A study of Sr isotopic characteristics and ore-search indicators of Gejiu Sn-bearing granites

Whole-rock Rb-Sr isochron dating of Sn-bearing granites and alkaline rocks from Gejiu, Yunnan Province has been conducted for their emplacement ages and initial⁸⁷Sr/⁸⁶Sr ratios. The Sr isotopic compositions of apatites from some basic rocks in Jiasha, and granite bodies also have been studied in detail. The genesis and evolution of Gejiu Sn-bearing granites as well as ore-search indicators are discussed on the basis of the available data in conjunction with the geochemical data on trace elements (such as Rb and Sr), Sr isotopic characteristics of the volcanic rocks, meta-diabase and host rocks and the isotopic features of ore leads.     

New igneous zircon Pb/Pb and metamorphic Rb/Sr ages in the Yaounde Group (Cameroon, Central Africa): implications for the Central African fold belt evolution close to the Congo Craton

Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and ²⁰⁷Pb/²⁰⁶Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624?±?2?Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The ⁸⁷Rb/⁸⁶Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. ⁸⁷Rb/⁸⁶Sr isochron ages differ from sample to sample (599?±?3, 572?±?4, 554?±?5, 540?±?5?Ma) yielding the oldest Neoproterozoic age (~600?Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (~540?Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The ⁸⁷Rb/⁸⁶Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (~540?Ma) more closely to the border of the Congo Craton.     

Age and strontium‐isotope geochemistry of differentiated rocks from the newer Volcanics,MT Macedon Area,Victoria, Australia

The Newer Volcanics Province of Victoria and South Australia consists of a major region of mainly alkaline basalts within which are two restricted areas containing strongly differentiated flow‐rocks. Typical alkalic basalts from this widespread province have K‐Ar ages from 4.5 to 0.5 m.y. and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7038 to 0.7045. Contrastingly, in the Macedon area of differentiated lavas, flow compositions range from limburgite to soda trachyte, with K‐Ar ages from 6.8 to 4.6 m.y. and initial ⁸⁷Sr/⁸⁶Sr ratios from 0.7052 to 0.7127. These differentiated rocks therefore are older, and some of them may have been contaminated by reaction with more radiogenic basement rocks during differentiation. Alternatively, the variation in initial Sr‐isotope composition may have resulted from varying isotopic composition of partial melts from the immediate source rocks. The most felsic of the differentiated rocks, soda trachyte, is extremely enriched with Rb relative to Sr; one of the three restricted outcrops of this rock (Camel's Hump) yields a total‐rock Rb‐Sr isochron age of 6.3 ± 0.6 m.y. with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7127. K‐Ar sanidine ages reported for the three outcrops of trachyte are identical to each other and to the Rb‐Sr isochron result.     

Geochronology of the mkushi gneiss complex,Central Zambia

Rb-Sr and K-Ar ages are reported from the Mkushi Gneiss Complex and its intrusives in central Zambia. Of the Rb-Sr whole-rock data of the Mkushi gneisses (three suites, 22 samples) only one suite (six samples) defines an isochron of 1777 ± 89 Ma with initial ⁸⁷Sr/⁸⁶Sr of 0.713 ± 0.0006 (errors 95% confidence level; λ⁸⁷Rb = 1.42·10⁻¹¹ a⁻¹). The other samples scatter below this isochron, but above a 1480 Ma line. Four hornblendes from amphibolitic dykes yield K-Ar ages between 864 and 804 Ma. Whole-rocks of the ‘red facies’ of the intrusive Mtuga Granite (nine samples) define an isochron of 607 ± 39 Ma with initial ⁸⁷Sr/⁸⁶Sr of 0.730 ± 0.005. The ‘white facies’ of the Mtuga Granite (four samples) and probably also the copper-bearing aplitic veins (seven samples) are of the same age, but with even higher initial ⁸⁷Sr/⁸⁶Sr ratios. Seven biotites from the Mkushi gneisses yield Rb-Sr and K-Ar ages between 469 Ma and 444 Ma. These data are interpreted to signal (1) about 1780 Ma ago: emplacement of the granitoid precursors of the Mkushi gneisses, derived from older continental material, (2) between about 1480 Ma and 860 Ma ago: formation of the Mkushi gneisses and the amphibolites (probably during the Irumide orogeny, about 1350 Ma ago); (3) between about 860 Ma and 800 Ma ago: closure of the hornblendes to K-Ar (490–550°C) after resetting (probably in relation to the c. 860 Ma old tectono-thermal event widespread in eastern and eastern-central Africa), (4) about 600 Ma ago: intrusion of the Mtuga Granite and the copper-bearing splites (post-tectonic Pan-African), possibly derived from the Mkushi gneisses, and (5) about 460-450 Ma ago: closure of the biotites to Rb-Sr and K-Ar (approximately 400°C), reflecting the final regional cooling (termination of the Pan-African events). Some regional implications are discussed.     

