川西北马脑壳金矿床流体包裹体Rb-Sr同位素组成：对矿床成因的制约

位于四川西北部的马脑壳金矿床为陕甘川金三角区内一重要矿床，许多学者对其进行过大量的研究，对其地质地球化学特征有了较为全面的认识，同时对矿床的成因也做了多方面的探讨。为了确定矿床的成矿时代，从层状矿体中采集了富含流体包裹体的石英矿物。通过测定包裹体的Rb-Sr同位素组成，应用ISOPLOT程序计算获得(210±35)Ma的成矿年龄，该年龄与容矿地层的形成时代相近。流体包裹体的锶同位素初始比值与三叠纪时期海水的锶同位素比值相一致。综合矿体产出特征、矿石组构及区域构造演化历史，认为作为矿床主体的层状矿体是同生沉积的产物，脉状网脉状矿体则是由后期造山运动体制下所产生的成矿热流体沿裂隙构造交代充填所形成。

威宁宣威组底部硅质页岩Rb-Sr古混合线年龄及其地质意义

在滇黔相邻地区，二叠纪宣威组下部发育一层硅质页岩和碳质页岩。页岩层位稳定、分布广泛，与下伏 峨眉山玄武岩组上部凝灰岩层呈假整合接触关系。用迭代法恢复硅质页岩的Rb-Sr同位索古混合线，得到硅 质页岩的年龄为(255 4-12)Ma。这个年龄不仅支持地层古地磁研究结果，而且符合地层年代学研究结果，表明 Rb-Sr同位索古混合线年代学方法在中生代地层能够适用。这个年龄与普遍认为的峨眉山玄武岩主喷发期258 Ma从时代关系上相符，而且有效地约束了峨眉山玄武岩喷发的上限。

An 40Ar/39Ar, Rb/Sr, and stable isotope study of micas in low-grade fold-and-thrust belt: An example from the Swiss Helvetic Alps

White micas in carbonate-rich tectonites and a few other rock types of large thrusts in the Swiss Helvetic fold-and-thrust belt have been analyzed by Ar-40/Ar-39 and Rb/Sr techniques to better constrain the timing of Alpine deformation for this region. Incremental Ar-40/Ar-39 heating experiments of 25 weakly metamorphosed (anchizone to low greenschist) samples yield plateau and staircase spectra. We interpret most of the staircase release spectra result from variable mixtures of syntectonic (neoformed) and detrital micas. The range in dates obtained within individual spectra depends primarily on the duration of mica nucleation and growth, and relative proportions of neoformed and detrital mica. Rb/Sr analyses of 12 samples yield dates of ca. 10-39 Ma (excluding one anomalously young sample). These dates are slightly younger than the Ar-40/Ar-39 total gas dates obtained for the same samples. The Rb/ Sr dates were calculated using initial Sr-87/Sr-86 ratios obtained from the carbonate-dominated host rocks, which are higher than normal Mesozoic carbonate values due to exchange with fluids of higher Sr-87/Sr-86 ratios (and lower O-18/O-16 ratios). Model dates calculated using Sr-87/Sr-86 values typical of Mesozoic marine carbonates more closely approximate the Ar-40/Ar-39 total gas dates for most of the samples. The similarities of Rb/Sr and Ar-40/Ar-39 total gas dates are consistent with limited amounts of detrital mica in the samples. The delta(18)O values range from 24-15%. (VSMOW) for 2-6 mum micas and 27-16parts per thousand for the carbonate host rocks. The carbonate values are significantly lower than their protolith values due to localized fluid-rock interaction and fluid flow along most thrust surfaces. Although most calcite-mica pairs are not in oxygen isotope equilibrium at temperatures of ca. 200-400 degreesC, their isotopic fractionations are indicative of either 1) partial exchange between the minerals and a common external fluid, or 2) growth or isotopic exchange of the mica with the carbonate after the carbonate had isotopically exchanged with an external fluid. The geological significance of these results is not easily or uniquely determined, and exemplifies the difficulties inherent in dating very fine-grained micas of highly deformed tectonites in low-grade metamorphic terranes. Two generalizations can be made regarding the dates obtained from the Helvetic thrusts: 1) samples from the two highest thrusts (Mt. Gond and Sublage) have all of their Ar-40/Ar-39 steps above 20 Ma, and 2) most samples from the deepest Helvetic thrusts have steps (often accounting for more than 80% of Ar-39 release) between 15 and 25 Ma. These dates are consistent with the order of thrusting in the foreland-imbricating system and increase proportions of neoformed to detrital mica in the more metamorphosed hinterland and deeply buried portions of the nappe pile. Individual thrusts accommodated the majority of their displacement during their initial incorporation into the foreland-imbricating system, and some thrusts remained active or were reactivated down to 15 Ma.

