Neoarchean and Rhyacian TTG-Sanukitoid suites in the southern São Francisco Paleocontinent,Brazil: Evidence for diachronous change towards modern tectonics

The southern portion of the Sao Francisco Palaeocontinent in Brazil is denoted by Archean nuclei and Paleoproterozoic magmatic arcs that were amalgamated during Siderian to Orosirian orogenic processes(ca.2.4-2.1 Ga).New isotopic U-Pb in zircon and Sm-Nd whole rock combined with major and trace element composition analyses constrain the crystallization history of the Neoarchean Piedade block(at ca.2.6 Ga) and the Paleoproterozoic Mantiqueira Complex(ca.2.1-1.9 Ga).These therefore display quite different magmatic histories prior to their amalgamation at ca.2.05 Ga.Sm-Nd and Rb-Sr isotopes imply a mixed mantle-crustal origin for the samples in both units.A complete Palaeoproterozoic orogenic cycle,from subduction to collision and collapse,is recorded in the Piedade Block and the Mantiqueira Complex.Rhyacian to Orosirian subduction processes(ca.2.2-2.1 Ga) led to the generation of coeval(ca.2.16 Ga)TTG suites and sanukitoids,followed by late(2.10-2.02 Ga) high-K granitoids that mark the collisional stage.The collisional accretion of the Mantiqueira Complex against the Piedade Block at 2.08-2.04 Ga is also recorded by granulite facies metamorphism in the latter terrane,along the Ponte Nova suture zone.The collisional stage was closely followed by the emplacement of within-plate tholeiites at ca.2.04 Ga and by alkaline rocks(syenites and enriched basic rocks) at ca.1.98 Ga,marking the transition to an extensional tectonic regime.The discovery of two episodes of TTG and sanukitoid magmatism,one during the Neoarchean in the Piedade Complex and another during the Rhyacian in the Mantiqueira Complex,indicates that the onset of subduction-related melting of metasomatized mantle was not restricted to Neoarchean times,as generally believed,but persisted much later into the Paleoproterozoic.     

Textural and chemical variation in plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens,USA

This study presents major- and trace-element chemistry of plagioclase phenocrysts from the 1980 eruptions of Mount St. Helens volcano. Despite the considerable variation in textures and composition of plagioclase phenocrysts, distinct segments have been cross-correlated between crystals. The variation of Sr and Ba concentration in the melt, as calculated from the concentration in the phenocrysts using partition coefficients, suggests the cores and rims crystallised from compositionally different melts offset by the plagioclase crystallisation vector. In both of these melts Sr and Ba are correlated despite the abundance of plagioclase in the 1980 dacites. We propose that rapid crystallisation of plagioclase upon magma ascent caused a shift in melt composition towards lower Sr and higher Ba, as documented in the rims of the phenocrysts. Although the cores of the phenocrysts crystallised at relatively shallow depths, they preserve the Sr and Ba of the deep-seated melts as they ascended from a deeper region. Further magma ascent resulted in microlite nucleation, which is responsible for a similar shift to even lower Sr concentration as observed in the groundmass of post-18 May 1980 samples. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

安徽淮北三铺地区镁夕卡岩矿床锶/钡比值分带

世界上含金镁夕卡岩矿床十分罕见。本文简单介绍了安徽淮北三铺地区含金多金属镁夕卡岩矿床的成矿基本地质条件、矿床富镁特征和交代矿化蚀变分带模式。重点研究锶、钡元素在矿床中的迁移规律以及锶／钡比值的分带性,在研究大量分析数据的基础上,分析Sr、Ba分别追随Ca、K元素迁移方向,发现Ba元素由内带向外带迁移、Sr元素由外带向内带的逆向迁移规律,首次指出该镁夕卡岩矿床中的Sr/Ba比值的分带性,确定出Sr／Ba比值0．3～3．0为矿（体）化交代蚀变带,可作为镁夕卡岩型矿床重要的地化找矿标志。     

东海中陆架泥质区及其周边底质中钙、锶、钡的地球化学特征

元素含量分析表明东海北部近岸泥质区以低Ｓｒ、相对低Ｃａ和最低的Ｓｒ／Ｂａ为特征;中陆架砂质区低Ｃａ、相对低Ｓｒ和相对低的Ｓｒ／Ｂａ;中陆架泥质区高Ｃａ、Ｓｒ、低Ｂａ和高的Ｓｒ／Ｂａ;外陆架砂质区具有最高的Ｃａ、Ｓｒ和Ｓｒ／Ｂａ。Ｃａ、Ｓｒ、ＰＩＣ三者关系密切,Ｂａ与Ｃａ、Ｓｒ、ＰＩＣ的相关性均差。Ｃａ、Ｓｒ具明显的亲自生性,主要赋存于海洋生源碳酸盐;Ｂａ具明显的亲它生性,主要赋存于硅酸盐矿物。Ｃａ、Ｓｒ、Ｂａ的地球化学特征受研究区物源和水动力环境的强烈制约。海洋生物地球化学过程对Ｃａ和Ｓｒ的地球化学特征有重要影响。中陆架泥质区是东海北部现代陆架Ｃａ、Ｓｒ的“汇”     

ICP-OES测定地球化学样品中La,Y,Ba

水系沉积物、土壤等试样用HF-HC1-HN03-HC1O4混酸消解后,以8%(V/V)HN03溶解干渣,建立了测定地球化学样品中La,Y,Ba 3种微量元素的电感耦合等离子体发射光谱(ICP-OES)方法.方法测定限(μg/L,10SD)为:La 6.0,Y 1.0,Ba 1.0.精密度(RSD%,n=11)为:0.20～1.50,经国家一级地球化学标准物质的分析验证,结果与标准值吻合,该方法现已用于区域化探试样的分析.     