微区-微量样品Rb-Sr同位素分析技术及其应用前景

利用微钻取样技术和微量样品Rb-Sr同位素分析方法,本文对出露在东秦岭造山带的中生代合峪花岗岩的自形钾长石巨晶进行微区-微量样品Rb-Sr同位素组成分析。分析结果表明,钾长石斑晶具有显著的Rb/Sr比值和Sr同位素组成变化,斑晶和基质钾长石均构成年龄为132~133Ma的Rb-Sr等时线,代表岩浆的后期冷却时代。钾长石晶体的初始⁸⁷Sr/⁸⁶Sr比值由边缘相到中心相没有明显的变化,代表花岗质岩浆结晶阶段的Sr同位素组成,暗示合峪花岗岩的钾长石巨晶为原生成因。以高空间分辨率为特征,微区取样技术已经广泛地应用在变质岩和深成岩浆岩的同位素年代学和成因研究。结合微量样品同位素分析技术,微区-微量样品Rb-Sr同位素方法有望在火山岩的成因和年代学方面得到应用。     

Geochronology of archean gneisses in the Lake Helen area,Southwestern Big Horn Mountains,Wyoming

The RbSr and UPb methods were used to study gneisses in the $\text{7}\text{1}\text{2}\text{-}\text{minute}$ Lake Helen quadrangle of the Big Horn Mountains, Wyoming. Two episodes of magmatism, deformation and metamorphism occurred during the Archean. Trondhjemitic to tonalitic orthogneisses and amphibolite of the first episode (E-1) are cut by a trondhjemite pluton and a calc-alkaline intrusive series of the second episode (E-2). The E-2 series includes hornblende-biotite quartz diorite, biotite tonalite, biotite granodiorite and biotite granite.A RbSr whole-rock isochron for E-1 gneisses indicates an age of 3007 ± 34 Ma (1 sigma) and an initial ⁸⁷Sr/⁸⁶Sr of 0.7001 ± 0.0001. UPb determination on zircon from E-1 gneisses yield a concordia intercept age of 2947 ± 50 Ma. The low initial ratio suggests that the gneisses had no significant crustal history prior to metamorphism, and that the magmas from which they formed had originated from a mafic source.A RbSr whole-rock isochron for E-2 gneisses gives an age of 2801 ± 31 Ma. The ⁸⁷Sr/⁸⁶Sr initial ration is 0.7015 ± 0.0002 and precludes the existence of the rocks for more than 150 Ma prior to metamorphism. The E-2 magmas may have originated from melting of E-1 gneisses or from a more mafic source.     

