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51.
Jane D. Sills David Savage Janet V. Watson Brian F. Windley 《Earth and Planetary Science Letters》1982,58(3):345-360
Layered ultramafic-gabbro bodies occur widely in the Archaean of northwest Scotland. They were metamorphosed at granulite or high amphibolite facies and were tectonically thinned and broken up during deformation. They comprise repeated ultramafic-gabbro layers, locally with Ni-poor sulphide-rich tops, each rhythmic unit showing decreasing MgO, Ni and normative anorthite with stratigraphic height. Major, trace and rare earth element data are presented for the range of rock types. In ultramafic rocks, MgO varies from 22 to 37 wt.%, Ni from 1000 to 2500 ppm and TiO2 from 0.08 to 0.40 wt.%, while the MgO content of the gabbros ranges from 14 to 6 wt.%. The REE patterns are flat to LREE enriched with no significant Eu anomalies. In ultramafic rocks REE are from 4 to 10 times chondrite, and in the gabbros LREE range from 8 to 30 times chondrite and HREE from 6 to 15 times chondrite. Study of incompatible elements (Ti, Zr, Y) which are relatively immobile during metamorphism shows that neither garnet nor hornblende were involved in fractionation. Trace element modelling shows it is improbable that the ultramafic rocks represent primary MgO-rich liquids even though their incompatible element contents are quite high. The chemical trends are interpreted in terms of olivine and pyroxene settling from a tholeiitic high-Mg magma with 15–20 wt.% MgO derived by 30–40% partial melting of an undepleted mantle. The ultramafic rocks are the cumulates and the gabbros the derived liquids. 相似文献
52.
H. R. Rollinson B. F. Windley M. Ramakrishnan 《Contributions to Mineralogy and Petrology》1981,76(4):420-429
The Archaean Karnataka craton of southern India contains Eastern and Western crustal blocks (separated by a major thurst) in which Sargur Schists occur as lenses within tonalitic Peninsular Gneisses. The Schist complex comprises pelites, quartzitic psammites, carbonates and calc-silicates, iron formations, and basic rocks, and thus provides many mineral assemblages ideal for the calculation of PT conditions. With their gneisses the Sargur rocks are unconformably overlain by the Dharwar greenstone belts, and are generally thought to be older than 3,000 my.In the Western block maximum metamorphic conditions are given by meta-basic rocks as 790±50° C and 13±2 kb, but adjacent meta-sediments give a pressure of 9 kb, suggesting that the differences in P and T recorded in this block mark a polychronic metamorphic geotherm related to the exhumation of the terrain by uplift and erosion. In the eastern block maximum temperatures were in the range 750°-850° C and maximum pressures were 7 kb. The rocks of the two blocks were sampled 100 km apart, and thus there was probably a regional pressure difference between the two blocks caused by differentiated crustal thickening prior to or during metamorphism.The shape of the geotherm from the Western block shows near-isothermal decompression over 20 km. Our data suggest that during Sargur metamorphism maximum crustal thicknesses were in excess of 45 km and that there was a minimum difference of 20 km in crustal thickness between the Eastern and Western blocks. 相似文献
53.
There is a linear relationship between the spacing of Pliocene-Pleistocene volcanoes and the thickness of the lithosphere and attenuated crust in the East African rift valley. Assuming that the physical-chemical properties of the Archaean and Cenozoic lithosphere and crust were broadly similar, we use the spacing of volcanic centres in the Abitibi greenstone belt of southern Canada to determine lithospheric and crustal thickness in the Archaean. The abitibi volcanoes have been deformed and so have elliptical cross-sections. In order to arrive at their original form we have removed the effects of tectonic strain by two alternative mechanisms of pure and simple shear which give comparable results. A mean original volcano spacing of 84–88 km suggests that the lithsophere was 80–90 km thick and that the crust was probably 35–45 km thick in this greenstone belt about 2700 m.y. ago. The crustal values are comparable with those determined by geochemical parameters and are consistent with the suggestion that greenstone belts formed in extensional marginal basins between crustal-thickened continental masses, deep sections of which are now seen in Archaean high-grade regions. 相似文献
54.
The Ballantrae ophiolite in southern Scotland includes a NEE–SWW-trending serpentinite mélange that contains blocks of mafic blueschist and high-pressure, granulite facies, metapyroxenite (Sm–Nd metamorphic age: 576 ± 32 and 505 ± 11 Ma). Tectonic blocks of mafic schist are less than 3 × 3 m in size, and have greenschist, blueschist or epidote amphibolite facies assemblages corresponding to the high-pressure intermediate-type metamorphic facies series.Adjacent rocks of the serpentinite mélange are hydrothermally-altered MORB-like ophiolitic basalt (prehnite–pumpellyite facies), dolerite (actinolite–oligoclase sub-facies) and gabbro (amphibolite facies), all with assemblages that are diagnostic of the low-pressure metamorphic facies series.The difference in metamorphic facies series and parageneses of minerals between the high-pressure mafic blocks and the adjacent, low-pressure ophiolitic meta-basic rocks suggests that the former were exhumed from > 25 km depth within a cold subducted slab, and were juxtaposed with the latter, the bottom of a MORB-like ophiolite in the hanging wall of a trench. An ENE–WSW-trending, 501 ± 12 Ma volcanic arc belt extends for 3 km south of the serpentinite mélange. We suggest that ridge subduction associated with a slab window created arc-related gabbro (483 ± 4 Ma) at Byne Hill and within-plate gabbro (487 ± 8 Ma) at Millenderdale. Final continental collision created the duplex structure of the Ballantrae complex that includes the HP blocks and serpentinite mélange. These relations define diapiric exhumation in the Caledonian orogen of SW Scotland. 相似文献
55.
