The Late Neoproterozoic granitoid magmatism of the Pelotas Batholith, southern Brazil

Geological mapping, petrography, geochemistry, and isotope studies enable the division of the Pelotas Batholith into six granitic suites: Pinheiro Machado (PMS), Erval (ES), Viamão (VS), Encruzilhada do Sul (ESS), Cordilheira (CS), and Dom Feliciano (DFS). The rocks of the PMS show a large compositional range (granite through granodiorite to tonalite), and the suite is considered pre- to syncollisional. Other suites show restricted compositional variations (granite to granodiorite) and are late to postcollisional. In general, the suites are metaluminous to slightly peraluminous (PMS, ES, and VS) or peraluminous (CS) or have alkaline tendencies (ESS and DFS). The magmatic evolution corresponds to high-K calc-alkaline to alkaline magmatism. The suites are enriched in K, Rb, and REE compared with rocks of typical calc-alkaline series. Initial ⁸⁷Sr/⁸⁶Sr ratios vary from 0.705 to 0.716, except in the CS, where they attain values of 0.732–0.740. Sm–Nd T_DM model ages vary between 0.98 and 2.0 Ga, with initial ε_Nd values ranging from −0.3 to −10. U–Pb zircon dates of samples from PMS, VS, and ESS suggest an age between 0.63 and 0.59 Ga for magmatism. Rb–Sr dates of samples of alkaline granites from DFS present ages between 0.57 and 0.55 Ga. The main tectonic controls of the magmatism of the Pelotas Batholith are high-dip sinistral shear zones.     

An Early Aged Ophiolite in the Western Kunlun Mts., NW Tibetan Plateau and Its Tectonic Implications

1 Introduction The Kuda ophiolite occurred in the western Kunlun Mountains, which lies about the intersection of longitude 77°10′ E and latitude 36°45′ N (Figs. 1, 2). The upper portion of the ophiolite mainly consists of a thick layer of basaltic pillow lavas, which was well exposed along the high way from Xinjiang Uygur Autonomous Region to the western Tibetan Plateau, and the middle-lower part, the mafic-ultramafic cumulates and upper mantle rocks occur at the top of the mountain n…     

The spatial distribution of trace elements in topsoil from the northern slope of Qomolangma (Everest) in China

The environment of Mt. Qomolangma (Everest) area is of great significance to the global environmental background and environmental change research. However, there are few studies on the content and distribution of soil trace elements in the area. About 130 soil samples were collected nearby the Rongbuk valley at the northern slope of the Qomolangma from 4,400 to 6,600 m elevations. Nine soil trace elements, Cr, Zn, Sr, Pb, Ni, Co, Cd, Mn, Cu, were analyzed with ICP-AES (inductively coupled plasma atom emission spectrometry). The results showed that soil trace elements content increased with altitude; the content of the Cd in this area was very high, which was 5.8 times of the average content of Chinese soil. There was a noticeable change point for soil trace elements content at the altitude of 5,800 m, and the content of Cd increased abruptly above 5,800 m. This point was just located at the boundary of two types of rocks. The Late Precambrian-Neoproterozoic granite–gneiss and metacryst migmatized interbedded with marble located below 5,800 m; black-dark slate and marl of Cambrian located above 5,800 m (including 5,800 m), the geochemical characteristic of different rocks was the main factors controlling the soil trace elements content in the northern slope of Qomolangma Mountain.     

The Neoproterozoic Kolet Um Kharit bimodal metavolcanic rocks, south Eastern Desert, Egypt: a case of enrichment from plume interaction?

Neoproterozoic metavolcanic rocks of Kolet Um Kharit (KUKh) in the southern Eastern Desert of Egypt have been traditionally regarded as a bimodal island-arc sequence. However, geological and geochemical arguments presented here make this interpretation doubtful. Geochemically, these rocks are classified into mafic (tholeiitic basalts) and felsic (high-K rhyodacites to rhyolites) groups. Both the KUKh mafic and felsic metavolcanic rocks show similar geochemical characteristics, implying a genetic link. They have comparable trace element ratios, such as Zr/Nb (27–30 vs. 20–36), Y/Nb (5.44–6.25 vs. 5.05–5.9), K/Rb (577–1164 vs. 573–937), Ba/La (4.29–25–9 vs. 11.4–16.2), Nb/Yb (1.82–2.03 vs. 1.76–1.99). Similarly both groups have parallel LREE-enriched patterns (La/Yb_CN=2.37–2.81 vs. 2.55–3.17); and negative Nb and Ta anomalies (Nb/La_pm=0.51–0.58 vs. 0.45–0.52 and Ta/La_pm=0.51–0.62 vs. 0.49–0.55). The observed negative Nb and Ta anomalies in the KUKh metavolcanic rocks cannot be attributed to crustal contamination or fractional crystallization. These rocks could represent either a remnant of break-up LIP or were derived from an enriched mantle source containing subduction components beneath an intraoceanic back-arc basin. The recognition of the KUKh rocks as derived from an enriched mantle source revives interest in models that involve enrichment from “plume” interaction during the evolution of the Arabian-Nubian Shield.     

