The first Himalayan Workshop held in Leicester University in 1985 was so successful that it was then agreed to hold the informal conference and workshop annually. Subsequently the Himalayan Workshops were held in Vandoevre-les-Nancy, France in 1986, London (1987), Lausanne, Switzerland (1988), Milan, Italy (1990) and Grenoble, France (1991). The upsurge in research throughout the Himalayan and Tibetan regions in the last ten years has warranted these annual meetings, and the dissemination of new research throughout the Himalayan community has been invaluable. This year the meeting was held in the Department of Earth Sciences, Oxford University, under the auspices of The Geological Society, London, from 6-8 April 1992. Over 120 participants from 15 countries attended. The Himalayan chain has long been recognized as the world's most spectacular example of a continent-continent collision belt resulting from the collision of the Indian plate with the central Asian landmass some 50 Ma ago. Since then India has moved northwards with respect to Asia by over 2000 km at an average speed of 50mm a −1 resulting in compressional tectonics not only in the Himalaya to the south of the main Indus suture zone, but also active compression in the mountain ranges north of the Indus suture: the Karakoram, Hindu Kush, Pamirs, Kun Lun and Tien Shan, as well as uplift of the 5 km high Tibetan plateau. Crustal shortening has mainly been accommodated by thrusting and folding in the Himalaya, by internal, diffuse lithospheric thickening within the Tibetan plateau, by limited, underthrusting of the
Abstract High‐ P granulites contained in two allochthonous tectonic units were thrust southwards onto the northern margin of the Zimbabwe craton during the Pan‐African Zambezi orogeny. In the lower sheet, the Masoso Metamorphic Suite contains mafic garnet granulite assemblages formed during a high‐ P‐T metamorphic event, although the suite as a whole is predominantly granitic. The garnet granulites occur as relicts within narrow mafic layers characterized by migmatitic and mylonitic fabrics. The annealed mylonites represent surfaces of deep‐crustal tectonic imbrication that formed immediately before the Pan‐African orogeny. Gabbros which intruded the granulites after the main phase of migmatization have formed corona textures that document a low‐ P‐T metamorphic event at mid‐crustal levels. The style of deformation then changed and the Masoso Suite with its mylonitic layers was folded and thrusted southwards onto the Archaean Zimbabwe craton.
The Caucasus mountain belt has a complex tectono-thermal history that is as yet poorly understood due to the scarcity of reliable geochronological information. Here, we report new monazite and zircon data from the Dzirula massif in the southern Caucasus, which permit development of a model for Variscan LP-HT (low pressure--high temperature) metamorphism in this region. Data are presented for two key lithologies of the Dzirula massif: 1) a group of variably deformed granitoid (mainly granodioritic-tonalitic) rocks and 2) metapelitic LP-HT cordierite-biotite-sillimanite migmatites and paragneisses. Electron-microprobe Th-U-total Pb monazite dating demonstrates that the LP-HT metamorphism in the Dzirula massif is Variscan, and occurred around 330 Ma. LA-ICP-MS zircon dating reveals that the granitoids include two different magmatic series. An older, mainly tonalitic, series is of Lower Cambrian age. Younger intrusions, including gabbros and diorites, but mainly tonalites and granodiorites, are Variscan in age. The older series show ductile deformation features and, along with the metapelites, experienced Variscan high-grade metamorphism that resulted in penetrative blastic recrystallization and anatexis. The younger series is generally undeformed, but often shows a magmatic foliation, and variable alteration under greenschist facies conditions. It is suggested that the Variscan intrusions facilitated the regional LP-HT metamorphic event at 330 to 340 Ma. Age and petrographic data from the Greater Caucasus imply a similar evolution as observed in the Dzirula massif. The Dzirula massif can thus be used as a proxy to model the evolution of the Caucasian Variscides.
B. E. Leake and P. W. G. Tanner. The Geology of the Dalradian and Associated Rocks of Connemara, Western Ireland: a Report to Accompany the 1:63 360 Geological Map and Cross-Sections Connemara. Dublin (Royal Irish Academy), 1994. 76 pp; 12 maps, 4 coloured geological maps (1:10560 &1:63 360). Price £15.00 - Volume 59 Issue 395
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ABSTRACT South of the Main Mantle Thrust in north Pakistan, rocks of the northern edge of the Indian plate were deformed and metamorphosed during the main southward thrusting phase of the Himalayan orogeny. In the Hazara region, between the Indus and Kaghan Valleys, metamorphic grade increases northwards from chlorite zone to sillimanite zone rocks in a typically Barrovian sequence. Metamorphism was largely synchronous with early phases of the deformation. The metamorphic rocks were subsequently imbricated by late north‐dipping thrusts, each with higher grade rocks in the hanging wall than in the footwall, such that the metamorphic profile shows an overall tectonic inversion. The rocks of the Hazara region form one of a number of internally imbricated metamorphic blocks stacked, after the metamorphic peak, on top of each other during the late thrusting. This imbrication and stacking represents an early period of post‐Himalayan uplift.
