culminated in separation of the Aravalli–Bundelkhand–Dharwar block in the east and the East African orogen in the west.  相似文献   

7.
The ∼2.7 Ga Sandur Superterrane (SST), of the western Dharwar craton, is a collage of greenstone terranes having distinct lithotectonic associations; volcanic associations are prevalent. Fine-grained metasedimentary rocks, which are optimal for provenance studies, are sparse in greenstone terranes of this craton. However, extensive shale sequences are present in the eastern volcanic terrane (EVT) and the eastern felsic volcanic terrane (EFVT) of the SST. Within the EVT, the black shales are stratigraphically associated with black cherts, metabasalt and banded iron formation (BIF), and underlain by greywackes. Shales have compositions of tholeiitic basalt in terms of TiO2, Cr, Co, Ni, V, and Sc contents, and plot near the arc basalt endmember on the Th/Sc versus Sc mixing hyperbola. In contrast, Archean average upper continental crust of Taylor and McLennan [Taylor, S.R., McLennan, S.M., 1985. The Continental crust: Its Composition and Evolution. Blackwell, Oxford, 307p.; Taylor, S.R., McLennan, S.M., 1995. The geochemical evolution of the continental crust. Rev. Geophys.33, 241-265], plots mid-hyperbola indicative of bimodal arc magma provenance. Accordingly, the Sandur shales likely had a catchment in an oceanic arc or back-arc dominated by tholeiitic basalts. Specifically, Nb/Th ratios 1.5-2.5 in shales are close to those of Archean arc basalts (1-4), so a plateau or ocean island basalt source, where Nb/Th >8, can be ruled out. Compositionally, cherts are shale highly diluted by silica, with positive Eu anomalies, and are interpreted to be hydrothermal sediments precipitated from reduced fluids during periods of limited siliciclastic input. In the shales, variable SiO2 and Fe2O3 contents, depletions of MnO, MgO, and Na2O, and positive to negative Eu anomalies, but gains of K relative to arc basalt compositions, are interpreted as due to hydrothermal alteration. Greywackes underlying the shales have two compositions. Type I is similar to the shales, whereas Type II has fractionated REE with negative Eu anomalies consistent with a cratonic granitoid catchment [Manikyamba, C., Naqvi, S.M., Moeen, S., Gnaneswar Rao, T., Balaram, V., Ramesh, S.V., Reddy, G.L.N., 1997a. Geochemical heterogeneities of metagreywackes from the Sandur schist belt: implications for active plate margin processes. Precambrian Res. 84, 117-138]. Collectively, the results are in keeping with an intraoceanic arc outboard of a continental margin. During transgression the trench has a low energy shale facies with dominant arc contribution, but for regression high energy greywackes are deposited from a cratonic provenance.  相似文献   

8.
The watershed in the central Guizhou Province (Guizhou Province is called simply Qian) (CQW) is a karstic area. Rare earth elements (REEs) of dissolved loads, suspended particulate material (SPM) and sediments of riverbed are first synthetically reported to investigate REE geochemistry in the three phases in karstic watershed during the high-flow season. Results show that the low dissolved REE concentrations in the CQW are attributed to these rivers draining carbonate rocks. The dissolved REE have significant negative Eu anomaly and coexistence of middle and light REE (MREE??PAAS-normalized La N /Sm N and Gd N /Yb N ; LREE??PAAS-normalized La N /Yb N )-enrichment, which are due to the dissolution of impure Triassic carbonates. REE concentrations in most of SPM exceed that of sediments in the CQW and the average continental crust (UCC). The SPM and the sediments show some common features: positive Eu, Ce anomalies, and MREE enrichment. The controls on the patterns seem to be from weathering profiles: the oxidation state, the REE-bearing secondary minerals (cerianite, potassium feldspar and plagioclase), which are also supported by the evidence of Y/Ho fractionations in the three phases.  相似文献   