Isotopic geochronology of mesozoic acid volcanic rocks in Zhejiang Province,China

Volcano-sedimentary series of the Upper Jurassic to the Lower Cretaceous are extensively developed in Zhejiang Province. But ages and stratigraphic correlation concerning these rocks have long been a controversial problem. Systematic sampling was made of volcanic rocks of the Laocun, Huangjian, Shouchang and Moshishan Formations in western Zhejiang considered thus far as the Late Jurassic. Isotopic age determinations show that U-Th-Pb zircon ages are approximately concordant with Rb-Sr isochron ages, whereas K-Ar biotite ages and K-Ar isochron ages are all slightly lower. It can therefore be established that the ages of volcanic rocks mentioned above range from 134±6 to 122±2 m.y., corresponding to the “transitional period” from Jurassic to Cretaceous. It can also be concluded that the rocks have not undergone apparent epigenetic metamorphism. The initial⁸⁷Sr/⁸⁶Sr ratio is about 0.7089–0.7121, on the basis of which it may be postulated that the volcanic magma seems to have originated from the upper mantle with contamination by sialic materials subsequent to differentiation. For age determinations of such acid volcanic rocks Rb-Sr isochron method is considered more suitable in view of its following advantages: the high reliability of results; wide applicability to different samples; smaller sample requirement and the possibility for further studies involving petrogenesis by use of initial⁸⁷Sr/⁸⁶Sr ratio.     

A K-Ar and Sr-isotopic study of the volcanic rocks of the island of principe,West Africa — Evidence for mantle heterogeneity beneath the Gulf of Guinea

K-Ar dating on a suite of volcanic rocks from the island of Principe gives the following chronology.

1. Basal palagonite breccia (30.6 ± 2.1 Ma).
2. Older Lava Series (OLS) basalt (23.6±0.7 Ma) and hawaiite (19.1±0.5 Ma).
3. Younger Lava Series (YLS) nephelinite (5.60±0.32 Ma) and basanite (3.51 ±0.15).
4. Intrusive phonolite (5.32±0.17 Ma, 5.48±0.19 Ma), tristanite (4.89±0.15 Ma) and trachyphonolite (6.93±0.68 Ma) plugs.

Phonolites and YLS samples plot on a 5.9±0.3 Ma Rb-Sr isochron. The tristanite-trachy-phonolite suite samples also lie on this isochron. This lends support to the suggestion that the YLS basanite magmas were parental to the phonolites but rules out a similar relationship between the OLS magmas and the tristanite-trachyphonolite suite. The mean initial ⁸⁷Sr/⁸⁶Sr ratio for the YLS nephelinites and basanites is 0.70297. The basalts and hawaiites of the OLS show a positive ⁸⁷Sr/⁸⁶Sr vs. Rb/Sr correlation which may be interpreted as a 244±43 Ma pseudoisochron. This could be the result of a large-scale heterogeneity generated in the mantle during the early stages in the break-up of Gondwanaland. The mean initial ⁸⁷Sr/ ⁸⁶Sr ratio (at 21 Ma) for the OLS (0.70326) is significantly higher than that for the YLS and implies an isotopically distinct mantle source.     

Evolution of a hydrothermal fluid-rock interaction system as recorded by Sr isotopes: A case study from the Schwarzwald, SW Germany

