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Precambrian Continental Growth and Tectonism
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A revised cross section through the Selkirk fan structure provides the basis for a new model for the Middle Jurassic tectonic evolution of the southern Omineca belt, Canadian Cordillera. Palinspastic restoration of this cross section shows that the southwest-verging structures along the west flank of the Selkirk fan structure formed as a result of tectonic wedging of distal North American strata (Clachnacudainn complex) beneath more proximal North American strata, and that the Selkirk fan structure developed outboard from a crustal ramp (Dogtooth high) inherited from Late Proterozoic-early Paleozoic rifting along the western margin of North America. The first episode of Mesozoic deformation in southeastern British Columbia occurred between 187 and 173 Ma and involved the northeastward juxtaposition of the Intermontane superterrane over the outer part of the North American continental terrace wedge. It resulted in deep burial (20-25 km) of the outer margin of North America. A crustal ramp, localized along the western edge of the Late Proterozoic-early Paleozoic Dogtooth high, impeded the northeastward propagation of the orogenic wedge comprising the Intermontane superterrane and the imbricate, underlying northeast-verging thrust sheets of North American supracrustal rocks. Tectonic wedging, involving southwest-verging deformation, occurred within the orogenic wedge, and the resulting crustal thickening established sufficient topography and gravitational potential to drive the propagation of the deformation eastward into the Dogtooth Range and the Rocky Mountains. The southwest-verging structures along the west flank of the Selkirk fan developed between approximately 173 and 168 Ma concurrent with synorogenic extension and ~10 km of exhumation. The initial subsidence of the foreland basin during Kimmeridgian time (ca. 154 Ma) provides the first indication of tectonic loading and lithospheric flexure of the North American plate. It is interpreted to mark the time at which the orogenic wedge overrode the crustal ramp of the Dogtooth high and advanced onto relatively thick and rigid continental lithosphere. The tectonic model proposed for the Selkirk fan structure illustrates how the configuration of the rifted margin influenced the style of crustal thickening during subsequent compressional deformation.
Here, we report the occurrence of kornerupine-bearing, quartz-free granulite from the Malial area of Karimna- gar district, Andhra Pradesh. It occurs as small e n- claves and pods within the granite-gneiss, associated with garnet-orthopyroxene-cordierite-biotite-gneiss. Its mineral assemblage includes kornerupine-cordierite- biotite-spinel, K-feldspar, ilmenite and magnetite. Kornerupine, a hydrated magnesium-aluminium sili- cate plots on the 4MgO-3Al2O3-4SiO2 (4 : 3 : 4) co m- position along the solid solution join b etween 4 : 3 : 4 and 1 : 1 : 1 (3.5MgO.3.5Al 2O3.3.5SiO2) compos itions. The relative XMg values among various minerals are as follows: cordierite > biotite > kornerupine > spinel. The deduced post-peak metamorphic pressure-temperature conditions of 5 -6 kbar and 650-750° °C for the korne- rupine-spinel-bearing quartz-free granulites are con- sistent with the experimental work on stability of kornerupine in the MgO-Al2O3-SiO2-H2O system.
The A-type granitoids can be divided into two chemical groups. The first group (A1) is characterized by element ratios similar to those observed for oceanic-island basalts. The second group (A2) is characterized by ratios that vary from those observed for continental crust to those observed for island-arc basalts. It is proposed that these two types have very different sources and tectonic settings. The A1 group represents differentiates of magmas derived from sources like those of oceanic-island basalts but emplaced in continental rifts or during intraplate magmatism. The A2 group represents magmas derived from continental crust or underplated crust that has been through a cycle of continent-continent collision or island-arc magmatism.