9.
The mineralogical and geochemical characteristics of the Upper Triassic Baluti shale from the Northern Thrust Zone (Sararu section) and High Folded Zone (Sarki section) Kurdistan Region, Iraq, have been investigated to constrain their paleoweathering, provenance, tectonic setting, and depositional redox conditions. The clay mineral assemblages are dominated by kaolinite, illite, mixed layers illite/smectite at Sararu section, and illite > smectite with traces of kaolinite at Sarki. Illite, to be noted, is within the zone of diagenesis. The non-clay minerals are dominated by calcite with minor amounts of quartz and muscovite in Sararu shale; and are dominated by dolomite with amounts of calcite and quartz in Sarki shale. Baluti shale is classified as Al-rich based on major and minor elements. The chemical index of alteration (CIA) is significantly higher in the Sararu than the Sarki shales, suggesting more intense weathering of the Sararu than the Sarki shales. The index of compositional variability (ICV) of the Sararu shale is less than 1 (suggesting it is compositionally mature and was deposited in a tectonically quiescent setting). More than 1 for Sarki shales (suggest it is less mature and deposited in a tectonically active setting). Most shale of the Baluti plot parallel and along the A-K line in A-CN-K plots suggest intense chemical weathering (high CIA) without any clear-cut evidence of K-metasomatism. Clay mineral data, Al enrichment, CIA values, and A-CN-K plot suggest that the source area experienced high degree of chemical weathering under warm and humid conditions, especially in Sararu. Elemental ratios critical of provenance (La/Sc, Th/Sc, Th/Cr, Th/Co, Ce/Ce*PN, Eu/Eu*PN, and Eu/Eu*CN) shows slight difference between the Sararu and Sarki shales; and the ratios are similar to fine fractions derived from the weathering of mostly felsic rocks. The Eu/Eu* CN, Th/Sc, and low K2O/Al2O3 ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source, consistent with a source from old upper continental crust. Discrimination diagrams based on major and trace element content point to a role of the felsic-intermediate sources for the deposition of Baluti Formation, and probably mixed with mafic source rocks at Sararu section. The chondrite-normalized rare earth elements (REE) patterns are similar to those of PAAS, with light REE enrichment, a negative Eu anomaly, and almost flat heavy REE pattern similar to those of a source rock with felsic components. The source of sediments for the Baluti Formation was likely the Rutba Uplift and/or the plutonic-metamorphic complexes of the Arabian Shield located to the southwest of the basin; whereas the Sararu shale was affected by the mafic rocks of the Bitlis-Avroman-Bisitoun Ridge to the northeast of Arabian Plate. The tectonic discrimination diagrams, as well as critical trace and REE characteristic parameters imply rift and active setting for the depositional basin of the shale of Baluti Formation. The geochemical parameters such as U/Th, V/Cr, V/Sc, and Cu/Zn ratios indicate that these shales were deposited under oxic environment and also show that Sarki shale was deposited under more oxic environment than Sararu.  相似文献   

10.
The rare-earth element （REE） concentrations of representative granite samples from the southeast of the Obudu Plateau, Nigeria, were analyzed with an attempt to determine the signatures of their source, evolutionary history and tectonic setting. Results indicated that the granites have high absolute REE concentrations （190×10^-6-1191×10^-6; av.=549×10^-6） with the chondrite-normalized REE patterns characterized by steep negative slopes and prominent to slight or no negative Eu anomalies. All the samples are also characterized by high and variable concentrations of the LREE （151×10^-6-1169×10^-6; av.= 466×10^-6）, while the HREE show low abundance （4×10^-6-107×10^-6; av.=28×10^-6）. These are consistent with the variable levels of REE fractionation, and differentiation of the granites. This is further supported by the range of REE contents, the chondrite-normalized patterns and the ratios of LaN/YbN （2.30-343.37）, CeN/YbN （5.94-716.87）, LaN/SmN （3.14-11.68） and TbN/YbN （0.58-1.65）. The general parallelism of the REE patterns, suggest that all the granites were comagmatic in origin, while the high Eu/Eu＊ ratios （0.085-2.807; av.=0.9398） indicate high fo2 at the source. Similarly, irregular variations in LaN/YbN, CeN/YbN and Eu/Eu＊ ratios and REE abundances among the samples suggest behaviors that are related to mantle and crustal sources.  相似文献   

