Preserved initial in apatite from altered felsic igneous rocks: A case study from the Middle Proterozoic of South Australia
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Felsic
Isotopes of strontium
Felsic rocks are minor in abundance but occur ubiquitously in International Ocean Discovery Program Hole U1473A, Southwest Indian Ridge. The trace element abundances of high-Ti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied felsic rocks support crystallization origin from highly-evolved melts, probably controlled by fractional crystallization. Based on geochemical criteria and texture of the mineral assemblage in felsic rocks and their relationship with host gabbros, they can be divided into three types: (1) Felsic rock with sharp boundaries is formed when felsic melt intrudes into fractures of host gabbros, resulting in minimal interaction between the melt and the wall minerals. (2) Replacive felsic rock, which is characterized by a pseudomorphic replacement of minerals in the host gabbro. This vein type is caused by the replacement of the host mineralogy by minerals in equilibrium with the felsic melts. (3) Felsic rock with diffused boundaries is formed either by infiltration of felsic melt into the solidifying gabbro body or crystallization of interstitial melts. Infiltration modes of felsic melts are likely controlled by the temperature condition of the cooling host gabbros.
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Melt inclusions
Fractional crystallization (geology)
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Abstract: I– and S‐type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S‐type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I‐type analogues. Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S‐types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S‐type granites are likely to be closely related in origin, with the former comprising both restite‐rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation. We propose a subdivision of I‐type granites into two groups, formed at high and low temperatures. The high‐temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low‐temperature I–type granites, the compositions extend to lower SiO 2 contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO 2 in the more mafic rocks. While the high‐temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low‐temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low‐temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite. High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H 2 O.
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Fractional crystallization (geology)
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Ultramafic rock
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Massive sulphide deposits are closely associated with felsic volcanism. This association is believed to be genetic and it forms the cornerstone for most exploration programs, but unfortunately not all felsic volcanic rocks contain ore. It seems likely that ore-bearing felsic volcanic rocks have a different genetic history from those that are barren and, if this is so, these differences should be reflected in their REE geochemistry.A preliminary study of REE in Archean felsic volcanic rocks has shown that those associated with ore have flat REE patterns with well-developed Eu anomalies whereas those from barren volcanic rocks have steep REE patterns with weak or absent Eu anomalies. The felsic volcanic rocks associated with ore can be subdivided into two types: tholeiitic and calc-alkaline. Kam-Kotia, Matagami, and South Bay are tholeiitic whereas Sturgeon Lake, Golden Grove, and Kuroko are calc-alkaline.The well-developed Eu anomalies in the ore-related felsic volcanic rocks indicate that the melt has undergone a high degree of fractional crystallization en route to the surface, suggesting the existence of a subvolcanic magma chamber below the orebody. The characteristic REE patterns of the ore-associated felsic volcanics should help mining companies in area selection for massive sulphide exploration.
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Fractional crystallization (geology)
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Felsic orthogneisses with tonalitic and granodioritic compositions are dominant in the ultrahigh-temperature (UHT) metamorphic terrane of the Archaean-early Proterozoic Napier Complex, East Antarctica. Antiperthitic or mesoperthitic ternary feldspar occurs in addition to Qtz, Opx with/without Cpx in these felsic orthogneisses. Such feldspar grains commonly display zonal structure with respect to the volume of exsolution lamella; lamellae-enriched core and lamellae-free rim. Pre-exsolution one-phase compositions have been recovered separately for the feldspar core (an:ab:or = 20-28:53-56:17-28 in tonalitic samples and an:ab:or = 15-17:37-48:36-48 in granodioritic samples) and for the whole feldspar grain (an:ab:or = 24-29:58-63:13 in tonalitic samples and an:ab:or = 18-20:45-56:24-37 in granodioritic samples). Application of feldspar thermometry for these recovered one-phase compositions and the feldspar cores yield a temperature range of 940-1100 °C, which is relatively higher temperatures than the whole feldspar grain giving > 850-1070 °C. These compositional differences between core and rim of single feldspar grains are probably due to either sub-solidus or melt-related processes. The compositional and textural information of ternary feldspars in felsic orthogneisses can be therefore potential tools for understanding UHT crustal processes.
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Alkali feldspar
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