The Median Tectonic Zone in Eastern Fiordland, SW New Zealand, comprises a tectonically disrupted belt of Mesozoic magmatic arc rocks related to subduction along the palaeo-Pacific margin of Gondwana. New ion microprobe (SHRIMP) U-Pb zircon ages confirm that the bulk of the plutonic rocks in eastern Fiordland range from Mid-Jurassic to Early Cretaceous (168-137 Ma) in age. Carboniferous age granitoids occur in SW Fiordland, along the western side of, and within the zone. Triassic plutonic rocks appear to be restricted to the eastern side of the zone. The Mid-Jurassic-Early Cretaceous igneous rocks (collectively referred to as the Darran Suite) are cut by several plutons of Na-rich granitoid (Separation Point Suite) that give ages of c. 124 Ma, slightly older than equivalent rocks in the NW part of the South Island. Early Cretaceous granulite facies orthogneisses (126-119 Ma) in western Fiordland (Western Fiordland Orthogneiss) are considered to be the lower crustal equivalent of the Separation Point plutons.The majority of the Darran Suite rocks are I-type, hornblende-bearing calc-alkaline igneous rocks, most likely derived from melting in the mantle wedge above a subducting slab of oceanic lithosphere. In contrast, the Separation Point-type plutons are Na-rich, alkali-calcic granitoids with high concentrations of Sr (typically >500 ppm and up to 1000 ppm) and low concentrations of Y (c5 ppm) and heavy REE (<10 times chondritic). Isotopic compositions are primitive, with 87 Sr/ 86 Sr initial ratios of c. 0.7038, and Nd values of c. +3 at 120 Ma. Their geochemistry is consistent with melting of a mafic protolith of garnet amphibolite mineralogy. Mafic Darran Suite rocks have the appropriate chemical and isotopic compositions to generate the Western Fiordland Orthogneiss and the higher level Separation Point type plutons. We suggest that the sudden appearance of large volumes of Na-rich magma during the Early Cretaceous was triggered tectonically, perhaps by thrusting of the Median Tectonic Zone arc beneath western New Zealand. Melting of basal arc underplate at depths of >40 km would then have generated Na-rich granitoids, leaving residues of garnet+clinopyroxene+amphibole.
The Early Cretaceous Separation Point batholith of the South Island, New Zealand, represents the final magmatic stage of an extensive arc system located on the SW Pacific margin of Gondwana during the Mesozoic. The batholith consists of Na-rich, alkali-calcic diorite to biotitehornblende monzogranite. The rocks are distinct from calc-alkaline subduction-related granitoids, but comparable with those of adakite and Archaean trondhjemite-tonalite-dacite suites.Primitive Sr and Nd isotopic ratios and the absence of inherited zircon, indicate that the granitoids experienced little, if any, interaction with felsic crust. Their geochemistry is consistent with melting of a basaltic protolith of amphibolite mineralogy, either young, hot, subducted oceanic crust or newly underplated material beneath a thickened continental arc. The latter model is preferred because Separation Point rocks do not posess MORB isotopic characteristics, and cannot be explained as mixtures of MORB-melt and continental crust. Most likely it involves melting of basal arc material in response to the collision and thrusting of the arc beneath the continental margin following subduction of a back-arc basin. On the basis of strong geochemical similarities, the Early Cretaceous Western Fiordland Orthogneiss of SW New Zealand is considered to be the lower crustal equivalent of the Separation Point batholith.
On 16 July 1945, the first atomic bomb was detonated at the Alamogordo Bombing range in New Mexico, USA. Swept up into the nuclear cloud was the surrounding desert sand, which melted to form a green glassy material called 'trinitite'. Contained within the glass are melted bits of the first atomic bomb and the support structures and various radionuclides formed during the detonation. The glass itself is marvelously complex at the tens to hundreds of micrometre scale, and besides glasses of varying composition also contains unmelted quartz grains. Air transport of the melted material led to the formation of spheres and dumbbell shaped glass particles. Similar glasses are formed during all ground level nuclear detonations and contain forensic information that can be used to identify the atomic device.At 05:29:45 am local time on Monday, 16 July 1945, the nuclear age began. In a most graphic example of Einstein's famous equation (E = mc 2 or the original form m = L/V 2 where L = mass and V = the speed of light) a plutonium bomb, referred to as the 'Gadget', with a yield of 21 kilotons (equivalent to the explosive power of 21 000 tons of TNT) was detonated ( Fig. 1) at the Alamogordo Bombing range, 210 miles south of Los Alamos, New Mexico. The genie was out of the bottle and has been our uneasy companion ever since. One of the products of this nuclear explosion was a green glassy material formed by melting of the surrounding desert sand. Robert Oppenheimer had chosen the name 'Trinity' for the nuclear test and the name 'trinitite' was adopted for this green glassy material. The nuclear explosionThere are two fissionable isotopes (U-235 and Pu-239) that can be used to make atomic (fission) bombs. In both cases the basic principle is the same. A neutron interacts with a nucleus which leads to an increase in atomic mass and an unstable nucleus that splits into several pieces (fission fragments) + neutrons with a concomitant release in energy due to a loss in mass. The 235 U fission reaction is: U-235 + neutron → U-236 → fission fragments + neutrons + energyThe ejected neutrons cause further fission events leading to a chain reaction. For an explosive nuclear reaction billions of fission events need to occur in microseconds. The key is to have a sufficient mass of U-235 or Pu-239 in close proximity for this explosive reaction to occur.Two types of nuclear weapons were developed near the end of World War II. One, the 'gun type' consisted of an enriched uranium (U-235) bullet that was fired at an enriched uranium spike. The impact of these two pieces of enriched uranium produced the necessary mass density for a thermal nuclear explosion. Physicists were convinced that this type of bomb would work and only one was built and subsequently dropped on Hiroshima, on 6 August 1945. Fig. 1. The only known colour photograph of the Trinity Test.
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