11.
The results of field, petrographic and geochemical work of the granitoids of Hutti-Gurgunta area in the northern part of Eastern Dharwar Craton (EDC) is presented in this paper. This crustal section comprises polyphase banded to foliated TTG gneisses, middle amphibolite facies Gurgunta schist belt and upper greenschist facies Hutti schist belt and abundant granite plutons. The focus of the present study is mainly on basement TTG gneisses and a granite pluton (∼ 240 sq km areal extent), to discuss crustal accretion processes including changing petrogenetic mechanism and geodynamic setting. The TTGs contain quartz, plagioclase, lesser K-feldspar and hornblende with minor biotite while the granite contain quartz, plagioclase, K-feldspar and hornblende. Late stage alteration (chloritisation, sericitisation and epidotisation) is wide spread in the entire area. A huge synplutonic mafic body which is dioritic to meladioritic in composition injects the granite and displays all stages of progressive mixing and hybridization. The studied TTGs and granite show distinct major and trace element patterns. The TTGs are characterized by higher SiO2, high Al2O3, and Na2O, low TiO2, Mg#, CaO, K2O and LILE, and HFS elements compared to granite. TTGs define strong trondhjemite trend whilst granite shows calc-alkaline trend. However, both TTGs and granite show characteristics of Phanerozoic high-silica adakites. The granite also shows characteristics of transitional TTGs in its high LILE, and progressive increase in K2O with differentiation. Both TTGs and granite define linear to sub-linear trends on variation diagrams. The TTGs show moderate total REE contents with fractionated REE patterns (La/YbN =17.73–61.73) and slight positive or without any significant Eu anomaly implying little amount of amphibole or plagioclase in residual liquid. On the other hand, the granite displays poor to moderate fractionation of REE patterns (La/YbN = 9.06–67.21) without any significant Eu anomaly. The TTGs have been interpreted to be produced by low-K basaltic slab melting at shallow depth, whereas the granite pluton has been formed by slab melting at depth and these melts interacted with peridotite mantle wedge. Such changing petrogenetic mechanisms and geodynamic conditions explain increase in the contents of MgO, CaO, Ni and Cr from 2700 Ma to 2500 Ma granitoids in the EDC.  相似文献   

12.
对采自北京地区中元古界下马岭组的页岩样品,进行了地球化学研究,结果表明:主量元素Al2O3、Ca O和Na2O含量明显偏低,K2O和Mn O含量稍微偏低,Si O2、Ti O2及Fe2O3T含量同上地壳基本一致。微量元素及其比值特征显示源岩为混合源岩。稀土元素总量较高,轻稀土富集、重稀土平坦,铕明显负异常、铈弱负异常,各样品稀土元素配分模式与大陆上地壳一致,显示了沉积物具有同源性。源岩为来自于内蒙古隆起的花岗岩类、闪长岩类及基性岩的混合。微量元素比值及铈弱负异常的特征表明中元古界下马岭组页岩为覆水较深的还原环境沉积,源区构造背景为大陆岛弧。  相似文献   

13.
五素火山岩位于哀牢山蛇绿岩带西侧,大地构造位置属于思茅地块。火山岩整合产于早石炭世地层中,厚约800m,具双峰式特征。野外见4层流纹岩和英安岩,且均与玄武岩呈互层产出。与N-MORB比较,五素玄武岩的Ti含量偏高,Ti/V、Zr/Y比值和LREE丰度较高,具有E-MORB的特征,说明五素玄武岩的原始岩浆可能类似OIB的特征,指示其产于板块拉张环境而非岛弧环境。五素玄武岩的Th>Ta,暗示玄武岩经历了陆壳混染作用。综合野外地质和地球化学证据说明,五素双峰式火山岩应当属于大陆溢流玄武岩(CFB),产于板块被动陆缘拉张减薄环境。   相似文献   

14.
Archean sedimentary rocks of very limited lateral extent from horizons within basaltic and ultramafic volcanic sequences at Kambalda, Western Australia, are extremely variable in major elements, LIL and ferromagnesian trace element compositions. The REE patterns are uniform and do not have negative Eu anomalies. Two samples have very low total REE abundances and positive Eu anomalies attributed to a very much greater proportion of chemically deposited siliceous material. Apart from these two samples, the Kambalda data are similar to REE abundances and patterns from Archean sedimentary rocks from Kalgoorlie, Western Australia and to average Archean sedimentary rock REE patterns. These show a fundamental distinction from post-Archean sedimentary rock REE patterns which have higher LaYb ratios and a distinct negative Eu anomaly.  相似文献   

15.
The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K2O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K2O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO2-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.  相似文献   

