Petrogenesis and metallogenesis of the Taihe gabbroic intrusion associated with Fe–Ti-oxide ores in the Panxi district, Emeishan Large Igneous Province, southwest China
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Layered intrusion
Large igneous province
Lithology
The remanent magnetization of the basic igneous rocks of Skye, the hypersthene gabbro of Ardnamurchan, and the layered gabbro of Rhum has been examined. All the rocks possessing stable magnetization were found to be reversely magnetized. This is believed to be due to the Earth's magnetic field being reversed during the half million years or so during which the rocks were formed.
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The Bushveld Complex, a layered mafic intrusion in South Africa, shows extreme vertical differentiation in terms of mineral compositions and modal proportions from dunite to ferrodiorite. In a continuous borehole core drilled through the uppermost 2·8 km of the intrusion, typical rocks range upwards from troctolite, through gabbronorite and ferrogabbronorite to ferrodiorite, with extreme examples of anorthosite, magnetitite and feldspathic pyroxenite. The An content of plagioclase has previously been determined for 420 samples and decreases upward from An78 to An36, with six minor, slow reversals. Variations in modal proportions of plagioclase have been calculated based on 2200 density determinations on whole-rocks. Forty-five anorthosite layers have been identified, ranging from 1 to 23 m thick. None of these layers is associated with the above-mentioned reversals in An content in plagioclase and nearly all have leucocratic rocks below and above, with more than the likely cotectic proportions of plagioclase. These observations argue against an origin for anorthosite related to magma addition or to supersaturation and oscillatory nucleation. Rhythmically pulsed crystallization, possibly associated with pressure changes, followed by crystal settling and sorting of minerals of different densities is a hypothesis consistent with all the observations. Twenty layers of magnetitite have been identified. There is a significant reversal in An content in the overlying plagioclase compared with the underlying sample across only one such layer. Again, this observation challenges hypotheses that such layers result from magma addition, but is consistent with a pressure-change hypothesis for triggering magnetite crystallization. The upper contacts of magnetitite layers that grade into anorthosite over many centimetres possibly also reflect settling and sorting. Rocks forming the uppermost 100 m of the intrusion contain the most sodic plagioclase compositions, demonstrating that there is no downward crystallizing roof facies. Furthermore, this uppermost 100 m section is depleted in plagioclase relative to its cotectic proportions. Hence, we find no evidence supporting flotation or prolonged suspension of plagioclase.
Anorthosite
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Fractional crystallization (geology)
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Large igneous provinces are exceptional intraplate igneous events throughout Earth's history. Their significance and potential global impact are related to the total volume of magma intruded and released during these geologically brief events (peak eruptions are often within 1–5 m.y. in duration) where millions to tens of millions of cubic kilometers of magma are produced. In some cases, at least 1% of Earth's surface has been directly covered in volcanic rock, being equivalent to the size of small continents with comparable crustal thicknesses. Large igneous provinces thus represent important, albeit episodic, periods of new crust addition. However, most magmatism is basaltic, so that contributions to crustal growth will not always be picked up in zircon geochronology studies, which better trace major episodes of extension-related silicic magmatism and the silicic large igneous provinces. Much headway has been made in our understanding of these anomalous igneous events over the past 25 yr, driving many new ideas and models. (1) The global spatial and temporal distribution of large igneous provinces has a long-term average of one event approximately every 20 m.y., but there is a clear clustering of events at times of supercontinent breakup, and they are thus an integral part of the Wilson cycle and are becoming an increasingly important tool in reconnecting dispersed continental fragments. (2) Their compositional diversity in part reflects their crustal setting, such as ocean basins and continental interiors and margins, where, in the latter setting, large igneous province magmatism can be dominated by silicic products. (3) Mineral and energy resources, with major platinum group elements (PGEs) and precious metal resources, are hosted in these provinces, as well as magmatism impacting on the hydrocarbon potential of volcanic basins and rifted margins through enhancing source-rock maturation, providing fluid migration pathways, and initiating trap formation. (4) Biospheric, hydrospheric, and atmospheric impacts of large igneous provinces are now widely regarded as key trigger mechanisms for mass extinctions, although the exact kill mechanism(s) are still being resolved. (5) Their role in mantle geodynamics and thermal evolution of Earth as large igneous provinces potentially record the transport of material from the lower mantle or core-mantle boundary to the Earth's surface and are a fundamental component in whole mantle convection models. (6) Recognition of large igneous provinces on the inner planets, with their planetary antiquity and lack of plate tectonics and erosional processes, means that the very earliest record of large igneous province events during planetary evolution may be better preserved there than on Earth.
