马达加斯加石墨矿地质特征及其成因分析

马达加斯加石墨矿资源丰富,找矿潜力大。对马达加斯加东部及西南部3个典型石墨矿床进行综合对比研究,发现其石墨矿成因类型为区域变质型,矿石工业类型以大鳞片晶质石墨为主,自然类型可分为片岩型、片麻岩型及变粒岩型,石墨矿赋矿地层为Graphite岩系。对含矿岩石进行主量元素分析,认为其原岩为富含有机质、富铝的砂质、粉砂质、泥质沉积碎屑岩;形成环境为活动大陆边缘区域和大陆岛弧区域。马达加斯加石墨矿床的形成均经历了多次变质作用,富含有机质岩石主要通过区域变质作用形成晶质石墨矿床,并在混合岩化及深层次韧性剪切作用下进一步富集成矿。     

First postcranial remains of Multituberculata (Allotheria,Mammalia) from Gondwana

Multituberculates (Allotheria) are generally regarded as the evolutionarily most successful and longest-lived (Middle Jurassic to late Eocene) clade of Mesozoic and early Paleogene mammals. Despite this “reputation” and the fact that the group is particularly well represented in both taxonomic diversity and relative abundance on Laurasian landmasses during the Cretaceous and Paleocene, multituberculates are exceedingly poorly represented on the southern supercontinent Gondwana. Previous records on Gondwanan landmasses have been based on fragmentary dental remains and all except the three most recently published (each represented by a single isolated tooth or fragment of tooth) have been disputed and allocated to either Haramiyida or Gondwanatheria. Furthermore, several previous records, disputed or not, are based on fragmentary dental remains of a type (plagiaulacoid) that has evolved independently several times in mammalian evolution.Here we place on record a multituberculate femur from the Upper Cretaceous (Maastrichtian) Maevarano Formation of the Mahajanga Basin, Madagascar. This specimen, although fragmentary as well, exhibits a number of features common to all multituberculate femora: neck cylindrical in cross section and set apart from shaft; greater trochanter prominent, extending proximally beyond head, inclined dorsally, and separated from neck by deep incisure; lesser trochanter prominent and protruding ventrally; posttrochanteric fossa present on ventral aspect, lateral to lesser trochanter; subtrochanteric tubercle present on dorsal aspect, distal to incisure between greater trochanter and neck; diaphysis straight, elliptical in cross section (slightly compressed dorsoventrally); and third trochanter absent. Three of these features (prominent, ventrally placed lesser trochanter; presence of posttrochanteric fossa; presence of subtrochanteric tubercle) are regarded as autapomorphies of Multituberculata. This specimen therefore not only independently and conclusively confirms the presence of the clade on Madagascar—previously based on a small molar fragment—but on the entire supercontinent as well.     

Polycyclic aromatic hydrocarbons in commercial fish and lobsters from the coastal waters of Madagascar following an oil spill in August 2009

Concentrations of polycyclic aromatic hydrocarbons were determined in species of commercial fish and lobsters following an oil-spill just off the protected Madagascan coastline. Samples were collected along the coastline within and outside the affected area. Summed PAH concentrations ranged from 1.9 μg kg⁻¹ to 63 μg kg⁻¹ wet weight, but with no higher molecular weight PAHs (>202 Da) being detected. All concentrations of benzo[a]pyrene, benz[a]anthracene and dibenz[a,h]anthracene were <0.1 μg kg⁻¹ wet weight, well within the EU and UK set limits for the protection of human health. Additionally, samples were calculated as the benzo[a]pyrene toxic equivalency quotient (TEQ) and found to be well below the level of concern in relation to health of human consumers. Evaluation of the biota PAH data indicated the origin of PAH was predominantly petrogenic with >80% arising from oil sources. Profile studies indicate a low-level multisource petrogenic contamination probably representing a pre-spill background for the area.     

Gemstone Mineralization in the Palghat-Cauvery Shear Zone System (Karur-Kangayam Belt), Southern India

We summarize here the occurrence of a number of semiprecious stones within a major gemstone belt in the Palghat-Cauvery shear zone system close to the northern margin of the Madurai Granulite Block, southern India. The gem mineralization in this belt includes different varieties of corundum (star ruby, sapphire), cordierite (iolite), feldspar (moonstones and sunstone of various hues), beryl (emerald, aquamarine), chrysoberyl (alexandrite), kornerupine, topaz, spinel, crystal quartz and amethyst, among others. Gem mineralisation has not been directly dated, but is associated with pegmatites that crosscut rocks metamorphosed in late Neoproterozoic/Early Cambrian times. Similar aged gem mineralization occurs south of the Achankovil Shear Zone in southern India, as well as in Sri Lanka and Madagascar and strengthen the view of a broad late Neoproterozoic-early Cambrian gemstone province in central Gondwana. The ruby mineralization within this belt can be correlated with similar occurrences in eastern Madagascar, supporting a correlation between the Malagasy Betsimisaraka suture zone and the Palghat-Cauvery shear zone system in southern India.     