Aplicação de métodos radiométricos (Rb-Sr e Sm-Nd) na análise de bacias sedimentares o exemplo da Bacia do Paraná

Os métodos radiométricos (Rb-Sr e Sm-Nd) têm sido aplicados, com sucesso, em rochas sedimentares visando a obtenção de idades deposicionais e informações sobre proveniência. Em muitos casos, apesar dos resultados geologicamente significativos, ainda persistem dúvidas em relação a extensão e interpretação dos dados obtidos. Isto ocorre porque as rochas sedimentares resultam da mistura de fragmentos detríticos de diversas proveniências com diferentes sistemas isotópicos, conseqüentemente, já contendo um registro isotópico da rocha fonte. No caso do método Rb-Sr, aplicado em rochas sedimentares de granulometria fina, pode-se obter idades absolutas para o evento deposicional. Isto é viável desde que este registro isotópico proveniente das rochas fonte seja apagado no momento da deposição, ou seja, deve ocorrer a homogeneização isotópica do Sr no ambiente sedimentar. Para tanto, devem ser observados alguns pré-requisitos em relação a amostragem, granulometria, ambiente deposicional e composição mineralógica das amostras, entre outros. No entanto, ainda existem questionamentos em relação a esta metodologia especialmente quanto à ocorrência e à extensão do processo de homogeneização isotópica do Sr em ambiente sedimentar.(Continua0 O método Sm-Nd, apesar de ser uma técnica relativamente nova quando aplicada a rochas sedimentares, tem se tornado uma ferramenta fundamental para auxílio na determinação de proveniência de rochas sedimentares. As maiores dificuldades estão na correta interpretação dos valores obtidos e sua associação com um ambiente sedimentar, ou seja, de baixa temperatura. Neste trabalho, foram aplicados os métodos radiométricos Rb-Sr e Sm-Nd em amostras de rochas sedimentares coletadas em diferentes contextos geológicos da Bacia do Paraná. Foram tentativamente testados os diferentes parâmetros que atuam no sentido da homogeneização isotópica do Sr e do comportamento do Nd. Os resultados obtidos permitiram aprimorar a metodologia radiométrica Rb-Sr e Sm-Nd quando aplicada em rochas sedimentares, bem como obter resultados sobre a idade deposicional de diferentes unidades sedimentares e, por vezes, sua proveniência.

Estudo geoquímico e geocronológico Rb-Sr e Sm-Nd em zonas de cisalhamento mineralizadas em ouro e suas relações com as rochas encaixantes e geocronológico Sm-Nd em mineralizações de scheelita na faixa Seridó