16.
Six graywackes from the Archean greenstone belt in the Wind River of Wyoming and 11 graywackes and three shales from the Archean Fig Tree Group, South Africa, were analyzed for the rare earth elements (REE). There are real deviations beyond analytical uncertainty among sediments from the same formation. The absolute abundance of the REE (∑REE) is somewhat lower than that of the North American shale composite (NASC) which is representative of younger sedimentary rocks. Still, the results confirm previous suggestions that the RE pattern in Precambrian sediments is the same as the NASC but that, relative to the heavy REE, graywackes are slightly enriched in the light REE, shales are depleted, and there is a slight enrichment in Eu relative to the NASC. The average abundance of Eu relative to the other REE for all the Precambrian sediments is the same as that in chondritic meteorites. Attesting to the complexity of graywacke genesis, there is no correlation between the variations in the REE parameters and the variations in major or minor element concentrations. In particular, there is no obvious correlation between the excess Eu and Sr abundance. The total amount of REE, the LaYb ratio, and the Eu enrichment factor, however, all increase in the graywackes with the amount granite-gneiss detritus in the rocks. In general, the REE distributions in Archean graywackes and shales appear to be related to the relative amounts of clastic feldspar, mica, and minor phases which concentrate the REE.  相似文献   

17.
The Archean Shawmere Anorthosite Complex, at the southern end of the Kapuskasing Structural Zone, consists dominantly of anorthosite (An65 –85) with minor gabbroic and ultramafic units, which are completely enclosed and cut by tonalites. Both the anorthosites and the tonalites are themselves cut by narrow dikes of gabbroic anorthosite. All of the rocks have undergone high grade metamorphism and are recrystallized so that few igneous textures remain.The anorthosites, gabbros and ultramafic rocks of this complex are cumulates which contain calcic plagioclase (An65–95) and have atomic Mg/(Mg + Fe2+) ratios (Mg#) greater than 0.6; less than 3 ppm Rb; 150–210 ppm Sr; and less than 60 ppm Ba. REE abundanees range from 0.2 to 10 times chondritic and exhibit both light-enriched and light-depleted REE patterns. The lower Mg# for the samples having more enriched light REE indicates substantial fractions of ferromagnesian minerals crystallized in addition to plagioclase during fractional crystallization, suggesting that the parent magma was basaltic, and not anorthositic. The ranges in Sr, Ba and REE abundances required for the magmas are typical of those for tholeiitic basalts from Archean greenstone belts. Thus the Shawmere Anorthosite Complex may represent cumulates of a crustal-level magma chamber which could have been the immediate source of basic Archean volcanics.One gabbroic anorthositic dike sample has a steeply fractionalted REE pattern with heavy REE abundances less than chondrites and a large positive Eu anomaly. The proposed interpretations is that this rock formed by partial melting of mafic cumulates, perhaps those of the Shawmere Anorthosite Complex itself.  相似文献   

18.
The Nagoundéré Pan-African granitoids in Central North Cameroon belong to a regional-scale massif, which is referred to as the Adamawa-Yade batholith. The granites were emplaced into a ca. 2.1 Ga remobilised basement composed of metasedimentary and meta-igneous rocks that later underwent medium- to high-grade Pan-African metamorphism. The granitoids comprise three groups: the hornblende–biotite granitoids (HBGs), the biotite ± muscovite granitoids (BMGs), and the biotite granitoids (BGs). New Th–U–Pb monazite data on the BMGs and BGs confirm their late Neoproterozoic emplacement age (ca. 615 ± 27 Ma for the BMGs and ca. 575 Ma for the BGs) during the time interval of the regional tectono-metamorphic event in North Cameroon. The BMGs also show the presence of ca. 926 Ma inheritances, suggesting an early Neoproterozoic component in their protolith.The HBGs are characterized by high Ba–Sr, and low K2O/Na2O ratios. They show fairly fractionated REE patterns (LaN/YbN 6–22) with no Eu anomalies. The BMGs are characterized by higher K2O/Na2O and Rb/Sr ratios. They are more REE-fractionated (LaN/YbN = 17–168) with strong negative Eu anomalies (Eu/Eu* = 0.2–0.5). The BGs are characterized by high SiO2 with K2O/Na2O > 1. They show moderated fractionated REE patterns (LaN/YbN = 11–37) with strong Eu negative anomalies (Eu/Eu* = 0.2–0.8) and flat HREE features (GdN/YbN = 1.5–2.2). In Primitive Mantle-normalized multi-element diagrams, the patterns of all rocks show enrichment in LILE relative to HFSE and display negative Nb–Ta and Ti anomalies. All the granitoids belong to high-K calc-alkaline suites and have an I-type signature.Major and trace element data of the HBGs are consistent with differentiation of a mafic magma from an enriched subcontinental lithospheric mantle, with possible crustal assimilation. In contrast, the high Th content, the LREE-enrichment, and the presence of inherited monazite suggest that the BGs and BMGs were derived from melting of the middle continental crust. Structural and petrochemical data indicate that these granitoids were emplaced in both syn- to post-collision tectonic settings.  相似文献   