Juxtaposition of India and Madagascar: A Perspective

Proterozoic terrains in South India and Madagascar provide important clues in understanding the Gondwanaland tectonics, especially the assembly of this mega-continent during the Pan-African period. The Archaean terrains in both Madagascar and India are characterized by N-S trending greenstone belts occurring within gneissose granitic rocks in the northern part. Extensive development of K-rich granitic rocks of ca. 2.5 Ga is also characteristic in both areas. Such a broad age zonation of younger Dharwar (ca 2.6–3.0 Ga) in the north and the older Sargur (ca 3.0–3.4 Ga) in the south as in South India remains to be identified in future studies from Madagascar. The occurrence of greenschist facies rocks in the northeastern part and higher grade rocks in most of other parts in the north-central terrain of Madagascar is comparable with the general tendency of increasing metamorphic grade from northwestern to southern areas ranging from greenschist to granulite facies in South India. The Proterozoic crystalline rocks in both continents show pronounced lithological similarity with the wide occurrence of graphite-bearing khondalite in association with charnockitic rocks. While the Archaean-Proterozoic boundary is well defined in southern India by the Palghat-Cauvery or the KKPT shear zones as recently identified, this boundary is ill-defined in Madagascar due to extensive Pan-African overprinting, as well as the development of the Proterozoic cover sequence, the Itremo Group. There is also a possible general correlation between the Mesoproterozoic cover sequences in Madagascar and India, such as between the Itremo Group of west-central Madagascar and the Kaladgi and Cuddapah sequences of South India. The Pan-African granulite facies metamorphism of ca. 0.5 Ga extensively developed in both India and Madagascar is generally comparable in intensity and extent. P-T conditions and P-T-t paths also appear comparable, with the general range of ca. 700–1000°C and 6–9 kb, and near-isothermal decompressional paths. A-type granite plutons and alkaline rocks including anorthosites and mafic plutonic rocks of ca. 500–800 Ma develop in both terrains, provide strong basis for the correlation of both terrains, and define a Pan-African igneous province within East Gondwanaland. Major shear zones in both continents are expected to play a critical role in the correlation, albeit are still poorly constrained. Detailed elucidation of the tectonic history of the shear zones, and the timing of various events along the shear zones would provide important constraints on the correlation of the two continental fragments.     

Gold in Madagascar

Gold is commonly found throughout the Precambrian basement exposed in the eastern two-thirds of Madagascar, composed of Archean and Proterozoic greenstone belts and metasediment belts. Gold occurs in four main kinds of deposits:- as a diffuse component of the crystalline basement rocks, - in concordant quartz veins within the metamorphic rocks of the Precambrian basement, - in recent discordant veins, - and in recent and ancient alluvial deposits.The initial gold mineralisation is probably in relation with the greenstone belt formation, and many have been formed and/or reworked during different metamorphic and magmatic events. It is therefore difficult to obtain an accurate age of the different kinds of deposits.     

Geological evolution of the Antongil Craton,NE Madagascar

The Antongil Craton, along with the Masora and Antananarivo cratons, make up the fundamental Archaean building blocks of the island of Madagascar. They were juxtaposed during the late-Neoproterozoic to early Palaeozoic assembly of Gondwana. In this paper we give a synthesis of the geology of the Antongil Craton and present previously published and new geochemical and U–Pb zircon analyses to provide an event history for its evolution.     

Comparison of the Jurassic and Cretaceous sedimentary cycles of Somalia and Madagascar: implications for the Gondwana breakup

The Jurassic and Cretaceous sedimentary history of northern Somalia and the Morondava Basin of south-western Madagascar have been studied. Both regions display an independent facial development; however, a comparison of the sequential evolution of the Mesozoic sedimentary successions in these two presently widely separated areas reveals a surprisingly high level of similarity, which probably reflects major events during the disintegration of Eastern Gondwana during the Jurassic and Cretaceous. Although in Jurassic times the onset of transgressions and regressions in both areas compares well with eustatic development, major deviations in combination with the tectonic activities of different degrees are observed in the Early and Late Cretaceous synchronously in both regions. Transgressions are observed in Toarcian, Bajocian (not dated in northern Somalia), Callovian, Valanginian (Madagascar only), Aptian and Campanian times. Tectonism is noted before the Aptian and Campanian transgressions in northern Somalia and the Morondava Basin of south-western Madagascar.     

Leimavo Revisited: Agrarian Land-Use Change in the Highlands of Madagascar

Describing and explaining land-use change is of critical concern in Madagascar, where land transformations such as deforestation and resulting environmental degradation currently capture widespread attention. While the eastern rain forest recedes in the face of swidden cultivators, the highlands demonstrate more constructive transformations. In this paper I present a case study of land-use change in Leimavo, a small village near Ambositra studied in the 1960s by Jean-Pierre Raison. Here, the twentieth century has seen a gradual reduction in irrigated rice cultivation and cattle husbandry, and a boom in market-oriented orange, vegetable, and grain production. In the long term, a historical landscape of grassy hills has been transformed into a productive cultural landscape with woodlots, anti-erosion benching, rice terraces, fruit groves, and diverse crops. Critical factors determining the trajectory of land-use change include regional population pressure, state policies, market incentives, climate variations, and access to land and water resources. These critical factors, or explanations, are linked in the discussion by the use of a simple heuristic device—the range of choice—as a theoretical framework.