Geological evolution of the Neoproterozoic Bemarivo Belt, northern Madagascar

The broadly east-west trending, Late Neoproterozoic Bemarivo Belt in northern Madagascar has been re-surveyed at 1: 100 000 scale as part of a large multi-disciplinary World Bank-sponsored project. The work included acquisition of fourteen U-Pb zircon dates and whole-rock major and trace element geochemical data of representative rocks. The belt has previously been modelled as a juvenile Neoproterozoic arc and our findings broadly support that model. The integrated datasets indicate that the Bemarivo Belt is separated by a major ductile shear zone into northern and southern “terranes”, each with different lithostratigraphy and ages. However, both formed as Neoproterozoic arc/marginal basin assemblages that were translated southwards over the north-south trending domains of “cratonic” Madagascar, during the main collisional phase of the East African Orogeny at ca. 540 Ma. The older, southern terrane consists of a sequence of high-grade paragneisses (Sahantaha Group), which were derived from a Palaeoproterozoic source and formed a marginal sequence to the Archaean cratons to the south. These rocks are intruded by an extensive suite of arc-generated metamorphosed plutonic rocks, known as the Antsirabe Nord Suite. Four samples from this suite yielded U-Pb SHRIMP ages at ca. 750 Ma. The northern terrane consists of three groups of metamorphosed supracrustal rocks, including a possible Archaean sequence (Betsiaka Group: maximum depositional age approximately 2477 Ma) and two volcano-sedimentary sequences (high grade Milanoa Group: maximum depositional age approximately 750 Ma; low grade Daraina Group: extrusive age = 720 to 740 Ma). These supracrustal rocks are intruded by another suite of arc-generated metamorphosed plutonic rocks, known as the Manambato Suite, 4 samples of which gave U-Pb SHRIMP ages between 705 to 718 Ma. Whole-rock geochemical data confirm the calc-alkaline, arc-related nature of the plutonic rocks. The volcanic rocks of the Daraina and Milanoa groups also show characteristics of arc-related magmatism, but include both calc-alkaline and tholeiitic compositions. It is not certain when the two Bemarivo terranes were juxtaposed, but ages from metamorphic rims on zircon suggest that both the northern and southern terranes were accreted to the northern cratonic margin of Madagascar at about 540 to 530 Ma. Terrane accretion included the assembly of the Archaean Antongil and Antananarivo cratons and the high-grade Neoproterozoic Anaboriana Belt. Late- to post tectonic granitoids of the Maevarano Suite, the youngest plutons of which gave ca. 520 Ma ages, intrude all terranes in northern Madagascar showing that terrane accretion was completed by this time

Geology of the Fujairah 1: 50 000 map sheet 50-4, United Arab Emirates

The Fujairah 1:50 000 geological map sheet covers a segment of the Hajar Mountains extending from the Gulf of Oman around the city of Fujairah in the east, to the desert plains of the Rub’ Al Khali in the west. The solid geology predominantly comprises rocks of the Oman-UAE ophiolite complex, with a small area in the centre-north underlain by medium- to high-grade metamorphic rocks (Bani Hamid Group). The ophiolitic rocks comprise an earlier magmatic suite of mantle harzburgite and dunite, overlain by a classic spreading-ridge crustal section of layered gabbro, through “high-level” gabbro (mainly isotropic to varitextured gabbro), to a Sheeted Dyke Complex and topmost pillow basalt. These were intruded by a later magmatic suite comprising some dunite, a predominantly ultramafic “Mixed Unit” (including peridotites and pyroxenites), voluminous wehrlite bodies, and a suite of younger gabbroic rocks and associated minor intrusions (various mafic dykes, tonalites and bodies of magmatic breccia). The metamorphic rocks occur as thin tectonic slices within a major north-west-trending fault zone. They fall within the P4 unit of the Bani Hamid Group of rocks and comprise quartz schists and quartzites, with minor amphibolites and calc-silicate rocks, metamorphosed under amphibolite facies conditions. The solid geology is partially covered by Miocene-Pliocene and Quaternary deposits. These are most extensive in the west, where the Hajar Mountains descend onto the desert plains and comprise palaeo-alluvial fan sands and gravels deposited on variably silicified serpentinite and mantle peridotites. These have been incised by Quaternary fluvial systems and are patchily overlain by Recent alluvial fan deposits. Elsewhere, the Quaternary deposits consist of alluvium and dissected river terraces, with scree and alluvial cones developed on the mountain slopes. Coastal zone deposits of the eastern seaboard include raised beaches and sabkhas, locally overlain by broad, low-angled alluvial fans issuing from the main wadis

