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In comparison to the eastern Dharwar Craton, the mantle-derived xenocrysts/xenoliths are extremely rare or even unreported from the western Dharwar Craton, southern India. A Neoproterozoic (ca. 800-900 Ma) lamprophyre cropping out in the Mysuru area of southern India contains chrome-diopside xenocrysts (Cr2O3 content varying from 0.2 – 1.23 wt%) which provide important evidence about the pressure-temperature conditions and lithospheric thickness beneath the western Dharwar Craton. Studied chrome-diopsides show compositional zoning which is lacking in the liquidus phases (amphiboles and feldspars) of the lamprophyre which additionally favors a non-cognate origin of the former. Based on the compositional zoning, all the chrome-diopside xenocrysts can be divided into three groups: (i) Group I- which are euhedral and show reverse zoning with increasing Cr-content from core to rim; (ii) Group II- which are characterized by fractures and resorption textures, show complex reverse zoning and display up to three distinct compositional layers, and (iii) Group III- which evidence the reaction of chrome-diopsides with lamprophyric melt and are marked by alteration phases, such as actinolite and chlorite, together with relicts of some unaltered xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of all the three varieties of chrome-diopside suggest them to represent disaggregated xenocrysts of mantle-derived garnet peridotite. Temperature-pressure estimates for chrome-diopside xenocrysts ranges from 895 - 1026 °C (± 30 °C) and 32 – 38 kbar respectively and correspond to depth range of 106 – 127 km. The study reveals that lithospheric thickness during the Neoproterozoic beneath the western Dharwar craton was at least ~115 km and is similar in composition to that of the cratonic lithosphere found in the other cratonic domains.
In comparison to the eastern Dharwar Craton, the mantle-derived xenocrysts/xenoliths are extremely rare or even unreported from the western Dharwar Craton, southern India. A Neoproterozoic (ca. 800-900 Ma) lamprophyre cropping out in the Mysuru area of southern India contains chrome-diopside xenocrysts (Cr2O3 content varying from 0.2 – 1.23 wt%) which provide important evidence about the pressure-temperature conditions and lithospheric thickness beneath the western Dharwar Craton. Studied chrome-diopsides show compositional zoning which is lacking in the liquidus phases (amphiboles and feldspars) of the lamprophyre which additionally favors a non-cognate origin of the former. Based on the compositional zoning, all the chrome-diopside xenocrysts can be divided into three groups: (i) Group I- which are euhedral and show reverse zoning with increasing Cr-content from core to rim; (ii) Group II- which are characterized by fractures and resorption textures, show complex reverse zoning and display up to three distinct compositional layers, and (iii) Group III- which evidence the reaction of chrome-diopsides with lamprophyric melt and are marked by alteration phases, such as actinolite and chlorite, together with relicts of some unaltered xenocrysts. High Cr2O3, moderate MgO and low Al2O3 content of all the three varieties of chrome-diopside suggest them to represent disaggregated xenocrysts of mantle-derived garnet peridotite. Temperature-pressure estimates for chrome-diopside xenocrysts ranges from 895 - 1026 °C (± 30 °C) and 32 – 38 kbar respectively and correspond to depth range of 106 – 127 km. The study reveals that lithospheric thickness during the Neoproterozoic beneath the western Dharwar craton was at least ~115 km and is similar in composition to that of the cratonic lithosphere found in the other cratonic domains.
SUMMARY. Based on nearly I5oo published amphibole analyses the maximum possible A1 vi in hornblendes is shown to increase with increase of A1 iV. New analyses of hornblendes from amphibolecorundum rocks, with and without anorthite, are given and after critical examination of the available data it is concluded that the maximum verified AlVi-rich calciferous amphibole that approaches the closest to hypothetical tschermakite comes from a kyanite-bearing aluminous high-pressure-crystallized schist from Lukmanier, Switzerland. Pure natural edenite or ferroedenite is unknown, but a new analysis of the nearest known natural edenite, from Mysore, India, agrees with the postulated view that extraordinarily low temperatures are needed for edenite-ferroedenite crystallization, much below that possible in magmas and only rarely achieved in metamorphic rocks containing amphiboles. The limit of the approach of igneous hornblendes to edenite-ferroedenite and tremolite-ferroactinolite is outlined.At least I IOO eC is required for complete expulsion of water from some amphiboles. Aluminous hornblendesA FEW years ago, after examining 936 published amphibole analyses, Leake (I965) suggested that the maximum possible AF i in calciferous and subcalciferous amphiboles increases regularly as A1 i~ increases, where calciferous amphiboles are defined as possessing Ca > I-5O in the half unit cell and subcalciferous amphiboles have Ca between I.OO and 1.5 O. A number of amphiboles that appeared to contain more than the suggested maximum A1 iv for their A1 iv values were re-analysed and the previous values were shown to be erroneous. At that time samples of rocks containing hornblende with either corundum or kyanite were not available to the writer and the line limiting the maximum possible A1 vi was perspicaciously extrapolated to values close to Si 6.00, A1 i~ 2.oo, AF t I-4 o, mainly based on the re-analysis of an amphibole from an anorthite-vesuvianite rock. If this extrapolation is confirmed, then the above value would be the nearest that natural amphiboles approach to the hypothetical pure tschermakite Ca2MgaA12Si6A12(OH)zO2z, which contains 2.oo A1 vi. Recently, Kostyuk and Sobolev (I969, p. 79) have claimed that 55o chemical analyses of amphiboles 'reveal a negative correlation between A1 iv and A1 vi, 'in contrast to the data of Leake 0965)'. However, Saxena and Ekstrom (r97o) using the data given by Leake (I968a) have shown a positive correlation of A1 iv with A1 vi.
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