南海北部晚第四纪高分辨率元素比值反映的东亚季风演变

选取大洋钻探ODP 184航次在南海北部钻取的1144站(20°3.18'N,117°25.14'E;水深2037m)上部岩芯为研究材料.通过X荧光光谱(XRF)岩芯扫描仪获取晚第四纪100ka以来时间分辨率高达10年的元素记录,并与德国"太阳号"95航次在同一站位获得的17940-2柱状样(20°07.0'N,117...     

辽南庄河—凤城断裂构造变形及其成矿作用

庄河－凤城断裂走向北东，形成于早中生代，属辽南区域性切壳断裂构造，规模大、变形强、活动期次多，对成岩成矿有重要控制作用。     

Barium Increasing Prior to Opal during the Last Termination of Glacial Ages in the Okhotsk Sea Sediments

Ba and Ti in a sediment core (10 m long) from the Okhotsk Sea, covering the last 120 kyrs, were measured. The authigenic Ba (Ba_ex) contents were calculated and compared with the opal. The correlation coefficient between Ba_ex and opal was quite small (r = 0.34), but it greatly grew larger (r = 0.90), if the Ba_ex contents were multiplied by a simple function increasing with depth, except for two intervals. This may be due to the gradual change in the sedimentation environment during the glacial ages. One of the exceptional interval is found at 60–170 cm in depth, corresponding to 10–17 calendar kyr ago, the last termination period of glacial ages, where the Ba_ex began to increase prior to opal. Since the calcium carbonate contents similarly increased prior to the opal increase, the Ba_ex may be also related to calcareous organisms besides siliceous ones. The other is the last interglacial period around 120 kyr ago when the opal contents were high, but those of the Ba_ex were not increased. This can be explained, if the Ba_ex was reduced to sulfide and dissolved away in a strongly anoxic environment during the biologically productive period. During the glacial ages, the mass accumulation rate (MAR) of lithogenic Ti was about twice the amount of that during the interglacial ages. For opal, however, the contrast between the glacial and interglacial ages was more remarkable in its MAR than in its concentration in sediments, due to the larger variation in the bulk sedimentation rates. This revised version was published online in July 2006 with corrections to the Cover Date.     

Gold-hosting high Ba-Sr granitoids in the Xincheng gold deposit,Jiaodong Peninsula,East China: Petrogenesis and tectonic setting

The Xincheng deposit is the only large gold deposit with a proven reserve of >200 t gold hosted by the Early Cretaceous granitoids in northwest Jiaodong Peninsula, East China. The granitoids hosting this ore deposit comprise an inner medium- to fine-grained quartz monzonite and an outer medium- to coarse-grained monzogranite with distinctive K-feldspar megacrysts. LA–ICP–MS zircon dating yields U–Pb ages of 128 ± 1 to 132 ± 1 Ma and 127 ± 2 to 129 ± 1 Ma, for the quartz monzonite and the monzogranite, respectively. The Early Cretaceous ages obtained in our study are comparable with the 126–130 Ma age range reported for the Guojialing granitic suite. The monzogranites, typical high Ba–Sr granites, possess high SiO₂ (70.89–73.35%), K₂O (3.85–4.32%), total alkalis (K₂O + Na₂O = 8.08–8.68%), Sr (634–888 ppm), Ba (1395–2111 ppm) and LREE (59.43–145.88), with low HREE and HFSE contents and insignificant Eu anomalies. The rocks display markedly high Sr/Y (114–297) and (La/Yb)_N (20–79) ratios. They have low MgO (0.23–0.62%), Cr (0.4–8.33 ppm) and Ni (0.47–2.92 ppm) contents. The typical high Ba–Sr signatures of the outer acidic monzogranites are also shared by the inner intermediate-acidic quartz monzonites, with a relatively higher abundance of these elements. The plagioclases in the quartz monzonites and monzogranites are oligoclase–andesine with An contents of 11.7–44.5%, and oligoclase with An contents of 12.9–29.3%, respectively, which both show the reverse zoning texture. The quartz monzonites have zircon ε_Hf(t) values of −21.3 to −13.9 (average −18.7), which are less negative and show larger variations than those of the monzogranites (ε_Hf(t) = −24.7 to −18.1, average −19.5). Detailed elemental, mineralogical and isotopic data suggest that the high Ba–Sr quartz monzonites and monzogranites were most likely generated by partial melting of the basement rocks of the Jiaobei terrane accompanied by crustal assimilation, with minor addition of the intermediate magma derived from the partial melting of juvenile mafic lower crust formed by the earlier underplating of mantle magma, and the quartz monzonites may represent the path of intermediate magma inputting into felsic magma. In combination with previous investigations, we suggest subduction of the paleo-Pacific slab beneath the North China Craton (NCC) and associated asthenosphere upwelling were most likely the mechanism associated with the generation of the high Ba–Sr granites.     

全新世黄土剖面Li/Ba值变化及其古气候意义

对陕西华县老官台全新世黄土和古土壤剖面的化学分析表明:Li和Ba两种化学元素含量受后期风化成壤作用的控制而发生分离,Li在风化剖面中残留而相对富集,表现为残留富集的特点;而Ba在整个剖面中的变化相当显著,随成壤强度的增强而大量淋失,导致其含量急剧减少,表现为极强的活动性。所以Li/Ba值能较好地反映风尘沉积物在后期经历的风化成壤强度。由于控制风尘沉积物成壤过程的主导因素是降水量的多少,而影响黄土高原及周边地区降水量的主导因素是东亚季风环流,因此Li/Ba值变化实际上反映了东亚季风系统的变化。Li/Ba值是再现古气候环境变化的良好替代指标,这在其与磁化率和Rb/Sr值良好的对应关系中得到有力的证实。