The formation of platy olivine spinifex, the texture that characterizes komatiite lavas, has long been enigmatic. A major
problem is that the dendritic morphology of the olivine resembles that of crystals grown in laboratory experiments at high
cooling rates (>50°C/h), but at the position where these textures form, up to several meters below the komatiite flow top,
the cooling rate cannot have been greater than 1–5°C/h. We performed experiments that demonstrate that the platy habit of
spinifex olivine or pyroxene is a consequence of slow cooling of ultramafic magma in a thermal gradient (7–35°C/cm). The charges
were cooled at rates between 2 and 1428°C/h and, even at the low cooling rates, the thermal gradient led to constrained growth
and the development of preferentially oriented dendritic crystals with morphologies like those in natural platy spinifex-textured
lavas. Under these conditions, olivine starts to crystallize at temperatures well below the equilibrium liquidus temperature
(37°C < −ΔT< 56°C) depending on the composition of the starting material. When the cooling rate is high, the thermal gradient has a negligible
effect on the texture and the crystals have a random orientation, like that in the upper parts of komatiite flows.
The Archaean southern Bastar Craton of India is an integral part of the Singhbhum protocontinent and includes a suite of geochemically diverse Neoarchaean mafic volcanics that occur as well-exposed extrusive masses capping hills of granite gneiss. All mafic volcanics have undergone greenschist facies metamorphic conditions, but most have preserved original igneous textures. Geochemical data indicate the mafic volcanic rocks can be divided into three distinct varieties; sub-alkaline basalt (SAB), basaltic andesite (BA), and boninite (BON). We interpret the geochemical data to indicate that these volcanics are genetically related through fractionation of BON to BA and SAB.Regional geology, metamorphism, distinctive sedimentary records, and small negative Nb anomalies in the mafic volcanic geochemistry suggest they were deposited in a stable continental rift environment. We interpret the geochemical characteristics of the mafic volcanics to most likely reflect variations in source characteristics, together with minor crustal contamination, rather than the process of volcanic-arc magmatism. Geochemical modelling suggest that the primary BON composition is consistent with about 15–20% batch-melting of a mantle source. BA and SAB represent fractional crystallisation of olivine from a high-Mg basaltic or boninitic magma. Compatible and incompatible trace element modelling suggests that all three rock types probably originated from a lherzolite mantle source.
Garnet is widely found as a minor constituent in rocks of the Antarctic Peninsula Volcanic Group (APVG) of Trinity Peninsula. It occurs as conspicuous megacrysts, or in xenoliths within volcanic rocks of andesitic-rhyolitic composition and as detrital grains in the associated terrestrial sediments. It is also found as an accessory mineral in many plutonic rocks from the E coast of the Antarctic Peninsula. Evidence is presented to show that the garnet can be divided petrographically and chemically into two main groups: Type A: almandine-rich primary igneous garnet, and Type B: less almandine- and more pyrope-rich garnet as xenocrysts or included in xenoliths within the volcanic rocks.
Comparison with published experimental data on garnet occurrence in acidic igneous rocks suggests that high almandine-low spessartine garnet in volcanic rocks is a remnant phase of high pressure crystallization from magma at pressures of >7 kbar. Garnet with a higher pyrope content is regarded as xenocrystal in origin, having been derived from garnet-bearing country rocks at depth, either as accidental inclusions or through direct partial melting (restite) of the lower crust, and implies that a considerable thickness (>25 km) of crustal material was in existence before the generation of the Mesozoic magmatic arc. The origin of these calc-alkaline magmas may therefore be due, at least in part, to partial melting of pre-existing sialic crustal material.
Most of the existing major- and trace element-based discrimination diagrams are characterized by major defects such as subjective field boundaries, the constant-sum problem, and inadequacy of samples in creating them. Although major advances toward the solution of all these problems have been recently achieved through major element-based discriminant function diagrams, their applicability to old, altered rocks may be questionable. We present new discriminant function diagrams based on immobile trace elements and log-ratio transformation of the data of basic and ultrabasic rocks. These new diagrams are extremely successful in distinguishing the four tectonic settings (island-arc, continental rift, ocean island, and mid-oceanic ridge) and especially the plate margin (island-arc and mid-ocean ridge grouped together) and intra-plate or plate interior (continental rift and ocean island combined together) settings. The overall success rate for these natural log-transformed ratio-based diagrams using five trace elements (La, Sm, Yb, Nb, and Th), i.e., using four ratios ln(La/Th), ln(Sm/Th), ln(Yb/Th), and ln(Nb/Th), varies from 78.8% to 96.4%. Moreover, transitional tectonic settings such as interaction of a mid-ocean ridge with an ocean island or subduction of a ridge can be identified because basalts formed in these settings display linear trends in the present diagrams. Finally, the application of our new discrimination diagrams to altered, metamorphosed rocks from four widely separated areas lends further support regarding the usefulness of our proposal.