Metasomatic and Sr-isotopic changes, associated with formation of zoned alteration halos along hydrothermal veins, are documented for a gneiss from the Artenberg quarry near Steinach (Kinzigtal, Schwarzwald, SW Germany). Veins are postorogenic, SW-NE-oriented, and cut straight through metaquartzdioritic Variscan gneiss, where flow of low-temperature fluids (～100–200°C) caused adularia-quartz-sericite-type alteration. Fluid-rock interaction occurred nearly 50 Ma after Variscan metamorphism, as constrained by a Rb–Sr multimineral isochron for unaltered gneiss of 327.1?±?3.1 Ma, and by two independent ages of 279.2?±?3.1 Ma and 274?±?13 Ma, based on Rb–Sr systematics of late-stage quartz from the veins. In a profile from unaltered gneiss towards a vein, alteration-induced mineralogical changes correlate with metasomatic net addition of K, Rb, and Cl to the alteration zone, combined with net loss of Na, Ca, and Sr. Strontium isotopes give a more detailed insight into the fluid-rock interaction process. ⁸⁷Sr/⁸⁶Sr ratios in a profile across the alteration zone are incompatible with simple Sr leaching but reflect partial replacement of the rocks’ Sr by fluid-derived Sr, the isotopic composition of which varied with time. Early fluids, with high ⁸⁷Sr/⁸⁶Sr ratios compared to unaltered gneiss, evolved into fluids with somewhat lower ratios, and finally reached a second maximum in ⁸⁷Sr/⁸⁶Sr ratios. This Sr-isotopic fluid evolution is equally revealed by the mineral sequence of the vein mineralization. It appears that the compositional evolution of the fluids correlates with the sequence of mineral breakdown reactions in the gneissic host rock, and that the Sr-isotopic evolution of the fluids can be fully explained as the result of internal, progressive reaction of fluid with the local rocks. Results also show that the spatial distributions of Sr isotopes in metasomatic alteration zones may reflect the complex evolution of fluid-rock interaction systems, and ultimately constrain the factors controlling both fluid compositions and alteration patterns.     

相山铀矿田西部地区深部多金属矿化成矿年代与成矿流体演化:Rb-Sr同位素体系的制约

江西相山铀矿田科学深钻3号孔在深部-700 m发现大量铅锌多金属矿化脉,垂向上呈"上铀下多金属"的分布特征。本文选取深部多金属矿脉主成矿阶段(S3)自形闪锌矿样品6件和不同阶段的毒砂、黄铁矿、方铅矿、方解石等样品12件,以及围岩全岩样品17件,进行了Rb、Sr同位素组成研究。结果表明:(1)由闪锌矿Rb-Sr等时线法确定的相山铀矿田深部多金属矿化形成于121. 0±3. 5Ma,与围岩火山岩存在较大时差,可能与晚于围岩的深部次火山有关。根据穿插关系,多金属矿化略晚于碱性交代铀矿化,但明显早于酸性交代铀矿化;(2)多金属矿化脉体中金属矿物的Rb和Sr含量分别介于0. 041×10~(-6)～1. 38×10-6和2. 35×10-6～23. 11×10-6之间,Sr同位素初始比值(87Sr/86Sr)i变化较大,介于0. 706114～0. 718814之间,平均值为0. 713579,暗示相山铀矿田深部多金属矿化的成矿物质主要来源于地壳。初始流体Sr同位素值(0. 718665)明显高于成矿时赋矿围岩(流纹英安岩为0. 714581,碎斑流纹岩为0. 714417)的Sr同位素组成,表明多金属成矿流体和物质并非来自围岩火山岩;(3)由早到晚阶段的(87Sr/86Sr)i呈明显降低的演化趋势,表明成矿流体演化过程中受到大气降水的不断稀释作用。相山矿田的铀矿和深部多金属矿化同形成于华南中生代板内伸展构造背景。     

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.     

On the age of the Onverwacht Group,Swaziland Sequence,South Africa

Some rocks of the Onverwacht Group, South Africa, have been analyzed for Rb and Sr concentrations and Sr isotopie composition. These rocks include volcanic rocks, layered ultramafic differentiates and cherty sediments. Whole rock data indicate that the Rb-Sr isotopie systems in many samples were open and yield no reasonable isochron relationships. However, the data of mineral separates from a basaltic komatiite define a good isochron of $\text{t = 3.50 ± 0.20}$ (2δ) b.y. with an initial Sr⁸⁷/Sr⁸⁶ ratio of $\text{0.70048 ± 5}$(2δ). The orthodox interpretation of this age is the time of the low grade metamorphism. Since the basaltic komatiite is stratigraphically lower than the Middle Marker Horizon (dated as $\text{3.36 ± 0.07}$ b.y. Hurley et al., 1972), and since it is commonly found that volcanism, sedimentary deposition, metamorphism and igneous intrusion in many Archean greenstone-granite terrain all took place in a relatively short time interval (less than 100 m.y.), it is reasonable to assume that the age of 3.50 b.y. might also represent the time of initial Onverwacht volcanism and deposition. The initial Sr⁸⁷/Sr⁸⁶ ratio obtained above is important to an understanding of the Sr isotopic composition of the Archean upper mantle. If the komatiite represents a large degree of partial melt (40–80 per cent) of the Archean upper mantle material, then the initial ratio obtained from the metamorphic komatiite should define an upper limit for the Sr isotopic composition of the upper mantle under the African crustal segment.     