19.
The ∼2.6 Ga Hutti greenstone belt is one of several Neoarchean greenstone terranes of the eastern Dharwar Craton. There are prevalent mafic volcanic flows with subordinate felsic volcanic units and siliciclastic sedimentary rocks. All lithologies show variable intensities of submarine hydrothermal alteration, polyphase deformation and greenschist to amphibolite grade metamorphism, yet pillow, cumulus, and other primary volcanic features are locally preserved. Well exposed interlayered metabasalts, Mg-andesites (MA), and felsic flows outcrop along an 11 km sector in the SE of the terrane. Based on combined petrographic and geochemical characteristics, two tholeiitic basalt populations have been identified within the metabasalts: (1) those with enriched LREE at 20-50 times chondrite, and (2) an depleted LREE population at 12-20 times chondrite. The former has fractionated LREE, where (La/Sm)N = 1.2-1.7, but flat HREE, and negative anomalies at Nb, P, and Ti relative to neighbouring REE. The latter has lower absolute abundances of compatible and incompatible elements, mildly fractionated LREE, smaller anomalies at Nb, P, and Ti, with (Gd/Yb)N = 1.1-1.6. Several samples have the “N-MORB” signature of LREE depletion coupled with positive Nb anomalies. On the Th/Yb vs. Nb/Yb discrimination diagram depleted basalts plot near the MORB field whereas enriched basalts overlap the backarc and arc fields, consistent with a paired arc-back-arc. Mg-andesites feature SiO2 57-61 wt.%, multielement pattens similar to enriched basalts, coupled with Cr, Co, Ni contents greater than “normal” andesites. Felsic volcanic rocks are characterized by low Y, high (La/Yb)N, and Zr/Sm, but low Nb/Ta, with zero to positive Eu anomalies, thus conforming to most of the compositional criteria of Archean and Phanerozoic adakites. Similar associations of enriched and depleted arc basalts, with adakites, are known from Neoarchean greenstone terranes of the Superior Province. During intraoceanic subduction, slab dehydration-wedge melting generated arc basalts whereas slab melting-wedge hybridization, generated adakites and Mg-andesites.  相似文献   

20.
位于喀喇昆仑山喀喇昆仑断裂(塔什库尔干断裂)西侧的阿然保泰一带发育一套中二叠统灰岩-凝灰岩-枕状玄武岩地层。枕状玄武岩分布在北西向长约12km,宽约4.5km范围内。该套玄武岩枕状构造十分典型,岩石具气孔、杏仁状构造。玄武岩SiO2含量为44.14%～48.81%、TiO2为1.11%～1.83%,在Si2O-(Na2O+K2O)图中落入苦橄玄武岩、玄武岩和碱玄岩交界区,属于碱性岩石。稀土元素含量较高(54.40×10-6～139.9×10-6),Eu、Ce无异常,(La/Yb)N比值为2.87～6.29,配分模式为右倾型。大离子亲石元素富集(K、Rb、Ba等),但含量变化较大,高场强元素(Nb、Ta、Zr、Hf和P)相对亏损,Ti出现弱的负异常。玄武岩的地球化学特征显示阿然保泰玄武岩具洋岛玄武岩特征,源区为尖晶石二辉橄榄岩,其形成构造环境为板内拉张环境。阿然保泰OIB型玄武岩的发现证实了喀喇昆仑阿然保泰地区属于古特提斯主洋盆一部分。  相似文献   

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