Abstract The Gara, Yalea, and Gounkoto Au deposits of the >17 Moz Loulo mining district, largely hosted by the Kofi series metasediments, are located several kilometers to the east of the 650-Mt Fe skarn deposits in the adjacent Falémé batholith. The Au deposits are interpreted to have formed through phase separation of an aqueous-carbonic fluid, which locally mixed with a hypersaline brine of metaevaporite origin. Recognition of an intrusive relationship between the Falémé batholith and Kofi series opens the possibility that the Fe skarns and Au deposits are part of the same mineral system. In this paper, we combine new δ13C, δ18O, and δ34S data from the Karakaene Ndi skarn, Au occurrences along the western margin of the Kofi series, and zircons within plutonic rocks of the Falémé batholith. We combine these with existing data from the Loulo Au deposits to model the contribution of magmatic volatiles to Au mineralization. C and O isotope compositions of auriferous carbonate-quartz-sulfide veins from the Loulo Au deposits have wide ranges (δ13C: –21.7 to –4.5‰ and δ18O: 11.8 to 23.2‰), whereas values from carbonate veins in Kofi series Au prospects close to the Falémé batholith and the Karakaene Ndi Fe skarn deposit have more restricted ranges (δ13C: –16.8 to –3.7‰, δ18O: 11.4 to 17.2‰, and δ13C: –3.0 ± 1‰, δ18O: 12.6 ± 1‰, respectively). Kofi series dolostones have generally higher isotopic values (δ13C: –3.1 to 1.3‰ and δ18O: 19.1 to 23.3‰). Pyrite from Kofi series Au prospects adjacent to the Falémé batholith have a wide range of δ34S values (–4.6 to 14.2‰), similar to pyrite from the Karakaene Ndi skarn (2.8 to 11.9‰), whereas δ34S values of pyrite and arsenopyrite from the Loulo deposits are consistently >6‰. Comparison of the C and O isotope data with water-rock reaction models indicates the Loulo Au deposits formed primarily through unmixing of an aqueous carbonic fluid derived from the devolatilization of sedimentary rocks with an organic carbon component. Isotopic data are permissive of the hypersaline brine that enhanced this phase separation including components derived from both Kofi series evaporite horizons interlayered with the dolostones and a magmatic-hydrothermal brine. This magmatic-hydrothermal component is particularly apparent in O, C, and S isotope data from the Gara deposit and Au prospects immediately adjacent to the Falémé batholith.
Mica and hornblende K‐Ar and Ar‐Ar data are presented from each of the three crustal components of the Himalayan collision zone in North Pakistan: the Asian plate, the Kohistan Island Arc, and the Indian plate. Together with U‐Pb and Rb‐Sr data published elsewhere these new data (1) date the age of suturing along the Northern Suture, which separates Kohistan from Asia, at 102–85 Ma; (2) establish that the basic magmatism in Kohistan, which postdates collision along the Northern Suture, predates 60 Ma, and that the later granite magmatism spanned a range of 60–25 Ma; (3) show that uplift amounts within Kohistan are greater toward the Nanga Parbat syntaxis than away from it and that rate of uplift near the syntaxis increased over the last 20 Ma to a current figure of about 5.5 mm a year; (4) show that much of southern Kohistan had cooled to below 500°C by 80 Ma and that the major deformation which imbricated Kohistan probably predated 80 Ma and certainly predated 60 Ma and was related to the Kohistan‐Asia collision rather than the Kohistan‐India one; (5) imply that uplift along the Hunza Shear in the Asian plate together with imbrication of the metamorphics in its hanging wall took place at about 10 Ma and was associated with breakback thrusting in the hanging wall of the Main Mantle Thrust; (6) suggest that the Indian plate has a lengthy pre‐Himalayan history with an early metamorphism at about 1900 Ma, major magmatism at 500–550 Ma and early Jurassic lithospheric extension or inversion; and (7) show that the Indian plate rocks were metamorphosed shortly after the collision within Kohistan, which occurred at circa 50 Ma, and subsequently cooled back through 500°C at circa 38 Ma and 300°C at 30–35 Ma with ages of cooling through 200° and 100°C (as determined by fission track data) locally controlled by Nanga Parbat related uplift tectonics.
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