锶、碳同位素演化在新元古代地层定年中的应用——以胶辽徐淮地层分区为例

根据Sr、C同位素地层学原理，利用胶辽徐淮地区晚前寒武纪地层Sr、C同位素数据，与国际上已有的Sr、C同位素演化曲线对比，结果表明，胶辽徐淮地区晚前寒武纪地层是同一时期的沉积物，沉积主体之间Sr、C同位素比值的对比性很好，沉积时限约在750-860Ma之间，并延续到震旦纪，为北方青白口纪及其后的沉积。同时也表明，Sr、C同位素演化相结合是解决缺乏大化石的晚前寒武纪地层对比的有效方法。     

阿尔金南缘榴辉岩带中花岗片麻岩的时代及构造环境探讨

阿尔金南缘江尕勒萨依一带榴辉岩的围岩花岗片麻岩具有高SiO2（〉70％）、高铝（Al2O3〉13％）和高碱（ALK=7．52-8．91）的特征，铝饱和度指数ASI=1．10～1．27，属于钙碱性过铝质花岗质岩石；岩石强烈富集大离子亲石元素Rb和Th，中等富集K和Ba，亏损高场强元素Nb，Zr，Hf，Y和Yb；具典型大陆碰撞型花岗岩相应的元素丰度特征；岩石的稀土元素含量较高，轻、重稀土分馏明显，总体呈右倾的“V”型稀土配分模式，显示壳源型花岗岩特征；在R1-R2构造环境判别图上落于同碰撞花岗岩区。锆石的阴极发光图像和微量元素分析结果显示了典型岩浆钻石的特征，利用LA-ICP-MS方法进行锆石微区U-Pb同位素定年，得到^206Pb/^238U加权平均年龄值为923±13Ma，表明江尕勒萨依花岗片麻岩的原岩形成时代为新元古代早期。阿尔金新元古代同碰撞花岗岩的确定，不仪明确了新元古代早期陆块汇聚在该区的存在，而且对区内新元古代早期造山作用的鉴别，重塑西部古大陆的演化历史、构造格架以及探讨它们之间的关系都有十分重要的意义，同时对于深入探讨区内高压-超高压岩石与围岩之间的相互关系提供了重要的约束资料。     

Geochemical evolution of magmatism in Archean granite-greenstone terrains

Evolution of Archean magmatism is one of the key problems concerning the early formation stages of the Earth crust and biosphere, because that evolution exactly controlled variable concentrations of chemical elements in the World Ocean, which are important for metabolism. Geochemical evolution of magmatism between 3.5 and 2.7 Ga is considered based on database characterizing volcanic and intrusive rock complexes of granite-greenstone terrains (GGT) studied most comprehensively in the Karelian (2.9–2.7 Ga) and Kaapvaal (3.5–2.9 Ga) cratons and in the Pilbara block (3.5–2.9 Ga). Trends of magmatic geochemical evolution in the mentioned GGTs were similar in general. At the early stage of their development, tholeiitic magmas were considerably enriched in chalcophile and siderophile elements Fe₂O₃, MgO, Cr, Ni, Co, V, Cu, and Zn. At the next stage, calc-alkaline volcanics of greenstone belts and syntectonic TTG granitoids were enriched in lithophile elements Rb, Cs, Ba, Th, U, Pb, Nb, La, Sr, Be and others. Elevated concentrations of both the “crustal” and “mantle-derived” elements represented a distinctive feature of predominantly intrusive rocks of granitoid composition, which were characteristic of the terminal stage of continental crust formation in the GGTs, because older silicic rocks and lithospheric mantle were jointly involved into processes of magma generation. On the other hand, the GGTs different in age reveal specific trends in geochemical evolution of rock associations close in composition and geological position. First, the geochemical cycle of GGT evolution was of a longer duration in the Paleoarchean than in the Meso-and Neoarchean. Second, the Paleoarche an tholeiitic associations had higher concentrations of LREE and HFSE (Zr, Ti, Th, Nb, Ta, Hf) than their Meso-and Neoarchean counterparts. Third, the Y and Yb concentrations in Paleoarchean calc-alkaline rock associations are systematically higher than in Neoarchean rocks of the same type, while their La/Yb ratios are in contrast lower than in the latter. These distinctions are likely caused by evolution of mantle magmatic reservoirs and by changes in formation mechanisms of silicic volcanics and TTG granitoids. The first of these factors was likely responsible for appearance of sanukitoid magmatic rocks in the Late Mesoarchean. Representative database considered in the work includes ca. 500 precision analyses of Archean magmatic rocks.     