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Established in 2000, the European Geoparks Network (EGN) aims to protect geodiversity, promote geological heritage to the general public and support sustainable economic development within the area of each geopark

Geology of the Bani Hamid 1:50 000 map sheet, 50-4, United Arab Emirates

The Bani Hamid 1:25 000 geological map sheet covers an area in the central parts of the Hajar Mountains, which lies within the United Arab Emirates and the western part of the Omani enclave of Madhah. The Bani Hamid district spans the boundary between the Khor Fakkan and Aswad blocks of the Oman-UAE ophiolite. This boundary is defined by the north-west–south-east-trending Wadi Ham Fault, a series of sub-parallel anastomosing ductile shear zones and brittle faults, which also contains tectonically-bounded slivers of metamorphic rocks (Bani Hamid Group). Indeed, the boundaries of this special sheet were defined with the specific purpose of focussing on the high-grade rocks of the Bani Hamid Group, which fall on two map sheets in the 1:50 000 scale geological series (Khor Fakkan and Fujairah). The sheet contains most of the outcrops of the Bani Hamid Group, which forms a north-west–south-east-trending series of outcrops in the western part of the sheet and in a north-east–south-west-trending mountain range to the north and east. The Bani Hamid Group is subdivided into four informal units, based on packages with differing proportions of quartzite, amphibolite and calc-silicate rocks/marble. A small part of the Aswad block of the Oman-UAE ophiolite is exposed in the south-west corner of the sheet. This is composed mainly of units from the Moho Transition Zone (MTZ) and the lower crustal section, while the larger part of the map, to the north-east falls within the Khor Fakkan block and is made up chiefly of mantle harzburgite, passing up through the MTZ and lower crust along the eastern margin of the sheet. The Bani Hamid Group rocks are tectonically interleaved between two major nappes of mantle harzburgite within the Khor Fakkan block, the southernmost of which is extensively invaded by bodies of S-type granite. The solid geology is partially covered by sparse, but locally thick, Quaternary and Recent deposits, mostly within the major wadi systems such as Wadi Ham

U-Pb zircon ages for the Loch Borralan and Flannan syenites, and their relationship to the Caledonian orogenic front [abstract]

The Caledonian Front has, for many years, been considered to be mid-Silurian in age. An important piece of evidence for this is the age of the Loch Borralan Pluton (430 ± 4 Ma; van Breemen et al., 1979), which lies within the Moine Thrust Zone. The Loch Borralan Pluton comprises early and late magmatic suites; the early suite is poorly exposed and its structural relationships are equivocal, but there is little doubt that the later suite post-dates thrust movement (Parsons and McKirdy, 1983, confirmed by recent BGS mapping). Rb-Sr and K-Ar dating of mylonites in the Moine Thrust Zone has yielded ages between c. 440 and c. 410 Ma (Freeman et al., 1998; Dallmeyer et al., 2001), suggesting that deformation at the Caledonian Front may extend into the Devonian. This evidence is in direct conflict with the accepted age and structural position of the Loch Borralan Pluton. We therefore set out to re-examine the age of both early and late magmatic suites. To date, we have analysed zircons from a sample of the late (postthrusting) suite, which has given a U-Pb age of 428.4 ± 0.4 Ma. A further sample, from Bad na h- Achlaise where syenites cut quartzites belonging to a thrust klippe (Parsons and McKirdy,1983), contains inherited zircon and has given only highly discordant zircon data. A third sample, from the early magmatic suite, awaits analysis. The new age of 428.4 ± 0.4 Ma represents a minimum age for movement on thrusts within the Moine Thrust Zone. In parallel, we have carried out a U-Pb zircon study of an alkaline igneous intrusion, the Flannan syenite, which intrudes crystalline basement 50 km to the west of the Outer Hebrides. This intrusion has been modelled geophysically as saucer-shaped, with a maximum width of 9 km and depth extent of the order of 1 km. Its U-Pb age (427.7 ± 0.3 Ma) and geochemistry indicate close similarities to the Loch Borralan leucosyenite. The location of the Flannan syenite is enigmatic, as it lies 135 km to the NW of the Moine Thrust Zone (after allowing for post-Caledonian extension), in an area normally considered to lie well to the west of the Caledonian Front. This suggests that the influence of the Caledonian Orogeny may extend significantly further to the NW than previously recognised