Response of U‐Pb Zircon and Rb‐Sr total‐rock and mineral systems to low‐grade regional metamorphism in proterozoic igneous rocks,mount Isa,Australia

A detailed Rb‐Sr total‐rock and mineral and U‐Pb zircon study has been made on suites of Proterozoic silicic volcanic rocks and granitic intrusions, from near Mt Isa, northwest Queensland. Stratigraphically consistent U‐Pb zircon ages within the basement igneous succession show that the oldest recognized crustal development was the outpouring of acid volcanics (Leichhardt Metamorphics) 1865 ± 3 m.y. ago, which are intruded by coeval, epizonal granites and granodiorites (Kalkadoon Granite) whose pooled U‐Pb age is 1862 ⁺²⁷ _‐21 m.y. A younger rhyolitic suite (Argylla Formation) within the basement succession has an age of 1777 ± 7 m.y., and a third acid volcanic unit (Carters Bore Rhyolite), much higher again in the sequence, crystallized 1678 ± 1 m.y. ago.

All of these rocks are altered in various degrees by low‐grade metamorphic events, and in at least one area, these events were accompanied by, and can be partly related to, emplacement of a syntectonic, foliated granitic batholith (Wonga Granite) between 1670 and 1625 m.y. ago. Rocks that significantly predate this earliest recognized metamorphism, have had their primary Rb‐Sr total‐rock systematics profoundly disturbed, as evidenced by 10 to 15% lowering of most Rb‐Sr isochron ages, and a general grouping of many of the lowered ages (some of which are in conflict with unequivocal geological relationships) within the 1600–1700 m.y. interval. Such isochrons possess anomalously high initial ⁸⁷Sr/⁸⁶Sr ratios, and some have a slightly curved array of isotopic data points. Disturbance of the Rb‐Sr total‐rock ages is attributed primarily to mild hydrothermal leaching, which resulted in the loss of Sr (relatively enriched in ⁸⁷Sr in the Sr‐poor (high Rb/Sr) rocks as compared with the Sr‐rich rocks).     

Geochemistry, U-Pb Geochronology and Sr-Nd-Hf Isotopes of the Early Cretaceous Volcanic Rocks in the Northern Da Hinggan Mountains

Whole-rock geochemical, zircon U-Pb geochronological and Sr-Nd-Hf isotopic data are presented for the Early Cretaceous volcanic rocks from the northern Da Hinggan Mountains. The volcanic rocks generally display high SiO2(73.19–77.68 wt%) and Na2O+K2O(6.53–8.98 wt%) contents, with enrichment in Rb, Th, U, Pb and LREE, and depletion in Nb, Ta, P and Ti. Three rhyolite samples, one rhyolite porphyry sample, and one volcanic breccia sample yield weighted mean 206Pb/238 U ages of 135.1±1.2 Ma, 116.5±1.1 Ma, 121.9±1.0 Ma, 118.1±0.9 Ma and 116.9±1.4 Ma, respectively. All these rocks have moderate(87Sr/86Sr)i values of 0.704912 to 0.705896, slightly negative εNd(t) values of –1.4 to –0.1, and positive εHf(t) values of 3.7 to 8. Their zircon Hf and whole-rock Nd isotopic model ages range from 594 to 1024 Ma. These results suggest that the Early Cretaceous volcanic rocks were originated from melting of subducted oceanic crust and associated sediments during the closure of the Mongol-Okhotsk Ocean.     