The high-grade metamorphic south Central Zone (sCZ) exposes an oblique crustal section through the internal parts of the Pan-African Damara belt in Namibia. The structural pattern of the south Central Zone is characterized by kilometer-scale, northeast-trending dome structures for which a number of different origins have been proposed. Detailed structural mapping of the Karibib and Usakos domes in the Karibib district indicates an origin of the two domes as large tip-line folds located above blind thrusts. The two domes are overridden from the southeast by a crystalline thrust sheet along the Mon Repos thrust zone and the thrusts and associated folds form part of a deeply-eroded, foreland- (northwest-) vergent fold-and-thrust belt. Intrusive relationships of syn- and post-tectonic granitoids constrain the timing of thrusting to between 550 and 540 Ma, corresponding to the main phase of collisional tectonics in the Damara belt.
The Eastern Ghats belt is a large high-grade terrane exposed along the east coast of India. The late Proterozoic orogen consists of N–S trending charnockite and metasedimentary belts. The younger E–W trending Godavari Rift divides the orogen into a northern and a southern segment. Textural and structural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. This orogenic belt represents an important part of the reconstructed global Southwest-United-States–East-Antarctica (SWEAT) orogen. The exact timing of metamorphism and tectonism in this large orogenic belt plays a key role in this global reconstruction. Textural and structural relationships indicate a complex thermo-tectonic evolution involving several episodes of metamorphism and deformation. In order to reconstruct parts of the regional thermal history of the Eastern Ghats belt, U–Pb ages were determined on monazite, allanite, zircon and sphene and 40Ar/39Ar ages on hornblende. Monazite, allanite and some sphenes provide evidence that in the central and eastern tectonic units north of the Godavari Rift the last high-grade metamorphism occurred at ca. 960Ma. In the most western unit, the Western charnockite zone, allanite and monazite from late pegmatites indicate a major thermal event around 1.6Ga. This is the first indication that there may be pronounced temporal differences in the thermal evolution of different tectonic or ltihological units in the Eastern Ghats belt. The majority of the sphenes from the Eastern Ghats belt are discordant and lie along a reference line extending from ca. 935Ma to 504Ma. This discordance is interpreted to be due to a thermal disturbance during a Pan-African deformation phase, which is dated by the emplacement of apatite–magnetite veins that contain zircon with a concordant U–Pb age of 516±1Ma. All hornblende 40Ar/39Ar ages from the central units of the belt provide Pan-African plateau ages. The partially reset sphene and completely reset hornblende ages indicate that the Pan-African metamorphism reached at least middle-amphibolite facies conditions in parts of the Eastern Ghats belt. This thermal overprint was not strong enough to cause retrogression of the high-grade mineral assemblages, but has so far only been recognized in the reset mineral ages and some late magmatic rocks. A hornblende 40Ar/39Ar age of ca. 1110Ma from an amphibolite south of the Godavari rift indicates that the Pan-African thermal overprint was weaker in the Western charnockite zone, if it affected this area at all. The new ages presented for the Eastern Ghats belt are similar to published ages for the Rayner Complex and the Prydz Bay region of Eastern Antarctica. This similarity in ages is strong support for these areas in East Antarctica and the Eastern Ghats belt being complementary parts of an extensive orogenic belt formed during the Grenvillian Orogeny around 1Ga. Both continental fragments also show evidence for a Pan-African overprint, however, this event is much more pronounced in Antarctica than in the Eastern Ghats belt.