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Flood basalt
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Mineral assemblages and associated textures in the low and middle part of the Panzhihua intrusion indicate that plagioclase, olivine and Fe-Ti oxides were crystallized at a similar temperature range. Thus, it is possible to estimate variation of temperature, oxygen fugacity (fO2) and magma composition during the formation of the magnetite gabbro and gabbro in different cycles of the Panzhihua layered intrusion by means of compositions of plagioclase and olivine. Electron microprobe data indicate that plagioclase has small compositional variation (An58.0-52.5) from the low zone to the middle zone. Whereas, forsterite percentages of olivine (Fo) decrease significantly upwards from magnetite gabbro to gabbro within a single cycle unit. These features indicate that the Panzhihua intrusion was developed by replenishment of many pulses of Fe-Ti enriched magmas. Small and regular composition variations of plagioclase suggest that variations of fO2 and Fe3+/Fe2+ of the magma have little effect on crystallization of plagioclase. Therefore, compositions of plagioclase can be used to estimate temperature of crystallization of Fe-Ti oxides. In contrast, large variations of Fo of olivine within a cycle unit suggest that the compositions of olivine depend on values of Fe3+/Fe2+ and Fe2+/Mg of the magma, from which the olivine crystallized. Thus, Fo of olivine was used to rebuild variation of fO2during the formation of the magnetite gabbro and gabbro. Crystallizations of plagioclase of the lower and middle zones of the Panzhihua intrusion occurred between 1079℃ and 1121℃, we estimate that Fe-Ti oxide probably crystallized at the same temperature range as well. On the other hand, decrease of Fo of olivine from magnetite gabbro to gabbro within a cycle unit indicate that fO2 decreased during fractional crystallization of a new replenished magma. This conclusion is consistent with previous experimental studies about the fO2 variations during fractional crystallization process at a system closed to oxygen.
Mineral redox buffer
Layered intrusion
Fractional crystallization (geology)
Magma chamber
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Layered intrusion
Large igneous province
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Layered intrusion
Large igneous province
Lithology
Igneous petrology
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Abstract The Eulogie Park Gabbro comprises approximately 65 saucer‐shaped layers consisting of olivine gabbros, ferrigabbros, troctolites, leucogabbros, gabbros and magnetite‐rich rocks. The intrusion is divisible into three main parts: a lower portion consisting mainly of ferrigabbros and magnetite‐rich layers, a middle portion composed of troctolites and olivine gabbros, and an upper portion consisting mainly of olivine gabbros and leucogabbros. The minimum stratigraphic thickness is 3,000 ft (900 m). Modal compositions and textures exhibit marked changes across the boundaries of the layers. Mineral compositions vary irregularly with height in the layered sequence, and small oscillations are common. Compositional data suggest that temperature increased during the first half of the crystallisation period, and then remained fairly constant. Chemical and petrographie studies of the iron‐titanium oxides indicate that the primary phase was an ulvöspinel‐magnetite solid solution. Subsolidus oxidation resulted in magnetite‐ilmenite intergrowths, which also contain exsolved pleonaste. The oxygen fugacity, which controlled the crystallisation and exsolution behaviour of the iron‐titanium oxides, is thought to have beeen affected by the fugacity of sulphur. Many features of this unusual intrusion are not obviously related to the conditions of crystallisation commonly supposed to exist in basic intrusions, and most of the anomalies have not been satisfactorily resolved. Undercooling and rhythmic concentration changes are believed to have been responsible for both the small‐scale rhythmic layering and the larger scale layering in the intrusion.
Layered intrusion
Fugacity
Layering
Mineral redox buffer
Ilmenite
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