The Neoproterozoic Hongliujing A-type granite in Central Tianshan (NW China): LA-ICP-MS zircon U–Pb geochronology,geochemistry, Nd–Hf isotope and tectonic significance

Located between the Turpan-Hami, Junggar and Tarim blocks, the Central Tianshan zone is an important component of the Central Asian Orogenic Belt (CAOB) and crucial linkage between the Siberian, Kazakhstan, Junggar, Turpan-Hami and Tarim blocks. The Hongliujing granite associated with Nb–Ta mineralization in the Central Tianshan zone, dated at ca. 740 Ma using zircon LA-ICP-MS dating, is the first reported Neoproterozoic intrusion with a reliable and precise age in the Chinese Central Tianshan. The Hongliujing granite shares all the characteristics of A-type granites. It contains predominant alkali feldspar, and is characterized by high contents of SiO₂, Na₂O + K₂O, K₂O and high field strength elements (such as Nb, Ta, Zr, Ga and Y), and low contents of CaO, MgO, Ba and Sr, with high FeO^t/(FeO^t + MgO) and Ga/Al ratios typical of A-type granites. Based on the geochemistry and zircon Hf isotope data, we propose that the Hongliujing granite was most likely produced by partial melting of basic rocks in the lower crust which may have been derived from mantle magmas. The Hongliujing granite belongs to A1-type granites, which indicate a rifting formation environment, suggesting that like the Tarim Block, the Central Tianshan zone recorded Neoproterozoic rift-related igneous events related to the breakup of the Rodinia supercontinent. Our study verifies that not only the Tarim Block is related to the breakup of the Rodinia supercontinent, but also it is true for some key blocks in CAOB such as the Central Tianshan. Our new geochemical and geochronologic data also support and strengthen the notion that the Central Tianshan zone may be a part of the Tarim Block.     

Syn‐rift mass flow generated ‘tectonofacies’ and ‘tectonosequences’ of the Kingston Peak Formation,Death Valley,California, and their bearing on supposed Neoproterozoic panglacial climates

The Kingston Peak Formation of the Pahrump Group in the Death Valley region of the Basin and Range Province, USA, is the thick (over 3 km) mixed siliciclastic–carbonate fill of a long‐lived structurally‐complex Neoproterozoic rift basin and is recognized by some as a key ‘climatostratigraphic’ succession recording panglacial Snowball Earth events. A facies analysis of the Kingston Peak Formation shows it to be largely composed of ‘tectonofacies’ which are subaqueous mass flow deposits recording cannibalization of older Pahrump carbonate strata exposed by local faulting. Facies include siltstone, sandstone and conglomerate turbidites, carbonate megabreccias (olistoliths) and related breccias, and interbedded debrites. Secondary facies are thin carbonates and pillowed basalts. Four distinct associations of tectonofacies (‘base‐of‐scarp’; FA1, ‘mid‐slope’; FA2, ‘base‐of‐slope’; FA3, and a ‘carbonate margin’ association; FA4) reflect the initiation and progradation of deep water clastic wedges at the foot of fault scarps. ‘Tectonosequences’ record episodes of fault reactivation resulting in substantial increases in accommodation space and water depths, the collapse of fault scarps and consequent downslope mass flow events. Carbonates of FA4 record the cessation of tectonic activity and resulting sediment starvation ending the growth of clastic wedges. Tectonosequences are nested within regionally‐extensive tectono‐stratigraphic units of earlier workers that are hundreds to thousands of metres in thickness, recording the long‐term evolution of the rifted Laurentian continental margin during the protracted breakup of Rodinia. Debrite facies of the Kingston Peak Formation are classically described as ice‐contact glacial deposits recording globally‐correlative panglacials but they result from partial to complete subaqueous mixing of fault‐generated coarse‐grained debris and fine‐grained distal sediment on a slope conditioned by tectonic activity. The sedimentology (tectonofacies) and stratigraphy (tectonosequences) of the Kingston Peak Formation reflect a fundamental control on local sedimentation in the basin by faulting and likely earthquake activity, not by any global glacial climate.