Great part of the gold mineralizations are associated with shearing zones through which circulate a great volume of fluids, that interact with the host rocks, originating leaching or precipitation of chemical elements, including gold. The studied mineralizations are inserted in the Seridó Belt. The tungsten mineralization in Brejuí Mine is hosted in calcsilicate rocks from Jucurutu Formation. The São Francisco auriferous mineralization has as host rocks mica-schists from Seridó Formation, while the Ponta da Serra and Fazenda Simpático mineralizations are hosted in orthogneisses of this fold belt basement. The research conducted on these mineralizations had the purpose of integrate the data of chemical elements behavior during the shearing/mineralizing event, and its influence on the isotopic systems Rb-Sr and Sm-Nd. The studies of chemical mobility in the auriferous mineralizations showed that elements that during the shearing displayed in general an immobile behavior were Al, Ti and Zr. Among the elements that were mobilized during the event, K and Rb showed mass gain in ali belts of transformed rocks, while the elements Ca, Na and Sr normally lost mass. Petrographic studies showed that the minerais biotite and plagioclase, in all investigated mineralizations, played an important role in the chemical reactions occurred in the transformed rocks to the generation of muscovite, cordierite and sillimanite, justifying the input of K to the formation of muscovite, and the release of Na and Ca from plagioclase to the fluid phase. In the São Francisco auriferous mineralization, the results of the Rb-Sr isotopic analysis yielded ages of 645 ± 19 Ma and 596 ± 17 Ma, with both samples, from original and transformed rocks. Two ages, 569 ± 20 Ma. and 554 ± 19 Ma., were obtained with samples frem the transformed rocks domain. These ages suggest that there were two metamorphic pulses during the emplacement of the mineralized shearing zone. The Sm-Nd data yielded TDM ages of 1,31 Ga and 1,26 Ga with 3Nd (0,6 Ga) of -0,26 e -0,40 for the original and final transformed rocks, respectively. In case of the orthogneisses of Caicó Complex, e.g. the Ponta da Serra and Fazenda Simpático mineralizations, the Rb-Sr data did not yield ages with geological significance. In the Ponta da Serra mineralization, the Sm-Nd isotopic data yielded T DM ages of 2,56 Ga and 2,63 Ga to the original rocks and of 2,71 Ga to the mineralized sheared rock, and values of 3Nd (2,0 Ga) between -3,70 e -5,42 to the original and sheared rock, respectively. In the Fazenda Simpático, Sm-Nd data yielded TDM between 2,65 and 2,69 Ga with values of 3Nd (2,0 Ga) between -5,25 e -5,52. Considering the Sm-Nd data, the TDM ages may be admitted as the age of the parental magma extraction, producer of the protoliths of the orthogneisses from Ponta da Serra and Fazenda Simpático mineralizations. The chemical mobility studies showed that in the basement hosted mineralizations, Rb achieved mass while Sr lost mass, as Sm as well as Nd were strongly mobilized. The Sm/Nd ratio remained constant, however, confirming the isochemical character of those elements. In the basement mineralizations, Rb-Sr ages are destituted of geological significance, because of the partial opening of the isotopic system during the tectono-metamorphic transformations. In the tungsten mineralization, the diagram Sm-Nd constructed with the whole-rock data of calcsilicatic and the high-temperature paragenesis (garnet, diopside and iron-pargasitic hornblende) indicated an 631 ± 24 Ma age, while with the whole-rock data and low-temperature paragenesis (vesuvianite, epidote and calcite), a 537 ± 107 Ma age was obtained. These ages, associated with the petrographic observations, suggest that there was a time gap among the hydrothernal events responsible by the formation of the high and low temperature paragenesis in the calcsilicatic rocks mineralized in scheelite

Rb, Sr, Sm and Nd concentrations and isotopic composition of basalts from DSDP Leg 74 (Table 1)

Basement intersected in DSDP holes 525A, 528 and 527 on the Walvis Ridge consists of submarine basalt flows and pillows with minor intercalated sediments. These holes are situated on the crest and mid and lower northwest flank of a NNW-SSE-trending ridge block which would have closely paralleled the paleo mid-ocean ridge (Rabinowitz and LaBrecque, 1979 doi:10.1029/JB084iB11p05973, Moore et al. (1983 doi:10.1130/0016-7606(1983)94<907:TWRTDS>2.0.CO;2). The basalts were erupted approximately 70 m.y. ago, an age equivalent to that of immediately adjacent oceanic crust in the Angola Basin and coraistent with formation at the paleo mid-ocean ridge (Moore et al., 1983). The basalt types vary from aphyric quartz tholeiites on the ridge crest to highly plagioclase phyric olivine tholeiites on the ridge flank. These show systematic differences in incompatible trace element and isotopic composition. Many element and isotope ratio pairs form systematic trends with the ridge crest basalts at one end and the highly phyric ridge flank basalts at the other. The low 143Nd/144Nd (0.51238), 206Pb/204Pb (17.54), 207Pb/204Pb (15.47), 208Pb/204Pb (38.14) and high 87Sr/86Sr (0.70512) ratios of the ridge crest basalts suggest derivation from an old Nd/Sm-, Rb/Sr- and Pb/U-enriched mantle source. This isotopic signature is similar to that of alkaline basalts on Tristan da Cunha but offset to significantly lower Nd and Pb isotopic ratios. The isotopic ratio trends may be extrapolated beyond the ridge flank basalts with higher 143Nd/144Nd (0.51270), 206Pb/204Pb (18.32), 207Pb/204Pb (15.52), 208Pb/204Pb (38.77) and lower 87Sr/86Sr (0.70417) ratios in the direction of increasingly Nd/Sm-, Rb/Sr- and Pb/U-depleted source compositions. These isotopic correlations are equally consistent with mixing of depleted and enriched end member melts or partial melting of an inhomogeneous, variably enriched mantle source. However, observed Zr-Ba-Nb-Y interelement relationships are inconsistent with any simple two-component model of magma mixing, as might result from the rise of a lower mantle plume through the upper mantle. Incompatible element and Pb isotopic systematics also preclude extensive involvement of depleted (N-type) MORB material or its mantle sources. In our preferred petrogenetic model the Walvis Ridge basalts were derived by partial melting of mantle similar to an enriched (E-type) MORB source which had become heterogeneous on a small scale due to the introduction of small-volume melts and metasomatic fluids.