Rb—Sr and U—Pb Dating of the Daguzhai and Luobuli Granitic Massifs in South China

Rb-Sr and U-Pb isotopic studies of the two contrasting granite types of the Daguzhai and Luobuli massifs in South China provide new constraints on the interpretation of isotopic age data for plutonic igneous rocks. A Rb-Sr internal isochron age of 146±7Ma for the Luobuli adamellite is interpreted to represent the age of magma crystallization, whereas the whole rock Rb- Sr isochron yields an older apparent age of 161±10Ma which is regarded as resulting from contamination processes affecting the petrogenesis of this adamellite. In the Daguzhai granite the marked scatter of whole- rock Rb-Sr data in isochron diagram is ascribed to the open system behavior of Rb during postmagmatic autometasomatism. Uniformity of initial⁸⁷Sr /⁸⁶Sr ratio in this granite is indicated in a plot of⁸⁷Sr versus⁸⁶Sr. The autometasomatism has also affected zircon U-Pb system, resulting in a spread of data along the concordia curve between 165 and 125Ma. This spread is regarded as indicating the duration of the autometasomatism.     

晋东北地区中生代次火山岩及金银成矿作用地球化学

晋东北地区是金银矿床成矿有利地区。本文通过该区次火山岩Ｒｂ－Ｓｒ同位素及次火山岩和金银矿的稳定同位素研究，揭示了次火山岩和金银矿的生成时代及它们成因的内在联系，并指出了找矿标志。     

A Rubidium-Strontium study of the Twilight Gneiss,West Needle Mountains,Colorado

The Precambrian trondhjemitic Twilight Gneiss (Twilight Granite of Cross and Howe, 1905b) of the West Needle Mountains, southwestern Colorado, and its interlayered amphibolite and metarhyodacite yield a Rb-Sr isochron of 1,805±35 m.y. A low initial Sr⁸⁷/Sr⁸⁶ ratio of 0.7015 implies that metamorphism of these rocks to amphibolite facies took place soon after their emplacement. The mild metamorphism of Uncompahgran age, prior to 1,460 m.y. ago, and Laramide volcanism did not affect the Rb-Sr system in the Twilight. Rb contents of 26.5 to 108 ppm, Sr contents of 114 to 251 ppm, and K₂O percentages of 1.23 to 3.64 in the Twilight Gneiss, in conjunction with high K/Rb ratios and the low initial ratio of Sr⁸⁷/Sr⁸⁶, lend support to geologic data that suggest the Twilight originated as volcanic or hypabyssal igneous rocks in a basaltic volcanic pile.Publication authorized by the Director, U.S. Geological Survey.     

Kar and RbSr whole-rock ages reset during pan african event in the sinai peninsula (Ataqa Area)

A low pressure metamorphic Pan African terrain, composed mainly of schists and hornfels, ranging from high greenschist to low amphibolite facies, is exposed west of Dahab, southeastern Sinai, located between the metamorphic Kid Complex and the Feirani Volcanics.The studied metamorphic unit was dated using RbSr and KAr methods. A RbSr whole-rock isochron (based on seven samples) yields an age of 602 ± 11 Ma with an initial (⁸⁷Sr/⁸⁶Sr) ratio of 0.7041 ± 4. The RbSr age is assumed to be the age of a metamorphic phase (600 ± 10 Ma) well known in the area. On the other hand, KAr whole-rock ages (based on nine samples) show different values ranging from 590 to 526 Ma. These low KAr ages are due to the resetting of the KAr system by a thermal event, before 530 Ma affecting the Ar behaviour without disturbing the RbSr system. The Ar escape took place mainly from K-bearing feldspars, which were not affected by later textural, crystallographic or mineralogic variations.     

