The dolerites of Tasmania are close kin to those of the Karroo and especially to those of Antarctica. They occur as sills, sometimes more than 1,000 feet thick, and as great dikelike intrusions a mile or more wide in the horizontally bedded Permo-Carbon-iferous and Trias-Jura sediments of the island. Their outcrops cover an area of more than 6,000 square miles and may previously have extended over twice that area. The sills, in particular, with their pronounced columnar structure, occupy a prominent place in the Tasmanian landscape. At the time of its intrusion the dolerite magma was completely liquid, and analyses of chilled margins from widely separated localities show that it was homogeneous in composition. It was a saturated or tholeiitic basalt magma but differed from typical tholeiites by being richer in SiO₂, A1₂O3, and CaO and poorer in FeO, TiO₂, Na₂O, K₂O, and P₂O₅. The only perfect match is provided by the Antarctic dolerites. The dolerites underwent fractional crystallization and gravitative differentiation after emplacement, which led to a progressive enrichment of the later-formed pyroxenes in iron as differentiation progressed. Two immiscible series of pyroxenes crystallized side by side throughout-at first, magnesia-rich augite and orthopyroxenes, later iron-enriched augites and stable iron-magnesia pigeonites ("plutonic" pigeonites). The enrichment of the pigeonites in iron progressed more rapidly than the enrichment of the augites in iron. This behavior and the immiscibility of the two series of pyroxenes can be explained by reference to their atomic structure and the much greater ease with which a small Fe++ ion can replace an Mg++ ion of similar ionic radius as compared with a much larger Ca++ ion. It follows from such an argument that the immiscibility gap found between magnesia-rich augites and magnesia-rich orthopyroxenes and clinoenstatites should extend between the iron-rich varieties as well. Supporting evidence that this is so is found in the behavior of the pyroxenes of the Mount Wellington sill, Tasmania, of the Skaergaard intrusive in East Greenland, of the extrusive rocks of the Huzi Volcanic Zone, Japan, and of the Palisade sill, New Jersey. From these data a new triangular diagram expressive of the courses of crystallization of pyroxenes is drawn. Identical fractional crystallization and gravitative differentiation have given a contrast in trends of differentiation in the dolerite sills compared with the dikelike bodies. This contrast arises from the different forms of the chambers in which differentiation occurred. In the closed chambers of the sills, sinking of early-formed magnesia-rich pyroxene caused a slight absolute enrichment in iron of the residual magma displaced into the upper parts of the sills. The iron-rich pyroxene that formed could not sink past the layer of accumulated early-formed minerals and so was retained in the upper parts of the sills to form peculiar iron-rich rocks. In the dikelike intrusions, which were subjacent bodies (in the sense denned by Daly), there was no layer of accumulated early-formed crystals to prevent the sinking of the later-formed iron-rich pyroxenes. As a result, the concentration of iron in the residual magma was dissipated, and the trend of differentiation in the dikelike masses gave rise to andesitic rocks that resemble in many respects the two-pyroxene andesites of Japan. This dependence of the trend of differentiation on the form of the differentiation chamber is of some significance in petrogenesis and reconciles apparently conflicting views like those held by C. N. Fenner and N. L. Bowen as to the ultimate effects of the enrichment of the residual liquids of basalt in iron-a trend which appears to characterize all basaltic magmas. Fenner's conclusion that this trend should give rise to absolute enrichment of the residual liquid in iron holds true for closed chambers of differentiation, of which the Skaergaard intrusion in East Greenland is the example par excellence. The same trend is found on a lesser scale in the dolerite sills of Tasmania and the Palisade sill of New Jersey. Bowen's conclusion that the enrichment of the residual liquids of basalt in iron leads merely to an enrichment of the later-formed ferromagne-sians in iron relative to magnesium but not to an absolute enrichment in iron holds true for basaltic magmas that differentiated in subjacent chambers. Thus an olivine-basalt magma gives rise to "ferro-gabbro" in a closed chamber, while a tholeiitic magma gives rise to peculiar iron-rich rocks in a closed chamber and to normal calc-alkaline rocks in a subjacent chamber. The relative volume of calc-alkaline rocks produced in this way is, however, no greater than the proportion of trachytic rocks produced from olivine-basalt, so that it seems unlikely that the great bulk of calc-alkaline rocks are derived simply by the differentiation of tholeiitic magma.
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A. B. Edwards (1942) studied this question.