T. Kawai B.F. Windley M. Terabayashi H. Yamamoto Y. Isozaki S. Maruyama 《Gondwana Research》2008,14(1-2):105
We report the first occurrence of ice-rafted dropstones in mid-oceanic sediments belonging to an ocean plate stratigraphy within a Neoproterozoic accretionary complex on Llanddwyn Island, Wales, UK. Dropstones of sandstone, chert, and basalt occur as matrix-supported exotic clasts in a 1 m-thick, hemi-pelagic mafic mudstone; the largest clast is 20 × 25 cm across. These dropstones occur specifically in hemi-pelagic mafic mudstone that is located at the structural top of ocean plate stratigraphy that records a ridge-trench transition; they are supplementary to dropstones associated with extensive tillites reported in shallow marine sequences of continental shelf facies and in back-arc basins. 相似文献
56.
Yichao Chen Brian F. Windley Ji’en Zhang Kefa Zhou Miao Sang 《International Geology Review》2017,59(9):1097-1115
ABSTRACTThe Tarbagatay Complex, located in northwest Junggar, is situated tectonically between the Zharma–Saur arc to the north and the Tacheng terrane and the Boshchekol–Chingiz arc to the south. This Complex belt is variably composed of ophiolitic mélange, sedimentary mélange, and coherent units of turbidites and shallow water sediments. These rocks crop out in fault-bound slices with fault-parallel asymmetric folds. Both the lithologies and deformation features of the Tarbagatay Complex suggest an accretionary origin generally with a top-to-the-south tectonic vergence, suggesting N-dipping subduction beneath the Zharma–Saur arc. The presence of a former ocean is indicated by the Ordovician ophiolite mélanges and related marine fossils. The time duration of the Tarbagatay Complex can be bracketed by detrital zircon ages of turbidites and shallow water sediments with a lower limit of major peak ages of 350–370 Ma, and an upper limit of middle Permian indicated by detrital zircon ages of 262.3 Ma. Based on these data, we suggest that the subduction of the Tarbagatay Ocean likely started in the Late Devonian and lasted until the middle Permian. Taking into account the formation of the northern part of the Kazakhstan orocline, which has a similar temporal-spatial framework, we propose a tectonic model for the western CAOB that involves accretion and amalgamation from the Ordovician to the middle Permian. 相似文献
57.
Kristoffer Szilas Vincent J. Van Hinsberg Alex F.M. Kisters J. Elis Hoffmann Brian F. Windley Thomas F. Kokfelt Anders Scherstén Robert Frei Minik T. Rosing Carsten Münker 《Gondwana Research》2013,23(2):436-451
The Tartoq Group, located in SW Greenland, consists of supracrustal rocks of mainly tholeiitic basaltic composition, including pillow lavas, sills/dykes and gabbros, as well as ultramafic rocks. Metamorphic grade ranges from greenschist facies to granulite facies. The Tartoq Group crops out as a series of blocks and slivers that are imbricated with originally intrusive Mesoarchaean TTG orthogneisses. The supracrustal rocks form part of a SE vergent fold and thrust belt consistent with the imbrication of TTG gneisses and supracrustal rocks along a convergent margin. LA-ICP-MS U–Pb zircon dating of an intrusive TTG sheet yields a minimum age of 2986 ± 4 Ma for the Tartoq Group. This age is consistent with MC-ICP-MS Lu–Hf and Sm–Nd isotopic whole-rock data for mafic samples from different blocks of the Tartoq Group, which yield errorchron ages of 3189 ± 65 Ma and 3068 ± 220 Ma, respectively. The mafic supracrustal rocks of the Tartoq Group have chondrite-normalized REE patterns with LaCN/SmCN of 0.67–1.96 and rather flat primitive mantle-normalized multi-element patterns, except for scattered LILE contents, and generally negative Nb-anomalies with Nb/Nb* of 0.26–1.31. Th/Yb varies between 0.06 and 0.47 and Nb/Yb between 0.45 and 4.4 indicative of an arc affinity when compared to rocks from modern settings. The similar geochemistry of the different lithological units, together with their coeval formation, as evident from trace element geochemical trends, supports a co-magmatic origin for the rock assemblage and their formation as imbricated relics of oceanic crust. Accordingly, we propose that the Tartoq Group represents remnants of Mesoarchaean oceanic crust, which formed in a suprasubduction zone geodynamic environment. 相似文献
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60.
Chunming Han Wenjiao Xiao Brian F. Windley Guochun Zhao Bexun Su Songjian Ao Jien Zhang Zhiyong Zhang Bo Wan Bin Cui Wenjun Qu Andao Du 《Resource Geology》2014,64(4):379-386
The Daheishan Mo deposit is located in the eastern part of the Central Asian Orogenic Belt, NE China. Rhenium and osmium isotopes of molybdenites from the Daheishan deposit were used to determine the age of mineralization. Rhenium concentrations in molybdenite samples are between 17 and 30μg g?1. Analysis of seven molybdenite samples yields an isochron age of 168.0 ± 4.4 Ma (2σ). Based on the geological history and spatial‐temporal distribution of the granitoids, it is proposed that the Mo deposits in eastern China were related to the subduction of the Paleo‐Pacific plate during Jurassic time. 相似文献