Metasomatic Epidote in Amphibolite-Replacing Rocks: An Indicator of Tectonic Processes Related to the Exhumation of Rocks of the Belomorian Belt. Petrography and Geochemistry
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Keywords:
Amphibole
Metasomatism
Hornblende
Titanite
Abstract Although the evolution of residual melts in magmatic systems controls their eruptability and ore-forming potential, their compositions are obscured in plutonic rocks by a protracted near-solidus evolution and the absence of interstitial glass. Here, we trace the evolution of residual melt compositions in rocks from the Strontian Intrusive Complex, Scotland, using the trace element chemistry of amphiboles, and titanites which are intergrown with amphibole rims. Laser ablation mapping reveals an abrupt change in certain trace elements in the amphibole rims, with sharp increases in Eu/Eu* and Sr/Y, and decreases in rare earth elements, Ta, Nb, and Ta/Nb ratios. Core-rim variations in these elements in titanite show the same variations as in amphibole, but are more gradual. By reconstructing the crystallisation sequence of the Strontian magmas using textural observations and thermobarometric estimates, we determine that amphibole cores crystallised prior to titanite saturation, but amphibole rims crystallised simultaneously with titanite. Using the trace element composition of the mineral phases and their modal abundance in the rock, with comparison to the whole-rock chemistry, we determine that titanite hosts the majority of the rare earth and high field strength element budget of the rocks. We therefore propose that the onset of titanite crystallisation had a profound effect on the trace element composition of late-stage residual melts at Strontian, which were inherited by the amphibole rims and subsequent titanites. This is supported by Rayleigh fractional crystallisation modelling, which demonstrates that the composition of amphibole rims cannot be explained without the influence of titanite. We therefore show that the saturation of trace element-rich phases in magmas represents a significant geochemical event in the petrogenesis of intermediate to silicic magmas. This has implications for provenance studies that attempt to reconstruct bulk rock compositions from mineral compositions, as the residual melts from which those minerals crystallise can be driven to significantly different compositions from the host magma by late-stage accessory phase crystallisation.
Titanite
Amphibole
Trace element
Incompatible element
Fractional crystallization (geology)
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Distinct equilibration temperatures, deformation and trace element characteristics are observed in amphibole-bearing and amphibole-free peridotite xenoliths from Nushan, Sino-Korean Craton, eastern China. Amphibole-free peridotites are predominantly deformed, fine-grained (∼1 mm) and equilibrated at 990–1110°C. Their cpx are characterized by either light rare earth element (LREE)-depleted or relatively flat REE patterns with only a slight depletion in high field strength elements (HFSE). LREE enrichment is generally associated with Fe-rich samples, consistent with 'wall-rock' metasomatism adjacent to basaltic veins. In contrast, amphibole-bearing peridotites are less deformed, coarse-grained (>3 mm) and display chemical zonation in the pyroxenes suggesting cooling from 1050 to 850°C. Their cpx show a large variation in LREE (Cen = 1·7–68) and almost constant heavy rare earth element (HREE) contents (Ybn = 9·8–11·6). The highest LREE contents occur in cpx from amphibole-rich samples, coupled with Fe enrichment, strong enrichment in Th and U, and marked depletion in the HFSE. These characteristics may be accounted for by combined 'wall-rock' and 'diffuse' metasomatism involving an agent rich in volatiles and incompatible elements. As such the Nushan xenoliths could represent samples from two spatially separate metasomatic aureoles. Conversely, the cryptic and modal metasomatism could be genetically related, because the amphibole-peridotites and Fe-rich amphibole-free samples show similar Sr–Nd isotopic ranges that are indistinguishable from those of the Cenozoic basalts from eastern China. The different metasomatic assemblage and the trace element composition can be accounted for in terms of P–T control on amphibole stability and progressive chemical evolution of asthenosphere-derived melts during upward migration. Trace element signatures and metasomatic assemblages, together with the fertile composition of the Nushan peridotites, suggest an origin as newly accreted lithosphere rather than as relic cratonic mantle. Metasomatism may have occurred after late Mesozoic lithospheric thinning, which marked a dramatic change in lithospheric architecture beneath the Sino-Korean Craton.
Metasomatism
Amphibole
Peridotite
Xenolith
Trace element
Rare-earth element
Primitive mantle
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Metasomatism
Amphibole
Trace element
Peridotite
Xenolith
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Amphibole
Metasomatism
Peridotite
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Amphibole
Metasomatism
Peridotite
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Amphibole
Hornblende
Closure temperature
Radiogenic nuclide
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Titanite
Pegmatite
Microcline
Massif
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Amphibole
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In connection with the revision of the Silurian area in Ireland, at present in progress by the staff of the Irish branch of the Geological Survey, a number of rock-specimens of the dykes occurring on the coast of Co. Down were lately petrographically examined by the writer, who detected in one of the slices a blue amphibole of secondary origin. As a mineral of this kind was not hitherto known to occur in sitû in Ireland, it may be of interest to put its discovery on record here and to give also a few details respecting its characteristics.
Amphibole
Hornblende
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The D/H ratios of hornblendes, epidotes, zoisites and white micas in the epidote-amphibolites from four localities in Japan have been determined together with the chemical compositions. The order of enrichment for deuterium in these minerals is as follows: epidote > white mica > hornblende > zoisite. A quite different situation for D/H ratios between coexisting zoisite and epidote is pointed out.
Hornblende
Tourmaline
Amphibole
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