Strontium isotopic geochemistry of the volcanic rocks and associated megacrysts and inclusions from Ross Island and vicinity,Antarctica

Twelve whole-rock samples of volcanic rocks and a composite of 11 basanitoid samples from Ross Island and vicinity, Antarctica show a narrow range of ⁸⁷Sr/⁸⁶Sr ratios from 0.7030₅ to 0.7033₉. This range is consistent with a model of differentiation from a single parent magma, but the data allow a 30% variation in the ⁸⁷Rb/⁸⁶Sr ratio in the source region if the average ratio is less than 0.057 and if the source region has existed as a closed system for 1.5 b.y. Megacrysts of titaniferous augite, kaersutite, and anorthoclase are isotopically indistinguishable from the host volcanic rocks and therefore are probably cogenetic with the volcanic sequence. A single trachyte sample is isotopically distinct from the rest of the volcanic rocks and probably was contaminated with crustal strontium.Ultramafic and mafic nodules found in association with basanitoids and trachybasalts have ⁸⁷Sr/⁸⁶Sr ratios ranging from 0.7027₅ to 0.7057₅. Several of these nodules exhibit evidence of reaction with the melt and are isotopically indistinguishable from their hosts, but data for seven granulite-facies nodules show an apparent isochronal relationship. Although this isochron may be fortuitous, the resulting age of 158±22 m.y. is similar to ages reported for the voluminous Ferrar Dolerites, and suggests isotopic re-equilibration within the lower crust and upper mantle. These nodules are not genetically related to the Ferrar Dolerites, as evidenced by their lower initial ⁸⁷Sr/⁸⁶Sr ratios.Three ultramafic nodules are texturally and isotopically distinct from the rest of the analyzed nodules. These are friable, have larger ⁸⁷Sr/⁸⁶Sr ratios, and may represent a deeper sampling of mantle rock than the granulite-facies nodules. They were, however, derived at a shallower depth than the alkalic magma. Thus they are not genetically related to either the magma or the granulite-facies nodules.     

A Back-arc Palaeotectonic Setting for the Augaro Neoproterozoic Magmatic Rocks of Western Eritrea

The Augaro volcano-sedimentary assemblages of western Eritrea are part of the Neoproterozoic, N-S trending belt of low-grade volcano-sedimentary and associated plutonic rocks. In contrast to the volcanic-dominated oceanic-arc assemblages in central Eritrea, the predominant rock types in the west are supracrustal sequences of sedimentary origin with subordinate volcanic rocks. These Augaro supracrustal rocks are overlain, unconformably, by a basin-fill metasedimentary succession known as the Gulgula Group. The Augaro metavolcanic rocks are tholeiitic and range in composition from basalt to basaltic andesite. Comparison of trace element characteristics and N-MORB-normalised spidergrams of these rocks with those of modern volcanic environments and age-comparable metavolcanic rocks of known tectonic association from the Arabian-Nubian Shield suggest that the volcanic assemblages from western Eritrea were generated in a back-arc tectonic setting.

Single zircon Pb-Pb evaporation and vapour-transfer U-Pb analyses of magmatic zircons from pre/syn-tectonic granites yield a mean ²⁰⁷Pb/²⁰⁶Pb age of 849±20 Ma and an upper concordia intercept age of 849±26 Ma. These ages are interpreted to represent the time of major magmatism in western Eritrea and are comparable to ages of early arc magmatism in central and northern Eritrea and in the southern Nubian Shield. Initial eNd values and initial Sr isotope ratios of whole-rock samples of magmatic rocks calculated for an age of 850 Ma range from +4.0 to +7.1 and 0.7026 to 0.7037, respectively. Single zircon ²⁰⁷Pb/²⁰⁶Pb ages, initial eNd value and Sr isotope ratio for a granitic clast in the Gulgula metaconglomerate suggest that the source area for the Gulgula metasedimentary rocks is similar to the surrounding Neoproterozoic rocks of western Eritrea.